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  • 1.
    André, Alann
    et al.
    Swerea SICOMP AB, Mölndal.
    Kliger, Robert
    Division of Structural Engineering, Chalmers University of Technology.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Compression failure mechanism in small scale timber specimens2014In: Construction and Building Materials, ISSN 0950-0618, E-ISSN 1879-0526, Vol. 50, p. 130-139Article in journal (Refereed)
    Abstract [en]

    Understanding the failure mechanism of wood loaded in compression parallel to the grain has been shown to be an important parameter in the design of timber beams strengthened with fibre-reinforced plastics (FRP). In this paper, a constitutive relationship for wood under uniaxial compression load parallel to the grain was determined experimentally. Several parameters, such as silviculture, moisture content and radial position in the log in relation to the pith from where the specimen was sawn, were considered. Small clear-wood specimens were used. The strain localisation in the failure region (kinkband) was monitored using the digital image correlation method. The results show that silviculture and moisture content are two very important parameters which influence the compression failure mechanism. Furthermore, there is a significant difference in behaviour between specimens from the juvenile region of the log and specimens from mature wood. Based on experimental results, two numerical models were built, considering either a global or a local constitutive relationship. It was demonstrated that both numerical models yield accurate results and that, depending on the experimental equipment available, a constitutive relationship could be extracted and used as input in these numerical models.

  • 2.
    André, Alann
    et al.
    Swerea SICOMP AB.
    Nilsson, Sören
    Swerea SICOMP AB.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Finite element delamination study of a notched composite plate under flexural loads2009In: ICCM 17, Edinburgh: 17th International Conference on Composite Materials ; 27 Jul 2009 - 31 Jul 2009, Edinburgh International Convention Centre, Edinburgh, UK, London: IOM Communications , 2009Conference paper (Refereed)
    Abstract [en]

    The delamination process in notched composite plates under flexural loading has been investigated using finite element analysis. Cohesive elements implemented in the commercial finite element package ABAQUS have been used in the region around the drilled-hole, and positioned between layers where delamination was observed during experiments presented in an accompanying paper. The delamination initiation and subsequent propagation was studied between the layers at the tension side separately and simultaneously. For all FE models, the load displacement curve was in good agreement with the one from experiments. However, the amount of damage reported from the fractography study was more extensive than that predicted by the models. Finally, it was shown that the models with only one cohesive layer show significantly different results to that of the model with four cohesive layers in terms of size of the degradation area.

  • 3. Asp, Leif
    Activity: International Conference on Composites Testing and Model Identification, COMPTEST 2008: the key to matrix dominated composites failure2008Conference paper (Other (popular science, discussion, etc.))
  • 4.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Finite element delamination study of a notched composite plate under flexural loads2010In: Journal of Materials Science and Engineering. A, ISSN 1934-8959, Vol. 4, no 8, p. 66-73Article in journal (Refereed)
  • 5.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Fracture and yield predictions for epoxies in composite-like stress state1994Licentiate thesis, comprehensive summary (Other academic)
  • 6. Asp, Leif
    Local models for NCF composite materials mechanical performance prediction2007In: CD-ROM proceedings of the Sixtennth International Conference on Composite Materials, July 8 - 13, 2007, Kyoto, Japan: A giant step towards environmental awareness: from green composites to aerospace / [ed] Kazuro Kageyama, Kyōto, 2007Conference paper (Refereed)
    Abstract [en]

    This paper presents models for NCF reinforced composite materials performance prediction developed within the European research project FALCOM. The research project was conducted during the years 2002 and 2005 and was led by QinetiQ, UK. In total nine European countries were represented in FALCOM. The models consider material heterogeneity on three scales. On the micro-scale homogenization of the fibre bundle is performed. On the meso-scale formulations of Representative Volume Elements with different degrees of sophistication are defined. Finally on the macro-scale models span from straightforward employment of laminate theory to full through-thickness 3D-representation of the meso-scale features. The paper illustrates the diversity of local performance models for NCF composite analysis presenting a selection of models developed in the FALCOM project.

  • 7. Asp, Leif
    Multifunctional composite materials for energy storage in structural load paths2013In: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 42, no 4, p. 144-149Article in journal (Refereed)
    Abstract [en]

    This paper presents an overview of the research performed to date by a Swedish interdisciplinaryteam of scientists striving to develop multifunctional composite materials for storage of electric energy in mechanical load paths. To realise structural batteries from polymer composites, research pursued on carbon fibres for use as negative electrode in the battery as well as on polymer electrolytes for use as polymer matrix in the composite is reported. The work on carbon fibres comprises characterisation of the electrochemical capacity of commercial carbon fibre grades and how this is affected by mechanical load. Co-polymers are studied for their multifunctional performance with respect to lithium ion conductivity and stiffness. Also, rational processing of these polymer electrolytes and the effect of processing on their properties are addressed

  • 8.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Structural battery materials2012In: Proceedings of the 15th European Conference on Composite Materials / [ed] Marino Quaresimin; Laszlo Kollar; Leif Asp, Venice, 2012Conference paper (Refereed)
    Abstract [en]

    Since four years Swerea SICOMP has been leading a team of Swedish researchers developing structural battery materials from polymer composites. The research performed in the Swedish project KOMBATT (Lightweight structural energy storage materials) is funded by the Swedish foundation for strategic research (SSF). The research addresses two technical challenges in particular. Firstly, solid polymer electrolytes that efficiently transfer loads in the composite and simultaneously transports lithium ions, while being electrically insulating, must be developed. Secondly, the ability of the reinforcement, i.e. The carbon fibres, to intercalate lithium ions as part of the chemical redox reactions, while maintaining its mechanical properties must be assured. This paper is the first in a series of papers at this conference from the KOMBATT project team and presents background and overview of the project.

  • 9. Asp, Leif
    The effects of moisture and temperature on the interlaminar delamination toughness of a carbon/epoxy composite1998In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 58, no 6, p. 967-977Article in journal (Refereed)
    Abstract [en]

    HTA/6376C composite has been investigated for influence of temperature and moisture content on the interlaminar delamination toughness in mode I, mode II and mixed mode conditions. Dry and moisture-saturated specimens were tested over the temperature range - 50 °C to 100 °C. Evaluation methods based on load/displacement and load measurements were employed. In pure mode II the critical strain-energy release rate drops with moisture content and increase in temperature. In mixed mode the critical strainenergy release rate also decreases with moisture content, but no general trends in the dependence on temperature is observed. The critical strain-energy release rate in pure mode I is unaffected by changes in moisture content and was found to increase slightly at elevated temperatures. During crack propagation, enhanced fiber bridging due to increases in temperature and moisture content promotes R-curve behavior in the pure mode I tests. The resulting mode mixity of the mixed-mode bending (MMB) tests is found to be severely affected by the evaluation methods. Methods based on load measurements only are considered to give unreliable strain-energy release rates as the measured compliance/displacement relationships were found to be non-linear even prior to crack growth. Further studies are needed to assess the mixed-mode ratio in the MMB test.

  • 10.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Transverse failure initiation in polymer composites1995Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Transverse failure is one of the most important failure modes in polymer composites. The phenomenon often causes the first deviations from nonlinear laminate behavior. Also, in pressure vessels and pipes, fluid leakage through a path of transverse cracks is often the limiting design criterion. In the present work, experimental and theoretical studies focused on the micromechanical level have been carried out. The objective was to investigate transverse failure initiation in the matrix. The other major mechanism of failure initiation, fiber/matrix debonding, was not considered. The triaxial nature of the matrix stress state in glass fiber/epoxy was confirmed by finite element analysis. Experimental results for glassy epoxies subjected to composite-like stress states demonstrated large reductions in strain to failure as compared with uniaxial loading. The triaxial stress state is therefore by itself a sufficient explanation for the low transverse strain to failure in polymer composites. Plastic yielding in the matrix was demonstrated not to be the cause of failure initiation. Instead cavity induced cracking was suggested as a failure mechanism. A criterion was proposed based on a critical value for the dilatational energy density. Comparison with experimental results for epoxies subjected to a variety of multiaxial load-cases supported the criterion. Additional support was obtained from comparison with experimental results in the literature for transverse failure of glass fiber/epoxy at different fiber contents. Although the epoxy matrix was different from those in the present study, general trends in data were supported by predictions based on the criterion and finite element analysis. Thermal residual stresses were found to be important for high fiber contents. Based on the criterion, a conservative estimate of composite strain to failure was obtained. This is reasonable since the criterion predicts initiation, not final failure. Based on the model, effects from changes in constituent properties were examined in a parametric finite element analysis. Fiber modulus was found to strongly influence transverse failure. Introduction of a third phase interphase between fiber and matrix was also investigated. Beneficial results on transverse failure strain caused by matrix initiation was observed for thin rubbery interphases.

  • 11.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    A biaxial thermomechanical disk test for glassy polymers1997In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 37, no 1, p. 96-101Article in journal (Refereed)
    Abstract [en]

    Failure criteria for polymers need to include effects from the stress state. For this reason, biaxial test results are of interest. However, biaxial test methods usually require expensive equipment. In the test method presented here, a disk of epoxy is bonded between a steel ring and a steel disk. The temperature is then lowered until fracture is observed. Experiments were performed on three different glassy epoxy polymers. The biaxial stress state was analyzed by finite element analysis and by an approximate analytical model. Experimental observations support the ability of the method to provide material property data. An approximate analytical model was found sufficiently accurate for stress analysis and determination of the stress state at failure

  • 12.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Gudmundsson, Peter
    Department of Solid Mechanics, Royal Institute of Technology.
    Effects of a composite-like stress state on the fracture of epoxies1995In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 53, no 1, p. 27-37Article in journal (Refereed)
    Abstract [en]

    The strain to failure of a transversely loaded composite is much lower than for the pure matrix in uniaxial tension. Several studies of composites suggest the triaxial matrix stress state as one of the explanations. In order to investigate this experimentally, a triaxial tensile test previously used for rubbers (the poker-chip test) was successfully applied to four epoxies in the glassy state. The chosen specimen geometry mimicked the most severe stress state in the matrix as determined by finite element analysis of a transversely loaded glass-fiber/epoxy composite. The poker-chip strains to failure in the primary loading direction were 0.5-0.8%, whereas uniaxial strains to failure were 1.8-7%. The triaxial stress state in composite matrices may therefore by itself be a sufficient explanation for low values of transverse composite strains to failure

  • 13.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Criterion for crack initiation in glassy polymers subjected to a composite-like stress state1996In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 11, p. 1291-1301Article in journal (Refereed)
    Abstract [en]

    Three epoxy systems of interest as composite matrix materials are examined for their yielding and failure behavior under uniaxial, biaxial and triaxial stress states. Yield criteria applicable to glassy polymers, i.e. accounting for the hydrostatic stress effect on the deviatoric stress to yielding, are assessed. It is found that under stress states resembling those in matrix constrained between fibers, e.g. equibiaxial and equitriaxial tension, yielding is suppressed while brittle failure, presumably caused by crack growth from cavitation, occurs. A criterion for this mode of failure is proposed as the critical dilatational strain energy density. Experimental data are found to support this criterion.

  • 14.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Effects of fiber and interphase on matrix-initiated transverse failure in polymer composites1996In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 45, no 6, p. 657-665Article in journal (Refereed)
    Abstract [en]

    Failure initiation in polymer-matrix composites loaded transverse to the fibers is investigated by a numerical parametric study where the effects of constituent properties, interphase properties and thickness are examined. Failure initiation in the matrix only is studied, interfacial debonding not being considered. Two modes of failure - yielding and cavitation-induced brittle failure - are examined. A criterion for the cavitation-induced brittle failure has been proposed previously and failure prediction based on this criterion was found to agree with experimental data for a glass-fiber-reinforced epoxy. The present study shows that the elastic modulus of fibers has a large effect on the stress and strain to failure initiation. A rubbery interphase material is found in most cases to have a beneficial effect. The site at which failure initiates and the governing mode of failure initiation are also affected by the fiber modulus and the interphase properties

  • 15.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars A.
    Luleå tekniska universitet.
    Talreja, Ramesh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Prediction of matrix-initiated transverse failure in polymer composites1996In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 56, no 9, p. 1089-1097Article in journal (Refereed)
    Abstract [en]

    A study has been conducted of failure in unidirectionally-reinforced fiber composites loaded in tension normal to the fibers. The case considered is when this failure is governed by failure of the matrix rather than fiber/matrix debonding. Both yielding and cavitation-induced brittle failure of the matrix are considered. The latter mode of failure was suggested previously as the likely mode to occur in epoxies under stress states that are purely or nearly hydrostatic tension. Three fiber packing arrangements (square, hexagonal and square-diagonal) with different fiber volume fractions were studied numerically by a finite element method to determine the local stress states. It is found that cavitation-induced brittle failure occurs much before yielding in all cases. Experimental data taken from the literature support this finding.

  • 16.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Berglund, Lars
    Luleå tekniska universitet.
    Gudmundson, P.
    Mechanical behavior of epoxy resins in uniaxial and triaxial loading1993In: High temperature ceramic matrix composites: 6th EuropeanConference on Composite Materials, 20 - 24 September 1993, Bordeaux; HT-CMC / [ed] M. Neitzel; J.C. Lambert, Cambridge: Woodhead Publishing Materials , 1993, p. 323-328Conference paper (Refereed)
  • 17.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Brandt, F.
    Effects of pores and voids on the interlaminar delamination toughness of a carbon/epoxy composite1997In: Proceedings / Eleventh International Conference on Composite Materials, Gold Coast, Queensland, Australia, 14th - 18th July 1997: ICCM-11 / [ed] Murray L Scott, Melbourne: Woodhead Publishing Materials , 1997, p. 322-331Conference paper (Refereed)
  • 18.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Edgren, Fredrik
    Sjögren, A.
    Effects of stitch pattern on the mechanical properties of non-crimp fabric composites2004In: From nano-scale interactions to engineering structures: ECCM 11, 11th European Conference on Composite Materials ; May 31 - June 3, 2004, Rhodes, Greece / [ed] Costas Galiotis, Rhodos: European Society for Composite Materials , 2004Conference paper (Refereed)
  • 19.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Juntikka, R.
    Swerea SICOMP AB, Mölndal.
    High velocity impact on NCF reinforced composites2006In: From the science of composites to engineering applications: the dawning future of composites: ECCM 12, 12th European Conference on Composite Materials, Biarritz, 29th August - 1st September 2006 ; [conference proceedings] / [ed] Jacques Lamon, Biarritz, 2006Conference paper (Refereed)
  • 20. Asp, Leif
    et al.
    Juntikka, Rickard
    Swerea SICOMP AB, Mölndal.
    High velocity impact on NCF reinforced composites2009In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 69, no 9, p. 1478-1482Article in journal (Refereed)
    Abstract [en]

    In the current paper, a series of high velocity impact tests using Φ50 and Φ25 mm ice spheres and 0.32 g granite stones on non-crimp fabric (NCF) composite plates are reported. The impact tests were performed using an air gun and velocities between 100 m/s and 199 m/s. The impact events were monitored using a high-speed camera, with a 20 million frames per second capacity, as well as by a displacement transducer for out-of-plane displacement measurements of the impacted plates. NCF composite plates of two different thicknesses were impacted. The composites were manufactured from carbon fibre and epoxy resin by vacuum infusion.Engineering type models were employed to predict impact response and impact damage formation. Comparison between predicted and resulting damage for the impact test validates the application of a semi-empirical model for predicting impact velocity thresholds for damage formation. Analytical models relying on the assumption of solid impact bodies cannot be employed for these types of impact.

  • 21.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Modelling stiffness and strength of non-crimp fabric composites: semi-laminar analysis2011In: Non-crimp fabrics composites: manufacturing, properties and applications, Cambridge: Woodhead Publishing Materials , 2011, p. 402-438Chapter in book (Refereed)
  • 22.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Olsson, Robin
    Swerea SICOMP AB, Mölndal.
    Multiscale modelling of non-crimp fabric composites2012In: Proceedings of the ASME International Mechanical Engineering Congress and Exposition--2012: presented at ASME 2012 International Mechanical Engineering Congress and Exposition, November 9-15, 2012 Houston, Texas USA, New York: American Society of Mechanical Engineers , 2012, Vol. 3, p. 581-590Conference paper (Refereed)
    Abstract [en]

    Damage initiation and evolution in NCF composites leading to final failure includes a multitude of mechanisms and phenomena on several length scales. From an engineering point-of-view a computational scheme where all mechanisms would be explicitly addressed is too complex and time consuming. Hence, methods for macroscopic performance prediction of NCF composites, with limited input regarding micro- And mesoscale details, are requested. In this paper, multi-scale modelling approaches for in-plane transverse strength of NCF composites are outlined and discussed. In addition a simplistic method to predict transverse tensile and compressive strength for textile composites featuring low or no fibre waviness is presented

  • 23.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Nilsson, Karl-Fredrik
    SICOMP AB, Swedish Institute of Composites.
    Delamination criticality in slender compression-loaded composite panels2002In: Key Engineering Materials, ISSN 1013-9826, E-ISSN 1662-9795, Vol. 221-222, p. 3-16Article in journal (Refereed)
    Abstract [en]

    In this paper, a damage tolerance model based on the assumption of delamination criticality in compression loaded slender composite panels is outlined. In particular, the verification of the model by comparison between numerical predictions and experimental results is reviewed. Growth of shallow delaminations in slender panels is shown to be promoted by the global buckling of the panel. Consequently, care must be taken if structures with delaminations are to be allowed to buckle. In the paper, application of the model for aircraft design is briefly discussed. The overall predicted panel behaviour agrees with observations for test coupons. However, very small geometrical changes are shown to have tremendous effects on the predicted behaviour. Consequently, in structural design one must consider the sensitivity of geometrical conditions on the predicted behaviour. Therefore, reduction of the structural item into a design element is suggested. To generate conservative designs the suggested design element is to represent the worst case

  • 24. Asp, Leif
    et al.
    Nilsson, Sören
    Aeronautical Research Institute of Sweden.
    Singh, Sunil
    Mechanical Sciences Sector, DERA Farnborough.
    An experimental investigation of the influence of delamination growth on the residual strength of impacted laminates2001In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 32, no 9, p. 1229-1235Article in journal (Refereed)
    Abstract [en]

    This study concerns an experimental investigation to establish data for validation of residual strength models for impacted composite panels. The work focuses on compression tests of panels with embedded artificial delaminations at various depths. Accompanying tests on undamaged and impact-damaged panels are reported and the relevance of the tests on artificially delaminated panels is assessed. In the experiments both the artificially delaminated and the impacted plates failed by delamination growth. Consequently, the same mechanism governed failure in the two cases. Hence, the artificially delaminated plate test is reliable for validation of methods developed for analysis of the residual strength of impact-damaged panels. However, for impacted plates, the load at global plate buckling was consistently 10% lower than that of the artificially delaminated plates and 20% lower than that of the undamaged plates. Hence, conservative prediction of the global buckling load of an impacted composite panel requires methods that consider influence of stiffness reduction of the damaged zone

  • 25. Asp, Leif
    et al.
    Sjögren, Anders
    SICOMP AB, Swedish Institute of Composites.
    Greenhalhg, Emile S.
    Delamination growth and thresholds in a carbon/epoxy composite under fatigue loading2001In: Journal of composites technology & research, ISSN 0884-6804, E-ISSN 1945-7537, Vol. 23, no 2, p. 55-68Article in journal (Refereed)
    Abstract [en]

    This paper presents a study on delamination growth in Mode I, Mode II and mixed mode under fatigue loading in an HTA/6376C composite. The computed slopes of the modified Paris plots were high. Therefore, threshold values of the strain energy release rate for delamination growth were determined. Low fatigue threshold values revealed a significant effect of fatigue loading. The largest effect was found for the ENF test (Mode II) for which the fatigue threshold value was only 10% of the critical strain energy release rate in static tests. Threshold values for MMB (mixed mode) and DCB (Mode I) tests were 15% and 23% of the static values, respectively. Fractographic evaluation revealed identical initial failure mechanisms in fatigue and static loading conditions for the ENF specimen. The ENF specimen failed by formation and coalescence of microcracks. The low fatigue threshold for the ENF specimen was explained by microscopical observations on the specimen edge. It was also shown that the fracture surfaces generated in static and fatigue DCB and MMB tests were similar

  • 26.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sjögren, B.A.
    Berglund, Lars A.
    Prediction of failure initiation in polypropylene with glass beads1997In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 18, no 1, p. 9-15Article in journal (Refereed)
    Abstract [en]

    The effect of glass bead content and residual stresses on failure initiation in isotactic polypropylene composites has been investigated by finite element analysis for the cases of interfacial debonding, plastic yielding, and cavitation. Residual thermal stresses are demonstrated to have a large effect on global failure initiation stress. Yielding and cavitation occur at higher global stresses than debonding. Modeling results, as well as previous experimental data, support debonding as the initial failure mechanism

  • 27.
    Asp, Leif
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Szpieg, Magdalena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Wysocki, Maciej
    Swerea SICOMP AB.
    Mechanical performance and modelling of a fully recycled modified CF/PP composite2012In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 46, no 12, p. 1503-1517Article in journal (Refereed)
    Abstract [en]

    A fully recycled carbon fiber reinforced maleic anhydride grafted polypropylene (MAPP)-modified polypropylene (rCF/rPP) composite material has been developed and characterized. This new composite was manufactured employing papermaking principles, dispersing the recycled carbon fibers (rCF) in water, and forming them into mats. Two layers of the recycled polypropylene (rPP) films manufactured using press-forming were sandwiched between three rCF preform layers in a stack. The stack was heated and press-formed resulting in a composite plate with a nominal thickness of 1.20 mm and a fiber volume fraction of 40%. A series of tensile tests using rectangular specimens cut in four different directions (0°, 90°, ± 45°) in the composite plate were performed to confirm in-plane material isotropy. Models to predict stiffness and strength of the short fiber rCF/rPP composite were also employed and validated using experiments. The models were found to be in good agreement with experimental results. Fiber length distribution measurements were performed before (unprocessed) and after (processed) composite manufacturing to investigate the influence of processing on fiber degradation. The results revealed a significant reduction in fiber length by the press-forming operation. To model the viscoelastic and viscoplastic responses of the composite an inelastic material model was employed and characterized using a series of creep and recovery tests. From the creep tests, it was found that the time and stress dependence of viscoplastic strains follows a power law. The viscoelastic response of the composite was found to be linear in the investigated stress range. The material model was validated in constant stress rate tensile tests and the agreement was good, even close to the rupture stress.

  • 28.
    Bachinger, A.
    et al.
    Composite Structures, Swerea SICOMP AB.
    Marklund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Composite Structures, Swerea SICOMP AB.
    Rössler, J.
    Hellström, Pär
    Composite Structures, Swerea SICOMP AB.
    Asp, Leif
    Composite Structures, Swerea SICOMP AB.
    Stiffness-modifiable composite for pedestrian protection2014In: 16th European Conference on Composite Materials, ECCM 2014: Seville, Spain, 22 - 26 June 2014, European Conference on Composite Materials, ECCM , 2014Conference paper (Refereed)
    Abstract [en]

    A novel functional material allowing stiffness-reduction upon external stimulation was developed. Implementation of such technology in the design of a car front has high potential to result in increased protection of vulnerable road users (VRUs). The composite material is obtained by coating carbon fibres with a thermoplastic polymer in a continuous process, followed by infusion with an epoxy resin. The process is scalable for industrial use. The coating process was optimized regarding coating efficiency, energy consumption, risks involved for operating personnel and environment, and tailored to gain the optimal coating thickness obtained from numerical calculations. A drastic decrease in transversal stiffness could be detected for the composite material by dynamic mechanical thermal analysis (DMTA), when the temperature was increased above the glass transition temperature of the thermoplastic interphase. The ability of the material to achieve such temperature and associated reduction in stiffness by the application of current was verified using a special 3-point bending setup developed for this task.

  • 29.
    Banco, N.
    et al.
    EPS-University of Girona.
    Gamstedt, E.K.
    Kungliga tekniska högskolan, KTH.
    Asp, Leif
    Costa, J.
    EPS-University of Girona.
    Mixed-mode delamination growth in carbon-fibre composite laminates under cyclic loading2004In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 41, no 15, p. 4219-4235Article in journal (Refereed)
    Abstract [en]

    Delamination growth under fatigue loads in real composite components generally develops in a non-constant propagation mode. The aim of the investigation described in this article was to develop a model capable of predicting the fatigue delamination growth in a general case, under varying mode mix conditions. The crack growth development in essentially unidirectional laminates of carbon-fibre reinforced epoxy was analysed in terms of the Paris law for different constant propagation modes: mode I (double-cantilever beam test), mode II (end-notched flexure test) and different mixed-modes I/II (mixed-mode bending test). The dependence of the Paris law parameters on mode mix is compared with the existing models in the literature. It is shown that these models do not reproduce the non-monotonic dependence on mode mix which has been observed in experimental data. Therefore, an improved phenomenological model is introduced and compared with the experimental data obtained by other researchers. To check the ability of the model to predict variable mixed-mode fatigue delamination, the mixed-mode end-loaded split test was employed and the experimental results were compared to the predictions of the model. The underlying mechanisms responsible for the dependency of the crack propagation rates on the degree of mode mix are also discussed on the basis of fractographic analysis.

  • 30.
    Berglund, Lars A.
    et al.
    Luleå tekniska universitet.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Fracture of epoxies subjected to uniaxial and triaxial stress fields1994In: Deformation, yield and fracture of polymers: 9th International conference : Papers, Institute of Materials , 1994Conference paper (Refereed)
    Abstract [en]

    The uniaxial fracture properties of dogbone specimens are often used to estimate the performance of polymers in other loading situations, e.g. composite matrices. Although triaxial stress states are likely to affect the fracture behavior, this effect is difficult to quantify. In the present study, a method previously used for rubbers (the poker-chip method) was used to subject epoxies to a triaxial tension stress field. The method was successfully used and its validity supported by stress analysis and fractography. Results showed the fracture behavior of the poker-chip specimen to be dramatically different compared to the uniaxial case. A general decrease in strain to failure was observed for the triaxial test data. Yield criteria, shown to predict yielding in epoxies for biaxial stress states, suggest some other mechanism to control failure of the poker-chip specimen

  • 31. Blanco, N.
    et al.
    Gamstedt, E.K.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Costa, J.
    Mixed-mode delamination in a carbon-fibre composite laminate under cyclic loading2003In: ICCM 14: 14th International Conference on Composite Materials : July 14-18, 2003, San Diego, California, USA, Dearborn, Mich: Arctic Monitoring and Assessment Programme, 2003Conference paper (Refereed)
  • 32. Blanco, N.
    et al.
    Gamstedt, E.K.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mayugo, J.A.
    Costa, J.
    On the cyclic delamination in mixed mode in a composite2002Conference paper (Refereed)
  • 33. Blanco, Norbert
    et al.
    Costa, Josep
    Gamstedt, Kristofer
    Kungliga tekniska högskolan, KTH.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Experimental analysis of variable miced-mode fatigue delamination in a carbon fibre-epoxy laminate2006In: Proceedings of the 3rd International Conference on Composites Testing and Model Identification: CompTest 2006, Faculty of Engineering, University of Porto, Portugal , 2006, p. 52-53Conference paper (Other academic)
  • 34.
    Carlson, Tony
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    An experimental study into the effect of damage on the capacitance of structural composite capacitors2013In: Journal of Multifunctional Composites, ISSN 2168-4246, Vol. 1, no 2, p. 91-97Article in journal (Refereed)
    Abstract [en]

    This paper presents the work to characterise the effects of tensile induced matrix cracks on capacitance of structural composite capacitor materials. The study is based on earlier work within the field of multifunctional materials where mechanical and electrical properties have been characterised. Effects of damage on electrical properties have, however, not been covered by earlier studies. The structural capacitor materials were made from carbon fibre/epoxy pre-pregs as structural electrodes with thermoplastic PET as the dielectric separator. NaOH etching was used as a route for improved adhesion between the epoxy and PET to ensure matrix cracking in the CFRP electrodes occurred prior to delamination between the electrodes and the PET separator. A method to induce and measure the effect of the matrix cracks on electrical properties was successfully developed and used in this study. The method is based on a simple tensile test and proved to be quick and easy to perform with consistent results. The structural capacitor material was found to maintain its capacitance even after significant intralaminar matrix cracking in the CFRP electrodes from high tensile mechanical loads.

  • 35.
    Carlson, Tony
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Carbon fibre composites capacitors for short term electric energy storage in structural applications2011In: 18th International Conference on Composites Materials, ICCM 2011: Jeju; South Korea; 21 August 2011 through 26 August 2011, 2011Conference paper (Refereed)
  • 36.
    Carlson, Tony
    et al.
    Swerea SICOMP AB, Mölndal.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Plasma treatment: a route for improved adhesion between pet and epoxy in multifunctional composite capacitors?2012In: Proceedings of the 15th European Conference on Composite Materials / [ed] Marino Quaresimin; Laszlo Kollar; Leif Asp, Venice, 2012Conference paper (Refereed)
    Abstract [en]

    This paper presents an approach towards realising novel multifunctional polymer composites. A series of structural capacitor materials made from carbon fibre reinforced polymers have been developed, manufactured and tested. The capacitors were made using three thicknesses of DuPont Mylar A thermoplastic PET as dielectric separator employing carbon fibre/epoxy pre-pregs as structural electrodes. Plasma treatment was used as a route for improved epoxy/PET adhesion employing a number of treatment times, 5, 10, 15, 20 and 25s. The manufactured materials have been mechanically and electrically tested to evaluate their multifunctional efficiency. Plasma treatment have been shown to give some improvements to the interlaminate shear strength but not to any significant degree

  • 37.
    Carlson, Tony
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Structural carbon fibre composite/PET capacitors: Effects of dielectric separator thickness2013In: Composites Part B: Engineering, ISSN 1359-8368, E-ISSN 1879-1069, Vol. 49, p. 16-21Article in journal (Refereed)
    Abstract [en]

    This paper presents an approach towards realising novel multifunctional polymer composites with combined structural and electric energy storing ability. A series of structural capacitors were made using three thicknesses of DuPont Mylar A thermoplastic PET as a dielectric separator employing carbon fibre/epoxy pre-pregs as structural electrodes. Plasma treatment was used as a route for improved epoxy/PET adhesion. The manufactured materials were mechanically and electrically tested to evaluate their multifunctional efficiency.The multifunctional materials developed show good potential for replacing steel, aluminium and other materials with lower specific mechanical properties but do not match the high specific mechanical and electrical performance of monofunctional composites and capacitors.

  • 38.
    Carlson, Tony
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ordéus, Daniel
    Swerea SICOMP AB.
    Wysocki, M.
    Swerea SICOMP AB.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    CFRP structural capacitor materials for automotive applications2011In: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 40, no 6/7, p. 311-316Article in journal (Refereed)
    Abstract [en]

    In this paper, an approach towards realising novel multifunctional polymer composites is presented. A series of structural capacitor materials made from carbon fibre reinforced polymers have been developed, manufactured and tested. The structural capacitor materials were made from carbon fibre epoxy prepreg woven lamina separated by a polymer film dielectric separator. The structural capacitor multifunctional performance was characterised measuring capacitance, dielectric strength and tearing force. The developed structural carbon fibre reinforced polymer (CFRP) capacitor designs employing polymer film dielectrics (PA, PC and PET) offer remarkable multifunctional potential.

  • 39.
    Carlson, Tony
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Ordéus, Daniel
    Swerea SICOMP AB.
    Wysocki, Maciej
    Swerea SICOMP AB.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Structural capacitor materials made from carbon fibre epoxy composites2010In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 70, no 7, p. 1135-1140Article in journal (Refereed)
    Abstract [en]

    In this paper an approach towards realising novel multifunctional polymer composites is presented. A series of structural capacitor materials made from carbon fibre reinforced polymers have been developed, manufactured and tested. The structural capacitor materials were made from carbon fibre epoxy pre-preg woven laminae separated by a paper or polymer film dielectric separator. The structural capacitor multifunctional performance was characterised measuring capacitance, dielectric strength and interlaminar shear strength. The developed structural CFRP capacitor designs employing polymer film dielectrics (PA, PC and PET) offer remarkable multifunctional potential.

  • 40.
    Edgren, Fredrik
    et al.
    SICOMP AB, Swedish Institute of Composites.
    Asp, Leif
    Approximate analytical constitutive model for non-crimp fabric composites2005In: Composites. Part A, Applied science and manufacturing, ISSN 1359-835X, E-ISSN 1878-5840, Vol. 36, no 2, p. 173-181Article in journal (Refereed)
    Abstract [en]

    In this paper, a study set on development and validation of constitutive models to account for out-of-plane fibre waviness in Non-crimp fabric (NCF) composites is presented. For this purpose, a mathematical model based on Timoshenko beam theory applied on curved beams, representing wavy tows in a NCF composite layer is employed. Stiffness knock-down factors operating at the ply level are established and introduced in laminate theory. The developed models are validated on laminates by comparison between predictions and experimental data as well as by comparison with numerical results for a cross-ply laminate. Application of the models on NCF composite laminates (cross-ply and quasi-isotropic) reveals that the models successfully predict laminate elastic properties.

  • 41.
    Edgren, Fredrik
    et al.
    Swerea SICOMP AB, Mölndal.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Determination of strength parameters for NCF composites2006In: Proceedings of the 3rd International Conference on Composites Testing and Model Identification: CompTest 2006, Faculty of Engineering, University of Porto, Portugal , 2006, p. 66-67Conference paper (Other academic)
  • 42.
    Edgren, Fredrik
    et al.
    SICOMP AB, Swedish Institute of Composites.
    Asp, Leif
    Bull, Peter H.
    Kungliga tekniska högskolan, KTH.
    Compressive failure of impacted NCF composite sandwich panels: characterisation of the failure process2004In: Journal of composite materials, ISSN 0021-9983, E-ISSN 1530-793X, Vol. 38, no 6, p. 495-514Article in journal (Refereed)
    Abstract [en]

    In the present study, non-crimp fabric (NCF) composite face sheet sandwich panels have been tested in compression after impact (CAI). Damage in the face sheets was characterised by fractography. Compression after impact loaded panels were found to fail by plastic fibre microbuckling (kinking) in the damaged face sheet. Studies of panels for which loading was interrupted prior to failure revealed extensive stable kink band formation at several positions and in numerous plies. Kink bands initiated and propagated within a wide region close to the point of impact. In addition, kink bands initiated in zones with high shear stresses, away from the impact centre line. Consequently, the fractographic results from this investigation do not support the assumption of modelling the impact damage as an equivalent hole. To achieve accurate predictions of kink band initiation, the stress field must be known. The results from this study imply that bending effects caused by remaining dent or material eccentricities in the damaged region must be considered.

  • 43.
    Edgren, Fredrik
    et al.
    SICOMP AB, Swedish Institute of Composites.
    Asp, Leif
    Joffe, Roberts
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Failure of NCF composites subjected to combined compression and shear loading2006In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 66, no 15, p. 2865-2877Article in journal (Refereed)
    Abstract [en]

    Earlier studies have shown that formation of kink bands is the mechanism that is likely to govern failure of compression loaded non-crimp fabric (NCF) composite laminates. Because of this, a failure criterion for prediction of failure caused by kinking under multiaxial (axial compression and shear) loading has been adapted to a NCF composite system. The criterion has been validated for compression tests of quasi-isotropic laminates tested in uniaxial compression. By performing compression tests of the laminate at different off-axis angles, it was possible to vary the ratio of compressive axial stress/shear stress in the specimens. The test results proved that the criterion works well for predictions of kinking governed failure for the present material system. Detailed fractographic studies confirmed that formation of kink bands was the mechanism responsible for specimen failure. Kink bands were also found to develop at loads significantly lower than load at specimen failure.

  • 44. Edgren, Fredrik
    et al.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Sjögren, A.
    Compression after impact strength predictions for NCF composite structures2004In: From nano-scale interactions to engineering structures: ECCM 11, 11th European Conference on Composite Materials ; May 31 - June 3, 2004, Rhodes, Greece / [ed] Costas Galiotis, Rhodos: European Society for Composite Materials , 2004Conference paper (Refereed)
  • 45.
    Edgren, Fredrik
    et al.
    SICOMP AB, Swedish Institute of Composites.
    Mattsson, David
    Asp, Leif
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Formation of damage and its effects on non-crimp fabric reinforced composites loaded in tension2004In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 64, no 5, p. 675-692Article in journal (Refereed)
    Abstract [en]

    Non-crimp fabric (NCF) composites, manufactured by resin infusion techniques are one of the most promising next generation composite materials. They offer large potential for application in primary structures as they give excellent performance at low production costs. However, before NCF composites will be efficiently used in design, detailed understanding of governing micro mechanisms must be accumulated and described by predictive models. In the present study, NCF cross-ply laminates have been tested in tension. Intralaminar cracks caused in the 90° fibre bundle layers and their effect on laminate mechanical properties have been monitored. Occurrence of ‘novel' type of cracks propagating in the load direction (longitudinal cracks) is explained by a thorough FE analysis using an Representative Volume Element (RVE) approach, revealing stress concentrations caused by 0° fibre bundle waviness. Effects of damage on mechanical properties are modelled using modified micro mechanical models developed for analysis of conventional laminated composites. The analysis reveals mechanical degradation to be ruled by the crack opening displacement (COD). However, unlike traditional composites, transverse cracks do not generally extend through the entire thickness of the 90° layer, but are rather contained in single fibre bundles, limiting the COD

  • 46.
    Edgren, Fredrik
    et al.
    Swerea SICOMP AB, Mölndal.
    Soutis, C.
    University of Sheffield.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Damage tolerance analysis of NCF composite sandwich panels2008In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 68, no 13, p. 2635-2645Article in journal (Refereed)
    Abstract [en]

    This paper concerns development and validation of impact damage representations in carbon fibre non-crimp fabric reinforced face sheets for damage tolerance analysis of sandwich panels loaded in compression. For this purpose, experimental data accompanied by fractographic observations have been employed to scrutinize numerical predictions by state-of-the-art notch strength models. As a result, equivalent hole representations of visible impact damage (VID) and, more surprisingly, of the subtle barely visible impact damage (BVID) are recommended for reliable damage tolerance prediction of the compression after impact (CAI) load case for the investigated panels. This recommendation relies on the identification of the mechanisms controlling failure resulting in reliable damage tolerance predictions employing a linear cohesive zone model.

  • 47.
    Ekstedt, Sofia
    et al.
    Swerea SICOMP AB.
    Wysocki, Maciej
    Swerea SICOMP AB.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Structural batteries made from fibre reinforced composites2009In: ICCM 17, Edinburgh: 17th International Conference on Composite Materials ; 27 Jul 2009 - 31 Jul 2009, Edinburgh International Convention Centre, Edinburgh, UK, London: IOM Communications , 2009Conference paper (Refereed)
    Abstract [en]

    Structural, fibre reinforced, battery prototypes with two types of electrolyte matrix material, a gel and a solid polymer, have been manufactured. This was to confirm the concept of using carbon fibres as current collector in the anode as well as providing a mechanical load-carrying functionality. As a result, functioning batteries with gel electrolyte have been produced and their properties have been characterised.

  • 48.
    Ekstedt, Sofia
    et al.
    Swerea SICOMP AB.
    Wysocki, Maciej
    Swerea SICOMP AB.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Structural batteries made from fibre reinforced composites2010In: Plastics, rubber and composites, ISSN 1465-8011, E-ISSN 1743-2898, Vol. 39, no 3-5, p. 148-150Article in journal (Refereed)
    Abstract [en]

    Structural, fibre reinforced, battery prototypes with two types of electrolyte matrix material (a gel and a solid polymer) have been manufactured. This was to confirm the concept of using carbon fibres as current collector in the anode as well as providing a mechanical load-carrying functionality. As a result, functioning batteries with gel electrolyte have been produced and their properties have been characterised.

  • 49. Greenhalgh, E.
    et al.
    Asp, Leif
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Singh, S.
    Delamination resistance, failure criteria and fracture morphology of 0°/0°, 0°/5° and 0°/90° ply interfaces1999In: Deformation and fracture of composites: 18 - 19 March 1999, The Institution of Mechanics Engineering, London, UK, London: IOM Comm. , 1999, p. 43-52Conference paper (Refereed)
  • 50.
    Ireman, T.
    et al.
    SAAB AB.
    Asp, Leif
    Efficient design and certification of advanced composite structures2001In: Air & Space Europe, ISSN 1290-0958, E-ISSN 1879-1921, Vol. 3, no 3-4, p. 126-128Article in journal (Refereed)
    Abstract [en]

    An overview of the work performed in the Brite-Euram programme EDAVCOS: ‘Efficient design and verification of composite structures' is given. The development and validation of design and analysis methods for sandwich panels, stiffened panels, shear webs and bolted joints are briefly described. The review of current airworthiness procedures is outlined and some results are presented. Some general conclusions from the programme are also given.

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