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  • 1.
    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

  • 2. Fällström, Karl-Evert
    et al.
    Gustavsson, Håkan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Molin, Nils-Erik
    Wåhlin, Anders
    Transient bending waves in plates studied by hologram interferometry1989In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 29, no 4, p. 378-387Article in journal (Refereed)
    Abstract [en]

    Propagating bending waves are studied in plates made of aluminum and wood. The waves are generated by the impact of a ballistic pendulum. Hologram interferometry, with a double pulsed ruby laser as the light source, is used to record the out of plane motion of the waves. Elliptic-like fringes visualize differences in wave speed for different directions in the anisotropic plate and circular ones are obtained for the isotropic plate. The experimental data for the isotropic plate compare favorably with analytical results derived from the Kirchhoff-plate equation with a point impact of finite duration. A similarity variable is found when starting conditions are modeled as a Dirac pulse in space and time, that brings new understanding to the importance of specific parameters for wave propagation in plates. A formal solution is obtained for a point force with an arbitrary time dependence. For times much larger than the contact time, the plate deflection is shown to be identical to that from a Dirac pulse applied at the mean contact time. A method for determining material parameters, and the mean contact time, from the interferograms is hence developed.

  • 3. Fällström, Karl-Evert
    et al.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Molin, Nils-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Transient bending waves in anisotropic plates studied by hologram interferometry1989In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 29, no 4, p. 409-413Article in journal (Refereed)
    Abstract [en]

    Propagating bending waves are studied in plates made of glass-fiber reinforced polyester. The waves are generated by the impact of a ballistic pendulum. Hologram interferometry, with a double pulsed ruby laser as light source, is used to record the out of plane motion of the waves. The interferograms have an elliptic-like symmetry for an orthotropic plate, while the wave pattern for a symmetric angle-ply reinforced plate has a symmetry about the axes of reinforcements. Experimental data are compared on one hand to analytical results obtained by assuming that the orthotropic plate can be described as if isotropic along the main axes, and on the other hand to numerical results from calculations using the finite-element method. The effective Young's modulus raised to power 1/4 is shown to be an important parameter for the description of the dispersive wave pattern. A defect in the plate alters the wave pattern in the interferograms significantly. This may have technical use.

  • 4.
    Giannadakis, Konstantinos
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Szpieg, Magdalena
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Varna, Janis
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Mechanical performance of a recycled carbon fibre/PP composite2011In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 51, no 5, p. 767-777Article in journal (Refereed)
    Abstract [en]

    A composite made of recycled carbon fibres in recycled polypropylene matrix is studied experimentally to describe the features of the elastic and time dependent nonlinear mechanical behaviour. The properties of the developed material have a large variability to be addressed and understood. It was found that the stress-strain curves in tension are rather nonlinear at low strain rate and the strength is sensitive to strain rate. The elastic properties' reduction for this composite after loading to high strains is rather limited. More important is that even in the "elastic region" due to viscoelastic effects the slope of loading-unloading curve is not the same and that at higher stress large viscoplastic strains develop and creep rupture is typical. The time and stress dependence of viscoplastic strains was analysed and described theoretically. The viscoelastic response of the composite was analysed using creep compliance, which was found to be slightly nonlinear

  • 5. Gozzi, Jonas
    et al.
    Olsson, A.
    Swedish Institute of Steel Construction.
    Lagerqvist, Ove
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Construction Engineering.
    Experimental investigation of the behavior of extra high strength steel2005In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 45, no 6, p. 533-540Article in journal (Refereed)
    Abstract [en]

    This paper comprises a study concerning the mechanical behavior of extra high strength steel. This is investigated by means of biaxial testing of flat cross-shaped specimens in the full σ1-σ2 plane, a concept developed earlier at Steel Structures, Luleå University of Technology. Furthermore, new specimen designs had to be developed to enable testing of a material with high yield strength and low ultimate over yield strength ratio, such as the extra high strength steel Weldox 1100. The tests are performed in two steps: one initial loading followed by unloading and a subsequent loading in a new direction. The test results, containing data from 15 biaxial tests, are characterized by a slightly anisotropic initial yield criterion where the proof stress in compression is consequently somewhat higher compared to the results in tension. In the subsequent loading the observed phenomena are a Bauschinger effect in the direction opposite the initial loading direction and that the transition from elastic to plastic state in subsequent loadings is gradual and direction-dependent.

  • 6.
    Lundberg, Bengt
    et al.
    Luleå tekniska universitet.
    Henchoz, A.
    Institut CERAC.
    Analysis of elastic waves from two-point strain measurement: strain, particle velocity, power transmission and related quantities can be determined at an arbitrary section of a cylindrical rod from measurement of strains at two different rod sections1977In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 17, no 6, p. 213-218Article in journal (Refereed)
    Abstract [en]

    Provided that the one-dimensional wave equation applies, strain measurement at two sections of a linearly elastic cylindrical rod makes it possible to determine a number of important quantities at an arbitrary section of the rod; for example, strain, particle velocity and power transmission. The equations needed are derived, and the design of an analogue real-time analyzer is presented. The influence of some principal sources of error is analyzed and it is shown that it should be possible to perform accurate evaluation (errors less than a few percent) during a time interval which is not very long compared to the travelling time for a wave between the two gage positions. Comparisons are made between direct measurement and digital evaluation of strain, and between digital and analogue evaluation of particle velocity and power transmission. The discrepancies are typically less than ten percent during a time interval of 20 travelling times between gages. Although these results do not represent what is achievable, the accuracy is sufficient in several applications and demonstrates the feasibility of the method used.

  • 7.
    Nässelqvist, Mattias
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Gustavsson, R.
    Vattenfall Power Consultant AB Mechanical and Process Engineering, Kyrkogatan 4, Gävle.
    Aidanpää, Jan-Olov
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Bearing load measurement in a hydropower unit using strain gauges installed inside pivot pin2012In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 52, no 4, p. 361-369Article in journal (Refereed)
    Abstract [en]

    To determine a machine's mechanical condition it is of importance to know the radial bearing forces in the machine. Radial forces are caused by magnetic pull forces in the generator, clamped shafts, mass unbalance and flow properties around the turbine. Measuring the shaft displacement in the bearing or the bearing housing acceleration is not sufficient for status determination of a vertical hydropower unit. It is the magnitude and frequencies of the radial forces in combination with structure properties which give information as to whether a measured value is harmful or not. This paper presents an alternative method for measurement of radial bearing load in a hydropower unit. The method presented in this paper is based on strain measurements on pivot pins. The pivot pins are placed behind the bearing pad and the radial loads acting on the pad propagate through the pivot pin. New pivot pins were purchased and equipped with strain gauges. The new pivot pins were calibrated and a transfer function between applied load and measured output voltage was identified for each pivot pin. After calibration the pivot pins were installed in a vertical hydropower unit. Measurements were performed for several different operating modes of the hydropower unit. To verify that the measured load levels were of right order of magnitude, the radial bearing loads were calculated from numerical simulations of bearing properties and shaft eccentricity measurements. The two methods for determining bearing load showed almost the same results. This indicates that either method can be used to determine bearing load

  • 8.
    Olofsson, Kenneth
    et al.
    Swedish Pulp and Paper Research Institute, STF1.
    Fällström, Karl-Evert
    Palágyi, P.
    Forsmarks kraftgrupp AB.
    Laser generated and recorded transient bending waves in composite tubes1996In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 36, no 3, p. 224-231Article in journal (Refereed)
    Abstract [en]

    Propagating bending waves are studied in three different composite tubes by holographic interferometry. A conical mirror is placed axially inside the tubes. Axial illumination and observation directions make it possible to view the circumference of the tube, with a high sensitivity to radial deformation. It is shown how the deformation field can be numerically evaluated using a phase stepping and unwrapping technique. Transient bending waves in the tubes are both generated and recorded by the same pulsed laser, which makes the experiments easy to perform. Finite element simulations of the impacted tubes are compared to corresponding experiments. Both the geometry and the material properties of the tubes affect the wave propagation. For unidirectional composite tubes, the 0-deg and 90-deg directions have different dynamic behavior. The proposed method could be used in nondestructive testing of tubes.

  • 9.
    Olofsson, Kenneth
    et al.
    Luleå tekniska universitet.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Holographic interferometry measurements of transient bending waves in tubes and rings1993In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 33, no 4, p. 308-313Article in journal (Refereed)
    Abstract [en]

    Propagation bending waves are studied in a tube of steel and in a ring of aluminum. The waves are generated by the impact of a ballistic pendulum. Holographic interferometry, with a double-pulsed ruby laser as light source, is used to record the waves. A conical mirror is placed axially inside the tube. Axial illumination and axial observation directions, make it possible to view all sides of the tube simultaneously with a high sensitivity to radial deformation. The interferograms, which have an unusual perspective, are captured with a CCD-camera and then spatially transformed into an unwrapped strip of the tube wall. This makes the interpretation of the measurements simpler. The geometry of the tube causes the wave pattern to propagate with different speed and amplitude along and across the tube, even when the material itself is isotropic. A finite-element simulation of the impact is compared to the corresponding experiment. An impact on a ring with a defect is performed in order to study the effect on the wave pattern. The proposed method could be used in nondestructive testing of pipes.

  • 10.
    Saldner, Henrik O.
    et al.
    Luleå tekniska universitet.
    Molin, Nils-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Applications of reciprocity, electronic holography and shearography in statics1994In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 34, no 4, p. 334-339Article in journal (Refereed)
    Abstract [en]

    This paper illustrates the use of reciprocity in electronic-holography experiments to determine unknown forces or couples acting on a mechanical structure. Electronic holography and electronic shearography are simple, fast and contact-free methods for the measurement of deformation fields or gradients of deformation fields, both for static and sinusoidal loadings. From a first measurement, the deformation field of the structure for a known force is determined. In a second experiment, the deformation field for the unknown quantity is measured. Reciprocity is then used to determine the unknown quantity. These optical methods are useful tools for the understanding of reciprocity.

  • 11.
    Sundin, Karl-Gustaf
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Jonsson, Mikael
    A stiff and compact impact-force transducer based on strain measurement1985In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 25, no 1, p. 48-53Article in journal (Refereed)
    Abstract [en]

    A stiff and compact transducer for both static and impact compressive forces has been developed. The principle of the transducer is based on measurement of tangential strain around the periphery of an axially loaded short and axisymmetric body. It is shown that for an appropriate choice of transducer geometry the output is nearly insensitive to the distribution of the force over the loaded surface. In an experimental investigation, a prototype of the transducer was subjected to both static and impact loads of different distributions. The results show only a few percent variation in transducer sensitivity for the load distributions used. The transducer consists essentially of a homogeneous body and is simple to make. The size and material can be chosen to fit special applications

  • 12. Talreja, Ramesh
    Cycle-dependent creep under random loading: Narrow-band random loading superimposed on a steady tensile load results in cycle-dependent strain accumulation leading to ductile failure of carbon-steel specimens1975In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 15, no 8, p. 311-315Article in journal (Refereed)
    Abstract [en]

    An important aspect of the response of a metal subjected to cyclic loading and a superimposed mean load is its capacity for progressive strain accumulation. In the present investigation, random loading with tensile mean loads is applied to carbon-steel specimens. The cycle-dependent creep properties are described and a phenomenological approach to predict the number of cycles to ductile failure is proposed. At lower stress values, the fatigue-type brittle failure occurs. To predict the failure lives, the familiar fatigue-damage theory is modified for the case of random loading, and the effect of mean stress is included

  • 13.
    Tan, X.C.
    et al.
    Luleå tekniska universitet.
    Forsman, Bo
    Luleå tekniska universitet.
    Buckling of slender string in cylindrical tube under axial load: experiments and theoretical analysis1995In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 35, no 1, p. 55-60Article in journal (Refereed)
    Abstract [en]

    Laboratory buckling tests of strings was reported and the results were compared with theoretical formulas. The findings implied that the process of string buckling can be divided into two stages, specifically plane buckling and spatial buckling. In the spatial buckling, the string-buckling configuration can be illustrated by helical buckling theories. The friction between the string and the wall of the confining tube rendered an irregular buckling shape and prohibited further buckling at some critical loads. The buckling theories do not account for this effects. Thus, an approximate formula for estimating the friction force was proposed, which provided more exact results.

  • 14.
    Tarigopula, Venkapati
    et al.
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Hopperstad, Odd Sture
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Langseth, Magnus
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Clausen, Arild Holm
    Norwegian University of Science and Technology (NTNU), Trondheim.
    Hild, F
    Universite Paris.
    Lademo, Odd-Geir
    SINTEF.
    Eriksson, Magnus
    SINTEF Materials and Chemistry, Trondheim, Norway.
    A study of large plastic deformations in dual phase steel using digital image correlation and FE analysis2008In: Experimental mechanics, ISSN 0014-4851, E-ISSN 1741-2765, Vol. 48, no 2, p. 181-196Article in journal (Refereed)
    Abstract [en]

    Large plastic deformation in sheets made of dual phase steel DP800 is studied experimentally and numerically. Shear testing is applied to obtain large plastic strains in sheet metals without strain localisation. In the experiments, full-field displacement measurements are carried out by means of digital image correlation, and based on these measurements the strain field of the deformed specimen is calculated. In the numerical analyses, an elastoplastic constitutive model with isotropic hardening and the Cockcroft - Latham fracture criterion is adopted to predict the observed behaviour. The strain hardening parameters are obtained from a standard uniaxial tensile test for small and moderate strains, while the shear test is used to determine the strain hardening for large strains and to calibrate the fracture criterion. Finite Element (FE) calculations with shell and brick elements are performed using the non-linear FE code LS - DYNA. The local strains in the shear zone and the nominal shear stress-elongation characteristics obtained by experiments and FE simulations are compared, and, in general, good agreement is obtained. It is demonstrated how the strain hardening at large strains and the Cockcroft - Latham fracture criterion can be calibrated from the in-plane shear test with the aid of non-linear FE analyses.

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