Additive manufacturing (AM) aims to improve resource efficiency during manufacturing. Some of the weldable nickel-based superalloys are frequently used for AM due to their good weldability. In the aerospace industry, nickel-based superalloys are used in high-temperature applications such as in the rear section of jet engines where superior mechanical properties are essential. Both grain structure and residual stresses must be carefully controlled during manufacturing to achieve the desired properties. The grain structure forms during solidification in AM and influences the mechanical properties such as the strength of the manufactured parts. The formation of residual stresses is unavoidable because of the non-uniform heating and cooling in AM. The residual stresses may distort the parts, reduce the fatigue performance, or decrease the load-bearing capacity. The aim of simulation in this work is to link process parameters in AM to the generated grain structure and the residual stresses in the part. This would further enhance the resource efficiency of AM since trial-and-error manufacturing can be reduced. Simulation requires models that are relatively fast and accurate to be useful in the aerospace industry. This work is divided into two papers where models for predicting the grain structure and residual stresses in AM are developed and evaluated. A Cellular Automata Finite Element (CA-FE) framework was developed to predict the grain structure in laser-based powder bed fusion (PBF-LB) manufactured alloy 625 in the first paper. The inherent strain method (ISM) was used to predict the residual stresses in PBF-LB manufactured alloy 718 in the second paper. Both alloys 625 and 718 belong to the group of nickel-based superalloys and the two alloys combine good weldability with excellent strength at high temperatures. The results showed that the CA-FE and the ISM model can capture the general trends of both the experimentally observed grain structure and the residual stress field in a wall component that was manufactured using PBF-LB in alloy 625 and 718, respectively. The CA-FE and ISM models are also relatively fast which makes them suitable for product development in the aerospace industry among other industries.

Additive manufacturing (AM) is rapidly emerging as a feasible manufacturing method. The material is deposited in a layer-by-layer approach, enabling complex-shaped parts to be manufactured. However, the printed parts are typically distorted to some extent due to the residual stresses from the manufacturing stage. These residual stresses are a result of the material’s temperature fluctuations during the layer-by-layer deposition: as the material is heated, it expands, and when cooled, it shrinks. To address this issue, models have been developed capable of simulating the entire AM process. By employing modeling techniques, process optimization can be achieved by identifying suitable process parameters, eliminating the need for time-consuming trial-and-error experimentation. One of the enabling technologies to these models is the finite element method (FEM) where the deposited material and build plate is treated as a continuum discretized by finite elements (FE). This work provides an overview of the finite element models for AM and the couplings that can be made to predict the residual stresses and distortions of printed components more accurately. The coupling can be made on different scales, including the micro-, meso-, or part-scale. Micro-scale finite element models coupled with cellular automata models (FE-CA), initially developed by Gandin and Rappaz (1997), can predict the grain texture after solidification. Meso-scale models such as thermal models can predict the temperature history from the chosen scanning strategy. Subsequently, thermo-mechanical models can predict residual stresses and distortions. When it comes to predicting the distortions on a part-scale level, thermo-mechanical layer-by-layer lumping and inherent strain method can be used to reduce the computational time. The coupled thermo-mechanical FE models can be used with a physically based material model which is based on the material’s dislocation structure. For example, Lindgren et al. (2017) developed a dislocation density-based plasticity model. Here, FE-CA models can provide information about the grain orientation and grain size which are important parameters involving the strain hardening in a physically based model where the Taylor orientation factor is needed, and the Hall-Petch effect where the grain size is needed. Phase composition can be provided by a thermo-metallurgical model. Furthermore, the change in dislocation density is proportional to the plastic strain rate which is obtained from the meso- or part-scale thermo-mechanical model itself. Additionally, thermal activation can assist dislocations when passing obstacles such as precipitates or solutes, hence the prediction of an accurate temperature field is an important input to the physically based model.

REFERENCES:

Gandin, Ch.-A., Rappaz, M., 1997. A 3D Cellular Automaton algorithm for the prediction of dendritic grain growth. Acta Materialia. pp. 2187-2195.

Lindgren, L.-E., Hao, Q., Wedberg, D., 2017. Improved and simplified dislocation density based plasticity model for AISI 316L. Mechanics of Materials. pp. 68-76.

The grain texture of the as-printed material evolves during the laser-based powder bed fusion (PBF-LB) process. The resulting mechanical properties are dependent on the obtained grain texture and the properties vary depending on the chosen process parameters such as scan velocity and laser power. A coupled 2D Cellular Automata and Finite Element model (2D CA-FE) is developed to predict the evolution of the grain texture during solidification of the nickel-based superalloy 625 produced by PBF-LB. The FE model predicts the temperature history of the build, and the CA model makes predictions of nucleation and grain growth based on the temperature history. The 2D CA-FE model captures the solidification behavior observed in PBF-LB such as competitive grain growth plus equiaxed and columnar grain growth. Three different nucleation densities for heterogeneous nucleation were studied, 1 × 1011, 3 × 1011, and 5 × 1011. It was found that the nucleation density 3 × 1011 gave the best result compared to existing EBSD data in the literature. With the selected nucleation density, the aspect ratio and grain size distribution of the simulated grain texture also agrees well with the observed textures from EBSD in the literature.

Cold pilgering is a complex forming process used to produce seamless tubes in terms of modeling due to the complexity in kinematic of tools, friction condition and material behavior. The process development has mostly been based on simple formulas and costly full-scale tryouts. The aim in this study is to develop validated Finite element models of cold pilgering to support design of a robust process.

A three-dimensional thermo-mechanical Finite element models of cold pilgering has been developed in the course of the work leading to this thesis. The commercial code MSC. Marcwas used in the simulations. General 3D models are needed to be able to capture asymmetric deformation in cold pilgering. Elastic deflections of tools and roll stand were included in the model via linear and nonlinear springs that were calibrated versus experiments. A temperature dependent Chaboche type plasticity model was employed in this simulation to mimic strain hardening and softening behavior under multidirectional loading. The model parameters were optimized using multi-directional compression and uni-directional tensile tests. Heat exchange between tools and lubricant was included in the simulation via heat convection films on the surfaces. The film parameters were calibrated using experimental data. Simulation predictions for hardening, rolling force, process temperature and geometry were compared with experiments for validation purposes. The predictions showed overall good agreement with validation experiments enabling the use of this model for understanding and improving the process.

Cold pilgering is a complex forming process used to produce seamless tubes, posing significant challenges in material mod- eling due to its non-proportional loading history and extensive accumulated plastic strain. In this study, a temperature- and plastic strain-dependent Chaboche model for 316 L stainless steel was developed and calibrated. To simulate the complex loading conditions, unique alternating compression-compression tests were conducted, and the model parameters were optimized accordingly. The calibrated model was integrated into a thermo-mechanical finite element simulation of the cold pilgering process, resulting in improved accuracy in predicting stress-strain responses and yield stress evolution. Close agreement with experimental tensile tests of the final tube was demonstrated, illustrating the model’s capability to predict hardening behavior during cold pilgering. Valuable insights and a practical modeling approach for enhancing the simulation and optimization of cold pilgering processes are provided by this work. 

The combination of different materials enables to achieve highly efficient structures in terms of lightweight and mechanical performance, as well as in terms of manufacturing costs. However, the weakest points of these structures use to be the joints. For this reason, in the last years, many studies have dealt with joining technologies for dissimilar materials. The Reinforce3D’s Continuous Fibre Injection Process (CFIP) technology delivers a unique method to join dissimilar materials. CFIP is based on injecting continuous fibers, such as carbon fibers, simultaneously with liquid resin into tubular cavities within the part. Then the resin is cured and the final composite part is obtained. This work focuses on the characterization of the mechanical properties of CFIP-made specimens and describes the potential lightweight benefits of the technology. Mechanical tests were performed under tensile and bending conditions following standardized methods. The lightweight potential is addressed by developing a representative case study by implementing finite element and topology optimization methods. The results of this case study were finally compared with a monomaterial equivalent component (aluminium) demonstrating the improvement that CFIP provides in terms of lightweight while keeping the strength.

Aluminum alloys are one of the widely used materials in the structural components of automobiles and especially aircraft. The valid prediction of the performance and life span of these components via running simulations requires the usage of sophisticated physics-based material models. Such models are based on the mechanical response and the underlying microstructure evolution at varied deformation conditions. With the above motivation in mind, the aim of the current doctoral thesis was to investigate and understand the relationship between processing, microstructure, mechanical and fracture behavior of AA7075-T651 alloy and recycled AlSi10MnMg(Fe) alloy.

First, the deformation behavior of AA7075-T651 alloy, which was initially drawn from extruded round bars, was studied through compression tests at low (0.01 and 1 s−1) as well as at high (1400 – 5300 s−1) strain rates and deformation temperatures ranging between room temperature (RT) and 500 °C. At low strain rate deformation, the alloy experienced more softening due to adiabatic heating arising from 1 s−1 strain rate up to 200 °C. Beyond 200 °C, the softening effect was taken over by dynamic recovery and dynamic recrystallization which were enhanced by 0.01 s−1 strain rate. The deformation at high strain rates and elevated temperatures led to the formation of adiabatic shear bands (ASBs) and cracks in the material. The feasibility of formation and growth of ASBs and cracks increased with increase in strain and temperature, neglecting any significant effect from the strain rate.

Secondly, the recycled secondary AlSi10MnMg(Fe) alloy, which was produced by high pressure die casting (HPDC), was investigated. The secondary alloy showed great potential of exhibiting the strength and ductility within the range exerted by its conventional primary counterpart, i.e., AlSi10MnMg alloy, however, its tensile properties were restricted by the inhomogeneously formed fine-grained skin layer on the casting surface. The said inhomogeneity in skin formation was corresponded to the “waves and lakes” type of casting defect. Such inhomogeneous skin layer limited the ductility of the secondary alloy by undergoing abrupt fracture due to its poor bonding with the adjoining matrix. Even if the AlSi10MnMg(Fe) alloy used in the current research contained an abundance of porosity, cold flakes and intermetallics, which are known to be the driving factors behind the fracture of HPDC processed materials, the effect from the inhomogeneous skin was found out to be predominant.

The current study investigated the evolution and effect of microstructure and casting defects on the ductility and fracture behavior of a high pressure die cast (HPDC) secondary AlSi10MnMg(Fe) alloy of varied step thickness. The HPDC process imparted fine-grained skin layer on the surface of 2- and 4-mm thick step parts, but not on the 6- and 10-mm thick ones. Moreover, the nature of the skin layer formed was significantly inhomogeneous. The absence and formation of inhomogeneous skin layer was credited to the complex melt flow pattern inside the die cavity. The weak bonding existing between the inhomogeneous skin and its adjoining matrix made it susceptible to crack initiation during the subsequent tensile tests. The effect of strain rate on the ductility behavior of secondary alloy was studied through tensile tests performed on 2 mm thick HPDC castings with strain rates of 0.001, 0.1 and 10 s^-1, which revealed no particular relation between the two. On the other hand, the large variation observed in case of ductility could be correlated to the formation of skin layer. When the castings possessed no or continuous skin, they displayed larger ductility. Whereas when the castings contained inhomogeneous skin, they failed abruptly at different lower elongations because of the delamination of skin layer induced by the weak bonding across its adjacent matrix. HPDC process also produced defects in the castings, such as cold flakes and pores, which increased in size and amount with increase in the casting thickness. The effect of porosity on ductility was investigated through tensile tests on 2-, 6- and 10-mm thick HPDC castings with a strain rate of 0.001 s^-1. Even though the 10-mm thick casting contained the largest amount of pores, the ductility in this casting was not restricted as much as it was in case of 2- and 6-mm thick ones which underwent abrupt fracture because of inhomogeneous skin and cold flake, respectively. From the current study, the inhomogeneity in skin pattern turned out to be the most harmful feature for the ductility of the HPDC processed secondary alloy.

The current study investigated the microstructure evolution, mechanical properties, and fracture behavior of a high pressure die cast (HPDC) novel secondary alloy. The as-cast microstructure comprised (i) Primary α-Al, (ii) α-Al15(FeMn)3Si2 intermetallics, and (iii) Al–Si eutectics. The microstructure starting from the surface through the depth of the HPDC casting consisted of (i) fine-grained skin at surface, (ii) increased Al–Si eutectics at intermediate location, and (iii) coarse α-Al dendrites at center. Accordingly, the hardness increased from skin to the intermediate section and then decreased toward the center of the casting. The formation of skin layer was highly discontinuous, which was attributed to the complicated fluid flow pattern inside the die cavity. The skin layer indicated to slightly improve the strength of the HPDC alloy; however, it restricted the ductility of the material with a large variation. Such ductility behavior resulted from a fracture mechanism triggered by the inhomogeneous skin because of its poor bonding with the adjacent matrix. Even though the secondary alloy contained casting defects and α-Al15(FeMn)3Si2 intermetallics that are known to be driving factors for the fracture in such materials, the effects from the inhomogeneous skin turned out to be predominant in the current study. 

In view of developing a physics-based constitutive material model for AA7075-T651, the mechanical behavior and microstructure evolution of the material has been studied through compression tests using Gleeble thermo-mechanical simulator. The tests were performed at wide range of temperatures (room temperature (RT), 100, 200, 300, 400 and 500 °C) with two constant strain rates (0.01 and 1 s-1). The true stress-strain curves depicted an increase in the flow stress with increase in the strain rate and decrease in the deformation temperature, with an exception at RT. The effects of softening mechanisms, such as adiabatic heating, dissolution of precipitates, dynamic recovery (DRV) and dynamic recrystallisation (DRX), on the flow stress level, strain rate sensitivity (SRS) and temperature sensitivity over the entire range of temperatures were analyzed. Pertaining to the microstructure analysis, the intermetallic particles present in the initial as-received (AR) material were identified as (Al,Cu)6(Fe,Cu) and SiO2 with the help of back-scattered electron (BSE) imaging and energy dispersive X-ray spectroscopy (EDS). The microstructure of the material after the deformation processes were analyzed and compared with that of the AR state using inverse pole figures (IPF), grain orientation spread (GOS) and grain boundary rotation maps generated from electron back-scattered diffraction (EBSD) scans. DRV was observed for deformation at 300 °C, whereas a combination of DRV and incomplete DRX took place for 400 and 500 °C depending on the strain rate. The fraction of recrystallized grains was higher in case of deformation at higher temperature and lower strain rate. Furthermore, the difference in microstructure evolution on different surfaces of the deformed samples as well as at different locations on individual surfaces was also investigated.

The current study presents the effects of strain and temperature on the mechanical response and microstructure evolution in AA7075-T651 at high strain rates. Compression tests have been performed at room temperature (RT), 200, 300 and 400 °C using a Split-Hopkinson pressure bar (SHPB) setup with strain rates ranging between 1400 and 5300 s⁻¹. For deformation at RT, the flow stress increases with increase in strain rate. Whereas deformation at elevated temperatures show a non-monotonous behavior of the flow stress with respect to the strain rate. This trait is attributed to the pronounced effects from the adiabatic shear bands (ASBs); namely, distorted shear bands (DSBs) and transformed shear bands (TSBs); and cracks resulting from the plastic deformation instability during hot deformation. The sequence of microstructure evolution is: inhomogeneity in the initial microstructure – DSB – TSB – crack –fracture. The feasibility of formation and growth of ASBs and cracks increases with increase in strain and temperature, neglecting any significant effect from the strain rate. During the compression tests, temperature of the material rises due to adiabatic heating. Considering a certain strain developed in the material, this adiabatic temperature rise decreases as the deformation temperature is increased. Furthermore, during individual deformation processes, the temperature rise increases with increasing strain. The adiabatic temperature leading to the formation of TSB is approximated to be 0.7 times of the melting temperature of the alloy. These results from the current study are to be used in developing a physics-based material model for the alloy.

Accurate computer simulations require the selection of suitable material models and precise prediction of their parameters. In the fields of impact engineering and plastic working, stress–strain relations that include the post-necking regime up to fracture are crucial for predicting the behavior correctly. However, obtaining suitable stress–strain relations after necking requires some form of correction and adjustment for stress and/or strain. This study applies a stepwise modeling method for post-necking characterization that only utilizes the local strain field obtained from tensile experiments to precisely measure stress–strain relations at high strain rates. The effects of the notch radius of specimens on stress–strain relations were examined to measure stress–strain relations with large strain near the stress triaxiality of 1/3. The study also discusses adequate resolution for precise stress–strain measurements. Subsequently, specimens with suitable notch radius were used to measure stress–strain relations of plate specimens of aluminum alloy 2024-T3 at high strain rates. The study also examined the effects of strain rate on the flow stress and fracture strain of aluminum alloy 2024-T3.

Aggregate breakage in unbound pavement layers can lead to pavement distresses that affect their functionality and service life. Thus understanding the mechanics and clarifying the factors affecting materials breakage resistance are important for ensuring adequate performance of these layers. In this study, aggregate breakage in unbound granular materials (UGM) is investigated experimentally and numerically. Experimentally, aggregate breakage under uniaxial compression is examined for two UGMs prepared with the same aggregate type but different gradations. To capture the experimentally observed influence of gradation and load magnitude on aggregate breakage, a Discrete Element Method (DEM) model was developed, based on granular mechanics particle contact and failure laws. A simple procedure to identify the contact and failure law parameters from experiments is proposed. With those parameters, the model’s capability of capturing the effect of gradation and loading on the aggregate breakage in UGM is evaluated. Based on comparison with experimental findings, it is shown that the model can capture macro-scale properties of UGM, such as its deformation response under uniaxial compression, as well as the amount of aggregate breakage in the material.

Induction heating is used in many industrial applications to heat electrically conductive materials. The coupled electromagnetic-thermal induction heating process is non-linear in general, and for ferromagnetic materials it becomes challenging since both the electromagnetic and the thermal responses are non-linear. As a result of the existing non-linearities, simulating the induction heating process is a challenging task. In the present work, a coupled transient electromagnetic-thermal finite element solution strategy that is appropriate for modeling induction heating of ferromagnetic materials is presented. The solution strategy is based on the isothermal staggered split approach, where the electromagnetic problem is solved for fixed temperature fields and the thermal problem for fixed heat sources obtained from the electromagnetic solution. The modeling strategy and the implementation are validated against induction heating experiments at three heating rates. The computed temperatures, that reach above the Curie temperature, agree very well with the experimental results.

Driven by current safety and weight reduction policies in the automotive sector, the development of new high strength sheet metal products has experienced unprecedented growth in the last years. With the emergence of these high strength materials, new challenges related to their limited ductility and higher cracking susceptibility have also raised. Accordingly, the development of new fracture criteria accounting for the material's cracking resistance has become unavoidable. In this work, a new cracking resistance index (CRI) based on fracture mechanics is proposed to classify the crack propagation resistance (i.e. the fracture toughness) of high strength metal sheets. The index is based on the fracture energy obtained from tensile tests with sharp-notched specimens. The procedure is very fast and simple, comparable to a conventional tensile test, and it may be used as routine testing for quality control and material selection. The CRI is investigated for several advanced high strength steel (AHSS) sheets of 0.8-1.6 mm thickness with tensile strengths between 800 and 1800 MPa. The results show that the proposed index is suitable to rank high strength steel sheets according to their crack propagation resistance and it can be correlated to the material's crashworthiness and edge cracking resistance.

Press Hardening offers the possibility to obtain a wide range of mechanical properties through microstructural tailoring. This strategy has been successfully applied in thin sheet components, for instance, through differential cooling strategies. The application of these added value features to truck components implies adapting the process to the manufacture of thick sheet metal. This introduces an additional layer of complexity, but also opportunity, in a process where the final microstructure and, thus the mechanical performance is generated in the press shop. This work presents a study on optimizing the crash worthiness and impact energy absorption on a press hardened thick 22MnB5 steel sheet. Different microstructure design strategies have been studied, including ferrite-Pearlite (representative of a differential heating and austenitization strategy), in-die generated Bainite (representative of differential cooling) and Tempered Martensite (generated through laser tempering), keeping a fully hardened martensite as a reference condition. The material performance has been compared in terms of the monotonic properties, useful for anti-intrusion performance, and Essential Work of Fracture, a well-suited parameter to predict the crash failure behavior of high strength steels. The results show that laser tempering offers properties similar to Bainite-based microstructures and can be a successful replacement in components where the sheet thickness does not allow for the fine control of the in-die thermomechanical evolution.

The generation of reliable data in the high cycle fatigue domain is crucial to support further metallurgic developments of fatigue optimized steel grades. Commonly employed for this aim, traditional standardized characterization methods are expensive and time-consuming. Thus, to circumvent these limitations, different accelerated fatigue testing methodologies have been proposed. In this work, the rapid fatigue test based on stiffness evolution is numerically reproduced using the finite element method for a specific grade of twinning-induced plasticity steel. A high cycle fatigue constitutive law grounded on the continuum damage mechanics framework is employed for this purpose. To adequately capture the material non-linear behavior observed in the experiments, a novel hardening–softening stress–strain curve for damage is proposed. The entire load history in the fatigue domain is modeled. A cycle-jump algorithm is used to improve the computational efficiency of the simulations. It is shown that a reduction of about 55% in the analysis elapsed time is reached by using this algorithm, while the result accuracy is maintained. Finally, the good agreement between numerical and experimental results, revealed by a maximum relative error smaller than 6.0%, evidences the potential of the present constitutive formulation to model the behavior of metals in the high cycle fatigue domain.

The present work investigates the influence of pre-strain on the fracture toughness of 3rd Generation Advanced High Strength Steels (AHSS). Specifically, a Carbide Free Bainitic (CFB) and a Quenching and Partitioning (Q&P) steel have been studied, the properties of which are crucial for lightweight vehicle construction. Fracture toughness, which is a key parameter for crash performance applications, is assessed using the Essential Work of Fracture methodology. The study investigates the pre-straining states of uniaxial tension, plane strain, and equibiaxial tension in 1.5 mm Q&P and 1.4 mm CFB sheet-form steels of 1180 MPa tensile strength. Overall, Q&P steel demonstrates superior fracture toughness compared to CFB steel. Remarkably, the specific essential work of fracture (we) remains unaffected by pre-straining across different strain states. Nevertheless, pre-straining exerts a notable influence on the non-essential plastic work (βwp) due to the plastic energy consumed during pre-deformation. These results suggest that prestrain has little or no influence on the fracture properties of AHSS, which is relevant for the design and manufacturing of high crash-performance and safety-related components.

Lightweighting of automotive and heavy-duty vehicle components is an important task that does not need any further motivation or background. It can be read in a large part of the technical papers in the field. A common approach for finding lighter solutions is to increase the material grade while decreasing the material thickness. Often in combination with design changes. For perfectly smooth components this is not an issue, but when cut edges from manufacturing processes are present the situation changes. One topic to address is that increased material grade often means increased notch and surface damage sensitivity. This has implications both on forming and fatigue. The reason for selecting a higher strength material is to allow for higher stresses in design. It has however been shown that for a given stress level the fatigue performance of a higher strength material could be worse than for a lower strength counterpart if punched holes or trimmed edges are present. This means that in the search of lower weight there is a risk of increasing stresses, and at the same time selecting a material that is less suited to handle this increase. Hence, engineers and developers are put in a position where these effects must be quantified to find the most efficient solution. This quantification is a cumbersome and expensive task, often including a considerable amount of testing. Important sources of fatigue life reduction in this context are the residual stresses in the loading direction and the surface roughness in the cut edge. This thesis aims to present an overview of metal fatigue in the context of shear cut components. Necessary knowledge regarding the shear cutting process is provided along with a description of numerical methods and considerations for process simulations. These findings are then applied to the presented papers where the first introduces a simplified approach for numerical simulation of shear cutting to obtain residual stresses. In this approach the simplification mainly lies in the failure model calibration. The second paper studies the possibility of using the obtained residual stresses together with measured values of surface roughness to quantify fatigue life reduction of shear cut specimens.  

Shear cutting, such as punching and trimming, of sheet metals is a widely used process in the automotive and heavy-duty vehicle industry due to low running costs and low cycle times. It's known that the edge properties obtained from this process has an impact both on formability and fatigue resistance of the formed part. The purpose of this contribution is to present a novel methodology for simulation of shear cutting processes taking a simplistic and phenomenological approach, resulting in an industry feasible simulation setup, simulation time and experimental workload. The task of simulating shear cutting processes includes high nonlinearities, large deformations, and crack propagation. A common approach is to use extensive material characterization to feed a material model including failure. Several specimen geometries are investigated to capture different stress states, resulting in a test and simulation matrix that could be overwhelming for industrial users. In this work it was investigated if a single tensile test and a punching test could be used for calibration of a material model including plastic flow and failure. Three different high strength materials were investigated using different cutting clearances and sheet thicknesses, one aluminium alloy and two complex phase steels. Characterization of the resulting cut edges were used for validation of the simulation results. It is shown that good agreement between simulation and experiments is achieved in terms of punch force and displacement. The main characteristics of the cut edge is also captured. Hence, using this simplified approach for simulation of shear cutting processes could reduce time to market and development cost for implementation of new materials by providing information about the process effects to a minimum of simulation and experimental effort.

Effect from manufacturing processes on fatigue properties of high-strength thick steel sheets have gained increased attention the recent years, due to new demands on the heavy-duty vehicle (HDV) industry to reduce the carbon footprint. The aim of this study is to add knowledge of the effect of shear cutting clearance on the fatigue behaviour of complex phase CP800 thick steel sheets. In addition, sandblasting and its effect on the fatigue properties are studied. Service loads are fluctuating loads acting on chassis component making fatigue an important failure mode. The fatigue strength usually follows the yield strength of the material and hence weight could in theory be saved by using steels of higher strength. However, in the presence of edge defects this relation does not necessarily hold, this leads to large safety factors of the design and under-utilization of the high-strength material. Thus, an increased knowledge about the effect from manufacturing processes on fatigue properties is important for the quest to achieve weight reduction. This is particularly true for thick sheets which, to the best of our knowledge, are less investigated than their thinner counterparts, but of high importance for the HDV development.

In this paper, empirical results from fatigue testing of complex phase steel CP800, subjected to punching and trimming, are presented. Results for different cutting clearances are compared as well as the effect of sandblasting. A fast fatigue testing method called Rapid fatigue test based on stiffness evolution is utilized. The results show the improvement obtained by using sandblasting as well as illustrating the effect of different cutting clearances. These results can be used as a guidance for design and production of HDV components, where cutting clearance is set. Furthermore, the results can be used as an input for discussions whether the extra costs associated with sandblasting is motivated or not for components made from high strength, thick steel sheets.

Shear cutting processes have a detrimental effect on fatigue of high strength metal components. The effect tends to increase with material grade, counteracting the task of reducing weight in chassis components using higher strength materials. Base material fatigue data are often available, but assessment of components with cut edges often require additional costly and time-consuming testing. This paper provides a methodology for estimation of fatigue life reduction by using residual stresses obtained from process simulations, and measured surface roughness in the cut edge. A stress relaxation criterion is applied to handle reduction of the initial local residual stresses. Two complex phase steels and one aluminum alloy are studied for validating the approach. Polished fatigue data is reduced to estimate S–N curves of trimmed and punched specimens at different load ratios. Good agreement between the model and test results are found for all cases. The needed data for the predictions are only a high cycle S–N relationship for polished material, uniaxial tensile properties, and the cut edge fracture surface residual stress and roughness without any parameter fitting, making it a convenient tool for estimating the reduction in fatigue life and for parameter studies.

This paper presents a validated finite element modeling approach for simulating shear cutting, needing a minimal amount of experimental characterization. Only one uniaxial tensile test and one force–displacement relationship from a punching experiment are needed for calibration, with maintained prediction accuracy compared to more experimentally demanding approaches. A key ingredient is the observation that the Lode angle parameter is close to zero in the fracture region, postulating that the fracture strain only depends on stress triaxiality, with one free calibration parameter. The true stress–strain behavior is provided from inverse modeling of the tensile test, whereas the fracture model is calibrated using the punching test. The model is verified for different materials by comparing force–displacement curves for punching experiments not used in the calibration. The prediction error for the intrusion is below 4%. A validation is made for two setups. The local residual stresses are measured using Focused Ion-Beam Digital Image Correlation (FIB-DIC). The simulated values are within the experimental bounds. Cut edge morphology and plastic strains obtained by nano-indentation mappings are compared to simulation results, showing a decent agreement. For trimming, the cut edge morphology prediction performance decreases at 17% cutting clearance while it is maintained over the whole range for punching. The predicted hardness values have a mean absolute percentage error below 7.5%. Finally, the effect of element size and remeshing is discussed and quantified. The minimal experimental characterization and simulation effort needed, enables an efficient optimization of the cutting process in the industry.

Legislative demands force the automotive industry to reduce greenhouse gas (GHG) emissions. At the same time, crashworthiness must not be compromised. A ve-hicle’s GHG emissions, such as carbon dioxide, is dependent on its fuel consump-tion. Lowering the vehicle weight, reducing fuel consumption, will therefor reduce emissions. Thus, high performance lightweight materials and structures are on demand. Several methods for achieving high-performance lightweight components are available. One of the most successful approaches has been replacing mild steels with press-hardened steels, e.g. ultra high strength steels (UHSS). In the press-hardening process, a low-alloyed boron steel blank is austenitized followed by simultaneously forming and cooling. By controlling cooling rates, a martensitic microstructure can be obtained, resulting in components with superior properties compared to mild steels. Other methods of achieving lightweight components in-clude the usage of sandwich structures where stiff skins are bonded to a low-density core. In the present thesis, several types of sandwich structures are studied both numerically and experimentally. A UHSS sandwich with a bidirectionlly corru-gated core, intended for stiffness application, is manufactured and evaluated in three-point bending. Finite element models are utilized to recreate the three-point bend test. A large amount of finite elements are required for precise discretization of the core. The number of finite elements are reduced by replacing the sandwich with an homogeneous, equivalent model with input data obtained from analyzing representative volume elements (RVEs) of the core, subjected to periodic and ho-mogeneous boundary conditions. Good agreement is found between experiments and finite element models. A UHSS sandwich with a partly perforated core is evaluated numerically for energy absorption applications. Several hole configu-rations for the core are evaluated with respect to specific energy absorption. A fracture criterion is utilized for the sandwich skins. Computational time is re-duced by homogenization of the core using a stress-resultant based constitutive model. It is found that the sandwich concept allows for an increase in specific energy absorption and that the computational time can be reduced while still be-ing able to predict energy absorption. An experimental methodology is developed for mechanical characterization of micro-sandwich materials. Tools are developed for loading the micro-sandwich in out-of-plane tension and shear, where digital image correlation is used for measuring displacements fields and fracture of the micro-sandwich core. Statistical methods are adopted for analyzing the variation in the mechanical properties of the micro-sandwich from which statistical means may be obtained. The experimental data is used as input for constitutive models, simulating the micro-sandwich material subjected to peeling, using a T-peel test. The numerical models are validated against experiments, found to agree within one standard deviation, suggesting that the experimental methodology produces robust data.The present work has thus presented methods, further increasing the usability of UHSS with regard to lightweighting, and explored how such components may be simulated numerically with adequate accuracy and reasonable computation time. Furthermore, the present thesis contributes by presenting methods for character-izing micro-sandwich materials, including statistical methods for analyzing scatter in mechanical properties, and how such sandwich materials may be modeled, tak-ing elasto-plasticity and damage into account. These results opens up possibilities for further development and optimization of lightweight constructions.

Sandwich structures are commonly used to increase bending-stiffness without significantly increasing weight. In particular, micro-sandwich materials have been developed with the automotive industry in mind, being thin and formable. In the present work, it is investigated if micro-sandwich materials may be modeled using commercially available material models, accounting for both elasto-plasticity and fracture. A methodology for calibration of both the constitutive- and the damage model of micro-sandwich materials is presented. To validate the models, an experimental T-peel test is performed on the micro-sandwich material and compared with the numerical models. The models are found to be in agreement with the experimental data, being able to recreate the force response as well as the fracture of the micro-sandwich core.

Lightweight components are in demand from the automotive industry, due to legislation regulating greenhouse gas emissions, e.g., CO2. Traditionally, lightweighting has been done by replacing mild steels with ultra-high strength steel. The development of micro-sandwich materials has received increasing attention due to their formability and potential for replacing steel sheets in automotive bodies. A fundamental requirement for micro-sandwich materials to gain significant market share within the automotive industry is the possibility to simulate manufacturing of components, e.g., cold forming. Thus, reliable methods for characterizing the mechanical properties of the micro-sandwich materials, and in particular their cores, are necessary. In the present work, a novel method for obtaining the out-of-plane properties of micro-sandwich cores is presented. In particular, the out-of-plane properties, i.e., transverse tension/compression and out-of-plane shear are characterized. Test tools are designed and developed for subjecting micro-sandwich specimens to the desired loading conditions and digital image correlation is used to qualitatively analyze displacement fields and fracture of the core. A variation of the response from the material tests is observed, analyzed using statistical methods, i.e., the Weibull distribution. It is found that the suggested method produces reliable and repeatable results, providing a better understanding of micro-sandwich materials. The results produced in the present work may be used as input data for constitutive models, but also for validation of numerical models.

Interlayers in asphalt pavements are potential structural damage initiators. In order to better understand the quantitative role of interlayer parameters, such as surface roughness, binder type, binder content and loading type on interlayer shear strength, this paper focuses on the effects of particle interlock and contact conditions on interlayer strength through experimental and numerical modelling. Experimentally, interlayer shear box strength tests on a model material consisting of stiff binder blended with steel balls are performed with and without normal force confinement. A Discrete Element method model of the test is developed using measurements of the model material for calibrating the contact law and for validating the model. It is shown that this model captures adequately the measured force-displacement response of the specimens. It is thus a feasible starting point for numerically and experimentally studying the role of binder and tack coat regarding interlayer shear strength of real asphalt layers.

The automotive industry is currently adapting to progessively more stringent emission and safety regulations imposed by governmental agencies. This introduces significant design difficulties due to the conflicting nature of passenger safety in automotive manufacturing, namely that increased crashworthiness generally leads to heavier vehicles, which in turn leads to more severe crashes. Significant industry effort to introduce lightweight materials into automotive Body-in-White (BIW) design has thus been introduced in recent years to reduce curb weight while improving crashworthiness. Third generation Advanced High Strength Steels (3rd-gen AHSS) and new generations of press hardening steels (PHS) has emerged as cost-effective and natural substitutes in the safety critical crush zones of the vehicle. The limited ductility of these higher strength materials can however make them more prone to cracking, which in turn make reliable deformation behaviour difficult in a crash event. Thus, predicting cracks in the material and its resistance to further propagate them are essential in evaluating crash performance of a design. Fracture toughness measured within the frame of fracture mechanics using the Essential Work of Fracture (EWF) has shown to correlate well with AHSS crashworthiness for steel sheets, making it an interesting parameter for further study in this area. EWF is however strain rate dependent, and most available EWF testing for AHSS is still performed using quasi-static loading rates, conditions completely different from common high-speed crash scenarios. Furthermore, since full-scale testing is a costly endeavor, numerical modelling is used in Computer Aided Engineering (CAE) to test designs before proceeding with a physical prototype. To promote the use of new high strength steel grades in the industry, reliable and properly characterised material models are thus necessary. These models then need to be validated with component experiments to ensure that the models are accurate enough. This is usually done using crash box components in an axial compression or three-point bending setup because of their similarity to real structural components used in crash zones. In this work, EWF at the higher loading rates common in crash scenarios is further investigated to contribute additional data regarding strain rate dependence of fracture toughness measured within the frame of fracture mechanics for AHSS sheets. Furthermore, the crashworthiness of dynamically loaded axially compressed AHSS and PHS crash boxes are evaluated both experimentally using full-field measurements and numerically using a commercially available damage model. The high-speed photography allow for a more efficient component crashworthiness evaluation with fewer components due to the possibility to track crack initiations and their propagation during the deformation. The results from the commercial damage model show that although the prediction of the first cracks is decent, the damage evolution is not captured accurately. These results show the need for further development of economically feasible (shell) damage models that take propagation energy into account in crash simulations. This would also help promote the use of fracture toughness in the automotive industry.

The automotive industry is currently adapting to a new reality, where anthropogenic emissions need to decrease significantly. To meet present and future demands of vehicle design, press harden-ing techniques to produce complex geometries with high strength and ductility as well as good precision are of great interest. New generations of hot forming steels enable both further weight reductions by using thinner sheets as well as better crash performance due to its ability to improve the structural integrity of the body-in-white. To promote the use of these new steel grades, it is important to study their performance using well-instrumented lab scale test of full-scale compo-nents. Since these tests are often time consuming and expensive, calibrating constitutive models with tensile specimens and using finite element analysis is a more cost-effective alternative. How-ever, these calibrated models should be validated against full-scale experiments to verify their effectiveness in predicting the material behaviour in complex crash environments. In this paper, a high-speed 3D digital image correlation experiment is performed on a crash box under axial com-pression. The material is a hot forming steel grade with a specified tensile strength of 1000 MPa. The axial crash tests are modelled based on a visco-plastic model calibrated by high-speed tensile tests. The computed results in terms of force response and obtained deformations agree well with the corresponding measurements.

To promote the use of newhigh-strengthmaterials in the automotive industry, the evaluation of crashworthiness is essential, both in terms of finite element (FE) analysis aswell as validation experiments. Thiswork proposes an approach to address the crash performance through high-speed imaging combined with 3D digital image correlation (3D-DIC). By tracking the deformation of the component continuously, cracks can be identified and coupled to the load and intrusion history of the experiment. The so-called crash index (CI) and its decreasing rate (CIDR) can then be estimated using only one single (or a few) component, instead of a set of components with different levels of intrusion and crushing. Crash boxes were axially and dynamically compressed to evaluate the crashworthiness of TRIP-aided bainite ferrite steel and press-hardenable steel. Acalibrated rate-dependent constitutivemodel, and a phenomenological damage model were used to simulate the crash box testing. The absorbed energy, the plastic deformation, and the CIDR were evaluated and compared to the experimentally counterparts. When applying the proposed method to evaluate the CIDR, a good agreement was found when using CI:s reported by other authors using large sets of crash boxes. The FE analyses showed a fairly good agreement with some underestimation in terms of energy absorptions. The crack formation was overestimated resulting in too high a predicted CIDR. It is concluded that the proposed method to evaluate the crashworthiness is promising. To improve the modelling accuracy, better prediction of the crack formation is needed and the introduction of the intrinsic material property, fracture toughness, is suggested for future investigations and model improvements.

Edge cracking has become a limiting factor in the use of some advanced high strength steels (AHSS) for high-performance automotive applications. This fact has motivated the development of a multitude of experimental tests for edge formability prediction over the last years. In this sense, the Hole Expansion Test (HET) according to ISO16630 has been established in the automotive industry as a standard procedure for edge cracking sensitivity ranking. However, whereas it may be useful for rapid material screening, the results are often not accurate and reliable enough. Consequently, alternative methods based on Digital Image Correlation (DIC) have been proposed aimed at improving the prediction of edge cracking occurrence during forming and obtaining useful strain data that can be implemented in forming simulations. This paper explores the applicability of different DIC-based methods, such as Half-Specimen Dome Tests, Sheared Edge Tensile Tests, and KWI hole expansion tests with a flat nosed punch, for characterizing the edge formability of three cold-rolled AHSS sheets. The results obtained from the different testing methods are compared and validated with a laboratory-scale demonstrator. Finally, the limitations and advantages of the different methods are discussed.

For the reduction of the environmental footprint of Heavy-Duty vehicles (HDV), lighter chassiscomponents can be considered. A lighter HDV chassis gives the opportunity of lower fuel consumption, increased payloads, and savings of material resources. One way of achieving this, is to reduce thicknesses of components in combination of using higher strength steels. For the aim of forming UHSS into complex geometries the need to characterize thick sheet metal at elevated temperatures arises. This work aims at expanding earlier research of characterization of thinner sheet metal and create a testing methodology for tensile tests of 7 mm thick steel sheets at elevated temperatures. An experimental methodology for evaluating high strength steel under warm conditions have been developed and demonstrated. A Digital Image Correlation system is used to extract strain fields for all three testing temperatures. This together with an automatized induction system pre-defined temperature cycles are applied. When the desired Hollomon-Jaffe constant is obtained the tensile test is executed. The methodology shows promising results with good repeatability of stress-strain curves. The methodology shows good stability and are promising for future development and investigations of high strength steels under warm forming conditions.