Sheet metal punching is an important process in manufacturing of heavy-duty vehicle chassis components. The cut edges have a detrimental effect on high cycle fatigue life in uniaxial- and in-plane-bending but the reduction is less pronounced in out-of-plane bending. This paper aims to explain the reduced process sensitivity in out-of-plane bending fatigue, to quantify the high cycle fatigue life reduction at different load ratios, and to propose a methodology for fatigue life estimation. This could enhance the possibilities to identify critical load cases of chassis components, to judge whether fatigue life improving post-processes are necessary, and to locate critical initiation sites for fatigue. Fatigue testing of punched and polished specimens was conducted, and the punching process and four-point bending were simulated using FEM. The results were used to estimate crack initiation site, fatigue life reduction, and for validating the predictions. Fatigue life reduction is found to increase with increased load ratio, but to a smaller extent than expected. A contributing factor the reduced tensile residual stresses due to plasticity during the first load cycle. The reduced process sensitivity as compared to uniaxial fatigue could be explained by the separate locations of crack initiation and high tensile residual stresses in the cut edge. Specimen orientation seems to have a minor influence on the fatigue life. Only improving the outer surfaces, and not the central parts of the cut edge, could increase the high cycle fatigue life for pulsating and reversed loading.

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 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.

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.

High-strength steels are widely used in vehicle body-in-white, offering a good balance between crashworthiness and lightweight design. The increased requirements of heavier electric vehicles, in terms of fatigue resistance and crashworthiness, highlight that chassis parts have remarkable lightweighting potential. However, applying these grades in chassis parts is not straightforward, as the forming processes, like trimming, may introduce surface defects that compromise the fatigue resistance of the component. This work presents a material selection strategy for the applicability of high-strength steels in chassis parts of electric vehicles. The proposed approach allows the evaluation of the key parameters of the chassis parts in a simple way. The crash performance is evaluated through fracture toughness using the essential work of fracture (EWF) methodology. The method is applied to thin high-strength steel sheets employing double-edge notched tensile specimens (DENT). On the other hand, fatigue performance is investigated in terms of fatigue resistance for both notched and unnotched specimens. The results for different complex-phase and dual-phase steels show a good agreement between the EWF and the fatigue notch factor. The method could help apply high-strength steel to chassis parts, as designers will have a tool to focus the expensive fatigue tests on the best material candidates.

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.

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.