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Generally linear finite element analysis (FEA) is used to predict fatigue life of spot welded joints in a vehicle body structure. Therefore, the effect of plastic deformation at the vicinity of the spot welded joints is not included on fatigue prediction. This study introduces a simple technique to include the plastic deformation effect without performing elastic-plastic finite element analysis. The S-N curve obtained from fatigue test results is modified to consider this effect. Tensile strength test results of spot welded joint specimens were utilized to find the load range for FEA equivalent to the applied load range for fatigue tests. To demonstrate the proposed approach, fatigue test results of advanced high strength steels (AHSS) for lap-shear and coach peel specimens were used. Both the specimen types were tested at various constant amplitudes with the load ratios of R=0.1 and 0.3.

In the North American automotive industry, various advanced high strength steels (AHSS) are used to lighten vehicle structures, improve safety performance and fuel economy, and reduce harmful emissions. Relatively thick gages of AHSS are commonly joined to conventional high strength steels and/or mild steels using Gas Metal Arc Welding (GMAW) in the current generation body-in-white structures. Additionally, fatigue failures are most likely to occur at joints subjected to a variety of different loadings. It is therefore critical that automotive engineers need to understand the fatigue characteristics of welded joints. The Sheet Steel Fatigue Committee of the Auto/Steel Partnership (A/S-P) completed a comprehensive fatigue study on GMAW joints of both AHSS and conventional sheet steels including: DP590 GA, SAE 1008, HSLA HR 420, DP 600 HR, Boron, DQSK, TRIP 780 GI, and DP780 GI steels.

Great efforts have been made to develop the ability to accurately and quickly predict the durability and reliability of vehicles in the early development stage, especially for welded joints, which are usually the weakest locations in a vehicle system. A reliable and validated life assessment method is needed to accurately predict how and where a welded part fails, while iterative testing is expensive and time consuming. Recently, structural stress methods based on nodal force/moment are becoming widely accepted in fatigue life assessment of welded structures. There are several variants of structural stress approaches available and two of the most popular methods being used in automotive industry are the Volvo method and the Verity method. Both methods are available in commercial software and some concepts and procedures related the nodal force/moment have already been included in several engineering codes.

In this study fatigue tests of GMAW (Gas Metal Arc Welding) welded joints were conducted on both 1.6mm body sheet (DQSK-GA, DP590-GA, DP780-GI, and TRIP 780-GI) and 3.4mm frame materials (SAE1008 HR 240MPa, HSLA420 HR, DP600 HR, and uncoated Boron). Further, mixed thickness joints were tested which combined 3.4mm SAE1008 HR with each of the 1.6mm separately – with the exception of DQSK. A number of different joint configurations were tested including single and double lap-shear, start-stop lap shear, butt weld, and perch mount. Great care was taken in this study to ensure that the geometry of the welds was consistent, not only within a given material lay-up, but between all of the specimens of a given type – this effort was made in order to substantially reduce life scatter and provide a better understanding of the role base material plays in the fatigue life of GMAW joints.

An experimental study was conducted to evaluate the effect of water absorption on tensile and fatigue behaviors of an impact-modified short glass fiber polyamide-6 and a short glass fiber polybutylene terephthalate. Specimens were prepared in the longitudinal and transverse directions with respect to the injection mold flow direction and immersed in water. Kinetics of water absorption was studied and found to follow the Fick's law. Tensile tests were performed at room temperature with specimens in the longitudinal and transverse directions and with various degrees of water absorption. Mathematical relations were developed to represent tensile properties as a function of water content. Load-controlled tension-tension fatigue tests were conducted in both longitudinal and transverse directions and correlations between tensile and fatigue strengths were obtained. Specimen fracture surfaces were also microscopically studied and mechanisms of tensile and fatigue failures were identified.

An experimental study was conducted to evaluate the variable amplitude fatigue behavior of a neat polymer (polypropylene impact co-polymer) and a polymer composite made of polybutylene terephthalate (PBT) with 30 wt% short glass fibers. Fatigue tests were conducted on un-notched and notched specimens at room temperatures. Plate-type specimens were prepared in the transverse direction with respect to the injection mold flow direction and a circular hole was drilled in the center of notched specimens. Two-step loadings (high-low and low-high) tests at two damage ratio of 0.2 and 0.5 at stress ratios of R = 0.1 and -1 were conducted to investigate load sequence effects and prediction accuracy of the linear damage rule. Different behaviors were observed for unreinforced and short glass fiber reinforced polymers under the two-step loading tests.

Joining technology is a key factor to utilize dissimilar materials in vehicle structures. Adaptable insert weld (AIW) technology is developed to join sheet steel (HSLA350) to cast magnesium alloy (AM60) and is constructed by combining riveting technology and electrical resistance spot welding technology. In this project, the AIW joint technology is applied to construct front shock tower structures composed with HSLA350, AM60, and Al6082 and a method is developed to predict the fatigue life of the AIW joints. Lap-shear and cross-tension specimens were constructed and tested to develop the fatigue parameters (load-life curves) of AIW joint. Two FEA modeling techniques for AIW joints were used to model the specimen geometry. These modeling approaches are area contact method (ACM) and TIE contact method.

Because of increasing fuel costs and environmental concerns, the automotive industry is under enormous pressure to reduce vehicle weight. One strategy, downgaging, substitutes a reduced gage (thickness) steel in place of a thicker one, and is usually accompanied by a material grade change to a higher strength steel. Thus, Advanced High Strength Steels (AHSS) are increasingly used for lightweight automotive body structures. The critical durability concern with steels is the spot welds used to join them, since fatigue cracks in body structures preferentially initiate at spot welds. Hence, the Auto/Steel Partnership (A/SP) Sheet Steel Fatigue Taskforce undertook an investigation both to study the fatigue performance of AHSS spot welds, and to generate data for OEM durability analysis. The study included seven AHSS grades and, for comparison, mild steels and a conventional High Strength Low Alloy grade, HSLA340.

Morphological features of voids were characterized for T300/924 12-ply and 16-ply composite laminates at different porosity levels through the implementation of a digital microscopy (DM) image analysis technique. The composite laminates were fabricated through compression molding. Compression pressures of 0.1MPa, 0.3MPa, and 0.5MPa were selected to obtain composite plaques at different porosity levels. Tension-tension fatigue tests at load ratio R=0.1 for composite laminates at different void levels were conducted, and the dynamic stiffness degradation during the tests was monitored. Fatigue mechanisms were then discussed based on scanning electron microscope (SEM) images of the fatigue fracture surfaces. The test results showed that the presence of voids in the matrix has detrimental effects on the fatigue resistance of the material, depending on the applied load level.

An experimental investigation was conducted to evaluate tensile and fatigue behaviors of two thermoplastics, a neat impact polypropylene and a mineral and elastomer reinforced polyolefin. Tensile tests were performed at various strain rates at room, −40°C, and 85°C temperatures with specimens cut parallel and perpendicular to the mold flow direction. Tensile properties were determined from these tests and mathematical relations were developed to represent tensile properties as a function of strain rate and temperature. For fatigue behavior, the effects considered include mold flow direction, mean stress, and temperature. Tension-compression as well as tension-tension load-controlled fatigue tests were performed at room temperature, −40°C and 85°C. The effect of mean stress was modeled using the Walker mean stress model and a simple model with a mean stress sensitivity factor.

Gas Metal Arc Welding (GMAW) is widely employed for joining relatively thick sheet steels in automotive body-in-white structures and frames. The GMAW process is very flexible for various joint geometries and has relatively high welding speed. However, fatigue failures can occur at welded joints subjected to various types of loads. Thus, vehicle design engineers need to understand the fatigue characteristics of welded joints produced by GMAW. Currently, automotive structures employ various advanced high strength steels (AHSS) such as dual-phase (DP) and transformation-induced plasticity (TRIP) steels to produce lighter vehicle structures with improved safety performance and fuel economy, and reduced harmful emissions. Relatively thick gages of AHSS are commonly joined to conventional high strength steels and/or mild steels using GMAW in current body-in-white structures and frames.