Development of mechanism-based models for resistance spot weld failure simulation of multi-material advanced high strength steel sheets

Abstract

The automotive industry is constantly involved in the development of new projects aimed at reducing weight, fuel consumptions and costs while improving passengers? safety. In order to achieve these increasing demands, Advanced High Strength Steels (AHSS) have been introduced in recent years reducing vehicle structure weights and improving the crashworthiness. With the increase in the bearing capacity of crash-relevant structural components, the sheet metal joining techniques such as adhesive bonding and resistance spot welding (RSW) become critical. In order to develop the vehicle structure in these new projects, full-vehicle crash finite element simulations are usually performed. Simplified beam-like models are currently used in these simulations (with thousands of spot welds) to represent RSW joints response. The maximum bearing force of these models are fitted using large experimental campaigns, considering all the main factors that have the highest influence on the fracture response of a welded joint. The objective of this thesis is twofold: (1) to develop a model that is able to partially replace the extensive experimental campaign in providing parameters for the crash simulation simplified spot weld models, and (2) to gain understanding of spot weld joints failure response in order to improve the current simplified models. To achieve these objectives, a detailed spot weld model for the prediction of spot weld failure in joints in AHSS sheets is presented. The presented model includes a definition of the local material properties as well as the geometry features of a spot weld. In addition, an industrially suited fracture criterion, i.e. robust and without a long-term calibration, is used for the prediction of maximum force. An energetic fracture criterion based on the use of elastic-plastic fracture-mechanics is identified as the better suited for the prediction of spot weld failure and joint bearing capacity. The J-integral is evaluated in the weld notch and this value is compared with a material parameter, the fracture toughness, in order to obtain the joint maximum force. The presented detailed FE spot weld model is validated to joints of two different steel grades of the AHSS family usually present in the current vehicle structure, a hot formed martensitic boron-alloyed steel (22MnB5) and a cold formed dual phase steel (DP 980). The validation is performed comparing the results obtained with the finite element model and the experimental results extracted from an extensive loading test experimental campaign where the main factors that have an influence in the spot weld fracture response are considered. The obtained simulated critical forces of the loading tests present good agreement with the experimental ones in all tested configurations. Finally, based on the presented finite element spot weld model, some recommendations are exposed for extending the model for new combinations and loading conditions. The proposed procedure can be used to reduce the long-term characterization campaigns used to calibrate the joints of a new AHSS grade, where fracture is triggered by stress concentration ahead of a notch. Furthermore, some recommendations for the future structure design are given taking into account the information obtained with the present model.