This paper was produced for the 2019 NAFEMS World Congress in Quebec Canada

Resource Abstract

When considering the design and analysis of motor vehicle such as cars, vans and trucks, there are two ways to treat static and dynamic fatigue problems. These are the time and frequency domain approaches. Time domain methods are the most common and most widely used, especially in the automotive industries and historically it has been the method of choice for the fatigue calculation of welded structures. In previous SAE and NWC papers the frequency approach has been successful applied showing a good correlation with the life and damage estimated using a time-based approach. In this paper the applicability of the method to both spot and seam welded structures is demonstrated.



Firstly, a new (and unique) frequency domain approach for spot welds is presented, which is closely based on the method of Rupp, Störzel and Grubisic [SAE Technical Paper 950711, 1995]. The original approach used a CBAR element to represent the spot weld. With later implementations, the spot weld was modelled using a HEXA (brick type) element joined to the sheet metal using RBE3 spiders. This is often referred to as the ACM2 or CDH method. The forces on the top and bottom faces of the HEXA are converted firstly into forces and moments on a hypothetical CBAR and then into radial stresses around the hypothetical spot weld circumference using Roark’s formulas for membrane stress (the original Rupp method). These stresses (at discrete points around the circumference) are then used to calculate fatigue damage using “reference” spot weld fatigue curves.



Secondly, a new (and unique) frequency domain approach for seam welds is presented which is closely based on the original work of Fermér, Andréasson and Frodin [SAE Technical Paper 982311, 1998]. The original method used grid point forces to calculate structural stresses. Later implementations, (following better element stress recovery) used corner or cubic stresses on CQUAD4 type elements as the structural stresses (at the weld toe position). These stresses are used to look up the fatigue life from an S-N data curve derived (through interpolation) from reference curves for stiff and flexible welds. The flexibility of each weld toe grid point is used for this interpolation.



This paper will highlight both the spot and seam weld methods with application to a typical automotive vehicle.