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Understanding total fatigue life of welded joints is crucial to developing durable products. Traditional fatigue analysis methods have focused independently on either crack initiation or crack growth. Each of these methods has strengths but neither method predicts the total life of the part from fabrication to fracture. Recently the SAE Fatigue Design and Evaluation committee evaluated and validated a fatigue analysis technique that can predict the total life of the weld, from microscopic crack initiation to crack growth and finally to fracture. This paper describes a finite element-based (FE) methodology for implementing this total life fatigue analysis in a CAE environment.

A new total fatigue life methodology was utilized to make fatigue life predictions, where total fatigue life is defined as crack initiation and subsequent crack propagation to a crack of known size or the component’s inability to carry load. Fatigue life predictions of an A36 steel T-joint geometry were calculated using the same total fatigue life methodology for both welded and machined test specimens that have the same geometry. The only significant difference between the two analyses was the inclusion of the measured weld residual stresses in the welded specimen life predictions. Constant amplitude tests at several load levels and R ratios were analyzed along with block cycle and variable amplitude loading tests. The accuracy of the life predictions relative to experimental test lives was excellent, with most within a factor of +/- two.

Durability and reliability are crucially linked in product validation testing. Typically the product’s life requirement is to be able to withstand specified loading for a given duration with desired reliability and confidence levels. Product validation or durability testing is then used to assess actual product life relative to these requirements. The goal of validation test is to demonstrate that the part is indeed capable of withstanding the loading that it will see in service. It is desirable that lab loading is representative of and correlates with service loading. Fatigue analysis techniques and material data like the stress-life (SN) curve can be used to define equivalent damage test specifications and accelerate tests so a long service life can be replicated quickly in the test lab.

Finite Element Analysis (FEA)-based structural simulations are typically used to assess the durability of automotive components. Many parts experience vibration in use, and resonance effects are directly linked to many structural problems. In this case, dynamics must be included in the structural analysis. Dynamic FEA can be more realistic than static analysis, but it requires knowledge of additional characteristics such as mass and damping. Damping is an important property when performing dynamic FEA, whether transient or steady state dynamics, as it governs the magnitude of the dynamic stress response and hence durability. Unfortunately the importance of damping is often overlooked; sometimes a default damping value is erroneously assumed for all modes. Errors in damping lead to errors in the stress response, which in turn lead to significant changes in the fatigue life estimates.