Axle Bracket Weld Fatigue Analysis Using Verity And Node Based Submodeling
Axle Bracket Weld Fatigue Analysis Using Verity and Node Based Submodeling Marcus Peng Chen & Jerry Chung Analytical Engineering, American Axle & Manufacturing
Table of Contents * Part I: Background & Challenge; * Part II: Submodeling & Case Study; * Part III: Summary & Discussion;
Axle Failure Mode Part I: Background & Challenge * Seam weld is extensively used on Beam/Banjo axles to attach brackets; * In addition to the global axle bending/distortion, brackets are subject to loading as well; * Therefore, weld fatigue failure is one of the most dominant failure modes in axles.
Limitation of Weld Stress Analysis Different weld mesh of the same model Part I: Background & Challenge Issue 1: a slight change in element size/orientation results in significant difference in the weld end stress. Issue 2: actual weld failure occurred on another weld with much lower stress.
Weld Fatigue Process & Common Failure Modes 1. Crack Initiation The initiation fatigue life is usually short in the welded structure because it can be assumed that sharp-edged discontinuities exist in a welded structure. [Almar 1985; Maddox 1991, Nykanen 1993] 2. Crack Propagation The fatigue life estimations for welded structure are often based on the fatigue life propagation using fracture mechanics. [Almar 1985; Niemi 1995] Toe Failure (1), (2), (3) Throat Failure (4) Root Failure The most common failure mode. Does not occurs very often, and is usually caused by weld process imperfections. [Verity® in fe-safe V2 User Manual] Part I: Background & Challenge
Verity Showed a Good Correlation Part I: Background & Challenge 1.47X cycles 0.21X cycles to crack penetration @ the arrow marked spot Test: X cycles to visible leakage * As shown above, Verity was able to pinpoint the failure location. * Prediction indicates crack penetration at one spot, which is not exactly equivalent to the visible leakage time seen from the test.
Challenges & Proposed Solution Part I: Background & Challenge Solution: Node Base Submodeling (Global/Local Method) * Eliminate the necessity of fine-meshing the whole axle so that more efforts could be focused on the interested areas (such as weld). * Avoid rebuilding full axle model if a bracket or weld has been redesigned. Interested area: Bracket Weld Toe Challenges : * Due to the special mesh requirements imposed by Verity, brick element mesh for the whole tube turned out to be labor-intensive and error-prone. In addition, the inclusion of numerous welds makes the meshing even more difficult. * Full axle model takes long time to solve. * With the inclusion of the Verity required Nodal Force output, the FEA output file can easily exceed 5 gb, which does not only consume more FeSafe File Loading/Solving time, but also causes FeSafe Reading error in some cases.
Submodel Setup: Case Study I Part II: Submodeling & Case Study Step 2: Local model (conforms to Verity mesh requirement in the weld toe area), which only includes the interested welds and brackets. And the Nodal Displacement of the Global model is used as the boundary condition for this model. The solve time for this local model is very short (1 second for this case). Step 1: Global model (1 st /2 nd Tet/Brick),which does not have to include welds and brackets. The Spring Seats are included in this model for Boundary Condition purpose. No special mesh treatment is needed.
Submodel v.s. Full Model: Case Study I Stress Comparison Part II: Submodeling & Case Study Full Model Submodel
Submodel v.s. Full Model: Case Study I Weld Fatigue Comparison Part II: Submodeling & Case Study 0.21X cycles to crack penetration @ the arrow marked spot 0.19X cycles to crack penetration @ the arrow marked spot Test: X cycles to visible leakage Full Model Submodel
Case Study II Weld Fatigue Correlation Part II: Submodeling & Case Study X cycles to tube fracture . [Wei Li, 2010] * Both test and FEA prediction indicate that the tube will crack from the same location shown above. * The prediction indicates crack penetration at one spot , which is not exactly equivalent to the visible tube fracture time seen from the test. * As shown above, the actual weld geometry is not well controlled, which does causes large variations in weld fatigue life. 0.38X cycles to crack penetration @ the arrow marked spot.
Advantages of Submodeling Combined with Verity Part III: Summary & Discussion Full Model Submodel FEA Pre-processing Difficult to mesh; time-consuming Easy to mesh the global model; weld toe mesh is treated separately FEA Solver Time A few hours Global model takes a few hours; submodel run for a few seconds FEA Output File Size A few GB A few GB for global model, a few MB for the submodel FeSafe Pre-processing 10-20 minutes to load the model; possible to exceed FeSafe limit Less than 1 minute to load the model FeSafe Solver Time 0.5-1 hours A few minutes Post-Processing Takes a few minutes to load the model Takes a few seconds to load the model Accuracy Better Less accurate Bracket Design Change Have to rebuild the full model Can use the same global model, and rebuild the submodel only