Document Type dissertation Author Name Lados, Diana Aida URN etd-0204104-125758 Title Fatigue Crack Growth Mechanisms in Al-Si-Mg Alloys Degree PhD Department Materials Science & Engineering Advisors Diran Apelian, Advisor Richard D. Sisson, Jr., Department Head Makhlouf M. Makhlouf, Committee Member Satya Shivkumar, Committee Member Peggy E. Jones, Committee Member Fred J. Major, Committee Member Keywords Microstructure Elastic-Plastic Fracture Mechanics Crack closure A356 J-integral Conventionally cast and SSM Al-Si-Mg alloys Residual stress Heat treatment Fatigue crack growth mechanisms Threshold stress intensity factor Plastic zone Paris law Fracture toughness Roughness Date of Presentation/Defense 2004-01-16 Availability unrestricted
Due to the increasing use of cyclically loaded cast aluminum components in automotive and aerospace applications, fatigue and fatigue crack growth characteristics of aluminum castings are of great interest. Despite the extensive research efforts dedicated to this topic, a fundamental, mechanistic understanding of these alloys’ behavior when subjected to dynamic loading is still lacking. This fundamental research investigated the mechanisms active at the microstructure level during dynamic loading and failure of conventionally cast and SSM Al-Si-Mg alloys. Five model alloys were cast to isolate the individual contribution of constituent phases on fatigue resistance. The major constituent phases, alpha-Al dendrites, Al/Si eutectic phase, and Mg-Si strengthening precipitates were mechanistically investigated to relate microstructure to near-threshold crack growth (Delta Kth) and crack propagation regimes (Regions II and III) for alloys of different Si composition/morphology, grain size, secondary dendrite arm spacing, heat treatment. A procedure to evaluate the actual fracture toughness from fatigue crack growth data was successfully developed based on a complex Elastic-Plastic-Fracture-Mechanics (EPFM/J-integral) approach. Residual stress-microstructure interactions, commonly overlooked by researches in the field, were also comprehensively defined and accounted for both experimentally and mathematically, and future revisions of ASTM E647 are expected.
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