Title page for ETD etd-0204104-125758

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

Abstract
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 K_th) 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.

Files
lados.pdf

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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
Abstract 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 K_th) 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.
Files	lados.pdf