Ratcheting and fatigue response of materials undergoing asymmetric multiaxial and multistep loading spectra in the absence and presence of notches

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Catastrophic failure in engineering components in-service annually costs over $150 billion dollars in the North America. Design of parts in various industries such as automotive, aerospace, power plant and pressure vessel are rigorously hinged upon materials integrity, complex geometry, and loading conditions. As the existence of stress raisers such as holes and notches is inevitable, reliable design of machinery parts must be adapted through choice of influential parameters; material property, cyclic hardening/softening, stress level, and stress raiser shape/ size to lessen ratcheting deformation and to improve fatigue life. Applied stress cycles are highly concentrated at the vicinity of notch root causing a severe accumulation of plastic strain over asymmetric loading cycles, referred as local ratcheting. Ratcheting and fatigue phenomena progressively promote damage when they are coupled. Safety issues and expenses associated with failure of engineering parts, necessitate a high degree of reliability in design and analysis through developing a robust ratcheting-fatigue approach. The research program develops a robust framework to assess multiaxial ratcheting of SS316 steel and Al7075-T6 samples undergoing asymmetric fatigue cycles at various stress levels, rates, steps, sequences, and holding-time events. To account for rate-dependency in ratcheting, the Ahmadzadeh-Varvani (A-V) model developed under supervision of the applicant will be employed along with the unified viscoplastic flow rule. An indispensable step in the proposed framework is to govern plastic strain over loading cycles through the hardening rule. An analytical approach will be developed to relate applied stress/ strain to local stress/ strain components at notch root over loading cycles. The proposed algorithm will enable to evaluate (i) ratcheting progress over stress cycles through the framework, and (ii) stress relaxation at the vicinity of notch root and for given plastic strain increments. Through the course of the proposed research, several ratcheting tests with different loading steps/ sequences will be conducted up to failure stage on unnotched and notched samples using the applicant's multiaxial fatigue testing facilities at Ryerson University funded by NSERC in 2000. The applicant believes that upon acceptance of the proposal, outcomes of the proposed research over the next five years will largely contribute to develop design guidelines, safety codes leading engineers in the field toward a more reliable design of load-bearing parts, and noticeably saves cost involved with premature failure of engineering components. The involvement of students throughout the proposed research under the applicant's supervision will train highly qualified personnel capable of serving in Canadian and world-wide industries including automotive, aerospace, pressure vessel, and pipeline. This will benefit people and society employing safe and durable manufactured goods and components.