A cumulative damage procedure based on smooth-specimen tests of 2024-T4 aluminum alloy and aircraft quality SAE 4340 steel is formulated in which the effects of prestrain and mean stress on fatigue life are investigated separately. It is assumed that life reductions due to prestrain or mean stress at a given strain level may be separately incorporated into a simple cycle-ratio damage summation. Small plastic prestrains are shown to cause considerable life reductions. Larger prestrains appear to have little additional adverse effect. It was found possible adequately to correlate results with and without mean stress by a parametric representation of mean stress and strain amplitude. Analysis of arbitrary stress-strain sequences indicates that the proposed damage-summation procedure gives adequate life predictions. Cycle-ratio summations were close to unity for both stress-controlled and strain-controlled tests in which sequence, number of blocks, fraction of life at mean stress, and the life fraction at which plastic straining occurred were all varied.

An anisotropic model of continuum damage mechanics has been developed to predict the creep-fatigue life of solder joints. With the help of the finite element method, the stress, strain, and damage fields of the time-dependent and temperature-dependent solder can be obtained. The main advantages of this model include: (1) It can predict the initial crack location and time and the subsequent crack growth paths; (2) The damage analysis is almost the same as in traditional viscoelastic finite element analysis; (3) It can be applied to a complex structure with any loading; (4) It provides a full-field damage investigation of the structure. This damage theory can be used for various solder joints and also can be applied to analyze the creep-fatigue problems of other ductile and temperature-dependent materials. Extensive experiments including uniaxial creep, uniaxial fatigue, tension-torsion, Moiré, and bimaterial tests were performed to validate the new model. These validations and comparisons indicate that this model can predict adequately crack growth paths and fatigue lives of solder joints.

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