Firstly, according to analyze the fatigue test result of the normalized carbon steel, a logarithmic expression is developed to describe residual strength degradation process, the definition and expression of fatigue damage under symmetrical she with constant amplitude are also given, and the expression 'can explain the non-linear property of fatigue damage.

Based on the study of strength degradation of material in the fatigue process,a strength degradation model is proposed.

A nonlinear and continuous strength degradation model is presented from the view point that fatigue damage process is the process during which material static strength degrades constantly.

Stress-strength interference reliability analysis considering stochastic multi-stress and strength aging degradation;

A stress-strength interference(SSI) reliability model taking into consideration stochastic stress and strength aging degradation and point estimation of unknown parameters in the model are presented.

In order to master the mechanical behaviors of vertical connection and correct design in prefabricated reinforced concrete (PRC) structures the strength degeneration and stiffness degeneration of the connection under repeated loading are analysed based on test results and theoretical analysis.