Abstract:

Many experimental investigations were conducted on cyclic stress strain curves for seven types of airframe materials TC4, LY12CZ, LC4CS, LC9CS, LC9CgS3, 30CrMnSiA, and 30CrMnSiNi2A. For each kind of material, four kinds of strain ratios, six strain or stress levels for each strain ratio are tested. Based on the above test data, the equivalent strain life curves for these materials are deduced. A new method is presented for describing the cyclic stress strain behavior at any strain ratio, strain level and number of cycles. And a comparison is made among predicted fatigue lives by using different options of fatigue property descriptions and fatigue life predicting approaches. As a result, this paper suggests a practical approach for fatigue life prediction. Some conclusions are drawn as: (1) for the materials above, there are no real "stable" hysteresis curves or "saturation" phenomena in cyclic transient stress strain relationship existing; (2)it is not reasonable to classify a kind of material as wholly cyclic hardening or softening, because it may have contrary behavior under different strain ratios, strain amplitudes and cycles; (3)with enough experimental data, fitting a series of formulas to describe the transient hysteresis curve at any condition is practicable; (4) it is not acceptable to adopt the widely used Manson Coffin formula to predict the fatigue life under asymmetric cyclic strain; (5)to predict the fatigue life by using the local stress strain method, the option of the transient stress strain curve combined with the equivalent strain life curve may be more reasonable at present.

Key words: yield strength, cyclic stress-strain, cyclic hardening/softening, fatigue life