Measurement and use of stress relaxation data for copper alloys in bending

A review of both stress-relaxation behavior and common test techniques for measuring this behavior are presented and some experimental results are reported. Particular attention is given to the cantilevered beam method. Stress-relaxation data are commonly reported at two levels of applied stress, 100% of the 0.01% offset yield strength and 75% of the 0.2% offset yield strength. These stress levels are equivalent for many electronic connector alloys, so that data from tests at these two levels can be compared directly. Mechanical relaxation resulting from the initial room temperature loading of relaxation test specimens causes overestimates of the thermal relaxation. Experimental results show that the stress decreases by about 1% of the applied stress for the beryllium-copper alloys, while the alloys without beryllium decrease by approximately 5%. Strip thickness and lot-to-lot variation of alloys with similar compositions and processing histories produce no significant differences in stress-relaxation behavior. Mechanical properties of beryllium coppers were observed to increase during long-time exposure at 150, suggesting better stress-relaxation performance, while alloys like C19400, C51000, C52100, C68800, C72500 and C72900 showed little or no change