James M. Twomey, Deng Y. Chen, Michael D. Osterman, and Michael G. Pecht
CALCE, Center for Advanced Life Cycle Engineering, Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20742, USA
Load cycle counting is an important tool for estimating the fatigue life of structures. Rainflow counting methods have been developed to count load reversals and extract load cycles from load time histories. Rainflow counting methods were originally developed to assess fatigue damage associated with mechanical cycling where time-dependent factors such as dwell time and loading rate were not considered significant contributors to fatigue failure. However, loading rate and hold times can be significant factors in some fatigue failures, such as solder interconnects under temperature cycling. In these cases, fatigue life models may require the load transition time and load hold times to account for time-dependent material changes, such as creep. This paper develops a new multi-parameter cycle counting algorithm that provides a range-based hold time estimation for use with time-dependent fatigue damage models. To show the applicability of the new method, the life of solder joints under a complex thermal cycling regime was estimated using the data extracted by the new multi-parameter cycle counting algorithm. This life estimate was then compared with test results and found to be within the estimated uncertainty bounds.