Effect of load pulse shape on predicted damage accumulation in wood

Load-duration (creep-rupture) effects in wood are often modeled using cumulative damage theory. In application, such as for duration of load reliability analyses, incremental damage owing to stochastic load pulse models is summed over an assumed reference period (e.g., 50 years), with failure defined in terms of the damage state variable. The effect of load pulse shape on cumulative damage is examined herein through the use of one such proposed damage accumulation model. An exponential damage rate model (EDRM) is used to calculate the damage owing to a rectangular load pulse (RECT), a triangular load pulse (TRI), and two trapezoidal load pulses (TRAP1 and TRAP2). The differences in the calculated damage predicted by the EDRM and resulting from the four load pulse shapes are presented and the effect of critical load parameters (e.g., load intensity and duration) are illustrated. Also, the effect of neglecting ramp loading and unloading is discussed; that is, the error in assuming RECT versus either TRAP1 or TRAP2 load pulses is assessed.