Subcritical Crack Growth in MnZn Ferrites

Abstract
In order to establish the influence subcritical crack growth (SCG) has on the mechanical behaviour of commercial MnZn ferrites, both constant stress rate (CSR) and constant load (CL) tests were performed on E 42 cores using the W test. All tests were done at room temperature and under a controlled humidity or submerged in water. Increasing humidity decreases strength and lifetime. In CSR tests with increasing stress rate, strength increases. In CL tests at two different humidities, a threshold at about 75 % of the inert strength seems to limit the effect of SCG. The strength of these cores in W tests depends on the behaviour of only two kinds of defects. The fraction of the cores failing on either kind depends on stress rate and humidity. These two factors also influence the fracture mode, in these tests increasing from 35 to 62 % transgranular fracture with increasing humidity and decreasing loading rate.