ON THE APPLICABILITY OF THE MAGNETIC EMISSION TECHNIQUE FOR DETERMINING DUCTILE CRACK INITIATION IN IMPACT TESTS

The magnetic emission (ME) technique is used to determine the onset of crack propagation in brittle and ductile steels under dynamic loading . Brittle fracture is directly indicated by the ME signal; however the onset of stable crack propagation can usually not be seen in this sig nal. A new result is that the integrated ME signal, which is proportio nal to the magnetic held at the site where the probe is located, is ca pable of detecting this event. Critical values of the J-integral deriv ed with this method, in different test series, are compared with conve ntional R-curve and stretch zone methods. The ME-derived critical dyna mic J-integral data fit very well to the stretch zone results which de scribe physical crack initiation. It is therefore concluded that the e asy-to-apply and low-cost ME technique is capable of indicating materi al failure even with stable crack propagation and can therefore provid e critical material parameters.