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 signal. A new result is that the integrated ME signal, which is proportional to the magnetic field at the site where the probe is located, is capable of detecting this event. Critical values of the J-integral derived with this method, in different test series, are compared with conventional R-curve and stretch zone methods. The ME-derived critical dynamic J-integral data fit very well to the stretch zone results which describe physical crack initiation. It is therefore concluded that the easy-to-apply and low-cost ME technique is capable of indicating material failure even with stable crack propagation and can therefore provide critical material parameters.