THE PROPAGATION OF STRESS CORROSION CRACKS IN ALPHA-BRASSES.

摘要

An investigation was conducted on the propagation of transgranular and intergranular stress corrosion cracks in commercial grade alpha-brasses tested in a tarnishing ammoniacal solution. Observation of the surface crack traces indicated that the transgranular cracks propagated discontinuously by the sudden appearance, in its entirety, of a fine crack extending several microns ahead of the previous crack tip. The appearance of this fine crack was often associated with the detection of a discrete acoustic emission (AE). By periodically increasing the deflection during these studies, crack front markings were produced on the resulting fracture surfaces and these showed that the crack front at the free surface did not lag behind the overall crack front. This indicated that the discontinuous propagation of the crack trace was representative of the subsurface cracking. The intergranular crack trace appeared to propagate continuously at a relatively blunt crack tip and was not associated with discrete AE.;Based on these observations, transgranular cracking is believed to occur by discontinuous mechanical fracture of an embrittled region around the crack tip. The specific mechanism of embrittlement is not understood. Intergranular cracking is believed to result from a different mechanism with cracking occurring via the film-rupture mechanism.;Load pulsing tests were also conducted to determine whether transgranular stress corrosion cracks propagated discontinuously and to compare values for the crack advance distance, (DELTA)x*, and the time between advances, (DELTA)t*, with those observed for the crack trace. When tested with a time between pulses, (DELTA)t (GREATERTHEQ) 3 s, the transgranular fracture surfaces always exhibited crack front markings which had a one-to-one correspondence with the applied pulses. The spacing between crack front markings, (DELTA)x, decreased linearly with (DELTA)t. The transgranular surfaces produced with (DELTA)t (LESSTHEQ) 1.5 s exhibited crack front markings in a one-to-one correspondence with the applied pulses only at relatively long crack lengths. At shorter crack lengths, there were fewer markings than applied pulses indicating that cracking was discontinuous and that (DELTA)t* was between 1.5 and 3 s. In this case, (DELTA)x = (DELTA)x* which approached a limiting value of 1 (mu)m. No crack front markings were observed on intergranular fracture surfaces produced during these tests suggesting that intergranular cracking resulted from a different mechanism than that responsible for transgranular cracking.