This rise is coincident with the initiation of microcracking as recorded by acoustic emissions (elapsed time∼2.5 h, Figure 3). The acoustic emission signal shows large steps indicative of large, discrete fracturing events. Small fracturing events below detection limits could be occurring between the larger acoustic emission events (Figure 3).During this phase, the 4He release rate is observed to increase up to 100% before any signs of macroscopic failure become evident.