The critical J-integral (J_(id))can be useful as toughness parameter at initiation of stable crack growth. Under the condition of the impact bending test the critical step for evaluating of J_(id) is the detection of the crack initiation point on the impact load displacement curve. In this paper the crack initiation is determined by using emission of acoustic waves (AE). The experimental techniques used to evaluate J_(id) include both instrumented impact testing based on 300 J pendulum impact tester with a piezoelectric broadband AE sensor within the impact tup and dynamic 3 point bending loading with a servohydraulic test machine and additional AE sensors on the specimen. Different kind of pulses of the AE signals can be observed. One of them corresponds with crack initiation. The evidence of initiation at this point was confirmed by single and multiple specimen methods.
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