Mechanical reasons for plasticity-induced crack closure under plane strainconditions

A mechanical explanation of plasticity-induced crack closure under plain st rain conditions is given first by means of dislocation mechanics and then b y the methods of continuum mechanics. In plane strain, the event of crack c losure is due to transport of material from the wake to the crack tip. It i s an elastic effect caused by the response of the matrix surrounding the pl astic wake. The transported material produces a wedge which follows the cra ck tip, and unlike the plane stress condition it does not leave a remaining layer on the crack flanks. The length of the produced wedge at the crack t ip is of the same scale as the plastic zone. It is then shown that in spite of its smallness this wedge is able to cause the experimentally observed s hielding effect. The results also suggest that the discrepancies concerning the interpretation of fatigue crack growth and closure experiments are lik ely to be due to differences in accuracy in the detection of such small but nevertheless effective wedges.