Recently Elber proposed that fatigue cracks close at positive stress levels during constant amplitude stress cycling. To investigate this behaviour, centrally cracked 2024-T3 A~umiRium ahoy sheets were tested. Crack opening displacements and strains near the fracture surface were measured for a propagate fatigue crack. The experimentai results confirm the presence of crack closure at significant positive stress levels during the unloading portion of the cycle.
Plastic deformation remaining in the wake of crack extension was considered to be the factor controlling crack closure. Basic displacement reiationships were estabhshed for a sheet containing a fatigue crack, and were used to develop a qualitative discussion of the conditions governing crack closure. half-crack length, m SYMBOLS crack opening displacement, m crack opening displacement at maximum load. m Young's Modulus of EIasticity, MN/m2 shear modulus, MN/m* imaginary part of function distance between reference plane and crack plane, m real part of function ratio of minimum to maximum applied stress stress, MN/m2 m~imum applied stress during cyclic loading, (based on gross area). MN/m* mean applied stress during cyclic loading MN/m* minimum applied stress during cyclic loading, MN/m2 Cartesian coordinates complex variable, z = x+ Q dispIacement, m crack opening displacement at zero toad, m displacement due to plastic deformation, m change in length along a line of length L, tn strain elastic strain (3-v)/<! +v) forplanestress. Poisson's ratio yield stress. MN/m? stress function IN THE STUDY of fatigue crack propagation, crack closure has been considered to occur only for compressive loadsll].
The reasoning that leads to this assumption was based on linear elastic analyses modified for plasticity at the crack tip. The plasticity leads to residual tensile deformation in front of the crack tip, thus preventing crack closure at zero load. On the other hand, Elber f3] said that a fatigue crack did close at a positive stress level. To explain this new opinion, it was pointed out [23 that previous analyses did not take account of the plastic tensile deformation left in the wake of crack extension. This deformation was said to reduce crack opening and therefore to produce crack closure at positive loads during unloading. Results in both[2 and 31 could be interpreted as supporting this statement.