Combined Mode I‐Mode III Fracture of Fatigue‐Precracked Alumina

The mixed‐mode fracture behavior of (cyclic) fatigue pre‐cracked ceramic specimens was studied in combined tension‐torsion loading. Circumferentially notched cylindrical rods of polycrystalline alumina were precracked in uniaxiai cyclic compression to introduce a concentric mode I fatigue crack. Subsequently, the rods were quasi‐statically fractured in pure tension, pure torsion, and various combinations of tensile and torsional stresses to obtain the mode I‐mode III fracture envelope. The introduction of torsional loads promotes severe abrasion between the crack faces. The critical stress intensity factor for fracture initiation increases by a factor of °2.3 as the loading mode is changed from pure tension to pure torsion. Fracture surface tortuosity and abrasion “shield” the crack‐tip from the far‐field tensile and torsional loads to cause an apparent toughening effect. The mechanisms of mixed‐mode fracture in alumina are examined and consequences of the breakdown of the similitude concept implicit in the nominal use of fracture mechanics are discussed.