NEAR-TIP STRESS AND STRAIN FIELDS FOR SHORT ELASTIC CRACKS

Recent experimental fatigue crack growth studies have concluded an app arent anomalous behavior of short cracks. To investigate the reasons f or this unexpected behavior, the present paper focuses on identifying the crack length circumstances under which the requirements for a sing le parameter (K-I or Delta K-I if cyclic loading is considered) charac terization are violated. Furthermore, an additional quantity, the T st ress, as introduced by Rice, and the related biaxiality ratio B are ca lculated for several crack lengths and two configurations, the single- edge-cracked and the centrally-cracked specimen. It is postulated that a two-parameter characterization by K and T (or B) is needed for the adequate description of the stress and strain field around a short cra ck. To further verify the validity of the postulate, the influence of the third term of the Williams series on the stress, strain and displa cement fields around the crack tip and in particular on the B paramete r is also examined. It is found that the biaxiality ratio would be mor e negative if the third term effects are included in both geometries. The study is conducted using the finite element method with linearly e lastic material and isoparametric elements and axial (mode I) loading. Moreover, it is clearly shown that it is not proper to postulate the crack size limits for 'short crack' behavior as a normalized ratio wit h the specimen width a/w; it should instead be stated as an absolute, or normalized with respect to a small characteristic dimension such as the grain size. Finally, implications regarding the prediction of cyc lic (fatigue) growth of short cracks are discussed.