Hsin Jen Hoh

Plastic zone size and crack tip opening displacement of a Dugdale crack interacting with a coated circular inclusion

An analytical investigation on the plastic zone size of a crack near a coated circular inclusion under three different loading conditions of uniaxial tension, uniform tension and pure shear was carried out. Both the crack and coated circular inclusion are embedded in an infinite matrix, with the crack oriented along the radial direction of the inclusion. In the solution procedure, the crack is simulated as a continuous distribution of edge dislocations. With the Dugdale model of small-scale yielding [J. Mech. Phys. Solids 8 (1960) p. 100], two thin strips of yielded plastic zones are introduced at both crack tips. Using the solution for a coated circular inclusion interacting with a single dislocation as the Greens function, the physical problem is formulated into a set of singular integral equations. Using the method of Erdogan and Gupta [Q. J. Appl. Math. 29 (1972) p. 525] and iterative numerical procedures, the singular integral equations are solved numerically for the plastic zone sizes and crack tip opening displacement.

Fatigue Monitoring of Double Surface Defects Using PZT Based Electromechanical Impedance and Digital Image Correlation Methods

Most of the engineering failures especially in mechanical and aerospace industry are due to the fatigue. Fatigue cracks and their propagation can be monitored by observing changes in the structural stiffness resulting from strength reduction as a function of the number of loading cycles. This monitoring can be observed using piezoceramic (PZT) transducer based electromechanical impedance (EMI) technique, and digital image correlation (DIC) system which uses variations on structural surface. The fatigue load usually aggravates the fracture if any defects pre-exist in the structure. The present work monitors multiple-crack emanating from electrode sparked multiple plane defects which are induced in to the specimens prior to the fatigue test, using EMI technique and DIC system. The fracture occurs much below the ultimate stress for the structures which already have defects as in the present case. EMI technique uses signature comparison of healthy and cracked state of the structure to depict crack growth. Initial detection of surface hair-line cracks from the corners of induced defects and their propagation till merging and subsequent failure were explained by signature variations and DIC techniques. Thus, a signal processing technique EMI, and image processing technology DIC were found to complement each other in prediction of early crack and their appearance on surface of the specimens.