A simple method for measuring surface strains around cracks

A simple system has been developed to measure surface strains that occur during in situ deformation of mechanical test specimens. The system uses photolithographically deposited displacement markers and computer image recognition routines to determine in-plane displacements and strains from digital images. The strain calculating routines are integrated into a simple mouse-driven software package that facilitates the transformation from digital images to useful strain field information. Additional routines have been developed to determine crack tip stress fields and J integrals. Crack tip stress intensities have been calculated from strain maps obtained for traction-free cracks in stainless steel. The J integrals were found to be independent of contour and consistent with applied stress intensities. Crack tip stress intensities were calculated for bridged cracks in lamellar TiAl. The toughening effect of the bridging zones was determined by including the bridged region in the contours. Resistance curves generated from strain maps were consistent with those measured during mechanical testing.

[1]  K. Hatanaka,et al.  A measurement of three-dimensional strains around a creep-crack tip , 1992 .

[2]  John W. Hutchinson,et al.  Singular behaviour at the end of a tensile crack in a hardening material , 1968 .

[3]  J. Rice,et al.  Plane strain deformation near a crack tip in a power-law hardening material , 1967 .

[4]  Ernest L. Hall,et al.  Computer Image Processing and Recognition , 1980, IEEE Transactions on Pattern Analysis and Machine Intelligence.

[5]  C. Shih,et al.  Small-Scale Yielding Analysis of Mixed Mode Plane-Strain Crack Problems , 1974 .

[6]  K. Chan.  Toughening mechanisms in titanium aluminides , 1993 .

[7]  Crack-Tip Displacement Fields and JR -Curves of Four Aluminum Alloys , 1992 .

[8]  J. Rice.  A path-independent integral and the approximate analysis of strain , 1968 .

[9]  Young-Won Kim,et al.  Microstructural evolution and mechanical properties of a forged gamma titanium aluminide alloy , 1992 .

[10]  The use of digital processing in studying stretch-forming sheet metal , 1992 .

[11]  Bongtae Han,et al.  Immersion interferometer for microscopic moiré interferometry , 1992 .

[12]  Michael A. Sutton,et al.  A study of stationary crack-tip deformation fields in thin sheets by computer vision , 1994 .

[13]  C. E. Taylor,et al.  An application of pattern mapping to plane motion , 1990 .

[14]  S. Timoshenko,et al.  Theory of elasticity , 1975 .

[15]  L. E. Malvern.  Introduction to the mechanics of a continuous medium , 1969 .

[16]  D. Shih,et al.  A high-toughness γ-titanium aluminide , 1992 .

[17]  Y. W. Kim,et al.  Relationships of slip morphology, microcracking, and fracture resistance in a lamellar TiAl-alloy , 1994 .

[18]  Daniel Post,et al.  Developments In Moire Interferometry , 1982 .

[19]  J. S. Sirkis,et al.  Displacement and strain measurement with automated grid methods , 1991 .