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DYNAMIC MEASUREMENT OF CRACK CLOSURE BEHAVIOUR OF SMALL FATIGUE CRACKS BY AN INTERFEROMETRIC STRAIN/DISPLACEMENT GAUGE WITH A LASER DIODE
Author(s) -
Akiniwa Yoshiaki,
Harada Shoji,
Fukushima Yoshihiro
Publication year - 1991
Publication title -
fatigue and fracture of engineering materials and structures
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.887
H-Index - 84
eISSN - 1460-2695
pISSN - 8756-758X
DOI - 10.1111/j.1460-2695.1991.tb00661.x
Subject(s) - materials science , crack closure , crack tip opening displacement , strain gauge , crack growth resistance curve , displacement (psychology) , composite material , structural engineering , fracture mechanics , stress (linguistics) , laser , optics , physics , engineering , psychology , linguistics , philosophy , psychotherapist
An interferometric strain/displacement gauge (ISDG) system with a laser diode was developed to measure dynamically the crack closure behaviour of a small surface crack. Two Vickers indentations, about 100 μ apart, were illuminated with the laser pulse synchronized with the load signal. The interference fringe patterns moved as the indentation moved apart. The 95% confidence limit, obtained on the basis of the difference between the strain measured by the ISDG and that by the resistance strain gauge (RSG), was ±6.0 × 10 −4 μ for stepwise loading. The standard deviations of the relation between stress and displacement under elastic conditions were 1.7 for stepwise loading at 0.067 Hz and 2.6, 2.6, 2.3 for dynamic loading at 5, 10 and 20 Hz respectively. The crack mouth opening displacement was measured dynamically as well as statically with the system. The crack opening stress was determined using the unloading elastic compliance method. The measuring frequency hardly affected the crack opening stress. Smooth specimens of a type 304 austenitic stainless steel were fatigue tested. The crack propagation behavior was observed and crack closure measurements monitored for crack lengths ranging from 0.12 to 0.75 mm. The aspect ratio of the surface crack was determined on the basis of the relation between the surface crack length and the elastic compliance. The relation between the crack propagation rate and the J integral range was unique, and agreed well with that calculated by the effective stress intensity range for long cracks.