Determination of crack-opening displacement by the method of caustics

A new experimental technique based on the method of caustics is presented for the measurement of the distance between the lips of a crack near the crack-tip. The two parts of the caustic formed by reflections from the front and rear faces of the specimen lie at a distance from each other. The gap between these parts depends on the total c.o.d. (crack-opening displacement), that is the initial opening and the opening due to loading, as well as on the optical and mechanical characteristics of the material By increasing the external loading of the cracked plate, the gap between the parts of the caustic was changed and this gap measured the instantaneous c.o.d. due to loading. The method was applied to the measurement of small c.o.d.s. due to small-scale loading, with satisfactory results. Therefore it can certainly be used to measure c.o.d.s at large loading steps, up to fracture, because the gap between the parts of the caustic becomes significant and easy to measure. Measurements with cracked plates made of p.m.m.a. (polymethylmethacrylate) and polycarbonate showed that the results obtained are in good agreement with theory. Thus, it has been proved that the method of caustics yields a very sensitive means for measuring c.o.d.s, especially in small-scale deformations, where measurement of c.o.d by conventional methods is inaccurate. A great advantage of the method is that it measures the c.o.d.s at a well defined region, which always remains near to the crack tip.

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Plastic relaxation at a crack tip by asymmetric slip

Proceedings of the Royal Society of London Series A - Mathematical and Physical Sciences ◽

10.1098/rspa.1988.0083 ◽

1988 ◽

Vol 418 (1854) ◽

pp. 261-280 ◽

Cited By ~ 7

Keyword(s):

Slip Band ◽

Crack Opening ◽

Crack Tip ◽

Friction Stress ◽

Small Scale ◽

Crack Tip Opening Displacement ◽

Opening Displacement ◽

Sharp Crack ◽

Scale Yielding ◽

Crack Tip Opening

Relaxation at a sharp crack tip by a single slip band is considered. It is shown that for mixed-mode loading of a plane crack in an isotropic medium there is a unique angle between the slip band and the crack for which the energy release rate (or stress intensity factor) of the crack can be reduced to zero. For such a slip-band calculations are made of the slipband length and the crack-opening displacement as a function of the loading, crack length and friction stress acting on dislocations in the slip band. For small-scale yielding, a simple model is discussed that gives a good approximation to the crack-tip opening displacement and slip-band angle.

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Test for Crack Opening Displacement of Steel Fiber Reinforced Concrete under Three-Point Bending

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.36.157 ◽

2010 ◽

Vol 36 ◽

pp. 157-161 ◽

Cited By ~ 2

Author(s):

Tin Gyi Zhang ◽

Yuan Bao Leng ◽

Dan Ying Gao

Keyword(s):

Crack Opening Displacement ◽

Crack Opening ◽

Mouth Opening ◽

Crack Tip ◽

Crack Mouth Opening Displacement ◽

Crack Tip Opening Displacement ◽

Opening Displacement ◽

Critical Crack ◽

Critical Crack Opening ◽

Crack Tip Opening

Based on the principle of electrical measurement method, the clip gauge was made to measure the crack opening displacement (COD).Through the three-point bending test on the specimens of steel fiber reinforced high strength concrete (SFHSC), the effect of the fiber volume fraction (ρf) upon the critical crack opening displacement (the critical crack tip opening displacement and the critical crack mouth opening displacement) was studied. The result shows that the effect of ρf on mouth-tip ratio (the ratio of critical crack mouth opening displacement to critical crack tip opening displacement) can reflect its effect upon the critical crack opening displacement. According to the geometrical relationship between the initial crack length and the critical crack opening displacement，calculation method for the initial crack length was proposed. Based on the test result, the formula was established for calculating the critical crack tip opening displacement.

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Controlled Initiation of Short Fatigue Cracks in 316L Steel

Volume 6: Materials and Fabrication ◽

10.1115/pvp2005-71298 ◽

2005 ◽

Author(s):

D. Stefanescu ◽

J. Marrow ◽

M. Preuss ◽

A. Sherry

Keyword(s):

Experimental Study ◽

Stainless Steel ◽

Crack Opening Displacement ◽

Crack Nucleation ◽

Fatigue Cracks ◽

Crack Opening ◽

Crack Tip ◽

Image Correlation ◽

Opening Displacement ◽

R Ratio

Validation of models for short crack behavior requires accurate measurement of crack opening displacement and crack tip strain fields. Development of reliable measurement procedures, using new techniques such as Image Correlation (IC), requires specimens containing cracks with a well defined geometry. In this paper, results of an experimental study concerning controlled initiation of short fatigue cracks at positive R-ratio in laboratory specimens made from 316L stainless steel are presented. Experimental techniques, including hardness testing and X-ray diffraction were employed in order to investigate the effect of surface preparation on the surface mechanical properties and residual stresses. Crack nucleation is difficult in smooth specimens of 316L austenitic stainless steel at positive R-ratio due to the high fatigue limit and low tensile strength. Specimens with a thin ligament were therefore developed to enable nucleation of a single short fatigue crack. An experimental study of the crack growth aspect ratio evolution was then carried out using a beach marking technique. The technique described in this paper enables single short fatigue cracks of well defined geometry to be nucleated under tensile cyclic loading. Stress corrosion cracks can be developed using the same specimen geometry. Miniature tensile specimens can then be extracted to perform in-situ measurements of the crack opening displacement and crack tip strain field by Image Correlation from Scanning Electron Microscopy observations.

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