Crack-tip caustics in mechanically anisotropic materials

Engineering Fracture Mechanics, 1991

Ahatraet-A thorough investigation of the three-dimensional region around the border of a straight through-crack in a plate was undertaken by the method of caustics. A series of experiments were performed on 209OAl alloy fatigue precracked double cantilever beam specimens of various thicknesses. The power and potential of the method of caustics in studying the ~r~dimension~ nature of the stress field in crack problems was demonstrated. It was found that in the immediate vicinity of the crack tip and for specimen thicknesses larger than a critical thickness, a region exists that is under strong "plane strain" inguence. The size of this region increases initially with specimen thickness and finally reaches a limiting value. Following the "plane strain" region which surrounds the crack tip, three-dimensional stress conditions hold in the vicinity of the crack tip up to a critical distance which depends on the thickness of the plate. Beyond this critical distance plane stress conditions dominate. The value of the effective thickness of the specimen which contributes to the formation of the caustic was found to increase by receding from the crack tip and it finally reaches the actuaI specimen thickness. The results of the present study shed light onto the complicated problem of the state of stress around cracks and establish the limits of applicability of the method of caustics in crack problems.

EPJ Web of Conferences, 2010

Regions with high stress gradients represent critical zones of engineering structures such as crack tip or vicinity of the contact zone. The optical method of caustics is one of the few experimental methods which provide applicable results in these locations. This method was originally developed for crack-tip measurements of stress intensity factors and J-integral for isotropic and then for anisotropic materials. Subsequently, it was extended to contact problems where the size and shape of caustics on the screen are related to the amount and the inclination of loading force. Here, the method of caustics is extended to the analysis of contact problems for mechanically anisotropic materials. This makes the caustics method widely applicable to the analysis of any high stress gradient locations in a structure.

Engineering Fracture Mechanics, 1986

A simple and powerful crack growth criterion under combined environmen~1 conditions and applied loads is fo~ulated for mixed-rn~e situations based on the method of caustics. It is established that the axis of symme~y of the caustic represents the crack growth direction, while the crack growth condition is expressed in terms of the diameter of the caustic. Caustics can easily be obtained by illuminating the specimen in the vicinity of the crack tip by a laser light beam and recording the optical pattern on a viewing screen at some distance from the specimen. An elegant procedure for the experimental study of environmental cracking is thus developed.

A new formula of experimental evaluation of the stress intensity factor was proposed depending on the area of the caustic. From series of experiments, which were performed in single edge-cracked specimens made of Lexan (PCBA), it was found that the values of the stress intensity factor KI,, which were based on the diameters of the caustics, were varied with the load, the crack length and the caustic shape because the caustic become oval in higher loads or higher crack lengths. This means that the stress intensity values which are evaluated by the two diameters of the caustics (Dt , Dl), are significantly differed. The accurate stress intensity factor KI Keywords can be experimentally evaluated by the area of the formed caustic. According to the new formula of the stress intensity factor of the stress optical constants were evaluated. The present work illuminates the complicated problem of the stress intensity factors evaluation by the caustic diameters and establishes the new accurate of the caustic area approach for the evaluation of the stress intensity factors at crack tip and the stress optical constants of the transparent materials.

Mechanics Research Communications, 1983

International Journal of Fracture, 1985

Engineering Transactions, 2013

The shape of the “initial curve”, i.e. the locus of material points, which if properly illuminated provide (under specific conditions) the “caustic curve”, is explored. Adopting the method of complex potentials improved formulae for the shape of the “initial curve” are obtained. Application of these formulae for two typical problems, i.e. the mode-I crack and the infinite plate with a finite circular hole under uniaxial tension, indicates that the “initial curve” is in fact not a circular locus. It is either an open curve or a closed contour, respectively, the actual shape of which depends also on the in-plane displacement field.

1995

The Open University's repository of research publications and other research outputs Crack Closure Measurement by the Optical Method of Caustics Thesis How to cite: Wallhead, Ian (1995). Crack Closure Measurement by the Optical Method of Caustics. PhD thesis The Open University. For guidance on citations see FAQs.

Procedia Materials Science, 2014

Glassy polycarbonate (PC) is a widely used engineering material in industries, since it has high strength and toughness as well as good transparency. However, these advantages of PC can be suppressed by physical aging, especially its dynamic fracture toughness. In addition, the material properties of PC can be changed dramatically after large plastic compressive deformation, and it can show obvious orthotropic behavior. Here, the combined effect of aging and plastic compressive deformation on dynamic facture of PC was investigated by the optical method of caustics. The dynamic reflective method of caustics for orthotropic materials was developed here to study the dynamic fracture property of PC with plastic deformation. The samples were prepared with different extents of plastic compressive strain up to approximately 50% engineering strain, followed by aging with various aging times up to 300 hours. The shadow spot patterns during the fracture processes were recorded by a high speed camera. Results show that the PC with plastic compressive deformation has different fracture toughness even quite different fracture modes on its different orientations. Aging affects the fracture property of PC with or without plastic flow in similar trend. Under large plastic compressive strain, PC has more strong anisotropy and dynamic fracture tolerance, which has great potential for optimal material design.

1997

The optical method of caustics for measuring the dynamic stress intensity factor in a transient process is investigated in this study. The transient full-field solutions of a propagating crack contained in an infinite medium subjected to step-stress wave and ramp-stress wave loadings are used to establish the exact equations of the initial and caustic curves. The results of the stress