Patch repair of cracked structures has become a rapidly grown technology. The major function of a repair is to reduce the stress intensity factor at the crack-tip. Calculation of stress intensity factor of a repaired crack has been performed by analytical and numerical methods. However, these methods are based on simplifying assumptions regarding material behavior and repair conditions. In the present paper an experimental and an numerical determination of mode-I stress intensity factor (SIF), K I at the tip of an edge crack reinforced with bonded patches is undertaken by using the optical method of caustics and the finite element analysis (FEA). The experimental method of caustics is simple in its application and has successfully been used for the solution of a host of crack problems of engineering importance. The experimental results are compared with the corresponding one obtained by FEA. The program ANSYS 11 was used for the FEA. The cracked used plates were made of Lexan (PCBA) and the patches were made of Plexiglas (PMMA).

In this complex study an attempt was made on the one hand to analyze and understand in a systematic way the exact nature of the formation of certain characteristic energy dissipationinduced fractographic features and patterns/markings revealed by scanning electron microscopy (SEM) in particulate epoxy systems under impact (dynamic) loading conditions, and on the other hand to correlate these patterns and features with relevant crack propagation effects. For this scope a combined approach consisting of a qualitative as well as a semiquantitative analysis was employed. In the qualitative analytical approach it was shown that depending on the actual velocity and direction of crack propagation the above observed fractographic entities can be correlated to certain highly localized energy dissipative processes at front-failures as well as to local inertial molecular mass effects.Depending on the changes in the velocity and direction of propagation, the associated effects may be controlled by two basic processes: the single crack front and the multiple crack front splitting. The first process seemed to be governed by a shear toughness-biased system, whereas the second one used a critical strain energy release rate subcracking mechanism. Under certain conditions both processes may be influenced by inertial molecular effects in promoting the formation of relative-smooth fracture surfaces.The increased presence of particles tends to restrict an increase in the surface roughness due to energy dissipation-induced crack retardation effects. The presence of the notch tends to lower the fracture surface roughness compared to notch-free specimens and also to suppress the occurrence of certain elastic as well as viscoelastic-plastic crack delay effects observed in notch-free specimens in function of particle volume fraction.Based on relevant kinematics-aided modeling and impact energy measurements it seems possible to explain, by a gross semi-quantitative approach, the above particles and notch effects. In this context it seems plausible that the existence of a defect-induced fracturing time spectrum of the propagating crack front, in combination with the 'notch-induced shift' behavior of this spectrum, can be valuable for some approximating explanations of the above notch effects and in general the 'kinematics' of the surface roughness formation.

The range of applicability of the optical method of caustics, which is based on plane stress conditions, around cylindrical holes in plane specimens is investigated. The position of the generatrix curve of the caustic and the conditions, which influence its size, are defined. The dimensions of the specimen, the ratio of the thickness to the diameter of the hole, the applied stress and the dimensions of the optical set‐up influence the position of the generatrix curve of the caustic. At a critical distance from the boundary of the hole the state of stress degenerates from three‐dimensional to plane stress. In this paper, the extent of this region was defined by finite element analysis, in case of uniaxial tension. The defined critical distance was correlated with the condition, which influence the caustic, thus the range of applicability of the method of caustic around hole was defined. The validity of this range was experimentally checked.

A study of the experimental evaluation of the stress-optical constants accurate values was undertaken by the method of caustics. A series of experiments were performed in single edge-cracked specimens made of Plexiglas (PMMA). It was found that the values of the stress-optical constants were varied with the load and the thickness of the specimens. This means that a three-dimensional stress conditions hold in the neighbourhood of the crack tip. The accurate stress intensity factor K I can be experimentally evaluated by accurate values of the stress-optical constants. The present work illuminates the complicated problem of the changing state of stress around cracks and establishes the limits of applicability of the method of caustics for the evaluation of stress intensity factors and the triaxiality factor at crack tip

Abstract:The study of this paper was to determine the plastic zone dimension at the crack tip by the experimental method of caustics. The caustics shape at the crack tip depends on plastic zone which is small for brittle materials and large for ductile ones. The plastic zone dimensions depends on the crack length, the plates thickness and the loading level. The experimental method of caustics is an excellent tool for the plasticity and stress intensity factors evaluation of loaded cracked structures.