Crack Tip Opening Displacement Criteria Research Articles

Abnormality diagnosis of cracks in the concrete based on double crack tip opening displacement criterion

The distinct characteristic of subcritical propagation requires at least two control parameters to completely express expansion of cracks in the concrete. The basic definition of crack tip opening displacement (CTOD) is corrected based on the monitored variable of crack mouth opening displacement (CMOD). The improved CTOD criterion, called the double CTODd criterion, for the abnormality diagnosis of cracks in the concrete is constructed based on the fictitious crack model and linear asymptotic superposition assumptions of the concrete. The double CTODd criterion is verified using two specimens of three-point bending notched beam. The verification results are reasonable and correct. The establishment of the double CTODd criterion lays a solid theoretical foundation for the abnormality diagnosis of cracks in concrete structures (such as concrete dams) based on CMOD.

Development and application of a crack tip opening displacement-based mixed mode fracture criterion

Consistent with experimental observations, a crack tip opening displacement (CTOD)-based, mixed mode fracture criterion is developed for predicting the onset and direction of crack growth. The criterion postulates that crack growth occurs in either Mode I or Mode II direction, depending upon whether the maximum in either the opening or the shear component of CTOD, measured at a specified distance behind the crack tip, attains a critical value. The proposed CTOD-based fracture criterion is implemented in two finite element codes to predict the stable tearing behavior of (a) a modified Arcan test specimen made of AL 2024-T3 and (b) a double cantilever beam (DCB) specimen made of AL 7050. Using the measured load-crack extension data as input, results from simulations of stable crack growth along experimentally measured crack paths for the Arcan specimen demonstrate that the CTOD values obtained in the simulations are in excellent agreement with the proposed criterion. Then, using the CTOD criterion as input, simulations of the Arcan and DCB specimens demonstrate that the CTOD fracture criterion successfully predicts (a) load-crack path data for both specimens, (b) load-crack extension behavior for the Arcan specimen and (c) load-load point displacement data for the DCB specimen.

A study of some issues in stable tearing crack growth simulations

This paper investigates several issues in stable crack growth simulations relevant to the application of a particular crack-growth criterion based on the crack-tip opening displacement (CTOD). Specifically, the effects of finite element size, CTOD location, and large rotation on the performance of the CTOD criterion are examined. Finite element simulations of actual crack growth tests on Arcan specimens are carried out under plane stress and Mode-I conditions using a custom code ZIP2DL. With regard to finite element predictions for the load-crack-extension curve, the results of the simulations suggest the following. First, when elements are small enough along the crack growth path to produce converged crack-tip opening displacement (not necessarily converged crack-tip stresses and strains), the size of the elements has negligible effect. Second, the fixed distance between the CTOD location and the current crack-tip should be chosen so that it is consistent with the choice of the element size along the crack path and of the critical CTOD value. Third, the effect of large specimen rotation can be dealt with by performing both the CTOD calibration and its application under small-deformation conditions.

Numerical characterisation of a cylinder with circumferential cracks under pressurised thermal shock

A numerical modelling of pressurised thermal shock (PTS) accidents has been performed. Crack growth and crack arrest are numerically simulated using the finite-element method. The analysis is done in two steps. In the first the observed crack growth phenomena has been used as input to compute parameters such as crack mouth opening displacement, J-integral, stress intensity factor, etc. These parameters are compared with the experimentally observed values and also with the values obtained through numerical investigations by other organisations. Crack growth and crack arrest behaviour are compared with the triaxiality parameter of the state of stress at that moment. The observed crack growth phenomena is plotted on an R-6 diagram to assess the importance of the failure assessment diagram in the evaluation of the integrity of the structure. Effort has also been directed towards studying crack growth using crack tip opening displacement criteria. In the second step, crack growth and arrest conditions have been numerically predicted using the material fracture toughness properties and compared with the experimentally observed data.

An assessment of the crack tip opening displacement criterion for predicting creep crack growth

From a set of finite element (FE) simulations of creep crack growth in compact tension specimens, the critical value of the crack tip opening displacement, CTOD, for creep crack growth has been generated for a Ni-base superalloy (Waspaloy) at 700°C. It was found that the critical value is independent of the initial crack length, amount of previous creep crack growth and the load level. Hence, the CTOD seems likely to be a viable criterion for use in creep crack growth rate analysis. Good agreement was also obtained between the experimental test results and FE predictions of the creep crack growth with time and between the crack growth rate, da/dt, versus the C * parameter based on load-line displacement rate.

Theoretical and experimental investigations of fracture by finite element and grating methods

A finite element technique allowing crack extension in elastic-plastic bodies is outlined, and a grating method for accompanying experimental investigations is presented. Using a special crack tip opening displacement criterion, the crack extension behavior of thin compact tension specimens was investigated numerically. Good agreement between experimental and numerical simulations was found only for three-dimensional calculations, comparing the strain and displacement distributions near the crack tip.

Mixed-Mode Fracture in Plastically Deforming Materials

A finite element method was developed for evaluating an angled crack in a thin plate. In the spirit of the Dugdale and Barenbaltt damage zone models, the method superposes cohesive stresses in a damage zone onto a linear elastic fracture analysis. The method uses incremental changes in the boundary conditions along the damage zone to model loss in stiffness due to yielding and stable crack growth. A crack tip opening displacement criterion is used for predicting stable crack growth and unstable crack growth. The resulting nonlinear analysis is used to predict the applied loads at failure. Numerical results compared favorably with experimental values.

A mathematical model to approach the fracture process of overconsolidated clay

Some soil and most rock masses contained defects (joints, cracks, bedding planes, fissures in clay, etc.). Beyond the peak shear strength of a mass of soil or rock, the stress falls as the strain increases. In this paper, the plane strain problem with double edge-cracks under shear loads is analysed based on linear elastic fracture mechanics but with finite stress concentration concept. We assumed that the fissured clay is a kind of strain-softening material and the growth of slip surface is along the maximum shear stress plane. The growth of slip surfaces in the progressive failure is expressed by the shear loads and slip displacement curves, which are obtained under the crack tip opening displacement criteria (CTOD c) or the energy release rate criteria ( J c ). The residual slip displacements are also taken into account by cumulating the elastic ones.

Strain and stress fields near the blunted tip of a crack under mixed mode loading and the implications for fracture

The strain distribution in the vacinity of a blunted crack-tip is analysed by slip line theory under the conditions of plane-strain, small-scale yielding, and mixed-mode loading of Modes I and II. A generalized crack-tip opening displacement is introduced by which the strain and stress fields near the blunted crack-tip are determined uniquely over a wide range of Mode I and II combinations. Also, coupled experimental and finite-element analyses under the condition of large-scale yielding reveal that the initiation of stable crack growth occurs when the generalized crack-tip opening displacement attains a critical value which is constant for the material tested. The finite-element analysis is based on the finite deformation theory of elastic-plastic materials. The generalized crack-tip opening displacement criterion is found to be superior to the J- integral and the usual COD for the characterization of the initiation of stable crack growth. The plastic work in a small circular region at the crack-tip is found to be equivalent to the generalized crack-tip opening displacement, as a fracture criterion.

Critical review of the fatigue growth of short cracks

Retirement for cause (RFC) has become a popular design/analysis philosophy because it facilitates continued use of components which would otherwise be retired based on a safe life philosophy. RFC permits this continued service on the basis that service-induced damage tracked by periodic inspections will not develop to a critical size within a future operating interval. Successful implementation of RFC requires fracture mechanics technology to predict in-service growth behavior. Recent observations suggest that nonconservative estimates of crack growth rate (and therefore inspection interval) and critical flaw size arise when conventional linear elastic fracture mechanics (LEFM) is applied to predict the growth of physically small cracks. This paper summarizes the results of an extensive limited-circulation critical review of the phenomenology and mechanics of short crack growth with a view to identifying when this anomalous growth makes RFC analyses untenable in terms of LEFM. Factors which control the growth of short cracks are enumerated. It is shown for unnotched samples that the apparent effect may be traced to microcrack closure and the metallurgical, mechanical, and environmental transients which develop in the transition from initiation to steady-state growth. For notch samples the anomalous growth of cracks is traced to the inelastic action that develops a displacement-controlled notch field, which, contrary to LEFM analysis, dominates crack extension. Mechanics analyses relevant to characterizing the growth of short cracks are discussed. A crack tip opening displacement criterion is indicated to be appropriate.

衝撃曲げ試験によるＰＭＭＡの動的破壊じん性値の測定

With an increasing use of polymeric materials in structural components, fracture prediction of these materials under various loading conditions is becoming of great importance. The authors reported previously that the crack-tip singular stress field of a viscoelastic body was characterized by three parameters: the stress intensity factor KI(t), the strain intensity factor TI(t), and the energy release rate J. In this paper, the results of impact fracture tests on PMMA specimens are presented to examine the applicability of these parameters.The three-point bend specimens were fractured at 20°C by a falling steel sphere or cylinder over the range of loading rates 103MN/m3/2s≤KI(t)≤104MN/m3/2s. The fracture initiation time was measured by a strain gauge mounted near the crack-tip or a painted electrical resistance circuit in the path of the crack. The contact force between the specimen and impactor was analyzed by applying Hertz's theory to the local deformation near the contact point, and the fracture mechanics parameters were determined by using the simple formulas which had been derived by the authors using Timoshenko's beam theory.The critical values of the fracture mechanics parameters at crack initiation were compared with the quasi-static results reported in the previous paper. It was found that the critical values of KI(t) and J increased with loading rate, while the critical value of TI(t) was constant over the range of loading rates 10-3MN/m3/2s≤KI(t)≤104MN/m3/2s. An examination was also made on the fractography of the specimens, and the critical value of the crack opening displacement was found to be constant. This fact is consistent with the above experimental results, since the strain intensity factor criterion is equivalent to the crack-tip opening displacement criterion.

A Dugdale model for crack opening and crack growth under creep conditions

Creep deformation ahead of stationary and propagating crack tips has been studied by means of a time dependent Dugdale model. Analytical expressions for the stresses, strains, and for the crack tip opening displacement (COD), are derived from a non-linear partial integro-differential equation. Combined with a crack extension criterion, e.g., the critical COD criterion, the stress analysis allows for the calculation of crack growth rates, dot a , and of crack growth initiation times, t i, both as functions of the applied stress intensity factor, K I. The prerequisites for a unique correlation between dot a and K I are steady state crack extension and small scale yielding conditions. These are investigated in the present paper.