Crack Growth Parameters Research Articles

Subcritical Crack Growth Life Prediction for Ceramic Components of Advanced Heat Engines

Advanced, high-strength ceramics are finding increasing application in advanced heat engines. To ensure the long-term reliability of components made from these materials, subcritical crack growth (SCG) from inherent flaws has to be taken into account, as this has been identified as the primary failure mode under sustained loading. In analyzing fast fracture data, data censoring is necessary to obtain estimates of the inherent strength distributions for competing failure-causing flaw populations. This is particularly important for ceramic designs, where size scaling is a necessary part of the design analysis. While data censoring has become common for fast fracture data, data censoring involving stress rupture data has yet to be widely applied. This paper describes fast fracture and stress rupture tests performed on an advanced silicon nitride ceramic, the test data and fractography results, censored data analysis for both types of data, derivation of the subcritical crack growth parameters, and application of these parameters to verification specimens. Implications of the findings and recommendations for future studies are also presented.

Crack growth of through and part-through thickness cracks under cyclic loading

Stress intensity factor expressions corrected for crack front curvature are used to derive the corrosion fatigue growth data for through and part-through thickness cracks subjected to cyclic loading. Results are obtained by application of the boundary element method for solving three-dimensional crack problems. Different radius of curvature is assumed using the open literature cyclic crack growth parameters for air and sea water. As is to be expected, crack growth increased with decreasing crack front curvature and in sea water environment when compared with that in air.

Effects of short cracks on fatigue life calculations

In this study, the potential impact of the short crack problem in En7A steel with a high content of elongated MnS inclusions has been observed. Experimental short crack data were compared first with the predicted S-N curves determined using long crack growth parameters and second with the calculated lives using only long crack data and using both short crack and long crack data. All these life predictions revealed that long crack data alone cannot be used in fatigue lifetime analyses that treat short crack growth, because they can all lead to dangerous over-predicted lives.

Evaluation of the crack-growth resistance of ceramics in the stage of subcritical crack growth

We propose a procedure for the evaluation of the parameters of crack growth in ceramic materials based on the application of soft and rigid loading to a standard beam specimen with lateral notch. The kinetic diagrams of fracture of aluminooxide ceramics obtained as a result enable one to predict the durability of products made of this material under specific service conditions.

Double-torsion testing a 3Y-TZP ceramic

Double torsion tests were performed to study subcritical crack growth of a 3Y-TZP ceramic. A significant variation of stress intensity factor K 1 with crack length was observed. In order to take into account this variation a correction factor is introduced into the well-known analytical expression of stress intensity factor in mode I. This has enabled us to obtain accurate crack growth parameters i.e. an accurate measurement of (i) the three propagation stages of the subcritical crack growth and (ii) a threshold value equal to K IO = 3.5 MPa√m. Analytical expression of K 1 based on a compliance analysis was shown to be unsatisfactory for slow crack growth analysis because of the influence of the unbroken ligament on the compressive side arising from the curved crack front.

Acoustic emission detection and monitoring of highway bridge components : Azimi, M. Dissertation Abstracts International, Vol. 50 No. 7, p. 3050-B (Jan. 1990)

Fatigue cracking is the most common problem in steel highways bridges. Early detection of cracks requires simple, inexpensive repairs, whereas late detection of cracks may cause catastrophic accidents. Currently, the bridges are inspected by visual examination, dye penetrant, magnetic particles, ultrasonics, and radiography. Low reliability and high inspection costs require a more efficient inspection system to be developed. Acoustic emission (AE) seems to be the solution to these problems. It has developed over the past two decades as a nondestructive testing technique with high sensitivity to detect progressive microscopic events in the material. Development of portable AE equipment has made it an even more practical method for bridge inspections. In a prior study, acoustic emission was applied to A588 weathering steel beams (rolled and welded sections), and the effective frequency range for monitoring highway bridges was established. In this study, research characterizing the parameters of cracks versus noise was conducted. A total of 18 beams made of A588 steel including welded, rolled and coverplated beams, were monitored. Half of the beams were exposed to severe atmospheric environments to investigate the effect of rusting on the AE signals. Also, a total of twelve small specimens made from the same material were monitored to establish a relationship between the AE signals and stress intensity factors. In addition, three small welded specimens were monitored for comparison to the coverplated beams. As a result of this study, a statistical range of variation of the AE parameters at various stages of crack growth has been established. This range can be used as a reference for bridge inspection. A model for predicting the stage of crack growth versus AE parameters has been discussed, and a systematic inspection procedure by AE has been recommended.

Effect of Growth and Coalescence of Multisurface Cracks on Static Fatigue Behavior of Glass Ceramics

In order to study the effect of the interaction and coalescence of multisurface cracks on the static fatigue behavior of brittle materials such as glass and ceramics, a single crack and three collinear cracks were introduced on the surfaces of plate specimens of glass ceramics by Vickers microhardness indentation and were subjected to a constant four‐point bending load. It was found that time‐to‐failure for a certain applied stress significantly decreased with the decrease in the crack center distance of indentation cracks, and the effect of interaction and coalescence of multicracks was shown to be significant. On the basis of the crack growth parameters for a single crack, time‐to‐failure for multicracks was predicted by taking the interaction and coalescence into consideration. It was demonstrated that the proposed multicrack coalescence model gives better predictions.

A new fracture mechanics method to predict the fatigue life of welded cruciform joints

The fatigue life of MIG welded cruciform joint failing from root (LOP) region was successfully predicted using new fracture mechanics equations. These equations were developed by combining Paris' law and ΔK i-endurance equation and incorporating an integral factor ( I p) obtained by integrating Paris' equation. In the numerical formulation of the equation, the initiation life ( N i) and propagation life ( N p) of the joint were accounted for to obtain the total fatigue life ( N f). The initiation life was obtained experimentally using crack “Initiation Criteria”. The propagation life was evaluated using the numerically developed equations. The method outlined successfully predicted the fatigue life of the joint from its crack growth parameters. To test the accuracy of the method, the predicted data were compared with the experimental data. It was found that the predicted data compared quite well with the experimental data. This method is simple and can be applied easily for other joint geometries.

An arbitrary Lagrangian description of 2D and 3D cracked structures

An arbitrary Lagrangian description is proposed for the moving crack analysis. The method is based on the use of a mapping of the moving domain to the initial stationary one (boundaries, shape and conditions may be very general). In the framework of elasticity, the energy release rate is expressed as surface integrals on the fixed domain, explicitly related to the current crack growth (or crack shape in 3D) parameter. Numerical results of an energy release rate analysis are presented for a large range of 2D and 3D crack extension. The advantages of this method are: firstly, it avoids remeshing or introducing special numerical parameters such as relaxation forces to simulate the crack growth or performing parametric analysis for different crack shape; secondly, to obtain exact derivatives of the potential energy.

Determination of Subcritical Crack Growth Parameters by In Situ Observation of Indentation Cracks

A technique to determine stress intensity factor—crack velocity (K—v) relationships for subcritical crack growth from in situ observation of indentation cracks is described. To minimize the effect of residual contact stresses and lateral crack interaction, measurements were made only on cracks that had undergone significant subcritical crack growth. Crack shapes were determined fractographically from crack‐arrest markings, produced by temporary unloading during the crack extension process. The subcritical crack growth parameters obtained by this technique were in excellent agreement with those determined from dynamic fatigue and previous studies.

Creep Crack Growth Prediction of Solder Joints During Temperature Cycling—An Engineering Approach

A model is developed which predicts the creep damage accumulation in solder joints during temperature cycling. The model relates the crack growth rate to the rate of creep energy density dissipated using the C* parameter of nonlinear fracture mechanics for extensive creep damage. For a eutectic tin-lead solder joint, the damage due to both grain boundary sliding and matrix creep is considered. The validity of the model is proved by correlating the predicted fatigue life of solder joints for 84 I/O leadless ceramic chip carriers with the published measured data for a number of test conditions. The published experimental conditions are simulated in a three dimensional, nonlinear, time and temperature dependent finite element analysis. For each test condition, both components of creep energy density, i.e., energy densities because of grain boundary sliding and matrix creep, are determined separately in the finite element analysis. The values are then used to determine the crack growth parameters and predict the crack growth rate in the solder joint using the crack growth model. The predicted results are found to have good correlation with the measured fatigue life for each test condition.

Indentation Crack‐Shape Evolution during Subcritical Crack Growth

Indentation crack‐shape evolution during subcritical crack growth was examined in soda‐lime glass. Crack‐arrest markings were generated on the fracture surfaces by temporary unloading during subcritical crack growth. Crack shapes were determined from these crack‐arrest markings by optical microscopy. The shapes were found to be semielliptical, but the ellipticity changed significantly with crack extension under an applied bending stress. Crack‐shape evolution was predicted by using the stress intensity factor equation for a semielliptical crack for the applied stress distribution and the subcritical crack growth parameters for the material/environment system. Experimental results were found to be in excellent agreement with the predictions. The influence of the initial crack size and shape, the loading configuration, and the subcritical crack growth exponent n on crack‐shape evolution was also investigated in the simulations. It was found that, for the particular loading configuration and material, there is an equilibrium shape associated with the subcritically growing crack. The variation in crack shape with its extension during subcritical crack growth was found to be an important factor in lifetime prediction.

Robust Approximate Methods for Estimating Inelastic Fracture Parameters

Abstract Robust approximate methods to estimate the inelastic energy release rate J, and the creep crack-growth parameter, C*, for cracked components are described in this paper. These methods use linear elastic finite element analysis in conjunction with the concepts of the generalized local stress strain (GLOSS) analysis and redistribution nodes (r-nodes), and are readily applicable to complex geometries and loadings. J-estimates obtained by the use of robust methods are found to be in good agreement with the results of elastic-plastic finite element analysis.

Computer‐Aided, Single‐Specimen Controlled Bending Test for Fracture‐Kinetics Measurement in Ceramics

Fracture testing of ceramics by using controlled crack growth is proposed to allow study of crack‐kinetics behavior under a given loading history. A computer‐aided, real‐time data acquisition system improves the quality of crack‐growth parameters obtained in a simple, single‐specimen bend test. Several ceramic materials were tested in the present study: aluminum nitride as a linear‐elastic material; and alumina and yttria‐stabilized zirconia, both representative of ceramics with microstructure‐dependent nonlinear fracture properties. Ambiguities in the crack‐growth diagrams are discussed to show the importance of accounting for crack‐growth history in correctly describing nonequilibrium fracture behavior.

Creep and creep-fatigue cracking behaviour of two structural steels

The present study deals with crack initiation and crack growth, not only under creep and creep-fatigue conditions but also under more complex thermomechanical cyclic loadings, in both 316L and 1Cr-1Mo-0.25V steel. In these creep ductile materials, most studies have focused on the creep crack growth rates, d a /d t and load-geometry parameter C∗ correlations. In this paper, the creep crack initiation time is defined as the time T i necessary for a defect to grow by a small critical distance X c ( X c ≈ 50 μ m for example). This initiation stage may represent a large part of the rupture life of a cracked component. The importance of such studies is discussed in the first part. In the second part, an attempt is made to present a simplified method based on the fracture mechanics of creeping solids to define the relevant load-geometry parameters for crack initiation and crack growth under creep-fatigue loadings. In particular, it is shown that d a /d N - ΔK correlations apply only when the hold time t h is smaller than the transition time t tr between small-scale and large-scale viscoplasticity. Conversely, for long hold times, it is suggested that the T i -C∗ correlation be used to predict the fatigue.

Fracture Toughness and Subcritical Crack Growth of Small Crack in Silicon Nitride at Room Temperature

The effect of microstructure in silicon nitride on fracture toughness and Subcritical crack growth (SCG) behavior at room temperature was investigated. Three kinds of silicon nitride materials containing 5 mass% of Y2O3 and Al2O3 were sintered under different conditions, and they showed different microstructures. Fracture toughness of the materials was obtained by single edge precracked beam (SEPB) method and controlled surface flaw (CSF) method with Knoop indentation-crack in order to investigate the dependence of the fracture toughness on microstructure. The fracture toughness data measured by the CSF method increased with increasing crack size for all kinds of the materials. The increment of the fracture toughness per crack length was more considerable for coarser grain sizes. It can be due to stress shielding at crack tip by such toughening mechanism as crack bridging. The static fatigue behavior of the materials was investigated by the dynamic fatigue technique at room temperature. The fatigue parameter n in SCG model (da/dt=AKIn) increased with increasing as fracture toughness and grain size increased. Based on stress shielding model at crack tip, the fatigue crack growth parameters in SCG model were calculated. The predicted value agreed to the experimental results.

Load history effects on creep crack growth

In this paper the effects of load history on the high-temperature creep crack growth process are studied through a combined experimental and computational approach. The general features of constitutive response during cyclic creep are reviewed. Next, fracture parameters for creep crack growth are reviewed, with special emphasis on integral parameters. Finally, examples comparing computational predictions of experiments which experience history dependent load histories are presented. This includes displacement time comparisons and fracture parameter comparisons.

Fatigue life estimate of CMn steel welded cruciform joints

The fatigue life of a manual metal arc welded cruciform joint failing from a root lack of the penetration region is estimated by the application of crack growth relations. A two-parameter relation was used. The initiation life and propagation life of the joint were taken into account to obtain the total fatigue life of the joint from the crack growth parameters. To test the accuracy of the method, the predicted data was compared with the experimental data for a CMn type steel welded joints. The results were in good agreement with the experimental data.

Estimation of endurance of structural ceramics by dynamic fatigue testing

Results of investigations on determining the level of the control load in certification of ceramic articles at specified operating loads are presented. The parameters of subcritical crack growth, required for the calculation, have been determined using the results of dynamic fatigue testing. It is demonstrated that the parameters of subcritical crack growth determined by dynamic fatigue testing are adequate for use in calculating control loads in cases of operating loads with a high subcritical stress intensity at structural defects (KI <KIc). For long-term conditions of operating loads, the estimates of the endurance made by the present method will be lower than the actual values. The effectiveness of control tests under the specified conditions of the operating load has been demonstrated in an experiment with two batches of specimens of compacted chemically bound silicon nitride (OTM 907). The results of dynamic fatigue testing of silicon nitride are also given.

Viscoelasticity of ceramics at high temperatures

The dependence of the fracture toughness, K IC, on the loading rate has been calculated. On the basis of linear elastic fracture mechanics (LEFM) a strong dependence of the fracture toughness on the loading rate is obtained if subcritical crack growth is taken into account. If the subcritical crack growth parameters n and B are sufficiently small, which correspond to a high velocity of crack extension, the fracture toughness should decrease at lower loading rates. This behaviour is similar to the well-known decrease of bending strength. The experimental results for alumina containing glassy phase as a model material, however, show a maximum in a certain regime of loading rates. A model is established, which combines LEFM and the viscoelasticity, and leads to a maximum of K IC at a certain loading rate dependent on the viscosity of the glassy phase.