Showing papers on "Crack closure published in 1971"

The Significance of Fatigue Crack Closure

Abstract: Al alloy sheet fatigue crack closure under cyclic tensile loading, deriving expression for crack propagation rate in terms of effective stress amplitude

Lattice Trapping of Fracture Cracks

Abstract: Continuum treatments of lattice defects such as dislocations and fracture cracks do not predict the resistance to the defect mobility which is due to the Peierls energy in the case of the dislocation. The discrete character of the lattice in the case of the fracture crack leads to a stress stability range for the crack above and below the Griffith stress over which the crack is stable or ``lattice trapped''. We develop here two essentially qualitative theoretical treatments of the lattice structure of cracks. In the first, we carry out a lattice sum of the bond energies of atoms facing each other across the crack plane under certain assumptions regarding the crack shape and atomic force laws. In the second, we introduce a one‐dimensional lattice model of a crack which can be solved exactly. In both of these treatments the range of stress over which the crack is lattice trapped appears to be of the order of magnitude of the Griffith stress itself. As in the case of dislocations the lattice trapping is a strong function of the ``width'' of the elastic singularity at the tip, and our prediction is that one should expect materials to exist in which lattice trapping is not only observable, but is an important effect. We also find that because of lattice trapping, the true surface energy is a lower bound to the mechanical surface energy as expressed in the Griffith‐stress relation.

Crack-growth criteria-and non-linear fracture mechanics

Abstract: Processes taking place in a certain region near a crack-tip play a dominant part for the initial growth of the crack. This region, called the end-region , is generally very small, and its state at the critical load is a material property. Outside the end-region, the material can be treated as a continuum, in which therefore d σ ij d e ij ⩾0. The state of the end-region is described by the value of the Rice integral J , which is a monotonic function of the load. When J reaches a critical value J c , the crack starts to grow. In the case of unstable growth, the criterion J = J c is also one of fracture. J may be determined very accurately, even in cases of large-scale yielding, by finite-element methods, without detailed knowledge of the situation near the crack-tip. The critical value J c may then be found by experiment. Thus, a non-linear fracture mechanics theory can be formulated.

Small scale yielding near a crack in plane strain - A finite element analysis

Abstract: Elastic-plastic finite element analysis of near crack tip stress and strain field structure

The Hertzian fracture test

Abstract: The crack growth in the non-uniform stress field due to the contact loading of a spherical ball on an elastic half-space is described quantitatively for the case of an isotropic brittle solid. This theoretical understanding provides a basis for the Hertzian test which may be used to measure three important surface properties of strong solids and consequently their strength. These are fracture toughness, surface crack size densities and residual stress. All examples of applications of the test are described in detail for glass but the application to a wider variety of strong materials is implied and gives the test a wider significance.

Extensive Study of Low Fatigue Crack Growth Rates in A533 and A508 Steels

Abstract: For two heats of ASTM A533 Grade B Class 1 pressure vessel steel and an ASTM A508 Class 2 forged steel, an extensive study of fatigue crack growth rate characteristics was conducted within the framework of linear elastic fracture mechanics. Experimental data obtained covered a wide variation in cyclic growth rates ranging from those associated with fatigue threshold (order of 0.000000001 in./cycle) up to rapid fracture. The bulk of experimental information obtained was at very low cracking rates, usually 0.000001 in./cycle and below. Considered in the investigation were: (1) an evaluation of the effect of stress ratio on very slow rates of fatigue crack extension; (2) an evaluation of temperature (75 to 650 deg F) on very slow rates of fatigue cracking; (3) an examination of the combined effects of temperature and stress ratio and their influence on fatigue crack extension; (4) an evaluation of the effect of a distilled water environment on fatigue crack propagation rates; (5) an examination of the effects of specimen thickness; and (6) a comparison of results for two separate heats of the same alloy. /Author/

Delay Effects in Fatigue Crack Propagation

Abstract: The fatigue crack propagation behavior resulting from variations in load is examined for 2024-T3 aluminum alloy, from both a macroscopic and a fractographic point of view. A peak load is found to cause retardation of the crack growth rate, which becomes more pronounced as the percentage overload or baseline stress intensity level or both is increased. The delaying effect of the overload is observed to exist for a calculated crack length increment equivalent to the plastic zone size formed during the peak load. Multiple overloads and high-low block loading sequences are found to result in additional retardation. It is observed that the macroscopic fracture surface appearance (that is, transition to plane stress) is a function of the crack growth rate. From fractographic examination it is found that the initiation of microvoid coalescence during fatigue occurs when plane stress conditions are achieved; this limits the extent of the stretch zone associated with an overload cycle. As a result, the stretch zone is found to be followed by striations in plane strain and by dimples under plane stress conditions. The size of the stretch band is observed to depend on the stress intensity level during the overload cycle. The usefulness of closure concepts in aiding the understanding of fatigue crack propagation under uniform and nonuniform loading conditions is considered. Evidence is given to demonstrate the general applicability of closure concepts for analysis of macroscopic and fractographic fatigue crack propagation results. /Author/

Application of fracture mechanics in the analysis of pavement fatigue

Abstract: THE PURPOSE OF THIS PAPER IS TO PREDICT THE FATIGUE LIFE OF PAVING MIXTURES IN TERMS OF MATERIAL CONSTANTS, GEOMETRY, BOUNDARY CONDITIONS, AND THE STATE OF STRESS. FATIGUE IS DEFINED IN TERMS OF CRACK INITIATION, INFLUENCES ON CRACK GROWTH, AND CRITICAL STRESS INTENSITY AT THE CRITICAL FAILURE POINT. CRACK GROWTH PARAMETERS WERE DETERMINED FROM BEAM LOADING EXPERIMENTS. FATIGUE LIFE ESTIMATES BASED ON THESE PARAMETERS AGREED CLOSELY WITH EXPERIMENTALLY DETERMINED VALUES.

Elastic-plastic mechanics of steady crack growth under anti-plane shear

Abstract: F or a crack with steady growth under anti-plane shear, analysis shows a primary plastic zone included in an angle of ±19.7° ahead of the crack tip, and two very thin secondary (reverse) plastic zones along the crack flanks, each included in an angle of 0.37°. Numerical solutions give the shape of the plastic zones which determine the active and residual plastic strains, and give the crack tip displacement, which is approximately 0.07 of that for monotonic loading without growth. The length of the primary plastic zone is almost the same as that without growth, but the thickness is about 3/5 as great. Coupled with ductile fracture criteria, the present results predict initially stable crack growth, whereas analyses based on the simplification of yielding on just one plane predict unstable fracture immediately following initiation.

Crack Propagation in a Linearly Viscoelastic Strip

Abstract: The tip velocity of a crack propagating through a viscoelastic material depends on geometry, applied load and its history, and material properties A consideration of the work done by the unloading tractions at the crack tip shows that, for a large crack propagating through an infinitely long strip under constant lateral strain, the rate of propagation can be calculated from a knowledge of the intrinsic fracture energy (a material constant), the material creep compliance, and an additional size parameter This parameter vanishes from the analysis if the material is elastic, and the familiar instability criterion is obtained in this case Comparison with experimental data is provided and the consequences of step loadings are examined

Fatigue-crack propagation behavior of type 304 stainless steel at elevated temperatures

Abstract: The fatigue-crack propagation behavior of Type 304 stainless steel was investigated within the framework of linear elastic fracture mechanics at temperatures of 75‡, 600‡ 1000‡ and 1200‡F. The cyclic frequency for the elevated temperature tests was 4 cpm. It was found that, in general, fracture mechanics concepts may be used to describe the crack propagation behavior at these temperatures, and that increasing the temperature had a significant effect in increasing the fatigue-crack growth rate.

Fatigue Crack Propagation in Polymethylmethacrylate; the Effect of the Mean Value of Stress Intensity Factor

Abstract: Polymeric components often contain structural defects which give rise to regions of internal stress concentration. As the engineering application of these materials expands, the need to understand their behaviour under various loading conditions becomes more necessary.This paper reports the results of part of a programme of research undertaken to study fatigue crack propagation phenomena in thermoplastics.A fracture mechanics approach is used and the effects of the mean stress intensity factor, Km, and the range of stress intensity factor, ΔK, on crack propagation phenomena in polymethylmethacrylate are studied.Based on the experimental data available, a relationship of the following form, between the cyclic crack growth rate d(2a)/dN and the tensile loading levels, has been proposed d(2a)/dN = β(Kmax2 – Kmin2) nwhere Kmax and Kmin are the maximum and minimum values of the stress intensity factor in each loading cycle. In tests at room temperature (21°C), in air, at a loading frequency of 5 Hz, n was foun...

Aspects of Stage II fatigue crack propagation in low-carbon steel

Abstract: The transition from structure sensitive to structure insensitive Stage II fatigue crack propagation, which has recently been observed in carbon steels, has been studied. The transition crack length is determined by the ratio of plastic zone radius at the crack tip to the grain diameter. An explanation of the transition in terms of constraint at the crack tip is suggested. No systematic change in fatigue crack propagation rate is observed with changes in grain size.

Mode II Fatigue Crack Propagation

Abstract: Fatigue crack propagation rates were obtained for 2024-T3 bare aluminum plates subjected to in-plane, mode I, extensional loads and transverse, mode II, bending loads. These results were compared to the results of Iida and Kobayashi for in-plane mode I-mode II extensional loads. The engineering significance of mode I-mode II fatigue crack growth is considered in view of the present results. A fatigue crack growth equation for handling mode I-mode II fatigue crack growth rates from existing mode I data is also discussed.

An exploratory study of delay in fatigue-crack growth

Abstract: Compression, creep, stress relaxation and overloading effects on delay in fatigue crack growth and structural life predictions

Elastic Crack Analysis by a Finite Element Hybrid Method

Abstract: The paper presents an efficient finite element method for evaluating the elastic stress intensity factors at the tip of a sharp crack. The method is based on a hybrid stress model for which special stress terms which represent the correct singularity behavior at the crack tip can be included. The magnitudes of such singular terms which are among the unknowns of the final matrix equations are, in fact, the stress intensity factors to be evaluated. Example solutions include plane stress cracks of both the opening type and in-plane shear type for isotropic materials and of the opening type for anisotropic materials. (Author)

The effect of environment on the mechanism of Stage I fatigue fracture

Abstract: Fatigue crack propagation in nickel-base superalloys at low and intermediate temperatures occurs predominantly in the Stage I mode, along {111} slip planes Cracking normally starts at an external surface and the Stage I fracture surface has a cleavage appearance Both of these factors indicate that the environment may play an important role in this mode of propagation To assess the role of environment in Stage I fracture and to determine the mechanism of failure, fatigue tests were run in air and vacuum on single crystals of low-carbon MAR-M200 The fatigue life at room temperature is significantly greater in vacuum than in air, and the improvement in life increases as the stress range is reduced Fatigue crack propagation in specimens tested in air and in vacuum is entirely in the Stage I mode, but only the specimens tested at low stress ranges in air have a cleavage appearance In vacuum and at high applied stress levels in air, fracture surfaces have a matte appearance with fewer fracture steps and river lines At high magnifications, a dimpled structure is observed on these fracture surfaces The fatigue life in air can be attributed to a faster rate of crack growth resulting from oxygen adsorption at the crack tip A model for Stage I fatigue crack propagation in planar slip materials is presented which is an extension of the Griffith-Orowan criterion to cases where localized cleavage occurs at a crack tip in fatigue

Fatigue Fracture in Polyethylene

Abstract: The application of non-classical fracture mechanics to the growth of dynamic fatigue cracks in a visco-elastic solid is discussed. The ideas developed are used to characterize the results of fatigue crack growth measurements on a range of low-density polyethylenes. The results can be expressed in the form [Note: See the image of page 57 for this formatted text]d$c$/d$N$ = B $\scr{J}^{n}$, where d$c$/d$N$ is the growth of the crack each cycle,[Note: See the image of page 57 for this formatted text]$\scr{J}$ is a fracture mechanics parameter and B, n are constants. Most of the materials studied reveal two distinct regions (with differing values of the constants) linked by a transition zone, and corresponding to brittle and ductile crack propagation respectively. The fatigue life of virgin specimens can be predicted from the crack growth characteristics assuming the existence of intrinsic flaws which (when the predictions are matched to actual fatigue data) are found to correspond in size to the polyethylene spherulites.