Showing papers on "Crack closure published in 1988"

Mechanisms of fatigue crack propagation in metals, ceramics and composites: Role of crack tip shielding☆

Abstract: Crack tip shielding phenomena, whereby the “effective crack-driving force” actually experienced at the crack tip is locally reduced, are examined with reference to fatigue crack propagation behavior in metals, composites and ceramics. Sources of shielding are briefly described in terms of mechanisms relying on the production of elastically constrained zones which envelop the crack (zone shielding), on the generation of wedging, bridging or sliding forces between the crack surfaces (contact shielding) and on crack path deflection and meandering. Examples are taken from the fatigue behavior of high strength lithium-containing aluminum alloys, aluminum alloy-aramid fiber-epoxy laminate composites, and zirconia ceramics. It is shown that, whereas crack tip shielding can provide a potent means of enhancing “resistance” to crack growth, such extrinsic toughening mechanisms can result in the apparently anomalous behavior of “small cracks” and to the susceptibility of brittle materials to fatigue failure.

A virtual crack-closure technique for calculating stress intensity factors for cracked three dimensional bodies

Abstract: A three-dimensional virtual crack-closure technique is presented which calculates the strain energy release rates and the stress intensity factors using only nodal forces and displacements from a standard finite element analysis. The technique is an extension of the Rybicki-Kanninen (1977) method, and it assumes that any continuous function can be approximated by a finite number of straight line segments. Results obtained by the method for surface cracked plates with and without notches agree favorably with previous results.

Short and long fatigue crack growth: A unified model

Abstract: In this model the growth of a crack is analysed in terms of the successive blocking of the plastic zone by slip barriers (e.g. grain boundaries) and the subsequent initiation of the slip in the next grain. The discontinuous character of the slip process (slip jumps) plays a fundamental role in the model. The factor governing the transfer of slip across a grain boundary is considered to be the stress concentration ahead of the plastic zone which, for a constant applied stress τ, is found to be dependent only on a parameter n = a/c defining the position of the crack tip relative to the grain boundary. The discrete behaviour of the slip has a strong influence in the short-crack period and hence cannot be neglected in the analysis of the crack growth rate. This period is characterized by large variations in the parameter n. In the long-crack period the slip jumps do not influence the overall description of the growth and the parameter n is almost constant. By making the crack extension per cycle prop...

Mechanics of Fatigue Crack Closure

Abstract: Papers are presented on plasticity induced crack closure, crack closure in fatigue crack growth, the dependence of crack closure on fatigue loading variables, and a procedure for standardizing crack closure levels. Also considered are a statistical approach to crack closure determination, the crack closure behavior of surface cracks under pure bending, closure measurements on short fatigue cracks, and crack closure under plane strain conditions. Other topics include fatigue crack closure behavior at high stress ratios, the use of acoustic waves for the characterization of closed fatigue cracks, and the influence of fatigue crack wake length and state of stress on crack closure.

Role of silicon carbide particles in fatigue crack growth in SiC-particulate-reinforced aluminum alloy composites

Abstract: A study has been made of the mechanistic role of silicon carbide (SiC) particles during fatigue crack propagation in powder metallurgy AlZnMgCu metal matrix composites reinforced with 20 vol.% SiC particulates (SiC p ), with varying sizes of reinforcement phase. Crack growth and accompanying crack tip shielding (principally by crack deflection, closure and bridging) are examined in peak-aged alloys over a wide spectrum of growth rates from 10 −12 to 10 −4 m cycle −1 and are compared with corresponding behavior in the unreinforced matrix alloy. Crack growth resistance in the composites is found to be both superior and inferior to that of the unreinforced alloy, depending on how the SiC and SiC-matrix interface fracture. At low stress intensity ranges ΔK, the predominant fracture of carbides close to the crack tip results in low levels of crack closure and rapid growth kinetics for fine SiC p distributions, whereas for coarse SiC p distributions the rougher fracture surface promotes crack closure from asperity wedging and improved crack growth resistance. With increasing ΔK, the fracture of large carbides farther ahead of the crack tip leads to the development of non-uniform crack fronts, thereby promoting crack bridging via uncracked ligaments and a small improvement in crack growth resistance. At high ΔK levels approaching K Ic , growth rates are faster in the composites owing to their low toughness. On the basis of these results, the overall fatigue crack growth performance of the SiC p Al composites compared with that of the traditional monolithic aluminum alloys is briefly discussed.

Analysis of Crack Closure Under Plane Strain Conditions

Abstract: The phenomenon of plasticity-induced crack closure is associated with the development of residual material on the flanks of an advancing fatigue crack. While it is easy to see that this residual material can come from the side faces of a specimen under plane stress conditions, it is difficult to discover the origin of this extra volume of material on the crack flanks when it is assumed that plane deformations occur and plastic flow is incompressible. The purpose of this paper is to determine whether plasticity-induced fatigue crack closure occurs in an elastic-perfectly plastic body under plane strain conditions.

Chemical and metallurgical aspects of environmentally assisted fatigue crack growth in 7075-T651 aluminum alloy

Abstract: A comprehensive study has been carried out on a 7075-T651 alloy to examine the influence of water vapor on fatigue crack growth. The kinetics of fatigue crack growth were determined as a function of water vapor pressure at room temperature and at 353 K. Detailed fractographic analyses and surface chemistry studies were carried out to identify the micromechanisms and to quantify the chemical interactions for corrosion fatigue crack growth in this alloy. Experiments were also carried out in ultra-high vacuum and in oxygen to provide for comparisons. Two regions of fatigue crack growth response were identified. In the low pressure region (below 67 Pa at 5 Hz), crack growth is controlled by the rate of transport of water vapor to the crack tip, and the response can be described by a model for transport controlled crack growth. At pressures above 67 Pa, additional increases in crack growth rate occurred, which are attributed to the further reactions of water vapor with segregated magnesium in this alloy. Different micromechanisms for crack growth have been identified for vacuum, oxygen, and water vapor. These micromechanisms are considered in relation to the environmental parameters through a modified superposition model for corrosion fatigue.

Effects of SiC reinforcement and aging treatment on fatigue crack growth in an AlSiC composite

Abstract: The effects of SiC reinforcement and matrix aging treatment on fatigue crack growth behavior in a powder metallurgy aluminum 2124 alloy-SiC whisker composite were investigated. The microstructures were designed such that the matrix of the composite material and the unreinforced control aluminum alloy 2124 with an identical processing history had similar hardness and precipitation characteristics. The threshold stress intensity factor range ΔK0 for tensile fatigue crack growth was obtained using a procedure whereby mode I precracks were initiated in uniaxial cyclic compression prior to tension fatigue. The composite exhibits a ΔK0 value which is about twice that of the control alloy. However, ΔK0 is relatively insensitive to variations in both the aging treatment and the mean stress of the fatigue cycle in both the composite and the control alloy. Possible mechanisms underlying this trend are discussed.

Influence of time and temperature dependent processes on strain controlled low cycle fatigue behavior of alloy 617

Abstract: Strain controlled low cycle fatigue tests have been conducted in air to ascertain the influence of strain rate(e = 4 × 10-6'to 4 × 10-3 s-1) and temperature(T = 750/850/950 °C) on LCF behavior of Alloy 617. A strain range of 0.6 pct and a symmetrical triangular wave form were employed for all the tests. Crack initiation and propagation modes were studied. Microstructural changes that occurred during fatigue deformation were evaluated and compared with the results obtained on isothermal aging. Deformation and damage mechanisms which influence the endurance have been identified. A reduction in fatigue life was observed with decreasing e at 850 °C and with increasing temperature at e = 4 × 10-5 s-1. Cyclic stress response varied as a complex function of temperature and strain rate. Fatigue deformation was found to induce cellular precipitation of carbides at 750 and 850 ‡. Dynamic strain aging characterized by serrated flow was observed at 750 °C (e = 4 × 10-5 s-1) and in the tests at higher e at 850 °C. Strengthening of the matrix due to dynamic strain aging of matrix dislocations by precipitation of M23C6 carbides led to fracture of grain boundary carbide films formed at 750 °C, producing brittle intergranular crack propagation. At 850 °C transgranular crack propagation was observed at the higher strain rates e≥4× 10-4 s-1. At 850 and 950 °C even at strain rates of 4 × 10-5 s-1 or lower, life was not governed by intergranular creep rupture damage mechanisms under the symmetrical, continuous cycling conditions employed. Reduction of endurance at lower strain rates is caused by increased inelastic strain and intergranular crack initiation due to oxidation of surface connected grain boundaries.

A simplified laboratory approach for the prediction of short crack behavior in engineering structures

Abstract: — Conventionally determined fatigue threshold information (ASTM E647) can lead to non-conservative estimates of fatigue lifetimes when these data are utilized in damage tolerant design assessments. The non-conservative nature of such data can be attributed primarily to the development of excessively large amounts of crack closure at low R-ratios, particularly at near threshold stress intensity factor levels. These high closure levels attenuate the effective stress intensity condition prevailing at the crack tip and confound attempts to predict the behavior of short cracks that exhibit limited crack closure. A modified test procedure, involving constant maximum stress intensity factor (Kcmax) test conditions, is described which identifies fatigue crack propagation (FCP) threshold behavior in the absence of detectable amounts of crack closure. These data have been generated with conventional long crack specimens for several aluminum, iron, and nickel-based alloys and which are shown to closely simulate the FCP response of short cracks in these engineering materials. As such, the modified threshold test procedure, incorporating constant Kmax loading conditions, represents a valuable tool in the prediction of the cyclic lifetime of engineering components. The stress-cyclic lifetime (S-N) curve for aluminum butt-welded beams was computed based on Kcmax data and found to be in excellent agreement with actual test results.

Fatigue crack growth resistant nickel-base article and alloy and method for making

Abstract: An article having improved fatigue crack growth resistance is provided through an improved nickel-base superalloy and an improved method which controls grain size and a strain rate found to be critical in processing. The alloy is selected to have a gamma prime content in the range of about 30-46 volume percent and a resistance to cracking upon rapid quenching from a selected supersolvus solutioning temperature to a selected quenching temperature. The article produced has an improved balance and combination of fatigue crack growth resistance and tensile, creep, and stress rupture properties.

Three-Dimensional Finite-Element Simulation of Fatigue Crack Growth and Closure

Abstract: A three-dimensional elastic-plastic finite-element analysis of crack growth and closure under cyclic loading has been performed in order to investigate the behavior of a crack in a finite-thickness middle-crack tension specimen. The cases of both constant-amplitude loading and a single-spike overload are considered. The calculated crack-opening stresses for the exterior of the specimen agree with previous plane-stress results, and those obtained for the interior of the specimen agree with previous plane-strain results.

Tension Crack Development in Soils

Abstract: A model is proposed for finite‐element modeling of tension crack propagation in soils. The essential features of this model are the splitting of a single node into two distinct nodes in the wake of an advancing crack tip to replicate separation of material on either side of the crack, and the use of a fracture mechanics criterion to predict crack propagation. The material parameter employed in this criterion, the critical energy release rate, is determined experimentally and shown to be reasonably constant over a range of crack lengths, thereby supporting the adoption of this parameter as a material constant for tension cracking in soils. The model is then applied to two classes of problems in which tension cracking is known to exercise significant influence, a stiff embankment on soft soil and an excavated slope. For both problems, stable and reasonable solutions are obtained, suggesting that fracture mechanics offers a feasible approach to the analysis of tension cracking in soils.

Influence of Stress State on Crack Growth Retardation

Abstract: Several mechanisms have been proposed in the literature to account for crack growth retardation following a single peak overload. In order to determine the dominant mechanism, overload tests were performed on thick and thin specimens made from BS4360 50B structural steel. The baseline stress intensity range was 25 MPa √m and the load ratio (= minimum load/maximum load of fatigue cycle) was 0.05, while the overload was of stress intensity range 50 MPa √m. It was observed that the crack growth and closure responses were different at the surface and in the bulk of the thick specimen; no such variations in behavior occurred along the crack front for the thin specimen. For both thicknesses of material, the crack growth rate predicted by measurements of the crack opening load was in agreement with the observed crack growth rate, except for the period when crack growth rates were recovering from the slowest transient growth rate to the post-overload stabilized value. This discrepancy was due to the phenomenon of discontinuous closure-the crack first closed at a location far from its tip. Fracture surface profiles showed that the crack path deviated by only a small amount after application of the overload; hence the retarded growth cannot be due to crack branching. It is concluded from these tests and from a critical examination of the literature that, at high baseline AK levels, retardation is due to plasticity-induced crack closure. At low baseline AK levels approaching the threshold value, retardation may be due to plasticity-induced crack closure or to irregularities of the crack front.