Stable Fatigue Crack Research Articles

The influence of loading history on fatigue in engineering plastics

Smooth bar rotating beam fatigue tests were conducted on acetal homopolymer (Delrin) and poly(methyl methacrylate) (PMMA) resins. Thermal failures due to hysteretic heating were encountered at high bending stress levels. Stable fatigue crack propagation (FCP) occurred in both materials at low stress levels. Interrupted fatigue tests of Delrin were performed by first load cycling until the surface temperature reached 65° C, then cooling to room temperature, and immediately recycling to 65° C. As expected, this intermittent testing procedure resulted in much longer fatigue lifetimes when compared to uninterrupted cyclic loading. Furthermore, each successive loading sequence required a shorter time to reach 65° C. It is suspected that this effect is related to cyclic-induced permanent changes in the material's viscoelastic damping response. Fractographic analysis of the Delrin samples revealed a duplex structure consisting of an annular surface region of microvoids combined with a flat, featureless central zone. The fracture surfaces of the PMMA samples were flat and generally nondescript, except at low bending stresses where some evidence of stable crack advance was seen.

Fatigue crack propagation. Report 1. Relationships governing unstable propagation

1. Fatigue crack propagation between the brittle jumps consists of the incubation period, during which the crack length is constant, and the period of continuous crack growth in which the crack propagates in each cycle. 2. With increasing maximum SIF of the cycle the duration of the incubation period and of the period of continuous FCP decreases. Depending on the ratio of the crack extension between the brittle jumps Δly and continuous crack extension Δlc in the block, we can determine the following sections: Δly/Δlc>1; Δly/Δlc=1; Δly=Δlc=0. 3. Stable fatigue crack propagation in 15Kh2MFA steel between the brittle jumps takes place as a result of fatigue damage cumulation.

Mechanisms of fatigue fracture in short glass fibre-reinforced polymers

Fatigue-crack profiles and fracture surfaces of several short glass fibre-reinforced polymers were examined to gain insight into the mechanisms of cyclic damage and fatigue-crack propagation in these materials. Several distinctly different features were noted between fracture surfaces generated by stable fatigue crack growth and those produced by monotonic or unstable fracture. Among the most significant differences were the higher degree of single and multiple fibre fracture generally observed on stable fatigue-crack growth fracture surfaces, and the variations in the interfacial failure site in well-bonded systems. While the former effect is attributed to the occurrence of crack closure and the build-up of compressive stresses in the crack-tip damage zone during unloading, the differences in the interfacial failure mode are related to the adverse effect of fatigue loading on the interfacial bond strength. No features could be identified that would allow a quantitative correlation between the applied stress intensity factor level or the crack growth rates and characteristic fracture surface details.

Effect of sea water on the fatigue crack propagation characteristics of welds for offshore structures

The fatigue crack propagation characteristics of ASTM A36 steel weldments produced by both conventional surface welding operations and underwater repair welding techniques were evaluated in both air and seawater. The resulting fatigue crack growth rates were shown to depend sensitively on pore density and pore distribution, factors that varied significantly with welding procedure and environment. Variations in growth rate were correlated with scanning electron fractographs of the stable fatigue crack zone. The results of this study show that underwater wet welding procedures produce fatigue-resistant weld metal that is adequate for use at low applied stresses in offshore structures.

Effect of carbon content in CrMoV hot working tool steel

The aim of this work was to find out whether a reduction of carbon content would reduce the amount of segregated eutectic carbides and improve ductility and toughness. Quenched and temperted specimens of steel X 40 CrMoV 5 1 (∼AISI H 13) were tested at carbon contents of 0.42 and 0.27%. Tensile and short time creep tests were carried out at elevated temperatures. Toughness was derived from unnotched, notched and precracked specimens over a temperature range. Stable fatigue crack growth was measured in CT specimens at room temperature and in thermal fatigue specimens.As expected, ductility and toughness are raised by reducing the carbon content. At the same time a favourable improvement of hot strength and creep resistance appears, causing a lower thermal fatigue crack velocity. These beneficial effects are due to the higher alloy content in solution after hardening.It is proposed to test a steel X 28 CrMoV 5 1 for intricate tools in practical application and learn more about its wear behaviour.

モードＩ・モードＩＩ組合せ応力負荷の下での疲労き裂進展挙動とき裂の発生・破断寿命

The mixed mode fatigue testing was carried out by applying the same phase and arbitrary ratio of torsional loading to tensile loading.The threshold stress intensity factor and fatigue fracture toughness decreased with an increase of ΔKII0 for initial crack.On the other hand, the stable fatigue crack growth region was found to be devided into IIa region of low growth rate and IIb region of accerated fatigue crack growth rate. Both regions were influenced by mixed mode loading.The crack initiation life Ni and propagation life Nf had similar characteristic in terms of (1+Δτ/Δσ)/√ΔKI02+3ΔKII02=(1+ΔKII0/ΔKI0)/√ΔKI02+3ΔKII02, where ΔKI0 and ΔKII0 are the stress intensity factor amplitude of initial crack. The value of Ni/Nf was found to be about 0.12.

Method of investigating the regularities of unstable crack development in cyclic loading with the application of acoustic emission

This paper presents a method of acoustic emission in assessing the jumplike development of fatigue cracks. The method makes it possible to investigate the regularities of unstable crack development in standard specimens, and also to check the process of fatigue crack development in a structural element with complex geometry and unknown K-calibration (K is the stress intensity factor) at places that are inaccessible for visual observation. The method was used for investigating steels designated 15Kh2MFA(I) and, in the embrittled state, 15Kh2MFA(II). The mechanical properties and heat-treatment conditions of both steels are presented. It is shown that embrittlement by heat treatment of the steel 15Kh2MFA substantially reduces its fatigue crack resistance, and that it leads to the transition from stable fatigue crack development.

Craze development in poly(methyl methacrylate) during stable fatigue crack propagation

AbstractIn order to obtain a more complete understanding of failure mechanisms in glassy polymers subjected to fatigue loading conditions, craze zone dimensions (i.e., length and thickness at the crack tip) were measured simultaneously with fatigue crack propagation data in poly(methyl methacrylate) (PMMA) by optical interferometry. Since the craze shape was observed to assume a wedge‐shaped configuration similar to the one described by the Dugdale plastic strip model, crazing stresses were inferred on the basis of this model. When varying the stress ratio (R = minimum load/maximum load) of the applied cyclic load in the range from 0.1 to 0.7, it was found that both craze length and craze thickness are essentially independent of the R‐ratio and can be correlated in terms of the maximum stress intensity factor only. On the other hand, significant variations in craze dimensions with test frequency occurred over the range from 0.1 to 250 Hz. The results are discussed in terms of the viscoelastic nature of the material and a competition between the effects of strain rate and hysteretic heating.

Discreteness and self-similarity of failure in stable fatigue crack growth

1. On the basis of the approaches of linear fracture mechanics and the theory of self-similarity developed by Sedov it has been shown that the effect of the external factors on the cracking resistance criteria can be reduced to a single factor, i.e., the degree of constraint of plastic deformation characterized by parameter Q = (σ11)max/σT. 2. As the criterion controlling the transition from quasielastic self-similar to elasto-plastic self-similar crack propagation, it is proposed to use the Huber-Mises criterion, i.e., the critical value of octahedral tangential stress τo = τo* = √2/3σT for the local stress field at the crack front. This makes it possible to determine the range of variation of the σnσT ratio (0.47 ⩽σn/σT ⩽ 0.82) within which self-similar elastoplastic crack growth takes place in the steel with the formation of a single striation per cycle at the minimum elastoplastic constraint Q = 3. 3. The KI*-m diagram linking the threshold values of KI* with coefficient m which characterizes the crack growth retardation capability has been proposed. The regions of the KI* values and of the corresponding values of m which correspond to self-similar elastoplastic crack growth have been defined. The boundaries of self-similarity have been verified by means of experiments, and the experimental results were found to be in agreement with theory.

Effect of load cyclicity on the fracture toughness characteristics of steels. Report 1

1. We studied the effect of teat temperature on fracture toughness characteristics and features of stable and unstable fatigue crack growth in the static and cyclic loading of steels 15Kh2MFA and 15Kh2NMFA (I, II). 2. It was shown that a reduction in test temperature leads to a substantial reduction in the fracture toughness characteristics K fc 1 , K fc k , and kIc of the investigated steels under static and cyclic loadings. Under certain conditions, for several of the materials the cyclicity of the loading substantially lowers the fracture toughness characteristics. For the steels investigated, at 293 °K loading cyclicity negligibly changes K fc 1 compared to kc. At low temperatures, k fc 1 changes substantially compared to KIc. For a given test temperature, the values of K fc 1 at which loading cyclicity has a significant effect depends on the initial value of KI at the beginning of the test and on the number of load cycles. 3. The laws of unstable fatigue crack growth in steels 15Kh2MFA and 15Kh2NMFA (I, II) under cyclic loading (number and sizes of crack jumps) are determined by the test temperature, the level of initial values of KI in the test, and the ratios of the fracture toughness characteristics K fc 1 , K fc k , and kIc. 4. A reduction in the test temperature leads to an increase in resistance to fatigue crack growth at low values of kI and a decrease in resistance to fatigue crack growth at higher KI. 5. A reduction in loading frequency ambiguously affects resistance to fatigue crack growth for different materials.

An introduction to fracture mechanics for engineersPart II: Using the stress-intensity factor tocharacterise fracture and fatigue crack growth

This article continues the discussion of the Stress Intensity Factor parameter by demonstrating how it can be used to characterise catastrophic (rapid) fracture and stable fatigue crack growth. Numerical values for the fracture toughness, crack growth ‘constants’ and threshold Stress Intensities are given for a wide range of engineering materials. The principles of the Fracture Mechanics approach to failure prevention are illustrated by worked examples.

An introduction to fracture mechanics for engineers Part II: Using the stress intensity factor to characterise fracture and fatigue crack growth

This article continues the discussion of the Stress Intensity Factor parameter by demonstrating how it can be used to characterise catastrophic (rapid) fracture and stable fatigue crack growth. Numerical values for the fracture toughness, crack growth ‘constants’ and threshold Stress Intensities are given for a wide range of engineering materials. The principles of the Fracture Mechanics approach to failure prevention are illustrated by worked examples.

The influence of crystallographic orientation on fatigue crack growth in strongly textured Ti6A14V

Fractographic features of fatigue crack growth in a strongly textured Ti-6Al-4V bar have been studied in order to correlate these features with the orientation of crystallographic deformation modes. The observations fell into two groups: flat fracture surfaces and striations normal to the overall crack growth direction when m values in the Paris equation were low (~ 2.5); and ridged fracture surfaces and striations at various angles to the overall crack growth direction when m values were high (3.1–4.1). A consideration of the active deformation modes, {101̄0} slip and {112̄1} twinning, showed that only when deformation occurred symmetrically about the tip of the crack plane, and thus satisfied the continuum flow field set up in plane strain, did stable fatigue crack growth occur (low m values). Higher growth rate dependences on ΔK occurred as a result of asymmetric deformation about_the crack tip by fatigue crack growth on a number of different fronts, sometimes arising due to 〈112̄3〉 slip, and monotonie fracture processes.

Dislocational structure of iron at the tip of a fatigue crack

1. Electron microscopy by transillumination established that fatigue crack propagation in iron was accompanied by the development at the tip of the crack of a zone of intense plastic deformation, in which there was a substantial (three-to fivefold) increase in dislocation density and a corresponding reduction (1.8–3.0 times.) in cell size by comparison with those in the rest of the testpiece. 2. Comparison of the parameters of dislocational structure according to the results of electron microscopy by transillumination and the parameters of fatigue fracture surface relief in the crack propagation zone under conditions of plane strain according to the results of microfractographic research showed that stable fatigue crack growth at minimum speed persists until the size of the crack microstep exceeds the size of the secondary cellular structure subgrain. 3. The size of the secondary substructure cell DIIcell≈0.4 μ is practically independent of crack length in the zone of pure fatigue failure and determines the cyclic fracture toughness.

21. The influence of crystallographic orientation on fatigue crack growth in strongly textured Ti6Al4V

Abstract Fractographic features of fatigue crack growth in a strongly textured Ti-6Al-4V bar have been studied in order to correlate these features with the orientation of crystallographic deformation modes. The observations fell into two groups: flat fracture surfaces and striations normal to the overall crack growth direction when m values in the Paris equation were low (~ 2.5); and ridged fracture surfaces and striations at various angles to the overall crack growth direction when m values were high (3.1–4.1). A consideration of the active deformation modes, {1010} slip and {1121} twinning, showed that only when deformation occurred symmetrically about the tip of the crack plane, and thus satisfied the continuum flow field set up in plane strain, did stable fatigue crack growth occur (low m values). Higher growth rate dependences on ΔK occurred as a result of asymmetric deformation about_the crack tip by fatigue crack growth on a number of different fronts, sometimes arising due to 〈1123〉 slip, and monotonie fracture processes.