The mechanisms of fatigue-crack propagation are examined with particular emphasis on the similarities and differences between cyclic crack growth in ductile materials, such as metals, and corresponding behavior in brittle materials, such as intermetallics and ceramics. This is achieved by considering the process of fatigue-crack growth as a mutual competition between intrinsic mechanisms of crack advance ahead of the crack tip (e.g., alternating crack-tip blunting and resharpening), which promote crack growth, and extrinsic mechanisms of crack-tip shielding behind the tip (e.g., crack closure and bridging), which impede it. The widely differing nature of these mechanisms in ductile and brittle materials and their specific dependence upon the alternating and maximum driving forces (e.g., ΔK andK
 max) provide a useful distinction of the process of fatigue-crack propagation in different classes of materials; moreover, it provides a rationalization for the effect of such factors as load ratio and crack size. Finally, the differing susceptibility of ductile and brittle materials to cyclic degradation has broad implications for their potential structural application; this is briefly discussed with reference to lifetime prediction.

Mechanisms of fatigue-crack propagation in ductile and brittle solids

Previous studies have shown that both threshold stress intensity factors and fatigue crack growth rates are dependent on crack size The average growth rates for very short cracks considerably exceed those given by conventional stress intensity-crack growth laws fitted to long crack data Elastic and elastic plastic fracture mechanics solutions are modified to predict this behavior of short cracks by introducing an effective crack length U into the solutions for intensity factors and the J integral method of analysis The threshold stress at a very short crack length approaches the fatigue limit of the ma­terial, and therefore the value of k can be obtained once the threshold stress intensity factor and the fatigue limit are known The accuracy of the term k in predicting crack growth rates for short cracks is found to be independent of the applied strain level It varies linearly with the grain size of material and can be considered at surface as a measure of the reduced flow resistance of surface grains due to their lack of con­straint

Fatigue Crack Propagation of Short Cracks

An irreversible cohesive zone model for interface fatigue crack growth simulation

A damage model for the simulation of delamination propagation under high-cycle fatigue loading is proposed. The basis for the formulation is a cohesive law that links fracture and damage mechanics to establish the evolution of the damage variable in terms of the crack growth rate dA/dN. The damage state is obtained as a function of the loading conditions as well as the experimentally-determined coefficients of the Paris law crack propagation rates for the material. It is shown that by using the constitutive fatigue damage model in a structural analysis, experimental results can be reproduced without the need of additional model-specific curve-fitting parameters.

Simulation of delamination in composites under high-cycle fatigue

Mixed mode fatigue crack growth: A literature survey

An attempt is made to apply the J-integral concept as an elastic-plastic criterion for fatigue crack growth. Compact tension fracture specimens of A533B steel are subjected to gross cyclic plastic deformations, and fatigue crack growth rates up to 0.01 in./cycle are obtained. The results show correlation with J-integral values estimated from load versus deflection hysteresis loops. Also, agreement is obtained with the extrapolation of linear elastic fatigue crack growth rate data.

Fatigue Crack Growth During Gross Plasticity and the J-Integral

A robotic sampling device (32) for cutting part of a tube wall (12) for sampling, containing a cutting head (36), a retrieval assembly (38) and a drive mechanism (40), is used to cut a window or hole (60) in the tube wall (12) and retrieve the tube wall sample (56), where the sample can be mounted onto a separate tube for testing the physical properties of the cut wall portion, and where a video probe (62) and the like can be passed through the window (60) to monitor conditions near the support plates (14) and tube sheets (16).

Remotely operated diagnostic tube sampling device and method of sampling

Charpy-V type samples either step-quenched from 1200 °C or directly quenched from the usual 870 °C temperature, fractured by a slow bend test procedure, have been fractographically examined. Their notch root radius,ρ, ranged from almost zero (fatigue precrack) up to 2.0 mm. The fracture initiation process at the notch differs according to root radius and heat treatment. Conventionally austenitized samples withρ values larger than 0.07 mm approximately (ρ
 eff) always display a continuous shear lip formation along the notch surface, whereas specimens with smaller notches do not exhibit a similar feature. Moreover, shear lip width in specimens withρ >ρ
 eff is linearly related to the applied J-integral at fracture. In high temperature austenitized samples similar shear lips are almost nonexistent. The above findings, as well as overall fractographic features, are combined to explain why blunt notch AISI 4340 steel specimens display a better fracture resistance if they are conventionally heat treated, whereas fatigue precracked samples show a superior fracture toughness when they are step-quenched from 1200 °C. Variations of fracture morphologies with the notch root radius and heat treating procedures are associated with a shift toward higher Charpy transition temperatures under the combined influence of decreasing root radii and coarsening of the prior austenitic grain size at high austenitizing temperatures.

The Influence of Notch Root Radius and Austenitizing Temperature on Fracture Appearance of As-Quenched Charpy-V Type AISI4340 Steel Specimens

Two fracture criteria for elastic-plastic behavior of metals have recently been proposed, the J-integral fracture criterion and the equivalent energy concept. Since both are basically energy criteria, speculation has developed that they are related and perhaps identical. This work shows that the two criteria do not give identical results for a general load-displacement curve. Special cases where the two are identical are discussed as well as examples where they are not identical.

A Comparison of the J-Integral Fracture Criterion with the Equivalent Energy Concept

Corrosion fatigue crack growth rate tests using compact tension specimens of Type 304 stainless steel and smooth bar corrosion fatigue tests of Inconel Alloy 600 were run in boiling (140 °C) 17.5M NaOH solution. Contrary to earlier results from smooth bar Type 304 specimens, the measured crack growth rates of Type 304 did not show a high accelerative environmental effect; there was also no significant effect of cyclic frequency (0.1 to 10 Hz),R ratio, or electrochemical potential. For Inconel 600 the boiling caustic solution increased fatigue life relative to life in air, and anodic passivation was beneficial. Sensitization, found beneficial in sodium hydroxide, was detrimental in air in spite of similar failure morphologies. A delay of crack initiation is proposed as primarily responsible for the effects of the solution and sensitization.

J. A. Begley

Papers

Fatigue Crack Growth During Gross Plasticity and the J-Integral

Remotely operated diagnostic tube sampling device and method of sampling

The Influence of Notch Root Radius and Austenitizing Temperature on Fracture Appearance of As-Quenched Charpy-V Type AISI4340 Steel Specimens

A Comparison of the J-Integral Fracture Criterion with the Equivalent Energy Concept

Corrosion Fatigue of Type 304 Stainless Steel and Inconel™ 600 in a Boiling (140 °C) Concentrated Caustic Solution