Fatigue Crack Propagation Threshold Research Articles

The threshold of micro crack propagation under mixed mode.

In order to investigate the condition of the threshold of micro fatigue crack propagation under mixed mode, plane bending and cyclic torsion tests, and the various combinations of them are carried out under the condition of the stress ratio R=-1 by using the annealed cracked specimens made of carbon steel. Based on the assumption that a crack begins to propagate when the circumferential principal stress reaches a critical value peculiar value peculiar to the material, the stress condition at the threshold of crack propagation under mixed mode can be explained by applying the test results of plane bending and cyclic torsion.

Effect of grain size on the near threshold fatigue crack propagation of a nickel base precipitation hardened superalloy

1. 1. By coarsening the grain size from 22 to 91 μm in Alloy 718, it was found that: i. near threshold crack growth rates were reduced and ii. ΔK th values increased. 2. 2. Roughness-induced crack closure may be used to explain the influence of grain size. The coarse grained material resulted in much rougher fracture surfaces at near threshold growth rates. 3. 3. At room temperature, a fracture morphology transition occurred which coincided with the concave upward ergion in the crack growth curves above ΔK th. Below the transition, the maximum plastic zone size was less than the grain diameter and the crack propagated by crystallographic cracking. Above the transition in which the maximum plastic zone size was greater than the grain diameter, crack extension occurred through the formation the formation of fatigue striations. This transition was shifted to higher ΔK values by coarsening the grain size.

Crack deflection: Implications for the growth of long and short fatigue cracks

The influences of crack deflection on the growth rates ofnominally Mode I fatigue cracks are examined. Previous theoretical analyses of stress intensity solutions for kinked elastic cracks are reviewed. Simple elastic deflection models are developed to estimate the growth rates of nonlinear fatigue cracks subjected to various degrees of deflection, by incorporating changes in the effective driving force and in the apparent propagation rates. Experimental data are presented for intermediate-quenched and step-quenched conditions of Fe/2Si/0.1C ferrite-martensite dual phase steel, where variations in crack morphology alone influence considerably the fatigue crack propagation rates and threshold stress intensity range values. Such results are found to be in good quantitative agreement with the deflection model predictions of propagation rates for nonlinear cracks. Experimental information on crack deflection, induced by variable amplitude loading, is also provided for 2020-T651 aluminum alloy. It is demonstrated with the aid of elastic analyses and experiments that crack deflection models offer a physically-appealing rationale for the apparently slower growth rates of long fatigue cracks subjected to constant and variable amplitude loading and for the apparent deceleration and/or arrest of short cracks. The changes in the propagation rates of deflected fatigue cracks are discussed in terms of thelocal mode of crack advance, microstructure, effective driving force, growth mechanisms, mean stress, slip characteristics, and crack closure.

The influence of strain aging on the fatigue crack propagation threshold

While the influence of strain-aging on the endurance limit and on the dislocation behaviour during cyclic deformation has received some attention in the literature, almost no attention appears to have been paid to the influence of strain aging on fatigue crack propagation. An influence of strain aging on fatigue propagation can be expected to manifest itself most clearly at the very small propagation rates near ..delta..K /SUB th/ , the fatigue crack propagation threshold. The present study was therefore performed to determine whether an influence attributable to strain aging could be detected near ..delta..K /SUB th/ during tests carried out on two mild steels having different sensitivities to strain aging.

Fatigue crack propagation in steels

From previous investigations of the mechanisms of both fracture and fatigue crack propagation, the static fracture model proposed by Lal and Weiss may be thought as reasonable for describing fatigue crack propagation in metals at both low and intermediate stress intensity factor ranges ΔK. Recent progress in fatigue crack propagation indicates that it is not only possible, but also necessary, to modify this static fracture model. Based on the modified static fracture model, the effective stress intensity factor range ΔK eff , which is defined as the difference between ΔK and the fatigue crack propagation threshold value Δ th , is taken as the governing parameter for fatigue crack propagation. Utilising the estimates of the theoretical strengths of metals employed in industry, a new expression for fatigue crack propagation, which may be predicted from the tensile properties of the metals, has been derived. The correlation between the fatigue crack propagation rate and the tensile properties is thus revealed. The new expression fits the test results of fatigue crack propagation of steels below 10 −3 mm/cycle and indicates well the effect of stress ratio on the fatigue crack propagation rate.

Effect of grain size and cold work on the near threshold fatigue crack propagation rate and crack closure in iron

Effect of grain size and cold work on the near threshold fatigue crack propagation rate and crack closure in iron

Fatigue crack propagation rates and threshold stress intensity factors for welded joints of HT80 steel at several stress ratios

Fatigue crack propagation rates and threshold stress intensity factors were measured for welded joints and base metal by using 200 mm wide centre-cracked specimens. The fatigue crack propagation properties of welded joints were similar in spite of the different zones in which the cracks propagated (ie, in the heat-affected zone and in the weld metal) and the different welding process used (submerged arc welding and gas metal arc welding). They were, however, inferior to those of the base metal. It was revealed by observation of the crack closure that the fatigue cracks were fully open during the whole range of loading, due to the tensile residual stress distribution in the middle part of the welded joints. This observation also explains the lack of a stress ratio effect on the fatigue crack propagation properties of welded joints, and their inferiority to those of the base metal.

Fatigue crack propagation thresholds for long and short cracks in René 95 Nickel-base superalloy

A study was made of the near-threshold fatigue crack propagation behaviour of a wrought nickel-base superalloy, René 95, with reference to the effect of crack size on the threshold stress intensity ΔK 0 no detectable crack growth. Measured threshold ΔK 0 values at low load ratios (R = 0.1) for physically short (0.01 – 0.20 mm) cracks were found to be 60% smaller than the corresponding ΔK 0 values for long (about 25 mm) cracks. However, short crack threshold values at R = 0.1 were found to be similar to long crack thresholds at R = 0.8. Such behavior is rationalized in terms of fatigue crack closure, specifically involving the role of fracture surface roughness from crystallographic crack growth in nickel-base alloys. The large difference observed between the threshold values for long and physically short cracks serves to illustrate the potential problems in applying conventional (long crack) fatigue data to defect-tolerant lifetime predictions for structural components containing small flaws.

Fatigue Initiation Study of TMT Eutectoid Steel

Eutectoid steel and 1060 steel samples were given a variety of thermomechanical treatments (TMT), described in Table I, which varied the pearlite interlamellar spacing, the cementite and inclusion orientation, the degree of cold-work in the ferrite matrix, and (for 1060 steel only) the proeutectoid grain boundary ferrite network. These samples were then evaluated as to resistance to fatigue crack initiation. The TMT designated, G, involves a subcritical anneal and results in a partially recrystallized condition which shows not only excellent resistance to fatigue crack initiation in the near threshold region but is second only to a fine pearlite microstructure, B, in fatigue crack propagation threshold value. It is believed that the excellent properties of the TMT, G, are related to the formation of subgrains in the interlamellar ferrite. On a scale normalized to tensile strength,(ΔK/√ρ)/σu, fine oriented pearlite in a soft ferrite matrix (rapid up-quench TMT such as E, F) shows the best resistance to fatigue crack initiation. A proeutectoid ferrite grain boundary network is poor at resisting fatigue crack initiation but good at resisting fatigue crack propagation. It should be emphasized that the combined high resistance of the subcritical TMT (G) toboth fatigue crack initiation and propagation, coupled with its much easier implementation relative to similar microstructures produced by difficult rapid up-quench TMT's (E, F) make it a very promising approach to improving overall fatigue resistance.

Effects of Stress Ratio and Stress History on the Threshold for Fatigue Crack Propagation

In the present paper, authors have performed an experimental study on fatigue crack propagation at threshold level to clear the effects of stress ratio and stress history upon the threshold stress-intensity factor (K-threshold). Fatigue tests were carried out under load controlled three-point bending condition using side notch type specimens.The K-threshold values under the different stress ratio conditions are obtained by load decreasing test method which can eliminate the influences of loading history. The results show that the K-threshold range decreases with the increasing of stress ratio. An empirical equation which represents the relation between K-threshold range and stress ratio is proposed based on the comparison among present results and other researchers' data.TheK-threshold values after crack propagation at constant stress range are obtained by load increasing test method under various stress ratio conditions. It is found that theKthreshold value after stress history increases with the increasing of stress range at stress history process. And this tendency is well characterized by using maximum stress-intensity factor at stress history process and at threshold. A simplified equation for the prediction of K-threshold value after given stress history of constant stress range is derived from the present study. Estimated K-threshold values by proposed equation show fairly good agreements not only with present results but also with other researchers' data.

The Fatigue and Toughness Properties of a C-Mn-Cb Plate Steel

The fatigue and toughness properties of a C-Mn-Cb plate steel, described both by ASTM A633C and A737B, are presented and compared to the properties of A516-70. Production plate steels made by both conventional steelmaking techniques and low sulfur-calcium treatment are evaluated. The superiority of A633C over A516-70 in transition and upper shelf fracture toughness is demonstrated in both Charpy-V-notch and Dynamic Tear impact testing, as well as J-Integral fracture toughness at both a slow and fast loading rate. There was no substantial difference in fatigue behavior between the A633C and A516-70 steels, except that A633C was found more sensitive to inclusions, thus there was more of a pronounced improvement shown by using calcium treatment. The fatigue behavior was assessed using S-N axial fatigue testing, fatigue crack propagation testing and by determining the threshold for fatigue crack propagation. The residual stresses in A633C in the quenched and tempered condition influenced the fatigue crack propagation and near threshold behavior of this steel.

Recent advances in notch analysis of fracture and fatigue

Neuber's micro- and macrosupport effect theories are applied to obtain on expression for the plastic work required for crack extension. On this basis analytical predictions for fracture toughness, thickness effects and fatigue crack propagation threshold are made and found in very good agreement with experimental results.