Threshold Fatigue Crack Growth Rates Research Articles

Illuminating the chemo-mechanics of hydrogen enhanced fatigue crack growth in aluminum alloys

The presence of elemental hydrogen is known to accelerate fatigue crack growth in aluminum alloys. However, a direct link between experimental data and the governing atomistic mechanisms has remained elusive. Here we present a series of computational studies, across multiple length scales, directly linking an atomistic mechanism to experimental data for a specific aluminum alloy. Starting with an ab initio investigation of hydrogen bonding near the (1 1 1) aluminum surface, we quantify the effects of hydrogen surface impurities on slip and decohesion. We then modify an aluminum-only interatomic potential to reproduce ab initio trends by strategically shielding critical surface bonds in accordance with the environmental exposure level. The strategic shielding approach is used within a coupled atomistic-continuum discrete dislocation framework to predict the effect of hydrogen on near threshold fatigue crack growth rates. The predicted trends agree with published experimental data, suggesting that hydrogen enhanced surface deformation is a key failure mechanism for aluminum alloys in humid environments. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The influence of corrosion pits on the fatigue life of 17-4PH steam turbine blade steel

Fatigue life tests on smooth and pre-pitted specimens and near threshold fatigue crack growth rate measurements are performed on 17-4PH steam turbine blade steel. The influence of environment and stress ratio is investigated. A decrease in the fatigue life and a slowdown of the crack growth rate in the near threshold regime is found in salt solution compared to air. The latter can be explained by oxide-induced crack closure. The critical stresses for fatigue failure in the presence of corrosion pits can be calculated using a methodology that was already successfully applied to 12% Cr steam turbine blade steel.

Influence of dissolved hydrogen on the fatigue crack growth behaviour of AISI 4140 steel

In this investigation, the influence of dissolved hydrogen on the fatigue crack growth behaviour of a low alloy steel (AISI 4140) was examined. The investigation also examined the influence of microstructure on the fatigue threshold and the fatigue crack growth behaviour of this material. Compact tension (CT) specimens prepared from AISI 4140 steel as per ASTM standard E-647; were tested in two different heat treated conditions, i.e., cold rolled and annealed as well as austempered condition. The specimens were precharged with hydrogen using ex situ cathodic charging technique at a constant current density for three different time periods ranging from 150 to 250h. The samples were then subjected to fatigue crack growth test in the threshold region as well as in the linear region. The near threshold fatigue crack growth rate, fatigue threshold and the crack growth rate in the linear region of these specimens were determined. The fatigue crack growth behaviour of the hydrogen charged samples were compared with specimens without any dissolved hydrogen. The current test results show that the near threshold crack growth rate increases with the increase in dissolved hydrogen content. It was also observed that the fatigue threshold decreases as the dissolved hydrogen content increases. The crack growth rate was found to be higher in the austempered samples compared to the annealed samples. An interesting phenomenon was observed in the case of annealed specimens charged with hydrogen for 250h. These specimens had unusually high fatigue threshold (ΔKth).

Secondary elastic crack tip stresses which may influence very slow fatigue crack growth—additional results

For near threshold fatigue crack growth rates, a high sensitivity to stress field analysis is often displayed. For example, the partial closure or adjusted compliance ratio (ACR) methods are required to obtain the best correlation of data. This paper presents additional results on secondary crack tip stress field terms, which may be of interest for more fully representing the elastic stress field results for various configurations. Several new configurations are treated including dislocation effects, concentrated force circumstances (useful for Green's Functions), and periodic arrays of cracks. These results may be used to go beyond T-stresses in analyzing behavior.

Near threshold fatigue crack growth behavior of austempered ductile cast iron (ADI) processed by a novel two-step austempering process

The influence of a novel two-step austempering process on the microstructure and the near threshold fatigue crack growth behavior of austempered ductile cast iron (ADI) were investigated. Cylindrical tensile and compact tension (CT) specimens (for fatigue threshold tests) were prepared from an alloyed nodular ductile cast iron as per ASTM standards and were austempered by both the conventional single-step and the novel two-step austempering processes at four different temperatures. The near threshold fatigue crack growth behavior of these samples was examined in room temperature and ambient atmosphere. Tests results indicate this two-step austempering process has resulted in higher hardness, higher yield and tensile strengths for ADI but higher near threshold fatigue crack growth rate and lower fatigue threshold, as compared to the conventional single-step austempering process. Results also demonstrate that fatigue crack growth behavior of ADI in the near threshold region is influenced by microstructural parameters, such as volume fraction of austenite ( X γ), carbon content in austenite ( C γ), ferritic cell size ( d) as well as total austenitic carbon, X γ C γ. SEM fractographs in all samples exhibit a combination of mechanisms, i.e. ductile striation with quasi-cleavage facet around graphite nodules.

Near threshold fatigue crack growth versus long finite life

ABSTRACT A brief review of the discovery of the fatigue crack growth threshold is provided with a discussion of the main variables, Kmax and ΔK, which control the threshold over the low and high load ratio ranges, respectively. The significant effect on near threshold growth rates as illustrated by Donald are shown, for an example the aluminium alloy, 2324‐T39. Attention is then turned to the ‘partial closure model’ as a means of correlating near threshold fatigue crack growth rates. This seems to be the most promising model with a physical basis. For this reason, the discussion goes on to present a new theoretical analysis of the load displacement record characteristics, which should accompany ‘partial closure’ of fatigue cracks. In addition it is concluded that secondary stress effects should be explored for near threshold fatigue crack growth rates.

Near threshold fatigue crack propagation behaviour of CrMoV turbine steel

This paper describes the influence of material toughness degradation, through reversed temper embrittlement (RTE) and mean stress on the near threshold fatigue crack growth characteristics of a CrMoV turbine bolting steel at ambient and elevated temperatures. It was established at ambient temperatures that strong effects of R-ratio and material condition (toughness) were observed on near threshold fatigue crack growth characteristics. At elevated temperatures it was shown that for the non-embrittled material that only under low R-ratio conditions did increased temperature increase the level of threshold stress intensity ΔK th, by some 20%. In the case of embrittled material, increasing the temperature increased ΔK th levels by around 30% and decreased near threshold growth rates by an order of magnitude at low to intermediate R-ratio levels. The effects of R-ratio on ΔK th for all material and mechanical testing conditions could be simply expressed by the difference between ΔK th at R = O and a constant B multiplied by R. Quantitative fractographic observations indicated that, generally, the incidence of intergranular failure prevalent in embrittled and non-embrittled steels exhibited a maximum at some specific ΔK level. Also in embrittled steels large effects of environmental assisted crack (EAC) growth were observed at near threshold fatigue crack growth rates. It was suggested that this was the result of the much reduced material cohesive strength which was caused by the presence of both impurity and hydrogen atoms.

Application of a new model for fatigue threshold in a structural steel weldment

A new model for fatigue threshold has been proposed. The model takes into consideration the influence of material strength, grain size, and load ratio on fatigue threshold. Fatigue crack growth behavior of a structural steel weldment (ASTM Grade A 514) was investigated to examine the applicability of this model. The investigation also examined the influence of welding procedure [submerged arc welding (SMAW) versus metal inert gas welding (MIG)] on the resultant microstructure and the near threshold fatigue crack growth behavior of the material in room temperature ambient atmosphere. Compact tension specimens were prepared from ASTM Grade A 514 structural steel weldment in such a way that cracks propagated through the heat affected zone (HAZ), base metal and weldments. This arrangement permitted the influence of HAZ on fatigue crack growth rate to be determined in both linear and threshold regions of the Paris curve in room temperature ambient atmosphere. Welding was accomplished by two different techniques, i.e. SMAW and MIG processes. The results of the present investigation demonstrated that the near threshold fatigue crack growth rate was lowest and fatigue threshold was highest when the crack propagated through the weldment. The fatigue threshold values predicted by the model were in good agreement with the experimentally determined values. The MIG welding process was found to be beneficial compared to the SMAW process as far as the fatigue threshold is concerned for weldment.

残留応力場における２段繰返し変動荷重下の下限界近傍疲労き裂進展挙動

In order to make clear the effect of residual stress on fatigue crack growth behavior, fatigue crack growth tests under constant amplitude and repeated two-step loadings were carried out on SM50A steel in the near threshold region by using the specimens with or without longitudinal residual stress. The crack length and crack opening point were measured by the unloading elastic compliance method.The results obtained are as follows.(1) Under varying loadings, a fatigue crack was found to grow in the residual stress field even when the low level effective stress intensity range (ΔKeff)L was less than (ΔKeff)th.(2) However, the fatigue crack growth rate under low level load (da/dn)*L was influenced by the number of cycles of low level load NL. When NL was rather large, the crack growth increment ΔaL became saturated and the averaged growth rate decreased.(3) The modified Miner type linear accumulation law of crack growth in terms of ΔKeff was confirmed to give a conservative estimation of near threshold fatigue crack growth rate under varying loadings even in the residual stress field.

The influence of tempering martensite containing microstructures on the near threshold fatigue crack propagation behaviour

Fatigue crack growth studies have been conducted in a humid air environment for various martensite containing microstructures under low and high R-ratio testing conditions. Tempering reduced the yield strength properties; the greater the amount of martensite the greater the reduction. Tempering generally (i) increased the threshold for fatigue crack growth ΔK th, and (ii) decreased near threshold fatigue crack growth rates for both low and high R-ratio testing conditions. Also tempering inhibited the occurrence of subcritical transgranular cleavage in the high R-ratio tests of the ferrite-martensite microstructures and consequently affected Stage II fatigue crack growth behaviour. Finally the fully martensitic microstructure did not exhibit cracking and the present ΔK th data exhibited good agreement with other data for steels in the tempered condition.

Effect of oxidation kinetics on the near threshold fatigue crack growth behavior of a nickel base superalloy

The influence of oxidation kinetics on the near threshold fatigue crack growth behavior of a nickel base precipitation hardened superalloy was studied in air from 427° to 649 °C. The tests were conducted at 100 Hz and at load ratios of 0.1 and 0.5. The threshold ΔK values were found to increase with temperature. This behavior is attributed to oxide deposits that form on the freshly created fracture surfaces which enhance crack closure. As determined from secondary ion mass spectrometry, the oxide thickness was uniform over the crack length and was of the order of the maximum crack tip opening displacement at threshold. Oxidation kinetics were important in thickening the oxide on the fracture surfaces at elevated temperatures, whereas at room temperature, the oxide deposits at near threshold fatigue crack growth rates and at low load ratios were thickened by an oxide fretting mechanism. The effect of fracture surface roughness-induced crack closure on the near threshold fatigue crack growth behavior is also discussed.

Effect of purity and dispersoid type on near threshold fatigue crack growth rates in AlZnMgCu alloys

Effect of purity and dispersoid type on near threshold fatigue crack growth rates in AlZnMgCu alloys