Effects of surface deformation and crack closure on fatigue crack propagation after overloading and underloading

In the case of a negative baseline stress ratio, the fatigue crack growth rate can actually accelerate after a tensile overload. This crack propagation behavior is related to the local bulging of the specimen in the thickness direction during compression and the resultant tensile residual stress distribution at the crack tip. In the present investigation, the effects of a single tensile overload as well as the effects of a tensile overload followed immediately by a compressive underload on subsequent fatigue crack growth were investigated. The extent of crack opening displacement influences the magnitude of residual stress as well as the crack-tip opening level, and consequently, the subsequent rate of fatigue crack propagation.     

RETARDATION AND ARREST OF FATIGUE CRACK GROWTH IN AISI 4340 STEEL BY INTRODUCING REST PERIODS AND OVERLOADS

Abstract— The aim of this study is to investigate the effects of an intermittent rest period at 300°C, an overload in tension, and the combination of an overload and a subsequent rest period, on fatigue crack growth in AISI 4340 steel. The intermittent rest period was found to stop fatigue crack growth completely near the threshold level of Δ K. The alleviation effect of a rest period on crack growth was more distinct at lower levels of Δ K. With overload, the higher overload ratio caused a greater effect on crack growth rate. The reduced stress intensity factor caused by crack branching and an enhanced roughness of crack surfaces probably contribute to the retardation and arrest of fatigue crack growth. The most distinct retardation of fatigue crack growth was found after the combined treatment of an overload and a subsequent rest period. Compressive residual stresses following an overload and strain-age hardening during the rest period at 300°C are suggested as assisting the arrest or retardation of fatigue crack propagation.     

Prediction of corrosion fatigue lives of aluminium alloy on the basis of corrosion pit growth law

Tension‐compression and rotating‐bending fatigue tests were carried out using aluminium alloy 2024‐T3, in 3% NaCl solution. The corrosion pit growth characteristics, and also the fatigue crack initiation and propagation behaviour were investigated in detail. The results obtained are summarized as follows: (i) Most of corrosion fatigue life (60–80%) is occupied with a period of corrosion pit growth at low‐stress amplitude. The corrosion pit growth law can be expressed as functions of stress amplitude σ_a and an elapsed time t. (ii) The critical stress intensity factor for crack initiation from the corrosion pit was determined as 0.25 . This value is the same as the threshold stress intensity factor range for crack propagation. (iii) Corrosion fatigue life can be estimated on the basis of corrosion pit growth law and crack propagation law. The estimated fatigue lives agree well with the experimental data.     

Transient effects in fatigue crack propagation

The transient fatigue crack propagation resulting from the sudden change of the stress intensity factor amplitude or from the change of the stress cycle asymmetry or from the application of the single overload cycle was measured on carbon steel specimens. To simplify the conditions and to increase the accuracy the shape of the specimens was chosen in such a way, that the stress intensity factor was independent of the crack length. It was shown that the transient effects can be qualitatively understood and quantitatively in the first approximation described solely on the basis of the steady state fatigue crack propagation data, provided that the threshold conditions of non-propagation are taken into account.     

Study of underload effects on the delay induced by an overload in fatigue crack propagation

Many engineering structures are subjected to random loading. The problem of predicting crack growth rates in this case cannot be solved without an accurate knowledge of load-time history occurring in service. There are many calculating models of crack propagation under spectrum loading, such as Wheeler model [Wheeler O. Spectrum loading and crack growth. J Basic Eng D 1972;94:181–86], Huang et al. [Huang XP, Zhang JB, Cui WC, Leng JX. Fatigue crack growth with overload under spectrum loading. Theor Appl Mech 2005;44:105–15] which use different approaches trying to explain fatigue crack growth.In this paper we use Decoopman’s [Decoopman X. Influence des conditions de chargement sur le retard à la propagation d’une fissure de fatigue après l’application d’une surcharge. Thesis, Université de Sciences et Technologies de Lille; 1999] model. He has developed an empirical model which describes the fatigue crack propagation after an overload cycle on 12NC6 steel in fatigue. This model describes how the crack growth rate evolves during the delay induced by the overload.Nevertheless, it is limited to overload cycles. But, many authors [[4], [5]; Huang XP, Zhang JB, Cui WC, Leng JX. Fatigue crack growth with overload under spectrum loading. Theor Appl Mech 2005;44:105–15; Paris P, Erdogan F. A critical analysis of crack propagation laws. J Basic Eng Trans Am Soc Mech Eng 1963; 528–34] have shown that an underload cycle occurring after an overload cycle reduces the delay.This study proposes to implement the underload effect in order to decrease the conservative results expected from this model. Decoopman’s model proposes a delay weighting factor after an overload cycle. In order to take into account of an underload cycle, we suggest an acceleration coefficient to correct the model. The main advantage of this model is that the delay weighting factor and the acceleration coefficient are only dependent on yield stress σ_Y, the crack length a, and the various plastic zone sizes. Many experimental results have been compared to simulated results. These comparisons show a good agreement.     

Fatigue crack propagation in welded joints under variable-amplitude loading in synthetic sea water

Fatigue crack propagation rates and the fatigue threshold in transverse butt-welded joints of centre crack type were measured in synthetic sea water under variable-amplitude loading. The fatigue threshold was about a half of that in air, and the propagation rates near the fatigue threshold were high compared with those in air. The propagation rate for each cycle under variable-amplitude loading could be estimated from the constant-amplitude data, which were obtained in the crack-closure-free condition.     

On the role of compression overloads in influencing crack closure and the threshold condition for fatigue crack growth in 7150 aluminum alloy

A study has been made of the effect of single compression cycles on near-threshold fatigue crack propagation in an I/M 7150 aluminum alloy. Based on experiments at a load ratio of R= 0.10 on cracks arrested at the fatigue threshold (δk_th) in under-, peak and overaged microstructures, large compression overload cycles, of magnitude five times the peak tensile load, were found to cause immediate reinitiation of crack growth, even though the applied stress intensity range did not exceed ΔK_th. Following an initial acceleration, subsequent crack advance was observed to take place at progressively decreasing growth rates until rearrest occurred. Such behavior is attributed to measured changes in crack closure which vary the effective near-tip driving force for crack extension (ΔK_eff). Specifically, roughness-induced closure primarily is reduced by the application of compressive cycles via a mechanism involving crack surface abrasion which causes flattening and cracking of fracture surface asperities. Closure, however, is regenerated on subsequent propagation resulting in the rearrest. Such observations provide further confirmation that the existence of a fatigue threshold is controlled principally by the development of crack closure and are discussed in terms of the mechanisms of closure in precipitation hardened alloys.     

Q345R钢多个过载作用下疲劳裂纹扩展行为研究

单一过载使得疲劳裂纹扩展速率减缓,可以提高疲劳寿命,但对于多个过载作用下疲劳裂纹扩展仍未明确,有待于进一步研究。针对Q345R标准紧凑拉伸试样,在常幅循环加载条件下引入多个拉伸过载,研究多个过载作用对疲劳裂纹扩展行为的影响。研究结果表明:在保持应力比、过载位置和过载比不变的情况下,随着过载间距增加,迟滞效应先增加后减小;过载间距循环数是单一过载迟滞循环数的一半时,da/dN_min达到最小,迟滞效应最明显。采用含有过载交互作用参数Ø_I的修正Wheeler模型对不同的过载间距疲劳裂纹扩展行为试验结果进行预测,预测的结果与试验结果能够很好的吻合。     

FRP补强疲劳损伤钢结构裂纹扩展研究

钢结构在服役过程中,因超载、疲劳等原因会引起损伤累积,极大地影响了结构的安全使用.对纤维增强复合材料(FRP)补强钢结构张开型裂纹疲劳扩展进行了实验研究.比较了不同的钢结构表面处理方式对FRP与钢结构粘接接头剪切强度的影响,根据剪切强度选择最佳表面处理工艺.对FRP补强后钢结构试样进行疲劳实验,与未补强试样的结果加以对比.结果表明,用FRP增强后钢结构FRP增强区疲劳裂纹扩展速率得到抑制、疲劳循环次数得到了提高.     

Fatigue Resistance of an Anodized and Hardanodized 6082 Aluminum Alloy Depending on the Coating Thickness in the High Cycle Regime

In this study, the high cycle fatigue behavior of an anodized 6082 aluminum alloy is investigated. Main focus is on the most relevant influencing factors for crack initiation and propagation under cyclic loading and damage mechanisms considering coating type, thickness, and residual stresses. The bare substrate is compared to anodized and hardanodized specimens with three coating thicknesses, for each coating type, in the range from 20 to 70 μm. Coating hardness and microstructure as well as residual stresses are analyzed. Fatigue and fracture behavior under alternating tension–compression loading is determined. Dependent on the coating thickness, the fatigue strength is reduced by 8%–50% after anodizing and by 50%–62% after hardanodizing. As the coating thickness is equal to the initial crack length from a fracture mechanical point of view, stress intensities at the crack tips are higher for thicker coatings respectively longer initial crack lengths. Therefore, propagation of fatigue-induced cracks from the coating into the substrate is promoted for a higher coating thickness resulting in premature failure. A significant correlation between the coating thickness and tensile residual stresses induced by both coatings in the subjacent substrate is not found and residual stress influence on the overall fatigue strength is only minor.     

Influence of Stress Ratio and Loading Condition on Near-threshold Fatigue Crack Growth Behaviour

Near-threshold fatigue crack propagation behaviour of a cold-worked copper tested with distinct fa-tigue testing systems under different stress ratios has been investigated.The emphasis was placed onthe evaluation of crack closure effect as well as the effective fatigue threshold,It was found that thefatigue threshold for R=0 is higher than those for R=-1 and R=0.4,for which almost the samethreshold value was derived.Compared with the conventional closure evaluation method,the pro-posed new closure evaluation method can generally interpret the dependence of the fatigue thresh-old on stress ratio and loading condition,and leads to a higher and relatively constant effective fa-tigue threshold of about 3 MPa·m~(1/2) for the cold-worked copper at different stress ratios andloading conditions.     

Hold-time effect of a single overload on crack retardation at elevated temperature

Many studies have shown that the application of a single overload cycle during constant amplitude load cycling will produce crack retardation or delay. At elevated temperature, the delay effect is time-dependent due to creep that occurs during the overload cycle. A simple method is described to estimate the number of delay cycles as a function of hold time. The reduced crack growth rate following an overload is modelled by a modified constant-amplitude crack propagation relationship. The modification consists in the replacement of the crack rate term by a fractional derivative; the order of the derivative being related to the overload ratio. Delay is assumed to be equal to the number of constant amplitude cycles required by the crack tip to traverse the creep-damage zone created by the overload. A simple notch-type analysis is developed to estimate the creep-damage zone. Predictions compared very favorably with experimental results obtained with a side-grooved DCB specimen with a constant-K characteristic for a fine-grain superalloy IN-100 at 1200°F. The applicability of LEFM for crack growth under creep/fatigue conditions is discussed together with the extension of the results to spectrum loading.     

Einfluß von Überlasten auf die Ausbreitung von Ermüdungsrissen in der Aluminiumlegierung 7075 T 7351

Influence of overloads on the propagation of fatigue cracks in the aluminium alloy 7075 T 7351 For realistic lifetime predictions the crack propagation behaviour under variable amplitude spectrum loads has to be known. In this work the influence of single overloads on the fatigue crack propagation in the alloy 7075 T 7351 was investigated at a constant loading level (K_max and ΔK = const.). The experiments showed, that a single overload causes a short crack acceleration followed by a pronounced deceleration of the crack propagation rate. After this deceleration at higher overloads a zone where the crack propagation rate is slightly increased (lost retardation) was observed. It has been shown, that the minimum crack propagation rate as well as the length of the influenced region depends on the height of the overload. The observed crack acceleration, the deceleration as well as the lost retardation can be explained by internal stresses caused by those overloads which have to be added to the external stresses.     

Effect of submillimeter size holes on the fatigue limit of a high strength tool steel

The very high cycle fatigue and small fatigue crack growth behaviour of a generic tool steel material for diesel fuel injector application are described. The small crack growth tests for the tool steel material with and without the hardening heat treatment revealed the mechanisms of crack propagation and threshold behaviour. Based on the small fatigue crack propagation threshold value, an elastic plastic fracture mechanics methodology for the prediction of the endurance limit of specimens with submillimeter holes is proposed. The advantages of the new methodology are discussed in relation to existing methodologies for endurance limit prediction of specimens with small holes.     

A material sensitive modified wheeler model for predicting the retardation in fatigue response of AM60B due to an overload

Application of an overload within an otherwise constant-amplitude loading scenario causes retardation in crack propagation. Several models have been proposed for predicting retardation in crack propagation due to an overload cycle. Among them, the widely used Wheeler model, assumes the “affected zone dimension” to be a function of the current and overloaded plastic zone radii. When one considers the actual shape of the plastic zone, however, one realizes that the affected zone dimension does not agree with that assumed by Wheeler.In this paper, the influence of a single overload (but by considering three different overload ratios) on the fatigue crack growth retardation of center-cracked AM60B magnesium alloy plates is experimentally investigated. The retardation effect on crack growth due to an applied overload within a random-amplitude loading scenario, using various “clipping levels”, is also investigated. The sensitivity of this material to overload is compared with the response of some other materials.The actual radius of the plastic zone is evaluated for various stress intensity factors, using the finite element method. The results indicate that depending on the material, the affected zone would be sometimes larger or smaller than that produced by Wheeler’s model. Subsequently, a new parameter, hereafter referred to as the “sensitivity parameter” (β), is introduced that enables one to evaluate the affected zone dimension more accurately. It is shown that the proposed modified model is more effective than the original one in predicting the retardation response of the alloy. The integrity of the modified model is also investigated by evaluating the retardation in some other materials.     

Fatigue behavior of concrete subjected to biaxial loading in the compression region

Concrete hollow cylinders subjected to combined compression and torsion were used to simulate concrete airport pavements subjected to biaxial fatigue loading in the compression region. It was found that the increase in the compliance in the post-peak period is due to the damage evolution of the specimen. The static failure mechanisms was explained by fracture mechanics. Similar failure was observed in fatigue loading. It was found that with the crack growth as a parameter, the static response acts as an envelope for the fatigue failure response. The rate of the crack growth under fatigue loading follows a two-stage process: a deceleration stage followed by an acceleration stage up to failure. In the deceleration stage, the growth is governed by the R-curve of the specimen. In the acceleration stage, it is governed by the Paris Law. The previously proposed model in the biaxial tension region was extended to the biaxial compression region. In the biaxial compression region, static and fatigue behaviors under pure compressive loading were modelled in terms of inelastic displacement, rather than crack length.     

Derivation of crack closure and crack growth rate data from effective-strain fatigue life data for fracture mechanics fatigue life predictions

This study deals with the behavior of short cracks growing out of notches. Three types of load histories are used: (a) a fully-reversed constant amplitude history; (b) a periodic compressive overload history consisting of repeated load blocks containing one fully-reversed constant amplitude yield–stress magnitude cycle (the overload) followed by a group of smaller constant amplitude cycles having the same maximum stress as the overload cycle; (c) and a service strain history. Procedures are presented for deriving crack closure data and crack growth rate vs effective stress intensity factor range data from data obtained by subjecting a small number of smooth laboratory specimens to simple periodic compressive overload tests to obtain closure-free strain-life data. These procedures are illustrated in an example in which fatigue life predictions are made for a service strain history applied to notched plate specimens. The fatigue life predictions based on the measured and the derived crack closure and crack growth rate data are in good agreement with the experimentally determined fatigue lives.     

Improved test method and analytical modelling for fatigue crack growth in coarse‐grain titanium alloy with rough fatigue surfaces

Fatigue crack growth rate properties are typically determined by experimental methods in accordance with ASTM Standard E647. These traditional methods use standard notched specimens that are precracked under cyclic tensile loads before the main test. The data that are produced using this approach have been demonstrated elsewhere to be potentially adversely affected by the test method, particularly in the threshold region where load reduction (LR) methods are also required. Coarse‐grained materials that exhibit rough and tortuous fatigue surfaces have been observed to be strongly affected by the tensile precracking and LR, in part because the anomalies caused by crack closure and roughness‐induced closure become more important. The focus of the work reported in this paper was to further develop methods to determine more accurate fatigue crack growth rate properties from threshold through to fracture for coarse‐grained, β‐annealed, titanium alloy Ti‐6Al‐4V extra low interstitial thick plate material. A particular emphasis was put upon the threshold and near threshold region, which is of strong importance in the overall fatigue life of components. New approaches that differ from the ASTM Standard included compression precracking, LR starting from a lower load level and continuing the test beyond rates where crack growth would otherwise be considered below threshold. For the threshold regime, two LR methods were also investigated: the ASTM method and a method where the load is reduced with crack growth such that the crack mouth opening displacement is held constant, in an attempt to avoid remote closure. Constant amplitude fatigue crack growth rate data were produced from threshold to fracture for the titanium alloy at a variety of stress ratios. Spike overload tests were also conducted These data were then used to develop an improved analytical model to predict crack growth under spectrum loading and the predictions were found to correlate well with test results.     

Overloads in ductile and brittle materials

The first part of the paper presents fatigue crack propagation experiments with single overloads at different overload ratios and specimen thickness in a very ductile austenitic steel. The results show that in the Paris regime in a ductile material, the overload effect can be explained solely in the framework of the change of the plasticity‐induced crack closure. Other effects such as strain hardening, blunting, additional damage, crack deflection and branching are not significant. Whether or not this behaviour can be observed in less ductile materials and also in the threshold regime is investigated in the second part. Periodic overload experiments were performed on a relatively ductile 2124, and a more brittle 359, particle‐reinforced aluminium alloy. In the Paris regime, the retardation in the 2124 reinforced alloy showed the expected behaviour for a ductile material, whereas in the 359 reinforced cast alloy, an acceleration of the mean growth rate was observed. Near the threshold the difference between the two alloys and the effect of the periodic overloads decreased.     

Influence of surface recrystallization on the low cycle fatigue behaviour of a single crystal superalloy

This paper investigated the effect of surface recrystallization (RX) on the low cycle fatigue (LCF) behaviour of a single crystal (SX) superalloy. LCF tests on both raw and recrystallized samples showed that fatigue life was significantly reduced by surface RX. Fractography indicated that fatigue cracks initiated from the casting defects in RX layer and multiple crack initiations were commonly observed. Moreover, RX grains exhibited predominantly transgranular cracking, in contrast to the intergranular fracture reported in literature. The fatigue crack propagation behaviour was discussed in light of fracture mechanics and crack growth life model. The fatigue cycles required to penetrate RX layer were estimated to be about one magnitude lower than that in forming an equivalent crack in SX specimens. It is suggested that the earlier crack initiation and promoted crack propagation in RX layer, as well as the trend of multiple initiations, are responsible for the fatigue degradation by RX.