Closure Level Research Articles

Fatigue crack initiation from defects at weld toes in steel

The mechanisms of fatigue initiation and microcrack growth at the toes of good quality manual and automatic T-plate welded joints in steel were investigated, using a compliance method to measure crack depth and closure levels, and three-stage ink staining, replica techniques and SEM microscopy to monitor crack growth. Minor undercuts, slag inclusions and surface spatter at the weld toes were detected as fatigue initiation sites in manual welds. Fatigue initiation in automatic welds was mainly associated with undercut-type defects. Details of their propagation and coalescence early in the fatigue process are documented. Delayed microcrack coalescence in the case of manual welds appears to be related to the more irregular toes of the manual welds, when compared to their automatic counterparts.

Comparison of two methods to measure crack closure in ultra-high vacuum

The crack closure behavior in vacuum was investigated with the direct current potential drop technique and the crack tip strain technique on ARMCO iron in the near threshold region. Significant differences between the two techniques were observed. The crack tip strain technique shows clearly a crack closure level which is caused by the plastic and roughness induced crack closure. The direct current potential drop technique indicates contacts between the fracture surface also in the range of amplitude where the crack tip strain technique indicates an open crack. This is explained by sliding (frictional) contacts and some contacts like a “hook”. These contacts make it impossible to measure the real length of the crack in vacuum with the direct potential drop technique and further produce a problem for a clear experimental determination of the effective range of stress intensity.

The effect of tempering temperature on near- threshold fatigue crack behavior in quenched and tempered 4140 steel

Fatigue crack growth in compact tension samples of high purity 4140 steel quenched and tempered to various strength levels was investigated. Tempering temperatures of 200, 400, 550, and 700 °C produced yield strengths from 1600 to 875 MPa, respectively. Crack propagation and crack closure were monitored inK-decreasing tests performed underR = 0.05 loading conditions in laboratory air. Results indicated that as the yield strength increased the crack growth rate increased at a given ΔK and ΔKth decreased. Threshold values varied from 2.8 MPa m1/2 (200 °C temper) to 9.5 MPa m1/2 (700 °C temper). Cracks in the 200 °C tempered samples grew by an intergranular mechanism following prior austenite grain boundaries probably caused by hydrogen embrittlement or tempered martensite embrittlement. Tempering above 200 °C produced transgranular fatigue crack growth. The level of crack closure increased with tempering temperature and with crack propagation in a given tempered condition. Crack closure was caused by a combination of plasticity-induced and oxide-induced mechanisms. The use of an effective stress intensity range based on crack closure consolidated the fatigue crack growth curves and the threshold values for all tempering temperatures except 200 °C.

Mechanisms for the retardation of fatigue cracks following single tensile overloads: behavior in aluminum-lithium alloys

Mechanisms for the transient crack growth retardation of fatigue cracks following single tensile overloads are examined in a high-strength aluminium-lithium alloy 2090-T8E41, with specific emphasis on the role of fatigue crack closure. Mechanically, it is found that following the application of the overload, an immediate acceleration in fatigue crack growth is observed, which results from a reduction in (far-field) crack closure levels due to crack tip blunting. Subsequent crack growth retardation, as the crack penetrates the overload plastic zone, is associated with enhanced crack tip shielding prompted by compressive residual stresses ahead of the crack tip, crack deflection and consequent (near-tip) crack closure induced by asperity wedging in the immediate crack wake. Aluminum-lithium alloys, by virtue of their marked anisotropy and planar slip characteristics which promote meandering crack paths and hence high levels of shielding, are found to show large load interaction effects, and thus rank superior to other high strength aluminum alloys under tension-dominated variable amplitude loading spectra. However, under conditions where specimen surface layers are removed, or where cycling is performed at high (positive) load ratios, or involves compressive overload cycles (all processes which tend to limit the development of crack closure), the superior performance of aluminum-lithium alloys under variable amplitude fatigue loading may be compromised.

A SIMPLIFIED LABORATORY APPROACH FOR THE PREDICTION OF SHORT CRACK BEHAVIOR IN ENGINEERING STRUCTURES

Abstract— Conventionally determined fatigue threshold information (ASTM E647) can lead to non‐conservative estimates of fatigue lifetimes when these data are utilized in damage tolerant design assessments. The non‐conservative nature of such data can be attributed primarily to the development of excessively large amounts of crack closure at low R‐ratios, particularly at near threshold stress intensity factor levels. These high closure levels attenuate the effective stress intensity condition prevailing at the crack tip and confound attempts to predict the behavior of short cracks that exhibit limited crack closure. A modified test procedure, involving constant maximum stress intensity factor (Kcmax) test conditions, is described which identifies fatigue crack propagation (FCP) threshold behavior in the absence of detectable amounts of crack closure. These data have been generated with conventional long crack specimens for several aluminum, iron, and nickel‐based alloys and which are shown to closely simulate the FCP response of short cracks in these engineering materials. As such, the modified threshold test procedure, incorporating constant Kmax loading conditions, represents a valuable tool in the prediction of the cyclic lifetime of engineering components. The stress‐cyclic lifetime (S‐N) curve for aluminum butt‐welded beams was computed based on Kcmax data and found to be in excellent agreement with actual test results.

Effect of prolonged high-temperature exposure on the fatigue and fracture behavior of aluminum-lithium alloy 2090

Effects of prolonged exposures of 100–1000 h at 163°C on fatigue crack propagation and fracture toughness behavior have been studied in commercial aluminum-lithium alloy 2090-T8E41, and results compared with behavior in similarly over-aged 2124 alloy. Over-aging in 2090, which led to decreases in strength and toughness principally through the formation of plate-like copper-rich grain boundary precipitates and associated copper-depleted and δ′ precipitate-free zones, was found to result in increased fatigue crack growth rates above ∼ 10 −9 m cycle −1; near-threshold growth rates were less affected. Such behavior is related to a diminished role of crack-tip shielding during crack extension in over-aged microstructures, from less crack deflection and lower crack closure levels (from asperity wedging), arising from more linear crack paths. Despite this degradation in fatigue crack growth properties following high temperature exposure, crack growth rates in the over-aged 2090 remain comparable or superior to traditional high strength aluminum alloys, such as 2124-T351 and 7150-T651, due primarily to higher overall levels of crack-tip shielding.

The effect of phase continuity on the fatigue and crack closure behavior of a dual-phase steel

The effect of phase continuity on the low cycle fatigue and fatigue crack growth behavior of a Fe-C-Mn dual-phase steel has been investigated. Two microstructures, one consisting of continuous ferrite and the other continuous martensite, were examined. Although there was no difference in the low cycle fatigue lives between the two microstructures, the continuous martensite structure exhibited an extremely high fatigue threshold value of 20 MPa m1/2, compared to 16 MPa m1/2 for the continuous ferrite microstructure. A major effect of phase continuity has also been found in the crack closure levels during fatigue crack propagation studied over three decades of crack growth rates. The continuous martensite microstructure exhibited much higher closure levels due to the martensite constraining the plastic deformation in the ferrite and bearing a larger portion of the applied cyclic load. This effect is similar to the extrinsic toughening phenomenon cited in the literature. After accounting for the closure levels the intrinsic or effective fatigue crack growth rates are similar for the two microstructures. These intrinsic thresholds are predicted by employing experimentally obtained low cycle fatigue parameters and the ferrite grain size.

Fatigue crack growth behavior of 4340 steels

Fatigue crack growth behavior of 4340 steels was investigated in four gaseous environments; laboratory air, wet hydrogen, dry hydrogen and dry helium. Specimen orientation does not affect crack propagation rate results. The effects of R-ratio (load ratio) and environment on crack growth rate properties are interrelated. Increasing R -ratio increases the rates of near-threshold crack propagation. Nevertheless, the effect of R-ratio on crack growth rates in air is much more significant than that in the two dry environments. Interestingly, the R-ratio effect in wet hydrogen is comparable to that in dry environments. At an R-ratio of 0.1, the rates of crack propagation in air are slower than those in dry environments while crack growth rates are essentially identical in wet hydrogen and dry environments. Increasing R -ratio was found to decrease the environmental effect. Furthermore, increasing yield strength from 700 to 1040 MPa does not affect crack propagation behavior. While surface roughness-induced crack closure is thought to be minimal in affecting gaseous-environment near-threshold crack growth behavior of 4340 steels, oxide-induced crack closure governs crack propagation kinetics. It is suggested that in moisture-containing environments, thick oxide deposits measured on fracture surfaces may not result in high crack closure levels. Nevertheless, oxide-induced crack closure rationalized the effects of R-ratio and environment on near-threshold crack growth rate properties. Furthermore, hydrogen embrittlement is believed not to play an important role in influencing wet-hydrogen environment near-threshold crack propagation behavior. At higher ΔK levels (⩾ 12 MPa √m), an “intrinsic” dry hydrogen effect seems to be present, and crack closure, however, cannot account for the environmental effect.

Analysis of fatigue crack closure behaviour by modified Dugdale model.

Crack closure characteristics in fatigue crack growth were examined using the modified Dugdale model developed by Newman. The model can leave elastic-plastic asperities in the wake of the growing crack tip. The analysis shows the influence of the rigidity, E, of the asperities, asperity length, asperity thickness and the distance from the crack tip. The results are compared with the experiments. It is noted that the experimentally detected closure level can't give correct informations regarding actual one under some conditions. Especially, under corrosive environment, the crack closuer level is overestimated than the actual level. On the other hand, near threshold region in air environment, it is underestimated.

Fatigue crack growth behavior in an Mn-Cr austenitic steel

In this paper, the effects of environment, test frequency, load ratio, specimen orientation and yield strength on fatigue crack growth behavior of an Mn-Cr austenitic steel are described. It was observed that decreasing frequency generally enhanced the effect of environment. Furthermore, the increasing and constant Δ K (stress intensity range) tests developed consistent crack growth rate data. The fracture morphology was transgranular regardless of the test conditions. Striation spacings correlated well with the macroscopic crack propagation rates. During Stage II fatigue crack growth, the crack closure level decreased with decreasing Δ K, in contrast with near-threshold crack propagation behavior.

Fatigue Crack Growth Threshold and Crack Opening of a Mild Steel

Abstract The effects of stress ratio, compressive peak stress, and cold rolling on the threshold stress intensity and crack closure level of a mild steel (SAE 1010) were investigated. As the stress ratio and the magnitude of the compressive peak stress were increased, the threshold level decreased linearly. The threshold stress intensity also decreased linearly as the yield strength was increased by cold rolling. Crack opening measurements showed that the measured threshold is composed of two parts: an intrinsic threshold stress intensity range, ΔKin, and an opening stress intensity, ΔKop. The opening stress intensity decreased with increasing magnitude of the compressive peak stress. The intrinsic threshold stress intensity range was not significantly affected by the compressive peak stress. Severe cold rolling decreased the opening stress in the near threshold region to near zero. In a constant amplitude load-controlled test, the ratio of the opening stress intensity to the maximum stress intensity decreased as the maximum stress intensity (crack length) increased, and no crack closure could be observed when the net stress approached the yield strength of the material.

On the influence of the ambient environment on the fatigue crack growth process in steels

A comparison of the crack opening and fatigue crack growth characteristics of three tempered martensitic steels; a modified 4135, 2 1/4Cr-1Mo and a modified 9Cr-1Mo; in ambient air and in vacuum (3 × 10 −5 torr) has been carried out. In vacuum, roughness-induced closure is responsible for closure in the near-threshold region and the level of closure is independent of ΔK. The presence of oxygen in the ambient environment (50% relative humidity) can increase the overall closure level by as much as 50% in the case of 4135 and 2 1/4Cr-1Mo steels, but does not contribute significantly to closure in the case of the 9Cr-1Mo steel. Roughness-induced closure increases with increase in tempering temperature, but the extent of the environmental effect on closure in a given alloy is much less dependent on the tempering temperature. Rewelding is not responsible for the high threshold levels found in vacuum. Oxide film rupture as well as hydrogen embrittlement increase the rate of crack growth in air over that observed in vacuum.

Fatigue crack growth and threshold behavior at elevated temperatures.

Fatigue crack growth and closure were investigated in both SUS304 stainless steel and SM41A steel at elevated temperatures up to 500°C°C. A quantitative analysis was made of the roughness and oxide of the fracture surface at the near-threshold region. The effects of the roughness and the oxide on the fatigue threshold behavior were examined. The value of the threshold stress intensity factor range increased with increasing test temperature in both steels. An extra crack closure was not observed at elevated temperatures in SUS304 steel, although fracture surface roughness increased significantly with increasing test temperature. On the other hand, an elevation in the crack closure level was observed in SM41A steel due to the effect of the oxide products on the fracture surface at elevated temperatures. It was found that oxide-induced-crack-closure played an important role in effecting the crack closure at elevated temperatures, while roughness-induced-crack-closure had a less important effect on crack closure at near-threshold.

Mechanisms of near-threshold fatigue crack growth in a low alloy steel

The near-threshold fatigue crack growth behavior of CrMoV steel was investigated in the temperature range of 24–427°C. At load ratios of 0.1 and 0.5, the value of threshold stress intensity range, ΔK th , experienced a minimum as test temperature increased from 24 to 427°C. Increasing load ratio generally decreased the effect of temperature on ΔK th . Oxide and roughness-induced crack closure offered an explanation for the influence of temperature on near-threshold crack propagation rate properties. Moreover, extensive oxide thickness and surface roughness measurements substantiated the explanation. At 24°C, oxide thickness increased with decreasing ΔK due to fretting oxidation. Furthermore, increasing load ratio decreased oxide thickness. In contrast, at higher temperatures of 149, 260 and 427°C, oxide thickness generally decreased with decreasing ΔK, due to thermal oxidation, irrespective of load ratio. While oxide thickness increased with increasing temperature, surface roughness decreased with increasing temperature. At 427°C, at low load ratios oxide-induced crack closure was responsible for the increased crack closure level with decreasing ΔK while at high load ratios, plasticity-induced crack closure was suggested to be the controlling mechanism in affecting the crack closure level. At a fixed ΔK value, the crack closure level increased with increasing load ratio.

FATIGUE CRACK CLOSURE IN ALUMINIUM ALLOYS UNDER VARIABLE-AMPLITUDE LOADS

During fatigue crack propagation,resiaual deformations are built up in front of the crack tip and left behind the propagating crack. As a result of these residual deformations the crack will close(at least partly) during unloading. At zero load the presence of these deformations excersises residual compressive stresses in the wake of the crack normal to the fractured surfaces. The load at which the crack closes is therefore tensile rather than zero o/ compressive. This phenomenon is refferedteascrack closure. The main objective of this study is to correlate fatigue crack gr-owth rates with crack closure under different types of loadings: Constant-amplitude with two different cycle ratio R,(R= E. / Er ),mln. max. Programmed block loading and A single tensile overload. The incidence of crack closure is examined and the concept of equ-ivalent constant-amplitude is applied. The delayed crack growth retardation after a single overload cycle isiiterpreted using crack closure concept. Aspects covered include microscopic and fracto-graphic observations confirming the crack closure to provide the fatigue striations on the surfaces ruptured by cyclic loads. Materials used in this study are two Aluminium alloys,widely used in aeronautical structures: 2124T351 (AU4G1) and 2618AT851(AU2GN). Several interaction effects might be explained by changes in the crack closure levels due to variable amplitude loads.

Crack opening and closure in fatigue

Experimental and analytical crack opening and closure levels are determined for cracks growing in notched members. Replica techniques and the scanning electron microscope were used in the experiments. The analytical procedure involves the determination of the reversed plastic zone size, the crack opening and closure stresses and is based on Budiansky-Hutchinson's theoretical study of closure. Results indicate consistently that crack opening levels are higher than crack closure levels. Small cracks in bluntly notched members under R = − 1 loading exhibited lower crack opening (closure) levels than longer cracks. However, for R = 0 loading the crack opening (closure) levels remained relatively unchanged with increasing crack length. The implications of crack opening and closure results on the behavior of small cracks are discussed. An ad hoc approach for determining crack closure levels is presented in the Appendix. The suggested method gives good estimates of crack closure levels in the absence of plasticity and short crack effects and may be useful to engineers and designers.

The effect of microstructure on fatigue crack growth behaviors of a low carbon steel and a spheroidal graphite cast iron.

It has been a serious problem whether the microstructure dependence of crack growth arises due to the change in crack closure level or intrinsic crack growth resistance. In order to investigate the effect of microstructure on fatigue crack growth behaviors at the threshold region, three kinds of ferrite base materials were fatigued at room temperature and slightly elevated temperatures. The fatigue results on the low carbon steel can be explained well by the ΔKeff conception, that is, the crack growth resistance is not affected by the structural factors, such as cyclic strain aging and precipitation hardening by low temperature quench aging. On the other hand, the results on spheroidal graphite cast iron cannot be explained only by ΔKeff conception. It may be considered that the graphite particles act as promoters at higher ΔK level and as obstacles at lower ΔK level to crack growth.

Crack closure and the growth of short and long fatigue cracks

A general framework for the analysis of both short and long cracks is presented which relies importantly on crack closure. The analysis indicates that the growth of very short cracks is length independent. Since closure decreases with an increase in mean stress level it should become more difficult to arrest cracks as the mean stress level increases. Since in steels the closure level decreases with an increase in strength level, the range over which arrested crack growth can occur will be decreased. In fact in the absence of crack closure arrested crack growth should not occur.

Analysis of Turbulent Gas-Solid Suspension Flow in a Pipe

The mixing length theory is extended to close the relevant momentum equations for two-phase turbulent flow at a first-order closure level. It is assumed that the mass fraction of the particles is on the order of unity, that the particle size is so small that the particles are fully suspended in the primary fluid, and that the relaxation time scale of the particles is sufficiently small compared with the time scale of the energy containing eddies so that the suspended particles are fully responsive to the fluctuating turbulent field. Bulk motion of the particles is treated as a secondary fluid flow with its own virtual viscosity. The proposed closure is applied to a fully developed gas-solid pipe flow in which the particles are assumed to be uniformly distributed across the pipe section. Predicted velocity profiles and the friction factors are in good agreement with available experimental data.

THE EFFECT OF ENVIRONMENT ON CRACK CLOSURE AND FATIGUE THRESHOLD*

Abstract— The threshold value for fatigue crack growth of a medium carbon steel was increased when the test‐environment was changed from air to an aggressive H2S‐containing brine. This increase in fatique threshold was shown to be caused by corrosion product‐induced crack closure. Further, the fatigue threshold and crack closure level were shown to be dependent on the growth rate history in approaching threshold. The differences in fatigue crack growth rate and fatigue threshold resulting from test procedure and growth rate history were significantly reduced by employing the effective stress intensity concept.