Constant Stress Intensity Factor Range Research Articles

Theoretical and experimental study on optimal cropping load control strategy of duralumin alloy AlCu4Mg1 based on linear elastic fracture mechanics

In this work, a metal bar fine-cropping system is designed to study the influence load control strategies based on theoretical and cropping experiments. The principle of the new cropping system and mechanical compositions system are introduced. Three different load control strategies are designed based on duralumin alloy AlCu4Mg1 bar, and the optimal loading order is determined under them. According to a new proposed cross-section quality evaluation method, the test results show constant stress intensity factor range of external load ([Formula: see text]), linear decreasing cyclic load of appropriate time coefficient ([Formula: see text]) can obtain better cropping effect and the best quality is obtained by the loading cycle ([Formula: see text]).

Theoretical and experimental investigation on influence of load control strategy of LY12 duralumin alloy bar fine cropping process

In present work, both the theoretical solutions and the corresponding cropping experiments are carried out to investigate the influence of the load control strategy on bar fine cropping process. The working principle of the new proposed cropping method is detailed introduced. Five different types of the loading cycles are designed basing on it. A new proposed load control strategy with a suitable assumed constant stress intensity factor range is obtained theoretically and verified by the corresponding cropping experiments. According to a new proposed cross-section quality evaluation method, the comparative results show that the section quality and the cropping time are greatly influenced by the cropping load. The best quality is obtained by the loading cycle (iii).

Threshold stress intensity factor range of a mechanically-long and microstructually-short crack perpendicular to an interface with plastic mismatch

In this study, we investigated long fatigue crack growth perpendicular to a soft-hard material interface comprising the same Young’s moduli but different yield strengths. We determined the degree of fatigue crack growth retardation attributable to the interface under the constant stress intensity factor range (ΔK) using the plasticity-induced crack closure analysis with the Dugdale model. We subsequently determined the threshold stress intensity factor range (ΔKth) under a constant stress amplitude (σa). Under a constant ΔK, the retardation degree was primary dependent on the two materials’ yield strength ratio. Under a constant σa, ΔKth was dependent on both the yield strength ratio and the distance between initial crack tip and interface.

Fracture mode identification of low alloy steels and cast irons by electron back-scattered diffraction misorientation analysis

The fracture modes of low alloy steels and cast irons under tensile and fatigue conditions were identified by electron back-scattered diffraction (EBSD) misorientation analysis in this research. The curves of grain reference orientation deviation (GROD) distribution perpendicular to the fracture surface were obtained by EBSD observation, and the characteristics of each fracture mode were identified. The GROD value of the specimen fractured in tension decreases to a constant related to the elongation of corresponding specimen in the far field (farther than 5mm away from the fracture surface). The peak exhibits in GROD curves of two smooth specimens and a notched specimen near the fracture surface (within 5mm away from the fracture surface), and the formation mechanisms were discussed in detail based on the influences of specimen geometries (smooth or notched) and material toughness. The GROD value of fatigue fractured specimen is close to that at undeformed condition in the whole field, except the small area near the crack path. The loading conditions (constant stress amplitude loading or constant stress intensity factor range △K loading) and the EBSD striation formation during fatigue crack propagation were also studied by EBSD observation parallel to the crack path.

Understanding Fatigue Crack Growth Behavior at Low Frequencies for a Mn–Ni–Cr Steel in 3.5 % NaCl Solution Under Controlled Cathodic Potential

Abstract Fatigue crack growth behavior at low loading frequencies for a Mn–Ni–Cr steel immersed in 3.5 % NaCl solution, with and without cathodic polarization, is investigated and presented in this paper. Frequency shedding method is used to estimate fatigue crack growth rate over a range of frequencies between 0.01 and 5 Hz at a constant stress intensity factor range of 15 MPa √m. The effectiveness of cathodic potential of −900 mV SCE in containing corrosion contribution to crack growth is estimated by comparing with published data on fatigue crack growth rate in lab air and 3.5 % NaCl solution without cathodic polarization. It is noted that there are three regions of crack growth rate as a function of applied frequencies when the data is plotted in log–log scale: linear crack growth rate, plateau crack growth rate, and high crack growth rate. The crack growth rate of steel in 3.5 % NaCl solution at 15 MPa √m is scanned for different cathodic potentials between −760 mV SCE and 1150 mV SCE at 0.01 and 0.1 Hz. The potentials at which the crack growth rates are the maximum and the minimum are found to be −760 and −950 mV, respectively. The optimum cathodic protection potential for the minimum corrosion fatigue crack growth rate ranges between −900 and −950 mV. This optimum potential for the minimum corrosion crack growth rate is more negative than the cathodic potential required for restraining the corrosion effects on fatigue strength of steel to normal ambient air behavior.

Statistical aspects of fatigue crack growth life of base metal, weld metal and heat affected zone in FSWed 7075-T651 aluminum alloy

The statistical aspects of fatigue crack growth life of base metal (BM), weld metal (WM) and heat affected zone (HAZ) in friction stir welded (FSWed) 7075-T651 aluminum alloy has been studied by Weibull statistical analysis. The fatigue crack growth tests were performed at room temperature on ASTM standard CT specimens under three different constant stress intensity factor range controls. The main objective of this paper is to investigate the effects of statistical aspects of fatigue crack growth life on stress intensity factor ranges and material properties, namely BM, WM and HAZ specimens. In this work, the Weibull distribution was employed to estimate the statistical aspects of fatigue crack growth life. The shape parameter of Weibull distribution for fatigue crack growth life was significantly affected by material properties and the stress intensity factor range. The scale parameter of WM specimen exhibited the lowest value at all stress intensity factor ranges.

Effect of low cyclic frequency on fatigue crack growth behavior of a Mn–Ni–Cr steel in air and 3.5% NaCl solution

Corrosion fatigue phenomenon involves the time dependent corrosion and cycle dependent fatigue loading. The intention of this study is to develop a test method to evaluate the crack growth rate of a material in a corrosive environment at low frequencies. The effect of cyclic frequency on crack growth rate of a Mn–Ni–Cr steel in lab air and 3.5% NaCl solution is evaluated using frequency shedding method where the cyclic frequency is decreased exponentially as a function of crack length at a constant stress intensity factor range. A Zn sacrificial anode coupled to the specimen slows down the rate of crack growth.

마찰교반용접된 7075-T651 알루미늄 합금 용접부의 피로균열전파율의 공간적 변동성에 미치는 균열 방향의 영향

Abstract: In this investigation, the effect of crack orientation on spatial randomness of fatigue crack growth rate (FCGR) in friction stir welded (FSWed) 7075-T651 aluminum alloy joints has been statistically analyzed by Weibull distribution. The fatigue crack growth tests are conducted under three different constant stress intensity factor range (SIFR) control at room temperature with R = 0.1 and frequency 10Hz on compact tension (CT) specimen machined at base metal (BM) and weld metal (WM). The experimental fatigue crack growth rate data were obtained for two types of specimens having LT and TL orientations. LT specimens both base metal and weld metal showed higher fatigue crack growth rate as compared to TL specimens. In the lower SIFR region, FCGR were found to be almost 3 times higher in higher SIFR region. The shape parameter of Weibull both LT and TL orientation for FCGR was increased with increasing SIFR, the scale parameter was also increased with increasing SIFR. The smallest value of the shape parameter was shown in weld metal specimens having LT orientation at lower SIFR region.

Thermally activated processes of fatigue crack growth in steels

Fatigue crack growth rates in steels at high and low temperatures have been investigated using Paris curves. The fatigue crack growth rates at high temperatures are quite different from those at low temperatures. Arrhenius plots between fatigue crack growth rate (da/dN) and test temperatures at constant stress intensity factor range (ΔKI) indicate a difference of the rate-controlling process for fatigue crack growth with temperature. Slip deformation at the crack tip governs fatigue crack growth at high temperatures, while hydrogen diffusion is associated with crack growth at low temperatures.

마찰교반용접된 7075-T651 알루미늄 합금 용접부의 피로균열전파율의 공간적 불규칙성 (LT 방향의 시험편에 대하여)

본 연구의 목적은 마찰교반용접된 7075-T651 알루미늄 합금 용접부에 대한 피로균열전파율의 공간적 불규칙성을 고찰하기 위한 것이다. 본 연구에서는 이전의 피로균열전파 실험 데이터를 활용하였다. 피로균열전파율의 공간적 불규칙성을 명확히 이해하기 위하여, 피로균열전파 실험은 일정 응력확대계수범위 제어하에서 수행되었다. 재질, 즉 마찰교반용접된 7075-T651 알루미늄 합금 용접부의 용접재와 모재에 대하여 피로균열전파율의 공간적 불규칙성을 조사하기 위하여 실험 데이터가 해석되었다. 결론적으로 용접재의 변동성이 모재의 변동성보다 높게 나타났으며, Weibull 통계 해석에 의하여 변동성을 평가할 수 있음을 알았다.

Al7075-T651의 마찰교반용접된 접합부의 피로균열전파율의 통계적 분포

This paper deals with the effects of driving force and material properties on statistical distribution of fatigue crack growth rate (FCGR) for the friction stir welded joints of Al 7075-T651 aluminum plate. In this work, the statistical probability distribution of fatigue crack growth rate was analyzed by using our previous constant stress intensity factor range controlled fatigue crack growth test data. As far as this study are concerned, the statistical probability distribution of fatigue crack growth rate for the friction stir welded (FSWed) joints was found to evaluate the variability of fatigue crack growth rate for base metal (BM), heat affected zone (HAZ) and weld metal (WM) specimens. The probability distribution of fatigue crack growth rate for FSWed joints was found to follow well log-normal distribution. The shape parameter of BM and HAZ was decreased with increasing the driving force, however, the shape parameter of WM was decreased and increased with increasing the driving force. The scale parameter of BM, HAZ and WM was increased with the driving force.

일정 응력확대계수범위 제어 시험하의 마찰교반용접된 7075-T651 알루미늄 합금 용접부의 피로균열전파 거동의 실험적 고찰 (LT 방향의 시험편에 대하여)

본 연구에서는 마찰교반용접재의 피로균열전파 거동의 공간적 불규칙성을 고찰하기 위한 연구의 일환으로써, 최적의 조건에서 마찰교반용접된 7075-T651 알루미늄 합금 용접부의 LT-방향의 각각 5개의 피로균열전파 시험편에 대하여 일정 응력확대계수범위 제어하의 피로균열전파 실험을 수행하여 마찰교반용접부의 교반용접부재(WM)와 열영향부재(HAZ) 그리고 모재(BM)에 대한 피로균열전파 거동을 실험적으로 고찰하였다. WM재의 피로균열전파율이 가장 빠르게 나타났으며, 그 다음 HAZ재와 WM재 순으로 나타났다. 게다가 시험편간 피로균열전파율의 변동성은 WM시험편에서 가장 높았고, 반면 BM재에서 가장 낮게 나타났다.

Fatigue Crack Growth of Alloy 7050-T7451 Plate in L-S Orientation

Experiments have been conducted to investigate the crack growth characteristics of 7050-T7451 aluminium plate in L-S orientation. Two loading conditions are selected, i.e. constant amplitude and constant stress intensity factor range (ΔK). The effects of ΔK-levels and stress ratios (R) on crack splitting are studied. Test data shows that crack splitting could result in the reverse of crack growth rate trend with the increasing R ratio at high ΔK-level. The appearance of crack splitting depends on both ΔK and R.

Probabilistic Fatigue Crack Growth in Compact Tension Specimens from FSWed 7075-T651 Aluminum Alloys

The process of fatigue crack growth is generally random nature, due to many sources of variability, such as external loading, material properties and geometry conditions. Therefore is is important to investigate the statistical variability of fatigue crack growth in order to development the methodology for probabilistic fatigue crack growth life prediction. This paper is concerned with the randomness investigation of fatigue crack growth for the FSWed 7075-T651 aluminum alloys. In this investigation, the fatigue crack growth tests were conducted for five compact tension (CT) specimens of the FSWed 7075-T651 aluminum alloys, base metal (BM), heat affected zones (HAZ) and weld metal (WM), especially three different constant stress intensity factor range (SIFR) controls.

The Effect of Loading History on Unstable Fracture of Austenitic Steel (304)

The present paper presents an analysis of the influence of loading history on unstable fracture of austenitic steel 304 (SS 304) using the J-integral and its applicability in situations where a loading history exists. A CT specimen is employed for the purpose. The loading history effect on the unstable fracture of SS 304 is studied by performing cyclic loading with different load histories at constant load range (ΔPi) and at constant stress intensity factor range (ΔKj). The results show that the plastic singularity is well described by the integral (J) for the different types of loadings used. Moreover, the real meaning of (JIC) as a representative of the maximum energy release rate is quite limited if it ignores the loading history. The experimental results show that the dissipated energy ΔJ is more active when the loading is done at constant ΔP. The evolution of ΔJ can be directly linked to the crack growth rate and to the extent of the plastic zone developed at the crack tip.

Predicting fatigue crack growth rate in a welded butt joint: The role of effective R ratio in accounting for residual stress effect

A simple and efficient method is presented in this paper for predicting fatigue crack growth rate in welded butt joints. Three well-known empirical crack growth laws are employed using the material constants that were obtained from the base material coupon tests. Based on the superposition rule of the linear elastic fracture mechanics, welding residual stress effect is accounted for by replacing the nominal stress ratio (R) in the empirical laws by the effective stress intensity factor ratio (Reff). The key part of the analysis process is to calculate the stress intensity factor due to the initial residual stress field and also the stress relaxation and redistribution due to crack growth. The finite element method in conjunction with the modified virtual crack closure technique was used for this analysis. Fatigue crack growth rates were then calculated by the empirical laws and comparisons were made among these predictions as well as against published experimental tests, which were conducted under either constant amplitude load or constant stress intensity factor range. Test samples were M(T) geometry made of aluminium alloy 2024-T351 with a longitudinal weld by the variable polarity plasma arc welding process. Good agreement was achieved.

Weld residual stress effects on fatigue crack growth behaviour of aluminium alloy 2024-T351

The interaction between residual stress and fatigue crack growth rate has been investigated in middle tension and compact tension specimens machined from a variable polarity plasma arc welded aluminium alloy 2024-T351 plate. The specimens were tested at three levels of applied constant stress intensity factor range. Crack closure was continuously monitored using an eddy current transducer and the residual stresses were measured with neutron diffraction. The effect of the residual stresses on the fatigue crack behaviour was modelled for both specimen geometries using two approaches: a crack closure approach where the effective stress intensity factor was computed; and a residual stress approach where the effect of the residual stresses on the stress ratio was considered. Good correlation between the experimental results and the predictions were found for the effective stress intensity factor approach at a high stress intensity factor range whereas the residual stress approach yielded good predictions at low and moderate stress intensity factor ranges. In particular, the residual stresses accelerated the fatigue crack growth rate in the middle tension specimen whereas they decelerated the growth rate in the compact tension sample, demonstrating the importance of accurately evaluating the residual stresses in welded specimens which will be used to produce damage tolerance design data.

フェライト/ベイナイト]ハイブリッド材の疲労き裂進展挙動に及ぼす硬度の影響

We have examined fatigue crack propagation behavior in the [ferrite/bainite]-Hybrid materials with different hardness under a constant stress intensity factor range. It was found that crack propagation in the ferrite phase decelerated with reaching to the [ferrite/bainite]-interface, probably due to supression of plastic deformation at the crack tip. This suppression tendency of the crack propagation became higher when hardness difference between ferrite and bainite was larger. In the case with the highest hardness difference, fatigue crack did not pass through the [ferrite/bainite]-interface. Lower crack propagation rate was also found in the bainite which showed higher work softening due to crack propagation.

Effects of Different Load Schemes on the Fatigue Crack Growth Rate

An experimental study was conducted on a nickel-based super alloy, Inconel 718, at an elevated temperature of +400°C. The basic question was how to conduct the experiments in order to obtain fatigue crack growth data as free from load history effects as possible in the stage II region. This was investigated by use of four different load sequences. An important question was to investigate if the so called constant KI,max-method provides an upper bound of the fatigue crack growth rate in the stage II region. The results indicate that the constant KI,max-method gives an upper bound in fatigue crack growth rate in the current measuring range but also gives a statistically significant lower exponent of the crack growth equation. A more reliable method to establish the fatigue crack growth parameters is to propagate the crack under constant stress-intensity factor range. This should be done at two different stress-intensity factor ranges for a few millimetres at each range. Also, ΔK-reducing procedure should not be used, when performing fatigue crack growth testing in the stage II region, in order to minimize the risk of transient crack closure effects.

High temperature fatigue of a polycrystalline nickel base superalloy

A high temperature fatigue crack growth study on an experimental disc alloy of approximate composition Ni-14.75Cr-14-19Co-4.75Mo-3Al-3.75Ti-1.75Ta-0.7Hf-0.06Zr-0.02C-0.0175B wt- has been undertaken. Comparison of constant load fatigue crack growth tests conducted in air and a vacuum at 725C and in air at room temperature indicate that an oxidising environment has a major influence on crack growth rates over a wide range of applied stress intensity range. In particular it contributes to enhanced embrittlement of grain boundary regions, promoting an early transition to intergranular failure along with a concomitant increase in growth rate. Constant stress intensity factor range tests at high R ratio showed that a decrease in frequency at 725C in both air and a vacuum caused an increase in the crack growth rate per cycle due to time dependent crack growth. This was most significant in air at 725C rather than in vacuo, although in both instances low frequency tests were accompanied by wholly intergranular crack growth. At lower R ratios the influence of time dependent processes is less pronounced, especially in a vacuum. For the purposes of prediction a straightforward linear summation model using sustained load crack growth results combined with high frequency growth rates was found to model reasonably the influence of frequency on crack growth rates for the air tests at 725C. This can be rationalised by the observation that in air intergranular failure proceeds by linking of prior cracked or embrittled grains some distance ahead of the crack tip. At lower values of stress intensity range a slight under prediction of growth rates was evident, but improvements could be made through the use of triangular waveform data. For the vacuum tests, the linear summation model was consistently found to over predict growth rates due to the transitory nature of the sustained load crack growth rates under repeated loading and was not generally deemed suitable. Under these conditions damage occurs close to the crack tip and direct interaction between time dependent and time independent mechanisms will occur. This is not taken into account by a summation approach and more accurate modelling of damage formation in the varying strain fields ahead of the crack tip is required to predict this.