Crack Growth Retardation Behavior Research Articles

Fatigue crack growth of butt welded joints subjected to mixed mode loading and overloading

Since welded joints in structures are known to be critical regions for crack initiation and growth, study of fatigue crack growth behavior of welded joints under various loading scenarios are essential. In present study, two groups of experiments were conducted. At first, fatigue crack growth behavior of Al5083 butt welded joint subjected to mixed mode loading with constant amplitude has been studied. Afterwards, fatigue crack growth behavior of butt welded joints subjected to mixed mode single overload has been investigated. The sensitivity of the crack growth retardation behavior to post-welding heat treatments was also studied. Considering the effect of residual stress on fatigue crack growth, a modification to the Wheeler model was proposed to improve the accuracy of retardation prediction after applying a mixed mode overload.

An online‐offline prognosis model for fatigue life prediction under biaxial cyclic loading with overloads

AbstractThis paper presents a robust online‐offline model for the prediction of crack propagation under complex in‐phase biaxial fatigue loading in the presence of overloads of different magnitudes. The online prognosis model comprises a combination of finite element analysis and data‐driven regression to predict the crack propagation under constant loading, while the offline model is trained using experimental data to inform the post‐overload crack growth retardation behavior to the online model. The developed methodology is validated by conducting biaxial fatigue experiments using aluminum AA7075‐T651 alloy cruciform specimens. A close correlation is observed between the experimental results and model predictions. The results show that the model successfully predicts the crack retardation behavior under the influence of overloads with different magnitudes occurring at different stages of fatigue crack growth. Error analysis is conducted to investigate the sensitivities of the number of training points and crack increments to the prediction accuracy. In addition, the error propagation with respect to the crack length is studied, which provides constructive suggestions for further model improvement.

DIC-based studies of the overloading effects on the fatigue crack propagation behavior of Ti-6Al-4V ELI alloy

In this paper experimental correlations between da/dN and ΔK and between a vs. N of an extra-low-interstitial titanium (Ti-6Al-4V ELI) alloy under four typical overload conditions (OL, UL, OL-UL and UL-OL) in an otherwise constant amplitude loading were obtained. By means of the digital image correlation (DIC) technique, the crack-tip deformation fields as well as the crack opening process corresponding to the four overloading patterns were measured. The results shown that both the crack propagation behaviors and the crack-tip strain distributions of the Ti-alloy were greatly influenced by the overloading conditions applied. The effect of the overloading pattern (sequence) on the fatigue crack propagation behavior of the Ti-alloy was then analyzed by combining the crack-tip deformation measurements with the fatigue crack propagation results from three possible mechanisms, i.e., the crack-tip blunting effect, the crack closure effect and crack-tip plastic zone effect. It was found that for the present investigated Ti-6Al-4V ELI alloy, the above three mechanisms made contributions in different extents to the fatigue crack growth retardation behavior.

Fatigue crack propagation under biaxial fatigue loading with single overloads

The crack propagation behavior and the governing crack growth micromechanisms in aluminum alloy under in-plane biaxial fatigue loading with single overloads, of different magnitudes and occurring at different fatigue crack lengths, is investigated. The microscale fracture mechanisms governing crack growth behavior under these conditions are identified through detailed fractography. Crack growth retardation behavior observed due to the occurrence of single overloads is correlated with overload magnitude, instantaneous fatigue crack length, crack-tip plasticity and fracture surface morphology. The results obtained provide insight into the relationship between macroscale crack growth behavior to microstructural mechanisms, which is essential to understanding the fatigue behavior of metallic materials under variable amplitude biaxial loading scenarios.

판재 Al 2024-T3 합금재료에서 나타나는 두께별 피로균열진전지연거동에 관한 ΔK환산법의 정량적분

Sheet aluminum alloys used in manufacturing of machine structures for transportation show the difference of crack growth speed depending on thickness under the constant fatigue stress condition. The referred thickness effect is a major fatigue failure property of sheet aluminum alloys. In this work, we identified the thickness effect in fatigue test of thick plate and thin plate of Al 2024-T3 alloy under the constant fatigue stress condition, and presented the thickness effect to a correlative equation, which is determined by the shape factor, thickness ratio, and the loading factor, equivalent effective stress intensity ratio depending on thickness, . And we analyzed quantitatively the crack growth retardation behavior in thin plate compared to thick plate by the thickness effect using conversion method. We obtained such values as decrement of thickness(DoT), decrement of stress intensity factor range, (DoS) and identified the relation between them to present the nature of thickness effect in this work.

Influences of the Factors on Fatigue Crack Growth Retardation Behavior due to Multiple Overloads

Constant K fatigue crack growth tests were performed by applying an intermediate multiple overloads for S45C steel. The purpose of this study is to investigate effects of specimen thickness at various baseline stress intensity factor range levels (Kb), the application position of the overload (a/W) and the application frequency of the overload (OLHz) on fatigue crack growth retardation behavior. The principal results are summarized as follows. The number of retardation cycles for the constant baseline stress intensity factor level (Kb ) decreases with increasing specimen thickness. The normalized number of retardation cycles ( Nd / Nc ) decreases with increasing specimen thickness. But, at Kb = 45 MPa(m)1/2, the cycle increases with increasing specimen thickness.

Acoustic Emission Behavior during Fatigue Crack Propagation in 304 Stainless Steel

This study investigated acoustic emission behavior during fatigue crack growth test under constant and variable amplitude loading in 304 stainless steel. To describe the acoustic emission behavior, counts rate(dη/dn) was related with stress intensity factor range (SIFR, ΔK) in log-log plot. As a result of test, the relationship was represented a curve, which forms rise and fall behavior in counts rate as the SIFR increases. AE response to a single overload was sudden drop and slow recovery in counts rate, which was similar to crack growth retardation behavior. Under block loading, counts rate of each loading block was same as that of constant amplitude loading. Overall experimental results indicated that stress intensity factor controls the counts rate (dη/dn) as well as crack growth rate (da/dn) regardless of load range or crack length.

An Experimental Study on the Factors that Affect Fatigue Crack Growth Retardation Behavior in S45C Steel

Constant ΔK fatigue crack growth tests were performed by applying an intermediate multiple overload for S45C steel. The purpose of the present study is to investigate effects of specimen thickness at various baseline stress intensity factor range levels (ΔK(sub)b), overload application position (a/W) and overload application frequency (OL(sub)HZ) on fatigue crack growth retardation behavior. The principal results are summarized as follows. The amount of retardation for a given ΔK(sub)b level is increased with increasing the baseline stress intensity factor range level for all specimen thickness. The normalized minimum crack growth rate is increased with increasing the specimen thickness, except for ΔK=45MPa√m. The retardation cycle is decreased with increasing a/W and increased with OL(sub)HZ.

A NEW METHOD FOR PREDICTING FATIGUE LIFE IN NOTCHED GEOMETRIES

The objective of this paper is to develop a notch crack closure model, called NCCM, based on plasticity‐induced effects and short fatigue crack growth in the vicinity of the notch, and to predict the fatigue failure life of notched geometries. By using this model the regime for non‐propagating cracks (n.p.c.) and the relationship between the fatigue strength reduction factor, Kf , and the elastic stress concentration factor, Kt , under mean stress conditions, can be determined quantitatively. A crack closure model is assumed to apply in the notch regime based on an approach developed to explain the crack growth retardation behavior observed in smooth specimen geometries after an overload. Notch plasticity effects are also applied in the NCCM model. Fatigue failure life is calculated from both short fatigue crack growth in the notch region where elastic–plastic fracture mechanics (EPFM) is applied and from long fatigue crack growth remote from the notch where linear elastic fracture mechanics (LEFM) occurs. This prediction is obtained using a quantity called the effective plasticity‐corrected pseudo‐stress. The NCCM can be used to account quantitatively for various observed notch phenomena, including both the relationship between Kf and Kt and n.p.c. The effects of the tensile mean stress on the Kf versus Kt relationship is investigated and leads to the little recognized but technologically important observation that mean stress conditions exist where Kf can be greater than Kt . The role of notch radius and tensile mean stress on n.p.c. behavior is also explored. The model is verified using experimental data for notch geometries of aluminum alloy 2024‐T3, alloy steel SAE 4130 and mild steel specimens tested at zero and tensile mean stress.

炭素鋼同種摩擦圧接継手接合部の疲労き裂進展抵抗について

In order to evaluate the resistance to growth of fatigue cracks propagating along the weld interface of friction welded butt joints composed of carbon steel/carbon steel, a series of fatigue crack growth tests were carried out on S25C/S25C and S45C/S45C joints under three different cyclic load conditions. To investigate the effect of residual stress distribution in the specimen introduced in friction welding process on the crack growth behavior, two types of CCT plate specimens with a thickness of 2mm were prepared for respective carbon steel joints; one was the specimen as welded and another was the residual stress relief treated specimen. And the results of these joint specimens were discussed in comparison with those of the annealed CCT base metal plate specimens. First of all, whole crack growth behavior was investigated under increasing ΔK condition, and then the dependence of crack growth rate on the weld interface location was studied under a constant ΔK condition, and finally the ductility of the weld interface was evaluated indirectly through the observation of crack growth retardation behavior induced by the application of single static over-load during crack growth under constant ΔK. The results indicate that the residual stress does not play an important role on the fatigue crack growth of the joints, and metallurgical feature of the weld interface governs the fatigue crack growth behavior of the joints. And, the decrease in crack growth rate of the joint specimens by the application of the over-load is less than that of respective base metals, which indicates that the ductility of the weld interface of both joints is less than that of respective base metals.

Fatigue crack growth retardation behavior of in-100 at elevated temperature

In this investigation the effect of temperature on the fatigue crack growth retardation behavior of IN-100, a nickel base superalloy, when subjected to a single peak tensile overload cycle was examined. Baseline constant load amplitude fatigue crack growth rate ( da/ dN ) and single peak tensile overload testing was conducted at room temperature, 649 and 732°C in laboratory air. As a result of this investigation it was concluded that retardation of fatigue crack growth due to a single peak tensile overload cycle does occur in IN-100 at elevated temperatures. In general, the number of delay cycles decreased as temperature increased. The overload testing was conducted at two baseline stress intensity factor ( K max ) values. It was found that for a given amount of overload, the observed retardation was less for tests conducted at the higher value of baseline K max . A new parameter, the Crack Growth Rate Ratio (( da/ dN ) at K OL /( da/ dN ) at K max , was dev was found that the influence of test environment on the number of delay cycles resulting from a peak tensile overload could be correlated with the ratio of crack growth rate at the overload value of stress intensity to the crack growth rate at the baseline K max . The utility of the Crack Growth Rate Ratio as a potential tool for predicting the retardation behavior of a material when operating in a complex environment is discussed.

Spectrum Crack Growth in Adhesively Bonded Structure

A method is presented by which crack growth of damage tolerance type flaws can be predicted in adhesively bonded structure subjected to spectrum loading. The method accounts for a laminate stress intensity correction factor, induced bending and axial load transfer between plies, as well as a crack growth analysis procedure to predict the crack growth retardation behavior under spectrum load. Analytical predictions are compared to several test results to show the applicability of the prediction method presented.

Fatigue crack growth specimen configuration with compressive loads present

In a recent issue of this Journal the author provided experimental data which indicated the great importance of applied compressive loads on fatigue crack growth following a single tensile overload [i]. The results showed that crack growth retardation following a single tensile overload was drastically reduced if subsequent small amplitude loading included compressive loads. Crack growth retardation behavior was provided for 2024-T3 and 7075-T6 aluminum using compact (CT) specimens with constant amplitude R ratios of 0, -1/2, -3/4, -i and -2 following single overloads. The larger the negative R ratio, the smaller the retardation. Additional extensive results with single overloads and negative R ratio have also recently been reported [2]. All data were obtained using modified CT specimens, Fig. la, loaded through a spherical monoball gripping system to assure axial ram loading.