Fatigue Crack Growth Test Data Research Articles

A novel multiscale model for mixed-mode fatigue crack growth in laminated composites

A multiscale model is developed to predict mixed-mode fatigue crack growth (FCG) behavior in laminated composites by considering the micromechanical effects of fiber bridging. The proposed model consists of a micromechanical part dedicated to the fiber bridging mechanism near the crack tip, a macroscopic FCG model based on the maximum principal strain criterion, and an effective crack tip processing zone (i.e., an equivalent critical distance) connecting the two parts of the model. The core novelty of the presented model is to connect two scales (micro and meso) through a measurable physical quantity – called equivalent critical distance – accounting for various micromechanical as well as macroscopic phenomena occurring at both scales. The suggested model is validated against FCG test data available in the literature for laminated glass fiber composites under mode I, mode II, and mixed mode I/II conditions. It is found that the proposed framework is superior to its conventional (pure macroscopic) version, which eliminates fiber bridging effects, in the prediction of the cyclic mixed-mode crack propagation behavior in laminated composites. The multiscale nature of the proposed FCG model offers less complexity than fully micromechanical approaches while providing higher accuracy as compared to pure macroscopic models that ignore micromechanical phenomena occurring at the fiber scale.

A robust probabilistic fatigue crack growth model based on walker’s crack growth rate equation for metallic materials

The fatigue crack growth data inherits statistical variation despite carefully controlled test procedures. The available probabilistic models rely extensively on fatigue crack growth test data of the coupon material under replicate loading conditions. In view of the often unavailability of such a database, this work proposes a new probabilistic model based on Walker’s crack growth rate equation and considers the variability in both the stress intensity factor range, ΔK and stress ratio, R. The optimum estimates of the model parameters are established using the maximum likelihood estimation (MLE) method. The model is examined using selected sets of measured fatigue crack growth curves of 7075-T6 and 2024-351 aluminum alloys. It demonstrates acceptable predictions for fatigue crack growth data on replicate and variable R-ratio loadings with failure probabilities in the range of 0.10 ≤pF≤ 0.90. The error on the observed fatigue crack growth rate data is normally distributed. The criterion of the minimum number of replicate tests for a valid probabilistic assessment of the fatigue crack growth data is established. Based on the mean squared error (MSE) parameter, the model provides a better representation of the mean fatigue crack growth rate behavior of the material for fatigue loading with variable R-ratios.

Measurement-driven, model-based estimation of residual stress and its effects on fatigue crack growth. Part 2: Fatigue crack growth testing and modeling

This paper assesses the accuracy of fatigue crack growth (FCG) predictions for high-strength aluminum samples containing residual stress (RS) and complex two-dimensional cracks subjected to constant amplitude load. FCG predictions use linear-elastic, multi-point fracture mechanics. A first prediction includes RS estimated by the model described in Part 1; a second prediction includes RS measured by the contour method. FCG test data show a significant influence of RS. Ignoring the RS results in a +60% error in predicted FCG life (non-conservative). Including RS improves predictions of crack growth significantly (errors better than +26% (estimated RS) and −14% (measured RS)).

On the fatigue crack growth behaviour of selective laser melting fabricated Inconel 625: Effects of build orientation and stress ratio

AbstractThe building of Inconel 625 material was carried out using the selective laser melting method, and its fatigue crack growth property at ambient temperature was experimentally investigated. Compact‐tension specimens with different building orientations were utilized to determine the stress intensity factor threshold and fatigue crack growth rate curves at different stress ratios (R). The results indicated that the fatigue crack growth properties in the near threshold stress intensity factor and Paris regions were greatly affected by the loading factor, as well as the orientation of the alloy. The mechanism of fatigue crack growth at different stages was observed and discussed using scanning electron microscopy. Finally, based on the framework of the linear elastic fracture, a new and applicable effective driving force factor range was introduced to replace the traditional stress intensity factor range (ΔK) with good accuracy for all of the fatigue crack growth test data, considering both the stress ratio and orientation.

Fatigue crack growth rates for offshore wind monopile weldments in air and seawater: SLIC inter-laboratory test results

The majority of fatigue crack growth (FCG) data sets available on steels in air and seawater environments are several decades old and may not be appropriate for structural integrity assessment of offshore wind turbine foundations, which are fabricated using contemporary materials and welding technologies. Therefore, the SLIC joint industry project was formed to investigate the fatigue crack initiation and growth behaviour in offshore wind welded steel foundations. The FCG test data from the SLIC inter-laboratory (round robin) test programme have been analysed using a new proposed shape function solution and the results are presented and discussed. The obtained FCG trends in air and seawater environments have been compared with the recommended trends available in standards. The Paris-law constants and ΔKth values obtained from this programme can be used for defect assessment and remaining life prediction of offshore monopile weldments in air and seawater environments. The results from the SLIC project show that for a given value of ΔK the fatigue crack growth rate, da/dN, is on average around 2 times higher in seawater compared to air for the base metal and weldments. This factor of 2 in the seawater environment is almost half of the crack acceleration factor recommended by standards.

Fatigue Crack Growth Under Variable Amplitude Loading Through XFEM

Predicting fatigue crack growth (FCG) rate and path under variable amplitude loading (VAL) is a crucial issue in damage tolerant design commonly used in aerospace industry. The aim of the current study is to predict FCG life under VAL through Extended Finite Element Method (XFEM) and to explicitly illustrate both FCG life and crack propagation. For this purpose an algorithm is developed and integrated in ABAQUS software to analyze 3D crack propagation under VAL using Modified Generalized Willen-borg (MGW) retardation model. The results are compared with NASGRO crack propagation software and experimental FCG test data on 7075-T6 aluminum alloy under various over load (OL) and over load-under load (OL-UL) conditions which exhibit a good agreement.

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.

An investigation on fatigue crack growth rates through a designed nonsymmetric crack growth test

A nonsymmetric fatigue crack growth (FCG) test is specially designed and performed to study whether FCG test data from nonsymmetric cracking which is unavoidable in practice can be used to determine valid FCG rate information for fatigue life evaluation. Two types of alloys were used in this test and the FCG data were obtained via visual measurements. The effects of boundary conditions and central hole were examined using FEA software and the results showed that within engineering allowable error margin, the theoretical solution of stress intensity factors (SIFs) under the remote uniform tension is valid for nonsymmetric crack in this test. As a result, a method for analyzing the FCG rate of nonsymmetric through-thickness crack is proposed. It is found that the FCG rate parameters in the stable stage have no difference between two crack tips and the crack growth data can be used together to determine the Paris parameters. The proposed method, if supported by further research, can be adopted to validate FCG test data involving nonsymmetric cracking in standard tests.

Observing Load Sequence Effects on Simulated and Experimental Fatigue Crack Growth Occurrence

Load cycle interactions can have a significant effect in fatigue crack growth (FCG) under variable amplitude loading. Studying of FCG and fatigue life calculation under spectrum loading is important for the reliable life prediction of engineering structures. Many models have been proposed, but yet no universal model exists. In this paper, a fatigue life predicted under various load spectra, using three different fatigue crack growth models namely, the Austen, modified Forman and NASGRO models. These models are validated with fatigue crack growth test data under various amplitude loadings. This application is performed with aids of three-point bend specimens of ASTM A533 steel material. The results clearly show the load sequences effect and the predicted results agree with some discrepancies between the different models as well as with the test data. Thus, neglecting the effect of cycle interaction in fatigue calculations under variable amplitude loading can lead to invalid life predictions.

Effects of Load Sequence on Fatigue Crack Growth in Pressure Vessels

Low alloy steels such as ASTM A508 and A533 and their equivalent materials have been extensively applied in fabricating pressure vessels due to their relatively excellent mechanical properties and moderately good weldability. The integrity of such materials governs the safety of the power plants. These vessels mainly are subjected to random loading in service and the load cycle interactions can have a significant effect in fatigue crack growth. Studying of fatigue crack growth rate and fatigue life calculation under spectrum loading is important for the reliable life prediction of vessels. Many models have been proposed, but as yet no universal model exists. In this paper, a fatigue life predicted under various load spectra, using three different fatigue crack growth models namely the Austen, modified Forman and NASGRO models. These models are validated with fatigue crack growth test data under various variable amplitude loadings. This application is performed with aids of three-point bend specimens. The results show clearly the load sequences effect and the predicted results agree with some discrepancies between the different models as well as with the test data. Neglecting, the cycle interaction effects in fatigue calculation under variable amplitude loading lead to invalid life prediction.

Fatigue crack growth simulation of aluminium alloy under spectrum loadings

Fatigue crack growth in structure components, which is subjected to variable amplitude loading, is a very complex subject. Studying of fatigue crack growth rate and fatigue life calculation under spectrum loading is vital in life prediction of engineering structures at higher reliability. The main aim of this paper is to address how to characterize the load sequence effects in fatigue crack propagation under variable amplitude loading. Thus, a fatigue life under various load spectra, which was predicted, based on the Austen, Forman and NASGRO models. The findings were then compared to the similar results using FASTRAN and AFGROW codes. These models are validated with the literature-based fatigue crack growth test data in 2024-T3 Aluminium alloys under various overload, underload, and spectrum loadings. With the consideration of the load cycle interactions, finally, the results show a good agreement in the behaviour with small differences in fatigue life compare to the test data.

Noise reduction for fatigue crack growth test data under random loading

The noise included in electrical signal significantly influences on the estimating results of crack length and crack opening point. The noise can be effectively reduced by averaging some consecutive loading cycles without distortion of the signal for constant amplitude loading tests, whereas the averaging technique cannot be applied to random loading tests due to the variation of load. In this study, a noise reduction technique was developed by using a load-based averaging technique, which can be applied to fatigue crack growth test data under random loading. Additionally, the developed noise reduction method was applied to random loading test data to verify the effectiveness of the method.

Computer simulation of fatigue crack propagation under random loading conditions

The aim of this study is to simulate fatigue crack propagation under random loading conditions using a simple algorithm based on the Wheeler model [ Wheeler O. Spectrum loading and crack growth. J Basic Eng D 1972;94:181–86]. To create the computer simulation, a model based on the mechanical properties of the material has been used. These properties include the yield stress ( σ y) and Paris’s constants C and m. The loading conditions (baseline loading ratio R, baseline stress intensity factor range Δ K and overload stress intensity factor K ol, R ol) are also required. The present model is validated with fatigue crack growth test data conducted on 12NC6 steel samples with four different heat treatments in order to have different types of mechanical behavior. The computer simulation and experimental results for crack propagation for different overload distributions (a single overload, a repeated overload, different overload magnitudes, random overload) are in good agreement.

Short cracks initiated in Al 6013-T6 with the focused ion beam (FIB)-technology

The fatigue crack growth behaviour of short corner cracks in the Aluminium alloy Al 6013-T6 was investigated. The aim was to determine the crack growth rates of small corner cracks at a stress ratio of R = 0.1, R = 0.7 and R = 0.8 and to find a possible way to predict these crack growth rates from fatigue crack growth curves determined for long cracks. Corner cracks were introduced into short crack specimens, similar to M( T) – specimens, at one side of a hole (Ø = 4.8 mm) by cyclic compression ( R = 20). The precracks were smaller than 100 μm (notch + precrack). A completely new method was used to cut very small notches (10–50 μm) into the specimens with a focussed ion beam. The results of the fatigue crack growth tests with short corner cracks were compared with the long fatigue crack growth test data. The short cracks grew at Δ K-values below the threshold for long cracks at the same stress ratio. They also grew faster than long cracks at the same Δ K-values and the same stress ratios. A model was created on the basis of constant K max-tests with long cracks that gives a good and conservative estimation of the short crack growth rates.

Limitations of elastic definitions in Al–Si–Mg cast alloys with enhanced plasticity: linear elastic fracture mechanics versus elastic–plastic fracture mechanics

Linear elastic fracture mechanics describes the fracture behavior of materials and components that respond elastically under loading. This approach is valuable and accurate for the continuum analysis of crack growth in brittle and high strength materials; however it introduces increasing inaccuracies for low-strength/high-ductility alloys (particularly low-carbon steels and light metal alloys). In the case of ductile alloys, different degrees of plastic deformation precede and accompany crack initiation and propagation, and a non-linear ductile fracture mechanics approach better characterizes the fatigue and fracture behavior under elastic–plastic conditions. To delineate plasticity effects in upper Region II and Region III of crack growth an analysis comparing linear elastic stress intensity factor ranges (Δ K el) with crack tip plasticity adjusted linear elastic stress intensity factor ranges (Δ K pl) is presented. To compute plasticity corrected stress intensity factor ranges (Δ K pl), a new relationship for plastic zone size determination was developed taking into account effects of plane-strain and plane-stress conditions (“ combo plastic zone”). In addition, for the upper part of the fatigue crack growth curve, elastic–plastic (cyclic J based) stress intensity factor ranges (Δ K J ) were computed from load–displacement records and compared to plasticity corrected stress intensity factor ranges (Δ K pl). A new cyclic J analysis was designed to compute elastic–plastic stress intensity factor ranges (Δ K J ) by determining cumulative plastic damage from load–displacement records captured in load-control ( K-control) fatigue crack growth tests. The cyclic J analysis provides the true fatigue crack growth behavior of the material. A methodology to evaluate the lower and upper bound fracture toughness of the material ( J IC and J max) directly from fatigue crack growth test data ( Δ K FT ( J IC ) and Δ K FT ( J max ) ) was developed and validated using static fracture toughness test results. The value of Δ K FT ( J IC ) (and implicitly J IC) is determined by comparing the plasticity corrected elastic fatigue crack growth curve with the elastic–plastic fatigue crack growth curve. A most relevant finding is that plasticity adjusted linear elastic stress intensity factor ranges (Δ K pl) are in remarkably good agreement with cyclic J analysis results (Δ K J ), and provide accurate plasticity corrections up to a Δ K corresponding to J IC (i.e. Δ K FT ( J IC ) ). Towards the end of the fatigue crack growth test (above Δ K FT ( J IC ) ) when plasticity is accompanied by significant tearing, the cyclic J analysis provides a more accurate way to capture the true behavior of the material and determine Δ K FT ( J max ) . A procedure to decouple and partition plasticity and tearing effects on crack growth rates is given. Three cast Al–Si–Mg alloys with different levels of ductility, provided by different Si contents and heat treatments (T61 and T4) are evaluated, and the effects of crack tip plasticity on fatigue crack growth are assessed. Fatigue crack growth tests were conducted at a constant stress ratio, R = 0.1, using compact tension specimens.

Load History Effects Resulting from Compression Precracking

Abstract Compression precracking (CPC) has seen renewed interest as a possible alternative procedure for generating fatigue crack growth threshold data with minimal load history effects, but recent testing confirms results from the literature that compression precracking does induce load history effects through residual stresses that influence subsequent fatigue crack growth test data. Using the CPC method, specimens are precracked with both maximum and minimum compressive loads. Compressive yielding occurs at the crack-starter notch, resulting in a local tensile residual stress field through which the fatigue crack must propagate. Although the tensile residual stress field contributes to the driving force for precracking, it also introduces the possibility of history effects that may affect subsequent fatigue crack growth. The tensile residual stress field elevates the local driving force at the crack tip, promoting higher crack growth rates than would be expected from the applied loading. This paper presents three-dimensional finite element results and experimental data for compact tension specimens that characterize the load history effects induced by compression precracking. The analysis results indicate that for low tensile loading levels near the threshold region, the residual stresses cause the calculated crack tip driving force to increase from the applied driving force by 25% or more. In addition, significant crack growth of about two times the estimated plastic zone size is needed to grow away from the residual stress field and reduce the calculated crack tip driving force to within 5% of the applied driving force. Experimental results show that growth of about two to three times the estimated plastic zone size is necessary to establish steady growth rates under constant ΔK loading for the materials and loading levels evaluated. Constant ΔK testing following compression precracking will demonstrate when residual stress effects are no longer significant and will ensure consistent growth rates.

Fatigue crack growth with overload under spectrum loading

Load cycle interactions can have a very significant effect in fatigue crack growth under variable amplitude loading. Studying of fatigue crack growth rate and fatigue life calculation under spectrum loading is very important for the reliable life prediction of engineering structures. In this paper, a fatigue life prediction model under various load spectra, using the strain energy density factor approach and the plastic zone size near crack tip as main parameters in calculating effective strain energy density factor, has been proposed. The present model is validated with fatigue crack growth test data provided by Ray under various variable amplitude and spectrum loading in 7075-T6 and 2024-T3 aluminum alloy, respectively. Predictions of present model are compared with those of the state-space model, FASTRN and AFGROW codes. The results show that the predicted results agree well with the test data.

On the conventional K calibration equations for single-edge-cracked tension specimens

The conventional K calibration equations for single-edge-cracked tension specimens were examined by conducting constant stress intensity range fatigue tests on a 2024-T3 aluminum alloy over a range of crack lengths from 0.5 to 15 mm [corresponding to crack length-to-specimen width ratios ( a W ) of about 0.02 to 0.5] in dry oxygen at two ΔK levels. The results showed that the equations are in considerable error, ranging from less than one to over ten percent from a W of 0.05 to 0.5. These errors are attributed to the effect of bending associated with rotation at the loading pins. An empirical correction to the K calibration equations was made based on the fatigue crack growth test data in oxygen. Tests were then conducted in a different test environment (namely, deaerated 0.5M NaCl solution) to validate the modified K calibration equation and showed very satisfactory results.

Improved structural reliability assessment using non-linear regression techniques to process raw fatigue crack growth test data

A new method is proposed to process raw fatigue crack growth (FCG) test data for estimating the parameters of any crack propagation (FCG) model. This method is based on iterative minimization techniques for non-linear regression, extensively available in the literature. The method presented here permits a significantly reduced contribution of the variability inherent in the usual raw FCG data processing techniques to the predictions of the crack length distributions derived by any analytical FCG model and, consequently, improved structural reliability assessment is achieved.