Fatigue Crack Growth Rate Data Research Articles

Microstructural causes and mechanisms of crack growth rate transition and fluctuation of additively manufactured titanium alloy

Wire and arc additive manufacturing (WAAM) enables rapid near-net-shape fabrications of large-size parts and in-situ remanufacturing in many industry sectors. A comprehensive understanding of the fatigue failure mechanism of WAAM titanium alloys is a prerequisite for their widespread use in critical structural components subject to fatigue load. Here, the fatigue crack growth behavior of WAAM TA15 material is investigated. Fatigue crack growth tests are performed using compact tension specimens sampled from different locations and with different crack orientations of the WAAM TA15 block. The fatigue crack growth rate (FCGR) data exhibit two governing rates separated by a transition stress intensity factor value, ΔKn, and the degrees of fluctuation of the FCGR data in the two regimes are notably different. A piecewise log-linear model is first proposed by incorporating the Heaviside step function and ΔKn into the classical Paris’ model, allowing for the transition ΔKn to be determined by the data. The potential causes of the transition ΔKn are phenomenologically inferred via fractography and surface roughness profiling results. The critical microstructure affecting the value of ΔKn is identified by relating the crack tip cyclic plastic zone size at ΔKn to the sizes of main microstructures. The cause of different degrees of fluctuations in the two regimes separated by ΔKn is inferred by examining the microstructures within the plastic zone. The microstructural mechanisms of the local FCGR reduction and fluctuation are further identified and explained.

The Influence of Residual Stress on Fatigue Crack Growth Rates in Stainless Steel Processed by Different Additive Manufacturing Methods

The Influence of Residual Stress on Fatigue Crack Growth Rates in Stainless Steel Processed by Different Additive Manufacturing Methods

Fatigue Crack Growth Evaluation of Pipeline Steels and Girth Welds

Abstract Fatigue test specimens were prepared and tested with an API 5 L X70 spiral welded pipe steel and girth weld. For a few selected specimens, two unloading compliance techniques (elastic compliance and back-face strain compliance) were applied simultaneously to a single specimen for direct comparisons of in situ crack size estimation. This paper also includes fatigue crack growth rate (FCGR) data of other pipe steels and welds available in the literature. It was observed that most FCGR curves of pipeline steels (X65–X100) remained within the BS 7910 mean and upper bound design curves in the Paris region. It was noted that the FCGR of austenitic stainless pipe steel and girth weld obtained from Arora et al. (2014), “Fatigue Crack Growth Studies on Narrow Gap Pipe Welds of Austenitic Stainless Steel Material,” Procedia Eng., 86, pp. 203–208) showed a very superior fatigue property.

Evaluation of residual stress reproducibility and orientation dependent fatigue crack growth in powder bed fusion stainless steel

The complex thermal gradients of additive manufacturing (AM) result in residual stress and distinctive grain morphologies that influence mechanical performance and contribute to concern regarding the fatigue properties of AM parts. In this study, residual stress, microstructure, and fatigue crack growth rate (FCGR) results were compared in AM Type 304L stainless steel produced by laser powder bed fusion (PBF) on different systems using similar process parameters. Residual stress measured in the build direction was remarkably consistent in all PBF builds. Backscatter electron large area images revealed similar grain morphologies in the different builds, all of which exhibited elongated grains in the build direction and inhomogeneous grain size and shape. Fatigue crack growth investigated both parallel and perpendicular to the build direction revealed higher measured crack growth rates in the near-threshold regime in both orientations of the PBF material compared to wrought material. The difference in near-threshold fatigue crack growth rates is attributed primarily to the influence of processing-induced residual stress quantified by the residual stress intensity factor. These values revealed consistent FCGR effects in each orientation across specimens extracted from different builds and were then used to reveal a convergence of the corrected FCGR data of all PBF and wrought specimens.

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.

Fatigue Crack Growth Rate Description of RF-Plasma-Sprayed Refractory Metals and Alloys

A fitting method capable of describing the fatigue crack growth rate (FCGR) data in all stages of crack propagation by a simple Forman-style analytical formula was developed. To demonstrate its robustness, this method was used to quantify the fracture behavior of RF-plasma-sprayed W, Mo, W-Mo composite, and four selected Ni-based tungsten heavy alloys (WHA). The fitted FCGR parameters categorized the studied materials into two distinct sets. W, Mo, and W-Mo composite deposits made from inherently brittle refractory metals that contained a range of defects inherent to plasma spray process represented the first class. This class was characterized by low fracture toughness and a relatively wide range of fatigue crack growth thresholds. The second class of materials was represented by WHA. Here, the deposit defects were suppressed by liquid state diffusion that formed a typical WHA structure consisting of a Ni-rich matrix and large spherical W reinforcement particles. The WHA generally showed higher fatigue crack growth thresholds, but differed in fracture toughness values based on the W particle concentrations. The obtained fracture mechanical data represent a reference dataset of plasma-sprayed refractory materials, and their classification into groups clearly demonstrates the capabilities of the developed method to capture a wide range of different types of FCGR behavior.

Effect of compression precracking on the near threshold fatigue crack propagation in an AISI 316L stainless steel

This work addresses an alternative test procedure based on fatigue precracking under compression to mitigate the potential effects of crack closure on near-threshold fatigue data and, thus, to obtain more consistent and reliable ΔKth-values. The method relies on performing a compression precracking, in which both maximum and minimum loads are compressive, thereby resulting in a tensile residual stress field at the crack tip. Fatigue crack growth (FCG) tests conducted on an AISI 316L stainless steel provide the experimental data upon which the near-threshold fatigue crack growth rate data for different test techniques can be determined. The experimental program includes FCG tests conducted on conventional C(T) specimens with crack size to specimen width ratio a/W=0.2, single edge notched SE(B) specimens under three-point bending with a/W=0.2 and two different span sizes, S, characterized by S/W=4 and 8. Overall, the present investigation shows that combining the compression precracking technique with the utilization of non-standard bend specimens having increased span improves evaluation of the near-threshold fatigue crack growth rate data for the tested AISI 316L steel, thereby providing more accurate ΔKth-values.

Effects of residual stress on orientation dependent fatigue crack growth rates in additively manufactured stainless steel

Localized heating and resulting temperature gradients during additive manufacturing (AM) create significant residual stress that influences mechanical behavior, such as fatigue performance. To quantify residual stress effects on fatigue crack growth in AM materials, crack growth rates parallel and perpendicular to the build direction in directed energy deposition (DED) Type 304L austenitic stainless steel were measured. The on-line crack compliance method was used to determine the residual stress intensity factor, Kres, while simultaneously collecting fatigue crack growth rate (FCGR) data. Constant applied alternating stress intensity factor (constant ΔKapp) tests revealed the primary influence on measured FCGR is the orientation dependent Kres. Critical analysis of the compliance data from decreasing ΔKapp tests was used to quantify Kres, which was then used to correct FCGR data in the near-threshold regime. Results demonstrated that the fatigue response of DED Type 304L is inherently similar to that of annealed wrought Type 304/304L.

Fractal Analysis for Fatigue Crack Growth Rate Response of Engineering Structures with Complex Geometry

A growing fatigue crack in metallic materials and structures exhibits multifractal features that inherit signatures of the crack growth rate behavior of the material. This study exploits the recently established multifractal fatigue crack growth model to quantify the characteristic fatigue crack growth rate response of the AISI 410 martensitic stainless steel using an L-shaped bell crank structure. The objective is to demonstrate that the fatigue crack growth rate response of the material could be established by quantifying the fractality of the growing crack. The fractal approach avoids the need of the crack geometry factor when calculating the crack tip driving force. The fractal analysis of the crack image employs the box-counting algorithm to determine the fractal dimension along the edge of the crack length. The analysis is confined to the power law crack growth rate stage (Paris crack growth regime). Results show that the fatigue crack growth path in the bell crank structure is dictated by the Mode I (opening) component of the crack loading. The distribution of fractal-based fatigue crack growth rate data is within the 99% confidence limit of the median crack growth response by the Paris equation. Thus, the model could be employed for prediction of the fatigue crack growth response of engineering structures where the crack geometry factor is not readily available.

Prediction of fatigue crack propagation behavior of AA2524 after laser shot peening

Laser shot peening is a relatively novel surface treatment technology generating deep compressive residual stress and further retarding fatigue crack growth rate (FCGR). Meanwhile, the load ratio, R, plays an important role in fatigue crack growth behavior. Evaluating the remaining life of components accurately is essential to damage tolerance design, many efforts have been applied to correlate the R-effect. In this paper, the effect of load ratio on the gain of AA2524′s fatigue life after laser shot peening have been thoroughly studied. Initially, the fatigue crack growth rate of AA2524 after laser shot peening was measured under different load ratios. Subsequently, a comparative analysis between the FCGR for AA2524 before and after laser shot peening was thoroughly conducted. Then, the crack closure model has been applied to correlate the R-effect of peened specimens, but the results are unsatisfied due to the ignore of compressive residual stress. To address this issue, a novel method of predicting crack growth life considering compressive residual stress was proposed. The artificial neural network model and support vector regression model based on particle swarm optimization algorithm were trained by the FCGR data, and the fatigue crack growth life is predicted by a cycle-by-cycle method. The result shows the artificial neural network model is more accurate than the support vector regression model, and this intelligent algorithm can be a potentially effective way to predict fatigue crack growth behavior.

The effect of crack length and maximum stress on the fatigue crack growth rates of engineering alloys

The fatigue crack growth rate (FCGR) curve of metallic alloys is usually divided into three regions. Region II is often referred to as the Paris regime and is usually modelled with a power law relationship with a single exponent. Regions I and III are located at the beginning and end of the FCGR curve, respectively, and are frequently modelled with asymptotic relationships. In this paper we hypothesize that fatigue crack growth is governed by power law behaviour at all crack lengths and all stress intensity factor ranges (ΔK). To accommodate for the changes in the FCGR slope at regions I - III mathematical pivot points are introduced in the Paris equation. Power law behaviour with the presence of pivot points enables direct fitting of the crack length vs. cycles (a-N) curve to obtain the FCGR as a function of ΔK. This novel approach is applicable to small and long crack growth curves and results in accurate multilinear FCGR curves that are suitable for reconstruction of the measured a-N curves. The method is subsequently applied to i) different alloys to show local changes in the FCGR curve for changes in alloy composition and heat treatments, ii) naturally increasing ΔK testing of microstructurally small cracks to obtain accurate small crack FCGR data. The comparison with accurate long crack data shows that small cracks are faster, but the transition from region I to region II occurs at a specific fatigue crack growth rate which results in an apparent shift in ΔK at the transition. iii) Long cracks, which show that the FCGR increases with maximum stress for a given ΔK and stress ratio when the maximum stress approaches the yield stress. The maximum stress phenomenon becomes important in the case of fatigue testing, where the initial crack lengths are usually small and maximum stresses are high. It is concluded that for long cracks the phenomenon explains why the Paris equation is applicable rather at low maximum stress, while the Frost-Dugdale equation is more applicable at high maximum stress.

Determination of fatigue crack growth in the near-threshold regime using small-scale specimens

A complete description of the experimental procedure for characterizing the intrinsic fatigue crack propagation threshold (ΔKth,eff) as well as the fatigue crack growth rate (FCGR) in the near-threshold regime using small-scale specimens is presented. A comparative study is carried out on the high strength steel S960QL considering different single edge notch bend (SENB) specimen geometries. On one hand, the reference dimensions of 6 mm thickness (B) and 19 mm width (W) are analysed and referred to as conventional specimens. On the other hand, small-scale specimens with B = 3 mm and W = 4 and 6 mm are also considered. Several loading configurations (3-, 4- and 8-point bending) are used and a load ratio R = 0.8 is applied to avoid crack closure effects. The direct current potential drop (DCPD) technique is used to monitor the crack length. The reduced dimensions of the small-scale specimens imply the necessity of implementing a modified testing procedure compared to the recommendations of current standards for the generation of FCGR data. The results show a good agreement between tests conducted on different specimen sizes, which opens new perspectives in the use of small-scale specimens for characterizing the fatigue crack growth properties in metallic materials. Recommendations and limitations of the procedure are provided and discussed.

On small fatigue crack growth rates in AA7085-T7452

Aluminium alloy 7085-T7452 is a material developed by Alcoa (now Arconic) for its favourable mechanical properties in thick, forged aluminium aerospace structural parts. Currently reported fatigue crack growth rate data for 7085-T7452 in the open literature does not fully characterise the small crack growth behaviour of this material. The growth rates of small fatigue cracks are important since they often govern the useful service life of large primary structural parts in the aerospace industry. In this paper, small fatigue crack growth rates were measured experimentally using two methods and compared with data available in the open literature for longer cracks. The fractography based method was found to develop fatigue crack growth data that better characterised the small crack regime and so these results were then developed into a fatigue crack growth model which was evaluated for its ability to predict an independent set of 7085-T7452 specimens tested with a complex aircraft wing bending plus buffet loading spectrum. The fractography method based small crack growth rate model was able to robustly predict fatigue crack growth under this complex variable amplitude sequence from small, realistic initial discontinuity sizes better than long crack data, yet it is not without need of further refinement.

A unified description of mechanical and actuation fatigue crack growth in shape memory alloys

The damage tolerance approaches to fatigue rely on fracture mechanics concepts to explicitly account for the fatigue crack growth process. These approaches typically adopt the Paris equation, which relates the crack growth per cycle to the range of variation of the stress intensity factor, ΔK, during cyclic loading by means of experimentally calibrated parameters. Conditions on the validity of a single parameter description of the near crack tip stress and strain fields restrict the applicability of ΔK to fatigue crack growth resistance in Shape Memory Alloys (SMAs) under general thermomechanical loading paths. These conditions are substantially relaxed for the cyclic ΔJ–integral approach, which can achieve similitude over a wider range of loading conditions and geometric configurations. Thus, ΔJ qualifies as a potential unified descriptor of thermomechanical fatigue crack growth resistance in SMAs, an assertion, which is tested in this work for data from both purely mechanical and actuation fatigue crack growth experiments of a high temperature SMA, Ni50.3Ti29.7Hf20. In the latter, compact tension specimens are subjected to thermal cycling under constant load between an upper and a lower cycle temperature that allows for the stable phase to alternate in each cycle. It is shown here that a Paris-type power-law crack growth expression based on the ΔJ-integral can fit fatigue crack growth rate data from both types of experiments with a single set of parameters. This new analysis method for both actuation and mechanical fatigue crack growth rates provides a unified description for fatigue crack growth in SMAs and can allow estimating actuation fatigue crack growth rates, laborious and challenging to measure, from easier to detect mechanical fatigue crack growth rates.

Fatigue crack growth behaviour of wire and arc additively manufactured ER70S-6 low carbon steel components

The new emerging Wire and Arc Additive Manufacturing (WAAM) technology has significant potential to improve material design and efficiency for structural components as well as reducing manufacturing costs. Due to repeated and periodic melting, solidification and reheating of the layers, the WAAM deposition technique results in some elastic, plastic and viscous deformations that can affect material degradation and crack propagation behaviour in additively manufactured components. Therefore, it is crucial to characterise the cracking behaviour in WAAM built components for structural design and integrity assessment purposes. In this work, fatigue crack growth tests have been conducted on compact tension specimens extracted from ER70S-6 steel WAAM built components. The crack propagation behaviour of the specimens extracted with different orientations (i.e. horizontal and vertical with respect to the deposition direction) has been characterised under two different cyclic load levels. The obtained fatigue crack growth rate data have been correlated with the linear elastic fracture mechanics parameter varDelta K and the results are compared with the literature data available for corresponding wrought structural steels and the recommended fatigue crack growth trends in the BS7910 standard. The obtained results have been found to fall below the recommended trends in the BS7910 standard and above the data points obtained from S355 wrought material. The obtained fatigue growth trends and Paris law constants from this study contribute to the overall understanding of the design requirements for the new optimised functionally graded structures fabricated using the WAAM technique.

Prediction of Fatigue Crack Growth Behaviour in Ultrafine Grained Al 2014 Alloy Using Machine Learning

The present work investigates the relationship between fatigue crack growth rate (da/dN) and stress intensity factor range (∆K) using machine learning models with the experimental fatigue crack growth rate (FCGR) data of cryo-rolled Al 2014 alloy. Various machine learning techniques developed recently provide a flexible and adaptable approach to explain the complex mathematical relations especially, non-linear functions. In the present work, three machine algorithms such as extreme learning machine (ELM), back propagation neural networks (BPNN) and curve fitting model are implemented to analyse FCGR of Al alloys. After tuning of networks with varying hidden layers and number of neurons, the trained models found to fit well to the tested data. The three tested models are compared with each other over the training as well as testing phase. The mean square error for predicting the FCG of cryo-rolled Al 2014 alloy by BPNN, ELM and curve fitting methods are 1.89, 1.84 and 0.09 respectively. While the ELM models outperform the rest of models in terms of training time, curve fitting model showed best performance in terms of accuracy over testing data with least mean square error (MSE). In terms of local optimisation, back propagation neural networks excel the other two models.

Effect of residual stress redistribution and weld reinforcement geometry on fatigue crack growth of butt welded joints

Redistribution of residual stress during crack growth in butt welded joint of Al5083-H111 was studied. Furthermore, the effect of post weld heat treatment and removing the weld reinforcements on fatigue crack growth rate was investigated. Three dimensional simulations of welding process and fatigue crack growth were conducted. It was shown that removing the weld reinforcements causes an increase in the residual stress. A correlation between the fatigue crack growth rate data of welded joints can be obtained with that of the parent metal by considering the weld geometry and redistribution of residual stresses in calculation of stress intensity factor.

Fatigue properties of aluminium triangular lattice plates

We propose a method to predict the fatigue life of triangular lattices by using fatigue data from tests on single lattice struts. The method was validated using fatigue tests on 6 identical triangular lattice plates divided into three groups with different initial crack lengths. The predicted fatigue lives of lattices agree very well with the experimental results. A three-stage mechanism of lattice fatigue was observed in the tests, and the crack propagated in the direction 30° to the horizontal axis in the upper half of lattice plate in all three groups. The fatigue crack propagation rate was also studied and calculated by using the effective mechanical parameters from the homogenized structure. Experimental fatigue crack growth rate data were compared with predictions, showing that the proposed method can also predict the crack growth rate in triangular lattices very well.

Fatigue crack growth rates in high pressure hydrogen gas for multiple X100 pipeline welds accounting for crack location and residual stress

Fatigue crack growth rates (FCGR) of multiple X100 pipeline steel welds and heat affected zones were measured in high-pressure hydrogen gas to investigate their behavior compared to lower strength pipeline welds. A total of five high strength welds and two heat affected zones (HAZ) were examined all of which were fabricated using the same X100 base material. Different welding wires and techniques were used to fabricate the welds to provide a variety of end products to evaluate susceptibility to fatigue in high pressure hydrogen gas. Residual stresses were measured for each weld and HAZ using the slitting method and the effect of residual stress on the stress intensity factor, Kres, was determined. Using Kres, the fatigue crack growth rate curves were corrected to remove the effects of residual stress by examining the influence of Kres on stress ratio, R. Comparisons were then made between the high strength welds, which were corrected for residual stress, and lower strength welds from the literature. It was found that the higher strength welds and heat affected zones exhibited comparable fatigue crack growth rates to lower strength welds, as the FCGR data of the high strength welds overlaid the lower strength welds. This suggests that despite distinct differences in strength and microstructure between the different welds, hydrogen-assisted fatigue crack growth susceptibility is similar. A comparison was made between the Kres measured in extracted coupons and residual stress estimates provided in relevant welded pipe assessment standards such as API 579-1/ASME FFS-1. It was found the residual stress values in the test coupons extracted from welded pipe were significantly lower than those expected in the intact welded pipes and highlights the importance in quantifying and removing coupon residual stresses when fatigue crack growth rates are measured and including expected weld joint residual stress when making structural assessments.

Development of a Novel Test Method to Characterize Material Properties in Corrosive Environments for Subsea HPHT Design

Abstract High pressure high temperature (HPHT) design is a significant new challenge facing the subsea sector, particularly in the Gulf of Mexico. API 17TR8 provides HPHT Design Guidelines, specifically for subsea applications. Fatigue endurance (i.e., S–N) and fracture mechanics design are both permitted, depending on the criticality of the component. Both design approaches require material properties generated in corrosive environments, such as seawater with cathodic protection and/or sour production fluids. In particular, it is necessary to understand sensitivity to cyclic loading frequency (for both design approaches), crack growth rates (CGR) (for fracture mechanics approach) as well as fracture toughness performance. For many subsea components, the primary source of fatigue loading is associated with the start-up and subsequent shutdown operation of the well, with long hold periods in-between, during which static crack growth (CG) could occur. These are the two damage modes of most interest when performing a fracture mechanics based analysis. This paper presents the preliminary results of a novel single specimen test method that was developed to provide fatigue crack growth rate (FCGR) and fracture toughness data in corrosive environments, in a timeframe that is compatible with subsea HPHT development projects. Test data generated on alloy 625+ in seawater with cathodic protection are presented along with a description of how the test method was developed. A crack tip strain rate based formulation was applied to the data to rationalize the effect of frequency, stress intensity factor range (ΔK), and maximum stress intensity factor (Kmax).