Study Of Crack Growth Research Articles

Probabilistic hydrogen-assisted fatigue crack growth under random pressure fluctuations in pipeline steels

The increasing demand for energy and the global threat of climate change have driven the search for alternative energy sources, with hydrogen emerging as a prominent substitute for fossil fuels. The fatigue behavior of pipeline steels under gaseous hydrogen is a critical problem that is impeding the industry's adoption of hydrogen into the current natural gas infrastructure. A brief review of existing hydrogen-assisted fatigue crack growth (HA-FCG) studies, which reveal several key gaps, is given first. Existing HA-FCG models predominantly address constant amplitude loading, while the realistic driving force is random loading in gas pipelines. Also, the current uncertainty quantification studies for HA-FCG focus on material randomness and overlook the large uncertainties associated with random pressure fluctuations. To address these issues, this study proposes a HA-FCG model that utilizes a time-based subcycle approach, allowing for direct application to random spectrum loads without the need for cycle counting. A model parameter as a function of hydrogen operating conditions is introduced to capture the different regimes in HA-FCG, and the model predictions are compared with ASME B31.12 code. Following this, statistical analysis of random pressure fluctuation data collected from natural gas pipelines at multiple locations is performed. The realistic industry pressure data shows distinct statistical features, and it is observed that the high-fidelity data (high sampling frequency) is beneficial for accurate fatigue life predictions. Uncertainty quantification and load reconstruction are performed by the Karhunen–Loève expansion with a post-clipping procedure, leading to a probabilistic HA-FCG analysis. The paper concludes with key findings and suggests directions for future research.

Effect of thermal ageing on fatigue crack growth behaviour of forged alloy 617M at elevated temperatures

ABSTRACT Fatigue crack growth (FCG) studies have been conducted on compact tension specimens of alloy 617M for as-received and aged for 1000, 5000 and 10,000 h at 710°C. Effects of thermal ageing on FCG behaviour of alloy 617M forged has been characterised at 27°C, 650°C, 710°C and 750°C. Results reveal that threshold stress intensity factor ( Δ K th ) decreases as temperature increases upto 710°C and then increases at 750°C for all the ageing conditions except for 1000 h aged. The Δ K th value also decreases with an increase in ageing time till 5000 h, but at 10,000 h a reverse trend is observed. The influence of thermal ageing and temperature reflects major deviations for Paris constant (C) and exponent (m). Additionally, thermal ageing and temperature influences on FCG, the effects of dynamic strain ageing, Cr-rich carbides and γ’-phase precipitations have been discussed and corroborated with fractographic and microstructural changes.

Fatigue crack growth study on rolled and cryogenically treated AZ31B Mg alloy plate

This paper investigates the fatigue crack growth rate (FCGR) of AZ31B Mg alloy after cryogenic treatments. Shallow and deep cryogenic treatments were carried out separately at −80οC and −196οC for 12 and 72 hrs, respectively. The microstructural characterization of the cryogenically treated AZ31B specimens was carried out by optical microscope (OM) and X-Ray diffraction (XRD). The small crystalline size structure generated in AZ31B alloy due to the cryogenic treatment further promoted the nucleation of Mg17Al12 phase. Accumulation of dislocation density was also observed in the cryogenically treated specimens. CT specimens were prepared for all three conditions to estimate the FCGR. The as-rolled specimen (R) had higher fatigue resistance than the cryogenically treated specimens. The R specimen’s threshold intensity factor (ΔKth) was higher than the cryogenically treated specimens. The exponent (m) value of the R specimen was lower than the shallow and deep cryogenic treated specimens. Hence, the crack growth resistance was marginally higher in the R specimen compared to the cryogenically treated specimens. However, the critical stress intensity factor (ΔKcr) of cryogenically treated specimens was slightly higher than the R specimen. The post-fracture characterization of the specimens was carried out at three different regions by SEM. The cleavage faces, tear ridges, and voids were noticed in all specimens. The fatigue failure of the R specimen was due to ductile and brittle mode, whereas the cryogenically treated specimen was dominated entirely by brittle fracture.

Study of fatigue crack growth of al 6061-T6 welds obtained by gas metal arc welding along longitudinal direction

Study of fatigue crack growth of al 6061-T6 welds obtained by gas metal arc welding along longitudinal direction

An innovative approach to planar mixed-mode fatigue crack growth study

Determining fatigue crack growth parameters under mixed-mode loading is crucial for damage tolerance design, particularly in assessing a component's residual life with defects to prevent disasters. This study introduces a novel methodology for determining fatigue crack growth rate. A digital microscope is utilized to track the fatigue crack tip with respect to local coordinate systems established on the specimen surface. A grid is drawn and subdivided into units with proposed nomenclature. Compact Tension and Shear (CTS) specimens, extracted from AA-7085 plates, undergo fatigue loading in a servo-hydraulic universal testing machine. A fixture proposed by Richard et al. induces planar mixed-mode loading cases in the specimen. During experiments, the digital microscope tracks the crack tip using grid unit corners as local origins. The local coordinates of the crack tip are transformed into the crack tip coordinates with respect to the pre-crack tip. Crack evolution data are used to determine the fatigue crack growth rate by analyzing tangents of a three-parameter fitted curve. The equivalent SIF ranges are computed at different crack lengths in an XFEM script written in Ansys. The modified Paris parameters are found and experimentally observed crack deflection angles are compared with the Maximum Tangential stress, Strain energy density, and Richard’s criterion.

Numerical Study of Crack Prediction and Growth in Automotive Wheel Rims.

Finite element analysis has become an essential tool for simulating and understanding crack growth. This technique holds significant importance in the field of mechanical engineering, where it finds wide application in the design and optimization of structural components and material properties. This work began with the identification of critical zones and estimated the number of load life repeats through fatigue analysis, specifically applied to automotive rims utilizing innovative finite element methods. To investigate crack behavior, we are used the Extended Finite Element Method (XFEM) with the volumetric approach to compute the Stress Intensity Factor (SIF). The results obtained by our study align closely with experimental tests in terms of detecting the critical zone where a crack can appear. Our findings contribute to the understanding of fatigue behavior in automotive rims, offering new insights into their structural integrity and performance under various load conditions.

A fractographic study of fatigue crack growth from a cold expanded fastener hole at an engineering “crack initiation” scale

AbstractThe effects of cold expansion on fatigue crack growth from fastener holes in aircraft structure are a well‐studied problem, with many contributions focusing on both residual stresses and “long crack” experimental data. In this study, the authors have used quantitative fractography techniques to measure fatigue crack growth from naturally occurring discontinuities on an engineering “crack initiation” scale. Fatigue crack growth rates were measured through the full range of crack depths, allowing the effects of beneficial residual stresses to be directly observed. The almost identical early crack growth rates in cold expanded and non‐cold expanded specimens were demonstrated up to crack depths of approximately 0.1–0.2 mm. Beyond this depth, fatigue crack growth in cold expanded specimens was markedly slowed to the point at which failure ultimately arose from cracks away from, or growing into, the fastener hole. The available life improvement factor for cold expansion is therefore maximized through a crack growth‐based lifing approach.

Dual boundary element method for comparative studies on fatigue crack growth models

Fatigue crack growth studies require models that accurately predict component life with low uncertainty. Despite the large number of proposed models, there is no clarity on their applicability, which justifies a comparative analysis between some of them. The dual boundary element method (DBEM) was applied for cracked bodies, whereby the stress intensity factors (SIF), the growth rate, and the number of cycles were computed. Three crack increment models were studied under constant amplitude fatigue loads: the Paris, the Klesnil-Lucas, and the Forman models. Results were validated with experimental literature and through the finite element method, indicating that each model represents a specific zone of the crack growth curve. Klesnil-Lucas model reproduces the region near the fracture threshold, Paris fits the controlled crack growth zone, whereas Forman’s model recreates the unstable fracture zone, i.e., when the stress intensity factor approaches the material’s fracture toughness. The J-integral with stress field decomposition gave errors below 0.8% for mode I. Results were similar for the propagation path and the number of cycles to those obtained with the finite element method, with errors of about 3% considering different K-effective approaches. Klesnil-Lucas accurately predicts the number of cycles with an error margin below 3%, considering the curved region in the growth rate at the propagation onset, while the Paris model becomes very conservative, predicting values up to 50% lower than experimental data. The Klesnil-Lukas model is advised for simulating the entire crack propagation.

Residual life assessment of deep rolled railway axles considering the effect of process parameters

Deep rolling is an efficient technique to increase the fatigue life of railway axles. The application introduces compressive residual stresses into the area near the surface, which are the most effective positive influence on fatigue behaviour and remaining service life in presence of a crack. This paper investigates the remaining service life of deep rolled railway axles. Previous research studies are used as basis for this work whereas, firstly, an elaborated crack growth model for common railway axle steels was developed. This model is based on experimental crack propagation results and can consider crack closure as well as load sequence effects and local residual stress conditions. Secondly, a numerical deep rolling simulation model validated with experimental residual stress measurements to determine the introduced residual stress state and to investigate the influence of various process parameters. In this study, the comprehensive outcome of both studies is connected in order to evaluate and compare the residual life of railway axles with the presence of different deep rolling-induced residual stress distributions in-depth. The crack growth study involves combinations of the deep rolling force as one of the most influencing process parameters, load scenarios as well as initial crack sizes and finally discusses the comprehensive results.

Investigating the fracture toughness of the self compacting concrete using ENDB samples by changing the aggregate size and percent of steel fiber

In reality, concrete structures are normally under various loadings, and results of different studies have shown that cracks in these structures and their materials, due to their nature as well as the loading type, do not develop along the crack plane (pure mode I); rather, they expand under mixed modes, making the crack growth studies under these modes a very important issue. In the crack growth phenomenon, the fracture toughness is a very effective parameter usually calculated by ENDB samples because they are easy to handle. In this study, several samples were made by changing the maximum aggregates size (dmax = 9.5, 12.5 & 19 mm) and the amount of hooked-end steel fibers (SF = 0.1, 0.3 & 0.5%), and tested under different loading modes (pure/mixed modes I and III) using the strain control jack device. According to the results, the lowest fracture toughness belonged to pure mode III, aggregates with dmax = 12.5 mm performed better in the self-compacting concrete reinforced with steel fiber, Also, the results show that the increasing trend of steel fibers does not have a positive effect on the fracture toughness performance.

Phase field study of crack growth in t′ yttria stabilized zirconia with initial domain structures

In this paper, the fracture behavior of t′ yttria stabilized zirconia (YSZ) with initial domain structures is studied with the phase field (PF) method. Firstly, a PF model is proposed to characterize the initial domain structure and the subsequent domain switching in YSZ. After that, the PF model for domain structure evolution is coupled with the variational PF fracture model to simulate crack growth in the multi domain YSZ. The toughening effect induced by the ferroelastic domain switching is evaluated by computing the energy dissipation rate (EDR) along with the crack growth. It is shown that the internal stress field of the initial domain structure has very trivial influence on the crack growth resistance. However, the domain switching in the crack growth process can have significant influences on the fracture resistance. The domain structure evolution can bring about a local toughening or deshielding effect on the crack growth, depending on the crystal orientation. The underlying mechanisms are analyzed in detail. The proposed PF approach is efficient to study the mesoscale fracture behavior of multi domain YSZ and is helpful to assess the integrity of thermal barrier coatings.

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.

Crack growth study of wood and transparent wood-polymer composite laminates by in-situ testing in weak TR-direction

Crack growth study of wood and transparent wood-polymer composite laminates by in-situ testing in weak TR-direction

Study of stress-driven cracking behavior and competitive crack growth in functionally graded thermal barrier coatings

Thermal barrier coatings' (TBCs) stability and toughness are essential for insulating aero-engine parts. However, during service, the thickness of thermally grown oxide (TGO) will have an impact on the growth of internal vertical and horizontal cracks as well as the cracking and spalling of the coating, which may ultimately fail as a result of the aforementioned factors. The competitive growth of vertical and horizontal fractures in three functional gradient thermal barrier coatings (FG-TBCs) created with varied gradient index P magnitudes is investigated in this study. Gaining a thorough grasp of how competitive growth affects coating failure under various compositional and gradient structural parameters is the goal. Additionally, this work explores the effects of various TGO thicknesses on early crack expansion as well as the stress characteristics within the FG-TBCs. The research takes into account how crack growth changes the stress condition as well as the implications of crack extension rate, route, and form on coating failure. Our results show that the stress step phenomena in the FG-TBCs occurs in various places for various P values. The expansion of cracks in the top coat (TC) is fueled by this stress phase. Furthermore, a greater vertical crack extension length is obtained with a higher P value, with further expansion being prevented by horizontal cracks along the extension path. Notably, thicker TGO coatings change the direction and rate of expansion of cracks, both vertical and horizontal.

Studying Crack Formation in Catalyst Layer Films for Proton Exchange Membrane Fuel Cells

The performance and durability of proton exchange membrane fuel cells (PEMFCs) can be affected by cracks formed in catalyst layers (CLs). While previous research has shown that CLs prepared with pre-commercial high oxygen permeability ionomer (HOPI) are more efficient and durable, they can lead to significantly more cracking features than CLs prepared with traditional perfluorosulfonic acid (PFSA) ionomer. Therefore, it is important to reduce cracking to further improve cathode performance, durability, and film quality. With this goal in mind, we analyzed ink drying and crack formation during the wet-film coating process via imaging with an optical microscope fixed above a film applicator. Transient quantitative crack growth studies were performed to evaluate the dynamic nature of the process. Alongside that, the crack density of post-coating films was quantified at higher resolutions using a compound microscope. Ink composition, Pt-based catalysts and relative humidity of drying environment were varied in order to understand which combinations resulted in fewer cracks.This material is based upon work supported by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office, Award Number DE-EE0008822.

A study on the effect of different Paris constants in mixed mode (I/II) fatigue life prediction in Al 7075-T6 alloy

Mixed-mode (I/II) fatigue crack growth experiments in Al 7075-T6 alloy have been performed using a newly proposed single edge cracked circular (SECC) specimen. An equivalent stress intensify factor (ΔKeq) model based on the generalized MTS criterion (GMTS) has been tested for the first time for mixed-mode fatigue crack growth rate correlation and prediction of the fatigue life. Along with this proposed model, other widely used ΔKeq models have also been considered to study the effect of Paris constants on the fatigue life prediction. Three different mode mixity configurations (30°, 45°, and 60°) have been considered for experiments with multiple specimens to study the reproducibility aspect. With the help of detailed fatigue crack growth finite element simulations and experimental results, the performance of the various ΔKeq models has been tested in terms of correlation with the fatigue crack growth rate and prediction of the fatigue life. The results of the present investigation show that using the mixed-mode Paris constants along with Tanaka’s and Richard’s ΔKeq models accurately and consistently predicts the fatigue life compared to other Paris constants. The GMTS criterion has been shown to exhibit excellent performance in the prediction of crack growth path and thus signifies the importance of T- stress under mixed-mode fatigue loading. Additionally, the results of the present investigation establish the potential application of the SECC specimen for mixed-mode fatigue crack growth studies of metallic materials. Furthermore, fractographic studies also confirm the experimental observations in Al 7075-T6 alloy.

Bridging macro to micro-scale fatigue crack growth by advanced fracture mechanical testing on the meso-scale

Fatigue and fracture mechanics testing is strictly regulated by standards that specify a minimum specimen size. However, when component sizes or material volume decrease below this limit, test methods need to be adapted for small scales. In fatigue crack growth studies, experiments become complex and challenging. Thus, there is lack of systematic scale-bridging studies from meso- to micro-scales on ductile materials. This study outlines a method to bridge this scale gap by performing meso-scale studies. Cantilver bending specimens with a cross section of 50x50µm2 and a lever arm length of 200µm of nanocrystalline nickel, as a model material, are milled by Xe plasma focused ion beam technique and tested following ASTM E647 standard. The gathered da/dN-curves are highly reproducible. The threshold stress intensity factor ΔKth is obtained by step-wise load reduction and discussed in context of crack closure. So, not only the gap between macro and micro-scaled testing was bridged, but also a pre-cracking step was shown to be essential for obtaining reliable data, and a new method to determine the intrinsic threshold stress intensity factor was introduced.

Prediction of SIF range for plain API 5L Grade X65 steel under corrosion using AI & ML models

Corrosion is one of the significant aspects of science & engineering and the effects of corrosion on various infrastructures due to fatigue loading are paramount. Experimental investigations to evaluate fracture parameters and fatigue crack growth life take a considerable amount of time and effort. Recently, the computational boon in the form of Artificial Intelligence (AI) and Machine Learning (ML) has paved the way for predicting the required fracture parameters through training with the available limited experimental data. In this paper, the fracture parameter, Stress Intensity Factor (SIF) of plain API 5L X65 grade steel is predicted using the experimental data of crack growth studies performed on the specimen with single edge notch, eccentrically loaded, subjected to inert and corrosive environments (2.0% and 3.5% NaCl). From the experimental data, the fracture parameter, SIF range is determined for various crack lengths corresponding to certain loading cycles. Multivariate Adaptive regression splines (MARS), Artificial Neural Network (ANN), and relevance vector machine (RVM) based models are developed by using the experimental data to predict the SIF range. The efficiencies of the developed models are verified through several statistical parameters. The studies show that the proposed models are capable of predicting the SIF range for plain API 5L X65 grade steel effectively with the available experimental data. The predicted SIF range is highly useful for the prediction of crack growth life and the evaluation of the residual strength of structural components.

Numerical study on high-cycle fatigue crack growth of sinusoidal interface based on cyclic cohesive zone model

The numerical study of the high-cycle fatigue crack growth for the double cantilever beam (DCB) specimen with sinusoidal interface under remote mode-I loading is carried out based on the discrete interface element and the cyclic cohesive zone model (CCZM). Compared with the continuous cohesive element, this discrete interface element is more suitable for complex interface, easier to implement, and has better computational convergence. According to the summarized laws by the systematic researches, the general process of calibrating CCZM is proposed. The accuracy of proposed method is verified by the benchmark models representing different modes fatigue crack. In the simulation results of fatigue crack growth for sinusoidal DCB specimens, the as-N (as is the crack growth length and N is the number of load cycles) curves exhibit periodic oscillation behavior. The constructed rate model indicates that, the oscillation behavior intensifies with the increase of aspect ratio A/λ (A = amplitude, λ = wavelength). Moreover, the sinusoidal interface can delay fatigue crack growth, and this effect is more obvious with the increase of interface roughness. Due to the stress concentration at the wave crests and troughs of sinusoidal interface, the fatigue damage is easier to accumulate, which is the mechanism of fast and slow alternating propagation. In addition, the results of this paper also demonstrate that, the periodic oscillation behavior will be more intense when the materials on both sides of interface are inconsistent.

Equivalent Stress Intensity Factor: The Consequences of the Lack of a Unique Definition

The concept of an equivalent stress intensity factor Keq is used in the study of fatigue crack growth in mixed-mode situations. A problem seldom discussed in the research literature are the consequences of the coexistence of several alternative definitions of mixed mode Keq, leading to rather different results associated with the alternative Keq definitions. This note highlights the problem, considering several Keq definitions hitherto not analyzed simultaneously. Values of Keq calculated according to several criteria were compared through the determination of Keq/KI over a wide range of values of KI/KII or KII/KI. In earlier work on Al alloy AA6082 T6, the fatigue crack path and growth rate were measured in 4-point bend specimens subjected to asymmetrical loading and in compact tension specimens modified with holes. The presentation of the fatigue crack growth data was made using a Paris law based on Keq. Important differences are found in the Paris laws, corresponding to the alternative definitions of Keq considered, and the requirements for candidate Keq definitions are discussed. A perspective for overcoming the shortcomings may consist in developing a data-driven modelling methodology, supported by material characterization and structure monitoring during its life cycle.