Plasticity-induced Crack Closure Research Articles

Plasticity-induced crack closure identification during fatigue crack growth in AA2024-T3 by using high-resolution digital image correlation

Plasticity-induced crack closure identification during fatigue crack growth in AA2024-T3 by using high-resolution digital image correlation

A fatigue crack tip field model considering residual stress and plasticity-induced closure for welded structures

In order to more accurately understand and predict the fatigue crack growth behavior in welded structures, a fatigue crack tip field model considering residual stress and plasticity-induced closure (RSPC model) is presented in this paper. Building upon the existing CJP model, which adequately explains plasticity-induced crack closure, the RSPC model further considers the influence of tensile residual stress induced by welding. The model, through a detailed derivation and analysis of the residual stress function, establishes the theoretical stress and displacement fields at the crack tip of the butt-welded joint. Integrating these fields with the CJP model, the theoretical framework of the RSPC model is formulated. Subsequently, through fatigue crack growth experiments and digital image correlation analysis, a comparison and validation of the predictive performance of the CJP and RSPC models are conducted. It is observed that the latter aligns more closely with experimental results in calculating plastic zone area, with a maximum error of only 10%. In comparison to the CJP model, the average error decreases by 19.143%, indicating higher predictive accuracy. Furthermore, the study focuses on the contribution of tensile residual stress in butt welded joints to the growth of the plastic zone at the fatigue crack tip and the underlying reasons for its variations. Finally, the research investigates the constrained relationship between plastic zone size and fatigue crack growth rate. Corresponding relationship functions are fitted, providing more accurate guidance for crack control and material design.

Fatigue crack growth in AA6082-T6/AA7050-T6 bi-materials: Effect of plastic zone ahead of crack tip

The main objective here is the understanding of the effect of plastic zone ahead of crack tip on fatigue crack growth (FCG). For that, a numerical simulation of FCG was made, based on cumulative plastic strain, considering a bi-material composed by the 6082-T6 and 7050-T6 aluminium alloys. The first one has a lower yield stress but a higher critical cumulative plastic strain. The values of da/dN are higher for the 6082-T6 which means that the effect of the easier plastic deformation dominates over the effect of the higher critical cumulative plastic strain. The bi-material 6082-7050 showed a pronounced decrease of da/dN near the interface, which starts when the monotonic plastic zone ahead of crack tip reaches the material transition. This indicates that the crack tip deformation is affected by the material inside the monotonic plastic zone. On the other hand, the bi-material 7050-6082 shows an accentuated increase of da/dN, which starts when plastic deformation is observed ahead of transition. The elimination of the contact of crack flanks reduces drastically the transient effects, which indicates that these are linked with plasticity induced crack closure effects. Crack tip blunting was proposed to be the mechanism of plasticity induced crack closure explaining the trends observed.

A multi-scale experimental investigation on fatigue crack propagation rate behavior of powder bed fusion-laser beam 316L stainless steel subjected to various heat treatments

In this study, three heat treatments referred to as heat treatment 1 (HT1), HT2, and HT3 were applied to tune the microstructure of powder bed fusion-laser beam (PBF-LB) 316L stainless steel. Furthermore, the fatigue crack propagation rate (FCPR) of as-built PBF-LB 316L specimens and specimens subjected to the three aforementioned heat treatments was thoroughly investigated. To this end, the FCPR was evaluated by a fatigue testing machine combined with an infrared thermal camera, and the Paris law was established to quantitatively assess the FCPR of PBF-LB 316L. In addition, a systematic investigation of microstructural evolution, residual stress, tensile properties, and fatigue crack propagation (FCP) fracture surfaces as well as their effect on the FCPR in function of various heat treatments were carried out. The results indicate that the release of residual stresses, the annihilation of the cellular structure, and the occurrence of recrystallization are obtained through the application of HT1, HT2, and HT3 respectively. Furthermore, it is shown that although all three heat treatments can improve the FCPR behavior of PBF-LB 316L to some extent, HT3 PBF-LB 316L presents the lowest FCPR mainly due to its most pronounced plasticity-induced crack closure effect, indicating its highest resistance to FCP even though the recrystallization obtained by HT3 results in the lowest yield strength (YS). Finally, some recommendations about adequate heat treatments concerning fatigue life are suggested.

Simulation of crack closure effect on cracked beam vibrations

Vibrations of beams containing cracks are usually investigated by using open or breathing crack models. Neither one of these approaches considers the plasticity induced crack closure (PICC) phenomena which occurs in fatigue crack growth. In this study, PICC is integrated into the vibration analysis of a cracked beam. First, fatigue crack growth at a notch is simulated with elastoplastic finite element analysis. Residual deformations, stresses and the contact pressures on crack faces are obtained. In the second phase, the model which includes these quantities is used for forced vibration analysis under harmonic loading by using implicit finite element method. For comparison, analyses are also running without PICC. Simulation results indicate that PICC has a significant effect on vibrations of cracked beams. For low load amplitude levels, cracked beam behaves like an intact beam. Nonlinear behavior appears only at rather high amplitude levels. It is concluded that PICC could suppress the crack breathing behavior and this could make it difficult to detect a fatigue crack in a beam from its nonlinear vibration response.

Multi-scale characterization of fatigue crack tip deformation and propagation in QP steel

A novel measurement method for multi-scale characterization of fatigue crack tip deformation and propagation behaviors in high strength and plasticity, multi-phase, fine-grain Quenching and Partitioning (QP) steel was proposed. The fatigue crack propagation (FCP) test system with the macro/micro images acquisition devices was built to measure the FCP rate and crack tip deformation simultaneously. First, at the mesoscale, the evolution characteristics of the crack tip deformation fields, plasticity induced crack closure (PICC) behaviors and plastic zone during the FCP process were analyzed using the sub-image stitching and matching digital image correlation (DIC) method. Then, the FCP models were established with special emphasis on the model considering the PICC effect based on the obtained crack opening load at the mesoscale. Moreover, the corresponding microscale surface crack and fracture morphologies, microstructure phase transformation and deformations were observed and analyzed by electron backscatter diffraction (EBSD) and scanning electron microscope (SEM) techniques.

Plasticity-induced crack closure in the presence of loading irregularities in short cracks initiated at interior defects

A crack propagation finite-element model of interior defect-initiated short cracks under near-threshold loading conditions is used to investigate the effect of plasticity-induced crack closure in the presence of a single overload and underload. The effect of an overload predicted by the present model is qualitatively very similar to what is commonly reported, but in this case an underload produces a differing result to the literature consensus, yielding a similar but weaker effect as the overload. Extended acceleration of crack propagation due to a single underload might not be attributable to plasticity-induced crack closure under plane strain conditions. A distinct crack profile shape consequent of crack tip blunting is present with both loading irregularities. A new method for examining crack closure for the entire crack surface is presented.

Solution to the problem of low sensitivity of crack closure models to material properties

The paper focuses on differences between crack closure obtained by numerical models and by experimental fatigue crack growth rates, namely for three different steels (bainitic steel, predominantly pearlitic steel and additively manufactured austenitic stainless steel). The experimental data revealed a load ratio effect different from that predicted by the most used plasticity-induced crack closure (PICC) models. The term “irreversibility of plastic deformation” was proposed in this work to be used as a material property to estimate how strong the PICC effect would be in a material. Two basic phenomena, which are usually omitted in other models, were considered in the explanation: (i) cyclic softening/hardening, (ii) brittle microcracking at the maximum load. The strip-yield model has the ability of conducting fast and accurate simulations under the variable-amplitude loading. It was demonstrated that the results of this model are practically independent of the material properties. This is caused by the consideration of only monotonic elastic–plastic material properties. To simulate real load ratio effects, the parameter β in the strip-yield model (constraint factor in compression) is proposed to be used as a variable. It enabled a generation of different ratios of monotonic and cyclic plastic zones, which in turn helped to reproduce the crack closure values observed experimentally. Discussion and explanations were provided regarding the material microstructure. The proposed approach considers unequal tensile and compressive yield stresses caused by the different irreversibility of plastic deformation, which explains the dissimilarities in the load ratio effect observed in the investigated materials. It can also improve the accuracy of residual fatigue life estimations.

An investigation into the influence of variable amplitude creep-fatigue loading on plasticity-induced crack closure

Finite element simulations of fatigue crack growth (FCG) and creep-fatigue crack growth (CFCG) under constant amplitude (CA) and variable amplitude (VA) loading waveforms are performed to investigate the mechanics of crack opening and closing, which has implications on crack growth rate. The material of study is the ferritic-martensitic steel 9Cr-1Mo (modified 9Cr-1Mo) at 625 °C. Two-dimensional finite element analyses of compact tension (CT) specimens are performed to simulate crack growth under several VA loading scenarios, considering elastic–plastic and creep deformations of the material at the crack tip. In this study, the relevant variable influencing crack growth rate is the crack opening load, caused by the phenomenon of plasticity-induced crack closure. A series of loading conditions were simulated using the finite element method, i.e., fatigue overload (OL) in either CA FCG or CFCG waveforms, creep-fatigue OL in CFCG waveforms, and underload (UL) in CFCG waveforms. The crack opening loads obtained from this study for CA loading in both FCG and CFCG, agree with data from published studies. This study also found that a significant reduction in crack opening loads is experienced by the growing crack when an OL or UL cycle was introduced. Immediately after the OL or UL cycle is applied, the crack opening load increases during both fatigue OL and creep-fatigue OL to values larger than those recorded during CA FCG and CFCG loading, whereas UL waveforms result in consistently lower crack opening loads. The increase in crack opening loads is proportional to both the extent of OL and the duration of hold time in the OL cycle. However, following the application of a UL, the crack opening loads in the subsequent cycles were not influenced by the magnitude of the UL or the duration of hold time in the UL cycle.

Effects of stress ratio on plasticity-induced crack closure through three-dimensional advanced numerical finite element models

The plasticity induced crack closure (PICC) is a local phenomenon that substantially influences crack growth. PICC is most commonly determined by numerical models. Specifically, the vertical displacements of the nodes behind the crack front have been extensively used to obtain the PICC and the crack opening loads (Pop/Pmax). Although this procedure has been demonstrated to be adequate for positive stress ratios, this work evidences the uncertainty of such procedure to obtain PICC under negative stress ratios. For this reason, the main objective of this paper is to investigate the effect of stress ratio on PICC by 3D advanced numerical methodology. The purposed method consists of evaluating the contact pressures of all nodes behind the crack tip, from the first to the last released row of elements included on a crack propagation finite element model. Moreover, this method is tested at positive stress ratios and results show a good agreement with the CTOD method.

Small crack propagation and closure behaviours and mechanisms in a powder metallurgy superalloy at high temperature in air

To investigate small crack propagation and closure mechanisms in a powder metallurgy superalloy FGH4096 at room and high (600 °C) temperatures in air, meso-scale digital image correlation (DIC) was applied. CODs of small cracks did not increase monotonically as cracks propagated, which was related to the potential role of subsurface crack morphology. Roughness induced crack closure was found to be the leading mechanism at times. Oxidation induced crack closure was negligible at 600 °C, resulting from the oxidation-resistance of FGH4096. Crack opening stress intensity factors were generally higher at 600 °C, which ascribed to the higher level of plasticity induced crack closure.

Mixed mode crack growth behaviour considering plasticity-induced and roughness-induced closure

A theoretical analytical solution of the Christopher-James-Patterson (CJP) model in mixed mode is presented in this paper, where the influence of roughness-induced crack closure (RICC) on fatigue crack growth is considered and a new effective stress intensity factor range (ΔKP-R) is proposed as the driving force for crack propagation. The solution showed that both the maximum tangential stress criterion (MTS) and the minimum strain energy density criterion (S) can reasonably well predict the crack deflection angle. Experimental testing on EA4T axle steel samples showed that both the 8 % compliance offset criterion in the conventional ASTM procedure and the parabolic-line method were suitable for the determination of the crack-opening force. The solution includes plasticity-induced crack closure (PICC) and RICC effects, therefore the crack propagation behaviour can be described in a more effective way. Compared with the traditional mixed model crack growth models with higher fitting correlation coefficient, the fitting effect of the CJP model has been improved by 12 % in this solution, and the residual stress in the pre-crack stage increased the crack-closure level. Due to the influence of residual stress, the rate of roughness value (RA) showed a fluctuation of 4.5 %, which indirectly affected the fatigue crack growth rate (FCGR) of the test.

The cyclic R-curve method for predicting fatigue crack growth threshold based on modified strip-yield model of plasticity-induced crack closure under fully reversed loading

A strip-yield model modified to leave plastically deformed stretch in the wake of the advancing crack tip is applied to simulate the buildup of plasticity-induced crack closure with crack extension from precracks with various lengths under fully reversed loading. Two types of precracks are examined: open precracks which remain open even under cyclic compression, and closed precracks which may close under compression. The effects of precrack type and length on fatigue thresholds were systematically examined. Because of a sharp increase of crack opening stress with crack extension, the effective stress intensity range ΔKeff drops initially and then turns to increase. Assuming a constant threshold value of ΔKeff for steels, new master curve equations are proposed for the cyclic R-curve. The effect of the precrack type on fatigue thresholds is successfully derived for different precrack lengths.

Hydrogen-Accelerated Fatigue of API X60 Pipeline Steel and Its Weld

In this work, the hydrogen fatigue of pipeline steel X60, its girth welds and weld defects were investigated through in situ fatigue testing. A novel in situ gaseous hydrogen charging fatigue set-up was developed, which involves a sample geometry that mimics a small-scale pipeline with high internal hydrogen gas pressure. The effect of hydrogen was investigated by measuring the crack initiation and growth, using a direct current potential drop (DCPD) set-up, which probes the outer surface of the specimen. The base and weld metal specimens both experienced a reduction in fatigue life in the presence of hydrogen. For the base metal, the reduction in fatigue life manifested solely in the crack growth phase; hydrogen accelerated the crack growth by a factor of 4. The crack growth rate for the weld metal accelerated by a factor of 8. However, in contrast to the base metal, the weld metal also experienced a reduction of 57% in resistance to crack initiation. Macropores (>500 µm in size) on the notch surface reduced the fatigue life by a factor of 11. Varying the pressure from 70 barg to 150 barg of hydrogen caused no difference in the hydrogen fatigue behavior of the weld metal. The fracture path of the base and weld metal transitioned from transgranular and ductile in nature to a mixed-mode transgranular and intergranular quasi-cleavage fracture. Hydrogen accelerated the crack growth by decreasing the roughness- and plasticity-induced crack closure. The worst case scenario for pipelines was found in the case of weld defects. This work therefore highlights the necessity to re-evaluate pipelines for existing defects before they can be reused for hydrogen transport.

Fatigue crack growth behavior of a nanocrystalline low Young's modulus β-type Ti–Nb alloy

Low Young's modulus β-type titanium alloys, such as the Ti–45Nb alloy, with elastic properties close to that of human bone containing non-toxic elements are very desirable for load bearing implants. Their relatively low hardness and strength can be significantly increased by severe plastic deformation (SPD) methods, for example high pressure torsion (HPT). To gain a better understanding of the fatigue characteristics, the fatigue crack growth (FCG) behavior of Ti–45Nb was investigated in various microstructural states (as-received, HPT-deformed, HPT + cold-rolled, HPT + heat-treated). Compact tension (CT) specimens with different crack orientations were tested at different load ratios under tension-tension loading. The microstructure of the HPT and HPT + cold-rolled material shows highly elongated grains parallel to the principal deformation direction with grain sizes in the nanocrystalline regime. Compared to the majority of studies on FCG of SPD-processed materials this titanium alloy exhibits a remarkably weak anisotropy in the FCG-behavior in all material states but especially in the HPT and the cold-rolled state. This is mainly attributed to the transcrystalline fracture mode. The threshold of fatigue crack growth was found to decrease with increasing load ratio. This is related to a lower contribution of plasticity-induced crack closure at higher load ratios.

Novel Characterizations of Effective SIFs and Fatigue Crack Propagation Rate of Welded Rail Steel Using DIC

The fatigue crack growth-rate test of rail head, waist, and bottom material for U71Mn welded rail was carried out. Digital image correlation (DIC) was used to capture the full-field displacement. The crack-tip position was accurately obtained based on the full-field displacement data, and an accurate crack-tip opening displacement (CTOD) measurement point was found. The CTOD values of the welded rail head under overloaded and unloaded condition were extracted, and the area size of elastic CTOD and plastic CTOD was obtained. According to COD data under different experimental conditions, the corresponding crack opening force was extracted, the crack opening function introduced based on the Elber model, and a calculation method of effective stress-intensity factors (SIFs) considering the plasticity-induced crack closure proposed. The results in this paper provide some references for accurately assessing the fatigue life of welded rail.

Residual opening of fatigue cracks due to the wake of plasticity

The concept of a breathing crack and the clapping mechanism, when a crack periodically opens and closes subject to a cyclic or harmonic excitation, are widely utilised in studies concerning contact acoustic nonlinearity (CAN) and nonlinear phenomena. The investigation of these phenomena is particularly important for the development of reliable non-destructive fatigue crack detection techniques. The aim of the current contribution is to demonstrate the characteristic behaviour of a fatigue crack under excitation loading using a simple analytical model that accounts for the wake of plasticity behind the crack tip, which leads to the plasticity-induced fatigue crack closure phenomenon. It is believed that incorporation of these features and more realistic behaviour of fatigue cracks can help to improve the methods for the detection of nonlinear phenomena associated with CAN. The results of the present work indicate that ultrasonic inspections based on the CAN using a bilinear stiffness model may underestimate the length of the detected crack(s) due to the plasticity-induced crack closure phenomenon. Despite many modelling assumptions, the present work can serve as a starting point for the development of accurate models for different crack geometries and load histories. The latter may help to develop effective nondestructive techniques for detection and identification of fatigue cracks and prevention of fatigue failures.

The role of plasticity-induced crack closure in the non-propagation prediction of surface defect-initiated cracks near fatigue limit

The purpose of present work is to deepen the understanding of crack propagation, non-propagation, and fatigue limit. Quantitative prediction of fatigue crack non-propagation in constant amplitude loading is attempted through extensive 3D elastic–plastic finite element analysis of near-threshold propagation of short cracks nucleating from surface defects. Level of plasticity-induced crack closure is used to estimate the effective stress intensity factor range, which is evaluated in terms of non-propagation potential. It is shown that plasticity-induced crack closure alone is adequate in producing a local minimum in the development of effective stress intensity factor range, which gives a prerequisite to crack arrest.

Fatigue resistance of the binder jet printed 17-4 precipitation hardened martensitic stainless steel

The unnotched fatigue behavior of the 17-4 PH martensitic (α’) stainless steel, additively manufactured using the binder jet printing technique (BJP), with varying porosity levels and in two different aging conditions are investigated and are compared to that of the conventionally manufactured (CM) alloy. As expected, fatigue strength is enhanced by the reduction in porosity, with hot isostatic pressing of the BJP alloy resulting in a fatigue strength that is similar to the CM alloy. Over-aging of the alloy improves its fatigue resistance further, which is caused by an enhancement in the threshold stress intensity factor range for fatigue crack initiation. These variations are rationalized by recourse to the analysis of the microstructure-crack interactions. It was found that the combination of plasticity induced crack closure mechanisms and transformation induced plasticity are dominant at the fatigue crack tip of the over-aged alloy. Fracture mechanics-based Kitagawa and Takahashi diagrams were utilized to illustrate the significance of pore size on the unnotched fatigue resistance of BJP 17-4 PH alloys. These results are discussed in terms of designing alloy fabrication using BJP specifically for cyclic loading conditions.

A 2D numerical modelling of plasticity induced crack closure on MT specimens

In many engineering fields, structures are often subjected to fatigue loading that initiate and propagate cracks. The development of fracture mechanics studies has enabled the tolerance of a substantial amount of damage and the ability to predict failure in fatigue damaged structures, extending their service life safely. This can bring social impacts from economics, public-safety, or an environmental point of view. These damage tolerance analyses can be performed using linear elastic fracture mechanics (LEFM), where the Stress Intensity Factor (SIF) is a chief key, or under elastoplastic fracture mechanics (EPFM), where plasticity induced crack closure (PICC) plays a major role on fatigue crack growth (FCG). This study deals with the computational modelling developed consisting in the simulation of a two-dimensional model to predict FCG under PICC conditions. Extremely fine meshes were generated around the crack tip, in order to predict PICC loads, SIF and plastic zone size. The results showed good agreement with the experimental model, allowing for accurate damage tolerance assessments.