Crack Growth Retardation Research Articles

Effect of laser heating on mechanical properties, residual stresses and retardation of fatigue crack growth in AA2024

AbstractLocal laser heating treatment using a defocussed laser beam was applied to the surface of 2‐mm‐thick AA2024‐T3 sheets. Two different treatment designs—namely, lines and circles—as well as the positioning and number of treatments were investigated regarding their potential to retard the propagation of through‐thickness fatigue cracks. The highest fatigue crack growth life extension of 285% was achieved by the application of four laser heating lines or two circles on each specimen side. The induced compressive residual stress field through the LH process is primarily responsible for an improvement in fatigue crack growth life. An emphasis was placed on investigating the effect of the treatment on the possible reduction of tensile and fatigue strength (S‐N life). If only one line was applied transverse to the loading direction or only one circle was used, the reduction of fatigue strength was comparable to the reduction of fatigue strength resulting from the stress concentration introduced due to the presence of a rivet hole.

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

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

Effect of periodic overloads and spectrum loading on fatigue life and microstructure in a Grade 900A rail steel

Fatigue life and crack growth retardation due to spectrum loading, which are superposed on constant amplitude cycles, have been investigated in this paper. For this purpose, numerical and experimental methods are used to investigate the behavior of fatigue crack growth in the rail specimens. A 3D nonlinear boundary element model has been applied to estimate the fatigue life of the rail specimens. Then, the effect of several parameters on the fatigue life in rail specimens is investigated using the suggested 3-D boundary element model and experimental tests. In the numerical analysis, the modified Paris model has been used. The analysis and experimental tests have been performed on C(T) specimens made of a pearlitic Grade 900A steel. In the experimental tests and numerical analysis, the periodic overloads have remained constant and the effect of overload ratio (OLR) and occurrence ratios (OCR) on the fatigue life has been investigated. Finally, microstructure analysis on microstructure morphology of the region where cracks grow for specimens was performed. Comparison between experimental results and numerical data indicates that the modified Paris model and the Forman_Newman_De Koning (FNK) model is actually accurate.

Dynamic crack propagation in the turtle carapace

The turtle carapace is a biological composite shield consisting of a hard boney layer covered with a soft keratin-collagen bi-layer. The unique microstructure enables the turtle carapace to protect the underlying soft tissues from predator assaults. Recent experiments have shown that when the turtle carapace is subjected to dynamic loading, brittle fracture of the boney layer occurs, which is accompanied by delamination at the keratin-collagen and collagen-bone interfaces. To unveil the role of interfacial delamination, we propose a model of fracture in the multilayered microstructure of turtle carapace, in which brittle cracking in the boney layer, delamination of the soft bi-layer and plastic deformation in the bi-layer are taken into account. Calculations are carried out for crack propagation in the turtle carapace under dynamic loading; it is found that low strength of the keratin-collagen interface can activate large plastic deformation in the collagen layer and promote spreading of damage region in the keratin-collagen interface, which delays fracture of the boney layer. High toughness of the keratin-collagen interface increases plastic deformation in the soft bi-layer and suppresses crack propagation in the boney layer. We further reveal the role of mechanical properties of the collagen-bone interface. It is identified that high strength of the collagen-bone interface enables enhanced plastic dissipation in the soft bi-layer, retarding crack growth in the boney layer. The collagen-bone interface with high toughness plays a role in enhancing plastic deformation in the soft bi-layer, which potentially increases the resistance to crack growth in the boney layer. In addition, we explore the effect of strain rate sensitivity of the soft bi-layer. We show that decreasing the degree of strain rate sensitivity of the keratin layer can mitigate fracture of the boney layer. The findings of this study shed new light on the protection mechanisms of turtle carapace.

Experimental Study on the Fatigue Crack Growth and Overload Effect in Medium Density Polyethylene

The fatigue crack growth behavior of medium-density polyethylene (MDPE) under constant amplitude load was experimented on plate specimens. The effect of additional imposed single overload was also researched. An optimized crack front marking method was developed to obtain the actual crack front shape of polymer materials in fatigue crack growth, which was successfully applied to the studied MDPE. The results showed that the retardation of fatigue crack growth evidently appeared after the tensile overload was applied, which was similar to metallic materials. The passivation of the wedge-shaped plastic zone was a primary cause to crack retardation. The theory based on residual stress was introduced to describe the retardation effect, and the plastic zone size of crack tip was calculated by the Dugdale model. The Wheeler model was successfully used to fit the decreased crack growth rate.

An interaction integral retardation model for predicting fatigue life under multi-step loading

This paper proposes an interaction integral retardation (IIR) model to predict fatigue life accurately for components subjected to multi-step loading (MSL) conditions. The IIR model is based on the cyclic interaction integral that considers the effect of elasticity and plasticity simultaneously at the crack tip. Elasto-plastic numerical simulations were carried out based on FEM to compute the cyclic interaction integral. The IIR model was verified against the experiments under different loading conditions including constant amplitude loadings, single tensile overloads, and MSL for different steel strengths. The fatigue lives calculated by the IIR model exhibit a very good agreement with those obtained by the experiments for the applied loading conditions and steels. The efficiency of the IIR model was manifested by comparing the calculated fatigue lives against those computed by different traditional and well-established fatigue crack growth (FCG) models. For the MSL condition, fatigue lives obtained by the IIR model revealed a better agreement against the experiment compared to those given by the traditional FCG models. The crack-tip driving force represented by the elasto-plastic cyclic interaction integral could explain the mechanics of crack growth retardation and acceleration due to the change in the applied loading. Further, the local material behavior ahead of the crack tip was examined rigorously for different crack lengths under variable cyclic loadings. It was found that the local material behavior has a significant influence on the state of crack growth (retardation/acceleration) which results in accelerating or retarding the crack growth rate.

Effectiveness of the ultrasonic impact treatment in the retardation of the fatigue crack growth for 2024-T3 Al alloy components

Previous research has investigated the possibility of improving the retardation of the fatigue crack growth (FCG) by means of different treatments and techniques. Ultrasonic impact treatment (UIT) emerges as an alternative technique to improve the resistance of metallic components, mainly developed in materials after welding. Experiments on 2024-T3 aluminium alloys are described to determine the effect of the UIT in the fatigue crack growth. Two working conditions were studied varying three parameters of the UIT process, which were amplitude under load, impact frequency and feed rate, while the operating frequency of the ultrasonic generator was set to 36 kHz. The standard M(T) geometry specimen was chosen to be employed in fatigue testing to determine the crack length, crack growth rate and the crack front shape by means of an optical technique. Results suggested that the manipulated UIT parameters allow the crack growth retardation to the 2024-T3 Al alloys. Findings also suggested that the less severe treatment, the more crack growth retardation. Treatment duration and amount of impacts during treatment are two key parameters to optimize in order to find better improvements in this direction.

Effect of loading sequence on fatigue crack retardation of misaligned laser welded T-joints

This paper investigates the effect of overload and misalignment on crack propagation of laser welded T-joints. The results showed that the crack growth rates of these joints are like those of base material. For single overload, the increase of ΔK from 8.5 to 13 MPam1/2 multiplied the average retarded crack growth rate by 7.5 for R = 0.4. The highest crack retardation was caused by multiple overloads with overload ratio of 2 and interval of 10,000 cycles between overload peaks. According to fractographies, the size of plastic zone ahead of the crack tip and the crack branching governed for crack retardation.

Near-tip strain evolution and crack closure of growing fatigue crack under a single tensile overload

Fatigue crack growth retardation of AZ31 alloy under a single tensile overload was investigated by 3D synchrotron radiation, 2D in-situ SEM and digital image correlation techniques. Results showed a consistent evolution of crack closure level by both 3D and 2D methodologies. The straining and plastic zone ahead of crack-tip, and crack opening displacement behind, which were quantitative indicators of crack-tip shielding, residual stress formation and relaxation, were correlated well with the retardation behavior. Mechanistically, compressive residual stress played a dominant but time-limited role while crack closure was secondary, based on an intrinsic load transfer concept during crack growth.

Evaluation of fatigue crack propagation in steel ESET specimens subjected to variable load spectra

This paper reports on an experimental study of fatigue crack growth in steel specimens. First, block loading tests (sequences of low and high stress intensity factor ranges ΔK) are discussed. Limited crack growth retardation occurs at transitions from low to high load ranges; significant retardation or crack arrest are observed in high-low transitions. Next, semi-random load spectra are created, processed using a peak-and-valley analysis and further reduced by removing the load ranges below the stress intensity factor threshold ΔKth. Rainflow counting is performed to obtain load profiles consisting of a sequence of blocks with constant ΔK. For the semi-random and the (reduced) peak-and-valley spectra no significant load interaction is observed. Pronounced crack growth retardation is observed in an ordered spectrum obtained by rainflow counting. The strong reduction in number of cycles of the (reduced) peak-and-valley spectra allows for exploration of accelerated fatigue testing. Experimental results of fatigue crack propagation are compared to results of calculations using a Python based numerical framework.

Fractographic analysis of the overload effect on fatigue crack growth in 2024-T3 and 7075-T6 Al alloys

The effect of single overload on the fatigue crack growth in 2024-T3 and 7075-T6 Al alloys was analyzed. Fatigue tests under constant-amplitude loading with overload peak were carried out on V-notched specimens. Fractographic analysis was used as a principal approach to explain the crack growth retardation due to the overload. Scanning electron microscopy (SEM) analyses were conducted on the fractured surface of failed specimens to study the retardation effect. The obtained results show that the overload application generates a plastic zone in both aluminum alloys. The generated plastic zone is three times larger in the case of 2024-T3 compared to 7075-T6, and thus, a significant crack retardation was induced for 2024-T3. The retardation effect due to the overload for 2024-T3 and 7075-T6 lasted for about 10 mm and 1 mm, respectively, from the point of overload application.

Fatigue crack growth behaviour in Ti6Al4V alloy specimens produced by selective laser melting

The current study presents the fatigue crack growth behaviour of titanium alloy Ti6Al4V parts manufactured by selective laser melting (SLM), obtained as standard 6 mm thick compact specimens (CT). Both the crack propagation under constant amplitude loading and the transient crack growth behaviour after the application of overloads were studied. The effect of the mean stress and the transient retardation behaviour were analysed using the crack closure parameter, obtained both by compliance and digital image correlation techniques. A reduced crack closure level for the stress ratio $$\hbox {R}=0$$ was detected and for $$\hbox {R}=0.4$$ no crack closure was observed. The digital image correlation technique showed better results in the Paris regime and during the transient retardation behaviour. The overload application produced crack growth retardation due to the increase of the crack closure effect. The failure surfaces showed a transgranular crack growth in $$\upbeta $$ phase contouring the martensitic $$\upalpha $$ phase.

Numerical simulation of quasi-static and fatigue debonding growth in adhesively bonded composite joints containing bolts as crack stoppers

ABSTRACT In this paper, the cohesive zone modeling (CZM) method has been employed to simulate static and fatigue debonding growth in adhesively bonded composite joints containing bolts as crack stoppers. The cracked-lap shear specimen (CLS) for static and fatigue loads and the wide single-lap shear (WSLS) structural element for static load have been modeled. In both cases, mixed-mode I+ II loading conditions are present in the bondline. Bonded and bonded/bolted configurations of the specimens have been analysed in order to evaluate the efficiency of bolts as crack stoppers. Numerical simulations have been performed using the LS-DYNA FE code. For the static debonding simulation, a standard bilinear traction-separation law has been applied while for the simulation of fatigue debonding growth a modified traction-separation law has been implemented using experimental data derived from fatigue tests. The modification has been realized by means of a user-defined subroutine (UMAT) having the capacity to account for crack divergence by making use of a crack front detection algorithm based on effective element’s length. A parametric study on the effect of distance between bolts and crack-tip has been performed. Additionally, the WSLS structural element has been subjected to impact load aiming to create a realistic initial disbond scenario. The residual strength of the element has been predicted by applying a quasi-static load after impact. The numerical results reveal that the presence of bolts under certain circumstances can cause crack growth retardation, under both static and fatigue loading conditions. This study emphasizes on the importance of fatigue crack growth simulation and aims to contribute to the numerical design of crack stopping features as a means of complying towards certification of adhesive bonding in primary composite aircraft structures.

Two engineering models for predicting the retardation of fatigue crack growth caused by mixed mode overload

The effect of mixed mode overload on the retardation of pure mode I cyclic loading has been investigated experimentally on Al5083-H111. Two models for extension of the Wheeler retardation model are proposed in order to calculate retardation of fatigue crack growth subjected to mixed mode overload. The first proposed model can predict retardation with high precision. The second proposed model is a simplified form of the first one and does not contain more experimental constants than the Wheeler model so, it may be more practical. The application of the proposed models has been examined for another material and testing conditions.

Fatigue Growth Behaviour of Two Interacting Cracks with Different Crack Offset.

Under cyclic fatigue load, multiple cracks would significantly deteriorate the service life of the components with respect to the case of a single crack owing to the crack interaction. The present study aims to explore the effect of crack interaction on the fatigue growth behaviour of samples with different crack offset. In this study, fatigue crack growth tests were performed for samples containing a single crack and non-collinear cracks of different crack offset in an aluminum–lithium alloy. It was shown that the two facing non-collinear cracks changed their growth direction when the cracks were overlapped, resulting in load mode transfers from mode I to I + II mixed mode. Then, the interaction behaviour was studied by establishing the finite element models to calculate the stress intensity factor K of samples with different crack offset. The results indicated that the K decreased, largely owing to the shielding effect as the two cracks overlapped, leading to retardation of crack growth in the position of overlap, especially for the specimens with a small crack offset. It was also shown that the interaction effect could change from positive to negative during the process of the multiple cracks’ growth, thus leading to the acceleration or deceleration of crack growth rates, suggesting that the influence of interaction on cracks’ growth behaviour could vary with the different stages of crack growth.

Effect of single tensile overload on fatigue crack growth behavior based on plastically dissipated energy and critical distance theory

The paper systematically investigates the fatigue crack growth (FCG) behavior of Q345 steel under a single tensile overload during constant amplitude cyclic loadings by combining the experimental, numerical and theoretical methods. Compact tension specimens are applied to explore the retardation effect of FCG rate after the overload application. A predictive technique based on plastically dissipated energy method, critical distance theory and node-release technology is developed to characterize the FCG behavior following the overload. Crack opening displacement and crack opening load level along the crack flank are obtained to explore the effect of overload on plasticity-induced crack closure. Additionally, the stress-strain response at critical distance and compressive residual stress field ahead of crack-tip are further explored to reveal the intrinsic mechanism and accumulative damage. Results show that the magnitude of residual stretched material wedge and value of maximal crack opening load level increase obviously with the increasing of overload; discontinuous closure takes place behind and before plastic wedge at overload location, which limits the stress/strain ranges at crack-tip and thereby retards crack growth. The magnitude of compressive residual stress and size of stress field ahead of crack-tip increase with increasing overload ratio, which aggravates the retardation effect after overload.

Critical investigation on the effect of steel strength on fatigue crack growth retardation including a single tensile overload

The effect of steel strength on fatigue crack growth (FCG) retardation was investigated considering the influence of a single tensile overload (OL) by means of FEM. A technique that combines a fracture mechanics approach and an elasto-plastic material model available in WARP3D, an open source code, was applied. The interaction integral method provided by WARP3D was used to calculate stress intensity factor solutions based on the local stress fields obtained by elasto-plastic analyses. Crack growth calculations were then performed using ΔK obtained by the numerical analyses. FCG material properties were proposed for the employed steels based on elasto-plastic analyses where the residual stress fields induced by local plasticity ahead of the crack tip was considered. Calculated fatigue lives under two different loading patterns were verified with experiments. The results revealed that steel strength has a significant influence on the size of the induced plastic zone, crack geometry and the behavior of ΔK in particular after the OL. In addition, it is found that steel strength has a pronounced effect on the induced local stress field ahead of the crack tip as well as it exhibits a considerable influence on FCG retardation due to the OL. It is also noticed that, after the OL, the higher the steel strength the smaller the size of the induced plastic zone ahead of the crack tip as well as the higher the produced ΔK. This leads to accelerate FCG rate and shorten the resulting FCG retardation in which it influences the component service life.

Mixed-mode I and II fatigue crack growth retardation due to overload: An experimental study

Environmental loads acting on structures usually cause fatigue crack propagation under variable amplitude mixed-mode conditions. In order to better estimate the remaining fatigue life, the fatigue crack growth behaviour under these conditions needs to be studied. In this paper, an experimental study where mixed-mode I and II overloads are applied to a fatigue crack propagating under mixed-mode I and II constant amplitude loading, is presented. The overloads are found to cause fatigue crack growth retardation, lasting over a length which is more than two times longer than the overload crack tip plastic zone extent estimated using a linear elastic solution.

On the role of Cu-Zr amorphous intergranular films on crack growth retardation in nanocrystalline Cu during monotonic and cyclic loading conditions

In the present study, molecular dynamics simulation was used to investigate the influence of amorphous intergranular films (AIF) and its thickness on possible crack retardation in nanocrystalline Cu under tensile and cyclic loading conditions. Structural analyses were carried out based on different methods such as centro-symmetry parameter, common neighbor analysis, and atomic strain. Results showed that specimens with AIF thickness as small as 0.5 nm can restrict the propagation of both perpendicular and parallel cracks during uniaxial tensile deformation. By increasing the AIF thickness, the crack propagation was found to be further retarded due to crack tip blunting. Atomic strain analysis also revealed that thicker AIF localized the strain distribution more strongly thus inhibiting the nucleation of multiple intergranular cracks and subsequent rapid fracture. In case of cyclic loading, the strain was accumulated through the formation of twins/stacking faults when the AIF thickness was 0.5 and 1 nm, which can interact with the AIF and eventually cause the nucleation of voids/cracks. However, increasing the AIF thickness to 2 nm can restrict such mechanism as the strain was mostly accommodated by the amorphous phase.

Residual stress affecting environmental damage in 7075-T651 alloy

Abstract The role of tensile overload superimposed on a constant amplitude cycling results in compressive residual stresses at the crack tip that cause crack growth retardation. The degree to which this effect manifests depends on whether the tests are done at a constant driving force (Kmax) or at a constant crack growth rate (da/dN). It is observed that depending on the magnitude of the overload at a given applied base stress intensity, these residual stresses can have significant effect on the crack growth in both the inert (vacuum) and the chemical (NaCl) environments. In general, cracks will grow only if the total crack tip driving force Ktotal exceeds the long crack intrinsic threshold K* max,th . ${\rm{K*}}_{{\rm{max,th}}}^{}.$ The crack growth retardation results can be attributed to the combined effects of the crack tip chemical reaction rates and the overload compressive residual stresses.