Crack Growth Retardation Research Articles

Experimental and modeling investigation of the thermal shock behavior of TiC-based self-healing coatings on AISI 321 stainless steel

The AISI 321 steels of structural components serviced at high temperature are usually subjected to oxidation and thermal shock damage during service. Therefore, to improve their high-temperature oxidation and thermal shock resistance, the self-healing coating consisting of Al2O3-13 %TiO2 layer, TiC layer and NiCrAlY layer was prepared for 321 steels in this work. The experimental and microstructural characterization results showed that the thermal shock resistance of such self-healing coating was remarkably improved as compared to the counterpart double-AT13 coating. This is because the volume increment resulting from the oxidation reaction between the TiC and oxygen decreased the porosity of the self-healing coating and retarded crack growth, which also led to the improved high-temperature oxidation resistance. Further, a thermal shock life model based on the crack growth model of Paris formula were developed. The modeling results not only agreed well with the experimental results but also indicated that thickening of thermal grown oxide (TGO) is the main cause of crack initiation and growth.

Fatigue and fracture in dual-material specimens of nickel-based alloys fabricated by hybrid additive manufacturing

The integration of additive manufacturing with traditional processes, termed hybrid additive manufacturing, has expanded its application domain, particularly in the repair of gas turbine blade tips. However, process-related defects in additively manufactured materials, interface formation, and material property mismatches in dual-material structures can significantly impact the fatigue performance of components. This investigation examines the low cycle fatigue and fatigue crack growth behaviors in dual-material specimens of nickel-based alloys, specifically the additively manufactured STAL15 and the cast alloy 247DS, at elevated temperatures. Low cycle fatigue experiments were conducted at temperatures of 950 °C and 1000 °C under a range of strain levels (0.3%–0.8%) and fatigue crack growth tests were conducted at 950 °C with stress ratios of 0.1 and −1. Fractographic and microscopic analyses were performed to comprehend fatigue crack initiation and crack growth mechanisms in the dual-material structure. The results consistently indicated crack initiation and fatigue fracture in the additively manufactured STAL15 material. Notably, fatigue crack growth retardation was observed near the interface when the crack extended from the additively manufactured STAL15 material to the perpendicularly positioned interface. This study highlights the importance of considering yield strength mismatch, as well as the potential effects of residual stresses and grain structure differences, in the interpretation of fatigue crack growth behavior at the interface.

A new method for prediction of fatigue crack propagation life under variable amplitude spectrum loading

This paper proposes a new method for predicting propagation lives for through-thickness cracks under variable amplitude mode-I spectrum fatigue loads. Only the crack opening, i.e., the increasing parts of the load cycles within spectrum profiles are considered for cycle counting and computing their stress ratios. Moreover, a cycle-by-cycle analysis is also performed throughout the spectrum to compute the so-called Spectrum Overload Index (SOL-Index), which is a multiplier applied on stress intensity factor range used within different crack growth retardation models, in which effects of stress ratio are also included. The SOL-Index is calculated using the average of peak loads for all cycles within the spectrum and their percent deviations from the peaks’ average value. Simulation results using the proposed cycle counting method, the integrated crack growth model and the SOL-Index are compared with those from multiple experiments under two different loading spectrums and another study in the literature. The results show that usage of Wheeler's model embodying the Forman or Walker equation and the SOL-Index together yields the closest results to the experiments.

Effect of cathodic protection on the corrosion fatigue crack growth behavior and fatigue life of XS-grade shipbuilding steel

Shipbuilding steels are subjected to corrosion fatigue damage phenomena due to conjoint effects of cyclic loading by sea waves and corrosive seawater environment, which often leads to premature failure of ship hull structures. In practice, the impressed current cathodic protection (ICCP) technique is applied to mitigate ship hull corrosion by electrochemically making it cathodic. It is implemented by impressing a direct current to minimize the electrode potential to a predefined protection potential. It is significant to study the effect of ICCP on the corrosion fatigue crack growth rate (CFCGR) behavior and fatigue life assessment of ship hull structures. In this study, CFCGR behavior of high-strength XS-grade shipbuilding steel was investigated in unprotected freely corroding (FC) and protected ICCP (at – 800 mV) conditions using compact-tension specimens at 0.1 Hz frequency in artificial seawater (3.5 wt.% NaCl solution). After crack-closure corrections, it was found that the ICCP-protected specimen (∆KTH = 18 MPa √m) exhibited higher sub-critical or threshold corrosion fatigue crack propagation resistance as compared to the FC specimen (∆KTH = 14 MPa √m). Fractography of the FC specimen revealed the debonding of non-metallic inclusions and formation of micro-pits/secondary cracks as dominant damage mechanisms. ICCP-protected specimen exhibited a signature pattern of parallel secondary cracks and a proportional increase in their density with increasing stress intensity levels during primary crack growth. This fracture morphology indicated the primary-crack branching and further arrest, leading to retarded crack growth rates under ICCP-protection. Fatigue life analysis also confirmed this finding, where the beneficial ICCP-protection effect resulted in at least two times enhancement in fatigue lives from the unprotected FC condition.

Phase field modeling of fatigue crack growth retardation under single cycle overloads

A zone-based crack retardation model that responds to single cycle overloads is applied to a phase field fatigue framework. Fatigue crack growth is incorporated through the degradation of fracture toughness in the phase field fracture model where a representative loading strategy is followed instead of explicit cyclic loading. The model is demonstrated to capture Paris-Erdogan law type behavior. Crack growth retardation following an overload cycle is simulated through a zone ahead of the crack tip taking inspiration from existing plastic zone-based retardation models. The retardation zone is described through a strain energy density limit, with the overload ratio controlling the extent to which the fatigue damage accumulation rate is slowed inside the retardation zone. The model is implemented utilizing user subroutines in Abaqus with coupled-temperature displacement elements where temperature acts as a stand in for the phase field parameter. The model is found capable of reproducing experimental levels of fatigue life gains and reduction in crack growth rate for compact tension and center-cracked tension panel specimens when various overload levels are applied. Furthermore, the model is demonstrated to be able to capture complex crack paths with great accuracy.

Adaptive phase-field modeling of fracture propagation in bi-layered materials

We study fracture propagation in bi-layered materials using an adaptive phase-field method. The method combines a phase-field representation of fracture with spatial adaptivity to provide mesh refinement in the smeared zone of damage, where higher accuracy is needed to resolve the solution. We place a particular emphasis on studying the competition between crack arrest, deflection, and penetration at the interface of two dissimilar materials. To explore this competition, we perform tension tests on various models with different combinations of material and geometric properties. We begin by validating the adaptive phase-field model through a uniaxial tension test. The material mismatch and locations of the initial damage for this example are chosen such that the fracture propagation behavior is identical to that in a homogeneous material. Next, we independently vary the mismatches in elastic stiffness and critical energy release rate to study their influence on crack growth. Our observations show that both mismatches contribute to an effective toughening of the model, but in different ways. Critical energy release rate mismatch causes crack deflection along the interfaces, while elastic stiffness mismatch results in retardation of crack growth. We also note that for a specific combination of material mismatch and location of initial damage, crack nucleation in the adjacent material is preferred over propagation from the existing notch. Finally, through models with inclined interfaces, we demonstrate the effect of the relative orientation between the approaching crack and the material interface on crack penetration, deflection, and nucleation. The numerical results from this study provide valuable insight into the various failure mechanisms in bi-layered materials.

Effect of laser peening without coating (LPwC) on retardation of fatigue crack growth in SM490 plates

Laser peening without coating (LPwC) is a well-known technique to improve high-cycle fatigue properties by introducing compressive residual stress (RS) near the surface of metal components. In this study, X-ray diffraction (XRD) and flexural fatigue tests were applied to pre-cracked 12 mm thick SM490A welding structural steel specimens that were subjected to LPwC nearly 20 years ago with a pulse energy of 200 mJ, a spot diameter of 0.8 mm and a pulse density of 36 pulse/mm2. XRD revealed that the compressive RS has remained stable to date, with approximately 400–500 MPa remaining at the surface and a compressive depth of approximately 0.9 mm from the surface, which is comparable to the data measured by XRD immediately after LPwC. In the flexural fatigue tests with a stress ratio of 0.1 and stress rages of 100, 150 and 200 MPa, LPwC extended the fatigue life by more than 1.6 times, depending on the stress range and individual specimens. Crack restarting cycles were significantly increased by a factor of at least 1.8, and the crack growth rate was suppressed by a factor of about 0.7 or less.

Grain-level residual stress distribution at dwell fatigue crack tips in a nickel-based superalloy

Dwell fatigue crack growth in nickel-based superalloys is influenced by a complex interplay between mechanical and chemical phenomena. The stress distribution at the crack tip is closely related to these phenomena and to the local microstructure. In this study we evaluate experimentally the microscale residual stress distribution ahead of crack tips in RR1000 alloy samples after interrupted high temperature dwell fatigue testing. In the crack opening direction the results show a compressive residual stress zone, the extent of which is linked to the plastic zone size and crack growth retardation behaviour. In the direction of crack propagation the residual stress is also influenced by crystallographic grain orientation. The results demonstrate a novel approach of experimentally evaluating crack tip stress distribution via focused ion beam – digital image correlation (FIB-DIC) ring-core measurements at the grain-level. This provides new insights into fatigue crack growth in microstructurally heterogeneous materials.

Crack layer modeling of overload-induced slow crack growth retardation of high-density polyethylene

The existing crack layer model theoretically mimics the discontinuous slow crack growth behavior of high-density polyethylene; however, it can only be applied to cyclic loads of constant amplitude or constant load conditions. Most load-bearing components, such as pressurized pipes, are subjected to variable-amplitude loads with unexpected overloads under their service conditions. Thus, the effect of variable amplitude loading on the slow crack growth behavior and lifespans should be understood. In this study, a modified crack layer model was developed to simulate the overload-induced retardation behavior of discontinuous slow crack growth of pipe-grade high-density polyethylene for the first time. The developed model was verified by conducting a sensitivity study on several input parameters and comparing the results with experimental results. The measured retardation results were successfully reconstructed using the proposed model in high accuracy. This study expands the applicability of the crack layer model for the reliable use of pipe-grade high-density polyethylene under various fatigue loading conditions, including unexpected overloads.

Unraveling the superlattice effect for hexagonal transition metal diboride coatings

Superlattice structures enable the simultaneous enhancement in hardness (H) and fracture toughness (KIC) of ceramic-like coatings. While a deeper understanding of this effect has been gained for fcc-structured transition metal nitrides (TMN), hardly any knowledge is available for hexagonal diborides (TMB2). Here we show that superlattices can—similarly to nitrides—increase the hardness and toughness of diboride films. For this purpose, we deposited TiB2/WB2 and TiB2/ZrB2 superlattices with different bilayer periods (Λ) by non-reactive sputtering. Nanoindentation and in-situ microcantilever bending tests yield a distinct H peak for the TiB2/WB2 system (45.5 ± 1.3 GPa for Λ = 6 nm) but no increase in KIC related to a difference in shear moduli (112 GPa). Contrary, the TiB2/ZrB2 system shows no peak in H, but for KIC with 3.70 ± 0.26 MPa∙m1/2 at Λ = 4 nm originating from differences in lattice spacing (0.14 Å), hence causing coherent stresses retarding crack growth.

On improved fatigue properties of aluminum alloy 5086 weld joints

In this work, we aimed to investigate the fatigue properties of AA5086-H321 and its weld joints. The AA5086-H321 plates were welded in a butt configuration using the friction stir welding. Load-controlled axial sinusoidal cyclic tests were conducted in the ambient conditions, at a frequency 20 Hz, and at different load ratios i.e., Rσ=-1, 0.1, and 0.5. The weld joints, in general, are known to have reduced properties. However, in this study, the base alloy and its weld joint had similar hardness and ultimate tensile strength, but with marginally reduced elongation (∼4.4%). The endurance limits of the weld joints were either higher (∼28% and ∼11% at R=-1 and 0.1, respectively) or same (at R=0.5) as compared to that of the base alloy. The difference in fatigue lives of base alloy and its weld joint kept diminishing with increasing R-ratio. Microscopic examination of fracture surfaces in weld joints revealed three types of crack initiations: voids, inclusions, and tool-marks, and two types of crack propagation mechanisms: intergranular and transgranular. However, in base alloys, only defect-free surface initiation with transgranular crack growth was observed. Mean stress, as expected, had a detrimental effect on fatigue properties, with no definitive control on the type of crack initiation or its propagation mechanism. An increase in the fatigue lives of weld joints was attributed to the crack growth retardation during the intergranular fatigue crack propagation due to frequent defection of crack path, which was absent in the base alloys. Most of the intergranular facets were observed near the surfaces (top or bottom) of the weld nugget zone and disappeared when the crack front reaches its interface boundary with the heat affected zone. All these aspects of fatigue damage in microstructurally heterogeneous weld joints and the plausible explanations for the observed mechanical fatigue properties are presented in this study.

On the strain energy release rate and fatigue crack growth rate in metallic alloys

The field of fracture mechanics started with Griffith’s energy concept for brittle fracture in 1920. In 1963, Paris et al. used a fracture mechanics’ parameter to introduce an equation for the fatigue crack growth rate in ductile materials and this equation is now commonly known as the ‘Paris law’. However, the Paris law and the semi-empirical models that followed ever since do not fully account for the main intrinsic and extrinsic properties involved with fatigue crack growth in metallic alloys. In contrast, here a dimensionally correct fatigue crack growth rate equation is introduced that is based on the original crack driving force as introduced by Griffith and the presence of plasticity in a metal to withstand crack propagation. In particular it is shown that the fatigue crack growth rate shows a power law relationship with the cyclic strain energy release rate over the maximum stress intensity factor. The new description corrects for the ratio between the minimum and maximum stress in a cycle during constant amplitude loading and for crack growth retardation under variable amplitude loading. The method has been successfully applied to variable amplitude crack growth with spectra that are representative of different fatigue dominated aircraft locations. As such, it allows for accurate predictions of variable amplitude fatigue crack growth life in aerospace structures.

Metal–Matrix Composites from High‐Pressure Torsion with Functionalized Material Behavior

In composites, outstanding properties of two materials can be combined. In particular, metal–matrix composites (MMCs) can combine the properties of a high‐strength ductile metallic matrix with special properties of embedded ceramic particles. This hybrid can be used to create a functional material. However, during consolidation, the thermal load of most common MMC‐processing routes is an obstacle for such functionalization, because the unique properties of the ceramic phases most likely degrade. Mechanical alloying, in this case, by high‐pressure torsion (HPT), can overcome this challenge. Herein, the attempt to obtain smart materials through HPT processing is aimed. For that purpose, Cu‐MMCs are produced from mixed powders with and (BTO) with the challenge to incorporate their functional phase. BTO can provide a sensing ability for internal stress and can provide a fatigue lifetime by a retarded crack growth. The amount of the stabilized phase is evaluated by X‐ray diffraction. Cu–BTO–MMCs exhibit a local piezoelectric effect when strained, shown by in situ scanning Kelvin probe force microscopy. Cu––MMCs feature a retarded fatigue crack initiation and an earlier crack closure during fatigue crack growth due to the volume expansion once transforms.

The influence of laser shock peening on corrosion-fatigue behaviour of wire arc additively manufactured components

The need for increased manufacturing efficiency of large engineering structures has led to development of wire arc additive manufacturing (WAAM), which is also known as direct energy deposition (DED) method. One of the main barriers for rapid adoption of the WAAM technology in wider range of industrial applications is the lack of sufficient performance data on the WAAM components for various materials and operational conditions. The present study addresses this essential need by exploring the effects of laser shock peening surface treatment on corrosion-fatigue crack growth (CFCG) life enhancement of WAAM components made of ER70S-6 and ER100S-1 steel wires. The experimental results obtained from this study were compared with the CFCG trends from nominally identical specimens without surface treatment and prove the efficiency of the examined surface treatment method for corrosion-fatigue life enhancement and crack growth retardation of WAAM built steel components, regardless of the material type and specimen orientation. Furthermore, the residual stresses in the WAAM built specimens with and without surface treatment were measured to validate the influence of beneficial residual stresses, arising from surface treatment, on subsequent CFCG behaviour of the material. The residual stress profiles show the beneficial compressive stress fields in the surface treated areas which result in CFCG life enhancement. The results from this study make significant contribution to knowledge by evaluating the suitability of WAAM built steel components for application in offshore environments.

Retardation effect on sheet steel under a mixed I–II mode overload revealed by experiment, DIC and FEM methods

As a high strength and commercial steel, Q420 is widely employed in many engineering applications. In this paper, a series of experiments was conducted to investigate the fatigue crack growth retardation mechanisms under mixed I–II mode overloads with loading angles 0, 30, 60 and 90°. The baseline of loading spectrum is a pure mode I cyclic loading with a mixed overload applied after pre-cracking. The results show that the crack path is always straight for each case. Both the retardation level and fatigue life decrease with the increasing loading angle. A combined model that considers the residual stress, plasticity-induced and roughness-induced crack closure effects was proposed to predict the fatigue crack growth rate and fatigue life. The predicted results agree well with several experimental results.

Modelling and predictions of time-dependent local stress distributions around cracks under dwell loading in a nickel-based superalloy at high temperatures

Finite element (FE) analysis modelling of crack configurations was used to provide insights into the role of time-dependent deformation on dwell fatigue crack growth (DFCG) in turbine disc alloys. In particular, the potential influence of time-dependent deformation on fatigue crack growth rates observed during dwell holding periods and the potential crack growth retardation phenomena that can occur for overload-dwell cycles were investigated. The FE model evaluated the mechanical stress state evolution in a two-dimensional cracked geometry subjected to standard dwell and overload-dwell stress waveforms for a given microstructural condition (based on its γ’ particle distribution). Crack-opening stress distributions as a function of dwell time and distance ahead of the crack-tip were interrogated, with the aim to investigate the potential influence of local crack-tip stresses on crack growth during dwell holding periods and the potential for crack growth retardation following overload cycles. Stress distribution profiles predicted by dwell-only simulations showed how quickly local crack-tip stresses relax due to time-dependent plasticity. Characteristic effects of overloads of dwell crack growth resistance observed in experiments were also reasonably well captured. A numerically-derived incubation time was successfully applied in predicting the general trends of achieving more pronounced retardation effects with larger overload factors and extended periods of overloading prior to periods of dwell. FE modelling predicted a significant difference in behaviour between an overload for a given overload factor with a hold time at peak load of 1 s and 10 s. This study emphasises the importance of studying local crack-tip stresses in regards to characterising DFCG behaviour of turbine disc alloys.

Effect of long periods of corrosion on the fatigue lifetime of offshore mooring chain steel

R4-grade mooring chain steel specimens were subjected to alternating phases of saltwater corrosion and air fatigue to replicate seasonal load variations on chains in service. No significant difference in fatigue lifetimes was registered between precorroded specimens subjected to continuous fatigue and those periodically interrupted by phases of accelerated corrosion. Moreover, eight months of natural corrosion of specimens with fatigue cracks did not have a large effect on their remaining fatigue lifetimes. Retardation of fatigue crack growth by crack tip blunting from corrosion is deemed unlikely based on observations from the current work and the literature. Corrosion accelerated by anodic polarization is demonstrated to be unsuitable for replicating natural corrosion of fatigue cracks.

Application of strain energy based approach for evaluation of fatigue crack growth retardation effect under random overload

Fatigue crack growth in structural components subjected to random amplitude loading is a significantly complex problem. Many models have been proposed to estimate the retardation effect due to a single overload, though for the random overload sequence, there is still a lack of effective quantitative description methods. In this research, a new time to crack initiation (TTCI) type for the calculation of equivalent initial flaw size (EIFS) is proposed based on the change in net-section strain energy density. For a certain number of fatigue cycles and current crack lengths, the value of EIFS can be obtained by solving the corresponding integral equation. The proposed EIFS calculation method was applied for the prediction of fatigue crack growth under single overload and random overload sequences. The prediction results agree well with the experimental data for different conditions. For a certain single overload or overload spectrum, the retardation effect is reflected in the decrease in EIFS, which appears to propagate from a smaller initial crack length. The proposed method shows that the new TTCI type of EIFS can be used as the quantitative characterization of the retardation effect due to the random overload sequence.

In-situ experimental study on the fatigue crack propagation behavior of 7075-T6 and 2024-T3 aluminium alloys under variable amplitude loading and retardation model modification

Fatigue crack propagation behavior of Al7075-T6 and Al2024-T3 under four typical variable amplitude loading (VAL) patterns are investigated by using in-situ testing. The near-tip strain fields and crack opening level are measured combining in-situ optical microscopy testing and digital image correlation (DIC) technique. The crack tip deformation process is directly observed by in-situ scanning electron microscope (SEM). The results show that both the crack propagation behaviors and the crack-tip strain distributions are greatly influenced by the VAL patterns. The shape effect of the overload-induced plastic zone may prolong the crack growth retardation. The crack-front plasticity plays a dominant role in controlling fatigue crack growth. The crack-front plasticity supplies a link between crack closure and crack growth. The crack tip resharpening caused by the underload applied immediately after the overload may reduce the retardation extent. Finally, a modified retardation model is proposed and gives reasonable prediction results of the crack growth rate under VAL.

Prediction of fatigue crack growth under variable amplitude loading by artificial neural network-based Lagrange interpolation

Artificial neural network has been several applications on fatigue crack, with prediction on fatigue crack growth life often serving as milestones. A typical difficulty in predicting the life curve of fatigue crack growth is the retardation of crack growth caused by the overload effect. Overload retardation, resulting in the lack of experiment data points in the retardation interval, is a long-term challenge for predicting fatigue crack growth. We improve the ability of back-propagation neural network to predict fatigue crack growth life by Lagrange interpolation, an interpolation for local data points. It combines the trend of whole data point to deal with the retardation interval, and interpolates multiple data points in the retardation interval when there is a lack of local data, and uses a neural network to construct the prediction formula of crack growth through self-optimization. In the study involving constant and variable amplitude fatigue crack growth experiments, the proposed method is proved to improve, with statistical significance, the predictive ability on the whole range of experiment data. The method is simple and accurate. Consequently, it is helpful to solve the problem of fatigue crack growth life of engineering structure.