Roughness-induced Crack Closure Research Articles

Short fatigue crack growth and retained austenite in steels processed via quenching and partitioning

Previous research has consistently found that introducing metastable retained austenite (RA) as a second phase retards the failure of steel under fatigue. However, the reasons for this benefit are not understood. Accordingly, the properties of RA most advantageous to resist fatigue are not known. Within this context, this paper examines the interaction between second-phase RA and short fatigue crack growth in a steel processed via quenching and partitioning, using quasi in situ electron backscatter diffraction experiments. Results show that most RA transforms into martensite under the plastic strain surrounding the crack. They also reveal various mechanisms whereby RA transformation delays short fatigue crack propagation; transformation-induced crack closure (TICC), crack deflection and branching, and roughness-induced crack closure (RICC). Crack deflection and branching are driven by a tendency of cracks to propagate towards transformed RA, which is against the previous assumptions in the literature. Furthermore, the impact of crack deflection/branching on retardation is more powerful than that of TICC acting alone. Microstructures including second-phase RA should avoid RA-lean areas and promote elongated RA grains, with relatively large size, and major axis normal to the preferential crack growth direction. Untransformed RA within the plastic zone (i.e. overstabilized) does not contribute to crack retardation.

Closure Effect of I + II Mixed-mode Crack for EA4T Axle Steel

The crack-closure effect is a crucial factor that affects the crack growth rate and should be considered in simulation analysis and testing. A mixed-mode I + II loading fatigue crack growth test was performed using EA4T axle steel specimens. The variation of the plastic-induced crack closure (PICC) effect and the roughness-induced crack closure (RICC) effect during crack deflection in the mixed-mode is examined in this study. The results show that the load perpendicular to the crack propagation direction hinders the slip effect caused by the load parallel to the crack propagation direction under mixed-mode loading, and the crack deflection is an intuitive manifestation of the interaction between the PICC and RICC. The proportion of the RA value change on the crack side caused by contact friction was reduced by the interaction between PICC and RICC. The roughness of the crack surface before and after the crack deflection is different, and the spatial torsion crack surface is formed during the crack propagation process. With the increase of the crack length, the roughness of the fracture surface increases. During the crack deflection process, the PICC value fluctuates around 0.2, and the RICC value is increased to 0.15.

Effect of loading conditions on short fatigue crack propagation behaviours in cast Al[sbnd]Si alloys

The effect of loading conditions on short crack propagation behaviours in cast AlSi alloys was investigated by a combination of photo-microscopy monitoring and post mortem analysis. The findings suggest that the path of short cracks is minimally influenced by loading conditions, whereas loading conditions significantly affect the fatigue crack growth rate (FCGR) and its statistical characteristics. Crack closure effect is observed under zero and negative stress ratio conditions. Roughness-induced crack closure occurs due to the coarse grain microstructure of cast AlSi alloys and the Stage-I mode growth of cracks within grains in the microstructural short crack regime. The high level of plasticity around the crack tip under negative stress ratio conditions has a significant impact on FCGR, resulting in a negative crack opening stress. When the stress ratio exceeds approximately 0.3, short cracks exhibit no crack closure. The load-level effect is observed only at high load conditions with negative stress ratios. Finally, a probabilistic short crack growth model was established using a Bayesian linear regression model to analyse the effect of stress ratio on the uncertainty of FCGR. After considering the crack closure effect, the difference in distributions of FCGR model parameters decreases significantly among different stress ratio conditions.

Relation of nonmetallic inclusions to the cyclic properties of steel 42CrMo4 after steel melt cleaning by filtration and related processes

Nonmetallic inclusions (NMIs) detrimentally influence the steel properties. The application of metal melt filter systems promises to reduce NMIs in steel. In the current work, NMIs are analyzed from metallographic sections after metal melt filtration or cleaning by a related process of the steel 42CrMo4 using immersion filtration, flow-through filtration as well as filters floating on top of the melt. Analyses by automated scanning electron microscopy (ASPEX) as well as light optical microcopy (LOM) revealed the chemistry, size distribution, and morphology of NMIs. The filter systems had a significant effect on these characteristics of NMIs. The threshold value ΔKth against fatigue crack propagation was found to depend on the amount of NMIs within the steel. A higher amount of NMIs caused less crack deflection leading to less roughness-induced crack closure and in turn resulted in lower threshold values at a stress ratio of R = 0.1. The comparison of the present analysis of NMIs with fatigue data from previous work indicated that the analysis of NMIs from metallographic sections can be used to approximate the fatigue strength of a batch in the same way as the analysis of NMIs from fatigue fracture surfaces for single specimens, as suggested by Murakami.

Roles of microstructures in high-cycle fatigue behaviors of 42CrMo high-strength steel under near-yield mean stress

In this study, high-cycle S-N characteristics and fatigue cracking behaviors under near-yield mean stress of 42CrMo steel with two different fine/coarse-grain tempered sorbite/bainite microstructures (FGM/CGM) were investigated. The results show that fatigue strength is greater for FGM than for CGM as expected, but fatigue crack growth (FCG) threshold is higher for the latter. Compared to FGM with a single crack source, CGM has multiple fatigue sources together with higher-degree ratchetting-fatigue damage, i.e., plastic-strain, dislocation cells and sub-grains, thus accelerating fatigue crack initiation. In the near-threshold regime, roughness-induced crack closure in coarse-microstructures of CGM leads to FCG retardation behavior, which increases FCG threshold. In the Paris regime, CGM presents a similar FCG rate to FGM. Microstructure barriers (i.e., crystal interfaces, carbides and interfaces with different orientations), secondary cracks, crack-path fluctuations, ratchetting and plastic strain jointly result in severer crack-tip stress shielding effects in CGM than in FGM. However, fatigue crack tends to propagate through the high Schmid factor region with {110}<111> slip system with lower slip resistance in FGM but not in CGM, which facilitates fatigue cracking of the former. Under the combined effects of the above factors, similar FCG rates are thus exhibited in both steels with different tensile strengths.

The effect of microstructure on the fatigue crack growth behavior of Ni-Cr-Mo-V steel weld metal

Fatigue crack growth (FCG) rate, which could significantly affect fatigue lifetime of thick wall components, has been studied extensively in forgings with homogeneous microstructure. However, there were few studies on the effect of inhomogeneous microstructure on FCG rate in weld metal. In this work, the effect of microstructure on FCG behavior of Ni-Cr-Mo-V weld metal was investigated. The FCG rate in the Paris regime was similar to that reported in the literature. However, in the near-threshold regime, the FCG rate decreased rapidly at first with the decrease of stress intensity factor range (ΔK) and then increased slightly. Finally, the FCG rate slowly decreased with the decrease of ΔK. It was found that when the monotonic plastic zone size around the crack tip was equivalent to the ferrite grain size in reheated grain zone at a certain ΔK, the crack propagated along the ferrite grain boundaries. The occurrence of intergranular fracture increased the fracture surface roughness, and enhanced the roughness-induced crack closure effect, which increased the FCG resistance and reduced the FCG rate drastically at the early stage. At a lower ΔK level, frequent deflection and crack branching were observed and the mechanisms were investigated by crystallographic analysis.

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.

Roughness induced crack Closure: A review of key points

In this article we review and discuss the key points on the roughness induced closure (RICC) that is considered to affect crack tip shielding. While the RICC has been suggested as a mechanical obstruction in the wake of the crack during cyclic unloading for planar slip alloys at the threshold region, the emphasis on the access of the environment to the crack tip for an environmental damage was not considered. The question is how much the role of RICC alone versus the environment that can affect the overall damage needs some discussion. Dislocation-based models have indicated that the crack tip shielding effect from a single asperity is small. In addition, the location of such an asperity (rigid or deformable) behind the crack tip and multiple asperities also affects the shielding. Existing examples of threshold ΔKth data in a good vacuum (less than10-5 Pa) suggest that RICC is not a significant factor to reduce ΔKth with R in planar slip alloys at ambient temperature. The same alloy in moist air/dilute chemical environment do exhibit ΔKth reduction with R. This comparative observation indicates that the factor of environmental access dominates in the crack opening behind the crack tip affecting the near threshold region for fatigue damage. Near threshold region is where the time dependent environmental damage is significant.

Anomalous fatigue crack propagation behavior in near-threshold region of L-PBF prepared austenitic stainless steel

Laser powder bed fusion (L-PBF) process produces a specific non-equilibrium microstructure with properties significantly different from those of conventionally processed materials. In the present study, the near-threshold fatigue crack propagation in austenitic stainless steel 304L processed by L-PBF was investigated. Three series of specimens with different orientation of initial notch with respect to build direction were manufactured in order to evaluate the effect of specimen orientation on the near-threshold fatigue crack propagation behavior. The results showed absence of the orientation dependence of the fatigue crack propagation behavior. In addition, abnormally low threshold stress intensity factor values were recorded, which were attributed to the absence of crack closure even at low load ratio (R = 0.1). In order to explain observed behavior, the specimens of selected orientation were heat treated to relieve build-in residual stresses (HT1) and to create recrystallized microstructure (HT2) comparable to conventionally processed (wrought) stainless steels. It was found that the characteristic microstructure produced by L-PBF is the main reason for the absence of crack closure and the low threshold values at low load ratios. As-built microstructure containing sub-micron dislocation cell-substructure is prone to cyclic instability. In the crack tip region, cyclic plasticity results in strain-induced phase transformation and continuous thin martensitic layer is formed in the crack vicinity. The induced martensite phase is softer compared to the austenite matrix strengthened by cell-substructure. Together with cyclic instability of the matrix, the macroscopic cyclic softening occurs as the result within the crack tip region. Under such conditions, the formation of the plasticity-induced and roughness-induced crack closure is significantly reduced and macroscopic resistance to the fatigue crack propagation is low.

Fatigue crack growth behavior in CoCrFeMnNi high entropy alloy with harmonic structure topology

Heterogeneous structured high entropy alloys (HEAs) exhibit good strength-ductility synergy. It is imperative to reveal the effects of grain structure heterogeneity on FCG behavior. The present work investigated the FCG rate and failure mechanism of harmonic structured (HS) CoCrFeMnNi HEAs at room temperature. Although the grain size decreases in the HS sample with shell fraction of 23.8 %, it still has the similar FCG resistance compared to the coarse-grained sample at low stress ratio (R < 0.3). The network core–shell structure generates the crack deflection, enhancing roughness-induced crack closure. The HS design may provide a good balance of mechanical properties and FCG resistance.

Fatigue crack growth behavior of 42CrMo high-strength steel with tempered sorbite/bainite microstructures: Roles of grain and constituent in microstructures

Microstructural differences through the thickness have always been an attention focus for large-scale steel structures, since they could significantly influence fatigue crack propagation (FCP) characteristics. Here, the effects of microstructure in FCP behaviors of 42CrMo steel were analyzed with emphasis on two different tempered sorbite/bainite (TS/B) microstructures: (i) fine TS/B structures (M42); (ii) coarse TS/B structures (M52). The results show that the FCP rate is relatively slower for M52 than for M42 in the regime through the later stage of near-threshold (I-L) to the early stage of Paris-regime (Ⅱ-E) with low ΔK, but is fairly similar for the two cases in the other ΔK regimes. Under relatively low ΔK, the FCP path of M52 is more tortuous with numerous micro-cracks in B and larger-sized crack deflections at the carbides and boundaries compared to M42, especially in the I-L where fatigue micro-cracks can deflect at the single coarse carbide, thereby resulting in roughness-induced crack closure and crack retardation behavior. Theoretical analysis shows that FCP behaviors are closely related to the cyclic plastic region size (Δrp) at the crack-tip. In the early stage of near-threshold region, Δrp is restricted within a grain-size scale for both steels, thus leading to a similar FCP rate. However, from the I-L to the Ⅱ-E, coarse B/ferrite-grains in M52 are always larger than the Δrp of fatigue crack-tip, and thereby lead to numerous micro-cracks formation and crack deflections at the carbides and B boundaries, inducing crack closure. In the later stage of Paris region, B/ferrite-grains in the two cases are much smaller than Δrp, so that FCP behaviors depend on the microstructure group effects, or overall strength and fracture toughness of materials, which cause the close FCP rate.

Direct observation of three-dimensional short fatigue crack closure behavior in Ti-6Al-4V alloy using ultra-high-resolution X-ray microtomography

Local 3D short fatigue crack closure behavior was investigated in Ti-6Al-4V alloy using ultra-high-resolution X-ray microtomography (XMT). The results show that the inhomogeneous distribution of plasticity-induced and roughness-induced crack closure is caused by the variation of crack path morphologies. These crack path morphologies exhibited heterogeneous plastic deformation at the crack-tips due to the anisotropic nature of α grains and varying extents of crack tilting and twisting caused by the interaction of the crack front with α/α boundary or α/α + β interface. It was revealed via surrogate-based statistical analysis that the Schmid factor has the strongest effect on crack growth rate.

The relationship between surface roughness and fatigue crack growth rate in AA7050-T7451 subjected to periodic underloads

The accurate prediction of fatigue crack growth rates is necessary to assist with optimising the design, maintenance, and availability of aircraft structures. Roughness induced crack closure is considered by many researchers as an important factor that can impact these growth rates. It has also been suggested that crack closure may be the governing factor behind the accelerative effects of underloads. As such, the focus of this study is on better understanding what correlation there is between the known accelerative effects of underloads, and the local fracture surface roughness of fatigue cracks. Here, specially designed loading spectra are used to quantify the effects of underloads on local fatigue crack growth. Photogrammetry is then used to quantify surface roughness local to the application of the underloads, in order to explore any potential relationship between observed crack growth acceleration and local crack surface roughness. In addition, a general fractographic analysis of the fatigue fracture surface was conducted to understand what correlation there may be between any observed features and the fatigue crack growth rate. It is found that there is little correlation between small-scale surface roughness and the observed acceleration from underloads. However, these is some evidence that large out-of-plane features, such as river patterns, correspond with changes in crack growth rate.

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.

Investigation on the Fatigue Crack Propagation of Medium-Entropy Alloys with Heterogeneous Microstructures

The behavior and the mechanism of fatigue crack propagation in CrCoNi medium-entropy alloys (MEAs) with heterogeneous microstructures were investigated in this paper. After cold-rolling and recrystallization annealing at different temperatures and times, five sets of heterostructured specimens were acquired with different recrystallization levels. Then, the structure characterizations of these five sets of specimens were carried out by nanoindentation testing and electron back-scatter diffraction (EBSD) mapping. Finally, the fatigue crack propagation tests were conducted on single edge crack specimens of these different heterogeneous microstructures. The experimental results indicate that the crack propagation rates of specimens with partial recrystallization microstructures are higher than those with complete recrystallization microstructures, and the effect on fatigue crack thresholds of these specimens is the opposite. The fatigue cracks grow along the slip planes or twin boundaries in recrystallization grains (RGs), which induced crack deflections and the roughness-induced crack closure effect. For this reason, the area percentage of recrystallization and the grain size of RGs have a great effect on the value of the fatigue crack growth threshold.

Effect of particle morphology on fatigue crack propagation mechanism of TiB2-reinforced steel matrix composites

Fatigue crack growth mechanisms of as-cast and hot-rolled TiB2-reinforced steel matrix composites (SMCs) fabricated by eutectic solidification are systematically investigated through in-situ scanning electron microscopy (SEM) fatigue experiment system. The experiment results show that the as-cast SMC exhibits a relatively superior fatigue crack growth resistance with respect to the hot-rolled SMC. A lower FCGR at the low and intermediate stress intensity factor (ΔK) range in as-cast SMC is found, which is related to the TiB2 particle morphology. Hexagonal and rectangular primary particles have a high Young’s modulus and strong interfacial bonding, which inhibits the advancing of plastic deformation in front of the crack tip. Three-dimensional interconnected eutectic particle phases composed of dendrites can produce a hardening effect at crack tip and restrain the generation of plastic deformation in this morphology region. The stress-strain field in front of the crack tip is changed by the hard TiB2 particles, which leads to the deflection of the crack path. Hence, the tortuous crack path created by facing these particles causes reduction in the effective driving force of the crack tip and slows the FCGR. The net mismatch between the surfaces lead to premature contact and roughness-induced crack closure further reducing the FCGR. In comparison, large number of hot-rolled micro-cracks in the hot-rolled SMC are detrimental to fatigue crack growth resistance through the coalesce mechanism of micro-cracks, thereby accelerating the FCGR.

Additive manufacturing of functionally graded inconel 718: Effect of heat treatment and building orientation on microstructure and fatigue behaviour

This paper addresses the effect of the post-process heat treatments on the microstructure and fatigue crack growth behaviour of the functionally graded (FG) laser powder bed fusion (L-PBF) Inconel 718 (IN718) superalloy. Sets of samples were additively manufactured (AM) altering the process parameters, namely the laser power, the laser scanning speed, layer thickness, hatch distance, and beam distribution function, resulting in distinctly different microstructures. Two categories of samples underwent heat treatment (HT) and hot isostatic pressing followed by HT (HIP+HT), while one category was kept in the as-processed (AP) condition to reveal the effects of the post-treatments. Additionally, to study the effect of microstructural anisotropy, samples were printed in horizontal (H) and vertical (V) building directions. To better understand the behaviour of the FG materials, non-graded (NG) L-PBF samples and wrought material were investigated as references. Significant variations in terms of porosity, grain size and elongation, crystallographic texture, and content of the strengthening precipitates or detrimental phases were found in different AM groups. The fatigue behaviour of the NG and FG materials was also studied by conducting three-point bending tests. Findings in terms of the role of different microstructures on the fatigue-crack initiation and fatigue crack growth rate are presented and discussed for all samples. The study demonstrated that heat treatments can enhance the damage tolerance of L-PBF IN718 to the level of wrought material. Interestingly, the effect of the roughness induced crack closure was found to be a function of build orientation, especially in the low stress ratio regime.

Near-threshold fatigue crack growth in laser powder bed fusion produced alloy 718

The role of microstructure in influencing fatigue crack growth behavior for laser powder bed fusion (LPBF) produced nickel superalloy 718 was examined. Two common post-build heat treatments were applied to produce two distinct microstructures, one which retained much of the solidification structure from additive manufacturing, while the other was mostly recrystallized. For both groups, residual stresses were assessed along the crack path by neutron diffraction. At low load ratios (R = 0.1), the non-recrystallized LPBF material had the lowest and most varied fatigue crack growth thresholds at 6–7.2 MPa m1/2. This is attributed to a reduction in crack path roughness induced crack closure and high superimposed residual stresses. The recrystallizing heat treatment coarsened and homogenized the grain structure substantially, thus increasing the threshold to 7.3–7.6 MPa m1/2 due to a substantially rougher crack path and completely relieved residual stresses. Both LPBF heat treatments showed negligible effect of build orientation and gave significantly lower fatigue thresholds compared to wrought material tested in the L-T orientation (∼11 MPa m1/2). At high load ratios, the difference in fatigue behavior between microstructures was reduced but still present and was attributed to differences in geometric shielding from crack path roughness, grain size, and degree of microstructural homogeneity.

Fatigue-crack propagation behavior in a high-carbon chromium SUJ2 bearing steel: Role of microstructure

High-carbon chromium martensite steels are commonly selected for bearing components in the power generation, automotive and aerospace industries where fatigue failure is a major concern. Accordingly, it is of importance to elucidate the structure-property relationships governing the fatigue properties of such bearing steels. Here, the role of microstructure in influencing the fatigue-crack propagation behavior of a high-carbon chromium SUJ2 bearing steel is examined with emphasis on three martensitic structures: (i) a fine-grained (∼10 μm) structure with intragranular (but no intergranular) carbides, (ii) a coarser-grained (∼25 μm) structure with discontinuous grain-boundary carbides primarily near triple junctions, and (iii) a coarse-grained (∼31 μm) structure with continuous grain-boundary carbides. Although growth rates were fairly similar above ∼10−8 m/cycle, significant differences in behavior were observed at lower, near-threshold growth rates; specifically at a load ratio of 0.1, the ΔKth fatigue threshold was increased from 3.8 MPa m½ in the fine-grained structure to 5.9 to 6.3 MPa m½ in the two coarser-grained structures. The dominant factor governing such near-threshold behavior was found to be crack-tip shielding from roughness-induced crack closure, which was most pronounced in the coarser-grained structures; additionally, shielding from crack deflection and branching was apparent in these structures as the crack propagated through the network of intersecting martensite lamellae within the coarse grains. Compared to high-strength stainless steels and low-strength carbon steels, the fine-grained SUJ2 steel displays an exceptional combination of high strength and fatigue-crack growth resistance and is considered to be a superior structural alloy for bearing applications.