Higher Stress Intensity Factor Research Articles

Effect fracture resistance on retardation of crack growth

Purpose – The prediction of fatigue crack growth behaviour is an important part of damage tolerance analyses. Recently, the author’s work has focused on evaluating the FASTRAN retardation model. This model is implemented in the AFGROW code, which allows different retardation models to be compared. The primary advantage of the model is that all input parameters, including those for an initial plane-strain state and its transition to a plane-stress-state, are objectively measured using standard middle-crack-tension M(T) specimens. The purpose of this paper is to evaluate the ability of the FASTRAN model to predict correct retardation effects due to high loading peaks that occur during variable amplitude loading in sequences representative of an aircraft service. Design/methodology/approach – This paper addresses pre-setting of the fracture toughness K R (based on J-integral J Q according to ASTM1820) in the FASTRAN retardation model. A set of experiments were performed using specimens made from a 7475-T7351 aluminium alloy plate. Loading sequences with peaks ordered in ascending-descending blocks were used. The effect of truncating and clipping selected load levels on crack propagation behaviour was evaluated using both experimental data and numerical analyses. The findings were supported by the results of a fractographic analysis. Findings – Fatigue crack propagation data defined using M(T) specimens made from Al 7475-T7351 alloy indicate the difficulty of evaluating the following two events simultaneously: fatigue crack increments after application of loads with maximum amplitudes that exceeded J Q and subcritical crack increments caused by loads at high stress intensity factors. An effect of overloading peaks with a maximum that exceeds J Q should be assessed using a special analysis beyond the scope of the FASTRAN retardation model. Originality/value – Measurements of fatigue crack growth on specimens made from 7475 T7351 aluminium alloy were carried out. The results indicated that simultaneously evaluating fatigue crack increments after application of the load amplitude above J Q and subcritical increments caused by the loads at high stress intensity factors is difficult. Experiments demonstrated that if the fatigue crack reaches a specific length, the maximal amplitude load induces considerable crack growth retardation.

Rate-dependent fracture modes in human femoral cortical bone

An accurate understanding of fracture in human bone under complex loading scenarios is critical to predicting fracture risk. Cortical bone, or dense compact bone, is subject to complex loading due to the inherent multi-axial loading conditions, which are also influenced by the anisotropy of the microstructure. When determining critical fracture parameters, bone is traditionally idealized as isotropic. This paper presents a method to examine rate-dependent mode mixity associated with cortical bone crack initiation. Four-point bend experiments have been conducted on cortical femoral bone samples from three human donors at quasi-static (slow), intermediate, and dynamic loading rates. Digital image correlation was used to obtain full-field displacement maps at the crack tip during the experiments. An over-deterministic least squares method is presented and used to evaluate Mode I (opening) and Mode II (shear) stress intensity factors (SIF) for fracture initiation at slow (10 $$^{-2}$$ MPa-m $$^{1/2}$$ s $$^{-1}$$ ), intermediate (15 MPa-m $$^{1/2}$$ s $$^{-1}$$ ), and high ( $$4.5^{4}\,\hbox {MPa-m}^{1/2}\,\hbox {s}^{-1}$$ ) stress intensity factor rates. Results show that under dynamic loading, the critical SIF in Mode I assuming material anisotropy is approximately 50 % lower than fracture toughness assuming isotropy. Additionally, critical Mode I and II SIFs had the lowest values at the highest rate of loading examined, decreasing to one third of the values under quasi-static loading. Crack growth in the low and intermediate SIF rates appears to be Mode II dominant, and shows a transition to completely mixed-mode at the high rate of loading. These results suggest that the conventional assumption of isotropy is a conservative estimate at low and intermediate rates, but overestimates fracture strength at dynamic rates.

Study on the fatigue crack growth rates of Ti–5Al–5Mo–5V–1Cr-1Fe titanium alloy with basket-weave microstructure

In the present paper, the fatigue crack growth behavior of Ti–5Al–5Mo–5V–1Cr–1Fe titanium alloy (Ti-55511) with different basket-weave microstructure features prepared by quasi-β forging and subsequent double annealing treatments are studied. The results show that the fatigue crack growth rates (FCGR) for five microstructures of Ti-55511 alloy vary obviously at low stress intensity factor range levels and then tend to be similar at high stress intensity factor range levels. According to analyses, the difference of the crack deflection effect caused by different basket-weave features is responsible for above phenomenon. It׳s found that the α platelet length and thickness are two critical basket-weave features influencing the crack deflection effect of Ti-55511 alloy. The long and thick α platelets are most favorable to deflecting the main crack and increasing the crack path tortuosity, which can obviously improve the fatigue crack growth resistance. In addition, at the end of the article, a generalized Paris model based on tensile properties is established for Ti-55511 alloy. Results show that this model can fairly describe the FCGR for Ti-55511 alloy when tested at a stress ratio of 0.1.

Effect of head wear and lateral forces on underhead radius crack propagation

This paper investigated the effect of rail head wear (HW) and lateral forces on underhead radius stress conditions and resulting stress intensity factors (SIFs) of a long transverse crack. The occurrence of tension spikes at the underhead radius of the rail as a result of localised vertical and lateral bending of the head-on-web was significantly exacerbated with increasing rail HW. The extended finite element method (X-FEM) modelling, which had previously been validated by comparison with in-track measurements to verify the prediction of tension spikes, was used to model a single rail on discrete elastic foundations. SIFs along the crack front were parametrically evaluated in terms of changes in the contact patch offset (CPO), the ( L/ V) ratio of lateral ( L) to vertical ( V) loads, the rail HW, and crack size and shape. The X-FEM results revealed that for a long transverse crack, extending the crack length to the underhead radius position results in higher SIFs across the underhead radius as a result of tensile bending stresses. These higher SIFs can contribute to a massively higher crack growth rate at this location, as was evidenced by the crack growth morphology at the underhead radius.

Fracture of a Finite Medium with a Circular Internal Crack under Hyperbolic Heat Conduction-Prescribed Crack Face Thermal Flux

This paper studies the fracture mechanics of a thermoelastic medium with an internal circular crack subjecting to a prescribed thermal flux. The time varying crack tip thermal stress intensity factor is solved. Solution for the infinite medium under steady heat conduction is given in closed form. Comparisons between the non-Fourier results and the classical Fourier results are made. Numerical results show that the non-Fourier heat model predicts considerable high transient thermal stress intensity factor than the Fourier model. This paper, together with our previous paper entitled fracture of a finite medium with a circular internal crack under hyperbolic heat conduction-prescribed crack face temperature, completes the analysis of a finite medium with a circular internal crack under hyperbolic heat conduction.

Sequence of deformation and cracking behaviours of Gallium–Arsenide during nano-scratching

GaAs {001} single crystals were scratched with constant normal loads (from 5 to 100 mN) using a Berkovich indenter tip with the edge moving parallel to the [110] direction with a constant velocity of 600 μm s−1. The scratching process generates both plastic deformation and various crack types, such as median, lateral and radial cracks. For the first time, an understanding of the different steps of the plastic deformation and cracking behaviour has been achieved. First, a localised dislocation cloud is formed below the surface and at the rear half of the indenter where the principal shear stress is maximal. Second, median cracks are nucleated by pile-up of converging dislocations below the surface along the (1¯10) plane. These median cracks propagate further as the indenter continues to increase the stress. Simultaneously, surface radial cracks are generated directly by the indenter tip. Then, deformation proceeds by plastic flow in the (111) plane. Fourth, lateral cracks are generated, and then radial cracks form off of these due to high stress intensity factor located at the sharp edges of the lateral cracks. The last phase is when lateral or radial cracks intercept each other, large chips are taken off from the surface. Apart from the sequence for the plastic deformation and cracking events, it is also demonstrated that the scratching depth, the residual pile-up, the residual scratch depth and all cracks follow a power-law dependence. Finally, it is found that the elastic recovery is more or less 15% which is explained by the value of the rheological factor X obtained.

Stress Corrosion Cracking in Al-Zn-Mg-Cu Aluminum Alloys in Saline Environments

Stress corrosion cracking of Al-Zn-Mg-Cu (AA7xxx) aluminum alloys exposed to saline environments at temperatures ranging from 293 K to 353 K (20 °C to 80 °C) has been reviewed with particular attention to the influences of alloy composition and temper, and bulk and local environmental conditions. Stress corrosion crack (SCC) growth rates at room temperature for peak- and over-aged tempers in saline environments are minimized for Al-Zn-Mg-Cu alloys containing less than ~8 wt pct Zn when Zn/Mg ratios are ranging from 2 to 3, excess magnesium levels are less than 1 wt pct, and copper content is either less than ~0.2 wt pct or ranging from 1.3 to 2 wt pct. A minimum chloride ion concentration of ~0.01 M is required for crack growth rates to exceed those in distilled water, which insures that the local solution pH in crack-tip regions can be maintained at less than 4. Crack growth rates in saline solution without other additions gradually increase with bulk chloride ion concentrations up to around 0.6 M NaCl, whereas in solutions with sufficiently low dichromate (or chromate), inhibitor additions are insensitive to the bulk chloride concentration and are typically at least double those observed without the additions. DCB specimens, fatigue pre-cracked in air before immersion in a saline environment, show an initial period with no detectible crack growth, followed by crack growth at the distilled water rate, and then transition to a higher crack growth rate typical of region 2 crack growth in the saline environment. Time spent in each stage depends on the type of pre-crack (“pop-in” vs fatigue), applied stress intensity factor, alloy chemistry, bulk environment, and, if applied, the external polarization. Apparent activation energies (Ea) for SCC growth in Al-Zn-Mg-Cu alloys exposed to 0.6 M NaCl over the temperatures ranging from 293 K to 353 K (20 °C to 80 °C) for under-, peak-, and over-aged low-copper-containing alloys ( ~0.8 wt pct), they are typically ranging from 20 to 40 kJ/mol for under- and peak-aged alloys, and based on limited data, around 85 kJ/mol for over-aged tempers. This means that crack propagation in saline environments is most likely to occur by a hydrogen-related process for low-copper-containing Al-Zn-Mg-Cu alloys in under-, peak- and over-aged tempers, and for high-copper alloys in under- and peak-aged tempers. For over-aged high-copper-containing alloys, cracking is most probably under anodic dissolution control. Future stress corrosion studies should focus on understanding the factors that control crack initiation, and insuring that the next generation of higher performance Al-Zn-Mg-Cu alloys has similar longer crack initiation times and crack propagation rates to those of the incumbent alloys in an over-aged condition where crack rates are less than 1 mm/month at a high stress intensity factor.

Beneficial Patch Repair Effect on Fatigue Crack Growth of Al-Alloy 7050

Fatigue problems become an important topic in the maintenance of aircraft structures. Efficient repair technique, called composite patch repair, was used to reinforce the damaged structures and extend the service life. In this paper, using empirical approach, the effect of composite patch repair on fatigue crack growth was investigated on 7050 aluminum alloy. In additional, loading parameters associated with patch repair (Graphite/Epoxy) was studied in order to show their influence on fatigue life and fatigue crack growth rate, namely stress ratio. Results provide an increasing in fatigue life and fatigue crack growth rate in increased stress ratio. A delayed in fatigue life and diminution of FCGRs is the results of composite patch repair. Effect of patch repair was shown highly at high stress intensity factor when maximum applied loading is kept constant.

ナノ結晶ニッケル電着薄膜の疲労特性に及ぼす膜厚の影響

Two types of free-standing nickel thin films were produced by electrodeposition using sulfamate solution : CC films made under constant current without brightener and CCally films with brightener. The effect of film thickness on the tensile and fatigue properties was studied by using films with 10 and 30μm in thickness. The grain size of CC films determined by X-ray line broadening was about 52nm on the substrate side and increased away from the substrate side resulting in coarser grain size of thicker films on the solution side. The grain size of CCally film was about 9 to 10nm, and the size was slightly larger on the solution side. The fracture strength and yield strength in tension tests were higher in thinner films, and followed the Hall-Petch relation irrespective of film thickness when plotted in terms of the grain size measured on the solution side. The fatigue strength was also higher in thinner films, and the Hall-Petch relation was observed when plotted in terms of the grain size measured on the solution side. The resistance to fatigue crack propagation was higher in thicker films. The threshold stress intensity factor increased linearly with the square root of the grain size irrespective of film thickness. The thickness-effect of thin films on fatigue properties comes from the distribution of the grain size throughout thickness and the coarser grain size made on the solution side controlled the strength of the films. The fatigue fracture surface near the threshold consisted of granular features whose size was larger for films with coarser grains. At high stress intensity factors, striations were observed on the fracture surface of CC films, while only fine granular feature was observed for CCally films.

Stress ratio effects on small fatigue crack growth in Ti–6Al–4V

A systematic study of the effects of stress ratio on small fatigue crack growth in Ti–6Al–4V was conducted. Cylindrical fatigue specimens were tested axially at room temperature under a maximum stress of 690 MPa and with stress ratios ( R) of 0.5, 0.1, and −1. Tests were periodically interrupted and a standard replication technique was used to monitor the growth of cracks artificially initiated from 30 to 40 μm micro-notches, which were milled into the specimen surface with a focused ion beam (FIB). Measurement of striation spacing from fracture surfaces was evaluated for determining small crack growth rates and showed good agreement with replication data, but is only possible for relatively high stress intensity factor ranges, Δ K, on the order of 10 MPa√m or greater. A significant small crack effect is observed in this alloy, consistent with previous observations, where small cracks grew at stress intensity factor ranges below the long crack threshold and at higher rates than long cracks for equivalent Δ K levels. While a modest effect of stress ratio is seen on small crack growth rates when plotted as a function of crack size (faster growth at lower mean stresses for a given maximum stress), no discernable effect of R is seen when plotting as a function of Δ K. Significant scatter is observed in the small crack growth rates, and the implications of data reduction methods are discussed.

Fatigue crack fusion in thin-sheet aluminum alloys AA7075-T6 using low-speed fiber laser welding

Reinforcing cracked aluminum structures with composite patches have been recognized as an efficient and economical method to extend the service life of cracked aluminum components. To further enhance the effectiveness of composite patches, it is envisioned that the crack can be first fused by laser welding to remove the high stress concentration at the crack front before applying the composite patch. In this paper, the feasibility of the envisioned fusion repair is investigated. A systematic approach for the fusion process design is proposed to overcome challenges related to alloy strength recovery, crack tracing, focusing position, welding speed, plate flatness, shielding gas pressure, thin-sheet factors, and skewed cracks. A thick-sheet, partial penetration model is first used to determine the starting point of laser welding conditions. A systematic approach to transfer the thick-sheet condition to successful thin-sheet welding is then presented. Based on successfully fused crack samples of AA 7075-T6, the ultimate tensile strength tests show that in average 74% of the alloy's original strength was recovered for a single-pass repair and 68% for a double-pass repair and the results are highly repeatable. It should be clear to see the benefit of the crack fusion because without crack fusion, the composite patch is bonded to a part with zero UTS at the crack region and with a high stress intensity factor at the crack front.

Fatigue Properties of Nano-Crystalline Nickel Electrodeposited Thin Films

Nickel nanocrystalline thin films were produced by electrodeposition using sulfamate solution. Three types of thin films with different grain sizes were produced : CC films were made under constant current, PC films under pulse current, and CC-ally films under constant current with grain refinement additive. The grain size gets smaller in the order of CC, PC, and CC-ally films down to nanometers. The fracture strength and yield strength in tension tests follows the Hall-Petch relation, and the elongation was largest for CC-ally films. The fatigue strength increased with decreasing grain size, following the Hall-Petch relation down to about 10nm. The resistance to fatigue crack propagation decreased for nano grain-sized films. The threshold stress intensity factor was the smallest for PC and CC-ally films. In the intermediate-rate range, the propagation rate increased with decreasing grain size when compared at the same stress intensity factor. The fatigue fracture surface near the threshold consisted of granular features whose size decreased with decreasing grain size. At high stress intensity factors, striations were observed on the fracture surface of CC films, while only fine granular feature was observed for CC-ally films. On the specimen surface near the fatigue fracture surface of CP and CC-ally films, the small grain boundary fracture facets were observed at high stress intensity factors, while not observed near the threshold. For CC films, slip bands were seen together with the grain boundary fracture facets.

Stress Ratio Effect on Fatigue Behavior of Aircraft Aluminum Alloy 2024 T351

AbstractAluminum alloy series 2xxx, 6xxx, 7xxxx and 8xxx enjoy the widest use in aircraft structural applications. Among these materials, aluminum alloy 2024 remains the most commonly used and especially in T351 temper situation. The fatigue crack propagation behaviour of aluminum alloy 2024 T351 has been investigated using V-notch specimen in four bending test. A series of stress ratios from 0.10 to 0.50 was investigated in order to observe the influence of stress ratio on the fatigue life and fatigue crack growth rate (FCGR). The increase in FCGR, which occurs as the stress ratio is increased from 0.10 to 0.50, is generally attributed to an extrinsic crack opening effect. In T-S orientation and at low stress intensity factor, the increasing of stress ratio increase the FCG. Experimental results are presented by Paris law when coefficients C and m are affected by stress ratio. Contrary, at high stress intensity factor, the effect of stress ratio is reversed. We notice a decreasing of fatigue crack growth rate with an increasing of stress ratio. This effect may be explained by microstructure effect in (T-S) crack growth. The analysis of stress ratio effect by Elber model, shown that this model gives bad interpolation in this situation and the parameter characterized the crack closure factor will be adjusted.

Weld residual stress effects on fatigue crack growth behaviour of aluminium alloy 2024-T351

The interaction between residual stress and fatigue crack growth rate has been investigated in middle tension and compact tension specimens machined from a variable polarity plasma arc welded aluminium alloy 2024-T351 plate. The specimens were tested at three levels of applied constant stress intensity factor range. Crack closure was continuously monitored using an eddy current transducer and the residual stresses were measured with neutron diffraction. The effect of the residual stresses on the fatigue crack behaviour was modelled for both specimen geometries using two approaches: a crack closure approach where the effective stress intensity factor was computed; and a residual stress approach where the effect of the residual stresses on the stress ratio was considered. Good correlation between the experimental results and the predictions were found for the effective stress intensity factor approach at a high stress intensity factor range whereas the residual stress approach yielded good predictions at low and moderate stress intensity factor ranges. In particular, the residual stresses accelerated the fatigue crack growth rate in the middle tension specimen whereas they decelerated the growth rate in the compact tension sample, demonstrating the importance of accurately evaluating the residual stresses in welded specimens which will be used to produce damage tolerance design data.

The effect of inclusion geometry according to forging ratio and metal flow direction on very high-cycle fatigue properties of steel bars

The objectives of the present paper are to clarify the effect of inclusion geometry according to the forging ratio and the metal flow direction on very high-cycle fatigue properties of steel bars and to propose an evaluation method for characteristic inclusion geometries observed at fracture origin. Specimens were made of steel plates rolled to three levels of thickness from steel bars of low alloy steel. Then the specimens were cut in both parallel and perpendicular directions to the metal flow of the steel plates. Fatigue tests were conducted in a long life regime by using an ultrasonic fatigue testing machine. The fatigue limit for 10 9 cycles decreased with the increase of the thickness of steel plate, i.e. a decrease of the forging ratio. The fatigue limit of specimens cut parallel to metal flow was higher than that of specimens cut perpendicularly to metal flow. Fish-eye fractures appeared on the fracture surface of most specimens. Especially, characteristic inclusions, which are clusters of oxides, sulfides and their combination, were mainly observed at the origin of fish-eye fractures in specimens cut perpendicularly from thick plates. In this paper, these characteristic inclusions were treated as the extended elliptical inclusion/crack from the point of view of fracture mechanics and the area parameter model. More careful inspection at the fracture origin revealed that the rough surface area called the optically dark area (ODA) appeared only around the edge of the short radius on the extended elliptical inclusion where the higher stress intensity factor is generated. This result suggests that the middle part of the extended elliptical inclusion dominates the formation of ODA and the very high-cycle fatigue property. Finally, based on the fracture mechanics approach, area L/ W=4 , which is the square root of the area of the surrounded part where length/width < 4 at the middle of the extended elliptical inclusion, is proposed as the evaluation parameter for area parameter model.

Threshold stress intensity factor, KIH for delayed hydride cracking of a Zr-2.5Nb tube with loading mode

Abstract The threshold stress intensity factors, KIH's, for delayed hydride cracking of a Zr-2.5Nb tube were determined using either load-increasing or decreasing modes over a temperature range of 160–280 °C with a step-wise load increase or decrease by 0 .5 MPa m , respectively. KIH's in the load-increasing mode were found to be higher than those in the load-decreasing mode. The striation spacing corresponding to the critical hydride length or lc showed a sharp drop between the 1st striation line to the 9th line and then leveled off to a constant value in the load-increasing mode. In the load-decreasing mode, it was rather constant from the 1st line to the 14th line due to a higher stress intensity factor KI with its gradual increase at a reduced KI closed to KIH. Supplementary tests demonstrated that a higher KIH in the load-increasing mode is due to the combined effects of a larger tip radius of the fatigue crack and creep working in the load-increasing mode. KIH had a negative temperature dependency in the load-increasing mode and it had no temperature dependency in the load-decreasing mode. To correlate KIH with twins, tensile tests were conducted on the Zr-2.5Nb tube and its textural changes during the tensile tests were investigated using an X-ray diffractometer. The number of twins increased over a temperature range of 150–300 °C and then decreased rapidly at over 300 °C with no twins at 350 °C. KIH of the Zr-2.5Nb tube with the loading modes was discussed in view of cracking of the hydrides by twining.

Super-Hard Carbon Layers Produced on the Al2O3/Al2O3+x%SiC (whiskers) Ceramic Cutting Edges

The aim of the present study was to examine how the cutting properties of tool blades are affected by super-hard ceramic (carbon) coatings produced by the RF PECVD method. The cutting plates were made of the gradient-type Al2O3/Al2O3 + x%SiCwhiskers composites. The highest density, the greatest bending strength and the highest stress intensity factor were obtained with the layered Al2O3/Al2O3 + x%SiCwhiskers composite. The phase composition of the coatings was examined and the deposited carbon layers were identified to be nanocrystalline diamond (NCD) contaminated with super-finegrained graphite. The thickness, roughness, adhesion to the substrate, friction coefficient and hardness of the coatings were examined. The results evidently show that, when tested in cutting hard aluminium alloys, the carbon coatings deposited on the Al2O3/Al2O3 + x%SiCwhiskers cutting plates by RF PECVD have advantageous cutting properties. The service life of the coated plates compared to that of uncoated plates increased by 65% with the NCD coatings.

OS4-2-4 Estimation of applied stress in the near threshold of fatigue crack propagation utilizing high frequency current impedance and hardness measurement

In the failure analysis of fatigue fracture accident of industrial machinery, the estimation of applied stress on the fatigue crack is an important issue in order to determine the cause of failure and to prevent another accident. The estimation of applied stress using fractographic technique has been already established. However, the estimation can be applicable in relatively high stress intensity factor region. Low stress intensity factor region, however, is much more important in the real engineering. The objective of this study is the development of estimation method of applied stress, which can be used in the near threshold region of crack propagation. High frequency current impedance method and fracture surface hardness method were examined. In the impedance method, there was a correlation between the high frequency current impedance of fatigue fracture surface and stress intensity factor range. In the hardness method, micro Vickers hardness was measured on the fatigue fracture surface. The hardness parameter of fatigue fracture surface was defined as the area of indentation mark in order to reduce the effect of surface roughness of the fatigue fracture surface. The hardness parameter had a correlation with the stress intensity factor range in the near threshold region. Each method can be used in the stress estimation in low stress intensity factor region.

最大応力拡大係数(K値)Kmax増による下限界K値範囲ΔKth漸減現象への環境効果の検討

最大応力拡大係数(K値)Kmax増による下限界K値範囲ΔKth漸減現象への環境効果の検討

Dynamic interfacial debonding initiation induced by an incident crack

When a crack propagates towards a weak interface, interface debonding may occur before the incident crack reaches the interface. This phenomenon refers to the “Cook–Gordon mechanism”. In this investigation, an equivalent dynamic Cook–Gordon mechanism is studied both experimentally and analytically. Two strength-based criteria incorporating dynamic fracture mechanics analysis are proposed to predict the initiation location of interface debonding ahead of a dynamic incident crack. As validation, a comparison is made between the analytical predictions and experimental measurements. Results show that the strength-based criteria can effectively predict the initiation of interface debonding. Meanwhile, effects of the stress intensity factor and the T stress of the incident crack, on the interfacial debonding initiation are investigated. It is concluded that high-stress intensity factors of the incident cracks will easily induce interfacial debonding initiation, and changing the T stress is an effective way to control interfacial debonding initiation. Furthermore, high-interfacial tensile strengths rather than shear strengths, tend to suppress interfacial debonding initiation induced by a mode-I incident crack.