Cyclic Fatigue Crack Growth Rates Research Articles

Effects of laser shock processing on high cycle fatigue crack growth rate and fracture toughness of aluminium alloy 6082-T651

The effects of laser shock processing without protective coating on high-cycle fatigue crack growth and fracture toughness were investigated. Laser shock peening treatment was performed on compact tension specimens from both sides perpendicular to the crack growth direction, followed by subsequent grinding. Fatigue crack growth tests were performed at frequencies between 116 and 146Hz, at R=0.1 and a constant stress intensity range during the fatigue crack initiation phase and K-decreasing test. A lower number of cycles was required to initiate a fatigue precrack, and faster fatigue crack growth was found in tensile residual stress field of LSP-treated specimens. The crack growth threshold decreased by 60% after LSP treatment. The fracture toughness decreased by 28–33% after LSP treatment. The fatigue-to-ductile transition boundary on fractographic surfaces show linear fatigue crack fronts in non-treated specimens and curves after LSP treatment.

Thermal fatigue life prediction of particle reinforced composites

This article proposes a thermal fatigue life prediction model for particle reinforced composites. First, a three-phase model was used to determine the effective eigen strain of a two-phase element formed by an ellipsoidal matrix shell surrounding an ellipsoidal particle, and then the disturbance strain in the two-phase element caused by thermal inconsistency was obtained. If the average stress in the effective medium was considered as vanished, the thermal stress field in the matrix and particles was obtained by considering random orientation of two-phase elements. Second, based on the interface cracks propagating around the ellipsoidal particles, the strain energy release rate was examined. When the strain energy release rate reached the critical value of the matrix, the debonding stress was determined. The subtending angle and initiation crack length were available. Finally, according to the low cycle fatigue crack growth rate formation, the thermal fatigue life was computed. The results show that the bigger the thermal stress is, the smaller the thermal fatigue life is, indicating that thermal fatigue life is an exponential function of crack initiation length and critical length.

Rubber abrasion at steady state

Blade abrasion devices have previously been used to investigate the abrasion resistance of different elastomers. Despite extensive investigation the exact mechanisms that are present during abrasion continue to be the subject of debate. This investigation examines the effect of the normal and frictional forces on the rate and direction of crack growth during the abrasion process. Comparison is drawn between the rates of material loss as measured experimentally on two different styrene butadiene rubber compounds under a range of test conditions. For each set of conditions the representative stored energy release rate is calculated using a finite element analysis technique on a model of a specific abrasion asperity contacting a sliding rigid abrader. It is therefore possible to plot a graph of the stored energy release rate versus crack growth rate for an abrasion process. This is compared with an independently measured cyclic fatigue crack growth rate versus stored energy release rate. The correlation between these two independent tests was excellent. From this work insight into the nature of abrasion is gained.

Cyclic Fatigue Crack Growth in Three‐Point Bending PZT Ceramics under Electromechanical Loading

This paper investigates experimentally and analytically the cyclic fatigue crack growth in piezoelectric ceramics under electromechanical loading. Cyclic crack growth tests were conducted on lead zirconate titanate (PZT) ceramics subjected to dc electric fields, and a finite element analysis was used to calculate the maximum energy release rate for the permeable crack model. Based on bending experiments using single‐edge precracked‐beam specimens, cyclic fatigue crack growth rates are found to be sensitive to the maximum energy release rate and applied dc electric fields. Possible mechanisms for crack growth were discussed by scanning electron microscope examination of the fracture surface of the PZT ceramics.

Evaluation of the Effect of Crack Tip Constraint on Fatigue Crack Growth Rate in Inconel 718

Abstract The objective of this work has been to characterize the fatigue crack growth rate of Inconel 718 in the elastic and elastic-plastic regimes. The major new contribution here is to develop fatigue crack growth rate data on this alloy using shallow crack specimens subjected to cyclic loadings that involve material plasticity exceeding what is allowed by the standard Linear Elastic Fracture Mechanics (LEFM) procedures of ASTM E 647. Compact (C(T)) and three point bend (SE(B)) specimen geometries were used in this investigation. The SE(B) specimens were used to obtain shallow crack data using crack to depth ratios (a/W) as small as 0.08 in standard bend specimens with W = 50.8 mm. Compliance methods were used to estimate the crack length during the cyclic testing. The C(T) specimens were used to investigate the effect of fully reversed loading, i.e., R = −1.0. These specimens were tested only in deep crack configurations with a/W > 0.3. Both C(T) and SE(B) specimens were used to obtain high cycle fatigue crack growth data as well as some component of the low cycle fatigue crack growth rate data. The cyclic elastic stress intensity range was used to characterize the crack growth driving “force” in the high cycle regime as defined in ASTM E 647. A cyclic J integral range, as originally utilized by Dowling and Begley [1], was used in the elastic-plastic regime. High cycle and low cycle fatigue crack growth data were successfully obtained from tests on shallow crack SE(B) specimens. High cycle fatigue crack growth was not affected by specimen geometry or by crack length ratio, even for a/W ratios as low as 0.08. The low cycle fatigue crack growth rate was similar, but not identical, to what one would get by extrapolating the high cycle fatigue crack growth rate, as proposed by Dowling and Begley [1]. Under conditions of increasing J range, the crack growth rate under elastic-plastic conditions was accelerated, while under decreasing J range conditions the crack growth rate was decelerated in comparison to the extrapolated high cycle fatigue crack growth rate measurements. The R ratio did not affect the crack growth rate for the two cases tested here, namely R = 0.1 and R = −1.0. More rapidly increasing J range conditions resulted in greater crack growth rate acceleration. In the elastic-plastic fatigue regime, shallow crack specimens demonstrated slower crack growth rates than deep crack specimens.

The effect of anelasticity and phase transformation on crack growth in Y-TZP ceramics

Crack growth behavior has been investigated under monotonic and cyclic loadings for Y-TZP that produces remarkable anelastic strain. Monotonic loading testing was carried out under the condition of various stress rates (8 × 102, 8 × 10−1 and 8 × 10−4 MPa/s) and temperatures (RT and 373 K). Resistance of crack propagation was observed at the lowest stress rate at elevated temperature. Cyclic fatigue crack growth rate was examined under the condition of different frequencies and stress waveforms. Crack growth rate clearly depended on stress waveform, which was explicable by exhaustion and restoration of anelasticity at the crack tip region. Experimental results make it clear that anelasticity works as strong resistance against crack growth. In this study, the effect of environment-induced tetragonal to monoclinic phase transformation on fracture strength was also investigated for pre-cracked sample. Aged (transformed) samples have shown extreme crack closure and considerable improvement in strength.

The Evaluation of Cyclic Fatigue Crack Growth Rate by Stress Intensity/Life Test in Silicon Nitride

The Evaluation of Cyclic Fatigue Crack Growth Rate by Stress Intensity/Life Test in Silicon Nitride

Cyclic fatigue crack growth behaviour in β-(Si-Al-O-N) at ambient and elevated temperatures

Four-point bending fatigue tests on a hot-pressed sintered Sm–β-(Si–Al–O–N) ceramic were conducted at room temperature, 900 °C and 1000 °C in air under different load ratios and cyclic frequencies. The growth of indentation cracks was measured during the fatigue tests. The results indicate that the cyclic fatigue crack growth threshold is lower and crack growth rates are higher, for given values of Kmax, at 1000 °C than those at room temperature. The cyclic fatigue crack growth behaviour at 900 °C is similar to that at room temperature. It was found that the crack growth retardation due to cyclic fatigue loading is much more pronounced at higher frequencies. An increase in cyclic frequency from 1 to 10 Hz cause a reduction of up to two orders of magnitude in crack propagation rates. High-temperature cyclic fatigue crack growth rates increased and threshold stress intensity factor ranges decreased with increasing load ratio. Possible mechanisms for cyclic crack growth are discussed.

Micromechanisms of creep-fatigue crack growth in α′-β′-SiAlON at 1200 °C

The cyclic fatigue crack growth characteristics of gas pressure sintered (GPS) Y-α′-β′-Sialon at 1200 °C were examined by ‘controlled’ surface cracking from Vickers indentations in bars loaded in four-point bending. The stress intensity factor for semi-elliptical surface cracks in bending was calculated in terms of the stress and dimensions of the subcritically growing cracks. For comparison, static fatigue crack growth experiments were also carried out on the material. Similar to the fatigue crack growth behaviour of Si 3N 4 at ambient temperature, the high-temperature static and cyclic fatigue crack growth rates showed three characteristic regions having a plateau with a lowest Paris exponent. In the lower stress intensity factor region, cyclic fatigue crack growth rate was slower than that of static fatigue at the same stress intensity factor. In the higher stress intensity factor region, the crack growth rates, both static and cyclic fatigue, were almost the same. Electron microscopy analysis in the crack-tip damage zone indicated that, in addition to the subcritical crack growth of the main crack assisted by oxidation, creep damage mechanisms such as cavity formation and microcracking may be responsible for static fatigue crack growth in present study. However, it is not significant in the case of cyclic fatigue. Besides, deformation process such as dislocation activity within the matrix grains appeared to play an active role in determining the crack growth kinetics for cyclic loading. It seems that the fatigue stresses will help aid the crystallization process of the amorphous phase and lead to increase the role of dislocation plasticity during high temperature fatigue fracture.

Comparison of the growth of individual and average microcracks in the fatigue of AlSiC composites : Chen, E.Y., Lawson, L. and Meshii, M. Metall. Mater. Trans. A (Dec. 1995) 26A (12), 3163–3176

The ambient-temperature subcritical growth behaviour of SiC-whisker-reinforced yttria-stabilized zirconia (SiCw/3Y-TZP) ceramic composites is investigated during cyclic fatigue loading. Based on long-crack experiments using four-point bending specimens, cyclic fatigue crack growth rates (over the range 10−10 to 10−5(m/cycle)) are found to be sensitive to the maximum stress intensity factor and load ratio. Similar to other ceramic materials, the long crack fatigue threshold, ΔKTH, is found to be of the order of 45% of KIc, and enhanced with increasing fracture toughness. A unique growth law strongly dependent on the Kmax and modestly on ΔK is established. Possible mechanisms for cyclic crack growth in whisker-reinforced zirconia composite materials are discussed.

Cyclic fatigue-crack propagation in SiC-whisker-reinforced Y-TZP composites

The ambient-temperature subcritical growth behaviour of SiC-whisker-reinforced yttria-stabilized zirconia (SiCw/3Y-TZP) ceramic composites is investigated during cyclic fatigue loading. Based on long-crack experiments using four-point bending specimens, cyclic fatigue crack growth rates (over the range 10 −10 to 10 −5(m/cycle)) are found to be sensitive to the maximum stress intensity factor and load ratio. Similar to other ceramic materials, the long crack fatigue threshold, ΔK TH , is found to be of the order of 45% of K Ic , and enhanced with increasing fracture toughness. A unique growth law strongly dependent on the K max and modestly on ΔK is established. Possible mechanisms for cyclic crack growth in whisker-reinforced zirconia composite materials are discussed.

Cyclic fatigue—crack growth along polymer/glass interfaces

AbstractDelamination of polymer/glass interfaces was studied under cyclic and monotonic loading using an interfacial, four‐point flexure sandwich specimen. Specifically, crack growth rates along epoxy acrylate/glass interfaces were characterized over a range of velocities from 10−9 to 10−6 m/s as a function of low (10–20% RH) and high (75–80% RH) humidities. For low humidities, interfacial crack growth rates under cyclic loading are almost two orders of magnitude greater than those under monotonic loading with energy release rates G = Gmax of the cyclic loading. At high humidities, interfacial crack growth rates under monotonic loading are approximately equal to cyclic fatigue crack growth rates at low humidity.

Cyclic fatigue and resistance-curve behavior of anin situ toughened silicon carbide with Al[sbnd]B[sbnd]C additions

The room-temperature crack-growth properties of anin situ toughened, monolithic silicon carbide are reported. Hot pressing was performed at 1900°C with 3 wt.% Al, 2 wt.% C and 0.6 wt.% B additions. Compared to a commercial SiC (Hexoloy SA), significant improvements in both the fracture toughness and cyclic fatigue-crack propagation resistance have been achieved through control of the β to α transformation. Using fatigue-precracked, disk-shaped compact-tension specimens, marked rising resistance-curve behavior was measured over the first ∼ 600 μm of crack extension, leading to a “plateau” fracture toughness ofKc ∼ 9.1MPa√m; this represents more than a threefold increase over the toughness of Hexology, where aKc value of 2.5 MPa√m was measured with no evidence of a resistance curve. Cyclic fatigue-crack growth rates in the toughened SiC were found to be faster than those under sustained loads (static fatigue) at the same stress-intensity level. The cyclic fatigue-crack growth resistance was found to be far superior to that of Hexology. Whereas cracking in the commercial SiC became unstable when the maximum stress intensityKmax exceeded ∼ 2 MPa√m, thresholds for fatigue-crack growth in thein situ toughened material exceeded aKmax of 7 MPa√m. Such dramatic improvements in the crack-growth resistance of SiC are attributed to a microstructure consisting of a network of interlocking, plate-like predominantly α-phase grains, which combine to both bridge and deflect the crack. Under cyclic loads, fatigue-crack growth is promoted by the cycle-dependent decay in such crack-tip shielding due to frictional-wear degradation of the zone of grain bridging ligaments in the crack wake. These results represent the first reported evidence of cyclic fatigue behavior in a monolithic silicon carbide and the first direct measurement of the resistance curve properties in this ceramic.

<i>R</i>-Curve and Fatigue Crack Growth Behavior in Silicon Nitride Ceramics

The R-curve and fatigue behavior were investigated for two kinds of silicon nitride with different microstructures. In these materials the toughening exponents determined by the indentation-strength-in-bending method are almost zero. Moreover, the values of fracture toughness obtained by the indentation crack size measurement and the single-edge-precracked-beam method are the same. Therefore, it is concluded that these materials show flat R-curves. On the other hand, it is found that a considerable cyclic-fatigue effect, distinct from static fatigue, exists in these materials. Cyclic-fatigue crack growth rates are higher in small grained silicon nitride than in large grained one. When the crack growth behavior is represented by a power law relationship including two terms of the stress intensity range ΔK and the maximum stress intensity Kmax; i.e. da/dN=C(Kmax)p(ΔK)q, significant differences in the values of the exponent p and q between these materials are not recognized. On the basis of the above results and other data available, the factors affecting the cyclic-fatigue behavior and cyclic fatigue mechanisms are discussed.

Elevated-temperature crack growth in polycrystalline alumina under static and cyclic loads

An experimental investigation has been conducted to study the crack growth characteristics of a 90% pure aluminium oxide in 1050 °C air under static and cyclic loads. It is shown that the application of both sustained and fluctuating tensile loads to the ceramic, tested in a precracked four-point bend specimen configuration, results in appreciable subcritical crack growth. The crack velocities under cyclic loading conditions are up to two orders of magnitude slower than those measured in static loading under the same maximum stress intensity factor. Cyclic crack growth rates are markedly affected by the loading frequency, with a decrease in test frequency causing an increase in the rate of crack advance. Detailed optical and electron microscopy observations have been made in an attempt to study the mechanisms of stable crack growth and the mechanistic differences between static fatigue fracture. Under both static and cyclic loads, the predominant mode of fracture is intergranular separation. The presence of a glass phase along the grain boundaries appears to have a strong effect on the mechanisms of crack growth. Apparent differences in the crack velocities between static and cyclic fatigue in alumina arise from crack-wake contact effects as well as from the rate-sensitivity of deformation of the glass phase. Our results also indicate that the cyclic fatigue crack growth rates cannot be predicted solely on the basis of sustained load fracture data. White stable crack growth occurs in the 90% pure alumina over a range of stress intensity factor spanning 1.5 to 5 MPa m1/2, such subcritical fracture is essentially suppressed in a 99.9% pure alumina, ostensibly due to the paucity of a critical amount of glass phase. Both static and cyclic fracture characteristics of the 90% pure alumina are qualitatively similar to those found in an Al2O3-SiC composite wherein situ formation of glass phases, due to the oxidation of SiC in high-temperature air, is known to be an important factor in the fracture process.