Crack Initiation Rate Research Articles

Statistical approach for evaluating SCC initiation and propagation behavior of sensitized Type 304 stainless steel

The crack initiation and propagation behavior of a sensitized Type 304 stainless steel in distilled water containing small amounts of Na 2 SO 4 or Na 2 B 4 O 7 impurities was investigated by using slow strain rate technique (SSRT) with an in-situ cracking observation system, and analyzed with a statistical approach. It was found that the crack initiation time for the steel in Na 2 SO 4 or Na 2 B 4 O 7 solutions obeyed the exponential distribution, while K ISCC and crack velocity obeyed the Weibull distribution. An increase in Na 2 B 4 O 7 concentration decreased the SCC susceptibility of the steel by delaying crack initiation time, reducing the crack initiation rate and increasing K ISCC . In contrast, an increase in Na 2 SO 4 concentration increased SCC susceptibility of the steel by shortening crack initiation time and accelerating crack propagation.

Cyclic deformation and near surface microstructures of shot peened or deep rolled austenitic stainless steel AISI 304

Cylindrical specimens of the austenitic stainless steel AISI 304 were shot peened or deep rolled with different peening intensities, and rolling pressures, respectively. The resulting near surface properties were characterized by cross sectioning transmission electron microscopy (TEM), residual stress and phase analysis as well as interference line half-width and microhardness measurements. Cyclic deformation curves were obtained by hysteresis measurements under stress control with zero mean stress. The microstructural alterations in the fatigued surface regions were again characterized by the above mentioned methods. The investigations revealed that both shot peening and deep rolling lead to a complex near surface microstructure, consisting of nanocrystalline regions, deformation bands and strain induced martensitic twin lamellae with high dislocation densities in the austenitic matrix. These microstructural changes severely influence the cyclic deformation behaviour: Plastic strain amplitudes and cyclic creep were drastically decreased by shot peening and especially by deep rolling. Both surface finishing methods were found to decrease crack initiation and propagation rate. Remarkably, the initial residual stress profile and surface strain hardening were not completely eliminated even by applying high cyclic stress amplitudes. This is due to the fact that during cyclic loading dislocation cell structures were only formed in greater depths whereas the nanocrystalline layer remained stable. In the case of deep rolled surfaces, the martensitic layer was even increased by fatigue-induced martensite formation.

Influence of Artificial Defect on Fatigue Limit in Austempered Ductile Iron

Rotary bending fatigue tests were carried out to investigate the influence of artificial defects on fatigue limit in annealed and austempered ductile iron. Obtained main results are as follows : (1) Artificial defect(micro hole type, dia. which is the critical defect size to crack initiation at artificial defect, of ADI is larger than that of annealed ductile iron. This shows that the situation of crack initiation at artificial defect in ADI is more difficult in comparison with annealed ductile iron. (3) One of the reasons for the low rate of crack initiation from artificial defect in ADI is that the resistance of matrix to crack initiation is higher than that of annealed ductile iron. (4) In case that the of artificial defect and graphite nodule is the same, the rate of crack initiation from graphite nodule is higher than that from artificial defect. This reason is that the serious ruggedness around graphite nodule is formed by austempering treatment.

Intergranular Stress Corrosion Cracking Initiation and Growth in Mill-Annealed Alloy 600 Tubing in High-Temperature Caustic

Abstract Stress corrosion cracking (SCC) crack initiation and crack growth rates (CGR) were measured in mill-annealed alloy 600 (UNS N06600) tubing as a function of the stress intensity factor (K) ...

Influence of temperature and carbon content on the cyclic deformation and fatigue behaviour of α-iron. Part II: Crack initiation and fatigue life

The crack initiation mechanisms and the fatigue life of α-iron polycrystals have been characterized in plastic-strain-controlled tests with small to intermediate plastic strain ranges Δ ϵ pl in the temperature range from 220 to 400 K. The fatigue life of pure decarburized α-iron cannot be described generally by means of a Manson–Coffin law, except under conditions under which distinct cyclic hardening occurs. A Basquin law is found to be valid, when the effective stress component σ∗ dominates (i.e. at low temperatures and small Δ ϵ pl). At low temperatures and up to room temperature, intergranular crack initiation is observed. At higher temperatures, mostly transgranular cracks develop in decarburized α-iron in the valleys of the surface rumpling, while in α-iron with small carbon contents persistent slip bands form which then act as sites of transgranular crack initiation. Crack initiation rates and crack growth rates are evaluated in a semi-quantitative manner from crack depth distributions.

A procedure for estimation of pipe break probabilities due to IGSCC

A procedure has been developed for estimation of the failure probability of weld joints in nuclear piping susceptible to intergranular stress corrosion cracking. The procedure aims at a robust and rapid estimate of the failure probability for a specific weld with known stress state. Random properties of the crack initiation rate, the initial crack length, the in-service inspection efficiency, and the leak rate, are taken into account. A computer realization of the procedure has been developed for user-friendly applications by design engineers. Some examples are considered to investigate the sensitivity of the failure probability to different input quantities.

A comparison of short and long fatigue crack growth in steel

In this study, the growth behaviour of short fatigue cracks in En7A steel with a high content of elongated MnS inclusions has been investigated by generating and evaluating short crack growth data for the six principal specimen orientations. The anomalous initiation and growth behaviour of short cracks was also compared with that of long crack data for the same material and loading conditions determined before. This comparison revealed the sub-threshold initiation and faster growth rate of short cracks compared with long cracks under the same stress intensity factor range. In order to simulate this discrepancy between short and long cracks, a crack closure model for long crack data was used.

Reliability Improvement of Aluminum Wirebonds in High‐Power Igbt Modules

AbstractWirebonding is the weakest area of device packaging of power IGBT modules. Accelerated thermal fatigue testing causes cracks to form and propagate in the aluminum wirebond at the foot area. This study examined the relationship of the wirebond reliability and the aluminum wire grain structure, which can be affected by post‐wirebond heat treatment. A series of wirebonded IGBTs were annealed at a temperature range from 280°C to 400°C for up to 60 minutes. Wirebond shear strengths versus temperature cycles were examined. Cross‐sectional SEM was used to examine both aluminum grain size development by annealing and crack initiation and propagation in the wirebonds after temperature cycling. It was found that aluminum grain size was increased by post‐wirebond annealing. With temperature cycling, the wirebond shear strengths of the as‐wirebonded samples decrease rapidly, and lifted wirebonds were present after 1500 temperature cycles. The lifted wirebonds typically break within the aluminum wire near the wire/metallization interface. The shear strength of the wirebonds with post‐wirebond annealing showed no significant change even after 5000 temperature cycles, and there were no signs of significant deterioration of the wirebonds either. The wirebond crack initiation and growth rates were depressed substantially by larger aluminum grains. Annealing of the aluminum wire after wirebonding provided increased aluminum grain size resulting in improved reliability of the wirebonds of high power modules.

The characterization of damage in SCC by an electrochemical impedance and statistical study of multiple cracking

To build a realistic predictive model, it is essential to understand how the damage is developed. The evolution of this damage on 304L steel in Scheil reagent during slow strain rate tests has been studied for 4 various strain rates. An observation of the distribution of crack traces (number, size and morphology) has shown that the damage is closely linked to the strain rate. Interrupted CERTs have been realised for two strain rates. For each strain rate, electrochemical impedance measurements at fixed frequency (100 mHz) and histograms of the distribution of the crack traces have been carried out. The linked use of two techniques to characterize the damage in stress corrosion studies gives very interesting results. The evolution of the phase of the electrochemical impedance is closely linked to the initiation rate of cracks. Density of cracks that have reached a given size could be a new definition of the critical damage in stress corrosion studies when multiple cracks exist.

Micromechanics of stress and strain-controlled, high cycle fatigue crack initiation of intermetallic compounds

Tests have shown that ordered intermetallic compounds favor planar slip and tend to have no cross-slip or multislip. The yield stress of ordered compounds is less than that of disordered compounds. However, the stress-controlled, high cycle fatigue life of ordered compounds is appreciably more than that of disordered compounds. This is considered to be caused by a low rate of crack initiation. In contrast, strain-controlled tests show that ordering shortens life by crack initiation. In this paper, we attempt to explain these observed behaviors by the micromechanic theory of high cycle fatigue crack initiation developed by Lin and coworkers. The effect of ordering on crack initiation under cyclic combined axial and torsional loadings is also shown.

Micromechanics of high-cycle initiation of fatigue cracking intermetallic compounds

Tests have shown that ordered intermetallic compounds favor planar slip and tend to have no cross-slip or multi-slip. This was explained by the long Burger's vector in these ordered compounds. The flow stress of the ordered has been found to be less than that of the disordered ones. However the high-cycle fatigue life of the ordered was found to be significantly more than that of the disordered. This is believed to be due to low rate of crack initiation. Cracks have been found to initiate exclusively at slip bands extrusions and intrusions in intermetallic compounds. This paper shows a micromechanic fatigue model. This model was used to calculate the extrusion growth of a single fatigue band and then multiple fatigue bands in the most favorably oriented crystal. It was found that extrusion growth in the ordered alloy is significantly less than that in the disordered ones. The S− N curves to give given plastic shear strains of the extrusion at the free surface were calculated for both the ordered and disordered alloys. It was found that the ordered alloy has significantly higher S− N curves as observed in experiments.

Crack initiation and propagation during high-temperature fatigue of oxide dispersion-strengthened superalloys

The mechanisms of crack initiation and propagation have been investigated in two oxide dispersion-strengthened (ODS) Ni-base superalloys under conditions of symmetric low-cycle fatigue (LCF) and creep-fatigue. The behavior of both ODS alloys is compared with that of conventional alloys of otherwise similar composition. While the improvement in fatigue resistance previously reported for ODS metals and alloys is confirmed by the present study for temperatures below about0.6T m, the potential advantage of dispersion strengthening is not being exploited by the current generation of ODS superalloys at higher temperatures; crack initiation is found to occur prematurely due to the presence of recrystallization defects in the form of fine grains. The mechanism of crack initiation at fine grains is creep-type cavitation on boundaries transverse to the applied stress. Experimental results indicating the influence of temperature, loading frequency, and waveshape on the crack initiation rate are presented and discussed in detail. A qualitative correlation between waveshape and creep-fatigue life is suggested based on the macroscopic inelastic strain rate which is determined by the waveform and limits in turn the rate at which cavity growth can be accommodated.

Influence of microstructure on fatigue behavior and surface fatigue crack growth of fully pearlitic steels

This work examined the influence of microstructure on the surface fatigue crack propagation behavior of pearlitic steels. In addition to endurance limit or S(stress amplitude)-N(life) tests, measurements of crack initiation and growth rates of surface cracks were conducted on hourglass specimens at 10 Hz and with aR ratio of 0.1. The microstructures of the two steels used in this work were characterized as to prior austenite grain size and pearlite spacing. The endurance tests showed that the fatigue strength was inversely proportional to yield strength. In crack growth, cracks favorably oriented to the load axis were nucleated (stage I) with a crack length of about one grain diameter. Those cracks grew at low ΔK values, with a relatively high propagation rate which decreased as the crack became longer. After passing a minimum, the crack growth rate increased again as cracks entered stage II. Many of the cracks stopped growing in the transition stage between stages I and II. Microstructure influenced crack propagation rate; the rate was faster for microstructures with coarse lamellar spacing than for microstructures with fine lamellar spacing, although changing the prior austenite grain size from 30 to 130 jμm had no significant influence on crack growth rate. The best combination of resistance to crack initiation and growth of short cracks was exhibited by microstructures with both a fine prior austenite grain size and a fine lamellar spacing.

Growth of Short Fatigue Cracks at Notches in Woven Fiber Glass Reinforced Polymeric Composites

The fatigue crack initiation and growth characteristics of short cracks emanating from blunt notches with root radius varying from 1.6 to 6.35 mm at various depths in woven fiber-glass reinforced polymeric composites have been investigated. It is demonstrated that the initiation and growth rate of short cracks emanating from blunt notches can be accurately described by an effective stress intensity factor range ΔKeff. The ply orientations studied, did not have any effect on the analysis. The results provide an adequate engineering approach for designing against failure from range of stress concentrations, at least for this composite system.

ＳＭ５０Ａにおける疲労き裂発生・進展の応力繰返し速度依存性

The effect of stress frequency on fatigue crack initiation and propagation in SM50A steel, which has strain rate dependence in the plastic region, was studied.Fatigue crack initiation and propagation tests were carried out at 0.02Hz, 0.2Hz, 1Hz and 20Hz, at R=0, at room temperature, and the number of cycles to crack initiation and the crack propagation rate were measured. The local stress-strain behavior at the part of stress concentration was analyzed and the simulation of fatigue crack propagation was performed by FEM employing the elasto/visco-plastic overlay model. The comparison between the experimental results and the visco-plastic strain behavior calculated by analysis was made.The following results were obtained.(1) The number of cycles to crack initiation and the crack propagation rate depend on the stress frequency in such a way that for lower frequency, a shorter life of the crack initiation and a higher crack propagation rate were obtained.(2) The dependences of the crack initiation life and the crack propagation rate on stress frequency can be explained by the visco-plastic strain range based on the material viscosity in the plastic region at the notch root and the crack tip.(3) A parameter closely related to fatigue crack initiation and propagation rate is the visco-plastic strain range at the notch root and the crack tip in visco-plastic materials.

Stress Intensity Factors for Small Cracks in the Rim of Disks and Rings Subjected to Rolling Contact

The influence of the residual stresses attending the plastic deformation of a rim on the cyclic crack growth driving force produced by repeated two-dimensional rolling contacts is evaluated. The residual stresses are estimated for disks and rings with different geometries. Values of the Mode II and Mode I stress intensity range, ΔKI and ΔKII, are derived for small, subsurface cracks from the variations in the stress intensity factors with position of the contact for rolling in the absence of surface shear tractions. The calculations take into account (i) steady-state contact stresses generated by a relative peak pressure of po/k = 5; (ii) the radial and circumferential residual stresses, (iii) the frictional resistance on the crack faces; (iv) the crack inclination, and (v) the relative ratios of the inner and outer ring radii. A prediction of the crack initiation and growth rate for high strength steel is illustrated.

AN EXPERIMENTAL AND STATISTICAL INVESTIGATION OF SURFACE FATIGUE CRACK INITIATION AND GROWTH

AbstractSurface fatigue crack studies were carried out and statistically analysed from rotating bend fatigue tests in the high cycle region for three metals, copper, stainless steel and mild steel with four ferrite grain sizes. Quantitative observations of the initiation and growth behavior, including coalescence of closely spaced cracks, are discussed and a limiting condition for coalescence is proposed. Statistical properties of initiation, propagation and propagation rate of surface cracks are examined and the effect of different metals and grain sizes is discussed.

Toughness criterion identification for edge-notched unidirectional composite laminates

Stable and/or unstable crack initiation and propagation to total failure is numerically simulated, for a series of double-edge-notched unidirectional (UD) composite laminate tension specimens of T300/914C carbon-epoxy (CE) material, with different ply orientations: [0] 16, [45] 16 and [90] 16. Several candidate fracture criteria are calibrated first on the [0] 16 configuration to reproduce the experimental crack initiation load and rate of stable crack growth. These criteria are next applied to the other configurations: [0] 16 for initially stable crack growth and [45] 16 and [90] 16 for immediately unstable crack growth. The fracture criteria are compared, and an improved calibration is proposed, applicable to all orientations.

Initiation of stress corrosion cracks in aluminum alloys

Studies on initiation of stress corrosion cracking (scc) were conducted on notched rods of aged Al-8.6Mg, Al-21.5Zn and Al-2.6Mg-6.3Zn alloys tested in aqueous solutions and water-containing ethanol solutions under free corrosion conditions. It was found that there is a definite scc initiation period (t) preceding crack propagation. The crack initiation rate (1/t) increased with increasing initial stress intensity (KI,) at the notch. The crack initiation rates were thermally activated with apparent activation energies of Q ≈ 40 to 60 kJ/mol in the ethanol based solutions and Q ≈ 109 kJ/mol in aqueous solutions. Cathodic hydrogen precharging prior to scc decreased the subsequent crack initiation period. The results were discussed in relation to a hydrogen mechanism of scc and shown to be entirely consistent with the lattice decohesion model developed by Oriani.

In vitro studies of the wear of articular cartilage II. Characteristics of the wear of articular cartilage when worn against stainless steel plates having characterized surfaces

As part of an attempt to elucidate the physical parameters that govern the wear of articular cartilage, its wear properties were studied with the tissue interfaced against stainless steel plates having characterized surface profiles. The studies were carried out in apparatus that was designed and constructed for these purposes. In addition measurements were made of (1) the geometric contact areas between the tissue and the plates as functions of time and load, (2) the changes with time and load of the average fluid contents of the cartilage, (3) the changes in dimension and profile of the surface of the tissue after it had been worn under various conditions and equilibrated for different times with dilute neutral buffered solutions, and (4) the fraction of the hexosamine and hydroxyproline contents of the debris that dissolved during the wear in the dilute neutral buffer used as a lubricant. Wear rates were found to be sensitive functions of the surface profile of the metal. The tissue initially wore against “smooth” surfaces at rates that decreased with time until a final rate, characteristic for a given surface and pressure, was attained. The extent of wear in this region of the wear-time curve could be described by linear functions of the pressure, plate speed and time. The data are interpreted as follows: (1) network breakdown and reorientation of the domains of the macromolecules and their aggregates towards the wear direction occur within a surface region of the tissue as it wears, and such reorientation is responsible for the initial decrease of the wear rates; (2) the rate of crack initiation governs the overall wear rate of the tissue in the linear region of the wear-time curve; (3) the formation of wear particles is accompanied by chain scission.