Showing papers on "Paris' law published in 1990"

Behavior of surface and corner cracks subjected to tensile and bending loads in Ti-6Al-4V alloy

Abstract: The behavior of part-through flaws with regard to failure under monotonic loading and their growth under fatigue loading was studied experimentally and analytically. Comparisons are made of experimental values of toughness obtained using surface and corner cracked specimens with those obtained using standard test specimens, and also experimental growth cycles were compared with numerical predictions using the NASA/FLAGRO computer program. Tests were conducted on various types of surface and corner cracks under tensile and bending loads. Room temperature lab air provided the test environment. The material used in this study was the Ti-6Al-4V alloy in the solution treated and aged (STA) and stress relieved condition. Detailed tabulation of the fracture toughness data and results of life prediction using the NASA/FLAGRO program are presented. Fatigue crack growth rates for the part-through cracked specimens are compared with a base curve fitted from the data obtained using standard specimens. The fatigue loading used in the crack growth testing was constant-amplitude sinusoidal type.

Influence of Deformation-Induced Martensite on Fatigue Crack Propagation in 304-Type Steels

Abstract: This research reports an investigation into the influence of mechanically induced martensitic transformation on the rate of fatigue crack growth in 304-type austenitic stainless steels. Two steels of different composition, 304L and 304LN, were used to test the influence of composition; two test temperatures, 298 and 77 K, were used to study the influence of test temperature; and various load ratios were used to determine the influence of the mean stress. It was found thadecreasing the mechanical stability of the austenite by changing composition or lowering temperature reduces the fatigue crack growth rate and increases the threshold stress intensity for crack growth. However, this beneficial effect diminishes as the load ratio increases, even though increasing the load ratio increases the extent of the martensite transformation. Several mechanisms that may influence this behavior are discussed, including the perturbation of the crack tip stress field, crack deflection, work hardening, and the relative brittleness of the transformed material. The perturbation of the stress field seems to be the most important; by modifying previous models, we develop a quantitative analysis of the crack growth rate that provides a reasonable fit to the experimental results.

SHORT AND LONG FATIGUE CRACK GROWTH IN A SiC REINFORCED ALUMINIUM ALLOY

Abstract: Fatigue crack growth behaviour in a 15 wt% SiC particulate reinforced 6061 aluminium alloy has been examined using pre-cracked specimens. Crack initiation and early growth of fatigue cracks in smooth specimens has also been investigated using the technique of periodic replication. The composite contained a bimodal distribution of SiC particle sizes, and detailed attention was paid to interactions between the SiC particles and the growing fatigue-crack tip. At low stress intensity levels, the proportion of coarse SiC particles on the fatigue surfaces was much smaller than that on the metallographic sections, indicating that the fatigue crack tends to run through the matrix avoiding SiC particles. As the stress intensity level increases, the SiC particles ahead of the growing fatigue crack tip are fractured and the fatigue crack then links the fractured particles. The contribution of this monotonic fracture mode resulted in a higher growth rate for the composite than for the unreinforced alloy. An increase in the proportion of cracked, coarse SiC particles on the fatigue surface was observed for specimens tested at a higher stress ratio.

Optimal Inspection Planning for Fatigue Damage of Offshore Structures

Abstract: A formulation of optimal design, inspection and maintenance against damage caused by fatigue crack growth is formulated. A stochastic model for fatigue crack growth based on linear elastic fracture mechanics is applied. Failure is defined by crack growth beyond a critical crack size. The failure probability and associated sensitivity factors are computed by first-order reliability methods. Inspection reliability is included through a pod (probability of detection) curve. Optimization variables are structural design parameters, inspection times and qualities. The total expected cost of design, inspection, repair and failure is minimized with a constraint on the life time reliability.

Fatigue crack growth and closure behaviour through a compressive residual stress field

Abstract: Behaviour of fatigue crack growth and closure through a compressive residual stress field is investigated by performing fatigue crack growth tests on welded SEN specimens of a structural steel (JIS SM50A). Depending on the type of the initial residual stress in the region of crack growth, the growth and closure of the crack show different behaviour. In particular, in the transition region from a compressive residual stress field to a tensile residual stress field, the fatigue crack growth rates cannot be described by the effective stress intensity factor range ΔKeff, based on the measured crack opening stress intensity factor Kop. Also it is found that the R'-method using the data of da/dN vs ΔK for residual stress-free specimens, with the effective stress ratio R'[=(Kmax+Kr)/(Kmin+Kr)], gives non-conservative predictions of the growth rates in the transition region. Observations of crack closure behaviour in this study indicates that partial opening of the crack occurs and this plays an important role in crack growth through a compressive residual stress field. Based on the concept of a partial opening point (defined and measured in this work), fatigue crack growth behaviour can be better explained.

Fatigue Crack Propagation in Ceria‐Partially‐Stabilized Zirconia (Ce‐TZP)‐Alumina Composites

Abstract: Fatigue crack propagation rates in tension-tension load cycling were measured in ZrO{sub 2}-12 mol% CeO{sub 2}-10 wt% Al{sub 2}O{sub 3} ceramics using precracked and annealed compact tension specimens. The fatigue crack growth behavior was examined for Ce-TZPs. The fatigue crack growth behavior was strongly influenced by the history of crack shielding via the development of the crack-tip transformation zones. Crack growth rates under sustained peak loads were also measured and found to be significantly lower and occurred at higher peak stress intensities as compared to the fatigue crack growth rates.

The effect of the welding process on the fatigue crack growth

Abstract: The effect of four offshore welding procedures on the fatigue crack initiation and propagation from the weld toe region of fillet welded joints was investigated. The joints were made of low-carbon controlled-rolled and accelerated-cooled steel plates with transverse fillet welded attachments. Forty-four cruciform and T-joint test specimens were tested under constant amplitude uniaxial loading Aa = 150 MPa (21.7 ksi). The four welds were fabricated using submerged arc welding (SAW), flux cored arc welding (FCAW) and shielded metal arc welding (SMAW) with two different electrodes. The procedures were taken from normal offshore fabrication practices. An advanced automatic crack monitoring system based on the ACPD method was used to measure crack depths during the fatigue process. The depth measurements were calibrated by ink penetrant marking of the cracks and additional information of crack shape was obtained. Measurements were carried out on the geometry variables such as weld toe geometry and the size of initial crack depths. Stress concentration factors and stress intensity factors were evaluated. Statistical information for crack shape paths and the time to reach given crack depths are presented. Finally, a fracture mechanics computation based on the Paris law was carried out to estimate the propagation parameters C and m compared with current values from building codes. An accurate characterization of weld toe geometry, crack shape development and HAZ material parameters is necessary when the Paris law is used to describe the propagation process from a crack depth of 0.1 mm to final failure. When this information is available for different welding procedures, the fracture mechanics approach has a great potential use in predicting the fatigue crack growth in welded joints.

Role of oxides in fatigue crack propagation

Abstract: Oxides and oxidation play a very important role in fatigue crack propagation for many materials, not only at elevated temperatures, but also at ambient temperature. The simplest approach to modelling environmental effects on crack growth is to attempt to add a purely mechanical crack growth increment to an environmental component: the oxidation rate multiplied by the cycle time. Limited success has been achieved with models of this type, both for high temperature fatigue in air and in high pressure water environments. One significant result has been to emphasise that at temperatures up to about half the melting point of the material, it is environmental effects (and thus, frequently, oxidation) rather than creep which give rise to accelerated fatigue crack growth rates. However, the problem with these simple models is that there is no account taken of the interaction between the environmental processes taking place. There are many types of interaction between the mechanical and the chemical proces...

The nature of the two opening levels following an overload in fatigue crack growth

Abstract: During the period of retarded fatigue crack growth following a single tensile overload, two crack opening levels are observed Different interpretations have been put forth concerning the nature of these opening events In one proposal, the two opening events are considered to occur in plane strain, first behind the crack tip and then at the crack tip In another proposal, the first opening event occurs in the plane strain portion of the crack front and the second in the plane stress portion of the crack front In this paper, critical experiments designed to clarify this issue are described In most of the experiments, the aluminum alloy 6061-T6 was used Additional tests were made using the aluminum alloy 7090-T6 and the titanium alloy IMI 829 The results of these experiments show that over the entire range of ΔK, including the near-threshold range, the two opening events are associated with a plane strain followed by a plane stress opening process The results are important in that they not only shed light on the mechanism of crack retardation following an overload, but they also provide a basis for the development of a rational method of analysis for the calculation of the number of delay cycles following an overload which includes the effects of thickness and ΔK level Based upon the findings of this study, a semiempirical method for the determination of the number of delay cycles following an overload is presented The number of delay cycles predicted by this method is found to be in good agreement with the results of the present investigation as well as with other results from the literature

The growth of short cracks in 4340 steel and aluminum-lithium 2090

Abstract: The objectives were to investigate crack initiation characteristics and short crack growth behavior for Al-Li 2090 and for 4340 steel and to evaluate the ability of a closure-based crack-growth model to predict fatigue crack growth rates and total fatigue lives for the steel. Single-edge-notched tension specimens of each alloy were used to obtain the short crack growth rate information via an acetate replica technique. In addition to constant amplitude loading, tests on the steel were conducted using the Felix/28 variable amplitude spectrum (a shortened form of a standard loading sequence for fixed or semi-rigid helicopter rotors). The short crack growth rates were compared to those for long cracks grown under similar loading conditions. Metallurgical features associated with crack initiation are discussed. For Al-Li 2090 under R = -1 loading, the short cracks grew well below the long crack threshold and grew at acute angles to the loading axis. For 4340 steel under constant amplitude loading at R = 0.5 and 0 and for the Felix/28 spectrum loading, short-crack growth rates agreed well with long-crack growth rates, even near the long-crack threshold. A slight short-crack effect, growth below the long-crack threshold, was observed at R = -1. Fatigue lives were found to depend on the size and type of initiation site, especially for the Felix/28 loading sequence. A semi-empirical crack-growth model incorporating crack-closure effects was used to predict crack growth rates and total fatigue lives of notched 4340 steel specimens. An initial defect size and shape typical of those identified in this steel was assumed for the life predictions. For all loading conditions, reasonable agreement was found between measured and predicted values for both crack growth rates and fatigue lives.

Fatigue fracture of nylon polymers: Part 1 Effect of frequency

Abstract: We determine the effect of frequency on fatigue crack propagation rates in unfilled nylon polymers. Specifically it was of interest to investigate the frequency dependence under conditions where hysteretic heat generation did not occur. For dry injection-moulded nylon the results demonstrate that a strong frequency dependence exists with higher crack propagation rates at lower frequencies. This indicates that the mechanism of fatigue crack growth at room temperature is primarily one of creep crack growth, especially at frequencies below 1.0 Hz. It is also noted that hysteretic heating causes fracture mode transitions to occur during stable; fatigue crack propagation in injection-moulded nylons, even at relatively low cyclic frequencies (5.0 Hz)

Effects of interfacial strength on fatigue crack growth in a fiberreinforced Ti-alloy composite

Abstract: The fatigue crack growth behavior of a Ti-6A1-4V composite with boron fibers was previously studied in the as-received and thermally exposed conditions. Fracture strengths of the composite, fiber, and interface were characterized together with fatigue crack growth rates and failure mechanisms. Utilizing the matrix and fiber properties as input, a recently proposed model was exercised to elucidate the effects of interfacial strength on crack growth rates in the composite. Comparison of experimental results with model calculations revealed that a weak fiber/matrix interface combined with a strong, high-modulus fiber led to interface debonding and crack deflection and produced the beneficial effects of increased threshold and reduced transverse crack growth rates.

Mechanics and micromechanisms of fatigue crack growth in brittle solids

Abstract: This paper is concerned with the mechanics and micromechanisms of stable mode I crack growth in brittle solids subjected to compression—compression fatigue and tension—tension fatigue loads. Constitutive models, results of finite element analyses, and experimental observations are described for monolithic ceramics and ceramic-matrix composites, plain concrete, and a transformation-toughened ceramic in an attempt to deduce a general theory on the origin of mode I fracture in notched plates under uniaxial cyclic compression at room temperature. An analysis of the residual stress field which develops at elevated temperatures in response to power law creep and far-field compressive cyclic loads is also presented. The principal “driving force” for mode I fracture in cyclic compression is the generation of a near-tip zone of residual tension, when the deformation at the notch-tip leaves permanent strains upon unloading from the far-field compressive stress. The results indicated that materials with very different microscopic deformation mechanisms, i.e., microcracking, dislocation plasticity, martensitic transformation, interfacial debonding/slip, or creep, exhibit a macroscopically similar, stable fracture under far-field cyclic compression because the zone of residual tension is embedded in material which is elastically strained in compression. It is shown that cyclic compression loading offers a unique method for fatigue precracking notched specimens of brittle solids prior to tensile fracture testing, whereby an unambiguous interpretation of the critical stress intensity factors for crack initiation and growth can be achieved. Fatigue crack growth characteristics of a transformation-toughened ceramic and a creeping ceramic composite under tension—tension fatigue loads are also discussed.

Rolling contact fatigue and fatigue crack propagation in 1C-1.5Cr bearing steel in the bainitic condition

Abstract: A commercial lC-1.5Cr steel used widely for bearing manufacture has been heat-treated to give either martensitic or bainitic microstructures. The resistance of the steel to rolling contact fatigue damage, to fatigue crack growth under different environmental conditions, and in both of these microstructural states has been evaluated. Rolling contact fatigue failure was due to surface pit formation by spalling, caused either by subsurface- or surface-initiated fatigue cracks, or surface wear, dependent on microstructure and lubrication conditions. The presence of water in the lubricant increased the rolling contact fatigue damage and reduced the life in all of the microstructures, although in confirmation with other studies, there was some evidence that softer microstructures were more tolerant of this less ideal lubrication. Fatigue crack growth rates under Mode I loading conditions were also measured, both as a function of microstructural condition and lubricating oil environment, and the superior crack growth resistance of the bainitic and softer martensitic structure was confirmed. The effect of the lubricating environment on bearing steel performance is discussed in terms of corrosion fatigue and hydrogen embritflement.

Fatigue behaviour of CMSX 2 superalloy [001] single crystals at high temperature II: Fatigue crack growth

Abstract: The fatigue crack growth behaviour of two-dimensional cracks was studied in [001] single crystals of CMSX 2 nickel-based superalloy at 650 °C. An influence of the crystallographic orientation of the crack growth direction on the crack growth kinetics was evident for short cracks tested in air. This influence was mainly attributed to differences in crack closure. Comparison between short and long cracks indicated that no short-crack effect exists in these single crystals. Tests in vacuum gave significant differences in crack growth rate and in fatigue fracture mode, especially at low rates. These results are discussed and used to rationalize previous observations.

Fatigue reliability of tubular joints in offshore structures - reliability analysis

Abstract: A reliability analysis methodology for the limit state of fatigue failure and the limit state of fatigue-fracture is presented. A probabilistic fracture mechanics fatigue crack growth model is used to quantify fatigue damage in a given period of time. Ligament fracture is assessed using the CEGB-R6 elasto-plastic fracture mechanics procedure. The important sources of uncertainty considered include environmental conditions, hydrodynamic loading, global structural analysis, local stress calculation at the tubular joints and fatigue crack growth using fracture mechanics. Initial defect size, the fatigue threshold, crack propagation parameters, fracture toughness, the yield strength of the material and welding induced residual stresses are treated as basic random variables. Suitable probabilistic models for the above variables are presented and a broad outline of the level 2 reliability analysis procedure based on the Hasofer and Lind reliability index is given. Two examples of reliability analysis showing the variation of reliability with time are presented and the important factors which govern the failure of the joint either by fatigue, fracture or plastic collapse are discussed.

Effects of temperature and environment on the tensile and fatigue crack growth behavior of a Ni3AI-base alloy

Abstract: The effects of temperature, frequency, and environment on the tensile and cyclic deformation behavior of a nickel aluminide alloy, Ni-9.0 wt pct Al-7.97 pct Cr-1.77 pct Zr (IC-221), have been determined. The tensile properties were obtained in vacuum at elevated temperatures and in air at room temperature. The alloy was not notch sensitive at room temperature or at 600 °C, unlike Cr-free Ni3Al + B alloys. In general, crack growth rates of IC-221 increased with increasing temperature, decreasing frequency, exposure to air, or testing at higherR ratios. At 25 °C, crack growth rates were slightly higher than for a previously investigated Cr-free Ni3Al alloy. However, at 600 °C, the crack growth rates for IC-221 were lower than for the Cr-free alloy. Substantial frequency effects were noted on crack growth of IC-221 at both 600 °C and 800 °C in both air and vacuum, especially at highK. The relative contributions of creep and environmental interactions to fatigue crack growth are discussed.