Showing papers on "Paris' law published in 1983"

Roughness-Induced Crack Closure: An Explanation for Microstructurally Sensitive Fatigue Crack Growth

Abstract: The concept of roughness-induced crack closure is utilized to explain the role of prior austenite grain size and pearlite interlamellar spacing on near-threshold fatigue crack propagation in fully pearlitic eutectoid steel tested at low and high stress ratio in lab air and purified helium. It is shown that at low load ratios, near-threshold growth rates are significantly reduced for coarse-grained microstructures, compared to fine-grained at constant yield strength, due to roughness-induced crack closure. Using roughness-profile microscopy, it was found that fracture surface roughness near threshold scaled with grain size and inversely with yield strength, macroscopic roughnesses at threshold being considerably larger than the conventionally calculated cyclic crack tip opening displacement. Auger analysis of near-threshold corrosion products showed it to be iron oxide; the oxide thickness was seen to be decreased by increased stress ratio. The significance of this model to near-threshold fatigue crack growth behavior, in terms of load ratio, microstructure, and environment is discussed.

Mode III and Mode I fatigue crack propagation behaviour under torsional loading

Abstract: Fatigue crack growth rates have been measured under cyclic torsional loading (R = −1, 1 Hz loading frequency) in AISI 4340 steel tempered at 650° C, with circumferentiallynotched specimens (12.7 mm specimen diameter). The Mode III fatigue crack growth curve for macroscopically flat fracture surface — being obtained by an extrapolation procedure which eliminates the “Mode III crack closure” influence — has higher crack growth rates and shows a greater slope than Mode I results in the stress intensity range of ΔKIIIeff = 18 to 50 MPa m1/2. In the range of ΔKIIIeff < 18 MPa m1/2, the fracture surface has “factory roof” morphology (Mode I). The difference between fatigue crack growth behaviour in Mode III and Mode I as well as mechanisms that can lead to a fracture mode change are discussed. A comparison of the Mode III crack closure influence for specimen diameters of 12.7 mm (this study) and of 24.5 mm (an earlier study) shows that this influence is not only dependent on the depth of the crack and applied torque but also on the specimen diameter. The extrapolated crack growth rates show good agreement with measurements for various specimen diameters.

Near-threshold fatigue crack propagation: a perspective on the role of crack closure

Abstract: In recent years, mechanistic and continuum studies on fatigue crack propagation, particularly at near-threshold levels, have highlighted a dominant role of crack closure in influencing growth rate behavior In this paper we review and model the various sources of closure induced by cyclic plasticity, corrosion deposits, irregular fracture morphologies, viscous fluids and metallurgical phase transformations It is shown that many of the commonly observed effects of mechanical factors, such as load ratio, microstructural factors, such as strength and grain size, and certain environmental conditions can be traced to the extrinsic influence of closure in modifying the effective driving force for crack extension The implications of such closure mechanisms are discussed in the light of constant and variable amplitude fatigue behavior, the existence of a threshold stress intensity for no fatigue crack growth and the validity of such threshold concepts for the case of short fatigue cracks 106 references, 21 figures

The cyclicJ-integral as a criterion for fatigue crack growth

Abstract: The definition of the cyclic J-integral is offered and its physical significance for fatigue crack growth is discussed using the Dugdale model on the assumption that the crack closure, cycle dependent creep deformation, and crack extension under cycling can be neglected. It is shown that the cyclic J-integral for small scale yielding is equivalent to theJ-integral for linear elastic crack independent of loading processes, while the value for large scale yielding varies with the loading processes. However, in both cases, the cyclicJ-integral remains constant during the reversal of loading under a constant stress range, if the first monotonic loading stage is excluded. In this situation, the cyclicJ-integral can be applied as a criterion for fatigue crack growth, since it is evaluated as a generalized force on dislocations to be moved or the energy flow rate to be dissipated to heat by the dislocation movements in an element just attached to the fatigued crack tip during one cycle of loading. It is suggested that the available experimental data of different materials for fatigue crack growth can be generalized to a unified formulation on the basis of the energy criterion. It is also deduced that the threshold ΔJ corresponding to ΔKth should be larger than 4γ where γ is the surface energy of the material. Finally the operational definition of the cyclicJ-integral on single loadversus displacement curves is given for center cracked plate with wide uncracked ligaments in tension.

Steel bridge members under variable amplitude, long life fatigue loading

Abstract: The research described in this report is intended to provide information on evaluating the fatigue resistance of welded attachments subjected to variable amplitude fatigue loading. The research consisted of laboratory studies of welded attachments under random variable amplitude load spectra defined by a Rayleigh-type distribution with most stress-cycles below the constant amplitude fatigue limit. (Some stress cycles exceeded this limit.) Eight full-size beams with web attachments and cover plates were tested during the program. Fatigue crack growth data were generated using random block variable amplitude stress spectra defined by a Rayleigh-type distribution. Also, nonload-carrying fillet-welded cruciform-type specimens were tested under simple bending using a random variable amplitude block loading to supplement the existing shorter life studies carried out on stiffener details. The results obtained from these variable amplitude tests are consistent with the previously reported constant amplitude test. However, the existence of a fatigue limit below which no fatigue cracks propagate is assured only if none of the stress range cycles exceed this constant amplitude fatigue limit. If any of the stress range cycles (as few as one per thousand cycles) exceed the limit, fatigue crack propagation will likely occur. The random variable test data from the beam specimens generally fell between the upper and lower confidence limits projected from constant cycle data. The smaller simulated details generally resulted in the random variable test data falling near the upper confidence limit of constant amplitude test results.

Short crack propagation and closure effects in a508 steel

Abstract: — Fatigue crack growth measurements are usually made on standard specimens containing long cracks (∼10 mm) although in most practical situations, a large part of the fatigue life is spent with much shorter dimensions. The purpose of the present study is a comparison of crack growth behaviour for long cracks (∼13–16 mm) in CT specimens and smaller ones (∼0.3–0.5 mm) in four point bend specimens. Large effects are noticed indicating that, at a given stress intensity factor amplitude, the crack growth rate is significantly higher in specimens with short cracks. Mouth displacement measurements for both specimen configurations show that the crack closure phenomenon accounts for the observed effect. Crack closure is likely to be associated with fracture surface roughness as shown by partly machining the material left behind the crack tip in CT specimens.

Fatigue of stainless steel in hydrogen

Abstract: The fatigue crack growth rates of two austenitic stainless steel alloys, AISI 301 and 302, were compared in air, argon, and hydrogen environments at atmospheric pressure and room temperature. Under the stresses at the crack tip the austenite in type 301 steel transformed martensitically to a’ to a greater extent than in type 302 steel. The steels were also tested in the cold worked condition under hydrogen or argon. Hydrogen was found to have a deleterious effect on both steels, but the effect was stronger in the unstable than in the stable alloy. Cold work decreased fatigue crack growth rates in argon and hydrogen, but the decrease was less marked in hydrogen than in argon. Metallographic, fractographic, and microhardness surveys in the vicinity of the fatigue crack were used to try to understand the reasons for the observed fatigue behavior.

The effects of b and zr on the creep and fatigue crack growth behavior of a Ni-base superalloy

Abstract: The effects of 0.006 wt pct B and/or 0.05 wt pct Zr on the 650 °C creep and fatigue properties of a nickel-base superalloy were evaluated. Additions of B and Zr markedly improved the smooth specimen creep properties and the threshold stress intensity values for creep crack growth. The additions had no effect on the crack growth rates in creep or fatigue. No changes in microstructure, fracture appearance, or grain boundary sliding behavior due to B and Zr could be seen. The benefits of B and Zr were much greater in tests in air than in helium, and the primary effect of these additions was to minimize the harmful effects of oxygen. The general results can be explained by postulating that B and Zr minimize the deleterious change in interfacial energy caused by oxygen, but a clearer understanding of the nature of creep threshold values is needed. S. FLOREEN,, and J. M. DAVIDSON,, are both with

A critical study of the crack closure effect on near‐threshold fatigue crack growth

Abstract: Crack closure in the near-crack-tip region has been considered to be an important contribution to the development of a crack-growth threshold for macroscopic cracks. Recent analytical work, however, has suggested that closure well back of the tip may be the controlling factor. In order to check on this possibility, material has been machined away far behind the crack tip in order to eliminate long-range closure. Removal of this material did not eliminate the threshold, but did lower the threshold level by approximately 10 percent for tests conducted on 6.3 mm-thick X7090-T6 powder metallurgy aluminum alloy.

A study of short fatigue crack growth behaviour in titanium alloy imi 685

Abstract: — The fatigue growth of short cracks in coarse grained IMI 685, having an aligned a microstructure, has been monitored using a two stage replication technique. Crack growth rates are presented in terms of the failure mechanism and compared with standard data obtained from through cracked, compact tension specimens. The maximum difference, of up to four orders of magnitude, between long and short crack growth rates is associated with separation along specific crystallographic planes resulting in a cleavage-like fracture appearance. The rate of short crack growth is also shown to be dependent on applied stress and a dwell at maximum load during the fatigue cycle.

Effects of load ratio and temperature on the near-threshold fatigue crack propagation behavior in a CrMoV steel

Abstract: The influence of temperature in the range of 24 to 260 °C and load ratio on the near-threshold fatigue crack growth rate behavior of a CrMoV steel was characterized. At all temperatures investigated, the threshold stress intensity range, ΔK th, for fatigue crack growth decreased with increasing load ratio. The near-threshold crack growth rates increased significantly at 149 °C when compared with the rates at room temperature. However, the crack growth rates at 260 °C were comparable to those at 149 °C. These observations are rationalized in terms of the concepts of roughness and oxide-induced crack closure. Extensive fracture surface characterization using SEM, oxide thickness measurements by Auger spectroscopy, and roughness measurements by light-section-microscopy were conducted to substantiate the explanations.

Influence of an overload on the fatigue crack growth in steels

Abstract: — This study is concerned with the influence of a single-peak overload and the overload ratio on the subsequent rate of growth of a fatigue crack in steels Retardation increases with increasing overload ratio The crack opening load was also measured during all tests It is shown that the Elber's crack closure concept is not able to explain the effect of overloads The importance of the material yield stress was evaluated by testing steels of different strength It seems that the residual stress state induced by the overload is the major factor causing retardation Two models are analyzed

Micro-mechanical modelling of mode III fatigue crack growth in rotor steels

Abstract: A study has been made of fatigue crack propagation in mode III (anti-plane shear) for A469 and A470 commercial rotor steels (tensile strength 621 and 764 MN/m2 respectively) using torsionally-loaded circumferentially-notched cylindrical specimens. For crack growth under both small-and large-scale yielding conditions, radial mode III crack propagation rates are observed to be similar in both steels and to be uniquely related to the plastic intensity range 163-1 per cycle, provided friction, abrasion and interlocking between sliding crack faces is minimized by the application of a small tensile mean load. Over the range studied (i.e., ≈10-6 to 10-1 mm/cycle), mode III growth rates (dc/dN)III are found to be independent of load ratio (for R=−1.0 and −0.5) and to be a power law function of 163-1 or the mode III cyclic crack tip displacement. When compared to mode I crack growth at equivalent cyclic crack tip displacements, however, crack propagation rates in mode III are seen to be two orders of magnitude smaller than in mode I. Based on fractographic evidence of elongated voids, parallel to the crack front, at the tip of the fatigue crack, several models for mode III crack growth are proposed utilizing the concept that mode III crack advance occurs by the initiation and coalescence of voids formed at inclusions directly ahead of the crack tip. By considering the linkage of these voids to take place by mode II shear parallel to the main crack front, expressions for the mode III crack propagation rate are developed based either on considerations of the local mode II crack tip displacementsor the mode II accumulated crack tip strain (computed from the Coffin-Manson damage relationship). Whereas both types of models predict mode III growth rates to be a small fraction of the cyclic crack tip displacements per cycle, the damage accumulation model in particular is found to provide excellent agreement with experimentally measured growth rates in the present rotor steels.