Stress-life Curves Research Articles

Delamination effects in fuselage crack patching : Gut, C. Society for the Advancement of Material and Process Engineering, 1161 Parkview Drive, P.O. Box 2459, Covina, CA 91722, U.S.A. 1996. 903–914 Conference: Materials and Process Challenges: Aging Systems, Affordability, Alternative Applications. Vol. 41-I, Anaheim, CA, U.S.A. 24–28 Mar. 1996

The present paper addresses the fatigue characteristics of two cellular foam core materials as used in load carrying sandwich structures. The fatigue loading studied is a constant shear stress which corresponds to the main type of loading that the core in a sandwich structure exhibits. Based on results from testing stress-life curves are presented for a number of stress ratios (R) and are fitted to a simple two-parameter Weibull function. The influence of the R-value is emphasized and standard Haigh diagrams are constructed. It is seen that the fatigue behaviour of the core materials investigated herein well can be described in manners similar to that of classical metal fatigue. A thorough investigation is performed on the behaviour of fatigue crack formation and growth.

The fatigue resistance of TiAl-based alloys

TiAl-based alloys are a class of intermetallic alloys that show potential as a high-temperature structural material. Many applications considered for TiAl-based alloys require certain damage-tolerance characteristics. In this article, the fatigue resistance of two-phase TiAl alloys is reviewed to assess the current understanding of the fatiguedamage processes in this class of emerging materials. Salient features of fatigue-crack growth and stress-life curves are summarized to identify the challenges these fatigue characteristics present in the life assessment of TiAl components.

Constant and variable amplitude loading of 6261 aluminium alloy I-beams with welded cover plates—Influence of weld quality and stress relief

A study of the constant amplitude and variable amplitude (VA) fatigue performance of 6261-T6 aluminium 1-beams with a welded rectangular cover plate is reported. The test programme involved >100 specimens and considered the effects of weld ‘quality’ (in terms of weld toe angle, leg length and heat input), and thermal and vibratory stress relief on stress-life curves under simple repeated two-level ] block loading. The number of high-to-low amplitude cycles in a single VA block was varied from 1/20, through 10/20 to 1/1000 and 1/50,000. Load amplitude increase percentages of 25%, 75% and 150% were considered. Observed fatigue lives were compared with the predictions of Miners rule and explanations are proposed for situations where Miners rule is non-conservative.

High cycle fatigue life improvement of polycrystalline alpha-iron modified by silver, chromium, aluminium, and yttrium ion implantation

HCF tests on polycrystalline alpha-iron with and without silver, chromium, yttrium, and aluminum ion implantation have been conducted. The stress-life curves obtained were best fit by using the power function. It was shown that the fatigue property had been improved to different extent after ion implantation. Enhanced strengthening of the implanted layer due to radiation induced defects, super solid solution, and formation of intermetallic compounds in implantation was the prominent mechanism for property improvement.

Powder synthesis, sintering, and characterization of Ba 1+xZr 6−2xSi 2xO 24 -A low thermal expansion system

HCF tests on polycrystalline alpha-iron with and without silver, chromium, yttrium, and aluminum ion implantation have been conducted. The stress-life curves obtained were best fit by using the power function. It was shown that the fatigue property had been improved to different extent after ion implantation. Enhanced strengthening of the implanted layer due to radiation induced defects, super solid solution, and formation of intermetallic compounds in implantation was the prominent mechanism for property improvement.

A theoretical study of the effect of weld geometry parameters on fatigue crack propagation life

In the present work, the effect of important butt weld geometry parameters e.g. tip radius of undercut at weld toe, weld toe radius, flank angle, plate thickness and edge preparation angle, and the effect of initial crack geometry on the fatigue crack propagation life have been studied by using Linear Elastic Fracture Mechanics (LEFM), Finite Element Analysis (FEA) and superposition approaches. A simple mathematical model has been developed to predict the co-influence effect of the above-mentioned weld geometry parameters on the fatigue stress range vs life (SN) curve. This model gives an explanation for the overall effect of weld geometry parameters as the main reason of scatter phenomenon in fatigue testing practice. The predicted scatter band of SN curves subject to the variation of all the weld geometry parameters is in good agreement with SN curves covered by design classes from D to W (BS 5400). The design class F which is commonly used for design of butt welded joints has fallen in the middle of the predicted scatter band of SN curves. It suggests that for the sake of safe design practice class W should be used for fatigue design of butt welded joints instead of class F.

Effect of partial coverage upon the fatigue fracture behaviour of peened components: Meguid, S.A. Fatigue Fract. Eng. Mater. Struct. 1991 14 (5), 515–530

Cyclic deformation and fracture behavior of the 321 stainless steel are investigated at 633 K under engineering-stress and strain controlled low cycle fatigue (LCF) cycling, and the fatigue life data are compared with ASME section-III Division-1 S-N curve. In stress-controlled LCF tests, ratcheting-strain accumulation is observed even under zero mean-stress, and the direction of ratcheting is found to strongly depend on initial-ramp direction of first-cycle. In addition, plastic-strain range and striation width are observed to be higher than those in strain-controlled tests. The stress-life curve obtained from stress-controlled tests is 2–3 orders of magnitude lower in comparison to stress-life curve derived from strain-controlled LCF tests as per the ASME procedure. The discrepancy is attributed to the differences in deformation and damage development in stress and strain-controlled cycling.

Finite element prediction of the fatigue limit of steel

The fatigue properties of a material are traditionally described by a stress-life (S-N) curve, i . e . , the relationship between the applied cyclic stress and the number of cycles required to cause failure. At low cyclic stresses, the fatigue life extends to 107 to 108 cycles and effectively defines a fatigue endurance limit or fatigue strength. Recently, we described a method tq of predicting this highcycle regime for precipitation-hardened aluminum alloys. The basic concept is that the rate-controlling process is the development of persistent slip bands (PSBs). We represented a PSB by a finite element model and calculated the strain field of the PSB and its environs under the action of a remote stress. We proposed that the elongation of a PSB is controlled by the plastic strain (ep) n e a r its tip and derived a fatigue damage (D) criteflon to describe the weakening of the matrix:

Influence of non-geometric effect of PSB on crack initiation in aluminium single crystal: Zhai, T.-G., Lin, S. and Xiao, J.-M.Acta Metall. Mater. Sept. 1990 38, (9), 1687–1692

Carburized specimens of AISI 4027 steel were tested in repeated bending at stress levels ranging from high-stress, low-cycle to low-stress, high-cycle, and the corresponding stress-life curve was determined. Considerable scanning electron microscopy was performed on fracture surfaces of failed specimens. Results indicate that a fatigue crack nucleates at the external surface of the carburized case, and propagates inward until it becomes large enough to cause rapid failure in bending. The crack surface appearance changes according to the growth rate of the crack and the hardness of the matrix material. A low-stress crack growth through the carburized case seemed to result in the well-established intergranular separation behavior. However, growth under high stress levels was rapid, causing a mixture of transgranular-intergranular fracture to result, the transgranular mode being more pronounced. The tough core, on the other hand, exhibited a more ductile mode of failure. The predominant mode consisted of microvoids, dimples of non-uniform size, and tear ridges with only occasional flat, quasicleavage step facets. Under no conditions did fatigue striations become apparent on either case or core material.

Near threshold fatigue crack growth behaviour under repeated two-step loadings in residual stress fields: Jono, M., Sugeta, A. and Iwahashi, H. J. Soc. Mater. Sci., Jpn. Oct. 1989 33, (433), 1194–1199 (in Japanese)

Cyclic deformation and fracture behavior of the 321 stainless steel are investigated at 633 K under engineering-stress and strain controlled low cycle fatigue (LCF) cycling, and the fatigue life data are compared with ASME section-III Division-1 S-N curve. In stress-controlled LCF tests, ratcheting-strain accumulation is observed even under zero mean-stress, and the direction of ratcheting is found to strongly depend on initial-ramp direction of first-cycle. In addition, plastic-strain range and striation width are observed to be higher than those in strain-controlled tests. The stress-life curve obtained from stress-controlled tests is 2–3 orders of magnitude lower in comparison to stress-life curve derived from strain-controlled LCF tests as per the ASME procedure. The discrepancy is attributed to the differences in deformation and damage development in stress and strain-controlled cycling.

Finite element prediction of high cycle fatigue life of aluminum alloys

A new method is described for calculating the long fatigue life (>105 cycles) portion of the stress-life (S-N) fatigue curve for precipitation-hardened aluminum alloys. It is based upon a finite element model of the deformation of a persistent slipband (PSB), and the only material parameter required is the ultimate tensile strength (UTS) of the alloy. The stress dependence of the plastic strain at the tip of a PSB is shown to be very pronounced and to closely match that of anS-N fatigue curve. Very good agreement is obtained for 6061-T6, 2014-T6, 2024-T4, and 7075-T6 aluminum, and the fatigue strength (at 108 cycles) is calculated to be 26 pct of the tensile strength of each alloy, in agreement with experimental data. By contrast, the plastic strain at a crack tip has a much weaker stress dependence. Thus, these calculations also confirm that the elongation of a PSB, and not crack growth, is the rate-controlling process in high cycle fatigue.

Factors Influencing the Fatigue Behavior of Ferrous Laminates

Laminates based on ultrahigh carbon steel were prepared and found to exhibit enhanced fatigue life as compared to a monolithic reference material. This result was achieved through the insertion of weak interlaminar regions of copper into the layered material during preparation of the laminates. The presence of these regions allowed for the operation of a delamination mechanism in advance of the propagating fatigue crack. The result was interlaminar separation and associated crack blunting. Stress-life curves show that an increase in life by as much as a factor of four is achieved for these materials when compared to monolithic specimens of similar processing history.

A Power Law Fatigue Damage Model for Fibre-Reinforced Plastic Laminates

Following an earlier comparative investigation of the tensile fatigue response, in terms of stress versus log life curves, of three epoxy-resin-based laminates of 0/90 lay-up and reinforced with carbon, glass and Kevlar-49 fibres, the authors now present a study of the accumulation of fatigue damage in terms of the residual strengths of the fatigued laminates. A stress-independent parametric relationship between normalized stress and normalized number of cycles is found to describe the behaviour of all three composites, suggesting a single major mode of fatigue damage despite the widely different mechanical characteristics of the different materials. The model is a two-parameter power law model, derived from a consideration of the appropriateness of various curve-fitting methods, and permits the incorporation of all modes of damage accumulation, from gentle ‘wear-out’ to ‘sudden death’ simply by the adjustment of this pair of power law exponents.

The effect of tufftriding on the rolling contact fatigue behaviour of low alloy steel cylindrical specimens

Investigations of the rolling contact fatigue behaviour of a low alloy steel are reported. Experiments were conducted on untreated and Tufftrided cylindrical specimens using Tellus 15 oil and contact stress-life curves were obtained. Both curves exhibit knee points with well-defined pitting limits. Tufftriding increases the pitting limit substantially. Wear loss studies indicated that Tufftriding leads to negligible wear loss under high contact stresses. Microhardness explorations established that strain hardening is exhibited by worked untreated specimens but no strain hardening was found with worked Tufftrided specimens. Tufftrided surfaces thus retain their original ductility and this contributes to their better performance under rolling contact fatigue conditions.

Fractography of fatigue fractures in carburized steel

Carburized specimens of AISI 4027 steel were tested in repeated bending at stress levels ranging from high-stress, low-cycle to low-stress, high-cycle, and the corresponding stress-life curve was determined. Considerable scanning electron microscopy was performed on fracture surfaces of failed specimens. Results indicate that a fatigue crack nucleates at the external surface of the carburized case, and propagates inward until it becomes large enough to cause rapid failure in bending. The crack surface appearance changes according to the growth rate of the crack and the hardness of the matrix material. A low-stress crack growth through the carburized case seemed to result in the well-established intergranular separation behavior. However, growth under high stress levels was rapid, causing a mixture of transgranular-intergranular fracture to result, the transgranular mode being more pronounced. The tough core, on the other hand, exhibited a more ductile mode of failure. The predominant mode consisted of microvoids, dimples of non-uniform size, and tear ridges with only occasional flat, quasicleavage step facets. Under no conditions did fatigue striations become apparent on either case or core material.

Protection of Aluminium Alloys against Stress Corrosion Cracking in Saline Water by Properly Oriented Anodic Coatings I. Influence of anodising conditions:I. Influence of anodising conditions

AbstractWith the object of protecting aluminium alloys against stress corrosion cracking in saline water, anodic γ1−, γ2−A12O3 and γl−, γ2−Al2O3.3H2O films were formed on aluminium−2·5 wt-% magnesium alloy specimens. The influence of anodising current density, film thickness and, above all, grain orientation relative to the subsequent loading direction, and also that of oxide ageing between oxidation and sealing, was studied. The specimens were tested under accelerated laboratory stress corrosion conditions, including the corrosive environment (1 M NaCl solution), circulation conditions and impressed anodic current. For each category of specimen, tensile stress vs specimen life (time to cracking) curves were established. Results obtained with this procedure showed that the conditions of formation of the oxides and their hydrates, and particularly the orientation of the grains, have a considerable influence on their protective properties and mechanical strength; some of these oxide or hydrate coatings were...