Showing papers on "Fatigue limit published in 1983"

Propagation and non-propagation of short fatigue cracks at a sharp notch

Abstract: — Sharply notched specimens of a structural low-carbon steel were fatigued under several ratios of the maximum to minimum loads. The growth behavior of a short fatigue crack near the notch tip was analyzed based on crack closure measurements. A fatigue crack first decelerates with increasing crack length, and then accelerates or becomes non-propagating depending on the applied stress. A similar deceleration is seen when the rate is correlated to the stress intensity range. The effective stress intensity range is a unique parameter in correlating the growth rate of a short crack for all the stress levels examined, and the relation is identical to that obtained for a long crack. By considering the increase in crack closure with crack length, a quantitative method is proposed for predicting the non-propagating crack length and the fatigue limit of notched specimens as a function of the applied stress and the notch geometry.

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.

A Fracture-Mechanics and System-Stiffness Approach to Fatigue Performance of Spot-Welded Sheet Steels

Abstract: To improve fuel economy, the automotive manufacturers have been gradually incorporating lighter gage, higher strength sheet steels. For the same fatigue loads, the operating stresses of these spot-welded lighter-gage sheets will be greater causing a concomitant reduction in fatigue life. This paper describes the results of a program using fracture-mechanics concepts to determine the fatigue life of spot-welded connections of various stiffness. A model is developed which predicts the fatigue life, and shows that life increases with increasing joint stiffness. Two types of fatigue cracking were observed and found to be competing throughout the fatigue life. The type of cracking dominating the final failure was also related to the joint stiffness. Above a critical stiffness, shear failure could occur. Below this critical stiffness, sheet failure resulted, and the fatigue life was related to the square root of the change in rotation.

Fatigue of case-hardened steels; role of residual macro- and microstresses

Abstract: For a description of the fatigue behavior of case-hardened steels, a simple model is presented which incorporates the contributions of both residual macrostresses and residual microstresses (related to material strengthening) to the fatigue-limit distribution within the workpiece. Fracture initiation and the resulting endurance limit on rotating—bending fatigue are discussed for unnotched and notched workpieces. The significance of residual macrostresses for the fatigue resistance has been generally overemphasized until now, whereas residual microstresses can be of decisive importance. Applications of the model to previously published rotating—bending fatigue data of case-hardened (nitrided) workpieces of carbon and alloy steels yield good results.

Distributions of Fatigue Life and Fatigue Strength in Notched Specimens of a Carbon Eight-Harness-Satin Laminate:

Abstract: The objective of this research is to investigate the distribution properties of fatigue life and fatigue strength in sharply notched specimens of a carbon fabric laminate, which was made of three layers of eight-harness-satin/epoxy prepreg sheets. Fatigue life distributions were obtained at six stress levels by carefully designed fatigue tests of four point plane bending under a constant temperature and humidity condition. Sample sizes were 30 for the three high stress levels and 12 for the three low stress levels. The amount of scatter and distributional form of fatigue life and fatigue strength are discussed from the test results. It is found that fatigue strength distributions are practically normal and their stand ard deviations are constant regardless of fatigue life.

Statistical summaries of fatigue data for design purposes.

Abstract: Two methods are discussed for constructing a design curve on the safe side of fatigue data Both the tolerance interval and equivalent prediction interval (EPI) concepts provide such a curve while accounting for both the distribution of the estimators in small samples and the data scatter The EPI is also useful as a mechanism for providing necessary statistics on S-N data for a full reliability analysis which includes uncertainty in all fatigue design factors Examples of statistical analyses of the general strain life relationship are presented The tolerance limit and EPI techniques for defining a design curve are demonstrated Examples usng WASPALOY B and RQC-100 data demonstrate that a reliability model could be constructed by considering the fatigue strength and fatigue ductility coefficients as two independent random variables A technique given for establishing the fatigue strength for high cycle lives relies on an extrapolation technique and also accounts for "runners" A reliability model or design value can be specified

Modeling stiffness loss in boron/aluminum laminates below the fatigue limit

Abstract: Boron/aluminum can develop significant internal matrix cracking when fatigued. These matrix cracks can result in a 40 percent secant modulus loss in some laminates, even when fatigued below the fatigue limit. It is shown that the same amount of fatigue damage will develop during stress or strain-controlled tests. Stacking sequence has little influence on secant modulus loss. The secant modulus loss in unidirectional composites is small, whereas the losses are substantial in laminates containing off-axis plies. A simple analysis is presented that predicts unnotched laminate secant modulus loss due to fatigue. The analysis is based upon the elastic modulus and Poisson's ratio of the fiber and matrix, fiber volume fraction, fiber orientations, and the cyclic-hardened yield stress of the matrix material. Excellent agreement was achieved between model predictions and experimental results. With this model, designers can project the material stiffness loss for design load or strain levels and assess the feasibility of its use in stiffness-critical parts. Previously announced in STAR as N82-24298

Fatigue failure mechanism of composite laminates

Abstract: Fatigue failure of homogeneous materials (like metals) is characterized by a single crack running through. Rarely does a single crack cause failure to composite laminates. In general, it has been observed that multidirectional laminates fail by progressive initiation and propagation of many cracks which accumulate until the final fracture. There are five different modes of failure which may occur, either separately, or simultaneously (two or more). The failure modes are: fiber failure, inplane tension, inplane shear, interlaminar tension and interlaminar shear. The progressive failure occurring within the laminate alters the initial state of stress in each lamina within the laminate and causes a cumulative fatigue situation, even when the external load amplitude, imposed on the laminate, is kept constant. Fatigue functions, associated with the five failure modes, enable one to describe the fatigue strength of the laminate. Viewing the laminae as having non-linear but continuous stress-strain relations up to failure, gives an adducate strain to failure when using the strength criteria. A method is shown on how to predict the successive changes in the laminae stress fields and to compute the fatigue life of the laminate. It was found that the mode of fatigue failure can be different for various stress amplitudes acting on the same laminate. In the experimental program, T300/5208 graphite epoxy laminates were used. Fatigue functions and strength parameters of the failure modes were determined and used in the computing schemes. Multidirectional laminates were tested for observation on the fatigue failure mechanisms and the determination of the fatigue life for comparison with the analysis predictions.

Biaxial Failure of GRP—Mechanisms, Modes and Theories

Abstract: Numerous failure theories have been proposed for GRP. Experimental observations under biaxial loading reveal scatter, multiple failure mechanisms and failure modes, which depend on material type and stress conditions. Biaxial failure theories can be represented as surfaces whose shape depend on both failure theory and the choice of single valued characteristic strengths. Experimental results suggest surfaces which are quadratic functions of the stresses and strengths and which often lie well inside the maximum normal stress boundaries. Failure theories which use a complex stress test to evaluate an interaction coefficient are generally unacceptable because the resulting surfaces are so sensitive to small changes in strength data. It is necessary to consider different classes of reinforcement (unidirectional, woven fabric, and random mat) separately in proposing failure theories. For unidirectional materials quadratic theories only appear to be well defined under tension-tension-shear conditions. For mats and fabrics adaptations of the early Norris theories fitted separately in each stress octant appear to be satisfactory. Failure theories only predict material failure as distinct from structural failure and should be treated with caution when applied to design.

A new mechanical joining technique for steel compared with spot welding

Abstract: Two mechanical joining techniques were compared with spot welding in a number of plain carbon and high-strength steels with or without various metallic and painted coatings. Spot welds had higher static strength than mechanical joints, but the new button-type (crack-free) joint had equivalent fatigue strength at long lives. Relationships were developed to estimate the strength of the button-type mechanical joint given base metal strength properties and the geometry of the joint.

Chemistry effects in corrosion fatigue

Abstract: In principle, there could be as many different types of corrosion-fatigue behavior as there are metal/environment combinations, since each will interact chemically and electrochemically in different ways. Pessimism in the face of such potential complexity would be justified if we could not, from our knowledge of the corrosion properties of the material in its environment, make worthwhile generalizations. The object of this paper is to identify those common threads running through the experimental observations that enable us to deduce which are the important chemical and electrochemical interactions with the environment influencing the fatigue strength, cyclic endurance, or fatigue crack propagation kinetics of a given material or class of materials. Interpretation of the effects of chemical and electrochemical variables is seen to be dependent on both our basic knowledge of surface corrosion and crevice corrosion phenomena and our perception of the relative importance of the crack initiation and propagation phases of corrosionfatigue failure in particular materials and test specimen geometries.

Size Effect on Rotating Bending Fatigue in Steels

Abstract: Rotating bending fatigue tests were performed for forged and cast steels of which specimen sizes are in the range from 8 to 40mm in diameter. The cast steel including originally many defects exhibits a decrease of about 27% in the fatigue limit with an increase in the size from 8 to 40mm, while such a decrease is about 13% in the forged steel of comparatively homogeneous substance. The measurement of the strain during tests manifests that a plastic strain of about 80×10-6 is produced at the fatigue limit in the forged steel, independently of specimen size, and on the other hand, the cast steel yields no macroscopic plastic strain at the fatigue limit of this material. From these results, it is suggested that in the former the size dependent plastic strain, which is caused by the difference in the stress gradient produced in the cross section of the specimen, and in the latter the scatter in the fatigue strength induced by the defects, are the dominant factors controlling the size effect on the fatigue limit of steels.

Stresses in a Harrington Distraction Rod: Their Origin and Relationship to Fatigue Fractures In Vivo

Abstract: As illustrated by the case study described in this paper, in-vivo fractures of distraction rods often occur by metallurgical fatigue at the junction between the smooth and ratcheted parts of the rod (i.e., at the first ratchet junction, FRJ). To clarify causative factors of fatigue at the FRJ, stresses are analyzed in a standard 11-in. rod using both experimental and theoretical methods. The analyses reveal how distraction force, eccentricity of loading, rod geometry and material determine the stresses at the FRJ. These stresses can exceed the fatigue endurance limit for certain clinically encountered conditions.

Fatigue and Overload Fracture of Carburized Steels

Abstract: Predictions resulting from a theoretical fracture mechanics model are reviewed and confirmed by experimental fatigue and fracture results from a variety of carburized steels. This comparison emphasizes the unique contribution of molybdenum to the fatigue strength of the carburized part through its role in maintaining case hardenability and a desirable residual stress profile. This analysis also indicates that the use of molybdenum in carburizing steels results in a superior fracture toughness profile and, hence, improved protection against overload-induced failures

High Cycle Fatigue of (Fe, Ni, Co)3V type alloys

Abstract: High cycle fatigue tests in vacuum have been performed on ordered (Fe, Co, Ni)3V alloys between 25 °C and 850 °C. Heat-to-heat variations in fatigue properties of a Co-16.5 wtpct Fe-25 pct alloy, LRO-1, appeared to be due to differing quantities of grain boundary precipitates. Modification of this alloy with 0.4 pct Ti, to produce an alloy designated LRO-23, reduced the density of grain boundary precipitates and increased ductility, resulting in superior fatigue strength at high temperatures. The fatigue lives of LRO-1 and LRO-23 decreased rapidly above 650 °C, and increased intergranular failure was noted. The fatigue resistance of a cobalt-free alloy, Fe-29 pct Ni-22 pct V-0.4 pct Ti (LRO-37), was examined at 25 °C, 400 °C, and 600 °C; there was little evidence for intergranular fracture at any of these temperatures. Fatigue behavior of the LRO alloys is compared to that of conventional high temperature alloys.

Influence of thermomechanical processing on low cycle fatigue of prealloyed Ti-6AI-4V powder compacts

Abstract: A wide range of microstructures was generated using various thermomechanical processing sequences in Ti-6A1-4V Rotating Electrode Process (REP) powder compacts of low contaminant content. Low cycle fatigue results were found to be superior to those in higher contaminant compacts tested in a previous program. All microstructural groups showed fatigue strengths equivalent to those found in wrought alloy, with the beta-annealed condition being lowest as expected. Alpha + beta work and solution treatment resulted in an excellent fatigue strength of 875 MN/m2 (127 ksi) at 105 cycles; 85 pct of the UTS. In the five conditions tested, the fatigue strength increased with increasing tensile strength, decreasing grain size, and increasing volume fraction of low aspect ratio primary alpha. Most crack initiation sites were observed at the specimen surface. Only alpha + beta worked and solution-treated material exhibited subsurface initiations, none of which was associated with any defect or with a lower fatigue life. Although compacts contained some tungsten particles, in no case were they associated with crack initiation sites, indicating that they were innocuous in the conditions evaluated.

Fatigue design. 2nd edition

Abstract: The book examines fracture control problems, the response of materials to fatigue, and how to determine an individually characteristic response. A comparison of methods of fatigue prediction shows the limitations of some accepted methods and the realism and validity of resulting solutions. The development of fracture mechanics as a design procedure, as applied to life prediction problems in constructional alloys, has extended linear elastic fracture mechanics into considerations of elasto-plasticity. This extension has introduced the availability of data with which to treat the low-strength, high-toughness, plane stress situation and a section of the book deals with this procedure as applied to heavy engineering structural applications. (TRRL)

Microcrystalline Iron-Base Alloys Made Using a Rapid Solidification Technology

Abstract: Initial results of investigation on selected iron-base alloys using a rapid solidification powder metal technology currently being developed at Marko Materials under a DARPA/AMMRC program are reported. The various alloys studied include commercial precipitation hardenable (PH) stainless steels, iron-aluminide-base alloys and high-speed steels. These alloys were modified with specific amounts of boron and processed by melt spinning as rapidly solidified powders followed by hot extrusion. The consolidated alloys were subjected to various heat treatment procedures for optimum mechanical properties. A 30–35% improvement in tensile strength combined with good ductility has been achieved in boron modified PH stainless compared to the conventional alloys. Rapidly solidified Fe-Al-B alloys show high specific strength and modulus of elasticity in the temperature range 1000–1400°F. The other noteworthy attributes of the iron-aluminide-base alloys are outstanding oxidation resistance, good low-cycle fatigue strength, and high thermal stability at elevated temperatures. Very high elongations, typically up to 280%, were achieved in these alloys at high temperatures (1600°F), indicating the possibility of superplastically hot forming these materials. A rapidly solidified boron-modified tool steel showed significant improvement in tool life in cutting (turning) tests, compared to its commercial counterpart.

Effect of Defects on Aircraft Composite Structures

Abstract: : This paper describes the effects of manufacturing and service-induced damage on the static and fatigue strength of aircraft composite structures. Seven manufacturing defects associated with mechanical fasteners were investigated; out-of-round holes, broken fibers on the exit side of drilled holes, porosity, improper fastener seating depth, tilted countersinks, interference fit, and multiple fastener installation and removal cycles. Both static and fatigue test results are described, along with correlation with analysis techniques. The interaction of the effects of these defects on hole wear, measured in fatigue tests of structural joints, is described. The effects of two types of service-induced damage are also described; low energy impact damage and 23mm HEI ballistic damage. The relative sizes of visible and non- visible damage as determined by visual and non-destructive inspection techniques are compared. An evaluation of stitching and the inclusion of glass or Kevlar fiber buffer strips to improve the damage tolerance of carbon/epoxy structures is included. Results of tests of carbon/epoxy panel structures are discussed. Correlation of experimental results with predicted residual static strength is good.

Fatigue properties of implant materials in hip prosthesis form: a standardized test.

Abstract: The determination of the fatigue properties on small specimens is not accurate. It does not take into account the considerable scatter in fatigue properties which may arise in the implants. Therefore, testing of the properties on the actual implant is needed. The working hypothesis of the present article is that four-point bending fatigue tests allow determination of the fatigue limit of the femoral component material in total hip prostheses. It eliminates the disadvantages of the previously proposed load on head test because (1) the reproducibility of the stress pattern is easy; (2) the fatigue testing is over the whole critical part of the stem; and (3) the stresses do not change during the test. Testing of a series of hip prostheses shows that (1) the measured fatigue limit is typical for the material tested; (2) the location of the fatigue fractures is over the critical medial third of the stem; and (3) the fractographic aspects correspond to those of in vivo failed prostheses. The four-point bending procedure is proposed as a standard method.