Showing papers on "Fatigue limit published in 1981"

Fatigue of Composite Materials: Damage Mechanisms and Fatigue Life Diagrams

Abstract: The basic fatigue damage mechanisms in composite laminates are reviewed. Based on these mechanisms a pattern in the fatigue-life diagrams is proposed. Several experimental data are shown to agree with this basic pattern. Fatigue ratio is defined in terms of strains, and fatigue limit is shown to exist for unidirectional, cross-plied and angle-plied laminates. The limitations to the fatigue performance of composite laminates are pointed out and suggestions for improving the fatigue resistance are made.

Fatigue growth threshold of small cracks

Abstract: The effects of grain size and crack length on the threshold condition of fatigue growth of small cracks are analysed both theoretically and experimentally. The theoretical model is based on the assumption that the threshold condition is determined by whether the crack-tip slip band blocked by the grain boundary propagates into an adjacent grain or not. By comparing the model analysis with the experiments of mild steel, it was found that the threshold stress estimated from the fatigue limit of the smooth specimen gave the effective components of the applied stress. When the threshold stress normalized by the fatigue limit of the smooth specimen was correlated to the crack length normalized by a certain crack length, the theoretical relation was found to agree well with the experimental data. From a further analysis of the published data of various materials, the theoretical normalized relation was found to give a lower bound of the threshold values of the stress and the stress intensity factor. A deviation from the theoretical relation seen in the cases of hard metals was explained through an extension of the model by regarding that hard metals contained initial, inherent flaws even in the smooth specimen.

Cyclic deformation and fatigue behaviour of α-iron mono-and polycrystals

Abstract: The reported studies are based on a series of cyclic deformation tests that were conducted at room temperature on decarburized high-purity α-iron specimens in mono-and polycrystalline form. The experimental data cover plastic strain ranges Δe pl in the regime 10−4 ≲ Δe pl ≲ 10−2 and variations in cyclic plastic strain rates έ pl between ∼10-5 and ∼10−2 s−1. In the case of single crystals, the effect of solute carbon (∼30 wt. ppm) was investigated as well. The mechanical data were supplemented by detailed studies of the dislocation arrangements by transmission electron microscopy and of the surface patterns by scanning electron and optical microscopy. Detailed accounts are given of the following topics: cyclic hardening and saturation, dislocation mechanisms, shape changes due to asymmetric slip of serew dislocations, cyclic stress-strain response and fatigue crack initiation. Under conventional conditions of “high” έ pl (≲10−4 s−1) the fatigue behaviour of α-iron at room temperature reflects the low mobility of the screw dislocations which is characteristic of the lowttemperature mode of deformation of body-centred cubic (b.c.c.) metals. As a consequence the behaviour exhibits significant differences with respect to that of fatigued face-centred cubic (f.c.c.) metals such as: strongly impeded dislocation multiplication below Δe pl ∼ 5 × 10−4, appreciable secondary slip at higher Δe pl leading to a cell structure (persistent slip bands do not form), shape changes due to asymmetric slip of screw dislocations and a relatively high effective stress level. The reduction of έ pl and the presence of solute carbon atoms modify this behaviour significantly, making it more similar to that of f.c.c. metals. In all cases it was found that only the athermal component of the peak (saturation) stress but not the latter itself represents a suitable measure of the properties of the dislocation substructure. On the basis of the cyclic deformation behaviour and of observations of trans-and intergranular fatigue crack initiation it was concluded that the fatigue limit of α-iron is an intrinsic property of the b.c.c. structure whose characteristics, however, are affected sensitively by interstitial impurity content and by the strain rate of the fatigue test.

Multiaxial Fatigue: A Survey of the State of the Art

Abstract: This paper surveys the current state of knowledge concerning multiaxial fatigue. Developments are presented in chronological order and are discussed so as to supplement existing reviews in this field. Emphasis is placed primarily on the criteria or methods of evaluation of fatigue strength under general multiaxial loading at room temperature. The survey indicates that the early development of the criteria was based on extensions of static yield theories to fatigue under combined stresses. These are stress-based criteria limited primarily to high-cycle fatigue. Most of the later criteria are strain-based. These criteria fall into two broad groups: the equivalent stress or strain type and the critical plane type. Most of these criteria commonly lack considerations of the cyclic stress-strain response. Their application to nonproportional loading suffers from difficulties in implementation or from inconsistencies with results of experiments. Recent approaches fall in the category of continuous damage evaluation methods. At present, these appear to be abstract or difficult to implement. All the above criteria are critically examined and compared. With this background, a new plastic work approach, proposed by the author, is discussed briefly.

Bolt-Bearing Fatigue of a Graphite/Epoxy Laminate

Abstract: Graphite/epoxy laminates (T300/5208) were tested under bolt-bearing loads for a range of bolt clampup torques and for several test conditions involving water. High clampup torque improved both the static strength and fatigue limit by about 100 percent compared to a simple pin-bearing case, which had no through-the-thickness constraint. The static strength improvement was explained in terms of failure modes. Bolt clampup also influenced the hole elongation that developed before failure. For both the static and fatigue tests, the hole elongations were much larger for the low-torque-clampup case than for the pin-bearing case or for tests with moderate clampup torques. Tests in water degraded static bearing strength only slightly, but reduced the fatigue limit about 40 percent below that for tests conducted in air.

Dependency of Fatigue Predictions on the Form of the Crack Velocity Equation

Abstract: A computer search technique was developed to analyze fatigue strength data using both exponential and power law forms of the subcritical crack velocity equation. All crack velocity equations would fit a given set of fatigue data equally well in the data range but failure predictions based on the different crack velocity equations diverge from each other outside the data range. The exponential form of the crack velocity equation best fit both the static and dynamic fatigue data of hot-pressed Si3N4 and optical glass fibers, whereas the power law form best fit the static and dynamic fatigue data of soda-lime glass and A12O3. To determine the most appropriate crack velocity equation for a given material/environment system, it is recommended that fatigue data be obtained under different loading conditions and the data numerically regressed using the computer search technique with each of the possible crack velocity equations to find which best fits the data.

Compression Fatigue Behavior of Graphite/Epoxy in the Presence of Stress Raisers

Abstract: Graphite/epoxy composites are being applied to aircraft structures because of their demonstrated capability to reduce weight and increase life. Although composites have better fatigue properties than metals, their behavior differs significantly. Unlike metals, composites exhibit excellent tensile fatigue behavior-the constant amplitude fatigue strength at 10 7 cycles being close to static ultimate. Compression and reversed loading fatigue behavior in the presence of stress raisers, however, has not been well characterized. This program was therefore undertaken to determine the characteristics of composites under these conditions. Two stress raisers were considered; the first was an open hole and the second was damage resulting from low velocity, hard object impact. For the open hole specimen, constant and variable amplitude tests were performed to determine the significance of compression fatigue and to investigate the failure mechanism. These tests indicated that the fatigue life under compression and reversed loading is less than for tension-tension loading and will be an important design consideration in future composite applications. The failure mechanism appeared to be local progressive failure of the matrix near the stress raiser resulting in delamination, and final failure by fiber buckling. The variable-amplitude loading results also demonstrated the unconservativeness of Miner's rule for making analytical predictions. In order to determine the nature and significance of impact damage to generic composite structural elements, low velocity impact and residual properties tests were conducted on solid laminate specimens of 42 and 48 plies thick and on honeycomb sandwich specimens with 12-ply composite face sheets. Results of post impact properties tests indicate subvisual damage can degrade compression static and fatigue strength, although subvisual damage will not propagate under moderate (0.003) cyclic strain.

Fatigue of glass and carbon fiber reinforced engineering thermoplastics

Abstract: Cyclic tension fatigue S-N curves are given for injection moleded Nylon 6/6, polycarbonate, polysulfone, polyphenylene sulfide, and poly(amide-imide) matrices with glass and carbon fibers as well as for unreinforced material. The S-N curves for most composites appear linear, with no evidence of a fatigue limit up to 106 cycles. Some nonlinearity is evident with the Nylon 6/6 composities, and these appear to fail at a cumulative strain similar to the ultimate static strain. The remainder of the composites appear to fail by a crack propagation mechanism. The glass reinforced materials all degrade at a similar rate in fatigue, while the carbon reinforced materials with brittle matrices degrade more slowly than do those with ductile matrices. The latter effect may be due to greater integrity of the cracked regions for brittle matrix systems.

Effect of residual stress on fatigue crack growth rate measurement

Abstract: Examples are given where influence of residual stress leads to erroneous interpretation of fatigue crack growth rate measurements made in accordance with ASTM Method E 647-78T. The experimental data presented form a basisfor modification of applicable ASTM documents to give recognition to problems caused by residual stress, and to suggest guidelines for minimization of their effect on fracture property measurement.

The engineering fatigue properties of wrought copper

Abstract: — The paper presents a comprehensive review, supplemented by original data, of the engineering fatigue behaviour of copper. Variations in manufacturing route and softening treatments are shown to have little effect on the fatigue of annealed copper but the high cycle fatigue strength is increased by cold work. The high strain fatigue behaviour is defined in terms of the plastic strain range and the cyclic stress-strain characteristics are documented. Fatigue behaviour in bending and torsion is defined by data and related to that in tension by simple design rules. Notches are found to reduce the laboratory measured fatigue strength of copper by ∼ 30% and the effect of surface finish, surface distortion and surface residual stress is defined in the literature. Fatigue crack growth is defined in terms of stress intensity factor range ΔK by an upperbound law and, together with the conditions for non-growth (ΔK0), shown to relate to the equivalent conditions for steels via the ratio of the respective elastic moduli. The effect of environment on the fatigue of copper has received scant attention in the literature, such results as exist suggesting little if any reduction in strength to be brought about by gaseous or aqueous environments. The most dramatic change is the improvement of about an order of magnitude which results when tests in vacuum are compared with equivalent tests in air. Results of fatigue tests on copper in ammoniacal environments are conspicuously absent from the literature. As the test temperature is reduced below room temperature there is a predictable increase in high cycle fatigue strength, a reduction in fatigue strength occurring above room temperature. High strain fatigue test results presented in terms of plastic strain range appear insensitive to temperature although at very low strain rates and high temperatures a reduction in fatigue strength occurs. A linear life fraction cumulative damage creep-fatigue law appears sometimes to be non-conservative but much more testing is needed to evaluate fatigue damage summation laws generally for copper. Numerical data are given in support of all the aspects of the engineering fatigue behaviour reviewed in the paper.

Low‐ and high‐cycle fatigue properties of various steels specified in jis for machine structural use

Abstract: —Low-cycle fatigue properties were investigated on four carbon steels and five low alloy steels specified in JIS (Japanese Industrial Standard) for machine structural use, which are the most commonly used in Japan. Several different heats from each of several representative manufacturers were sampled so as to represent the average fatigue characteristics of current materials. The cyclic deformation behaviour of material was denned by comparing the monotonie yield stress on the extrapolated tensile work hardening curve with the cyclic yield stress in the cyclic stress-strain curve determined by incremental step test. The normalized ferrite-pearlitic steels cyclically hardened, while the quench-tempered martensitic were cyclically stable or softened. The S–N relations derived from the strain-controlled low-cycle tests were compared with the results obtained by load-controlled high-cycle tests. The extrapolated S–N curves based on the cyclic stress-strain curve predicted the fatigue strength in the high-cycle range to be stronger for cyclic-hardening steels, but weaker for cyclic-softening steels. The predicted S–N curves for stable steels coincided with the high cycle test data. The fatigue limit had a proportional relationship with cyclic yield stress, slightly depending on the cyclic deformation behaviour. On the other hand, the cyclic yield stress was found to exhibit a very good linear correlation with the monotonie tensile strength, independent of cyclic deformation behaviour. This explains the empirical law that the fatigue limit is approximately proportional to the tensile strength.

Corrosion fatigue of surgical stainless steel in synthetic physiological solution

Abstract: Fatigue tests conducted both in air and synthetic physiological solution show that the fatigue strength of surgical stainless steel in synthetic physiological solution is about 10% lower than the strength in air for a given endurance level. It is proposed that surgical stainless steel which is normally passive in physiological solution suffers corrosion fatigue because of susceptibility to crevice corrosion which occurs at extrusions and intrusions (crevices) on the surface thereby shortening the crack initiation time and the fatigue life.

Dislocations and the fatigue strength of metals

Abstract: Recent work has shown that cyclic straining induces well-defined dislocation structures in ductile crystals. The structures seem to result from the cross-slip of screw dislocations and from the mutual trapping of edge dislocations. A brief review is given of recent experimental work on these structures, and an attempt is made to explain them in terms of dislocations. There is a pattern of long-range internal stress associated with the structures, and the significance of this for crack nucleation is stressed. It seems possible that the fatigue endurance limit of metals is the stress level above which screw dislocations on different slip planes cross-slip and annihilate each other rather than trap each other without annihilation.

Torsional Fatigue Strength of a Shrink Fitted Shaft

Abstract: Torsional fatigue strength of several types of shrink fitted shaft couplings is estimated based on the results of stress analysis, in which the changes of frictional coefficient and contact pressure during cyclic loading are taken into account. Then these estimated strengths are confirmed by torsional fatigue tests. The results are summarized as follows. The firictional coefficient in the slip region increases steadily with an increase of cycles and saturates for more than 104 cycles. The saturated value of it is about 0.7. Torsional fatigue strength reduction factor of shrink fitted shaft is found to be Kf=1.28, which is far smaller than the rotational bending fatigue strength reduction factor (Kf=2∼3). Anti-fretting measures such as groove, taper are found to be not so effective for torsionalfatigue as for rotational bending fatigue.

Influence of sub-zero and shot-peening treatments on impact and fatigue fracture properties of case-hardened steels

Abstract: High-cycle fatigue endurance limits and impact fracture strengths were determined for carburized SAE 4028 steel with and without a shot-peening treatment and for carbonitrided SAE EX55 steel with and without a sub-zero treatment at -85 °C (-120 °F). The shot-peening and the sub-zero treatments did not influence the impact fracture strengths of the steels; the impact fracture strengths of carburized SAE 4028 and carbonitrided EX55 steels were 2250 MPa (325 ksi) and 3220 MPa (468 ksi), respectively. The treatments had marked influence on the fatigue endurance limits of the steels, however. The fatigue endurance limit of carburized SAE 4028 was 730 MPa (105 ksi) without the shot-peening treatment and 1035 MPa (150 ksi) in the peened condition. The carbonitrided EX55 steel had endurance limits of 965 MPa (140 ksi) without the sub-zero treatment and only 415 MPa (60 ksi) after the treatment. The differences in fatigue endurance limits were explained through changes in residual stress patterns in the hardened cases. Shot-peening induced a compressive residual stress in the surface of carburized SAE 4028 steel, which was as high as 650 MPa (94 ksi). The residual stress in the case of EX55 without the sub-zero treatment was compressive for both the martensite and austenite phases, whereas the stress existing in the austenite after the sub-zero treatment was highly tensile. Differences in residual stresses among individual phases within the microstructure were used to explain observed changes in fracture mode.

Analytical and Experimental Results for Bonded Single Lap Joints with Preformed Adherends

Abstract: A theoretical and experimental study is conducted to investigate the load transfer of a single lap joint in which the adherends are preformed so that the angle between the line of action of the applied in-plane force and the bond line is reduced. The preforming of the adherend reduces the moment resultant in the ad here nd at the edge of the overlap region, which reduces both the maximum peel and shear stresses in the adhesive and gives a more uniform shear distribution in the adhesive. An increase in static load transfer of up to 120% is shown, and several orders-of-magnitude increase in fatigue life are achieved with modest preform angles. Thus, sizable benefits can be obtained in the fatigue life or additional load capacity of bonded single lap joints by preforming the adherends. Results are presented from an analytical and experimental investigation to determine the effect that preforming the adherend has on the stress distributions and strengths of bonded single lap joints. The analytical investigation used a closed-form analysis similar to that of the classical analysis of Goland and Reissner3 and a two-dimensional quasi-nonlinear finite-element analysis similar to that of Ref. 4 to study in detail the stress distributions in the ASTM D 1002-72 specimen5 with a 1.27-cm (0.50-in.) overlap and preformed adherends. The finite element analysis is used to check the accuracy of the closed-form analysis. A parametric study is conducted using the closed-form analysis to determine the optimum adherend preform angle for typical adherend and adhesive material properties and joint geometries. The ex- perimental investigation includes numerous tension-to-failure tests to determine the effect that preforming the adherends has on the ultimate static strength of typical bonded joints. A photoelastic test is also conducted to show the changes in the stress distribution in the joint due to preforming the adherends. Due to the high stress concentrations near the edge of the overlap region in the adhesive, joints are usually designed for fatigue rather than static strength. In fact, design-allowable stresses for single lap joints are generally taken to be 30% of the static ultimate strength because of fatigue. The reductions in stress concentrations obtained by preforming the adherends of single lap joints should significantly improve the fatigue strength. Thus, fatigue tests are also conducted to determine the effect of preforming the adherends on the fatigue strength of typical bonded single lap joints. Because of program and time constraints, only a limited number of low-cycle fatigue tests are performed. Test variables include both preform angle and overlap length.

Fatigue Properties of Unnotched, Notched, and Jointed Specimens of a Graphite/Epoxy Composite

Abstract: Within a continuing program on high tensile graphite/epoxy composite, stress-strain, axial fatigue, and compliance behavior of unnotched, notched (3-mm diameter hole), and jointed specimens made of [O 2 /′45/O 2 /′45/90] s T300/914C laminates (177°C curing temperature) have been studied. In addition, the behavior of unnotched specimens cut from (1) the same laminate but with the longitudinal specimen axis now perpendicular to the zero-degree fiber direction, and (2) the high modulus fiber laminate with the same build-up was investigated. Stress-strain curves, S-N curves, and increase-in-compliance versus percentage-of-total life curves were determined for all specimen types for stress ratios, R, ranging from R = +5.0 (compression-compression cycling, C-C) to R = +0.1 (tension-tension cycling, T-T). An overall comparison of results from specimens with different stress raisers shows that the stress raisers diminish fatigue strength in the low-cycle range, but in the high-cycle range their influence has vanished. Effective stress concentrations were found to be different for compression and tension. During T-C cycling, increase of compliance was lowest for the fastener-filled no-load transfer joint and largest for the single-shear load transfer joint. The large compliance changes of the load-transfer specimens were attributed to increased bearing damage. In general, the scatter in static and fatigue strength was found to be comparable with that for similar features in metals. When the plain material was loaded transversely instead of longitudinally, static and fatigue strength were lower by a factor of about 3.

Fatigue behavior of steel light poles

Abstract: Standard light poles fabricated to California Department of Transportation standards were tested in fatigue. Comparative results were obtained from testing eight poles fabricated of ASTM A283 Grade D steel and six poles fabricated of ASTM A595 Grade A steel. The first six specimens of ASTM A283 had 45 deg fillet welds at the base of the arm and base of the pole while the ASTM A595 poles had unequal leg fillet welds. Since this difference in geometry affected the fatigue test results, two additional poles were fabricated and tested of ASTM A283 steel with unequal leg welds. The fatigue strength of the ASTM A283 poles with 45 deg fillet welds was below Category E' while the fatigue strength of ASTM A595 poles with unequal leg weld was equal to Category E'. The two ASTM A283 poles with unequal leg weld had a fatigue strength equal to Category E'. Fatigue cracks appeared at the toe of the weld at the base of the arm and at the base of the pole at approximately the same number of cycles. Cracks were very difficult to detect because of the galvanized coating. (FHWA)