Showing papers on "Fatigue limit published in 2001"

Fatigue of structures and materials

Abstract: Preface. Frequently used symbols, acronyms and units. 1. Introduction to Fatigue of Structures and Materials. Part 1: Introductory Chapters on Fatigue. 2. Fatigue as a Phenomenon in the Material. 3. Stress Concentrations at Notches. 4. Residual Stresses. 5. Stress Intensity Factors of Cracks. 6. Fatigue Properties of Materials. 7. The Fatigue Strength of Notched Specimens. Analysis and Predictions. 8. Fatigue Crack Growth. Analysis and Predictions. Part 2: Load Spectra and Fatigue Under Variable-Amplitude Loading. 9. Load Spectra. 10. Fatigue under Variable-Amplitude Loading. 11. Fatigue Crack Growth under Variable-Amplitude Loading. Part 3: Fatigue Tests and Scatter. 12. Fatigue and Scatter. 13. Fatigue Tests. Part 4: Special Fatigue Conditions. 14. Surface Treatments. 15. Fretting Corrosion. 16. Corrosion Fatigue. 17. High-Temperature and Low-Temperature Fatigue. Part 5: Fatigue of Joints and Structures. 18. Fatigue of Joints. 19. Fatigue of Structures. Design Procedures. Part 6: Arall and Glare, Fiber-Metal Laminates. 20. The Fatigue Resistance of the Fiber-Metal Laminates Arall and Glare. Subject index.

Structure, metallurgy, and mechanical properties of a porous tantalum foam.

Abstract: This study evaluated a porous tantalum biomaterial (Hedrocel) designed to function as a scaffold for osseous ingrowth. Samples were characterized for structure, Vickers microhardness, compressive cantilever bending, and tensile properties, as well as compressive and cantilever bending fatigue. The structure consisted of regularly arranged cells having struts with a vitreous carbon core with layers of CVI deposited crystalline tantalum. Microhardness values ranged from 240-393, compressive strength was 60 +/- 18 MPa, tensile strength was 63 +/- 6 MPa, and bending strength was 110 +/- 14 MPa. The compressive fatigue endurance limit was 23 MPa at 5 x 10(6) cycles with samples exhibiting significant plastic deformation. SEM examination showed cracking at strut junctions 45 degrees to the axis of the applied load. The cantilever bending fatigue endurance limit was 35 MPa at 5 x 10(6) cycles, and SEM examination showed failure due to cracking of the struts on the tension side of the sample. While properties were variable due to morphology, results indicate that the material provides structural support while bone ingrowth is occurring. These findings, coupled with the superior biocompatibility of tantalum, makes the material a candidate for a number of clinical applications and warrants further and continued laboratory and clinical investigation.

S–N curve characteristics and subsurface crack initiation behaviour in ultra‐long life fatigue of a high carbon‐chromium bearing steel

Abstract: The S–N curve obtained from cantilever-type rotary bending fatigue tests using hour-glass-shaped specimens of high carbon-chromium bearing steel clearly distinguished the fracture modes into two groups each having a different crack origin. One was governed by crystal slip on the specimen surface, which occurred in the region of short fatigue life and a high stress amplitude level. The other was governed by a non-metallic inclusion at a subsurface level which occurred in the region of long fatigue life and low stress amplitude. The inclusion developed a fish-eye fracture mode that was distributed over a wide range of stress amplitude not only below the fatigue limit defined as the threshold for fracture due to the surface slip mode but also above the fatigue limit. This remarkable shape of the S–N curve was different from the step-wise one reported in previous literature and is characterized as a duplex S–N curve composed of two different S–N curves corresponding to the respective fracture modes. From detailed observations of the fracture surface and the fatigue crack origin, the mechanisms for the internal fracture mode and the characteristics of the S–N curve are discussed.

Cyclic behavior of ultrafine-grain titanium produced by severe plastic deformation

Abstract: The cyclic behavior is investigated of commercial purity massive ultrafine-grained titanium obtained by severe plastic deformation through equal channel angular pressing (ECAP). Both stress- and strain-controlled experiments were carried out to access the fatigue performance in terms of Wohler diagram, fatigue limit, Coffin–Manson plot, cyclic hardening curves and cyclic stress-strain curves in a range of plastic strain amplitudes from 7.5×10 −4 to 10 −2 . A significant enhancement of fatigue limit and fatigue life in the ultrafine-grained state is found under constant stress testing. No cyclic softening and degradation in the strain-controlled experiments is noticed in titanium contrary to wavy slip materials such as Cu subjected to ECAP. A simple one-parameter dislocation-based model is proposed to account for experimental results. It is shown that many cyclic properties of severely predeformed materials with fine grains can be rationalized in terms of Hall–Petch grain boundary hardening and dislocation hardening.

Delamination Growth and Thresholds in a Carbon/Epoxy Composite under Fatigue Loading

Abstract: This paper presents a study on delamination growth in Mode I, Mode II and mixed mode under fatigue loading in an HTA/6376C composite. The computed slopes of the modified Paris plots were high. Therefore, threshold values of the strain energy release rate for delamination growth were determined. Low fatigue threshold values revealed a significant effect of fatigue loading. The largest effect was found for the ENF test (Mode II) for which the fatigue threshold value was only 10% of the critical strain energy release rate in static tests. Threshold values for MMB (mixed mode) and DCB (Mode I) tests were 15% and 23% of the static values, respectively. Fractographic evaluation revealed identical initial failure mechanisms in fatigue and static loading conditions for the ENF specimen. The ENF specimen failed by formation and coalescence of microcracks. The low fatigue threshold for the ENF specimen was explained by microscopical observations on the specimen edge. It was also shown that the fracture surfaces generated in static and fatigue DCB and MMB tests were similar.

High-cycle fatigue and durability of polycrystalline silicon thin films in ambient air

Abstract: To evaluate the long-term durability properties of materials for microelectromechanical systems (MEMS), the stress-life ( S / N ) cyclic fatigue behavior of a 2-μm thick polycrystalline silicon film was evaluated in laboratory air using an electrostatically actuated notched cantilever beam resonator. A total of 28 specimens were tested for failure under high frequency (∼40 kHz) cyclic loads with lives ranging from about 10 s to 34 days (3×10 5 to 1.2×10 11 cycles) over fully reversed, sinusoidal stress amplitudes varying from ∼2.0 to 4.0 GPa. The thin-film polycrystalline silicon cantilever beams exhibited a time-delayed failure that was accompanied by a continuous increase in the compliance of the specimen. This apparent cyclic fatigue effect resulted in an endurance strength, at greater than 10 9 cycles, of ∼2 GPa, i.e. roughly one-half of the (single cycle) fracture strength. Based on experimental and numerical results, the fatigue process is attributed to a novel mechanism involving the environmentally-assisted cracking of the surface oxide film (termed reaction-layer fatigue). These results provide the most comprehensive, high-cycle, endurance data for designers of polysilicon micromechanical components available to date.

Thermal cycling analysis of flip-chip solder joint reliability

Abstract: The reliability concern in flip-chip-on-board (FCOB) technology is the high thermal mismatch deformation between the silicon die and the printed circuit board that results in large solder joint stresses and strains causing fatigue failure. Accelerated thermal cycling (ATC) test is one of the reliability tests performed to evaluate the fatigue strength of the solder interconnects. Finite element analysis (FEA) was employed to simulate thermal cycling loading for solder joint reliability in electronic assemblies. This study investigates different methods of implementing thermal cycling analysis, namely using the "dwell creep" and "full creep" methods based on a phenomenological approach to modeling time independent plastic and time dependent creep deformations. There are significant differences between the "dwell creep" and "full creep" analysis results for the flip chip solder joint strain responses and the predicted fatigue life. Comparison was made with a rate dependent viscoplastic analysis approach. Investigations on thermal cycling analysis of the temperature range, (/spl Delta/T) effects on the predicted fatigue lives of solder joints are reported.

Microstructure and intermetallic growth effects on shear and fatigue strength of solder joints subjected to thermal cycling aging

Abstract: Microstructure development of eutectic solder alloy (63Sn/37Pb) after thermal cycling aging and its impact on the shear and fatigue failure of the solder joint has been investigated. The solder microstructure changes with the reflow process and subsequent thermal cycling environments in solder joint reliability tests from −40 to 125°C. In service, solder joints are subjected to thermal cycling aging, corresponding to power on–off cycling of the electronic equipment or cyclic environmental temperature loading, leading to thermal fatigue failures. Thus, it is important to study the effect of the microstructure changes, mechanical strength and fatigue resistance of solder before and after thermal cycling aging. A new specimen design has been developed to closely resemble the actual electronic packaging assembly condition. The joint is made simply by soldering a solder ball between two FR-4 substrates with copper pads using the reflow process. The study shows that the solder microstructure coarsened and intermetallic compound layers grew after 500, 1000 and 2000 thermal cycles. The shear and fatigue strength of the solder joint decreased with increased exposure to thermal cycling aging effects.

Laboratory evaluation of fatigue damage and healing of asphalt mixtures

Abstract: The changes in the stiffness of two asphalt concrete mixtures due to temperature, fatigue damage growth, and healing during rest periods are evaluated using the impact resonance method. The impact resonance method is a means of determining the dynamic modulus of elasticity of a specimen nondestructively. The dynamic modulus of elasticity decreases as temperature increases and as microcrack damage growth occurs in the specimen due to fatigue. The impact resonance method also detects increases in dynamic modulus of elasticity after the application of rest periods. A gain in flexural stiffness was also observed from measurements and is attributed to closure of microcracks or healing during the rest period. The amount of healing or stiffness gain appeared to increase when specimens were subject to a higher temperature during the rest period. A qualitative study of the two asphalt mixtures showed that there is a difference between the two with respect to healing performance.

Overview of fatigue properties of fine grain 5056 Al-Mg alloy processed by equal-channel angular pressing

Abstract: The fatigue performance of the fine-grain 5056 Al-Mg alloy processed by severe plastic deformation through equal-channel angular pressing (ECAP) is assessed in both stress- and plastic strain-controlled experiments. Compared to its conventional counterpart, the ECAP material exhibits a high tensile and low-cyclic fatigue strength under constant stress amplitude. However, its fatigue life under strain-controlled conditions is notably shorter than that of the O-temper specimens. Despite severe pre-straining of the specimen during ECA-pressing, cyclic softening was found to be rather small. It is shown that the mechanical characteristics obtained after ECAP can be significantly improved during short time annealing at moderate temperatures (150°C, 15 min) after fabrication. Such heat treatment is supposed to recover partially the grain boundary region, which has been most heavily distorted during processing. Mechanisms of fatigue in ECA-processed materials are discussed within a framework of a simple one-parametric model of dislocation kinetics.

Influence of casting technique and hot isostatic pressing on the fatigue of an Al-7Si-Mg alloy

Abstract: The influence of the casting filling technique and hot isostatic pressing (hipping) on the fatigue-life distribution of Al-7Si-Mg alloy castings has been studied. To vary the number density and size of oxide-film defects in the castings, test bars were cast using bottom-gated filling systems with and without filtration. Some unfiltered castings were subjected to a hipping treatment of 100 MPa at 500 °C for 6 hours. Test pieces were machined from the castings and were fatigue tested in pull-pull sinusoidal loading, at maximum stresses of 150 and 240 MPa under a stress ratio of R=+0.1. The fatigue lives at any probability of failure and Weibull statistical parameters of the filtered castings were higher than those of the unfiltered and nonhipped castings, illustrating the importance of the casting technique. However, the unfiltered but hipped castings exhibited higher performance. It is proposed that the significant improvement in fatigue life after hipping is due to the deactivation of entrained double oxide-film defects as fatigue-crack initiators.

Off-Axis Fatigue Behavior of Unidirectional Carbon Fiber-Reinforced Composites at Room and High Temperatures

Abstract: High-temperature off-axis fatigue behavior of unidirectional T800H/Epoxy composite has been studied. Effects of different temperature on off-axis fatigue strength are also discussed. Tension-tension fatigue tests are performed at room temperature and 100°C, respectively, on six kinds of off-axis plain coupon specimen. The absolute off-axis fatigue strengths are significantly reduced as the test temperature becomes higher. Irrespective of test temperature, almost linear S-N relationships are obtained for all off-axis angles over the range of fatigue life up to 106 cycles. Normalizing the maximum fatigue stress with respect to the static tensile strength at test temperature, the fatigue data for all off-axis angles eventually fall on a single S-N relationship. It is shown that a theoretical fatigue strength ratio can be defined as a non-dimensional effective stress based on the classical static failure theory and it succeeds in coping with the directional nature of off-axis fatigue strength. A fatigue damag...

Fatigue Resistance of Welded Details Enhanced By Ultrasonic Impact Treatment (UIT)

Abstract: Enhancement of the fatigue resistance of welded transverse stiffeners and cover plate details by ultrasonic impact treatment (UIT) was evaluated in 18 full-scale W27×129 rolled beam specimens. Fatigue tests were conducted under constant amplitude loading at various stress range levels and at two minimum stress levels simulating the effect of sustained load. The test specimens were investigated for fatigue crack initiation and propagation. Distributions of residual stresses adjacent to the weld toe were determined before and after the treatment. Test results indicated that UIT enhanced the fatigue performance of all treated details by improving the weld toe profile, changing microstructure and introducing beneficial compressive residual stresses at the treated weld toe. The treatment effectively elevated the fatigue crack growth threshold and the fatigue limit without changing the slope of the S–N curve.

The effect of environment on the fatigue of bonded composite joints. Part 1: testing and fractography

Abstract: In this work, the effect that test environment and pre-conditioning had on the fatigue behaviour of CFRP/epoxy lap–strap joints was investigated. It was shown that the fatigue resistance of the lap–strap joints did not vary significantly until the glass transition temperature, Tg, was approached, at which point a considerable reduction in the fatigue threshold load was observed. It was also noted that absorbed moisture resulted in a significant reduction in the Tg of the adhesive. This must be taken into account when selecting an adhesive to operate at elevated temperatures. The locus of failure of the joints was seen to be highly temperature dependent, transferring from primarily in the composite adherend at low temperatures to primarily in the adhesive at elevated temperatures. It was also seen that as the crack propagated along the lap–strap joint, the resolution of the forces at the crack tip tended to drive it into the strap adherend, which could result in complex mixed mode fracture surfaces.

Cyclic deformation and fatigue behaviour of the magnesium alloy AZ91

Abstract: In this report, the cyclic deformation behaviour of the die-casting magnesium alloy AZ91 was investigated at constant total strain amplitudes between 1.4×10 −3 and 2×10 −2 at room temperature and at 130 °C. The fatigue life data at both temperatures can be described well by the laws of Manson-Coffin and Basquin. The microstructural investigations performed show the strong influence of several microstructural features on the initiation and propagation of fatigue cracks. In order to understand the fatigue crack propagation behaviour, fatigue tests were interrupted at certain numbers of cycles in order to make replicas of the surface of the samples. It could be verified that crack propagation occurs mainly by the coalescence of smaller cracks. Furthermore, unloading tests, performed within closed cycles, were carried out in order to capture the changes of stiffness during a closed cycle with the aim to ascertain the damage evolution occurring during the fatigue tests and to determine the stresses at which the cracks open and close. The results obtained in this study form the microstructural basis of a life-prediction concept, reported earlier.

Static and Fatigue Analyses of RC Beams Strengthened with CFRP Laminates

Abstract: Extensive testing has shown that externally bonded carbon fiber reinforced polymer (CFRP) laminates are particularly suited for improving the short-term behavior of deficient reinforced concrete beams. Accelerated fatigue tests conducted to date confirm that fatigue response is also improved. This paper describes an analytical model for simulating the static response and accelerated fatigue behavior of reinforced concrete beams strengthened with CFRP laminates. Static and fatigue calculations are carried out using a fiber section model that accounts for the nonlinear time-dependent behavior of concrete, steel yielding, and rupture of CFRP laminates. Analysis results are compared with experimental data from two sets of accelerated fatigue tests on CFRP strengthened beams and show good agreement. Cyclic fatigue causes a time-dependent redistribution of stresses, which leads to a mild increase in steel and CFRP laminate stresses as fatigue life is exhausted. Based on the findigns, design considerations are suggested for the repair and/or strengthening of reinforced concrete beams using CFRP laminates.

The effects of machined workpiece surface integrity on the fatigue life of γ-titanium aluminide

Abstract: Tension–tension tests on turned, electro-chemical machined (ECM) and electro-discharge textured (EDT) specimens made from Ti–45Al–2Nb–2Mn+0.8 vol% TiB 2 alloy, showed the turned specimens to have a higher fatigue strength ∼475 MPa. It is likely that this was due to the presence of highly compressive surface residual stresses caused by the turning operation.

Influence of loading frequency on high cycle fatigue properties of b.c.c. and h.c.p. metals

Abstract: The influence of the loading frequency on the high cycle fatigue properties of two b.c.c. metals, commercially pure (c.p.) niobium and c.p. tantalum in annealed and cold worked condition, and of two annealed h.c.p. titanium and of Ti–6Al–7Nb alloy were investigated. Endurance data in the regime of 105 to 2×108 cycles to failure obtained with rotating bending and ultrasonic fatigue testing equipment (loading frequencies 100 Hz and 20 kHz, respectively) coincide within the ranges of scatter for niobium and Ti–6Al–7Nb alloy. The mean endurance limits at 2×108 cycles of these metals are ≈60% of the respective yield stress of the as produced material. The high loading frequency leads to prolonged lifetimes and increased mean endurance limits for tantalum and (less pronounced) for titanium. Fatigue crack initiation in tantalum changes from a preferentially ductile and transgranular mode at 100 Hz to a more brittle, crystallographic and intergranular mode at 20 kHz. The mean endurance limit of tantalum is above the yield stress of the as produced material, and high initial rates of plastic deformation therefore result. Cold working of tantalum and niobium improves their static strength properties, but is of only minor importance for the high cycle fatigue behaviour.