Showing papers on "Fatigue limit published in 1998"

A notch intensity factor approach to the stress analysis of welds

Abstract: In the context of linear elastic stress gradients that are present in welded joints, a stress field approach based on notch stress intensity factors is presented with the aim of describing stress distributions in the neighbourhood of weld toes, since fatigue strength is dependent on such distributions. This paper summarizes the analytical fundamentals and gives an appropriate definition of the parameters for stress components under opening and sliding modes. Then, by comparing the expected results with those obtained by numerical analysis, the contributions of the symmetric and skew-symmetric loading modes are quantified for different geometries, and summarized into concise expressions which also take into account the influence of the main geometrical parameters of the welded joint. The range of validity and the application limits of this field approach in the presence of weld toe radii are discussed. Finally, a synthesis of experimental fatigue strength data based on the new field parameters is reported.

Effect of SiC volume fraction and particle size on the fatigue resistance of a 2080 Al/SiCp composite

Abstract: The effect of SiC volume fraction and particle size on the fatigue behavior of 2080 Al was investigated. Matrix microstructure in the composite and the unreinforced alloy was held relatively constant by the introduction of a deformation stage prior to aging. It was found that increasing volume fraction and decreasing particle size resulted in an increase in fatigue resistance. Mechanisms responsible for this behavior are described in terms of load transfer from the matrix to the high stiffness reinforcement, increasing obstacles for dislocation motion in the form of S’ precipitates, and the decrease in strain localization with decreasing reinforcement interparticle spacing as a result of reduced particle size. Microplasticity was also observed in the composite, in the form of stress-strain hysteresis loops, and is related to stress concentrations at the poles of the reinforcement. Finally, intermetallic inclusions in the matrix acted as fatigue crack initiation sites. The effect of inclusion size and location on fatigue life of the composites is discussed.

Fatigue damage characterization in carbon fibre composite materials using an electrical potential technique

Abstract: Changes in electrical resistance during static and fatigue loading of unidirectional and cross ply carbon fibre reinforced polymer composites have been studied. The carbon fibres in the study were T300 and the matrix resins were Hexcel 914 and 920. It was found that changes in resistance during static tensile testing were about three per cent of the original resistance of the samples, while fatigue testing caused resistance changes of up to 10% of the original resistance, immediately prior to final failure. The initial linear portion of the resistance increase on static testing was reversible and could be attributed to reversible elastic strains in the fibres; later non-linear changes were a consequence of fibre fracture and were irreversible. Changes in resistance during fatigue also contained both reversible and non-reversible components. It was found that during fatigue testing the initial changes in resistance caused by the first few thousand cycles could be correlated with the eventual life. Samples with large initial resistance change had reduced lives compared with those with small changes in resistance. Fatigue lives of composite laminates may thus be predicted from monitoring of initial resistance changes. Many of the results could be explained via the parallel resistance model of conduction in composite laminates.

Statistical analysis of defects for fatigue strength prediction and quality control of materials

Abstract: A wide range of studies and experimental evidence have shown that the lower bound of fatigue properties can be correctly predicted by considering the maximum occurring defect size. The estimate of this dimension can be done by analysing the defect sizes using the statistics of extremes. The scope of this paper is to discuss and investigate the two key points in a successful application of this technique: the first is the choice of statistical method for the analysis of data; the second is the knowledge of the minimum number of defects needed to obtain a good estimate of extreme defects. The results obtained in this study allow one to formulate a procedure for estimating the extreme defects with a precision suitable for fatigue strength prediction.

Mechanical fatigue characteristics of Sn-3.5Ag-x (X=Bi, Cu, Zn and In) solder alloys

Abstract: In our previous study, the fatigue life of Sn-3.5Ag-Bi alloy was found to be dominated by the fracture ductility of the alloy and to obey a modified Coffin-Manson’s law: (Δep/2D)· Nfα, where Δep is plastic strain range, Nf is fatigue life, and α and=C are nondimensional constants. In this study, copper, zinc, and indium are selected as the third element, and the effect of these elements on the isothermal fatigue properties of Sn-3.5%Ag alloy has been investigated. The relationship between fatigue life and crack propagation rate estimated from load drop curve during fatigue test is also discussed. The addition of copper, indium, and zinc up to 2% slightly decreases the fatigue life of Sn-3.5Ag alloy due to the loss of ductility, while the life still remains higher than that of tin-lead eutectic alloy. The modified Coffin-Manson’s equation can also be applied to ternary Sn-3.5Ag-X. It is found that both ductility and fatigue life are significantly responsible for the load drop rate of the alloy, which reflects the extent of crack propagation. The fatigue life of Sn-3.5Ag-X alloy is therefore dominated not by the kinds and amount of third element but by the ductility of each alloy.

Improvement in fretting wear and fatigue resistance of Ti–6Al–4V by application of several surface treatments and coatings

Abstract: Application of surface modification methods is expected to be an ideal solution to mitigate fretting damage. In this study, our aim was to improve the fretting wear and fretting fatigue resistance of titanium alloys by using several types of surface treatments and thin films, including shot-peening, ion-beam-enhanced deposition (IBED) CrN films, shot-peening+IBED CrN films as well as IBED CuNiIn films. Results showed that with the application of all the above surface coatings and treatments, the fretting wear and fretting fatigue resistance of Ti–6Al–4V were improved. However, the mechanisms and effects of several surface modification methods to mitigate the fretting damage were quite different. IBED CrN film exhibited the best fretting fatigue performance while the duplex treatment by shot-peening/IBED CrN film exhibited the highest fretting wear resistance. There are four mechanisms which can be used to explain the different fretting performance of these surface treatments and coatings: (1) to induce a compressive residual stress; (2) to decrease the coefficient of friction; (3) to increase the hardness; (4) to increase the surface roughness.

Continuous-glass-fibre-reinforced polypropylene composites II. Influence of maleic-anhydride modified polypropylene on fatigue behaviour

Abstract: This study investigates the fatigue behaviour of continuous-glass-fibre-reinforced polypropylene composites, under longitudinal, shear and transverse loadings. In order to study the tension-tension fatigue behaviour under different failure modes, specimens are used with fibres oriented at 0, 10, ± 45 and 90° with the loading direction, respectively. The experiments are carried out on composites based on an isotactic-polypropylene (PP) and a blend of this homopolymer and maleic-anhydride modified polypropylene (MA-PP), the latter showing better interfacial bonding with the silanized E-glass fibres. Since the fatigue performance of composite materials is strongly determined by the matrix, the fatigue behaviour of pure matrix material is also investigated. Results indicated that improved adhesion in the glass/MA-PP composites has a positive effect on the damage development as measured by stiffness reduction during fatigue life. The fatigue sensitivity, i.e. the degree of reduction of fatigue strength with increasing life, is however not significantly affected by the matrix/interphase modification.

Fatigue characteristics of McKibben artificial muscle actuators

Abstract: The McKibben artificial muscle is a pneumatic actuator whose properties include a very high force to weight ratio. This characteristic makes it very attractive for a wide range of applications such as mobile robots and prosthetic appliances for the disabled. Typical applications often require a significant number of repeated contractions and extensions or cycles of the actuator. This repeated action leads to fatigue and failure of the actuator, yielding a life span that is often shorter than its more common robotic counterparts such as electric motors or pneumatic cylinders. In this paper, we develop a model that predicts the maximum number of life cycles of the actuator based on available uniaxial tensile properties of the actuator's inner bladder. Experimental results, which validate the model, reveal McKibben actuators fabricated with natural latex rubber bladders have a fatigue limit 24 times greater than actuators fabricated with synthetic silicone rubber at large contraction ratios.

Self-monitoring of fatigue damage and dynamic strain in carbon fiber polymer-matrix composite

Abstract: Self-monitoring of static/fatigue damage and dynamic strain in a continuous crossply [0/90] carbon fiber polymer-matrix composite by electrical resistance (R) measurement was achieved. With a static/cyclic tensile stress along the 0° direction, R in this direction and R perpendicular to the fiber layers were measured. Upon static tension to failure, R in the 0° direction first decreased (due to increase of degree of 0° fiber alignment and fiber residual compressive stress reduction) and then increased (due to 0° fiber breakage), while R perpendicular to the fiber layers increased monotonically (due to increase of degree of 0° fiber alignment and delamination). Upon cyclic tension, R (0° decreased reversibly, while R perpendicular to the fiber layers increased reversibly, though R in both directions changed irreversibly by a small amount after the first cycle. Upon fatigue testing at a maximum stress of 57% of the fracture stress, R (0°) irreversibly increased both in spurts and continuously, due to 0° fiber breakage, which started at 15% of the fatigue life, while R (perpendicular to the fiber layers) irreversibly increased both in spurts and continuously, due to delamination, which started at 33% of the fatigue life. The peak R (0°) in a cycle irreversibly decreased, while the minimum R (perpendicular to the fiber layers) at the end of a cycle irreversibly increased during the first 0.1% of the fatigue life, due to irreversible increase in the degree of 0° fiber alignment. R (0°) became noisy starting at 87% of the fatigue life, whereas R (perpendicular to the fiber layers) became noisy starting at 50% of the fatigue life. For a [90] unidirectional composite, R (0°) increased reversibly upon tension and decreased reversibly upon compression in the 0° direction, due to piezoresistivity.

Adhesively-bonded repairs to fibre-composite materials I. Experimental

Abstract: The performance of carbon fibre/epoxy repair joints, bonded using an epoxy film adhesive, under static and fatigue loading has been investigated. The repair joints were immersed in distilled water at 50°C for periods of up to 16 months and the effect of the hot/wet environment on the static and fatigue strengths was evaluated. Residual strength tests, where repairs were subjected to fatigue followed by static loading, were also performed. The mechanical properties of the substrate and the adhesive forming the joint were determined. All tests were undertaken at room temperature. It was found that there was no major effect of the conditioning on the above properties and that the repair joints had a similar static strength to that of the parent material. In some cases, a video camera fitted with a macro-lens was used to record the repair during static loading; cracks of an average length of 2.6 mm were visible in the composite just before catastrophic failure took place. In contrast to the static properties, the fatigue behaviour of the repair joints was significantly inferior to that of the parent material. Finally, fatigue tests were also performed on relatively large repair carbon fibre/epoxy panels with centrally-placed repairs. The fatigue results obtained from the repair panels were in close agreement with the fatigue results obtained from the repair joints.

Torsional fatigue of a medium carbon steel containing an initial small surface crack introduced by tension–compression fatigue: crack branching, non‐propagation and fatigue limit

Abstract: In order to examine the threshold condition for the fatigue limit of materials containing a small crack under cyclic torsion, reversed torsional fatigue tests were carried out on 0.47% C steel specimens containing an initial small crack. Initial small semi-elliptical cracks ranging from 200 to 1000 μm in length were introduced by the preliminary tension-compression fatigue tests using specimens containing holes of 40 μm diameter. The threshold condition for the fatigue limit of the specimens containing artificial small defects under rotating bending and cyclic torsion are also reviewed. Crack growth behaviour from an initial crack was investigated. The torsional fatigue limit for a semi-elliptical small crack is determined by the threshold condition for non-propagation of Mode I branched cracks. The torsional fatigue limit of specimens containing an initial small crack can be successfully predicted by the extended application of the √area parameter model in combination with the σ θmax criterion.

Fatigue Performance in Flexure of Fiber Reinforced Concrete

Abstract: An experimental investigation of the behavior of fiber reinforced concrete under cyclic flexural loading is presented. One type of polypropylene and two types of steel fibers in two different volume concentrations are studied. Load-deflection response is obtained for constant amplitude fatigue loading as well as for static loading. The damage level is recorded under static and fatigue loading using acoustic emission techniques. Data are presented in terms of complete load-deflection diagrams (for static loading) and in terms of S-N diagrams (for fatigue loading). Damage evolution is described in terms of acoustic emission activity as a function of deflection (static loading) or cycles (fatigue loading). The test results show that the addition of steel fibers increases the flexural fatigue strength considerably. Compared with plain concrete, the fatigue strength for 2 million cycles is changed from 60% to 90% of the ultimate flexure strength when the steel fiber content is one volume percent. High-fiber volume concentrations (2%) further increase absolute fatigue strength; however, fatigue performance measured relative to the static strength is decreased compared with the lower fiber volume concentration. Furthermore, the results show that the accumulated damage level at failure in the static test of unreinforced concrete is of the same order of magnitude as in the fatigue testing of the same material. However, using fiber reinforced concrete, the accumulated damage level in fatigue testing is 1-2 order of magnitude higher than the level reached in static testing of the same material. Finally, the tests show that the deflection at failure of the fiber reinforced concrete specimens under constant stress range fatigue loading can be predicted using the static load-deflection curve, provided the testing time is short enough to neglect creep effects.

Fracture and fatigue behavior of single crystal silicon microelements and nanoscopic AFM damage evaluation

Abstract: Quasi-static bending and fatigue tests of single-crystal silicon microelements fabricated by photoetching were performed. The microelements were subjected to simple bending and three-point bending with two-support roll length of 1.5 mm. The tests were conducted by using a specially designed electromagnetic actuator based testing machine (load range: 0.1 mN–5 N, accuracy: 0.02 mN), which enables mechanical testing including fatigue of microelements. Mechanical testing including fatigue of microelements could be performed with sufficient precision. Single-crystal silicon microelements deformed elastically until final catastrophic failure, showing a brittle nature. The influence of specimen size on quasi-static fracture behavior was investigated: fracture strength increased with a decrease in sample width, and the maximum fracture strength reached 7.7 GPa. The influence of water on fatigue strength was discussed. The fracture surface and sample surface were examined using an atomic force microscope. Nanoscopic damage during testing was evaluated, and the fracture mechanisms were discussed.

Fracture characteristics of fatigued Ti–6Al–4V ELI as an implant material

Abstract: Titanium alloys exhibit an excellent combination of properties, including greater strength, lower modulus of elasticity and excellent biocompatibility. Only Ti–6Al–4V ELI and Ti–6Al–7Nb have been standardized for biomaterials in ASTM among various titanium alloys. Ti–6Al–4V ELI is a representative titanium alloy used as an alternative artificial metallic material for failed hard tissue. The implant materials for alternative use to failed hard tissue are often used under cyclic loading. The principal cause of implant failure is fatigue fracture. The fatigue strength is one of the most important properties for implant materials. There is a possibility for the implant materials to be fractured under monotonic loading conditions during fatigue, consequently the prediction of the fatigue life of the implant materials is important. An evaluation of the fracture characteristics of fatigued implant materials will offer information about the residual fatigue life and the residual fracture resistance of these materials. The relationship between fatigue damage and mechanical properties in Ti– 6Al–4V ELI with equiaxed α structure were investigated in this study. The tensile strength of fatigued specimens of Ti–6Al–4V ELI with equiaxed α structure increases rapidly within the low-cycle-fatigue region. On the other hand, the elongation of fatigued specimens tested at various fatigue increments decreases rapidly within the low-cycle-fatigue region. The energy absorbed by fatigued specimens during various fatigue steps decreases rapidly in the late stages of the low-cycle-fatigue region. Furthermore, a hardness gradient develops initially from the surface to the inside of the specimens, and then in the later stages of fatigue, the internal hardness is equal to the surface hardness. The sub-structures both near and far from the surface of fatigued specimens, observed after various fatigue steps, show an increased dislocation density. However, at the late stages of low-cycle-fatigue, the dislocation density far from the specimen surface, also increases dramatically.

Break-up of sea ice by ocean waves

Abstract: The manner in which sea ice breaks up determines its floe-size distribution This, together with any redistribution due to ocean currents or winds, alters the fluxes between the atmosphere and the underlying ocean Many materials fail at stresses well below their flexural strength when subject to repeated bending, such processes being termed fatigue in some materials a stress exists below which the material will maintain its integrity even if subjected to an infinite number of load cycles This stress is termed the endurance limit We report a scries of field experiments to investigate the fatigue behaviour of first-year sea ice that subjected in situ cantilever beams to repeated bending with zero mean stress These tests suggest that an endurance limit exists for sea ice, and that it is approximately 60% of the flexural strength Using theory and data from wave experiments performed in similar conditions to the fatigue experiments, estimates are made of the conditions under which wave-induced break-up occurs These indicate that fatigue may be a neglected ingredient of sea-ice failure due to wave-induced motion

Thermomechanical properties of P/M β titanium metal matrix composite

Abstract: A high performance β titanium metal matrix composite (TMC) reinforced with in situ TiB particles has been developed via a low-cost blended elemental (BE) powder metallurgy process. The TMC shows anomalous mechanical properties as a titanium based material in tensile and fatigue strength, Young's modulus and wear resistance. Besides the high performance, the TMC has the potential to be fabricated at a lower cost far below that of the practical titanium alloys made by the conventional ingot metallurgy process. The as-sintered density of the TMC reached to 99% of theoretical with a unique activation sintering mechanism and has an excellent hot-workability superior to that of the conventional unreinforced Ti–6Al–4V alloy.

Fatigue crack growth analysis of fiber reinforced concrete with effect of interfacial bond degradation

Abstract: This paper presents an analytic model for fatigue crack life prediction in fiber reinforced concrete (FRC). The model elucidates fatigue crack growth in cementitious matrix material under the influence of external cyclic load and fiber and aggregate bridging. It is shown that fiber-matrix interface damage must be accounted for in order to properly simulate the three experimentally observed stages of matrix crack growth, involving a decelerated stage, a steady state stage, and an accelerated stage towards final fracture failure. This model, which explicitly accounts for the positive role of fibers on fatigue life, predicts the existence of the well-known fatigue limit load in standard S-N curve tests of FRCs. The basic framework of this analysis can be used to address fatigue life of FRC structures subjected to high cycle fatigue loading, as well as providing a basis for material design of fatigue resistant FRC at the microstructure level.

Mechanical Properties of Low Yield Point Steels

Abstract: The vibration of buildings due to earthquake or wind can be reduced by hysteresis damping of structural members, and low yield point steel is one of the most efficient damping materials. One conventional mild steel, one modified mild steel, and two low yield point steels were low-cycle fatigue tested. The following findings were obtained: (1) the low-cycle fatigue strength (i.e., the relationship of strain to number of cycles to failure) of the low yield point steels can be expressed by the Manson-Coffin equation; (2) the strain rate exerts a small effect on the low-cycle fatigue properties of the low yield point steels; (3) the cumulative energy absorption capacity of the low yield point steels to failure varies from type to type (that of the mild steels is superior to that of the low yield point steels with strain amplitudes of larger than 0.8%); and (4) with strain amplitude of less than 0.7%, the cumulative energy absorption capacity of the low yield point steels is much higher than that of the mild s...

Fatigue characteristics of high performance fiber-reinforced concrete

Abstract: This paper provides a summary of a study on fatigue behavior undertaken as part of SHRP project C-205 on the fresh and hardened properties of high early strength fiber-reinforced concrete (HESFRC). HESFRC was defined as achieving a minimum target compressive strength of 5 ksi (35 MPa) in 24 h. Several properties of HESFRC were investigated: for the fresh mixture, the air content, inverted slump test, temperature, and unit weight; for the hardened composite, the compres-Sive, bending, and tensile properties. Optimum mixtures that satisfied the minimum compressive strength criterion, and showed excellent values of modulus of rupture, toughness indices in bending, and split tensile behavior, were selected for fatigue testing. In this paper a description is given of key results of the fatigue bending tests only. The experimental program included a total of 24 fiber-reinforced concrete flexural specimens, ten of which were control specimens tested under static flexural loading, and the remaining 14 specimens were tested under fatigue loading. Two mixes containing 2 vol% of hooked steel fibers were selected. For each mix, three different target load ranges were applied: 10–70%, 10–80%, and 10–90% of the ultimate flexural capacity, as obtained from the corresponding control static test with fibers. A typical relation between maximum fatigue stress and number of cycles to failure was derived, suggesting a fatigue endurance limit of the order of 65% even if specimens are in a cracked state.

Mechanical surface treatments on Ti–10V–2Fe–3Al for improved fatigue resistance

Abstract: Shot peening as well as deep-rolling of circumferentially V-notched (kt=2.5) specimens was performed on fully aged Ti–10V–2Fe–3Al with 15% equiaxed primary-α content. To evaluate optimum conditions for fatigue life improvement, the Almen intensity and rolling force were varied to a wide extent for shot peening and deep-rolling, respectively. While both shot peening and deep-rolling significantly improve the fatigue performance of the notched electropolished reference, best results are found for deep-rolling. Under optimum process conditions, the fatigue strength can be increased by a factor of 2.5, i.e. the geometrical notch factor (kt) is fully counterbalanced.