Showing papers on "Fatigue limit published in 2006"

Damage accumulation and fatigue life of particle-abraded ceramics.

Abstract: Purpose This investigation compared initial and fatigue strengths of particle-abraded ceramics to those of as-polished alumina and zirconia ceramics in crown-like layer structures. Materials and methods Alumina or zirconia plates bonded to polycarbonate substrates were subjected to single-cycle and multi-cycle contact (fatigue) loading. Cementation surfaces of the ceramic were damaged by controlled particle abrasion, indentation with a sharp diamond at low load, or a blunt indenter at high load. The stresses needed to initiate radial fractures were evaluated. Results The strengths of specimens were lowered by fatigue loading. After the equivalent of 1 year of occlusal contacts, the strengths of undamaged specimens degraded to approximately half of their single-cycle values. In particle-abraded specimens, an additional 20% to 30% drop in strength occurred after several hundred load cycles. Particle abrasion damage was approximately equivalent to damage from sharp indentation at low load or blunt indentation at high load. Conclusion Damage from particle abrasion, not necessarily immediately apparent, compromised the fatigue strength of zirconia and alumina ceramics in crown-like structures. In fatigue, small flaws introduced by particle abrasion can outweigh any countervailing strengthening effect from compression associated with surface damage or, in the case of zirconia, with phase transformation.

Properties and failure mechanisms of z-pinned laminates in monotonic and cyclic tension

Abstract: The effects of through-thickness reinforcement of carbon/epoxy laminates with thin pins on the in-plane tensile properties, tensile fatigue life and failure mechanisms are investigated. Tensile studies in the 0 fibre direction are performed on unidirectional and quasiisotropic laminates reinforced with different volume contents and sizes of fibrous composite z-pins. Microstructural analysis reveals that z-pinning causes several types of damage, including out-of-plane fibre crimping, in-plane fibre distortion, mild dilution of the in-plane fibre volume fraction due to laminate swelling, and clusters of broken fibres. In unidirectional composites, resin pockets form around pins and coalesce into continuous resin channels at higher z-pin contents. Young’s modulus falls only a few percent at most, due partly to in-plane fibre dilution and partly to fibre waviness. Monotonic tensile strength is degraded more significantly, falling linearly with both pin content and pin diameter. Comparison with prior data shows that the rate of degradation is evidently a strong function of the particular pin insertion method used. Failure mechanisms include fibre rupture, presumably affected by broken fibres, and, in unidirectional laminates, longitudinal splitting cracks emanating from resin pockets. Whereas non-pinned laminates show very modest fatigue effects, the pinned laminates exhibit strong fatigue effects, with strength falling by as much as 33% at 10 6 cycles. The slope of the fatigue life (S–N) curve tends to increase in magnitude with pin content and density. Limited evidence and prior literature suggest that the dominant fatigue mechanism may be progressive softening and fibre damage in misaligned segments of in-plane fibres. 2005 Elsevier Ltd. All rights reserved.

Dissipated Energy Approach to Study Hot-Mix Asphalt Healing in Fatigue

Abstract: The healing phenomenon has been noted by pavement engineers for years, but its relation to hot-mix asphalt (HMA) fatigue behavior is still far from clear. This study conducted an analysis of healing and HMA fatigue behavior by introducing a specifically designed fatigue-healing test. These results help explain the differences in fatigue behavior at normal and low strain levels. An approach using the ratio of dissipated energy change, which is based on energy concepts, is used in this study. The results show that healing does exist, and its effect on fatigue life can be indicated by an energy recovery per second of rest period. The effect of healing is more prominent at low strain levels or in very long rest periods. At low strain conditions, the dominance of healing compared with the very low external load damage, considering the energy equilibrium, can result in full damage recovery. This full recovery of energy explains the existence of a fatigue endurance limit, below which HMA materials tend to have extraordinarily long fatigue lives that, as is shown, can be related to healing. The testing conducted clearly shows why polymer modification may extend the fatigue life in the field even though laboratory testing may show minimal differences compared with the neat binder test results.

Fatigue strength, microstructural stability and strain localization in ultrafine-grained copper

Abstract: Fatigue lifetime under stress control of ultrafine-grained Cu of 99.9% purity prepared by equal channel angular pressing is shown to exceed that of conventionally grained cold worked counterparts by a factor of 1.7 in the low-, high- and very-high-cycle region. The electron back scattering diffraction technique did not reveal changes of bulk microstructure due to cyclic loading. Minor changes of dislocation microstructure were detected by transmission electron microscopy. Qualitative change from moderate cyclic hardening to cyclic softening was observed with increasing stress amplitude. Comparison of S–N data with those available in literature shows substantially higher lifetime of the material studied in this work in the high- and very-high-cycle region. This effect is attributed to the high stability of the grain structure and lower purity of the examined ultrafine-grained copper.

Experimental study on ratchetting-fatigue interaction of SS304 stainless steel in uniaxial cyclic stressing

Abstract: The ratchetting behaviour and fatigue failure, as well as their interaction were investigated by uniaxial cyclic stressing tests for SS304 stainless steel at room temperature. The ratchetting strain and fatigue life of the material were measured in different loading levels. The effects of mean stress, stress amplitude and stress ratio on the ratchetting strain and final failure life were discussed. Simultaneously, the variations of responded strain amplitude with the number of cycles were illustrated to discuss the interaction of ratchetting behaviour and fatigue failure. The experimental results show that the ratchetting strain and fatigue life of the material depend greatly on mean stress, stress amplitude and stress ratio of uniaxial cyclic stressing, and two kinds of failure modes (i.e., ratchetting failure with obvious necking due to large ratchetting strain and fatigue failure due to low-cycle fatigue with nearly constant responded strain amplitude) take place respectively, depending on the stress level prescribed in the tests.

Effect of extrusion conditions on grain refinement and fatigue behaviour in magnesium alloys

Abstract: This paper describes the grain refinement due to controlled extrusion and the fatigue behaviour of the extruded materials in three magnesium alloys, AZ31B, AZ61A and AZ80. First, in order to investigate the effect of working temperature, billets of AZ31B and AZ61A were extruded at an extrusion ratio of 67 under controlled conditions with three different working temperatures. It was found that grain refinement was attained in both alloys whose grain size decreased with decreasing working temperature. Rotary bending fatigue tests were performed using smooth specimens. In AZ31B, fatigue strength increased with decreasing grain size, but in AZ61A, it did not depend on grain size. Fatigue crack initiation and subsequent small crack growth were examined in AZ31B. Consequently, grain refinement improved both crack initiation resistance and small crack growth resistance, resulting in the increase in fatigue strength. Furthermore, it was indicated that fatigue strength was expressed properly by the Hall–Petch relationship in AZ31B, but not in AZ61A. Then, in order to investigate the effect of extrusion ratio, billets of AZ61A and AZ80 were extruded at three different extrusion ratios under controlled conditions. It was found that grain size decreased with increasing extrusion ratio in both alloys, but mechanical properties were not affected significantly by extrusion ratio, i.e. grain size, and the grain size dependence of fatigue strength was different between alloys. In AZ61A, fatigue strength was higher in the material extruded at lower extrusion ratio (coarser grain), but in AZ80, in the material extruded at higher extrusion ratio (finer grain). Fatigue crack initiation behaviour was consistent with the tendency of fatigue strength, where crack initiation delayed in the material extruded at lower extrusion ratio in AZ61A and in the material at higher extrusion ratio in AZ80. Furthermore, the observed grain size dependence of fatigue strength in both alloys was discussed on the basis of a texture formed by extrusion and the presence of inclusions from which cracks generated.

The effects of rod contouring on spinal construct fatigue strength.

Abstract: Study design In vitro fatigue loading using a corpectomy model outfitted with posterior pedicle screw instrumentation. Objective The purpose of this study was to detect differences in fatigue resistance of titanium and stainless steel spinal constructs that use rods contoured using a French Bender, and to compare differences in fatigue resistance of contoured and straight titanium rods. Summary of background data Instrumentation failure is generally thought to be caused by fatigue or cyclic loading. Intraoperative contouring of the posterior rods is almost always required to match the native kyphotic (thoracic) or lordotic (cervical or lumbar) spinal curvature. How bending these rods affects their overall fatigue resistance is not well described. In addition, changes in fatigue resistance may be a function of material type. Methods Spinal constructs were evaluated using the ASTM F1717-01 model. Two different titanium-based rods (Ti6AL4V and CpTi) and two different steel-based rods (Orthinox and 316L stainless steel) were evaluated in this study (n = 6 for each group). Rods were contoured at two points using a French Bender and were rigidly coupled to polyaxial pedicle screws within UHMWPE vertebral bodies. Constructs were cycled at a load ratio of 10 between a minimum and maximum loading regime of -250 N/-25 N and -700 N/-70 N at a frequency of 4 Hz. Estimated maximum nominal stresses at various points of interest in the spinal constructs were calculated using beam theory. Effects of the rod material, load, and stress on the number of cycles to failure were analyzed using Cox proportional hazards regression. Results All of the spinal constructs with contoured CpTi rods and contoured Ti6Al4V rods failed at one of the bends in the rods. Almost all of the spinal constructs with straight CpTi rods and straight Ti6Al4V rods failed where the blocker screw fastens the rod to the coupler of the polyaxial screw head. Contoured titanium constructs demonstrated significantly lower fatigue life than contoured 316L constructs. Contouring tended to lower the fatigue life of both the Ti6Al4V and CpTi constructs. Conclusion Intraoperative rod contouring using a French Bender significantly reduces the fatigue life of titanium spinal constructs.

Effect of cold spray deposition of a titanium coating on fatigue behavior of a titanium alloy

Abstract: The deposition of titanium on a titanium alloy substrate is being examined for potential use as a surface treatment for medical prostheses. A Ti6Al4V alloy was coated with pure titanium by cold gas dynamic spraying. Coatings were deposited onto samples with two different surface preparation methods (as-received and grit-blasted). The fatigue life of the as-received and grit-blasted materials, both before and after coating, was measured with a rotating-bend fatigue rig. A 15% reduction in fatigue endurance limit was observed after application of the coating to the as-received substrate, but no significant reduction was observed on its application to the grit-blasted substrate. The reduction in fatigue endurance limit has been related to the substrate-coating interface properties, the elastic modulus, and the residual stress states.

Effect of Root Flaws on the Fatigue Property of Friction Stir Welds in 2024-T3 Aluminum Alloys

Abstract: The fatigue experiment of friction stir (FS) welds in 2024-T3 aluminum alloys were performed to investigate the influence of root flaws on the fatigue strength and life of FS welds. The test results of welds with flaws (flawed welds) were compared with the results suggested by the International Institute of Welding (IIW) recommendations and the welds without root flaws (flaw-free) in the published research reports. It was found that there was always existed flaws at the roots of FS welds because of unsuitable welding parameters and the vertical length of the flaws is about 0.31–0.33 mm for the FS butt-welded joint of 4 mm in thickness. The fatigue life of flawed welds is 33–80 times shorter than that of flaw-free welds, and the fatigue characteristic values have decreased from 120.6 MPa for flaw-free welds to 54.7 MPa for flawed welds at 2 × 106 cycles.

Chances in wind energy: A probalistic approach to wind turbine fatigue design

Abstract: Wind is becoming an ever more important source of renewable energy: installed wind turbine power now stands at 60,000 MW worldwide, providing 0.6% of world electricity demand. Still it is important that the cost of wind energy is brought down further, which means that wind turbines must be designed to be exactly as strong as necessary, but no stronger. Hence there is a need to investigate whether the conventional design procedure results in the right degree of conservatism, and if not, how it may be improved. The ideal is to make the design just conservative enough, i.e. to exactly attain the target failure probability. Because wind turbines tend to be located in remote areas, the target value is primarily determined by economic considerations, rather than by public safety issues. The aims of this work are: To quantify total uncertainty in the design procedure, and to find the relative importance of stochastic parameters influencing fatigue loads and strength. To conduct a comparative review of calculation models where necessary. To derive partial safety factors giving minimum unit electricity cost. The scope of the present research is limited to fatigue issues, since extreme loads have been investigated previously to some degree. An inventory of stochastic parameters is made; for each of the parameters the distribution is estimated, and the models currently used in wind turbine design (i.e. the procedures used to estimate characteristic parameters and how to use them in calculations) are reviewed. A limit state function is derived using the concept of life fatigue damage equivalent load range. With the First Order Reliability Method (FORM) and Monte Carlo simulation, yearly failure probabilities due to fatigue are estimated for a wind turbine that is designed exactly according to the standard, and installed following common site admission rules. A simple economic model is used to establish optimal partial factors. The partial factor values found for blades are somewhat smaller than in the standard, while values for hub, nacelle and tower are higher. The explanation for the latter is that two things are currently not taken into account in design calculations according to the standard: firstly, variation and bias in fatigue life prediction; secondly, the fact that a combination of many critical locations (for example in the tower) yields a larger failure probability than just one location. The sensitivity of the partial factor optimisation to changes in various assumptions is investigated. The main conclusions of the work are threefold: Given available data, a larger partial (load or material) factor should be used in fatigue design for cast iron and weld seams. However, the effect of this on design might be limited since hidden safety exists in the construction: material quality and hence fatigue strength are better than assumed, wind turbines are placed in climates that are more benign than they were designed for, and finally, dimensions may be determined by stiffness or extreme load considerations rather than by fatigue. The variation of the limit state function is determined mainly by uncertainty on fatigue strength and fatigue life prediction. Therefore the way forward is to accurately establish fatigue properties and calibrate fatigue life predictions for materials exactly as used in wind turbines. In this way variation may be reduced (and bias removed), and failure probability estimates may be refined. If better information is available, hidden safety may be removed and smaller partial factors used in calculations. The number of critical locations and correlation of loads and fatigue strength at different locations must be taken into account in calculations to establish failure probabilities, and must have influence on the partial factors to be used. Variation and bias of fatigue life predictions must be an explicit input to fatigue design calculations.