Showing papers on "Fatigue limit published in 2010"

Improvement of fatigue life of Ti–6Al–4V alloy by laser shock peening

Abstract: The aim of this paper was to address the effects of multiple laser peening and laser peening intensity used in laser shock peening (LSP) on the residual stress, micro-hardness and three-point bending fatigue performance of Ti–6Al–4V alloy. The multiple laser peening was accomplished by using the successive laser shocks at the same spot and the laser peening intensity was changed through changing the number of overlapped laser spots. The microstructure, which was characterized by highly tangled and dense dislocation arrangements due to high strain rate, can be found near the surface of the laser-peened specimen. By comparing with the as-received specimen, high micro-hardness and compressive residual stress were introduced at the surface of the laser-peened specimen. With increasing the number of overlapped laser spots, the fatigue life of the laser-peened specimen increased, reached a local maximum and then decreased. The specimen treated by using three overlapped laser spots exhibited the highest fatigue life. When the number of overlapped laser spots was kept to be three, the LSP treatments with one single laser shock and two successive laser shocks respectively provided a 22.2% and 41.7% increase in the fatigue strength as compared with the as-received specimens.

The effects of texture and extension twinning on the low-cycle fatigue behavior of a rolled magnesium alloy, AZ31B

Abstract: The effect of texture on the low-cycle fatigue behavior of a rolled magnesium alloy, AZ31B, was studied at room temperature. It is shown that the Coffin–Manson and Basquin relationships can be used to describe the fatigue resistance of the alloy. The alloy loaded along the rolling direction exhibits only slightly better low-cycle fatigue resistance than that loaded along the transverse direction, due to the in-plane texture symmetry. The in-plane cases exhibit better fatigue behavior than the through-thickness loading. Neutron diffraction and synchrotron diffraction were employed to assist in making mechanistic understandings for the findings. The fundamental difference in the low-cycle fatigue behaviors between the in-plane and through-thickness loadings is attributed to the different activation sequences of twinning and detwinning mechanisms involved and, particularly, the greater requirement for c-axis compression of the grains during the through-thickness tests. The different activation sequences are essentially determined by the initial crystallographic texture, such that the inverted hysteresis-loop shapes are observed.

Microstructure and mechanical properties of laser welded DP600 steel joints

Abstract: To reduce fuel consumption and greenhouse gas emissions, dual phase (DP) steels have been considered for automotive applications due to their higher tensile strength, better initial work hardening along with larger elongation compared to conventional grade of steels. In such applications welding and joining have to be involved, which would lead to a localized alteration of materials and create potential safety and reliability issues under cyclic loading. The aim of this investigation was to evaluate microstructural change after laser welding and its effect on the tensile and fatigue properties in DP600 steel. The welding resulted in a significant increase of hardness in the fusion zone, but also the formation of a soft zone in the outer heat-affected zone (HAZ). While the ductility decreased after welding, the yield strength increased and the ultimate tensile strength remained almost unchanged. Fatigue life at higher stress amplitudes was almost the same between the base metal and welded joints despite slightly lower fatigue limit after welding. Tensile fracture and fatigue failure at higher stress amplitudes occurred at the outer HAZ. Fatigue crack initiation was observed to occur from the specimen surface and crack propagation was characterized by the characteristic mechanism of striation formation. Dimples and deformation bands were observed in the fast propagation area.

The Effects of Laser Peening and Shot Peening on High Cycle Fatigue in 7050-T7451 Aluminum Alloy

Abstract: The high cycle fatigue performance of 7050-T7451 aluminum was investigated for untreated as-machined, laser peened, and shot peened conditions Constant amplitude, smooth (Kt = 1) fatigue tests were conducted in four-point bending at a stress ratio of R = 01 Results show that laser peening induces a layer of compressive residual stress more than three times deeper than for shot peening Both treatments significantly increase fatigue performance At a moderate level of stress, peened specimens outlast as-machined specimens, by a factor of 79 for laser peening and 29 for shot peening At higher stress, life improvements are lower, a factor of 33 for laser peening and 21 for shot peening At a 100,000-cycle lifetime, fatigue strength of laser peened specimens is 41% higher than as-machined specimens and the fatigue strength of shot peened specimens 30% higher than as-machined A form of pitting was noted on the laser peened surfaces and follow-on tests assessed the effects of the pitting on fatigue performance Results indicate that the pitting does not significantly influence fatigue performance

Surface modification of titanium and titanium alloys by ion implantation.

Abstract: Titanium and titanium alloys are widely used in biomedical devices and components, especially as hard tissue replacements as well as in cardiac and cardiovascular applications, because of their desirable properties, such as relatively low modulus, good fatigue strength, formability, machinability, corrosion resistance, and biocompatibility. However, titanium and its alloys cannot meet all of the clinical requirements. Therefore, to improve the biological, chemical, and mechanical properties, surface modification is often performed. In view of this, the current review casts new light on surface modification of titanium and titanium alloys by ion beam implantation.

Influence of temperature and thickness on the off-axis behaviour of short glass fibre reinforced polyamide 6.6 – cyclic loading

Abstract: This paper investigates the anisotropic properties of short glass fibre reinforced polyamide 6.6 (PA66-GF35) under tension–tension and tension–compression cyclic loading. Tensile fatigue tests were carried out on dog-bone specimens, machined out from injection-moulded plates 80 × 80 mm, of three different thicknesses t (1 and 3 mm) at three different nominal fibre orientation angles θ (0°, 30° and 90°). The tests were carried out at RT as well as at 130 °C. The Tsai–Hill failure criterion, modified to account for cyclic loading, is applied to the fatigue data for estimating the fatigue strength parameters of the material under investigation. Results are compared to the strength parameters obtained under quasi-static loading in a previous part of this work [De Monte M, Moosbrugger E, Quaresimin M. Influence of temperature and thickness on the off-axis behaviour of short glass fibre reinforced polyamide 6.6 – quasi-static loading. Composites: Part A, 2010;41(10):1368–79]. The experimental results highlight how specimen thickness remarkably affects mechanical properties: the thinner the specimen the higher will be the degree of anisotropy. Also temperature strongly reduces the fatigue strength under cyclic loading. The Tsai–Hill criterion allows for an adequate fitting of experimental data at the investigated temperatures and load ratios.

Improvement of fatigue life by incorporation of nanoparticles in glass fibre reinforced epoxy

Abstract: The fatigue properties of glass fibre reinforced epoxy laminates modified with small amounts (0.3 wt.%) of nanoparticles (fumed silica SiO 2 and multi-wall carbon nanotubes (MWCNT)) were evaluated by means of static (90°-tensile and stepped tensile) and dynamic fatigue tests. For the MWCNT-modified matrix, the electrical conductivity was measured in situ . The addition of nanoparticles lead to increases in inter fibre fracture strength of up to 16%. More significantly, the high cycle fatigue life is increased by several orders of magnitude in number of load cycles. The increased inter fibre fracture strength could be correlated to the improved fatigue behaviour, as final failure in high cycle fatigue is strongly correlated to matrix cracks. For the MWCNT-modified composites, the state of load and damage state was monitored by conductivity measurements. A correlation between the onset of matrix cracking and increase in electrical resistivity could be drawn enabling self sensing capabilities.

Effect of anisotropy on the low-cycle fatigue behavior of rolled AZ31 magnesium alloy

Abstract: Low-cycle fatigue (LCF) tests were carried out to investigate the effect of anisotropy on the fatigue resistance of rolled AZ31 magnesium (Mg) alloy. The alloy showed a strong basal texture indicating that most of the basal planes of hexagonal close-packed lattice were located parallel to the rolling direction. Specimens loaded parallel to the rolling direction (RD) were easy to yield in compression, while those loaded normal to the rolled plane (ND) were easily yielded in tension by twinning. This anisotropy caused superior fatigue resistance of ND specimen by introducing the beneficial compressive mean stress. An energy-based model considering the effect of the mean stress was used for the prediction of fatigue life and the predicted results showed a good agreement with experimental data.

Effects of Ultrasonic Nanocrystal Surface Modification (UNSM) on Residual Stress State and Fatigue Strength of AISI 304

Abstract: The effects of a new mechanical surface treatment method, called ultrasonic nanocrystal surface modification (UNSM), on near-surface microstructures and residual stress states as well as on the fatigue behavior of an austenitic steel AISI 304 are investigated and discussed. The results are compared with consequences of other mechanical surface treatment methods such as deep rolling or shot peening.

S-N Lines for Welded Thin Joints — Suggested Slopes and FAT Values for Applying the Notch Stress Concept with Various Reference Radii

Abstract: An assessment of thin welded structures with actual IIW design lines results in an overestimation of fatigue lives (strengths) at high load levels and a conservative estimation at low load levels, in many cases, independently of the applied fatigue assessment approach (nominal, structural or notch stress). This is mainly due to the slopes of the design S-N line k = 3.0 for normal and k = 5.0 for shear stresses, which are valid for thick and stiff structures. To overcome this inconsistency for welded thin and flexible structures, the slopes k = 5.0 for normal and k = 7.0 for shear stresses are suggested, keeping the already known FAT values derived for the notch stress concept variants with rref = 1.0, 0.3 or 0.05 mm, respectively. However, the slope is not only determined by plate thickness; it is the result of an interaction between thickness, local geometry (stress concentration), structural stiffness, loading mode and last but not least residual stresses. This complexity makes it difficult to identify the driving parameters and to predict the slope of the S-N line in many cases.

Fatigue Performance Prediction of North Carolina Mixtures Using the Simplified Viscoelastic Continuum Damage Model

Abstract: Fatigue performance modeling is a major topic in the field of asphalt concrete modeling work. Currently, the only standard fatigue test available for asphalt concrete mixtures is the flexural bending fatigue test, AASHTO T-321. Several issues are associated with flexural fatigue testing, the most important being that the stress state is not uniform but varies with the depth of the specimen, and that the beam specimen fabrication equipment is not widely available. Viscoelastic continuum damage (VECD) fatigue testing is a promising alternative to flexural fatigue testing. Different researchers have successfully applied the VECD model to asphalt concrete mixtures using constant crosshead rate direct tension tests. However, due to the load level limitation of the newly released Asphalt Mixture Performance Tester (AMPT) testing equipment, there is an immediate need to develop a model that can characterize fatigue performance quickly using cyclic test data. In this study, a simplified VECD model, developed at North Carolina State University, is applied to various North Carolina mixtures that are part of the NCDOT project, Local Calibration of the MEPDG for Flexible Pavement Design. A failure criterion that is based on pseudo stiffness is developed from the test data. The application of the VECD model using this failure criterion results in very good agreement between the measured and predicted fatigue life for the eleven mixtures. In addition, a completely independent verification study is conducted for the FHWA ALF mixtures, including both unmodified and modified mixtures. Again, it is found that the simplified VECD model predicts the fatigue life of the ALF mixtures well. It is shown that the simplified VECD model based on the data from a single temperature and a single strain level can predict fatigue test results fairly accurately under various temperature conditions and at various strain levels. It is also shown that the model can be utilized further to simulate both controlled strain and controlled stress direct tension fatigue testing and gain insight into the impact of various mixture design variables, such as asphalt content, binder grade, NMAS, and the inclusion of RAP materials, on the fatigue performance.

Fatigue Behavior of Stainless Steel 304L Including Strain Hardening, Prestraining, and Mean Stress Effects

Abstract: This paper discusses cyclic deformation and fatigue behaviors of stainless steel 304L and aluminum 7075-T6. Effects of loading sequence, mean strain or stress, and prestraining were investigated. The behavior of aluminum is shown not to be affected by preloading, whereas the behavior of stainless steel is greatly influenced by prior loading. Mean stress relaxation in strain control and ratcheting in load control and their influence on fatigue life are discussed. Some unusual mean strain test results are presented for SS304L, where in spite of mean stress relaxation fatigue lives were significantly longer than fully-reversed tests. Prestraining indicated no effect on either deformation or fatigue behavior of aluminum, while it induced considerable hardening in SS304L and led to different results on fatigue life, depending on the test control mode. Possible mechanisms for secondary hardening observed in some tests, characterized by a continuous increase in the stress response and leading to runout fatigue life, are also discussed. The Smith-Watson-Topper parameter was shown to correlate most of the experimental data for both materials under different loading condition.

A combined model for initiation and propagation of damage under fatigue loading for cohesive interface elements

Abstract: A model for the simulation of damage initiation and subsequent propagation under cyclic loading is proposed. The basis for the formulation is a cohesive law that combines phenomenological SN-curves for damage initiation with a fracture and damage mechanics approach for crack propagation. The evolution of the damage variable is expressed as a function of fatigue cycles. The model is independently calibrated for mode I and mode II loading using SN-curves and Paris-law coefficients obtained from simple coupon tests. The model was applied to three initiation-driven cases: Bending of 90° laminates, the Short Beam Shear test and the Double Notched Shear test. The predictions for the first two cases showed an excellent correlation with experimental data. Some modifications to the model were required when applying it to the Double Notched Shear test.