Improvement Of Fatigue Behavior Research Articles

Static and Fatigue Performance of RC Beams Strengthened with CFRP Laminates

The paper presents the results of a study involving the static and fatigue performance of reinforced concrete beams strengthened with externally bonded carbon-fiber-reinforced plastic (CFRP) sheets. The main parameters in the static test study were the concrete compressive strength, the number of CFRP laminates, and the placement of CFRP reinforcement. The static test program shows that the application of CFRP to reinforced concrete beams results in increased strength and enhanced performance. Accelerated fatigue testing is performed on several specimens receiving various amounts of the CFRP lamination system, including one member that is fatigued for over half the expected fatigue life, then rehabilitated with the CFRP, and fatigued again until failure. Comparisons are made for the standard section and equivalent sections with two and three layers of CFRP involving the improvements in fatigue behavior, stiffness, and capacity. The results from the fatigue study indicate that fatigue life of reinforced concrete beams can be significantly extended through the use of externally bonded CFRP laminates.

Simulation of residual stresses by shot peening

Shot peening is a cold working process in which small spherical parts called shots are blasted on a surface of a metal work-piece with velocities up to 100 m/s. This treatment leads to an improvement of fatigue behavior due to the developed compressive residual stresses and also to an increase of the surface hardness due to the cold working effect of shot peening. The aim of the paper is to simulate this process with the help of the finite element method in order to predict the development, magnitude and distribution of the residual stresses in accordance with the parameters, which are, e.g. shot velocity, shot diameter and material parameters. In the first step an axisymmetric model is used to simulate the perpendicular impact of an elastic sphere against an elastic–plastic surface. Due to the large plastic deformations, 9/3 mixed formulation elements are used instead of the usual nine-node displacement formulated elements, which are not well suited for this task. In a second step it is shown how the results of the axisymmetric approach can be transformed into a three-dimensional model in order to investigate the effect of adjacent shots.

Current trends in laser shock processing

The physical and mechanical aspects of laser shock processing (LSP) with plasma confinement were investigated with the aim of establishing the state of the art and assessing possible applications. Using a pulsed Nd :glass laser, fundamental processing aspects were studied, such as the precise role played by surface protective coatings and laser impact area on shock loading. For instance, it was shown that Al based coatings could enhance the peak stress values experienced by iron based surfaces by up to 50%. Residual stresses were investigated as the main contributor for improving surface behaviour. Lastly, the beneficial effects of LSP were demonstrated through two applications of the process: improvement offatigue behaviour and pitting corrosion resistance of steels.

The improvement of fatigue behaviour of sintered steels by surface rolling

In this investigation fatigue-life curves of two low- and high-strength powder metallurgy steels Fe-1.5Cu ( ϱ=7.15 g cm −3) and Fe-8.0Ni-1.0Mo-0.5C ( ϱ=7.35 g cm −3) are compared for three different notch factors in as-sintered and surface-rolled conditions under constant-amplitude loading as well as under variable-amplitude loading. The results are interpreted on the basis of the microstructural features, microhardness and residual-stress distributions. Statistically based design data are presented for the improvement of the fatigue strength by surface rolling of powder metallurgy parts taking into account the stress concentration and loading mode.

Mechanical properties of a transformation-toughened glass-ceramic

The fracture toughness of a transformation-toughened glass-ceramic containing 12 vol% t-ZrO2 was measured. Heat-treatment conditions were selected to produce from the same glass composition a glass-ceramic which contained 12 vol% m-ZrO2 in a nearly identical matrix. The transformation-toughened material was found to have a fracture toughness 65% greater than the m-ZrO2-containing material at room temperature. However, at 77 K both materials were found to have approximately the same fracture toughness. Additionally, the susceptibility of the two glass-ceramics to fatigue was determined. No improvement in fatigue behaviour of the transformation-toughened glass-ceramic over the other glass-ceramic was observed.

Effect of processing conditions and heat treatments on mechanical properties of single-crystal superalloy CMSX-2

It is shown that the mechanical properties of single-crystal superalloys can be optimized through appropriate solidification conditions and heat treatments. A comparison of the creep behaviour of low- and high-gradient processed single crystals of CMSX-2 alloy shows that minimizing porosity improves the creep strength at intermediate temperatures. The optimum creep resistance is achieved by aligned γ′-precipitates, about 0·5 μm in size. It is shown that in high-cycle fatigue the solidification conditions have a dramatic effect on the fatigue strength. The improvement in fatigue behaviour is attributed to the extremely low density and small size of pores achieved by using a very high-gradient process. The strain–controlled low–cycle fatigue properties of CMSX-2 at intermediate temperatures are also greatly improved by using the high-gradient process. Elimination of porosity in ‘industrial’ single crystals by hot isostatic pressing restores their creep and especially their fatigue performance to the level of the high-gradient solidified material.MST/224

IMPROVEMENT OF FATIGUE LIFE OF 1080 STEEL BY THERMOMECHANICAL PROCESSING

Abstract— Many thermomechanical treatments (TMT) have been applied in the past to alloys in order to improve the balance between various static mechanical properties. The current research has applied a given type of TMT to the improvement of fatigue behavior in eutectoid steel. The TMT involved the cold rolling of annealed eutectoid steel to a reduction in thickness of 75% followed by rapid heating in liquid lead to a temperature just above the A1 temperature for a short time period and an air cool to room temperature. This type of TMT produces very oriented cementite in recrystallized ferrite. Plane bending cantilever fatigue tests with constant maximum load were run at 30 Hz and zero mean stress (R=−1·0). In terms of S–N fatigue curves the annealed condition is inferior to both the TMT and cold rolled conditions, however, there is no apparent advantage of the TMT over the cold rolled condition. There is, however, a considerable advantage in the TMT which becomes evident with normalization of all S–N curves with respect to the ultimate tensile strength. One thus finds that the TMT is quite superior, on an equivalent strength basis, to both the cold rolled and annealed conditions. The same was true on a basis of hardness normalization. This means that by TMT one can produce fatigue properties in a material of much higher formability which are equivalent to or better than those in a cold rolled material. Samples oriented in the cross‐roll direction had total fatigue lives longer than for those oriented in the roll direction. This is explained in terms of mechanical fibering of pearlite colonies and inclusions, and by a crystallographic texturing of the ferrite matrix. TMT strength is a result of austenite grain size refinement, reduction of interlamellar spacing, fiber strengthening and both solid solution and precipitate hardening.