Papers by Keyword: Stress Ratio

Abstract: The fatigue crack initiation life at various stress ratios for GH4133B superalloy specimen with different circular notch radios are studied at ambient temperature and atmospheric pressure utilizing the fracture mechanics method. It is shown that a controlling parameter ΔK_I/ρ^0.5 can be adopted to characterize the fatigue crack initiation life, and the effect of notch radius on fatigue crack initiation life increases with increasing control parameter, while the fatigue crack initiation life firstly increases and then decreases with increasing stress ratio. The fatigue fracture surfaces of circular notched compact tension specimens of GH4133B superalloy for fatigue crack initiation tests at various stress ratios are investigated using a scanning electron microscopy, and the fracture surface morphologies in the fatigue source region are analyzed. It is found that the separation between carbide inclusion on the surface or subsurface and matrix, or the separation between nickel-rich second phase particle and matrix, or the interaction between persistent slip bands and inclusion, mainly results in the fatigue crack initiation.

Abstract: The dominant mechanics and mechanisms of fatigue crack propagation in approximately 500-nm-thick freestanding copper films were evaluated at three stress ratios, R = 0.1, 0.5, and 0.8. The fatigue crack propagation rate (da/dN) versus stress intensity factor range (ΔK) relation was dependent on the stress ratio (R): da/dN increased with increasing R. Plots of da/dN versus the maximum stress intensity factor (K_max) exhibited coincident features in the high-K_max region (K_max 4.5 MPam^1/2) irrespective of R, indicating that K_max was the dominant factor in fatigue crack propagation. In this region, the fatigue crack propagated in tensile fracture mode, or chisel-point fracture, irrespective of the R value. In contrast, in the low-K_max region (K_max < 4.5 MPam^1/2), da/dN increased with decreasing R. In this region, the fracture mechanism depended on R. At the higher R value (R = 0.8), the fatigue crack propagated in the tensile fracture mode similar to that in the high-K_max region. On the other hand, at the lower R values (R = 0.1 and 0.5), a characteristic mechanism of fatigue crack propagation appeared: within several grains, intrusions/extrusions formed ahead of the crack tip along the Σ3 twin boundaries, and the fatigue crack propagated preferentially through the intrusions/extrusions.

Abstract: The aim of this study is to discuss an effect of stress ratio and loading mode on high cycle fatigue performances of extruded magnesium alloys. Axial loading fatigue tests under three conditions of stress ratio, R, of 0, -1 and-1.5, and also rotating bending fatigue tests have been performed in laboratory air at room temperature using hourglass shaped specimens of AZ31, AZ61, AZ80 and T5-treated AZ80 alloy. From the experimental results, some materials showed a specific stepwise S-N curve on which two knees appear. The shape of S-N diagram depended on a kind of tested materials, applied stress ratio and loading mode. It was suggested from the detail observation of fracture surface that fatigue crack initiation mechanism changed from a twin-induced failure mode at high stress amplitude level to a slip-induced one at low stress amplitude level. This transition was determined with the relation between the minimum stress during a fatigue cycle and the compressive yield stress at which deformation twin occurs.

Abstract: Based on the fatigue crack propagation experiments did by A.-L. Gloanec et al., the fatigue crack propagation rates of TiAl alloy of two processing routes, namely casting and PM, and stress ratios had been tested, in order to find out the effects of microstructure and stress ratio. An improved fatigue crack propagation formula for region Ⅱ (the expansion region) was derived according to Paris formula. The specific values of the constants in the formula were calculated. Fatigue crack propagation resistance of nearly fully lamellar microstructure is superior to that of equiaxed γ grain. The experimental results present that both microstructure and stress ratio has a significant influence on fatigue crack growth rate.

Abstract: Biaxial compression tests are performed on 450mm*450mm*450mm cubic specimens of big aggregate concrete at five kinds of stress ratios,0:-1, -0.25:-1, -0.5:-1, -0.75:-1 and-1:-1 after exposure to freeze-thaw cycles of 50, 100, 150, 200, 250 and 300 times by employing a large static-dynamic true triaxial machine. Failure modes of the specimens are observed and described. The two principally static compressive strengths are measured. Based on the test data, the influences of the freeze-thaw cycles and stress ratios on the biaxial strengths of big aggregate concrete after exposure to freeze-thaw are analyzed respectively. The relationships between the ultimate compressive strength and freeze-thaw cycles, stress ratios are given respectively. The unified failure criterion with consideration of the influence of freeze-thaw cycles and stress ratios is proposed, which provides the experimental and theoretical foundations for strength analysis of big aggregate concrete structures subject to complex loads in cold environment. Key words: big aggregate concrete; freeze-thaw cycle; stress ratio; biaxial compressive strength; failure criterion

Abstract: Based on the experiments did by B.K.Parida and T.Nicholas [1], the fatigue crack propagation rate of TiAl alloy under different stress ratios had been tested in order to find out the role of stress ratio and to derive an improved fatigue crack propagation formula for region II(the expansion region) according to Paris formula and to calculate the specific values of the constants in the formula. The experimental results reveal that stress ratio has a significant influence on fatigue crack growth rate.

Abstract: The application of infrared thermography to obtain the external surface temperature during the application of cyclic loading, allows to evaluate the dynamic behavior of an element and to determine the fatigue limit. In this contribution, the reliability of the fatigue limit provided by the Risitano et al. and Canteli et al. methodologies is investigated in order to check their validity for practical applications. With this aim, an experimental program on aluminium specimens (AL 2024) under load control using a stress ratio minus one is performed. The fatigue limit for AL 2024 is derived, first from the Wöhler curve and then, compared with both Risitano et al. and Canteli et al. methodologies.

Abstract: Steel-concrete composite beams have been widely used in the high-rise buildings and large span steel structure. Composite beams effective flange width has been further investigated in domestic, as well as the force and deformation within elastic stage and plastic stage. In this paper, we give out the formula derivation of steel-concrete composite beams stiffness amplified coefficient. Combined with a practical engineering, we analysis the impact of stiffness amplified coefficient on the whole structure by SAP2000 software and compare the results back and forth. In the end, we put forward some suggestions for construction engineers.

Abstract: Fatigue crack growth (FCG) rates of compact tension (CT) specimens of Aluminium alloy 6065-T4 were investigated at room temperature and constant amplitude loadings. Standard CT specimens with pre-cracked according to ASTM E647-E11 were subjected to mode I loading with three R-ratio (0.1, 0.3, and 0.5). Paris law has been used to model the stress ratio effect. A stereozoom microscope was used to observe the microstructure changes before and after the tests. The results indicated that the higher the stress ratio value (from 0.1 to 0.5), the faster the FCG rates on CT specimen of AA 6065-T4.

Abstract: The test on fatigue crack growth rate of ADB610 steel is completed. The standard compact tension specimen is loaded by cyclic loading, whose stress ratio is constant at a value of R=0.1. Crack growth rates in the form of Paris formula at different survival levels are obtained by using three methods. After a simple comparison it is clear that crack growth rates obtained by three methods are generally same and the method based on (da/dN, delta K) has better effects on data analysis.