Papers by Keyword: Stress Ratio

Abstract: Performance of cracked fibre metal laminate (Glare) under fatigue load differs from that of cracked plain aerospace aluminum alloy. Load transfer towards stronger fibres in Glare shields crack tips in aluminum layers that retards fatigue crack growth rates. The paper experimentally investigates the fatigue behaviour of Glare at variable stress ratio’s in an ambient environment for comparison with that of plain aerospace aluminum alloy.

Abstract: The paper deals with a comparison of fatigue life calculations, obtained on the basis of classical Basquin diagrams and approximated diagrams acquired with the use of the section method, which allows us to fit the diagrams shape to real material properties. While comparing the calculation results, literature data concerning fatigue tests of welded cruciform specimens from high performance low-alloy steel HSLA-80 presented by Kihl and Sarkani as well as of smooth specimens out of the aluminum alloy 75S-T6 by Grover et al. has been used. It has been noticed that the calculations performed with the use of fatigue diagrams approximated using the section method reflect the true behavior of the material. The models by Niesłony-Böhm and Smith-Watson-Topper compensating the influence of the mean stress gave similar results.

Abstract: Modeling of fatigue crack growth (FCG) under variable amplitude (VA) loading has been a topic of intensive research for decades. This paper proposed an efficient numerical simulation procedure of the FCG process under VA loading via extended finite element method (XFEM). The procedure can describe the integrative effect of several factors (specimen thickness, stress ratio, Poisson’s ratio and so on) simultaneously with a general model. In order to improve the computation efficiency, the whole continua domain is divided into two parts by the level set functions and the global stiffness matrix is divided into four parts accordingly. Then, the inverse operation of global stiffness matrix can be calculated rapidly with the LDU decomposition and the equilibrium equation can be solved effectively. Several sets of fatigue test data in 2024-T351 and 7075-T6 aluminum alloys are used to verify the procedure, the predictions are in good agreement with the test data.

Abstract: The fatigue crack growth tests for nickel-based GH4133B superalloy used in turbine disk of a type of aero-engine are carried out at room temperature. The stress intensity factor ranges and the fatigue crack growth rates at various stress ratios are measured, and the corresponding threshold stress intensity factor ranges are determined. Using the Paris formula, the experiment data of fatigue crack growth are analyzed. It is shown that the fatigue crack growth rate increasing with increasing stress intensity factor range and stress ratio, and a modified Paris formula considering threshold stress intensity factor range can describe the fatigue crack growth behavior precisely. The fracture surface morphologies are investigated using a scanning electron microscope. It is shown that in the crack initiation region, steady growth region and rapid growth region, the fracture surface exhibits a cleavage fracture mode, fatigue striations and an intergranular fracture mode, respectively. Finally, the von Mises stresses and stress intensity factors at the crack tip of specimen of GH4133B superalloy at various external loads and crack lengths are simulated using the finite element method, and the threshold stress intensity factors under different maximal external loads at a certain crack length are calculated. The comparison between test and simulation indicates that the stress intensity factors at the crack tip calculated by the finite element method agree well with experimental data.

Abstract: surface formula based on stress ratio was proposed in consideration of the effects of stress ratio on fatigue crack growth rate and the scatter of fatigue crack growth. The parameters of surface formula were estimated by using the linear principle of no-linear function. The surface formula was applied on GC4 steel to analyze statistically the effects of stress ratio on fatigue crack growth rate. The results showed that the crack growth rate will increases as the stress ratio increases in the stable stage of fatigue crack propagation for GC4 steel. The increase trend is more obvious when higher reliability is demanded.

Abstract: An experimental investigation was performed on fatigue crack growth behavior of a 16MnR pressure vessel steel. Standard compact tension (CT) specimens with three specimen thicknesses and notch sizes were subjected to Mode I constant amplitude loading with several stress ratios and loading amplitudes. The results revealed that the stress ratio had an insignificant influence on the fatigue crack growth of the material. The stable fatigue crack growth rate (FCGR) was accelerated as specimen thickness increased. The fatigue crack was extended in terms of the curve crack shape. The crack front at the surface was retarded compared to that at the interior along thickness direction, and the crack front at the mid-thickness plane reached the maximum value of the crack length. The similar curve crack shape was obtained in the stable crack growth stage. The maximum difference of the crack front along thickness direction was increased with the increasing of the specimen thickness. The early crack growth from the notch was effected by the size of the notch, the stress ratio and loading amplitude.

Abstract: This paper applies the fracture-mechanics-based analysis and the crack-closure concept to naturally occurring small cracks and large crack growth, and to make total fatigue life predictions solely based on crack growth from the assumed initial materials defect. The equation of total fatigue crack propagation life under constant amplitude loading is presented. And the total fatigue propagation lives of LY12-BCZYU aluminium alloy SENT specimens by this equation and validated by experimental results. Validation against calculations by the model and experimental data shows a good agreement.

Abstract: With the developments of the design theories and the researches, the quantity and the technology of space truss systems in our country have reached the international leading level. In the meantime, the construction methods need to achieve even higher standard. In the article, the rhombic space truss roof of the north station building of Suzhou is selected as the research object. Each step of the sliding and unloading construction is simulated by FEM. The stability and rigidity of the truss are checked in detail. It gives an representative exsample for complicated construction design of large-span space trusses.

Abstract: According to standard test method for fatigue crack growth rates of metallic materials, the crack growth rate of 30NCD16 at three stress ratio (R=0.1, 0.3 and 0.5) were measured. Based on linear elasticity fracture mechanics theory, the fatigue crack growth rate was studied through the nonlinear least squares fitting method. The Paris model parameters at steady growth region and near threshold growth region and NASGRO model parameters were obtained. The effective stress intensity factors versus curves at three stress ratios were determined by crack closure effect. The results show that the Paris equation can preferably describe relations at steady growth region. At this region the model parameter m lies 2.5-4. This result is consistent with the known statistical facts of most metallic materials. NASGRO equation can preferably describe relations from near threshold growth region to high values region. all the test data at three stress ratio was able to correlate and . Crack closure was the major factor in correlating stress ratio and crack growth rate, the degree of crack closure weaken with increasing stress ratio.

Abstract: Fatigue failure of steel occurs when cracks form in a component and continue to grow to a size large enough to cause fracture. In order to understand the strength of a steel component, it is important to locate these cracks. We developed a scanning Hall probe microscope (SHPM), equipped with GaAs film sensors to observe fatigue cracks at room temperature in air while they were growing. In our previous works [1,2], the correlation between crack growth and magnetic field in high carbon tool steels (JIS SKS93 and JIS SUJ2) were determined. We also reported the sensitivity of the SHPM equipped with a three-dimensional line-probe that was developed to decrease the sensor gaps. By using the line-probe sensor we succeeded to measure the magnetic flux density distributions in very close proximity to the specimen’s surface. However, in order to further understand the relation between magnetic flux density and crack growth, other materials, microstructures and fatigue test conditions should be evaluated. In the present work, we focus on the effect of stress ratios on the changes of the magnetic flux density in annealed carbon tool steel.