Advanced Materials Research Vols. 33-37

Abstract: Bonding prestressed CFL to strengthen reinforced RC structures can improve the bending capability of structures and the working efficient of CFL. Base on the fatigue experiments of 4 RC beams strengthened by prestressed CFL, the present paper analyzes the fatigue properties of prestressed CFL reinforced RC beams. Comparing with the experimental results of strengthened RC beams without prestress, it can be concluded that the prestressed technology can improve the reinforcing effect and fatigue lives.

Abstract: The problem of multiple site damage (MSD) has got more attention in ageing structures. Cumulative effects of interacting cracks may significantly degrade the damage tolerance capacity of structures. Cracks caused by MSD are extremely difficult to detect and greatly reduce the residual strength, fatigue life and overall structural integrity of aircraft panels. This paper presents a simple numerical method, which use the principles of fracture mechanics and the computation results, to predict the fatigue crack growth life of MSD structure. Comparing with calculating crack growth life cycle by cycle, this method will save much time. To verify the validity of the proposed method, experiment was conducted and reported with simulation specimen of representative MSD structure with 5 details. The comparison between the calculated a-N curves and the crack growth lives and the test results shows that the prediction result with this fast method is acceptable. A discussion was carried out by numerical analysis; in with typical MSD structures with different initial crack length were adopted. Crack interaction effect was found obviously, but it occurred mainly in the last part of the crack growth lives. The relative size of MSD cracks depends significantly on the distribution of the initial cracks.

Abstract: Low cyclic fatigue testing of 45 medium carbon steel with the annular V-notch indicates that the propagation of crack under variable amplitude increase by degrees overloading is faster than that under constant amplitude overloading. Some problems which include the effect of constant amplitude overloading to propagation speed of crack, the suitable loading process of accelerating propagation of fatigue crack, and the fracture figure in suitable loading, should be discussed. Based on the experimental results and the analysis of the fracture surfaces of the specimens, the mechanism of propagation of fatigue crack under the overloading is explored. It is proposed that at the moment of the application of the overloading, the obvious instantaneous increase of crack growth rates is due to crack blunting. Then an adequate way of loading to make propagation of crack quickly is found that provide a basis for the engineering application of extra-low cycle fatigue fracture.

Abstract: Since mechanical interaction between multiple cracks affects the rate of crack growth due to fatigue and stress corrosion cracking, it is important to consider its influence when predicting growth. In this study, a procedure predicting the growth of interacting surface cracks was developed. First, using the results of fatigue crack growth tests performed in a previous study, the transient growth behavior during coalescence and growth under interaction was evaluated based on area of crack face. It was shown that the area is a representative parameter of the growth of interacting surface cracks as well as independent cracks. The growth in area showed good correlation with the crack driving force defined using size of area. Then, in order to investigate the relationship between growth of interacting cracks and their relative spacing, crack growth simulations were carried out. The body force method was used to evaluate the change in stress intensity factors (SIF) during crack growth under interaction, and the simulation could reproduce the crack configurations obtained in the fatigue crack growth test. SIF of an interacting crack tip converges to that of a coalesced crack as the distance between cracks decreases. It was concluded that when the distance between cracks is small enough, the cracks can be replaced with a semi-elliptical crack of the same area of crack face for a growth evaluation. The threshold offset distance for the replacement was suggested to be less than 0.1Rx, where Rx is the span length of two cracks on the surface.

Abstract: In order to predict the fatigue life of matrix-dominated polymer composites, a new model based on stiffness degradation concept was proposed. The effect of off-axis was considered by defining a non-dimensional modified stress level, and the expression of fatigue stress limit could be provided in the new model. Based on the fatigue tension-tension experiment of PI/SiO2 hybrid thin films under different stress levels, the simulated results was confirmed to be well agreeable with the experimental data.

Abstract: Directionally solidified (DS) superalloys, which have elongated large grains, are used for gas-turbine blades. Since the grain size is not small enough in comparison with the crack size observed often in a real component, the inhomogeneous microstructure due to the aligned grains may strongly affect the crack propagation property. Center-cracked-plate specimens with three different orientations, TP0 with the parallel DS axis to the load, TP90T with the perpendicular DS axis to the load and the crack propagation direction, and TP90L with the perpendicular DS axis to the load and parallel to the crack propagation direction, are subjected to high-temperature (1143K) fatigue. The specimens after the tests show the transgranular cracking perpendicular to the load axis in TP0, intergranular one parallel to the DS axis in TP90L, and intergranular / transgranular one parallel to the DS axis in TP90T. The crack propagation rate da/dN shows a good correlation with effective stress intensity factor range
Keff in each cases. However, the magnitude of da/dN at a same
Keff in TP90L is relatively higher than that in TP0, and that in TP90T remarkably fluctuates. A microscopic observation of TP90T reveals that the high da/dN is caused by the intergranular cracking, while the low da/dN is observed on the transgranular cracking. A procedure for the prediction of crack propagation is proposed on the basis of the two types of cracking; transgranular and intergranular cracks perpendicular to the load axis. The validity of the proposed procedure is discussed by the comparison of tested and predicted results of crack propagation in a fatigue condition.

Abstract: To reduce the cost and time of transport due to the different track gauges(narrow, standard, broad) in the Eurasian railroad network such as TKR(Trans-Korea Railway), TCR(Trans-China Railway) and TSR(Trans-Siberia Railway), it is very necessary to develop and adapt the gauge-adjustable wheelsets system. The freight trains’ with gauge-adjustable system could operate on the different track gauges in the transcontinental railway. Therefore, to assure the safety of the newly developed gauge-adjustment wheelsets system, it is essential to evaluate integrity of locking parts in the system by using fatigue analysis. In this study, it was performed that contact stress analysis of locking parts by using FEM(Finite Element Method) in the case both the gauge changeover operation and freight trains' service in the curved track, respectively. Besides, to consider the variation of fatigue data, the crack initiation life was statistically evaluated.

Abstract: This paper presents a probabilistic approach for fracture mechanics analysis of cracked structures. The objective of this work is to calculate the rigidity of cracked structures based on failure probability. The methodology consists of cracked structures modelling, finite element analysis with adaptive mesh, sampling of cracked structure including uncertainties factors and probabilistic analysis using Monte Carlo method. Probabilistic analysis represents the priority of proceeding either suitable to repair the structures or it can be justified that the structures are still in safe condition. Therefore, the combination of finite element and probabilistic analysis represents the failure probability of the structures by operating the sampling of cracked structures process. The uncertainty of the crack size can produce a significant effect on the probability of failure, particularly for the crack size with large coefficient of variation. The probability of failure caused by uncertainties relates to loads and material properties of the structure are estimated using Monte Carlo simulation technique. Numerical example is presented to show that probabilistic analysis based on Monte Carlo simulation provides accurate estimates of failure probability. The comparisons of simulation result, analytical solution and relevant numerical results obtained from other previous works shows that the combination of finite element analysis and probabilistic analysis based on Monte Carlo simulation provides accurate estimation of failure probability.

Abstract: The fatigue behaviors of a directionally solidified (DS) nickel base superalloy, coated with a MCrAlY coating (NiCrAlYSi) were studied. Two kinds of tests were performed. One kind of tests are low cycle fatigue (LCF) test under strain control at different temperatures, another kind of tests are stress controlled LCF test with SEM-servo hydraulic testing machine for in situ cracking observation. The results show that the effect of coating on LCF life of coating/substrate system was rather different according to different strain levels and temperatures. The coating has no or less effect on LCF life under high strain range and the LCF life is governed by fatigue behavior of substrate in spite of the difference of temperature. However, when strain range is smaller, crack initiation and propagation are observably affected by temperature, which leads to a shorter LCF life of coating/substrate system at 500°C and a longer LCF life at 760°C or 980°C. This means the failure of coating/substrate system is dominated by the cracking of surface coating under low strain range. The brittleness at 500°C lower than DBTT results in rapid stage II crack propagation. The crack initiation from coating surface was in situ observed at room temperature and 700 °C and it was found that cracks usually initiated from the surface roughness of coating and then propagate to failure. The brittleness and surface roughness are the basic acceptable causes leading to the early damage of a coating/substrate system.

Abstract: The present study investigates the relations between distributions of strain with those of martensitic volume fraction around fatigue cracks in SUS304 stainless steel. Specimens having random pattern created by spraying on their surfaces were fatigued at room temperature (297K) in air. The pictures around fatigue crack were taken during fatigue crack propagation experiments. Distributions of strain were calculated by the digital image correlation (DIC) method with these pictures. Distributions of martensitic volume fraction Vα’ were measured around fatigue cracks with ferrite scope. The distributions of strain resembled those of Vα’. The higher the strain becomes, the more the α’ martensitic phase was induced by the plasticity-induced transformation around fatigue cracks. It was revealed that the same value of Vα’max is induced by strain regardless of static or fatigue.