Advanced Materials Research Vols. 118-120

Abstract: In order to confirm the structural integrity of pressurizer surge line affected by thermal stratification and thermal shock, the thermo-mechanical fatigue (TMF) behavior of the material used for surge line was investigated based on the real situation in the pressurized water reactor (PWR). Smooth, hollow specimens were subjected to in-phase (IP) and out-of-phase (OP) cycling in air under a mechanical strain control mode. For the sake of comparison, low cycle fatigue (LCF) tests were also performed at the peak temperatures of TMF cycling. The Nano Hardness Tester was used to test the nano hardness of the sample on the cut section surface. The results are shown that there is no significant difference between the IP, OP and IF lives in the investigated temperature ranges. The fracture analysis reveals that the crack initiation and propagation occurred in a transgranular mode under OP, IP and IF cycling condition, and a harden layer occurrence may be the cause of the crack initiation.

Abstract: In order to understand the generation of rail oblique crack, based on vehicle-track coupling dynamics theory, the coupling dynamics system with vehicle, rail, sleeper and roadbed was constructed. By using new fast explicit numerical integration method, movement differential equations of system were solved, and wheel-rail forces and contact geometry relation were obtained when power car crossing curve track. Results of calculation have shown that when power car is crossing curve track, lateral forces of outer rail at all wheelsets are pointed to outer rail, longitudinal forces are pointed to forward direction of wheelsets, and their resultant forces are pointed to the second quadrant. Wheel-rail contact points of outer rail at the first and third wheelset are located within 13mm radius of railhead, but at the second and fourth wheelset they are located within 80mm radius of railhead. Degree of wheel-rail interaction at the first and third wheelset is severer than that at the second and fourth wheelset. According to direction of resultant forces and location of wheel-rail contact points, it can be learnt that greater wheel-rail force at the first and third wheelset will easily cause generation and growth of rail oblique crack when power car crossing curve track.

Abstract: . The corrosion damage data are obtained through the pre-corrosion accelerated experiment which is performed under three different time levels: 24h, 48h and 72h. Based on the corrosion damage data, a three-dimension (3D) solid model is created by CAD. An analysis procedure is developed using CAD and finite element analysis to predict stresses and residual strength due to corroded pits. The residual strength results of specimens are obtained from the model based on the corrosion damage data. The simulation results are compared with the true testing static intensity results and the method is proved that it is reliable and effective in engineering.

Abstract: Fatigue S-N relations are investigated experimentally on the grade B cast steel for Chinese railway freight car bogie frames. Group test method was employed. Results reveal that the fatigue cracks were initiated from the material cast shrinking cavities with hot ties. Distinct fatigue striations, river-like flowers, and second cracks appeared on the fracture surface in perpendicular to the fatigue crack path. Lots of dimples in the transient fracture district indicate that present material is ductile. Considering both the scattered S-N relations and no fatigue limits under variable loads, the S-N relations are measured probabilistically in terms of continuous declined probabilistic S-N curves including super-long life regime (SLLR) of >107 cycles. Affordable novel approach, i.e. concurrent probability extrapolation method, was applied. The test S-N data in mid-long life regime (MLLR) and conventional fatigue limits are used. The curves are established by extrapolating the conventional S-N curves crossing the fatigue limits, in a concurrent probability manner, into the SLLR. Appropriate measurements have been given for the test data of the present material.

Abstract: In this paper, numerical simulation for hydraulic expanding connection of tube to tubesheet was performed. Residual contact preesure on the contact surface between tube and tubesheet as well as residual expansion stress in the tubesheet were investigated. It is seen the distribution of residual contact pressure is not uniform. Instead, near the two tubesheet surfaces, there are two tightness bands on which the residual contact pressure is high. Based on the fact that residual expansion stress could be a driving force for the tubesheet cracking, it is suggested that the expansion pressure should not be too large but enough to completely form the tightness bands on the contact surface. With this criterion, expansion pressures for typical tube materals and size are given.

Abstract: In order to accurately and conveniently simulate spot-weld in FEA, based on the typical specimen of stainless steel, three typical finite element models (FEM) of spot-welds have been created as analysis objects. Firstly, based on the calculation theory of the spot-weld fatigue, two sets of analytical solution, which were the maximum principle stress of the specimen around the spot-weld nugget, were obtained under the action of stretching and shearing, respectively. Then, their linear elastic FEM were established, and the stress states around the spot-weld nugget and the adjacent sheets were analyzed. Finally, according to the results compared between the analytical solution and simulation solution, it can be seen that, when the specimen was stretched, using the brick model to simulate the nugget can get the smallest absolute error of the principle stress, so its spot-weld fatigue damage is the most consistent with the actual situation, a stiff beam model followed, and an umbrella model is the worst; when the specimen was sheared, using the brick model to simulate the nugget can also get the smallest absolute error of the principle stress, so its spot-weld fatigue damage is also the most consistent with the actual situation, the umbrella model is followed, and the stiff beam model is the worst. The results show that appropriate selection of spot-weld FEM is extremely essential for fatigue life prediction under the initial design of products.

Abstract: Under the complex loads, the injector guide pillar (IGP) used in injection machine was failure only after 1.5-year-service. In order to determine the cause of the fracture, the numerical simulation technology was applied to analyze the mechanical properties of the IGP. The contact between the mating surfaces of the clamping mechanism was modeled; nonlinear multi-region contact of surface-surface was applied to establish the contact model of FEA. The constraint of tie was used for modeling thread joint. The simulated results indicated that the smaller area of contact surface, the higher value of stress in the neck of IGP. Electronic measurement was also used to check the results of stress in IGP obtained by FEA. It was found that the experimental data agreed well with simulated results. Based on the numerical analysis and experimental study, the structure of IGP was improved by adopting a smoother double-round neck. The fatigue life of the improved structure was longer than that of the original machine.

Abstract: By representing the uncertain parameters as interval numbers, the reliability index equations of bars structures were obtained. A modified matrix affine arithmetic polynomial evaluation method plus recursive derivative information was proposed in this paper, which keeps all powers of noise symbols without approximation. Based on the nature that affine forms and intervals variables could transform each other, affine forms of bounded uncertain variables and modified affine arithmetic including derivative information for interval univariate polynomial evaluation were introduced into modeling and calculating non-probabilistic reliability index. An extended beam example and a ten-bar truss structure example were provided to illustrate the validity and feasibility of the presented procedures.

Abstract: Damage of metals due to the influence of hydrogen is quite frequent and leads to dangerous failures. The characteristics of the hydrogen embrittlemnt of the 65Mn steel were evaluated with small punch test. With the increment of the amount of the hydrogen absorbed into the alloy at room temperature, the strength and the toughness of the material reduce. From the small punch experimental results, it is found the total impact energy, the fracture strain and the fracture stress decrease with the increment of the cathodic hydrogen charging time. The fracture surfaces change from the typical ductile fracture with big voids to the typical intergranular brittle fracture mode after hydrogen absorbed in the specimens with higher charging current density.

Abstract: For structural system with multiple modes simultaneously involving random variables, interval variables and fuzzy variables, a logical deduction algorithm is proposed to deal with propagation of uncertainties. Corresponding to assumed membership level in the membership level interval [0,1], the membership interval of fuzzy variables can be obtained. Firstly the fuzzy variables and the corresponding design points that optimize the respective mode will be calculated by iteration. Secondly, with the logical deduction, the interval of upper bound and lower bound of the system reliability with multiple modes can be solved by the fourth moment algorithm based on the point estimate, then the average of the interval will be used to approximate the true value of the upper bound and lower bound. Several numerical examples are presented to demonstrate the proposed method’s advantages both in efficiency and accuracy.