Key Engineering Materials Vols. 353-358

Abstract: On the basis of the dynamic equations of the Jeffcott rotor-stator model with imbalance, the reliability of the rotor-stator systems with rubbing is examined. A statistical fourth moment method is developed to determine the first four moments of system response and state function. The distribution function of the system state function is approximately determined by the standard normal distribution functions using the Edgeworth series technique. The reliability is obtained and the effect on reliability of shaft stiffness and damping, stator stiffness and damping, radial clearance and stator radial stiffness is studied. Numerical results are also presented and discussed.

Abstract: The failure analysis of uncertain multi-degree-of-freedom (MDOF) nonlinear vibration systems with uncorrelated failure modes subjected to random excitation is examined. An earlier version of the statistical fourth-moment method is extended to vector-valued and matrix-valued functions and is developed to determine the first four moments of the system response and state function. Random variables and system derivatives are conveniently arranged into 2D matrices by means of Kronecker products. The distribution function of the system state function is approximately determined by the standard normal distribution functions using Edgeworth series technique and its failure probability is obtained. The method is based on matrix calculus. Kronecker algebra is used in the mathematical development.

Abstract: Hydrogen-induced corrosion is a common damage in petrochemical industry. It can rise crack and bubbling on the surface of metal materials when the hydrogen corrosion is in the stage of severity. In the present, it is hard to evaluate the material conditions nondestructively under hydrogen attack. In this paper, the possibility of the relationship between ultrasonic propagation properties and the content of hydrogen corrosion was investigated. We have found that the surface wave velocity is a parameter, which can be directly correlated with the hydrogen content. The sensitivity has been found to be acceptable.

Abstract: A typical mechanical character of rock is that the tensile strength is far less than the compressive strength. Meanwhile, the test data of tensile strength is very dispersive. Because the direct tensile tests always result in failure due to the difficulty in clamping the rock sample, the splitting test is used to determine the tensile strength of rock. There are four kinds of loading modes in the splitting test in actual laboratory test: angle pad splitting, round pad splitting, aclinic loading platen splitting, arc loading platen splitting. In this paper, the direct tensile test, the splitting test and the influence of different loading modes on rock tensile strength were studied. In order to study the stress distribution, the progressive splitting failure process was numerically modeled under the four kinds of loading cases by the Realistic Failure Process Analysis code (RFPA2D). Results show that the stress states under angle pad splitting, round pad splitting are similar to the stress states under diametrical compressive state. Regarding that the round pad splitting test is easy to implement, and its numerical results are also stable relatively, the round pad loading mode was suggested to be adopted.

Abstract: Since the number of fatigue crack propagation data is related to cost and time and the crack propagation data are generally finite, adopting median rank or average rank as the probability value of a test datum will result in significant error. On the basis of analyzing disadvantages of traditional methods to identify distribution functions, which begin with overall fitting effect, this paper proposed a method by applying numerical simulation and fuzzy linear regression. It can be used to determine the distribution function of crack propagation data. Through fuzzy linear regression, the result that crack propagation size under certain load cycles are in logarithmic normal distribution can be acquired. In the same way, the number of load cycles is also in logarithmic normal distribution when the crack propagates to a certain size.

Abstract: The reliability estimation of pipeline is performed in accordance with the probabilistic methods such as the FORM (first order reliability method) and the SORM (second order reliability method). A limit state function has been formulated with help of the FAD (failure assessment diagram). Various types of distribution of random variables are assumed to investigate its effect on the failure probability. It is noted that the failure probability increases with the increase of the dent depth, the operating pressure and the outside radius, and the decrease of the wall thickness. Furthermore it is found that the failure probability for the random variables having the Weibull distribution is larger than those of the normal and the lognormal distributions.

Abstract: In this paper, by employing a three dimensional geomechanics model test whose scale is 1:250, the dam cracking process was observed, and integrity stability was analyzed and evaluated under overloading test. The experimental results show that: the stress and strain of the dam can basically meet the requirements of dam safety under normal water load, and the dam failure mode of the left abutment is different from that in the right side because of different types of dam figure and geological conditions. Firstly, the local slip fracture is formed on the foundation, then extends along the bottom of dam, and leads to instability and final failure. In overloading process, crack initiate from dam heel under twice normal water pressure, and form a small cracking zone, then yielding continuously and crack band appearing in downstream surface under 4.5th normal water pressure, and overall failure under 8.5th normal water pressure. The integrity stability is shown strong enough with experimental dam shape (XLD03 Figure) and the configuration of its foundation.

Abstract: With the increasing needs of the more convenient transportation, wheelchairs are often used by people with mobility disabilities and the elderly people. However, as wheelchairs are primarily designed for the mobility assistive devices, not for the vehicle seats, wheelchair users are exposed to risk factors for neck and upper body injury caused by automobile accidents. In this paper, in accordance with the ANSI/RESNA WC-19, a fixed vehicle mounted wheelchair occupant restraint system (FWORS), wheelchair integrated restraint system (WIRS), and wheelchair integrated x-bend restraint system (WIXRS) are evaluated using computer simulations for the case when a wheelchair is subjected to the frontal impact (20 g, 48 km/h).

Abstract: In recent years, several electronics manufacturers have been working toward introducing lead-free solder and halogen-free print circuit boards (PCBs) into their products. The key drivers for the change in materials have been the impending environmental legislations, particularly in Europe and Japan as well as the market appeal of ‘green’ products. The reliability of the new materials is an important determinant of the pace of adoption. Fairly extensive mechanical fatigue reliability data is also available for micro-joining soldered joint such as Ball Grid Array (BGA) with tin-lead solder. However, similar data is not available for BGAs assembled with lead-free solder. Mechanical reliability is a critical indicator for phone and BGA survival during repeated keypress, and to some extent during drop. In this paper, the mechanical bend fatigue of BGAs with tin-lead and lead-free solders on halogen-free substrates are examined respectively. A tin-silver-copper alloy was used as lead-free solder due to its increasing acceptance, and the results were compared to those from samples assembled with Sn63Pb37 solder. The reliability was examined at both low cycle and high cycle fatigue. Results show that the mechanical bend fatigue reliability of BGA assemblies with lead-free solder is higher than that of BGA assembly with tin-lead solder. Cross section and failure analysis indicated two distinct failure modes - solder joint and PCB failure. A 3-D parametric finite element model was developed to correlate the local PCB strains and solder joint plastic strains with the fatigue life of the assembly. The intermetallic compoumd (IMC) of micro-joining joint interface was analysised in the future in order to study on the effect of IMC on the reliability.