Applied Mechanics and Materials Vol. 750

Abstract: Both assembly force and temperature play an important role in the proton exchange membrane (PEM) fuel cell performance. In this paper, contact pressure between bipolar plate and gas diffusion layer (GDL) in a PEM fuel cell under various assembly forces and at different temperatures was studied numerically. Considering the coupling effects of assembly force and operating temperature on contact pressure, a three-dimensional finite element model of the PEM fuel cell was established and the contact pressure between the GDL and the bipolar plate was studied using commercial code ABAQUS. In order to verify the simulated results, the experimental study was conducted to investigate the contact pressure distribution between the bipolar plate and the GDL. The experimental results are in good agreement with the finite element method (FEM) results. The simulated and experimental results reveal that the contact pressure increased with the increase of assembly force and temperature. It is found that the contact pressure distribution between the bipolar plate and the GDL had the best uniformity under the applied torque of 3.0N·m and at the operating temperature of 80 °C in this work.

Abstract: The crack growth behaviors loaded in mode I under strain and stress control at different temperatures were presented in α-Fe by atomistic simulations using LAMMPS code. The interatomic bonds of atoms were characterized using the embedded atom method interatomic potential. The simulation models were built with initial edge crack subjecting to cyclic uniaxial constant strain rate and constant stress. A temperature range from 100 K to 1200 K was considered to probe the influence of the temperature on crack growth. The crack growth mechanism and the radial distribution function (RDF) during crack growth were investigated. The results indicated that the crack propagation mechanisms were sensitive to temperature and the boundary conditions. By proposed image adjusting technology the dislocation slip bands can be more clearly displayed on screen. In order to include the effect of temperature on crack growth, a temperature factor defined as a function of temperature in exponential form was introduced to modify the theoretical expressions based on thermal activation theory. Its coefficient and index can be determined by the RDF peak value obtained from atomistic simulations. For cyclic loading the crack growth process was dependent on both temperature and cyclic loading period in terms of simulations.

Abstract: The study of residual stress in engine block has the potential to provide the necessary infrastructure for a wide range of scientific and technological developments concerning the automobile industry. Such study can act as centre of excellence for scientific studies in the field of materials science and machinery as well as for industrial applications. The basic principle of measuring residual stress with grooving method and the main measuring steps were introduced in this paper. Groove cutting in the internal surface of engine block surrounds a given area in which the remaining stress is released, and then the strain results are recorded through strain gauge. The residual stress test data were compared at different positions of each engine block. It is found that engine blocks A and D on the sides of engine develop higher residual stresses than blocks B and C in the centre of engine. The stresses can be higher than 200MPa in tension in engine block A, depending on the geometry, size, microstructure, subsequent welding process, and cutting sequence. In addition, residual stress at the bottom of engine is higher than that at the top because of the structure of the engine. The residual stress is also calculated by numerical modeling method from which it shown that the condition is commendably fit the results which the distribution of stress.

Abstract: In this study, virtual and physical experimental investigations were performed on the resistance spot weldability of hot-formed ultra high strength steels (UHSS). The sheets were joined by the resistance spot welding as lap or cross joint. The mechanical properties were assessed in terms of cross tension strength (CTS) and tensile shear strength (TSS). The micro structural profile and weld nugget diameter were also investigated. Both virtual and physical methods yielding results are in good agreement. The results are discussed in the light of earlier work published in the literature and contribute to a better understanding on the evaluation of resistance spot weldability.

Abstract: Plastic pipe reinforced by cross helically wound steel wires (PSP) is a new plastic-matrix steel composite pipe developed in China recently. To deeply understand the mechanical properties of buried PSPs, a finite element model of PSP subjected to non-axis symmetric load is proposed. The model is verified by replacing the parameters of PSP by those of steel pipe. A good agreement shows that the model can be used to analyze mechanical behavior of buried PSP. Finally, the influences of internal pressure, properties of soil, geometry parameters of PSP and magnitude of load, etc. are discussed.

Abstract: Electromagnetic acoustic transducers (EMATs) has overcome many disadvantages of the traditional piezoelectric ultrasonic sensors and can be applied in elevated temperatures and on-line inspection. In an EMAT testing system, the tested material and EMAT itself make up a whole system. So the performance of EMAT is closely depended on some physical parameters of tested material. On the other hand, characteristic parameters of metallic materials are closely related to temperature, such as density, modulus of elasticity, electrical conductivity and magnetic permeability. As the temperature increases, the density of the metal material is reduced, the elastic modulus is reduced, the electrical conductivity is decreased, the magnetic permeability has a consistent increasing trend until the Curie point. Thus the EMAT detection performance affected by the change of temperature should be considered. This paper studied the effects of temperature on EMAT testing. Three materials of 20 # steel, Q235 and 16MnR were investigated. Testing temperature varies from 26 °C to 500 °C. The results show that under elevated temperature condition, EMAT echo signals still have a good waveform and stability. Meanwhile, signals are attenuated less than 2dB. It proved that the EMAT technique has a good steady performance in elevated temperature.

Abstract: The creep behavior of 2.25Cr-1Mo-0.25V ferritic steel was investigated using a set of physically-based creep damage constitutive equations. The material constants were determined according to the creep experimental data, using an efficient genetic algorithm. The user-defined subroutine for creep damage evolution was developed based on the commercial finite element software ANSYS and its user programmable features (UPFs), and the numerical simulation of the stress distribution and the damage evolution of the semi V-type notched specimen during creep were studied. The results showed that the genetic algorithm is a very efficient optimization approach for the parameter identification of the creep damage constitutive equations, and finite element simulation based on continuum damage mechanics can be used to analyze and predict the creep damage evolution under multi-axial stress states.

Abstract: In the paper double-cone seal of ammonia converter is analyzed by means of the finite element method. The contact nonlinear FEM is adopted to simulate the process of double-cone seal in the pretension condition and the operation condition. The curve of contact stress versus time is generated based on the simulation. The stress distribution of plat head and seal ring identified that the standard radial clearance design in GB150-2011 is reasonable and can meet the seal requirements. The deformation and stress distribution evaluation of the entire double-cone seal is further conducted. Bolts load obtained from analysis result and theoretical value of the two conditions are in good agreement, and the deviation is less than 2.1%, which proves that the simulation result is reliable.

Abstract: Residual stresses inherently occur in welds and through-thickness distributions in an extra thick weld is unclear to date, though it is important in various steel, plant, and shipbuilding industries. Here introduce a recent complementary technique to determine residual stress measurement methods. Spatial variations of residual stresses were determined through the thickness of 70-mm thick ferritic steel welds created using low heat-input (1.7 kJ/mm) one-pass electro-gas welding (EGW) and high heat-input (56 kJ/mm) multi-pass flux-cored arc welding (FCAW). The results show that significant tensile stresses (about 90% of yield strength) occur along the weld centerline near the top surface (within 10% of the depth) in the FCAW specimen. Meanwhile, in the EGW weld, the peak stress moved towards the heat-affected zone at a depth of about 40% of the thickness due to the high heat-input. Secondly, large-scale (1 x 1 m) brittle fracture experiments have been performed to examine the brittle crack-arrest toughness and crack propagation path in each case. The fracture behavior compares to the observations of the grain size, hardness, and Charpy impact energy of the localized crack region in each weld. This study provides an insight in terms of the relevance between residual stresses and the brittle fracture behavior.

Abstract: This paper describes an approach to identify the influence of mechanical properties of the materials under the condition of containing residual stress. The numerical method of simulating small punch test (SPT) is used to determine the material response under loading. The simulated material behavior of the specimen is based on the ductile elastoplastic damage theory of Gurson, Tvergaard and Needleman (GTN). The residual stress can be prefabricated on the specimen by loading and unloading. By comparing the original specimen with the specimen contains residual stress, the change of the mechanical properties of the materials can be studied. The results of simulation indicate that the material properties decrease with the increase of the residual stress.