Advanced Materials Research Vols. 33-37

Abstract: Due to lots of hypothesis, the theoretical analytical solution of the creep parameters inversion can not reflect the in-situ conditions actually. In order to simulate the process of the compressive creep tests of the in-situ bearing plate, the affection of the stratum distribution and the influence of the geological status in site actually, FLAC3D is used and numerical back analysis method of the creep parameters at dam site is set up. Based on the in-situ compressive creep tests’ data of the diabase rock masses at Dagangshan dam site, creep parameters are got with this method. Results indicate that the numerical calculated displacements of the compressive creep are similar to the in-situ monitoring displacements. It reveals that with this numerical method creep parameters can be backing analyzed logically. This supplies technical assurances to stability estimate and analysis of rock masses in slope projects.

Abstract: Cracking due to the restrained shrinkage stress has been frequently observed at early age in concrete construction. The early-age shrinkage cracking reduce the structural performance of concrete and may accelerate deterioration to shorten the service life. Two kinds of strategies and instrumentation have been developed to assess early age cracking: ring tests with a restraining core, longitudinal tests where the restraint is at the end grips of the specimen. Early-age concrete exhibits creep or relaxation behavior that is a typical response of viscoelastic materials. Viscoelastic constitutive model should be used to analyze stress development of the restrained concrete. The assessment of the risk of cracking can be based on the model with input of the estimated concrete deformation.

Abstract: In structural welded joints after long-term service under elevated temperature, fracture occurred mainly in the heat affected zone (HAZ). Recently, the nucleation and growth of creep voids in the fine-grained HAZ of weldments, recognized as Type IV fracture, has become an important problem for ferritic heat resisting steel. In this paper, a new computational model was presented to analyse the void growth induced creep damage development in HAZ. The new constitutive model based on continuum damage mechanics (CDM) equations is combined with a micromechanism-based model in order to account for the void growth process, which is different from the previous studies of creep damage. Material properties used for the creep damage computations are fitted from actual creep test data. Basic benchmark tests were performed to verify the new computational model. Then the model was used to study the creep damage development in the welded joints where four different material properties, base material, coarse-grained HAZ, fine-grained HAZ, and weld material, are taken into account. The numerical simulation results for creep lifetimes agreed well with the experimental results.

Abstract: The plate impact experiments have been conducted to investigate the dynamic behavior of 91W-6.3Ni-2.7Fe. Lagrangian analysis technique was introduced to discuss the mechanical properties of the tungsten alloys under high strain rate and the stress-strain curves of the tungsten alloys were given. Based on the experimental observations, the three-dimensional finite element models of projectile and tungsten alloy target are established by adopting ANSYS/LS-DYNA, Dozens of cases were performed to investigate the dynamic mechanical behavior of tungsten alloy target under impact loading. A good agreement between numerical predictions and experimental results was obtained, which suggests that the finite element model is efficient and credible to simulate the mechanical properties of tungsten alloys.

Abstract: The structural change and properties of W6Mo5Cr4V2 alloy (M2 steel) inoculated by addition of rare earth (RE)-Ti were investigated. The results indicated that the impact toughness and fracture toughness were increased distinctly due to network eutectic carbides elimination, matrix structure refinement, and well-distribution of eutectic carbides. The hardness of modified M2 steel hardly changed. In addition, the high temperature wear resistance was improved.

Abstract: Influence of quenching temperature and cooling speed on the structures and properties of cast Fe-B-C alloy containing more than 1.0%B and lower than 0.2%C was researched. The results showed that the structures of Fe-B-C cast alloy changed from a great of pearlite + a small of martensite 􀄗 a great of martensite + a small of pearlite 􀄗 martensite and the hardness increased with the increase of quenching cooling speed. In the condition of water cooling, higher or lower quenching temperatures were not advantageous to obtaining single martensite. Quenching at 950~1000oC, cast Fe-B-C alloy could obtain the compound structures of fine lath martensite. The hardness and impact toughness of cast Fe-B-C alloy excelled 55HRC and 15J/cm2 respectively.

Abstract: The multi-axial constitutive behavior of poled and unpoled ferroelectric ceramic PZT53 is measured under different proportional tension-torsion loading and compression-torsion loading until destroy. Deformation is measured using strain gauges. These results are used to map out “Switching” and “destroy” surfaces. The two surfaces are found to be a function of the polarization. Switching surface is similar to Mises and Tresca yield surface. Destroy behavior corresponds to maximal principal normal stress rule. For poled PZT53, the effect of bias electric field on its multiaxial behavior has also been exploited.The results indicated that the “Switching” surface disappear with the increased bias electric. The ultimate goal is to develop an experimentally validated micromechanics based phenomenological constitutive model for ferroelectric materials.

Abstract: This paper describes phase field simulations of the rafting behavior of γ’ phase with a simple interfacial dislocation network model. The interfacial dislocation network model accounts for the effect of the network on the lattice misfit between γ and γ’ phases and the subsequent rafting behavior. The model is implemented into the phase field simulation to see the dependence of the rafting behavior of γ’ phases on the interfacial dislocation network. Without the dislocation network model, the amount of the rafting was negligibly small. On the other hand, with the dislocation network model, the γ’ phases shows a large amount of rafting, which is in good agreement with the results of the experimental observations. Therefore, the combination of the phase field method and the simple interfacial dislocation network model developed in this work is appropriate for the simulation of the rafting of γ’ phases.

Abstract: To quantify the effect of structural through-thickness reinforcement in foam core sandwich composite panels, an experimental study was carried out which included three-point bending tests, core shear tests, flatwise tensile and compression tests, as well as edgewise compression tests. Standard test procedures based on ASTM guidelines are followed to test the behavior of the stitched panels with reinforcement at 90 degree orientation with respect to the sandwich faces. The test specimens were manufactured by using polyurethane foam Rohacell 71 IG and carbon fiber reinforced composite facesheets. The dry perform facesheets and foam core were then assembled in a dry lay-up already stitched. Kevlar 29 yarn was used to stitch both sets of panels. The results showed a significant effect of the stitching on the in-plane Young’s modulus which was attributed to local displacements of the in-plane fibers and changes in the fiber volume fraction. Stitching of sandwich panels significantly increases the maximum failure loads under flexure, core shear, flatwise tensile, flatwise compression, and edgewise compression loading. A finite element based unit-cell model was developed to estimate the elastic constants of structurally stitched foam core sandwich composite panels taking into consideration the yarn diameter, the stitching pattern and direction as well as the load direction. Depending on these parameters, local changes of the fiber volume fraction as well as regions with undisturbed and disturbed fiber orientations within the laminate plies are taken into account. A good match between the finite element modeling and the experimental data was obtained. The present work should be considered as a step towards developing a more sophisticated numerical model capable of describing mechanical behavior of sandwich structures.

Abstract: This paper investigates the subsurface deformation of two types of aluminium alloys with differing nature; A2124 (precipitation-hardened) and A5056 (work-hardened), sliding against an M2 tool steel slider. With block-on-ring configuration, the wear test was carried out at different loads ranging from 23 - 140N, in a dry sliding condition. Detailed secondary electron microscopy (SEM) performed on the longitudinal cross sections of the worn alloys indicates that the subsurface deformed layer beneath the worn surfaces is composed of a number of distinct layers like the mechanically mixed layer (MML), the shear deformed and bulk layers with increased hardness as the surface was approached. Deformation below the MML followed the expected behaviour of an exponential decay of strain with depth. In contrast to other studies in the literature, a linear relationship between depth of deformation and specific wear rate was not found for both alloys. This was believed to a result of the MML, which occupied a great proportion of the total depth of deformation. The relationship between the characteristics of the MMLs especially on the work hardening as a function of alloy type is also discussed.