With a view to examine the axial crushing behavior of thin-walled tubes, which form the parts of transports such as aircrafts and automobiles, under dynamic compressive loads, a series of numerical simulations for thin-walled circular tubes of various dimensions was carried out by using the explicit FEM code LS-DYNA. The energy absorption efficiency and the bifurcation conditions of the collapse modes of these tubes subjected to dynamic axial compression were primarily investigated. In order to confirm the validity of the simulations, first, some calculated results such as the load - displacement relation, the collapse behavior, the absorbed energy per unit volume, and so forth by using the axisymmetric FE model were compared with those obtained by way of experiment under the corresponding condition. Here, the absorbed energy per unit volume was formulated on the basis of Alexander's theory and the Johnson-Cook model, and the results obtained by this formulation were compared with the corresponding results obtained by the simulations and experiments for many tubes of various dimensions. Next, the effects of the initial imperfection and the inertial force on the bifurcation of the axial-collapse mode were examined by using the 3D-shell model. It was clarified that the mode of the dynamic progressive buckling was affected by the natural vibration mode.
The motion of the tennis ball after impact is relative to impact area, impact initial conditions (impact angle, impact velocity, and rotational speed), and the elastic characteristic of racket. Thus, the phenomenon of the impact with a spin is very complicated. To solute this problem, a model of racket-ball impact was developed in the present research based on the numerical method. Impact experiments are conducted, and the entire process of impact between the tennis ball and the racket is recorded by high speed camera. In the experiments, the racket is divided into four areas : central area, top area, side area, and near area. It was found that the each impact area has different effect on the motion of the tennis ball after impact.
Split Hopkinson Pressure Bar (SHPB) technique has been used to evaluate dynamic fracture toughness values of metals, polymers and composites. There are some attempts for such SHPB toughness testing that conventional static equation is applied to evaluate toughness under high speed loading condition by introducing gradually increasing load to the specimen. In this study, dynamic finite element analysis was performed to simulate SHPB type toughness testing with use of rubber buffer. A 2-D model including viscoelastic behavior of polymer specimen was developed, and it was found that the dynamic behavior experimentally obtained was well simulated using the model. Limitation of the use of the static analysis for impact toughness evaluation was also examined by considering acceleration effect on the stress intensity factor. The analytical result exhibited that the applicability of the static analysis may be assessed by considering the effect of acceleration on each of the test configurations.
To identify the yielding phenomena often observed in tensile test at high strain rates even for the constitutive equation of material without any yielding phenomena, numerical simulations using the dynamic FEM analysis have been performed to study the effect of specimen geometry and loading configurations in the Split-Hopkinson Pressure Bar apparatus. Stress distributions in the specimens and the loading bars are clarified, and it is found that the three-dimensional stress wave propagation from the specimen to the loading bar dominates the yielding phenomena due to radial oscillations associated with, even though a slight effect of the specimen geometry is involved to some extent. It is also found that the relevant shape of loading bar end is served to suppress the yielding phenomena.
In this research, the effect of pre-fatigue on the impact tensile properties of the laser welded butt joint of high strength steel plates with tensile strength levels of 590MPa and 780MPa (denoted by HR590 and HR780, respectively) were investigated by the split Hopkinson bar apparatus. Quasi-static tensile tests were also performed to examine the effect of strain rate on the strength and the elongation. It was found that the effect of pre-fatigue on the tensile strength of welded butt joints is quite small if the applied stress in the pre-fatigue is less than the apparent yield stress of the welded butt joints. In the results of impact tensile tests for the welded butt joint of HR780 steel, however, the fracture strain of the specimens subjected to high cycle pre-fatigue was smaller than that of the virgin specimens. This may be caused by interaction of a number of cracks introduced by pre-fatigue with high strain rate.
The interlaminar shear strength (ILSS) of a unidirectional carbon/epoxy (T700/2521) laminated composite under impact loading is determined using the conventional split Hopkinson pressure bar. Double-notch shear (DNS) specimens with lateral constraint from a supporting jig are used in the static and impact interlaminar compressive shear tests. Short-beam shear specimens are also used under static 3-point bending. Finite element analyses are performed to determine the shear stress and normal stress distributions on the expected failure plane in the DNS specimen using the MSC/NASTRAN package. The effects of deformation rate at failure on the ILSS and failure mode are investigated. It is observed that the ILSS is independent of the deformation rate at failure up to nearly 1.5m/s. The validity of the test results is confirmed by microscopic examinations of both static and impact failure surfaces for the DNS specimens.
The effect of short time annealing on the high-cycle fatigue behavior of an ultrafine grained Al-Mg-Sc alloy produced by equal channel angular pressing (ECAP) has been studied. Transmission electron microscopy revealed that small Al3Sc precipitate particles initially restricted grain growth and then the Al3Sc particles coarsened, allowing homogeneous grain growth during annealing at 350°C. The as-ECAP specimen exhibited high tensile strength, but low ductility. The annealed ECAP specimens showed slightly smaller values of 0.2% proof stress and ultimate tensile stress, and fairly larger values of elongation to failure than the as-ECAP specimen. In the as-ECAP specimens, cyclic softening was observed, whereas the annealed ECAP specimens exhibited no cyclic softening. The existence of Al3Sc particles produced by the annealing enhanced the uniform deformation and the absence of cyclic softening in the annealed ECAP specimens. These resulted in increase of remarkably prolonged high-cycle fatigue lives in comparison with the as-ECAP specimen.
Rotating bending fatigue tests were carried out for a shot peened maraging steel in order to investigate the influence of reverted austenite on the internal fracture. Fracture initiated from an internal inclusion regardless of the formation of reverted austenite in long life region, while fracture occurred from a rough surface of specimen in short life region. Although fatigue strength was decreased when surface fracture occurred, that for an internal fracture was increased conversely by the formation of reverted austenite. Many intergranular cracks generated after the formation of flat facets around an inclusion and then fractured by the growth of a surface crack in case of no reverted austenite, while by the formation of reverted austenite intergranular cracking was suppressed and the growth process of an internal crack presented after intergranular cracking. Consequently, an orbit trace was formed clearly at the boundary between the growth of an internal crack and that of a surface crack.
Fatigue behavior under variable amplitude loading was studied on a high carbon chromium steel, JIS SUJ2. Special attentions were paid on the evaluation of fatigue damage by the Palmgren-Miner rule and the damage mechanisms under variable amplitude loading accompanied by the failure mode transition in high cycle regime. Tests were performed using cantilever-type rotating bending fatigue testing machines in laboratory air. Load sequences employed were two-step multiple block loadings (low ↔ high). When two stress levels, under each of which the failure mode was the same, were applied, the failure mode was subsurface-type. Fatigue damages were evaluated successfully by the Palmgren-Miner rule. On the contrary, when the failure modes under both stress levels were different, the failure mode was surface-type. In this case, the cumulative damage at each stress level was independent of each other. Based on SEM observation, the morphologies of fish-eye formed under two-step multiple block loadings were the same as those under constant amplitude loadings.
Super duplex stainless steels were welded using a new flash butt welding technology of temperature controlling. Flash butt welding consists of two stage processes. First stage is a flashing action. The specimen produced a flashing or arcing across the interface of the two butting ends of the specimens. Fine particles of metals near the surface were burned out towards the opposing surface of the specimen irregularity and then the melted particles were deposited on the surface. The second stage is a resistance welding. The solid state bonding was performed in the region around the deposited particles. The cross sectional microstructure of the weld bond region was observed using a scanning electron microscopy. The microstructure showed two regions : a deposited fine particles region and a solid state bonding region. The grain growth was hardly observed in the weld region and the heat-affected zone. The impact energy increased with increasing heating time up to 1373K because of the increasing fine grained deposited metal.
Thermal barrier coating (TBC) was deposited on Ni based superalloy specimens, where bond coat was NiCoCrAlY by high velocity oxygen fuel (HVOF) spraying and top coat was 8mass% yittria stabilized zirconia(YSZ) by atmospheric plasma spraying. Edge indent tests of the specimens were carried out at a temperature of 1073K by using indenters with apex angles of 85° or 120° and under different displacement speeds. In addition, the specimens were heated and held at 1273K for long time, and the edge-indent tests of the specimens were performed both at 1273K and room temperature. The results showed that the delamination load Pd and the delamination energy Ed measured at 1073K after short time holding were larger than those measured at room temperature, and were almost the same irrespective of loading speed. The Pd and Ed at 1073K were large when the apex angle of the indenter was large. The Pd and Ed at 1273K increased with increasing holding time and reached a maximum at 1400ks. With further increase in holding time, the Pd and Ed largely decreased. The Pd and Ed at 1273K were smaller than those measured at room temperature, which means that the coating with sufficient delamination energy at room temperature can be delaminated more easily at high temperature. The edge-indent method is effective for quantitative evaluation of delamination energy of coating at high temperature as well as room temperature.
The strength prediction method for 2D-C/C composite components is proposed in this paper. The proposed method is based on the combination of competing risk model and FEM stress analysis. In the strength prediction, tensile and compressive fracture to lamina direction, tensile and compressive fracture transverse to lamina direction and shear fracture were introduced as basic fracture modes. In order to investigate the applicability of the proposed method, the fracture test with structural element test specimens was performed, and the result was compared with prediction result by the method. From the comparison, it was shown that the proposed prediction method has a slightly higher but more precise prediction than the other traditional fracture models. As a result of this study, it is concluded that the proposed method was applicable to arbitrary shaped 2D-C/C composite components.
This paper presents the multi-scale stress analysis of trabecular bone by the homogenization method bridging nano-micro-macro scales. Three-dimensional microstructure of trabeculae is obtained by the X-ray CT and the image-based modeling technique. Biological apatite (BAp) crystallite orientation is considered in the microstructure model by means of the anisotropic mechanical properties. The c-axis of BAp is set up as the maximum principal stress direction under the long term macroscopic stress condition. These properties are automatically assigned to each voxel element. To determine appropriately the microstructure model, the trabeculae morphology is analyzed and quantified as the trabecular density distribution. The proposed method is applied to pig's femur. It was revealed by the morphology analysis and homogenized macroscopic properties that the trabecular bone has plate-like characteristics. The predicted anisotropic level of the macroscopic properties was quantitatively coincident with the measured value by the X-ray diffraction analysis.
Octacalcium phosphate (OCP) has a unique layered structure. The layered structure is feasible for novel organic-inorganic hybrids. We treated OCP powder with a silane-coupling agent, vinyltrimethoxysilane (VS), and examined its thermal properties. The VS was not incorporated in the hydrated layer of OCP but reacted only with the surface. The non-treated OCP (Pure-OCP) and treated OCP (OCP-VS) showed the almost same dehydration behavior when they were examined by thermogravimetry and differential thermal analysis. However, the different behavior in 010 diffraction peaks was observed in X-ray diffraction patterns when they were heated at 150°C. The treatment with VS retarded the decomposition of the layered structure of OCP. Some of the VS molecules in OCP-VS might have undergone the condensation reaction among the silanol groups in VS. Due to this VS network on the surface of OCP, the framework of the layer of OCP becomes stable. When Pure-OCP and OCP-VS heated at 150°C were aged under ambient conditions for 1 week, rehydration occurred in both samples. The VS introduced on OCP may provide a novel organic modification of OCP.