Since titanium alloy is one of the most promising structural materials for the high velocity vehicles such as aircrafts and automobiles, the impact tensile strength of the material at various temperatures is presently investigated. Three kinds of aging treatments were performed on the beta-titanium alloy, and the stress strain relations were identified in the wide range of the temperature from 77K to 573K, and the strain rate from 10-3S-1 to 103S-1. Thermally activated process concept was applied to explain the temperature and the strain rate effect on the flow stress, and the stress-strain relations at high strain rates were well understood by the concept with taking account of adiabatic heating effect during deformation. The stress strain curves depend on the microstructures produced by the different aging treatments, while the temperature and the strain rate effects are almost identical for the materials investigated.
In order to improve durability of building structure at a disastrous earthquake, Rolled Steels for Building Structure (JIS G 3136-1994) were established. In general, such a violent earthquake is followed by several aftershocks of the same level of the earthquake. Remaining mechanical properties of building components and materials under dynamic tension are essential to evaluate the durability at aftershocks and next disastrous earthquake. In this paper, material damage at a disastrous earthquake is simulated by tensile pre-straining at a quasi-static strain rate, 1×10-3s-1. The pre-strained SN490B steel and SM490A steel (one of the conventional steel for building structure) are characterized in tension with a universal material testing machine at the strain rates of 1×10-3s-1 and 1×10-1s-1 and with a high velocity loading machine of a horizontal slingshot type, adopting the one bar method, at 1×103s-1. The difference between the steels is only the level of lower yield stress, that is, yield ratio. No irregular pre-strain and strain rate dependence of any remaining characteristics is found. The feasibility of interchangeability between Charpy absorbed energy and absorbed energy per unit volume in tension is discussed, three problems (the differences of loading mode, deformed volume and impact velocity between them) are specified.
This paper presents a practical viscoplastic constitutive model covering a wide range of strain rates, large deformation, and applicable to various grouped metals. The essential new feature of the constitutive model is employment of a specific function between quasistatic yield strengths and strain rate sensitivities of grouped metals in the strain rate range where thermally activation for dislocation motion is the dominant mechanism of the material strain rate sensitivity. This new feature enables us to use unified or common material constants to describe the dynamic behavior of grouped materials in the strain rate range mentioned above. In the model, formulations for the strain rate sensitivity at higher strain rates of up to 104-105s-1, and for the temperature dependence of flow stress are also made so that the strain rate sensitivity in the wide range of strain rates, and at various temperatures are successfully described. In order to verify the validity of the proposed constitutive model, simulations of stress-strain curves at an elevated strain rate and variations of stress with the logarithm of strain rate at various temperatures are compared with the corresponding experimental results. It is found that the simulations based on the proposed constitutive model are in fairly good agreement with experimental results.
Impact tensile strength of concrete (mortar) and its measuring method is discussed. The experiment is conducted by means of a Hopkinson bar technique using an air-gun arrangement. The measuring method is based on the superposition and concentration of tensile stress waves reflected both from the free ends of a striking bar and a specimen bar. The impact tensile experiment of mortar specimen bars was performed as well as static strength tests. The experimental data in relation to those strengths were analyzed statistically by a Weibull distribution. The impact tensile strength was significantly influenced by loading rates, and it was found to be about 1.5 times of the static value. For examining the adequacy of the present measuring method, numerical simulations on the stress waves propagated in the specimen bars were carried out under the condition of two-dimensional axisymmetric model using the FEM code and some reasonable results were found.
The dynamic strengths of adhesively bonded dissimilar butt joints were investigated numerically and experimentally. Stress distributions and the variation of stress with time in the joint were analyzed by the finite element method. A stress singularity parameters approach was applied for evaluating the dynamic tensile strength of adhesively bonded dissimilar butt joints, composed of PMMA and Al alloy plates. The dynamic strength of the joint under impact loading was calculated by a new simple method proposed in this study. The statistical analysis of the dynamic tensile strength of the joints by using the Weibull distribution was presented. It was found that the dynamic tensile strength of the joints increased significantly with increasing loading rate.
An impact fracture test method has been developed to measure fracture toughness under mixed mode I/II loading using an edge-cracked half-disk specimen. Using this method, dynamic and quasi-static tests were performed and mixed-mode fracture toughness for graphite was measured. The results were compared with those that the authors had obtained using center-notched disk specimens. It was found that fracture toughness measured with edge-cracked half-disk specimens was comparatively good agreement with MHS (maximum hoop stress) or MERR (maximum energy release rate) criteria. On the other hand, using center-notched disk specimens, mode II components of mixed mode fracture toughness were estimated larger. It seems that one reason of this was the effect of the second order, nonsingular term in the series solution for the crack-tip stress field.
A simple experimental method was developed in order to evaluate the mixed mode (I+II) interlaminar fracture toughness of polymer matrix composite laminates under low-velocity impact loading. The MMF (Mixed Mode Flexure) specimen was employed for determining the critical energy release rate at the onset of macroscopic crack growth. The SHPB (Split Hopkinson Pressure Bar) system was used for measuring the dynamic load and displacement. The dynamic response of the specimen was studied on the basis of computational results using finite element method. The dynamic stress field in the vicinity of the crack tip was similar to the static stress field when a slow-gradient ramped incident stress wave applied to the specimen. However, the dynamic stress field was largely oscillated with time when a stepped incident stress wave was applied. In the previous case, a simple formula based on static beam theory and compliance method could be applied for calculating the dynamic energy release rate. The experimental results agreed well with the above computational results.
The deformation and transformation behavior of a maraging TRIP (TRansformation Induced Plasticity) steel (MAVAL X12) is studied experimentally. The stress holding test under cooling and the strain-controlled isothermal loading test are performed with the thin-walled tubular specimens in order to get the dilatation curves under a constant hold stress and the stress-strain curves. The existence of the back stress is confirmed since the dilatation curves do not close after a heating/cooling thermal cycle. Its value in the axial direction is determined to be 28.4MPa by means of the dilatation curves under applied hold stresses. The transformation-start condition is identified to be a linear Clausius-Clapeyron line both under tensile/compressive and shear stresses. The temperature-dependence of the yield stress determined by means of the tension, compression and torsion tests is charactered by the following: In the higher temperature range the measured values mean the yield stress of the parent phase, whereas in the low temperature range they exhibit the “composite effect, ” since the microstructure is consisted of the hard thermally-and stress-induced martensite phase and the soft parent phase. In the middle temperature range the measurement shows the result of the competition of both effects.
The crack in spalling failure of steel making backup rolls propagates mostly in Mode II. Mode II crack growth rate (ΔKII-da/dN) and Mode II threshold stress intensity factor range (ΔKIIth) for a backup roll steel were measured by using the newly developed testing system with a conventional closed-loop type tension-compression fatigue testing machine. The Mode II fatigue crack fracture surface morphology of the specimen was very similar to that of an actual backup roll. The crack growth behavior of the fractured backup roll was investigated based on fracture mechanics by using the laboratory test data of ΔKIIth for the backup roll steel.
Fatigue crack growth behavior of aluminum alloy under three-step varying load conditions was investigated. The load patterns used in this study consist of high low two-step load (KH/KL) and additionally inserted middle level (KM). The crack growth behavior was observed fractographically, and the results were correlated with two typical phenomena observed under two-step loads; one is the retardation phenomenon due to load change, and another is the steady crack growth behavior where linear cumulative law of crack growth rate based on ΔKeff can be applied. It was revealed that the crack growth law derived under two-step load could be applied to the estimation of crack growth rate at KM. But, the crack growth rate at KL depends on the K-values at KH and KM, although the trend of the crack growth behavior at KL under three-step load is qualitatively similar to the result observed at KL under two-step load. Accordingly the analytical crack growth law derived in authors previous papers under two-step load conditions was extended to make it applicable to the results obtained in the present paper. The estimated results based on the modified method well agree with the experimental results.
Multiple cracks distributed in a plane affect one another in growth. This interference effect accelerates the crack growth. Some studies have developed the Monte-Carlo simulation models, which simulate the crack growth considering this interference effect of multiple cracks. In these studies, the interference effect of multiple cracks was considered by introducing a simple rule of crack coalescence or growth arrest in their growth process, which was determined by their relative location. In this study, a simulation model was developed, which considered this interference effect by using the finite element method. This model used a finite element mesh generator for distributing multiple cracks in a voluntary position over a plane. The mesh generator was able to evaluate J value of each crack tip considering the interference effect in every timely step, and crack growth rate of each crack was calculated with the evaluated J value. This simulation model enabled us to assess the interference effect of multiple cracks and the failure condition accurately. With this simulation model, the effect of crack initiation behavior in a lifetime was examined, and the fact that increase of crack initiation accelerated the crack growth and reduced the lifetime under not only constant load condition but also constant strain condition were found. Furthermore, the crack length distribution showed the compound Weibull distribution form when the interference effect of multiple cracks acted to a great extend.
A number of fitting methods for fatigue strength data have been proposed to evaluate the fatigue characteristics quantitatively. It is, however, still unknown what is the best method for estimating the fatigue strength or the fatigue life of materials. This paper was focused on the review on the conventional fitting methods, and discussion on the model validity. Furthermore, a new fitting technique was proposed to analyze the database which included the S-N data under various stress ratios. It was finally found that the method proposed in this study was applicable to all the data stored in the JSMS fatigue database, and was more effective in fitting the fatigue data than the conventional fitting methods.
The experimental results of fatigue test near the stress levels around fatigue limit have large scatter. In this paper, we propose the statistical method for decision of the S-N curve considering an effect of them. The procedure of proposed method is as follows. Firstly, all fatigue test data are separated into an inclined part and a horizontal part with an arbitrary number of cycles as a boundary. Secondly, the distributions of fatigue strength are estimated with computational probability paper, and agreement of the distribution of both parts is searched by changing the boundary number of cycles. Finally, the optimum separation is the one for which the agreement of distribution reaches its maximum. Because each fatigue data is transformed into normalized fatigue strength, this method has the advantage point that the distribution of fatigue strength and the fatigue limit can be found by relatively small number of samples. As an example, the proposed approach has been applied to the fatigue test data for some materials. The well accuracy of the proposed S-N curves was proved by comparison of the obtained fatigue limit and calculational result by Probit analysis. Furthermore, the approach can be also applied to the materials which doesn't show obvious fatigue limit, such as CFRP, because non-existence of fatigue limit can be judged.
In order to clarify the extreme strength of spinning fracture, the high-speed spin tests were carried out for two ceramic circular disks till they burst. On the other hand, the spinning fracture strengths were evaluated by theoretically analyzing the centrifugal stress distribution and by using the unified estimation method for ceramic strength. Also, the residual stresses on the surface of the ceramic circular disks were estimated by using the indentation flaw method (IF method) and their influences on the spinning fracture strength were studied from the viewpoint of fracture mechanics. The extreme spinning fracture strengths were successfully estimated from the 4-point bending strength by using the unified strength estimation method. The residual stress had the negligible influence on the spinning fracture strength. The ceramic circular disks were clarified to have sufficient reliability against fracture in practical service of textile machinery.
The risk of fracture does exist even in the carefully designed ceramics components under excess stress unexpected in design procedure such as high stresses caused by restraining the deformation at the contact region of the component. To assess the safety and the reliability of ceramics components under such circumstances, it is indispensable to take the damage tolerance of the material into consideration. Ceramics is thought to be perfectly brittle and have no damage tolerance. But, though very little, it has the damage tolerance. The important problems open to us now are to estimate the extent of the damage tolerance of ceramics quantitatively and to clear the relation between the damage tolerance of the material and the reliability of the component. In this paper, we demonstrate the existence of the damage tolerance in the ceramics by tests using specimens of porous cordierite. The nonlinear stress-strain curve of the brittle material is thought to be a reflection of the damage tolerance of the material and it is shown that the nonlinear stress-strain curve obtained can be simulated by the distributed micro cracks model developed here. The relation between the damage tolerance and the reliability is also discussed through several simulation works. These simulations show that when the damage tolerance of the material is getting larger, the reliability of the component becomes higher. This conclusion is supported by the fracture test results using notched specimens of porous cordierite where the notch sensitivity is shown to be very little in this material. This means that this material has the high reliability under the stress concentration.
In this study, temperature differences in crack initiation of thermal barrier coating layer were examined by the thermal shock tests which is conducted by what the pencil-type specimen is dropped into the water. Crack initiation strengths of thermal barrier coating were estimated based on both of the temperature differences and the thermal stress distribution which is obtained from the closed form of thermal stress solution. Obtained results are summarized as follow. (1) It was found to be able to conduct the good thermal shock test without a gas on the specimen surface by using the pencil-type specimen with a tip angle 30°. Resistance of crack initiation in thermal barrier coating layer with a thin thickness was very high. (2) Crack initiation strength normalized by volume of the coating layer was defined newly for removing the effect of the volume on the strength. The normalized crack initiation strength corresponded to the strength of the coating materials. These results show us to be able to estimate a crack initiation strength of thermal barrier coating with a various coating thickness under a thermal shock by using the normalized crack initiation strength.
During fabrication of semiconductor devices, the stresses on a silicon substrate sometimes cause dislocations and worsen the electric characteristics of the device. Therefore, a dislocation-free fabrication process needs to be developed to improve reliability. In this paper, a process design method using stress singularity parameters (K and λ) is proposed. In order to prevent the dislocation generation, the process parameters, such as the device structure, materials and process temperatures, are set to keep the stress singularity parameters (K and λ) under the critical values of dislocation generation Kdc. These parameters are predicted using FEM method considering the internal stress of thin films. The process design was applied to a bipolar transistor and a MOS transistor and the experimental results agreed very well with the prediction. It was confirmed that this new process design method is effective in improving device reliabirity.
To improve tribological properties of polyacetal (POM), blends with polyethylene (PE) were prepared by extrusion and studied. Three types of PE, i.e., low density polyethylene (LDPE), high density polyethylene (HDPE), and high molecular weight high density polyethylene (HMWHDPE) were employed in the present study. Each specimens in the shape of a pipe containing one of these PE types were rubbed against a rotating carbon steel (S45C) slider under unlubricated conditions. Observation with an electron probe X-ray microanalyzer (EPMA) revealed that the specimens had sea-island structures in which polyethylene particles were finely dispersed in the POM matrix. An optical microscope observation indicated that these polyethylene particles mainly bore the load applied to the frictional surface. The wear volume in both the case where LDPE was blended and that where HDPE was blended showed a remarkable decrease, the blending effect of HDPE being much greater than that of LDPE. The coefficient of friction in both of these cases became smaller, the values indicated becoming nearly the same. On the other hand, contrary to expectations, blending with HMWHDPE resulted tribological properties which were handly improved.
Viscoelastic properties of organic hybrid materials prepared from various types of phenylbenzotriazole compounds (FB) as additive and chlorinated polyethylene (CPE) as matrix are investigated by means of dynamic mechanical analysis. When 60.5vol% of 2-[2-hydroxy-3, 5-bis (α, α' dimethylbenzyl) phenyl]-2H-dibenzotriazole (FB990) is loaded, the maximum tanδ of the organic hybrid material increases 2.7 times and the temperature at maximum tanδ shifts more than 30°C, compared with those of CPE. At the temperature range of 20-35°C, FB990 increases both storage modulus and tanδ of the organic hybrid material. On the other hand, 2-(5-methyl-2-hydroxyphenyl) benzotriazole (FB100) decreases tanδ, since it exists in the state of crystal in the polymer matrix. Long alkyl and phenyl groups tend to prevent crystallization of FB and to increase tanδ of the organic hybrid material. Further, Cl group of FB improves the compatibility between additive and CPE. The study of the glass transition temperature of CPE (-17°C), the organic hybrid materials and FB990 (35.5°C) by using differential scanning calorimeter shows that there is slightly negative interaction between the FB990 and CPE, leading to the formation of micro agglomeration of FB990 in the polymer matrix. Alumina, which is loaded to organic hybrid material as inorganic filler, increases storage modulus and loss modulus. At this time, the tanδ of the composite can be calculated from the tanδ and volume fraction of the organic hybrid material.
In order to investigate the degradation of AlN substrate in hydrothermal environments, AlN ceramics coated with two kinds of electroless plating films, such as Ni-P and Cu, were exposed in water at 373K up to 10 days. The depth and the size of individual defects in the plating film were measured by a microscope and the maximum depths were estimated by the statistics of extreme. The maximum depths of early film defects in the plating films obeyed the double exponential distribution. In the case of Ni-P/AlN, the degradation of AlN substrate was caused by early film defect in the plating films such as pinhole and not by corrosion. The degradation of the AlN substrate with Cu plating film occurred by corrosion not by early film.