The morphology observed on the fracture surfaces of polystyrene (PS), high-density polyethylene (HDPE), polyphenyleneether (PPE) and polycarbonate (PC) was studied. The stresses were added by cyclically mechanical load whose frequencies are from 0.02 Hz to 1.4Hz and thermal stress load where the temperature ranges were from −55 to 125°C and from −40 to 90°C. The frequency at the cyclically mechanical test was decided from actual response experiments that had been carried out prior to the fatigue test. The specimen for the thermal stress fatigue test was bound between two copper boards and the inner stress was caused by the difference of the coefficient of the thermal expansion. The fatigue lives of the four polymers were in the range from around 400 to 9000cycles, and the less the entanglement density, the shorter the life. The four polymers showed characteristic patterns on the fracture surface. The pattern of the most fragile polymer, PS, was mirror-like and it did not change in the wide range of frequencies. The fracture surfaces of ductile polymers such as PE, PPE and PC were complicated but regular. In particular, the dimpled pattern of the PE surface and the striated one of PPE were noteworthy because the patterns suggested the mechanism of fatigue fractures. In the case of PE, small crystal slides by the cyclic load and microscopic destruction occur. In PPE, the strong entanglement controls the collapse in a wide area and the striation and other regular patterns are due to the destruction in a limited area.
Epoxy resin is used for structural engineering applications such as adhesives and matrix of composite materials. However, epoxy resin have one undesirable property with a poor resistance to crack initiation and growth. Recently many attempts have been made to improve in the fracture toughness of epoxy resin by forming interpenetrating polymer network (IPN) with acrylate. In this study, the mechanical properties and the microscopic structure of Epoxy/Acrylate IPN and its glass fiber reinforced plastics (GFRPs) were investigated. The mode I and II interlaminar fracture toughness GIc and GIIc of the GFRP were improved by introducing the IPN into the matrix resin. The shear deformation can be observed on the fracture surface of the epoxy/acrylate matrix composites. The increase in the ductility of the matrix resin would be contributed to the enhancement of the fracture toughness.
The influence of heterogeneous structures of molten ethylene-propylene block copolymers (EPBC)/ethylene-propylene rubber (EPR) blends on rheological properties were investigated. A slow mechanical relaxation, characteristic of heterogeneous liquids, was observed in the long time region of the angular frequency versus dynamic modulus plots of the EPBC/EPR blends. The mechanism of the relaxation is associated with the shape recovery of the deformed dispersed phases. Stress undershoot in a course of stress development under steady shear flow is observed for EPBC/EPR blends with the EPR content of higher than 30wt%. The time required for the stress to reach the undershoot minimum increases with increasing EPR content in the blends. Comparing the blends with the same EPR content, the time required becomes longer as the molecular weight of EPR increases. The longest relaxation time of the blends, evaluated from the stress relaxation after cessation of steady shear flow, is the same as that of the pure EPBC, when the samples have lower EPR compositions than 30 wt% with a lower molecular weight EPR and all compositions with a higher one. The blends containing a lower molecular weight EPR with higher than 40wt% show the longer relaxation time than the pure EPBC. These results indicate that the change in the longest relaxation time for the blends originates from, whether or not the three-dimensional network structure is formed as a result of the elongation of dispersed EPR phases in the shear flow field, or the difference in morphology when the network structure is really formed.
Structural changes of dispersed domains in immiscible polymer blends under steady shear flow are directly observed by using cone-plate and coaxial cylinder geometries to analyze the orientation angle and the detailed shape of the domains. It was observed that the orientation angle have rather broad distributions even at the high shear rate. The ellipsoidal domains are not ellipsoid of revolution. At a certain steady state, domains having larger volume than the average volume are more elongated in the flow direction than the smaller ones. Shear stress and first normal stress difference cannot be calculated from average shape of observed domains.
Yielding of glassy poly(methyl methacrylate) (PMMA) was studied by means of dynamic mechanical spectroscopy (DMS). Specimens prepared by either slow cooling or quenching through the glass transition temperature Tg were uniaxially stretched up to various amounts of strain. For these stretched and also unstretched specimens, temperature dispersion curves of the dynamic viscoelasticity were measured by DMS. For slowly cooled specimens, the loss modulus E" at temperatures between β- and α-relaxations of the polymer increased progressively with increasing imposed strain. The strain-induced higher values of E" showed a frequency dependence similar to that of quenched specimens without stretching history. These results indicate that the strain-induced increase of E" reflects an extent of strain-induced structural change. Strain-induced increments of E" of quenched specimens were much smaller than those of slowly cooled specimens. This is probably because the quenched specimens are in highly non-equilibrium by nature. Yield point on stress-strain curves was observed more definitely for slowly cooled specimens compared to that of quenched ones. Thus, it is concluded that yielding in glassy polymers is a phenomenon caused by strain-induced structural change.
Effects of shear flow history on the degree of crystallization of high density polyethylenes with broad molecular weight distributions are examined by conventional DSC measurements in connection with slight differences in molecular weight distributions. It was observed that the shear rate where the degree of crystallization starts to increase is related to the shear rate where the shear thinning is observed. By comparing the data for samples showing almost the same viscosity behavior but having slightly different molecular weight distributions, it was observed that the degree of crystallization for the sample containing small amounts of longer chains become slightly higher, which is consistent with the idea that the small portions of long chains become crystalline precursors. It was also observed that a long time, longer than the mechanical relaxation time, is needed to erase the effects of flow.
Porous ceramics is basically categorized as brittle material. However, it shows a little nonlinear stress-strain behavior and has damage tolerance. To evaluate the extent of damage tolerance of porous ceramic, we have proposed the distributed-micro-crack model and applied it to nonlinear stress-strain behavior analysis of porous ceramics. In the previous studies, the distribution of initial crack sizes was not considered in the model. Though the model without initial crack size distribution can predict the initial stage of deformation (i.e. quasi linear stress-strain region) of porous ceramics, it can't evaluate the final stage of deformation near fracture. It can be considered that this discrepancy is due to the localized damage mechanism that can't be estimated by the model without initial crack size distribution. Therefore, initial crack size distribution is introduced into the model in present studies. In this paper, results from analyses by the modified distributed-micro-crack model with initial crack size distribution are presented. Weibull distribution is employed as the initial crack size distribution profile. It is concluded that this modification can simulate localized damage and not only initial stage of stress-strain relationship but also near fracture stage of it obtained by the model shows good agreement with experimental data.
In recent years, the wider width and the smaller number of main girders for the highway bridges have been adopted in order to abridge the constructional and maintenance works reasonably. The metal hangers on the girder to construct the timbering, for the painting work and so on are indispensable, but there are some problems from the point of view of attractive scene and fatigue of main girder ; in addition, the increase of total cost inevitably results from such a metal hangers, too. Particularly, in the case of construction of concrete slab for the bridge pavement, the concrete sludge leaks out of the opening between the upper flange of main girder and the form of slab and it stains not only the girder itself, but also it becomes a structural weak point from the viewpoint of the durability. Therefore, the special leakage-proof packing has been developed in order to overcome those difficulties. The present paper, particularly deals with the verification of the effect of the sludge leakage out of the opening part on the concrete strength by virtue of the structural models.
Recently, a method has been proposed by the authors for the measurement and evaluation with pychometric function using Probit method. The standard sample for the experiment of measuring the performance of panel was designed by means of paired comparison and constant stimuli. In order to evaluate the visual sensory properties of panel in the practical inspection task, the psychophysical experiment was performed to obtain the psychometric curves to evaluate the detection probability of target object. Three kinds of attributes such as size, aspect ratio, and degree of gray scale for the inspection image of FRP panel was prepared in experiment. Moreover, we propose an efficient method using Staircase method for the experiment and evaluation. The detection probability of Staircase method is compared with that of Probit method. The efficeince and accuracy in quality management are also discussed. In conclusion, our method can estimate the parameter of pychometric function efficiently.
In the recent competitive market, benefits from plant operation and maintenance are the crucial target in asset management. Risk-based engineering (RBE) could be an effective tool for the purpose, with various approaches proposed for optimizing maintenance planning. One approach is the probabilistic risk assessment (PRA) which defines the risk as the product of probability of failure (POF) and consequence of failure (COF). Taguchi method is introduced into the optimization procedure with the net income gain function as the operation income subtracting maintenance cost and risk cost. S/N ratios are calculated using L9 orthogonal array of net income gain function for several control factors and noise factors. Noise factors include material creep life scatter which is calculated from the NIMS creep data sheet. The trial calculation results for assumed high temperature power plant show that this combined approach of PRA and Taguchi method is effective for selecting robust and optimum plan for operation and maintenance of plant against all case of noise factors.
Internal fractures with the fish-eye were recognized in high cycle fatigue for Cr-Mo low alloy steel at elevated temperatures. To understand this phenomenon, the fish-eye patterns were examined in detail. Stepwise S-N curves were obtained for rotating bending and uniaxial loading fatigue tests and internal fractures with the fish-eye were observed at low stress and high cycle region. Stress amplitudes to obtain S-N curves were bigger than 0.2% proof stress for both types of fatigue tests and it was recognized that specimens hardened during the fatigue tests. Ligaments were observed on the fracture surface between the fish-eye and the specimen surface. In the relationship between the width of the ligament and the plastic zone size at tip of the fish-eye estimated by using cyclic 0.2% proof stress as yield strength, there is no good correlation between them on the whole. But the data plots could be classified into three types, that is, one where good correlation was recognized between the two and the others where the plastic zone sizes were estimated bigger or smaller comparing with the width of the ligament. A range of histogram of hardness measured at a depth of 40μm from the specimen surface at the cross section of a fatigued specimen expanded to higher levels of hardness than that at a depth of 150μm. The hardened region at the surface layer of the fatigued specimen was considered to be a crystal grain where slip deformation occurs easily depending on the relationship between crystallographic orientations and loading axis, and hardening due to dynamic strain aging occurs effectively. Crystal grains at the specimen surface where fatigue crack should form basically hardened during the fatigue test, and then, it became difficult for fatigue crack to initiate from the grains. During this period fatigue crack initiated at an internal defect such as a nonmetallic inclusion and internal fracture with the fish-eye occurred.
A simplified crack initiation analysis of piezoelectric ceramics under cyclic loading was performed by using the double cantilever beam (DCB) model based on the continuum damage mechanics. A constitutive equation of piezoelectric ceramics and a damage evolution equation that have been proposed by the authors were applied to the DCB model. The crack initiation in the DCB model was dealt within the framework of the continuum damage mechanics ; i.e. damage development in piezoelectric ceramics was described by the evolution equation of a damage variable and the criterion of the crack initiation was based on the value of the damage variable at the crack tip (so-called local approach). The analysis was intended for a crack initiation time under mechanical and electric cyclic-loading. By using the present model, the effect of applied electric field on the crack initiation time was elucidated and variations with process of time in distributions of stress, strain and damage variable in front of the crack tip were predicted. Furthermore, dependence of the crack initiation time on the discretizing size in the numerical analysis was discussed.
Unfrozen water content (UWC), which affects the mechanical behavior of frozen soil, was investigated over a continuous temperature range using an isothermal adiabatic calorimeter. The employed soil materials were fine sand, clay and a mixture of fine sand and clay. Water, with or without NaCl, was added to the soil materials. Specimens were obtained by freezing the saturated soils rapidly. The UWC increased with salt concentration and content of fine soil particles. A temperature rise increased the UWC in all frozen soils. A rapid increase in UWC was observed around the eutectic point of saline frozen sand. However, the rapid increase was not observed for saline frozen clay. Total UWC was divided into UWC in brine and absorbed UWC. It was found that the UWC in brine decreases with the content of fine soil particles. Then, the flexural strength of frozen soils was examined for various characteristics of UWC. The dependence of flexural strength on salt concentration was agreement with the dependence on UWC in brine, and the dependence of flexural strength on the content of fine soil particles was in agreement with the dependence on absorbed UWC. The flexural strength of frozen soils, including those containing both salt and fine soil particles, was not determined by the total UWC, but predominantly by UWC in brine.
The purpose of this study is to develop mechanical properties of concrete that using sewage sludge molten slag as coarse aggregate. As sewerage system spreads, sewage sludge that comes out from process of sewerage disposal is increasing. However remaining capacity of final disposal site is almost in the limit, and it is difficult to secure new disposal site. To build the recycling society is important to build sustainable development, and at present 70 percent sewage sludge is recycled. Sewage sludge slag is recycled as construction materials, fertilizer and fuel. The following were clarified by this research. Amount of elusion of heavy metals from sewage sludge molten slag were lower than environmental standards, and strength of molten slag was lowerthan natural coarse aggregate. Strength, durability factor of freezing thawing and compressive fatigue strength were decreased as the slag substitution ratio was increased, but drying shrinkage was not influenced from the slag substitution ratio.