The aircraft structural integrity should be demonstrated by considering every factors which degrade its strength. Recent helicopter structural components such as rotor hub and blades are manufactured mainly by uni-directional composite materials. The reasons of the usage of the composite material are the low notch sensitivity and high damage tolerance. Aramid composite has extremely high fatigue strength and high toughness. Then the fatigue strength characteristics of a uni-directional glass composite and aramid composite has been studied by high cycle fatigue tests with open hole. The test result of aramid composite showed the high notch sensitivity at high cycle region. In case of the aramid composite, the crack expanded laterally to the load direction from the hole, on the other hand, in case of glass composite the crack expand parallel to the load direction from the hole. Some coupon tests about those materials and FEM analysis have been carried out. From this study, following results were obtained. 1) In case of aramid composite, high stress concentration occurs, because of low shear modulus of the fiber that is almost same as the epoxy matrix. Then, no matrix crack occurs around a hole, so fiber breakage occurs and propagates. 2) In case of glass composite, the stress concentration is relieved by small cracks around a open hole because of shear modulus difference of the fiber and matrix. Then, no fiber breakage occurs. The conclusion of this study is that the appropriate resin system design should be placed on the shear modulus difference between fibers and matrix.
Silicone gel provides good electrical insulation and has high resistance against humidity, high temperature, low temperature and chemical reactions. So, it is often applied to electrical devices to prevent them from corrosion. In such devices, the bonding wires for electrical connection are sometimes fatigue damaged, because under a vibration environment the gel vibrates the bonding wires. To prevent this fatigue damage, we must develop an evaluation method of vibration behavior of a bonding wire protected with gel. However, it is difficult to analyze the vibration behavior of a wire in gel structures. We thus developed a vibration analysis method that takes into account the viscoelasticity of gel and the mechanical correlation between bonding wires and the gel. By comparing these analytical and experimental results, we can confirm the validity of the evaluation method of vibration behavior for a bonding wire protected with silicone gel.
Damage in rolling contact fatigue was observed using polycarbonate which is a transparent material. The rolling contact fatigue tests were performed using a set of one polycarbonate and one carbon steel S45C rollers under lubricated condition. In the tests, damage occurred only in the polycarbonate. With the carbon steel was set as the drive side, experiments in which the rollers were rotated in a single direction and those in which the direction of rotation was changed at a frequency of every 3×105 times were carried out. In the former case, arrowhead shaped surface cracks were initiated, and pits were subsequently formed under the conditions of maximum principal stress criterion. In the latter case, pits were formed which appeared elliptic from a surface view. The cracks then grew from the pits bottom toward the center of roller. When the polycarbonate was set as the drive side, internal cracks initiated in shear mode, then the surface layer of the polycarbonate peeled off. From those experiments, it was found that damage mechanisms of rolling contact fatigue changed depending on the loading conditions. These mechanisms are influenced by the friction of the roller surfaces and the pressure of the oil which enters the cracks.
This paper develops a damage model for evaluation of low cycle fatigue lives under complex cyclic multiaxial loadings. The author has proposed the equivalent strain parameter for the life prediction of the nonproportional low cycle fatigue. This strain parameter could evaluate the dependence of fatigue lives on strain history and material, and correlate the fatigue lives within a small scatter band under 15 kinds of proportional and nonproportional strain paths. However, the parameter was applicable to the life prediction under the limited nonproportional strain history, so that some modifications were required. In this study, a simple damage model for the life prediction by combining the equivalent strain parameter with Miner's law in order to apply the life prediction under more complex nonproportional loadings. The applicability of the proposed model was examined for life evaluation of nonproportional low cycle fatigue for different materials; type 304 stainless steels, copper, aluminum alloys, chromium-molybdenum and carbon steels, which were obtained from different research institutes. The model could correlate most of all the fatigue data within a factor of two scatter band and has a possibility to become a good damage model for nonproportional low cycle fatigue.
The rolling contact fatigue tests with specific sliding 0% and 20% measuring tangential force, the pulsating compression fatigue tests and the completely reversed torsion fatigue tests were carried out using six structural metals. The relationships of the fatigue limit among the rolling contact fatigue, the pulsating compression fatigue and thecompletely reversed torsion fatigue were examined on the basis of the stress analyses under the contact surface. Then the fatigue limit ps under the rolling contact fatigue with specific sliding 0% and 20% could be estimated from the tensile strength σB, the yield stress σy and Young's modulus E as p0%=0.07σB√E/σy and p20%=0.05σB√E/σy respectively.
The growth behavior of crack was investigated under cyclic torsion with and without axial static stress using pre-cracked steel. After shear mode crack growth, branching of the crack was observed on both sides of the crack. The length between the branching points was dependent on loading conditions. This length was longer when the applied shear amplitude and the static axial stress level were higher. The crack propagation curves have turning points. Before the turning point, shear mode crack growth was observed and the crack propagation rate had good correlation with the branching behavior. It is found that the crack opening and the initiation of micro cracks help the shear mode crack growth After the branching of crack, loading method was changed from cyclic torsion to push-pull. When applied normal stress was higher, the crack grew under the condition of maximum shear stress criterion. On the other hand, when applied stress was lower, the crack grew under the condition of maximum principal stress criterion. Those behaviors are also related to slip band density and crack opening.
Titanium nitride (TiN) and aluminum nitride (AlN) films of 1-26μm thickness were deposited on pure titanium specimens by reactive RF magnetron sputtering method. Tensile tests were carried out for the specimens to evaluate the fracture strength of films and the interfacial fracture toughness between film and substrate. With increasing load the film was cracked repeatedly, and the partial delamination of the film started under the plastic deformation of substrate. The fracture strength of films σc were determined by the extrapolation method using the relationship between inverse of crack intervals and tensile stress of substrate. The σc of TiN and AlN films was almost the same irrespective of film thickness, but the σc of AlN film was small when the film thickness was 26μm. The interfacial fracture toughness Gc12 of both films increased with increasing film thickness, and the Gc12 of TiN film was larger than that of AlN film at the same film thickness.
It is well-known that dense SiC matrix composites reinforced by SiC fibers with BN coating can be fabricated by subsequent reaction sintering with molten Si. In this experiments, it was confirmed that the mechanical properties of continuous Si-C fibers (Hi-Nicalon) were not affected by the thickness of the BN coating using CVD process and the bending strength of reaction-sintered SiC increased from 500 to 1000MPa with decreasing the size of residual silicon. The effect of SiC matrix strength on the matrix cracking stress in unidirectional fiber-reinforced SiCf/SiC composites was investigated by use of these data. As a result, it made clear that the matrix cracking strength in SiCf/SiC composites increased with increasing the SiC matrix strength. There are the upper limits of the SiC matrix strength deduced from the SiC fiber strength and the fiber volume fraction to achieve the apparent ductile fracture of the SiCf/SiC composites.
Lately, the FRP adhesion method has become a subject of special interest. The bond strength between the FRP sheets and concrete influences the structural properties of reinforcement by this method. The influence of temperature on bonding properties between the FRP sheet and concrete should be investigated, since the properties of plastics are sensitive to an increase or decrease in temperature. The present article describes a study of the influence of bonding and curing temperatures, and the testing temperature on the cleavage bonding properties between the CFRP sheet and concrete. In order to obtain the cleavage bonding properties, compact tension tests were conducted, varying the bonding and curing temperatures, the testing temperature and the type of resin. The bonding and curing temperatures were 20 and 5°C, the testing temperature was selected from five levels within the range of -15 to 60°C. Epoxy resin and MMA resin were compared in the experiments. The relations between the load and the crack mouth opening displacement were measured as well. The cleavage bond strength, the bond softening diagram, the fracture energy and the toughness index were derived from the data as the cleavage bonding properties. The conclusions obtained from this study are as follows: 1) The cleavage bonding properties between the CFRP sheet and concrete are influenced remarkably by the testing temperature, although they are slightly influenced by the bonding and curing temperatures, and the type of resin. 2) The maximum value of the cleavage bonding strength and the fracture energy is obtained at a testing temperature of 20°C. Those properties decrease as the testing temperature increases or decreases. 3) The bond softening behavior becomes more ductile as the testing temperature increases. On the contrary, it becomes more brittle as the temperature decreases.
In the present study, sugi (Cryptomeria japonica D. Don) and karamatsu (Larix kaempferi Carriere) logs were smoke-heated with different temperatures and times using a modified food smoker. After smoke heating, several wood qualities were examined, and then the effects of treatment temperature and time of smoke heating on wood quality were discussed. Moisture contents (MC) were decreased with increase of temperature and time in each treatment. The difference in MC between heartwood and sapwood became small due to great decrease of MC in sapwood with increase in time, resulted in uniform distribution of MC around fiber saturation point. However, prolonged treatment caused tension stress at surface layer due to drying, which led to the increase in frequent occurrence of surface checks. In the treatment at 60°C, relative degree of crystallinity (RDC) increased by smoke heating over 40 hours in karamatsu wood, but not changed in sugi wood. However, RDC of both species increased with increase in time at 80 and 100°C treatments. Equilibrium moisture content (EMC) did not change in the treatment at 60°C. In the treatments at 80 and 100°C, however, EMC decreased with increase in treating time. This trend corresponds to increase of RDC. In both species, almost no change of sapwood color occurred in the treatment at 60°C. However, total color difference increased with increase in treating time at 80 and 100°C; the values showed more than 5, which means appreciable color change. Bending properties showed almost no change in the treatments at 60 and 80°C in both species. In karamatsu wood, however, specific Young's modulus increased with increase of time in the treatment at 100°C. These results suggest that no thermal degradation of wood occurred by smoke heating within 100 hours at a temperature inside the log below 100°C.
The X-ray diffraction method was applied to measure the change of the lattice strain and domain switching in rhombohedral lead zirconate titanate (PZT) due to poling and applied strains. The lattice strain was determined from the linear relation between the diffraction angle and sin2ψ. The lattice strain measured by X-rays is at most 15% of the macrostrain determined from the dimensional change due to poling. A major part of the macrostrain was caused by domain switching. External loading induced domain switching and lattice strain. The lattice strain induced by external loading was at most 25% of the applied strain. The amount of domain switching was evaluated by the change of the intensity ratio of 222 diffraction to 222 diffraction. The intensity ratio for normal diffraction (ψ=0°) was decreased with the applied strain, because the spontaneous poling direction, 222 direction, turned to the loading direction. The broadening of X-ray diffraction profiles obtained from the diffraction plane perpendicular to the poling direction was the maximum, indicating the largest microstrain in the poling direction.
Quantitative estimation of damage in concrete is investigated, introducing acoustic emission (AE) and damage mechanics. The damage of concrete is quantitatively evaluated by carrying out AE measurement in the uni-axial compression test. It is clarified that the process of damage accumulation could be evaluated by the rate process analysis. A damage parameter is determined from damage mechanics, applying Løland's model to a stress-strain relation. Correlating the variation of the rate with the damage evolution, a procedure to estimate the damage parameter is developed. The feasibility of the procedure is demonstrated by concrete samples damaged due to the freezing and thawing process.
A novel test procedure is suggested for the exploration of interfacial bonding strength under combined stress states using a test piece in which a single sphere is embedded in its center to avoid a generation of stress singularity at the edge of bonded interface between two materials. The combined stresses are applied due to combinations of tension, torsion and bending loads for a tensile specimen, and by shifting the position of a sphere from a center for a three point bending specimen. Quantitative analyses of AE signals combined with finite element calculations in which thermal residual stresses are taken into account, lead to local interfacial debonding criteria under combined stresses. According to the criteria obtained, shear stress necessary for debonding increases linearly with an increase in compressive normal stress at interface. That is the Mohr-Coulomb criterion is predominant while quadratic and elliptical interaction criteria such as the Hoffman's rule are not applicable. This is attributed to the difference in the test procedures. In the conventional procedures using laminated cylinder specimens, the generation of stress singularity at the free edge of bonded interface between two materials reduces apparent shear stress necessary for debonding, whilst in the present procedure, the actual shear stress is exactly evaluated.
Atmospheric corrosion test data for silver were analyzed which were obtained in various sites in Japan. A new corrosion life assessment method was developed. The method took into account the interactive effect of multiple environmental factors on the decrease in metal weight during corrosion tests. The simple and practically useful formula for corrosion life prediction was established in terms of environmental indices and the formula could enable the evaluation of time dependent loss of metal weight based on the environmental assessment.
A method for measuring surface strains of real specimens is proposed using the scattering photoviscoplastic analysis with polyester coating. A major advantage of this experimental technique is that interference fringes in the vicinity of the adhesion plane bounding the real specimen and the coating can be observed. The difference in principal strains on the symmetric section of real specimen in two dimensional problems can be calculated using two kinds of scattered-light fringe patterns obtained by two different incidences of polarized light. For the demonstration of effectiveness of this method, surface strains of unnotched and semicircular notched plate specimens made of aluminum alloy were measured under uniaxial tension. For the unnotched specimen, measured longitudinal strains were consistent with ones calculated from the elongation of the gauge length. For the notched specimen, normal strains on the notched section were measured and the distributions were consistent with FEM analyses.
The long fiber reinforced thermoplastic (LFRTP) pellet has better mechanical properties than short fiber reinforced pellet and better moldability than stampable sheet. At present, injection molding method is mainly used for molding LFRTP pellets because of its high productivity. However, the long fiber of LFRTP pellet, whose length is same as pellet length, is degraded during processing if conventional injection molding machines are used, and as the result, the mechanical properties are not improved as expected in many cases. Therefore, a new molding process is required to make good use of LFRTP pellets. Recently, a new molding process has been developed appropriate for LFRTP pellets. It is possible for this new molding machine to make fiber length longer in the molding than universal injection molding one. However, the deterioration of the filamentization becomes the disturbance of the improvement of the mechanical property. Then, the quantification of the filamentization was tried in this study. In addition to it, the experimental strength was compared with the theory strength that considers fiber length, fiber concentration and fiber orientation. As the result, we were able to prove the effectiveness of quantification technique of the filamentization. Then, it was possible to grasp the relationship between tensile strength and filamentization, and the importance of filamentization was confirmed.
Recently, the position sensitive proportional counter (PSPC) has been becoming popular as a detector for the X-ray stress measurement. However, little information is available in the literature regarding the effects of the systematic errors on the stress measurement using PSPC. In this paper, the collimator misalignment is discussed using a model and a simulation method of the Ω assembly X-ray stress measurement. As one of the results, it was found that the translation and/or the rotation of collimator from the normal position yield error in stress measurement depending on the misalignment conditions. The error in stress measurement due to the collimator misalignment is expressed as a linear relation of the ratio t/R0 or κ/R0 (t: translation, κ: rotation, R0: goniometer radius) for the translation or the rotation, respectively. If the collimator is misaligned under the combination of translation and rotation, the error can be expressed as the sum of the error caused individually by translation and rotation. If the stress is measured under the combination of specimen mis-setting and collimator misalignment, it was found that the error is the sum of the error caused individually by specimen mis-setting and collimator misalignment.