This paper describes experimental developments for a concise preparation method of flexible interphase in commonly used GFRP and determination methods of the finite interphase thickness. The preparation scheme of the flexible interphase basically comprises surface coating of glass fibres with toughened epoxy resin. The finite thickness of the flexible interphase is determined by means of scanning electron microscopy equipped with energy dispersive X-ray element analyser and visco-elasticity image analysis featured in a scanning probe microscope. Certain evidences are successfully observed for the flexible interphase to be created by means of the proposed preparation method. However, it is found that the interphase thickness obtained from each method exhibits different values due to the particular sample status. The model composite used in the X-ray method contains a platinum layer as a boundary-indicator between the flexible interphase and the matrix resin. The platinum layer may hinder the interacting diffusion between the interphase and the matrix during the curing process, and this probably causes the broader interphase to remain in the model composite used in the X-ray method. Both methods are found to be useful to evaluate the interphase thickness around the single glass-filament at the microscopic level. However, those results show relatively large scatters; this means that more efforts are still needed to create the interphase uniformly around the glass fibres using more sophisticated surface treatment techniques.
Six different types of ZrO2/Ni composite materials were fabricated by means of the slip casting method to study the effects of composition and dispersiveness on their deformation and fracture properties. The result of three-point bend testing indicated that the composition had significant influence on the bend strength and absorbed fracture energy with sufficient dispersiveness, whereas there was little effect on these properties with insufficient dispersiveness. The result of mode I fracture testing also exhibited that the critical mode I energy release rate, GIc, depended upon the composition, and the sufficient dispersiveness resulted in better GIc than the insufficient dispersiveness. The microstructures and fracture behaviors of the composites were examined using a laser microscope, an optical microscope with an image analysis technique and a scanning electron microscope (SEM). The result of the image analysis showed that the compositions of the sintered materials were different from those of the compacts. The SEM observation also exhibited that in the materials with insufficient dispersiveness, cracks tended to propagate along the interfaces of Ni aggregations, resulting in the degradation of the fracture properties.
This study is concerned with the analysis of relationship between surface structure and the micro voids that are formed by stretching the composite polymer sheet having incompatible polymer particles. Now we tried to simulate the void formation near surface and surface structure considering the interfacial delamination between Poly (ethylene terephthalate) (PET) and dispersed incompatible polymers, and to compare the numerical results by finite element method (FEM) with experimental ones. The following results were obtained; (1) Decreasing interfacial tension coincided with increasing the height of protrusion that was formed by stretching experimentally. Also the critical stress of delamination coincided with increasing the height numerically. (2) Increasing interfacial tension or decreasing the critical stress of delamination coincided with enlarging the aspect ratio of the void near surface. These results are similar to the result of study that was intended for the inside of void expression. (3) It is supposed that a basin would be expressed on surface with void expression from the experimental and numerical results quantitatively.
The purpose of this research is likely to apply biomimetic designed composites to artificial structure. Looking at the bio-joint mechanism, the solid-fluid composite structure of cancellous bone is likely to play a great important role in the load transmission. Therefore, the solid-fluid composite structure of the cancellous bone was modeled to the solid-air biomimetic composites with aluminum honeycomb. The numerical analysis was carried out using the FE model, and it was clarified that the bulk modulus of air and the dimensions become effective for load dispersion.Consequently, the bulk modulus and the dimensions are the important parameters in the design of solid-air composite with the excellent load dispersion characteristic.
Tapered FRP tubes have high energy absorption performance under axial compressive load. FRP will be useful for structural material of a shock absorber in the various industries. And in order to evaluate the behavior of this material, several experimental tests and numerical simulations using FEM are carried out recently. However, it is difficult to simulate properly the behavior of tapered FRP tubes, so called “Progressive Crushing” with FEM, because the fracture mechanism contains various kinds of fractures such as delimitation, fiber fracture and so on. In this study, we proposed a special FEM model based on experimental data that is useful for the crush-worthiness analysis of FRP tube.
Corrosion tests were carried out by using GFRP notched bar specimen in order to establish the guideline of FRP bolt design. Pure water, sulfuric acid solution and sodium hydroxide solution were used as corrosive environment.The change of weight and retention of tensile strength were investigated for these environments. The degree of the degradation was remarkable in order of sodium hydroxide solution, sulfuric acid solution, and pure water.Microscopic observation indicated that glass fiber/resin interfacial debonding was generated by sulfuric acid solution. In sodium hydroxide solution, dissolution of the glass component from glass fiber and decomposition of matrix resin were caused. Colored layer that observed in sodium hydroxide solution was identified with corroded layer that was formed by hydrolysis of esters. The thickness of corroded layer was affected by the orientation of fibers. The effects of notch number (1-5) and pitch of notches (2.5-12.5mm) on the retention of tensile strength was discussed. The degradation of specimens with small number of notches was more noticeable than that with many notches. On the other hand, the decrease of tensile strength was smaller for smaller pitch of notches. These results were supported by the geometry of corroded layer near notches.
This paper presents the effect of post-cure on mechanical properties of CF/Epoxy composites. At first, the shrinkage strain and the mechanical properties such as Vickers hardness of resin, transverse modulus and strength are measured for carbon/epoxy unidirectional laminates after four-hour thermal treatment (post-cure process) at temperatures ranging from 70°C to 200°C. Then, the effect of post-cure temperature on transverse cracking behavior in a cross-ply laminate is experimentally investigated. The change in the residual stress due to post-cure is also estimated for the cross-ply laminate. Finally, the fracture surfaces of transverse cracks are observed using the scanning electron microcopy (SEM) to reveal the effect of post-cure on fracture behavior. It is found that the unidirectional laminate shrinks in only the transverse direction after the post-cure process. The hardness of the resin increases with increasing post-cure temperature below 150°C while transverse modulus and strength are independent of post-cure temperature below 150°C. Transverse cracking in post-cured cross-ply specimens initiates at lower stresses than the critical stresses in virgin specimens. The transverse crack density is predicted by a Weibull's probabilistic failure model. It is deduced from SEM observation of fracture surfaces that the fiber/resin interface is weakened by thermal treatment that may lead to reduction in the transverse strength at 200°C.
The friction and wear properties are investigated in the combination state of pitch carbon fiber reinforced PPS (Polyphenelene Sulfide) composite materials and SUJ2. The tribological tests under the rolling-sliding condition are carried out for the pure PPS resin and three types of PPS matrix composites, which short carbon fiber diameters are varied to 18, 13, 9μm. And, these composites varied to the fiber weight content 15, 30, 45wt% are used as the same test specimens. As a result, it became clear that the different worn patterns on the surface of test specimen were appeared and these patterns changed due to the pitch carbon fiber diameters and the fiber weight content. It was also found that both the coefficient of friction and the specific wear rate decreased and the rolling distance became longer with the increase of the fiber weight content and the decrease of the fiber diameter. Moreover, it was showed that the ratio of interfacial area was an effective parameter in the case of evaluating the rolling distance and the specific wear rate on various composites.
For the application of the Molecular Dynamics to the real system, the accuracy of the empirical interatomic potential is of key issue. In this paper, for the description of the dynamic system including small number of atoms such as gas phase and surface reaction, the empirical bond order potentials for silicon cluster structures are proposed. Since the poor description of Tersoff potential for clusters is considered to be responsible for the angle and coordination dependences of the bond order term, new original forms of the bond order term are proposed. The bulk cohesive energy, lattice parameter, the energy and especially its order of various cluster structures are set as fitting parameters. In addition, effective fitting technique by using the structural energy difference method is also proposed. As a result, the energy order and global minimum structures of Si3-Si5 clusters and the bulk properties can be accurately described by S1b and S2b model. These potentials are excellent as compared with Tersoff type 2 and type 3 potential except for the bulk elastic constants of Tersoff type 3. The potential describing larger than Si6 cluster are created (S1c and S2c model), however it is found that it is impossible to express both bulk and large cluster properties within our scheme. It is considered that the deficiency is caused by the lack of the incorporation of the environmental effect, that is the environmental difference between cluster and bulk, by using our empirical potential forms. Moreover, it is found that the error of structural difference method is very small (averaged error is 1.2%), therefore our proposed technique is proved to be effective for fitting procedure.
This paper estimates the internal stresses of two joined materials which have different initial size displacements. Material [I] is an infinite plate having an elliptical hole. Material [II] is an elliptical inclusion. They have a discrepancy between [I] and [II] of the elliptic boundary. Furthermore, their materials have different mechanical properties. For continuous connection of both boundaries, we provide the boundary tractions and analyze them. The related report published by previous authors did not consider an initial size displacement of both materials boundaries and had several errors at derived formulas. In this paper we show correct results. Several numerical results are given by graphical representation.
Rotors and discs used in low-pressure turbines must be made of a material possessing high strength, high toughness and be capable manufacturing of extremely large components. For these reasons, 3.5% NiCrMoV steel is widely used. It has been previously reported that intergranular stress corrosion cracking in this material is initiated in wet deaerated steam when the temperature reaches approximately 400K. As no rotor and disc material with high strength and high toughness that can serve as a substitute for 3.5NiCrMoV steel has been found, efforts to resolve this problem have been conducted to reduce material strength to lower the susceptibility to stress corrosion cracking. The purpose of this study was to elucidate a relationship between material strength and intergranular stress corrosion cracking susceptibility. SSRT was performed in a neutral atmospheric environment at a test temperature of 403K. The results were as follows: (1) The SSRT found that, as the 0.2% offset strength decreased, the area of the intergranular fracture was reduced. For the test specimen with an 0.2% offset strength of 777MPa, only a tiny intergranular fracture was observed; at 0.2% offset strengths of 697MPa and below, cracking disappeared completely. (2) From the precipitation status of the two carbides M3C and M7C3 precipitated at the grain boundaries, it was found that the disappearance of the M3C carbide coincided with the disappearance of the intergranular fracture. From this observation, it is thought that differences in composition of these carbides have an effect on the susceptibility of 3.5% NiCrMoV steel to stress corrosion cracking. (3) It was confirmed that the M3C carbide disappears when the 0.2% offset strength is approximately 700MPa or below, and that these materials achieve the target value for increased resistance to intergranular stress corrosion cracking in neutral environments.
The ductile/brittle transition condition in an indentation of brittle materials is derived to utilize the prediction of the ductile mode abrading conditions. By modeling the action of an abrasive as a semi? spherical indentation, the ductile/brittle transition condition is represented by a critical radius (ac) of a spherical indenter: at a larger radius than that, a penny-shaped pre-existing crack extends as a median crack in the material. The stress intensity factors (KIC's) of the penny-shaped pre-existing cracks which are located at various depths with radii (c's) under the indentation are analyzed by using the stress fields derived by Chiang et al. The crack length-dependent critical stress intensity factor is applied to the evaluation of crack extension. The critical radii (ac's) of typical structural ceramics such as silicon nitride, silicon carbide, and alumina are deduced to be from 0.5-2.5μm. Although these materials have wide ranged characteristic constants such as Young's modulus, hardness (Hv), fracture toughness (KIC), Poisson's ratio, and mean grain diameter (dg). ac is given by the following rather simple equation under the assumption that the size of the pre-existing crack is the same as that of the mean crystalline grain. ac/dg=0.075·103.7KIC/(Hv√dg) The critical radius ac, which is experimentally evaluated in the view of preventing the bending strength deterioration of indented Si3N4 specimens, is fairly coincident with the analytical one.
Nature of the ctitical crack, which triggers hydrogen assisted fracture of the high strength bolt steel ASTM A490 has been studied with an emphasis on the metallurgical observation of the subsurface quasi-cleavage crack. The macroscopic fracture facet, appeared on the SEM photograph of the fractured surface, is formed by coalescence of a number of neighbouring cracks, which have different initiation sites. Detailed examination with TEM and optical microscope revealed that the initiation and early stage of propagation prefer transgranular mode.The preferencial crack path in this type of high-strength steel traces mostly along the lath or packet boundary of the prior austenite grains.
The flexible disc of Ti-6Al-4V alloy is one of the most significant member in the structural design of superconducting generator rotor. It is necessary to establish the multiaxial fatigue criterion considering the effect of mean stress in order to estimate the fatigue life of flexible disc under the combined load conditions using the operation. In this study, multiaxial fatigue tests of tension-compression and torsional load were carried out by use of thin walled tubular specimens of Ti-6Al-4V alloy to investigate the effect of the amplitude and mean components of axial and torsional stresses on the fatigue strength. Moreover, multiaxial fatigue tests of the flexible disc model specimens under the operating load conditions are performed by use of the exclusive testing machine and the stress conditions are calculated by FEM. The test results are analyzed by means of the equivalent stress involving the effect of mean stress introduced from the Kawata's criterion based on the concept of internal friction theory. The validity of fatigue life prediction by this approach is confirmed for the structural design of superconducting generator rotor.
The objectives of this study are to clarify the electrodeposition mechanism and to select the best electrodeposition conditions for closing cracks of reinforced concrete on land. The contents of this paper are as follows; first, with regard to electrodeposition mechanism, two factors, namely chemical and electrochemical factors were investigated. Next, considering the above mechanism, the best conditions leading to crack closure and carbonate resistance in reinforced concrete on land were selected. The results of this study indicated that the effects of the above factors were summarized and the best electrodeposition conditions were selected.
The applicability of non-destructive corrosion detection using AC impedance method to the specimens is investigated, in which test specimens have been exposed to seashore environment for seven years at three different sites, Matsumae in Hokkaido, Nagoya and Ishikawa in Okinawa. The followings are main conclusions derived from the tests results: a) Steel corrosion in concrete and cracks in concrete cover due to steel corrosion were observed after seven years exposure. Steels in the specimens exposed to tidal condition in Nagoya corroded considerably. On the other hand, steels in the specimens exposed to seaside area in Matsumae and Okinawa corroded a little. b) The corrosion detection technique using AC impedance method can be well used to estimate amounts of steel corrosion of specimens irrespective of environmental conditions.