The surface images of softwood pulp fibers were investigated with an atomic force microscope (AFM). Microfibril-like structures were observed on the fibers. Periodic domains were observed on the fibrils, and they lay in a row at every 70nm. The surface structures were discussed in terms of the autocorrelation function calculated from the AFM images. The correlation length, ξ, obtained from the function was 70nm. This value corresponds to that directly confirmed in the original AFM images. The autocorrelation function can be converted into the power spectral density (PSD). The PSD decayed versus the absolute value of wave number vector, k, according to a power-law. The exponent value was-2, which indicates that no fractal structures exist on the fiber surface. The other power-law decay was observed for the small-angle x-ray scattering (SAXS) intensity of the fiber. The intensity followed Porod's inverse fourth power law. These two results indicate that the surface structure of the fiber is definitely bordered. The coincidence of the two different analyses implies that the PSD obtained from the AFM images can be interpreted as a scattering image of surface under the ambient condition.
Silicate layer was prepared on the nylon 6 film by sol-gel method. Effect of reaction conditions on the various properties, such as layer thickness, surface morphology and the water vapor permeation rates were examined. Increasing sol-gel reaction time gives a increasing silicate layer thickness. However because of the roughness of the surface produced due to the 3-dimensional siloxane network formation in sol-gel reaction system occurred at a longer reaction time, the water vapor permeation rate was high. Addition of DMF in sol-gel reaction system controlled the drying of coated film and suppressed the shrinkage of silicate layer resulting in the smooth crack-free layers. When UV was irradiated on the sol-gel reaction system, the surface of the coated film was rather smooth and the coated film gave a fairly high barrier to water vapor, although the thickness of the layer was rather small.
To study the effect of aging on the mechanical strength of uniaxially thermoformed thermoplastics, we carried out uniaxial tensile test of thermoformed poly (vinyl chloride) (PVC) aged for varios time periods. The ductility of thermoformed specimens in the direction transverse to that of stretching was reduced by aging. At a high elongation ratio, the ductility in the direction of stretching was not reduced by aging. Thus, the ductility is probably dominated by not only the relaxation from the quenched structure but also the relaxation of strongly stretched polymer chains. These data bring us to predict the thermoforming condition avoiding the reduction of ductility for aging.
Tensile strength of epoxy resin specimens subjected to various mechanical and thermal treatments was studied: The mechanical treatment was imposition of finite tensile strains in either glassy or rubbery state and the thermal treatment was annealing or quenching. Specimens quenched from 180°C (> the glass transition temperature) were the most ductile, whereas specimens strained at 180°C and subsequently quenched were a little less ductile than the specimen quenched only. Fully annealed specimens showed the least ductility. Specimens strained at 140°C (< the glass transition temperature) and quenched were more ductile than the fully annealed specimens, yet less ductile than the specimens strained at 180°C and quenched. The variation in ductility of the specimens was discussed in terms of the degrees of their thermodynamic structural equilibrium and storage of strain energy caused by the mechanical and thermal treatments.
Polymer reaction of polycarbonate (PC) as a model of active protective materials has been studied. The specimen molded by injection was decomposed with steam at 120°C and subsequently treated with Na2CO3 at 130°C for 22 days for a preliminary experiment. The molecular weight increased slightly by the treatment. The effects of the deterioration period, temperature, the amount of Na2CO3 and the polymerization methods on the recovery of molecular weight and the kinetics have been conducted. It suggested that (1) phenyl and phenoxy groups at the end of the polymer chains are indispensable to re-combine the ends, (2) the reaction rate of the specimen decomposed for a longer time was higher, (3) Na2CO3 could accelerate the recover reaction, and (4) the deterioration and the recover reactions co-exist. The system of the self-repair and the concept of “passive protection” and “active protection” are discussed. So-called “intelligent materials” are more sophisticated than the self-repairing materials studied here because the intelligent materials have the ability of understanding the reason as the definition of “intelligent”.
The molding condition and mechanical properties of the unidirectional UHMW-PE fiber reinforced linear lowdensity polyethylene (LLD-PE) composites were investigated. The most important fabrication parameter of thermoplastic materials used for such a composite was molding temperature. First differential scanning calorimetry (DSC) (thermal analysis) was performed and the melting point for UHMW-PE fiber and PE matrix film was measured. Mechanical properties of UHMW-PE fiber by high heat treatment were investigated. Then, simulate DSC measurements were performed for specimens subjected to several types of thermal history. Time-temperature profile in simulate DSC measurements referred to the actual processing conditions. The effects of molding temperature on mechanical properties were investigated. At the molding temperature observed re-fusion peak of UHMWPE fiber in DSC measurements, mechanical properties in longitudinal direction was maintained. On the other hand, mechanical properties in transverse direction were highest value. From these results comparing with the theoretical value by Law of mixture, it is considered that interfacial properties of PE/PE composites were changed by the molding temperature. PE/PE composites with optimum molding condition could be fabricated by using the suitable molding temperature between PE fiber and matrix.
In order to develop the new composite material with an additional function, a new concept of affordable interphase has been applied to vinyl-ester resin composite. Fabrication methods of glass woven fabric vinyl-ester resin composite with affordable interphase were proposed and the mechanical properties were investigated. The method using alkyd of an unsaturated polyester resin which has low modulus and high fracture toughness for the resin of affordable interphase was the most appropriate. The alkyd was applied on the glass fabric, and then the laminates were fabricated by hand lay up method using vinyl-ester resin as matrix. Tensile test for the specimens with different interphase conditions was performed and the crack propagation behavior was observed. The tensile strength of the specimen with affordable interphase was higher than that of the specimen without affordable interphase. The strain at which the first transverse cracks observed became higher on the specimen with affordable interphase, because the cracks hardly propagated into high toughness resin area. Consequently it results in the high tensile strength.
Structural information of the styrene-ethylene glycol dimethacrylate (St-EGDM) cross-linked copolymers obtained by Pyrolysis-gas chromatography (Py-GC) was compared with those obtained by conventional dynamic mechanical analysis and swelling measurement. The monomer ratios of St/EGDM in the highly cross-linked polymers determined by Py-GC were in fairly good agreement with the monomer feed ratios, while those estimated indirectly by the conventional methods were considerably higher than the feed ratios. Furthermore, the St contents estimated by Py-GC for the polymer samples obtained in low conversion were found to be extremely lower than the feed ratios suggesting that the cross-linking domains would be formed at the early stage of the polymerization. These observations suggest that the network structures of the cross-linked polymers in this work would be inhomogeneous to a great extent mainly due to the potential presence of the cross-linking domains in which EGDM monomer units are localized.
In order to study the effect of as-cast surface on the fatigue properties of cast aluminum alloy, 4 point bending fatigue tests were carried out in air and sprayed 3.5vol% salt water. As the results, no effect of the as-cast surface on the fatigue properties was found both in air and corrosion environment. This was mainly caused by the effect of shrinkage cavity in surface layer, which was responsible as a fatigue crack initiation site: The very thin surface layer hardly showed resistance to fatigue crack initiation. That means that the presence of the as-cast surface brought no improvement of the fatigue strength.
Impact tension was applied to a PMMA (polymethylmethacrylate) strip specimen with a notch. An inclined notch was introduced by milling specimens before the experiments. The fracture strains εnf and εsf were measured near the notch root and at the smooth portion, respectively. The strain concentration factor defined by the ratio Kf=εnf/εsf was a constant to the inclination angle of notch, θ. However, the tendency of factor, Ke, in the elastic region for θ was a curvy. The factor Ke of the short specimen was larger than that of the long one. Numerical analysis showed similar results. The impact strength of the strip with a notch could be estimated by the simple strain rate near the notch root.
The effects of microstructure and constraint on the process of micro-void coalescence type of fracture and toughness of α/β titanium alloys with equiaxed α phase were investigated. Materials tested were 3 types of titanium alloys (Ti-6Al-4V, Ti-4.5Al-3V-2Fe-2Mo, and Ti-10V-2Fe-3Al alloy) and two or three different heat treatments were conducted for these three alloys in order to obtain different α grain size. Fracture toughness testing was performed and JIC and Tmat values were evaluated. Correlation between the toughness and the ductility under the high triaxial stress state was investigated to clarify the dominant factors constructing the fracture toughness. It was shown that the refinement of microstructure leads to the reduction of ductility and fracture toughenss because the most of micro-voids in α/β titanium alloys were nucleated at α/β interface. However, near-β titanium alloys with ultra-fine particle of α (<1μm) have exceptionally a different tendency from other α/β titanium alloys. About the size effects, the difference of specimen thickness hardly affects on JIC value but Tmat in paricular material was significantly influenced by the specimen thickness, corresponding to the dependence of triaxiality on the ductility.
Dynamic branching fracture phenomena are often observed in brittle materials. Prediction of such fracture path is extremely important not only for academic interest, but also to prevent catastrophic failure of structures and to establish a safety design methodology. In previous studies, we elucidated the governing condition of dynamic branching fracture phenomena from experimental results, and also succeeded the generation-phase simulation for dynamic branching fracture. In this study, we develop a fracture-path prediction simulation method for dynamic branching fracture phenomena. In these simulations, the dynamic fracture mechanics parameters for shortly bifurcated cracks are accurately evaluated by a switching method of the path independent dynamic J integral. By using the maximum hoop stress criterion and the local symmetry criterion, the fracture-path prediction simulations are carried out. The results of both prediction theories excellently agree with the experimentally observed actual fracture paths.
The aim of this paper is to investigate the mixed mode fracture criteria of an interface crack based on a cohesive interface model proposed by Ma and Kishimoto (1996). The stress intensity factors of an interface crack are influenced by the definition of the characteristic length. By the numerical analysis under the small scale yielding condition, the characteristic length has been related to the crack tip radius. In this study, the mixed mode fracture tests for the various types of adherent specimens were carried out. The mixed mode dependence of the fracture toughness and the fracture boundary curves based on the stress intensity factors were discussed. The material parameters of the Ma-Kishimoto model were decided from the experimental results. This constitutive relation was embedded at the crack tip of the interface and the propagation of the interface crack was simulated by the Finite Element Method. The interface fracture toughness and the interface stress intensity factors obtained by the numerical calculation were in good agreement with the experimental results.
Free-edge stress singularity develops near the intersection of the interface and the free-surface of bonded dissimilar materials. Fracture at bonded dissimilar materials may therefore occur from an interface crack near the intersection of interface and free-surface. In this study, the relationship between the stress intensity factor of a small edge crack on interface of bonded dissimilar materials and free-edge stress singularity under external loading was investigated numerically by using the boundary element method. The relationship was also investigated theoretically by using the principle of superposition. The results of numerical analyses were compared with those of theoretical analyses. It was found that stress intensity factors K1, K2 were proportional to the intensity of free-edge stress singularity Kσ. The numerically determined proportional coefficient between K1 and Kσ agreed well with the theoretical one, and was not affected by crack length.
Cyclic softening and hardening behaviors are observed in metallic materials which undergo continuous stress. As a typical example, it is known that a cyclic softening behavior will be accepted in the material strengthened by hot treatment etc. and a cyclic hardening behavior will be accepted in the annealing material. In this study, we examined the relation formula between the X-ray parameter and the fracture mechanics parameter which undergo variable loading in the SS400 steel as a cyclic hardening material. In addition, we examined the HW685 steel, a cyclic hardening material having the same carbon content in the similar way for comparison. Furthermore, we summed these data up and examined the relationship between the X-ray parameter and the fracture mechanics parameter when setting the experiment constant value α to 0.12 from the engineering standpoints. As a result, we obtained the significant viewpoints through this study and we will report them.
In order to clarify the influence of the thickness of specimen on the fatigue crack propagation properties in spherical graphite cast iron, fatigue crack propagation test was carried. Spherical graphite cast iron (FCD450) was used as specimen material. Therefore, this material was conducted two kind of heat treatment process. One is the austemper treatment (ADI) and another is quenching and tempering (QT). As-cast material is added to those, and three kinds of materials are used for the experiment. The fatigue crack propagation test is carried out on the stress ratio R at 0.05 in the room temperature. The specimen is used 1CT type fatigue specimen, which followed ASTM Standard. The thickness of specimens are 12.5mm, 8mm and 4mm·And, it is investigated the correlation between the fatigue crack propagation and measured crack closure of each specimen. The obtained results are as follows: (1) The influence of the thickness specimen is caused from threshold stress intensity factor range, ΔKth to II a stage. The crack propagation speed increase, as the thickness of specimen is less, and ΔKth is lowers. (2) The results of various thickness of specimen agree well in da/dN-ΔKeff curve. Therefore, the influence of the thickness of specimen is mainly depended on the crack closure. The difference of crack closure with thickness is not depended on graphite or fracture surface roughness. It is influenced by the presence of the oxide on fracture surface. (3) It will be cosidered that crack tip shape is influenced on the thickness of specimen.
β-spodumene glass-ceramics containing 30vol% Ni or 30vol% Co were prepared. Their mechanical properties were studied in comparison with the glass and monolithic glass-ceramics with different mean grain size. The R-curve behavior and the COD (Crack Opening Displacement) profiles for these composites were evaluated and the mechanism for fracture resistance was examined. For the composite containing Co, fracture resistance showed remarkable R-curve behavior from KIO=1.6MPa×m1/2 to KR=4.0MPa×m1/2 within short crack extension of 1.0mm. The composite was notably toughened. This result was discussed in terms of the microstructure. The COD measurements in glass showed that the interaction between fracture surfaces was not observed. In the COD profiles of glass-ceramic/metal composites and monolithic glass-ceramics, on the other hand, the experimental values were found to be lower than the theoretical values (Irwin parabola) near the crack tip because the interaction enhanced. Especially, it was obvious that external stress shielding effects acted more strongly near the crack tip in glass-ceramic/metal composites. The increase in the grain size and addition of metal were found to enhance the interaction between fracture surfaces. The relationship between the applied stress intensity (Kappl) and the stress intensity at the tip of crack (Ktip) as a function of Kappl/KIC indicated that metal particles acted as elastic bridging elements in glass-ceramic/metal composites. The size of bridging zone in COD profile for each specimen was clarified to correspond to that of rising extension in R-curve.
The tensile and fatigue tests of 3D-woven Tyranno fiber reinforced SiC matrix composites were conducted at room temperature. The tensile strength was not dependent on loading rate, but the strain at failure increases with a decrease in loading rate from 10 to 0.02MPa/s. The effect of frequency on fatigue behavior was studied between 20 and 0.02Hz with a sine wave form and a stress ratio of 0.1. It was shown that the fatigue failure was cyclic-dependent between 20 and 0.2Hz, but time-dependent between 0.2 and 0.02Hz. The reduction of Young's modulus and sliding stress of interface during fatigue was analyzed. The cyclic-dependent behavior was dominated by wear of interphase and damage on the surface of Si-Ti-C-O fiber during cyclic loading-unloading. The time-dependent behavior was controlled by stress corrosion cracking of fiber and matrix.
This paper describes the effect of thermal aging at 870°C for 8000h in air on mechanical properties of aluminized CoCrAlY coatings and directionally-solidified Ni-base superalloy substrates of an advanced gas turbine blade. Small punch tests in a range from room temperature to 950°C in air have shown a strong temperature dependence of mechanical properties of various coating regions and substrate. The near-surface coating region of unaged and thermally aged blades showed low ductility. The internal and near-interface coating regions and substrate softened at room and elevated temperatures by the thermal aging. Ductility of the internal and near-interface coating regions and substrate at elevated temperatures was substantially degraded. Low cycle fatigue tests at room temperature using a small punch technique have shown that the fatigue lives of the near-surface and internal coating regions were reduced by the thermal aging. In the case of the small punch test in vacuum, ductility at 870°C of the thermally aged coating regions and substrate showed higher ductility than in air. The mechanical degradation at elevated temperatures of the coating is discussed in light of the metallurgical evolution and environmental embrittlement.
Rotating bending fatigue tests were performed in distilled water and in a 3%NaCl solution using smooth specimens of 6063 aluminium alloy. The corrosion fatigue strengths were evaluated and compared with those of 2024 and 7075 alloys. Fracture mechanisms were discussed on the basis of detailed observation of crack initiation and small crack growth behaviour. The fatigue strength of 6063 alloy decreased with increasing aggressiveness of environment, but was nearly the same as those of 2024 and 7075 alloy in a 3%NaCl solution, indicating a considerably low susceptibility to the corrosive environment of 6063 alloy. There was a strong propensity that fatigue cracks were initiated at grain boundaries as environment became more aggressive. Such intergranular crack initiation was attributed to corrosive dissolution of grain boundary precipitates and a precipitation free zone. The crack growth rates of small cracks were enhanced by corrosive environments; particularly remarkable at the early stage of small crack growth following crack initiation in the 3%NaCl solution, where extensive intergranular facets were seen. Therefore, both premature crack initiation and enhanced small crack growth were responsible for the reduction in fatigue strength in the corrosive environment.
In this paper, the surface strain of an Al2O3/Ni-Cr atmospheric plasma thermally sprayed SUS304 steel specimen during the fatigue test (σmax=173MPa, R=0, 873K) was measured with an electronic speckle pattern interferometry (ESPI) system. The relationship between surface strain and crack initiation/delamination behavior was discussed. The strain value obtained from the ESPI system was confirmed to be almost the same as that obtained from a strain gauge on an un-sprayed specimen when tensile stress was loaded at 293K. Thermal expansion deformation and stress deformation at high temperatures were easily measured with the ESPI system. The presence of cracks and delamination on a sprayed coating can be detected in situ and the strain concentration or disappearing can be detected without damage. The sprayed specimen surface strain was almost the same as the un-sprayed specimen at 873K, which indicates that deformation of the sprayed coating is always associated with substrate surface deformation at high temperature. The maximum surface strain was lower after 1×105 cycles fatigue than after 2 cycles fatigue. Surface cracks occurred but stopped at the inner Ni-Cr layer after 2 fatigue cycles at 873K. Many surface cracks and delaminations along the interface between the Ni-Cr layer, and substrate were confirmed after 1×105 fatigue cycles.
The purpose of this investigation is to clarify the influence of an internal fatigue crack on surface actual stresses in the circumferential distribution. The cantilever rotary bending fatigue tests were conducted on shot-peened ductile cast iron specimens. The actual stress distributions in the surface at the maximum tensile applied stress were dynamically measured by the X-ray stress measurement based on single exposure technique during fatigue testing. The circumferential distribution changed with increase in the number of cycles. Residual stress distributions were also measured under unloading. The possibility to detect the internal crack using change in these distributions was discussed. As a result, from observation of fracture surfaces, it was confirmed that the peak position in the actual stress distribution agreed with the internal crack position. The internal crack causes the peak in the distribution. The method proposed in this study is useful for detecting a position of an internal crack on specimen circumference. On the other hand, the residual stress distributions showed no significant change even where the internal crack existed, whereby the internal crack could not be detected by change in the residual stress distribution.
A new method is proposed to estimate nondestructively the distribution of the equibiaxial residual stress beneath the surface by using an Eulerian cradle type goniometer. By maintaining a constant value of the X-ray incident angle, the probing depth of X-rays is kept nearly constant, even though sin2ψ is changed. From the slope of the linear relation between the diffraction angle and sin2ψ, the stress value is calculated for a given probing depth. This new method is applied to estimate the residual stress of a shot-peened steel sample, whose compressive zone extended about 150μm below the surface. High energy X-rays of 72keV from synchrotron radiation source, SPring-8, was used for stress measurements. The residual stress distribution estimated by the new method agreed well with that determined by the conventional sin2ψ method with Cr-Kα radiation combined with the method of the successive removal of the surface layer by electropolishing.