Kohji Minoshima

Tensile and fatigue fracture behavior and water-environment effects in a SiC-whisker/7075-aluminum composite

Abstract An investigation has been carried out of the tensile and fatigue fracture behavior and of the influence of water environments on fatigue strength of a SiC whisker reinforced high strength Alue5f8Znue5f8Mg alloy composite (SiC w /7075-T6) fabricated by a powder metallurgy process. Apart from the elongation at failure, the mechanical properties of the composite were superior to those of an unreinforced 7075-T6 alloy. The fatigue strength in air of the composite at a stress ratio of 0·1 was higher than that of an unreinforced aluminum alloy. At lower stress level, a water environment decreased the fatigue strength of the composite, although the strength was superior to that of an unreinforced aluminum alloy in air. A fatigue crack initiated at a whisker cluster or a crack normal to the loading axis directly initiated at the specimen surface. In water, a fatigue crack was initiated at a corrosion pit.

Characterization of the aramid/epoxy interfacial properties by means of pull-out test and influence of water absorption

Abstract Single fiber pull-out tests were carried out to investigate the influence of water absorption on the interfacial properties of aramid/epoxy composite. The fiber/matrix interfacial strength was severely decreased between 4 and 7 week immersion time in deionized water at 80xa0°C, and thereafter showed a plateau. This change with immersion time did not correspond with that of the water gain of the pull-out specimens, because the water gain did not reflect the one in the fiber/matrix interface. As a result of the degradation of the fiber/matrix interfacial strength, the pulled-out fiber surfaces of 7, 10 and 13 week wet specimen were smooth. In situ observations of interfacial crack propagation by a video microscope and an analysis of acoustic emission (AE) signals showed that AE signals obtained during the pull-out process were classified into four types according to fracture modes. AE signals detected at final unstable crack propagation and fiber breakage had high amplitude and long duration.

The influence of vacuum on fracture and fatigue behavior in a single aramid fiber

Abstract To perform micro-mechanical tests including fatigue in microelements, a specially designed fatigue testing machine has been developed. The testing machine was designed to perform fatigue tests in microelements under simple- or three-point bending as well as under uniaxial loading. The testing machine is equipped with an environmental chamber to control the testing environment, and thereby the testing can be done in controlled environments such as vacuum or (humid) gases. Using the system, quasi-static and fatigue tests on aramid single fibers (Kevlar 49®, diameter: about 12 μm) were performed. Kevlar 49 is the trademark of DuPont. The tests showed that the strength of the aramid fibers is strongly influenced by environment. Water absorption and vacuum conditioning decrease the quasi-static fracture strength. Under fatigue loading, the aramid fiber has excellent properties compared with metallic materials, showing gentle slopes in S–N curves, although large scatter bands exist. The fatigue strength in vacuum is higher than that in air. The fiber breaks with fiber splitting. The fiber surface damage induced by fatigue loading in air and in vacuum was examined by using an atomic force microscope.

Effects of stress waveform and water absorption on the fatigue strength of angle-ply aramid fiber/epoxy composites

Abstract The influences of stress waveform and water absorption on the tension–tension fatigue fracture behavior were investigated in ±45° angle-ply laminates of aramid fiber reinforced epoxy matrix composite. For dry specimens, the fatigue strength under negative pulse waveform was higher than that under the positive pulse waveform. Rotation of fibers to the longitudinal direction, which resulted from creep deformation caused by the cyclic loading superimposed on the maximum stress hold time, decreased the compliance, thereby increasing the fatigue life under the negative pulse waveform. Water absorption degraded the fiber/matrix interfacial strength and caused the swelling of the matrix, which resulted in decreases in the static tensile strength and fatigue strength. Although the strength under the negative pulse waveform was slightly higher than that under the positive one, the influence of stress waveform on fatigue strength was smaller in wet specimens.

Evaluation of stress corrosion resistance and corrosion fatigue fracture behavior of ultra-high-strength P/M Al–Zn–Mg alloy

Abstract Quasi-static tensile tests in air and slow strain rate tests (SSRTs) in a 3.5% NaCl solution were conducted in an ultra-high-strength P/M Al–Zn–Mg alloy fabricated through powder metallurgy. Attention is also paid to fatigue strength and fatigue crack growth behavior in laboratory air and in a 3.5% NaCl solution. The alloy has extremely high strength of about 800xa0MPa. However, elongation at break remains small, at about 1.3%. The final fracture occurs by a macroscopically flat crack normal to the tensile axis, with little reduction in area and little shear lip on the periphery of a smooth sample. However, it fails microscopically in a ductile manner, with dimples. Dimple size is less than 1xa0 μ m, because the grain size of the alloy is extremely small. Strengthening mechanisms operating in the alloy are: small grains, sufficient metastable η ′ phase in a matrix, and intermetallic compound acting as a fiber reinforcement. The SSRT strength in a 3.5% NaCl solution decreases slightly at a very low strain rate, that is smaller than those observed in aluminum alloys sensitive to stress corrosion. This means that the crack initiation resistance to stress corrosion is superior. However, under cyclic loading, the corrosion fatigue strength becomes lower than that conducted in air, because pitting corrosion on a sample surface acts as a stress concentrator. Crack initiation site of quasi-static and fatigue failure of the alloy is at inclusions, and hence, it is essential to decrease inclusions in the alloy for the improvement of the mechanical properties. Fatigue crack resistance of the alloy is inferior to conventional Al–Zn–Mg alloys fabricated by ingot metallurgy, because the fatigue fracture toughness, or ductility, of the alloy is inferior to other Al alloys, and intergranular cracking promotes crack growth. However, no influence of 3.5% NaCl solution on corrosion fatigue crack growth is observed, although an investigation is required into whether stress corrosion crack growth occurs or not, and at the same time, and of corrosion fatigue crack growth behavior at lower stress intensity. The fracture surface and crack initiation sites are closely examined using a high-resolution field emission type scanning electron microscope, and the fracture mechanisms of the alloy are discussed.

Tensile and Tension-Tension Fatigue Fracture Behavior of γ-Al 2 O 3 /Al Metal Matrix Composite at Room and Elevated Temperature

Tensile and tension-tension fatigue (stress ratio: 0.05) fracture behavior of a continuous γ-Al 2 O 3 fiber reinforced aluminum (Al-0.5Ba) matrix metal matrix composite (fiber volume fraction: 50%) manufactured by a squeeze casting process was investigated in laboratory air at room and elevated temperature (573 K). Special attention was paid to fracture processes: in situ observation of static fracture processes was also made in a scanning electron microscope (SEM). Fracture surfaces were closely examined with an SEM, and we have discussed the tensile and fatigue fracture mechanisms. The fiber reinforcement improved the material properties, including tensile strength, elastic modulus and fatigue strength of both [0°], [0°/90°], and [′45°] composites, compared with those of unreinforced equivalent matrix materials, 1XXX series aluminums. The in situ observation in the SEM showed that the static fracture of [0°] and [0°/90°) composites was controlled by failures of 0° plies, and the fracture initiated at broken fibers at the specimen surface. In the case of [′45°] composites, intralaminar failures at the middle position in thickness occurred, leading to interlaminar deformation and failures, and finally unstable fracture. The tensile strength of [0°] composites at 573 K was higher Than that at room temperature. In the case of [0°/90°] composites, there was little difference in strength between room temperature and 573 K. However, strength at 573 K of [′45°] composires decreased from that at room temperature. Both at room temperature and at 573 K, few fiber pullouts were observed and pullout fibers were covered with matrix. Failures of [90°] composites occurred in matrix near fiber/matrix interfaces and fiber splitting also existed. These indicated that the composite had excellent fiber/matrix interfacial strength. Failure mechanisms of fatigue in [0°] composites differed depending on stress level; at high stress levels, final failures were brought about by a macroscopic transverse crack, whereas at low stress levels, longitudinal crack first initiated and propagated in the longitudinal direction, that is, loading axis direction. This longitudinal crack reached the gripped position, and then a macroscopic transverse crack was initiated and propagated, resulting in final failure. In the case of [′45°] composites, macroscopic failures occurred in the transverse direction, irrespective of stress level, and the S-N curve was very flat.

Genetic algorithms for high-precision reconstructions of three-dimensional topographies using stereo fractographs

A software is developed which enables reconstruction of the 3D shape of fracture surfaces without human assistance. It is based upon computer image processing and pattern recognition techniques by using a stereo-pair of scanning electron micrographs. The processing consists of two subprocesses: searching the matching points between two images and computation of heights using het relative shift of the matching points. By using the previously developed system, some mismatches were inevitable in the search process, in particular, for low-contrast SEM images such as striations, intergranular facets, and so on.

Development of an Expert System for Fractography of Environmentally Assisted Cracking

An expert system that diagnoses the causes of failure of environmentally assisted cracking (EAC) based upon fractography has been developed. The system uses the OPS83 programming language, expressing rules in the manner of production rules, and is composed of three independent subsystems, which respectively deal with EACs of high-strength or high-tensile-strength steel, aluminum alloy, and stainless steel in dry and humidified air, water, and aqueous solutions containing Cl, Br, or I ions. The concerned EAC issues cover stress corrosion cracking (SCC), hydrogen embrittlement, cyclic SCC, dynamic SCC, and corrosion fatigue as well as fatigue and overload fracture. The knowledge base covers the rules relating to not only environments, materials, and loading conditions, but also macroscopic and microscopic fracture surface morphology. In order to deal with vague expressions of fracture surface morphology, fuzzy set theory is used in the system, and the description of rules about vague fracture surface appearance is thereby possible. Applying the developed expert system to case histories, accurate diagnoses were made. We discuss the related diagnosis results and usefulness of the developed system.

Fatigue and Fracture Behavior of Carbon Fiber Reinforced Plastic Under Combined Tensile and Torsional Stress and Influences of Water Absorption

Static and fatigue tests of thin-walled, [+45] filament-wound tubular specimens of a carbon fiber reinforced plastic, T300/827, under combined axial tensile and torsional loading have been conducted, and the influences of water absorption on the failure behavior have been investigated. The tests were conducted under a load controlled condition keeping the combined stress ratio, α = τ/ σ. The static strength of dry specimens, which had been held in air, shows good agreement with the Tsai-Hill failure criterion. The static strength of wet specimens, which had been preconditioned in water for two months, at α = 0 - 1 decreased by water absorption, whereas at α = 2 - ∞, where compressive strength in the fiber direction dominates over the failure, a decrease in the strength of wet specimens from dry ones was small. Fatigue strength of dry specimens at α = 0 - 1 was smaller than those of α = 2 - ∞. This indicates that a combined loading mode influences the fatigue strength. A decrease in fatigue strength of wet specimens from air data at α = 0 - 0.5 was higher than those at α = 2 - ∞.

TENSILE AND FATIGUE FRACTURE BEHAVIOR AND WATER-ENVIRONMENT EFFECTS IN SiC WHISKER/7075 COMPOSITE

ABSTRACT The mechanical strength except elongation and the fatigue strength in laboratory air of SiC whisker/7075 composite were superior to an unreinforced 7075-T6 alloy. At lower stress level, water environment decreased the fatigue strength of the composite. AE signal analysis and fracture surface observation by SEM were conducted, thereby discussing fracture mechanisms and environmental influence of water.