Kenjiro Komai

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 AlZnMg 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.

Hygrothermal degradation and fracture process of advanced fibre-reinforced plastics

Abstract An investigation has been carried out on the effects of water absorption on the mechanical properties and fatigue strength of unidirectionally reinforced carbon-epoxy (CFRP) and aramid-epoxy (AFRP). T-1/347 normal-type CFRP absorbed more water than MM-1/982X heat-resistant-type CFRP. After a long immersion of about 7 months, however, MM-1/982X composites absorbed water rapidly and the fatigue strength was considerably decreased. In T-1/347 wet specimens preconditioned in water for 2 months a high amount of water absorption degraded the matrix and decreased the strength of the fibre-matrix interface, thereby worsening the mechanical properties. For MM-1/982X composites a small amount of water absorption moderately decreased the interfacial strength and increased the ductility of the matrix, thereby improving the mechanical properties. However, the fatigue lives of wet specimens of both CFRPs were decreased in water. Water absorption lowered the yield strength of the epoxy resin, thereby changing the shape of the stress-strain curves of AFRP. This had no effect on the tensile strength of the composites. Water absorption increased the fatigue strength of both Duponts Kevlar-49 and Teijins Technora AFRP. The water absorption increased the ductility of the matrix and enabled local realignment of the fibres. The Technora composite had a stronger interface than the Kevlar-49 composite. Water absorption lowered the interfacial strength of both AFRPs and changed the failure mechanisms. In these tests the synthesized evaluation of acoustic emission (AE) signals using several AE parameters was carried out and the fracture mechanisms were discussed in terms of the observations of internal damage by a scanning acoustic microscope and of fracture surfaces by a scanning electron microscope.

Failure analysis and prevention in SCC and corrosion fatigue cases

Failure cases concerning stress corrosion cracking (SCC) and corrosion fatigue (CF) have been surveyed, and the overall trend of the cases and correcting measures were summarized. 60% of SCC occurred in stainless steels, and special attention should be paid when stainless steels, above all austenitic stainless steels, are used. In most cases of SCC, residual stress induced by welding and chloride environment were important, and the time to SCC failure was 1 to 2 years. The most important corrective measure in SCC cases was material change. Half of the stress sources in CF cases were mechanical stress, but special attention must be paid that the remaining 45%, which were caused by resonance and/or vibration and cyclic thermal stress. The most important corrective measure in CF cases was structural modification to lower the stress concentration and design stress.

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.

CORROSION-PRODUCTS INDUCED WEDGE EFFECT AND FATIGUE CRACK GROWTH OF STAINLESS STEELS

ABSTRACT In martensitic type stainless steel (SUS410) crack closure behavior was affected by solidlike and viscoelastic corrosion-products induced wedge effect; the load sharing capacity of the former was exclusively effective resulting in a raised stress intensity to close the crack, thereby reducing growth rates. In austenitic type SUS304 no wedge effect was observed. da/dN of the former was accelerated by stress-assisted dissolution and hydrogen embrittlement. da/dN of the latter was accelerated by water molecule adsorption induced reverse slip suppression, in addision to the preceding two factors.

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.

Influence of Water Absorption on Impact Fracture Behavior and Residual Strength after Impact of CFRP/Aramid Honeycomb Core Sandwich Panels

An investigation has been carried out concerning the influence of water absorption on the impact fracture behavior and the residual strength after impact of CFRP/Aramid honeycomb core sandwich panels. Some specimens were manufactured by a hotpress molding method and others by an autoclave one. Low velocity impact tests were conducted using a falling weight tester. When the applied impact energy was higher than the threshold one, below which no damage of honeycomb core was introduced, the percentage of the absorbed energy became large. Although the SEM observation showed the degradation of the fiber/matrix interfacial strength by water absorption, no influence of water absorption on the impact induced delamination area was observed. The residual strength after impact was evaluated by compression test and four point bending test. Due to voids and poor adhesion between facing and core materials of the hotpress specimens, the residual compressive strength of the hotpress specimen was smaller than that of the autoclave one.

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.

Influence Of Vacuum And Water On Tensile Fracture Behaviour Of Aramid Fibres

This paper presents the influence of vacuum and water absorption on the tensile fracture behaviour of four different types of aramid fibres: Kevlar 29. Kevlar 49 and Kevlar 149. manufactured by DuPont and Technora by Teijin Co. Ltd Single fibre tensile tests were conducted in air and in vacuum for fibres preconditioned in air, water at SOT for two months and in vacuum for about four hours The mechanical properties of aramid fibres, longitudinal elastic modulus and fracture strength, decreased by water absorption. The fibre fracture was associated more with fibre splitting, when preconditioned in water. When tested in vacuum, the air-preconditioned fibre strength decreased form that conducted in air. Moreover, the strength of the fibres conditioned in vacuum further decreased when tested in air, compared with that of the air-conditioned fibres tested in vacuum The fibre surfaces were closely examined with an atomic force microscope (AFM). The AFM has nanometric resolution and thereby the fibre surface damage caused by water absorption, high temperature and preconditioning in vacuum could be visualised and the degradation and fracture mechanisms of the fibres were discussed