Terufumi Machida

Tensile fatigue of a laminated carbon–carbon composite at room temperature

Abstract The tensile fatigue behavior of a cross-ply carbon–carbon (C/C) laminate was examined at room temperature. Tension–tension cyclic fatigue tests were conducted under load control at a sinusoidal frequency of 10 Hz to obtain stress–fracture cycles (S–N) relationship. The fatigue limit of the C/C was found to be 213 MPa (93% of the tensile strength), and no fracture was observed at over 104 cycles. The residual tensile strength of specimens that survived fatigue loading was enhanced with increase in fatigue cycles and applied stress. Observations of the fatigue-loaded specimens revealed that the formation of micro-cracks at the fiber–matrix interfaces was facilitated during fatigue loading. These interfacial cracks were concluded to protect the fibers from being damaged by matrix cracks and this behavior was considered to be the governing mechanism of strength enhancement by fatigue loading.

Effect of Stress Concentration on Tensile Fracture Behavior of Carbon-Carbon Composites:

The effect of stress concentrations on tensile fracture behavior of carbon-carbon (C/C) composites was investigated using circularly holed specimens and double-edge-notched (DEN) specimens. As for the circularly holed specimens, the tensile fracture stress was much higher than that estimated from the maximum stress criterion, which suggest that major stress relaxation mechanisms should exist. On the other hand, the linear elastic fracture mechanics can be applied to the DEN specimen, which means the damaged zone should be small enough compared with the notch length. In order to discuss the magnitude of the stress relaxation, damaged regions of the two kinds of testing geometry were estimated using the point stress criterion. The estimation led to remarkable difference in the size of the damaged regions, which will explain the difference in the magnitude of the stress relaxation. Through the observations of fractured specimen, it was deduced that not only the shear deformation but delamination along fiber bundles and opening of transverse crack would relax the stress concentrations. The other mechanism was also proposed based on the testing results, that is strength increase in the damaged region.

Measurement of the Mechanical Properties of the Tympanic Membrane with a Microtension Tester

The mechanical properties of guinea pig and human tympanic membranes were measured by a microtension tester newly developed by us. Guinea pig tympanic membranes were tested under various conditions. When the tensile rate was 4.17 x 10(-4) m.sec-1 fresh specimens were 8.8 times stronger for a tension applied parallel to the radial fibres than for that applied at a right angle to them. No difference in tensile strength was seen when the tension was applied at a right angle to the radial fibres. When the tensile direction was parallel to the radial fibres, the posterior part was stronger than the anterior part. Two human tympanic membranes were measured under formalin fixation. The measured values were calculated into those of the fresh specimens based on the results of the guinea pig material. The adjusted values for fresh human specimens were 2.56 kgf.mm-2 at a right angle to the radial fibres and 3.28 kgf.mm-2 parallel to them. Applying the thin cylinder and shell theory, the breakage pressure of the human tympanic membrane was calculated to be 1.02-22.4 atmospheric pressure.

The Physical Strength of the Membranous Labyrinth and Its Relation to Endolymphatic Hydrops

Pathological findings of Meniere’s disease were first reported by Hallpike and Cairns [1]. The main finding was an enlarged endolymphatic space of the cochlear duct and saccule which was called “endolymphatic hydrops.” Subsequent histopathological studies on temporal bones with Meniere’s disease have confirmed the presence of endolymphatic hydrops in the cochlea and saccule, although some investigators reported enlargement of the utricle or deformity of the semicircular canal [2, 3].