Hiroshi Hatta

Comparison of 2D and 3D carbon/carbon composites with respect to damage and fracture resistance

Abstract The static mechanical responses of two- and three-dimensionally reinforced carbon/carbon composites (2D- and 3D-C/Cs) were compared. The mechanical properties examined included tensile and shear stress–strain (S–S) relations, and fracture behavior using compact tension and double edge notch configurations. Compared with 2D-C/Cs, 3D-C/Cs were shown to possess a similar tensile S–S relation, lower shear strength, higher ultimate deformation in shear, and much higher fracture resistance. The differences in shear and fracture resistance were shown to be derived from a weaker fiber/matrix interface and weaker bonding between fiber bundles in the 3D-C/Cs. These weak interface characteristics of 3D-C/Cs are due to the high value of residual stresses caused by the three-dimensional fiber constraint of 3D-C/Cs.

Strength improvement by densification of C/C composites

Abstract The tensile strength of carbon/carbon composites (C/Cs) was examined as a function of the density in the range above ρ=1.6 g/cm3. Three processing routes of preformed yarn, resin char, and HIP processes were adopted to densify the C/C composites. The density was increased to 1.95 g/cm3 through these routes. The interfacial strength between the fiber and matrix was significantly and unexpectedly varied by repeating the HIP process, though the detailed source mechanisms could not be identified. The tensile strength of the densified C/Cs was shown to be dependent on the processing routes. In particular, opposite tendencies were obtained as a function of ρ, i.e. the monotonic strength increases for the C/Cs with the preformed yarn and resin char treatments, while a monotonic decrease was observed using the HIP process. However, after the tensile strength was re-examined, the ultimate tensile strain was found to be expressed by a monotonic decreasing function of the interfacial strength. This implies that weakening fiber interfaces is a key factor for obtaining C/C composites with the higher tensile strength.

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.

SiC/C multi-layered coating contributing to the antioxidation of C/C composites and the suppression of through-thickness cracks in the layer

Abstract Silicon carbide/carbon multi-layered coatings have been formed on the surfaces of C/C composites. The multi-layered coatings were deposited by the CVD method at 1200°C using SiCl 4 , C 3 H 8 and H 2 gases. The principal idea of the present multi-layered coatings is to suppress the through-the-thickness coating cracks by making the thickness of each SiC and carbon layer lower than the corresponding critical thicknesses, h c s. The values of h c s for the SiC and carbon coatings were 0.2 μm and above 15 μm, respectively. The role of the SiC layers is oxidation protection and that of the carbon layers is to mechanically isolate each SiC layer. Based on theoretical and experimental discussions, the SiC/C multi-layered coating was shown to successfully suppress the through-thickness coating cracks.

A Comprehensive Model for Determining Tensile Strengths of Various Unidirectional Composites

Simultaneous fiber-failure (SFF) model that determines tensile strengths for various systems of unidirectional composites comprehensively is presented. The SFF derives the strength of unidirectional composites as a function of the single-fiber strength distribution, interfacial shear strength, and matrix strength. The point of the SFF is that a fiber group which is considered to be experiencing simultaneous fiber failures triggered by neighboring fiber failures is assumed to fail when the weakest fiber in the fiber group fails. We discuss a method to determine the magnitude of the fiber group for various systems of composites on a basis of whether a crack located near a bi-materials interface penetrates into another material or deflects along the interface. The SFF is established by integrating the magnitude of the simultaneous fiber failures into the conventional Global load sharing model.

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.

Interfacial shear strength of C/C composites

Abstract Fiber-bundle push-out, single-fiber push-in, and single-fiber push-out tests were conducted in order to examine the applicability of these methods for determining the interfacial shear strength of carbon–carbon composites. The fiber-bundle push-out test resulted mostly in fractures along the fiber/matrix interface but created a small amount of fractures in the matrix. Hence, the evaluated strength was regarded as an approximate value. In order to precisely evaluate the interfacial strength, push-in and push-out tests for a single fiber were performed using a micro-Vickers indentation tester. In these tests, the load has to be placed within a target fiber, and the indentation should not extend to the matrix. This condition restricted the load that could be applied to a carbon fiber. Within this limit, a single carbon fiber could not be pushed-in. For the sake of load reduction, single-fiber push-out tests were conducted using thin specimens. The thickness appropriate for a single-fiber push-out specimen was estimated based on the interfacial shear strength obtained by the bundle push-out tests. Below this thickness, single-fiber push-out tests could be successfully performed.

Effects of dispersed microvoids on thermal expansion behavior of composite materials

Abstract In our earlier papers [T. Takei, H. Hatta, M. Taya, Mater. Sci. Eng. A131 (1991) 133; T. Takei, H. Hatta, M. Taya, Mater. Sci. Eng. A131 (1991) 145], we found that the coefficients of thermal expansion (CTEs) of particulate composites are lowered by dispersed microvoids in the matrix. In order to prove this void dispersion effect by an analytical model and to utilize this effect for wide control of composite CTE, an analytical parametric study is conducted in this paper. It is found in this study that the combination of fiber type reinforcement with high elastic modulus and low CTE and disk-shaped voids was identified to remarkably lower the composite CTE. This CTE control technique is based on the mechanism that microvoids in the matrix are compressed by compressive stress field developed around reinforcement when the composite is subjected to temperature rise. In order to confirm experimentally this void dispersion effect on CTEs, three types of composite materials with dispersed voids in their matrices were fabricated, particulate, short fiber, continuous fiber (3D) reinforced composites, and CTEs of these composites were measured. It was concluded from comparison between the predictions and experimentally observed CTEs that the CTE reduction by the void dispersion actually occurs and this effect is analytically predictable up to some limit volume fraction, which depends on a composite type.

Thermal Conductivity of Hybrid Short Fiber Composites

A combined analytical/experimental study has been undertaken to in vestigate the effective thermal conductivity of hybrid composite materials. The analysis utilizes the equivalent inclusion approach for steady state heat conduction (Hatta and Taya, 1986), through which the interaction between the various reinforcing phases at finite con centrations is approximated by the Mori-Tanaka (1973) mean field approach. The multiple reinforcing phases of the composite are modeled as ellipsoidal in shape and thus can simulate a wide range of microstructural geometry ranging from thin platelet to con tinuous fiber reinforcement. The case when one phase of the composite is penny-shaped microcracks is studied in detail. Multiphase composites consisting of a Kerimid matrix and Al 2O3 short fibers and Si3N4 whiskers were fabricated and after a careful study of their microstructure, their thermal conductivities were measured. Analytical predictions are shown to be in good agreement with experimental results obtained for the Al2O 3/Si3N4/ Kerimid short fiber composites.

Shear fracture of C/C composites with variable stacking sequence

Abstract Cross-ply laminated carbon–carbon composites with changing ply ratio of 0° and 90° were fractured under shear using several test methods. In these tests, special attention was placed on the understanding of the damage mechanisms. The ±45° off-axis shear test yielded the most precise shear stress–strain relation and was recommended to measure the shear modulus G 12 . On the other hand, for shear strength measurement, the Iosipescu test was superior. The non-linearity in the shear stress–strain curve was found to be solely due to matrix cracking, while the peak shear stress was determined by fibre failure. In order to measure the shear strength by the Iosipescu test, two fracture planes normal and parallel to the loading axes have to be considered. The shear strength increased linearly with fibre volume fraction normal to the corresponding plane. The overall highest shear strength was obtained for the (0/90) n laminate.