Shojiro Ochiai

Failure behavior of an epoxy matrix under different kinds of static loading

The yield and fracture behavior of an unreinforced epoxy resin has been investigated. The parabolic Mohr failure criterion was applied to experimental results under different loading conditions. From a plain resin slab, specimens for tensile, torsion and compression tests were manufactured and the failure behavior of the resin was tested and discussed in detail. The results of the mechanical tests and a fractographic study of the fracture surfaces were correlated with the stress-state-dependent strength and fracture stress of the epoxy resin. This plays an important role in fiber-reinforced composites because just after cooling to room temperature the resin matrix is under a tri-axial residual stress state. From the mechanical properties of the plain resin and by using the parabolic failure criterion it is possible to explain the low strain to failure of unidirectional laminates under transverse tensile loading.

Glass fibre strength distribution determined by common experimental methods

The tensile strength of brittle fibres is routinely described by the Weibull distribution. The parameters of the distribution can be obtained from tests on single fibres and fibre bundles or from model composite tests. However, there is growing evidence that the distribution parameters obtained by different experimental techniques differ systematically. In order to investigate the possible causes of such discrepancies, single-fibre tension, fibre bundle, and single-fibre fragmentation tests are employed in this study to obtain strength distribution of commercial E-glass fibres. The results reveal parameter dependence on the approach used to extract the distribution parameters from experimental data. Particularly, in the case of single-fibre tension tests, the shape parameter obtained from average fibre strength vs. length data is larger than that obtained at a fixed gauge length. It is assumed that the apparent fibre strength scatter is caused by both the inherent flaw structure along a fibre and by the fibre-to-fibre strength variability within a batch, due to slightly differing processing and handling history of the fibres. Fibre fragmentation test results are used to derive the Weibull distribution parameters applicable to the fibre batch. The strength distribution obtained is compared with strength data for the single fibres, and reasonably good agreement is observed.

Strain concentration factors for fibers and matrix in unidirectional composites

Abstract One of the methods of calculating the stress disturbances due to broken fibers in unidirectional composites is the so-called shear lag analysis. This method has been developed with the approximation that only the fibers carry the applied stress, not the matrix, and that the matrix acts only to transfer stress to the fibers. As a consequences of this approximation, the application of the method has been limited only to composites in which matrix stiffness is low in comparison with that of the fibers, and the volume fraction of fibers is high. In the present work, the ordinary shear lag analysis was modified to introduce the influence of the matrix stiffness. In this modified method, the tensile stress concentration in the fibers and matrix adjacent to cut fibers and matrix and shear stresses at the interface between fibers and matrix were estimated. The influence of interfacial debonding on the strain concentration was also studied.

Finite-element modeling of initial matrix failure in CFRP under static transverse tensile load

The failure of transversely loaded unidirectional CFRP has been investigated by the use of mechanical and thermo-mechanical test methods and finite-element analysis. The case considered here is characterized by a high interfacial strength between fiber and matrix, so that matrix failure governs the fracture process of the composite. On the basis of the experimental results, the parabolic and other failure criteria were applied to the FE calculations. The failure dependence of the resin on the actual stress state could be described. Furthermore, the influence of thermal residual stresses on the initial matrix failure has been investigated, and the actual stiffnesses and thermal expansion changes of the epoxy resins and the composites as a function of temperature have been determined experimentally. The results of the mechanical and thermo-mechanical tests performed on the pure resins and on the composites were incorporated into a finite-element analysis and compared with the transverse tensile properties of the composite laminates. In the FE analysis, the local fiber-volume fraction was varied over a wide range in order to investigate its influence on the thermal residual stresses and transverse composite strength. The results could explain the low strain to failure of transverse laminates under tensile loading.

Deformation and fracture behavior of an Al2O3/YAG composite from room temperature to 2023 K

Abstract The deformation and fracture behavior of a newly developed Al 2 O 3 /YAG composite, fabricated by the unidirectional solidification of a eutectic composition, was investigated by means of bending tests between room temperature and 2023 K, on specimens whose longitudinal directions were parallel (L specimen) and perpendicular (T specimen) to the direction of solidification. At low temperatures, the composite fractured in a brittle manner at all displacement speeds. At high temperatures, it fractured in a brittle manner at high displacement speed, but in a ductile manner with accompanying plastic deformation at low speed for both L and T specimens. The brittle-ductile transition temperature became higher at higher displacement speeds in both L and T specimens, while it was slightly higher in the latter specimen. The crack propagated dominantly through YAG with lower ductility than Al 2 O 3 , as a consequence of which the fraction of YAG in the fracture surface was higher than the structural value estimated from a polished surface. The strength at room temperature of both L and T specimens was maintained up to 2023 K at high displacement speeds. At low displacement speeds, the strength decreased beyond 1900 K as a result of the enhanced plastic deformation. The stress exponent for plastic deformation at high temperatures (1823–2023 K) was 5–6, suggesting that the plastic deformation is controlled by a dislocation mechanism during bending as well as in compressive and tensile loading. The fracture toughness tended to increase with increasing temperature and with decreasing displacement speed, especially in L specimens.

Rate dependence of mode I fracture behaviour in carbon-fibre/epoxy composite laminates

Abstract The rate dependence of mode I interlaminar fracture behaviour in unidirectional carbon-fibre/epoxy composite laminates has been investigated over a wide range of loading rates from quasi-static (displacement rate, δ = 0.01–500 mm min−1) to impact (δ = 5–20 mm see−1) at room temperature. Impact fracture tests were performed by the WIF (wedge-insert-fracture) method with a SHPB (split Hopkinson pressure bar) system for accurate measurement of impact fracture toughness, while quasi-static fracture tests were performed by the DCB (double-cantilever-beam) method with a screw-driven testing machine. In the present composite laminates, the fracture toughness decreased stepwise with increasing loading rate showing a distinct rate-sensitive transition region and two rate-insensitive regions above and below. As a consequence of this stepwise characteristic, the crack growth behaviour varied with loading rate: in and below this transition region, the crack grew unstably accompanied by high-speed propagation and arrest; but above the transition region, the crack grew stably and continuously. This trend was well explained by a simple model incorporating the rate dependence of fracture toughness and the contribution of kinetic energy in the specimen during unstable crack propagation.

Influences of interfacial bonding strength and scatter of fibre strength on tensile behaviour of unidirectional metal matrix composites

The influences of interfacial bonding strength and scatter of strength of fibres on tensile behaviour of unidirectional metal matrix composites, whose matrix has low yield stress in comparison to the strength of fibres, were studied using the Monte-Carlo simulation technique using two-dimensional model composites. The following results were found. The strength of composites increases with increasing bonding strength, especially when the bonding strength exceeds the shear yield stress of the matrix and then remains nearly constant. The strength of composites is very sensitive to bonding strength when the scatter of fibre strength is large, but not when it is small. The fracture mode varies from non-cumulative to cumulative with increasing scatter of fibre strength for both cases of weak and strong interfacial bondings. The fracture surface becomes irregular when bonding strength becomes low and scatter of fibre strength becomes large. The applicability of the Rosen and Zweben models and the rule of mixtures to predict the strength of composites was examined.

Work instability and its influence on the critical current density of silver sheathed Bi2223 tapes

The characteristic microstructure which develops during the preparation of silver sheathed Bi2223 tapes by the powder-in-tube method is explained. The critical current density (Jc) increases with decreasing tape thickness. During reduction of the thickness, densification and texturing of the microstructure takes place. However, due to the heavy reduction, the degree of variation of oxide layer thickness becomes abruptly large, while Jc degrades. This phenomenon is called work instability. A correction of Jc that eliminates the influence of work instability has been successfully carried out from a statistical viewpoint.

The influence of thermal residual stresses on the transverse strength of CFRP using FEM

The failure of transversely loaded unidirectional CFRP has been investigated using mechanical and thermo-mechanical test methods as well as finite element analysis (FEA). The FEA analysis consists of two cases: a high interfacial strength between fiber and matrix, so that matrix failure governs the fracture process of the composite, as well as a weak interface, so that fiber matrix debonding is the dominating failure process of the composite. The failure dependence of the resin on the actual stress-state could be described. Furthermore, the influence of the thermal residual stresses on the initial matrix failure has been investigated, and the actual stiffness as well as the thermal expansion change of the epoxy resins and the composites as a function of temperature have been determined experimentally. The results of the mechanical and thermo-mechanical tests performed on the neat resin and on the composites were incorporated into FEA and compared with the transverse tensile properties of the composite laminates. In the FE-analysis, the local fiber volume fraction was varied over a wide range in order to investigate its influence on the thermal residual stresses and transverse composite strength. The results can explain the low strain to failure of transverse laminates under tensile loading. The calculated interfacial shear strength (ISS) and the interfacial normal strength (INS) are in good agreement with values found in the literature.

Influences of matrix ductility, interfacial bonding strength, and fiber volume fraction on tensile strength of unidirectional metal matrix composite

A trial to predict the influences of ductility of matrix, interfacial bonding strength, and volume fraction of fiber on the tensile strength of unidirectional metal matrix composites was attempted by means of a Monte Carlo computer simulation method. The main results are summarized as follows. (1) The strength of strongly bonded composites increased with increasing ductility of matrix and then remained nearly constant. (2) When the matrix was ductile, the strength of composite increased with increasing interfacial bonding strength and then remained nearly constant. When the matrix was not ductile, the strength increased but then decreased with interfacial bonding strength. In this case, there was an optimum bonding strength, for which the strength of composite was highest. (3) Concerning the strength of composite as a function of volume fraction of fiber, there arose the case where it is approximately described by the rule of mixtures and also the case where it is not described by this rule, depending on the ductility of matrix, interfacial bonding strength, and scatter of strength of fiber.