Masaki Hojo

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.

Effect of stress ratio on near-threshold propagation of delimination fatigue cracks in unidirectional CFRP

Abstract The effect of the stress ratio on near-threshold growth of delamination fatigue cracks was investigated with unidirectional laminates made from Ciba Geigy 914C prepegs (T300/914) and from Toray P305 prepegs (T300/#2500). Tests of delamination fatigue crack propagation were carried out under mode I opening loading by using double cantilever beam specimens. The normalized gradient of energy release range was controlled in load-shedding tests. In the region of crack growth rates above about 5 × 10−10 m/cycle, the growth rate was expressed as a power function of fracture mechanics parameters. Below this region, there existed a growth threshold. The influence of the stress ratio became smaller when the rate was correlated to the energy release rate range than when the rate was correlated to the stress intensity range or the maximum energy release rate. A controlling fracture mechanics parameter is discussed on the basis of fractographic observation and mechanism consideration. A new phenomenological law of fatigue crack propagation is derived.

Prestandardization study on mode II interlaminar fracture toughness test for cfrp in japan

Abstract This paper summarizes results from a series of interlaboratory round robin tests (RRTs) performed in order to establish a JIS standard for mode I interlaminar fracture toughness test using double cantilever beam (DCB) specimens. For the case of unidirectional laminates, brittle and toughened CF/epoxy, and CF/PEEK systems were used. Only a brittle CF/epoxy system was used for woven laminates. The round robin tests were conducted with two main aims: first, to examine the influence of starter films and the precracking condition on the initial mode I fracture toughness values; and second, to establish the definition of initial fracture toughness. Polyimide starter films stuck to the epoxy matrix, and caused unstable crack growth from starter films. Comparison of the tests with and without mode I precracks from starter films indicated that tests with precracks gave lower values of initial fracture toughness. The definition of initial fracture toughness values was discussed, based on the reproducibility. A 5% offset point was recommended as the initial fracture toughness from the RRT results. The influence of loading apparatus, data reduction methods, etc. was also discussed.

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.

Computer simulation of trabecular remodeling in human proximal femur using large-scale voxel FE models : Approach to understanding Wolff's law

Ever since Julius Wolff proposed the law of bone transformation in the 19th century, it has been widely known that the trabecular structure of cancellous bone adapts functionally to the loading environment. To understand the mechanism of Wolffs law, a three-dimensional (3D) computer simulation of trabecular structural changes due to surface remodeling was performed for a human proximal femur. A large-scale voxel finite element model was constructed to simulate the structural changes of individual trabeculae over the entire cancellous region. As a simple remodeling model that considers bone cellular activities regulated by the local mechanical environment, nonuniformity of local stress was assumed to drive the trabecular surface remodeling to seek a uniform stress state. Simulation results demonstrated that cell-scale ( approximately 10microm) remodeling in response to mechanical stimulation created complex 3D trabecular structures of the entire bone-scale ( approximately 10cm), as illustrated in the reference of Wolff. The bone remodeling reproduced the characteristic anisotropic structure in the coronal cross section and the isotropic structures in other cross sections. The principal values and axes of a structure characterized by fabric ellipsoids corresponded to those of the apparent stress of the structure. The proposed large-scale computer simulation indicates that in a complex mechanical environment of a hierarchical bone structure of over 10(4) length scale (from approximately 10microm to approximately 10cm), a simple remodeling at the cellular/trabecular levels creates a highly complex and functional trabecular structure, as characterized by bone density and orientation.

Comparison of test configurations for determination of mode II interlaminar fracture toughness results from international collaborative test programme

AbstractThis paper presents a summary of the tests performed within a Versailles Project on Advanced Materials and Standards collaborative test programme to examine the measurement of mode II interlaminar fracture toughness using four different test methods based on end notched flexure, stabilised end notched flexure, end loaded split, and four point end notched flexure carbon fibre reinforced epoxy specimens. Tests were performed by members of the European Structural Integrity Society, the Japan Industrial Standards group, and the American Society for Testing and Materials.

Calcium response in single osteocytes to locally applied mechanical stimulus: differences in cell process and cell body.

It is proposed that osteocytes embedded in the bone matrix have the ability to sense deformation and/or damage to the matrix and to feed these mechanical signals back to the adaptive bone remodeling process. When osteoblasts differentiate into osteocytes during the bone formation process, they change their morphology to a stellate form with many slender processes. This characteristic cell shape may underlie the differences in mechanosensitivity between the cell processes and cell body. To elucidate the mechanism of cellular response to mechanical stimulus in osteocytes, we investigated the site-dependent response to quantitatively controlled local mechanical stimulus in single osteocytes isolated from chick embryos, using the technique of calcium imaging. A mechanical stimulus was applied to a single osteocyte using a glass microneedle targeting a microparticle adhered to the cell membrane by modification with a monoclonal antibody OB7.3. Application of the local deformation induced calcium transients in the vicinity of the stimulated point and caused diffusive wave propagation of the calcium transient to the entire intracellular region. The rate of cell response to the stimulus was higher when applied to the cell processes than when applied to the cell body. In addition, a large deformation was necessary at the cell body to induce calcium transients, whereas a relatively small deformation was sufficient at the cell processes, suggesting that the mechanosensitivity of the cell processes was higher than that of the cell body. These results suggest that the cell shape with slender processes contributes to the site-dependent mechanosensitivity in osteocytes.

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.