Foam core sandwich panel structures are an excellent structural concept to realize full potential capabilities of composite materials owing to its high strength to weight and high stiffness to weight ratio together with suitable formability for integral structures. However, degradation of strength due to the interfacial crack initiated from the damaged area is one of serious problems of this concept. Authors proposed the interfacial crack suppression method named the crack arrester. This concept is to install materials with higher stiffness on the crack propagation path and induce the redistribution of the load between the foam core area near the crack tip and the leading edge of the crack arrester in order to decrease the energy release rate at the crack tip less than the interfacial fracture toughness. A crack suppression effect of the crack arrester were analytically estimated and experimentally validated under mixed mode of mode I and mode II, and mode III type loading conditions. As for the analyses, it was confirmed that the energy release rates at crack tip decreased as the crack tip approached the leading edge of the arrester under constant loadings. In the experimental validation, fracture toughness tests were conducted and the apparent fracture toughness increased as crack tip approached the leading edge of the arrester. Based on the acquired apparent fracture toughness derived from test data, a crack onset judgement method was proposed and a crack onset judgement diagram was prepared in order to apply this concept to structural designs

View full abstract

This paper is concerned with the statistical time and temperature dependent static and creep strengths in the fiber direction of unidirectional CFRP under tension and bending loads. First, the formulation of statistical time and temperature dependent static and creep strengths of CFRP are proposed based on the viscoelasticity of matrix resin. Second, the tensile and flexural static strengths in the fiber direction of unidirectional CFRP are measured at various temperatures. Then, the creep failure times of unidirectional CFRP are predicted statistically using the tensile and flexural static strengths measured at various temperatures and viscoelastic behavior of matrix resin. Third, the tensile and flexural creep failure times of unidirectional CFRP at a constant load and temperature are measured experimentally for comparison with the predicted ones. Furthermore, the characteristics of statistical tensile and flexural creep failure times of unidirectional CFRP are compared with each other.

View full abstract

In this study, elastic properties of the plain-weave fabric composite laminates are investigated through the finite element analysis. In the analysis, the intralaminar inhomogeneity caused by the architecture of woven yarns is taken into consideration. Using the homogenization procedure, the effective plate stiffness of the fabric composite laminate is calculated. Assuming the number of laminated plies is infinite, the effective three-dimensional continuum elastic properties of the fabric composite are also calculated. Then, the effect of the number of plies on the in-plane (in-plane tension and in-plane shear) elastic properties and out-of-plane (bending and twisting) elastic properties of the composite laminate is investigated. Through the numerical investigation, it is found that the in-plane tensile stiffness and the twisting stiffness of the composite laminate, particularly of a laminate with the small number of laminated plies, are overestimated when they are evaluated with the assumption that each lamina consists of a homogeneous material with the effective three-dimensional continuum elastic properties.

View full abstract

The aim of this paper is to evaluate the influence of the alternate dipping condition on the tensile strength of HAp/collagen composite fibers. We prepared the collagen fibers using the bio-inspired method, where the fibrosis and the cross-linking are generated at the same time. Here, 1-ethy-3carbodiimide hydrochloride (EDC) was selected as the cross-linking agent. Then, the HAp crystals were deposited on the surface of the prepared collagen fibers using the biomimetic deposition method, where the collagen fibers are dipped into the pseudo body fluid alternately. The tensile tests were carried out using the micro-tensile testing device developed by the authors previously. As a result, the tensile strength of the collagen fibers was maximized under the EDC concentration of 5 [mmol/L]. The amount of the deposited HAp crystals were increased with the increase of the alternate dipping time, resulting in the increase of the tensile strength of the HAp/collagen composite fibers, within the range covered in this paper. The size of the deposited HAp crystals were decreased with the decrease of the alternate dipping temperature, resulting in the increase of the tensile strength of the HAp/collagen composite fibers, within the range covered in this paper. It can be suggested that this is attributed to the uniform HAp deposition. As a next step, it is desired to develop the bundling method of the HAp/collagen composite fibers and investigate its optimum condition.

View full abstract

The open hole tension (OHT) and open hole compression (OHC) static and fatigue tests were conducted for the interlaminar toughened and un-toughened quasi-isotropic CFRP laminates using benzoxazine resin as the matrix. The influence of toughened interlayer on the OHT and OHC static and fatigue strengths of quasi-isotropic CFRP laminates was discussed. As results, the fatigue strength as well as the static strength by OHT loading is clearly larger than that by OHC loading. The strengthening by toughened interlayer is found in OHT loading, however this strengthening is not found in OHC loading. It was clarified by the observation of damages that the delamination onset and growth before the final fracture during fatigue loading are suppressed by the toughened interlayers in the case of OHT loading.

View full abstract

This paper describes a numerical study of welding strength between a thermoset carbon fiber-reinforced plastic (CFRP) and a thermoplastic resin, which forms a complicated interface (prepared by welding technology). Plane stress conditions were assumed and a finite element method (FEM) with continuum damage mechanics (CDM) and cohesive zone model (CZM) was applied for the numerical study. Shear strain was applied until the model fractured, or the strain reached 4% for several cases, in which the position of the interface between the thermoset resin (TS) and thermoplastic resin (TP) differed. A structure in which the TS/TP interface overlaps with the carbon fiber (CF) layer resulted in an increase in welding strength because the CF that lies along the TS/TP interface bears the shear stress. For validation, the numerical study results were used to explain the experimental results. A lap shear test was conducted to evaluate the interfacial strength. An increase in lap shear strength was achieved using the welding technology, and a complicated fracture surface especially fiber breakage at the surface of the welding was observed, indicating that the CF bears the stress, which is an important factor in welding technology. The increase in lap shear strength by the integrated structure can be explained without fiber breakage using the results of numerical simulation.

View full abstract