Hisashi Kumazawa

Analysis and Experiment of Gas Leakage Through Composite Laminates for Propellant Tanks

The mechanism of the through-thickness gas leakage of carbon fiber-reinforced plastics (CFRP) laminates is investigated in view of propellant leaks for composite tanks of reusable launch vehicles. In this study analysis of leakage caused by the existence of matrix cracks acting as the chain of leakage paths is developed under the simple assumption that conductance for leakage is a function of crack-opening displacements. The analytical results in consideration of mechanical and thermal loads are compared with experimental results, which are measured as helium gas leaks through carbon fiber-reinforced plastics laminates containing matrix cracks at room temperature. Good agreement between the analytical and experimental data is confirmed. Numerical analysis based on the proposed method can be used to evaluate the influence of mechanical loads on propellant leak through CFRP cross-ply laminates. The analytical calculations show that the increase of propellant leakage can be caused by the enlargement of crack-opening displacements caused by mechanical and thermal loads, increase of crack density, and decrease of temperature.

Numerical Modeling of Gas Leakage Through Damaged Composite Laminates

A computational fluid dynamics (CFD) model was developed to predict leakage through a damaged laminate. Crack opening displacements (COD) from a 3D finite element model were fit using a polynomial function. This curve fit was then used in defining the openings in the CFD model. Delamination length at crack intersections was estimated based on experimental data for specimens subjected to thermal loads only. Leakage under combined thermal and biaxial mechanical loadings was predicted with the estimated delamination length. Considering the sensitivity of the predictions to many variables, the calculated leakage under combined thermal and biaxial mechanical loadings was estimated with reasonable accuracy.

Effects of an Open Hole on the Biaxial Strengths of Composite Laminates

Experimental biaxial data are essential to validate failure criteria for composite laminates. In this study an optimized design of cruciform specimen used for in-plane biaxial loading was proposed. The uniformity of strain field in the gauge region of the smooth specimen under biaxial loadings was confirmed via finite element analysis. Both smooth and open-hole specimens fabricated by π/4 quasi-isotropic carbon epoxy laminates were tested under three different biaxial stress schemes, where stresses {σx, σy} are expressed in normalized forms as {1, 0}, {1, 1}, and {1, -1}, respectively. The relations between far-field stresses and global biaxial loadings were established based on test data, and the strengths of both smooth and open-hole specimens under different biaxial loading schemes were acquired. Digital image correlation (DIC) technique was employed to measure strain field in the neighborhood of the central hole of the open-hole specimens during test, and the measurements were compared with strain distribution obtained from numerical simulations. Good agreement between DIC images and FEA results was achieved. First-ply-failure (FPF) envelopes based on micro-mechanics of failure (MMF) for both smooth and open-hole cases were drawn and compared with test data.

Gas Leakage Evaluation of CFRP Cross-ply Laminates under Biaxial Loadings

In this study, leak characteristics of CFRP (carbon fiber reinforced plastic) cross-ply laminates are experimentally investigated under biaxial loadings using a specially prepared in-plane biaxial testing system. Permeability through the damaged laminate under biaxial stresses was measured with a leak detection system. In order to evaluate the effect of damage on leakage, ultrasonic C-scan was used to inspect the matrix cracking in the specimens. Experimental results reveal that leakage through the damaged laminate is in correlation with the amount of damage and depends not only on load level but also on biaxial load ratio. Leak analysis, which had been previously developed, was employed and calculations of permeability were in good agreement with the experimental data. The results indicate the validity of the assumption of domination of crack openings on permeability, and the dependency of constants of leak conductance on damage susceptibility in the leak analysis.

Mechanical Properties of Coated Plain Weave Fabrics under Biaxial Loads

In this study, biaxial tensile tests of cruciform specimens with open hole were conducted for evaluating the strength of the coated plain weave fabrics composed of high specific tensile strength fibers. In addition to the biaxial tensile tests, uniaxial tensile tests of strip specimens were also carried out to obtain uniaxial fundamental properties of the fabrics. As the results of the biaxial and uniaxial tensile tests, open hole tensile strength of the fabric under biaxial loadings is approximately equal to the strength under uniaxial load irrespective of biaxial load ratio, and warp directional strip specimens exhibit higher strength than that of weft directional specimens in spite of the same densities of yarns in both directions. Experimental results exhibit that misalignment of weft yarns of the membrane partly contributes the loss of the strength in weft direction. The results of observations by microscope and tensile tests of single yarns reveal that the difference of the strength in warp and weft directions is caused by the degradation of weft yarns by heating process of polymer film coating on the fabrics in addition to the misalignment of yarns.

Generalized Quasi-Three Dimensional Finite Element Analysis of Laminates.

Quasi-three dimensional finite element method (Q3-DFEM) has been widely recognized as an effective method in analyzing symmetric laminates loaded in the longitudinal direction. However, when laminates are not symmetric or when bending moment is applied, Q3 DFEM is not sufficient to analyze composite laminates. A generalized Q3 DFEM (GQ3 DFEM) accounting for bending and twisting of unsymmetric laminates has been developed. In this paper, GQ3 DFEM based on the Kirchhoff hypothesis for laminates is formulated based on deformations separated into two parts, which are the global displacements corresponding to CLT, and the local displacements representing local states. As the local displacements can be separated from the global displacements based on CLT, the effect of local geometrical irregularities is directly evaluated. GQ3 DFEM is used to determine displacements, stresses and energy release rates. Finally it is shown that GQ3 DFEM based on the Kirchhoff hypothesis is useful for predicting the behavior of composite laminates.