Masamichi Matsushima

Experimental Confirmation of the Theory of Elastic Moduli of Fabric Composites

Some experiments for measuring the elastic moduli of fabric composites are con ducted in order to evaluate the theoretical predictions developed by the authors in their previous work. Experimental results coincide very well with the theory for 8 harness satin carbon/epoxy systems. There exists some discrepancy between theories for con strained and unconstrained local warping in the case of plain weave composites. Ex perimental results are bounded by these two theories. The dependency of elastic moduli on laminate ply number is discovered in plain weave composites. The ratio of ply thickness to thread width is also a very important parameter, which strongly affects the elastic moduli of plain weave composites. The in-plane shear modulus of fabric composites is mainly influenced by fiber volume fraction, which varies linearly with ng . Off-axis properties of fabric composites are measured and compared with theories. The experimental results essentially confirm the validity of the authors theoretical predictions.

Some experimental findings in compression-after-impact (CAI) tests of CF/PEEK (APC-2) and conventional CF/epoxy flat plates

Abstract Compression-after-impact (CAI) tests have been conducted for quasi-isotropic thick plates with 48 plies by using the NASA method and on plates with 32 plies by using the SACMA method. Specimens are made of CF/PEEK (APC-2) and conventional CF/epoxy for the NASA plates and CF/epoxy for the SACMA plates. In the NASA CAI tests, the sequence of delamination buckling and its propagation is clearly revealed through various experimental techniques. One major technique is moiretopography, and the other is thermo-mechanical stress analysis with a high-accuracy infrared sensor. The arrest of delamination propagation just before catastrophic failure due to high fracture toughness is clearly captured by the moirecamera. This behavior provides good CAI values of CF/PEEK. The initial buckling properties of the delaminated region by the impact are then extensively discussed. Numerical predictions of initial buckling stress have been obtained by modelled geometry of the delaminated region simplified from its precise structure clarified by ultrasonic C-scanning. They agree fairly well with the experimental results. The in-plane stress distribution in the delaminated region before initial buckling is measured by an infrared stress graphic system. This compared favorably with finite element predictions. Two types of symmetric buckling modes with respect to the central plate surface, twin and single peak ones, are experimentally captured.

Hardening non-linear behaviour in longitudinal tension of unidirectional carbon composites

Uniaxial tensile tests of unidirectional carbon-epoxy coupons are conducted in the longitudinal direction. It is observed that the longitudinal modulus increases with axial stress or strain up to the intermediate level of tension. A fractional constitutive relation with a quadratic denominator is derived by the method of the theory of non-linear elasticity. This equation adopting the estimated higher-order compliance coefficients exhibits an excellent agreement with the experimental results. An empirical strain-based equation is also proposed as a simpler alternative. Averaging formulae for both types of relation are provided for a practical application. The present phenomenon includes the behaviour in a low-stress region discovered by some early work. The consideration of the present non-linear behaviour improves the correlation between theory and experiments in stress-strain relationships of fabric composites with carbon fibres.

Geometrical and material nonlinear properties of two-dimensional fabric composites

The formerly developed nonlinear analysis of fabric composites is expanded to include longitudinal hardening in unidirectiona l carbon/epoxy composites and the deformed state in single-layer plain-weave composites. A simple constitutive equation is proposed to describe the hardening property. The full material nonlinear solution of the 8H satin carbon/epoxy system is in favorable agreement with the experimental results. It is confirmed that this material property is crucial in predicting a macroscopic behavior of fabric composites if warping is fully suppressed. An iterative procedure of the geometrical nonlinear analysis accounting for the warping is developed. The predicted stress-strain relations and warping coincide very well with the corresponding experimental results of carbon/epoxy plain-weave composites. The warping contributes greatly to the apparent hardening of this material at low stress levels. Nomenclature Ajj(x)9A jj = local and averaged in-plane stiffness afj (x) ,dfj = local and averaged in-plane compliance a* * = in-plane compliance in the warping-free state a - width of a thread au _ = length of a crimp part Bij(x),BiJ = local and averaged bending/stretching coupling stiffness bfj (x) ,b*j = local and averaged bending/stretching coupling compliance DIJ (x) ,Dij = local and averaged bending stiffness d*j(x),dfj = local and averaged bending compliance h = thickness of a fabric composite plate h j (x) = shape function of thread crimp h2 (x) = shape function of a warp section ht —

Improved correlation of predicted and experimental initial buckling stresses of composite stiffened panels

Abstract Experimental and numerical investigations are conducted for rigorous correlation of initial buckling properties of stiffened panels made of carbon fiber/poly-ether-ether-ketone (CF/PEEK) and CF/epoxy. Decreasing longitudinal elastic modulus of unidirectional CF composites lamina in compression plays a key role for better numerical predictions. A consideration of end fixtures in finite element modeling plays another key role in correlation and both initial buckling stress and mode for the short panels used here. A quarter finite element model with an end fixture by hypothetical symmetry is considered as the irreducible minimum in the modeling. A careful setting of lamina thickness is also important for better predictions. Initial imperfection close to the real shape of CF/PEEK panels is taken into account and an improvement in correlation of predicted and experimental results is reached. A conventional Rayleigh-Ritz approach considering only a local buckling mode of skin is developed. This analytical prediction compares fairly well with the numerical and experimental results in the preceding work in which their stringers are stiff enough.

Comparison of numerical analysis of linear and post-buckling behavior of CFRP T-stiffeners with experimental results

Numerical predictions of linear buckling and post-buckling behavior of CFRP T-stiffeners are conducted by Finite Element Analysis. Such behavior is crucially important in aircraft structural design. Comparison between numerical predictions and experimental behavior is executed in detail. Width-to-thickness ratio in flange or web mainly governs linear buckling stresses. Inserted filler along the intersection line between flange and web also considerably affects the linear buckling behavior. By taking realistic initial imperfection into account, geometrical nonlinear analysis provides fairly descriptive post-buckling deflection behavior in flange and web. It is also possible to predict the final failure stress by assuming simple failure criteria and non-progressive failure.

Experiments and numerical analysis of compression after impact (CAI) behavior of CF/PIXA stiffened panels for HSCT structure

Compression after impact (CAI) behavior of stiffened panels using T-shaped stringers made of heat resistant thermoplastic composites, CF/PIXA, was obtained by tests at room temperature and 180°C. Excellent impact resistance in terms of delamination area was verified for the present composite panels due to the special fusion bonding film. Room temperature CAI strengths of the stiffened panels were found to be better than CF/PEEK panels in a rough comparison, without regard to differences in stacking sequence and geometry. Relationships between temperature and CAI strengths for CF/PIXA stiffened panels were compared with the results for flat plates (SACMA) and a compatibility between flat plate and stiffened panel data was observed. It is clarified that damage tolerance properties of CF/PIXA stiffened structure are so remarkable at room temperature that compression strengths are quite insensitive to relative impact energy. Its damage tolerance capability is also excellent at high temperature for High Speed Civil Transport structures. Linear and nonlinear buckling analyses based on simple models without delamination and interlaminar stresses were conducted using a finite element code. The results agreed well basically with experimental results. In the case of prediction of the final strength, several points of improvement are identified. The present prediction provides the initial step to discuss the possibility of post-buckling strength design at high temperature stiffened panels.