Kyeongsik Woo

Global/Local Finite Element Analysis for Textile Composites

Conventional finite element analysis of textile composite structures is impractical because of the large, complex microstructure. Global/local methodology combined with special macro elements is proposed herein as a practical alternative. Initial tests showed dramatic reductions in the computational effort with only a relatively small loss in accuracy for stresses away from the global/local boundary.

Three-Dimensional Failure Analysis of Plain Weave Textile Composites Using a Global/Local Finite Element Method

Textile composites are known to have improved out-of-plane properties and impact resistance. However, detailed analysis of textile composites is very difficult to perform due to the geometric complexity. In the present study, a practical computational procedure based on a global/local finite element method was developed for detailed analysis of textile composites. This procedure utilizes two problem levels: global and local levels. At the global level, an initial global solution was obtained using a coarse global mesh. At the local level, a small portion of the textile composite was modeled with a refined local mesh. For the global analysis, macroelements were used since the use of effective engineering properties are not in general accurate for the larger microstructural scale found in textile composites. Results indicated that the failure behavior of plain weave textile composites was sensitive to the fiber bundle waviness. The initial failure mode was the fiber bundle separation when the waviness ratio was large while it was the fiber bundle fracture when the waviness ratio was small. It was also found that the stress distributions near the free surface were different from those inside. The stress level was much higher inside than near the free surface. However, the boundary region was shallow and was limited to the outer-most layer.

Effects of fiber tow misalignment on the engineering properties of plain weave textile composites

Effects of fiber tow phase angle of the adjacent layers on the engineering properties of plain weave textile composites were studied. Unit cell analyses were performed for two-layer unit cell models with different fiber tow phase angles, and multi-field macroelements were used to reduce computer resource requirements. Results indicated that the engineering properties of plain weave textile composites can vary significantly according to the manner in which the layers are stacked.

Macro Finite Element for Analysis of Textile Composites

The analysis of textile composites is complicated by the complex micro-structure. It is not practical to account for this microstructure directly using traditional finite elements. A new type of finite element was developed to efficiently account for microstructure within a single element. These new elements, which are referred to herein as macro elements, performed well in initial tests.

Enhanced direct stiffness method for finite element analysis of textile composites

Abstract Traditional homogenization techniques are not useful when the microstructural scale of a material is of the same order of magnitude as the structural scale of a component. Such is the case for many textile composites. Since discrete modeling of the microstructure throughout a component is prohibitively expensive, continuum finite elements are needed which account for the microstructure within a single element. This paper describes a simple substructuring technique for formulating these special elements.

Boundary Effects in Woven Composites

Two dimensional finite elements were used to study boundary effects in plain weave composite specimens subjected to extension, shear, and flexure laods. Effective extension, shear, and flexural moduli were found to be quite sensitive to specimen size. For extension and flexure loads stress distributions were affected by a free surface, but the free surface boundary effect did not appear to propagate very far into the interior. For shear load the boundary effect appeared to propagate much further into the interior.

A post-processor approach for stress analysis of woven textile composites

In this study, macro-elements have been extended for the post-processing of detailed stresses of woven-textile composites. A symmetric plain-weave unit-cell configuration was analyzed to evaluate the performance of the post-processors. The stress results by macro-element post-processors were compared to those by conventional analyses. Results indicated that the post-processors were able to predict detailed stresses to a reasonable level of accuracy for woven-textile composites. The usefulness of the post-processor was demonstrated for a 5-harness satin-weave configuration.

Effective Modulus of Creased Thin Membranes

In this study, the effective moduli of creased membranes were calculated by finite element analyses. Geometrically and materially nonlinear contact analyses were performed to simulate the entire process of creasing and uni-axial tensile test. First, the creased geometry of thin membranes was predicted from creasing simulation. The creased membranes were then subjected to a series of numerical uni-axial tensile tests to calculate the effective moduli. The creased and tensile geometries were also obtained by experiments and theoretical computations, respectively, and the results were compared. Numerical specimens with various crease gauges were considered to study the effect of the amount of creasing. The size effect of the specimen was also investigated.

Low Degree of Homogeneity Due to Phase Shifts for Woven Composites

In this study, variation of effective properties due to fiber tow phase shifts was investigated for woven textile composites. Plain weave unit cells were modeled with randomly selected phase shifts. Effective properties were calculated using macro finite elements and the results were assessed statistically. It was found that the effective properties depended strongly on the tow phase shifts for thin plain weave composites. The variation in the effective modulus and Poissons ratio was large, and it showed skewed distributions. As the number of layers increased, however, the average properties converged and the coefficient of variation decreased.

Transverse vibration analysis for partly wrinkled membranes

The development is reported of a membrane-based wrinkle algorithm to analyze the transverse vibration behavior of partly wrinkled, annular, and three-sided membranes. This membrane-based wrinkle algorithm was implemented in the nonlinear finite element code ABAQUS, providing implementation of a true membrane constitutive model. First, the models were validated either with analytical solutions or experimental results. Then, the transverse vibration behavior of unwrinkled membrane models was studied. Finally, the presented constitutive model was incorporated in ABAQUS to inspect how wrinkling affects the vibration behavior of membranes. The frequency and mode shapes were investigated between the unwrinkled and wrinkled membranes. It was observed that even small amounts of wrinkling can significantly affect the modal frequencies of membranes. We offer physical explanation for these results.