Lee W. Kohlman

Effects of hygrothermal cycling on the chemical, thermal, and mechanical properties of 862/W epoxy resin

The hygrothermal aging characteristics of an epoxy resin were characterized over a one-year period, which included 908 temperature and humidity cycles. The epoxy resin quickly displayed evidence of aging through color change and increased brittleness. The influence of aging on the material’s glass transition temperature (T g) was evaluated by Differential Scanning Calorimetry and Dynamic Mechanical Analysis. The T g remained relatively constant throughout the year-long cyclic aging profile. Chemical composition was monitored by Fourier Transform Infrared spectroscopy, where evidence of chemical aging and advancement of cure was noted. The tensile strength of the resin was tested as it aged and this property was severely affected by the aging process in the form of reduced ductility and embrittlement. Detailed chemical evaluation suggests many aging mechanisms are taking place during exposure to hygrothermal conditions.

Analytical Model and Numerical Analysis of the Elastic Behavior of Triaxial Braided Composites

AbstractThis paper is concerned with elastic behavior of a triaxial braided composite by using a three-dimensional analytical model and mesoscale finite-element (FE) analysis, in conjunction with experimental observations. The analytical method and FEM take into account the actual fabric structure by considering the fiber undulation and actual architecture parameters. A representative unit cell model of the triaxial braided architecture is first identified based on fiber volume ratio, specimen thickness, and microscopic image analysis. Detailed geometric parameters for axial and bias fiber bundles are obtained, which provide precise information to enable the development of analytical and FE models. A general three-dimensional analytical model based on realistic architecture is developed with consideration of axial and bias fiber undulation. A typical study on the effect of axial fiber undulation is presented through the analytical model and axial tensile test. The prediction of effective elastic constants...

Improved Subcell Model for the Prediction of Braided Composite Response

Abstract In this work, the modeling of triaxially braided composites was explored through a semi-analytical discretization. Four unique subcells, each approximated by a “mosaic” stacking of unidirectional composite plies, were modeled through the use of layered-shell elements within the explicit finite element code LS-DYNA. Two subcell discretizations were investigated: a model explicitly capturing pure matrix regions, and a novel model which absorbed pure matrix pockets into neighboring tow plies. The in-plane stiffness properties of both models, computed using bottom-up micromechanics, correlated well to experimental data. The absorbed matrix model, however, was found to best capture out-of-plane flexural properties by comparing numerical simulations of the out-of-plane displacements from single-ply tension tests to experimental full field data. This strong correlation of out-of-plane characteristics supports the current modeling approach as a viable candidate for future work involving impact simulations.

Single Ply and Multi-Ply Braided Composite Response Predictions Using Modified Subcell Approach

AbstractIn this work, the modeling of triaxially braided composites was explored through a subcell approach using an improved semianalytical discretization scheme. The unit cell of the braided composite was divided into four unique subcells, each approximated by a mosaic stacking of unidirectional composite plies and modeled through the use of layered-shell elements within the finite-element model. Two subcell discretization schemes were investigated: a model explicitly capturing pure matrix regions, and a model which absorbed pure matrix pockets into neighboring tow plies. Differences in the mesostructure between single-ply and multi-ply braid coupons were addressed through modifications to the subcell discretization. The absorbed matrix model simulated the unique out-of-plane deformations observed experimentally in single-ply tensile tests with acceptable moduli predictions. An investigation of single-shell versus multi-shell coupons for the analysis of multi-ply braids revealed the through-thickness mo...

Characterization and Analysis of Triaxially Braided Polymer Composites under Static and Impact Loads

In order to design impact resistant aerospace components made of triaxially braided polymer matrix composite materials, a need exists to have reliable impact simulation methods and a detailed understanding of the material behavior. Traditional test methods and specimen designs have yielded unrealistic material property data due to features such as edge damage. To overcome these deficiencies, various alternative testing geometries such as notched flat coupons have been examined to alleviate difficulties observed with standard test methods. The results from the coupon level tests have been used to characterize and validate a macro level finite element based model which can be used to simulate the mechanical and impact response of the braided composites. In the analytical model, the triaxial braid architecture is simulated by using four parallel shell elements, each of which is modeled as a laminated composite. Currently, each shell element is considered to be a smeared homogeneous material. Simplified micromechanics techniques and lamination theory are used to determine the equivalent stiffness properties of each shell element, and results from the coupon level tests on the braided composite are used to back out the strength properties of each shell element. Recent improvements to the model the incorporation of strain rate effects into the model. Simulations of ballistic impact tests have been carried out to investigate and verify the analysis approach.