Guillaume Seon

Failure Predictions for Carbon/Epoxy Tape Laminates with Wavy Plies

Accurate matrix-dominated constitutive properties can be a key to accurate failure models for polymer-matrix composites. This work shows the effect of nonlinear interlaminar shear stress-strain relations on delamination failure predictions for thick IM7/8552 carbon/epoxy tape laminates with wavy plies. Nonlinear finite element model (FEM) predictions and subsequent test correlations are presented. The interlaminar shear stress-strain relations are generated using short-beam shear tests and a digital image correlation full-field strain measurement technique. Test data for the wavy-ply coupons show that nonlinear shear stress-strain response is required for accurate failure prediction.

Simulation of damage in composites based on solid finite elements

Solid finite element‐based techniques are attractive for simulation of the matrix-dominated failure modes in composites. This work shows the ability of such techniques to capture the initiation and growth of matrix ply cracks and delaminations in carbon/epoxy laminates. Three-dimensional (3D) finite element models for open-hole tensile test articles are developed. Such models account for the micromechanical damage in the matrix through nonlinear interlaminar stress‐strain relations. Stress-based and fracture mechanics‐based failure criteria are used to predict the matrix-dominated failures. Material stiffness loss consistent with the failure criteria is implemented in the 3D solid finite element material definition procedure forsimulationofthematrix-plycracks.Initialdamageandapredefinedpatharenotrequired.Also,cohesive-zonemodelsare used to capture delamination. Damage initiation and growth simulations for the open-hole tensile articles are accomplished. Damage sequence, surface strains, and failure loads are verified with tests.

Structural analysis of composites with porosity defects based on X-ray computed tomography:

Advanced structural analysis methods that account for manufacturing defects in composite parts are needed to enable accurate assessment of their capability and useful life and to enhance current design and maintenance practices. In particular, porosity/voids are typical defects in carbon/epoxy and glass/epoxy composite aircraft flight-critical components. High-fidelity nondestructive evaluation by X-ray computed tomography allows accurate defect measurement and automatic conversion to structural models to assess the effects of defects on structural properties. This study presents a comprehensive structural analysis methodology, which includes nondestructive detection and finite element modeling of the defects in composites. Effects of porosity/voids on interlaminar tensile and shear strength of unidirectional carbon/epoxy composite specimens are investigated. Failure predictions and subsequent test correlations are presented.

Methods for assessment of interlaminar tensile strength of composite materials

Among the mechanical properties of polymer-matrix composite materials, the interlaminar tensile strength is among the most difficult to characterize. ASTM Standard D 6415 uses a curved-beam configuration for measuring interlaminar tensile strength. Not only the manufacturing process to produce curved-beam coupons with uniform radius and thickness could be challenging but also the curved-beam strength data typically exhibits large scatter. One question is whether ASTM D 6415 curved-beam interlaminar tensile strength data are coupon-specific, that is the curved-beam strength is not really a coupon-independent material property, suggesting that ASTM D 6415 is not adequate to measure interlaminar tensile strength. The objective of this work is to develop efficient and accurate methods to capture interlaminar tensile strength of composites. The authors expand a recently developed short-beam method coupled with the digital image correlation full-field deformation measurement technique to measuring the interlaminar tensile strength. The interlaminar tensile strength data are presented for IM7/8552 tape composite system. However, average curved-beam strength value is significantly lower compared to the short-beam test results. Micro-focus CT measurements show that porosity in the radius area is the reason for the low average strength value and the large scatter in the curved-beam strength test data. Once the stress concentration effects of porosity are captured through transfer of CT measurements into three-dimensional finite element model, the short-beam and the curved-beam test results agree. The short-beam method, which measures the interlaminar tensile strength for a pristine material, and the refined curved-beam method which accounts for manufacturing defects, represent more complete interlaminar tensile strength assessment methodology for composite structural designs.

Structures perspective for strength and fatigue prognosis in composites with manufacturing irregularities

Recent advances in understanding deformation and failure mechanisms of polymermatrix composites used in rotor structures enable accurate and efficient measurement of material stiffness, strength, and fatigue characteristics based on testing small unidirectional laminate specimens. Successful failure predictions increased our confidence in the development of virtual test methods replacing some of the standard tests of multi-directional laminated composite materials with three-dimensional models accurately predicting deformation, damage topography, strength, and cycles to failure. However, the remaining key questions are related to the ability of transitioning the material-scale virtual test information to larger composite structures. This work presents results of the feasibility assessment targeting the scaling of knowledge and methods acquired at the material scale, to larger structural elements.

DIC Data-Driven Methods Improving Confidence in Material Qualification of Composites

This work presents initial results of research activities focused on utilizing the Digital Image Correlation technique for measurement of the complex deformation of composite materials towards the development of consolidated common material qualification processes. These common processes will enable reduced material qualification test matrices potentially accommodating substitution of resin types and other modifications of the material systems. In particular, data-driven methods based on full-field DIC strain measurements and finite element analysis developed at the University of Texas Arlington Advanced Materials and Structures Lab, are verified on carbon-fiber reinforced/untoughened and toughened polymer-matrix composite material systems. The DIC data-driven methods allowed verification of simplifying assumptions, increasing our confidence in material allowables. The methods also allowed identifying standing challenges in lamina characterization, including inconsistencies of certain ASTM standard test fixtures and analysis methods, preventing accurate measurement of lamina characteristics key to laminate analysis.