Andrew Makeev

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.

A Test Method for Assessment of Shear Properties of Thick Composites

Transverse stress—strain constitutive relations are required for structural analysis of thick-section composites. A standard technique for assessment of transverse shear properties is the V-notched beam method. Such technique is based on strain gage measurements, which require a large specimen thickness for strain gage placement and impose tight geometry tolerances to minimize variations of strain at the gage location. A full-field strain measurement capability could enable simpler test specimen designs. A method for assessment of shear stress—strain relations using a short-beam shear (SBS) test and a digital image correlation (DIC) technique is presented in this work. The DIC technique is based on quantifying locations of a random texture on a surface to measure surface shape and deformation. V-notched-beam and SBS test results are compared for a glass/epoxy tape composite. To illustrate accuracy of SBS stress calculations, finite element results are obtained. Highly nonlinear interlaminar stress—strain relations are documented.

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.

Finite element mesh generation for composites with ply waviness based on X-ray computed tomography

A method for automated generation of finite element meshes for unidirectional composites with waviness defects is proposed. Images used as input for mesh generation are recorded with X-ray computed micro-tomography. Quality and contrast of the scanned images is such that fiber directions cannot be detected everywhere. To generate finite elements mesh that follow fiber directions it is suggested to interpolate available fiber slope data using radial basis functions and to create mesh nodes by integrating ordinary differential equations for fiber slopes. Examples demonstrate practical steps of detecting waviness from volume slice images and generation of meshes that model waviness defects with acceptable accuracy.

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.

Fatigue Life Assessment for Composite Structure

This work presents some of the most recent advances in the technologies which could enable accurate assessment of useful life for composite aircraft fatigue-critical, flight-critical components and structure. Such technology advances include: (1) nondestructive subsurface measurement shift from just detection of defects to three-dimensional measurement of defect location and size; (2) material characterization methods ability to generate 3D material allowables at minimum time and cost; and (3) fatigue structural analysis techniques ability to capture multiple damage modes and their interaction. The authors summarize their recent results in all three subjects.

A comparison of predictions from probabilistic crack growth models inferred from Virkler's data

The significant variability observed in fatigue crack growth experiments requires application of probabilistic modeling techniques to fatigue analyses. In this work, philosophical and practical concerns for probabilistic modeling are discussed. Several probabilistic fatigue models are derived from the Virkler data and then used to make predictions under a different loading condition. This comparison of predictions demonstrates their sensitivity to the choice of model. In this effort a method for inference of random process parameters and corresponding confidence limits from highly statistically dependent observations is developed.

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.

An Iterative Method for Solving Elasticity Problems for Composite Laminates

An iterative method for approximate analytical solution of elasticity problems in composite laminates is presented. The stress analysis is performed for laminates in the three-dimensional strain state independent of the longitudinal direction. Predictions of the method are compared with results from existing analytical and numerical models. Simple and accurate approximations for stresses are obtained.

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.