David Mollenhauer

Strength Prediction in Open Hole Composite Laminates by Using Discrete Damage Modeling

The present paper addresses the issue of direct simulation of complex local failure patterns in laminated composites. A model capable of discrete modeling of matrix cracking, delamination, and the interaction of these two damage modes is proposed. The analytical technique uses the eXtended Finite Element Method (X-FEM) for the simulation of matrix crack initiation and propagation at initially unknown locations, as well as a cohesive interface model for delamination. The model is capable of representing the complex kinematics of crack networks in composite laminates without previous knowledge of the crack locations or user intervention. An important feature of the technique is that it uses independently measured standard ply-level mechanical properties of the unidirectional composite (stiffness, strength, fracture toughness). Failure simulations of composites containing open holes are presented. Although the process of crack initiation is impossible to capture precisely due to local material variations, the proposed method exhibits excellent agreement with experimental data for matrix crack growth in unidirectional graphite-epoxy composites.

Micro-Geometric Modeling of Textile Preforms with Vacuum Bag Compression: An Application of Multi-chain Digital Element Technique

[Abstract] Increasing demands on composite technology necessitate structural materials possessing both great ductility and extreme strength. To fully understand the mechanical behavior of 3-D textile composites, it is essential to perform analyses such as prediction of effective material properties and characterization of damage initiation and growth based on highly accurate fabric geometry. Most textile composites used in the aircraft industry are made by laminating multiple fabric layers, which are nested randomly, flattened, border stitched, compacted by rigid molding or flexible vacuum bags. The effects of the forming process on the yarn geometry are investigated. In this paper, we present a novel numerical approach to predict fabric geometry under vacuum bag compression. Textile fabric layers were modeled as multiple digital chains and nested randomly. The vacuum bag was represented as a fishing-net-like structure. The contact elements between the vacuum bag and fabrics were established. The vacuum pressure was applied to each knot on the net structure evenly. The results show that the textile fabrics are indeed deformed dramatically during the vacuum bagging compression.

Simulation of Mode I Fracture at the Micro-Level in Polymer Matrix Composite Laminate Plies

A technique for simulating evolving matrix damage through a polymer matrix composite ply was developed. This method simulates disbonding between fiber and matrix and cracking within the matrix at the micro-scale. The aim of the study is to develop a methodology whereby the mode I traction-separation law (cohesive zone) for a given lamina could be obtained by simulation only using the fiber and matrix constituent properties as inputs. The results obtained with randomly spaced fibers representing approximately 1/3 of a ply thickness are encouraging.

Full-field singular stresses in a composite laminate weakened by a cylindrical cavity: theory and experiment

Analytical and experimental investigation of strain fields arising in composite laminates in the vicinity of an open hole and ply interfaces was performed. The singular stress fields, arising at the ply interface and hole edge intersections, were analytically examined by using a novel technique based on superposition of an asymptotic and spline approximation solution. The moiré interferometry technique, providing high spatial resolution and allowing rapid strain variations across a laminate thickness to be recorded, was utilized to experimentally measure the strain distributions throughout the thickness of a [+302/-302/904]3s laminate. The singular solution and the experimentally measured strain showed good agreement for most strain components with some discrepancy for the interlaminar normal strain component.

Mechanism-Based Direct Simulation of Tensile Failure in Composite Laminates

High-fidelity mechanistic modeling of deformation and failure in composite materials is a critical step toward increasing their application across the aerospace industry. The present paper is devoted to development of such tools geared toward laminated composite applications. Matrix cracking and delamination initiation, propagation, and interaction prediction in laminated composites without any prior knowledge and/or meshing of matrix cracking surfaces was accomplished by combining stress and fracture mechanics-based constitutive modeling within a mesh independent crack-modeling framework. Tensile loading of quasi-isotropic and angle ply laminates was performed and captured the correct delamination sequence, evolving from automatically generated matrix cracks, as well as the failure load values.

Application of Discrete Damage Modeling to Laminated Composite Overheight Compact Tension Specimens

Damage progression in laminated Overheight Compact Tension specimens was modeled using discrete representations of individual cracks and delaminations. Matrix cracking and delamination initiation, propagation, and interaction, without any prior knowledge and/or meshing of matrix cracking surfaces, is accomplished by combining stress and fracture mechanics-based constitutive modeling within a mesh independent crack-modeling framework. Simulation results for a specimen with a [452/902/-452/02]s stacking sequence were compared with load-displacement curves and 3D X-ray micro computed tomography results from tested specimens. Excellent correlation was shown between the simulated and experimental load-displacement curves including proper representation of both the curve non-linearity and peak load. Similarly, remarkable correlation between simulated and experimental damage extent was shown. Delamination extent and shape were quite close. Crack distribution and extent was close but differed between the two data sets more than the comparison of delamination damage.

Image Reconstruction Based Modeling of 3D Textile Composites

[Abstract] Innovative weaving and braiding processes open up a new opportunity for making 3-D textile composites that give significantly damage-tolerant structural response with design flexibility for durable joints, near-net shape processing, etc. To fully understand the mechanical behavior of 3-D textile composites, it is essential to perform analyses to predict effective material properties and damage initiation and growth. In this paper we present a new approach to generating 3D textile composite geometric models based on image processing techniques. The main objectives are to visualize, manipulate, and reconstruct textile internal structures based on multidimensional image data for the purpose of further mechanics analysis. A software code called ImageScan is developed to generate geometry models from a set of image slices of a textile composite based on image reconstruction technology. The images from an optical microscope or other source can be segmented into objective constituents and reconstructed into 3D geometry, which can be input into an appropriate mechanics model to predict the material properties and mechanical deformation under a specific boundary condition and loadings.

Detailed Morphology Modeling and Residual Stress Evaluation in Tri-axial Braided Composites

Local strain fields in tri-axial braided composites arising in the vicinity of a saw cut due to the release of thermal processing stresses were experimentally measured by using moire interferometry technique and compared to those obtained by 3D stress analysis. The independent mesh method (IMM) and digital chain methods were used to perform the 3D stress analysis and the composite tow morphology modeling respectively. A significant degree of morphological detail was required to achieve good comparison with experimental data. Three degrees of refinement were produced in direction of matching the actual morphology of the tri-axial braded composite which was tested experimentally. These levels of refinement included (i) correct tow path angle and curvature variation based on braid parameters (ii) addition of the effect of compaction process during the cure stage and (iii) addition of the surface sanding affects during moire test preparation stage. The results obtained by using IMM were able to capture sharp variations of the strain components observed by using the moire interferometric technique both in terms of spatial distribution and magnitude and provide accurate evaluation of the residual strain levels in the triaxial braided composites.

Modeling of Complex Fiber Architecture Composites Using the Independent Mesh Method

[Abstract] An independent mesh modeling approach is proposed for complex multiple connected configuration of matrix in composite materials with three-dimensional fiber architecture. The approach is based on yarn surface geometry definitions imported from an image reconstruction and/or a predictive tool describing the yarn placement in the composite. The phase boundary between the yarn and the matrix is described as an approximation of the Heaviside step function with higher order shape functions. Polynomial B-spline approximation functions are used in the present paper. Fiber yarns are modeled by using displacement based spline approximation and yarn shape geometry. The displacement continuity condition between the yarns and the matrix is imposed by using the penalty function method. A model solution for plates with inclusions was obtained and examined to evaluate the accuracy of the method. An advantage of the proposed technique is that it appears to tolerate small errors in yarn geometry definition, such as interpenetrations. Tensile loading of a triaxial braided composite was considered.