Endel V. Iarve

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.

Three-dimensional stress analysis in laminated composites with fasteners based on the B-spline approximation

Abstract A three dimensional contact problem describing the interaction between a composite plate with a circular hole and elastic fastener has been solved by using the displacement spline approximation and Lagrangian multiplier method. An algorithm capable of adjusting the nonuniform through the thickness contact zone was developed. The converged contact conditions: radial stress and displacement continuity are shown in each ply of a [−45/90/45/0] s laminate loaded in bearing through an elastic fastener. An asymptotic solution of the three body contact problem appearing at the elastic fastener hole edge in a composite laminate is formulated and solved. The power of singularity at a ± 45 ply interface in an AS4/3501-6 laminate with a filled hole was calculated for different stiffnesses of the fastener. At most circumferential locations the power of singularity rapidly increased with increasing fastener stiffness. The behavior of all stress components has been studied in the vicinity of a titanium fastener hole edge and ply interface in a [45/ − 45] S AS4/3501-6 laminate under uniaxial tension. Good agreement between the singular term of the asymptotic solution and spline variational solution for all stress components was observed in the filled hole tension problem.

Local Field Assessment inside Multiscale Composite Architectures

We introduce asymptotic expansions for recovering the local field behavior inside multiscale composite architectures in the presence of residual stress. The theory applies to zones containing abrup...

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.

Asymptotically exact stresses in laminates with a rigid fastener

Abstract A method of superposition of a hybrid and displacement approximation has been extended to provide accurate stress fields in a multilayered composite laminate including the singular neighborhood of the ply interface and the edge of a hole containing a rigid fastener. Asymptotic analysis was used to derive the hybrid stress functions. Multiple singular terms at each interface were incorporated. The displacement approximation is based on polynomial B-spline functions. The method provides determination of the coefficient of the singular term along with convergent stress components, including the singular regions. Reissners variational principle was employed. Coefficients of the singular term of the asymptotic expansion were determined for a [45/-45]s laminate under bearing loading introduced through a rigid fastener.

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.

Mesh-Independent Modeling and Moiré Interferometry Studies of Damage Accumulation in Open-Hole Composite Laminates

A three-dimensional ply-level modeling of multiple matrix cracking near an open hole in a quasi-isotropic composite laminate was performed. A mesh-independent displacement discontinuity modeling method based on higher-order shape functions was constructed for this purpose. The mesh configuration is dictated by the boundaries of a specimen, such as the presence of a hole, whereas the matrix cracking surfaces are aligned with the fiber direction in a given ply. The surface of the displacement jump associated with matrix cracking was defined in terms of the domain Heaviside function approximated by using higher-order polynomial B-splines. Several matrix cracks in each ply of a [0/45/90/–45]s composite were modeled, and their effect on the fiber-direction stress magnitude in the 0° ply was examined. Up to 35% relaxation of the fiber-direction strain amplitude due to matrix cracking (splitting) in the 0° ply was predicted. The moir? interferometry was used to experimentally determine the strain and displacement fields in the surface layer of the same composite, previously prestressed beyond the damage initiation load. A good correlation between the experimental data and the stress redistribution predicted by the mesh-independent damage modeling technique was observed.

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.