Three-dimensional analysis of anisotropic spatially reinforced structures

The material-adaptive three-dimensional analysis of inhomogeneous structures based on the meso-volume concept and application of deficient spline functions for displacement approximations is proposed. The general methodology is demonstrated on the example of a brick-type mosaic parallelepiped arbitrarily composed of anisotropic meso-volumes. A partition of each meso-volume into sub-elements, application of deficient spline functions for a local approximation of displacements and, finally, the use of the variational principle allows one to obtain displacements, strains, and stresses at anypoint within the structural part. All of the necessary external and internal boundary conditions (including the conditions of continuity of transverse stresses at interfaces between adjacent meso-volumes) can be satisfied with requisite accuracy by increasing the density of the sub-element mesh. The application of the methodology to textile composite materials is described. Several numerical examples for woven and braided rectangular composite plates and stiffened panels under transverse bending are considered. Some typical effects of stress concentrations due to the material inhomogeneities are demonstrated

Stochastic damage evolution in textile laminates

A probabilistic model utilizing random material characteristics to predict damage evolution in textile laminates is presented. Model is based on a division of each ply into two sublaminas consisting of cells. The probability of cell failure is calculated using stochastic function theory and maximal strain failure criterion. Three modes of failure, i.e. fiber breakage, matrix failure in transverse direction, as well as matrix or interface shear cracking, are taken into account. Computed failure probabilities are utilized in reducing cell stiffness based on the mesovolume concept. A numerical algorithm is developed predicting the damage evolution and deformation history of textile laminates. Effect of scatter of fiber orientation on cell properties is discussed. Weave influence on damage accumulation is illustrated with the help of an example of a Kevlar/epoxy laminate

The effects of specimen width on tensile properties of triaxially braided textile composites

The objective of this study was to examine the effect of the unit cell architecture on the mechanical response of textile reinforced composite materials. Specifically, the study investigated the effect of unit cell size on the tensile properties of 2D triaxially braided graphite epoxy laminates. The figures contained in this paper reflect the presentation given at the conference. They may be divided into four sections: (1) a short definition of the material system tested; (2) a statement of the problem and a review of the experimental results; (3) experimental results consist of a Moire interferometry study of the strain distribution in the material plus modulus and strength measurements; and (4) a short summary and a description of future work will close the paper

Monitoring of acoustic emission damage during tensile loading of 3D woven carbon/epoxy composites

Registration of acoustic emission (AE) events during tensile loading of fiber-reinforced composites allows the damage caused by these events to be defined and monitored, including damage initiation and progression thresholds. It also provides frequency-based recognition of different types of damage and comparison of its intensity in materials with different reinforcement architectures. The paper reports results of AE registration for 3D non-crimp orthogonal woven (3DNCOW) carbon/epoxy composites. The observed repeatability and spatial distribution of AE events confirm that damage initiation and development are uniform over the tensile sample. The damage characterization by AE is compared with the morphology of damage observed on the specimen cross-sections at characteristic stages of the damage development. The main parameter distinguishing damage mode obtained from the AE registration is the AE energy. It has however been found that the peak frequency of the AE events does not correlate directly with the sequence of the observed damage modes. AE events of high peak frequency, assumed to be related to fiber fracture, suggest that it starts at a later stage than predicted by the Weibull statistics of fiber strength.KU Leuven (2009

Fatigue tensile behavior of carbon/epoxy composite reinforced with non-crimp 3D orthogonal woven fabric

An experimental study of the in-plane tension-tension fatigue behavior of the carbon fiber/epoxy matrix composite reinforced with non-crimp 3D orthogonal woven fabric is presented. The results include pre-fatigue quasi-static test data, fatigue life diagrams, fatigue damage progression, and post-fatigue quasi-static test data for the warp- and fill-directional loading cases. It is revealed that the maximum cycle stress corresponding to at least 3 million cycles of fatigue life without failure, is in the range of 412-450 MPa for both loading directions. This stress range is well above the static damage initiation threshold and significantly above the first static damage threshold (determined by the onset of low energy acoustic emission). The second static damage threshold, determined by the onset of high energy acoustic emission and related to the appearance of local debonds and intensive transverse matrix cracking falls within this range. The established correlation between a 3000,000 cycle fatigue stress limit on one side and the second static damage threshold stress on the other is of a high practical importance, because it will significantly reduce the amount of future fatigue tests required for this class of composites. Surprisingly, for equal maximum cycle stress level, the fatigue life under fill-directional loading appears about three times shorter than that under warp-directional loading. The 100,000 cycle, 500,000 cycle and 1000,000 cycle fatigue loading with 450 MPa maximum cycle stress has resulted in so high variations of post-fatigue static modulus, strength and ultimate strain, that no consistent and statistically meaningful trends could have been established; further extensive experimental studies are required to reliably quantify this effect. (C) 2011 Elsevier Ltd. All rights reserved.KU Leuve

Quasi-static tensile behavior and damage of carbon/epoxy composite reinforced with 3D non-crimp orthogonal woven fabric

This paper presents a comprehensive experimental study and detailed mechanistic interpretations of the tensile behavior of one representative 3D non-crimp orthogonal woven (3DNCOW) carbon/epoxy composite. The composite is tested under uniaxial in-plane tensile loading in the warp, fill and +/- 45 degrees bias directions. An "S-shape" nonlinearity observed in the stress-strain curves is explained by the concurrent contributions of inherent carbon fiber stiffening ("non-Hookean behavior"), fiber straightening, and gradual damage accumulation. Several approaches to the determination of a single-value Young's modulus from a significantly nonlinear stress-strain curve are discussed and the best approach recommended. Also, issues related to the experimental determination of effective Poisson's ratios for this class of composites are discussed, and their possible resolution suggested. The observed experimental values of the warp- and fill-directional tensile strengths are much higher than those typically obtained for 3D interlock weave carbon/epoxy composites while the nonlinear material behavior observed for the +/- 45 degrees-directional tensile loading is in a qualitative agreement with the earlier results for other textile composites. Results of the damage initiation and progression, monitoried by means of acoustic emission, full-field strain optical measurements, X-rays and optical microscopy, are illustrated and discussed in detail. The damage modes at different stages of the increasing tensile loading are analyzed, and the principal progressive damage mechanisms identified, including the characteristic crack patterns developed at each damage stage. It is concluded that significant damage initiation of the present material occurs in the same strain range as in traditional cross-ply laminates, while respective strain range for other previously studied carbon/epoxy textile composites is significantly lower. Overall the revealed advantages in stiffness, strength and progressive damage behavior of the studied composite are mainly attributed to the absence of crimp and only minimal fiber waviness in the reinforcing 3DNCOW preform.K.U. Leuve

Shock loading response of sandwich panels with 3-D woven E-glass composite skins and stitched foam core

Sandwich composite are used in numerous structural applications, with demonstrated weight savings over conventional metals and solid composite materials. The increasing use of sandwich composites in defense structures, particularly those which may be exposed to shock loading, demands for a thorough understanding of their response to suc highly transient loadings. In order to fully utilize their potential in such extreme conditions, design optimization of the skin and core materials are desirable. The present study is performed for a novel type of sandwich material, TRANSONITE® made by pultrusion of 3-D woven 3WEAVE® E-glass fiber composites skin preforms integrally stitched to polyisocyanurate TRYMERTM 200L foam core. The effect of core stitching density on the transient response of three simply supported sandwich panels loaded in a shock tube is experimentally studied in this work. The experimental program is focused on recording dynamic transient response by high-speed camera and post-mortem evaluation of imparted damage. The obtained experimental results reveal new important features of the transient deformation, damage initiation and progression and final failure of sandwich composites with unstitched and stitched foam cores. The theoretical study includes full 3-D dynamic transient analysis of displacement, strain and stress fields under experimentally recorded surface shock pressure, performed with the use of 3-D MOSAIC analysis approach. The obtained theoretical and experimental results for the transient central deflections in unstitched and two stitched foam core sandwiches are mutually compared. The comparison results reveal large discrepancies in the case of unstitched sandwich, much smaller discrepancies in the case of intermediate stitching density, and excellent agreement between theoretical and experimental results for the sandwich with the highest stitching density. The general conclusion is that further comprehensive experimental and theoretical studies are required in order to get a thorough understanding of a very complex behavior of composite sandwiches under shock wave loading. © 2008 Elsevier Ltd. All rights reserved