I. M. Daniel

Characterization and modeling of mechanical behavior of polymer/clay nanocomposites

Abstract Polymer/clay nanocomposites consisting of epoxy matrix filled with silicate clay particles were investigated. These particles consist of 1 nm thick platelets or layers with an aspect ratio in the range of 100–1000. Recent and ongoing research has shown that dramatic enhancements can be achieved in stiffness and thermal properties in these nanocomposites with small amounts of particle concentration. The resulting nanocomposite properties are intimately related to the microstructure achieved in processing these materials. The ideal situation of full exfoliation, dispersion and orientation is not usually achieved. A more common case is partial exfoliation and intercalation. The latter is a process whereby the polymer penetrates the interlayer spaces of the clay particles, causing an increase in layer spacing (d-spacing). A three-phase model, including the epoxy matrix, the exfoliated clay nanolayers and the nanolayer clusters was developed. The region consisting of matrix with exfoliated clay nanolayers or platelets was analyzed by assuming near uniform dispersion and random orientation. The properties of intercalated clusters of clay platelets were calculated by a rule of mixtures based on a parallel platelet system. The Mori–Tanaka method was applied to calculate the modulus of the nanocomposite as a function of various parameters, including the exfoliation ratio, clay layer and cluster aspect ratios, d-spacing, intragallery modulus, matrix modulus and matrix Poissons ratio. With appropriate parameters obtained from experiments, model predictions were in good agreement with experimental results.

Processing of clay/epoxy nanocomposites by shear mixing

Abstract A three-roll mill was used to disperse/exfoliate the clay nanoparticles in an epoxy matrix. The compounding process was carried out with varying clay contents (1–10 wt.%). The technique was found highly efficient and environmentally friendly in achieving high levels of exfoliation and dispersion within a short period of time.

Progressive Transverse Cracking of Crossply Composite Laminates

A simplified shear lag analysis using a progressive damage scheme is pro posed for crossply composite laminates under uniaxial tensile loading. Closed form solu tions for stress distributions, transverse crack density and reduced stiffnesses of damaged plies as well as the entire laminate can be obtained as a function of applied load and prop erties of the constituent plies, where the shear lag parameter is only a function of geom etry and shear moduli of the lamina. The analysis can also take into account the residual stresses in the laminate. Experimental and predicted results for reduced stiffness as a func tion of crack density are in good agreement for various crossply composite laminates. Pre dicted results for crack density as a function of applied load, as well as stress-strain curves to failure are shown and are in good agreement with experimental results for a crossply laminate under monotonic tensile loading conditions. The proposed theory can be used as a basis for development of a damage accumulation model.

EFFECT OF FIBER WAVINESS ON STIFFNESS AND STRENGTH REDUCTION OF UNIDIRECTIONAL COMPOSITES UNDER COMPRESSIVE LOADING

An investigation has been conducted of the effect of fiber waviness on stiffness and strength reduction of unidirectional composites under compressive loading. Analytical models have been developed for determining the elastic properties and compressive strength as a function of fiber waviness for three types of wavy patterns: uniform, graded and localized waviness. Compression tests were conducted to verify the predictions. Experimental results were in good agreement with predictions based on the analytical models. It is shown that in unidirectional composites both major Youngs modulus and compressive strength are degraded seriously with increasing fiber waviness. Material anisotropy is also shown to influence the degree of stiffness and strength reduction. Interlaminar shear failure was found to be the dominant failure mechanism for unidirectional wavy composites under compressive loading.

Failure Modes of Composite Sandwich Beams

An investigation was conducted of failure modes and criteria for their occurrence in composite sandwich beams. The initiation of the various failure modes depends on the material properties of the constituents (facings and core), geometric dimensions and type of loading. The beams were made of unidirectional carbon/epoxy facings and aluminum honeycomb and PVC closed-cell foam cores. The constituent materials were fully characterized and in the case of the foam core, failure envelopes were developed for general two-dimensional states of stress. Sandwich beams were loaded under bending moment and shear and failure modes were observed and compared with analytical predictions. The failure modes investigated are face sheet compressive failure, adhesive bond failure, indentation failure, core failure andfacing wrinkling.

Elastic properties of composites with fiber waviness

An investigation was conducted of the effects of fiber waviness on the elastic properties of composite materials theoretically and experimentally. Unidirectional and crossply carbon/epoxy composites with uniform, graded and localized fiber waviness were studied. An analytical constitutive model was developed to determine the elastic properties as a function of fiber waviness. Compression tests were conducted to verify the constitutive relations. Experimental results were in good agreement with predictions based on the constitutive model.

Fabrication, testing and analysis of composite sandwich beams

The objective of this work was to determine experimentally the flexural behavior of composite sandwich beams and compare the results with predictions of theoretical models. Sandwich beams were fabricated by bonding unidirectional carbon/epoxy face sheets (laminates) to aluminum honeycomb cores with an adhesive film. All constituent materials (composite laminates, adhesive and core) were characterized independently. Special techniques were developed to prevent premature failures under the loading pins and to ensure failure in the test section. Sandwich beams were tested under four-point and three-point bending. Strains to failure in the face sheets were recorded with strain gages, and beam deflections, and strains in the honeycomb core were recorded by using moire techniques. The beam face sheets exhibited a softening non-linearity on the compression side and a stiffening non-linearity on the tension side. Experimental results were in good agreement with predictions from simple models which assume the face sheets to behave like membranes, neglecting the contribution of the honeycomb core, and accounting for the non-linear behavior of the face sheets.

Effects of Strain Rate on Delamination Fracture Toughness of Graphite/Epoxy

The objective of this paper is to evaluate various experimental techniques and analysis methods for the characterization of interlaminar fracture toughness, and to determine the effects of strain rate on that property for a graphite/epoxy composite. Mode I interlaminar fracture was investigated by means of a double-cantilever beam (DCB) specimen for AS-4/3501-6 graphite/epoxy. Hinged tabs were used to insure unrestrained rotation at the free ends. Specimens were loaded at quasi-static deflection rates of up to 8.5 mm/s corresponding to crack extension rates of over 51 mm/s. Crack extension was monitored by means of strain gages mounted on the surface of the specimen, or a conductive-paint circuit attached to the edge of the DCB specimen. Continuous records were obtained of load, deflection, and crack extension for determination of the strain energy release rate. The latter was expressed as a power law of the crack extension velocity. Results indicate that the strain energy release rate increases with crack velocity by up to 28 percent for the range of rates considered.

Strain Rate Effects on the Transverse Compressive and Shear Behavior of Unidirectional Composites

Methods for dynamic characterization of composite materials were extended and applied to the study of strain rate effects under transverse compression as well as shear. Falling weight impact and Split Hopkinson Pressure Bar systems were developed for dynamic characterization of composite materials in compression and shear at strain rates up to 1800 sO. Strain rates below 10 s5l were generated using a servohydraulic testing machine. Strain rates between 10 sol and 300 sol were generated using the drop tower apparatus. Strain rates above 500 s-l were generated using the Split Hopkinson Pressure Bar. Seventy-two and forty-eight ply unidirectional carbon/epoxy laminates (IM6G/3501-6) loaded in the transverse direction were characterized. Off-axis (15°, 300, 450 and 600) compression tests of the same unidirectional material were also conducted to obtain the in-plane shear stress-strain behavior. Strain rates over a wide range, from 10-4 s-1 (quasi-static) up to 1800 s-1, were recorded. The 90-degree properties, which are governed by the matrix,show an increase in modulus and strength over the static values but no significant change in ultimate strain. The stress-strain curve stiffens as the strain rate increases. This stiffening behavior is very significant in the nonlinear region for strain rates between 10-4 s-I and 1 s51. For strain rates above 1 s-1, the stress-strain behavior continues this stiffening trend until it is almost linear at a strain rate of 1800 s-1. The shear stress-strain behavior, which is also matrix-dominated, shows high nonlinearity with a plateau region at a stress level that increases significantly as the strain rate increases.

Size effect on compression strength of fiber composites failing by kink band propagation

The effect of structure size on the nominal strength of unidirectional fiber-polymer composites, failing by propagation of a kink band with fiber microbuckling, is analyzed experimentally and theoretically. Tests of novel geometrically similar carbon–PEEK specimens, with notches slanted so as to lead to a pure kink band (not accompanied by shear or splitting cracks), are conducted. They confirm the possibility of stable growth of long kind bands before the peak load, and reveal the existence of a strong (deterministic, non-statistical) size effect. The bi-logarithmic plot of the nominal strength (load divided by size and thickness) versus the characteristic size agrees with the approximate size effect law proposed for quasibrittle failures in 1983 by Bažant. The plot exhibits a gradual transition from a horizontal asymptote, representing the case of no size effect (characteristic of plasticity or strength criteria), to an asymptote of slope -1/2 (characteristic of linear elastic fracture mechanics, LEFM). A new derivation of this law by approximate (asymptotically correct) J-integral analysis of the energy release, as well as by the recently proposed nonlocal fracture mechanics, is given. The size effect law is further generalized to notch-free specimens attaining the maximum load after a stable growth of a kink band transmitting a uniform residual stress, and the generalized law is verified by Soutis, Curtis and Flecks recent compression tests of specimens with holes of different diameters. The nominal strength of specimens failing at the initiation of a kink band from a smooth surface is predicted to also exhibit a (deterministic) size effect if there is a nonzero stress gradient at the surface. A different size effect law is derived for this case by analyzing the stress redistribution. The size effect law for notched specimens permits the fracture energy of the kink band and the length of the fracture process zone at the front of the band to be identified solely from the measurements of maximum loads. The results indicate that the current design practice, which relies on the strength criteria or plasticity and thus inevitably misses the size effect, is acceptable only for small structural parts and, in the interest of safety, should be revised in the case of large structural parts.