Micromechanical analysis of stress relaxation response of fiber-reinforced polymers

The paper describes a micromechanical method to determine the stress relaxation response of polymer composites consisting of linearly viscoelastic matrices and transversely isotropic elastic fibers. A representative unit cell is subjected to some prescribed axial and shear loadings to study and quantify the time-dependent behavior of composite materials. Closed-form analytical expressions are derived describing the anisotropic viscoelastic response of composite materials as functions of matrix and fiber properties. The present analytical expressions are employed to determine the stress relaxation behavior of a graphite/epoxy composite and the results are compared with the finite element analysis of the micromechanical model. Very good correlation between analytical expressions and numerical results is illustrated for the linearly anisotropic viscoelastic response of composite materials.     

A method for measurement of multiple constitutive properties for composite materials

Accurate stress–strain constitutive properties are essential for understanding the complex deformation and failure mechanisms for materials with highly anisotropic mechanical properties. Among such materials, glass-fiber- and carbon-fiber-reinforced polymer–matrix composites play a critical role in advanced structural designs. The large number of different methods and specimen types currently required to generate three-dimensional allowables for structural design slows down the material characterization. Also, some of the material constitutive properties are never measured due to the prohibitive cost of the specimens needed. This work shows that simple short-beam shear (SBS) specimens are well-suited for measurement of multiple constitutive properties for composite materials and that can enable a major shift toward accurate material characterization. The material characterization is based on the digital image correlation (DIC) full-field deformation measurement. Two key elements show advantage of using DIC in the SBS tests. First, tensile, compressive, and shear stress–strain relations are measured in a single experiment. Second, a counter-intuitive feasibility of closed-form stress and modulus models, normally applicable to long beams, is demonstrated for short-beam specimens. The modulus and stress–strain data are presented for glass/epoxy and carbon/epoxy material systems.     

A THERMOELASTICITY THEORY FOR DAMAGE IN ANISOTROPIC MATERIALS

A thermoelasticity theory for damage in anisotropic materials is developed. This theory can be applied to evaluate the damage parameter D, the normalized effective mess density ρ^e/ρ, and the effective modulus E^e quantitatively in some metals and composites by incorporating the thermographic stress analysis method (TSA; also, SPATE method). The effective moduli due to fatigue damage and static loads in a glass fiber/epoxy laminate obtained by the TSA method are compared with the values measured by an extensometer. The correlation between the measurements using the two methods is very good.     

Influence of date palm fibre and graphite filler on mechanical and wear characteristics of epoxy composites

In this article, mechanical and tribological performance of the epoxy composites based on graphite filler and/or date palm fibre are comprehensively discussed. The influence of the date palm fibre and/or graphite filler on the microstructure of the materials, tensile fracture samples, and worn surface of tribological samples are examined using scanning electron microscopy. The results revealed that interfacial adhesion of the date palm fibre with the epoxy is the key of the mechanical and tribological performance of natural fibre/polymer composites. The addition of the graphite is highly recommended for the natural fibre/polymer composites which can assist to reduce the friction which in turn enhances the wear characteristics of the polymer composites; however, the high content of the graphite deteriorates the mechanical properties.     

Blast impact response of aluminum foam sandwich composites

Military and civilian structures can be exposed to intentional or accidental blasts. Aluminum foam sandwich structures are being considered for energy absorption applications in blast resistant cargo containers, ordnance boxes, transformer box pads, etc. This study examines the modeling of aluminum foam sandwich composites subjected to blast loads using LS-DYNA software. The sandwich composite was designed using laminated face sheets (S2 glass/epoxy and aluminum foam core. The aluminum foam core was modeled using an anisotropic material model. The laminated face sheets were modeled using material models that implement the Tsai-Wu and Hashin failure theories. Ablast load was applied using the CONWEP blast equations (*LOAD_BLAST) in LS-DYNA. This paper discusses the blast response of constituent S2-glass/epoxy face sheets, the closed cell aluminum foam core as well as the sandwich composite plate.     

Purity-dependent structures of Al nanoclusters on HOPG observed by STM

Nanoclusters with a diameter of 2–10 nm were deposited from highly pure Al and Al–50%Cu targets onto highly oriented pyrolitic graphite (HOPG) by pulsed laser ablation. The structures and atomic configurations of these nanoclusters were then investigated with atomic resolution using a scanning tunneling microscope. On the pure Al nanoclusters, hexagonal two-dimensional configurations were found over a large area around a three-dimensional nanocluster. These two-dimensional structures were oriented at 30° to the graphite lattice and the atomic distance was estimated to be 0.424 nm. On the other hand, two-dimensional atomic configurations were not observed around the Al–50%Cu nanoclusters and the atomic distance was almost the same as that of highly oriented pyrolytic graphite visible in the STM. Thus, the initial growth and structures were completely different between the two materials.     

建筑膜材料在双轴拉伸作用下的特性

通过对PVC膜材料进行7种应力比下的双轴拉伸试验(比值分别为0∶1、1∶5、1∶2、1∶1、2∶1、5∶1和1∶0),对膜材料在双轴拉伸作用下的拉伸特性进行了研究。在试验过程中施加了3次循环荷载,以使得经过循环作用后的膜材料的性能更接近实际使用的情况。结果表明膜材料具有明显的正交异性和非线性。该文推导了双轴拉伸作用下经纬向应变之间的关系,结合试验所得的变形特性,就经纬向应力比C≤0.5时,经向弹性常数为负这一现象作了分析。根据已有的理论,对膜材料双轴向弹性常数进行了分析,当C=1时,膜材料的经向和纬向同时具有最大的弹性常数。     

Finite element simulation of low velocity impact on shape memory alloy composite plates

Delamination of composite materials due to low velocity impacts is one of the major failure types of aerospace composite structures. The low velocity impact may not immediately induce any visible damage on the surface of structures whilst the stiffness and compressive strength of the structures can decrease dramatically.

Shape memory alloy (SMA) materials possess unique mechanical and thermal properties compared with conventional materials. Many studies have shown that shape memory alloy wires can absorb a lot of the energy during the impact due to their superelastic and hysteretic behaviour. The superelastic effect is due to reversible stress induced transformation from austenite to martensite. If a stress is applied to the alloy in the austenitic state, large deformation strains can be obtained and stress induced martensite is formed. Upon removal of the stress, the martensite reverts to its austenitic parent phase and the SMA undergoes a large hysteresis loop and a large recoverable strain is obtained. This large strain energy absorption capability can be used to improve the impact tolerance of composites. By embedding superelastic shape memory alloys into a composite structure, impact damage can be reduced quite significantly.

This article investigates the impact damage behaviour of carbon fiber/epoxy composite plates embedded with superelastic shape memory alloys wires. The results show that for low velocity impact, embedding SMA wires into composites increase the damage resistance of the composites when compared to conventional composites structures.     

Non-linear current-voltage characteristics in anisotropic epoxy resin-graphite flake composites

The conductivity of anisotropic composite materials constituted of single-crystal graphite flakes in an epoxy resin and subjected to a variable d.c. electric field has been studied. Non-linearities in the current-voltage characteristics have been found and are discussed within the framework of two theoretical models, the non-linear random resistor network and the dynamic random resistor network. We show that our materials can be modelled by the non-linear random resistor network, which, to our knowledge, is the first time that the predictions of this model seem so well supported in a real, three-dimensional composite. This revised version was published online in November 2006 with corrections to the Cover Date.     

Machining-induced surface texture effects on the flexural properties of a graphite/epoxy laminate

The effects of surface texture induced by secondary processing on the flexural properties of a graphite/epoxy laminate were evaluated. Test specimens were machined by three methods including the abrasive waterjet, circular diamond saw and shaper planer mounted with polycrystalline diamond tool inserts. Machined surface topography was evaluated by surface profilometry as well as scanning electron microscopy. Flexural strength and modulus were obtained from fourpoint flexure loading to failure and Weibull statistics were used to evaluate characteristic strength and modulus. Although the textures of the machined surface representative of each process were different, no difference in bulk strength of the graphite/epoxy laminate was noted under bending loads.     

Deformation and fracture in graphene nanosheets

Graphene nanosheets (GNSs) are flake-like materials composed of few-layer graphene sheets. GNSs are similar to multi-walled carbon nanotubes (CNTs) in graphene structures and in layer numbers. However, GNSs and CNTs behave very differently in deformation and fracture. In this study, natural graphite flakes were employed to make expanded graphite (EG), which is composed of partially connected GNSs. Both sonication and three-roll milling were used to separate the GNSs and to disperse them into an epoxy resin. By compacting EG, the GNSs inside were compressed and deformed. By breaking the GNS/epoxy composite, most GNSs on the cracked surfaces were fractured. Both SEM and TEM have been used for microscopic observations. The micrographs revealed that folding and wrinkling are the major modes of deformation, while tearing and peeling are the dominant modes of fracture. These modes are virtually non-existent in CNTs. The factors to cause the different behavior are discussed.     

Friction and Wear Behavior of Resin/Graphite Composite under Dry Sliding

The friction and wear behavior of resin/graphite composite has been investigated using a pin-on-disc configuration under dry sliding condition. The results showed that the resin/graphite composite exhibited much better mechanical and tribological properties compared with the unimpregnated graphite. The friction coefficient was reduced by addition of furan resin, which could also prevent the"dusting" wear at loads more than 15 MPa. The steady and lubricated transfer film was easily formed on the counterpart surface due to the interaction of furan resin and wear debris of graphite, which was useful to reduce the wear rate of the resin/graphite composite. The composite is highly promising for mechanical sealing application and can be used at high load for long time sliding.     

Stress-wave displacement polarizations and aftenuation in unidirectional composites: Theory and experiment

The wave propagation mechanism of changes in displacement polarizations was studied in unidirectional graphite/epoxy composite materials. Change in Displacements can be large enough to cause a transition in the mode or displacement polarizations from longitudinal to transverse. These unusual mode transitions are a result of the peculiar elastic anisotropy observed in only a few crystals and unidirectional graphite/epoxy composities at high-fiber volume fractions Theoretical calculation of these mode transitions were compared with experimental measurements Mode transitions occur when the wave vector orientation is varied from 51.9° to 74.4° in unidirectional samples of T300/5208 graphite/epoxy composite with a 0.6°-fiber volume fraction. Energy flux deviation and particle displacement directions and amplitudes also were compared with theory. To show this mode transition, an attenuation study was performed. The attenuation coefficient, measured in units of reciprocal time, does not appear to depend on the wave vector orientation and the wave type (quasi-transverse and quasi-longitudinal waves) at 5-MHZ frequency. But the attenuation coefficient, expressed in units of reciprocal length, does depend on the wave type and the wave vector orientation because the wave velocity is included in the calculation of this coefficient. Previous studies have focused on how anisotropy and attenuation influence the stress wave speed (eigenvalues), but in this study we focused more on how the same parameters influence the displacement polarizations (eigenvectors) of the same propagating waves. Because eigenvalues and their corresponding eigenvectors are both solutions of the same eigenvalue problem, more attention should be given to measurement of the eigenvectors.     

Electronic Shearography for Detecting Disbonds in Lattice/Skin Structures

Aluminum structures with an integral lattice (rib system) and skin currently are used for a variety of large space structures (Titan and Delta series) due to their structural efficiency. Various components of space structures will be manufactured out of graphite/epoxy composites in the future. Interstages of rockets, decks of small satellites (MightySat program), payload shrouds, solar cell substrates, rocket motor casings, etc., are being fabricated at the Air Force Phillips Laboratory. Tests have shown that the principal mode of initial failure in these structures involves separation between the skin and the ribs, during fabrication or loading. The redundancy of the rib system in these structures leads to alternate load paths that make it difficult to detect such defects. The fabrication, integration, and testing often are carried out in different parts of the country. It is therefore important to have a method of detecting and possibly quantifying the intergrity of the rib/skin interface. The objective of this paper is to demonstrate that electronic shearography (ES) can be used to detect disbonds in rib/skin ``Isogrid' structures. This laser-based interferometry technique provides fringe patterns that represent full-field displacement gradients. The ruggedness and portability of the system make it a prime candidate for in-service inspection of large structures. The observed fringe patterns change dramatically for disbonded ribs that form a basis for rapidly detecting disbonds over a large area. The expected fringe patterns can be quantified and compared with results from finite-element (FEM) analyses of the structure. A commercial FEM code was used with orthotropic material properties that are representative of the composites used.     

Preparation and Surface Characterization of Highly Ordered Polymer/Graphite Nanosheet Composites

Graphite nanosheets obtained from sonicating expanded graphite were coated chemically with magnetite nanoparticles. The modified graphite nanosheets were dispersed in epoxy or unsaturated polyester prepolymer, and subjected to a magnetic field before the suspension solidified. Composites with graphite nanosheets of highly ordered alignment have been prepared via the described approach. Several techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical microscopy were used to characterize the modified graphite nanosheets and the polymer/graphite nanosheet composites.     

Surface Wave Utility in Composite Material Characterization

Abstract

A surface wave velocity measurement technique is used to supply supporting measurements in the computation of elastic constants for practical nondestructive evaluation of composite materials. Theoretical modeling work is carried out to illustrate the surface wave velocity changes as a function of angle with respect to the axes along the fibers of a unidirectional graphite epoxy composite material for a variety of different problems, including porosity (PC) changes, fiber volume fraction (FF) changes, and delamination. Experiments are conducted on two unidirectional reinforced composites and a (0–90)_s cross ply graphite epoxy laminate to illustrate the surface wave velocity measurements and the inverse computation procedure for evaluation of the stiffness coefficients. Variations of the feature values in the stiffness matrix are also discussed for inhomogeneities, delaminations through cracking, and large defects.     

Graphite/epoxy honeycomb core sandwich permeability under mechanical loads

The air permeability of two honeycomb core sandwich materials as a function of applied shear stress was characterized. The honeycomb core sandwich specimens were provided to Analytical Services and Materials, Inc. by The Boeing Company for evaluation. The core material for the test specimens was either Hexcel HRP-3/16-8.0 or DuPont Korex-1/8-4.5 and was nominally 1.27 cm (0.5 in.) thick. The facesheets where made of Hercules' AS4/8552 graphite/epoxy (Gr/Ep) composites and were nominally 0.15 cm (0.059 in.) thick. The permeability of the sandwich specimens during both static (tension) and dynamic (reversed and non-reversed) shear loads were measured. The permeability was measured as the rate of air flow through the core from a 2.54-diameter circular area of the core exposed to an air pressure of 68.9 kPa (10.0 psig). In both the static and dynamic testing, the Korex core experienced sudden increases in core permeability corresponding to a core catastrophic failure, while the HRP core generally experienced a gradual increase in the permeability prior to a bond line failure. The Korex core specimens failed at lower loads than the HRP core specimens both in the transverse and ribbon directions.     

Mechanical properties of potentially-smart carbon/epoxy composites with asymmetrically embedded shape memory wires

Embedding Shape Memory Alloy (SMA) wires in composite structures enables controlling of their mechanical properties. The main aim of this study is to characterize experimentally the mechanical properties of two-layer smart composite structures which are made of one layer of carbon fibers epoxy laminate and one layer of epoxy embedded with SMA wires. A carbon/epoxy layer was first fabricated using vacuum infusion method. Then a SMA/epoxy layer was prepared separately and then laid over the completely cured carbon/epoxy layer using the hand lay-up process. The final structure is smart and has potential of being specifically bent under controlled thermal loading, due to the embedded pre-strained SMA wires. However the temperature was kept constant and there was no thermal excitation of the SMA wires in this experimental study. The configuration of the material constituents through the thickness of the structure renders the cross-section to be unsymmetrical. The specimens were tested in a specially developed unsymmetrical tensile testing machine. From the readings of force from the testing machine and strain gages, the tensile and shear stress–strain relations of the composite materials were obtained. The elastic and shear moduli and also Poisson’s ratio of the composite materials were defined and it was observed that, the effective moduli increased with increasing density of SMA wires in the layer. It is concluded that, due to the asymmetrical material variation, finding the mechanical properties via conventional testing machine is not accurate and a special testing machine is needed.     

Suggestions regarding thermal diffusivity measurements on pyrolytic graphite and pyrolytic boron nitride by the laser pulse method

The laser pulse method can be successfully applied to the measurement of thermal diffusivity of isotropic materials subject to some assumptions. For anisotropic materials, this method is applicable to the measurement of principal thermal diffusivity only on the condition that there is no difference in direction between the principal axis and that of the temperature gradient. After analyzing the heat conduction process in an anisotropic solid, it has been shown that large errors in the measurement of thermal diffusivity would exist if the direction of the principal axis deviates inconspicuously from that of the temperature gradient. The experimental results of thermal diffusivity of highly oriented pyrolytic graphite (HOPG) samples with various deviation angles have been compared with the analytical results. The laser pulse method is not applicable to measurements on semitransparent pyrolytic boron nitride (PBN). We adopted a two-layer composite sample to measure the thermal diffusivity of PBN in the c direction and a particular graphite-PBN composite sample has been prepared which has a very low thermal resistance at the interface. The thermal diffusivity and thermal conductivity of PG (below 2300°C) and PBN (below 1000°C) are given.Invited paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Hierarchical electromagnetic absorption in micro-waveguide stuffed with magnetic media for resin-based composites of graphene nanosheets and manganese oxides

Waveguide configurations of hierarchical system are proposed as new microstructures for composites in absorbing enhancement. Supercritical fluid (SCF) one-pot exfoliation of layered graphite and manganese oxide mixing materials is developed to obtain a hierarchical system, containing graphene nanosheets (GNS) and exfoliated manganese oxides (EMO) in different sizes. Composites with GNS–EMO embedded in epoxy resin matrix are produced for a design of dielectric and magnetic loss integrated absorber. Volume fraction of GNS–EMO in composites is given for an optimal quantity of resin epoxy in fixation and formation. The effect of mixing ratios between electric and magnetic components is provided for the design of dielectric and magnetic loss integrated absorbers. Frequency shifting phenomena are revealed in the component adjusting course. Excluding the offsetting sizes, reflection loss of composites is enhanced as thickness increases. Synergistic effect of electric and magnetic coordinated materials demonstrates the superiority of micro-waveguide structures in GNS–EMO composite absorber.