Composites Engineering最新文献

Alumina-reinforced aluminum 6061 alloy composites containing 10, 15 and 20 volume percentage particulates were tested using the standard tensile and Charpy impact toughness tests. A statistical approach, known as the Taguchi method, was implemented to identify heat treatment conditions for improved tensile and impact toughness properties. Modulus of elasticity, yield strength, and ultimate tensile strength all increased while impact toughness decreased as a function of the volume percentage of reinforcement for the alumina/aluminum composites in the T6 heat treated condition. Taguchi analysis results indicate that, on average, a 4% increase in yield strength and a 7% increase in ultimate tensile strength over the standard T6 heat treatment can be attained using a heat treatment of 6 h of solutionizing at 529°C and 12–18 h of aging at 160°C. Similarly, using a heat treatment of 2 h of solutionizing at 529°C and 12 h of aging at 127°C results in an impact toughness improvement of more than 87% over the standard T6 heat treatment.

Considering the increasing applications of composites as structural components, the reliability of these materials is an important issue. In this paper, stress-strength interference theory is used to determine the reliability of a component that is characterized having a Weibull strength distribution and is under the effects of an applied load that is not deterministic, but follows a probability distribution. The assumption that failure is caused by the maximum of a sequence of applied loads is the basis for describing the applied load distribution as a Gumbel Type 1 extreme value distribution. Reliability plots are given for a class of strength parameters that are typical of graphite/epoxy laminates. A numerical example for a pin-loaded composite laminate is shown using experimentally obtained data.

Characterization of unidirectional fiber reinforced glass/epoxy pultruded composites using ultrasonics (high frequency, 1-5 MHz) and the impulse-frequency response vibration (intermediate frequency, 50-100 Hz) technique, is demonstrated here. This paper compares the response of both of these non-destructive test techniques to the changes in the pultrusion process variables and to the indeed contaminants introduced during manufacturing. The ultrasonic methods use multi-mode techniques of wave velocity and attenuation measurements to measure the viscoelastic constants of the pultruded composite while the vibration technique provides the dynamic flexural modulus and loss factor (damping) measurements. Quasi-destructive assays were also performed using a low frequency (1-50 Hz) Dynamic Mechanical Analyser (DMA) to verify the state of pultruded samples with induced contaminants (simulated porosity and interfacial debonding) and the results compared with the non-destructive measurements. Mathematical models to describe the influence of porosity and debonding agents on the material properties were derived based on statistical regression analysis procedures. Results indicate that the peak damping value of the tan δ curve obtained from the DMA is a sensitive parameter to detect abnormalities in the finished product. The ultrasonic velocity and dynamic flexural modulus measurements provide useful information on the stiffness characteristics while the attention and loss factor can be related to the anomaly-sensitive damping properties of the material.

Three computational approaches for 3-D analysis of laminated composite plates are presented. These are displacement assumed approach, displacement assumed/equilibrium approach and mixed two-field approach. The fundamental concept in all of the three approaches is that “deficient” polynomials have to be used for the through-the-thickness displacement approximation. Bernstein polynomials, having discontinuous first derivatives at the interfaces, are used as approximation functions in the through-the-thickness direction. The examples of a double-sine distributed surface load on 3-ply, 9-ply, and sandwich plates examined by Pagano are solved. It is shown that the results obtained with all three present approaches agree excellently with the closed form solution, for the length-to-thickness aspect ratios ranging from 2–100. A comparison between the results obtained with the deficient and with the continuously differentiable displacement approximations is also presented. This reveals that polynomial approximations that are continuously differentiable through the whole thickness of a laminate, provide qualitatively incorrect transverse stresses at the interfaces. By increasing the degree of the polynomial, one gets two distinct from the top and from the bottom limits for any transverse stress component on the interfaces. On the contrary, when using deficient approximation functions one obtains the same from the top and from the bottom limits for the transverse stresses on the interfaces.

Delamination is a phenomenon which involves complex degradation of both layers and inter-laminar connections. To take into account these degradations, the composite laminate is modeled at a meso scale as a stacking of homogeneous layers connected by interfaces. Layers and interfaces may be damaged. Both onset of delamination and its propagation on a short distance are predicted. Two applications are presented, the numerical simulations are compared with experimental results: (i) delamination in the vicinity of a straight edge of a specimen under tension or compression, (ii) delamination near the hole of a perforated plate under tension.

Filament winding of composite structures generally causes fiber bands to weave or undulate throughout the structure in a non-periodic fashion, except for hoop-only winding. This causes the filament-wound fiber bands, made from multiple groups of filaments, called tows, to fail below the fiber strength of straight fibers. This fiber failure strength reduction is investigated by performing a finite element stress analysis and developing a strength-of-material type closedform solution for curved fibers. The stress analysis results from both methods are compared to actual test results conducted on single and double type weave patterns. The predicted failure stress from the analysis closely matches the experimental results. Application of this work to filament-wound composite pressure vessels is discussed.

The torsion of bars with inhomogeneous shear moduli is considered, under different assumptions on the cross-sectional geometry, and the spatial variation law for the moduli. In particular, bars of arbitrary geometry, and modulus that varies in the section only through a function of the coordinates are considered, under the additional assumption that the modulus is constant on the sectional boundary. Bounds on the torsional rigidity are established. As special cases, laminates are discussed, and novel solutions for the torsion of a circular cylindrical bar with angular symmetry are presented. The problem of flexure of inhomogeneous cylindrical bars is considered, allowing for arbitrary variation of the moduli but with constant Poisson's ratio. Specific solutions are exhibited for the case of angular and radial variations of the shear modulus. Finally, the derived solutions are employed to determine the effective properties of the graded structures.

This paper considers the transient dynamic response of a composite medium with arbitrary shaped voids. A boundary integral equation method is used to compute the displacement histories due to a loading pulse applied to the surface of a composite medium. The present methodology can also be used to study the response of a medium with near-surface cracks and elastic impurities. Numerical solutions are presented for graphite/epoxy, glass/epoxy and isotropic media. The influence of the degree of anisotropy of a medium, orientation and geometry of a void, time history of the loading pulse and the interaction between multiple voids are examined. It is found that the near-field displacements of a damaged medium differ substantially from an undamaged medium. The results reported here can be used for qualitative and quantitative nondestructive evaluations and also for computing the acoustic material signature of a composite medium with defects. The present results are also useful in the validation of inverse solution algorithms developed for material characterization.

Novel jet vapor deposition (JVD) processes offer considerable promise for the inexpensive synthesis of functionally graded (composite) materials (FGMs). Here, we explore microstructure-mechanical property relationships for a model Al/Cu metal-metal system and an Al/Al₂0₃ metal-metal oxide multilayered nanocomposite system fabricated by the JVD process. The 10μm thick $\text{Al}\text{Cu}$ multilayers were deposited on silicon wafers at a substrate temperature of ∼140°C. The A1 and Cu layers were of approximately equal thickness and were systematically varied from ∼20 to ∼1000 nm. The 20μm thick $\text{rmAl}\text{Al}{}_2\text{O}{}_3$ multilayers were deposited on glass slides at ∼250°C. The oxide layer thickness was held constant in the ∼2–6 nm range, whilst the Al layer thickness was systematically varied from ∼3 to ∼50 nm. The structure of the Al/Cu multilayers was polycrystalline and had a strong [111] texture, whereas the Al/Al₂O₃ multilayers consisted of amorphous aluminum oxide layers and polycrystalline metal layers with randomly oriented grains. The yield strength of the Al/Cu multilayers exhibited an inverse dependence upon layer thickness when the layer spacing exceeded ∼50 nm. When the $\text{Al}\text{Cu}$ layer spacing was thinner than ∼50 nm, the strength was better predicted by a Koehler image force model. A similar phenomenon was also found in the Al/Al₂O₃ multilayers. In this case the critical metal layer thickness for the transition from an Orowan to a Koehler type behavior was approximately 25 nm. This is consistent with theoretical predictions which indicate that the critical layer thickness of the low modulus consistuent decreases as the difference in shear moduli between the two constituent layers increases.