Composites Engineering最新文献

Progressive failure of a crossply composite laminate under uniaxial tension was characterized by correlating ultrasonic and acoustic emission measurements with damage in real time. Both ultrasonic backscattered energy and acoustic emission gave consistent results and proved to be sensitive indicators of matrix cracking and other failure mechanisms. The acoustic emission signals were analyzed by investigating the amplitude and count frequency and the corresponding applied strain. Loading/unloading tests showed that no additional damage occurs upon reloading up to the previous peak load.

In the present study the exact vibration frequencies of multi-span laminated beams are found using the exact element method, including the effect of rotary inertia and shear deformations. The effect of shear in laminated beams is more significant than in homogeneous beams, due to the fact that the ratio of extensional stiffness to transverse shear stiffness is high. The exact dynamic stiffness matrix is derived, and then any set of boundary conditions including elastic connections, number and length of spans, can be solved as in the classical direct stiffness method for framed structures. The natural frequencies of vibration of a structure are those values of frequency that cause the dynamic stiffness matrix to become singular, and one can find as many frequencies as needed from the same matrix. In the paper several examples are given, and compared with results from the literature.

An analytical study of optimal fibre direction for improving the lateral buckling strength of thin-walled composite open-section members is presented. Based on a Vlasov-type linear hypothesis, beam stiffness coefficients, which account for cross-section geometry and for the material anisotropy of the section as well as the geometrical characteristics of columns, are obtained. Uniformly distributed load, transverse concentrated load, unequal end moments, tip-loaded cantilever and columns with different types of loading, are considered. The results show that, in some cases, the web fibre angle makes a remarkable contribution to increasing the buckling load compared with the unidirectional orientation of the pultrusion process.

A non-linear finite element method is used to investigate the mechanics of the test procedure of single-fiber fragmentation test of AS4/Epon 828 composite containing an interphase. Stress transfer between the fiber and matrix for fragments created at different matrix strain state is evaluated. The ineffective fiber length was found to vary with the matrix strain state. It is found that the stress transfer and local stress fields remain unchanged for constant, linear and power-law interphase property gradients as long as the average interphase properties are the same. Causes and effects of debonding and debond propagation are examined through the evaluation of the stress fields near the fiber breaks. The effect of thermal residual stresses is examined. Stress transfer for fragments of shorter length than the critical length is studied. Physical meaning of the critical length is discussed.

The main objectives of this research are to characterize the fiber content of composite materials using nondestructive methods, to characterize fiber, matrix and fiber-matrix interface degradation in a quantitative manner and to predict the elastic behavior of the composite material using ultrasonic techniques.

We can achieve the above objectives if we can experimentally compute the stiffness matrix that can be derived by either destructive or nondestructive methods. We focus on nondestructive methods to generate the stiffness matrix of ceramic matrix composites (CMC) using ultrasonic techniques. The use of ultrasonic waves in measurement of the dynamic elastic moduli of solids is well known (Truell et al. (1969). Ultrasonic Methods in Solid State Physics, Academic Press). If the density and elastic anisotropy of a solid are specified then the elastic moduli can be deduced from wave speed measurements of shear and longitudinal waves propagating in particular directions in the solid. The relations between wave speed and moduli follow from the theory of small amplitude elastic wave propagating in an anisotropic solid (Musgrave (1970). Crystal Acoustics, Holden-Day).

In this paper, we will discuss the experiments conducted on three CMC (CAS-Nicalon) unidirectional blocks with varying fiber fractions estimated at 31, 42 and 51%, using ultrasonic longitudinal, transverse and surface acoustic wave (SAW) velocities. Techniques to improve and automate data acquisition are discussed along with the experimental results.

This paper discusses the effect of laminated composite patch with different stacking sequences on repairing an inclined central cracked plate under biaxial loads. By means of the finite element method and the strain energy density theory, the results show that the optimal ply orientations of the patch are the three directions 90° and ± 45° related to the crack direction. Also, the use of different stacking sequences for the patch does not affect the energy distribution near the crack tip significantly.

A technique combining the modal expansion with the finite element representation of a finite region containing defects is presented here to analyse the transient response of a laminated composite plate. As an illustration, a model problem of a laminated plate containing a normal surface breaking crack is studied in detail. Numerical results are presented showing the effect of the crack depth on the transient response.

The response of long, thin-walled, cross-ply composite tubes subjected to pure bending was studied analytically. The formulation includes three parts: pre-buckling response, material failure and bifurcation buckling. The pre-buckling response is analyzed using nonlinear kinematics to accommodate the ovalization of the cross-section. The formulation is based on the principle of virtual work and is used to generate a numerical solution procedure. The Tsai-Wu failure criterion is used to detect material failure in the pre-buckling response. The maximum stress criterion was also considered for comparison. Finally, the buckling analysis considers the possibility of bifurcation into modes containing periodic displacements along the axis of the tube. The tubes are assumed to be geometrically perfect and free of residual stress. Three materials-AS3501 graphite-epoxy, Kevlar 49-epoxy, and E-glass-epoxy-and three diameter-to-thickness ratios-50, 100 and 400-are considered. The moment-curvature response of the tubes is non-linear due to the ovalization of the cross-section (Brazier effect) which induces a limit moment instability. Either material failure or bifurcation buckling always occurs prior to the limit moment in the cases considered. Little difference was observed between the failure loads predicted by the Tsai-Wu and the maximum stress criteria. Tubes with plies of circumferentially oriented fibers in the outermost and innermost positions in the wall proved superior in strength compared with the other cases considered.

The success of modern continuum mechanics in modelling problems in solid mechanics is truly remarkable. For instance, the general theories of elasticity, plasticity, and viscoeleasticity all rely on the continuum hypothesis. However, while continuum mechanics has provided a powerful means of studying the physics of deformation of composite materials, there are situations when the continuum hypothesis is simply inadequate. These problems are generally associated with inelastic behavior and are mainly attributed to the necessity to homogenize two distinctly different materials into a single continuum.

In this paper, we introduce a multicontinuum theory designed specifically for the analysis of composite material systems. The chief attribute of the theory is its ability to do structural analysis while allowing each constituent to retain its own identity. Major analytical and numerical advances in the theory originally developed by Hansen et al. [Hansen, A. C., Walker, J. L. and Donovan, R. P. (1994). A finite element formulation for composite structures based on a volume fraction mixture theory. Int. J. Engng Sci. 32, 1–17.] are presented. The utility of the theory is demonstrated by using constituent information to predict the yield surface of a unidirectional boron/aluminum composite in the course of an analysis carried out at the structural level.

The relatively high specific strengths and moduli of advanced composite materials make them an attractive option for use in high speed industrial equipment. Hence, a potential market exists for integrating composites into existing machinery to replace critical metallic components. An application of such is the replacement of a high speed reciprocating steel bodymaker ram with one fabricated from a filament wound carbon fiber tube with bonded steel end fittings. Due to the dynamic forces which are inherent in the operation of high speed machinery, components are often subjected to impact, fatigue, and combined high cycle impact-fatigue. The behavior of composites as well as adhesives subjected to either impact or fatigue is well established. However, the combined impact-fatigue behavior of adhesive joints between filament wound carbon-fiber reinforced epoxy tubes and steel end fittings has not been investigated and is the focus of this study.

An impact-fatigue testing machine was designed and fabricated specifically for these tests. The impact pulses generated by the machine closely resemble those of a conventional drop weight impact test machine. In addition, the impact-fatigue machine is capable of completing high cycle impact-fatigue tests (10⁶ impacts) within a relatively short period of time by operating in excess of 10 impacts per second. Tests were performed at several impact load levels ranging from 15% to as much as 40% of the joint $\text{U}$ltimate $\text{S}$tatic $\text{C}$ompressive $\text{F}$ailure -$\text{L}$oad. These impact load levels were monitored throughout the specimen lifetime. Furthermore, three bondline thicknesses were investigated to attain an initial indication of the sensitivity to bondline thickness variation.

Results indicate that the specimens exhibit an initial plateau region for a number of cycles during which time no decrease in load carrying capacity is measured. After a critical number of impacts, damage becomes apparent as the sample is no longer capable of maintaining the initial load. At this point, these constant-displacement tests show a load drop and a corresponding compliance change is noted as the sample begins to show less resistance to impact. Post-impact compression test results also show this drop in strength and modulus. Further, the percentage of the $\text{U}$ltimate $\text{S}$tatic -$\text{C}$ompressive -$\text{F}$ailure Load being applied dynamically determines the length of the plateau as well as the rate of degradation. Thus, the determination of the performance of composite tube/metal end fitting bonded joints using this impact-fatigue test approach gives information critical fo