Thermoelastic stress analysis applied to fully reversed bending fatigue

The thermoelastic effect has been used to study stress distributions in a number of in-plane loading problems. Analysis of the temperature distribution has been largely limited to isotropic one-dimensional approximations with heat transfer through the thickness of the specimen. In sonic fatigue, specimens undergo fully reversed bending with a stress gradient along the length of the specimen as well as through the thickness. This has also been modeled as a one-dimensional heat transfer problem with negligible heat transfer along the specimen length. The authors solve this as a two-dimensional problem for an isotropic material to determine the effect of heat transfer.     

Critical factors for frequency-dependent fatigue processes in composite materials

Several fatigue-test parameters, including cyclic frequency, prefatigue material conditioning (preloading and step loading) and test-control modes (strain control and load control) are investigated and their effect on the fatigue response of composite materials is discussed. A conceptual model based on the test results is offered to aid in the understanding of fatigue processes in composite materials and the effect of frequency on fatigue response.     

A method for fatigue analysis of metallic and composite materials under asymmetric high-cycle loading

A new method of plotting limit stress diagrams is set forth. The method is based on the hypothesis of unified limit diagram invariant to the number of cycles to failure. The unified diagram is given by a transcendental power function whose exponent is considered an additional material constant characterizing the sensitivity of the material to cycle asymmetry (stress ratio). The equations derived on the basis of this function encompass all forms of limit stress diagrams, including convex, nearly rectilinear, and concave ones. The method is tested for a wide range of metallic and composite materials subjected to asymmetric tension-compression, bending, and torsion.Translated from Prikladnaya Mekhanika, Vol. 40, No. 11, pp. 106–116, November 2004.This revised version was published online in April 2005 with a corrected cover date.     

Natural convection in composite systems with phase change materials

In this study, we carried out a numerical simulation of transient heat transfer in a composite passive system consisting of air–phase change material–air, arranged as a rectangular enclosure. The vertical boundaries of the enclosure are isothermal and the horizontal ones adiabatic. The enthalpy formulation with a fixed grid is used to study the process of phase change with liquid–solid interface zone controlled by natural convection. The flow in this zone is simulated by a model based on the Darcy porous medium. The numerical solution of the mathematical model is done using finite difference–control volume algorithm. The influence of the geometrical and thermal parameters is studied. It is found that subcooling coefficient is the most important parameter influencing heat transfer, and for a given subcooling, there is an optimum phase change partition thickness.     

A crack tip tracking algorithm for cohesive interface element analysis of fatigue delamination propagation in composite materials

A novel approach is proposed for the use of cohesive elements in the analysis of delamination propagation in composite materials under high-cycle fatigue loading. The method is applicable to delamination propagation within the Paris-law regime and is suitable for the analysis of three-dimensional structures typical of aerospace applications. The major advantages of the proposed formulation are its complete independence of the cohesive zone length – which is a geometry-dependent parameter – and its relative insensitivity to mesh refinement. This is only possible via the introduction of three nonlocal algorithms, which provide (i) automated three-dimensional tracking of delamination fronts, (ii) an estimation of direction of crack propagation and (iii) accurate and mesh-insensitive integration of strain energy release rate. All calculations are updated at every increment of an explicit time-integration finite element solution, which models the envelopes of forces and displacements with an assumption of underlying constant cyclic loading. The method was implemented as a user-defined subroutine in the commercial finite element software LS-Dyna and supports the analysis of complex three-dimensional models. Results are presented for benchmark cases such as specimens with central cut plies and centrally-loaded circular plates. Accurate predictions of delamination growth rates are observed for different mesh topologies in agreement with the Paris-laws of the material.     

Dynamic modeling of a revolute-prismatic flexible robot arm fabricated from advanced composite materials

A dynamic model for a two degree-of-freedom planar robot arm is derived in this study. The links of the arm, connected to prismatic and revolute joints, are considered to be flexible. They are assumed to be fabricated from either aluminum or laminated composite materials. The model is derived based on the Timoshenko beam theory in order to account for the rotary inertia and shear deformation. These effects are significant in modeling flexible links connected to prismatic joints. The deflections of the links are approximated by using a shear-deformable beam finite element. Hamilton's principle is implemented to derive the equations describing the combined rigid and flexible motions of the arm. The resulting equations are coupled and highly nonlinear. In view of the large number of equations involved and their geometric nonlinearity (topological and quadratic), the solution of the equations of motion is obtained numerically by using a stiff integrator.The digital simulation studies examine the interaction between the flexible and the rigid body motions of the robot arm, investigate the improvement in the accuracy of the model by considering the flexibility of all rather than some of the links of the arm, assess the significance of the rotary inertia and shear deformation, and illustrate the advantages of using advanced composites in the structural design of robotic manipulators.     

Strength of composite materials

Here we consider the class of composites in which the strength of the contact between the materials is less than the strength of the components. It is found that the strength of such a material is independent of the size of the initial defect within certain limits but is determined by the shape and size of the most hazardous [weakest] inclusion. A theoretical relationship is deduced for the strength in relation to the size of the largest inclusion, which agrees well with experiment [1], This mechanism probably plays a part in the failure of steel and may be one reason for the scale effect in steel.     

On short-beam shear tests for composite materials

Interlaminar beam tests in the form of three-point and four-point flexure are examined both experimentally and analytically. Experimental data are obtained on unidirectional composites. Photomicrographs of actual failure modes and results of a stress analysis based on classical theory of elasticity are utilized to supplement the experimental data. Complex failure modes in the presence of extremely high combined stress gradients are observed and cast serious doubts on the usefulness of interlaminar-beam experiments for characterizing the delamination resistance of composite materials. Further difficulties are encountered with ductile-matrix-resin composites.Paper was presented at V International Congress on Experimental Mechanics held in Montreal, Quebec, Canada on June 10–15, 1984.     

M-integral analysis for microcrack-damaged anisotropic composite materials

Summary  This paper presents an M-integral analysis for the microcracked anisotropic composite materials. By using an elementary solution derived for a single finite crack subjected to a concentrated force on crack faces, the problem of strong interacting, arbitrarily oriented and located microcracks in an anisotropic composite materials is reduced to a system of Fredholm integral equations. The crack-tip fracture parameters, such as the stress intensity factors, are evaluated from a numerical solution of the system of integral equations. Its dependence on the coordinate system, calculation, and physical interpretation of the M-integral are discussed in the interaction problem. Finally, a numerical example of the damage evaluation by the M-integral analysis is given. Received 24 September 1999; accepted for publication 8 February 2000     

A micromechanics based damage model for composite materials

The predictive capacity of ductile fracture models when applied to composite and multiphase materials is related to the accuracy of the estimated stress/strain level in the second phases or reinforcements, which defines the condition for damage nucleation. Second phase particles contribute to the overall hardening of the composite before void nucleation, as well as to its softening after their fracture or decohesion. If the volume fraction of reinforcement is larger than a couple of percents, this softening can significantly affect the resistance to plastic localization and cannot be neglected. In order to explicitly account for the effect of second phase particles on the ductile fracture process, this study integrates a damage model based on the Gologanu–Leblond–Devaux constitutive behavior with a mean-field homogenization scheme. Even though the model is more general, the present study focuses on elastic particles dispersed in an elasto-plastic matrix. After assessing the mean-field homogenization scheme through comparison with two-dimensional axisymmetric finite element calculations, an extensive parametric study is performed using the integrated homogenization-damage model. The predictions of the integrated homogenization-damage model are also compared with experimental results on cast aluminum alloys, in terms of both the fracture strain and overall stress–strain curves. The study demonstrates the complex couplings among the load transfer to second phase particles, their resistance to fracture, the void nucleation mode, and the overall ductility.     

A tensile impact test apparatus for composite materials

A tensile impact test apparatus capable of applying a pure axial tensile loading to even a highly orthotropic composite material, e.g., a unidirectionally reinforced composite, was designed and constructed. Existing impact test methods such as Charpy, Izod and plate impact induce very complex stress states, making the interpretation of results difficult. Details of the apparatus design, and instrumentation problems which had to be overcome, are discussed.was Graduate Student, Composite Materials Research Group, P.O. Box 3295, University of Wyoming, Laramie, WY 82071.     

Explicit cross-property correlations for anisotropic two-phase composite materials

Explicit correlations between two groups of anisotropic effective properties—conductivity and elasticity—are established for two-phase composite materials with anisotropic microstructures (non-randomly oriented inclusions of non-spherical shapes). The correlations are derived in the framework of the non-interaction approximation. The elasticity tensor is expressed in terms of the conductivity tensor in closed form. Applications to realistic microstructures, containing mixtures of diverse inclusion shapes are given. Compliance/stiffness contribution tensors of an inclusion, that characterize the inclusion's contribution to the overall elastic response, are derived in the course of analysis; these results are of interest on their own.     

Three-dimensional instability problems for viscoelastic composite materials and structural members

Investigations of viscoelastic composite materials and structural members carried out using the TDLTSDB and initial-imperfection method are reviewed. The investigations address the internal and surface loss of stability of layered and fibrous composites, the loss of stability of plates, and the delamination (buckling) of plates with cracks. Each of these problems is reviewed separately. New areas of further research are proposed. The review focuses on the investigations carried out by the author and his students Published in Prikladnaya Mekhanika, Vol. 43, No. 10, pp. 3–27, October 2007.     

An experimental investigation of losipescu specimen for composite materials

A detailed experimental evaluation of the losipescu specimen tested in the modified Wyoming fixture is presented. Moiré interferometry is employed to determine the deformation of unidirectional and cross-ply graphite-epoxy specimens. The results of the moiré experiments are compared to those from the traditional strain-gage method. It is shown that the strain-gage readings from one surface of a specimen together with corresponding data from moiré interferometry on the opposite face documented an extreme sensitivity of some fiber orientations to twisting. A localized hybrid analysis is introduced to perform efficient reduction of moiré data, producing whole-field strain distributions in the specimen test sections.     

Carbonate-salt-based composite materials for medium- and high-temperature thermal energy storage

This paper discusses composite materials based on inorganic salts for medium- and high-temperature thermal energy storage application. The composites consist of a phase change material （PCM）, a ceramic material, and a high thermal conductivity material. The ceramic material forms a microstructural skeleton for encapsulation of the PCM and structural stability of the composites; the high thermal conductivity material enhances the overall thermal conductivity of the composites. Using a eutectic salt of lithium and sodium carbonates as the PCM, magnesium oxide as the ceramic skeleton, and either graphite flakes or carbon nanotubes as the thermal conductivity enhancer, we produced composites with good physical and chemical stability and high thermal conductivity. We found that the wettability of the molten salt on the ceramic and carbon materials significantly affects the microstructure of the composites.