J. J. Luo

Processing and characterization of epoxy/clay nanocomposites

Nanocomposite materials consisting of an epoxy matrix and silicate clay particles have been processed and characterized mechanically. The clay material used was a modified natural montmorillonite. The clay particles consisted of 1 nm thick layers with aspect ratios in the range of 100–1000. The clay particles were mixed with acetone and sonicated, then mixed with the polymer, deaerated and cured. The ultimate objective of processing was to produce a polymer/clay nanocomposite with separated (exfoliated) platelets, dispersed as uniformly as possible. Samples were prepared with clay concentrations of up to 10 wt%. The process used resulted in limited exfoliation but mostly intercalation, i.e., infusion of polymer between the silicate layers and increase of interlayer spacing. The characteristics of the nanocomposite were assessed by transmission electron microscopy and x-ray diffraction. Results from these observations show that the basal spacing of clay platelets increased from an initial pre-processing value of 1.85 nm to 4.5 nm. Enhancement of mechanical properties was measured by tensile testing of coupons. Stiffness increases of up to 50% over that of the unfilled epoxy were measured for clay concentrations of 5 wt%. Strength increases were also measured for low clay concentrations and low strain rate loading. Micromechanics modeling of mechanical behavior is discussed as a function of clay platelet dispersion.

Modeling of fatigue damage in a polymer matrix composite

In recent years, advanced polymer matrix composites (PMC) are increasingly used in high-speed transport airframe structures and aircraft engine components due to their light weight, high strength, and high stiffness properties. One critical issue for structural designers and material developers is the long term behavior of these PMC and their associated damage and failure mechanisms under fatigue loading. In this study, fatigue damage of a polymer matrix composite proposed for high temperature applications was characterized both at room and high temperatures. The objective of this research is to predict the damage development under specific load and temperature. A Monte Carlo technique was applied to simulate the nondeterministic transverse cracking based on the stress-life curves, a simple damage accumulation model and stress analysis model. The probability density function of transverse crack spacing was obtained and a theoretical model was proposed to predict this damage development in terms of crack densities versus number of cycles. It is shown that both the simulation and the theoretical model agree well with experimental results at various load levels and temperatures.

ACOUSTIC EMISSION MONITORING OF FATIGUE DAMAGE IN METALS

Acoustic ernission (AE) consists of high frequency stress waves generated by the rapid release of energy due to fracture, plastic deforrnation, wear or interfacial friction [ 1]. Acoustic ernission monitoring is a very sensitive method with a wide dynarnic range and can be used as a diagnostic means of continuous assessment of darnage in materials and components. Acoustic ernission methods can be applied to metallic components and specimens subjected to monotonic or fatigue loading. In general, acoustic ernission can be used to monitor crack initiation and propagation and to locate the source of the ernission.

Statistical damage analysis of transverse cracking in high temperature composite laminates

High temperature polymer composites are receiving special attention because of their potential applications to high speed transport airframe structures and aircraft engine components exposed to elevated temperatures. In this study, a statistical analysis was used to study the progressive transverse cracking in a typical high temperature composite. The mechanical properties of this unidirectional laminate were first characterized both at room and high temperatures. Damage mechanisms of transverse cracking in cross-ply laminates were studied by X-ray radiography at room temperature and in-test photography technique at high temperature. Since the tensile strength of unidirectional laminate along transverse direction was found to follow Weibull distribution, Monte Carlo simulation technique based on experimentally obtained parameters was applied to predict transverse cracking at different temperatures. Experiments and simulation showed that they agree well both at room temperature and 149 °C (stress free temperature) in terms of applied stress versus crack density. The probability density function (PDF) of transverse crack spacing considering statistical strength distribution was also developed, and good agreements with simulation and experimental results are reached. Finally, a generalized master curve that predicts the normalized applied stress versus normalized crack density for various lay-ups and various temperatures was established.

Characterization of Constitutive Behavior of Satin-Weave Fabric Composite

As the use of woven fabric composites increases, understandingtheir elastic behavior is essential. For this study, an experimental approach wasundertaken to determine the elastic properties of a woven fabric composite, witha focus on determining stiffness coupling terms that are often neglected in analyticaland FEA models. Specimens were fabricated with a 5-harness satin-weave carbon–epoxy composite and were subjected to three loading conditions: tension, bending,and torsion. The approach taken to calculate the stiffness (or compliance) matrixterms is presented, along with symmetry properties that provide some relationshipsbetween these terms. The results indicate that the minor coupling terms are difficultto quantify due to the small strains that need to be measured. The results indicatethat the minor coupling terms are very sensitive to fiber misalignment. Carefulfiber alignment along the longitudinal (warp) direction of test coupons givescoupling terms corresponding to that direction which are much less sensitive to thefiber misalignment. The major in-plane and out-of-plane compliance terms were alsomeasured by the tests conducted.

Analysis of acoustic emission waveforms from propagating fatigue crack

The objective of this study is to investigate and develop/adapt acoustic emission methods for detection and characterization of fatigue damage growth in metals. The work consisted of developing techniques for acoustic emission (AE) data acquisition, simultaneous measurement of physical damage and processing, analysis and correlation of acquired data with physical damage. The material investigated was 4340 steel. Center-notched specimens were subjected to tension-tension fatigue loading at various initial peak stress intensity factors. Acoustic emission parameters and waveforms were recorded by two 150 kHz resonant transducers symmetrically placed on either side of the crack. The crack length was monitored with crack propagation gages. Whereas crack length increases smoothly with fatigue cycles acoustic emission counts occur irregularly showing occasional jumps. Examination of the waveforms showed that they are distinct at different points along the AE count curve and are not related to rate or cumulative ...

A cylinder model for characterization of deformation and damage development in a unidirectional composite

Abstract A cylinder model is proposed to provide the framework of stress analysis for multiple-fiber or matrix-fracture problems in unidirectional composites under longitudinal tension. On the basis of assumptions of axisymmetric displacement fields, the principle of virtual work is applied and governing equations are derived. The equations lead to a general correlation between macroscopic and microscopic deformation of damaged composites. The results are independent of fiber/matrix interface bonding conditions. The theory is applied to the damage development of unidirectional brittle-matrix composites. The transverse strain reversal phenomenon is explained by the model as the direct result of underlying mechanics of damaged composites, and thus can be used to help identify the microscopic damage development. The calculation suggests that interface debonding occurs along with matrix cracking, possibly with some delay, followed by interface crack opening after debonding.

Analysis of Acoustic Emission Output from Propagating Fatigue Crack

Acoustic emission (AE) is a very useful approach in detecting and characterizing fatigue damage growth in general and crack initiation and propagation in particular. Numerous studies have been conducted dealing with correlations of AE output and fracture mechanics and fatigue damage parameters [1–9]. It has been shown that under monotonie loading AE can detect yielding and that the cumulative AE output from a notched specimen is directly related to the stress intensity factor [10]. However, such correlations between AE and damage may not be easy to establish because of the influence of loading, material, geometric and noise factors. In application of the AE method, discrimination between signal and noise is of paramount importance.

Mechanical characterization of graphite platelet nanocomposites through molecular mechanics and continuum modeling

Mechanical properties of nanocomposites consisting of epoxy substrate reinforced by randomly oriented graphite platelets are studied with Mori-Tanaka method in collaboration with molecular mechanics. Elastic constants of graphite nanoplatelets which are the inclusion phase of the micromechanical model are calculated based on their molecular force field. The calculated elastic constants are well compared with the both experimental data and other theoretical predictions in literatures. The results from Mori-Tanakas method based on the graphite modulus calculation from molecular mechanics are found that nanocomposite moduli have strong dependence on the aspect ratios of reinforcing particles, but no direct size dependence. The predicted nanocomposite moduli compare favorably with modulus measurement of several graphite particles of various aspect ratios and sizes. The experimental data also shows that particle sizes have very weak effect on nanocomposite moduli.© 2006 ASME

Deformation of Inhomogeneous Elastic Solids With Two-Dimensional Damage

A general correlation is derived between macroscopic stresses/strains and microscopic deformation on the damage surfaces for inhomogeneous elastic solids with two-dimensional damage. Assuming linear elastic behavior for the undamaged materials, the macroscopic deformation associated with nonlinear strains, or damage strains, is shown to be the weighted sum of the microscopic deformations on the damage surfaces, For inhomogeneous materials with periodic structures (laminated composites, for example) and various identifiable damage modes, simples relations are derived between the macroscopic deformation and microscopic damage. When the number of identifiable damage modes is less than or equal to the number of relevant measurable macroscopic strains, the correlation can be used to evaluate the damage progression from simple macroscopic stress and strain measurements. The simple case of a unidirectional fiber-reinforced composite under longitudinal load is used to show how the results can help detect and characterize the damage using macroscopic measurements, without resorting to assumptions of detailed microscopic deformation mechanisms.