D. W. Radford

Separating sources of Manufacturing distortion in laminated composites

Manufacturing distortion in composites has been a problem for many years and continues to impede the development of composite materials. Previous efforts have shown these manufacturing distortions to be related to differences in the in-plane and through-thickness properties. Often tooling is modified in an attempt to compensate for these fabrication induced distortions; however, the various sources of the problem have not been clearly identified, measured, nor accepted. Earlier efforts have suggested that both the cooldown from process temperature and shrinkage during cure play a role, yet no single work has measured the relative contributions of these effects. Further, researchers have suggested numerous other factors as being responsible for all, or some portion, of the measured distortion. In this work, distortion in autoclave-cured laminated composites has been experimentally measured, at temperatures ranging from 240C to 1780C, to determine the reversible and irreversible contributions to manufacturing warpage and to attempt to separate the distortion into components related to different mechanisms. Three mechanisms, anisotropy, material property gradients, and stress gradients are described, and are further separated into thermoelastic and non-thermoelastic components. The measured reversible response closely matches computed distortion, based solely on the in-plane and through-thickness coefficients of thermal expansion. Further, it seems that the irreversible contributions can be separated into material dependent effects, such as cure shrinkage, and process related effects, such as part/tool interactions. Calculations, based on anisotropy, indicate that both temperature change and cure shrinkage contribute significantly to manufacturing distortion; but, their contribution does not account for the total distortion. The irreversible contribution is much greater than predicted, indicating the action of a mechanism which only involves a non-thermoelastic contribution, such as a part/tool interaction. Trends in the experimentally observed distortion indicate the presence of other non-thermoelastic process related contributions and show that these other contributions seem to diminish as the thickness, corner radius and included angle increase.

Shape Instabilities in Composites Resulting from Laminate Anisotropy

Experimental results show that mid-plane symmetric composite laminates of semi-cylindrical, curved geometry warp during manufacture and continue to change shape with changing temperature throughout their service life. A mathematical model is presented which predicts the observed shape change and yields results which compare closely with the experimentally obtained values. Further, a method of modifying the lami nate stacking sequence to counter the environmentally induced shape instability is sug gested and a predictive model, based on classical laminated plate theory, is developed. Measured instabilities of optimized laminates indicate that the proposed technique does modify the shape stability of test laminates as predicted. Thus, this technique allows the design and manufacture of curved composite laminates which are shape stable over a wide temperature range.

COMPOSITE REPAIR OF TIMBER STRUCTURES

Abstract An approach, using pultruded composites, to rejuvenate low aspect ratio timber beams, which model railroad bridge span timbers, is described. The approach focuses on overcoming the loss of shear properties by inserting fiberglass pultruded rods from the bottom to the top of the beam, through areas of damage. The concept includes the incorporation of an adhesive during the process of insertion, which not only bonds the reinforcing rods in-place, but also, fills adjacent cracks. Scale beam testing, with a variety of reinforcement cases, has been performed and the overall results are extremely positive, with test beams showing strong recovery of flexural properties and improvement in the strain to failure.

Cure Shrinkage Induced Warpage in Flat Uni-Axial Composites

Autoclave cured uni-axial carbon fiber/epoxy samples are observed to warp during production even though classical laminated plate theory predicts that such composites should have no such tendency. Particularly in thin laminates the observed convex up curvature can be quite pronounced, but measurable warpage does exist even in thicker composites. Distorted uni-axial laminates produced by top bleed autoclave cure have been analyzed using quantitative optical microscopy and have shown volume fraction gradients related to laminate through-thickness position. The results of these preliminary quantitative observations indicate that many laminates are measurably resin-rich at the composite/tool interface, and resin-poor at the laminate top surface near where bleeding takes place. However, the present metallographic technique does not yield a precise through-thickness resin fraction profile. Modeling of the magnitude of this warpage has been undertaken to investigate the likely through-thickness volume fraction profiles. The approach used has included a number of volume fraction variations in a classical laminated plate analysis and determines the mid-plane curvatures predicted based on general composite thermal expansion and matrix shrinkages. The results of this analysis for long uni-axial carbon fiber/epoxy sample strips of varying thickness match the curvature experimentally observed and substantiate the idea that the volume fraction gradient is not linear through the laminate thickness, but rather is locally resin-rich near the tooling, uniform through most of the thickness, and resin-poor at the top surface adjacent to the bleeder. Further, the results show that volume fraction gradients induced during top bleed autoclave cure are an important component of composite part warpage.

Volume fraction gradient induced warpage in curved composite plates

Abstract Curved composite right-angle brackets have been shown to warp during manufacture due to the response of the part geometry to the large differences in the in-plane and out-of-plane shrinkage. In addition, it has been suggested that warpage in flat, uniaxial composites can be related to through-thickness volume fraction gradients which yield a significant degree of laminate asymmetry. The combination of this geometry effect and a graded volume fraction is investigated in an effort to explain differences in the amount of warpage in curved right-angle brackets produced on convex tooling versus on concave tooling. A computerized metallographic image analysis technique is used to measure the laminate homogeneity and the volume fraction gradients are determined for composites produced on each tooling geometry. Based on these experimentally measured values of curvature and volume fraction gradient, it is shown that the observed tooling geometry dependent differences in warpage noted in many components can be explained. In addition to explaining the tooling geometry dependent manufacturing warpage, this improved understanding of the mechanisms involved suggests the use of concepts of functionally graded materials to help minimize component distortion.

Viscoelastic Characterization of Transversely Isotropic Composite Laminae

Understanding and designing for damping in composite laminates has become a topic of great interest; unfortunately, only limited viscoelastic property data is presently available. Direct experimental measurement of the three-dimensional viscoelastic properties is not simple to implement and, thus, an approach leading to the complete 3-D viscoelastic characterization using a reduced number of measured parameters is desirable. To address the difficulties related to direct measurement of properties, this work proposes a reduced number of material coefficients, which allow the specification of the viscoelastic constitutive relationships for a transversely isotropic material, based on only five independent dynamic stiffness parameters and three independent damping loss factors. Further, using this model, a method is developed, based on energy equations, which allows the viscoelastic properties to be evaluated from experimental data, collected from three bend-beam oscillatory tests and two measured Poisson’s ratios. The approach is validated through the use of both published data and an experimental investigation conducted using a Dynamic Mechanical Analyzer.

Determination of the Elastic Constants of a Transversely Isotropic Lamina Using Laminate Coefficients of Thermal Expansion

Three-dimensional elastic constants for a fiber-reinforced lamina are often difficult to obtain from traditional mechanical tests. The relationships developed in thiswork enable the determination of any three of the five independent elastic constants, of a transversely isotropic lamina, from measurements of the remaining two elastic constants and the coefficients of thermal expansion (CTE’s) of [(+θ/−θ)n]s and unidirectional laminates. The approach is used to predict the elastic propertiesof a carbon fiber reinforced epoxy material and the predicted properties are shown to be in good agreement with quoted values. Further, the developed relationships can be used to determine elastic constants over a range of temperatures and, potentially, to study the effects of manufacturing on elastic properties. Therefore, this technique constitutes an additional method for evaluating the elastic constants that characterize a transversely isotropic, fiber-reinforced lamina.

Inorganic Polymer Matrix Composite Strength Related to Interface Condition

Resin transfer molding of an inorganic polymer binder was successfully demonstrated in the preparation of ceramic fiber reinforced engine exhaust valves. Unfortunately, in the preliminary processing trials, the resulting composite valves were too brittle for in-engine evaluation. To address this limited toughness, the effectiveness of a modified fiber-matrix interface is investigated through the use of carbon as a model material fiber coating. After sequential heat treatments composites molded from uncoated and carbon-coated fibers are compared using room temperature 3-point bend testing. Carbon-coated Nextel fiber reinforced geopolymer composites demonstrated a 50% improvement in strength, versus that of the uncoated fiber reinforced composites, after the 250 °C postcure.

Time and Temperature Dependence of the Viscoelastic Properties of PEEK/IM7

Understanding viscoelastic properties of composite materials is essential for the design and analysis of many advanced structures. However, experimental viscoelastic characterization of anisotropic materials can be complicated because of the number of independent parameters to be evaluated. Recently, an approach leading to the 3-D viscoelastic characterization of transversely isotropic materials using a reduced number of measured parameters has been developed. The model reduces the number of independent parameters to describe the viscoelastic behavior of transversely isotropic materials, which greatly simplifies the experimental procedures. Based on this recently developed model, the present article evaluates time and temperature effects on the viscoelastic properties of a fiber reinforced lamina. The experimental investigation is conducted on sub-scale specimens loaded in flexure, using Dynamic Mechanical Analysis (DMA) equipment. Ultimately, the approach presented in this work will allow the construction of master curves for the independent viscoelastic parameters that characterize a transversely isotropic material. Therefore, the experimental technique presented in this work provides a means for the study of viscoelastic properties of fiber reinforced composites, and constitutes a valuable contribution to the understanding of time and temperature dependence of these mechanical properties.

Elastic properties of PEEK/IM7 related to temperature

An approach using laminate Coefficient of Thermal Expansion (CTE) measurements and two measured elastic constants is applied to predict the elastic properties of PEEK-IM7 prepreg laminae. This study assesses the feasibility of the use of this method to compute elastic properties over a range of temperatures. Three-dimensional elastic constants of PEEK/IM7 are determined for the temperature range of 20°Cto 120°C. The results indicate good agreement with quoted values measured by standard techniques, at room temperature. Moreover, the technique allows the study of the temperature dependence of the elastic properties over a range of temperatures. Thus, the technique demonstrates an ability to generate elastic properties data for a transversely isotropic material, over a wide temperature range, which ultimately can be used for improved performance prediction of structural composites.