The stress state in graphite epoxy tension specimens subjected to pure tension, to combined tension, and to out-of-plane bending is analyzed. A finite element solution technique is used that includes full representation of the wedge-action grip mechanism of the testing machine as well as end tabs bonded to the test laminate with a finite thickness adhesive layer. Results of the study indicate that stresses are generally lower for the tension specimens with tapered tab geometry. Peak stresses in the specimens are found to be sensitive to tab length. thickness, and layer orientation, as well as test laminate stiffness. Out-of-plane specimen bending resulting from grip misalignment also resulted in significant increases in the specimen stresses.

The off-axis tension test was examined experimentally to obtain actual displacement fields over the surface of graphite-polyimide coupon specimens; the experimental results were compared with the approximate analytical solution of Pagano and Halpin and newly generated finite-element results. A new optical method of high-sensitivity moiré interferometry was used to determine the actual displacements to high precision. It is shown that the approximate analytical solution and the finite-element results compare very favorably with the measured centerline displacements in the test section, and the finite-element displacement fields provide excellent agreement with the moiré displacements throughout the specimen. Results are presented for a 15° fiber orientation and coupon aspect ratios of 5 and 15.

One component of Lawrence Livermore National Laboratory's (LLNL) flywheel program is the development of new resin systems to serve as matrix materials for fiber-reinforced composites. A resin matrix for filament-wound flywheels must have a combination of characteristics that is normally difficult to achieve and, at times, may be almost mutually exclusive. The needs most prominent are low viscosity for fiber penetration, a long pot life for handling ease, low toxicity, and low cost. Very few resin systems satisfactorily meet each of these criteria. An additional constraint lies within the service temperatures where the flywheel must operate. Three distinct temperature regimes have been defined at 65, 100, and 120°C or higher. For the low-temperature use, a room-temperature-curable resin system with a glass transition temperature (Tg) of approximately 65°C seems satisfactory [1]. For intermediate-temperature uses, a rubberized epoxy resin cured with an aromatic amine (Tg = 104°C) looks promising [2].

The creation of new types of fibers has, in the past, stimulated the development of equally new composites. Besides the most commonly used glass and graphite fiber types, a number of newer ceramic fibers have been introduced: Fiber FP® by DuPont and 3M Company's Nextel® are two recent examples. In addition, a silicon carbide yarn is currently available from the Nippon Carbon Company of Japan. This last fiber, called Nicalon®, has created a great deal of interest because of its unique suitability for reinforcement of resin, metal, and ceramic matrix composites. I will briefly describe some of the characteristics of this ceramic yarn and the composites that have been fabricated with it. While these data are of interest, it is probably more important that this fiber has stimulated wide enthusiasm for fibers and ceramics created from organometallic precursors.

The instrumented drop weight impact test has been used to study the relative impact damage tolerance of Kevlar®-49 and Kevlar-29 aramid fibers, Thornel®-300 graphite fiber, and E-glass as reinforcements in epoxy-matrix composite materials. Under the conditions of the test, the energy absorbed to failure, defined as the first through crack or the maximum of the load-displacement trace, is proportional to the tensile strain energy to failure of the reinforcing fiber. Thus, the Kevlar fibers, and in particular Kevlar 29, absorb significantly more energy than Thornel 300, while E-glass is intermediate between the two Kevlar fibers. Hybrids with Kevlar 49 and Thornel 300 also absorb more total energy than do all graphite.

The failure of composite laminates is invariably preceded by the cracking of matrix and interface, which appears as the cracking of unidirectional plies along the fibers and as delamination between plies. Since the ply cracking and delamination appear as macroscopically self-similar crack extension at least within each ply, they are amenable to the linear elastic fracture analysis.

The tensile failure stresses of polycrystalline alumina filaments (DuPont Fiber FP) were experimentally measured and statistically evaluated for several different gage lengths 1.27, 2.54, 5.0, 7.6, 12.7, and 25.4 cm. Static and dynamic tensile elastic moduli results also are reported, and a method for determining the true fiber gage length is described.

Life estimation is crucial in designing fiber composite structures for long-term applications. To study this subject, in 1969 we started an experimental program using simple epoxy composites from S-Glass®, aramid (1971), and some graphite fibers. These impregnated fiber strands were tested for lifetimes (also called stress-rupture or static fatigue) under tension. We selected the strand specimens because they truly represented the basic building blocks of composite structures. As data became available, they were analyzed by different schemes and published during the last decade. In January 1980, a major earthquake in northern California destroyed most of our test specimens. For all practical purposes, this ended our approximately 10-year experiment. The purpose of this article is to review and analyze the available data on the S-Glass/epoxy composite and to make some final comments with regard to the long-term performance of this composite. Plots for life estimation and reliability assessment will be provided for engineering use.

Composite materials continue to play an especially important role in structures used in space travel and space research vehicles. High stiffness, low weight, and extreme dimensional stability are the properties which are most important for such applications. Ease of fabrication is an additional advantage in many cases. Three applications are briefly outlined below. Further information can be obtained from the NASA offices involved or from Hercules Incorporated, the manufacturer of the composite structures discussed.

This article reports the shear characteristics within 1% strain of three typical glass fiber/epoxy laminates commonly used in engineering. By using acoustic-emission and microstructure analysis, correlation of the stress-strain curve with the microstructure of interface cracks and fiber fractures can provide experimental data for the determination of design allowables.

Environmental degradation of composite materials is of serious concern for many long-term applications. This is particularly true in the case of aramid/epoxy composites because both the reinforcing fiber and the matrix are organic materials. Over the past decade, we have collected limited data on no-stress aging of these materials, which we summarize here. We aged the test specimens in three environments: (1) 20 to 23°C, 20 to 55% relative humidity (RH), no fluorescent lighting; (2) 20 to 28°C, 24 to 37% RH, with fluorescent lighting; and (3) outdoor weathering in San Ramon, CA. When the composite strands and vessels were stored in the dark (indoors), we found no noticeable strength degradation in ambient environment for up to ten years.

The increasing use of Kevlar 49/epoxy composites in critical structural applications has led to a concern for their ability to maintain structural integrity over long periods. In order to predict their durability with confidence, we must understand their microscopic deformation and failure modes as well as the roles played by the fiber, epoxy matrix, and fiber-matrix interface in composite performance.

A stress separation technique using an approximate strain-optic law has been developed for photo orthotropic elasticity. The technique is similar to the shear difference method of stress separation used in photoelasticity of isotropic materials and involves stepwise numerical integration. The technique has been illustrated through its application to the analysis of a circular disk of orthotropic material under diametral compression.

Fatigue loading is a common and important engineering problem in applications involving loads that vary in amplitude with time. Cyclic loads, a common type of variable load, can be characterized as cyclic stress amplitudes superimposed on “steady” stresses. It is well known that fatigue-load amplitudes of roughly half the ultimate strength of a given material can cause the material to fail after a large number (on the order of 106) of load cycles. The weakness of such a loaded material indicates a corresponding degree of damage development. The amount of damage and its consequences must be great to reduce the strength of long-life specimens to the low level of applied stress corresponding to most engineering applications.

An accelerated weathering test (2 to 0.67 cycle) has been applied to glass-fiber reinforced polyester (GRP) to evaluate the effect of constituents on the surface durability of GRP. The nature and rate of induced surface degradation was assessed by scanning electron microscopy. For general-purpose GRP (GRP (GP)), replacement of chopped glass roving reinforcement with chopped glass mat reinforcement did not change the nature and rate of surface degradation. Incorporation of a glass surfacing veil, to provide a resin-rich surface layer, however, improved the durability of GRP (GP) to fiber pop out by 100%. Moreover, replacement of styrene with a styrene-methyl methacrylate cross-linking agent enhanced the resistance of GRP (GP) to fiber pop out, erosion, and surface microcracking an average of 70%. Substitution of an ultraviolet absorber Tinuvin P with Tinuvin 327 did not alter the durability of ethylene glycol-based GRP, while replacement of Tinuvin P with Tinuvin P/Tinuvin 144 in neopentyl glycol-based GRP improved the resistance to fiber pop out by 17% but decreased the durability to erosion and surface microcracking by 19 and 12%, respectively.

In the process of fabricating specimens for a fatigue investigation, we found that the panels had a systematic thickness and density variation. Because this could bias test results, we conducted an experimental study to assess the consequences of the observed variability within the panels. Moreover, we obtained data on the effect of specimen width on the static strength.

Current mechanical design and analysis requirements are such that advanced techniques, and the finite-element method in particular, are used routinely, often by people with little or no theoretical background or training. Selecting a software package for dissemination to a user community is a significant task, especially given the initial cost, the cost of user training, and the impact that use or abuse of the package will have on future products and manpower requirements.

Fractographic techniques have been increasingly applied to failed specimens in attempts to define micromechanical failure mechanisms in composite materials. This effort can provide useful information relevant to all phases of studies of composite materials. For example, fractographic data obtained from laboratory control specimens can be used in material selection and optimization. Similarly, efficient structural design requires a detailed understanding of the fracture behavior of the material and the modes of failure of the component. Finally, as a production tool, microscopy can be applied to assess the quality of incoming material once key accept/reject features have been identified.