T.T. Chiao

Measurement of shear properties of fibre composites

Abstract Six methods have been evaluated of measuring the shear properties of a fibre composite consisting of a 65-volume % aramid fibre in an epoxy. The shear data obtained from each of the methods was compared with the 90°-wound thin-walled tube torsion test, which was used as the standard. Based on that comparison, it is recommended, for the present, that the ± 45° off-axis laminate tensile shear test be used for determining the shear properties of composites. The method is simple, inexpensive, and reliable, and its shear stress/strain response is the closest to that of the 90° wound thin-walled tube torsion test.

Stress-Rupture of S-Glass Epoxy Multifilament Strands

The stress-rupture behavior of filamentary composites is of great concern in many long-term applications. We have begun a program to collect reliable stress-rupture data on very simple composites such as various fiber strands impregnated with an epoxy matrix. We de veloped a tensile test method for fibers, designed a stress-rupture test apparatus and verified its performance, and installed 400 test apparatuses in a controlled-environment building. This paper dis cusses the data collected to date on single-end S-glass/epoxy strands at three load levels: 83.8, 74.5, and 65.2% (based on the average breaking load). Using at least 100 specimens at each load level, we found that the time-to-break values consistently varied by a factor of 1000. The data clearly indicate a shorter life at higher loads. We are collecting data at three lower load levels: 50, 40, 33%. We are attempting to characterize the distribution function of the stress- rupture life but are not yet able to draw any conclusions.

Stress-Rupture of Simple S-Glass/Epoxy Composites

The stress-rupture of S-glass/epoxy composites has been studied. A 40 ksi increase in stress reduces the life of an S-glass/epoxy composite by a factor of 10. An empirical extrapolation of 15,000 hours of testing implies that S-glass/epoxy composites can sustain an equivalent fiber stress of 200 ksi for 10 years. The S-glass/epoxy stress-rupture distributions from over 1300 tests are described by an exponential model and are related to the applied stress by a power law in time.

Tensile properties of PRD-49 fiber in epoxy matrix.

Recently a high-modulus and high-strength organic fiber, identified as PRD-49-III by the manufacturer, became commercially available. To assess the fiber performance and its potential as a reinforcement for fiber composites, we have determined the tensile properties of the fiber in an epoxy matrix. During this study we made over 6000 fiber/epoxy strands using the filament winding process; these strands were representative samples taken from over 60 lb of single-end fiber. We were interested in the fiber uniformity, its strength distribution at room and LN2 temperature, stress-strain characteristics, and the strain rate effect on the fiber strength.

A Tensile Test Method for Fibers

Considerable confusion exists in the reported tensile properties of fibers as there has been a lack of differentiation between single-filament data and fiber strand data as well as inadequate test methods and specimen preparation procedures. Until recently fiber tensile strength has been based on testing single filaments, but most fibers are produced and used in strand form. It is a known fact

Engineering design data for an organic fibre/epoxy composite

Abstract A complete series of mechanical characterization tests has been conducted on a filament wound composite of approximately 60 vol% organic fibre in an epoxy matrix. Using, for the most part, flat-plate specimens, quasi-static mechanical properties (including elastic constants and ultimate strengths) in tension, compression, and shear were obtained. The longitudinal tensile strength was found to be equal to or above the 1380 MPa generally quoted for similar mat materials. Although tensile and compressive elastic constants were the same, the material proved to be several times stronger in longitudinal tension than in longitudinal compression, and several times weaker in transverse tension than in transverse compression. The low transverse strength (12.35 MPa) and the high in-plane shear strength (38 MPa) of this composite compared to other organic fibre/epoxy composites indicate that it may be usable only in applications where failure is controlled by longitudinal tensile properties.

Acoustic Emission Produced During Burst Tests of Filament-Wound Bottles

Acoustic emission was recorded during burst tests of filament-wound, composite pressure vessels. Organic and graphite fibers were tested, and two different epoxy resin systems were used: one with a low and another with a relatively high cure temperature. Acoustic emission was studied for the effects of different winding patterns, artificial flaws, winding-induced fiber fraying, different resins, and different fibers. Small effects produced in the vessels by changes in these variables were greatly magnified when they appeared as changes in acoustic emission. They would, in fact, be difficult or impossible to detect by other test means.

Stress-Rupture Behavior of Strands of an Organic Fiber/Epoxy Matrix

The stress-rupture behavior of strands of an organic fiber impregnated with an epoxy matrix has been studied to evaluate the long-term performance of this class of composites. An increase in the applied stress of approximately 138 MN/m 2 (20 ksi) reduces the life of the organic fiber/epoxy strands by a factor of 10. This rate of degradation is about one half that of the S-glass/epoxy composite under identical nonhostile environmental conditions. A three-parameter Weibull function can describe the stress-rupture behavior. A Weibull extrapolation of data from more than 400 tests implies that this material has a survival probability of 0.9 for ten years while sustaining an equivalent fiber stress of 2200 MN/m 2 (320 ksi).

Strength Retention of S-Glass/Epoxy Composites

For this study we used only one spool of single-end glass (SCG l50-l/0-lZ-HTS 901, 204 filaments). When not in use, the spool was kept in a container with a drying agent. A flexible epoxy formulation of 40% acetone solution (Dow DER 332/Union Carbide ERL-4206/Celanese Epi-Cure 855,t 70/30/40 parts), was used in the program. This low-viscosity resin, which has higher elongation-to-break than the fiber,

Strain Rate Effect on the Ultimate Tensile Stress of Fiber Epoxy Strands

The strain rate effect on composite performance has been the subject of many arguments. This note points out some of the problems involved in measuring strain rate effect and presents our tensile data on some simple unidirectional composites.