Commercial PPTA fibers ( Kevlar 49® and Twaron 1055®) and copolyaramid fibers (Trevar®) are subjected to various hydrolytic and chemical treatments. Tensile modulus, tensile strength, and elongation at break are measured, and mechanical property deterioration is compared. Copolyaramid fibers show improved hydrolytic stability and chemical resistance compared to PPTA fibers. The time dependence of degradation processes can be described by two decreasing exponential functions. WAXS measurements detect only slight differences in the crystalline structure and superstructure of the treated fibers. Thus, the main origin of mechanical degradation is the destruction of intercrystalline links such as tie molecules or tie crystallites. ' Technora® and Trevar® are registered trademarks of Teijin Limited and Hoechst, respectively.

Aramid and aramid copolymer fibers are used in a wide variety of military and civilian applications; however, the long-term effects of environmental exposure on tensile properties are still not well understood. The current effort investigates the effect of hygrothermal conditioning on the tensile properties of Kevlar® KM2 ®, Twaron®, and the newly available Russian copolymer, Armos® high performance fibers. Moisture uptake studies show that at room temperature, water diffuses more slowly into the copolymer Armos ® (D = 8.7 × 10-13 cm2/s) compared to the Kevlar® KM2® and Twaron® homopolymers (D = 2.16 × 10-12 cm2/s and D = 1.8 × 10 -12 cm2/s, respectively). Tensile properties have been measured for these aramid fibers that have been conditioned in water at 40°C, 60°C, 80°C, and 100°C for periods of 17 and 34 days. For both aramid and aramid copolymer fibers, hygrothermal conditioning did not significantly change fiber tensile properties below 80°C. At the most extreme condition of 100°C, 34 days, aramid fibers showed significant loss of tensile strength (58% for KM2 and 34% for Twaron®), while a reduction in tensile strength of 13% (Armos®) was observed for aramid copolymer (Armos®) fibers. Conditioned fibers exhibited no significant change in mass as a result of the conditioning procedure and FTIR spectroscopy results did not indicate signs of chemical or thermo-oxidative change due to hygrothermal conditioning. These results suggest that in aramid fibers, the primary mechanism of degradation at temperatures between 80°C and 100 °C is due to the ingress and egress of moisture in the highly ordered core structure of the fiber. The presence of water in the intercrystalline regions of the fiber core enable segmental chain motion that can relax tie molecules, alter crystal orientation, and change apparent crystallite size. Because of differences in molecular architecture and phase morphology, the aramid copolymer, Armos®, is less susceptible to degradation of tensile properties under these conditions.

Translation of tensile properties from high-performance fibers to end-use fabrics is sensitive to weaving-induced filament defects and environmental exposure. In this effort, isolated and combined effects of hygrothermal exposure and curvature-induced kink bands on tensile strength of Vectran TM HT (polyarylate liquid crystal polyester fiber) and Kevlar Õ KM2 are studied. Hygrothermal conditioning was conducted at temperatures ranging from 40 C to 100 C in water for 30 days. Curvature-induced defects were created by wrapping tows around stainless steel rods of different diameters (0.25 mm to 5 mm) to create kink bands. Combined effects were evaluated by conditioning tows with kink bands at 100 C for 30 days. All conditioned samples were dried and tested at room temperature. Hygrothermal aging showed that tensile properties for Vectran fibers were not appreciably affected below 100 C ($12% reduction), while KM2 fibers dropped continuously with increasing temperatures ($48% at 100 C). The influence of curvature on kink band density was established for each fiber type. The isolated effect of kink band density on residual strength was approximately 15% for both Vectran 1670/600 and KM2-600. Combined effects of curvature-induced kink bands followed by hygrothermal exposure showed significant reductions in tenacity up to $96% for KM2 and 60% for Vectran HT1670/600. Inspection of the microstructure within the kink bands reveals extensive micro-cracking and fibril failure due to accelerated moisture ingress.

This article experimentally investigates the tensile strength distributions of 600 den Kevlar KM2 yarns under quasi-static tensile loading. The strength distributions were best characterized using the 3-parameter Weibull and generalized Gamma distributions. In order to assess the effects of weaving on the strength distributions, Kevlar yarns were tested from a spool and then compared to yarns extracted from greige and scoured Kevlar fabrics. The weaving process and treatments caused various levels of strength degradation which shifted the strength distributions toward lower strengths. The warp yarns were degraded to a greater extent than the fill yarns. The scouring process induced further strength degradations in the woven fabric. Length-scale effects were studied by using gage lengths varying between 25.4 and 381.0 mm. The strength distributions were observed to shift toward lower strengths with increasing gage lengths. A new distribution function based on a modification to the 3-parameter Weibull distribution is proposed to account for length-scale effects. In addition to the strength distributions, the experimental load-extension plots are used to compute the strain energy density or work-to-break values normalized by the yarn volume, which are then statistically characterized and analyzed in a similar manner.

This article describes a new technique for automated measurement of crack initiation, growth, and propagation in composite materials during mode I double cantilever beam (DCB) testing. The proposed method uses time-domain reflectometry (TDR) to detect changes in geometry and electromagnetic properties (dielectric or magnetic) along a transmission line that can be embedded in or bonded to the surface of the specimen. Two types of transmission line TDR sensors are evaluated (IM7 carbon fiber and ARACON) during DCB tests. A P-SPICE transmission-line simulation model is used to verify the baseline signal response for the DCB sensor and the sensitivity for crack detection, with good agreement. Comparison with standard visual methods in DCB testing showed excellent correlation in crack location, crack propagation (LC), and the interlaminar fracture toughness (GIC) values. The TDR sensor design and model-based parametric studies are carried out to determine optimal sensor geometry and configuration. The results demonstrate that the TDR-based method can measure crack propagation parameters at high resolution and accuracy, in an automated manner using low-cost sensors.