Effect of Prior Heat Treatment on the Strength of Glass Fibers Measured at Room Temperature

Abstract

s 500 s fn ! 4 0 0 , fn W 3 0 0 ' 0 0: NUMBER of papers' -3 have been published concerning the A deleterious effect of post-production heating on both virginfibers and pristine rods. The glasses discussed in these papers showed a noticeable decrease in room-temperature strength after heating in air in the range 50" to 220"C, the strength reduction increasing with rising temperature. The cause of the strength reduction is still not clear although work by Sakka2 indicates that a degradation of the surface is involved.The purpose of this note is to demonstrate that lightly damaged fibers, when heated in air, undergo strength reduction similar to that of virgin fibers and that this strength reduction is not affccted by a heating atmosphere of dry inert gas.A continuous E-glass filament, with a nominal diameter of 0.00037 in., spirally wound on a Bakelite tube so that adjacent spirals were not in contact, was supplied by the Owens-Corning Fiberglas Corporation. It was anticipated that the filament would be damaged slightly by contact with the Bakelite; however, further mechanical contact was prevented. Four groups of specimens, tested directly from widely separated segments of the filament, gave mean tensile strengths of 459, 472, 473, and 473 kips/ima (ksi). The 113 specimens comprising these four groups h:td an overall mean strength of 467 ksi with a coefficient of variation of 13.4%. The specimen preparation and testing procedures have been detailed el~ewhere.~The lengths of filament to be heated were wound loosely on aluminum frames and placed in a gastight box having inlet and outlet valves. For air-heated fibers, both valves were opened and the box was placed in a resistance furnace and heated for 40 to 60 rnin. For inert gas heated fibers, a cylinder of pressurized argon gas was connected to the inlet valve by a copper coil, the box was flushed with argon,* and then the outlet valve was adjusted to give a 1 psi gage pressure of argon in the box. The box was then placed in the furnace with the copper coil which heated the argon before it entered the box. After heating for 40 to 60 min., the box was quickly cooled almost to room temperature and the filament was removed and made into tensile specimens. The strengths after heating are shown in Fig. 1. Each data point represciits 28 to 35 specimens; all strength tests were conducted a t 75°F and 45y0 rh a t a strain rate of 0.7 in./in./min. The group heated in air a t 66°C was stronger than the unheated group. This may be due to a healing of pre-existing flaws by mild corrosion coupled with the likelihood that the section of filament from which this group was made was initially less damaged than the other lengths removed from the Bakelite tube. For the heating times and temperatures studied, there is no difference between the fibers heated in air and those heated in argon. * 0°5 0 +------0 Haated in olr 0 Heated in orgon 2 0 0 1 i 95% conffdance limits on mean 100

Glass Fibre Strength—A Review with Relation to Composite Recycling

“…The maximum strength loss was 60% after 65 h of conditioning at 273˝C, and the strength loss after 4 h of conditioning was approximately 40%, which was also very similar to that reported by Thomas [24]. In further work [26] Cameron showed that when working with commercially-produced E-glass fibres (rather than laboratory-spun pristine fibre), the effect of thermal conditioning on room temperature strength was equally significant. Using heating times of 40-60 min, he measured a 50% drop in strength after conditioning at 300˝C.…”

“…At higher conditioning temperatures, this minimum value is reached within a very short time. Small sample sizes in these experiments prevented statistically-significant conclusions from being drawn, but these trends were further researched by others [25][26][27][28][29].…”

Understanding physical changes and strength loss of E-glass fibres following exposure to elevated temperatures

“…The effect of the atmosphere in which fibre HT was conducted was a considered variable in work by Cameron. 57,62,63 He determined that changes in the atmosphere did not produce a significant effect on retained strength, a conclusion verified by the findings of Lund and Yue 52 who treated unsized fibre in either air or nitrogen. Tentative evidence from Feih et al 7 suggests that an inert atmosphere may retard strength loss of sized fibre for short durations of HT and that this is likely to be related to slowing of the thermal degradation of the sizing.…”

“…The earliest investigations into strength loss of heat-treated E-glass were carried out over 55 years ago 8,57,61–63 and established many of the phenomena that remain relevant today. Both Thomas 8 and Cameron 57,62,63 showed that retained tensile strength decreases with increasing HT temperature, for all temperatures investigated up to 600°C.…”

The Tensile Properties of a Thermoset Composite using Heat Cleaned Glass Fibres as Reinforcement

“…Following textile processing the starch size is often removed by heating so that a new, resin compatible, size can be applied. Several researchers [3][4][5][6][7] have reported that exposing glass fibres to heat can decrease their strength at room temperature by causing a deterioration of the fibre surface. The primary objective of this part of the work has been to establish the extent of any degradation and its relationship to process conditions during heat treatment.…”