J. L. Thomason et aL/Composites: Part A 61 (2014) 201-208 Fibre diameter analysis. Unsized 1761 Standard deviation (um) Minimum diameter(um) 15.05 Maximum diameter (um 23.06 9巴 1.5 n the measured diameters of glass fibres emphasise termine individual fibre diameters prior to each sin- t and not use average or datasheet values Treatment Temperature('C) 3. 2. Single fibre testing results Fig 3 Influence of heat treatment temperature on room temperature glass fibre average failure strain (a water sized, o APS sized) The results for the average single fibre strength of silane sized and water sized fibres after heat treatment are shown in Fig. 2 strength reduction after elevated temperature conditioning it and the fibre strain at failure is shown in Fig 3. The results indicate seems probable that there also exists a strength reduction mecha that thermal conditioning caused a considerable strength reduc nism related to a fundamental change in the glass itself. This con- tion for both silane-sized and water-sized fibre samples, with a loss clusion is supported by the results of a deeper investigation of the of over 70% of the original strength in the case of conditioning at relationship between the strength of thermally conditioned glas 600C. It can be seen that both glass fibre types reduce in strength, fibres and the degradation of the silane coating which has recently with the silane sized glass falling by a greater percentage of its ori- been reported (181 ginal strength. The water sized fibres exhibit a fairly linear reduc It is well known that the commonly used method to determine tion in room temperature strength with increasing conditioning single fibre modulus delivers results that are directly dependent on temperature. The strength of silane-sized glass appears relatively the gauge length tested. The modulus dependence on gauge length stable at low temperatures but exhibits a threshold (at appre is a phenomenon related to the use of the testing machine cross- mately 250C) above which a precipitous reduction in fibre head position to approximate the fibre strain that does not take strength and strain to failure occurs. These results appear to agree into account the contribution of the strain of the other components well with results from other investigators [8-12 ].Consequently it in the testing system. All modulus values reported here have been appears that the average strength of glass fibres is strongly influ- corrected for the test system compliance using a value of enced by thermal conditioning at temperatures and times which .0346 mm/GPa, obtained from Fig 4 using data from a recently re- nay commonly be experienced during the thermal processing of ported gauge length study of the same fibres [16]. The results for end-of-life composite materials in ord er to obtained red the average room temperature fibre modulus (error bars show 7]. These effects appear to be occurring at temperatures signifi- 95% confidence limits)of unsized and silane sized glass fibres are cantly lower than the 1150-1250C temperature range for the shown as a function of the sample conditioning temperature in manufacture of glass fibre and also well below 759C the glass Fig. 5. It can be seen that both samples show a significant trend transition temperature of boron free E-glass 19 for room temperature modulus being increased by a short elevated Further important conclusions that can be drawn from Figs. 2 temperature conditioning. It is also clear in Fig. 5 that both samples and 3 include the clear role of sizing in protecting the strength let show an approximately linear increase in room quent loss of protection, during thermal conditioning, of the Furthermore, although both samples started with approximately organic part of the polysiloxane layer on the fibre surface, very the same level of tensile modulus. the modulus of the water ikely contributes to the loss of average fibre strength in the silane fibres was found to be significantly higher(at 95% confidence level) sized fibres However since the water sized fibres also exhibit a y=00130x+0.0343 y=0.0127x+0.0349 Treatment Temperature('C) ge Length(mm) of heat treatment temperature on perature glass fibre Fig, 4 Gauge length dependence of fibre modulus for compliance correction factor (▲ water sized.● APS sized) (▲ water sized,· APS sized)large range in the measured diameters of glass fibres emphasises the need to determine individual fibre diameters prior to each sin￾gle fibre test and not use average or datasheet values. 3.2. Single fibre testing results The results for the average single fibre strength of silane sized and water sized fibres after heat treatment are shown in Fig. 2 and the fibre strain at failure is shown in Fig. 3. The results indicate that thermal conditioning caused a considerable strength reduc￾tion for both silane-sized and water-sized fibre samples, with a loss of over 70% of the original strength in the case of conditioning at 600 C. It can be seen that both glass fibre types reduce in strength, with the silane sized glass falling by a greater percentage of its ori￾ginal strength. The water sized fibres exhibit a fairly linear reduc￾tion in room temperature strength with increasing conditioning temperature. The strength of silane-sized glass appears relatively stable at low temperatures but exhibits a threshold (at approxi￾mately 250 C) above which a precipitous reduction in fibre strength and strain to failure occurs. These results appear to agree well with results from other investigators [8–12]. Consequently it appears that the average strength of glass fibres is strongly influ￾enced by thermal conditioning at temperatures and times which may commonly be experienced during the thermal processing of end-of-life composite materials in order to obtained recycled fibres [7]. These effects appear to be occurring at temperatures signifi- cantly lower than the 1150–1250 C temperature range for the manufacture of glass fibre and also well below 759 C the glass transition temperature of boron free E-glass [19]. Further important conclusions that can be drawn from Figs. 2 and 3 include the clear role of sizing in protecting the strength lev￾els in glass fibres. It seems likely that the degradation and conse￾quent loss of protection, during thermal conditioning, of the organic part of the polysiloxane layer on the fibre surface, very likely contributes to the loss of average fibre strength in the silane sized fibres. However, since the water sized fibres also exhibit a strength reduction after elevated temperature conditioning it seems probable that there also exists a strength reduction mecha￾nism related to a fundamental change in the glass itself. This con￾clusion is supported by the results of a deeper investigation of the relationship between the strength of thermally conditioned glass fibres and the degradation of the silane coating which has recently been reported [18]. It is well known that the commonly used method to determine single fibre modulus delivers results that are directly dependent on the gauge length tested. The modulus dependence on gauge length is a phenomenon related to the use of the testing machine cross￾head position to approximate the fibre strain that does not take into account the contribution of the strain of the other components in the testing system. All modulus values reported here have been corrected for the test system compliance using a value of 0.0346 mm/GPa, obtained from Fig. 4 using data from a recently re￾ported gauge length study of the same fibres [16]. The results for the average room temperature fibre modulus (error bars show 95% confidence limits) of unsized and silane sized glass fibres are shown as a function of the sample conditioning temperature in Fig. 5. It can be seen that both samples show a significant trend for room temperature modulus being increased by a short elevated temperature conditioning. It is also clear in Fig. 5 that both samples show an approximately linear increase in room temperature mod￾ulus with conditioning temperature above approximately 200 C. Furthermore, although both samples started with approximately the same level of tensile modulus, the modulus of the water sized fibres was found to be significantly higher (at 95% confidence level) Table 1 Fibre diameter analysis. Unsized APS sized Average diameter (lm) 18.18 17.61 Standard deviation (lm) 1.28 1.53 Number of fibres 537 719 Confidence level (95.0%) 0.11 0.11 Minimum diameter (lm) 15.05 13.88 Maximum diameter (lm) 23.06 23.52 0.5 1.0 1.5 2.0 2.5 0 100 200 300 400 500 600 Single Fibre Strength (GPa) Treatment Temperature (°C) Fig. 2. Influence of heat treatment temperature on room temperature glass fibre average strength (N water sized, d APS sized). 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 100 200 300 400 500 600 Single Fibre Failure Strain (%) Treatment Temperature (°C) Fig. 3. Influence of heat treatment temperature on room temperature glass fibre average failure strain (N water sized, d APS sized). y = 0.0130x + 0.0343 y = 0.0127x + 0.0349 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0 20 40 60 80 Length/Modulus (mm/GPa) Gauge Length (mm) Fig. 4. Gauge length dependence of fibre modulus for compliance correction factor (N water sized, d APS sized). J.L. Thomason et al. / Composites: Part A 61 (2014) 201–208 203