Strain to break

Figure 1 OB. Dependence of strain to break on the butadiene content for various...

Figure 9. Left stress-strain curves of MEA after 100 cycles from 30 to 80% RF1 at 65°C pictures of broken sample are shown in the inserts microcracks were observed on the surface of the sample right the average and the minimum strain-to-break of MEA as function of the number of RH cycles [reprinted with permission from reference 34].34...

Figure 10. RH cycling results in steady degradation of membrane strain-to-break, but little effect on the limiting current due to hydrogen crossover.36...

Figure 18. Stress-strain curves showing the effect of RH cycling and chemical degradation on the stress-strain behavior of the MEA (left) and drastic reduction of membrane strain-to-break as a result of chemical [reprinted with permission from reference 34].34...

Experiments indicate that the critical strain-to-failure is also affected by the average molecular weight and by material processing history. McGrath40 reported strain-to-break of a non-crystalline ionomer (a poly(arylene-ether) random copolymer, biphenyl sulfone in H form, or bi-phenyl sulfone in H form (BPSH)) is proportional to the length of the chain. We found in our laboratory that the casting procedure also affects the strain-to-break of the solution-cast ionomer film. As shown in Fig. 19, a Nation film cast at near room... 

Figure 19. The strain-to-break of solution-cast Nafion (EW = 850) films cast at RH and 50° C.

Figure 20. Strain-to-break for Nafion 117 (left) and NRE-212 (right) membrane samples before and after Fenton degradation test.

Calculations. The stress-strain curves for the silk fabric were plotted automatically from the data obtained with the Instron. The initial modulus was determined from a suitable straight line portion of the stress-strain curve. The strain-to-break was then calculated with an effective gauge length determined from extrapolation of the initial modulus. The energy-to-break was calculated from the integrated area under the corrected stress-strain curve to the break point. 

Initial Characterization. Modern silk was subjected to tensile testing to evaluate the test procedure to be used, the number of samples required and the expected precision. Five fabric samples tested in the warp direction were found to give a coefficient of variation of less than 5% for breaking-load and less than 10% for strain-to-break. 

To determine the effect of coating thickness on tensile properties, coating thicknesses of 0.25, 0.5, 0.75, 0.85, 1.0, 1.25 and 1.5 im of Parylene-C were applied to modern silk fabric. In Table 2, the breaking-load and strain-to-break are shown as a function of coating thickness, and a linear dependence is evident. A similar effect was observed when paper was coated with Parylene-C (5). 

Coating Thickness (Urn) Breaking-Load mean(Ni Strain-to-Break mean(% ... 

Thermal Exposures. The thermally Induced changes in tensile properties of coated and uncoated silk fabric, expressed as percent retained breaking-load, strain-to-break and energy-to-break in Figures 2,3, and 4, respectively, are shown with the lines representing the calculated exponential decline. 

As can be seen in Table 4, the "activation energy" derived from the series of exposures that includes silk fabric heated at 150°C predicts a half-life for the strain-to-break of 85 years at 20°C. The half-life at 20°C calculated from the series of exposures that covers the 70°C to 110°C range is markedly different, only 32 years. Such a large effect produced by inclusion of only one data point indicates that results obtained at this temperature may be suspect. By definition, the activation energy must be independent of temperature over the range of extrapolation. 

Figure 10. Effect of light on strain-to-break of silk broadcloth.

Effect of Parvlene-C Coating on Historic Silk Fabric. The results of tensile testing of coated and uncoated historic silk fabric are given in Table 5. For modern, strong silk (original, uncoated breaking-load >250 N, strain-to-break >15%), a 0.75 )Un thick layer of Parylene-C increased the tensile properties, with the exception of the initial modulus, as was shown in Table 1. 

For the cream-colored fabric, coating with Parylene-C resulted in both increased initial modulus and an increased breaking-load. However, no increase in the strain-to-break was observed. This was found to be the case for both 1 and 5 )Im coating thicknesses. 

For the blue fabric, the results showed an increased breaking-load upon coating, but a decreased strain-to-break, which resulted in an actual reduction in the ability of the fabric to elongate and absorb shock, and therefore, little increase in the energy-to-break. However, the initial modulus also was increased. 

Historic fabric samples that were further deteriorated by heat or light exposures prior to coating with Parylene-C showed the same trends increased breaking-load and initial modulus with no added strain-to-break. The absolute increase in breaking-load was small when compared with the increase that resulted when modern silk fabric was coated, but the percentage increase was higher due to the initially low absolute values of the tensile properties. 

The composite with epoxy-matrix exhibits mechanical properties which are sensitive to its thermal history. Figure 7 shows that the as-received specimens exhibit by far the greatest degree of toughness. For the as-received materials, ultimate tensile strength averaged 162.60 0.76 MPa (95% confidence intervals) whereas strain-to-break averaged 2.44 + 0.74 %. 

Fig. 10. Strain-to-break measured by axial extensometer as a function of sub-Tg annealing time...

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