Performance evaluation of NASC

1.7 Mechanical performances of non-asbestos sealing composites

The properties of NASC specified in current standards include mainly density, transverse tensile strength, plasticity, oil resistance, and so on. The performances of non-asbestos sealing gaskets can be divided into two types. One is related to the performance indexes of gasket products, which are the main items of product inspection prescribed in gasket product standards. These performance indexes are the important basis for evaluating the quality of gasket products. They involve compressibility, resilient rate, stress relaxation rate and leakage rate (AQSIQ, 2003; AQSIQ, 2008). The other is correlated with the gasket constants in current code design guidelines, such as gasket yield factor y which is defined as the minimum gasket stress required to cause the gasket material to deform into the flange face irregularities and gasket factor m which is defined as the ratio of the minimum gasket stress needed to hold the joint sealed under the operating conditions to the internal pressure p (ASME, 2007). In this section, the theoretic prediction and experimental evaluation of some performances of NASC and gasket products are presented.

1.7.1 Test apparatus and procedure

A series of tests for tensile property, compressibility, resilient rate, and stress relaxation property were conducted on an INSTRON 3367 universal testing machine. Samples were made of the developed NASC reinforced with aramid fiber, as mentioned in section 3.2.

For tensile tests, the dumbbell-shaped samples were adopted, and the tests were performed according to the Chinese standard GB/T 1447-2005 "Fiber-reinforced plastics composites/determination of tensile properties". The test conditions are listed in Table 8.

Sample Quantity	Sample Dimension (mm)	Fiber Length (mm)	Fiber Content (wt%)	Tensile speed (mm/min)	Temperature (K)
36	20x10x2.5	1-3, 4-6,7-9	5, 10, 15, 20, 30, 40	200	293, 373, 423

Table 8. The conditions for tensile tests

The compressibility and resilient rate tests were carried out according to the Chinese standard GB/T 12622-2008 "Standard test method for compressibility and recovery of gaskets for pipe flanges". The test conditions are listed in Table 9.

Sample Quantity	Sample Dimension (mm)	Fiber Content (wt%)	Loading Speed (mm/min)	The Maximum Stress (MPa)	Temperatur e (K)
8	10x10x2.5	20, 40	200	10, 20	293

Table 9. The conditions for compressibility and resilient rate tests

For the tests of the stress relaxation property, the samples were placed in the chamber of the testing machine and heated to the desired test temperatures. After the temperature remained constant for about 5 minutes, the compressive loads were applied to the samples. The change in compressive stress of the samples was measured over time under the constant sample deformation. The test conditions are listed in Table 10.

Sample Quantity	Sample Dimension (mm)	Fiber Content (wt%)	Loading Speed (mm/min)	The Maximum Stress (MPa)	Temperature (K)
8	10x10x2.5	20, 40	200	10, 20	293, 423

Table 10. The conditions for stress relaxation tests

1.7.2 Results and discussions

Figs. 17 and 18 show the stress-strain curves of the samples at room temperature with different fiber mean aspect ratios X and fiber volume contents Vf, respectively.

Fig. 17. Stress-strain curves of NASC with different fiber aspect ratios

Fig. 18. Stress-strain curves of NASC with different fiber volume contents

Fig. 19 illustrates the stress-strain relationships of the samples at different test temperatures with a known fiber mean aspect ratio and a given fiber volume content. In these figures, symbols "A, □, o" represent experimental data, and the curves are plotted according to the prediction formulae proposed in section 5.2. It can be seen from the Figs. 17-19 that the slope of stress-strain curves of NASC increases with the increase of fiber mean aspect ratio and fiber volume content. Aramid fiber has a good reinforcing effect on elastomer matrix composites. The larger the fiber mean aspect ratio, the more obvious is the reinforcing effect. Because the modulus of aramid fiber is much larger than those of elastomer matrix material and compatibilization fiber like sepiolite fiber, the increase in the aramid fiber content results in the significant augment of the strength of NASC.

The error between the experimental data and values predicted using the derived constitutive equation increases with increasing strains, and all the predicted stresses are larger than those obtained by experiments. The essential reason for the error of prediction is that the difference between the aspect ratios of various fibers was neglected. The changes in the transverse tensile strength of NASC with fiber mean aspect ratio and fiber volume content are shown in Figs. 20-21.

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so

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0>

Рч

CO

2 0

0 100 200 300 400

Fiber mean aspect ratio X

Fig. 20. Relationship between transverse tensile strength and fiber mean aspect ratio

0

0.0 0.1 0.2 0.3 0.4

Fiber volume content Vf

Fig. 21. Relationship between transverse tensile strength and fiber volume content

Fig. 22 illustrates the effect of test temperature on the transverse tensile strength. In these Figures, symbol "A" represents experimental data, and the curves represent the prediction results. The transverse tensile strength increases with the increase in the fiber mean aspect ratio and the fiber volume content. The transverse tensile strength of NASC is interrelated with temperature, and it deceases with increasing temperature. The temperature induces thermal stresses in the fiber, matrix and fiber-matrix interface, which affect the temperature – dependent tensile strength of the composite finally. Because the coefficient of thermal expansion of the elastomer matrix is much lager than that of the fiber, big thermal stresses both in the fiber and in the matrix will be generated with variation of temperature, and a small shear stress in the fiber-matrix interface is produced as well (Zhu et al., 2008). These factors result in the decrease of transverse tensile strength with increasing temperature.

The compressibility and resilient rate of NASC obtained by tests and those predicted using Eqs. (43) and (46) are listed in Table 11. It can be found that the compressibility of NASC increases with increasing the maximum compressive stress and decreasing the fiber volume content, while the resilient rate decreases with the increase in the maximum compressive stress and the fiber volume content.

Fiber Content (wt%)	The Maximum Compressive Stress (MPa)	Compressibility (%)	Resilient Rate (%)
Test Data	Prediction Result	Relative Error (%)	Test Data	Prediction Result	Relative Error (%)
21	10	31.1	30.1	3.3	69.3	62.5	10.3
21	20	36.2	33.8	6.9	62.7	55.3	12.5
41	10	23.1	21.6	6.7	54.8	48.0	13.2
41	20	28.3	25.9	8.9	51.2	46.6	9.4

Table 11. Compressibility and resilient rate of NASC

Figs. 23-25 present the test and prediction results of stress relaxation property of NASC, in which symbols "A, □" represent experimental results, and the curves are plotted according to the prediction values of stress relaxation property using Eq. (51). The stress relaxation rate increases with increasing initial stress and temperature and decreasing fiber volume content. The fiber mean aspect ratio and orientation have less evident influence on the stress relaxation property of NASC. The experimental and prediction results are basically identical, and the maximum error is about 8.5%.

Fig. 23. Stress relaxation curves of NASC under different maximum stresses

Fig. 24. Stress relaxation curves of NASC at different temperatures