Hydrostatic pressure tensile testing

The bursting strength is the hydrostatic pressure required to mpture a specimen when it is tested in a specified instmment under specified conditions. It is the pressure required to produce mpture of a circular area of the paper (30.5-mm dia) when the pressure is apphed at a controlled rate (TAPPI T403). It is related to tensile strength and extensibiUty and is used extensively throughout the industry for packaging and container grades. 

From this relatively simple test, therefore, it is possible to obtain complete flow data on the material as shown in Fig. 5.3. Note that shear rates similar to those experienced in processing equipment can be achieved. Variations in melt temperature and hypostatic pressure also have an effect on the shear and tensile viscosities of the melt. An increase in temperature causes a decrease in viscosity and an increase in hydrostatic pressure causes an increase in viscosity. Topically, for low density polyethlyene an increase in temperature of 40°C causes a vertical shift of the viscosity curve by a factor of about 3. Since the plastic will be subjected to a temperature rise when it is forced through the die, it is usually worthwhile to check (by means of Equation 5.64) whether or not this is signiflcant. Fig. 5.2 shows the effect of temperature on the viscosity of polypropylene. 

Mechanical properties of hydrogenated titanium alloys are strongly dependent on the applied stress tensor, especially on its hydrostatic component. This was illustrated by the high-pressure tensile and extrusion tests on the Ti-6Al-2.5Mo-2Cr alloy and the same alloy hydrogenated to a = 0.15 wt.%H. Tests were carried out using the apparatus at the Institute of Metal Physics UD RAS operating at hydrostatic pressures of machine oil to 15 kbax and temperatures to 250°C. 

Test methods will logically include tensile testing to measure strength, and hydrostatic testing at a pressure with a liquid to test for leaks. Pressures not exceeding 300 mm Hg pressure (internal heart fluid pressure) could be employed for this purpose. 

Figure 14.8 shows stress-strain curves for polycarbonate at 77 K obtained in tension and in uniaxial compression (12), where it can be seen that the yield stress differs in these two tests. In general, for polymers the compressive yield stress is higher than the tensile yield stress, as Figure 14.8 shows for polycarbonate. Also, yield stress increases significantly with hydrostatic pressure on polymers, though the Tresca and von Mises criteria predict that the yield stress measured in uniaxial tension is the same as that measured in compression. The differences observed between the behavior of polymers in uniaxial compression and in uniaxial tension are due to the fact that these materials are mostly van der Waals solids. Therefore it is not surprising that their mechanical properties are subject to hydrostatic pressure effects. It is possible to modify the yield criteria described in the previous section to take into account the pressure dependence. Thus, Xy in Eq. (14.10) can be expressed as a function of hydrostatic pressure P as... 

In the burst test a flat sheet of paper is clamped by a cireumferential ring and a small rubber diaphragm underneath is gradually inflated with fluid, foreing the sheet to bulge until it ruptures. The hydrostatic pressure at the moment of failure is measured. The virtues of the burst test are its simplicity and the speed with which it can be undertaken. The reeorded hydraulic pressure offers a quantitative measure of bonding between fibres. It is linearly related to tensile strength. 

In order to understand the behavior of composite propellants during motor ignition, we conducted a study of the mechanical and ultimate properties of a propellant filled with hydroxy-terminated polybutadiene under imposed hydrostatic pressure. The mechanical response of the propellant was examined by uniaxial tensile and simple shear tests at various temperatures, strain rates, and superimposed pressures from atmospheric pressure to 15 MPa. The experimentally observed ultimate properties were strongly pressure-sensitive. The data were formalized in a specific stress-failure criterion. 

The aim of this work is to provide both experimental information and a corresponding formalization in order to elucidate structural propellant grain safety during ignition. The experimental data were obtained from uniaxial tensile tests and simple shear tests performed with an imposed hydrostatic pressure varying from atmospheric pressure to 15 MPa. It is well established that the materials studied exhibit time-temperature and pressure-sensitive properties. The ultimate properties reported here are formalized in a proposed stress-failure criterion capable of including the pressure effect. 

Figure 8. Results of tensile tests performed under different imposed hydrostatic pressures for T = -60 °C and Vc = 5 mm/min.

The saturation pressure seems to be proportional to the elastic modulus of the propellant (Figure 10). Although there is no solid experimental evidence, it is suspected that the positive hydrostatic pressure acts as a retardation parameter in cavitation or in debonding of particles from the polymeric matrix, as described by Gent and co-workers. Qualitatively, the effect of applied pressure during a tensile test is believed to delay the occurrence of vacuoles and to decrease their number. This assumption may be sustained by simultaneous volume-expansion measurements taken during tensile tests under different pressures. For an increase in pressure, the relative measured volume decreases (see Figure 11). 

GghI (1970). Gent (124) proposed a model in which the hydrostatic tensile stress at an inclusion or local heterogeneity increases the free volume and therefore effectively reduces the Tg of the material. At a sufficiently high stress concentration, the reduction in Tg is sufficient to reduce the local Tg to the test temperature. The reduced yield stress of the material in this mbber-like phase and the hydrostatic tensile stress then leads to cavitation and craze initiation. Implicit in this free-volume approach is that an imposed hydrostatic pressure will tend to prevent the formation of crazes in accordance with experimental observation. The criterion is summarized in the equation for the critical applied stress for initiation,... 

It is very difficult to carry out tensile experiments at constant volume to obtain the partial derivatives in Equation 13.7. Most experimental tests are carried out at constant pressure (atmospheric), and in general, there is a change in volume with tensile straining. Fortunately, Poisson s ratio is approximately 0.5 for rubbers, so this change in volume is small, and also Equation 13.7 is approximately valid for tensile deformation at constant pressure. For precise work, the hydrostatic pressure must be varied to maintain V constant or theoretical corrections applied to the constant-pressure data to obtain the constant-volume coefficients [1,2]. In pure shear experiments, V should be constant and Equation 13.8 should be valid. 

In a recent attempt to bring an engineering approach to multiaxial failure in solid propellants, Siron and Duerr (92) tested two composite double-base formulations under nine distinct states of stress. The tests included triaxial poker chip, biaxial strip, uniaxial extension, shear, diametral compression, uniaxial compression, and pressurized uniaxial extension at several temperatures and strain rates. The data were reduced in terms of an empirically defined constraint parameter which ranged from —1.0 (hydrostatic compression) to +1.0 (hydrostatic tension). The parameter () is defined in terms of principal stresses and indicates the tensile or compressive nature of the stress field at any point in a structure —i.e.,... 

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