Decompression tests

Texture has a number of component attributes, and some of them can be assessed by mechanical means. The texture or firmness of cooked potatoes is evaluated by subjecting each sample to a compression test using a universal testing machine equipped with a load cell. Cooked potato cylinders are compressed in a single-cycle compression-decompression test. Uniaxial compression is measured with an Instron machine with a lOON load cell. Measurements are performed on hot potato cylinders (depth 12 mm, height 10 mm) from 15 potatoes immediately after cooking, at a deformation rate of 20 mm/min. Stress and strain at fracture are calculated by the Instron series IX version 7.40 software and means of 15 repetitions are calculated. 

C) for several months. The precipitated agarose, from which the glycopeptide surfactant had been extracted, retained its capacity to form gels and these were later used in decompression tests (see below). 

Values from high-pressure permeation tests can give useful information regarding the selecting of elastomer types to withstand potential explosive (rapid gas) decompression damage (Sections... 

FIGURE 23.17 Styrene-butadiene copolymer (SBR) peel adhesion test piece after first appl3nng a gas decompression (GD) procedure and then peel testing. 

The tensile strength of compacts [30] also provides useful information. Excellent specimens of square compacts are necessary to conduct the tensile testing. For this reason, a split die [31 ] (Fig. 2) is used to make compacts that are not flawed. The split die permits triaxial decompression, which relieves the stresses in the compact more uniformly in three dimensions and minimizes cracking. These specimens are then compressed with platens 0.4 times the width of the square compacts in the tensile testing apparatus. (Fig. 3). Occasionally nylon platens and side supports are used to reduce the tendency to fail in shear rather than tension. The force necessary to cause tensile failure (tensile forces are a maximum... 

Oilfields in the North Sea provide some of the harshest environments for polymers, coupled with a requirement for reliability. Many environmental tests have therefore been performed to demonstrate the fitness-for-purpose of the materials and the products before they are put into service. Of recent examples [33-35], a complete test rig has been set up to test 250-300 mm diameter pipes, made of steel with a polypropylene jacket for thermal insulation and corrosion protection, with a design temperature of 140 °C, internal pressures of up to 50 MPa (500 bar) and a water depth of 350 m (external pressure 3.5 MPa or 35 bar). In the test rig the oil filled pipes are maintained at 140 °C in constantly renewed sea water at a pressure of 30 bar. Tests last for 3 years and after 2 years there have been no significant changes in melt flow index or mechanical properties. A separate programme was established for the selection of materials for the internal sheath of pipelines, whose purpose is to contain the oil and protect the main steel armour windings. Environmental ageing was performed first (immersion in oil, sea water and acid) and followed by mechanical tests as well as specialised tests (rapid gas decompression, methane permeability) related to the application. Creep was measured separately. 

Explosive decompression in pressurized hose and seals can result in damage only after several decompression cycles, i.e. as a result of fatigue. Tests were made42 which produced fracture surfaces similar to those from explosive decompression and the importance of maximum strain, temperature, void size and void position was highlighted. 

The ISO and the ASTM oxygen method and the ASTM air pressure method call for the pressure to be released slowly at the end of the exposure but ISO does not point out that this is to avoid porosity. Presumably, in the worst case it could be a test for explosive decompression. 

Each chamber was filled with agarose solution to a depth of 4 mm. After gelation, the agarose samples were exposed to 100-fsw (feet sea water) pressures (i.e., 44.5 psig) for 40 min at 21°C, and then decompressed to atmospheric pressure in accord with one of the seven different decompression schedules tested (see Section 8.1.1). Only bubbles formed in the bottom 3 mm of a given agarose sample were counted, so that the total volume of gel examined in each sample amounted to 0.27 ml. 

The separate decompression tables of the French Navy, the U.S. Navy, and the Japanese Department of Labor, which are all based on the Haldane-ratio principle (ref. 408-410), require total decompression times for the test dive which are much shorter than those required by other military and commercial tables (Table 8.1). The first stop during decompression with either the FN, USN, or Japan tables occurred at a 10-ft depth, and the mean bubble counts ( S.E.M.) within the 0.27-ml agarose samples just prior to termination of this first (and only) stop were 127.25 9.39, 111.88 17.64, and 98.75 10.72, respectively. Of these three Haldane-ratio-principle tables, the FN table required the shortest total decompression time and the Japan table the longest time, so that the mean bubble number at the 10-ft depth was inversely related to the total decompression time. (In these three cases, the total decompression time essentially represented the sum of the initial... 

The amorphous phase appearing above 20 GPa at room temperature (see above) has also recently been studied by X-ray diffraction [135] and Raman scattering [132,133]. Serebryanaya et al. [135] identify the structure as a three-dimensionally polymerized Immm orthorhombic lattice, but find that compression above 40 GPa gives a truly amorphous structure. In contrast to the orthorhombic three-dimensional polymer structure discussed in the last section, the best fit here is found for (2+2) cycloaddition in two directions, with (3+3) cycloaddition in the third, and thus some relationship to the tetragonal phase. From the in situ X-ray data a bulk modulus of 530 GPa is deduced, about 20% higher than for diamond. Talyzin et al. [132, 133] find that this phase depolymerizes on decompression into linear polymer chains, unless the sample is heated to above 575 K under pressure. A strong interaction with the diamond substrate is also noted, such that only films with a thickness of several hundred nm are able to polymerize fully [ 132]. Hardness tests were also carried out on the polymerized films, which were found to be almost as hard as diamond and to show an extreme superelastic response with a 90% elastic recovery after indentation [133]. 

David and Augsburger [146] were the first to try this method of analysis. Further tests, for example, determination of complex functions based on methods of numerical mathematics, were performed by the research group of Muller [147, 148], Although the results were helpful, for exact description the models became rather complicated and the derived parameters were complex. According to Bauer [149], these models have to be three dimensional for a reasonable description. Another similar approach was used by the research group of Rippie [12,13]. They described the structure evolution in the tablet during decompression by the aid of vectorial 3D models and concluded that fracture and stress contribute to the final structure of the tablet. 

In this chapter, we present results of the testing of a broad spectrum of polymers in carbon dioxide over a range of temperatures and pressures and evaluation of the effect of the high pressure carbon dioxide on the chemical/physical properties of materials tested. The testing was performed in a static manner with four controlled variables, namely temperature, pressure, treatment time and decompression time. The evaluation of the interaction of high pressure carbon dioxide with polymers included sorption and swelling behavior, solubility issue, plasticization and crystallization, and mechanical properties. The results of these evaluations are discussed in three sections Sorption, Swelling and Dissolution of Carbon Dioxide in Polymers at Elevated Pressure, Thermal Properties, and Mechanical Properties. ... 

Linear profiles are the simplest profiles to use for powder compressions. Typically, a sawtooth or v-shaped profile is used where the punch is extended at a constant velocity and retracts at a constant velocity. In theory, during a sawtooth profile, the punch reverses its motion instantaneously between the compression and a decompression strokes. At low speeds (e.g.. <1 mm/sec), the hydraulic response system can easily accommodate this discontinuity. However, at high speeds (>100mm/.sec), the control system may show a small lag in the position-time waveform (<10 milliseconds) as it attempts to rapidly reverse the direction of punch. The sawtooth waveform is commonly used for more fundamental compression studies (e.g.. Heckel analysis), where the desired powder volume reduction is proportional to time. It is also u.seful when evaluating instrument performance during factory acceptance testing. 

---