Testing speed

Fig. 1. Stress—strain curves for ionomer and polyethylene resins. Test speed is 5 cm/min. The reference matedal is high molecular weight conventional...

The preceding experience did lead to a lack of confidence, and it was concluded that an impeller-by-impeller performance check should be carried out theoretically at test speed with the test gas. This idea was carried out on a cold methane compressor with nine impellers. The results of this were fruitful as can be seen from Figure 10-5, and the expected performance at various test speeds calculated in accordance with the code is shown in Figure 10-6. As a result of this work, two test speeds were... 

Tensile and flexural properties were studied with an Instron 4204 testing machine. Tensile tests were performed on the drawn strands at a test speed of 3 mm/ min, while three-point-bending tests (ISO 178) at a speed of 5 mm/min were applied to the injection molded specimens. Charpy impact strength was measured of the unnotched samples with a Zwick 5102 pendulum-type testing machine using a span of 70 mm. The specimens (4 X 10 X 112 mm) used for three-point-bending tests were also used for the impact tests. It should be noted that neither the tensile tests for the strands nor the impact tests were standard tests. The samples were conditioned for 88 h at 23°C (50% r.h,) before testing. 

Unless otherwise mutually agreed upon, the test speed shall be within 3 percent of the rated speed shown on the pump data sheet (see Appendix B). Test results shall be converted to anticipated results at the rated speed. 

Test environments used were NaOH solution with various concentrations of 10 to AOwt.%. Immersion tests were carried out at temperatures of 20 to lOA C for up to 3000 hrs, and after immersion weight measurements and flexural tests were performed at room temperature (Testing speed 2mm/min, Span 40mm). Optical and scanning electron microscopes and infrared spectroscope (IR) were further used to study the degradation mechanism of the resins. 

One problem with some of the technical data sheets on thermoplastic compounds, is lack of uniformity. The variations which exists between suppliers are in the size of the test specimens, the speed at which the particular test is performed, and in some cases the temperatures in which the test is performed. To accomplish increased uniformity, it is suggested that the size of test specimens be standardized for a particular test and not be varied to display a more impressive number. Test speeds should be established for the test and not for a particular type of thermoplastic. Also, standardization of temperatures to be used if data are reported at other than 73°F. 

Either for quality control purposes or to estimate degree of cure, a variety of non-standard experimental procedures have been reported. Generally, the intention is to simplify the test, speed up quality control or to use very small non-standard test pieces. An example which has gained acceptance as a... 

The basic method is a closely specified test intended for quality control or the comparison of bonding systems, but could readily be extended to investigate the effects of test piece dimensions, peeling angle, test speed, etc. 

In terms of tonnage, polyolefins are by far the most important polymeric materials for structural applications, and there is consequently enormous interest in optimising their fracture properties. A rational approach to this requires detailed understanding of the relationships between macroscopic fracture and molecular parameters such as the molar mass, M, and external variables such as temperature, T, and test speed, v. Considerable effort is therefore also devoted to characterising the irreversible processes (crazing and shear deformation) that accompany crack initiation and propagation in these polymers, some examples of which will given. 

In most thermoplastics, transitions from ductile to brittle behaviour may be induced by increasing the test speed. For the reasons already invoked in the introduction, this is of particular concern in iPP, whose impact proper-... 

Fig. 15 Variation of Kc with test speed in CT specimens of iPP with Mw of 455 kg mol 1 and a polydispersity of 5.4, tested in air, along with an indication of the mode of crack propagation [19]...

Fig. 21 The critical stress intensity for mode I crack initiation at different temperatures as a function of test speed in a iPP with Mw of 248 kg mol1 and a polydispersity of 5.2 and a similar material containing approximately 80 wt% y3 phase. The arrows mark ductile-brittle transitions in the y3 modified specimens [24]...

FEG-SEM of crack-tip deformation in a [3 nucleated CT specimen and Fig. 23 shows the corresponding TEM images. These confirm deformation to be less localised than in a spherulites at comparable test speeds, with diffuse regions of cavitation and lamellar shear coexisting with relatively well-defined crazes. The craze structures themselves are nevertheless similar to those in a-iPP. 

A typical test speed is 32 kph at a load of 55 kg. The surface temperature of the wheel can be measured using an infrared thermometer. Failure will be due to hysteresis work, and the part will decompose from the inside out. 

In Charpy-like tests, a severe triaxial stress state prevails at the sample crack tip. Softer modes of loading can be of interest for specific applications. Biaxial tests like impact falling weight are therefore often used. Plaques of defined dimensions are impacted with a hemispherically tipped dart of given diameter at selected test speed(s) (often 3 ms-1 and more). 

The mechanical performance of a polymer is governed by its intrinsic molecular architecture (e.g. molecular weight, tacticity, polydispersity,...) and by extrinsic parameters such as the test speed, the test temperature, the stress... 

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