Flex resistance tests

Neoprene is the generic name for polychloroprene rubber. It has been produced commercially since 1931 and had rapid and wide acceptance because it is much superior to natural rubber for heat and oil resistance. Heat resistance is far better than NR, BR or SBR. but less than EPDM. When heated in the absence of air, neoprene withstands degradation better than other elastomers which are normally considered more heat resistant, and retains its properties fifteen times longer than in the presence of air. Compression set at higher temperature is better than natural rubber and 100°C is typically the test temperature rather than 70°C. Abrasion resistance is not as good as natural rubber but generally better than most heat resistant and oil resistant rubbers. This is also true for tear strength and flex resistance. 

Pinhole flex resistance is the ability of a plastic film to resist the formation of pinholes (tiny holes) during repeated folding. Films having a low value of pinhole flex resistance will tend to generate pinholes at the fold line during repeated flexing. The test is described in ASTM Standard F456. Pinhole flex resistance is related to... 

Pinhole Flex Pinhole flex is the testing by which flex resistance of a specimen ean be determined. Pinhole formation is the criterion presented for measuring failure. 

In dynamic testing of ozone resistance, a fabric-backed vulcanized rubber specimen is continuously flexed in the ozone chamber over a roller. The fabric backing is in the form of a belt. Any protective chemical films (e.g., certain waxes and antiozonants) that might build up on the surface of the specimen in static testing are quickly broken by the continuous flexing. ASTM test method D 1149 covers static testing and D 3395 covers dynamic testing in a controlled ozone atmospheres. 

The flex resistance of these materials was tested in accordance with ASTM F 392-74 (Gelboflex test). The flexing action consisted of a twisting motion, following by a horizontal motion. 

Figure 49 One kind of fatigue resistance test on a bonded specimen. The machine flexes the test specimen to a certain extent for a very large number of alternate flexions.

For dynamic water resistance, the Maeser Test, ASTM D2099 (53) measures the number of flexes and water penetration. The flex imparted to the leather is a magnification of the flex given the vamp of a shoe ia actual wear. 

Low Temperature Properties. Medium hardness compounds of average methyl acrylate, ie, VAMAC G, without a plasticizer typically survive 180° flex tests at —40° C. Such performance is good for a heat-resistant polymer. Low temperature properties can be greatly enhanced by the use of ester plasticizers (10). Careful selection of the plasticizer is necessary to preserve the heat resistance performance of the polymer. Plasticized high methyl acrylate grades lose only a few °C in flexibiUty, compared to grades with average methyl acrylate levels. 

Further properties which a detonating fuse should have are the ability to initiate blasting explosives (tested with suitable relatively insensitive mixtures usually of TNT and ammonium nitrate) resistance to low temperatures without cracking on flexing and to hot storage without desensitisation and toughness to prevent damage from stones, etc. The fuse must always be waterproof and must often withstand diesel oil, which can separate from ANFO. 

Because of the extreme importance of regularity and freedom from failure which must characterise safety fuse, detailed and exhaustive tests must be carried out on the product. Certain controls are obvious, namely, measurement of powder charge and of burning speed, both before and after immersion in water. Other tests usually carried out include resistance to cracking on flexing at low temperatures round a mandrel, coil tests in which the fuse is bunched into flat or complex coils and freedom from failure after immersion in water is determined. New types of fuse are usually tested to indicate the amount of smoke produced and also to determine the adequacy of the end spit. The end spit is the projection of particles of burning powder from a cut end and is of importance because it provides the mechanism which enables the fuse to ignite a detonator. 

Copolyesterether elastomers have excellent resistance to flex fatigue. For example, in the Ross flex test (ASTM D1052), all of the samples listed in Table 13 resisted cut-growth over 300,000 cycles. 

The permeable barrier was composed of a steel frame that was constructed to hold the SMZ and to allow for media replacement. The frame was constructed of 5-cm steel angle iron and 2.5-cm and 7.6-cm square steel tube. The frame had solid floor and end walls (1.3-cm-thick steel plates) to divide it into three distinct cells. Each cell had perforated metal walls (0.16-cm thick perforated steel sheets with 0.64-cm holes covering 50% of the surface area) transverse to the direction of flow. The perforated metal was installed on both the inside and the outside of the steel tube skeleton, resulting in a 7.6-cm-wide annulus between the inner and outer walls of the frame. The entire frame assembly was professionally painted with high-quality, rust-resistant paint. The barrier frame was placed in the pilot-test tank in three sections on top of a 1-m depth of aquifer sand that had been previously added to the tank in lifts. The physical and chemical properties of the sand are described later in this chapter. The three frame sections were bolted together after applying a silicone caulk (Sika-Flex ) for sealing. The end of the barrier in contact with the side of the tank was sealed to the... 

The abrasion tests were performed with a Stoll flex abrader. The yarn was wound on a card, taped In a group, and cut to the dimensions indicated In Figure 7. The yarn treated with conventional starch exhibited significantly greater abrasion resistance qualities than the yarn treated with degraded starches. 

---