Tear tests

Figure 8.9 Schematic illustration of a pendulum type tear testing device...

Why do we perform tear testing on polymeric films What creates anisotropy in a film s mechanical properties at a molecular level Why is this anisotropy so much more important in films than in thicker parts ... 

Figure 2. Tear test (a). Sketch of torn cross-section (b).

It is not surprising that, given the importance of tearing and the different levels of result obtained from different geometries, a considerable number of tear tests have been devised which, in part, reflect the different stress concentrations found in various products. The arbitrary nature of the geometries means that, in general, the measured tear strength is not an intrinsic property of the material and it is difficult to directly correlate the results of laboratory tests with the performance of products in service. 

Figure 8-19. Forms of tear test piece. Solid line, original shape broken line, shape after...

Tear test pieces are normally stamped from sheet with an appropriate die (see Chapter 4) and then a nick cut if required, although with the Delft test piece both operations are usually done simultaneously. 

Figure 8-21. Cross section of die-cut tear test pieces, (a) Nicked crescent test piece (b) Delft test piece, w = effective width of untom portions.

Tear test pieces are gripped and stretched to break in a machine of the same type as used for tensile tests and, hence, the comments made in Section... 

The quantity directly measured in a tear test is the force on the test piece during testing. Particularly with the trouser method, this force will fluctuate along the tear path. ISO 34 takes the maximum force reached in the case of crescent, angle and Delft methods but for the trouser method a median force is determined in accordance with procedures given in ISO 6133147, Analysis of multi-peak traces obtained in determinations of tear strength and adhesion strength. 

There are three main configurations for the tear test. In the first two a tensile force is applied to the sample. In one configuration, the force is vertical to the plane of the sample, and in the second it is at right angles. In the third style, an element of shear is also introduced. Figure 9.3 shows the basic configuration of the tear test pieces. 

API Appendix SR 6 requires 40% minimum shear in a dropweight tear test on 50% of the heats for 20 in. (508 mm) diameter API X52 or stronger pipe ... 

Canadian Standards Association Z184 requires 50 to 75% shear average, 35% minimum in a dropweight tear test. 

Dr op weight tear testing on a 20 In. (50 a mm) dimeter and larger pipe shall be carried out at 32 F (oaC) in accordance with Appendix SR-6 of API 5L however, igg percent Of the heats shall exhibit 85 percent or more shear fracture area. 

Mechanical Testing. Tensile strength tests were performed with an Instron tensile tester as per the American Society for Testing and Materials (ASTM) D-1682, and tear testing was done with an Elmendorf apparatus as per ASTM D-1424 at 21 °C and 65% rh, after conditioning the specimens for at least 24 h. Results reported are the average of three tests. 

Interesting results are obtained by atomic force microscopy (AFM, adhesive mode) after tribological wear and tear tests like the disc on wheel system. The surfaces of the disc and also the wheel were investigated by adhesive mode to determine the influence of PTFE. 

Tear resistance of irradiated sample, subjected to the notch-tear test (ASTM D 1004) was found to be unchanged after doses from 0 to 64 Mrad [430]. 

The phenomena observed in rubber as it nears rupture and subsequently have not been studied with the same attention as behaviour at lower stresses4. Considering a vulcanizate as a single uniform material, rupture has been studied principally from by means of the tensile test, the tear test in its various modification, abrasion testing and hysteresis effects. 

The most popular method involves 2-thiobarbituric acid (TBA) two molecules of 2-thiobarbituric acid are condensed with malonaldehyde. The emergent chromogen — the two tautomeric structures of the red TBA-malonaldehyde adduct — is determined at 532 nm, and also often at 450 nm, to determine aUcenals and aUcanals, respectively. The qualitative Kreis test was based on a similar principle it involved detection of the epihydrine aldehyde — a tautomeric malondialdehyde — in a color reaction with resorcine or phloroglucinol. The popularity of the TBA test stems from a correlation between the results and sensory evaluations. Paradoxically, this is related to the most important drawback of the TBA technique — its lack of specificity. In addition to the reaction with malonaldehyde, TBA forms compounds of identical color with other aldehydes and ketones, products of aldehyde interaction with nitrogen compounds, and also with saccharides, ascorbic acid, creatine, creatinine, trimethylamine oxide, trimethylamine, proteins, and amino acids. For this reason, the TBA test may even be treated as a proteolysis indicator (Kolakowska and Deutry, 1983). Recently, TBA-reactive substances (TEARS) were introduced, primarily to stress that the reaction involves hydroperoxides in addition to aldehydes. Due to the nonspecificity of the TEARS test, its results reflect the rancidity of food better than other conventional methods, especially off-flavor, which is caused by volatiles from lipids as well as being affected by products of lipids interaction with nitrogenous compounds. 

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