Tear strength measuring

As mentioned above, a large number of different geometries have been used for tear strength measurements. Often they have been chosen with more thought for the convenience of testing than the significance of the results. 

Copolymer test specimens were prepared by pressing slabs for 0.5-3 min at temperatures approximately 20°C above the copolymer melting point, followed by cooling under pressure to room temperature in 5-10 min. Test samples were conditioned at 24°C and 50% relative humidity for at least 48 hr before testing. Stress-strain and tear-strength measurements were made on 0.6 to 0.8-mm-thick specimens. Thicker specimens yield lower values for tensile strength and elongation at break. The test methods used were ... 

Elmendorf Tear testing can be performed on three types of standard samples however, the constant radius sample is the preferred test sample for plastic films. (See Fig. 2.) This sample provides a constant radius from the start of the tear strength measurement—useful for materials where the tear may not propagate directly up the sample as in-tended.f ]... 

Tear Strength, Initial (Graves) Initial Tear Strength measures the force required to initiate tearing on a specimen. 

Tear-test specimens are generally pulled on a tensile tester at a cross-head rate of 8.5 mm/sec (20 in/min). The maximum force required to initiate or propagate tear is recorded as force per unit thickness. Tear strength measurements can indicate gross differences in performance in some applications. They are also useful in production control. 

Flexural or tear strength. Tear strength measures the resistance to growth of a nick or cut when tension is applied to a test specimen. Tear strength is critical in predicting an elastomer s working fife in demanding and abusive applications. 

FIGURE 1.17 Effect of blow up ratio on the film tear strength measured along the machine direction (MD) and transverse direction (TD). 

The second parameter, tear strength, describes the film resistance to tear propagation. It is measured with a special apparatus, the Ehnendorf Tear Tester (ASTM D1922), and defined as the weight of a loaded pendulum capable of tearing a notched piece of film. Two values are usually measured for each film sample. One determines tear propagation in the machine direction of the film, the other in the transverse direction. 

Tables 4—6 Ust ASTM methods used for the characterization of PB and PMP. A number of specialized methods were developed for testing particular articles manufactured from polyolefins several of these determine the performance of PB and PMP film, including the measurement of the film s dart impact strength and tear strength. Dart impact strength is measured by dropping a heavy dart with a round tip on a stretched film. Tear resistance, which reflects the film s resistance to tear propagation, is measured with the Ehnendorf tear tester. Two values for the tear strength are usually reported, one in the machine dkection of the film and the other in the transverse dkection. Pipes manufactured from PB are tested by pressurizing them internally with water the time-to-burst failure is determined at various temperatures (46). The standard test method for haze and luminous transmittance (ASTM D1003) is used for the measurement of PMP optical characteristics.

Characterization of Hydrogel Films. Mechanical testing was conducted in buffered saline on an Instron instrument, according to the modified ASTM D-1708 (tensile) and D-1938 (tear) and were reported in g/mm2 for modulus and g/mm for tear strength. The water contents and the amount of extractables were measured gravimetr ica1ly. 

Tear strength is only applicable to flexible materials and is very little used to monitor ageing simply because tensile strength will serve perfectly well. There are circumstances where compression stress-strain properties would be relevant but the relatively bulky test pieces will be subject to the limitation of oxygen diffusion in any accelerated tests and changes can probably be estimated from tensile measurements. Similarly, shear stress-strain is very rarely used for monitoring ageing. 

Tensile and Tearing Behavior. As shown in Figure 4, the tensile and tear strengths increased with increasing PMMA content measurements of tear strength were not feasible at PMMA contents > 30%. In any case, the incorporation of PMMA at even relatively low levels greatly improves the rather low strengths of the unmodified PU. 

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. 

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. 

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