Testing tear propagation resistance

FIGURE 3.72 Test for tear propagation resistance of plastic film and thin sheeting. Tear resistance (gf/cm/mil) = force required to tear sample (gf/cm)/thickness offilm (mil). Standard test methods ASTM D1922, BS 2782 method 308B. 

D 1790 Test method for Brittleness Temperature of Plastic Film by Impact D 1938 Test method for Tear Propagation Resistance of Plastic Film and Thin Sheeting by a Single-Tear Method D 2240 Test Method for Rubber Property - Durometer Hardness D 2702 Standard Practice for Rubber Chemicals - Determination of Infrared Absorption Characteristics... 

ASTM D1938, Test Method for Tear-Propagation Resistance (Trouser Tear) of Plastic Film and Thin Sheeting by a Single-Tear Method, 2002. 

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.

The trapezoid test method (ASTM Dll 17) is recommended for determining the tear resistance of nonwoven fabrics. An outline of a trapezoid is marked on a 7.5 x 15 — cm specimen, and the nonparallel sides are clamped in the jaws of the tensile-testing machine. The load is applied to the specimen in such a way that the tear propagates across the specimen width. The value of the breaking load is obtained from the load—elongation curve and is determined primarily by the bonding or interlocking of the fibers of the composite structure. 

In tensile strength determinations, the material has to completely break through the cross-section, whereas the tear strength test indicates the resistance to the propagation of a defect, such as a nick, in the polyurethane. The way that elastomers tear under different conditions has led to a number of different tests. 

The two most commonly specified tests are the Die C test and the "trouser tear" (D470) test. Both of these tests measure resistance to tear propagation. Other test shapes include the crescent tear with or without end tabs. This sample shape is often nicked. The "Delft" (IS0816) sample with its internal nick is often used from samples taken from finished products. The exact size of the nick does not appear to be critical to the result. 

The most popular method used for quality control uses a pendulum device (Fig. 6.3) to measure the resistance to tear propagation (ASTM D1922). For this test, specified test samples are die cut and include a slit at one end. The tabs formed on either side of the slit are clamped into the device so that when the pendulum is released a so-called trouser leg tear force is applied along the specimen in the direction of the slit. The test device reports the maximum resistive force carried in the film. Commonly, ten specimens in both the machine and transverse direction of the film are tested. 

The tear strength, or the capability to resist tear propagation of nonwoven fabrics under various tearing conditions, is evaluated using the following standard tests ... 

For both the tongue and Elmendorf test methods, it is important to observe the behavior of the specimen as the tear is propagated. In cases where the yams in the test direction are much stronger than the perpendicular yams, it is sometimes difficult or impossible to propagate the tear in the desired direction. In this case, a crosswise tear results. Tear resistance is primarily a function of fabric constmction. Loose, open weaves such as cheesecloth tend to resist tear, whereas tight weaves tend to tear easily. In the open weave, the concentrated force field at the point of tear is dissipated by the compliance of the fabric stmcture to accommodate the stress field, thereby distributing the force over a greater number of yams. 

There are several ways to measure a material s resistance to tearing. In these tests, the applied force is not distributed over the entire specimen but is concentrated on a slit or notch and the tear strengtii is reported as the force required to propagate a tear from this point. For urethane elastomers and foams, the most... 

If conditions of service are such that there is a probability of objects being present with sufficient force behind them to cause cutting even in high friction conditions, it might be argued that cuts are to be expected and it is resistance to catastrophic propagation of these cuts which is of importance and, hence, the appropriate test is one for tear strength. 

Distinction can be made between the force to initiate a tear and the force to propagate a tear. Both are important, as even when a tear has started, for example because of an accidental cut. the resistance to propagation will determine whether the damage becomes catastrophic. The discontinuity at which a stress concentration is produced is formed either by a cut, a sharp reentry angle, or both. Most. standard test pieces involve an artificially introduced cut, and only in a method with a sharp angle and no cut would any measure of tear initiation force be possible. 

The existing test standards on the resistance of these materials to crack propagation concentrate on determining the tear strength values. Some of these standards arc specific to plastics film and sheeting [202 208], fabrics coated with rubber or plastics [209-212]. non-woven textile fabrics [213-216], and woven textile fabrics [217-218]. 

Propagation Tear Resistance of Plastic Film and Thin Sheeting by Pendulum Method, Elmendorf Tear. Primary Film Test Method ASTM D1922. Tear strength is the force necessary to eontinue tearing a sample after a nick has been made it is reported in grams. 

Standard test method for propagation tear resistance of plastic film and thin sheeting by pendulum method, ASTM Standard D1922-09, ASTM International, West Gonshohocken, PA, 2011. DOL10.1520/ D1922-09. 

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