Testing initial tear resistance

Initial Tear Resistance of Plastic Film and Sheeting. Primary Film Test Method ASTM... 

All application verification and soil samples must be individually labeled with unique sample identification (ID) and other identifying information such as study ID, test substance name, sample depth, replicate, subplot and date of collection, as appropriate. Proper study documentation requires that sample lists and labels be created prior to work commencing in the field. Water- and tear-resistant labels should be used since standard paper labels may become water-soaked and easily torn during sample handling. Sample lists should have the same information on them as the labels and are a convenient place to record plot randomization, initials of the individual who collected the sample, and date of collection. As such, the sample list is important in establishing chain of custody from the point of sample collection until its arrival at the laboratory. 

The fact that the correlation equation contains a constant can be explained by considering the action of the two testing machines. The constant strain machine records only the maximum force exerted on the specimen. For flexure tests, this is usually the point where the core first cracks. Additional resistance to breaking occurs as the paper tears and in the case of a board containing glass fiber, as the fibers pull out of the core. The force required for this second break may even exceed that for the initial core crack. 

Puncture resistance BS 4812 1972 (93) ISO 3036. A triangular pyramid puncture head is attached to a pendulum. It is released to swing onto a test piece. The energy required to force the puncture head right through the piece, i.e. to make the initial puncture and to tear and bend open the test piece, is measured. 

Vacuumised metal coatings are gradually improving. Tests indicate that these substantially increase the protection offered by plastic materials but do not equate with a ply of foil. Two plastic plies, each with a vacuumised foil, when laminated in direct contact with one another, can give excellent barrier properties. The barrier properties achieved by metallisation may reduce somewhat once the material becomes creased. Protection from some of these creasing effects can be improved by the incorporation of a more flexible ply (e.g. LDPE), i.e. PET metallised/LDPE. PET is very resistant to tear, hence needs a tear initiation feature. It also confers child-resistance. 

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. 

Two foam insulation systems prepared by the A. D. Little Company of Cambridge, Massachusetts were also tested. These systems had two vapor barriers, one on the outer surface (Z/Z x and one at Z/Zj j x 0.62 (where Z is the distance into the insulation from the outer surface and Z the insulation thickness). Each vapor barrier was a laminate composed of one layer of mylar [0.013 mm (0.0005 in) thick], two layers of aluminum [0.025 mm and 0.013 mm (0.001 in and 0.0005 in) thick], another layer of mylar [0.013 mm (0.0005 in)], and a layer of dacron woven fabric [33.9 g/m (1.0 oz/yd )]. The two layers of mylar offer tensile strength, the two layers of aluminum resist gas diffusion, and the dacron cloth resists tearing. Both systems used foams which had chopped fiberglass added for reinforcement. One system used Sta-foam AA1602 (a toluenedi isocyanate), whereas the other used Upjohn 452 (a pol3nnetric isocyanate). As can be seen from Fig. 13b, both systems had fair thermal performance Initially, but the performance deteriorated rapidly with thermal cycles. 

Let us turn our attention to the deformation curve encountered in sample testing in the uniaxial extension at a constant relative strain rate, de/dt. Let the curve be plotted in e-a coordinates, where o is the stress applied to the initial section of the sample (Figure 5.10a). If the material is brittle, the failure can occur already within the initial Hookean portion of the curve characterized by a linear dependence between e and a. The collapse will then occur before the proportional limit, Op, has been reached. If the material is plastic, the increase in the stress slows down and the transition to a plastic flow occurs once the yield point, Of, has been reached. The increase in the stress then progresses at a slower rate, and the so-called strain hardening takes place. A subsequent fast increase in a in these coordinates corresponds to the formation of a neck. After that, o reaches a particular maximum and tear takes place. This stress maximum (per initial cross section) is referred to as the temporary resistance to degradation, Og. This parameter can be determined experimentally under the most simple and reproducible conditions, and for this reason, it is used as the standard characteristic in a number of strength theories. In addition to this characteristic, one also often uses the maximum deformation, 5 = and the thinning, that is, the decrease in cross-sectional area, [Pg.200]

---