Tear failure

The mosaic structure of a crystal is intimately connected with its mechanical strength. If we consider the lattice theory of a simple ionic crystal, such as sodium chloride, it is easy to calculate the stress necessary to rupture the crystal by separating it into two halves against the forces of interionic attraction. Such calculations lead to estimates of the tensile strength which are hundreds or thousands of times greater than those actually observed. If, however, the crystal possesses a mosaic structure the mechanism of fracture will be different. The two halves of the crystal will not now be separated simultaneously at every point instead there will be local stress concentrations at which the crystal will fail, the stress concentrations will then be transferred to other points and ultimately the crystal will break in two. The process may be likened to the tearing of a sheet of paper it is not easy to sever a piece of paper by means of a uniformly applied stress, but if a tear is started the stress is concentrated at the end of the tear, failure at that point takes place and the tear is rapidly propagated across the sheet. 

Surface of TPE samples irradiated to 500 kGy showed two types of cracks — a number of broad cracks interconnected by a network of microcracks. Thomas etal. [1986] attributed these to embrittlement of the bulk as a result of cross-linking, and to radiation-induced surface oxidation. The tear failure surface of non-irradiated TPE showed a rough surface with sinusoidal foldings and closely spaced vertical striations. The development of these patterns was attributed... 

There is another type of tear failure known as knotty tear, where the tear tends to deviate from the intended tear path and circles around almost perpendicularly, as shown in Figure 5.4. The tearing force-time chart corresponds to curve (c) in Figure 5.3. 

When the surface is put into tension (by bending, or by the application of a tensile test) cracks form readily in the embrittled layer and often propagate into the interior, leading to failure (Figure 1). Even though the interior may be relatively undegraded chemically, a crack in the embrittled layer may provide sufficient stress concentration to cause brittle fracture or a ductile tearing failure instead of deformation and draw-ing. 

Turvey G J and Zhang Y (2005a), Tearing failure of web-flange junctions in pultruded GRP profiles. Composites Part A Applied Science and Manufacturing, Vol. 36, No. 2, pp. 309-317. 

For instance, a 1" wide joint utilizing a sealant with capacity of 25% indicates that the sealant can withstand the joint s compression to I" or its expansion to 1 " without either the adhesive failure of its bond or the cohesive (tearing) failure of the sealant. 

The ability to join thin substrates If thin pieces of, for example, aluminium are joined using rivets, the final component is susceptible to a tearing failure through the substrate which emanates at the rivet and its associated hole in the substrate. Any tensile load across the riveted joint (as seen in Fig. 19) is maximised at the point where the rivet enters the skin. Thus, when die substrate is thin, this can readily lead to crack and/or tear initiation and failure of the structure. Using bonding technology within these components eliminates these bearing stresses and minimises the chance of premature failure. 

Abrasive wear in polyethylene occurs when the surfaee of a sample is removed by contact with a counterface with which it is in relative motion. The surfaces of the polymer and the counterface are always rough to some extent, either by design or due to the inescapable consequences of fabrication. Thus there are always asperities that protrude above the level of the surrounding surface. It is these asperities that make contact and are sites for ductile tearing failure. Asperities may be sharp and incisive, as in the case of those found on inorganic counterfaces, such as stainless steel and emery paper, or rounded and deformable, as in the case of those found on polymer surfaces. Sharp asperities cut and scour surfaces smooth ones act by adhesion to viscoelastically shear the surface. 

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 low temperature which, if the materials come into contact with the body, can cause severe tissue burns. Flesh may stick fast to cold uninsulated pipes or vessels and tear on attempting to withdraw it. The low temperatures may also cause failure of service materials due to embrittlement metals can become sensitive to fracture by shock. 

There are other types of impact tests for shock loading where energy is required to cause complete failure is reported. Each has their specific behaviors that can be related to specific product performance requirements. Tests include ball burst, ball or falling dart using different weights and heights, bag drop, bullet-type instantaneous impact, Charpy, dart drop, Mullen burst, tear resistance, and tub (2). 

In TPE, the hard domains can act both as filler and intermolecular tie points thus, the toughness results from the inhibition of catastrophic failure from slow crack growth. Hard domains are effective fillers above a volume fraction of 0.2 and a size <100 nm [200]. The fracture energy of TPE is characteristic of the materials and independent of the test methods as observed for rubbers. It is, however, not a single-valued property and depends on the rate of tearing and test temperature [201]. The stress-strain properties of most TPEs have been described by the empirical Mooney-Rivlin equation... 

The area of the tear region (03 00) shows evidence of ductile failure, as shown in Figure 46(f). To perform the analysis it was necessary to sever this connection between the partially fractured tube pieces. Therefore, the two pieces of tubing were pulled apart. The expected result is the ductile failure that was in fact... 

PVC/NBR polymer blends can be produced as colloidal or mechanical blends, the former generally giving superior properties. Commercially available blends have PVC contents ranging from 30-55%. The blends have reduced elasticity, which gives improved extrudability, but they also exhibit superior ozone resistance, improved oil swell resistance, and tensile and tear strength this, however, is achieved at the expense of low temperature flexibility and compression set. The ozone resistance of such blends is, however, only improved if the PVC is adequately distributed and fluxed. This is harder to achieve in mechanical blends, but if it is not achieved failure due to ozone attack can occur. 

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