Plastics bending

Another special factor in ionic crystals is that dislocation cores in them acquire net charge. As a result, plastic bending of an ionic crystal causes the top and bottom regions to become charged relative to the middle. This is easily demonstrated because such specimens preferentially attract fine insulating powders. It has been studied in some detail by Li (2000). 

Fig. 6.7. A model for plastic bending of fiber and fragmentation of matrix during fracture of randomly oriented fiber composites. After Helfet and Harris (1972) and Hing and Groves (1972).

The effect was first observed after uniaxial deformation, but such deformation is not restricted to pure tension and compression. Plastic bending, for example, causes true macrostress (Fig. 16-2), but the deformation mode is predominantly a tension or compression of layers parallel to the neutral axis of the beam. The longitudinal residual stress indicated by x-rays is therefore the sum of true macrostress and pseudo-macrostress, and the x-ray result will be numerically larger at either surface than the result obtained by dissection. 

J.G. Williams, H. Hadavinia, B. Cotterell, Anisotropic elastic and elastic-plastic bending solutions for edge constrained beams. Int. J. Solids Stmct. 42, 4927-4946 (2005)... 

DIN 53435 Testing of plastics - Bending test and impact test oti dynstat test pieces... 

Figure 14.19. (a) Elastic bending causing detachment of fitm. (b) Plastic bending. 

Fig. 2. Glass and Plastics Bended with UV-curable Aerobic Acrylic.

Peel test Assuming the strain in the tab is negligible (e.g. if (1) the peel forces are very low or (2) a fabric- or plastic-backed rubbery adhesive or a relatively thick metallic substrate is the peeling member) and plastic bending of the tab does not occur, then adhesive fracture energy (critical strain energy release rate) is given by... 

Strip heaters present most of the same construction varieties as the band heaters. Strip heaters are commonly used to apply surface heating to dies and molds. Sometimes they act as a radiant heat source in thermoforming or plastic-bending applications. 

Fig. 4.5 SiCNW morphologies of the elastic bend (a, c) and the final plastic bend of (b) and (d). The location indicated by the white arrow in (c) is the triggering point of plastic defcmnation. e is an enlarged TEM toight field image. The black arrows in (e) show the top and bottom deformation-induced amcnphous zones. Re ninted with permission from Han et al. [5]. With kind permission of the American Chemical Society...

Figure C.2 Elastic-plastic and fully plastic bending of a beam.

When Afo > Af then elastic-plastic bending occurs which may be analysed using plastic bending theory, and it is convenient to describe the analysis via the non-work hardening case [28-30,44] in which any bending moment M > M may be written as ... 

As Oq- 0,Gc G and for 9q —> n/2, then Gc G . (The addition of G arises from the laige ko assumption which is not true as 0 -> 0o ) Thus o is crucial to finding Gc and if it can be measured directly the correction for plastic bending of the peel arm can be made by finding ko from Eq. 53 and hence dUa/bda from Eq. 50. However, often 6q is small and difficult to measure and, since G is generally much greater than G, its value is vital and must be estimated. 

Firstly, Moore and Williams [55J have reported results from T-peel tests using the five-layer structure laminate discussed above. The values of Gc were obtained from T-peel tests via the above analytical method, allowing for plastic bending of the peel arms. (Thus, measurement of the angles 9 and 62, see Fig. 3, was also undertaken since they are required for the plastic-bending analysis.) The results from the T-peel and the fixed-arm peel test, where the peel angle was varied as discussed above, were not significantly different. 

D, was applied to conform the peeling strip to the roller. A force, P, was then applied to an unbonded strip to determine the value of P necessary to simply bend the strip and overcome friction. When a bonded strip was then peeled, the value of Gc was determined by the difference in the applied peel forces. This scheme has recently received a good deal of attention [60-62] and has been extended to include plastic bending around the roller and a more complete calibration scheme. The analysis given here is a somewhat extended version of that given in [60]. 

For the unbonded strip which is used in the calibration tests, then Gc = 0 and for elastic bending dr = 0 . However, for the case of plastic bending then, again Gc = 0, but now G will increase as D is increased until it becomes constant at a value determined by r, say Gj. Thus, the calibration curve becomes ... 

The probabilistic model of plastic bending capacity M j is described by a log-normal distribution. The statistical parameters are expressed in terms of the nominal capacity (Table 2), which is the design variable. 

Table 2. Statistical parameters of the plastic bending capacity (Schleich et al, 2002).

All standard impact testers can be instrumented to provide a complete load and energy history of the specimen. Such a system monitors and precisely records the entire impact event, starting from the acceleration (from rest) to the initial impact and plastic bending to fracture initiation and propagation to the complete failure. The instrumentation is done by mounting the strain gauges or load cell onto... 

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