Deformation residual

If an initially unloaded elastie body is subjeeted to surfaee or body forees, it will be deformed. If the forees are removed, the deformation will disappear, and the body will be returned to the same state it was in before the forees were applied. Certain materials are observed to behave elastieally for limited deformations, but, if those deformations are exeeeded, then the internal strueture of the material may be altered and inelastie deformation may oeeur. The stresses in the body may be different than those that the material would have experieneed if it had responded elastieally, i.e., the stresses may be affeeted by the change in material strueture. When the forees are removed, some residual deformation may remain. These qualitative observations suggest a set of constitutive assumptions whieh will be stated in mathematieal terms below. 

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

Influence of the ZnCFO contents (3,0 5,0 7,0 phr) on crosslink kinetics of the modelling unfilled rubber mixes from NBR-26 of sulfur, thiuram and peroxide vulcanization of recipe, phr NBR-26 - 100,0 sulfur - 1,5 2-mercaptobenzthiazole - 0,8 stearic acid - 1,5 tetramethylthiuramdisulfide - 3,0 peroximon F-40 - 3,0, is possible to estimate on the data of fig. 7. As it is shown, the increase of ZnCFO concentration results in increase of the maximum torque and, accordingly, crosslink degree of elastomeric compositions, decrease of optimum cure time, that, in turn, causes increase of cure rate, confirmed by counted constants of speed in the main period (k2). The analysis of vulcanizates physical-mechanical properties testifies, that with the increase of ZnCFO contents increase the tensile strength, hardness, resilience elongation at break and residual deformation at compression on 20 %. That is, ZnCFO is effective component of given vulcanization systems, as at equal-mass replacement of known zinc oxide (5,0 phr) the cure rate, the concentration of crosslink bonds are increased and general properties complex of rubber mixes and their vulcanizates is improved. 

Compression set The residual deformation of a material after removal of the compressive stress. 

Fig. 7 a and b. Scheme of the thermomechanical behaviour of a well phase-separated thermoelasto-plastic. Stress-strain (or time) curves. Plots of heat effects versus time. First loading (ABC) and unloading (CD) cycle. Second loading (AC) and unloading (CD) cycle. The yielding point occurs at B. AD indicates the residual deformation after the first cycle. AB on the dQ/dT-time curve is the endo-effect resulting from the initial small-strain deformation AB U9)... 

In Fig. 24(a) the purely elastic deformation and the plastic elastic flow processes are plotted and hatched in a different manner. Figure 24(b) shows the dependence of stress on time. It can also be seen, that with discharge at time t0 the purely elastic residual deformation disappears at once, whereas the plastic-elastic portion does so gradually (diffusion processes). 

The following symbols are used in Table 1.3 ot> is strength limit in extension e is defoimation at break E is modulus of elasticity Er is residual deformation (after elastic recoil) Hb is Shore hardness Eel is rebound elasticity T, is glass transition temperature... 

ISO 4600 details a ball or pin impression method for determining the ESCR. In this procedure, a hole of specified diameter is drilled in the plastic. An oversized ball or pin is inserted into the hole, and the polymer is exposed to a stress cracking agent. The applied deformation, given by the diameter of the ball or pin, is constant. The test is multiaxial, relatively easy to perform, and with not very well-defined specimens, and the influence of the surface is limited. Drawbacks are the small testing surface and the undefined stress state. After exposure, tensile or flexural tests may be performed on the specimens. This leads to the determination of either the residual tensile strength or the residual deformation at break. 

The occurrence of these two groups of values (below and above 5% residual deformation) can be explained by the strain-induced a p polymorphic transition in PBT. As stressed above, it is well known (Yokouchi et ai, 1976) that up to 5% deformation the a polymorphic modification characterized by higher microhardness // , dominates in the samples. Furthermore, for 12-15% deformation (for homo-PBT), the a p transition is essentially completed (see Fig. 6.11(a)) and the samples show predominantly the polymorphic modification, which has a lower microhardness < // . However, after removal of the load (a = 0) the samples contract (e.g. after a deformation of e = 5-10%, the plastic deformation is around 1% and after s = 15-20% the plastic deformation is around 3%). In all these cases the plastic... 

C). This picture could resemble a plastic deformation of the bead of linear polystyrene. However, the residual deformation of the hypercrosslinked network cannot be regarded as plastic. 

Figure 7.43 Effect of the pretreatment of the network obtained by crosslinking styrene-0.57% DVB copolymer with monochlorodimethyl ether to 100% on the position and form of thermomechanical curves (1) control sample (2) the sample heated up to 136°C under a loading of 400 g and relaxed at 164°C for 2h without pressure (3) the sample heated up to 136°C and then cooled under a loading of 400g (12% residual deformations), then subjected to swelling and drying (4) the sample heated up to 136°C and then cooled under a loading of 400 g (10% residual deformations). (Reprinted from [202] with permission of Wiley Sons, Inc.)...

Figure 9.3 Residual deformation of a sandwich panel after impact, (a) Stiff skin, weak bond, (b) flexible skin, strong bond.

Serviceability limits are considered to determine performance of the product when subjected to service loads and environments. Service conditions represent those maximum or limiting conditions that are expected in service. Examples of serviceability limits that should be considered in the design of RPs include residual deformation, buckling or wrinkling, deflection and deformation, thermal stress and strain, crazing, and weeping. 

Spontaneous elastic deformations result from the successive overlapping of spring and damper deformation in response to load application and relaxation, there is residual deformation due to the damper. 

The elastomer-filler composite is partly destroyed during deformation, then partly restored. This results in a change in the stress-strain curve and depends on the prestressing level. Relaxation leaves a residual deformation. Under dynamic load applications, the shear modulus depends on the stress amplitude [14, 15]. 

Compression Set. Compression set is the residual deformation of a material after removal of an applied compressive stress. For good performance in many applications, compression set values should be low. A low compression set valne indicates that a material recovers much of its original height after compression and release of the compressive force. 

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