Strain induced load

In a simplified approach the first step in analyzing any product is to determine the loads to which it will be subjected. These loads will generally fall into one of two categories, directly applied loads and strain-induced loads. Directly applied loads are usually easy to understand. They are defined loads that are applied to defined areas of the product, whether they are concentrated at a point, line, or boundary or distributed over an area. The magnitude and direction of these loads are known or can easily be determined from the service conditions. Figure 3-2 shows examples of directly applied loads. 

Frequently, a product becomes loaded when it is subjected to a defined deflection. The actual load then is a result of the structural reaction of the product to the applied strain. Unlike directly applied loads, strain-induced loads are dependent on the modulus of elasticity and, with TPs, will generally decrease in magnitude over time. Many assembly and thermal stresses could be the result of strain-induced loads. They include metal insert press fits in the plastic and clamping or screw attachments. 

In a simplified approach the first step in analyzing any part is to determine the loads to which it will be subjected. These loads will generally fall into one of two categories, directly applied loads and strain-induced loads [2]. 

Strain-gauge load cells are sensitive to temperature gradients induced by, for example, radiant heat from the sun or resulting from high temperature wash down. Load cells should be shielded from such effects or given time to stabilize before use. 

In contrast to pipelines and harbor installations, platforms are dynamically loaded. Therefore in the choice of steels, in addition to strength and types of machinability, the risk of corrosion fatigue and strain-induced stress corrosion must be taken into account in combination with cathodic protection (see Sections 2.3.3 to 2.3.5). 

Testing mode Basically material fatigue failure is the result of damage caused by repeated loading or deformation of a structure. The magnitudes of the stresses and strains induced by this repeated loading or deformation are typically so low that they would not be expected to cause failure if they were applied only once. 

Creep is the time-dependent strain induced by a constant mechanical loading. The strain is a function of the stress level, the time for which the stress is applied, and the temperature. The results can be presented graphically in various ways by combining these three parameters or in quantified forms creep modulus and creep strength, for example. 

Isolated keratinocytes subjected to cyclic strain exhibit a significant increase in cell proliferation, DNA synthesis, and protein synthesis compared to stationary or constantly loaded cells, which appear to involve changes in cyclic AMP. Takei et al. (1997) reported a strain-induced reduction in the levels of cyclic adenosine monophosphate, protein kinase A (PKA), and prostaglandin E2 (PGE2) as compared to stationary controls. Takei et al. (1997) also studied the effects of cyclic strain on protein kinase C (PKC) activation and translocation in cultured keratinocytes. 

Stretch-induced mechanical loading also appears to effect secondary messenger activation in airway smooth muscle cells. A 20% single static stretch of rat pulmonary smooth muscle cells increases both Ca+2 influx and efflux. Mechanical strain rapidly increases tyrosine phosphorylation of ppl25FAK and paxillin in airway smooth muscle cells cultured in type I collagen matrices. Tyrosine kinase inhibitors hindered strain-induced reorientation and elongation of airway smooth muscle cells. 

The study of the strain-induced polymorphic transitions by microhardness measurement offers the opportunity to gain additional information on the deformation behaviour of more complex polymer systems such as polymer blends. Since polymer blends are usually multicomponent and multiphase systems the question arises of how the independent components and phases react under the external load. The polymorphic transition will reflect the behaviour of the crystalline phases provided strain-induced polymorphic transition is possible. 

The result shown in Fig. 6.8 that the two species of crystallites respond to the mechanical field in sequence - first the homo-PBT crystallites and later those arising from PEE, means that the homo-PBT crystals are probably dispersed within PEE in such way that they experience the mechanical field from the very beginning of loading. Moreover, one can assume that in the blend some internal stress and/or strain pre-exists since the strain-induced polymorphic transition starts even at lower... 

Thus, the observation of two sharp well defined, and clearly separated on the deformation scale, strain-induced polymorphic transitions convincingly demonstrates that the two populations of PBT crystallites of differing origin, undergo the mechanical loading not simultaneously but in two steps first those comprising homo-PBT (at s = 2-3%), followed by crystallites belonging to PEE (at s = 25%). 

In summarizing the results from the last three sections, one can conclude that the systematic variation of microhardness under strain performed on (a) homo-PBT (Section 6.2.1), (b) its multiblock copolymer PEE (Section 6.2.2) and (c) on blends of both of these (this section) is characterized by the ability of these systems to undergo a strain-induced polymorphic transition. The ability to accurately follow the strain-induced polymorphic transition even in complex systems such as polymer blends allows one also to draw conclusions about such basic phenomena as cocrystallization. In the present study of a PBT/PEE blend two distinct well separated (with respect to the deformation range) strain-induced polymorphic transitions arising from the two species of PBT crystallites are observed. From this observation it is concluded that (i) homo-PBT and the PBT segments from the PEE copolymer crystallize separately, i.e. no cocrystallization takes place, and (ii) the two types of crystallites are not subjected to the external load simultaneously but in a sequential manner. 

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... 

The deformed shape resulted from shock induced plastic strain is presented in Fig. 6 by plotting the deformed shape of a slice within the RVE. The deformed shape shows the formation of bands in the region where dislocation microbands are formed. This indicates that dislocation activities under high strain rate loading can be considered as somces for shear band formation. 

However, when compared with pure copolymer, the highly stretched nanocomposite exhibited a higher amount of unoriented crystals, a lower degree of crystal orientation, and a higher amount of 7-crystals. This behavior indicated that polymer crystals in the filled nanocomposite experienced a reduced load, suggesting an effective load transfer from the matrix to MCNF. At elevated temperatures, the presence of MCNF resulted in a thermally stable physically cross-linked network, which facilitated strain-induced crystallization and led to a remarkable improvement in the mechanical properties. For example, the toughness of the 10 wt% nanocomposite was found to increase by a factor of 150 times at 55°C. Although nanofillers... 

The middle, secondary, stage of the curve (Figure 10.28) is linear and is called secondary or steady-state creep. In this case, the internal relaxation in the solid is balanced by the strain induced by the load. This linear portion of the curve is described approximately by an equation... 

In a creep test, the load on the test piece is kept constant, and the dimensional changes that this brings about are monitored as a function of the lime. Internationally, there are two standards available for the evaluation of creep properties of plastics ISO 899-1 [108], which deals with tensile creep, and ISO 899-2 [109] (formerly ISO 6602), which deals with flexural creep. In both of these standards the initial stress, i.e., the stress based on the original (unloaded) dimensions of the test piece is assumed (as it is for the corresponding short-term tensile and flexural strength tests), and the strain is similarly defined as for the short-term strength tests, except, of course, that in this case, it is the strain as a function of time that is of paramount interest (Fig. 22a). The term creep strain is used to differentiate this type of strain from the short-term strain induced by classical tensile and flexural tests. The modulus is defined as the ratio of the initial stress to the creep strain and is referred to as the "creep modulus." Since the stress is constant and the strain increases with time, it follows that the creep modulus decreases as lime increases. Other properties of interest when conducting creep tests arc... 

Secondary stress. The basic characteristic of a secondary stress is that it is self-limiting. As defined earlier, this means that local yielding and minor distortions can satisfy the conditions which caused the stress to occur. Application of a secondary stress cannot cause structural failure due to the restraints offered by the body to which the part is attached. Secondar) mean stresses are developed at the junctions of major components of a pressure vessel. Secondary mean stresses are also produced by sustained loads other than internal or external pressure. Radial loads on nozzles produce secondary mean stresses in the shell at the junction of the nozzle. Secondary stresses are strain-induced stresses. 

Low cycle corrosion fatigue for strain-induced fatigue corrosion with low-frequency load cycles on materials and protective layers... 

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