Plastic stress loads

These tests provide a basis for conclusions concerning potential stress corrosion cracking under a completely static load. In plastic stress loads, especially in case of low elongation rates, the softer base material in comparison to the hardened welding seam zones may be at risk for hydrogen-induced stress corrosion cracking [25, 67-69]. 

Creep the dimensional change of a plastic under load with time followed by the instantaneous elastic or rapid deformation at room temperature permanent deformation caused by prolonged application of stress below the elastic limit. 

Ceramic, plastic and other non-metal tower shells are used quite often (Figures 9-3, 4, and 5). It is important to consider in ceramic construction that the main inlet or outlet nozzles or any other large connections should be oriented 90° to each other to reduce the possibility of cracking the walls, as most cracks go one-half diameter. Preferably there should only be one nozzle at any one horizontal plane. The nozzles should never carry any piping or other stress load. 

The tests are performed under carefully controlled stress (load), temperature, time, and creep (elongation) conditions. To save time, tests for different constant loads are performed simultaneously on different specimens of the same material. Creep tests may be rather extensively conducted, as for example when developing creep data prior to the design and fabrication of the first all-plastic airplane (41). The usual procedure is to plot the creep versus time curve, but other combinations are possible. 

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)... 

Stiffness properties of RPs are used (as with other materials) for the usual purpose of estimating stresses and strains in a structural design, and to predict buckling capacity under compressive loads. Also, stiffhess properties of individual plies of a layered flat plate approach may be used for the calculation of overall stiffiiess and strength properties. The relationship between stress and strain of unreinforced or RPs varies firom viscous to elastic. Most RPs, particularly RTSs are intermediate between viscous and elastic. The type of plastic, stress, strain, time, temperature, and environment all influence the degree of their viscoelasticity. 

In designing RPs, as reviewed certain important assumptions are made so that two materials act together and the stretching, compression, twisting of fibers and of plastics under load is the same that is, the strains in fiber and plastic are equal. Another assumption is that the RP is elastic, that is, strains are directly proportional to the stress applied, and when a load is removed, the deformation disappears. In engineering terms, the material obeys Hooke s Law (Hooke s law states, it... 

Otherwise, the safety coefficients are not selected only on the basis of stress load type, form and processing conditions, but also depend on the plastic material itself. A notch-sensitive plastic that is hard and brittle requires higher safety coefficients than one that is hard and tenacious. Within this framework, the semicrystalline thermoplastics generally show a more favorable behavior than the amorphous ones. 

Considering the mechanical stress loads acting upon the plastic in addition to the attacking medium, it is apparent that they would further exacerbate the damaging effects. Therefore, the chemical resistance of different plastics, a factor that must be known for pipelines in particular, can be determined in time tests. 

Mineral, organic and metallic fibers, and the surfaced materials made from them such as fleece mats, textiles, and weaves, not only make possible economical manufacturing of materials with specifically targeted physical property improvements based on standard plastics and technical molding compounds, but also help manage high mechanical stress loads, which are often direction-dependent and show local variations, with anisotropic composite structures. 

When simulating the mold-filling process, the designer of a plastic part must therefore select the feed points such that the flow lines are not within regions of maximum stress load. 

They used an anisotropic stress loading on the material and achieved a more anisotropic material response. While an anisotropic, nonlinear elastic-plastic model would be best to model skin, the preceding may be used as an intermediate step in FEA. 

The change in dimensirm of a plastic under load over a period of time (excluding the initial instantaneous elastic deformation). Owing to viscoelastic nature, a plastic subjected to a load for a period of time tends to deform more than it would from the same load released immediately after application. The degree of this deformation depends oti the load duration. Creep is the permanent deformation resulting from prolonged application of stress below the elastic limit. Data obtained in creep test are presented as creep vs. time, with stress and temperature cmistant. Slope of the curve is the creep rate, and the end point of the curve is the time for rupture. Creep at room temperature is called cold flow (ASTM D674). 

Designing plastic parts involves very specific expertise. There are basic principles the designer follows for the best performance, e.g., minimum stress in the part (where the stress load is as low as possible) and uniform stress in the part (where the stress should be distributed uniformly). Consistent wall thicknesses fecditate achieving low and uniform stress. However, when the design requires varying thicknesses, then gradual transitions are essential. 

This chapter has covered some of the stress loading situations other than simple tensile, compressive and bending loads encountered by plastics parts. The unique characteristics of plastic materials of certain types make them especially suited to resist the specific stress conditions. With proper part design, material selection and modification the designer can make parts that will perform well under unique stress conditions. 

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