As the curve transitions from the elastic to plastic deformation typically there is a peak stress. For polymer materials, this peak stress is identified as the yield stress. As the material is pulled further, fracture occurs. The stress value when fracture occurs is defined as the tensile strength for polymer materials.
The tensile strength can be greater than, equal to, or less than the yield strength. When a material is placed under a constant stress, the response observed initially will be a function of the stiffness, or modulus, of the material. Modulus is expressed as the applied stress divided by the resulting strain. If stress is continuously applied to a thermoplastic, strain will continually increase.
As a result, the calculated modulus at a point later in time will appear to have decreased. However, the stiffness of the material is not actually decreasing. Apparent modulus is the perceived stiffness of the material and is a mathematical artifact describing the effect of a constant stress and the corresponding increase in strain over time.
A similar reduction in modulus is observed within plastic materials as the temperature is increased. In this way, time and temperature act on plastics in similar ways. Experimentation using dynamic mechanical analysis DMA illustrates this Figure 2. Creep is the tendency of a solid material to deform permanently under the influence of constant stress: tensile, compressive, shear, or flexural. It occurs as a function of time through extended exposure to levels of stress that are below the yield strength of the material.
Creep is the result of the inherent viscoelastic nature of polymers that causes time dependency. Prolonged static stresses lead to a decay in apparent modulus that is associated with localized molecular reorganization of polymer chains. At stresses below the yield point, stress-relief molecular reorganization proceeds through disentanglement, as there is no opportunity for yielding.
Stresses above the yield point result in plastic deformation, which is not fully recovered after removal of the stress. This macro response takes place through permanent molecular rearrangement. There are two main factors that affect polymer viscoelasticity, and accordingly, the creep behavior of a plastic part: temperature and strain rate. As the temperature is increased, the polymer chains are further apart, there is more free volume and kinetic energy, and they can slide past one another to disentangle more easily.
As the strain rate is increased, the polymer chains do not have enough time to undergo yielding, and disentanglement is favored over yielding. Failure within a plastic article through creep loading can occur either through excessive deformation that renders the part dimensionally unusable or through cracking associated with creep rupture. Cracking within plastics occurs as a response to stress; it takes place as a stress-relief mechanism through disentanglement of the polymer chains.
GPPS : General purpose polystyrene, also known as crystal-clear polystyrene, is a fully transparent, rigid and rather brittle low cost thermoplastic made from styrene monomer. GPPS is a solid product manufactured in the form of mm pellets. HIPS is a graft copolymer having polystyrene sidearms. The grafting occurs when some of the radicals react with the double bonds of the polybutadiene. EPS : Expandable polystyrene consists of micro-pellets or beads containing a blowing agent usually pentane.
The expanded or foamed polystyrene is thermally insulating, has high impact resistance and good processability. Styrenic Copolymers and their blends are considered engineering thermoplastics because their properties can be tailored over a wide range for a large number of applications with a broad range of processing methods which permits the manufacture of high quality, very durable plastic products suitable for many demanding applications.
Polystyrene is a polymer that is cheap and easy to process. A large variety of products are formed by injection molding including dining utensils, plastic cups, housewares, toys, CD cases, cosmetic containers, covers and fixtures.
Expandable polystyrene, either crystal polystyrene 2 or styrene copolymers soaked with a blowing agent usually pentane , are used to produce various foamed products, like disposable drinking cups, egg cartons, trays, fast-food containers, cushioned packaging, and thermal insulation for the construction market.
Crystal polystyrene is not crystalline; it is completely amorphous. The opposite of strain hardening is strain softening. Amorphous polymers, when physically aged, exhibit soemtimes true strain softening. Miehe et al. When the temperature exceeds the glass transition temperature, the plastic material abruptly changes its mechanical behavior. In this region, the ultimate extension can be very high at rather low loads. In some cases, it can exceed several hundred percent before failure occurs.
The behavior before break will depend on the crosslink and entanglement density; materials that are lightly crosslinked will undergo large elastic deformation prior break, whereas uncrosslinked polymers will show viscoelastic behavior. The brittle-ductile transition temperature, T b , is not always identical with the glass transition temperature, T g. Strain hardening is sometimes erroneously called "strain softening" when the shear or tensile resistance decreases with increasing strain. The observed drop in stress beyond the yield point is caused by a reduction of the cross section area necking.
The true stress, however, which is the load divided by the actual cross-sectional area, typically increases. Miehe, S. Goektepe and J. Mendez Diez, Int.
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