Stress-strain property

The spring constant, E, in this equation is known as the Young s modulus, or the elastic modulus, of the material and it gives an indication of the resistance of a material to tensile forces. The higher the value of E, the greater the resistance of the material to tensile forces. A material which obeys Hooke s law for all stresses is called linear elastic for almost all polymeric systems invoking the assumption of linear elasticity is usually only an approximation. 

Phenol-formaldehyde resin Cast epoxy resin Nylon 6,6 

A final important parameter to be discussed is the strain energy density, defined by Eq. (4)  

The well-known lap shear test (ASTM D1002) attempts to put a shearing force onto an adhesive. 

Not all of the components of the tensor are independent a number of them must be equal since otherwise the body would rotate. Thus Si2 = 521, 23 = S32, S31 = i3 and Eq. (8) may be rewritten as Eq. (9). 

The term elastomer is normally used to describe a material that will stretch when placed under load and will retract to approximately the original shape when the load is removed. The solid polyurethanes described so far have this elastic property. The elasticity differs from that of ordinary rubbers due to the nature of the bonds in the polyurethane matrix. 

Polyurethane, like rubbers, has its tensile strength calculated using the initial cross-sectional area and not the area at break, as with metals. Therefore, the ultimate strength is much higher than conventionally quoted. 

With metals, the modulus is stress divided by strain (Young s modulus) and is both a ratio and a constant. In the case of polyurethanes, the load defection curve is linear only over the first few percent. The Young s modulus is calculated in this area. As the curve passes through the origin, the modulus is the same in compression as in tension. Work has also shown that the Young s modulus is three times shear modulus (Wright and Cummins, 1969). 

Hardness is a stiffness measurement. Stiffness is a stress/strain relationship. In load-carrying applications the stiffness is a bulk property, whereas measuring hardness is only a surface measurement. Two polyurethanes with completely different chemistries can have the same hardness. There are up to six completely different chemistries that are in common use (as shown in Table 7.1) that can give the same hardness. 

To measure the hardness, one needs to have a sample that is at least 5 mm thick and has a flat-enough surface for the base of the Durometer to rest on. For the most accurate measurements, a dead load instrument should be used. 

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Fig. 2. Typical stress—strain properties of staple fibers at 65% rh and 21°C. Rate of elongation is 50%/min. To convert N/tex to gf/den, multiply by 11.3.

Post-Curing. Whenever production techniques or economics permit, it is recommended that compounds based on terpolymer grades be post-cured. Relatively short press cures can be continued with an oven cure in order to develop full physical properties and maximum resistance to compression set. Various combinations of time and temperature may be used, but a cycle of 4 h at 175°C is the most common. The post-cure increases modulus, gready improves compresson set performance, and stabilizes the initial stress/strain properties, as chemically the polymer goes from an amide formation to a more stable imide formation. Peroxide-cured dipolymer compounds need not be post-cured. 

B 1,481.2 1,4-Polybutadiene (low vinyl) 1,2-Polybutadiene (high vinyl) Polyethylene Polybutylene Improved material stress-strain properties... 

Average viscosity (poise) (4°C) Stress-strain properties 7400 3800 3800 3800 90 165... 

Both tear resistance and hysteresis increase on incorporation of silica, but the effect is less pronounced as compared to the stress-strain properties. Tension set of the ZnO-neutralized m-EPDM system is low (around 20%) and incorporation of filler causes only a marginal increase in set due to chain slippage over the filler surface, as previously discussed. Measurement of physical properties reveal that there occurs an interaction between the filler surface and the polymer. Results of dynamic mechanical studies, subsequently discussed, support the conclusions derived from other physical properties. 

This effect was estimated from the experimental comparison of the stress-strain properties in three sample series which were brought to different phase contents by means of heat treatment. All samples were hydrogen-alloyed to a = 0.35 at T = 1053 K, then furnace cooled. Before straining, samples of the first series were maintained at the test temperature for 0.5 h. Series 2 samples were heated to the j9-phase, T = 1163 K, for 15 min, then cooled to the test temperature and treated like series 1 samples. The phase content in the third series was equilibrated by heating to 1163 K and slow cooling to 903 K before the test temperature was fixed. 

The high-pressure study on the Ti-H alloys showed stress-strain properties are not the only ones which are affected by pressure, but phase equilibria are also strongly dependent on this parameter . A new ( -phase and a corresponding single phase region in the isobaric T — C section of the T — P — C phase diagram appear in the... 

The table data show that the stress/strain properties of compositions are improved by additional dispersion (mixing). Ultrasonic analysis is sufficiently reliable and informative as a means of mixing quality assessment. The very small change of the characteristics for filled compositions (chalk + kaolin) can be due to the fact that these fillers are readily distributed in the matrix as they are. 

In TPE, the hard domains can act both as filler and intermolecular tie points thus, the toughness results from the inhibition of catastrophic failure from slow crack growth. Hard domains are effective fillers above a volume fraction of 0.2 and a size <100 nm [200]. The fracture energy of TPE is characteristic of the materials and independent of the test methods as observed for rubbers. It is, however, not a single-valued property and depends on the rate of tearing and test temperature [201]. The stress-strain properties of most TPEs have been described by the empirical Mooney-Rivlin equation... 

Pedemonte E., Alfonso G.C., Dondero G., De C.F., and Araimo L. Correlation between morphology and stress strain properties of three block copolymers. 2. The hardening effect of the second deformation. Polymer, 18, 191, 1977. 

Stress-strain properties (Cure 160°C/t9o) Hardness, Shore A 70 69 70 66... 

FE simulations of the stress-strain properties of fiUer-reinforced elastomers are an important tool for predicting the service live performance of mbber goods. Typical examples are the evaluation of rolling resistance of tires due to hysteresis energy losses, mainly in the tire tread or the adjustment of engine mounts in automotive applications. 

Figure 8. Comparison of the stress-strain properties of the press-quenched films of HBIB to those from the homopolymers HB and HI. Composition of each polymer is denoted by the butadiene content next to the graph.

There are various test methods, one being the De Mattia Flex Test method which is suitable for rubbers that have reasonably stable stress-strain properties, at least after a period of cycling, and do not show undue stress softening or set, or highly viscous behaviour. The results obtained for some thermoplastic rubber should be treated with caution if the elongation at break is below,... 

Stress-strain diagram, 27 721 Stress-strain instrument, 27 744 Stress-strain properties, of styrene plastics, 23 359-362... 

In measurement of tensile stress-strain properties, a test piece is stretched to breaking point and the force and elongation are measured at different stages. Tensile strength, elongation at break or work to failure (the area under the stress-strain curve) provide... 

The determination of tensile stress-strain properties is conducted in accordance with ISO 527 [4] and the values that can be obtained are illustrated in Figure 7.1. For weathering tests where cabinet space is restricted some workers have used a tensile impact dumbbell from ISO 8256 [5] with a square central section which allows test pieces to be exposed edge on. The considerable disadvantage is that modulus cannot be measured as there is no parallel gauge length. 

Tear strength is only applicable to flexible materials and is very little used to monitor ageing simply because tensile strength will serve perfectly well. There are circumstances where compression stress-strain properties would be relevant but the relatively bulky test pieces will be subject to the limitation of oxygen diffusion in any accelerated tests and changes can probably be estimated from tensile measurements. Similarly, shear stress-strain is very rarely used for monitoring ageing. 

ISO 37 1994 Rubber, vulcanized or thermoplastic - Determination of tensile stress-strain properties... 

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