Tensile strain recovery

The tensile strain recovery and the work recovery % are also calculated. The procedure for crimped specimens is much the same, except that the test specimen is first pretensioned to take out the crimp and the extension shown on the recording chart eliminated by rezeroing the recorder. The location of the cross-head is locked in the memory at this point, so that on recovery the cross-head does not reenter the crimp range. 

Tensile strain recovery % 25.4 cmmin , butene-1 28% strain 100 (7)... 

Branchings Uniformity. Comparison of uniformly and nonuniformly branched ethylene—1-butene copolymers of the same density (Table 4) shows that uniformly branched resins are much more elastic, their tensile modulus is lower, and their strain recovery is neady complete. 

The tensile data can be applied to the design of short-term (such as 1 or 2 hour duration) or intermittent loads in a product provided the use temperature, the humidity, and the speed of the load are within 10% of the test conditions outlined under the procedure. The intermittent specification merely indicates that there be sufficient time for strain recovery after the load has been removed. 

The fiber industry has long been aware of PTT s good tensile elastic recovery [3], Ward et al. [4] studied the deformation behavior of PET, PTT and PBT fibers and found the tensile elastic recoveries were ranked in the unexpected descending order of PTT > PBT > PET. Chuah [47] found that the PTT elastic recovery and permanent set nearly tracked that of nylon 66 up to 30% strain (Figure 11.12). 

Plastics generally have intermediate tensile moduli, usually 0.5 x 1O to 4 x 1O psi (3.5 X 10 to 3 X 10 MN/m ), and Iheir breaking strain varies from a few percent for brittle materials like polystyrene to about 400% for tough, semicrystalline polyethylene. Their strain recovery behavior is variable, but the elastic component is generally much less significant than in the case of fibers (Fig. I-3c). Increased temperatures result in lower stiffness and greater elongation at break. 

BS 4029, 1978, specifies a method of test for the measurement of tensile elastic recovery of all types of single filaments or fibers when subjected to tensile strain applied by a CRE type machine. No special pretreatments such as mechanical cycling or relaxation of internal stresses by immersion in boiling water are specified, although it is acknowledged that they may be required for some special purposes. The determination of elastic recovery from a given stress (specific stress) is not included. 

When a specimen is stretched less than the breaking elongation and allowed to relax by removal of the strain according to a predetermined procedure. ( = tensile recovery curve, work recovery curve, and strain recovery curve.)... 

X. Wu and J. W. Holmes, Tensile Creep and Creep-Strain Recovery Behaviour of Silicon Carbide Fiber/Calcium Aluminosilicate Matrix Ceramic Composites, J. Am. Cerartu Soc. 76, 2695-2700 (1993). 

Strain recovery curve See tensile hysteresis curve. 

Elastic Recovery. The fiber industry has long been aware of PTT having the best tensile elastic recovery and resiliency among the three aromatic polyesters (45). This property is very desirable for fiber applications such as in carpets and textile apparels. The driving force for PTT s good elastic recovery comes from the response of its molecular chain to the applied strain. 

In the course of tensile creep, the form of the time dependence of strain (as expressed by the stretch ratio X, for example) depends on the magnitude of tensile stress at high stresses." " Recovery is considerably more rapid than would be predicted from the Boltzmann superposition principle, as illustrated in Fig. 13-23 for polyisobutylene of high molecular weight. " The course of recovery is predicted successfully by the theory of Bernstein, Kearsley, and Zapas. 2 - 22 -pije stress-dependent recoverable steady-state compliance D = which is equal to Z) at low stresses, decreases with increasing Ot- This effect, moderate when the tensile strain e is defined as X — 1, is more pronounced when it is replaced by the Hencky strain, defined as In X. The stress dependence of steady-state compliance in shear will be discussed in Chapter 17. The reader is referred to the review by Petrie" for more details. 

Strain Recovery Curve See Tensile Hysteresis Curve. 

Figure 6.17 Schematic tensile stress-strain diagram showing the phenomena of elastic strain recovery and strain hardening. The initial yield strength is designated as o-y. is the yield strength after releasing the load at point D and then upon reloading.

Figure 15.23 shows both the tensile stretching curve and the recoveiy curve of a typical fiber. The stretching curve shows the typical stress-strain behavior, which includes both elastic and plastic deformations. The recovering curve shows only the elastic deformation (or strain) is recovered after the removal of the applied tensile stress. The elastic recoveiy, or strain recovery, ean then be defined as ... 

In order to enhance the stiffness and strain recovery of this copolymer, a tensile condition process was developed previously. In this process, films were drawn in the semi-solid state so that the resulting morphology and molecular orientation closely resembled spun fibers of the same material. Correlations between strain and fiber draw-down and temperature were described. 

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