Tensile creep modulus

Polymers of this type have exceptional good values of strength, stiffness and creep resistance (see Table 18.13). After 100 h at 23°C and a tensile load of 70 MPa the creep modulus drops only from 4200 to 3(K)0 MPa whilst at a tensile load of 105 MPa the corresponding figures are 3500 and 2500 MPa respectively. If the test temperature is raised to 150°C the creep modulus for a tensile load of 70 MPa drops from 2400 to 1700 MPa in 100 h. 

In various experiments different elastic constants are being determined with a torsion pendulum, for instance, the shear modulus, G, is measured, with creep or vibrations in elongation or in bending the Young s modulus, (tensile modulus), E. For an isotropic material the relation between E and G is as follows ... 

The carbon chain is in a planar zigzag orientation and forms an orthorhombic lattice with interpenetration of adjacent chains.61 As a result of this structure, ETFE has an exceptionally low creep, high tensile strength, and high modulus compared to other thermoplastic fluoropolymers. Interchain forces hold this matrix until the alpha transition occurs at about 110°C (230°F), where the physical properties of ETFE begin to decline and more closely resemble perfluoropolymers properties at the same temperature. Other transitions occur at -120°C (-184°F) (gamma) and about -25°C (-13°F) (beta).62... 

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... 

The variation of the 100 second tensile creep modulus with 100 second tensile strain is presented in Fig. 16. The behaviour of a specimen cut from an isotropic sheet (which had been subjected to the same thermal cycle as the drawn sheet) is included for comparison. It is apparent that all specimens exhibited non-linear viscoelastic behaviour, but there is little anisotropy of non-linearity. Furthermore the degree of non-linearity exhibited by the specimens from the drawn sheet is similar to that of a specimen from the isotropic sheet. At any chosen creep strain the anisotropy of modulus for the drawn sheet is relatively low. 

Long-term Loading - Tensile Creep Modulus - data... 

Table 4.21 Tensile-creep modulus of thermoplastic materials. ...

Fig. 4.136 Tensile-creep modulus of aromatic polyester at different temperatures and stress levels [98Dom].

Fig. 4.139 Tensile-creep modulus of polyamide 612 at different stress levels at room temperature [12Els].

Fig. 4.140 Tensile-creep modulus of different reinforced polyamides at different stress levels [12Els].

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