Polymers 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 both polymers, creep of compression-molded specimens is caused mainly by crazing, with shear processes accounting for less than 20% of the total time-dependent deformation. Crazing is associated with an increasing creep rate and a substantial drop in modulus. The effects of stress upon creep rates are described by the Eyring equation, which also offers an explanation for the effects of rubber content upon creep kinetics. Hot-drawing reduces creep rates parallel to the draw direction and increases the relative importance of shear mechanisms. 

Similarly, Figure 6 summarizes the creep behavior of glass-and mineral-filled polyphenylene sulfide under three sets of conditions 24°C/5,000 psi flexural load, 66°C/5,000 psi, and 1210C/3,000 psi. Table III compares the per cent loss In apparent creep modulus at 1,000 hours and at 10,000 hours for each of these conditions using the apparent creep modulus at one hour as a basis. These data Indicate that the creep resistance of the glass- and mineral-filled polymer Is similar to that of the 40% glass-filled resin. 

Flexural creep modulus - data Polymer Solids and Polymer Melts C. Bierdgel, W. Grellmann The foUowing Table 4.22 shows a summary of available flexural-creep modulus values of thermoplastics materials according to the demands of ISO 899-2 or other relevant standards. Table 4.22 Flexural-creep modulus of thermoplastic materials. ... 

Fig. 4.163 Flexural-creep modulus of glass fiber reinforced liquid-crystal polymer at different temperatures and stress levels [12Els].

Polylethylene terephthalate), PET, 252, 254 Polyisoprene, 221 Polymers, 5 amorphous, 5-7, 76 creep, 77, 259 creep modulus, 84 creep rupture, 11-12, 284 crystalline, 5-7, 76 crystallinity, 322... 

Figure 8.13 A 10 s isochronal creep modulus, measured at room temperature, as a function of draw ratio for a range of quenched (open symbols) and slowly cooled (closed symbols) samples of linear polyethylene drawn at 75 °C. ( ), Rigidex 140-60 (A, A), Rigidex 25 ( , ), Rigidex 50 (o, ), P40 (0, ), H020-54P. (Reproduced with permission from Capaccio, Crompton and Ward, J. Polym. Set, Polym. Phys. Ed., 14, 1641 (1976))...

In this book, we use the term creep only for the time-dependent plastic deformation. In the context of polymer science, the time-dependent elastic deformation is also frequently called creep , but this is not done here, except for using the term creep modulus . Creep (i. e., viscoplasticity) can also occur in metals and ceramics at high temperatures and will be discussed in chapter 11. 

In the context of polymers, the time-dependent elastic deformation (retardation and relaxation, see section 8.2.1) is frequently denoted as creep as well. To avoid confusion, this is not done in this book, excepting the standard term creep modulus . 

Usually, creep models for engineering materials are based on the use of the strain-time response of the material. Other quantities, such as deflection and creep modulus, can be obtained using the corresponding relationships. Figure 4.4 shows the general strain versus time response of rPET polymer concrete subjected to constant stress and temperature. 

The stress level is an important variable that influences the response of rPET polymer concrete. However, at relatively low design stress levels, stress is likely to have little influence because the material behaviour is almost linear viscoelastic that is, the average creep modulus is essentially the same at any stress level, as detailed in the previous chapter. The response of rPET polymer concrete under design loads is always likely to be linear viscoelastic. 

Clustering of sulfonic acid groups and water helps elucidate the large and dramatic changes in proton conduction, water transport, elastic modulus, polymer creep, and stress relaxation that occur with Nafion at low water activity and high temperature. The identification of a clustering transition facilitates the choice of processing conditions to erase the memory of Nafion and better control the mechanical and transport properties. 

Many polymeric materials are susceptible to time-dependent deformation when the stress level is maintained constant such deformation is termed viscoelastic creep. This type of deformation may be significant even at room temperature and under modest stresses that lie below the yield strength of the material. For example, automobile tires may develop flat spots on their contact surfaces when the automobile is parked for prolonged time periods. Creep tests on polymers are conducted in the same manner as for metals (Chapter 8) that is, a stress (normally tensile) is applied instantaneously and is maintained at a constant level while strain is measured as a function of time. Furthermore, the tests are performed under isothermal conditions. Creep results are represented as a time-dependent creep modulus E t), defined by ... 

In order to predict the creep behavior and possibly the ensuing failure a number of approaches have been proposed. These are based respectively on the theory of viscoelasticity — including the concept of free volume — or on empirical representations of e(t) or of the creep modulus E(t) = ao/e(t). The framework of the linear theory of viscoelasticity permits the calculation of viscoelastic moduli from relaxation time spectra and their inter conversion. The reduction of stresses and time periods according to the time-temperature superposition principle frequently allows establishment of master-curves and thus the extrapolation to large values of t (cf. Chapter 2). The strain levels presently utilized in load bearing polymers, however, are generally in the non-linear range of viscoelasticity. This restricts the use of otherwise known relaxation time spectra or viscoelastic moduli in the derivation of e (t) or E (t). 

Creep data are invaluable for predicting die long-term functional behavior of a material or product. However, the current body of data (seldom, if ever, reported on product data sheets) cannot be compared for a series of material candidates. Polymers must be tested exactly the same way ( test mode, initial stress level, time, and temperature) in order to have a valid comparison, without relying on mathematical adjustments. The concept of the use of creep modulus has been widely adopted. However, one must still adhere to the paradigms of linear viscoelasticity to allow valid comparisons of different polymeric materials. 

While the standard linear model does not precisely describe creep or stress relaxation behavior because of the assumption of a single relaxation time, the above arguments still ly to actual polymer behavior, where Dc (t) < Dr ) . Thus, for constant load applications, the creep compliance or its inverse, the so-called effective creep modulus should be used, whereas for constant displacement (e.g., a plastic nut and bolt), the relaxation modulus should be used. 

In summary, unreinforced polymers will have a creep modulus in tension that is less than the creep modulus in compression or a creep compliance in tension that is larger than the creep compliance in compression. For continuous fiber polymeric composites, the situation is reversed. 

The material in use as of the mid-1990s in these components is HDPE, a linear polymer which is tough, resiUent, ductile, wear resistant, and has low friction (see Olefin polymers, polyethylene). Polymers are prone to both creep and fatigue (stress) cracking. Moreover, HDPE has a modulus of elasticity that is only one-tenth that of the bone, thus it increases the level of stress transmitted to the cement, thereby increasing the potential for cement mantle failure. When the acetabular HDPE cup is backed by metal, it stiffens the HDPE cup. This results in function similar to that of natural subchondral bone. Metal backing has become standard on acetabular cups. 

Polymers are a little more complicated. The drop in modulus (like the increase in creep rate) is caused by the increased ease with which molecules can slip past each other. In metals, which have a crystal structure, this reflects the increasing number of vacancies and the increased rate at which atoms jump into them. In polymers, which are amorphous, it reflects the increase in free volume which gives an increase in the rate of reptation. Then the shift factor is given, not by eqn. (23.11) but by... 

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