Load-time/viscoelasticity

Figure 1.2 Highlighting load-time/viscoelasticity of plastics (1) stress-strain-time in creep and (2) strain-stress-time in stress relaxation.

International Rubber Hardness. The International mbber hardness test (ASTM D1415) (2) for elastomers is similar to the Rockwell test ia that the measured property is the difference ia penetration of a standard steel ball between minor and major loads. The viscoelastic properties of elastomers require that a load appHcation time, usually 30 seconds, be a part of the test procedure. The hardness number is read directly on a scale of 0 to 100 upon return to the minor load. International mbber hardness numbers are often considered equivalent to Durometer hardness numbers but differences ia iadenters, loads, and test time preclude such a relationship. 

Viscoelasticity It is the plastics respond to stress with elastic strain. In the material, strain increases with longer loading times and higher temperatures. 

Since v is a viscoelastic quantity, it is expected to increase with the loading time in the case of static loading (Theocaris and Hadjijoseph, 1965). 

Craze growth at the crack tip has been qualitatively interpreted as a cooperative effect between the inhomogeneous stress field at the crack tip and the viscoelastic material behavior of PMMA, the latter leading to a decrease of creep modulus and yield stress with loading time. If a constant stress on the whole craze is assumed then time dependent material parameters can be derived by the aid of the Dugdale model. An averaged curve of the creep modulus E(t) is shown in Fig. 13 as a function of time, whilst the craze stress is shown in Fig. 24. 

Fig. 4. Load/time trace from a typical test. Viscoelastic delay of strain recovery in the PMMA Hopkinson bar is compensated for by dynamic calibration.

Injection mouldings are usually thin walled to minimise the cooling part of the cycle time. Explain why they are more likely to fail under a compressive load by viscoelastic buckling, than by uniaxial compressive yielding. 

Under practical aspects and by considering the viscoelasticity of polymeric materials, the testing conditions such as loading time (speed) and temperature are of special interest. The following Figs. 5.10 and 5.11 demonstrate the influence of the pendulum hammer speed and the testing temperature by using filler-reinforced EPDM and SBR vulcanizates, respectively. 

Between the abovementioned two extreme conditions, bitumen behaves as a viscoelastic material and such conditions are those that exist on project sites. Therefore, in service, the bitumen behaves as a viscoelastic material and its mechanical properties depend on both temperature (T) and stress (a) loading time (t). 

For linear viscoelastic materials, the secant modulus obtained for different values of a, at the same temperature depends on the loading time, t, only (CEN EN 12697-26 2012). 

During loading time t), the bituminous mixture deforms the deformation (51) and, thus, the strain (e) increase rapidly at the beginning and then become quasi-constant. After removal of applied stress (rest period), part of the total deformation is recovered instantaneously (the elastic deformation), another part of deformation is recovered gradually and is time dependent (viscoelastic deformation) and another part of the deformation cannot be recovered (viscous or plastic or permanent deformation). Figure 7.15 explains the above, in terms of strain, which is known as creep behaviour. As it can be seen, the above behaviour is similar to the viscoelastic behaviour of the bitumen (see Section 4.21.1). 

Creep is defined as a long-term property measured by deforming a sample at a constant stress and monitoring the flow (strain) over a period of time. Viscoelastic materials flow or deform when subjected to loading (stress). In a creep experiment, a constant stress is applied and the resulting deformation is measured as a function of temperature and time. Just as stress relaxation is an important property to structural engineers and polymer scientists, so is creep behaviour. 

The time-dependence of deformation renders the parameters measured in simple tensile tests (see section 3.2) much less important than they are in metals, for instance. Although they do describe the behaviour at short-termed loads, time-dependent parameters obtained, for example, from isochrones -have to be used to design polymer components. Viscoelastic and viscoplastic effects can be neglected only if strains and loading times both are small. 

Viscoelasticity It is the plastics respond to stress with elastic strain. In the material, strain increases with longer loading times and higher temperatures. A material having this property is considered to combine the features of a perfectly elastic solid and a perfect fluid representing the combination of elastic and viscous behavior of plastics. 

The testing of polymers requires unique understanding of the viscoelastic nature of polymers. For example in a creep test it is required to suddenly apply a constant tensile, compression, or torsion stress to a bar of material. The most common description of a uniaxial tensile creep test is shown in Fig. 5.5(a). Several questions may arise one of which is How is the load to be applied suddenly without causing dynamic effects. One answer is for the load to be applied as ramp input as shown in Fig. 5.5(b). Obviously, the latter case is not a correct creep test. How big an error is involved A solution of the differential equation representative of the material for the ramp input of Fig 5.5(b) can be obtained and it can be shown that the error in the strain output is negligible if the loading time, to, is small compared to the retardation time of the material, x. 

Figure 6.9 Creep compliance as a function of loading time for a viscoelastic polymer melt.

Figure 15.5 (a) Load versus time, where load is applied instantaneously at time L and released at t,. For the load-time cycle in (a), the strain-versus-time responses are for totally elastic (fc), viscoelastic (c), and viscons d) behaviors. 

---