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1 Introduction

Figure 1.19: A force (strain) always has an effect (strain, elongation) on a material. In the ideal case, stress

and strain are linearly related. The conversion factor between stress and strain, the modulus, is a measure

of the material’s resistance to strain, its strength. The calculations for different forces (tensile or elongation,

shear and pressure) are analogous to each other.

larger stresses will permanently deform and/or the effects of stress and strain will take

time. The time-dependence of a stress or strain is usually likened to a viscous flow (Fig-

ure 1.20).

Yes, even for solids it is considered a flow, just a very slow one. For example, when

old window glass sags, it might have taken 100 years to flow, but this movement is

still considered a flow. All materials can thus be described as having viscosity. For real

materials, you need to combine the effects and equations of strength with the time-

dependent effects and equations of viscosity to fully describe their viscoelastic behavior

(Figure 1.21). The time-dependent part of the stress is called creep, the time dependent

part of the strain is called recovery. In addition, real materials beyond the linear, ideal

region of stresses will always end up with some deformation. What is important is that

the material can handle the amount of deformation without breaking.