Page 46 - Engineering Plastics Handbook

20 Introduction

allow us to perform a vast number of activities that would not be possible
without engineering thermoplastics. Engineering thermoplastics are often
associated with load-bearing and semistructural products, but their uses
in other applications are increasing, such as wire coatings and clear sheet-
ing, which are included in this handbook.
Product designs are essential to the ongoing success of engineering plas-
tics products. Underlying the design considerations for every engineering
thermoplastic is the effect of viscoelasticity on long-term applications under
applied loads at elevated temperatures. Spring-and-dashpot models pro-
vide an understanding of the important subject of viscoelastic behavior.
Spring-and-dashpot models are described in this chapter in a logical
sequence, starting with the hookean spring element and newtonian dash-
pot element and progressing to multielement models. The spring element
indicates short-time tensile properties, and the viscous dashpot indicates
time-dependent properties. Below the elastic limit where there is a linear
relationship between stress and strain, a thermoplastic material returns
to its initial form for small strains. Beyond its elastic limit, a thermoplas-
tic does not return to its initial form. In fact, at a constant applied stress,
deformation (strain) continues to increase. Steel and aluminum are elas-
tic, not viscoelastic, materials. The significance of thermoplastic behavior
beyond the elastic limit relates to the viscous properties for long-term appli-
cations under stress. Engineering thermoplastics are typically (not always)
used for long-term applications and at elevated temperatures. Short-term
test methods provide data for initial selection and elimination of resins and
compounds and a starting point for application-specific compounds.
Solutions for beam and flat plate formulas are based on traditional engi-
neering design and require safety factors to comply with the effects of vis-
coelasticity. Beam bending calculations consider the second moment of
inertia and stiffness. I beams, T beams, and hollow-center beams have a
high second moment of inertia and allow the use of less material [1]. The
second moment of inertia is an area moment of inertia. It is the second
moment of a cross-sectional area around a given axis, and it is a measure
of a beam’s capacity to resist bending. The larger the second moment of
inertia, the more resistant the beam is to bending [2, 3]. The bending
moment of a section M is directly proportional to the second moment of
inertia I. The polar moment of inertia J of the cross section of a beam is
z
a measure of a beam’s capacity to resist torsion. The larger the J the
z,
more resistant the beam is to twist.
Sandwich panels constructed of a cellular or honeycomb core and lam-
inated outer skins are treated as I beams to calculate the moment and mod-
ulus. The laminate skin is the I beam flange, and the core is the beam shear
web in calculations. Under a bending load, the flexural stiffness of a sand-
wich panel is proportional to the cube of its thickness [4]. For these rea-
sons, modified “I beams” constructed as a foam core–outer skin sandwich