SHORT-TERM MECHANICAL PROPERTIES

This section gives some commonly used criteria to define and describe the short-term strength mechanical behavior of thermoplastic materials. Specific property data for LANXESS materials can be found in the

CAMPUS© database system for plastics, and in LANXESS’ Property Guides. Consult the publication Material

Selection for information on the

various test methods and property data used for thermoplastics engineering resins. These publications are available through your sales representative.

Figure 3- 8

Typical stress-strain behavior of unreinforced plastics.

Figure 3- 7

These curves illustrate the characteristic differences in the stress-strain behavior of various plastics.



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Tensile Properties

Tensile properties are measured in a device that stretches a molded test bar between two clamping jaws. The jaws separate at a steady rate, and the device records the force per cross-sectional area (stress) required to stretch the sample from 0% elongation to break. The results are often graphed as stress versus percentage elongation (strain). Figure -7 shows the kinds of stress- strain behavior exhibited by plastics. Rigid plastics exhibit a nearly linear behavior similar to metals. Ductile materials display a more complex behavior.

Figure -8 identifies the transitional points in the stress-strain behavior of ductile plastics. Point A, the

proportional limit, shows the end of

the region in which the resin exhibits linear stress-strain behavior. Point B is the elastic limit, or the point after which the part will be permanently deformed even after the load is removed. Applications that cannot tolerate any permanent deformation must stay below the elastic limit. Point C, the yield point, marks the beginning of the region in which ductile

plastics continue to deform without a corresponding increase in stress.

Elongation at yield gives the upper

limit for applications that can tolerate the small permanent deformation that occurs between the elastic limit and the yield point, but not the larger deformation that occurs during yield. Point D, the break point, shows the strain value when the test bar breaks.

Tensile Modulus

Commonly used in structural

calculations, tensile modulus measures material stiffness. Higher values indicate greater stiffness. Because of plastic’s viscoelastic behavior, determining tensile modulus is more subjective and less precise for plastics than it is for metals and most other materials. Mathematically, you can determine the tensile modulus by taking the ratio of stress to strain as measured below the proportional limit on the stress-strain curves. When dealing with materials with no clear linear region, you can calculate the modulus at some specified strain value, typically at 0.%. For some applications, buckling analysis, for example, it may be more appropriate to derive a modulus from the slope of a line drawn tangent to the curve at a point on the stress-strain diagram (tangent modulus).

Tensile Stress at Yield

Tensile stress at yield, the stress level corresponding to the point of zero slope on the stress-strain curve, generally establishes the upper limit for applications that can tolerate only small permanent deformations. Tensile- stress-at-yield values can be measured only for materials that yield under test conditions.

Tensile Stress at Break

Tensile stress at break is defined as the stress applied to the tensile bar at the time of fracture during the steady-deflection-rate tensile test. Data for tensile stress at break establish the upper limits for two types of applications: one-time-use applications that normally fail because of fractures, and applications in which the parts can still function after undergoing permanent deformation.

Ultimate Strength

Ultimate strength measures the highest stress value encountered during the tensile test. This value should be used in general strength comparisons, rather than as a design criterion. Ultimate strength is usually the stress level at the breaking point in brittle materials. For ductile materials, it is often the value at yield or break.

Poisson’s Ratio

As a plastic specimen stretches longitudinally in response to tensile loading, it narrows laterally. Poisson’s ratio measures the ratio of lateral to longitudinal strains as the material undergoes tensile loading. Poisson’s ratio usually falls between 0.5 and 0.40 for engineering resins (see table -). Some elastomeric materials approach the constant-volume value of 0.50.

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Coefficients of Friction (Static) Ranges Table 3-2 for Various Materials

Compressive Properties

Under equivalent loading conditions, plastics tend to fail in tension rather than compression. For this reason it is more common to test tensile properties rather than compressive properties. As a rule of thumb, plastics tend to be approximately 0% stronger under compressive loading. Consult your LANXESS representative if you require detailed analysis in a compressive mode. Assuming that the compressive strength equals the tensile strength usually results in a conservative design.

Flexural Modulus

Defined as the ratio of stress to strain in the elastic region of a stress-strain curve derived from flexural testing, flexural modulus measures a resin’s stiffness during bending. A test bar subjected to bending loads distributes tensile and compressive stresses through its thickness. The flexural modulus is based upon the calculated outer-fiber stress. Test values for tensile modulus typically correlate well with those of the flexural modulus in solid plastics, but differ greatly for foamed plastics that form solid skins.

Coefficient of Friction

The coefficient of friction is the ratio of friction force, the force needed to initiate or maintain sliding, to normal force, the force perpendicular to the contact surfaces. Coefficients are commonly listed for two types of friction: static friction, the forces acting on the surfaces to resist initial movement, and dynamic friction, the forces acting between surfaces that are already sliding. Table - lists typical values for common plastics.



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LONG-TERM MECHANICAL