Impact Loading Plastics

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Impact loading plastics

Significance of high deformation rates

When plastics or composite materials are used in machines or vehicles, in explosions, in crash situations or even during high-speed machining of these materials, sudden impact forces can occur, which usually cause locally greatly increased deformation rates and are further intensified by the presence of stress peaks at sharp notches and/or low temperatures [1].

Since the molecular relaxation mechanisms in plastics require sufficient reaction time, the strength, deformation and fracture behaviour are significantly altered at high impact speeds or very low temperatures, as the material does not have the time required to react to the impact loading. The decisive factor for the effect of impact loading on the material is therefore the generated strain rate dε/dt or the characteristic exposure time.

Classification of test methods according to test duration and deformation rate

In principle, impact test methods are classified according to the impact frequency or test duration or strain rate as follows (see Fig. 1):

• 10^-1 – 10 s^-1 or Hz – impact loading
• > 10² – elastic or elastic-plastic stress waves with sound propagation velocity

Fig. 1:

Classification of test methods according to test duration and deformation rate

Dependence of dynamic yield strength on deformation rate

With the increase in the deformation rate or the decrease in the test or inspection time, a transition from isothermal to adiabatic testing occurs, as the heat generated cannot be dissipated to the environment within the short test duration. As a result of this, the characteristic values of the dynamic yield strength (see also: static yield stress), the tensile strength and the modulus of elasticity increase (Fig. 2), whereas the deformation characteristics show a decrease. Of particular significance here is the reduction in impact strength (see: impact test), which promotes deformation-free critical brittle fracture.

Fig. 2:

Dependence of the dynamic yield strength of a polymer blend consisting of polycarbonate (abbreviation: PC) and acrylonitrile butadiene styrene (abbreviation: ABS) on the deformation rate

Factors promoting brittle fracture

Plastics in particular are subject to a multitude of testing, manufacturing, stress-related and material-related factors that promote brittle fracture in the respective polymer material. These testing and geometry-related influencing factors are essentially (see Fig. 3):

• low testing or operating temperatures,
• stress crack-promoting environmental media,
• multiaxial stress states and stress concentrations (see: fracture mechanics),
• residual stresses in the test specimen or component,
• notches and sharp edges, and
• internal cavities (gas bubbles, vacuoles, micropores), porosities and cracks.

Fig. 3:

Geometric influencing factors (a), stress concentrations and multiaxial stress state (b) and residual stresses in a motorcycle helmet visor (c)

Due to these influencing factors, testing under sudden loading is often carried out on notched or cracked test specimens, varying the test speed and test temperature. Material-related influencing factors include, for example:

• the morphology and structure of the materials,
• frozen orientations or orientations of fillers,
• the degree of crystallinity and
• the spheroidal size in semi-crystalline plastics.

High orientations can lead to higher impact strength in amorphous plastics and reduced impact strength in semi-crystalline materials.

Test methods for characterizing toughness under impact loading

The test methods commonly used to characterise the impact strength of plastics range from 1 to 10 m s^-1 and are carried out using the following methods, although various technical variants exist:

• Impact test and notched impact test,
• tensile impact test and notched tensile impact test,
• instrumented Charpy impact test,
• instrumented tensile impact test and
• instrumented puncture impact test (free-falling dart test).

If higher test speeds are required, as shown in Fig. 4, more technically sophisticated test methods are used (see Fig. 1).

Fig. 4:

Overview of achievable test speeds for impact loading depending on the test speed

See also

• Impact loading free-falling dart test
• Impact loading high-speed testing
• Impact loading pendulum impact tester
• Instrumented puncture impact test
• Instrumented Charpy impact test (ICIT)
• Instrumented tensile impact test (ITIT)

References

[1]	Grellmann, W.: Impact Loading. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 143–156 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56690-807-5; see AMK-Library under A 22)