Tensile Test Control
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Tensile test control
Influencing factors
The characteristic value level of plastics depends to a large extent on the test speed and temperature, which manifests itself in the creep and relaxation tendency in the viscoelastic properties. The cause can be found in the test conditions of the conventional tensile test on plastics, as the local and integral strain rate are not constant due to numerous influencing factors, such as orientation and the internal residual stress state. This affects the deformation behaviour and the absolute value of the characteristic values. These unavoidable influencing factors can be avoided or minimised by using controlled tensile tests, which, however, are not standardised for plastics, unlike the testing of metallic materials [1]. There are two basic types of these controlled tensile tests: force control and strain control.
In conventional testing with constant crosshead speed, the primary control loop must guarantee the constancy of the crosshead speed vT regardless of the applied test load. In the case of force or strain control, the crosshead speed serves as the control variable for generating a constant force increase dF/dt or a constant strain rate dε/dt within the sensor length of the strain measurement sensor. This means that the speed of the testing machine is not a constant variable.
Force control
In force- or stress-controlled tensile testing, the specified or target value is the force or stress increase per unit of time dF/dt or dσ/dt. To determine the setpoint, a conventional tensile test is performed up to a desired limit value (Fig. 1), which must not exceed the yield stress or tensile strength, as this would cause instabilities in the control loop, leading to the controlled test being cancelled.
| Fig. 1: | Determination of the set-point value for force control in the tensile test |
The secant from the zero point to the limit value describes the set-point dF/dt or ΔF/Δt of the controlled variable through its rise and corresponds to a ramp function. The closed control loop can be implemented with an analogue function generator (Fig. 2a) or a digital software-supported control system (Fig. 2b). Depending on the sensor used (electro-mechanical force transducer or clip-on extensometer (see: tensile test path measurement technique)), the actual value (input variable) and the set-point value can be the force or stress as well as the elongation or strain. The PID controller settings depend on the selected strain or stress rate and the modulus of elasticity of the material under investigation. In particular, P values (proportional gain) that are too small lead to non-compliance with the target values, while P values that are too large cause control loop instabilities.
| Fig. 2: | Analogue (a) or incremental (b) force or strain control in tensile testing |
Strain control
In an elongation- or strain-controlled tensile test, the specified or target value is the increase in elongation or strain per unit of time dΔL/dt or dε/dt. To determine the target value, a conventional tensile test is performed up to a specified limit value (Fig. 3), which must not exceed the yield stress or the tensile strength, as this could cause instability.
| Fig. 3: | Determination of the set-point value for strain control in the tensile test |
The measured elongation or strain of the sensor in the measuring interval L0 can only be used as a control variable in the area of uniform elongation (elongation without necking), unless locally resolving optical strain measurement techniques are used. The secant from the zero point to the limit value describes the set-point dΔL/dt or Δε/Δt of the control variable through the rise and corresponds to a ramp function. The closed control loop can be implemented with an analogue function generator (see Fig. 2a) or a digital software-supported control system (see Fig. 2b).
Influence of the control type on the stress–strain relationship
A comparison of tensile tests with and without control shows that the force- or stress-controlled test forces a constant increase in force or stress regardless of the other test conditions and thus represents the most stringent condition for the plastic material under investigation in terms of the speed dependence of these materials. Due to the variable test speed in the tested volume, the conventional tensile test lies between the stress- and strain-controlled tests in terms of property level (Fig. 4).
| Fig. 4: | Comparison of tensile tests with different control types for an unreinforced polypropylene (abbreviation: PP material) |
The strain-controlled tensile test best takes into account the requirements regarding relaxation and creep behaviour, which is why the lowest stress level, but the highest strains are recorded in the test. In this specific example, the different stress–strain behaviour for a polyamide 6 with 20 M.-% short glass fibres is shown (Fig. 5). Figure 5a shows the deformation behaviour in a conventional tensile test with a constant crosshead speed and a nominal strain rate of 1 %/min. The normative strain rate recorded with a clip-on extensometer shows significant changes during the tensile test and only reaches the required strain rate at one point at approx. 2 % of the strain. As expected, the tensile test with integral control of the normative strain rate shows a constant normative strain during the measurement interval (Fig. 5b).
| Fig. 5: | Comparison of stress-strain diagrams and normative strain rates for PA6-GF 20 for a) the conventional and b) the strain-controlled tensile test |
Figure 5 shows that, as a result of the more favourable relaxation conditions, a lower stress level is achieved in the controlled test and a greater elongation at break occurs due to creep during the tensile test. These effects are also associated with a low dispersion of the characteristic values of the tensile test [2].
See also
- Laser extensometry – local strain control
- Tensile test compliance
- Laser cross-unit
- Creep behaviour – recovery test
- Creep behaviour – tensile creep test
References
| [1] | ISO 6892-1 (2019-11): Metallic Materials – Tensile Testing – Part 1: Method of Test at Room Temperature |
| [2] | Bierögel, C.: Tensile Tests on Polymers. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 106–123 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-806-5; see AMK-Library under A 22) |