Thermoelastic Effect
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General
In many materials, including plastics, a phenomenon known in technical literature as the thermoelastic effect is observed, particularly in quasi-static tensile and compressive stress on test specimens.
Physical fundamentals
At the start of a tensile test on plastics in the elastic or viscoelastic deformation range, a slight cooling of the test specimen occurs (Fig. 1), which can be recorded with a thermographic camera. Once a certain amount of strain or stress has been exceeded, the test specimen heats up again and forms a thermal hotspot at a necking front or at an introduced notch. In the compression test, the behaviour is reversed, and a slight heating is observed at first [1, 2].
| Fig. 1: | Temperature change of a test specimen under tensile or compressive loading |
Although in the case of ideal elastic deformation of a test specimen with a loading and unloading phase (Fig. 1), the sum of the mechanical work performed, and the heat is zero (energy elasticity), a temperature gradient caused by entropy changes occurs at the surface of the object (entropy elasticity).
Basically, these thermoelastic effects are generated by heat input or heat extraction at components or test specimens and cause changes in the local temperature as well as the temperature gradient, which can be measured on the surface of objects. This results in an interaction between the local thermal properties and the local stress tensor (see: thermal conductivity).
When heat is applied to a solid, e.g. a test specimen, gas or liquid, elastic deformation (change in length or volume) occurs, which is also referred to as thermal expansion (see: thermal expansion coefficient). The absolute amount of deformation that occurs depends on the elastic constants, such as elastic modulus E and Poisson's ratio µ. The reverse effect occurs when mechanical energy (tensile or compressive stress, hydrostatic pressure) is applied to solids, gases or liquids, causing a change in temperature referred to as thermoelasticity. Under constrained conditions, such as fixed clamping or closed vessels, a change in the stress state may also occur [3, 4].
Under adiabatic mechanical tensile stress, which occurs without any heat or energy exchange, prismatic test specimens undergo a temperature change ΔT which, according to Eq. (1), depends on the increase in stress (uniaxial stress state) Δσ [5].
| (1) |
with
| c | heat capacity/volume | |
| αTh | linear thermal expansion coefficient | |
| T | temperature |
Example of temperature change on the surface due to the thermoelastic effect
If the coefficient of expansion of the test specimen in question is positive, then the increase in stress in the tensile direction (positive) results in a decrease in temperature, which can be detected using an infrared thermal camera. The absolute value of the temperature change is very small for most materials and depends on the elastic constants of the material under investigation. Examples of a decrease in temperature are metallic materials and plastics (Fig. 2). When these materials are subjected to compressive stress (negative stress), an increase in temperature is observed on the surface of the test object.
In the case of negative linear expansion coefficients, such as in elastomers and rubber, an increase in temperature is recorded under tensile stress, which also occurs when gases are compressed by means of pumps. When the load is removed, cooling back to the previous equilibrium state occurs. However, due to the time dependence of heat conduction, these processes occur with a delay relative to the mechanical stress.
| Fig. 2: | Changes in surface temperature due to the thermoelastic effect on (a) a notched PA 6 test specimen with 10 M.-% GF and (b) an elongated PA 6 test specimen |
See also
- Thermography
- Tensile test
- Entropy elasticty
- Thermomechanical analysis (TMA)
- Thermal expansion coefficient
References
| [1] | Riegert, G.: Induktions-Lockin-Thermografie – ein neues Verfahren zur zerstörungsfreien Prüfung. Dissertation, Universität Stuttgart, (2007) (https://doi.org/10.18419/OPUS-1734) [1] (last access on February 1, 2026) |
| [2] | Mungenast, D.: Charakterisierung des thermoelastischen Effektes von amorphen Polymeren. Bachelorarbeit, Montanuniversität Leoben (2011) |
| [3] | Christ, H.-J.: Wechselverformung von Metallen – Zyklisches Spannungs-Dehnungs-Verhalten und Mikrostruktur. Springer Berlin, (1991) (ISBN 978-3-540-53962-9; see AMK-Library under L 9-1) |
| [4] | Bergmann, W.: Werkstofftechnik 1 – Struktureller Aufbau von Werkstoffen – Metallische Werkstoffe – Polymerwerkstoffe – Nichtmetallisch anorganische Werkstoffe. Carl Hanser, Munich, 7th Edition (2013) (ISBN 978-3-446-43536-0) |
| [5] | Redinger, S., Arendts, F. J.: Thermoelastische Spannungsanalyse von Metallen und Verbundwerkstoffen: Stress Pattern Analysis by Thermal Emission. Institut für Flugzeugbau, Universität Stuttgart (1992) |
Additional references
- Strauch, I.: Thermische Effekte in der Materialmodellierung von Polyamid 6 bei kurzzeitdynamischen Belastungsvorgängen. Dissertation (2019). Otto-von-Guericke-Universität Magdeburg. [2] (last access on February 1, 2026)