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{{Language_sel|LANG=ger|ARTIKEL=Thermoelastischer Effekt}}
{{PSM_Infobox}}
A-Bild-Technik
__FORCETOC__

==General==

In many [[Material & Werkstoff|materials]], including [[Plastics|plastics]], a phenomenon known in technical literature as the thermoelastic effect is observed, particularly in [[Quasi-static Test Methods|quasi-static]] [[Tensile Test|tensile]] and [[Compression Test|compressive stress]] on [[Specimen|test specimens]].

==Physical fundamentals==

At the start of a tensile test on plastics in the [[Deformation#Elastic deformation|elastic]] or [[Viscoelastic Material Behaviour|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|stress]] has been exceeded, the test specimen heats up again and forms a thermal hotspot at a necking front or at an introduced [[Notch|notch]]. In the [[Compression Test|compression test]], the behaviour is reversed, and a slight heating is observed at first [1, 2].

[[File:Thermoelastic_Effect_Fig-1.jpg|500px]]
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px" |Temperature change of a test specimen under [[Tensile Test|tensile]] or [[Compression Test|compressive]] loading
|}

Although in the case of ideal elastic [[Deformation|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|energy elasticity]]), a temperature gradient caused by entropy changes occurs at the [[Surface|surface]] of the object ([[Entropy Elasticity|entropy elasticity]]).

Basically, these thermoelastic effects are generated by heat input or heat extraction at [[Plastic Component|components]] or [[Specimen|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|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|thermal expansion coefficient]]). The absolute amount of [[Deformation|deformation]] that occurs depends on the elastic constants, such as [[Elastic Modulus|elastic modulus]] ''E'' and [[Poisson's Ratio|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 Δ''T'' which, according to '''Eq. (1)''', depends on the increase in stress ([[Uniaxial Stress State|uniaxial stress state]]) Δ''σ'' [5].

{|
|-
|width="20px"|
|width="500px" | <math>\Delta T=\frac{\alpha_{th}T}{c_{\sigma}}\Delta\sigma</math>
|(1)
|}

with
{|
|-
|''c''
|width="15px" |
|heat capacity/volume
|-
|''α''Th
|
|[[Thermal Expansion Coefficient|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 [[Material & Werkstoff|materials]] and depends on the elastic constants of the material under investigation. Examples of a decrease in temperature are metallic materials and [[Plastics|plastics]] ('''Fig. 2'''). When these materials are subjected to [[Compression Test|compressive stress]] (negative stress), an increase in temperature is observed on the [[Surface|surface]] of the test object.

In the case of negative linear expansion coefficients, such as in [[Elastomers|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|stress]].

[[File:Thermoelastic_Effect_Fig-2.jpg|450px]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px" |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|Tensile test]]
* [[Entropy Elasticty|Entropy elasticty]]
* [[Thermomechanical Analysis (TMA)|Thermomechanical analysis (TMA)]]
* [[Thermal Expansion Coefficient|Thermal expansion coefficient]]

==References==

{|
|-valign="top"
|[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)
|-valign="top"
|[2]
|Mungenast, D.: Charakterisierung des thermoelastischen Effektes von amorphen Polymeren. Bachelorarbeit, Montanuniversität Leoben (2011)
|-valign="top"
|[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)
|-valign="top"
|[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)
|-valign="top"
|[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)

[[Category:Deformation]]
[[Category:Compression Test]]
[[Category:Tensile Test]]

Thermoelastic Effect - Revision history