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What is accelerated Thermal Ageing Test?

What is Thermal Ageing?

Ageing of the materials with temperature could happen and differs for various materials. Important safety systems/equipments need to be evaluated for the effect of temperature on the materials used in the system. Even storage for long years shall also be considered for some materials like polymers to estimate the ageing effect.

Ageing effect couldn’t be estimated practically for operating conditions or storage conditions as the duration is very high (tens of years) and the qualification of product couldn’t happen. Hence, Ageing effect due to temperature could be evaluated by exposing the unit to higher temperature for accelerated time. This is called as Accelerated Thermal ageing test.

How to calculate time required for Thermal Ageing of an instrument at accelerated temperature?

There are different methodologies to do Thermal Ageing test. Most commonly used methodology is application of Arhenius equation. Usually, this methodology is applicable for Polymer/Organic materials for which temperature effect is more.

Thermal Ageing is due to the chemical process induced by the temperature, which could lead to change the bulk properties of the material over a time.

The Arhenius methodology has been developed to simulate the accelerated ageing mechanism. As per this methodology, deterioration of materials in service is due to chemical reaction. These occur internally, sometimes between components of the material, and sometimes with compounds in the environment such as oxygen or water vapor. Chemical reactions occur more rapidly at higher temperatures.

Arrhenius showed that temperature dependence of chemical reactions follows an exponential equation. He postulated a consistent correlation between the amount of physical change and chemical reaction so that the time to reach a selected amount of physical change will vary according to an equation. The rate of thermal aging is the slope of the graph using the Arrhenius equation.

Accelerated Time calculation for Thermal Ageing Test

As per Arhenius equation,

where,

‘E’ is activation energy for the material under consideration; in eV

‘K’ is Boltzman Constant i.e. 8.617 x 10-5 eV.K-1

‘t1’ is ageing time required at accelerated temperature ‘T1’.

‘t2’ is service life time operating temperature ‘T2’.

‘T1’ is accelerated temperature

‘T2’ is Operating temperature

For instance, let us consider the cross linked polyolefin cables (XLPO cables) being used for an application where the normal operating temperature is 50 oC and the expected service life is 40 years. Let us calculate the accelerated time required for thermal ageing test at accelerated temperature of 135 oC.

i) Activation energy (E) for XLPO cable is considered as 0.78 eV

[Ref: An Experimental Study on Dielectric Response of the Cross linked Polyolefin (XLPO) Insulation of the Unshielded Cable Under Electric Field and Heat Flux; Publisher: IEEE]

ii) t2 = 40 years

iii) T1 = 135 degC

Time required for accelerated Thermal Ageing test for accelerated temperature of 135 oC is ~43 days.