Meaning
Thermal error that occurs when the electrical current passing through a resistive sensor increases the local temperature of the plastic substrate being measured. Strain gauge self heating is a substantial problem when measuring the deformation of polymers, as they have very low thermal conductivity and are highly sensitive to temperature changes. The heat generated by the gauge cannot escape easily, causing the local area to expand and the material to soften.
This localized heating creates a false strain reading that does not reflect the actual mechanical load on the part. It can lead to a misunderstanding of the material’s properties and the performance of the design. The effect is most pronounced when using high excitation voltages or small gauges on thin plastic sections.
Thermal Dissipation
In traditional metal testing, the high thermal conductivity of the specimen allows the heat from the strain gauge to be dissipated quickly. However, plastics act as thermal insulators, trapping the heat directly under the sensor. As the temperature of the polymer increases, the material undergoes thermal expansion, which is picked up by the gauge as an apparent strain.
This expansion is often much larger than the mechanical strain being measured, leading to massive errors in the data. The softening of the polymer also changes its modulus, making it more compliant and further distorting the results. To minimize this effect, researchers must use very low excitation voltages and choose gauges with a higher resistance.
This reduces the amount of power being converted into heat at the sensor location.
Measurement Error
Self heating can cause the strain readings to drift over time, even when the load on the part is constant. This drift makes it difficult to conduct long term tests like creep or fatigue analysis, as it is impossible to separate the thermal effect from the mechanical deformation. The magnitude of the error depends on the power density of the gauge, the local thickness of the plastic, the surrounding environmental conditions and the excitation voltage.
If the test is conducted in a still air environment, the heat buildup will be even more severe. Engineers must carefully calibrate their setup to account for this heating or use alternative measurement techniques like digital image correlation. Without proper compensation, the data from a strain gauge on a polymer part is often unreliable.
Every test must be carefully designed to ensure that the sensor does not influence the behavior of the material.
Compensation Method
Several techniques can be used to mitigate the effects of strain gauge self heating and improve the accuracy of the measurements. One common approach is to use a dummy gauge on an identical piece of material that is not under load. Both gauges are connected to a bridge circuit, which cancels out the common thermal expansion and leaves only the mechanical strain.
Another method is to use a pulsed excitation voltage, which only applies power to the gauge for a fraction of a second when a reading is taken. This gives the heat more time to dissipate between measurements and noticeably reduces the temperature rise. Moulders and test engineers also try to use the largest possible gauge size to spread the heat over a wider area.
These precautions are essential for obtaining high quality mechanical data from plastic components. Every report should state the excitation voltage and the steps taken to minimize thermal errors.
