Thermal Stress Analysis of Metals
High‐End‐Thermography with ImageIR® 8300 hp

Initial situation
Numerous chemical and physical effects affect metals whereby their material properties such as tensile strength canchange. Analyses on the degree of change are crucial for assessing whether or not a component or a complete device
can still carry out its function without restriction. Tensile testing provides information for such assessments.
During this procedure external mechanical pressure is applied to solid state objects resulting in stress changes in theirinterior. The changed surface temperature is a parameter that describes the sum of all internal stress changes of thebody which reach the surface. As a result of positive changes in pressure solid bodies heat up and cool down
accordingly during negative changes in pressure. The inner stress change Δσ thus behaves directly proportional to themeasured temperature change ΔT. The example of a test for tensile specimen measurement demonstrates thatmetallic solid bodies can be tested for such stress changes with the help of thermographic cameras.
Test object
Two aluminium specimen with a typical shape for tensile testing are used for the analysis. These specimen are coatedwith a high infrared emitting varnish on one side.
Testing equipment
An ImageIR® 8300 hp with a 100 mm lens is used. The lens allows an adequate distance with a suitable image detailfor small test objects. The camera which is equipped with a detector in the format of (640 x 512) IR pixels works in thesub‐windowing mode to solve the task. With an image size of (320 x 256) IR pixels the image frame rate can beincreased to the necessary 600 Hz. The corresponding data are captured with the software IRBIS® 3.1 professional andthen analysed with the software IRBIS® 3 active.

© InfraTec 02/2017 (All the stated product names and trademarks remain in property of their respective owners.)
Headquarters

InfraTec GmbHInfrarotsensorik und Messtechnik
Gostritzer Straße 61 – 6301217 Dresden / GERMANY

Page 1USA office

Phone
Fax
E‐mail+49 351 871‐8630+49 351 871‐8627
thermo@InfraTec.de
www.InfraTec.eu

InfraTec infrared LLC
5048 Tennyson Pkwy.
Plano TX 75024 / USAPhone
E‐mail+1 844‐226‐3722 (toll free)thermo@InfraTec‐infrared.com
www.InfraTec‐infrared.com

Thermal Stress Analysis of Metals
High‐End‐Thermography with ImageIR® 8300 hp
Test procedure
Clamped in a resonance tensile testing machine the samples are stretched periodically within their mechanical elastic
range. During the entire procedure the ImageIR® 8300 hp acquires the surface temperature of the sample and sends
the data in real time to a laptop for storage. In addition to the IR data an analogue signal of the tensile testingmachine is also recorded. This signal corresponds to the current excitation frequency and its amplitude allows
conclusions to be drawn as to the actual tensile force applied.
ResultsSample 1:
At the beginning of the test sample 1 is subjected to a static mechanical pre‐stress of 2.25 kN. Afterwards, aperiodically load was applied slowly increasing from zero to a signal of +/‐ 2.25 kN resulting in an absolute load
between 0 and 4.5 kN to the sample. The excitation frequency is around 100 Hz and is dependent on the combinationof sample and resonance tensile testing machine.
As expected the temperature profile shows an increase in temperature along the vertical sample axis at the narrowest
region of the sample. However, significant temperature differences occur between the left and right side. Apparently,
higher stress is built up on the left side.

Fig. 1‐3: Lock‐In amplitude image of sample 1. The asymmetric stresscurve in the sample is clearly visible. At the bottom left the 3D
visualization is shown with the same scaling.

© InfraTec 02/2017 (All the stated product names and trademarks remain in property of their respective owners.)
Headquarters

InfraTec GmbHInfrarotsensorik und MesstechnikGostritzer Straße 61 – 63
01217 Dresden / GERMANY

Page 2
USA office

Phone
Fax
E‐mail+49 351 871‐8630+49 351 871‐8627
thermo@InfraTec.de
www.InfraTec.eu

InfraTec infrared LLC
5048 Tennyson Pkwy.
Plano TX 75024 / USAPhone
E‐mail+1 844‐226‐3722 (toll free)thermo@InfraTec‐infrared.com
www.InfraTec‐infrared.com

Thermal Stress Analysis of Metals
High‐End‐Thermography with ImageIR® 8300 hp
Sample 2:
The second sample has to withstand similar load conditions. However, the stress changes here periodically from thebeginning with a signal of +/‐ 2.25 kN. Moreover the sample is loaded until breakage in this case. Overall, it seems tobe more homogeneous compared with the first sample since the stress distribution appears more symmetrical. Just
before the breakage a load peak occurred on the left edge within the narrowest region. During persisting load thispeak shifts with the resulting crack further towards the middle of the sample. The changing stresses are reflectedaccordingly in the temperature changes with regard to the measurements.

Fig. 4‐6: Lock‐In amplitude result of sample 2 at the start of the load
cycle. A relatively symmetrical temperature (stress) distribution can berecognised over the sample cross‐section. At the bottom left the 3D
visualization is shown with the same scaling.

© InfraTec 02/2017 (All the stated product names and trademarks remain in property of their respective owners.)
Headquarters

InfraTec GmbHInfrarotsensorik und Messtechnik
Gostritzer Straße 61 – 6301217 Dresden / GERMANY

Page 3USA officePhoneFax
E‐mail+49 351 871‐8630+49 351 871‐8627
thermo@InfraTec.de
www.InfraTec.eu

InfraTec infrared LLC
5048 Tennyson Pkwy.
Plano TX 75024 / USAPhone
E‐mail+1 844‐226‐3722 (toll free)thermo@InfraTec‐infrared.com
www.InfraTec‐infrared.com

Thermal Stress Analysis of Metals
High‐End‐Thermography with ImageIR® 8300 hp

Fig.7‐8: With progressive breakage of sample 2 the stress peak shifts towards to the middle of the sample. On the right the 3D visualization is shownwith the same scaling.

Summary
Lock‐In thermography is suitable for qualitative stress measurement both for the elastic stress range and for thetransition to the plastic stress range. Using suitable formulas for the relationship of the temperature change to thestress change within the material quantitative results can certainly be calculated. Particularly in the case of poor heat
conductors such as steel inhomogeneity within the material such as surface corrosion with considerable penetration
depth and the associated material cross‐section weaknesses can be recognised and their influence on the tensile
strength determined.

© InfraTec 02/2017 (All the stated product names and trademarks remain in property of their respective owners.)
Headquarters

InfraTec GmbHInfrarotsensorik und Messtechnik
Gostritzer Straße 61 – 6301217 Dresden / GERMANY

Page 4
USA office

Phone
Fax
E‐mail+49 351 871‐8630+49 351 871‐8627
thermo@InfraTec.de
www.InfraTec.eu

InfraTec infrared LLC
5048 Tennyson Pkwy.
Plano TX 75024 / USAPhone
E‐mail+1 844‐226‐3722 (toll free)thermo@InfraTec‐infrared.com
www.InfraTec‐infrared.com