A number of physical and chemical effects affect metals whereby their material properties such as tensile strength can differ. Analyzes on the degree of change are essential for evaluating whether or not a component or a complete device can still perform its function without restriction. Tensile testing supplies information for such assessments.
During this process, external mechanical pressure is applied to solid state objects resulting in stress variations in their interior. The changed surface temperature is considered to be a parameter describing the sum of all internal stress changes of the body which reach the surface. As a result of positive changes in pressure, solid bodies heat up and then cool down accordingly during negative variations in pressure. The inner stress change Δσ thus acts directly proportional to the measured temperature change ΔT. The example of a test for tensile specimen measurement shows that metallic solid bodies can be tested for such stress variations with the help of thermographic cameras.
Two aluminum specimens with a typical shape for tensile testing are employed for the analysis. These specimens are coated with a high infrared emitting varnish on one side.
An ImageIR® 8300 hp with a 100 mm lens is employed. The lens permits an adequate distance with an ideal image detail for small test objects. The camera which is provided with a detector in the format of (640 x 512) IR pixels functions in the sub‐windowing mode in order to solve the task. With an image size of (320 x 256) IR pixels, it is possible to increase the image frame rate to the necessary 600 Hz. The equivalent data are captured with the software IRBIS® 3.1 professional and then examined with the software IRBIS® 3 active.
The samples, clamped in a resonance tensile testing machine, are periodically stretched within their mechanical elastic range. During the entire process, the ImageIR® 8300 hp attains the surface temperature of the sample and then sends the data in real time to a laptop for storage. Besides the IR data, an analog signal of the tensile testing machine is also recorded. This signal corresponds to the present excitation frequency and its amplitude allows conclusions to be drawn as to the actual tensile force applied.
At the start of the test, sample 1 is subjected to a static mechanical pre‐stress of 2.25 kN. Later, a periodically 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 about 100 Hz and is dependent on the combination of sample and resonance tensile testing machine.
As anticipated, the temperature profile presents an increase in temperature along the vertical sample axis at the narrowest region of the sample. However, vital temperature differences take place between the right and left side. Higher stress is apparently built up on the left side.
Figure 1-3. Lock‐In amplitude image of sample 1. The asymmetric stress curve in the sample is clearly visible. At the bottom left the 3D visualization is shown with the same scaling.
The second sample must withstand similar load conditions. However, the stress changes here occasionally from the beginning with a signal of +/‐ 2.25 kN. Furthermore, the sample is loaded until breakage in this case. Overall, it seems to be more homogeneous compared with the initial sample since the stress distribution appears more symmetrical.
Prior to the breakage, a load peak occurred on the left edge within the narrowest region. During persisting load, this peak shifts with the resulting crack further towards the sample’s middle. The changing stresses are reflected consequently in the temperature changes with regard to the measurements.
Figure 4-6. Lock‐In amplitude result of sample 2 at the start of the load cycle. A relatively symmetrical temperature (stress) distribution can be recognized over the sample cross‐section. At the bottom left the 3D visualization is shown with the same scaling.
Figure 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 shown with the same scaling.
Lock‐In thermography is ideal for qualitative stress measurement both for the transition to the plastic stress range and for the elastic stress range. Using ideal formulas for the relationship of the temperature change to the stress change within the material quantitative results can surely be calculated. Mainly in the case of poor heat conductors such as steel inhomogeneity within the material such as surface corrosion with significant penetration depth and the associated material cross‐section weaknesses can be recognized and their influence on the tensile strength can be determined.
This information has been sourced, reviewed and adapted from materials provided by InfraTec GmbH.
For more information on this source, please visit InfraTec GmbH.