Daniel Türler

Laboratory procedures for using infrared thermography to validate heat transfer models

Infrared (IR) imaging radiometers, which measure relative levels of thermal radiation energy, can be used for noninvasive surface temperature measurements of building thermal envelope components undergoing steady-state heat flow in laboratory thermal chambers. One advantage of IR measurement is that it provides large contiguous sets of surface temperature data which are useful for validating the accuracy of complex computer models that predict heat flow through thermally insulated systems. Because they give such detailed information about surface temperature, IR measurements complement hot-box measurements of heat flow. This paper recommends general procedures for reliable quantitative thermographic measurements in chambers operated for winter heating conditions. Actual surface temperature depends on heat flow, surface emittance, and environmental conditions such as air temperature, air flow field, and background thermal radiation. The infrared temperature measurements are affected by many of the same factors including surface emittance, air temperature, background thermal radiation, and air humidity. Equipment specifications for the absolute accuracy of infrared temperature measurements are typically ′1° to ′2°C. Measurements that use a temperature-controlled reference emitter to remove error appear to show accuracies of ′0.5°C for flat specimens with low temperature gradients.

Predicting the geometry and location of defects in adhesive and spot-welded lap joints using steady-state thermographic techniques

Development of nondestructive evaluation (NDE) methods for spot-welded and adhesive-bonded sheet metal joints is essential for widespread use of lightweight materials and new construction techniques in automotive applications. An important objective of research in progress is development of NDE methods to identify and characterize critical flaws in welded and adhesive-bonded joints. We used steady-state heat- flow and thermographic imaging techniques to test welded and adhesive-bonded lap joints in steel and aluminum samples and in adhesive-bonded composite panels and to identify defective spot welds. The resulting surface-temperature maps or thermograms were used to detect voids and areas where the adhesive was not bonded. To better characterize defects in welds and adhesive layers, algorithms have been developed to post process temperature data, producing more accurate definition of the geometry and location of defects than in previous images. Classic heat-transfer theory was used to calculate the heat-flux equilibrium for each individual pixel on the thermograms. Convective and radiative surface heat- transfer coefficients were applied to compensate for the heat exchange between the sample and the environment. This post processing permits us to determine the locations of spot welds and the sizes of the weld nuggets in welded joints, and to clearly image voids in adhesive layers between joints. The effectiveness of the image-processing algorithms was investigated using data from laboratory experiments on test specimens with flaws of known size and location. In addition, the images of the defects produced with the new method were compared to results of two-dimensional heat transfer simulations through the same samples. The simulations were also used to determine boundary conditions for post-processing of images.

Thermographic and Acoustic Imaging of Spot-Welded and Weld-Bonded Joints

The work presented here is part of a research effort focused on developing nondestructive evaluation (NDE) and testing techniques that are sufficiently fast, robust, accurate, and cost effective for on-line inspection of automotive structures. A series of laboratory experiments was conducted to assess the feasibility of thermographic and acoustic methods for evaluating the quality of individual spot welds and the structural integrity of spot-welded and weld-bonded joints. Emphasis was placed on identifying structurally weak “stick” welds, which are much more difficult to detect than broken welds. After nondestructive evaluation, the samples were subjected to mechanical tests to determine the strength of individual spot welds. Analytical and numerical models are also being developed to help interpret results of the laboratory experiments. The insight gained from the data and modeling results are essential in moving from qualitative techniques that identify flaws to quantitative methods that assess the severity of defects.

Imaging Flaws in Adhesive Joints Using Acoustic Techniques and Infrared Thermography

The ability to ensure the integrity and reliability of adhesive joints is essential to increased use of lightweight materials in the automotive industry [1]. For manufacturing applications, on-line inspection techniques must be fast, accurate with limited access to parts, robust in manufacturing environments and available at reasonable cost. The work presented here is part of a project in which nondestructive techniques are being evaluated to access their ability to detect and characterize flaws in adhesive joints in metals and composite materials. The long-term goal of the project is development of a prototype on-line inspection system for automobile structural members.