Infrared thermal imaging method was applied for the development of a non-destructive inspection technique to
determine the quality of resistance spot welds. The current work is an initial feasibility study based on post-mortem
inspection. First, resistance spot welds were fabricated on dual phase steel sheets (DP 590 steel) with carefullycontrolled
welding parameters. It created welds with desirable and undesirable qualities in terms of nugget size,
indentation depth, and voids and cracks. Second, five different heating and cooling methods were evaluated. The heating
or cooling source was applied on one side of the weld stack while the surface temperature change on the other side of the
weld was recorded using an infrared camera. Correlation between the weld quality and the "thermal signature" of each
weld was established. Finally, a simplified thermal finite element analysis was developed to simulate the heat flow
during inspection. The thermal model provided insight into the effect of the nugget size and indentation depth on the
peak temperature and heating rate. The results reported in this work indicate that the IR thermography technique is
feasible for weld quality inspection due to the distinguish temperature profiles for different welds and the repeatability
and consistency in measurement.
Adhesive bonded electrical heaters have been used in outside rearview mirrors of automobiles in order to act as defrosters. Entrapment of air pockets between the heater and the mirror can affects the performance and structural integrity of the mirror assembly. Since painting over the mirror is not an option in the production environment, the biggest challenge for IR imaging is to minimize surface reflection. Looking through a smooth, highly reflective first-surface mirror and a 2 mm thick glass without picking up other heat sources in the room, such as people, electronics equipment and the camera itself, requires careful planning and effective shielding. In this paper, we present our method of avoiding mirror reflection and IR images of the heated mirror in operation. Production heaters and heaters with artificial defect were studied. The IR imaging method has shown to be an effective tool for heater quality control and performance studies.
We conducted a preliminary IR imaging study of blood circulation in patients with peripheral vascular diseases. Abnormal blood flow is common in older adults, especially those with elevated blood lipids, diabetes, hypertension, and a history of smoking. All of these conditions have a high prevalence in our population, often with more than one condition in the same individual. The differences in blood flow is revealed by temperature differences in areas of the
extremities as well as other regions of the body. However, what is needed is an imaging technique that is relatively inexpensive and can reveal the blood flow in real time. The IR imaging can show detailed venous system and small tempearture changes associated with blood flow. Six patients with vascular diseases were tested in a clinic set up. Their legs and feet were imaged. We observed large temperature differences (cooling of more than 10° C) at the foot, especially toes. More valuable information were obtained from the temperature distribution maps. IR thermography is potentially a very valuable tool for medical application, especially for vascular diseases.
Laser welds in light-weight transit bus panels were studied by IR imaging. The corrugated structural panels were made from 3mm thick stainless steel. The panels were welded by a high power laser with lines of equally spaced 1” stitches. After discovering problems by visual and tapping inspections, the panels were sent to our laboratory for further investigation. The IR thermography method was chosen to study the welds because of its non-contact nature and potential for large area, high-speed inspections. We used thermophysical properties of the panels and finite element modeling to predict temperature variations of “good” and “bad” welds. Surface heating and “heat leak” methods were used to inspect each weld. The IR images clearly showed characteristic temperature signatures of “good” and “bad” welds. We also discovered a number of partial welds and questionable welds. In the follow-up destructive inspection, the welds interfaces were imaged and related to the infrared images. None or partial penetrations were found on a number of bad and partial welds. Even in the good welds, the weld stitch appeared to be discontinuous. This study helped the bus manufacturer to assess their welding process and make necessary improvement.
An infrared microscope was used to study the surface temperature profiles of power transistor arrays in integrated circuits (IC) during operation. Each transistor array was set to conduct current for 20-50 microseconds. The integration time of the IR camera is adjusted to be between 2 and 10 microseconds. A thorough study of the camera's timing characteristics allows its precise synchronization to transient thermal events in the transistor arrays. Progressively adding incremental delay times to the synchronization pulses allows the complete characterization of the thermal transients as a function of time and location. The IR microscope timing characteristics were determined by imaging an incandescent lamp filament during pulsed operation. Examples of heat pulses in a lamp filament and power transistors are given.
In a temperature-modulated differential scanning calorimetry (TMDSC) system, temperatures are measured by thermocouples under the sample and reference furnaces. TMDSC helps to accurately measure temperature during a DSC measurement. It also helps the researcher to establish a more realistic model to calculate heat capacity of various materials. This study examined assumption of temperature gradient in TMDSC characterization. An infrared camera was used to obtain surface temperature maps of DSC cells during temperature sweeps. TMDSC units from Perkin-Elmer and TA Instrument were studied using different heating and cooling rates. Temperature gradient exists between the top and bottom of the sample. IR images showed that temperature distributions within the sample and reference cells exist. Phase lags between the top and bottom temperatures were also observed.
Thermal transients caused by electrical switching devices and fast optical or electrical pulses only exist for a very short time (less than 1 millisecond). If the transient happens in a high thermal conductivity material, e.g. silicon wafer, the thermal transient will disappear in a few milliseconds or faster. Most modern IR cameras have external triggering capability. However, the IR camera often runs on its own clock. If the triggering of data acquisition is not synchronized with the camera, there is often an uncertainty of timing, e.g. 16.7 ms for camera running at 60Hz. Even when the IR camera is running at 500 Hz, there is 2 ms uncertainty. Depending on when you push/click the button, the high-speed thermal transients can be missed and they will disappear in the next image. We developed a technique to synchronize the IR camera to capture the thermal transients. By using a delay function, we can control the data acquisition very precisely to study the thermal transients.
High-speed infrared (IR) imaging provides researchers a noncontact and nondestructive tool of studying fatigue behavior of materials. In this study, ULTIMETTM superalloy manufactured by Haynes International, Inc. was used. High-cycle fatigue tests (20 Hz) were conducted at the University of Tennessee using two material test system machines. An IR camera recorded temperature evolution during the fatigue test. Four distinct temperature regions were observed: (1) the initial temperature rise was found to be dependent on the damage condition of the specimen. A pre- damaged specimen showed a slower rate of initial temperature rise than the as-received specimen; (2) the equilibrium temperature of a specimen depends on the applied stress, test frequency, and thermophysical properties of the material. The equilibrium temperatures varied from 23 degree(s)C to 50 degree(s)C; (3) further temperature increase was detected 5,000 cycles before final failure, indicating the onset of a fatal crack; and (4) final temperature drop was found due to the separation of the test sample. High temperature regions were observed at the crack tip as a result of stress concentration. High-speeding IR imaging also revealed the thermoelastic effect during cyclic loading.
High Temperature X-ray Diffraction (HTXRD) is a very powerful tool for studies of reaction kinetics, phase transformations, and lattice thermal expansion of advanced materials. Accurate temperature measurement is a critical part of the technique. Traditionally, thermocouples, thermisters, and optical pyrometers have been used for temperature control and measurement, and temperature could only be measured at a single point. Infrared imaging was utilized in this study to characterize the thermal gradients resulting from various sample and furnace configurations in a commercial strip heater furnace. Furnace configurations include a metallic strip heater, with and without a secondary surround heater, or surround heater alone. Sample configurations include low and high thermal conductivity powders and solids. The IR imaging results have been used to calibrate sample temperatures in the HTXRD furnace.
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