·Table of Contents
·Methods and Instrumentation
Full-field Thermoelasticity: A New Generation of an Optical Method Showing Directly the Effects Produced by Mechanical StrainsMichel Honlet
Honlet Optical Systems GmbH
Bradley R. Boyce
Stress Photonics Inc.
Madison, Wisconsin, USA
The drawbacks of scanning measurements are evident:
All these drawbacks contributed to a progressive abandonment of the SPATE systems. Even the manufacturers' efforts to provide improvements to the system were not able to eliminate the major drawbacks of a scanning system. In order to refresh the techno-logy major changes had to come improving dramatically the measurement time.
This new generation of measurement devices can be used like a video camera, allowing to analyze structures of almost any size. Thanks to the elimination of the major drawbacks, notably the scanning process, the gain in time is enormous.Example : Comparison between data acquired by SPATE (scanning system) and by DELTATHERM (system using focal plane array infrared camera, FPA).
|Pic 1: A metallic object as seen through an infrared camera (here: DELTATHERM)||
Pic 2: Scanning system (SPATE)|
Measuring time: 6 hours
Pic 2: FPA Camera (DELTATHERM)|
Measuring time: 1 minute
Comparison of the results:
The data acquired by SPATE (picture 2) and by DELTATHERM (picture 3) comes from the same object, stressed under the same loading conditions with a hydraulic actuator. Note the differences between the acquisition time and the resolution of both unfiltered pictures.
|Pic 4: Stress concentrations of an aircraft door under repeated load, simulating the opening and closing of the door. The locations where the stress concentrations appear are exactly those where fatigue cracks will appear. (Document: NASA Langley Research Center)|
Pic 5 & 6: Stress concentrations (left) on the front face of a motor block (right)
under vibrating load. This was a test to verify the calculations of the
stresses in a motor block under normal regime condition. Beyond the
calculated stress concentrations some unpredicted stresses were
detected. (Document: FORD Motor Company)
Powerful digital signal processing and high frame rate cameras (>300 Hz frame rate at 320 x 256 pixels) have made it a simple matter to analyze structures undergoing random dynamic loading. Even in the situation of multiple loading channels the digital data processing is able to separate the influences of individual loads. Such a system can be integrated into a testing bench and is able to resist to violent movements of several "g" during the measurement of an object or a structure moving in a range of several centimeters.
Since TE appeared, it has been used on objects being harmonically loaded. Only since a few years it has become possible to use the TE technique on randomly loaded. Even higher performance is also achievable, when this TE technique is used in a multiple channel random loading configuration, like road simulation test benches or flight stress simulators.
In the following example (pictures 7 and 8) the high sensitivity of such a thermoelastic analysis system and the capability to quickly produce results is shown:
Pic 7: Detail of a forklift seat hinge. As fatigue damage occurred in this area, the driver was asked to simulate fatigue load by bouncing during 20 seconds on his seat while a TSA measurement took place.
|Pic 8: Result of the measurement on a forklift seat hinge, loaded by its driver bouncing on his seat during 20 seconds. The data results correlate with the fatigue of the materials experienced after several years of operation.|
But STRESS PHOTONICS recently introduced a useful new capability of its latest DELTATHERM systems, the ability to collect data in a triggered random mode. This high performant and unique feature is called: Transient Loading Thermoelastic Stress Analysis. By using a threshold trigger on the reference signal to key the collection of a fixed length segment of thermoelastic signals, amazing results can be obtained. A good example of this type of measurement is the automotive door slam testing, see Pictures 9-12.
The complicated impact of an automotive door against the door jam has challenged designers and stress analysts. Most companies have resorted to tests in which the acceptable durability is confirmed by attaining a given number of calibrated door slams without a failure. The design challenge is exacerbated by the many door confi-gurations required and the rapid pace of product development.
In Pictures 9-12 (next page) a transient loading example is shown. Here, calibrated door slams of different energies are compared. The images are the result of a small number of slams (1-5) averaged together for each load level. The quick identification of trouble spots for a variety of door configurations helps speed the design process by eliminating the need for expensive and lengthy durability testing.
|Pic 9: 1.36 J (10 ft-lb), window down, max value:130||
Pic 10: 2.7 J (20 ft-lb), window down, |
max value: 270
Pic 11: 2.7 J (20 ft-lb),window down,|
max value: 270
Pic 12: 2.7 J (20 ft-lb),window up, |
max value: 150
Comments about the transient loading results:
A "door-cannon" accurately sets door slam energy levels to assure consistent testing. Pictures 9 and 10 show how a compa-rison of energy levels produces scaled results of the maximum value in the "hot-spot", while Pictures 11 and 12 show a comparison of two different configurations of the door (window down vs window up).
Because the calculation power of PCs has dramatically increased the instrumentation for TE has now evolved to a software based system. The consequence is that new features and capabilities are mostly added through software, accelerating the rate of maturation of the technology. All the while this is happening, the software foundation is providing a launching point for the incorporation of facilities for new applications, making the technology more globally applicable. Typical examples are applicationsusing thermal NDE (nondestructive evaluation or testing) for the detection of subsurface defects. On top of this the modularity of new instrument designs allows for the rapid integration of new advances in the fundamental components of sensors and processors.
Meanwhile, the development of the technology itself continues. A significant trend is the integration of TE with other measurement technologies. Full-field measurement technologies for the characterization of materials and structures are starting to find common ground, and the technology is now coming into common use that will allow integration. The cooperation of technologies in the current engineering environment is essential to meet the pace set by customer demands and competition.
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