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ABSTRACT--This paper presents a newly developed laserbased noncontacting strain sensor suitable for temperatures up to t200~ which was adapted to a comrnercial tensile testing machine equipped with an electrical furnace. The principle of the strain sensor is based on tracking laser speckles through a digital correlation technique. Furthermore, the description of the signal processing and the optical arrangement is presented. Based on the experimental data, it can be shown that this simple, laser-based strain sensor can be used successfully for the determination of mechanical and thermal strains up to temperatures of about 1200~ Using a special data-processing procedure, it was feasible to minimize decorrelation effects caused by changes in the specimen surface due to, for example, slipband and microcrack formation, surface oxidation and phase transformation and, thus, measure large mechanical strains. The strain resolution for the selected setup was about 20 microstrains depending on the testing parameters.KEY WORDS--Strain measurement, digital speckle correlation technique, thermal strain, large mechanical strain, high test temperatureIn the field of mechanical testing of materials at elevated temperatures, it is for various reasons desirable to determine mechanical and thermal strain values with noncontacting optical methods. Both standard clip gages and strain gages are very limited in their operational temperature range, and even if applicable, they may cause local damage of the specimen surface or cause chemical surface reactions.Most of the published optical systems aim for the determination of strain fields. A survey of the most commonly used optical methods is given in Refs. 1-3 presenting their applicability, resolution and limitations.The best documented methods, which are used mostly for room temperature tests, are the holographic techniques, 4 laser speckle interferometry, digital speckle shearography, 5 moir6 methods,6'7 speckle correlation techniquesl'8 and laser inter- Original manuscript submitted: September 10, 1998. Final manuscript received: September 13, 1999 ferometer systems 9,10 In particular, for high-temperature applications the following methods are most often described in the literature: the computer vision technique, electronic laser speckle interferometry (ESPI) and the digital laser speckle correlation technique.Lr 11 used ESPI to measure in-plane deformation fields up to about 1500~ reporting problems with turbulence, changes in the microstructure of the specimen surface, background radiation and mechanical stability. The thermal strain fields of metal ceramic compounds were determined using digital speckle pattern interferometry up to about 300~ In the framework of the joined European action (COST Action 510), t2 both an ESPI system and a laser speckle correlation-based system (to be described in this paper) are being developed for applications in mechanical testing at very high temperatures. Preliminary experimental results are reported in Refs. 13 and 14.Speckle interferometric ...

In recent years, the focus on quality control in the steel industry has shifted from offline to inline non-destructive testing in order to detect defects at the earliest possible stage in the production process. The detection and elimination of such defects is vital for sustaining product quality and reducing costs. Various measurement principles (e.g. ultrasonic testing, electromagnetic acoustic transducer, x-ray inspection) were analyzed and their advantages and disadvantages are discussed regarding their usability in a steel plant. Based on these findings a magnetic method combined with a new sensor concept was chosen. By using highly sensitive sensors based on the giant magnetoresistive effect, it is possible to detect magnetic flux leakage variations on the surface of a magnetized steel strip caused by defects or inhomogeneities inside the material. Based on promising measurement results of preliminary tests and simulation results obtained by finite element method-models, a prototype is now being built for offline measurements and the optimization of the measurement method. In the event that the development of this second prototype is successful, an inline configuration will be implemented.

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