TENSILE TESTING AT HIGH STRAIN RATES USING DIC METHOD. Martina MAREŠOVÁ, Pavel KONOPÍK

Transcription

1 TENSILE TESTING AT HIGH STRAIN RATES USING DIC METHOD Martina MAREŠOVÁ, Pavel KONOPÍK COMTES FHT a.s., Průmyslová 995, Dobřany, Czech Republic, EU, Abstract As prediction of material behaviour by Finite Elements Analysis is nowadays an integral part of material engineering, the necessity to obtain relevant material data with highest accuracy possible became crucial. Usually, tensile tests are performed under quasi-static conditions, which mean that the loading of specimen corresponds with strain rate about hundredths s -1. In reality, however, materials are often subjected to much higher strain rates (up to thousands s -1 ) and their properties can change rapidly with rising strain rate. Material testing at high strain rates is important especially for materials that are used in applications where impact damage is probable, such as in pressure vessels, vehicles etc. For this paper, individual tests were performed at strain rates from hundredths s -1 up to hundreds s -1, recorded using standard or high speed camera and evaluated by Digital Image Correlation (DIC) method. This method is based on recognition of change of pattern in sequence of images. Stochastic pattern is applied on the surface of the specimen prior testing. Under the load, the specimen is deformed and so is the applied pattern. Comparing the images, changes in the pattern are registered and displacements and strains are calculated. The results are discussed. Keywords: Tensile tests, high strain rates, digital image correlation (DIC). 1. INTRODUCTION To obtain good FEM simulation results the provided input material data have to encompass not only behaviour of investigated material under standard quasi-static testing conditions, but also under service loading conditions. This means that the influence of strain, strain rate, stresses and temperatures have to be assessed and taken into consideration. For many years, one of the traditionally used tests is tensile test, which is still widely used with very small change in samples geometries or tests execution procedures itself. Significant improvement, however, have been achieved in the possibilities of measurements with new testing equipment and evaluation procedures. Usually, tensile tests are performed under quasi-static conditions, which mean that the loading of specimen corresponds with strain rate about hundredths s -1. In reality, however, materials are often subjected to much higher strain rates and their properties can change rapidly with rising strain rate. Material testing at high strain rates is important especially for materials that are used in applications where impact damage is probable, such as in pressure vessels, vehicles etc. At the moment, high strain rate testing is not standardized, however, several documents with recommended procedures exists even for sheet tensile testing (one of them is for example [1], but number of others can be found). What used to be very difficult or almost impossible were measurements of actual samples dimension in the course of tests and even local strain measurements. These measurements are possible at present with the use of laser extensometers allowing longitudinal and transversal strain measurements, videoextensometers, high speed cameras with sophisticated evaluation software and most recent Digital Image Correlation (DIC) systems that made it possible to measure local strains during tests. These new methods of strain evaluation applied on tests at high strain rate of hundreds s -1 provide results with high precision.

2 2. TENSILE TESTING AT HIGH STRAIN RATES As mentioned previously, tensile testing is standard, widely used method of obtaining data for description of material behaviour during its loading and damage. Recorded force versus displacement curves are further converted into true stress true strain diagrams, that are later on used for material models parameters fitting. The force measurement is well established for most of realistic cases, but sample deformation or rather strain measurement is more complicated issue. Traditionally used mechanical extensometers attached to the sample can successfully measure longitudinal and transversal strains; however, their use at high strain rate testing is mostly impossible. Firstly, there often is not enough data acquired from the extensometer during the dynamic event, and secondly, dynamic test could be destructive to the extensometer itself. Especially for high strain rate testing, optical extensometers proved to be more suitable. There are two methods of optical measurement available for strain measurement laser extensometers and videoextensometers. As its name shows, laser extensometer uses laser beam that is reflected from sample surface or a marker and strain is evaluated in a way similar to that of mechanical extensometers. Videoextensometers use images of the test specimen recorded by video-camera. Test specimen carries contrasting marks whose position in recorded images is analyzed and evaluated by supporting software either in real time or after the test. Basically, video-extensometer acts as a strain meter by directly calculating the measured extension as a percentage of the original length; to obtain actual extension values it is necessary to provide some sort of calibration (methods of calibration are specific for any given system). Video-extensometer automatically measures the initial gauge length is at the start of a test with the same accuracy as the specimen extension during testing itself. Therefore, it is not essential to accurately control the initial gauge length as required when using conventional, mechanical measuring gauges. Also, as the setting of the initial gauge length can be (and most often is) done after the test itself, it is placed in such a way to contain the necking area. Latest development in the field of deformation measurements are methods that calculate and evaluate deformation on the whole recorded surface of the specimen full field deformation measurements. One of such methods is Digital Image Correlation (DIC) method. The principle of Digital Image Correlation (DIC) method is known since 1970s ([2], [3] and many others). It is based on recognition of change in sequence of images. Stochastic pattern is applied on the surface of the specimen prior testing. The test itself is then recorded by one (2D in-plane deflection measurement) or two (3D) cameras. Under the load, the specimen is deformed and so is the applied pattern. Comparing the images, changes in the pattern are registered and displacements and strains are calculated. Systems based on this method enables measurements of either testing samples or real components. In years , within the project Ductile damage parameters identification for nuclear power plants FR-TI2/279 sponsored by Ministry of Industry and Trade of Czech Republic tensile tests at various strain rates were performed on specimen manufactured from, among others, Zirconium sheets. To reach widest possible range of strain rates, two testing machines were used during the testing; servo-hydraulic system MTS 810 (see Fig. 1 left) was used for tests up to 300 mm s -1 loading velocity and impact tester IM30T (3 m high Drop Weight Tower - DWT, see Fig. 1 center) where tests with impact velocity from 1000 to 5000 mm s -1 were performed. The same geometry of samples (see Fig. 1 right) was used for all tests. Because the main interest was the moment of crack initiation, the samples were machined with round notches of various radii to concentrate the crack in specific area. All tests were recorded by high-speed camera and recorded images subsequently evaluated.

3 Fig. 1 Left) system MTS 810, center) impact tester IM30T, right) geometry of sheet specimen 2.1 Video-extensometer In the first batch of tests, done in 2011, the notches were of radius R = 1.25 mm, 1.50 mm and 1.75 mm and strain rates 0.03 mm s -1, 30 mm s -1, 300 mm s -1 and 1000 mm s -1. The software used for evaluation was ImpacqtV3, provided with the DWT. The strain measurement, in this case, is done the way of videoextensometer. On the specimen, the initial gauge length is marked. In the software, two tracking points L1, L2 are placed automatically on a distinct pixel near the markers (see Fig. 2 left). Position of the points is tracked from picture to picture by the software. In the same way, the changes in the sample s width can be tracked for subsequent true stress true strain calculations. The pink line used for calibration, as initial width of the sample was measured beforehand. Fig. 2 Left) Initial placement of tracking points, right) UTS and YS dependence on loading velocity The first tests proved that values of Ultimate Tensile Strength (UTS) and Yield Stress (YS) of sheet zirconium showed rising trend with higher strain rate. However, the video also showed that tested radii of notches were too small, as the fracture was initiated at the sides and not from the center of the sample, which was needed for further investigation. For this reason, second set of tests was performed.

4 2.2 Digital Image Correlation These tests were performed on samples without notch (R0) and notches of radii R = 5 mm, 10 mm and 15 mm. Loading velocities were 2 mm s -1 (using MTS machine) and impact velocities 2000 mm s -1 and 5000 mm s -1 using IM30T. The strain measurement was done using DIC method based ARAMIS system [4]. Because the tests are fast (ones to tens of milliseconds), the tests are recorded using high-speed camera. Recorded images are then uploaded to the evaluation software ARAMIS and calibrated (initial width of samples was measured pre-test using optical microscope). Values of UTS and YS confirmed the trend of rising with increasing loading velocity. The colour maps in Fig. 3 and Fig. 4 show the distribution of major strain (strain in direction of highest achieved strain) over the sample. The influence of the notch radius is clearly visible; while in Fig. 3 the crack is initiated from the sides of the sample and the resulting fracture is in typical 45 angle to the direction of major strain, the sample with 15 mm radius notch in Fig. 4 shows standard necking with crack initiated from the center of the sample. Fig. 3 Colour map of major strain of sample with radius R = 5 mm 3. CONCLUSION Precise strain measurement is the main problem of high strain rate testing. Optical measurement using highspeed camera proved to be very suitable in tests of zirconium sheets described in this paper. Recorded images were evaluated using digital image correlation method resulting in precise measurement of sample elongation. Also, because all visible surface of the sample can be evaluated by DIC method, the map of local surface strains can be created. Combination of DIC and high strain rate testing promises the possibility to ensure highly precise input data for computer modeling and material behavior prediction.

5 Fig. 4 Colour map of major strain of sample with radius R = 15 mm ACKNOWLEDGEMENT This paper includes results created within the project Ductile damage parameters identification for nuclear power plants FR-TI2/279 sponsored by Ministry of Industry and Trade of Czech Republic. Also, support of TAČR program TA is gratefully acknowledged. LITERATURE [1] OPBROEK E. at al. Recomendations for dynamic TensileTesting of Sheets Steels. International Iron and Steel Institute, [online] [2] KEATING T.J., WOLF P.R., SCARPACE F.L. An Improved Method of Digital Image Correlation. Photogrammetric Engineering and Remote Sensing 1975, roč. 41, č. 8, s [3] SUTTON M.A., ORTEU J.-J., SCHREIER H. W. Image Correlation for Shape, Motion and Deformation Measurements. New York: Springer Science+Bussines Media, [4] GOM: ARAMIS System. [online]