Strain measuring accuracy with splitting-beam laser extensometer technique at split Hopkinson compression bar experiment

Abstract

Abstract. An accuracy problem of strain measurement at compression split Hopkinson compression bar experiments with a splitting-beam laser extensometer was considered. The splitting-beam laser extensometer technique was developed by Nie et al. to measure strain of a specimen during its tension under a high strain rate loading condition. This novel concept was an inspiration for the authors to develop own laser extensometer system, which allows for simultaneous and independent measurement of displacement of bar ends between which a compressed material specimen is placed. In order to assess a metrological property of this measuring system, a wide range of high strain rate experiments were performed, including tests with various sample materials (Al 5251, Cu OFE) with different rate of strain, and with the use of two bars material. A high accuracy of the developed laser extensometer was found in measurement of specimen strain, for which uncertainty is not greater than 0.1% and, for a typical specimen dimension, the maximum permissible error is 4.5 μm.Key words: SHPB test, strain rates, non-contact strain measurement, laser extensometer. The simplest approach is to use a high speed digital camera to photograph the specimen deformation [10,11]. The camera is focused on the entire gauge section and part of the nongauge section at both ends to achieve a sufficient image with the highest resolution possible. Due to motion analysis of the selected characteristic points, defining the strain gage length, a strain-time relationship can be determined. However, due to the limited frame rate and its corresponding resolution of a contemporary high speed camera, the obtained results did not provide sufficient data points to construct a full stress-strain curve [8]. Strain measuring accuracy with splitting-beam laserThe high-rate digital image correlation (DIC) is a further development of the above described techniques [12,13]. In this case, information about specimen deformation is given by motion analysis of a large number of points (speckle) spreading over the surface of the specimen gauge length [14,15]. A random speckle pattern is generated by a spraying black paint and a white base layer directly onto the surface of the specimen. Thus, DIC provides highly valuable information on dynamic full-field strain measurements in the specimen, however, the above-mentioned limitations resulting from the usage of a contemporary high speed camera and an extreme difficulty in acquiring a large quantity of high-resolution images at very high frame rates do not allow for constructing a precise stressstrain curve [8].Another strain measuring technique is the interferometric-strain-gauge method, which is one of the first diagnostic techniques applied to straightforward non-contact measurement of specimen strain in SHPB testing. Sharpe et al. [16] used this method for evaluation of strain uniformity in the compressed specimen by comparison of surface strain at the specimen midpoint to the average strain calculated from the conventional Kolsk...

Dynamic compressive response and fracture mechanisms of ZrB2–SiC ceramic based on discrete element method

“…Figure 10A shows the cumulative damage fraction curve and numerical analysis fitted by the previous equation, and the consistency between the fitted curve and the numerical benchmark can be evaluated by the standard deviation. With the increase of strain rate from 613 to 2861 s −1 , the standard deviation is .999, .998, .997, .998, and .997 in turn, and it proves that the fitted cumulative damage fraction curve is in good agreement with the numerical reference curve where a and b are two parameters that determine the shape of the cumulative distribution function in Equation (11), and corresponding parameters are shown in Figure 10B. With the increase in strain rate, b decreases from 6.601 to 4.745, b decreases, indicating that the distribution becomes flatter.…”

“…7,8 Several new technologies capture the response of materials to impact loads. [9][10][11] New technologies such as ultrahigh-speed X-ray imaging, 9 high-speed cameras, 10 and beam splitting-laser extensometer 11 have been applied to SHPB testing. There is still a lack of micro-quantitative research.…”

“…The results show that UHTCs have an evident strain rate dependence, and the strain rate has an essential effect on the strength and failure patterns 7,8 . Several new technologies capture the response of materials to impact loads 9–11 . New technologies such as ultrahigh‐speed X‐ray imaging, 9 high‐speed cameras, 10 and beam splitting‐laser extensometer 11 have been applied to SHPB testing.…”

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“…The model contains all main components of the arrangement: a pulse shaper, slide bearings and a barrel. In simulation, dimensions of all elements of SHPB are the same as in the experiments (Panowicz et al, 2017).…”

Effects of Sample Geometry Imperfections on the Results of Split Hopkinson Pressure Bar Experiments

“…In addition to the main part of the setup (bars, sample, and striker), the model included a pulse shaper, slide bearings and a barrel. In the simulation, the dimensions of all the elements of the SHPB were the same as in the experiments [13].…”