Abstract
This paper describes the experimental procedure to reduce the load ringing phenomenon in dynamic tensile tests. Reduction and prevention of oscillations in the force signal is an important criterion in the selection of the test jigs. A slack adaptor type jig system has been newly developed for acceptable response at high strain rates. Dynamic tensile tests were conducted using a servo-hydraulic machine with high strength steel sheets at the strain rate of 200 s−1. A piezo-electric load cell is used to measure the force. To confirm and determine the proportion of oscillations from bending effects on a specimen, two strain gauges are attached on the both sides of the gauge section for measurement of the difference in signals with deformation. A digital image correlation (DIC) method is employed to measure the strain during tensile tests. Experimental results show that the load ringing phenomenon in raw data of measured load signals has remarkably diminished with the slack adaptor jig newly developed at a strain rate of 200 s−1.
Keywords
35.1 Introduction
Tensile testing of sheet steels at high strain rates is becoming more important due to the need for the assessment of crashworthiness of car structures in the automotive industry. It is essential to obtain the dynamic material properties of sheet steels because auto body structures are likely to undergo high speed deformation during a crash event [1–3]. Stress–strain curves determined at the quasi-static state are not enough to describe the behavior of the structure under the dynamic load. Accurate numerical simulations of car-crash need stress–strain curves determined at high strain rates. There are many different types of testing techniques to obtain the material properties under dynamic conditions such as the servo-hydraulic system, the split Hopkinson (Kolsky) bar system and the drop weight system. Among these systems, the servo-hydraulic system can normally cover the strain rate range from 0.1 to 500 s−1, while the split Hopkinson bar type system covers the strain rate range of 1000 s−1 or higher. So the servo-hydraulic system is suitable to characterize the mechanical behavior of steels under dynamic conditions for accurate crashworthiness analysis. The servo-hydraulic system for dynamic testing needs a special clamping method, load measurement system and strain measurement system due to different conditions from the quasi-static strain rate. These issues are critical to the quality of testing results, especially the load ringing which is the oscillation of acquired load signals.
This paper will present the experimental results of tensile tests using a servo-hydraulic machine on a high strength steel sheet, DP590, at a strain rate of 200 s−1. To reduce the load ringing phenomenon in tensile tests at high strain rates, a new slack adaptor jig system has been carefully developed for smooth response during dynamic tensile tests. A piezo-electric load cell is used to measure the force signal. The direct measurement of strain at high strain rates is carried out with the use of a digital image correlation (DIC) method which is a non-contact optical measurement technique.
35.2 Development of a Slack Adaptor Jig System
For tensile testing on the quasi-static condition, a specimen is initially clamped at its both ends to the testing apparatus. However, for tensile testing at higher strain rates using servo-hydraulic systems, a special clamping system is necessary to grip a specimen instantly with the actuator when the stroke jig reaches the designated speed after the jig travels freely by some distance without loading the specimen. To satisfy this requirement, a slack adaptor type jig system has been newly developed. A schematic diagram of the slack adaptor is shown in Fig. 35.1a, b to describe the employment of the slack adaptor in a servo-hydraulic test machine, which is an in-house high-speed material testing machine (HSMTM) with a maximum stroke velocity of 7.8 m/s, a maximum load of 30 kN and a maximum displacement of 300 mm [4–7].
The slack adaptor was manufactured with titanium alloy grade 5 for light weight in order to reduce the impact force on the specimen. A sliding bar of a cylindrical rod has a conical shaped adaptor at its end, which has the contact angle of 60° between sliding bar adaptor and upper slack adaptor to relieve the impact force. The length of the sliding bar is determined as 150 mm to ensure a sufficient acceleration distance.
The dimensions of the test specimen in this paper are shown in Fig. 35.2 where the gauge length is 9.53 mm and the width of the gauge section is 3.18 mm. These dimensions were determined based on ASTM D638 type V [8, 9] and referred to recommendation of ISO/DIS 26203-2 [10] which guides a tensile testing method at high strain rates for metallic materials.
To assess any bending component on the specimen and check the alignment between the jig and the specimen, an ultrahigh-elongation strain gauge was applied on each side of the specimen gauge section. The strain was measured at both sides of the gauge section independently. Figure 35.3a shows how to attach the strain gauge on each side of the gauge section and Fig. 35.3b shows setting of the HSMTM and the slack adaptor to assess the bending effects during the tensile tests.
Figure 35.4 shows the comparison result of strain gauge signals between the front and the back of the specimen gauge section. The load data and the signals from the strain gauges were acquired simultaneously using the same DAQ board for synchronization. The tensile test was conducted at a strain rate of 100 s−1. The signals from the strain gauges on the front and back side of the specimen gauge section are exactly the same, which ensures that there is no bending effect on the specimen. The peak value in signals at the back side of specimen is due to breakage of a strain gauge.
35.3 Tensile Tests Results Using Developed Slack Adaptor Jig at Strain Rate of 200 s−1
Tensile tests of DP590 with the thickness of 1.2 t were carried out using the slack adaptor jig developed at a strain rate of 200 s−1. The axial strain was measured by a DIC method from images acquired from the FASTCAM SA4 high speed camera. The alignment of a test specimen was checked using the high speed camera during specimen deformation. Captured digital images were recorded with a resolution of 256 × 976 pixels at a frame rate of 12,500 frames/sec. An AF Micro Nikkor 60 mm F2.8D lens and a fiber optic light were used during tests to reduce temperature elevation. Sequential deformed shapes of the specimen gauge section at a strain rate of 200 s−1 are shown in Fig. 35.5. The slack adaptor developed ensures good alignment during tensile tests. The load–time curve at a strain rate of 200 s−1 is plotted as raw data in Fig. 35.6. The displacement of the actuator measured by an LVDT and the load data measured by a piezo-electric type load cell was synchronized with the same DAQ system. Tensile testing results often show oscillation in load signals due to the load ringing noise at strain rates higher than about 100 s−1 and the smoothing or filtering process is necessary for usable smooth data, especially to determine the yield stress [11–14]. In the figure, no load ringing phenomenon is observed in raw data at a strain rate of 200 s−1 and stress–strain curves were obtained more accurately using the slack adaptor developed than that in general cases [14].
35.4 Summary
A new slack adaptor was developed for the high speed material testing for sheet steels. The slack adaptor jig system proposed was prepared with titanium alloy for light weight and a conical shaped adaptor was employed to reduce the impact force on the specimen. In order to investigate the proportion of oscillations resulting from bending effects, two ultrahigh-elongation strain gauges were attached on the both side of the specimen gage length to measure each signal separately. It is noted that there are no bending effects on the specimen using the slack adaptor developed. Tensile tests at a strain rate of 200 s−1 were conducted with high strength steel sheet of DP590 with the thickness of 1.2 mm. It is also noted from experimental results that the slack adaptor developed remarkably reduces the load ringing in raw data of load signals. Consequently, the slack adaptor developed diminishes the load ringing phenomenon in the load data during dynamic tensile tests at a strain rates of 200 s−1 and improves the quality of the dynamic material properties of sheet steels.
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Kwon, J.B., Huh, H., Ahn, C.N. (2016). An Improved Technique for Reducing the Load Ringing Phenomenon in Tensile Tests at High Strain Rates. In: Song, B., Lamberson, L., Casem, D., Kimberley, J. (eds) Dynamic Behavior of Materials, Volume 1. Conference Proceedings of the Society for Experimental Mechanics Series. Springer, Cham. https://doi.org/10.1007/978-3-319-22452-7_35
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