PSI - Issue 37

Pedro J. Sousa et al. / Procedia Structural Integrity 37 (2022) 826–832 Sousa et. al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Composites and other high-performance materials are currently seeing increased usage in the aviation and aerospace industries, among others. These materials may often be subjected to significant loads in short periods of time, leading to an increased importance of testing in high-strain rate conditions (Kuhn and Medlin, 2000). These test procedures are capable of providing crucial parameters for design engineers in the earlier steps of project development. In this way they are able to predict unexpected dangerous behaviour and to ensure that the necessary usage safety requirements can be safely met in sudden events. A common approach for this type of material testing is centred on the use of the Split Hopkinson Pressure Bar (SHPB) apparatus (Adorna et al., 2018; Dave et al., 2018; Gong et al., 2019; Smerd et al., 2005; Zwiessler et al., 2017). In the Advanced Monitoring and Structural Integrity unit of INEGI, these tests are performed with a custom-built pneumatic setup, comprising two compression and two tension bars (Shamchi et al., 2020, 2019). The two main variants, shown schematically in Fig. 1 and 2 for tension and compression respectively, work in a very similar way, by analysing the strain measured in the incident and transmission bars, between which the specimen is placed.

Fig. 1. Schematic of the Split-Hopkinson Pressure bar for tension testing, using a tube-shaped striker

Fig. 2. Schematic of the Split-Hopkinson Pressure bar for compression testing, using a bar-shaped striker

After firing the striker, there is a strain wave passing through the incident bar, which will then affect the specimen, breaking it. In the meantime, part of this wave proceeds to the transmission bar, and the rest is reflected back to the incident bar. By analysing these three waves, it is possible to obtain the stress the specimen was subjected to (Gray, 2012). Originally, the bars at INEGI were manually controlled using a couple of buttons and an analogue pressure gauge, which meant that obtaining consistent firing pressures was not a trivial matter. This made the system somewhat prone to user error. It also often required manual triggering of multiple acquisition systems, which usually corresponded to the stress signals for both bars, acquired using strain gauges, complemented by full field strain measurements obtained using contactless techniques such as Digital Image Correlation. The main goals of the developed control system included not only being able to define a particular firing pressure and remove the user-factor error contribution from that equation, but also making independent trigger pulses available, to be used to synchronize the different acquisition systems. Additionally, for example during the pressurization phase, noise may be induced on the bars’ strain gauges, resulting in small pulses, which may trigger the acquisition system at improper times and cause lost data, wasting valuable specimens and time. This can be mitigated with more complex trigger conditions, such as a pulse width condition, which increases the setup complexity for each experiment. The synchronization pulses provided by the control system can also be used to trigger the data acquisition with simpler trigger conditions by enabling the use of higher threshold voltages. In short, the goal of this work is to improve the usability and repeatability of Split Hopkinson Pressure Bar experiments, by implementing an automated control system with trigger signals for data acquisition.