The solution: Genesis HighSpeed
To ensure that all of these requirements could be met, NIST developed an integrated software system called the Split Hopkinson Bar Data Processing And Distribution System (PADS). HBK’s Genesis HighSpeed data acquisition equipment supplies Split Hopkinson Bar measurement data to Data PADS. HBK was able to meet NIST’s requirements because the Genesis HighSpeed is a robust DAQ system capable of capturing all of the relevant data from the three key points during the test –
- the initial compressive impact,
- the reflected compression stress wave
- and the ongoing tensile wave –
in the very short space of time available. This, combined with HBK’s international metrology expertise, means that it could provide all of the sensor, data acquisition and data processing equipment needed by researchers wishing to build similar systems – from strain gauges through to the analysis software. An advantage of HBK’s Genesis HighSpeed DAQ is that it can also accurately record the low amplitude voltage because it is fitted with a high speed A/D board. The DAQ system needs to have a high frequency response to record the signal that usually lasts less than one millisecond. Generally the minimum frequency response of all components in the data acquisition system should be 2 MHz for strain gauge data and 100 kHz for heating data. Strain gauges have evolved as the standard technique to measure bar strains in the NIST Split Hopkinson Bar experiments. Usually two strain gauges are attached symmetrically on the bar surface across the bar diameter. With strain gauges mounted at the midpoints of the Incident and Transmission Bars and using one-dimensional elastic wave analysis, the sample’s stress against strain response can be obtained. The signals from the strain gauges are conditioned with a Wheatstone bridge. Typically the voltage output from the Wheatstone bridge in these experiments has small amplitude of the order of millivolts. The two sets of standard linear strain gauges are arranged in a full bridge circuit. The upstream strain gauges have a dual purpose because they measure the incident strain on the material and the reflected strain that is returned from the loading pulse in the equipment striking the sample. The downstream strain gauge records the transmitted strain that is passed through the material and into the second section of the NIST Split Hopkinson Bar. Another advantage of the Genesis HighSpeed DAQ equipment is that it is easy to use and the software can easily be modified to meet the particular needs of any application. The combination of Genesis HighSpeed data acquisition equipment with HBK’s knowledge of strain and strain gauges enable HBK to provide customers a unique combination of expertise.
Data analysis made easy
Historically, engineers have analyzed data from similar experiments by using static, procedural scripts in languages such as Fortran or Matlab. However, Genesis HighSpeed and HBK’s associated software with excellent graphical user interfaces (GUI) have given engineers new tools to utilize. Also engineers tended to perform tests, analyze the data, and then make only the processed data available. However, HBK provides an extensive relational database enabling researchers to store original data, along with processing parameters, and reprocess the data in real time as needed. This is very useful when producing stress/strain curves from Split Hopkinson Bar data. Utilizing the Genesis HighSpeed equipment means the data can be accessed over a network via a file server or installed on a laptop PC and used in stand-alone mode enabling the complete test record to be readily retrieved. An example of the usefulness of this capability is Data PADS, which interactively recomputes stress/strain relationships and other data curves under various assumptions by storing both the raw strain gauge data and metadata describing how the strain gauge data should be processed. The NIST-developed Data PADS also includes a database containing visible high speed video, thermal camera video, high speed pyrometer data, sensor data on the projectile’s velocity, as well as technical papers and associated information. In addition the software has to be able to permit access by multiple users who might be performing different types of tests with the system in various configurations. In addition to the normal one-dimensional data such as calibration constants and test conditions, the NIST Split Hopkinson Bar equipment produces a wide range of both two-dimensional data such as current, temperature, projectile position and strain gauge data vs. time. Three-dimensional data such as thermal images and visible light images against time can also be produced to give a comprehensive range of data.