NIST: Foundations for developing new industrial applications

US manufacturing utilizes rapid data acquisition to improve production capability

 

As manufacturing companies across the globe seek to improve the quality of their products, researchers in the USA have utilized HBM’s DAQ equipment to ensure international competitiveness. In its efforts to remain globally competitive, US manufacturing is implementing fundamental changes that are focused on designing and building complex, highly customized, high quality goods. Global demand means these must be built rapidly to meet changing market demands. Manufacturers need to shorten product development cycles and increase flexibility and speed of production systems along with supply networks. This must be completed while reducing any energy requirements and environmental impact. One major challenge facing US manufacturing is the ability to accurately predict the best machining parameters for a particular process and work material. This has been the focus of major research in the USA where the National Institute of Standards and Technology (NIST) has developed advanced process metrology methods and tools to increase the scientific understanding of existing manufacturing processes.

As part of the research NIST has developed a precision-engineered split-Hopkinson bar – or split-Hopkinson pressure bar – combined with a rapid heating capability. The device has been developed to provide materials properties information to improve finite-element modeling of high speed machining processes. The NIST split-Hopkinson bar comprises two 1.5 m long, 15 mm diameter high-strength steel bars that are mounted on bearings to enable easy axial sliding of the bars while resisting bending in other directions. These are called the Incident Bar and the Transmission Bar. In addition there is a third bar – called the Striker – made of the same steel as the two main bars. This is much shorter but has the same diameter as the two main bars and can move easily. When conducting experiments a cylindrical sample of the material under test is fitted between the two bars and carefully aligned for axial symmetry. This means that, ignoring radial effects, any data gathered can be analyzed using one-dimensional wave theory. An air gun is then fired at various high speeds to launch the Striker against the Incident Bar. The impact of the Striker sends a compressive stress wave traveling through the Incident Bar and the sample. Using this approach means that the sample is rapidly impacted by the compressive stress wave. When the compressive wave arrives at the sample, the impedance difference between the bar and the sample splits the input wave into two parts. One part is a tensile wave that is reflected back along the Incident Bar. The second part continues as a compressive wave that – rapidly and permanently – plastically deforms the sample. The compressive wave then propagates into the Transmission Bar. The NIST split-Hopkinson bar is designed so that only the sample is affected by plastic deformation.

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). HBM’s Genesis HighSpeed data acquisition equipment supplies Split Hopkinson Bar measurement data to Data PADS. HBM 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 HBM’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 HBM’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 HBM’s knowledge of strain and strain gauges enable HBM to provide customers a unique combination of expertise.