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.
The research intends to form the foundations for developing new industrial applications that will strengthen the USA’s international competitiveness.
Several processes have been developed that are expected to be of critical importance for multiple manufacturing processes and NIST feels that these demand generic metrology approaches.
- The common process phenomena that NIST has been looking at include a better understanding of forces, temperatures, material properties, and material transformations particularly at tool interfaces.
- Another focus has been to look at ways of reducing the impact of tool wear from performance, friction and vibrational considerations in the manufacturing process.
The impact of these changes demands a transformation from manufacturing practices based on human experience towards scientific-based modeling, decision-making, and production. The NIST program is developing fundamental measurements, standards, and tools to help US manufacturers make this transformation.
Samples in a split-Hopkinson bar
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.
Additionally, the NIST device has been combined with a controlled resistive-heating system to provide a unique perspective on the metrology of the test sample.
The heating system was originally developed to measure the physical properties of metals at high temperature, such as the critical point at melting of a pure metal. This has now been improved to pre-heat an experimental sample extremely rapidly using precisely controlled pulses of DC electrical current once the sample has been placed for testing.
The NIST split-Hopkinson bar can heat a test sample to temperatures of around 1,000ºC in less than a second. These high temperatures can be held for several seconds, prior to rapidly shutting off the current , to enable dynamic compression tests on rapidly heated samples to be performed at strain rates of up to 104 per second.
This combination of the NIST split-Hopkinson bar with a pre-heated sample enables researchers to investigate the influence of heating rates and the time at any designated temperature on the flow stress of carbon steels so that high speed machining operations can be simulated.
Although the pulse-heating rate is less than that normally found in high speed machining processes, it is much more rapid than pre-heating samples using more traditional methods. This is an advantage because it enables flow stress measurements to be obtained since there is less time for thermally activated microstructural processes – such as dislocation annealing, grain growth, and solid-state phase transformations – to take place.
It is critical that the NIST Split Hopkinson Bar is monitored by high-performance data acquisition and analysis equipment since the primary aim of the tests is to gather data that will facilitate the development of advanced process metrology methods and tools.
Generally five to ten tests are done on any single material so the tests must be repeatable and the DAQ must be capable of ensuring ready comparison of the data from different tests. In addition each test is destructive as the samples are quite thin and permanently deformed during the experiment.
This placed a number of constraints on the design of the system to ensure that all relevant data was correctly captured and subsequently analyzed, as the time interval during the tests is only a few milliseconds.
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).
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.
Historically, engineers have analyzed data from similar experiments by using static, procedural scripts in languages such as Fortran or Matlab. However, Genesis HighSpeed and HBM’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, HBM 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.
Using the NIST Split Hopkinson Bar apparatus has enabled simulations to be conducted of the fundamental problem of chip formation. During chip formation, the work piece interacts with the cutting tool under extreme conditions of pressure and temperature. This results in large plastic deformation taking place at a very high rate of strain, both in the thin primary shear zone, and in the secondary shear zone along the tool/work piece interface as the freshly cut material slides up the face of the tool.
In some materials, the temperature during high-speed cutting can also get close to the melting temperature. Although modeling and simulation has made marked progress because user-friendly finite element software packages have been developed, the development of the NIST Split Hopkinson Bar apparatus helps fill the need for improved material properties, increasing the precision and reliability of the modeling and simulation of machining processes.