Intermediate Strain Rate Material Characterization with Digital Image Correlation
Summary
Here we present a methodology for the dynamic characterization of tensile specimens at intermediate strain rates using a high-speed servo-hydraulic load frame. Procedures for strain gauge instrumentation and analysis, as well as for digital image correlation strain measurements on the specimens, are also defined.
Abstract
The mechanical response of a material under dynamic load is typically different than its behavior under static conditions; therefore, the common quasistatic equipment and procedures used for material characterization are not applicable for materials under dynamic loads. The dynamic response of a material depends on its deformation rate and is broadly categorized into high (i.e., greater than 200/s), intermediate (i.e., 10−200/s) and low strain rate regimes (i.e., below 10/s). Each of these regimes calls for specific facilities and testing protocols to ensure the reliability of the acquired data. Due to the limited access to high-speed servo-hydraulic facilities and validated testing protocols, there is a noticeable gap in the results at the intermediate strain rate. The current manuscript presents a validated protocol for the characterization of different materials at these intermediate strain rates. Strain gauge instrumentation and digital image correlation protocols are also included as complimentary modules to extract the utmost level of detailed data from every single test. Examples of raw data, obtained from a variety of materials and test setups (e.g., tensile and shear) is presented and the analysis procedure used to process the output data is described. Finally, the challenges of dynamic characterization using the current protocol, along with the limitations of the facility and methods of overcoming potential problems are discussed.
Introduction
Most materials demonstrate some degree of strain rate dependency in their mechanical behavior1 and, therefore, mechanical testing conducted only at quasistatic strain rates is not suitable to determine the material properties for dynamic applications. The strain rate dependency of materials is typically investigated using five types of mechanical testing systems: conventional screw drive load frames, servo-hydraulic systems, high-rate servo-hydraulic systems, impact testers, and Hopkinson bar systems1. Split Hopkinson bars have been a common facility for the dynamic characterization of materials for the past 50 years2. There have also been efforts to modify Hopkinson bars to test at intermediate and lower strain rates. However, these facilities are typically more suitable for the high strain rate characterizations of the material (i.e., usually greater than 200/s). There is a gap in the literature on the strain rate characterization of material properties at intermediate strain rates in the range of 10−200/s (i.e., between quasistatic and high strain rate results obtained from split Hopkinson bars3), which is due to the limited access to facilities and a lack of reliable procedures of intermediate strain rate material testing.
A high-speed servo-hydraulic load frame applies load to the specimen at a constant and predefined velocity. These load frames benefit from a slack adaptor, which, in tensile tests, allows the crosshead to reach the desired velocity before the loading starts. The slack adaptor allows the head to travel a certain distance (e.g., 0.1 m) to reach the target velocity and then starts applying the load to the specimen. High-speed servo-hydraulic load frames typically perform tests under displacement control mode and maintain a constant actuator velocity to produce constant engineering strain rates3.
Techniques for measuring specimen elongation are generally classified as either contact or noncontact techniques4. Contact techniques include the use of instruments such as clip-on extensometers, while laser extensometers are employed for noncontact measurements. Since contact extensometers are prone to inertia influences, they are not suitable for dynamic tests; noncontact extensometers do not suffer from this problem.
Digital image correlation (DIC) is an optical, non-contact, full-field strain measurement technique, which is an alternative approach to strain gauging to measure strain/load and overcome some of the challenges (e.g., the ringing phenomenon) associated with dynamic material characterization5. Resistance strain gauges can suffer from limitations such as a limited area of measurement, a limited range of elongation, and limited mounting methods, whereas DIC is always capable of providing a full-field strain measurement from the specimen surface during the experiment.
The presented procedure describes the use of a high-speed servo-hydraulic load frame along with DIC and can be used as a complementary document to the recently developed standard guidelines6 to clarify the details of the experimental procedure. The section on the servo-hydraulic load frame can be followed for a variety of test setups (e.g., tensile, compressive, and shear) and even with common quasistatic load frames as well, and, therefore, covers a vast range of facilities. Furthermore, the DIC section may be applied separately to any type of mechanical or thermal tests, with minor modifications.
Protocol
Representative Results
Discussion
The raw data obtained from the experiment is influenced by the specimen geometry and strain gauges location on the specimen. The load data in low strain rate dynamic tests acquired by a piezo-electric load washer incorporated into the load frame at higher strain rates (Bruce et al.3 suggested > 10/s, while for Wang et al.9 reported this limit to be 100/s) typically suffer from large amplitude oscillations due to dynamic waves associated with the loadin…
Disclosures
The authors have nothing to disclose.
Acknowledgements
The authors acknowledge the great assistance from Dmitrii Klishch, Michel Delannoy, Tyler Musclow, Fraser Kirby, Joshua Ilse and Alex Naftel. Financial support by the National Research Council Canada (NRC) through the Security Materials Technology (SMT) Program is also appreciated.
Materials
| Camera Lens | Opto Engineering | Telecentric lens 23-64 | |
| High Speed Camera | SAX Photron Fastcam | ||
| High Speed DAQ | National Instruments | USB-6259 | |
| High Speed Servo-Hydraulic Load Frame | MTS Systems Corporation | Custom Built | |
| Jab Bullet Light with diffuser | AADyn JAB BULLET | 15° diffusers | |
| Strain gauge | Micro-Measurements | Model EA-13-062AQ-350 |
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