Intermediate Strain Rate Material Characterization with Digital Image Correlation

The mechanical performance of materials depends on the rate of load application. Our protocol can be used to capture material behavior at intermediate strain rates. The main advantage of this technique is that the materials can be characterized while using a non-contact, full field strain measurement technique to capture surface strains with high-speed cameras.

The intermediate strain rate characterization of materials often requires dealing with undesirable oscillation called ringing in the results, which should be avoided through proper specimen design, and test protocol. Before beginning the procedure, prepare dog bone-shaped tensile specimens, according to International Organization for Standardization parameters. Next, paint the surface of the specimen to exhibit high contrast features.

Matching the contrast pattern to the camera image sensor size such as each dark contrast feature is composed of approximately three pixels or more, then, set the specimen aside, to let the paint dry. When the specimen is ready, turn on the power to the control console, and confirm that the isolation valve, from the pump to the high rate frame is open, before turning on the computer. From the desktop, start the controller application, selecting the high rate calculate displacement cfg configuration, and clicking reset, to clear interlock one.

Start the hydraulic pump, and open the service manifolds to one at a time, waiting until the low indicator stops flashing, before pressing the high indicator for each manifold. Start the test design software, and confirm that the hydraulic pump, and high service manifold one, are on. Then click, file, new, test from template and custom templates, and select tension test.

For strain gauge set up, set the switch of the load frame crosshead control, to the low rate, and match the wires of the specimen strain gauge, to the wires of the strain gauge box inside the test chamber according to the wire colors. Then, in the controller application, under auxiliary inputs, and strain one, select the maximum range of the strains. Next, activate the manual control, and enter the position of the head to the full extension at minus 125 millimeters.

Turn off the enable manual command checkbox, and uncheck the exclusive control box. An elastic cord may be used to hold the slack adapter in a retracted position to make room for installing the coupon. Use the mounting fixture to align the coupon inside the grips.

Press the key icon to active the handset and confirm that the exclusive control box in the software is unchecked. Make sure the top grip is loose to prevent an undesirable load application to the specimen, and remove the elastic cord. Press the wheel icon to activate the controller, and slowly roll the wheel to bring the head down until the bottom arm of the slack adapter is almost fully retracted.

Press the key icon again, to deactivate the handset and check the exclusive control box using the manual control to bring the head to exactly minus 125 millimeters. Use a wrench and a key to rotate the slack adapter to tighten the top grips without twisting the coupon, and check the spiral washers between the slack adapter and the intermediate cross head to make sure that the washers are tight, and that there is no axial clearance along the load train. Then, return the frame to the high rate and make sure that the enclosure doors are tightly closed.

To set the instrument up for digital image correlation, open the high speed imaging viewer software and click detect before saving the set up. Click camera option and select the input-output tab to set the external signals. To set the frame rate and frame resolution, click variable and set the camera frequency and the data acquisition box acquisition rate to the same number as the high speed data acquisition system in the load frame.

Then, open the high speed data acquisition in the high speed imaging viewer, and select the required channels and samples per frame. To run the test, open the tension test and select new test run to enter a valid file name. Modify the fields as necessary and click okay.

For the ramp rate, select the nominal desired head velocity and click okay. A series of prompts will appear reminding that the key hardware should be checked after which the test can be initiated by clicking on the run icon. On the control console, press and hold the arm charge accumulator switch to arm the system.

Then, press fire to complete the test. The speed of the test should be calculated based on real life scenarios for example for a car crash simulation speed of around eight meters per second can be applied. For data analysis, export the raw data from the load frame computer into the post-processing software of choice and determine the location on the gauge section where the specimen failed.

Restrict the strain field to a local area of the vicinity of the failure section and measure and record the strain in the local area. Then, draw a stress strain curve obtained from the measurements. Both of these test were performed properly but the load data directly extracted from the load frame force link could not be used in this test in an alternative load measurement technique such as tab section strain gauging, was necessary.

In this test however, the raw load data from the load frame was in good agreement with the strain gauge loads. A typical example of digital image correlation results for a dog bone aluminum specimen with a strain field evolution with time on the entire gauge section is shown. The uniform strain within a given cross section of the specimen illustrates a proper loading and data analysis during the test.

And the loss of digital image correlation in the last image was due to severe necking that resulting in paint flaking and was unavoidable immediately before the failure at the vicinity of the failure zone. These stress strain curves obtained from digital image correlation and from the load frame cross head displacement data, illustrate and average stress strain for the entire gauge section and demonstrate the validity of the technique and a good agreement between the results. A comprehensive evaluation of the results ensures that the protocol is carried out within the boundaries of valid assumptions.

Such as the inertia dominant regime or non-slip conditions and grips. Following this procedure a range of different mechanical characterization tests such as sheer, bending, puncture, or compression tests can be performed on a variety of materials. This technique fills the gap, between quasi static tests and ultra high strain rate characterization techniques.

Enabling us to fully characterize material behaviors as a function of strain rate.