In this experimental work we use Phase Shifted Moire Interferometry to measure local displacements and strains directly on the surface of test specimens. Our motivation is to study the cyclic stress-strain behavior of Nitinol and to develop improved material models and methods for calibrating them. We are specifically interested in the transient effects during initial loading, quantifying their evolution and developing best practices for computational modeling of medical implants using available commercial simulation technology.
The specimens were manufactured from medical grade Nitinol tubing and processed into a simple dog bone shape for testing in tension or bending. You can request copy of our paper below if you would like more details. Below is a picture of the experiment setup. You can see the interferometer, high speed test system, environmental test chamber, optical table, beam block, magnification lens and video camera.
The next series of images gives you a hint at the level of detail available with our technique. In the first image, the specimen is at rest and there is no load applied. In the second image, the specimen has been stretched to induce a localized phase transformation of the material that appears as a diagonal band of increased compliance—the stretching is concentrated—the material within the band experiences much greater stretch than the surrounding material. In the final image, the load has been completely removed, but notice that there is a uniform field of fringes that remain.
How are we able to make such amazing measurements?
Custom software was written to integrate the cameras capabilities into the phase shifted moir\’e interferometry software system. The interface integrates control of the camera and fiber phase shifter with two digital control lines from the dynamic test machine. The analog actuator output signal is rectified and used to hardware-trigger the camera, allowing frames to be captured at the min and max fatigue loading conditions. It means we can study transient and functional fatigue effects DURING cyclic loading. A user-controllable digital output can also be to allow the fatigue testing control software to trigger taking phase shifted moir\’e interferometry data while the fatigue test machine is paused. Using the block configuration mode within the fatigue test machine software, the machine could fatigue the specimen for a given time, pause the machine, start phase shifting and acquiring image data, and automatically restart cycling after the data was recorded.
What’s the point?
Various test methods are available to obtain the cyclic stress-strain behavior for a given material (e.g., Manson et.al.). In all methods, a specimen is cycled between fixed load or displacement conditions until some criteria of stability is met. The cyclic stress-strain curve is determined from the stabilized load or stress plotted as a function of strain. Three test methods are considered in this study: The multiple step, incremental step and single step tests. Below is a schematic depicting two loading histories.
Our approach is unique because the local strain at any selected point is available during the experiment, or can be retrieved through post-processing. The next plot is an example of local strain history measurements from an incremental step test. The application of five blocks of loading are shown and local strain measurements are made at the peak loads during each block. Inelastic strain is recorded at the end of each block at load steps 11, 21, 31, 41 and 51 when the specimen has been unloaded to 5N. This data was taken over a period of about 10 hours—the time required to cycle through five blocks of incremental step testing at 0.5N/s.
This is a very simple application of our technique. Things get a lot more interesting when we replace the simple smooth specimen with one containing a notch and even more so when we switch to bending.
Get a copy of Local Strain Analysis of Nitinol During Cyclic Loading: