Test configuration
Disk (Intervertebral)
The intervertebral disc is a heterogeneous, cartilaginous structure that contributes to flexibility and load support in the spine.
About this material
The intervertebral disc is a heterogeneous, cartilaginous structure that contributes to flexibility and load support in the spine. Biomechanically, the disc is subjected to different types of load and stresses such as static, dynamic and torsional loads, as well as tensile, compressive and shear stresses. Intervertebral disc degeneration is an important problem concerning lower back pain. Mechanical properties of intervertebral discs is of particular interest in understanding the patterns and mechanics of disc pain.
Test configurations (15)
Test configuration
Tension
Test configuration
Unconfined Compression
Test configuration
Indentation (Stiffness)
Test configuration
Friction
Test configuration
3D Thickness Mapping
Test configuration
Torsion
Test configuration
Indentation (Hardness)
Test configuration
Confined Compression
Test configuration
Temperature Control
Test configuration
Puncture
Test configuration
External Analog Sensors
Test configuration
Camera Feed
Test configuration
Positioning (Manual)
Test configuration
Positioning (X Auto)
Case studies (1)
Related resources (5)
FAQ
What is an aggregate modulus?
The aggregate modulus (Ha) corresponds to the stiffness of the material at equilibrium when fluid has ceased flowing through it in a confined compression test configuration. The aggregate modulus can be calculated from Young's modulus (E) and the Poisson ratio (v) using the formula Ha=E(1-v)/[(1+v)(1-2v)].
How does the creep function works?
The Creep function moves a stage until a predefined load variation (Load Target (relative to initial load)) is reached and maintains this loading through real-time closed-loop PID adjustment of the stage position. This function can be used, for example, to study the deformation of a sample under constant loading. Note that tuning a PID requires a reasonable amount of closed-loop system understanding. A set of PID parameters optimized for a particular experimental configuration will probably be inappropriate for a different configuration and could even create system instabilities (oscillations) that can result in damage to the system.
Parameters:
STAGE AXIS - On multi-axis systems, the user must specify which axis will perform the “Creep”.
DIRECTION - Specify if the stage has to start moving in the positive or negative direction. With a positive displacement, a vertical stage will move down (compression), a horizontal stage will move right, and a rotation stage will move clockwise. Enter a negative value for stage displacements in opposite directions.
STAGE VELOCITY, MM/S - Specify the speed at which the stage will move. Refer to the specifications of your Mach-1™ model to learn about the velocity range allowed. In Mach-1 Motion, the speed is always specified in absolute value terms.
RELATIVE LOAD TARGET, GF OR N - Specify the relative amount of load to be applied and maintained on the sample. When a value is entered, the stage will initiate its movement until a change in load of that magnitude is reached. The value for Relative Load Target is specified in absolute value terms.
CREEP TIME, SEC - This is the time period during which the relative loading will be maintained. Note that after this period, the final stage position will be maintained.
P GAIN - This is the Proportional gain of the PID control loop. This is the main parameter that drives the feedback loop. Our experiments revealed that a good starting point for the optimization of this parameter is given by P = 0.001 (and I = 0), then raising P in logarithmic increments (0.01, 0.1, 1) until a satisfactory amplitude in reached. The higher the P value, the faster the error will be corrected, however if the P is made too large, the error can go negative (actual load > desired load) which may start a process where the stage starts to oscillate between speeding up and slowing down. This oscillation can result in the motor control system going unstable. A simple way to eliminate this instability would be to decrease P until the oscillation stops.
I GAIN - This is the Integral gain of the PID control loop. Integral control sums the error from each position update then multiplies the sum by the Integral gain, I. Our experiments revealed that for most creep tests, best results are achieved with I = 0. If a residual error exists while holding a load, the I value can also be incrementally increased until the error disappears.
What is viscoelasticity?
Property of a material to exhibit both viscous and elastic characteristics when undergoing deformation. Such material will show time-dependent mechanical behavior.
LAB MODULE – Torsion of Disk-Shaped Samples – Student Version
What is the elasticity?
Property of a material to deform under load or strain and to return to its original shape when the stress is released.
LAB MODULE – Unconfined Compression of Hydrogel Disks – Student Version