Fatigue testing is a method commonly used to characterise the survival probability of a wide variety of materials in particular ceramics, composites and those relating to dental restoratives . The machine (Dartec HC10 as shown in the image below) we have in place can be configured to apply a range of force within the constraints of the load cell suited to carry out a particular test. It comes equipped with three interchangeable loads cells each having a different limiting force (100N, 1kN and 10kN). The user can choose to set the appropriate limits within the software in order for the machine to cycle in either load or stroke (displacement) mode. The Dartec can be easily adapted to perform cyclic tests in compression, tension, bending and flexure depending on the test design. Variables such as testing under wet conditions can also be introduced to mimic the physiological environment.
- The Dartec HC10 consists of a test frame fitted with a movable crosshead. The crosshead can be adjusted to a suitable height in order to accommodate a specifictest design.
- The piston head is fitted with an attachment that enables the user to interchange between load cells (100N, 1kN and 10kN).
- To enable the application of cyclic loads or static loads the machine is equipped with a Dartec 9610 control unit which interfaces between the software and theactuator (piston-load cell).
- The test frame is engineered as such that the force applied by the load cell during a test is driven by hydraulic pressure supplied by a hydraulic unit.
- A servo valve fitted on to the crosshead provides a close loop flow or pressure response as a result of programmed values in the control loop application available in the software. This enables the machine to control the output pressure whilst testing.
- There is a large bolt fitting at the bottom of the test frame which runs along the axis of the actuator. A number of test designs can be manufactured and adapted to be bolted onto the bottom of the test frame. These can include models that are specifically designed to characterise the insertion and removal force of ‘O’ rings ondental implants over a given number of cycles.
- There is also an environment chamber available which has been manufactured to be bolted directly onto the test frame and to also accommodate testing jigs. Theenvironment chamber can be attached to a unit comprising of a pump and a temperature controller that can apply a dynamic flow of water at regulated temperature around the environment chamber. This enables the user to characterise the behaviour of materials under wet dynamic conditions in much the same way as implant materials do under physiological conditions.
The Main Unique Aspects Of The Machine
- The user is able to set up the limits and parameters of their test using the cyclic generator application under the toolkit 96 software. Upper and lower limits need tobe set in either in load or stroke mode in order to carry out a cyclic test. In reality these limits would be the definitive parameters of the test, however, as the machine is subject to some given error the user can define error bands or a region of error around these limits in which the actuator is constrained to.
- Safety features are available in the software that can be set which allows the machine to act automatically. Such actions can involve shutting down the hydraulicpressure or subjecting the machine to operate in set up mode (idle mode) once a particular safety limit has been exceeded.
- The Toolkit 96 software has a number of data collection and analysis applications. The data acquisition tool can be set to collect data over a very short period of time i.e. over 10 or 20 seconds. It can also be set up to automatically collect a very short range of data at a given cycle number. Alternatively the user can set the software to collect data by continuous logging data points for the duration of the test.
Examples Of Work Undertaken At The Institute
A number of projects have been carried out measuring the changes in the mechanical behaviour of materials over a given number of cycles. For example we have measured the changes in the insertion and removal forces of ‘O’ rings on dental implants as a function of two variables; the number of cycles and the degree of inclination for insertion and removal. The diagram below (left) illustrates some data exhibiting typical insertion and removal forces for ‘O’ rings on dental implants collected over 10 seconds at around 250 cycles.
We have also carried out numerous studies involving the biaxial flexure behaviour of ceramic materials such as zirconia (3M) under wet conditions. The graph below (right) illustrates the distribution of biaxial flexure strength for a batch of zirconia samples as a function of number of cycles.