To meet the demands of specific applications, steel and other alloys are produced in a wide variety of material conditions and structures. In order to evaluate the suitability of individual batches of material for any particular application samples may undergo a number of destructive and non-destructive tests. Creep testing is one method of material evaluation which is widely used in the following markets:
- Aerospace and Automotive
- Power Generation
- General Engineering
The “Creep Test” is performed on an alloy sample to determine its physical characteristics at typical component operating temperatures. In very simple terms, a sample is heated to a material dependent stable temperature between 300°C and 1200°C. A load is then applied to the sample to exert a longitudinal force on the grain structure of the alloy. The sample is maintained in this state for the period of the test or until the component ruptures. During the test, data is continuously monitored and recorded to qualify the stability of the temperatures, loading and sample extension.
It is important to maintain tight control of the temperature across the entire sample with 0.2 degrees centigrade being typical for the sample uniformity tolerance. To achieve this many furnaces employ highly accurate control devices with three zones of heating. Tests may run for many months and occasionally years, it is important therefore that control systems are designed to accommodate power fail and abort strategies which will allow tests to be continued after unplanned interruptions.
Large creep laboratories may have several hundred furnaces, carrying out simultaneous testing of samples. The results from the tests form part of the audit trail of data for the eventual components and for convenience many users rely on communicating control systems to provide automation to the process of data management:
- Isolated thermocouple inputs
- Easy adjustment of Setpoint
- Power fail and recovery routines
- Long term process stability
- Measurement of “creep” and detection of sample break
- Digital communications and Data management solutions
In a typical Eurotherm multi loop solution the master loop controls the temperature at the centre of the sample with two slave loops controlling the top and bottom zones. User wiring is employed to ensure the working setpoint for the master loop is also used by the slave loops. Temperature is measured by thermocouples attached to the samples. Heating control is most often via a solid state relay.
Creep (or strain), can be taken into the control system either via an analogue input from a suitable transducer or through master communications from other digitally communicating devices.
The control system can also access digital inputs and outputs to detect sample rupture and to control and monitor load beam leveling.