Creep of Aluminum Alloys
Test aluminum alloy creep
Conventional testing of samples on stretching are performed in a continuous increase of the load, and, Consequently, voltages. It is believed, that the respective deformation under a given stress are independent of time. If, However, voltage of sufficient magnitude, but lower tensile strength exert continuously for a long time, then the deformations will continuously increase (Figure 1). If the voltage is large enough and long enough to be attached, the sample was destroyed approximately as smooth as when the load increases.
Fig. 1 – Creep Curves for an Aluminium-Copper Alloy Stressed at 110 MPa [1]
This phenomenon is called creep. Creep - a slow increase in plastic deformation of the material at the time of load occurs, lower, than those, which can cause permanent deformation under normal tensile tests. The intensity of the phenomenon with increasing operating temperature increases.
aluminum creep tests consist in, that the test sample for a long time exposed to a constant tensile force and constant temperature. At this time, all the sample elongation is recorded.
test results on creep is expressed not only by a numerical value of the voltage, and the operating temperature, duration and amount of loading resulting strain. The figure shows such creep curves for aluminum alloy Al-Cu.
Such data are essential for the design of structures, which will be subject to the power and heat resistant. This can be, for example, hot tar transport vehicle (required service life 1000 hours) or process pressure vessel (required service life 100000 hours). It may also be necessary to calculate the service life of the structure in an emergency, for example, preservation of structural integrity, surrounded by fire (30 minutes) or even a very short service life to destruction, as needed for the carrier rocket shell (2 minutes). In all these cases, the time to failure for a given voltage level and temperature is design parameters.
Information, which uses the constructor, It is the result of a certain number of tests, sometimes more, Tensile samples with different voltage levels and a constant temperature, as well as at different temperatures and a constant voltage. Wherein Test duration can be up to several thousand hours. Thus was prepared a family of curves of creep, such as in the picture.
Four aluminum creep
On each of these curves, deformation increases in four stages:
– first elastic (from point O to point A);
– then with a deceleration of the deformation rate (from A to B);
– the third portion of the curve from B to C - almost a constant strain rate, which can be negligible at low voltages and temperatures;
– and, finally, the section from C to D, when it is increasing the strain rate leads to destruction of the sample.
In the figure, only the upper curve has a last stage. It indicates the way the designer of the data application in the design of products.
One way - to limit the total creep at a given temperature, let us say, value 0,1 % for 10000 hours when exposed allowable workloads.
Another way - to limit the working voltage to the level of, wherein the final stage of the creep curve is not achieved during the life of the product and aluminum in which the total deformation at the end of this period will still acceptable.
Time, is required to obtain data on creep, It can be very large. It is generally considered impractical to continue to creep test more than a few thousand hours. However, designers of pressure vessels may require the destruction during the duration of the test data more 100000 hours. In such cases, special methods of data interpolation is often used.
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