Stress-rupture test design

A considerable amount of mechanical properties design data on Alloy 800 has already been produced for SPX1, but certain additional work, especially long creep tests were necessary for confirmation. These long term creep/stress rupture tests include the effects of titanium and aluminium additions and welding evaluations. Work has been carried out mainly on pre-pressurised tubes tested at temperature 525 and 550 C. 

Voorhees, Sliepcevich, and Freeman (204) have presented a procedure for calculating the time of rupture from creep and stress-rupture data normally available to a designer. Prior to the work of Voorhees the design of thick-walled vessels at high pressures and elevated temperatures was usually based upon the maximum principal stress and an allowable stress determined from creep and stress-rupture test data. This is the current method recommended by the ASME code (11) for vessels operating at pressures up to 3000 psi. 

Experience has also shown that in cases such as stress rupture and thermal ageing the test parameters may have to be designed progressively. Shorter tests at higher loads (or temperatures) are set up first and the times to failure measured. The test conditions for longer lifetimes are then set on the basis of these results. This is particularly important where the validity of the final result depends on obtaining a failure within a particular time interval, e.g., over 5,000 h as in IEC 60216 [2], or where the measurements must be completed within a set time. 

Chiao, T.T., et al. Stress Rupture of Strands of an Organic Fiber/Epoxy Matrix, in Composite Materials Testing and Design (Third Conference), ASTM STP 546 (1974) Philadelphia, ASTM... 

As we saw in the preceding discussion, several mechanical parameters can be derived from stress-strain tests. Two of these parameters are of particular significance from a design viewpoint. These are strength and stiffness. For some applications, the ultimate tensile strength is the useful parameter, but most polymer products are loaded well below their breaking points. Indeed, some polymers deform excessively before rupture and this makes them unsuitable for use. Therefore, for most polymer applications, stiffness (resistance to deformation under applied load) is the parameter of prime importance. Modulus is a measure of stiffness. We will now consider how various structural and environmental factors affect modulus in particular and other mechanical properties in general. 

Preparing the important creep rupture data for the designer is similar to that for creep except that higher stresses are used and the time is measured to failure. It is not necessary to record strain. The data are plotted as the log stress vs. log time to failure. In creep-rupture tests it is the material s behavior just prior to the rupture that is of primary interest. In these tests a number of samples are subjected to different levels of constant stress, with the time to failure being determined for each stress level. 

Engineering design often requires engineers to predict material properties at high temperatures where no experimental data are available. The creep deformation rate can be so slow that it might require 10 years test time to reach 1% deformation. Reliable predictions based on accelerated test data obtained over a shorter period of time are essential. Several theoretical parameters were proposed to predict long-term metal creep or stress rupture life based on short-term test data. One of the most utilized parameters is the Larson-Miller parameter, as defined by Equation 4.20 ... 

FIGURE 16.26 Plot showing the risk of rupture after proof testing to the ratio of proof-test stress to design stress. 

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