Experimental results

This section presents some typical results obtained from experimental works on the P91 and P92 materials. The effects of the strain-controlled cyclic loading and the holding period in the isothermal tests, on the material behaviour, are described. The data from these two tests were used in the constitutive model development; this will be explained further in Chapter 4. Results obtained from the thermomechanical fatigue tests and the load-controlled notched bar tests are also presented; these data will be used to verify the material model performance.

3.6.1 Strain-controlled cyclic loading tests

Figure 3.15 shows a typical stress-strain hysteresis loop for a strain-controlled fatigue test. Important parameters such as stress range, , plastic (or inelastic) strain range, p, total strain range, t, (which is the controlled parameter in the test) and elastic strain range, e, can be identified from the stress-strain hysteresis loop as shown in Figure 3.15. In the case of the P91 specimens with ±0.5% strain amplitude, significant plasticity was observed from the hysteresis loop where the width of the loop at the mean stress (plastic strain range) represents approximately 60 percent of the total strain range.

The first cycle of the hysteresis loop, at different temperatures, shows the effect of the temperature on the resultant response. The slope of linear stress-strain data in tension and compression, as shown in Figure 3.16 for P91 material, depends on temperature, i.e., the slope decreases as temperature increases. The test at the higher temperature also produced a lower peak stress level as well as a higher plastic strain range compare with the others at the lower temperatures. Similar trends in the results were observed for the P92 parent and weld materials, as shown

in Figure 3.17. The figure compares the stress level at each test temperature for both P92 parent and weld materials. It can be seen that the stress-strain behaviours of both materials are practically the same during the first cycle. The P91 parent material data is compared with the P92 parent material data at 500 and 600°C, in Figure 3.18; this indicates a slight difference in the maximum and minimum stress levels attained in the tests with the same strain range and strain rate. The P92 material has slightly higher stress values, by about 30 MPa, than those of the P91 steel. Similar trends in stress level comparison for P91 and P92 materials have also been reported by Vaillant et al. (2008).

The P91 and P92 steels exhibit cyclic softening behaviour. Generally, the behaviour in fatigue loading tests is associated with the increase of plastic strain range in a material as a result of continuous cyclic loading. Figure 3.19 shows the evolution of plastic strain range observed in the tension-compression test of P91 and P92 materials. It can be seen from the graph that the plastic strain ranges increase rapidly during the initial stage and then become stable after approximately a quarter of the total number of cycles. In some P92 results, particularly at 600°C, the plastic strain range increases during the last stages of the test. The figure also shows that the plastic strain ranges recorded at higher temperature are always higher than the values at lower temperature, which may have a contribution due to the greater creep which occurs at higher temperature.

Figure 3.20 shows another aspect of cyclic softening behaviour. For a strain-controlled fatigue test, it can be seen that the stress range decreases as the cycles increase. There are similar trends for all tests performed at different temperatures, see Fig. 3.20, where the stress range evolves in three different stages. The stress range levels decrease rapidly in the initial stage before slowly decreasing in the

_____________ Experimental work middle region of the test. In the latter stages, the stress levels continue to rapidly decrease until failure occurs. The second stage seems to have the largest portion of the cyclic period. From Figure 3.20, it is also clear that the number of cycles to failure decreases when temperature increases between 400 and 600°C for both P91 and P92 materials. However, the number of cycles to failure for P92 at 675°C was found to be higher than the failure cycles of the same material at 600°C. The recorded stress-strain loops at the end of the tests at different temperatures for P92 are shown in Figure 3.21. Interestingly, it can be seen at the bottom of compression data for the three loops that there is a “hysteresis loop tail”, which indicates the occurrence of cracks (Dunne et al., 1992). For example, a visible crack was observed on the P92 parent material test at 1042 cycles and 600°C with a 90µm crack width as shown in Figure 3.13(a).

Figure 3.15: Example of typical stress-strain hysteresis loop and the parameters determined from the loop for an isothermal strain-controlled test

-500 -300 -100 100 300 500

-0.006 -0.004 -0.002 0 0.002 0.004 0.006

S

S MP

P91P_400C P91P_500C P91P_600C

Figure 3.16: The stress-strain loop for the P91 parent (P91P) material during the first complete cycle of a tension-compression test at various temperatures

-500 -300 -100 100 300 500

-0.006 -0.004 -0.002 0 0.002 0.004 0.006

S

S MP

P92P_500C P92P_600C P92P_675C P92W_500C P92W_600C P92W_675C Figure 3.17: The stress-strain loop for the P92 parent (P92P) and the weld (P92W)

during the first complete cycle of a tension-compression test at various temperatures

_____________ Experimental work

-0.006 -0.004 -0.002 0 0.002 0.004 0.006

S

Figure 3.18: Comparison of the stress-strain behaviour of P91 and P92 parent materials for isothermal test at 500 and 600°C

0.45 0.55 0.65 0.75

0 400 800 1200 1600

Cycle, N

Figure 3.19: The evolution of plastic strain ranges in isothermal tension-compression tests indicating the cyclic softening behaviour