An experimental investigation of the influence of plasticity on creep degradation rate

Intrinsic soil properties, such as the Atterberg limits, are essential factors influencing the mechanical behaviour of the fine-grained soils. In this study, a series of long-term multiple-stage loading oedometer tests were performed on alluvial organic soils to investigate the creep behaviour. The plasticity ratios ranged from 0.4 to 0.63. The smaller value of the plasticity ratio Rp indicated higher soil plasticity. The results showed that the coefficient of secondary compression Cαe of alluvial organic soils was stress- and strain-rate-dependent. The coefficient of secondary compression change index m was derived using a double-logarithmic approach for a creep degradation and was related to the plasticity and clay percentage to fines. Based on the results, it was found that high plasticity soils exhibit slow creep degradation rate during one-dimensional straining under normally consolidated state. The results show that the higher soil plasticity expressed by the plasticity index, plasticity ratio and clay percentage to fines, smaller the coefficient of secondary compression change index. Moreover, the correlations among a soil plasticity properties and creep parameters for the alluvial soils have also been proposed.

Change in the Strength of Steel Grade 11 Loaded at Room Temperature

Steel grade 11 is widely used to fabricate superheater pipe in the thermal power plant. Since the steam has a high temperature (above 300oC) and pressure (max. 20 MPa), lifetime of the tube/pipe decreases or the failure occurs during the operation. Problems of the steel include micro-crack, creep degradation, pit corrosion, mechanical failure, etc.; and increasing working time raises the failure potential and decreases the economy if the thermal plant has to stop operating. This research investigated the change in the mechanical properties of steel grade 11 (ASTM A335) loaded constantly at room temperature. The steel specimens were cut from a new pipe P11, hanged under various stresses (6.45 – 9.68 – 12.9 MPa) for 2160 and 4320 hours. After the loads were removed, the steel specimens were tensile tested and observed by optical microscopy and SEM. The results shown that strengths of the steel increased with increased the stress for 2160 hours, but slightly decreased for 4320 hours. There was no significant change of the microstructure including 87 vol.% ferrite and 13 vol.% pearlite, and the grain sizes were estimated as about 18 μm for all conditions. It was considered that a deformation hardening occurred in the beginning stage of loading, but the creep degradation started as the loading time was lengthened. The fact that a significant change in the microstructure was not observed in the present steels revealed that high temperature played an important role in degradation of the practical steels.

Detection of Creep Degradation on Collapsed Membrane Wall From P265GH Pressure Purpose Steel by Ultrasonic Testing

Creep failure of the pressure vessels operated at high temperatures in power and petrochemical industry is a frequent reason of its collapse. Some ultrasonic properties, as the ultrasonic waves’ velocity, are sensitive to change of material properties due to thermally induced degradation processes, especially plastic deformation. The article discusses possible applications of ultrasonic testing to detect creep degradation process based on results of measurements performed on collapsed membrane wall from P265GH pressure purpose steel. The methodology may help with selection of potentially critical areas on pressure equipment under operation prior its further investigation while performing e.g. residual life assessment.