Huifeng Jiang

Different Structural Integrity Assessment Methods for Pipe Containing Circumferential Through-Thickness Crack

Heretofore, several kinds of codes are applicable to the structural integrity assessment for pipe containing defects, i.e. API 579, R6 and BS 7910 etc. In this paper, different methods from API 579-1/ASME FFS-1: 2007 and R6-2000 were employed to assess the integrity of pipe containing a circumferential through-thickness crack. However, there was a significant difference between the calculated load ratios by these two codes, although the calculated fracture ratios were very close. To verify these results, elastic-plastic finite element analysis was carried out to calculate the limit load and the load ratio. Additionally, the experimental results and our previous engineering experience were also referred to. The final results imply that the larger load ratio obtained from R6-2000 rather than API 579 code is more reasonable for the pipe with good fracture toughness.© 2011 ASME

Creep-Fatigue Behavior and Life Prediction of 316L Stainless Steel Under Different Loading Levels

Stress controlled creep-fatigue interaction tests were carried out for 316L stainless steel under different loading levels at 550°C. Cyclic properties such as material life and mean strain were investigated for 316L stainless steel. When the maximum stress is keeping fixed, with increasing minimum stress, the material creep-fatigue life increases first and then decreases. When the stress amplitude is equal to 167.5MPa (minimum stress is equal to 50MPa), material creep-fatigue life reaches its maximum value. Moreover, under stress control mode, dynamic strain aging was found to manifest itself macroscopically as displacement or mean strain abrupt jumps during cyclic deformation. When the minimum stress is less than 0MPa, abrupt displacement jumps occur at the early stage of cyclic deformation and there are many jumps during the whole process. While the minimum stress is larger than 0MPa, displacement only jumps once at the end of deformation. Additionally, by employing a life prediction method on the basis of the Hull-Rimmer creep cavity growth theory, the creep-fatigue life was assessed for 316L stainless steel at 550°C under different loading levels. The predicted lives were compared with the tested ones and a good agreement was found between them.Copyright

Creep Fatigue Behavior and Life Prediction Including the Load History Effect Under High Temperature

Creep fatigue interaction is one of the main failure modes for high temperature pressurized equipment. In practice, the amplitude of load on structure often changes which results in the load history effect on cyclic behavior and life of material. Therefore, 316L stainless steel 1-step and 2-step creep fatigue test under high temperature was conducted, the influence of two kinds of load history on material behavior was analyzed. The results showed the first step load with different amplitude and life fraction would lead to different hardness or softness of material and affect the cyclic behavior under the second step load. There is also much difference of material behavior at the time of load change for high-low and low-high load case. A uniform nonlinear creep fatigue damage model based on “disorder” and a modified failure rule coupled with the load history effect were presented in the paper. The creep fatigue life under 2-step load at high temperature was evaluated by the damage model and the modified rule. The predicted results were in good agreement with the experimental data.Copyright

A New Empirical Life Prediction Equation for Stress-Controlled Fatigue-Creep Interaction

A new empirical life prediction method is developed. The equivalent radius of cavities at grain boundary is adopted as the damage parameter. Similar with Nam’s model, in this paper, it is also assumed that cavities only nucleate during fatigue cycles and further grow with the development of creep. Then the number of cavities nucleated in a cycle is proportional to the fatigue effect, i.e. the amplitude of loading stress. As the creep process is composed of the static creep and the cyclic creep during the fatigue-creep interaction, then the equivalent stress causing cavity growth should be proportional to the maximum hold stress (static creep) and the mean stress (cyclic creep). Therefore, this model is applicable to stress control mode and includes the effects of fatigue, static creep and cyclic creep during the fatigue-creep interaction. By employing this method, the fatigue-creep lives are assessed for 1.25Cr0.5Mo steel at 520°C and 540°C. The predicted lives are compared with the tested ones and a good agreement is found between them. Moreover, it is found that the coefficient of the mean stress is 3-order larger than that of the stress amplitude, which means the effect of static creep is much punier than that of cyclic creep. Considering the detailed test parameters, the short hold duration for peak load may be responsible for this.Copyright