Applied Mechanics and Materials Vol. 750

Abstract: Strain-based failure assessment methods have been usually used in recent years to propose the fitness-for-service flaws. In this paper, the stress-based and strain-based failure assessment diagram (SB-FAD) approaches were reviewed firstly. Then, SB-FAD approach was used to the material commercially pure titanium TA2 with high strain-hardening coefficient. Several failure assessment curves obtained from strain-based method for four typical specimens, i.e., CCP (center-cracked plate), DECP (double edge cracked plate), SECP (single edge cracked plate) and CT (compact tension) were proposed. It was found that the geometries and crack sizes had obvious effects on the failure assessment curves. By comparing with the stress-based failure assessment curves, the traditional strain-based failure assessment curves could lead to more conservatism in the stage after yielding. However, the SB-FAD modified by Ainsworth can be used.

Abstract: This study is concerned with assessing the integrity of cracked steam turbine rotors components which operate under cyclic loading conditions. Damage accumulation and growth have occurred on the inner surface of slot fillet of key and in a disk and blade rivet attachment. Full-size stress-strain state analysis of turbine disk was performed for different stage of lifetime under considering loading conditions. As a result accumulated damage in critical zones of turbine disks depending on time of loading is defined. The tensile tests were performed for determination the main mechanical properties of disk’s material after loading history. The smooth and notched specimens were cut out from critical zones of turbine disk with given operating time. The low-cycle fatigue tests were performed with the harmonic test-cycle. Additional tests were performed on special designed program test-cycle, which equivalent to start-stop cycle of turbine disk. An engineering approach to the prediction of residual lifetime of turbine disks which is sensitive to the loading history at maintenance is proposed. Approximate estimations of carrying capacity are presented for the different stress-strain state of steam turbine disks at the operation.

Abstract: This paper presents an investigation of the effect of load variation on gear tooth surface micropitting, for an application in planet gears in a wind turbine gearbox. To study the effect of load variation, two methods are employed: an experimental testing of gear micropitting under variable loading and a probabilistic analysis of gear contact stress and specific lubricant film thickness variations using the ISO Technical Report ISO/TR 15144-1:2010. The load variation of wind turbine gearbox is derived from SCADA (Supervisory Control and Data Acquisition) data recorded in operation. Both experimental and analytical results show that high levels of contact stress, load variations and repeated load cycles are determinant factors for the initiation and propagation of micropitting of gear tooth surfaces.

Abstract: One potential challenge to the integrity of the reactor pressure vessel (RPV) in a pressurized water reactor is posed by pressurized thermal shock (PTS). Therefore, the safety of the RPV with regard to neutron embrittlement has to be analyzed. In this paper, the procedure and method for the structural integrity analysis of RPV subjected to PTS is presented. The FAVOR code is applied to calculate the probabilities for crack initiation and failure by considering crack distributions based on cracks observed in the Shoreham and PVRUF RPVs in the U.S. A local approach to fracture, i.e. the σ*-A* model is used to predict the warm prestressing (WPS) effect on the RPV integrity. The results show that the remaining stress contributes to the WPS effect, whereas the increase of fracture toughness is not completely attributed to the remaining stress. The modeled load paths predict a material toughness increase of 30-100%.

Abstract: The high cycle fatigue of super duplex stainless steel of SAF 2507 was investigated by rotating bending fatigue test in both air and 3.5% NaCl environment. The results showed that there is no much reduction of the fatigue life in corrosive environment, which is 90% of the air fatigue strength. In air fatigue, failure happens in ductile mode with austenite grains having finer and straighter fatigue striations than ferrite grains. Width and spacing of striations vary with the orientation and locations when the second cracking occurs. It is not reliable to identify the phase by morphology of striations. In 3.5% NaCl environment, the fracture exhibits a mixed mode of cleavage and quasi-cleavage in ferrites and ductile in austenite grains.

Abstract: Strain-controlled low cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests of K447A are conducted at 760oC in order to investigate the effects of different dwell times and strain ratios on the fatigue behavior and life. For the cases of stain ratio R_ε=-1 with balanced hold time, the tensile and compressive mean stresses will generate. For the case of stain ratio R_ε=-1 with compressive holding 60s, the tensile mean stress will produce. For the case of stain ratio R_ε=-1 with tensile holding 60s, the compressive mean stress will produce. For the cases of stain ratio R_ε=0.1 and R_ε=-1with no hold time, the tensile mean stress will produce. The cyclic damage accumulation (CDA) method and modified CDA method were employed to predict the fatigue life for K447A, respectively. The fatigue life predicted by CDA method is within the scatter band of 18.2X. The fatigue life predicted by the modified CDA method agrees very well with the experimental life and the predicted life is well within the scatter band of 3.1X, which means that the modified CDA method is able to consider the influences of dwell time and strain ratio on the fatigue life of K447A.

Abstract: As a new type of smart material, shape memory alloy (SMA) is widely used in the aerospace, biomedicine, civil engineering and so on due to its unique mechanical and physical properties such like the shape memory effect, pseudoelasticity and good biological compatibility. This paper investigates the numerical simulation and application of the SMA component: A SMA strip, which has been pre-curved in the room temperature and is expected to extend upon heating and shorten on cooling along the curve. According to the investigation of the thermal respond of the SMA strip component, it can be seen that the temperature of the phase transformation can increase with the initial transformation strains and the process of the phase transformation in the boundary is more complex due to the complicated stress state.

Abstract: This paper presents the reliability analysis of the creep rupture strength and long-term life of a directionally solidified nickel-base superalloy, DZ125, which is widely used for manufacturing turbine blades of advanced aeroengines. A number of creep rupture experiments of DZ125 alloy are carried out at 760^oC, 850^oC, 980^oC and 1040^oC. Larson-Miller (L-M for short) parameter is used to predict the long-term creep life and a master curve in the L-M parameter is obtained. A “Z-parameter” model is employed to characterize the magnitude of the deviation of the creep rupture data to a master curve. The “Z-parameter” of DZ125 alloy followed Gaussian distribution which is verified by Chi-square testing. Based on the “Z-parameter” model, the curves of Stress-TTP (i.e. Time-temperature parameter)-Reliability, Stress-Life-Reliability and Allowable Stress-Temperature-Reliability are obtained to predict the creep rupture strength and life for DZ125 alloy.

Abstract: By means of heat treatment and creep property measurement, an investigation has made into the creep behaviors of a containing 4.5% Re nickel-base single crystal superalloy at high temperature. Results show that the elements W, Mo and Re are enriched in the dendrite arm regions, the elements Al, Ta, Cr and Co are enriched in the inter-dendrite region, and the segregation extent of the elements may be obviously reduced by means of heat treatment at high temperature. In the temperature ranges of 1070--1100 °C, the 4.5% Re single crystal nickel-based superallloy displays a better creep resistance and longer creep life. The deformation mechanism of the alloy during steady state creep is dislocations slipping in the γ matrix and climbing over the rafted γ′ phase. In the later stage of creep, the deformation mechanism of alloy is dislocations slipping in the γ matrix, and shearing into the rafted γ′ phase, which may promote the initiation and propagation of the micro-cracks at the interfaces of γ/γ′ phases up to the occurrence of creep fracture.

Abstract: In this study, experimental and the finite element (FE) methods are utilized to investigate residual stresses of a multi-pass butt-welded 12Cr1MoV/P91 dissimilar pipe. X-ray technique is used to validate the FF results. Both methods show that residual stresses are significantly unsymmetrical distributed with respect to weld center line (WCL), and peak stresses exist on the inner surface of P91 part. The simulated results are especially in good greement with experimental ones within the weldment and its vicinity. Results for repair welds covering different circumferential angular are also presented to investigate the influence of repair length on residual stresses. It shows the shorter repair develops higher residual stresses, and this effect is more significant within the repairing area than the other regions.