Experimental study of remaining creep life of SA-304L stainless steel using small punch creep test

In this paper, the remaining creep life of SA-304L stainless steel at elevated temperatures is studied. The small punch creep tests (SPCT) were performed on SA-304L virgin and aged materials at constant loads. Times to fracture and the minimum deflection rate in SPCTs were recorded, and the time-temperature parametric analysis was performed, based on experimental results. The constants of the Larson–Miller parameter and the Monkman–Grant relationship were obtained for different consumed creep life ratios, and eventually the equations were established to explain the variation of constants of the Larson–Miller parameter and the Monkman-Grant relationship with respect to the consumed creep life ratio.     

Predicting the tensile creep of concrete

Over a four year period, six phases of testing were performed to observe the influence of age at loading, applied stress level, mix composition and relative humidity on the tensile creep of concrete. From these investigations it was possible to develop a model which allowed the prediction of tensile creep based on a knowledge of the compressive strength of the concrete (determined at the age of loading), the applied stress level and the relative humidity. Subsequently, this model was validated using the results from three independent investigations. Compressive creep as well as tensile creep was also obtained. This allowed a comparison of compressive creep with tensile creep and illustrated that on the basis of equal stresses, tensile creep is on average between 2 and 3 times greater than compressive creep (the maximum ratio is in excess of 8). For this investigation, however, on the basis of stress/strength ratio the difference between tensile and compressive creep is less significant. Considering a simply supported flexural reinforced concrete element, the investigation suggests that it is unwise to consider actual compressive creep equal to actual tensile creep as is often the case in design practice.     

羽绒棉纤维的性能测试及其静态热性能研究

测试了羽绒棉纤维(细旦涤纶)的直径、纤度、强力,分析了纤维的应力松弛和蠕变性能对时间的依赖性及定伸长变形性,比较了其与木棉纤维的静态热性能,结果表明:羽绒棉纤维纤度平均值为0.8 dtex;60 s内应力松弛和蠕变率增加,对时间依赖性强;羽绒棉纤维的塑性变形率高;与木棉纤维相比,羽绒棉纤维在较高温度区域的导热性比木棉纤维的好,而降温速率明显低于木棉纤维。     

Static fatigue of Hi-Nicalon™-S fiber at elevated temperature in air,steam, and silicic acid-saturated steam

A facility for testing SiC fiber tows in static fatigue and creep at elevated temperatures in air and steam was developed. Static fatigue of Hi-Nicalon™-S fibers was investigated at 800°C-1100°C at applied stresses between 115 and 1250 MPa in air, in Si(OH)₄(g)-saturated steam, and in unsaturated steam. Fibers tested in Si(OH)₄(g)-saturated steam and in air had silica scales throughout the test sections, but those tested in unsaturated steam did not develop scales near the steam injection point. Fiber lifetimes in static fatigue were shortest in unsaturated steam, intermediate in Si(OH)₄(g)-saturated steam, and longest in air. Failure strains did not exceed 0.3%. Steady-state strain rates and static fatigue lifetimes are modelled empirically by the Monkman-Grant relationship. Failure mechanisms are discussed.     

Study on the failure behavior of the current interrupt device of lithium-ion battery considering the effect of creep

The working condition of the CID (current interrupt device) has an important impact on the safety of the prismatic lithium-ion batteries. One of the important factors that causes the failure of the prismatic power battery is the overturning of CID due to material creep. The tensile and creep tests of the MFX2 aluminum alloy which is the material of CID were designed. Based on the experimental data the isotropic hardening multilinear elastic-plastic constitution and creep constitution at the battery working temperature were established. The constitutions were also verified by the notch test. The finite element model of the CID and the cap was established to analyze the effect of creep on the failure behavior of the CID throughout life. The simulation tests in the constant temperature and constant load of CID is designed. The simulation results showed that the constitution characterizes the basic mechanical properties of the CID reasonably. During the life of the CID, the creep behavior would go through the fast stage firstly and then fall into the slow steady-state stage. The creep effect of the material would cause the CID to overturn in advance, overturning pressure would be reduced, the life of the power battery would also be reduced.     

Effect of interface morphology on the residual stress distribution in solid oxide fuel cell

Solid oxide fuel cell directly and efficiently converts chemical energy to electrical energy. However, the necessity for high operating temperatures can result in mechanical failure. Fuel cell is a multilayer system and its stress distribution is greatly affected by the interface morphology. In this work, cosine interfaces with different amplitudes are used to approximate the fluctuation of actual interface. The effects of interface morphology on stress state, energy release rate of crack and creep behavior have been investigated. The results show that if the interface is planar, the residual normal stress component is zero on the interface, while the nonplanarity of interface can cause the normal stress S_n and shear stress S_t on the interface. When the amplitude is relatively small, the max values of S_n and S_t on the interfaces vary linearly with increasing amplitudes in both anode and cathode. Above a certain value, nonlinearity of the interface becomes important. Max tensile S_n always occurs at the peak of convex interface, but the position of max compressive S_n varies. Max shear stress is prone to occur at 1/4 of the wavelength at small amplitude and moves towards 1/2 of the wavelength when the amplitude increases. Fracture mechanics analysis shows that the surface crack possibly penetrates into the anode function layer and then is constrained by the stiff electrolyte. On the other hand, the horizontal crack likely penetrates into the electrolyte layer when the interface is not planar. Creep analysis shows that 11 800 hours of continuous operation at high temperature cannot remove stress undulation introduced by nor-planar interface but can make max value of Sn and St decrease around 30%.     

Influence of initial ovality on creep life of P92 pipe bends subjected to in-plane bending

Initial ovality is an inevitable problem in the process of pipe bends manufacturing which results in the stress redistribution of the pipe bends working at high temperature. In order to study the influence of ovality on creep life of pipe bends, full-size creep experiment of P92 pipe bend subjected to in-plane bending has been conducted. The creep strains and outside diameters of dangerous positions have been measured. The microstructures of three different positions of the pipe bend were compared through SEM and the results showed the number and size of the carbide precipitation were the largest at the flank of the pipe bend, which indicated that the creep damage developed fastest at the flank. The modified Kachanov–Robatnov constitutive equations were used to stimulate the creep of P92 pipe bends with FEA software. The representative stress, damage and multiaxiality distributions of the pipe bends have been discussed. The FEA results were consistent with the experimental results and the influence of initial ovality on creep life of P92 pipe bends were analyzed. The results showed that creep life of pipe bends reduced by the increase of ovality and their relationship coincided with the parabolic law.