Papers by Keyword: Creep Fracture

Abstract: Damage and fracture processes in high temperature creep of an investment cast B1914 Ni-based superalloy with the increased amount of boron to 0.08wt.% for high temperature applications were analysed. Constant load creep tests in tension were conducted at temperatures from 800 to under applied stress ranging from 150 to 700 MPa. The microstructure of fractured specimens was investigated by scanning electron microscope Tescan equipped with an electron-back scatter diffraction. Microstructure investigation showed that the microstructure of the B1941 superalloy consists of a gamma (γ) phase with a dendritic structure and gamma prime (γ ́) phase with a cuboidal shape. Precipitates of γ ́and a lamellar eutectic, composed of γ/(Mo,Cr,Ni)₃B_2, were identified in the interdendritic region. Creep damage and fracture are closely connected with decohesion of the interface between M₃B₂ boride and matrix.

Abstract: The microstructure and creep behaviour of the welded joints of P92 steel pipe were investigated in order to determine the influence of orbital heat welding technology on the creep resistance. Creep specimens were machined from the welded joints. Tensile creep tests of welded joints were performed at 873 K using different stresses. The microstructure of tested specimens was investigated by scanning electron microscope Tescan equipped with an electron-back scatter diffraction. The creep results showed that the creep fracture strain of the welded joints decreases with decreasing value of applied stress. Microstructure investigation showed that fracture behaviour of welded joints is influenced by an enhanced cavity formation at grain boundaries in the heat-affected zone causing lower fracture ductility.

Abstract: Interfacial creep fracture behavior of sandwich composites consisting of E-glass/ unsaturated polyester (UPE) resin and E-glass/vinyl ester (VE) resin facings over PMI foam core has been experimentally investigated. Digital image correlation (DIC) method was employed in the double cantilever beam (DCB) test to measure the creep displacement field surrounding the crack tip. Crack opening displacements (COD) at various creep time were extracted from the creep displacement field and mode I stress intensity factors were determined using the small region COD-based linear extrapolation method. Significant increments of COD and nominal stress intensity factor are found in specimen with UPE resin and specimen with VE resin after 24 hours creep test. It is also found that specimen with UPE resin has a better interfacial creep fracture resistance than that of specimen with VE resin.

Abstract: By means of creep properties measurement and microstructure observation, an investigation has been made into the damage and fracture mechanism of a nickel-based single crystal superalloy during creep at moderate temperature. Results show that the deformation mechanism of the alloy in the latter stage of creep is that the primary-secondary slipping systems are alternately activated, and the micro-crack is firstly initiated on the γ′/γ phases interface in the intersection regions of two slip systems. As creep goes on, the micro-crack is propagated along the γ′/γ interface, which is perpendicular to stress axis, to form the square-like cleavage plane on the (001) plane. Thereinto, the propagation of the cracks on (001) plane is intersected with {111} cleavage plane which is secondly activated, which may terminate the propagation of the crack to form the cleavage plane with square-like feature on (001) plane along the <110> directions. Due to the multi-cracks may be propagated on different cross-section of the alloy during creep, and the tearing edge or secondary cleavage plane are formed along the direction with bigger shearing stress at the crack tip, which makes the multi-cracks connected each other until the occurrence of creep fracture, this is thought to be the main reason of the creep fracture having the uneven and multi-level cleavage characteristics.

Abstract: Fracture morphology analysis is an important way to understand material fracture process, find the reason of fracture and improve its performance. Nickel based powder metallurgy (P/M) superalloy is an important material which is widely used in the modern aeronautic and astronautic industry. In order to determine the fracture characteristic morpholgies and mechanisms of the P/M superalloy fractured at different conditions, the fracture surfaces of the high temperature low cycle fatigue,high cycle fatigue, high-low cycle complex fatigue, tension and creep specimens from the P/M superalloy were studied by scanning electron microscopy and stereo-microscopy. The mechanisms and influencing factors of high temperature fatigue crack initiation and propagation were emphatically analyzed, and creep and tension fracture mechanisms of the P/M superalloy were also discussed. The application of the fracture surface analysis for the fracture failure analysis of the P/M superalloy turbine disk were introduced finally.

Abstract: The mechanical behavior of rock interior creep fracture evolution and the state are difficult to observe. Combined with the laboratory rock compressive-creep test, the uniaxial creep numerical test based on FLAC3D was carried out. The different positions of the model displacement, stress, strain and damage zone were analyzed under different loading. The mechanical behavior of evolution process and characteristics of rock creep fracture were revealed under the different loading. The results show that: the data of numerical simulation and laboratory test are identical by comparing with laboratory test, and numerical test on simulated rock creep mechanical behavior evolution and methods is feasible and effective. It is a new method that can improve laboratory test of the microscopic creep, provide basis for evaluation of engineering rock mass stability too.

Abstract: The standard power law approaches widely used to describe creep and creep fracture behavior have not led to theories capable of predicting long-term data. Similarly, traditional parametric methods for property rationalization also have limited predictive capabilities. In contrast, quantifying the shapes of short-term creep curves using the q methodology introduces several physically-meaningful procedures for creep data rationalization and prediction, which allow straightforward estimation of the 100,000 hour stress rupture values for the aluminum alloy, 2124.

Abstract: Polymers are vital materials in better performance of specific strength. However their application can be restricted by the lower glass transition temperature, Tg. Some polymers have been developed as engineering plastics for the high temperature applications. We examined the high temperature strength of polymers at constant applied stress. The creep rupture and viscoelastic behavior were scrutinized for PC (polycarbonate) and PMMA (polymethyl methacrylate), which were quite different in the molecular structures. The former contains benzene rings and the latter is a single polymer. Tg is 423 K for PC and 378 K for PMMA. The large difference in the creep behavior was observed near Tg. The creep life strongly depends upon the applied stress just below Tg. The creep life is a function of the applied stress as follows. n life t − µ s . The stress exponent, n depends upon the temperature. Mechanical models were applied to evaluate the viscoelastic properties of the polymers at high temperatures. The viscosity rapidly decreased near Tg , regardless of the smaller decrease in the elastic constant. The results would be due to the difference in the molecular structures. The benzene ring could contribute to the higher resistance against the creep deformation through the higher viscosity.

Abstract: The tensile creep behavior of NiAl-9Mo eutectic alloy has been investigated over a stress range of 50 to 100MPa at the temperatures ranging from 850 to 950°C. All of the creep curves exhibit the very long steady-state stage. The creep parameters and TEM observations indicates the kinetics of the steady-state creep deformation is governed by dislocation climb in the NiAl matrix phase. The crack origination and development at the colony boundary results in the onset of tertiary creep stage and final fracture of the alloy.