SMALL CRACK GROWTH BEHAVIORS AND CLOSURE EFFECTS IN A NICKEL-BASE POWDER METALLURGY SUPERALLOY AT HIGH TEMPERATURE

Crack closure effects play an important role in dominating small crack propagation behaviors, which were rather less well investigated, especially at high temperature in the air. Based on photomicroscopy and digital image correlation, small crack growth behaviors and growth rates are investigated both at 600℃ and RT in air for a Powder Metallurgy superalloy, then the crack displacement fields are measured. Two max. stress levels and two stress ratios are considered in order to understand their effects on small crack growth behaviors. The experimental results reveal the crack growth behaviors ranged from 80 m to ~1000m. With the help of EBSD at the grains of the crack growth path, links of this particular growth behaviors with the microstructure features, such as the orientation, grain boundary, are discussed. Using DIC-measured crack opening displacement with the crack growth, the roles of oxides and roughness induced crack closure in early small crack propagation at high temperature are analyzed. Finally, a crack closure model is proposed including the combined effects of oxide-induced, roughness-induced and plasticity-induced crack closure (OICC, RICC and PICC).
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20 KHZ CRACK GROWTH RATE TESTING IN ADVANCED HIGH STRENGTH TOOL STEELS

High nitrogen chromium alloyed PM tool steels display, among other attractive properties, high strength and improved corrosion resistance. Also, powder metallurgy is a manufacturing process that allows the fabrication of near net shape complex geometries and high-quality components in an economical way. Nitrogen acts in an effective way replacing the carbon by the formation of hard carbonitride phases and permits higher amounts of chromium in solid solution. The low carbon and high nitrogen contents, where most of the carbon is replaced by nitrogen, suppresses the formation of metal carbides is in favor of the formation of metal nitrides (MX) and carbonitrides (M2X) allowing a higher nominal amount of chromium in solid solution. As a result of the PM rout well dispersed and fine distribution of nitrides and carbonitrides is created in a martensitic matrix. The microstructure obtained controls the mechanical properties of the steel grade, and a balance of high strength and high toughness is required. In the present study the influence of the nitride and carbonitride distributions on the fatigue properties, and in particular the stress intensity threshold and the early crack growth, was investigated using a 20 kHz ultrasound test equipment.
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MIXED FINITE ELEMENT METHOD FOR FRACTURE MODELING OF PIEZO- AND FERROELECTRIC MATERIALS WITH STRAIN GRADIENTS (FLEXOELECTRICITY) [Keynote]

Following the continuous miniaturization of the microelectromechanical systems (MEMS), a size-dependent phenomenon of flexoelectricity starts to play an essential role at the micro- and nanoscale. Direct flexoelectricity is an electromechanical coupling of strain gradients, which are inversely proportional to the length scale, and electric field. Due to the application of strain gradients, the centrosymmetry of the unit cells is broken, allowing a wider choice of dielectrics to be used in applications. In the proposed research, the nonlinear ferroelectric material behavior is further enhanced with strain gradients and applied to fracture problems with naturally occurring gradients of electromechanical fields near the crack tip. Or in another way, it is the incorporation of the remanent strains and polarization into the flexoelectric formulation. Our solution demonstrates the strong influence of the gradients on the ferroelectric domain switching behavior, leading to modified electromechanical fields close to the crack tip compared to the well-known ferroelectric problems.
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CHARACTERIZATION OF ICE ADHESION: MODES OF LOADING AND MICROSTRUCTURE

We present fracture mechanics-based approaches to characterize interfacial fracture parameters for the tensile and shear behavior of a typical ice/aluminum interface. An experimental framework employing single cantilever beam, direct shear, and push-out shear tests were developed to achieve near mode-I and near mode-II fracture conditions at the interface. Both analytical (beam bending and shear-lag analysis), and numerical (finite element analysis incorporating cohesive zone method) models were used to extract mode-I and II interfacial fracture parameters. The combined experimental and numerical results, as well as surveying published results for the direct shear and push-out shear tests, showed that mode-II interfacial strength and toughness could be significantly affected by the test method due to geometrically induced interfacial residual stress. As a result, the apparent toughness of the zero-angle push-out test could reach an order of magnitude higher than those derived from direct shear tests. Moreover, it was found that the interfacial ice adhesion is fracture mode insensitive and roughness insensitive for tensile and shear modes, for the observed modes of failures in this study.
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GROWTH AND COALESCENCE OF MULTIPLE CRACKS – EXPERIMENTS AND FRACTURE MECHANICS BASED MODEL

Short crack growth tests are carried out on the coarse-grained nickel-based cast alloy Iconel 100 (IN100) and two microstructures of the austenitic stainless steel AISI 347 using the replica technique. IN100 is tested under TMF and AISI347 isothermally. For both materials, several cracks are found which grow together to form the final main crack. Atypically, the final main crack length does not develop exponentially. To describe the damage evolution of the final main crack, a model is developed based on inelastic fracture mechanics, which includes the different crack driving forces along the crack front, and applied to the test results.
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CHARACTERIZATION OF THE DAMAGE TOLERANCE OF NANODESIGNED COATINGS BASED ON HIGH ENTROPY ALLOYS

Coatings are primarily designed to offer excellent wear and corrosion resistance. However, these properties are adjusted at the expense of the damage tolerance of the materials applied. By introducing the concept of high entropy alloys new property combinations are expected. In this contribution coatings composed of purely refractory HEA nitride as well as coatings containing non-refractory elements such as Al or Si will be presented. The focus is on a comparison of different experimental strategies to evaluate the damage tolerance of these coatings.
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A HYBRID EXPERIMENTAL AND NUMERICAL INVESTIGATION ON THE FRACTURE PROPERTIES OF ZIRCONIUM WITH MAX PHASE COATINGS COVERING A WIDE RANGE OF STRESS STATES

This work aims to carry out a hybrid experimental and numerical investigation on the fracture properties of the zirconium cladding tube coated with Cr2AlC, which belongs to the group of MAX phase materials. A macroscopic failure criterion is finally developed based on the experimental and numerical simulation results, thus contributing to the design of the accident-tolerant fuel system (ATFs) in nuclear power plants. A series of in-situ bending tests involving various sample geometries covering a wide range of stress states are carried out under a quasi-static condition. Oxidized samples and samples aged in hot water under high pressure are also involved to consider the aging and oxidation effect on material failure. The modified Bai-Wierzbicki (MBW) damage model and the analytical Yoon2014 model are coupled in the simulation so the damage and strength differential effect can be considered in modeling material failure. By transferring boundary conditions between the micro- and macroscopic model as a weak macro-micro coupling, homogenization is achieved so that a micromechanical sub-model can also be developed and the micromechanical simulation and macroscopic simulation can be cross-scale bridged.
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