Mechanical Properties and Microstructure of Shotcrete under High Temperature

High temperature is recognized as one of the extreme environments in the application of shotcrete which significantly deteriorate the performance of shotcrete. This paper reviews the mechanical properties and microstructure of shotcrete under high temperature. First of all, this paper reviews the cause of formation of high ground temperature. Based on these causes, the author establishes a heat transfer model with a spiral shape by introducing a multidimensional morphological formula into the heat conduction process. Then, the paper reviews the influence of high temperature on the mechanical and micro properties of shotcrete, the cooling technology under high temperature, and the optimization research technology of shotcrete. The author discusses the influence of high temperature on the thermal parameters and the deformation of shotcrete from the perspective of thermodynamics. Multiple studies have shown that the irregular movement and disorderly overlapping of molecules in the shotcrete caused by the high temperature environment result in the premature termination of the hydration reaction of cement in shotcrete. Finally, the author suggests the challenges of high-temperature shotcrete in term of the process structure, performance optimization, and application in special engineering fields. The research in this paper intends to give guidance to those conducting shotcrete research under high temperature, and to promote the further development of shotcrete technology.

Rolling Ironstones from Earth and Mars: Terrestrial Hydrothermal Ooids as a Potential Analogue of Martian Spherules

High-resolution images of Mars from National Aeronautics and Space Administration (NASA) rovers revealed mm-size loose haematite spherulitic deposits (nicknamed “blueberries”) similar to terrestrial iron-ooids, for which both abiotic and biotic genetic hypotheses have been proposed. Understanding the formation mechanism of these haematite spherules can thus improve our knowledge on the possible geologic evolution and links to life development on Mars. Here, we show that shape, size, fabric and mineralogical composition of the Martian spherules share similarities with corresponding iron spherules currently forming on the Earth over an active submarine hydrothermal system located off Panarea Island (Aeolian Islands, Mediterranean Sea). Hydrothermal fluids associated with volcanic activity enable these terrestrial spheroidal grains to form and grow. The recent exceptional discovery of a still working iron-ooid source on the Earth provides indications that past hydrothermal activity on the Red Planet is a possible scenario to be considered as the cause of formation of these enigmatic iron grains.

Study of the nature and causes of damage of 40KhN2MA steel propshafts

The nature and character of the destruction of the high-loaded airscrew shaft of the gas turbine engine made of steel 40KhN2MA are studied using the methods of scanning electron and optical microscopy and X-ray spectral microanalysis. We determined the chemical composition, microstructure and character of destruction of the flange part of the shaft. It is shown that chemical composition of the shaft material meets the requirements of the regulatory documents and defects of metallurgical origin are absent both in the metal of the shaft and in the fatigue fractures. All the identified cracks have been formed during operation, and developed according to the fatigue mechanism. Zones of fretting-wear and fretting-corrosion damage to the surface in the stud holes are the foci of the fatigue crack birth in the airscrew shaft. The cause of formation and development of the fatigue cracks in the shaft flange is a high level of contact stresses in the flange body, attribute to a poor quality of the interface of the splined joint due to significant fretting corrosion damage to the splines and vibration damages. A set of recommendations and measures aimed at elimination of the adverse factors is elaborated. To exclude the formation and development of fatigue cracks in the splined flange upon operation, we recommend to: optimize the tightening torque of the splined joint; introduce quality control of the interface between contact surfaces of the splined joint when replacing the airscrews; address the issue of dynamic balancing of the airscrews upon their production and repair.