Papers by Keyword: Thermal Damage

Abstract: Nonlinear Ultrasonic Spectroscopy (NUS) methods investigates nonlinear phenomena that occur when the sound or ultrasound waves pass through the material. This paper focuses on the use of these methods for nondestructive testing of reinforced concrete damaged by high temperature. For this research, reinforced concrete beams with one steel rod were made and the NUS measuring equipment was assembled. An appropriate nonlinear parameter was selected to assess the damage. The nonlinear behavior of thermally damaged reinforced concrete was manifested by the deformation of ultrasound waves passing through the measured samples, resulting in nonlinear effects in the frequency spectra of the recorded signal.

Abstract: The paper presents the results of the experimental work which was focused on the use of the Impact-echo method for testing of concrete specimens damaged by high-temperature. The test specimens were heated to 600 and 800 °C for sixty, one hundred twenty and three hundred minutes. After heating, the samples were continuously cooled to room temperature and tested by Impact-echo method.

Abstract: The article discusses experimental research and simulation testing on prototypical foil bearings. All experimental tests were conducted on a special test rig which makes it possible to operate in various conditions and within a wide range of speeds. As a result of the study, it turned out that adverse operating conditions caused almost instant bearing damage, accompanied by a significant rise in temperature. The main factors affecting the durability of foil bearings were material covering mating surfaces, bearing geometry, way of assembling the bearing, rotational speed and load. To better understand the physical phenomena occurring in foil bearings, a numerical model has been developed which allowed carrying out thermal analyses. The analysis of heat flow in the bearing's structure showed that, because of the system geometry, significant problems with proper removal of large amounts of heat continued to be experienced, which may have led to an accelerated rate of fatigue damage and shorter bearing life. This phenomenon can occur in bearings operating under tough conditions (e.g. at low speeds or under heavy loads). The research showed that the development of a new foil bearing is a very difficult task and requires many aspects to be taken into account, including the aspects directly related to the operation of the bearing itself, as well as those related to the rotor's operation and characteristics of the machine.

Abstract: The new processing technique of vacuum evaporative pattern casting (V-EPC) process was explored to fabricate the cast-iron bonded cBN grinding wheels in this paper. The influences of pouring temperature, degree of vacuum, percentage and size of the cBN grits on the microstructures, distribution and thermal damage of cBN grits in the grinding layer, as well as the surface quality of the grinding layer were investigated. The experimental results revealed that thermal damage of the cBN grits and severe damage of nodulizer occurred when the pouring temperature was around 1480°C. The optimized pouring temperature for fabricate the austenite-bainite ductile cast-iron bonded cBN grinding wheels was about 1400°C. The appropriate degree of vacuum was 0.06Mpa. Too high or too low level of degree of vacuum would result in low surface quality of the grinding layer. The uniformity of the distribution of cBN grits in the metal matrix improved with the increase of the percentage and size of the cBN grits.

Abstract: Engineering ceramics have received significant attention in the recent years owing to their exceptional mechanical properties, which are expected to be beneficial for engineering applications. However, it has always been a great challenge to realize ductile-mode grinding in engineering ceramics, with one of the critical obstacles being the heat generation that limits the removal rate. As a result, thermal damages are often observed on the ground surfaces. This paper presents the ductile-mode grinding. In the process the grinding wheel is excited along the radial direction by applying an ultrasonic vibration of frequency is 38.5 kHz and amplitude of 0-2 μm. The wheel comes in contact with the Al₂O₃ ceramic at constant forces 18-24 N. Experimental results indicate that the ground surface is devoid of thermal damages, when removal rate of the vibration-assisted process is approximately 1.5 times higher than without vibration.Keywords: ductile-mode grinding, radial directional vibration, hard and brittle material, engineering ceramic, thermal damage

Abstract: High temperature in grinding can cause thermal damage to the workpiece. In order to reduce the grinding zone temperature, a method about enhancing heat transfer based on the rotation heat pipe technology is proposed. In this paper, the heat transfer effect of heat pipe grinding wheel (HPGW) under different conditions such as the rotating speed of grinding wheel, the kinds of working fluid and the film thickness are analyzed. The results show that the HPGW has great superiority of heat transfer under certain conditions.

Abstract: Based on the theory of the thermal conduction and the thermal elastic equations, does some researches in view of the thermal Damage of K9 glass by finite element analysis. The distribution of temperature field and thermal stress field of the ideal K9 glass which irradiated by 100W pulsed-Nd: YAG laser are simulated with finite element software. The result shows that the temperature of the surface of glass doesnt reach the melting and the thermal stress of it doesnt get to the requirement for fracture strength. Then, the model of K9 glass containing inclusions has be built. The simulation result indicates that inclusions may affect the distribution of temperature field and thermal stress field of the K9 glass, which makes the injected laser beams energy centralize on a small area around the inclusions owing to the existence of the strong absorbing, thus leading to K9 glass laser damage threshold reduction and easier to thermal damage.

Abstract: In order to investigate the effect of temperature (from normal temperature to 850°C) coursed by fire on the strength damage of concrete, thermal compression tests for concrete specimens named C35 have been performed under different temperature conditions. Emphasis is laid on the relationship between temperature and thermal damage strength; and the relevant formula is proposed in this paper. The regularity and mechanism of thermal damage evolution in concrete on strength under high temperature are analyzed. Combined the result of tests with the residual strength thermal damage model, we obtain the specific damage variable value D under different temperatures. Finally, we compare the fitting formula curve to relevant reference; there are some important conclusions which can be partly applied to fire design of concrete structure.

Abstract: Damage mechanics is introduced into the fire response calculation of the concrete structure. The damage mechanics equations for fire response calculation are established. They are the damage evolution equation based on “residual strength” theory, heat conduction equations, and elastic mechanical equations. The fire response calculation of a concrete slab under external load and fire is shown. ANSYS is selected for calculating. The temperature field and stress field are obtained, the damage and failure process are described using the technique of killing or activating elements in ANSYS, and the fire resistance of the slab is obtained.

Abstract: Laser cladding act as a remanufacturing technology used more and more but the study of the fatigue properties of such products falls relatively behind. To work out the influence fatigue damage and thermal damage have on the fatigue life of laser cladding remanufactured products, this paper deals with the study of the characteristic of metal magnetic memory (MMM) signals of components experienced with fatigue test and a succeeding laser surface burning. The MMM signals was acquired for three times, that is before fatigue test, after fatigue and after laser treatment, respectively. Statistic investigation of the experiment result showed that the magnetic signal was closely related to the state of fatigue damage and thermal damage of the specimen. Based on such result, it is expected to quantify the study by statistical analysis of large samples so as to find a correlationship with the last fatigue life.