Study of the stress-strain and temperature fields in cutting tools using laser interferometry

OBRABOTKAMETALLOV MATERIAL SCIENCE Том 23 № 3 2021 EQUIPMEN . INSTRUM TS Vol. No. 4 2021 The method involving deposition of brittle coatings on the object under study is unsuitable for recording dynamic deformations, has signi fi cant measurement errors, and can only be used for qualitative analysis. The grid method [3] is dif fi cult to be used with high-strength tool materials because it is dif fi cult to determine changes in the grid node distances caused by the limited deformation of these types of materials. The noncontact mirror method evaluates the effect of grid line distortion due to re fl ection from the deformable body, but has suf fi cient sensitivity only when measuring bending deformations. The moiré fringes method is labor-intensive because obtaining and using gratings is a complex process. Digital image correlation [4] makes it possible to automate analysis of the results, but the relatively low sensitivity of this method means that it can only be used for studying deformation of the workpiece material. The photoelasticity method [5] does not allow experiments with real tool materials, and can be applied only at an extremely low cutting speed when processing workpieces are made of soft materials, owing to the low heat resistance of optically active materials. The photoelastic coating method partially solves these problems, but the peeling of coatings in areas with a high deformation gradient leads to increased measurement errors. The shadowmethod (the caustic method) [6] is distinguished by the complexity of decoding the resulting pattern, especially in the case of a complex stress state. The holographic interferometry method [7] is highly sensitive and can be applied to objects with complex shapes. When using double exposure, the measurement accuracy is high, but continuous dynamic processes cannot be recorded. When implementing the real-time method, it is necessary to isolate the study object from external vibrations. The method of electronic speckle pattern interferometry [8] makes it possible to measure deformations not only in the direction normal to the surface of the study object, but also along its plane [9]. However, at the same time, the resolution and minimum dimensions of the investigated surface are signi fi cantly limited. The laser interferometry method makes it possible to register strain and stress fi elds with a high gradient, not only on transparent models, but also on real objects. The disadvantages of this method are the complexity of registering rapidly changing interference patterns while studying dynamic processes and the problems related to their interpretation. The general advantages of optical methods are that they are contactless, highly sensitive, and inertia-free measurement processes. In studying tool temperature states, contact methods using various types of thermocouples are the most widely used. However, it is dif fi cult to measure temperatures near the tool-chip contact zone using embedded arti fi cial thermocouples. Cut or running thermocouples [10] can only be used to determine the nature of the temperature distribution on the tool faces at low cutting speeds. The semi-arti fi cial thermocouple method is time-consuming, and the use of a split tool distorts the temperature fi eld [11]. The natural (tool-work) thermocouple is applicable only to conductive materials. It allows only the average value of the temperature in the cutting zone to be determined, requires initial calibration, and has low accuracy. Using fi lm microtransducers that are based on resistance thermometers [12] does not solve the problem of obtaining a temperature fi eld due to the dif fi culty of positioning a large number of sensors on the cutting tool. Methods that assess the appearance of oxide layers in the air (tempering colors) and irreversible structural changes in the material, including microhardness, make it possible to record only the maximum temperature that appears during the experiment. In addition, changes in the microstructure of heated tool materials, such as cemented tungsten carbide, are insigni fi cant. Thermosensitive coating methods ( PVD fi lms [10] and thermal paints [13]) have high inertness and low accuracy in measuring temperature fi elds due to differences in the thermophysical characteristics of the coating material and the study object material, and the processes of heat transfer between them. The accuracy of infrared thermometry (thermography) methods depends primarily on the accuracy of the experimentally determined emissivity coef fi cient of the study surface, which can be affected by increasing temperature, surface roughness, and degree of oxidation [14]. The problem of determining and taking into account the change in the emissivity coef fi cient during heating can be partially solved by using two-color thermometry [15]; however, the in fl uence of surface quality and the degree of oxidation on measurement accuracy remains unaffected. Because the study surface interferes with the oxide fi lms, a false