PSI - Issue 81

Viktor Kovalov et al. / Procedia Structural Integrity 81 (2026) 297–304

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Fig. 1. Main types of cutting tools used on heavy-duty lathes

Such a variety of designs is due to the need to ensure a balance between tool rigidity, chip removal efficiency and ease of insert replacement in difficult cutting conditions. The most common materials processed on heavy-duty lathes are alloy and carbon steels, which account for about 60% of all cases. They are characterised by high hardness and strength (35 – 45 HRC), which causes intensive wear of the cutting edge due to increased friction and cutting temperature. Typical machining conditions are: cutting depth a(p) = 10 – 19 mm; feed f = 0.8 – 1.0 mm/rev and cutting speed v = 75 – 110 m/min. Under these conditions, the cutter is subjected to a combination of mechanical impact and thermal loads, which accelerate degradation processes. Statistical analysis of tool failures has shown that most failures are caused by wear or sudden destruction of the carbide plate. The variability of stability indicators is determined by three main groups of factors: 1. Random fluctuations in cutting force associated with unstable chip formation and vibrations during machining; 2. Microstructural heterogeneity of the hard alloy, causing local stress concentrations and the formation of microcracks ; 3. Technological errors in tool installation, including cutter misalignment or uneven coolant supply, which affect the thermal regime. The stochastic nature of these factors leads to significant dispersion of results: the coefficient of variation of stability reaches 0.9, which indicates a high degree of randomness in durability indicators even for cutters of the same design and material. Thus, deterministic approaches to reliability prediction are insufficient, and probabilistic modelling is required for an adequate description of wear and failure processes. Typical types of failures of carbide inserts in severe turning conditions include: complete or partial chipping of the cutting edge; - multiple thermal cracks; plastic deformation or rounding of the cutting zone; sudden plate breakage; delamination of the carbide layer or coating. The combination of high mechanical loads, vibrations and uneven temperature distribution creates a random operating environment for cutting tools. These factors justify the need for a probabilistic approach to reliability modelling. 2.2. Probabilistic model of cutting tool reliability A probabilistic reliability model for cutting tools has been developed to describe the wear and failure processes of assembled turning tools during operation under severe turning conditions. It combines analytical and stochastic methods, allowing the probability of failure-free operation of the tool to be assessed, taking into account random load fluctuations, material properties and degradation processes. A composite turning tool is considered as a technological system consisting of a body that may experience random mechanical failures due to overload or material fatigue, and a replaceable carbide plate prone to abrasive wear, chipping, and thermal cracks. The total reliability of the tool is determined as a combination of the reliabilities of these elements and is stochastic in nature. The degradation process of the cutting part is described by a random variable X(t), which increases with operating time under the influence of thermomechanical loads. Failure occurs when X(t) exceeds the limit value X cr . The probability of failure-free operation at time is defined as R(t)=P[X(t) ) = 1 − ∫ ∞ 0 ( ) ( ) where f S (S) - is the load distribution density, and F R (S) — is the integral function of the material resistance distribution. The stochastic nature of the change in the operating states of the assembled cutter is described by a semi-Markov process with three states: serviceable state (the tool is fully operational), degradation state (partial wear or microdamage is observed), and failure state (the tool is unsuitable for operation). The probabilities of transitions between these states depend on the load parameters, cutting temperature and operating time and are determined by the ratio Q ij (t)=P{X n+1 =j,T n+1 − T n ≤t ∣ X n =i}.