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Aramid fiber–reinforced polymer composite (AFRPC) is popular in aerospace and defense industries owing to its superior thermal and mechanical properties. However, its intricate hexagonal cellular structure and the material’s heterogeneous, soft, and brittle characteristics lead to significant surface defects, such as burr formation, wall tearing, roughness, dimensional inaccuracies, and uncut fibers during traditional machining. Such poor machining quality issues notably affect the operational lifespan and functional performance of its sandwich structural components. To address these issues, the rotary ultrasonic assisted machining (RUSAM) process has been introduced. To thoroughly investigate the RUSAM of AFRPC using various cutting tools, a 3D finite element model was developed and validated. This paper mainly investigates the effect of various machining parameters such as vibration amplitude (VA), cutting width (CW), feed rate (FR), and spindle speed (SS) on the cutting force, surface morphology, burr formation, and burr height during RUSAM of AFRPC structure by plane and toothed disc cutters. The burr height was found to decrease with the increase of spindle speed (60.82% and 71.00%) and vibration amplitude (78.15% and 82.32%), whereas increase with cutting width ( 149.81 % and 321.16%) and feed rate (156.53% and 314.83%) during RUSAM by plane and toothed disc cutters, respectively. The pattern of variation of burr height with machining parameters was found similar to that of the cutting force. Significance analysis based on 4 levels, 4 factors orthogonal L 16 ( 4 4 ) experiments revealed the cutting width to be the most influential parameter on the burr height and cutting force followed by the spindle speed, feed rate, and vibration amplitude during RUSAM of the AFRPC core by the disc cutters. Up to 62.54 % reduction in burr height was realized by rotary ultrasonic assisted machining compared to the conventional machining. Under specified operating conditions, the disc cutter generates a higher but less number of burr as compared to the toothed disc cutter without any tearing defects. 3–10% and 5–20% burrs were observed during rotary ultrasonic assisted machining compared to 20–50% and 40–70% burrs during conventional machining of AFRPC structure by plane and toothed disc cutters, respectively. This experimental research will be extremely useful to comprehend the burr formation mechanism and optimize the machining parameters for enhanced surface morphology of AFRPC structures.

When shield tunnelling is constructed in complex geological conditions using a tunnel boring machine, the disc cutter in the cutterhead easily wears to the failure state, particularly when the ground conditions are heterogeneous. This paper summarises the failure modes of the disc cutter in heterogeneous ground conditions into three categories, based on the observed wear data from field: (1) uniform disc cutter wear, (2) non-uniform disc cutter wear, and (3) breakage of cutter ring. Subsequently, the stress state of a disc cutter in the heterogeneous ground was analysed and the effective factors were investigated. The relationships between friction energy during cutting, working status of the machine and the characteristics of the geological conditions were evaluated. Based on the stress analysis and friction energy, a prediction model was proposed. The proposed model was applied to two field case studies: pertaining to uniform and mixed-face ground conditions, for which the empirical coefficient k for energy transfer was also determined. The preliminary results from this research indicated that the proposed model was valid for both homogeneous and heterogeneous ground conditions. Further case studies provided by co-operators are expected to improve the effectiveness of the proposed model.

With the increasing number of long tunnelling and urban subway constructions, mixed-face ground conditions are frequently encountered. Rock fragmentation mechanism under disc cutter cutting in TBM tunneling through the mixed-face ground is complex and can lead to engineering difficulties. During TBM tunneling in mixed-face ground with soft rock in upper layer and hard rock in the lower layer, reduction of the advance rate and reduced rotational speed of cutter head occur compared with homogeneous ground. As a result, the muck in the working chamber cannot be replaced timely, leading to the formation of mud cake. Additionally, the disc cutters cannot rotate normally and are worn eccentrically and severely. Finally, the cutters collide with hard rock periodically at the interface between soft and hard rock, thus being subject to a huge impact load, even overload on some cutters, resulting in chipping of the cutter ring and damage to the cutter holder. This paper presents numerical analysis of the disc cutter cutting process considering the difference of rock-cutting behaviors of disc cutters in the mixed-face ground with the aid of PFC3D code. Based on the forces imposed on the disc cutter and rock crack propagation, TBM tunneling in the mixed-face ground is investigated. The decrease of the mean rolling force of the disc cutter causes rotation hindering in the disc cutter in soft rock stratum leading to flat cutter wear. The gap of the normal force between the soft rock and hard rock generates the overturning moment of the cutter head, which causes the eccentricity and vibration of the cutter head.

Disc cutter wear is one of the comprehensive results of the rock–machine interaction in tunnel boring machine (TBM) tunneling. The replacement of the disc cutter is a time-consuming and costly activity that can significantly reduce the TBM utilization ( U ) and advance rate (AR), and has a major effect on the total time and cost of TBM tunneling projects. Therefore, the importance of predicting the cutter life accurately can never be overemphasized. Most cutter wear prediction models are only suitable for 17-in. or smaller disc cutters. However, use of large-diameter disc cutters has been an irresistible trend for large-section hard rock TBMs. This study attempts to reveal the genuine wear rule of a 20-in. disc cutter and develop a new empirical model for predicting the cutter life in granite based on field data collected from a water conveyance tunnel constructed by the TBM tunneling method in China. The field data including the actual cutter wear and the geological parameters along the studied tunnel were compiled in a special database that was subjected to statistical analysis to reveal the genuine wear rule of a 20-in. disc cutter and develop the reasonable correlations between some common intact rock parameters and the disc cutter life. These equations were developed based on data from massive to very massive granite with a UCS range of 40–100 MPa, which can be applied for the assessment of the cutter life of a 20-in. disc cutter in similar hard rock projects with similar rock strengths and rock abrasivities.

Determination of the TBM disc cutter performance at the optimum rock cutting condition is very important, but the existing theoretical, laboratory, numerical or empirical methods still have some shortcomings. Full-scale rock cutting test is regarded as the most accurate and reliable method in the laboratory, but it still requires large rock blocks and specific testing equipment, which make the use of this method very limited and costly. Thus, by collecting, analyzing and formulizing the results of many full-scale linear cutting tests, this study partly overcomes the shortcoming of the full-scale rock cutting test and proposes some new empirical formulas to predict TBM disc cutter performance in a much easier and less costly way. First, this study reveals the general rules to characterize the change of rock cutting results with the increase of rock uniaxial compressive strength and cutter penetration depth by conducting full-scale linear cutting tests on five different rock types. Then, by analyzing a large amount of full-scale linear cutting test results, this study correlates the TBM disc cutter performance at the optimum rock cutting condition with rock uniaxial compressive strength and a widely used semi-theoretical prediction model. The new proposed empirical formulas in this study only use rock properties, cutter geometries and cutting geometries to design the machine specifications and select the TBM operation parameters, which is a much easier and less costly method.

This paper focuses on comparisons between the experimental and semi-theoretical forces of CCS disc cutters acting on different rocks. The experimental forces obtained from LCM tests were used to evaluate the prediction accuracy of a semi-theoretical CSM model. The results show that the CSM model reliably predicts the normal forces acting on red sandstone and granite, but underestimates the normal forces acting on marble. Some additional LCM test data from the literature were collected to further explore the ability of the CSM model to predict the normal forces acting on rocks of different strengths. The CSM model underestimates the normal forces acting on soft rocks, semi-hard rocks and hard rocks by approximately 38, 38 and 10%, respectively, but very accurately predicts those acting on very hard and extremely hard rocks. A calibration factor is introduced to modify the normal forces estimated by the CSM model. The overall trend of the calibration factor is characterized by an exponential decrease with increasing rock uniaxial compressive strength. The mean fitting ratios between the normal forces estimated by the modified CSM model and the experimental normal forces acting on soft rocks, semi-hard rocks and hard rocks are 1.076, 0.879 and 1.013, respectively. The results indicate that the prediction accuracy and the reliability of the CSM model have been improved.

As the primary rock-breaking tool of the tunnel boring machine (TBM), the disc cutter experiences significant wear from the immense cutting load. Given the intricate geological conditions in TBM excavation, accurately assessing the extent of wear on disc cutters solely through experience and predictive models is challenging. Therefore, implementing synchronous wear monitoring when a disc cutter breaks rock to accurately assess the amount of wear on the disc cutter and determine the point at which a tool change is needed is extremely important. In this work, a disc cutter wear test system is designed on the basis of the principle of eddy current testing and the variation in disc cutter wear during cutting is revealed through experiments. The radial reduction rate (Dr) is used as an index of disc cutter wear to analyze the variations in penetration, cutting speed, the cutting resistance coefficient (Cc), and specific energy consumption (SE) in relation to disc cutter wear. The experimental results indicate that the measurement error of the disc cutter wear testing system is 8.6%, which allows disc cutter wear level to be obtained. With increasing cutting displacement, the Dr of the disc cutter first increases rapidly but then slows. The Cc of the disc cutter increases exponentially with increasing Dr under different penetration depths. When the cutting displacement of the disc cutter reaches 40 m, Dr exhibits an exponential growth trend with increasing cutting speed. The critical value of Dr is 0.084. When Dr exceeds this value, the cutting performance exhibits a sharp increase in torque, which significantly impacts the driving efficiency. The research results of this paper provide a reference for creating a disc cutter replacement plan and extending the service life of the disc cutter.HighlightsOptimized design of a disc cutter wear test system based on eddy current detection.The radial reduction rate is proposed as an indicator of disc cutter wear.The critical value for disc cutter replacement is explored and revelled.

Rock penetration is the most important function of tunnel boring machines (TBMs). Based on a detailed review of TBM rock penetration research, this study introduces a rarely reported full-scale experimental cutterhead system that combines the advantages of in situ penetration tests and laboratory rock-breaking tests. The main focus of this study is to investigate TBM penetration performance using this experimental cutterhead system. Nine groups of penetration tests were conducted on an integral concrete specimen with cutterhead rotational speed and net penetration varying from 1.9 to 5.9 r/min and 2.5 to 8.8 mm/r, respectively. The cutting force and chipping performance of each cutter were monitored, examined, and analyzed considering boreability and mechanical efficiency. The results indicate that the cutter normal force is unaffected by the cutter installment radius and cutterhead rotational speed. However, the muck weight and specific excavation rate increase in perfectly fitted exponential functions with increasing cutter position number, indicating that the cutting efficiency increases with cutter position number. Muck sieving results show that face cutters produce larger and more elongated chips than center cutters, as the extent of cutter side sliding declines with increasing cutter installation radius. The boreability index decreases in a perfectly fitted power function with increasing net penetration, indicating that the critical threshold for cutterhead net penetration is approximately 5 mms/rev. The proposed models predicting the cutter normal force and boreability index were compared with 13 sets of in situ penetration test data. This study can guide TBM excavations encountering rocks with equivalent strength and intactness.HighlightsA rarely-reported experimental cutterhead system is introduced to study TBM rock-breaking performances.The cutter normal force is unaffected by the cutter installation radius and cutterhead rotational speed.The disc cutter rock-breaking efficiency increases with the cutter position number.The critical threshold of the penetration rate is approximately 5 mm/rev for the studied sample.

Water content is an important factor affecting the rock-breaking efficiency of tunnel boring machine (TBM) disc cutters. However, limited efforts have been made to study the fracturing mechanism of sandstone excavation by TBM disc cutters under varying water content conditions. To investigate the breakage behavior of water-soaked sandstone by TBM disc cutters, five sets of penetration tests on sandstone specimens with different water content levels were performed. The tests were conducted using a modified RYL-600 computer-controlled rock shear rheometer. An acoustic emission (AE) monitoring system was utilized throughout the entire process to track the AE activity of the specimens. The force–depth curves of the penetration process at various water content levels were investigated. The effects of water content on AE characteristics, rock fracture properties, and specific energy were analyzed. The results indicate that AE activity can be divided into three stages: quiet period, slow rise period, and active period. With increasing water content, peak penetration force, consumed energy, and specific energy decrease gradually, while chip volume increases. Water promotes mutual penetration of surface and internal cracks of the specimen, resulting in the formation of larger chip volumes. These findings provide theoretical guidance for designing and improving TBM cutter head parameters in water-rich soft rock formations.

Rock excavation by disc cutters is widely employed in tunnel boring machine (TBM). Rock-breaking impact load acted on disc cutters especially when cutting heterogeneous rock, which if not properly dealt with, may induce cracking of disc cutters. The impact load is excitation source of the TBM dynamic response and it is feasible to analyze the law of impact load based on dynamic response. In this study, a set of real-time dynamic response monitoring and analysis system was installed in a slurry TBM of a water delivery tunnel in Beijing to analyze the impact load action law that induced cracking of the cutter ring, and a series of in-situ tunneling experiments were carried out. The time-domain and frequency-domain characteristics of acceleration response were deeply investigated. The results show that compared with the static parameters, such as thrust and torque of the TBM, the acceleration response is extremely sensitive to changes of the driving parameters (advance speed, cutterhead rotating speed and penetration rate) and rapidly attenuates from the cutterhead to the shield tail of the machine. In the full section quartzy sandstone stratum, the acceleration response of the cutterhead is significantly increased with the slight improve of the penetration rate and the rotating speed and the effect of the penetration rate is larger. The acceleration response is small during the TBM tunneling with low penetration rate–high rotating speed, which is also applicable to the broken stratum. The results of dynamic response test indicate that the impact load resulting in cracking of cutter ring can be effectively reduced by three methods: reducing penetration rate, reducing rotating speed and tunneling with low penetration rate–high rotating speed. The statistical data of cutter wear validate this conclusion. The findings in the engineering practice can provide valuable references for similar TBM construction.HighlightsIn situ tests were conducted to analyze the effect of driving parameters on TBM dynamic response.The dynamic load of disc cutters was investigated from the perspective of TBM dynamic response.Three measures were proposed to reduce the cracking of disc cutter ring.The measures were verified by the statistical analysis data of cutter failure.