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• Closed form prediction of flutter occurrence in a generalised structure with damping. • Closed form prediction of flutter amplitude with damping, bilinear lift law and offset angle of attack. • Quantified prediction of the effect of structural and aerodynamic damping. • Identification of a type of dynamic divergence as a necessary condition for flutter. • Identification and quantification of viscous and stiffness mode coupling in flutter. • Efficient analytical predictions of necessary criteria for the occurrence of chaotic flutter. Flutter prediction of its occurrence, limit cycle amplitude and chaotic behaviour is investigated using a general binary (two degree of freedom) coupled modal model to provide new closed form analytical predictions and insight into the aeroelastic phenomenon. The generalised unsteady flutter model, based on flutter derivatives (for any cross-section) and an approximate continuous bilinear lift curve, is used in a mass decoupled form to formulate and develop analytical criteria for the onset and amplitude of flutter under damping and offset angle of attack. This is based on modal coupling through stiffness and damping terms, to clearly identify and clarify two forms of flutter based on stiffness and viscous mode coupling. Closed form analytical criteria for flutter onset are developed including a new necessary dynamic divergence criterion (analogous to spragging in brake squeal) based on flutter derivatives for any structure. The model is first validated with the well-known classical numerical and experimental airfoil results of Theodorsen and analytical results of Pines, which are also shown to correspond to stiffness mode coupling. In addition, the new modelling enables the detection and corresponding conditions of viscous mode coupling flutter which does not require closely spaced modes. The occurrence and conditions of both stiffness and viscous mode coupling flutter is verified numerically with results highlighting important structural parameters i.e. damping factors which could cause flutter outside classical conditions. In particular, the conditions for viscous mode coupling are newly identified and found to occur at flow velocities substantially lower (up to approximately 10%) than classical stiffness mode coupling values. Dynamic divergence is shown to be the lower bound for the critical flutter speed under a range of conditons. The viscous mode coupling is shown to occur under high levels of torsional damping (20 times the nominal 1% of critical), for the airfoil case, compared to vertical damping, but does not require the modal frequencies to be close. The efficient analytical prediction for flutter amplitude is then numerically verified for Theodorsen's airfoil. The results show that flutter amplitude is primarily dependent upon flow velocity and stall conditions without structural nonlinearity. Finally, a further nonlinear investigation of flutter identifies useful analytical criteria for the occurrence of chaotic flutter which is confirmed numerically. These results, based on a bilinear lift coefficient model, highlight that the occurrence of chaotic flutter can be due to simplified aerodynamics alone i.e. without nonlinear structural stiffness. The closed form analyses provide critical insight into; the occurrence and underlying mechanisms of flutter based on stiffness and viscous mode coupling, dynamic divergence as well as the nonlinear amplitudes and behaviour under an offset angle of attack and stall conditions. [ABSTRACT FROM AUTHOR]

This work characterises wear debris/particles generated naturally in grease-lubricated spherical roller bearings (SRBs) used in urban rolling stock. Morphological examination revealed the most active wear in grease lubricated SRBs is predominantly associated with the rolling/sliding contact and adhesive mechanisms, resulting in chunk, laminar and spherical wear debris found most frequently. Morphological transitions in spherical and chunky wear debris associated with bearings' service life was seen for the first time in railway SRBs. Interestingly, the formation of chunky wear debris was not necessarily governed by rolling contact fatigue and crack propagation mechanisms seen in oil lubricated systems, but rather associated with the collapse of larger spherical wear debris. Additionally, the surface complexity observed on individual debris, associated with rolling-sliding wear is also discussed. The findings add to the sparse knowledge of wear debris formation and their contribution to grease degradation, particularly for railway SRBs. Furthermore, an estimate of the contamination level of the SRBs was obtained by carefully adapting the fluid lubricant standards. The combination of these results will allow for improved wear prediction and the optimisation of bearing maintenance practices. • Seven types of wear debris naturally generated in grease lubricated rolling stock SRBs was identified and studied. • Formation of wear debris in the railway SRBs is mostly associated with rolling/sliding contact mechanism. • Morphological evolution of spherical debris linked with service life was seen for the first time in grease lubricated SRBs. • Chunk wear debris found in grease lubricated SRBs were governed by collapse and/or fracture of large spherical wear debris. [ABSTRACT FROM AUTHOR]

Incremental sheet forming (ISF) is an emerging flexible manufacturing technique. However, it is currently facing the challenge of popularization in industry since the poor geometric precision of the finished parts cannot meet the engineering standard. Aiming at enhancing product geometric precision, online path planning algorithms had been proposed by researchers. However, the existing algorithms had been applied successfully in manufacturing products with very simple geometries only and had limited capabilities of being generalized to complex products, which prevented the widespread application of ISF. To solve this long-lasting problem, more powerful and generic path planning algorithm needs to be proposed for the ISF process. In this study, an adaptive MPC-based path planning algorithm was developed for the ISF process, aiming at enhancing the geometric accuracies of products with more complex geometric features. The algorithm was formulated based on the state-space modelling of the process, the learning-based parameter identification approach, and quadratic programming optimization. The performance and the generality of the present adaptive MPC algorithm were experimentally validated in manufacturing parts with several complex geometric features that were typical in industrial parts using closed-loop ISF processes. In the first test, the present adaptive MPC algorithm showed a significant performance improvement of 56% in the reduction of maximum error on the base compared to the non-adaptive MPC. In the other two tests, the present algorithm also received satisfactory performances. Compared to the non-adaptive algorithm, the test results indicated that the present adaptive algorithm had a better performance and a higher generality. [ABSTRACT FROM AUTHOR]

Four commercial greases with various thickeners and base oils were experimentally examined to compare their false brinelling wear resistance in a test rig simulating roller bearings during rail/sea transportation for the first time. Greases containing zinc dialkyl dithiophosphates (ZDDP) showed superior false brinelling reduction, evidenced by no visible wear mark in the raceways. The mechanism for false brinelling mitigation was shown to be from a ZDDP-induced tribofilm which decreases the friction and wear coefficient in the contact area. Surface chemical analysis showed that for grease lubricated fretting contacts, ZDDP-derived tribofilms can be generated in the presence of micro-sliding motions and energy dissipation at the contact interface at low frequency (i.e., 4–8 Hz), due to the mechanochemical reactions. For greases without ZDDP, false brinelling wear was reduced by 97% when using grease with a more abundant and less viscous oil, which bleeds readily from an open structured thickener. The results highlight the ability of ZDDP as an additive in grease to better protect roller bearings against false brinelling during rail/sea transportation. [ABSTRACT FROM AUTHOR]

Experiments are conducted for investigating the effect of normal pressure, sliding amplitude and oscillating sliding frequency on friction, wear coefficient and threshold energy for wear activation of 52100 high carbon bearing quality steel (ASTM A295) during a false brinelling test. The cylinder-on-cylinder set-up is used. The investigation showed that the friction coefficient decreases with increasing normal pressure and increases with increasing sliding amplitude; however, there is no influence of an increase in oscillating sliding frequency. Similarly, the wear coefficient decreases with increasing normal pressure and increases with increasing sliding amplitude; however, unlike the friction coefficient, it increases with an increase in oscillating sliding frequency. Different conditions do not have a significant influence on the threshold energy for wear activation. By incorporating a varying friction and wear coefficient with changing sliding parameters, the analytical prediction for false brinelling are improved by 10–50% over the constant parameter model. • The effect of varying conditions on friction, wear coefficient was investigated. • The effect of different parameters on threshold energy for wear activation was investigated. • The results were then used to improve and validate the false brinelling models developed previously. [ABSTRACT FROM AUTHOR]

Model predictive control (MPC) is an advanced control algorithm that has been successfully implemented in the on-line path optimization of computer-numerical-control (CNC) machines for performing single-pass incremental sheet forming (ISF) processes, aiming at precision enhancements of products. Multi-pass ISF technique is usually employed to manufacture parts with extremely large wall angles which cannot be achieved without failure using the single-pass ISF. However, existing studies show that the geometric precision of the products of multi-pass ISF is usually very poor, which has significantly hampered the widespread application of this technique in industry. In the present study, two control algorithms based on the MPC framework were proposed, targeting at achieving a more precise geometry of multi-pass ISF products. MPC-1 was an extension of the control algorithm for the single-pass ISF process reported in our previous work, and MPC-2 was developed for the first time. The developed MPCs were comparatively validated by a group of physical benchmark test. Compared to MPC-1, MPC-2 reduced the maximum geometric error in the wall, conjunction, and base zones on the same part by 26%, 51%, and 73%, respectively. The test results show that MPC-2 had a better performance in multi-pass ISF processes. [ABSTRACT FROM AUTHOR]

With the increasing application of the advanced high strength steel material in the automobile industry, the thickness reduction of the bending area has attracted more and more attention since the product strength is highly influenced by the quality of the bending region. In this paper, three major factors, the thickness reduction, the variation of the local bending radius within the bending zone, and the tooling mark on the product's surface, are investigated through three different loading patterns for a free U-bending profile numerically and experimentally. The results demonstrate a thinning pattern consists of three peaks over the bending region for large bending ratio (R/t = 2.14) and only one peak for small bending ratio (R/t = 0.5). Corresponding valleys for the local radius are found to match the thinning pattern. Further, the use of finite element simulation can successfully predict the location and the severity of the wear on the product. From the experiment results, even if the metal blank only experienced one stroke, the tooling mark contains both adhesive and abrasive wear. A better understanding of the characteristics of the bending zone is achieved, and the findings can help in improving the design process for forming strategies. [ABSTRACT FROM AUTHOR]

This study presents a novel and non-destructive method for quantitatively analysing grease lubricated bearing wear by using X-ray microcomputed tomography (μCT). The efficacy of μCT to measure rolling contact wear via wear debris analysis was demonstrated, as was quantifying grease contamination and wear debris identification. Test rig experiments of grease lubricated spherical roller bearings, were performed to simulate rolling contact wear. Six grease samples were collected from two bearings at three different intervals to investigate wear progression. Results showed μCT measured wear volume increased exponentially with increasing bearing operation times. Grease cleanliness (particle counts per ml) worsened nonlinearly due to the exponential growth of wear debris over longer bearing operational times. Comparing the conventional ISO281 bearing assembly and operating condition assessment, a particle-count based method via μCT analysis provided a less subjective and dynamic assessment for quantifying grease contamination. Grease contamination was found to increase with longer operational timeframes due to the combination of wear debris growth and decreased lubrication film thickness. Finally, laminar, chunk and spherical wear particles, associated with rolling contact wear, were identified by the morphometric parameters measured by μCT. This study highlights the potential of μCT as a wear monitoring and grease quality assessment technique for railway bearing applications. • Microcomputed tomography is pioneered to measure wear in test rig bearings. • The proposed method is nondestructive and requires no sample manipulation. • The efficacy in wear measurement is proven through debris analysis in grease. • Quantitative analysis of 3D wear debris images provides wear mechanism assessment. • Grease contamination worsens with wear debris growth exponentially and lubricant film thinning. [ABSTRACT FROM AUTHOR]

Wheel squeal noise is a common problem that occurs in railway systems especially on curved tracks. These tonal noises are usually found at high frequency, typically over 1 kHz. The cause of the squeal noise remains unclear and could be different under varied conditions. Several mechanisms that could cause wheel squeal, including falling friction and mode coupling, have been proposed in literature. Most existing lab experimental investigations were focused on the squeal noise caused by falling friction with vertical contacts. As a result, the squeal noise generated by mode coupling could be missed due to the absence of a non-vertical contact angle. In this work, a two-wheel test rig has been designed with curved wheel profiles to achieve adjustable contact angles with lateral components. Experimental and numerical modal analysis of the two-disk system was conducted to determine the dynamic characteristics for exploration of mode coupling effects. Using a set of benchmark parameters predicted to be the cause of significant mode-coupled instability, squeal noise was found at frequencies higher than the existing falling friction experimental tests as reported in the literature. The frequency and amplitude measured in the experiments agreed with the predictions of the analytical mode coupling model. Further experiments varying control parameters, contact angle, angle of attack and direction of rotation were performed, confirming that the new squeal noise was caused by mode coupling. • The first experimental verification of mode coupling wheel squeal mechanism using two-disk testrig. • Experimental generated squeal noise level and frequency agreed well with the analytical model prediction. • Additional experiment with reversed rotation confirmed the identified squeal noise was caused by mode coupling. • Further parametric experiments varying contact angle and angle of attack were conducted, and the results were compared with the analytical model prediction and literature. [ABSTRACT FROM AUTHOR]

In Incremental Sheet Forming (ISF) of metal sheets, it is desirable to develop a quick method to predict the mechanical behaviour during forming, and ultimately, to optimise the process. The aim of this paper is to propose a Rapid Kinematic Method (RKM) for a quick update of the positions of the material points in two-point Incremental Sheet Forming (TPIF) of metal sheets. Based on this method, the strain and thickness distributions in the part can be determined and its tendency to fail assessed. The developed RKM is used to simulate single-stage and three-stage ISF of an ellipsoidal shape made of thin aluminium sheets, and the RKM predictions are compared with Finite Element Analysis simulations and experiments. The change in the deformation mechanism introduced by multi-staging has been carefully examined and a strategy for enhancing the sheet formability by structural means identified. The RKM presented here is applicable to parts of general shapes and the updated positions of the material points during ISF thus established can be used as an initial guess for a more rigorous model of mechanics. [ABSTRACT FROM AUTHOR]

• A hybrid constitutive model is developed to describe the stress-strain behaviors under ultrasonic vibration. • Finite element models are developed and verified for the ultrasonic-assisted ISF process. • The affecting mechanism of ultrasonic vibration on material deformation behavior is revealed. • Ultrasonic-actuated dynamic recovery is observed through microstructure examination. Ultrasonic vibration has been applied in the incremental sheet forming process and shows the potential to reduce the forming force and increase the formability, but the constitutive behavior and deformation behaviors of material under the high-frequency vibration are still not completely explained. In the present work, a hybrid constitutive model which combines the phenomenological, thermal activation and dislocation annihilation models is established and experimentally calibrated. Then, a finite element model is developed for the ultrasonic-assisted incremental sheet forming (UISF), in which the hybrid constitutive model and a user defined subroutine were incorporated to describe the ultrasonic effect and to simulate the high-frequency vibration, respectively. Compared with the experimental forming force, the FE analysis with the hybrid constitutive model shows good predictability with errors less than 10 %. Based on the simulation results, the improvement of formability was observed and the material deformation behavior during ultrasonic-assisted incremental sheet forming was analyzed in detail. Moreover, the material microstructural features were analyzed through Electron Back Scatter Diffraction (EBSD). The occurrence of relative larger grains and the increase of low angle grain boundaries (LAGBs) indicates the occurrence of ultrasonic-actuated dynamic recovery of the material. [ABSTRACT FROM AUTHOR]