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Chaotic motion in a fluttering wind turbine blade is investigated by the development of an efficient analytical predictive model that is then used to suppress the phenomenon. Flutter is a dynamic instability of an elastic structure in a fluid, such as an airfoil section of a wind turbine blade. It is presently modelled using generalised two degree of freedom coupled modes of a blade airfoil section (pitch and plunge) combined with local unsteady aerodynamics, based on flutter derivatives and a continuous bilinear lift curve under damping. The mode coupling causes instability and limit cycle flutter due to a Hopf bifurcation. Following the critical flutter speed, the response can transition to chaos through successive other bifurcations like period doubling. New closed-form conservative analytical conditions for chaos following blade flutter are identified and discussed for the wind turbine section taking into account the blade geometry and optimal design of the wind turbine. These predictions are numerically verified for a range of conditions including stall slope and damping. The results confirm that chaos following blade flutter can occur due to nonlinearities in the aerodynamics, i.e. due to a bilinear lift law. This phenomenon is then suppressed to unrealistically high wind speeds and/or eliminated by quantified variation of system parameters using the predictive model. The results show that small changes in tip speed ratio (−15%), and stall slope factor (−17%) can eliminate or suppress chaos following flutter, while, in general, larger magnitude changes in dynamic parameters (i.e. mass, inertia > 81%, stiffness > 97%, damping > 100%) are required to achieve the same, by detuning the coupled plunge and pitch natural frequencies or damping out overlapping parametric resonances. These results also highlight that the analytical predictions can remarkably be generalized to any parameter set and provide almost instantaneous calculations representing many thousands of numerical simulations from many bifurcation diagrams (computational acceleration factor of 107 times). General insight is also provided into the occurrence and suppression of airfoil chaos following flutter in aeroelastic structures like wind turbines. [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]

Chaotic instability in a vibration phenomenon, known as brake squeal, is investigated including the combined effects of falling friction, sprag-slip and mode coupling. Brake squeal is a high-pitched noise that occurs sometimes when a vehicle is decelerated using disc brakes. The equations of motion for the two dominant coupled modes of the brake system reduce to two autonomous coupled nonlinear second-order systems. The mode coupling instability via friction causes limit cycle behaviour via a supercritical Hopf bifurcation. This limit cycle is shown to break up into chaotic motion characterised by a Poincare map with a less than one-dimensional attractor, similar to that found in a forced dry friction oscillator. For the first time, conservative analytical conditions for brake squeal chaos are developed and verified numerically over a range of sprag angles and brake pressures for fundamental and real brake systems. The predictive model is then used to identify and quantify means to suppress brake squeal chaos to unlikely, excessive friction levels. The results provide predictive insight into conditions under which brake squeal chaos occurs and its suppression. [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]

Two-point incremental forming (TPIF) shows higher forming limits than single-point incremental forming (SPIF) especially for producing complex industrial functional components. However, unexpected failure still occurs occasionally under certain forming conditions in TPIF. In this paper, the applicability of six damage models for failure prediction in TPIF is evaluated. The procedure of damage parameter calibration and finite element modelling are presented. The predicted failure depth and location with these damage models are evaluated under different loading conditions. Furthermore, the stress and strain and damage evolution in TPIF are provided and discussed in detail. It is concluded that the failure location and depth can be predicted more accurately by the Ayada model than others in this work. It is noted that once double-sided contact occurs in TPIF, it reduces the variation of triaxiality through the thickness, altering the accumulation of damage. [ABSTRACT FROM AUTHOR]

The occurrence of chaotic motion in a railway instability phenomenon known as wheel squeal is investigated. The analysis is motivated and applied to predicting the large amplitude friction excited oscillations of the coupled wheel and rail motion. The equations of motion reduce to two autonomous coupled nonlinear second-order systems. Instabilities of the wheel and rail motions are shown to be due to the friction coupling which at low amplitudes causes limit cycle behaviour via a Hopf bifurcation. When the amplitude grows large enough, full nonlinear creep oscillations are shown to occur causing oscillations about positive and negative sliding conditions. Chaos is shown to occur when the motion meanders and jumps between large positive and negative creep and is characterised by a Poincare map with fractal nature. The route to chaos is shown to be via quasiperiodicity. Based on this insight, necessary analytical conditions for wheel squeal chaos are developed and verified using numerical simulations over a range of angles of attack and wheel/rail contact angles. Conditions under which chaotic instability is more likely to occur are identified and discussed, including mode coupling and parametric excitation. The results may describe why some very loud occurrences of wheel squeal are not characterised by a pure tone. [ABSTRACT FROM AUTHOR]

The geometrical error of the formed part is one of the most significant limitations that restricts the widespread application of incremental sheet forming (ISF) in aerospace industry. The geometry of ISF parts is dependent upon the tool path, so its correction can improve the part precision. Previous research has utilized model predictive control approach to achieve this, but the method was restricted to simple convex shapes. In this study, the tool path and the formed shape were parameterized and the analytical models of geometry responses relative to tool perturbations were proposed. Then, a model predictive control algorithm was developed, aiming at reducing the geometrical errors of the parts with complex non-convex shapes in the ISF process. Experimental validation of the developed control algorithm was carried out by forming a complex shape by single-point incremental forming. The results show that the developed control algorithm greatly reduced the geometrical error in the closed-loop process. [ABSTRACT FROM AUTHOR]

Residual stresses in metallic components greatly affect the geometrical accuracy of advanced high-strength steels (AHSS) products used for automotive industry. Chain-die forming is a promising method to fabricate AHSS products which can minimize the residual stress induced. This paper aims to understand the residual stress in chain-die-formed dual-phase (DP) steels through analytical and numerical modelling together with experimental validation. Analytical and numerical models were firstly employed to predict the stress development and associated residual stresses induced by chain-die forming in consideration of an as-received residual stress field and a comprehensive material-hardening model. The residual stresses in different regions of formed parts were practically measured using the neutron diffraction method to verify the developed models. Same as the mesh size of the examining points of finite element analysis (FEA) models, the averaged over the same sampling size is measured to guarantee the accuracy. It seems to be consistent with the simulation and experimental results in consideration of both factors. However, compared with as-received residual stress, the kinematic factor has a more dominating impact on the final residual stress. The results presented should contribute to the prediction of geometric errors and minimize product defects caused by the residual stresses in chain-die forming or equivalent sheet metal-forming industry. [ABSTRACT FROM AUTHOR]

With an increasing demand to reduce the body weight of motor vehicles, advanced high strength steel (AHSS) is widely applied in the automobile industry. However, high residual stresses and redundant deformation caused by the cold roll forming process are still outstanding problems. Under such circumstances, Chain-die forming technology is developed. Through enlargement of the effective roll radius, the deformation length is significantly stretched, leading to a significant reduction in residual stresses and redundant strain. In this study, Chain-die formed AHSS U-channel products are investigated experimentally and numerically. The influences of a number of features such as material strength, sheet metal thickness, vertical forming gap, and effective virtual roll radius are discussed regarding average flange angle and the variation of flange angle along the longitudinal direction. The results show that the average flange angle decreases with the increase of material strength, vertical forming gap, and effective virtual roll radius. Furthermore, among all four controllable variables, effective virtual roll radius has the most significant impact on the variation of the flange angle along the longitudinal direction. [ABSTRACT FROM AUTHOR]

Chain-die forming is a new sheet metal forming technology proposed which has the capability to fabricate advanced high-strength steel (AHSS) variable cross-section profiles. The existing research reveals that the control of longitudinal plastic strain and web-warping are two significant issues which determine the success of the fabrication of AHSS variable cross-section profiles. In this paper, both analytical and numerical methods are employed to analyse the longitudinal plastic strain and web-warping in Chain-die-formed AHSS sections with variable widths. By contrast to the existing analytical modelling in regards to flexible roll forming of irregular profiles, in this study, as Chain-die forming can be considered a continuous stamping process, the analytical models are purely established on the basis of geometric relationships between the pre-cut sheet and forming tools. In theory, no longitudinal plastic strain, id est maximum web-warping, could occur. No web-warping, id est maximum longitudinal plastic strain, could also arise. The real situation is included between such two extremes, and they are strongly dependent on some critical geometric parameters of the pre-cut sheet and forming tools. This is seen to be consistent with the simulation and experimental results. Subsequently, due to different forming mechanisms and loading processes, the severity and characteristics of the longitudinal plastic strain and web-warping of chain-die-formed AHSS sections with variable widths show distinct differences compared with those of flexible-rolled-formed profiles. Finally, Chain-die forming has been further proved to maximally reserve the material's elongation as the redundant deformation is almost avoided during the forming process. This study provides an effective mathematical expression to analyse and predict longitudinal plastic strain and web-warping of chain-die-formed AHSS variable cross-section profiles. It is expected to be useful to tooling designers to quickly assess the severity of a Chain-die forming process in the tooling design stage in order to shorten the tooling development time. [ABSTRACT FROM AUTHOR]

Incremental sheet forming (ISF) is an emerging forming technology that promises high flexibility and formability. These properties make it suited for small-scale and customised production. However, the poor geometric accuracy of ISF limits the wide application of this flexible forming technology. This paper presents a two-directional toolpath correction approach to enhance ISF forming accuracy using a model predictive control (MPC) algorithm. A toolpath optimisation method for vertical toolpath correction has been validated in our previous work (Lu et al., Int J Adv Manuf Technol 72:1-14, 2015), and it helps to reduce errors in the base of the test shapes to a suitable level while its major limitation is that horizontal geometric errors are relatively large. This paper extends our previous work (Lu et al., Int J Adv Manuf Technol 72:1-14, 2015) by augmenting the vertical control module with a new control module for horizontal toolpath correction. The proposed control algorithm was experimentally validated in single-point incremental sheet forming (SPIF) using two forming case studies. In the first case study (a truncated pyramid), two control approaches with different assumptions for the horizontal springback distribution along the horizontal cross-sectional profile were tested and compared. Then, the developed MPC control algorithm was applied to form a more complex asymmetric shape. The results show that the developed strategy can reduce the forming errors in the wall and base of the formed shape compared to the existing works. The ISF process with MPC control leads to significant accuracy improvement in comparison with the typical ISF process that is without toolpath control. [ABSTRACT FROM AUTHOR]

Chain-die forming is a promising manufacturing technology that is emerging as an alternative to roll forming and stamping in fabricating advanced high-strength steel (AHSS) products. The maximum edge longitudinal membrane strain (MELMS) and the springback are regarded as the crucial indicators for evaluating the quality of fabricated AHSS products. As Chain-die forming is a newly developed sheet metal fabricating method, the evaluations and predictions of the effects of the independent product parameters on MELMS and springback are of high interest. This paper selects three important quantitative product parameters (tensile strength, flange height, and thickness) to investigate experimentally the effects of different product parameters on the quality of Chain-die-formed U-profile AHSS products. Experimental verification on a new group of experimental samples using the established mathematical expressions is also performed successfully. This study shows that response surface methodology with Box-Behnken design can be efficiently applied for evaluating and predicting the MELMS and springback of Chain-die-formed AHSS products. It is found that the most influential parameter which determines the severity of the MELMS is the linear effect of flange height. This study also indicates that the springback is largely determined by the linear effect of tensile strength. The findings are expected to help designers and researchers to identify the formability range and predict the quality of Chain-die-formed AHSS products in advance of the quality control and fabrication stages. [ABSTRACT FROM AUTHOR]

Incremental sheet forming (ISF) is a promising manufacturing technology in which complex 3D shapes can be formed with one simple tool. Compared to conventional forming processes, for complex shapes, it is more flexible and economical with higher formability and shorter lead time. Therefore, ISF is ideally suitable to rapid prototype and small-batch production, especially in the aerospace and biomedical sectors. Over the last decade, although the process has been experimentally studied extensively, the associated deformation mechanics is still unclear and intensive investigation is needed. The purpose of this study is to provide further knowledge of the deformation mechanics of the sheet and clarify the deformation mechanism in a typical cone-forming process through finite element (FE) simulation approach. In particular, comprehensive FE models with fine solid elements are utilised, which allow the investigation of deformation modes including stretching, bending and shearing. The FE model is firstly validated with experimental results in terms of forming forces, and then, the evolution history of all the strain components along with the effective strain is presented. The contribution of each strain component to the effective plastic strain during the cone-forming process is discussed. Moreover, the characteristic of each strain component is investigated in detail. It is confirmed from the FE simulation that the deformation modes in the ISF process are a combination of shearing, bending and stretching, although the quantitative contributions in each direction are varied. The effect of step-down size on material plastic deformation as well as formability is also investigated. [ABSTRACT FROM AUTHOR]

Incremental sheet forming (ISF) is a highly flexible sheet forming process, but it suffers from poor geometric accuracy. This paper presents a feedback control strategy using model predictive control (MPC) to obtain improved geometric accuracy in ISF. Based on the incremental deformation mechanism of ISF, a novel model for shape state prediction has been proposed. The step depth of the toolpath was optimised based on shape state feedback during the forming process. Two different cases were studied for experimental validation of the developed control strategy. Comparisons between formed parts in both controlled and uncontrolled ISF processes were implemented in terms of cross-sectional profiles, formed depth at the bottom area and global deviation distribution. Results show that the geometric accuracy in ISF with feedback control has been greatly improved at the bottom area of the formed parts compared with a standard ISF approach without control. Improved geometric accuracy has been achieved on the wall of the parts as well. [ABSTRACT FROM AUTHOR]

Incremental sheet forming (ISF) is a promising manufacturing process that features benefits of reduced forming forces, enhanced formability and greater process flexibility. It also has a great potential to achieve economic payoff for rapid prototyping applications and for small quantity production in various applications. However, limited research has been conducted from the sustainability point of view, particularly for energy consumption. More consumed energy will generate more heat and affect tool and product wear. Also, geometric accuracy is still one of the dominant limits for the further development and commercialization of the ISF technology. Therefore, the aim of this study is to investigate how different process parameters affect the consumed energy during the forming process and also find the optimal working condition for lower deformation energy with higher geometric accuracy. A Box-Behnken design of 27 tests for pyramid-forming processes have been performed for a multi-objective optimisation that considers four factors: step down, sheet thickness, tool diameter and wall angle at three levels. The deformation energy during the forming process was calculated based on the measured forming forces. It was found that the deformation energy heavily depends on the sheet thickness because of higher plastic energy required to deform the material. Increasing step-down size within a limited range or decreasing the wall angle is also an effective approach to reduce the deformation energy. Moreover, the effects of various process parameters on the global geometric accuracy have also been investigated. The geometric error has been empirically predicted by quadratic equations giving the influence of the most influential forming parameters. It was concluded that the geometric quality is largely determined by the quadratic effect of wall angle, the linear effect of sheet thickness and the interaction effect of thickness and step down. Finally, the optimal working conditions for both independent and simultaneous minimisation of deformation energy and geometric error during the pyramid-forming process are provided. [ABSTRACT FROM AUTHOR]

Incremental sheet forming (ISF) is a new promising manufacturing technology with high potential to shape complex three-dimensional parts by using a relative small tool. However, ISF is presently limited due to non-uniform thickness distribution of formed parts, especially excessive thinning on severely sloped regions. This typically leads to fracture and hence limits process formability. This paper aims to tackle this issue by developing appropriate tool path strategies for multi-pass deformation design in ISF. More specifically, two more complex shapes are designed to validate the proposed multi-pass deformation design methodology by finite element simulations and experiments. The interactions of different tool path strategies on material thinning of the final part are evaluated in terms of process formability and thickness strain distribution given the same multi-pass design. Furthermore, the movement of material points is traced to explain the material flow mechanism in multi-pass forming compared to single-pass forming. It is concluded that both deformation passes (intermediate shapes) and tool path generation strategies would influence the thickness strain distribution as well as process formability. Appropriate tool paths should be devised to further reduce material thinning and improve the process formability by taking the geometrical features of the designed part into account. Additionally, the proposed multi-pass deformation design enables sufficient material to flow into the deformed region from the outside region so as to allow a compressive deformation state to develop and steeper shapes to be formed. [ABSTRACT FROM AUTHOR]

Incremental sheet forming (ISF) has been attractive during the last decades because of its greater flexibility, increased formability and reduced forming forces. However, traditional finite element simulation used for force prediction is significantly time consuming. This study aims to provide an efficient analytical model for tangential force prediction. In the present work, forces during the cone-forming process with different wall angles and step-down sizes are recorded experimentally. Different force trends are identified and discussed with reference to different deformation mechanisms. An efficient model is proposed based on the energy method to study the deformation zone in a cone-forming process. The effects of deformation modes from shear, bending and stretching are taken into account separately by two sub-models. The final predicted tangential forces are compared with the experimental results which show an average error of 6 and 11 % in respect to the variation of step-down size and wall angle in the explored limits, respectively. The proposed model would greatly improve the prediction efficiency of forming force and benefit both the design and forming process. [ABSTRACT FROM AUTHOR]

Incremental sheet forming (ISF) has demonstrated its high potential to shape complex three-dimensional components without using specific tooling, thus enabling product customization, cost reduction and efficiencies. This paper presents an experimental campaign to investigate AA7075-O aluminum alloy sheet forming using single point incremental forming (SPIF) technology and understand the forming process mechanism. Firstly, tensile tests were carried out to characterize the mechanical properties of AA7075-O sheets with three different thicknesses. Then, the effects of tool path types with different incremental steps on the maximum part draw angle and the successful part height were evaluated to clarify the formability. Additionally, a fracture forming limit diagram was developed to give the design limits for strain. Finally, the trends in forming forces were analyzed considering the influence of different draw angles, sheet thicknesses, step-down sizes and sheet orientations. Experimental results showed that formability is affected by the part draw angle and incremental steps of the tool path. However, the likelihood of successful forming also depends on other geometrical design parameters such as the part height. In addition, the influences of process parameters on tool forces provide further insights into the deformation mechanics of AA7075-O sheets. The slope of the force curve after the peak value can be identified as a forming failure prediction indicator regardless of the tool path types when using a truncated cone as a benchmark. Also the influence of sheet orientations on forming forces was investigated in SPIF. The vertical forming force is found to be smaller in a sheet orientation 45 to the rolling direction than that of a sheet orientation 0 for three different sheet thicknesses. [ABSTRACT FROM AUTHOR]