Development of Fixture Layout Optimization for Thin-Walled Parts:A Review

An increasing number of researchers have researched fixture layout optimization for thin-walled part assembly during the past decades.However,few papers systematically review these researches.By analyzing existing literature,this paper summarizes the process of fixture layout optimization and the methods applied.The process of optimization is made up of optimization objective setting,assembly variation/deformation modeling,and fixture layout optimization.This paper makes a review of the fixture layout for thin-walled parts according to these three steps.First,two different kinds of optimization objectives are introduced.Researchers usually consider in-plane variations or out-of-plane deformations when designing objectives.Then,modeling methods for assembly variation and deformation are divided into two categories:Mechanism-based and data-based methods.Several common methods are discussed respectively.After that,optimization algorithms are reviewed systematically.There are two kinds of optimization algorithms:Traditional nonlinear programming and heuristic algorithms.Finally,discussions on the current situation are provided.The research direction of fixture layout optimization in the future is discussed from three aspects:Objective setting,improving modeling accuracy and optimization algorithms.Also,a new research point for fixture layout optimization is discussed.This paper systematically reviews the research on fixture layout optimization for thin-walled parts,and provides a reference for future research in this field.

Constitutive Model of Friction Stir Weld with Consideration of its Inhomogeneous Mechanical Properties

In practical engineering, finite element（FE） modeling for weld seam is commonly simplified by neglecting its inhomogeneous mechanical properties. This will cause a significant loss in accuracy of FE forming analysis, in particular, for friction stir welded（FSW） blanks due to the large width and good formability of its weld seam. The inhomogeneous mechanical properties across weld seam need to be well characterized for an accurate FE analysis. Based on a similar AA5182 FSW blank, the metallographic observation and micro-Vickers hardness analysis upon the weld cross-section are performed to identify the interfaces of different sub-zones, i.e., heat affected zone（HAZ）, thermal-mechanically affected zone（TMAZ） and weld nugget（WN）. Based on the rule of mixture and hardness distribution, a constitutive model is established for each sub-zone to characterize the inhomogeneous mechanical properties across the weld seam. Uniaxial tensile tests of the AA5182 FSW blank are performed with the aid of digital image correlation（DIC） techniques. Experimental local stress-strain curves are obtained for different weld sub-zones. The experimental results show good agreement with those derived from the constitutive models, which demonstrates the feasibility and accuracy of these models. The proposed research gives an accurate characterization of inhomogeneous mechanical properties across the weld seam produced by FSW, which provides solutions for improving the FE simulation accuracy of FSW sheet forming.

Effect of Dynamic Pressure Feedback Orifice on Stability of Cartridge-Type Hydraulic Pilot-Operated Relief Valve

Current research on pilot-operated relief valve stability is primarily conducted from the perspective of system dynamics or stability criteria,and most of the existing conclusions focus on the spool shape,damping hole size,and pulsation frequency of the pump.However,the essential factors pertaining to the unstable vibration of relief valves remain ambiguous.In this study,the dynamic behavior of a pilot-operated relief valve is investigated using the frequency-domain method.The result suggests that the dynamic pressure feedback orifice is vital to the dynamic characteristics of the valve.A large orifice has a low flow resistance.In this case,the fluid in the main spring chamber flows freely,which is not conducive to the stability of the relief valve.However,a small orifice may create significant flow resistance,thus restricting fluid flow.In this case,the oil inside the main valve spring chamber is equivalent to a high-stiffness liquid spring.The main mass-spring vibration system has a natural frequency that differs significantly from the operating frequency of the relief valve,which is conducive to the stability of the relief valve.Good agreement is obtained between the theoretical analysis and experiments.The results indicate that designing a dynamic pressure feedback orifice of an appropriate size is beneficial to improving the stability of hydraulic pilot-operated relief valves.In addition,the dynamic pressure feedback orifice reduces the response speed of the relief valve.This study comprehensively considers the stability,rapidity,and immunity of relief valves and expands current investigations into the dynamic characteristics of relief valves from the perspective of classical control theory,thus revealing the importance of different parameters.

Experimental study on the failure factors of the O-ring for long-term working

The O-ring in the rotary dynamic sealing system of gas meters should have outstanding long-term sealing performance under the conditions of high and low temperature,which can ensure the safety of gas meters.In this paper,based on the O-ring sealing mechanism,taking the compression set as the main sealing index,a finite element simulation method is used to analyze the failure charac-teristics under different conditions according to the constitutive relation of nitrile rubber.Then,using the orthogonal test method for the O-ring sealing performance index,the main factors which affect the law of long-term test are analysed,and the orthogonal regression mathematical model that can predict the compression set is obtained.Finally,the O-ring is improved according to the model,and the test results show that the performance of the new O-rings with high and low temperature resistance is greatly improved and meets the expected target requirements.

Necking of Q&P Steel during Uniaxial Tensile Test with the Aid of DIC Technique

A lot of research has been focused on the necking process during the plastic deformation of sheet metals, but the localized necking is rarely distinguished form diffused necking by experiments, due to the limit of measurement equipment and method. Quenching and Partitioning (Q&P) steel is a 3rd generation advanced high strength steel (AHSS). Its good combination of high strength and ductility ensures potential application in automobile industry. Uniaxial tensile tests of QP980 steel sheet at five strain rates are performed to investigate the necking process and the effect of strain rate on necking behavior of Q&P steel. Digital image correlation (DIC) method is applied during tensile tests, and evolutions of major strain, minor strain and normal strain distributions along gauge section of the tensile specimens are obtained. The diffused and localized necking strains are determined according to SWIFT necking theory and HILL necking theory respectively. The test results indicate that with the increasing of strain rate in the investigated range, the diffused necking strain decreases from 0.152 to 0.120 and localized necking strain decreases from 0.245 to 0.137. Meanwhile, the difference of the two strains decreases form 0.096 to 0.017. Thus it can be concluded that strain rate has an influence on both necking strains during the deformation of QP980 steel sheet. Diffused and localized necking strains are determined by uniaxial tensile tests with the aid of DIC technique and the effect of strain rate on necking strains is evaluated.

Scheduling algorithm of dual-armed cluster tools with residency time and reentrant constraints

To solve the scheduling problem of dual-armed cluster tools for wafer fabrications with residency time and reentrant constraints,a heuristic scheduling algorithm was developed.Firstly,on the basis of formulating scheduling problems domain of dual-armed cluster tools,a non-integer programming model was set up with a minimizing objective function of the makespan.Combining characteristics of residency time and reentrant constraints,a scheduling algorithm of searching the optimal operation path of dual-armed transport module was presented under many kinds of robotic scheduling paths for dual-armed cluster tools.Finally,the experiments were designed to evaluate the proposed algorithm.The results show that the proposed algorithm is feasible and efficient for obtaining an optimal scheduling solution of dual-armed cluster tools with residency time and reentrant constraints.

Kinematics and Path Following Control of an Articulated Drum Roller

Automatic navigation of an articulated drum roller, which is an articulated steering type vehicle widely used in the construction industry, is highly expected for operation cost reduction and improvement of work effi- ciency. In order to achieve the path following control, considering that its steering system is articulated steering and two frames are articulated by an active revolute joint, a kinematic model and an error dynamic state-space equation of an articulated drum roller are proposed. Besides, a state- feedback control law based on Lyapunov stability theory is also designed, which can be proved to achieve the purpose of control by the analysis of stability. What＇s more, to evaluate the performance of the proposed method, simu- lation under the MATLAB/Simulink and experiments using positioning algorithm and errors correction at the uneven construction site are performed, with initial dis- placement error （-1.5 m）, heading error （-0.11 tad） and steering angle （-0.19 rad）. Finally, simulation and exper- imental results show that the errors and steering angle can decrease gradually, and converge to zero with time. Meanwhile, the control input is not saturated. An articu- lated drum roller can lock into a desired path with the proposed method in uneven fields.

Prediction of Cross-Tension Strength of Self-Piercing Riveted Joints Using Finite Element Simulation and XGBoost Algorithm

Self-piercing riveting(SPR)has been widely used in automobile industry,and the strength prediction of SPR joints always attracts the attention of researchers.In this work,a prediction method of the cross-tension strength of SPR joints was proposed on the basis of finite element(FE)simulation and extreme gradient boosting decision tree(XGBoost)algorithm.An FE model of SPR process was established to simulate the plastic deformations of rivet and substrate materials and verified in terms of cross-sectional dimensions of SPR joints.The residual mechanical field from SPR process simulation was imported into a 2D FE model for the cross-tension testing simulation of SPR joints,and cross-tension strengths from FE simulation show a good consistence with the experiment result.Based on the verified FE model,the mechanical properties and thickness of substrate materials were varied and then used for FE simulation to obtain cross-tension strengths of a number of SPR joints,which were used to train the regression model based on the XGBoost algorithm in order to achieve prediction for cross-tension strength of SPR joints.Results show that the cross-tension strengths of SPR steel/aluminum joints could be successfully predicted by the XGBoost regression model with a respective error less than 7.6%compared to experimental values.

Parametric analysis of thermal effect on hydrostatic slipper bearing capacity of axial piston pump

Hydrostatic slipper was often used in friction bearing design, allowing improvement of the latter's dynamic behavior. The influence of thermal effect on hydrostatic slipper bearing capacity of axial piston pump was investigated. A set of lumped parameter mathematical models were developed based on energy conservation law of slipper/ swash plate pair. The results show that thermal equilibrium clearance due to solid thermal deformation periodically changes with shaft rotational angle. The slipper bearing capacity increases dramatically with decreasing thermal equilibrium clearance. In order to improve the slipper bearing capacity, length-to-diameter ratio of fixed damper varies from 3.5 to 8.75 and radius ratio of slipper varies from 1.5 to 2.0. In addition, the higher slipper thermal conductivity is useful to improve slipper bearing capability, but the thermal equilibrium clearance is not compromised.

Effect of hot-humid exposure on static strength of adhesive-bonded aluminum alloys

The effect of hot-humid exposure(i.e., 40 C and 98% R.H.) on the quasi-static strength of the adhesive-bonded aluminum alloys was studied. Test results show that the hot-humid exposure leads to the significant decrease in the joint strength and the change of the failure mode from a mixed cohesive and adhesive failure with cohesive failure being dominant to adhesive failure being dominant. Careful analyses of the results reveal that the physical bond is likely responsible for the bond adhesion between L adhesive and aluminum substrates. The reduction in joint strength and the change of the failure mode resulted from the degradation in bond adhesion, which was primarily attributed to the corrosion of aluminum substrate. In addition, the elevated temperature exposure significantly accelerated the corrosion reaction of aluminum, which accelerated the degradation in joint strength.

Opening Angle Rules of the Aerodynamic Brake Panel

In order to get the relationship between aerodynamic brake effect and the opening angle,based on a high-speed train model in CFD software FLUENT,the aerodynamic force properties from brake panels with different opening angles were analyzed as well as the flow field's variation laws. Six cases were researched,taking opening angles 45°,60°,75°,80°,85° and 90° respectively.Three-dimensional Reynolds-average Navier-Stokes equation combined with k-ε turbulence model was utilized. The control equation was discretized and solved by finite volume method.SIMPLE method was also considered to couple the pressure and velocity fields and search the numeric solutions. Conclusions can be achieved from the results which are shown as follows. When the opening angle increases from 45° to 75°,the aerodynamic forces and the central area with larger pressure increase fast,and the flow field distribution changes greatly; when the opening angle increases from75° to 90°,the aerodynamic forces and the central area with larger pressure increase slowly, and the flow field distribution changes slightly; considering train boundary and opening performance of the wind resistance brake mechanism,the opening angle should be 75°.

Try and error-based scheduling algorithm for cluster tools of wafer fabrications with residency time constraints

To improve the productivity of cluster tools in semiconductor fabrications,on the basis of stating scheduling problems,a try and error-based scheduling algorithm was proposed with residency time constraints and an objective of minimizing Makespan for the wafer jobs in cluster tools.Firstly,mathematical formulations of scheduling problems were presented by using assumptions and definitions of a scheduling domain.Resource conflicts were analyzed in the built scheduling model,and policies to solve resource conflicts were built.A scheduling algorithm was developed.Finally,the performances of the proposed algorithm were evaluated and compared with those of other methods by simulations.Experiment results indicate that the proposed algorithm is effective and practical in solving the scheduling problem of the cluster tools.

Characteristics of physical blocking on co-occupant's exposure to respiratory droplet residuals

Existed evidences show that airborne transmission of human respiratory droplets may be related with the spread of some infectious disease,such as severe acute respiratory syndrome(SARS) and H1N1 pandemic.Non-pharmaceutical approaches,including ventilation system and personal protection,are believed to have certain positive effects on the reduction of co-occupant's inhalation.This work then aims to numerically study the performances of mouth covering on co-occupant's exposure under mixing ventilation(MV),under-floor air distribution(UFAD) and displacement ventilation(DV) system,using drift-flux model.Desk partition,as one generally employed arrangement in plan office,is also investigated under MV.The dispersion of 1,5 and 10 μm droplet residuals are numerically calculated and CO2 is used to represent tracer gas.The results show that using mouth covering by the infected person can reduce the co-occupant's inhalation greatly by interrupting direct spread of the expelled droplets,and best performance can be achieved under DV since the coughed air is mainly confined in the microenvironment of the infected person.The researches under MV show that the two interventions,mouth covering and desk partition,achieve almost the same inhalation for fine droplets while the inhalation of the co-occupant is lower when using mouth covering for large droplets.

Vehicle OHT Dispatching Performance Analysis of an AM HS in 300 mm Semiconductor FABs

To improve the performance of an automated material handling system (AMHS) in 300 mm semiconductor fabrication plants (FABs), an overhead-hoist-transport (OHT) vehicle dispatching problem was described for semiconductor FABs. An original wafer lot dispatching policy was proposed. To minimize costs due to transportation logic, a dispatching rule based on an adapted Hungarian algorithm was presented, and six factors were considered. In addition to the recurring parameters, two original parameters of the vehicles utilization and wafer lot priority were considered to evaluate system performance. To obtain a balanced efficiency regarding the FAB output factors, simulation and sensitive analysis were used to find the best weight parameters of the cost matrix. In particular, a high rate of priority wafer lots (greater than 20%) and vehicles utilization (greater than 75%) are obtained without penalizing the efficiency of the FABs. The results indicate that the proposed dispatching policy is valid and practical.

Buffer allocation method of serial production lines based on improved ant colony optimization algorithm

Buffer influences the performance of production lines greatly.To solve the buffer allocation problem(BAP) in serial production lines with unreliable machines effectively,an optimization method is proposed based on an improved ant colony optimization(IACO) algorithm.Firstly,a problem domain describing buffer allocation is structured.Then a mathematical programming model is established with an objective of maximizing throughput rate of the production line.On the basis of the descriptions mentioned above,combining with a two-opt strategy and an acceptance probability rule,an IACO algorithm is built to solve the BAP.Finally,the simulation experiments are designed to evaluate the proposed algorithm.The results indicate that the IACO algorithm is valid and practical.

Mathematical Model of Steady State Operation in Jet Pipe Electro-Hydraulic Servo Valve

Jet pipe electro-hydraulic servo valve is the heart of feedback control systems,and it is one of the mechatronic components used for precision flow control application.It consists of several precision and delicate components.The performance of the jet pipe servo valve depends on many parameters.During the developmental stage,it is very difficult to ascertain the function parameters.The steady-state analysis of jet pipe electro-hydraulic servo valve has been made to simulate its fluid characteristics(flowin,flow-out,leakage flow,recovery or load pressure,etc.)by mathematical modeling.Theoretical model was conducted on various affecting parameters on the pressure,the main flow rate of fluid,or leakage flow through the receiver holes.The major parameters studied are jet pipe nozzle diameters,receiver hole diameters,angle between the two centre-lines of receiver hole,nozzle offset,and nozzle stand-of distance.In this paper the research is important to determine and optimize the structural parameters of jet pipe servo valve.Thus,equations of the pressure and flow characteristics are set up and the optimal structural parameters of jet pipe are established.

Wear Mechanism of Cemented Carbide Tool in High Speed Milling of Stainless Steel

Adhesion of cutting tool and chip often occurs when machining stainless steels with cemented carbide tools. Wear mechanism of cemented carbide tool in high speed milling of stainless steel 0Cr13Ni4 Mo was studied in this work. Machining tests on high speed milling of 0Cr13Ni4 Mo with a cemented carbide tool are conducted. The cutting force and cutting temperature are measured. The wear pattern is recorded and analyzed by high?speed camera, scanning electron microscope(SEM) and energy dispersive X?ray spectroscopy(EDS). It is found that adhesive wear was the dominant wear pattern causing tool failure. The process and microcosmic mechanism of the tool’s adhesive wear are analyzed and discussed based on the experimental results. It is shown that adhesive wear of the tool occurs due to the wear of coating, the a nity of elements Fe and Co, and the grinding of workpiece materials to the tool material. The process of adhesive wear includes both microcosmic elements di usion and macroscopic cyclic process of adhe?sion, tearing and fracture.

Queuing-Based Performance Analytical Model for Continuous Flow Transporters of AMHS

To quickly and accurately estimate the expected work-in-process (WIP)of material intersection points in continuous automated material handling systems (AMHSs) ,a queuing-based performance analytical model was presented for continuous flow transporters (CFTs) . In the modeling procedure which considered layout of crossovers and the variability of service time of turntables, an M /G /1 queuing model with multi-class customers and a non-preemptive priority M /G /1 queuing model with multi-class customers were introduced to accurately present the queuing WIP of each material intersection point and perform the analytical model. Finally,300 mm wafer fabrication facilities (fabs)with 24 bays were applied to evaluating the proposed model. Compared with results of an Arena simulation, the model performs well in evaluating the number of queuing WIP of the intersection points and overall system of CFTs in AMHSs.

Investigation on Yield Behavior of 7075‑T6 Aluminum Alloy at Elevated Temperatures

Aluminum alloys have drawn considerable attention in the area of automotive lightweight.High strength aluminum alloys are usually deformed at elevated temperatures due to their poor formability at room temperature.In this work,the yield behavior of 7075 aluminum alloy in T6 temper(AA7075-T6)within the temperature ranging from 25°C to 230°C was investigated.Uniaxial and biaxial tensile tests with the aid of induction heating system were performed to determine the stress vs.strain curves and the yield loci of AA7075-T6 at elevated temperatures,respectively.Von Mises,Hill48 and Yld2000-2d yield criteria were applied to predicting yield loci which were compared with experimentally measured yield loci of the AA7075-T6.Results show that yield stress corresponding to the same equivalent plastic strain decreases with increasing temperature within the investigated temperature range and the shape of yield loci evolves nearly negligibly.The experimental yield locus expands with an increase of equivalent plastic strain at the same temperature and the work hardening rate of AA7075-T6 exhibits obvious stress-state-dependency.The nonquadratic Yld2000-2d yield criterion describes the yield surfaces of AA7075-T6 more accurately than the quadratic von Mises and Hill48 yield criteria,and an exponent of 14 in the Yld2000-2d yield function gives the optimal predictions for the AA7075-T6 at all investigated temperatures.

Conflict-Free Routing Scheduling of OHTs Based on Multi-agent Intelligent Control System Framework

Overhead-hoist-transporters (OHTs) have become the most appropriate tools to transport wafer lots between inter-bay and intra-bay in united layouts of automated material handling systems (AMHSs) in 300 mm semiconductor wafer fabrications. To obtain a conflict-free scheduling solution, an intelligent multi-agent-based control system framework was built to support the AMHSs. And corresponding algorithms and rules were proposed to implement cooperation among agents. On the basis of the mentioned above, a time-constraint-based heuristic scheduling algorithm was presented to support the routing decision agent in searching the conflict-free shortest path. In the construction of the algorithm, the conflicted intervals of the k-shortest-route were identified with the time window theory. The most available path was chosen with an objective of the minimum completion time. The back tracking method was combined to finish the routing scheduling. Finally, experiments of the proposed method were simulated. The results show that the multi-agent framework is suitable and the proposed scheduling algorithm is feasible and valid.