Process-scheme-driven automatic construction of NC machining cell for aircraft structural parts

In order to enhance the NC programming efficiency and quality of aircraft structural parts （ASPs）, an intelligent NC programming pattern driven by process schemes is presented. In this pattern, the NC machining cell is the minimal organizational structure in the technological process, consisting of an operation machining volume cell, and the type and parameters of the machining operation. After the machining cell construction, the final NC program can be easily obtained in a CAD/CAM system by instantiating the machining operation for each machining cell. Accordingly, how to automatically establish the machining cells is a key issue in intelligent NC program- ming. On the basis of the NC machining craft of ASP, the paper aims to make an in-depth research on this issue. Firstly, some new terms about the residual volume and the machinable volume are defined, and then, the technological process is modeled with a process scheme. Secondly, the approach to building the machining cells is introduced, in which real-time complement machining is mainly considered to avoid interference and overcutting. Thirdly, the implementing algorithm is designed and applied to the Intelligent NC Programming System of ASP. Finally, the developed algorithm is validated through two case studies.

DELAY-TIME MODEL BASED ON IMPERFECT INSPECTION OF AIRCRAFT STRUCTURE WITHIN FINITE TIME SPAN

According to the failure characteristics of aircraft structure, a delay-time model is an effective method to optimize maintenance for aircraft structure. To imitate the practical situation as much as possible, imperfect inspections, thresholds and repeated intervals are concerned in delay-time models. Since the suggestion by the existing delay-time models that the inspections are implemented in an infinite time span lacks practical value, a de- lay-time model with imperfect inspection within a finite time span is proposed. In the model, the nonhomogenous Poisson process is adopted to obtain the renewal probabilities between two different successive inspections on de- fects or failures. An algorithm is applied based on the Nelder-Mead downhill simplex method to solve the model. Finally, a numerical example proves the validity and effectiveness of the model.

Corrosion Fatigue Life Prediction of Aircraft Structure Based on Fuzzy Reliability Approach

Material performance of LY12CZ aluminum is greatly degraded because of corrosion and corrosion fatigue, which severely affect the integrity and safety of aircraft structure, especially those of lbe navy aircraft structure. The corrosion and corrosion fatigue failure process of aircraft structure are directly concerned with many factors, such as load, material characteristics, corrosive environment and so on. The damage mechanism is very complicated, and there are both randomness and fuzziness in the failure process. With consideration of the limitation of those conventional probabilistic approaches for prediction of corrosion fatigue life of aircraft structure at present, and based on the operational load spectrum obtained through investigating service status of the aircraft in naval aviation force, a fuzzy reliability approach is proposed, which is more reasonable and closer to the fact. The effects of the pit aspect ratio, the crack aspect ratio and all fuzzy factors on corrosion fatigue life of aircraft structure are discussed. The results demonstrate that the approach can be applied to predict the corrosion fatigue life of aircraft structure.

Digital Simulation of Full Scale Static Test of Aircraft

Full scale aircraft static test is a very important process of aircraft design, it is costly and time consuming. The testing accuracy and validity mainly depend on the rationality of the test scheme design. When the aircraft is being tested, the specimen＇s safety mainly depends on monitoring and understanding the testing data by way of evaluating the coherence with the digital simulation data synchyononsly. The test digital simulation can aid realizing above requirements and improving the test efficiency significantly during test scheme design stage or testing stage respectively. The key technologies and the solving methods of test digital simulation are presented and the application example is given.

Influence of environmental factors on corrosion damage of aircraft structure

Corrosion is one of the important structural integrity concerns of aging aircraft, and it is estimated that a significant portion of airframe maintenance budgets is directed towards corrosion-related problems for both military and commercial aircraft. In order to better understand how environmental factors influence the corrosion damage initiation and propagation on aircraft structure and to predict pre-corrosion test pieces of fatigue life and structural integrity of an effective approach, this paper uses the cellular automaton (CA) method to character the effect of electrolyte concentrations, dissolution probabilities, and temperature on the corrosion damage of a metal structure exposed to an aggressive environment, and the procedure for applying the local rules between the cells of the CA to simulate the corrosion damage evolution is formulated. The simulation results show that the different electrolyte concentrations, dissolution probabilities and temperature, the laws of the dissolution current with time are investigated and they obey the time power function.

INVESTIGATIONS ON LASER SHOCK-PROCESSING (LSP) TO IMPROVE FATIGUE LIFE OF SMALL HOLE IN AIRCRAFT STRUCTURES

Laser shock-processing (LSP) is of particular advantage for improving fa-tigue behavior of small holes and blind holes. Because there are not good accessibility andpassage, these holes cannot be treated by shot peening or cold extrusion. The fatigue livesof aircraft aluminum alloy 2024-T62 are increased greatly by means of optimization oflaser shocking parameters. With 95 % confidence, the mean fatigue life of LSP specimensis 4. 35~7, 75 times larger than that of the un-shocked ones.

THE RELIABILITY ANALYSIS OF AIRCRAFT STRUCTURES

This paper presents a set of fundamental equations for reliability analysis of single-critical-point aircraft structures.The mathematical model is formulated by considering the following factors:the static strength of structure,initial crack,the initiation and propagation and unstability of fatigue crack,the residual strength of structure,the statistical distribution of load,the periods of overhaul,accident damage,the communication of damage among fleets etc.Based on this mathematical model,the influence of these factors on the reliability can be quantitatively analyzed,and various criteria for fatigue design of aircraft structures can be evaluated from the aspect of reliability.

REQUIREMENTS AND DESIGN METHOD FOR MATCHING AIRCRAFT GUNS WITH THEIR SUPPORTING STRUCTURE

In aircraft structural dynamic design the matching of guns with their supporting structure is one of the most important tasks on which hinges the success or failure of the structural design. The design curves for matching guns with their supporting structure can be obtained from response calculations of the plate-spring system supporting the gun on the ground,the model structure tested on the ground and the actual structure.A set of matching curves is given for engineering application.Then,the matching design can be accomplished by means of impact load spectrograms so as to perform an optimal structural design and to make further improvements on dynamic design program.

Sectorization Model of Terminal Airspace with Arrival and Departure Separation

Terminal airspace(TMA)is the airspace centering several military and civil aviation airports with complex route structure,limited airspace resources,traffic flow,difficult management and considerable airspace complexity.A scientific and rational sectorization of TMA can optimize airspace resources,and sufficiently utilize the control of human resources to ensure the safety of TMA.The functional sectorization model was established based on the route structure of arriving and departing aircraft as well as controlling requirements.Based on principles of sectorization and topological relations within a network,the arrival and departure sectorization model was established,using tree based ant colony algorithm(ACO)searching.Shanghai TMA was taken as an example to be sectorizaed,and the result showed that this model was superior to traditional ones when arrival and departure routes were separated at dense airport terminal airspace.

ARRAY PULSED EDDY CURRENT IMAGING SYSTEM USED TO DETECT CORROSION

A theory model is established to describe the voltage-current responsefunction. The peak amplitude and the zero-crossing time of the transient signal is extracted as theimaging features, array pulsed eddy current (PEC) imaging is proposed to detect corrosion. The testresults show that this system has the advantage of fast scanning speed, different imaging mode andquantitative detection, it has a broad application in the aviation nondestructive testing.

Cold expansion technology of connection holes in aircraft structures:A review and prospect

As one kind of key anti-fatigue manufacture approaches with simplicity and effectiveness, the hole cold expansion technology satisfies the increasing needs for light weight and durability of aircraft structures.It can improve the fatigue life by several times at no additional weight conditions.The hole cold expansion technology has been widely used in manufacturing and repairing of both fighters and commercial aircraft, and has become a research hotspot in the strengthening technology.In recent years, hole cold expansion process methods, residual stress around expanded holes, the behavior of fatigue crack initiation and propagation, and fatigue lives after cold expansion are researched extensively through lots of experiments and finite element simulations.A review on the hole cold expansion technology research status in the last twenty years is presented in this paper.Via the analysis of the current characteristics and defects of the hole cold expansion technology, combined with the actual needs in design and manufacture of new-generation aircraft, development trends and novel research directions are presented for realizing precise and high-efficiency anti-fatigue manufacture.

Application of wireless sensor networks to aircraft control and health management systems

Use of fly-by-wire technology for aircraft flight controls have resulted in an improved performance and reliability along with achieving reduction in control system weight. Implementation of full authority digital engine control has also resulted in more intelligent, reliable, light-weight aircraft engine control systems. Greater reduction in weight can be achieved by replacing the wire harness with a wireless communication network. The first step towards fly-by-wireless control systems is likely to be the introduction of wireless sensor networks (WSNs). WSNs are already finding a variety of applications for both safety-critical and nonsafety critical distributed systems. Some of the many potential benefits of using WSN for aircraft systems include weight reduction, ease of maintenance and an increased monitoring capability. This paper discusses the application of WSN for several aircraft systems such as distributed aircraft engine control, aircraft flight control, aircraft engine and structural health monitoring systems. A brief description of each system is presented along with a discussion on the technological challenges. Future research directions for application of WSN in aircraft systems are also discussed.

Successes and challenges in non-destructive testing of aircraft composite structures

Composite materials are increasingly used in the aerospace industry.To fully realise the weight saving potential along with superior mechanical properties that composites offer in safety critical applications,reliable Non-Destructive Testing(NDT)methods are required to prevent catastrophic failures.This paper will review the state of the art in the field and point to highlight the success and challenges that different NDT methods are faced to evaluate the integrity of critical aerospace composites.The focus will be on advanced certificated NDT methods for damage detection and characterization in composite laminates for use in the aircraft primary and secondary structures.

Design of aircraft structures against threat of bird strikes

In this paper, a method to design bird-strike-resistant aircraft structures is presented and illustrated through examples. The focus is on bird strike experiments and simulations. The explicit finite element software PAM-CRASH is employed to conduct bird strike simulations, and a coupled Smooth Particles Hydrodynamic（SPH） and Finite Element（FE） method is used to simulate the interaction between a bird and a target structure. The SPH method is explained, and an SPH bird model is established. Constitutive models for various structural materials, such as aluminum alloys, composite materials, honeycomb, and foam materials that are used in aircraft structures,are presented, and model parameters are identified by conducting various material tests. Good agreements between simulation results and experimental data suggest that the numerical model is capable of predicting the dynamic responses of various aircraft structures under a bird strike,and numerical simulation can be used as a tool to design bird-strike-resistant aircraft structures.

Microstructural characterization and mechanical behavior analysis of 7075-T6 aluminum subjected to simulated lightning strikes

Aircrafts damages caused by lightning strikes have been known since the early days of aviation.However,the physical effects on the aircraft structure are still being investigated.This work seeks to evaluate the lightning strike effects in the aluminum alloy 7075-T6.Samples were submitted to lightning strike simulation in laboratory and the damages evaluated through characterization techniques.Ultrasound and profilometry tests have shown material loss to 0.272 mm depth in the damaged region.In addition,it was detected the material accumulation occurrence in the damage vicinity of the region.Below the damage,it was found a region where metallurgical changes were identified.The tensile and microhardness tests results have shown reduction in the percentage elongation and hardness increasing in the material affected by lightning.These results are corroborated by the X-Ray Diffraction(XRD)and Rietveld Method(red line)that indicated an increasing in dislocation density and micro-deformation in the material matrix.Optical microscopy results have shown the presence of microcracks on the normal and cross-section surface of the samples damaged.The Energy Dispersive X-Ray Spectroscopy(EDXS)and Electron Backscattered Diffraction Test(EBSD)found coarse intermetallic phases and precipitates compounds with dimensions greater than 1 lm in length.They were responsible for nucleation of the microcracks that propagate along the material grain boundaries.

Control techniques of tilt rotor unmanned aerial vehicle systems： A review

The tilt rotor unmanned aerial vehicle（TRUAV） exhibits special application value due to its unique rotor structure. However, varying dynamics and aerodynamic interference caused by tiltable rotors are great technical challenges and key issues for TRUAV＇s high-powered flight controls, which have attracted the attention of many researchers. This paper outlines the concept of TRUAV and some typical TRUAV platforms while focusing on control techniques. TRUAV structural features, dynamics modeling, and flight control methods are discussed, and major challenges and corresponding developmental tendencies associated with TRUAV flight control are summarized.

Simplification approaches for multi-level load spectra by using equivalent damage rule

Reasonable and effective fatigue load spectrum is key of aircraft structural fatigue test.Two kinds of load spectra simplification approaches based on statistical consistent fatigue damage model are proposed in the paper.One method is to simplify the original multi-level load spectra to spectral load with fewer level and was verified by the experimental data of five levels and seven levels load spectra.The equivalent damage here is not equivalent fatigue life,but equivalent number of spectral blocks.Moreover,it was proved that a section of low stress load cannot be converted into the highest stress load considering equivalent damage.The other is to convert the life distribution under multi-level load spectra into that of constant amplitude spectrum according to a certain principle,that is,equivalent damage is achieved through equivalent fatigue life,and two groups of three levels spectral data were used for experimental verification.The results show that equivalent damage simplification approach of multi-level load spectra is feasible,and it achieve that the damage of one loading block before and after the simplification is equal in probability statistics.The life distribution of load simplified as constant amplitude spectrum is consistent with test data.The simplification method presented in this paper can be generalized and is a predictive engineering method which does not depend on experimental data.