Shifeng Wen

Determination of reduced Young's modulus of thin films using indentation test

The flat cylindrical indentation tests with different sizes of punch radius were investigated using finite element method (FEM) aimed to reveal the effect of punch size on the indentation behavior of the film/substrate system. Based on the FEM results analysis, two methods was proposed to separate films reduced Youngs modulus from a film/substrate system. The first method was based on a new weight function that quantifies films and substrates contributions to the overall mechanical properties of the film/substrate system in the flat cylindrical indentation test. The second method, a numerical approach, including fitting and extrapolation procedures was put forward. Both of the results from the two methods showed a reasonable agreement with the one input FE model. At last, the effect of maximum indentation depth and the surface micro-roughness of the thin film on the reduced Youngs modulus of the film/substrate system were discussed. The methods proposed in the present study provide some new conceptions on evaluating other properties of thin films, e.g. creep, for which a flat-ended punch is also employed.

Effect of Surface Quality of Open Holes on Fatigue Life

Much of the structural loading in an aircraft is transferred through fuselage skin panels to riveted fastening holes at lap joint of these panels. Stress concentration caused by these fastening holes lead to a large loading of cycle tensile stress on these holes, and then bring in fatigue fracture. According to statistics, fatigue fracture of fastening holes accounts for 90% of the whole fatigue fracture of the aircraft, which becomes the main cause of aircraft structure failure. It is a significant research on how to reduce effects of stress concentration to improve aircraft anti-fatigue in design material and manufacture. In this chapter, how to determine the effects of hole surface quality to the stress distribution and life of aluminum perforated panel by finite element software and test are introduced.

Anti-fatigue Strengthening Technology of Holes

In order to improve the resistance of the hole in the body structure, to make up for the defects caused by the drilling process, and to ensure the safety of the service life of the aircraft, people often use cold expansion, impression and hammering, and other anti-fatigue technology to further improve the fatigue resistance of the hole. The strengthening mechanism is mainly to introduce the residual compressive stress, reduce the stress peak, or average stress of the hole edge to achieve the purpose of improving its fatigue life [1]. In this chapter, the effects of cold expansion, impression and hammer strengthening techniques on the residual stress distribution, and the fatigue life of the hole are studied by means of finite element analysis and related experimental techniques.

Outline of Fatigue and Fracture Mechanics

Fatigue is a branch of research comprised of many disciplines, requiring a broad knowledge not only of elastic mechanics, plastic mechanics, fracture mechanics, vibration, and stress analysis, but also mathematics, metallurgy, machinery, metal physics, and other inextricably linked disciplines. It includes fields of research in the microscopic range, macroscopic modelling, mechanics, and theory and analysis of experimental science. This chapter briefly introduces some of the basic knowledge that is often used in anti-fatigue design and analysis of aircraft structures.

Fatigue Test and Analysis of Box Section

Due to requirements for weight reduction and structural safety, thin-walled structure is widely used in the aircraft, such as stiffened panels, spars, and box sections composed by skin, beams, and ribs. The most common failure mode of thin-walled structures is strength failure [Meizheng et al. Comprehensive design of modern aircraft structure. Xi’an: Northwestern Polytechnical University Press, 2001 1, Lu et al. China Mechanical Engineering 20(17): 2055–2058 2]. In order to ensure the safety of the aircraft, the corresponding strength check is needed. The component-level strength test of the building block system was carried out in the box section. The purpose of this study is to evaluate the correlation analysis method, and to provide support for the validation test of the aircraft structure.

Shot Peening Strengthening Technology

As the requirements of high speed, high maneuverability, lightweight, long lifetime, and high reliability of aircraft become higher and higher, integrated structure design is widely used, such as integral frame, integral panel, thin-gauge skin, etc. Besides, the proportion of monolithic structure which usually has shot peening strengthened after processing for life requirements in aircraft components is increasing year by year. Adopting the method of finite element analysis and related experimental techniques, this chapter analyzed the influence of shot peening on fatigue life and residual stress field.

Anti-fatigue Design and Analysis of Joints

Aircraft wings consist a variety of connection joints, which are often the most dangerous places where strength failure or fatigue damages occur. Although people are trying to develop the integrated structure of the aircraft to avoid the joints, a large number of aircraft structures are still using the traditional form of lap joints to realize the transmission and distribution of load among different parts due to the requirements of design, process, and economic constraints (Cheng in Aircraft construction process. National Defense Industry Press, Beijing, 1990, [1]). Different parts, such as body skins, wing panels, etc., often use different forms of joints. The fatigue quality of the joints depends on the type of fasteners, the form of assembly, the quality of the hole, the size of the hole diameter, and other factors (Aircraft Design Manual in Aircraft design handbook: load, strength and stiffness. Beijing: Aviation Industry Press, 2001, [2]).