Advanced Materials Research Vols. 33-37

Abstract: Riveting is a procedure widely used for fitting together two or more elements of a structure, that could be of the same or different material. In these assemblies the stress field is complex and a number of parameters, including effect of the geometrical discontinuities, contact between elements, tightening, material properties and applied load must be considered. The current work focuses on the study of fretting fatigue crack formation in common 6XXX aluminum alloys, used in land transportation equipments, and the determination of the characteristic crack initiation sites by means of both experimental and numerical methods. 3D finite element models were validated by the experimental results obtained with strain gauges. The influence of the contact friction coefficient at the fretting surface and fastening forces on the initiation of cracks, are discussed by the comparison of the different numerical results.

Abstract: Recent development in structure optimization offers the potential for significant improvements in the design of more durable structures. The present paper reveals the importance of structural optimization with crack propagation life of integrally stiffened panels. In the full paper, we explain in detail how to optimize structural fatigue life and design the structure of integrally stiffened panels which has the optimization life. The first topic is: the review of existing structural optimization design method. The second topic is: optimization methodology with crack propagation life. In our optimization methodology, the RSM (Response Surface Methodology) and GA (Genetic Algorithm) are successfully applied for structural optimization design with crack propagation life. The third topic is: damage tolerance optimization of integrally stiffened panels with crack propagation life. In this paper, structural parameters: the height and location of stringer, are the design variables. The structural weight is a fixed value. Through analyzing, the optimization structure with maximum life can not simply be chosen, and the maximum life would not increase all ways while the high of stringer increased. At last, the optimization structure, which has maximum crack propagation life, is given on the integrally stiffened panels.

Abstract: Casing drilling technique which has been dramatically developing is a revolution in petroleum industry and has aroused great concerns. The fatigue failure of casing thread connection is a critical issue for using. Therefore, to study the fatigue failure of casing connection is an important issue for understand the life of casing drilling. For the notched element, how to estimate the life and which parameter (equivalent stress, equivalent strain ,or the strain in the root )represent the damage under fatigue condition is still a problem. The purpose of this paper is to investigate the fatigue life of notch element under multiaxial stresses and to find out the damage parameter so as to predict the life of notch element. First specimen were machined with the same notch geometer dimension as the casing thread connection, fatigue tests with tension and torsion loading were carried out by fatigue test machine , for stress levels designed to obtain S-N lifetime curve. The stress and strain for the connections subjected to proportional loading were analyzed by elastic-plastic finite element method. The stress-strain state for notched specimens subjected to constant amplitude proportional multiaxial loadings was also calculated and analyzed by the finite element model. Take the equivalent stress, equivalent strain and the strain by FEM in the root into the prediction model, the strain by FEM has a good agreement with the experiment.But the results from the equivalent stress and equivalent strain also in good agreement with the experiment and is thought to be a simple prediction way.

Abstract: The effects of five single and three mixed corrosive environments on the fatigue crack growth and residual strength of steel 30CrMnSiNi2A were experimentally studied. The crack growth rates in corrosive environments, obtained by using Paris equation, were compared with crack growth rate in lab air. The results showed that the interactions of aggressive environments with fatigue loads caused the accelerations of fatigue crack growth rates in steel 30CrMnSiNi2A. But the effects of various environments on the fatigue crack growth rate are different. Among the environments the most detrimental one was oil-box zone, followed by cookroom&washroom, tank seeper, 3.5%NaCl, moist air, high altitude and dried air. Also, the test data showed the less effect of various corrosive environments on critical crack length, that is, no direct infection of corrosive environments on residual strength capability dominated by fracture toughness.

Abstract: Metal free reeds are used for musical instruments like harmonica. Free reeds are small, thin cantilevers, and oscillate by blowing air. It is reported that free reeds break due to fatigue during play. In order to elongate the life of free reeds, the fatigue properties should be investigated and a motion analysis method should be developed. The experimental and analytical research on metal free reed, however, has been rarely reported. In this study, two types of fatigue testing machines were developed to obtain basic fatigue characteristics. The fatigue testing machines are designed for bending fatigue of actual free reeds whose thickness is less than 400 μm. An S-N diagram is successfully obtained up to 107 cycles by using the developed fatigue testing machines. The fracture surfaces of fatigued specimens are in good agreement with those of free reeds failed in use. Then, an analytical method for the self-excited oscillation of free reeds was developed based on a mass-damper-spring model. The proposed method can take account for the shape of free reed. The self-excited oscillation of free reeds with different shape are analyzed and in good agreement with experimental results.

Abstract: The crack closure phenomenon has attracted great attention in the prediction of fatigue crack growth. The finite element analysis of fatigue crack growth has been conducted by many researchers mainly emphasized on the technique implementation of the simulation. In this paper the behavior of plasticity induced fatigue crack closure was analyzed by the elastic-plastic finite element method for middle crack tension (MT) specimen. The material was assumed as linear-kinematic hardening. The crack growth was simulated by releasing the “bonded” node pairs ahead of crack tip in stepwise. The calculations focused on the effects of load cases and crack length on crack opening/closure levels. For constant amplitude cyclic loadings with different load ratios, the crack opening/closure levels increases for a while and then decreases continuously, with the increase of crack length. For the loadings with invariable maximum stress intensity factors (briefly the constant-K loading), however, the crack tip plastic zone sizes at different crack lengths remain unchanged and the crack opening and closing load levels normalized by the maximum load levels keep constants as well. The results indicate that the crack length does not affect the relative opening and closure levels and numerical analysis for the constant-K loading case should play a key role in characterizing the fatigue crack growth behavior.

Abstract: The use of multi-type joints, such as rivet joints, adhesive joints, lap joints, L-shape joints, etc., has been driven by the need for stronger and lighter structures, particularly in bridges, aerospace structures, pipeline systems, automobiles industry. Among the multi-type joints, lap joints and L-shape joints possess a considerably important position. Moreover, in many real-field situations, it is not accessible to such joints, and thus it gives additional difficulties to detect damages. Fortunately, the electro-impedance method based on the use of smart sensors provides special opportunities for damage detection of such joints, which are not easy or impossible to be accessible. The piezoelectric-ceramic sensors which simultaneously act as an actuator and sensor are widely used for structural health monitoring. In the high frequency range, the electro-impedance-based technique using a piezoelectric-ceramic patch is very sensitive for the evaluation of the incipient and small damages. A large amount of experiments were executed and several conditions were imposed to simulate real-time damage, such as the bolt loosening and bolt absence. The different indices are discussed and executed to efficiently quantify the damage conditions. The theory behind this technique and the experimental investigations are presented in this paper. The analytical results strongly show the detectability and reliability of this method.

Abstract: Beam structures are a common form in many large structures, and therefore the real-time condition monitoring and active control of beams will improve the reliability and safety of many structures. This paper presents a damage assessment method which combines the impedance method and guided wave method. The combination enabled to improve the damage detection efficiency. The impedance method is used first to detect whether the damage occurs or not and judge the damage extent. The guided wave then is introduced to accurately localize damages. The improved method provides possibility for more accurately identifying and localization damages compared to that conventional method. A powerful wavelet transform is used to extract the signals efficiently. Additionally, with using the general function generator to excite the piezoceramic (PZT) patches to generate the guided wave, the guided wave propagates along with the beam structures with PZT patches bonded, and the real-time signals are recorded. Damages are indicated by a change of response signals when compared with a template undamaged condition. The wave attenuation and mode conversion is sufficient to detect various types of defects. The results show considerable ability for identifying and localization of the simulated damages.

Abstract: Widespread fatigue damage (WFD) is an important concern of aging aircrafts. Residual strength of stiffened panels with widespread fatigue damage was evaluated by an engineering approach and a finite element method respectively. Nine stiffened panels with three types of damage were tested for their residual strength. The predictions are in good agreement with the experiment results and it is shown the methods could be used in an engineering practice for the residual strength evaluation with the acceptable accuracy. It can be seen from this research that WFD could result in significantly reduction in the residual strength of stiffened panels.

Abstract: During the development of an aircraft structure design, designers draw out a skin-stringer panel with a skin pad on the basis of a conventional skin-stringer panel(skin is directly connected with stringer by rivets or there was a sheet betweeen skin and stringers) to reduce the manufacturing cost. In this paper, we calculated the SIF of three kinds of skin-stringer panels by FE. And then, we analyze the damage tolerance of these structures by fortran program. Especially we carried out a crack propagation experiment of a skin-stringer panel with a skin pad and compared it with the results of a static analysis. Finally we researched the influence of the thickness of a skin pad on the damage tolerance. According to the researches above we concluded the advantages of the skin-stringer panel with a skin pad on the damage tolerance, and presented some suggestions about the thickness of a skin pad.