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Bolt tightening torque has an influence on the fatigue life of the multi-bolted joints which cannot be ignored, but there are few methods to predict the fatigue life of the multi-bolted composite structure by considering the bolt tightening torque. A fatigue life prediction method of the multi-bolted composite structure by considering the bolt tightening torque is proposed. It can take into account the various damages of bolts and laminates and accurately calculate fatigue life and damage evolution of multi-bolted composite structure. The multi-bolted composite structure being composed of T300/BMP-316 composite laminates and TC4 titanium alloy bolts is used to verify the present method. The logarithmic error between the predicted fatigue life and the experimental is 4.36%, and the failure mode is consistent with the experimental. Through the present method, the influence of the bolt tightening torque is explored, in which it is found that with the increasing of bolt tightening torque, the fatigue life increases firstly and decreases, and there is an optimal tightening torque. By observing the damage evolution diagram, it is found that when the tightening torque is below the optimal tightening torque, the damage propagation cannot be effectively restrained, which will lead to the reduction of fatigue life. On the contrary, when the tightening torque is more than the optimal tightening torque, it will cause the initial damage to the test piece, which will also lead to the reduction of fatigue life. In engineering, to choose the best tightening torque of the structure for assembly can effectively improve the fatigue life of the structure.

The stiffened plate is one type of representative thin-walled structure consisting of plates reinforced by varying stiffeners to strengthen its resistance to buckling deformation. However, the mechanism by which stiffeners could efficiently improve the buckling resistance of such structures has not been well reported. Therefore, to investigate the mechanism and predict buckling behavior of the structures accurately, a refined Reddy-type global-local stiffened plate model (RGLS) is established firstly, which can fulfill continuous condition of in-plane displacement and transverse shear stress among the adjacent layers between the plate and stiffeners. In conjunction with the established model, a refined triangular element which can accommodate the continuous condition of transverse shear stresses at the interface between the plate and the stiffeners is constructed. Subsequently, on the basis of the unidirectional stiffened sandwich structures, the symmetric grid-stiffened sandwich structures are proposed. The buckling behavior of grid-stiffened structures is further explored by the present element. A full experimental study and a system of numerical examples have been performed to assess the reliability of the established elements. These examples demonstrate that the buckling loads could be captured by the developed element with high computational efficiency and accuracy. However, the models will significantly overrate buckling loads of the stiffened sandwich structures without considering the continuum condition of the transverse stresses at the plate-stiffener interface. [ABSTRACT FROM AUTHOR]

A novel damage tolerance design method of discrete interleaved toughening for laminated composite was proposed. In this paper, discrete thermoplastic Polyamide-6,6 (PA 66) films were deployed at between the selected adjected layers, and three toughening dimensions were considered. Besides, three impact positions of specimens were tested under 5 J, 10 J, and 15 J, respectively. The impact behavior and compression after impact (CAI) of specimens were discussed and compared. Experimental results showed that delamination damage was suppressed in the process of propagation outwards across the toughened region, and the path of delamination crack propagation was changed and swerved. Therefore, the delamination damage projected area (DDPA) of all specimens were reduced by 21.09%–62.85%, compared with the non-toughened plate (base plate). In addition, the finding demonstrated that CAI strength is influenced by the delamination position and DDPA. The proposed method could improve the CAI strength by suppressing delamination and swerving the propagation path of delamination. Moreover, the CAI strength of all toughened plates were improved by 16.35%–61.94% in contrast to the related base plates. [ABSTRACT FROM AUTHOR]

A finite element (FE) model based on fiber kinking and a transversal fracture angle damage model with cohesive elements are proposed to simulate the low-velocity impact (LVI) and compression after impact (CAI), and build a relationship between LVI energy and CAI strength of composites. The proposed FE model is validated by a comprehensive experimental work conducted using a high strength carbon fiber/epoxy material system i.e. CCF300/BA9916II and underwent LVI and CAI experimentation. The relative errors between numerical and experimental results of LVI damage area, maximum impact force, impact time, as well as CAI strength are less than 5%. The FE analysis results of LVI show that the dominant damage mode is delamination, and the CAI results demonstrate a brittle behavior with almost no loss of stiffness before failure. It is further deduced that the relationship of LVI energy and damage induced is directly proportional initially; however, after a threshold level of impact energy, the curve turns horizontal so that the increase in further impact energy does not increase the damage area substantially. A similar relationship is developed between impact energy and CAI strength. [ABSTRACT FROM AUTHOR]

Transverse shear stresses have a significant influence on the buckling analysis of laminated structures. Thus, the models violating interlaminar continuity of transverse shear stress will encounter difficulty for the buckling analysis. The postprocessing method by integrating three-dimensional equilibrium equation is also invalid, as the transverse shear stresses obtained from the postprocessing method are unable to be involved in the strain energy. In this paper, a Reddy-type higher-order zig-zag theory is firstly proposed, in which the in-plane displacement field is obtained by superposing the developed zig-zag function on the displacement field of Reddy's theory. To predict accurately the buckling behaviors of laminated composite and sandwich beams, a new equilibrium approach in conjunction with the Hu-Washizu (HW) mixed variational principle has been developed to produce the improved transverse shear stresses. Moreover, the accurate transverse shear stresses can be involved in the strain energy which can actively impact the accuracy of buckling stresses. To assess the performance of the proposed method, the critical loads of the laminated composite and sandwich beams with various configurations have been analyzed. Agreement between the present results and the solutions of layerwise are very good, and the proposed model only includes the four displacement parameters which can illustrate the accuracy and effectiveness of the present model. In addition, new results using all the models considered in this paper have been presented which can serve as a reference for future investigations. [ABSTRACT FROM AUTHOR]