Strength/weight ratio improvement for hybrid frame-panel car bodies using joints with controlled shear stiffness

The paper presents a new approach for design of long-length car bodies with predetermined fatigue strength, which utilizing bonded joints with low-modulus adhesives and transversal reinforcement. Ready-to-use composite sandwich panels with GFRP skins are bonded into a metal frame during the production process of a frame-panel car body. Joints used for this process can be classified as single-lap ones. High requirements for strength/weight ratios of modern car bodies demand smoothing the loads on panels and a frame, especially in the case of steel frames with significant deviations from ideal design geometry (alternating gaps between frame and panels). Smoothing can be satisfied by means of stiffness control of bond lines with alternating thickness. The new conception of stiffness control by using of small diameter self-tapping screws is proposed. The conception implemented by usage of developed nomograms, which relates bond line thickness, reinforcement pitch (distance between reinforcement elements) and shear stiffness per unit length of lap joints. The nomograms allow determining reinforcement pitch in respect with given both a bond line thickness and a shear stiffness of joints. The mentioned design approach is demonstrated on the example of a low-floor tram car body. After the determination of a reinforcement pitch the appropriate estimation of loads on joints and frame is carried out. Loads obtained from simplified FE car body model and refined by usage special adjustment factors, which calculated on the base of detailed FE modeling of typical frame-panel structures. The static and fatigue strength analysis performed by verified FE models of bonded/screwed joints and analytical experimental dependencies respectively.