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Aircraft structures for engineering students - part 6

Tấm cánh do đó thường xuyên công gắn với stringers từ các tấm vững chắc của vật liệu, như là các xương sườn cánh. Hình 7,11 cho thấy xương sườn cánh cho Airbus của châu Âu trong đó stiffeners web, bích lỗ nhẹ nhàng và lugs đính kèm da đã được công trọn vẹn từ rắn. Phương pháp này không thể tách rời xây dựng liên quan đến các nguyên tắc thiết kế không có mới và có những ưu điểm | 9.2 General stress strain and displacement relationships 291 3 Fig. 9.13 Distribution of direct stress in Z-section beam of Example 9.3. deform the beam section into a shallow inverted s see Section 2.6 . However shear stresses in beams whose cross-sectional dimensions are small in relation to their lengths are comparatively low so that the basic theory of bending may be used with reasonable accuracy. In thin-walled sections shear stresses produced by shear loads are not small and must be calculated although the direct stresses may still be obtained from the basic theory of bending so long as axial constraint stresses are absent this effect is discussed in Chapter 11. Deflections in thin-walled structures are assumed to result primarily from bending strains the contribution of shear strains may be calculated separately if required. 9.2 General stress strain and displacement relationships for open and single cell closed section thin-walled beams We shall establish in this section the equations of equilibrium and expressions for strain which are necessary for the analysis of open section beams supporting shear loads and closed section beams carrying shear and torsional loads. The analysis of open section beams subjected to torsion requires a different approach and is discussed separately in Section 9.6. The relationships are established from first principles for the particular case of thin-walled sections in preference to the adaption of Eqs 1.6 1.27 and 1.28 which refer to different coordinate axes the form however will be seen to be the same. Generally in the analysis we assume that axial constraint effects are negligible that the shear stresses normal to the beam surface may be neglected since they are zero at each surface and the wall is thin that direct and shear stresses on planes normal to the beam surface are constant across the thickness and finally that the beam is of uniform section so that the thickness may vary with distance around each section but is .