Tham khảo tài liệu 'modeling and simulation for material selection and mechanical design part 17', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | with rolled thread after heat treatment depends significantly on the preload level reason strain hardening and residual stresses from thread rolling are not compensated by a new grain structure from heat treatment so nonlinear profiles from loading stresses and residual stresses are superposed . The test principle for determining axial fatigue load saspermt2 is defined in ISO 3800 30 or more detailed in DIN 969 10 for threaded fastening elements. Normally the screw shank is the location of fatigue failure but the clamped part or nut thread component can also end in fatigue failure . thin sheet metals as clamped part and a screw head with locking teeth. If the location of fatigue failure is at screw head fillet significant bending of screw is probable see also Fig. 54 . Minimizing fatigue problems can be realized by reducing the screw stressing . larger screw size lower additional force for screw in fastening system proper screw section design see Fig. 2 . sufficient radius at head-to-shank-fillet perpendicularity of screw axis and head support smooth transition of each discontinuity at screw shank such as different diameters of screw running out of thread to unthreaded shank no overlapping of stress concentrations . chamfer at clamped part under head or at Figure 53 Typical cross-section of screw M18 X failed in fatigue result of testing procedure according ISO 3800 stress amplitude ơa 80 MPa mean stress 555 MPa symmetric axial force without bending moment. Copyright 2004 by Marcel Dekker Inc. All Rights Reserved. first nut thread flank avoiding corrosive pittings at thread flanks or at head-shank-transition . The most established actions to increase the fatigue limit of the screw itself are discussed in Fig. 41. Figure 53 contains a cross-section of a screw M18 X which has failed in fatigue. A plane fracture zone can be seen at outer regions of the cross-section area of crack initiation and crack propagation and an unplane fracture area in
