资源描述
单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Chapter 3 Failure Theories and Material Strength,Key Terms:,bulk strength,surface strength,brittle material,ductile material,Outlines,Theories of Failure,Bulk Strengths of the Machine Components,Surface Strength of the Machine Components,3.1 Theories of Failure,Key Terms:,uniaxial,stress,biaxial stress,triaxial,stress,yield strength,ultimate strength,shear strength,3.2 Bulk Strength of the Machine Components,3.2.1,Loads and Stresses,1,）,Loads: force,bending moment,torque,static load,repeated load,impact load,2) Stresses: tension stress ,compression stress,shear stress,static stress,varying (fluctuating) stress,repeated alternating (,pulsant,) stress,r=0,completely reversed (symmetrical) stress r=-1,Reversed Stress,Pulsant,Stress,Fig. 3.1 Typical Stresses,3) Parameters of fluctuating Stresses,max, maximum stress,min, minimum stress,m, mean stress,a, stress amplitude,r, stress ratio,3.2.2 Stress-Strength Design Method,the maximum stress condition:,2) the safety factor condition:,Typical Stress-strain Diagram for Steel,sp,Proportional limit,se,Elastic limit,ss,Yield limit,sb,Ultimate strength,Typical Stress-strain Diagram for Brittle Material,Feature,：,No yield,No plastic strain,3.2.3 Strength Design Under Static Stresses,1,）,Design for ductile materials,2,）,Design for brittle materials,3.2.4 Strength Design under Varying Stresses,1,、,Fatigue Failure,Fatigue failure of a cross section,2,、,Fatigue Tests,3,、,Fatigue Curve (,-N diagram),Fig. 3.3 Typical Fatigue Curve of Steel,Fatigue limit,Endurance limit,N,rN,循环基数,m: index,N0: the critical number of cycles,kN,: life coefficient,N0: the critical number of cycles,4 Simplified Fatigue Diagram,Influence factors:,size factor :,stress concentration factor :,surface condition factor :,combined influence factor : (,)D,5,、,Allowable fatigue design diagrams,6,、,Fatigue life prediction with stable alternating stresses,For biaxial Stable Stresses:,7 Prediction of fatigue life with regularly varying stresses,Linear cumulative-damage rule (Miners rule),3.3 Surface Strength of the Machine Component,3.3.1 Surface Contact Stresses and Strength,1,、,Contact Stress (Hertz Stress),H,F: normal force,b: cylinder length,1,2: Poissons ratio,E1,，,E2,：,elastic modulus,1,2: radius of curvature,H:,pulsant,stress,Pitting or,spalling,contact strength design criterion:,3.3.2 Surface Extrusion Stresses and Strength,extrusion stresses,P,extrusion strength design criterion :,3.3.3 Surface Wear Strength,The endThanks!,
展开阅读全文