Mechanical Failure(파괴) Fatigue Failure of crankshafts Associate Professor Su-Jin Kim School of Mechanical Engineering Gyeongsang National University

Ductile, Brittle Failure (연성, 취성 파괴) Ductile fracture (Al) Brittle fracture (Cast Iron) Plastic deformation warning before fracture Little or no deformation warning • Ductile fracture is usually more desirable than brittle fracture!

Fracture Toughness (파괴인성) The ability (of material containing a crack) to resist fracture. Kc (MPa√m)   Fracture toughness is a property which describes the ability of a material containing a crack to resist fracture Fracture toughness is a quantitative way of expressing a material's resistance to brittle fracture when a crack is present. If a material has much fracture toughness it will probably undergo ductile fracture. Brittle fracture is very characteristic of materials with less fracture toughness. The subscript Ic denotes mode I crack opening under a normal tensile stress perpendicular to the crack, since the material can be made deep enough to stand shear (mode II) or tear (mode III).   a Crack

Fracture Toughness Graphite/ Ceramics/ Semicond Metals/ Alloys Composites/ fibers Polymers 5 K Ic (MPa · m 0.5 ) 1 Mg alloys Al alloys Ti alloys Steels Si crystal Glass - soda Concrete Si carbide PC 6 0.7 2 4 3 10 <100> <111> Diamond PVC PP Polyester PS PET C-C (|| fibers) 0.6 7 100 Al oxide Si nitride C/C ( fibers) Al/Al oxide(sf) Al oxid/SiC(w) Al oxid/ZrO (p) Si nitr/SiC(w) Glass/SiC(w) Y O /ZrO Material: Kc S40C : 54 Al7075: 24 Nylon : 2.2 Glass : 0.7

Stress Concentration (응력집중) Stress at crack, sm t = radius of curvature so = applied stress Kt = stress concentration factor Crack propagates if crack-tip stress (sm) exceeds a critical stress (sc) t Stress concentrated at crack tip

Fatigue(피로) Failure Fatigue is failure under repeated loading Crack initiation Fatigue crack propagation Catastrophic rupture 1. Crack initiation 2. Crack growth 3. Rapid fracture Fatigue is the weakening of a material caused by repeatedly applied loads. It is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values that cause such damage may be much less than the strength of the material typically quoted as the ultimate tensile stress limit, or the yield stress limit.

Fatigue Life (피로수명) Fatigue life : the number of stress cycles before failure S-N Curve : a graph of a stress amplitude S - against the number of cycles to failure N Effected by material, surface defects, mean stress No fatigue if stress < Fatigue limit (Endurance limit) Stress amplitude, S unsafe safe Number of cycles, N Al2014 S45C Fatigue limit 300 100 500 MPa 103 106 109 time Stress σ amplitude ASTM defines fatigue life, Nf, as the number of stress cycles of a specified character that a specimen sustains before failure of a specified nature occurs. S-N Curve is a graph of the magnitude of a cyclic stress (S) against the logarithmic scale of cycles to failure (N).

Improving Fatigue Life Eliminate surface defects (high surface finish) Impose compressive surface stresses (to suppress surface cracks from growing) : short pining, carburizing Remove stress concentrators : fillet concave corner Decrease mean and amplitude stress C-rich gas Compression stress C Fillet

Creep (크리프) Creep is elongation under a static load for a long time though stress is smaller than yield stress σ < σy Occurs at elevated temperature, T > 0.4 Tm (in K) time t strain ε T3 > T2 > T1 ε/t=   creep rupture