Monotonic versus fatigue fracture: Beach marks and striations indicate fatigue, but their absence does not confirm fracture from monotonic loads. Fracture surfaces from fatigue do not always reveal beach marks and fatigue striations Macroscale ductile versus brittle fracture: Macroscale ductile fracture is revealed by obvious changes in cross section of the fracture part and/or by shear lips on the fracture surface. Macroscale brittle ractures have fracture surfaces that are perpendicular to the applied load without evidence of prior deformation Macroscale fracture surfaces can have a mixed-mode appearance(brittle-ductile or ductile- brittle). The brittle-ductile sequence is more common on the macroscale, while the appearance of the ductile portion is typically microscale in a ductile-brittle sequence Microscale ductile versus brittle fracture: Microscale ductile fracture is uniquely characterized by dimpled fracture surfaces due to microvoid coalescence Microscale brittle fractures are characterized by either cleavage(transgranular brittle fracture)or intergranular embrittlement Table 1 Macroscale fractographic features Mark/Indication Visible distortion Plastic deformation exceeded yield strength and may indicate instability(necking, buckling)or post-faill ure damage I Visible nicks or gouges Possible crack initiation site Fracture surface orientation relative to Helps separate loading modes I, Il, Ill component geometry and loading Identifies macroscale ductile and brittle fracture. (see Fig. 2) conditions Both flat fracture and shear lips present Crack propagation direction parallel to shear lips on fracture surface Mixed-mode fracture(incomplete constraint) Tightly closed crack on surface Possible cyclic loading Possible processing imperfection, e. g, from shot peening, quench cracks Radial marks and chevrons(v-shape) Point toward crack initiation site Show crack propagation direction(see Fig. 5, 6) Crack arrest lines(monotonic loading) Lines point in direction of crack propagation (u-shape) Indicate incomplete constraint(see Fig. 15) Crack arrest lines(cyclic loading) Indicates cyclic loading (beach marks, conchoidal marks) Propagation from center of radius of curvature Curvature may reverse on cylindrical sections as crack propagates(see Fig. 40) Ratchet marks More likely in cyclic loading Indicates initiation site(s)(see Fig. 40, 41) Adjacent surface and or fracture surface May indicate corrosive environment discoloration May indicate elevated temperature Oxidized fingernail on fracture surface Possible crack initiation site Fracture surface reflectivity Matte: ductile fracture or cyclic loading Shiny: cleavage likely Faceted ("bumpy )and shiny; intergranular fracture in large grain size Fracture surface rough Increase in surface roughness in direction of crack growth· Monotonic versus fatigue fracture: Beach marks and striations indicate fatigue, but their absence does not confirm fracture from monotonic loads. Fracture surfaces from fatigue do not always reveal beach marks and fatigue striations. · Macroscale ductile versus brittle fracture: Macroscale ductile fracture is revealed by obvious changes in cross section of the fracture part and/or by shear lips on the fracture surface. Macroscale brittle fractures have fracture surfaces that are perpendicular to the applied load without evidence of prior deformation. Macroscale fracture surfaces can have a mixed-mode appearance (brittle-ductile or ductile￾brittle). The brittle-ductile sequence is more common on the macroscale, while the appearance of the ductile portion is typically microscale in a ductile-brittle sequence. · Microscale ductile versus brittle fracture: Microscale ductile fracture is uniquely characterized by dimpled fracture surfaces due to microvoid coalescence. Microscale brittle fractures are characterized by either cleavage (transgranular brittle fracture) or intergranular embrittlement. Table 1 Macroscale fractographic features Mark/Indication Implication Visible distortion Plastic deformation exceeded yield strength and may indicate instability (necking, buckling) or post-failure damage Visible nicks or gouges Possible crack initiation site Fracture surface orientation relative to component geometry and loading conditions · Helps separate loading modes I, II, III · Identifies macroscale ductile and brittle fracture. (see Fig. 2) Both flat fracture and shear lips present on fracture surface · Crack propagation direction parallel to shear lips · Mixed-mode fracture (incomplete constraint) Tightly closed crack on surface · Possible cyclic loading · Possible processing imperfection, e.g., from shot peening, quench cracks Radial marks and chevrons (v-shape) · Point toward crack initiation site · Show crack propagation direction (see Fig. 5 , 6) Crack arrest lines (monotonic loading) (u-shape) · Lines point in direction of crack propagation · Indicate incomplete constraint (see Fig. 15) Crack arrest lines (cyclic loading) (beach marks, conchoidal marks) · Indicates cyclic loading · Propagation from center of radius of curvature · Curvature may reverse on cylindrical sections as crack propagates (see Fig. 40) Ratchet marks · More likely in cyclic loading · Indicates initiation site(s) (see Fig. 40 , 41) Adjacent surface and or fracture surface discoloration · May indicate corrosive environment · May indicate elevated temperature Oxidized fingernail on fracture surface Possible crack initiation site Fracture surface reflectivity · Matte: ductile fracture or cyclic loading · Shiny: cleavage likely · Faceted (“bumpy”) and shiny; intergranular fracture in large grain size Fracture surface roughness · Increase in surface roughness in direction of crack growth