ig. 1 Schematic view of a component that has fractured in multiple pieces. If chevrons are visible on the fracture surface, the sequence of crack formation can be used to obtain the crack formation sequence and the location of the initiation site. Source: Ref 25 The fracture surface orientation relative to the component geometry may also exclude some loading conditions (axial, bending, torsion, monotonic versus cyclic) as causative factors. For example, crack initiation is not expected along the centerline of a component loaded in bending or torsion, even if a significant material imperfection is present at that location because no normal stress acts at the centerline. (There is a shear stress at this location in bending, but in a homogeneous material, it is too small to initiate fracture. That might not be the case for a laminated structure loaded in bending Alternatively, brittle torsion failure is readily identified at the macroscale in cylindrical sections because of the unique helical nature of the fracture surface(Fig. 2)(Ref 26) Fig 2 Wolfs ear helical fracture due to torsion loading.(a) Schematic of brittle torsion fracture of chalk. (b) Helical tensile fracture of oxygen-free high-conductivity copper bar prestrained in torsion to a shear strain of 4. 3x. Source(b): Ref 26 Surface roughness and optical reflectivity also provide qualitative clues to events associated with crack propagation. For example, a dull/matte surface indicates microscale ductile fracture, while a shiny, highly reflective surface indicates brittle cracking by cleavage or intergranular fracture. In addition, when intergranular fracture occurs in coarse-grained materials, individual equiaxed grains have a distinctive rock-candy appearance that may be visible with a hand lens Surface roughness provides clues as to whether the material is high strength(smoother) or low strength (rougher) and whether fracture occurred as a result of cyclic loading. The surfaces from fatigue crack growth are typically smoother than monotonic overload fracture areas. The monotonic overload fracture of a high strength quenched and tempered steel is significantly smoother overall than is the overload fracture of a pearlitic steel or annealed copper. Also, fracture surface roughness increases as a crack propagates so the roughest area on the fracture surface is usually the last to fail ( Fig. 3)(Ref 27). Fracture surface roughness and the likelihood of crack bifurcation also increase with magnitude of the applied load and depend on the toughness of the material(Fig 4)(Ref 28)Fig. 1 Schematic view of a component that has fractured in multiple pieces. If chevrons are visible on the fracture surface, the sequence of crack formation can be used to obtain the crack formation sequence and the location of the initiation site. Source: Ref 25 The fracture surface orientation relative to the component geometry may also exclude some loading conditions (axial, bending, torsion, monotonic versus cyclic) as causative factors. For example, crack initiation is not expected along the centerline of a component loaded in bending or torsion, even if a significant material imperfection is present at that location because no normal stress acts at the centerline. (There is a shear stress at this location in bending, but in a homogeneous material, it is too small to initiate fracture. That might not be the case for a laminated structure loaded in bending.) Alternatively, brittle torsion failure is readily identified at the macroscale in cylindrical sections because of the unique helical nature of the fracture surface (Fig. 2) (Ref 26). Fig. 2 Wolf's ear helical fracture due to torsion loading. (a) Schematic of brittle torsion fracture of chalk. (b) Helical tensile fracture of oxygen-free high-conductivity copper bar prestrained in torsion to a shear strain of 4. 3×. Source (b): Ref 26 Surface roughness and optical reflectivity also provide qualitative clues to events associated with crack propagation. For example, a dull/matte surface indicates microscale ductile fracture, while a shiny, highly reflective surface indicates brittle cracking by cleavage or intergranular fracture. In addition, when intergranular fracture occurs in coarse-grained materials, individual equiaxed grains have a distinctive rock-candy appearance that may be visible with a hand lens. Surface roughness provides clues as to whether the material is high strength (smoother) or low strength (rougher) and whether fracture occurred as a result of cyclic loading. The surfaces from fatigue crack growth are typically smoother than monotonic overload fracture areas. The monotonic overload fracture of a high￾strength quenched and tempered steel is significantly smoother overall than is the overload fracture of a pearlitic steel or annealed copper. Also, fracture surface roughness increases as a crack propagates so the roughest area on the fracture surface is usually the last to fail (Fig. 3) (Ref 27). Fracture surface roughness and the likelihood of crack bifurcation also increase with magnitude of the applied load and depend on the toughness of the material (Fig. 4) (Ref 28)