Ductile and Brittle Fracture Identification

When a ductile metal is loaded to failure, the fracture surface reveals the history of a process that began at a microscopic scale. The first stage is void nucleation. Voids form at second-phase particles or inclusions, either by decohesion of the particle-matrix interface or by fracture of the particle itself. The density and spacing of inclusions in the microstructure controls when this starts.

Once voids have nucleated, they grow as the surrounding matrix stretches and thins under continued loading. When neighbouring voids grow close enough, the ligament between them becomes unstable and ruptures, linking two voids into one. This void coalescence continues across the cross-section until the part separates. The resulting fracture surface, examined under SEM, is covered with bowl-shaped depressions called dimples. Each dimple is the imprint of one half of a void; the matching half is on the opposing fracture face. The dimple floor often contains a small particle: the original nucleation site.

Dimple morphology gives additional information. Equiaxed dimples form under pure tensile loading, where voids open symmetrically. Elongated or parabolic dimples form under shear loading, where one face drags past the other. Tear dimples are elongated in the opposite direction on opposing faces. Recognising these patterns lets an analyst determine whether the load was tensile, shear, or a combination, which speaks directly to what force caused the failure.

Brittle fracture by cleavage is the opposite of the void-growth picture. The crack moves through the grain along a specific low-index crystallographic plane, almost without any plastic work. In iron and low-carbon steel the preferred cleavage plane is {100}. The crack front propagates at a fraction of the speed of sound, crossing the component in milliseconds.

At the grain scale the fracture looks flat and glassy. But grains are not all oriented the same way, and adjacent grains have their {100} planes tilted slightly relative to each other. When the crack front jumps from one grain to the next, it has to deflect slightly to stay on the cleavage plane in the new grain. This deflection creates a step on the fracture surface. Many of these steps join up to form river marks, lines that flow across the grain surface converging in the upstream direction, toward the crack origin. Following river marks back to their convergence point takes the examiner to the fracture origin.

At the macroscopic scale, cleavage fractures in thick sections often show chevron marks, broad V-shaped or herringbone patterns visible to the naked eye. These also point toward the fracture origin. The combination of macroscopic chevron marks to find the approximate origin location and microscopic river marks to locate it precisely is the standard fractographic workflow for brittle fracture investigation.

In normal circumstances grain boundaries in metals are stronger than the grain interiors, so cracks go through grains (transgranular fracture) rather than around them. Intergranular fracture happens when this relationship reverses, when the boundary has been weakened to the point where it becomes the preferred crack path. This inversion is a flag that something has gone wrong with the material or its environment.

• Temper embrittlement: in alloy steels, slow cooling through or prolonged holding in the 375-575 degree Celsius range segregates phosphorus and other trace elements to grain boundaries, reducing boundary cohesion. An otherwise acceptable steel can be made brittle by the wrong heat treatment.
• Hydrogen embrittlement: atomic hydrogen diffuses into high-strength steel and accumulates at grain boundaries and at the crack tip. It reduces the cohesive strength of the boundary and can produce intergranular fracture at stresses far below the yield strength. High-strength bolts, landing gear, and spring steels are particularly susceptible.
• Stress-corrosion cracking: certain alloy-environment combinations produce intergranular cracking under sustained tensile stress. Austenitic stainless steel in chloride solution and brass in ammonia are classic examples. The crack moves along boundaries sensitised by prior corrosion.
• Liquid-metal embrittlement: a liquid metal in contact with a stressed solid can penetrate grain boundaries and cause rapid intergranular cracking. Brass in liquid mercury and aluminium in contact with liquid gallium are standard examples; the mechanism is relevant to fire investigations where low-melting alloys were present.

Under SEM, intergranular fracture has a faceted, rock-candy appearance. The grain surfaces are relatively smooth, reflecting the rounded shape of the grains, and adjacent facets meet at angles reflecting the grain boundary geometry. Energy-dispersive X-ray spectroscopy (EDS) can detect segregated embrittling elements on the grain faces, providing direct chemical evidence of the embrittlement mechanism.

The scanning electron microscope resolves fracture surface features at scales impossible with optical microscopy. Its large depth of field means complex topography stays in focus across the image, and its back-scattered and secondary-electron imaging modes reveal both topographic and compositional contrast. But SEM fractography only pays off if the sample preparation is done correctly.

1. Macroscopic examination and photography
   Before any cleaning or sectioning, photograph the fracture surface at natural scale with a calibrated scale bar. Mark the approximate origin location from chevron or river marks, identify any pre-existing crack regions (darker, oxidised) versus final overload regions (brighter, fresh). Record location, orientation, and any visible corrosion or mechanical damage.
2. Cleaning
   Remove loose corrosion products that would obscure fine features using acetone or isopropyl alcohol and a soft brush. If tenacious corrosion is present, a 5-10% ammonium citrate solution removes iron oxides without attacking the underlying metal. Never scrub: fracture surface topography is irreplaceable evidence.
3. Sectioning
   Cut the sample to a size that fits the SEM specimen stub, using a low-speed diamond saw and keeping the blade well clear of the fracture surface. Avoid bending the fracture surface: mating halves should be kept together and handled separately from the start.
4. Sputter coating (if needed)
   Non-conducting samples such as ceramics, polymers, and heavily corroded metals require a thin gold or carbon coating to prevent charge build-up. Steel and aluminium alloys are usually conductive enough to image without coating.
5. SEM imaging at multiple magnifications
   Start at low magnification (50-200x) to locate the origin and map the fracture zones. Step up in magnification to confirm the fracture mechanism: dimples confirm ductile fracture, cleavage facets and river marks confirm brittle fracture, fatigue striations confirm cyclic loading. Capture EDS spectra from any unusual particles or grain-boundary deposits.

At 18:30 on 27 March 1980, the semi-submersible drilling rig Alexander Kielland capsized in the Ekofisk field in the North Sea during a force-6 gale. Of the 212 people on board, 123 died. The Norwegian government commission that investigated the accident produced one of the most detailed fracture analyses in offshore engineering history.

The rig consisted of five pontoons supporting five buoyancy columns, connected by bracings. Bracing D-6 connected column D to the central platform. An external hydrophone had been attached to D-6 by a fillet weld through a hole drilled in the bracing tube wall. This attachment was not shown on the original design drawings. The weld created a severe stress concentration at the edge of the hole.

Fatigue cracks initiated at the weld toe, driven by the cyclic wave-loading the bracing experienced during every sea state. The cracks grew over what the commission estimated as approximately two years of service. When they reached a critical size, D-6 fractured. The fracture surface showed flat brittle cleavage over the pre-existing fatigue crack region, confirming that final failure was by brittle fracture once the remaining ligament was too small to sustain the applied stress. The sudden loss of D-6 overloaded the adjacent bracings, which failed in rapid succession within seconds, and the platform capsized.

The ductile-brittle distinction is cleaner in a textbook than in a failure laboratory. Real fractures frequently show mixed zones: a central flat cleavage region surrounded by ductile shear lips, or a fracture surface that transitions from dimple fracture in warm service to cleavage at a stress concentration where local temperatures are lower. Recognising mixed-mode fracture is important because the blend of features carries its own diagnostic information.

One category deserves special mention: quasi-brittle fracture in high-strength alloys. An aerospace aluminium alloy or a high-strength steel fastener can fracture with a flat surface and little visible deformation, and it can look brittle macroscopically. But SEM reveals dimples, just very small ones, because the material did deform plastically, just over a much smaller scale than mild steel would. Calling this fracture brittle without the SEM evidence leads to an incorrect failure analysis. The implication is that quasi-brittle fracture is a high-energy process, similar to overload rather than embrittlement, which changes how liability is assessed.