Fracture Surface Features and What They Mean
A fracture surface records how a crack travelled, and the marks on it can be read at low magnification, under the microscope and in cross section. This page collects the macroscale and microscale features the source tabulates, what each one implies, and the rules for locating a crack origin from the fracture itself.
Where the crack started and where it went
The initiation site and the propagation direction come first, because every other reading depends on them. Chevrons, radial marks, ratchet marks and arrest line curvature all contribute.
How the metal separated
The mechanism and the path, ductile or brittle, transgranular or intergranular, with the microscopic features naming the process.
Whether the load was monotonic or cyclic
Beach marks and striations settle it when they are present, and their absence does not rule fatigue out.
The environment
Discoloration, reaction products, flutes and corrosion deposits record what the surface was exposed to, before and after the fracture.
The constraint the part worked under
Shear lips, the orientation of the surface, and whether arrest lines appear at all, describe how constrained the crack tip was.
Whether a fabrication imperfection was involved
Nicks, seams, machining marks and inclusions are recorded on the fracture, and the question of whether one of them caused the failure is a separate step.
- Seldom can one feature be tied to one unique cause. Macroscale examination on its own is often inconclusive, and microscale examination without the macroscale and microstructural picture can produce a wrong conclusion. The source describes the working method as a series of photographs at increasing magnification, each marking the region that the next photograph enlarges.
- The fracture surface only records crack propagation. Events before the crack nucleated, such as a shape change from prior deformation, show up in the part around the fracture or in the microstructure away from it, not on the fracture face.
| Mark or indication | What it implies |
|---|---|
| Visible distortion | Plastic deformation exceeded the yield strength, and it may point to an instability such as necking or buckling, or it may be damage that happened after the fracture. |
| Visible nicks or gouges | A possible crack initiation site. Look at the mark before deciding the fracture started somewhere else. |
| Orientation of the fracture surface | Read against the component geometry and the loading conditions. It helps separate the three loading modes and it identifies macroscale ductile and brittle fracture. |
| Flat fracture together with shear lips | The crack propagated parallel to the shear lips, and the mixed appearance indicates incomplete constraint. |
| Tightly closed crack at the surface | Possible cyclic loading, and also a possible processing imperfection such as shot peening damage or a quench crack. |
| Radial marks and chevrons | The v-shapes point back toward the crack initiation site and show the direction of crack propagation. |
| Arrest lines from monotonic loading | The u-shapes open in the direction of crack propagation and indicate incomplete constraint. |
| Arrest lines from cyclic loading | Beach marks or conchoidal marks indicate cyclic loading, and they grow from the centre of their radius of curvature. On a cylindrical section the curvature can reverse as the crack moves. |
| Ratchet marks | More likely under cyclic loading, and they mark the initiation site or sites where several cracks joined. |
| Discoloration of the surface or the fracture | May indicate a corrosive environment, or an elevated temperature that the surface saw before or after separation. |
| Oxidized fingernail | A patch of oxide on the fracture surface that can mark the crack initiation site. |
| Reflectivity of the fracture | Matte points to ductile fracture or to cyclic loading. Shiny points to cleavage. Faceted and shiny together points to intergranular fracture in a coarse grained steel. |
| Roughness of the fracture | Roughness rises in the direction of crack growth, though bending can reverse the pattern when the crack moves into the compressive region. A smooth region beside a rough one indicates cyclic loading, a rough matte fracture is ductile, and a sharp change in texture marks the transition from fatigue growth to overload. |
| General rubbing | May indicate vibration, and it can show the final direction of separation. A swirl pattern indicates torsion. |
| Localized rubbing | May indicate crack closure under cyclic loading, and it can wipe out beach marks that were there earlier. |
| Deformed draw marks and rolling scratches | A twisted appearance indicates torsion loading. |
| Machining marks | Marks running normal to the component axis are not distorted by torsion loading, which separates torsion from a bending load. |
| Variable roughness along the fracture edge | In brittle bending the rough side is the tension side. |
| Artifacts, tire tracks | Common in cyclic loading and caused by particulate matter trapped between the two faces. An artifact, not a mechanism. |
Source: ASM Handbook, Vol. 11, Failure Analysis and Prevention (ASM International), the macroscale fractographic features table in the article Fracture Appearance and Mechanisms of Deformation and Fracture.
| Mark or indication | What it implies |
|---|---|
| Dimpled surface | Ductile overload fracture at that location, from microvoid coalescence. |
| Faceted surface | Brittle cleavage fracture, and also possible in stress corrosion cracking or in low delta K fatigue. |
| Intergranular, smooth grain boundaries | Likely either improper thermal processing or environment assisted fracture at high temperature or in a corrosive environment. Low delta K fatigue is a less common cause. |
| Intergranular, dimpled grain boundaries | Decohesive rupture close to the melting point, or improper processing that left a denuded zone beside the grain boundary. |
| River or fan pattern | Cleavage fracture. The crack runs down the river, and the fan rays point to the initiation site inside the grain. |
| Tongues | Twinning deformation during rapid crack propagation. |
| Flutes on a transgranular surface | A corrosive environment together with ductile fracture. The crack propagates parallel to the flutes. |
| Striated or ridged surface | Fatigue striations from cyclic loading. Constant spacing points to a constant stress amplitude, variable spacing to a variable amplitude or block loading. A striated look can also come from second phases in the microstructure, so the surrounding features have to agree. |
| Grooves or flutes | Stress corrosion cracking. |
| Artifacts, mud cracks | Dried liquid on the surface, which may simply mean incomplete cleaning. In the as-received condition it can point to service fluids and to stress corrosion cracking, so the deposit should be analysed. |
Source: ASM Handbook, Vol. 11, the microscale fractography features table in the same article.
- Intergranular fracture is the clearest case of a feature that points at a family of causes rather than at one. It can come from thermal processing, from an environment assisted mechanism, or from low delta K fatigue, which is why the source cross-references it to a dedicated article.
- The mechanisms that produce a brittle looking fracture without an overload are collected on the steel embrittlement types page, and the six damage modes with their signatures at each scale are read against the damage mode identification chart.
| Load case | Where cracking is expected to start |
|---|---|
| Plain bar under axial tension, no notch | Along the centreline, unless a surface stress raiser is present. The near-surface and centreline regions are the two candidates, and the centreline wins when nothing else raises the local stress. |
| Three-point bending | At the point of maximum bending moment. A crack that starts somewhere else means a geometric or material imperfection moved the local maximum stress. |
| Rolling contact | Below the surface, where the maximum stress develops. A surface origin in a rolling contact usually means something else is at work, such as a dent, a lubricant film breakdown or a handling mark. |
| Torsion | The orientation of the fracture and the way any machining marks survive or distort separates torsion from bending. |
| Prior processing left residual stress | The residual field adds to or subtracts from the applied load, so the origin moves. It has to be considered together with the applied stress rather than on its own. |
Source: ASM Handbook, Vol. 11, the crack initiation section of the same article, and the metalworking article for the initiation site rules.
- Crack branching and bifurcation locate the origin when the part came apart in more than one piece and chevrons are visible. The branching pattern opens away from the origin, which is how a large fracture face is reduced to one area to examine closely.
- A discontinuity is not automatically a defect. Manufactured material carries imperfections, and an imperfection becomes a service defect only when it interferes with the function and the expected life of the part. Whether a flaw on the fracture surface caused the failure, or was simply present while something else did the damage, is a decision that quantitative analysis settles.
- That distinction is what keeps a failure investigation honest, and it is worked through case by case on the tool and die failure analysis page. The defects that reach a bench in bar stock form are catalogued on the steel bar defects page.
A crack that was already there before service
Temper color or scale on a fracture face dates the crack to a thermal cycle rather than to the load. How a quench crack is told apart from a service crack is set out on the quench cracking page.
A crack that started at a machined mark
Rough tool marks, stamp marks and spark eroded surfaces all act as stress raisers, and the layer a spark eroded surface leaves behind is set out on the EDM white layer article.
When the fracture looks brittle and no overload explains it
That group of failures usually comes from a change in the steel rather than from the load, and the temperature window behind it is normally visible in the process record.
When the surface hardness is part of the question
Hardness readings at the origin, at the surface and in the core are the cheapest test in the sequence, and the scales and their conversion are covered on the hardness testing methods page.
Confirm before quoting
This page is a reading aid for general reference. A fracture conclusion rests on the part itself, its manufacturing and heat treatment record and its service history. Confirm the finding with a metallurgical examination before changing a grade or a process.
Fracture surfaces are read at several magnifications in practice, and a conclusion drawn from one photograph can mislead. Keep the specimen and the record together.
Related reference pages
Tool steel heat treatment troubleshooting · Why D2 tool steel chips and cracks · Tool steels for cracking resistance · Tool steel quality verification · Cold work tool steels · Tool steel supply
Source: ASM Handbook, Volume 11, Failure Analysis and Prevention (ASM International), article Fracture Appearance and Mechanisms of Deformation and Fracture by W.T. Becker and S. Lampman, the macroscale and microscale fractographic feature tables and the fracture surface information section. Reference data for interpretation only. Confirm the finding on the actual part. Aobo Steel supplies tool steel in the annealed condition.