Failure Analysis Data

Damage Mode Identification Chart

A failed part has to be classified before it can be explained, and the classification is read from the fracture and from the damage around it. This chart sets out the six damage modes the source works with, what each one looks like at three levels of examination, and the brittle and ductile signatures that hold at every scale.

The six damage modes and how each one reads Instantaneous modes separate the metal at the moment of loading. Progressive modes consume the part over time, and they leave a region of slow growth that can be measured. The three examination columns below are the ones the source uses, and the order matters, because a part can be sectioned only once.

Mode	Visual, 1 to 50 times on the fracture surface	Scanning electron microscopy, 20 to 10,000 times	Cross section, 50 to 1000 times
Instantaneous failure modes
Ductile overload	Necking or distortion in the direction the load was applied, a dull fibrous fracture, and shear lips at the edges. The part has visibly changed shape before it separated.	Microvoids or dimples, elongated in the direction of loading. One crack with no branching. Slip bands emerging at the surface.	Grain distortion and flow beside the fracture, with an irregular transgranular path. The section shows how far the metal moved before it tore.
Brittle overload	Little or no distortion, a flat fracture, and a bright coarse crystalline texture. Radial marks or chevrons point back to the origin.	Cleavage or intergranular separation. The origin area often carries an imperfection or a stress concentrator such as a notch, a stamp mark or an inclusion.	Little distortion, with an intergranular or transgranular path. May relate to a notch at the surface or to a brittle phase inside the section.
Progressive failure modes
Fatigue	A flat progressive zone carrying beach marks, a final zone consistent with the direction of loading, and ratchet marks where several origins joined. A smooth region beside a rough one marks the change from fatigue growth to the final overload.	A worn, flat appearance in the progressive zone, with striations at magnifications above about 500 times. The final zone may be ductile or brittle.	A progressive zone that is usually transgranular with little apparent distortion. The final zone may be either ductile or brittle.
부식	General wastage, roughening, pitting or trenching. Stress corrosion and hydrogen damage can produce multiple cracks that look brittle rather than corroded.	A path that may be irregular, intergranular or through a selectively attacked phase. Energy dispersive spectroscopy can identify the corrodent.	General or localized surface attack, selective phase attack, and scales whose thickness and morphology record the exposure.
입다	Gouging, abrasion, polishing or erosion. Galling or scoring runs in the direction of motion, and fretting leaves roughened areas holding compacted powdered debris.	Wear debris and abrasive particles can be characterized by morphology and composition. Rolling contact fatigue looks like wear in its early stages.	Localized distortion at the surface consistent with the direction of motion, and embedded particles that identify the abrasive.
Creep	Multiple fissures that appear brittle, with external and internal fissures carrying reaction scale. Fracture follows a limited change in dimensions, and the damage builds over long exposure.	Multiple intergranular fissures covered with reaction scale, and grain faces that may show porosity.	Microstructural change typical of overheating, multiple intergranular cracks, voids on grain boundaries, and wedge shaped cracks at grain triple points. Some cold flow appears in the last stage.

Source: ASM Handbook, Vol. 11, Failure Analysis and Prevention (ASM International), the damage mode identification chart in the article Determination and Classification of Damage and the fracture mode identification chart in the article Overload Failures. The two charts use the same modes and the same three levels of examination, and they are merged here.

Beach marks and striations prove cyclic loading, and their absence does not rule it out. Fatigue surfaces do not always show either one, so a clean, flat fracture with a smooth to rough transition still deserves a fatigue reading. The individual marks and what each one implies are listed in the fracture surface features chart.
The modes combine. A fatigue or creep crack usually ends in a ductile or brittle overload, and the final separation tells you how the part let go rather than why the crack started. Proportion is the evidence, so the relative area of slow growth and final fracture is what dates the failure.
Wear and corrosion are progressive in the same way, and the classification covers them because a worn or corroded section loses net section until the last remaining metal overloads. That final overload is a consequence, so recording it as the cause reverses the finding.

What is usually behind each mode The mode narrows the list of mechanisms and ranks what is left. The second column below is the mechanism the mode points to, and the third is what the analyst checks to confirm it.

Mode	What is usually behind it	What the analyst checks
Ductile overload	The load exceeded the strength of the part. A short term rupture at high temperature and high stress also has a ductile appearance, so it has to be separated from creep.	Hardness, chemical analysis or destructive testing to confirm the alloy and its processing, and the loading direction to see whether the fracture was secondary to something else.
Brittle overload	The load exceeded the dynamic strength of the part. Low temperature moves the same material toward brittle behaviour.	Alloy and processing, plus toughness and grain size, and whether the loading direction points to an impact or to a secondary event.
Fatigue	Cyclic stress above the endurance limit. Prior mechanical or corrosion damage can start the crack, and alignment, vibration and balance feed the load.	Strength, surface finish, assembly and operation. A large fatigue zone with a small final zone points to high cycle low stress, and the reverse to low cycle high stress.
부식	The attack morphology and the alloy have to be assessed together, because severity depends on the pair rather than on either one alone.	pH, temperature, flow rate and dissolved oxidants, and whether the exposure has become more severe than the original design assumed.
입다	For gouging and abrasive wear the source of the abrasive decides the fix. Lubricant performance and failed seals or filters are the other two common feeders.	Hardness and microstructure of the worn surface, the abrasive itself, and the lubrication record.
Creep	Mild overheating, or mild overstressing at elevated temperature, over a long period. Unstable microstructures and a small increase in grain size raise the creep rate.	The service temperature against the design temperature, the alloy specification, and the exposure time. Voids and triple point cracks in a replica taken from the surface settle it in the field.

Source: ASM Handbook, Vol. 11, the contributing factors rows of the two identification charts.

A component that can fail by a progressive mode can often be replaced or repaired before it separates, which is why the classification matters for maintenance as much as for investigation. Steam piping, for example, is examined at set intervals for creep voids in the heat affected zones of longitudinal seam welds, and void formation at grain boundaries is the early warning.
Field metallography and replication, together with penetrant, boroscopic, ultrasonic and radiographic inspection, are the tools that find these characteristics on a part that is still in service. A part that has already cracked is read the same way with the section on a bench.

Brittle and ductile at every scale Brittle and ductile behaviour is not a property of the material alone, and the same fracture can look brittle at one magnification and ductile at another. The table below fixes the reading to the scale of observation, from the design calculation to the transmission electron microscope.

Scale of observation	Brittle	Ductile
Structural engineer	Applied stress at failure is below the yield stress	Applied stress at failure is above the yield stress
By eye, 1 times	No necking, shiny facets, crystalline and granular look	Necked, fibrous, woody look
Macroscale, below 50 times	Low reduction of area, low ductility	Medium to high reduction of area
Scanning electron microscopy, 100 to 10,000 times	Brittle microprocess, cleavage or intergranular separation	Ductile microprocess, microvoid coalescence
Transmission electron microscopy, above 10,000 times	May still show a large amount of local plasticity	A high amount of plasticity across the whole section

Source: ASM Handbook, Vol. 11, the article Overload Failures, Table 2, distinguishing characteristics of brittle versus ductile behaviour depending on the scale of observation.

The scale table explains why two analysts can describe the same fracture differently. A part that failed below its yield stress by the design calculation can still show local plasticity under the microscope, and a part that necked visibly can carry cleavage facets inside.
A fracture that looks brittle at every scale usually points at one of the embrittlement mechanisms rather than at a load. The steel embrittlement types page lists the temperature window and the steels for each one.

Start with the part history, not the fracture

The same damage mode can come from several steps, and the manufacturing and service record is what separates them. The stage by stage causes and the examination sequence are set out on the tool and die failure analysis page.

When the crack was present before service

A crack that already carried temper color or scale when the tool first ran came from the quench or from an earlier thermal cycle, and reading it as a service failure sends the fix to the wrong place. The proof is collected on the quench cracking page.

When the same mode keeps returning

A mode that repeats across tools points at the process rather than at the individual part. Damage that shows up as a heat crack network belongs with the thermal fatigue grades, and damage that shows up as scoring or material transfer belongs with the adhesive wear grades or the abrasive wear grades.

When the mode is a fracture without much warning

Brittle overload is the mode that ends a tool before it has worn, and it is usually a toughness or a heat treatment answer rather than a wear answer. The grades chosen for that load case are set out on the cracking resistance page.

Confirm before quoting

This chart is a classification aid for general reference. A conclusion on a specific failure rests on the part itself, its manufacturing record and its service history. Confirm the finding with a metallurgical examination before changing a grade or a process.

Fracture surfaces are read at three levels in practice, and a single photograph at one level can mislead. Keep the specimen, the record and the readings together.