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12.2.1 Analysis of Fracture Surfaces and Cracking

 Analysis of fracture surfaces and cracking

Fracture surfaces and the location of cracks can provide important clues regarding the cause and progress of damage. Conclusions can be made on the basis of the following characteristics:

  • Crack location
    • Cause of cracking: influences (e.g. corrosion, overstress)
    • Weak point: cause (e.g. design, material, production, operation, handling) and type (e.g.notch, inhomogeneity, crack)
    • Influence of the weak point on the fatigue resistance (crack growth)
    • Time at which cracking occurred (e.g. signs of overheating or corrosion, deposits, etc.)
  • Crack growth:
    • Direction: type and direction of loads (e.g. torsion, tension, flexure)
    • Growth rate
    • Load type (e.g. static, dynamic)
    • Material properties (e.g. oriented structure, inhomogeneities)
    • Load strength (e.g. stress amplitudes)
    • Load duration (e.g. from crack growth)
    • Temporal load progress (e.g. crack growth), identification of temporally-classifiable characteristics
    • Time at which cracking occurred (e.g. reverse calculation)
    • Construction (e.g. structural notch)
    • Surface influence (e.g. brittle coating, cracking)
  • Operating fracture/size of residual fracture:
    • Loads at residual fracture
    • Material properties (e.g. brittleness or ductile behavior)

 Fracture symptoms show type and progression of stresses

Figure "Fracture symptoms show type and progression of stresses" (Ref. 12.2-1): The macroscopic analysis of cracks and fractures can yield especially useful clues regarding the origin of cracks and the causal loads. If a microscopic inspection (e.g. SEM) is no longer possible due to damage to the fracture surface (e.g. corrosion or oxidation), macroscopic evaluation may be the only means of analyzing the fracture surface.

Macroscopic evaluation is necessary for extracting useful samples for microscopic analysis in which the sample is observed in the direction of crack progress, as well as for inspection of the crack zone. Technical literature (Ref. 12.2.1-6) that predates the widespread use of microscopic fracture surface analysis can be extremely useful in this process (up to about 1970).

The diagram shows examples of cracking in dynamically stressed parts.
In the hollow shaft of a low-pressure turbine in a small fan engine (top diagram), an undeformed crack was discovered that ran at an angle of less than 45° to the shaft axis (Ref. 12.2.1-2). The shaft material was a ductile steel. These characteristics indicate that this was a dynamic fatigue fracture. This type of crack progress is typical for fractures caused by dynamic torsion loads (also see Fig. "Fractures reveal initiating forces (dynamic)" and Ref. 12.2.1-9).
The bottom left diagram shows the complete fracture of a round rod due to one-sided dynamic flexural loads. The one-sided crack indicates this type of load. This assumes that there was no flaw in the crack area (“A”) that could have considerably lowered dynamic strength. The limited, arc-shaped dynamic fatigue fracture surface (“B”), which is relatively small compared to the residual force fracture, shows that the cross-section was subjected to powerful static prestress (average stress). This caused the crack to become instable at a small critical crack depth (Ills. 12.2-1 and 12.2-3).
In the center of the diagram, the circular cross-section with the opposing symmetrical cracks and arc-shaped limited dynamic fatigue fractures is the typical result of fatigue crack development under alternating flexural loads.
The diagram shows a shaft section with a crack at an angle of less than about 45°, which is typical for a dynamic torsion fracture, as in the hollow shaft above.

 Fractures reveal initiating forces (static)

Figure "Fractures reveal initiating forces (static)" (Refs. 12.2.1-6 and 12.2.1-7): The direction of force is an important factor that determines the crack progress and shape of the fracture. Other important influencing factors are the ductility/plastic deformability of the material, and the geometry (e.g. notches) of the part (Fig. "Fractures reveal initiating forces (dynamic)"). The appearance of violent fractures is also determined by fracture-mechanical factors such as cross-section thickness, crack size, and environmental influences (e.g. corrosion; Fig. "Forced fracture behavior by section thickness").
The violent fractures attributed to the damaging loads (left frame) are cleavage fractures (perpendicular to the normal stress) and shear fractures under shear stress. Except for cleavage fractures not occurring under shear stress, cleavage fractures or shear fractures are possible under all force directions, depending on the part shape and material properties (right frame). Combinations of both fracture types frequently occur, depending on the relevant fracture-mechanical conditions. In this way, fractures of sufficiently thin cross-sections under tension loads near the surface will generally be shear fractures (shear lips). Cleavage fractures will occur in the inner cross-section (also see Figs. "Forced fracture surface characteristics" and "Identifying expansion direction of a forced fracture ").

 Fractures reveal initiating forces (dynamic)

Figure "Fractures reveal initiating forces (dynamic)" (Ref. 12.2.1-3, 12.2.1-4 and 12.2.1-6): The widely varying appearances of cracks and fractures can provide experts with important clues regarding the causes and progression of damage. The damage symptoms are influenced in many different ways:

  • Loads:
    • Direction
    • Size
    • Type (dynamic, static)
    • Distribution in cross-section (e.g. residual stress)
  • Materials:
    • Ductility/brittleness
    • Orientation of structure (deformationduring production of unfinished stock, e.g. forging, rolling)
    • Directional flaws and fouling (e.g.machine material)
    • Surface changes
    • Coatings
    • Flaws and weak points
    • Location of flaws (surface, volume)
    • Embrittlement (e.g. hydrogen, aging)
  • Environmental influences:
    • Corrosion (e.g. stress corrosion cracking, see Volume 1, Chapter 5.4.2.1)
    • Oxidation
  • Crack development:
    • Crack location
    • Crack size

The diagrams not only show the typical appearance of the fractures, but also the expected crack development with its location and direction on the part surface.

 Fracture can show if a notch is result or cause

Figure "Fracture can show if a notch is result or cause ": Consequential damages from blade fractures often include impact notches at the edges of the fracture of the primary failed blade. On the other hand, dynamic fatigue fractures often result from damage-causing foreign object strikes. The question, then, is whether an impact caused the dynamic fatigue fracture, or whether it should be classified as consequential damage. If the fracture surface in the impact area is analyzable, then this question can usually be answered.
Dynamic fatigue fractures occur in the HCF range with no noticeable plastic deformations. Therefore, deformation of the fracture surface at the point of impact is an indication that it occurred after the fracture surface was created. This means that the notch is consequential damage (middle diagram). If the dynamic fracture surface (lines of rest, crack progress lines) at the notch at the fracture origin is not plastically deformed, then it can be assumed to be primary FOD damage (bottom diagram).

 Forced fracture surface characteristics

Figure "Forced fracture surface characteristics": Brittle or damaged fractures may no longer be visually and microscopically assessable, making it very difficult to accurately determine the fracture type. The identification of residual forced fractures and their delineation from the primary fracture (e.g. dynamic fatigue fracture) is of great importance for determining the fracture cause and/or estimating the static loads (mean stress, residual fracture size relative to primary fracture size).
If the fracture surface can still be analyzed with a microscope, SEM analysis can identify the forced fracture by means of typical forced fracture characteristics (usually dimples, top detail). The direction of deformation of the dimples can reveal the direction of the loads when the fracture occurred.
If the fracture surface can no longer be analyzed (deformed, oxidized, corroded), it is still possible to identify the loads that caused the damage:
In a metallographic cross-section, plastic deformations that locally eliminate the possibility of a dynamic fatigue fracture appear as flow lines (bottom left detail). Typical curved grain boundaries below the fracture surface (arrows) are a sign of a forced shear fracture.
Macroetching can be used to determine damage-causing fracture mechanisms, even in parts with heavily damaged fracture surfaces, such as occur in shafts. If the so-called “fibers” (tolerable, material-specific directional inhomogeneities created by the unfinished-stock production process) are bent in a typical manner in the fracture area, it indicates that a ductile forced fracture occurred in this area. One might consider that the fracture surface may have been deformed in the course of damage, creating a mistakable characteristic. However, experience has shown that this is not to be expected even in cases with heavily damaged fracture surfaces. The direction of deformation of the grain boundaries and “fibers” indicates the direction of loads when the crack occurred.

 Identifying expansion direction of a forced fracture

Figure "Identifying expansion direction of a forced fracture ": The progress direction of forced fractures can usually be determined by macroscopic and microscopic characteristics. This is especially important when trying to limit the location of a damage-causing crack or understand a damage process. In the inner section of the cleavage fracture (detail), the direction of crack progress is in the direction of the open pinnate structure. If Wallner lines have formed on the fracture surface of brittle materials (Fig. "Fracture characteristics of brittle materials"), their concentric arrangement around the fracture`s point of orgin indicates its location (similar to lines of rest).

 Fracture characteristics of brittle materials

Figure "Fracture characteristics of brittle materials" (Refs. 12.2.1-2 and 12.2.1-5): Brittle fracture surfaces, such as in ceramics and glass, are surprisingly suitable for analysis to determine the direction of crack progress. This is typically revealed by the Wallner lines on the fracture surfaces of ceramic materials. Although they resemble the lines of rest of dynamic fatigue fractures, the mechanism by which they are created is completely different. They are formed by the reflection and interaction of elastic waves in the material ahead of the crack tip. The line pattern indicates the point where the crack originated (model of wave development of a stone thrown in a pond, top diagram). In ideal cases, the appearance of the fracture also reveals the stress distribution. The side with compressive stress may be marked by a finely-structured “pinnate” fracture (top right diagram).
The crack pattern can also provide information regarding stress distribution and stress levels. One consequence of the reflection of the elastic wave ahead of the crack tip in a flexure-stressed cross-section (middle left diagram) is that the crack plane ahead of the pressure side undergoes an arching shift.
If fragments can be reassembled like a puzzle (bottom right diagram), the crack pattern is a clear indicator of the crack progress and order of crack development. When cracks grow, they can only branch out at slight angles. This means that the order of crack development in the depicted example must be 1, 2, 3, 4 (Ref. 12.2.1-4).
If a traversing crack borders several cracks, and therefore also runs along several individual fragments, it can be assumed that the traversing crack is the primary crack.
In brittle materials, the fragment size (middle right diagram) can supply information regarding the stress levels at the time of damage, provided the fragmentation is not dominated by subsequent impacts that shattered the primary fragments. This process can be explained as follows:
If the crack growth rate reaches the material-specific acoustic velocity, the crack will branch. The higher the tensile stress accelerating the crack in the cross-section is, the shorter the distance required for the crack to reach its critical growth rate. This means that as stress levels increase, the resulting fragments become smaller.
Sufficient experience allows one to come to conclusions regarding the stress levels when damage occurred based on the appearance of the fracture surface at the fracture origin (bottom left diagram and Fig. "Informations from brittle fracture surface"). In brittle materials, an arc-shaped fracture mirror forms concentrically around the crack origin. This is bounded by a pinnate fracture zone and the so-called “mist”. The stress levels can be estimated on the basis of the radius of the fracture mirror (Ref. 12.2.1-8). A prerequisite is that the necessary relationships in the affected material have been determined through experiments. The stress levels at the time of fracture can provide information about any stress peaks that may have briefly occurred. Delayed fractures of parts that are not externally stressed (e.g. stress corrosion cracking in glass in moist air, Fig. "Unexpected effects of new technologies") may indicate the presence of residual stresses.

 Estimating dynamic loads during crack growth

Figure "Estimating dynamic loads during crack growth": Understanding damage often requires knowledge of the loads during crack growth. This is especially true if unusually high operating loads are suspected. If it is possible to analyze the striations, then attempts can be made to estimate the loads during crack progress. In some cases, fracture mechanics can help determine the load levels (stress amplitude Ds) during crack growth through microscopic analysis of the spacing of fracture striations (Fig. "Development of fracture surface features") on a fatigue fracture surface. This can be accomplished as long as the crack growth is in stage II (Fig. "Development of fracture surface features") and is not noticeably influenced by additional effects such as environmental factors (e.g. corrosion) or creep (e.g. dwell times; Figs. "Types of crack growth under corrosion" and "Growth behaviour of small cracks"). The critical fracture surface analysis and the estimation of falsifying effects requires a great deal of experience to be practical and useful.
The distance “D a” of the striations (top right detail, Ref. 12.2.1-4) in a fatigue fracture is determined in relation to the crack size “a”. “D a” corresponds to a load cycle and therefore also the crack growth da/dN (Fig. "Development of fracture surface features"). If the Paris diagram (Fig. "Characteristic crack growth") for the affected material is known (diagram), then the corresponding amplitude of the stress concentration D K can be determined. The provided formula gives a rough approximation of the corresponding stress amplitude Ds.

 Informations from brittle fracture surface

Figure "Informations from brittle fracture surface": The introduction of materials that behave brittly, at least in a certain temperature range, into engines makes analysis of the fracture surfaces of these materials increasingly important (ceramics, glass, intermetallic phases). With a bit of luck, the fracture surface of a brittle material can be analyzed to estimate the stress levels when both the brittle fracture (Fig. "Fracture characteristics of brittle materials", Ref. 12.2.1-2) and the crack-causing failure occurred. Knowledge of the crack location (surface, volume) is especially important with brittle materials. Often, this can not be directly identified macroscopically. Only proper sample selection can make the necessary SEM analysis a success.
The left diagram shows the hub and shaft socket of a monolithic ceramic SiC turbine disk after a bursting test (Ref. 12.2.1-5). The detail shows the area of the fracture mirror with the clearly recognizable fracture-causing surface flaws.
The critical flaw size “ac” (Fig. "Influence of conditions at the crack tip"), which results in spontaneous fractures, can be used with the relationship KIc= s (p . ac))1/2 to determine the fracture stress s . The fracture ductility KIc of the SiC used in this case is about 3.5 MN/m3/2. In comparison, KIc in heat-treatable steels is about 100 MN/m3/2.

 Cracks and surface structure identify damage processes

Figure "Cracks and surface structure identify damage processes": The way in which a crack influences or is influenced by the surface topography, can indicate the temporal progress and causal influences of crack development. Part surfaces usually have typical characteristics from production and repair:

  • Machining grooves (turning, grinding, milling). Crack direction relative to grooves
  • Signs of etching on the surface and in the crack
  • Signs of peening (calottes), deformation of crack edges
  • Erosion from abrasive treatment (e.g. oxide blasting)
  • Deposits in the crack (e.g. from coatings)

Whether or not a crack has been influenced by production factors, e.g. pressed shut (see left detail) or widened (e.g. etched, middle detail), shows whether it was created before or after the machining process.

“A”: The crack runs through a machining groove (indicates it was created during later operation) and is influenced by a angled groove that is attributed to the installation process. The direction of crack deformation at the surface shows whether the crack was already present during installation (e.g. after an overhaul).

“B”: Cracks that run diagonally relative to the machining grooves may have been created before the chipping machining process. This crack pattern indicates that the damage was not causally influenced by the machining process.

“C”: This crack pattern indicates loads that are independent of the notch effect of the machining grooves. Possible causes are flaws in the unfinished stock, grinding cracks (typically run perpendicular to the grooves), or LCF cracks/thermal fatigue cracks (e.g. in the hub bores of a disk).

“D”: A crack deformed by an impression (e.g. FOD, handling; see left detail). The time when the impression occurred can reveal information about the crack development.

“E”: The presence of several short cracks in neighboring parallel grooves indicates that the grooves should not be considered as the cause of damage. A more likely cause is a high dynamic load that led to simultaneous cracking.

“F”: This crack, which traverses a prominent groove, indicates that the groove was causally related to the damage.

 Informations by gaping cracks

Figure "Informations by gaping cracks": Cracks can widen for various reasons, which can indicate damage-relevant influences.
The following typical mechanisms can cause cracks to gape open:

  • Tension residual stresses after crack development
  • Plastic deformation due to compressive stress during crack formation (e.g. thermal fatigue)
  • Plastic deformation after crack formation (e.g. consequential damage)
  • Material-removing influences (e.g. etching, erosion, corrosion)
  • Oxidation (e.g. influence of hot gas, Fig. "Influence of hot gases on crack growth")

The crack base and edges can help determine the cause of widening.
If the crack base is widened and rounded off, and especially if it has a thick oxide coating (right detail), it indicates slow, delayed crack growth. This is typical for thermal fatigue cracks (Fig. "Symptoms of thermal fatigue cracks").
Compressed crack edges indicate compressive stress in the load cycles. This characteristic has also been observed during thermal fatigue (Fig. "Cracks in integral turbine wheels").
Gaps can only widen due to plastic deformation if the material behaves in a ductile manner. This phenomenon in materials that become ductile at high temperatures indicates the temperature levels during crack development, and therefore also indicates the damage-relevant state of operation.
Closed cracks with signs of brittle fractures indicate temperature ranges in which the material behaves brittly. These can be very low (in intermetallic phases) or very high temperatures. Brittle fractures are observed at high temperatures when the crack occurs in the range of the solidus temperature (hot crack; Volume 2, Ill. 7.2.2-9.2). Even with dynamic cracks in the HCF range, no noticeable plastic deformations (i.e. widening) occur. In the case of LCF cracks, especially in the hub bores of disks, the plastic deformations induced by the cyclical deformation process lead to residual compressive stress, which closes the cracks (Fig. "Local plastic deformations during LCF"). Experience has shown that this pressing-together of the crack edges can even make penetrating inspections difficult.

 Crack symptoms give hints at causes

Figure "Crack symptoms give hints at causes": Cracks have many different appearances. Their pattern in the cross-section can often be characteristic and provide information regarding the damage mechanism and the crack growth (e.g. oxidation, Fig. "Influence of hot gases on crack growth"). In this diagram, typical crack patterns are attributed to damage mechanisms. However, it must be pointed out that the crack pattern does not permit universal or certain classification, and the accuracy of the evaluation depends largely on the experience of the individual conducting the analysis.

Typical characteristics of crack patterns are:

  • Crack direction relative to the grains(intercrystal, transcrystal, and mixed)
  • Straight and/or jagged
  • Crooked and/or diagonal to thenominal stress
  • Traversing and/or branching
  • Single crack or crack field
  • Crack length
  • Crack widening
  • Crack filling: e.g. foreign metal (Ill.12.4-4), refrozen own material melt, oxides
  • Coating of crack edges: e.g. mediafrom the production process (e.g. bronzing)
  • Reaction zones along the crack edges: depleted alloy components, enriched diffusion zones

References

12.2.1-1 W.H. Parker, “Engine Life Assessment Test Case TF41 LP Compressor Shaft Torsional Fatigue”, Agard Advisory Report 308 (AGARD-AR-308) of the “Propulsion and Energetics Panel Working Group 20 on Test Cases for Engine Life Assessment Technology”, 1991, Chapter 6, pages 95-137.

12.2.1-2 “Metals Handbook Ninth Edition Volume 12 Fractography”, ASM, Metals Park Ohio“, 1987, pages 118, 119, 112, 113, 744.

12.2.1-3 “Metals Handbook Ninth Edition Volume11 Failure Analysis and Prevention”, ASM, Metals Park Ohio”, 1986, page 111.

12.2.1-4 L. Engel, H. Klingele, “Rasterelektronenmikroskopische Untersuchungen von Metallschäden”, Carl Hanser Verlag Munich Vienna, 2nd edition 1982, ISBN 3-446-13416-6, pages 15, 36, 42-46, 80, 81-97.

12.2.1-5 A. Rossmann, “Schadenuntersuchung und Schadenverhütung an Bauteilen der Ingenieurkeramik”, from J. Grosch “Schadenskunde im Maschinenbau”, Expert Verlag, 1st edition 1980, ISBN 3-8169-1202-8, pages 76-96.

12.2.1-6 E.J. Pohl, “Das Gesicht des Bruches metallischer Werkstoffe”, Allianz Versicherungs AG und Münchner Rückversicherungs-Gesellschaft,1956, Volumes 1 and 2.

12.2.1-7 E.J. J. Broichhausen, “Schadenskunde, Analyse und Vermeidung von Schäden in Konstruktion, Fertigung und Betrieb”. Hanser Verlag Munich,Vienna, 1985, ISBN 3-446-13409-3, page 110.

12.2.1-8 D.W. Richerson, “Modern Ceramic Engineering”. Verlag, Marcel Dekker Inc., New York and Basel, pages 325-375.

12.2.1-9 D.J. Wulpi, “Torsional Fractures of Shafts”. periodical “Metal Progress”, August 1981, pages 25-31.

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