1. DR. AL EMRAN ISMAIL
FRACTURE MECHANISMS

2. Fracture Mechanisms in Metals
Three micro-mechanisms of fracture in metals: (a) ductile fracture, (b) cleavage (intergranular), and (c) intergranular fracture.

3. Fracture Mechanisms in Metals
Three of the most common fracture mechanisms in metals and Alloys:
Ductile materials (Figure (a)) usually fail as the result of nucleation, growth, and the coalescence of microscopic voids that initiate at inclusions and second-phase particles.
Cleavage fracture (Figure (b)) involves separation along specific crystallographic planes. Note that the fracture path is transgranular. Although cleavage is often called brittle fracture, it can be preceded by large-scale plasticity and ductile crack growth.
Intergranular fracture (Figure (c)), occurs when the grain boundaries are the preferred fracture path in the material.

4. Voids growth and coalescence
Uniaxial tensile deformation of ductile materials.

5. Fracture Mechanisms in Metals
Figure above schematically illustrates the uniaxial tensile behavior in a ductile metal.
The material eventually reaches an instability point, where strain hardening cannot keep pace with the loss in the cross-sectional area, and a necked region forms beyond the maximum load.
In very high purity materials, the tensile specimen may neck down to a sharp point, resulting in extremely large local plastic strains and nearly 100% reduction in area.
Materials that contain impurities, however, fail at much lower strains.
Microvoids nucleate at inclusions and second-phase particles; the voids grow together to form a macroscopic flaw, which leads to fracture

6. Fracture Mechanisms in Metals
The commonly observed stages in ductile fracture are as follows:
Formation of a free surface at an inclusion or second-phase particle by either interface decohesion or particle cracking.
Growth of the void around the particle, by means of plastic strain and hydrostatic stress.
Coalescence of the growing void with adjacent voids.

7. Voids growth and coalescence
Once voids form, further plastic strain and hydrostatic stress cause the voids to grow and eventually coalesce.
Figures (a) and (b) are scanning electron microscope (SEM) fractographs that show dimpled fracture surfaces that are typical of microvoid coalescence.
Figure (b) shows an inclusion that nucleated a void.

8. Voids growth and coalescence
(a) (b)

9. Voids growth and coalescence
Void nucleation, growth, and coalescence in ductile metals: (a) inclusions in a ductile matrix, (b) void nucleation, (c) void growth, (d) strain localization between voids, (e) necking between voids, and (f) void coalescence and fracture.

10. Voids growth and coalescence

11. Voids growth and coalescence

12. Voids growth and coalescence

13. Voids growth and coalescence

14. Surface fracture apperances
Figure above shows Schijve’s model that provides details of a fracture surface exhibiting ductile fracture features.
These features are affected by applied stress condition, specimen geometry, flaw size, mechanical properties and environment.
Some solid materials can show a shear lip as an indicative of ductile fracture and as a result the fracture surface exhibit a slant (SL) area.
Some material exhibit double-shear lips which are indications of ductile fracture due to an overload.
Conversely, a brittle fracture surface is normally flat without shear lip.

15. Surface fracture apperances

16. CLEAVAGE
Cleavage fracture can be defined as the rapid propagation of a crack along a particular crystallographic plane.
Cleavage may be brittle but it can be preceded by large-scale plastic flow and ductile crack growth.
The fracture path is transgranular in polycrystalline materials.
The propagation crack changes direction each time crosses a grain boundary (the crack seeks the most favorable oriented cleavage plane in each grain)

17. CLEAVAGE
Figure (a) shown SEM fractographs of cleavage fracture in a low-alloy steel.
The multifaceted surface is typical of cleavage in a polycrystalline material; each facet corresponds to a single grain.
The “river patterns” on each facet are also typical of cleavage fracture.
These markings are so named because multiple lines converge to a single line.

18. CLEAVAGE
Figure (a) SEM fractographs of cleavage in A 508 Class 3 alloy.

19. Surface fracture apperances
Environmental effect, such as low and high temperatures and corrosive media can have a significant impact on the mechanical behavior and fracture appearances of solid bodies.
For instance, a ductile steel alloy may become brittle at relatively low temperatures.

20. Surface fracture apperances
Stage II fatigue failure: This is due to a change in crack growth direction of stage I in which the crack in a polycrystalline material advances along crystallographic planes of high shear stress.
The characteristics for fatigue failure of stage II can be summarized as in Figure below:

21. Surface fracture apperances

22. Surface fracture apperances
The fatigue events are described as:
Crack growth occurs by repetitive plastic blunting and sharpening of the crack front.
Shear deformation direction reverts to complete a full cycle in compression. This event may cause cleavage fracture.
If rapid crack growth rate occurs, then rapid failure takes place and beach marks and striations may be absent, regardless if the material is ductile or brittle.
Intercrystalline fracture is possible, particularly at the lower range of stress.

23. Surface fracture apperances
Formation of striation depends on the nature of the materials such as aluminum and Al alloys. However, steel may exhibit cleavage mechanism as a dominant fracture mode.
Striations indicate the changing position of the crack front with each new cycle of loading.
Ripple (annual ring) patterns can form on the fracture surface.
The domain of high-cycle fatigue prevails during stage II.

24. Surface fracture apperances
Figure (b): Model for the formation of striations –
Broek’s possible mechanisms for the formation of
striations in certain materials such as aluminum
alloys and in some strain-hardened alloys.

25. Surface fracture apperances
The possible stages during the formation of striations:
(Stages 1 & 2): Slip formation occurs at the crack tip due to a stress concentration. Slips form in the direction of maximum shear stress as explained by the Cottrell-Hull mechanism (Figure above).
(Stage 3): Other slip planes are activated and consequently, cross-slip may occur.

26. Surface fracture apperances
(Stage 4): Crack tip blunting occurs due to strain hardening which may activate other slip planes.
(Stage 5): The crack re-sharpens due to plastic deformation (plastic zone) embedded in the elastic surroundings. During load release, the elastic surroundings excerpt compressive stresses on the plastic zone. This reversed plastic deformation process closes and re-sharpens the crack tip.
(Stages 6 & 7): Crack closure and re-sharpen occur due to repeated loading, leading to more crack growth (extension).

27. Surface fracture apperances
Wavy slip lines in niobium after static deformation

28. Surface fracture apperances
Figure below shows the characteristic fatigue striations of an Al-alloy having a modulus of elasticity approximately equal to E = 72GPa.
Fatigue striations on a crack surface of
an Al-alloy (12000x)

29. Surface fracture apperances
These striations are ripples on the fractured surface caused by perturbations in the cyclic stress system.
The width of a striation depends on the fatigue stress but it is in the order 10-4 mm or less.
For instance, the apparent stress intensity factor range is related to striation spacing as empirically proposed:
where;
E = Modulus of elasticity (MPa)
x = Average striation spacing (m)

30. Surface fracture apperances
The striation spacing is a measure of slow crack growth per stress cycle and it may be constant for constant stress amplitude.
However, striations may not form when the stress range and the maximum stress are relatively large, leading to fast fatigue crack growth rate.

31. Surface fracture apperances
Determine the apparent SIF range and the fatigue crack growth rate for the aluminum alloy fracture surface shown in Figure below? (E = 72GPa)
Fatigue striations on a crack surface of
an Al-alloy (12000x)

32. Surface fracture apperances
Solution: The solution to this problem requires that the actual average striation spacing be calculated using the magnification given. The average striation spacing: x = 4 mm / 12,000 = m. ∆K=E(x/6)=17MPam

33. Surface fracture apperances
A stainless steel plate containing a 6 mm single edge crack was subjected to a constant cyclic loading with a stress ratio of R = 0. The plate was 5 mm thick, 20 mm wide and sufficiently long. Calculate the apparent SIF range and the maximum load. The crack growth rate as per Paris equation was:
da/dN=10-12(∆K)3.5

34. Surface fracture apperances

35. Surface fracture apperances

36. Surface fracture apperances

37. Surface fracture apperances