Root Cause Failure Analysis. Trinath Sahoo

5.6 Brittle fracture in a mild steel.

      Source: Callister, William D. and Rethwisch, David G. (2009). Materials Science and Engineering: An Introduction, 8e. Wiley.

Example: Failure of a Pipe

      Figure 5.7 Ductile failure: ‐one piece ‐large deformation (after some amount of plastic deformation).

      Source: Colangelo, Vito J. and Heiser, Francis A. (1987). Analysis of Metallurgical Failures. 2e. Wiley.

Figure 5.8 Brittle failure: ‐many pieces ‐small deformation, (even when the stress is within the elastic range).

      Depending on the ability of material to undergo plastic deformation before the fracture two fracture modes can be defined – ductile or brittle. Ductile fracture is characterized by large amounts of plastic deformation. Proportionally large amounts of energy will therefore be required to induce this fracture. When designing a structure, it is usually preferable for the material to fail in a ductile manner as there will be sufficient warning (evidence of deformation) before the final failure occurs. In a brittle fracture, little plastic deformation and low energy absorption before fracture take place.

      Ductile fracture‐ most metals (not too cold):

       Extensive plastic deformation ahead of crack.

       Crack is “stable”.

       Resists further extension unless applied stress is increased.

Stages in Ductile Fracture

       Necking

       Cavity formation (micro cracks)

       Crack formation

       Crack propagation

       Fracture

Brittle Fracture Characteristics

       Takes place without any appreciable plastic deformation and by rapid crack propagation.

       Direction of crack motion is nearly perpendicular to the direction of applied tensile stress.

       Flat surface.

      Brittle fracture‐ ceramics, ice, cold metals:

       Relatively little plastic deformation.

       Crack is “unstable.”

       propagates rapidly without increase in applied stress.

Origin of Fractures (Ductile and Brittle)

      Every structure has a load limit beyond which it is considered unsafe. An applied load that exceeds this limit is known as overload. When a component fails because due to a single application of a load greater than the strength of the component, it is termed as overload failure. The nature of fracture arising due to overload failure could either be ductile or brittle or a combination of the two.

      In general, ductile fractures are associated with metal flow at failure zone due to plastic deformation and fibrous‐surface appearance. In brittle fractures, plastic deformation is almost absent and the surface shows irregular bright facets of a cleavage type. Establishing the origin of a fracture is essential in failure analysis, and the location of the origin determines which measures should be taken to prevent a repetition of the fracture. The fracture‐surface characteristics that show the direction of crack propagation (and conversely, the direction toward the origin) include features such as chevron marks, crack branching, and river patterns. Features that help identify the crack origin include concentric fibrous marks, radial marks, and beach marks. By a study of these features, crack progress can be traced back to the point of origin, and then, it can be ascertained whether the crack was initiated by an inclusion, a porous region, a segregated phase, a corrosion pit, a machined notch, a forging lap, a nick, a mar, or another type of discontinuity, or was simply the result of overloading.

      Some of the questions that should be raised concerning the nature, history, functions, and properties of the fractured part, and the manner in which it interacts with other parts, to find out root cause of failure are‐

       Loading. Where the nature, rate, and magnitude of the applied load correctly anticipated in the design of the part? Were repeated or cyclic loadings involved? What was the direction of the principal stress relative to the shape of the part? Where residual stresses present to an undesirable degree?

       Material. Was the recommended alloy used? Where its mechanical properties at the level expected? Where surface or internal discontinuities present that could have contributed to failure? Did the microstructure conform to that prescribed?

       Shape. Did the part comply with all pertinent dimensional requirements of the specification? Did the part have sufficient section thickness to prevent local overloading? Where fillets formed with sufficiently large radii? Where there adequate clearances between interacting parts? Where any of the contours deformed during service? Was there evidence of mechanical surface damage?

       Environment. Was the part exposed to a corrosive environment or to excessively high or low temperatures? Was the surface of the part suitably protected? Where the properties of the part altered by the exposure? Was there interaction (for example, galvanic) between the material of the part and that of adjacent components?

Fatigue Failures

      So far we’ve talked about the gross overloads that can result in immediate, almost instantaneous, catastrophic failures. Another type of failure occurs by means of progressive brittle cracking under repeated alternating or cyclic stresses of an intensity considerably below the normal strength. Although the fracture is of brittle type, it may take some time to propagate, depending on both intensity and frequency of the stress cycles. The number of cycles required to cause fatigue failure at a particular peak stress is quite large but it decreases as stress increases. A very important distinction is that fatigue cracks take time to grow across a part. In a fatigue failure, an incident of a problem can exceed the material’s fatigue strength and initiate a crack that will not result in a catastrophic failure for millions of cycles. There were cases where fatigue failures in 1200 rpm motor shafts that took less than 12 hours from installation to final fracture, about 830 000 cycles. On the other hand, there are cases the crack growth in slowly rotating process equipment shafts has taken many months and more than 10 000 000 cycles to fail.

      Fatigue fracture results from the simultaneous action of repeated or fluctuating cyclic stress, tensile stress, and plastic strain. No fatigue crack starts or grows in the absence of any of these three active components. Cyclic stress initiates the crack and tensile stress produces the crack growth. There are many variables in service that influence the fatigue behavior or characteristic pattern. These include the magnitude and frequency of application of the fluctuating stress, the presence of a mean stress, temperature, environment, part size and shape, state of stress and residual stresses, surface finish, surface damages, and microstructure. The occurrence of fatigue may be considered as a three‐stage process.

      1 Initiation of the surface or sub‐surface fatigue crack under a fluctuating load.

      2 Crack