Root Cause Failure Analysis. Trinath Sahoo

Figure 5.1 Stress–strain diagram of a medium‐carbon structural steel.

      Rapture Strength

      Rapture strength is the strength of the material at rupture. This is also known as the breaking strength.

      Elastic Deformation

      Elastic deformation occurs when stiffness of the component is less and the same is primarily determined by modulus of elasticity and cross section. Elastic deformation can lead to the failure of mechanical components especially in high precision assemblies and machinery where even small elastic deformation under operating conditions is not acceptable.

      Plastic Deformation

      Excessive plastic deformation of the mechanical components can lead to the failure in two conditions (i) externally applied stress is beyond the yield strength limit and (ii) component is subjected to applied stress lower than yield stress but exposed to high temperature conditions enough to cause creep.

      To avoid the failure by plastic deformation owing to externally applied stress more than yield strength, the cross section should be designed after taking proper factor of safety and considering the yield strength of materials of which component is to be made. For mechanical components that are expected to be exposed in high temperature, creep resistant materials should be selected so that under identical load condition, low steady‐state creep rate of creep‐resistant materials can allow desired longer creep life.

      Identification of Types of Failures

      Failure analysis is separated into two distinct parts, the first being the mode of failure, and second, the cause of failure. The mode is the failure process, and the cause is the part that can be altered or changed to prevent future occurrence. Some commonly recognized failure modes are as follows:

       Fracture (Ductile and Brittle)

       Fatigue (mechanical and thermal)

       Stress Corrosion

       Hydrogen Damage

       Corrosion

       Wear and Erosion

      Fracture – Ductile Overload vs. Brittle Overload Failures “Ductile failure” is one where there is a great deal of distortion of the failed part. Commonly, a ductile part fails when it distorts and can no longer carry the needed load, like an overloaded steel coat hanger. However, some ductile parts break into two pieces and can be identified because there is a great deal of distortion around the fracture face, similar to what would happen if you tried to put too much load on a low carbon steel bolt.

      The term “brittle fracture” is used when a part is overloaded and breaks with no visible distortion. This can happen because the material is very brittle, such as gray cast iron or hardened steel, or when a load is applied extremely rapidly to a normally ductile part. A severe shock load on the most ductile piece can cause it to fracture like glass.

An important point about failures is that the way the load is applied, i.e., the direction and the type, can be diagnosed by looking at the failure face. A crack will always grow perpendicular to the plane of maximum stress. Below we show examples of the difference in appearance between ductile overload and brittle overload failures.

      We know we can look at an overload failure and knowing the type of material, tell the direction of the forces that caused the failure. Common industrial materials that are ductile include most aluminum and copper alloys, steels and stainless steels that are not hardened, most nonferrous metals, and many plastics. Brittle materials include cast irons, hardened steel parts, high strength alloyed nonferrous metals, ceramics, and glass.

      One note of caution is that the type of fracture, ductile, or brittle should be compared with the nature of the material. There are some instances where brittle fractures appear in normally ductile materials. This indicates that either the load was applied very rapidly or some change has occurred in the material, such as low temperature embrittlement, and the material is no longer ductile. An example of this was a low carbon steel clip used to hold a conduit in position in a refrigerated (−50 °F) warehouse. The clip was made from a very ductile material, yet it failed in a brittle manner. The investigation showed it had been hit by a hammer, a blow that would have deformed it at normal temperatures.

      In a brittle overload failure, separation of the two halves isn’t quite instantaneous, but proceeds at a tremendous rate, nearly at the speed of sound in the material. The crack begins at the point of maximum stress, then grows across by cleavage of the individual material grains. One of the results of this is that the direction of the fracture path is frequently indicated by chevron marks that point toward the origin of the failure.

Brittle vs. Ductile Fracture characteristics

      Ductile materials‐ extensive plastic deformation and energy absorption (“toughness”) before fracture.

      Brittle materials‐ little plastic deformation and low energy absorption before fracture.

      Figure 5.2 Stress–strain curve of brittle and ductile material.

      Figure 5.3 Ductile vs brittle fracture. (a) Very ductile, soft metals (e.g., Pb, Au) at room temperature, other metals, polymers, and glasses at high temperature. (b) Moderately ductile fracture, typical for ductile metals. (c) Brittle fracture, cold metals, ceramics.

      Ductile Fracture

      Figure 5.4 Different stages before ductile fracture. (a) Necking (b) formation of microvoids (c) coalescence of microvoids to form a crack (d) crack propagation by shear deformation (e) fracture.

Figure 5.5 Cup and cone fracture in Al.

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

      Brittle Fracture

       No appreciable plastic deformation

       Crack propagation is very fast

       Crack propagates nearly perpendicular to the direction of the applied stress

       Crack often propagates by cleavage – breaking of atomic bonds along specific crystallographic planes (cleavage planes).