Interviews at OEMs and Research labs – Engineering Failures

An OEM or a research interview is designed to test the ability of the candidate to solve a problem by applying physics driven thinking skills. Unless the interviews are for people who would develop mechanical methods/mathematical models for high-end research interviews mostly check fundamentals, application and thinking skills.

Let us choose the topic "Engineering Failures" for the following three levels of interview

Entrant Level- Typical OEM

Expectation: Understanding of basic loads and ability to calculate stress in simple components

Interviewer

A hollow shaft made of ductile being subjected to a torsion. Find the stress concentration factor Kt for a through hole on the surface of the shaft as shown.

Candidate

Analysis: Whenever we need to evaluate the stress concentration factor, we need to relate it to standard stress state, uniaxial, biaxial, either with only tensile or combination of tensile and compressive stress-state.

The current stress state is pure shear and hence equivalent to a biaxial stress state with equal compression and tension

Here we compute stress concentration due to tensile stress state and superpose it with compressive stress state. That is the principle of superposition

For the current loading 𝝈=𝝉, 𝒕𝒐𝒓𝒔𝒊𝒐𝒏𝒂𝒍 𝒔𝒉𝒆𝒂𝒓 𝒔𝒕𝒓𝒆𝒔𝒔

It is obvious that maximum stress is 4 times that of the applied torsional stress. Hence maximum stress concentration factor is 4.

 

Entrant Level- Typical Research

Expectation: Understanding of basic loads and ability to observe unobvious facets

Interviewer

Explain how elevated operating temperatures affect the components and joints?

Candidate

Step1

What is the major effect of temperature on strength? Be it stiffness, yield strength or UTS.

Observation: In order to analyse the effect on component or joint we need to first list various possible modes of strength and structural failures.

Engineers' note:

Some times high temperatures may not lead to corrosion but thermal cycling does!

Step 2 - Candidate must quantify the problem understanding to conclude

  • “Some of the consequences are thermal softening, reduction in Yield strength and UTS, Creep, oxidation Visco elastic and Visco plastic behavior. Hence capturing worst temperature is very important” during product operation.
  • Assuming typical values for P (Larson-Miller constant), we find that creep life is reduced by 40% for a small value or 8% increase in temperature.
  • S-N curve falls as the temperature increases reducing the fatigue strength.
  • Natural frequencies decrease by 11% if Young’s modulus reduces by 20%.
  • Buckling capacity reduces by 20% if the Young’s modulus reduces by 20%.
  • Joint capacity reduces, be it a bolted joint or be it an interference joint.

Engineers' note:

Combined Creep LCF damage is more than the sum of individual damages

For thermal fatigue one should not use mechanical S-N curve

Fairly experienced

Expectation: Critical thinking skills for product design

Interviewer

Explain various possible failures of the piston ring and what it leads to?

Candidate

Step 1 - Comprehend the problem definition.

Observation: In order to understand the failures we should not only consider the loads but also it’s assembly and operating conditions.

Comprehension: Generally piston ring is mounted onto an external square grove of the tube. The pressure differential exists in one direction (from right to left). As the tube spins the piston ring also expands centrifugally.

Step 2 - Candidate must quantify the problem understanding

ISSUE one (Failure mode)
As the piston ring grows due to centrifugal load and due to temperature and since it is stopped by the outer ring the ring may fail by buckling

ISSUE two Failure mode and reparability)
Due to vibration, the ring may continuously rub the sealing tube and the wall thickness of the outer tube is critical and must be carefully designed. If it becomes too thin reparability is affected

Let us assume that P is the critical external pressure seen by the ring. If the growth of the ring is restricted, then we have:

Step 3 - Analysis

A tube is inside a tube  the air should not leak across the out springing piston ring works when the engine is running as tube is also rotating and has centrifugal load it seals but when engine is shut down piston ring slides to the middle

Did you know?

  • What are off-design failures? Give examples from auto, aero and propulsion industry products
  • Could we use dual piston rings in the above case?
  • Could we phosphate piston rings to increase ant scuffing properties?
  • Could we extend the idea of ring buckling to cylinder buckling if yes or if not explain why?

Learn with us most complex design facets!!

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