# Interviews at OEMs and Research labs – Connecting Theory to Practice

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.

### Entrant Level- Typical OEM

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

### Interviewer

Give one practical example for each mode of fatigue crack growth

### Candidate

Analysis: Crack propagation and loads have a direct correlation.
For example mode one is obvious as perpendicular loads try to open the crack and torsional load leads to tearing.

Note the second example is not obvious but if we write the stress state, it becomes easy to reckon that it is in mode II

### Entrant Level- Typical Research

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

Mixed crack propagation

### Interviewer

Give a practical example for mixed mode and explain how to compute stress intensity factor.

### Step1

Candidate must write the stress state to reckon if more than one mode is involved

Observation:
Mixed-mode
Three situations are shown for each type of loading. There could be a mixed-mode that would have a combination of the two modes. For example, the cylinder is subjected to axial pulling and torsion, mode I and mode III could be coupled. Stress intensity factors are defined for each mode and are designated as K1 K2 and K3.

### Step 2 Quantification

Box crack propagation is because of the mixed mode because of axial load it is opening mode and because of the torque it is  second mode.

If the box is subjected to a perpendicular force and also an axial torque then the situation becomes a mixed-mode situation

Irwin’s equation could be used for mixed-mode by the following formula, which is based on energy conservation.

This is because we assume, K1C≅K2C. As nature takes the path of least resistance, in a mixed-mode situation crack propagates in a direction that is of maximum hoop/tangential /circumferential

### Fairly experienced

Expectation: Critical thinking skills for product design

Determine the life of the component in seconds. Details of the situation are tabulated below:
Machine component has the following details

### Interviewer

Compute for what period of time the component survives?

### Step 1

We must calculate the final crack length beyond which the crack propagation rates become practically unacceptable

Observation: Linear fracture mechanics is very close to practice. Though we calculate the cycles ideally using Paris law some times large loads bring about crack tip plasticity hence propagation may be halted.

### Engineers' note:

As crack always progresses in the direction of least resistance as do components vibrate to dissipate max energy in the simplest modes.

### Step 2 - Candidate must quantify the problem understanding

Note: Using the fracture toughness, find the maximum crack length reached before failure, meaning, the rate of crack propagation becomes large

### Step 3 - Analysis

• The Paris law is used to study the fatigue crack propagation. The rate of crack extension per cycle of loading is a function of stress intensity range
• It is easy to see that as the R ratio decreases, the rate of crack propagation decreases
• Like endurance limit, for crack propagation to incept the threshold, stress intensity range (SIR) is needed. This again depends significantly on R ratio given an environment of operation

### Did you know?

• What are the various mechanisms that are could bring about crack closure?
• Could you justify the following conclusions:
• For certain R ratios, Kmin could be larger than Kopen, Crack does not close
• For certain R ratios, Kmin could be smaller than Kopen, Crack closes

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