Factor of safety (FoS) is defined as:

Working stress divided by material strength Material strength divided by working (design) stress Maximum load divided by average load Elastic modulus divided by yield stress

A bolt is rated at a yield strength of 500 MPa. An engineer specifies it for a joint where the working stress will be 125 MPa. What is the factor of safety?

0.25 2.0 4.0 375

Static failure of a machine component occurs when:

Small repeated loads cause cracks to grow slowly over many cycles A single, large load causes the material to yield or fracture Operating temperature exceeds the material's melting point Vibration causes the part to loosen and separate from its mounting

Fatigue failure is responsible for approximately what fraction of real engineering failures?

About 10% About 50% About 90% About 25%

What is unique about fatigue failure compared with static failure?

Fatigue only occurs in brittle materials; ductile materials do not fatigue Fatigue can cause fracture at stresses far below the material's static yield strength, after many load cycles Fatigue always produces visible external cracks before final fracture Fatigue failure is always preceded by large plastic deformation

The endurance limit (fatigue limit) of a material is:

The maximum stress at which yielding begins under static load The stress amplitude below which the material can theoretically withstand an infinite number of cycles without fatigue failure The stress at which a crack first becomes visible to the naked eye The minimum stress needed to initiate plastic deformation during one cycle

Resonance in a mechanical structure occurs when:

The structure vibrates at a frequency higher than any applied force The frequency of an applied force matches the natural (resonant) frequency of the structure, causing oscillation amplitude to grow dramatically Two parts of a machine collide periodically due to excessive clearance A material reaches its melting point from internal friction losses

The Tacoma Narrows Bridge collapsed in 1940 due to resonance. Which design change in modern suspension bridges prevents similar failures?

Replacing steel cables with polymer ropes that absorb vibration Using stiffened (box section) bridge decks and aerodynamic cross-sections that resist torsional flutter at wind speeds within the design range Installing sensors that detect resonance and send evacuation alerts Designing the bridge to be flexible enough to absorb any wind frequency

Critical speed of a rotating shaft is the speed at which:

The bearing temperature exceeds the oil flash point, causing sudden seizure The shaft torque exceeds the yield strength of the shaft material The rotational frequency matches the shaft's natural (lateral) frequency, causing large-amplitude whirling vibration Centrifugal forces cause the shaft to elongate beyond its elastic limit

Design for Manufacturability (DFM) aims to:

Maximise the strength of every part regardless of cost or complexity Reduce manufacturing cost and complexity by designing parts that are easy and economical to produce with available processes Ensure every part can be 3D-printed without tooling Make parts as small as possible to save material costs

Design for Assembly (DFA) would most likely recommend which of the following design practices?

Use as many different fastener types as possible to suit each joint Design each part to require precise manual adjustment during assembly Reduce the number of separate parts by combining functions into single components and use self-locating features Specify very tight tolerances on all parts to ensure perfect fit

A stress concentration factor (Kt) greater than 1 at a notch or hole means:

The material at the notch is stronger than the surrounding material The notch reduces stress by distributing load over a larger area The actual local stress at the notch exceeds the nominal (average) stress by a factor of Kt, making it the most likely site for fatigue crack initiation The part will fail immediately upon loading regardless of the applied force