Understand the materials and technologies behind modern engineering.

A stress concentration is a local increase in stress near a notch, hole, sharp corner or other discontinuity. Its severity is often described by a stress-concentration factor: the peak elastic stress divided by nominal stress. Smooth transitions and larger radii usually reduce it.

The heat-affected zone (HAZ) is base material that does not melt during welding but experiences a thermal cycle sufficient to alter its microstructure and properties. Its behavior depends on alloy chemistry, peak temperature, cooling rate and prior condition.

Fatigue crack growth is the incremental extension of a crack under cyclic loading. Growth rate is often related to the range of stress intensity. Engineers integrate growth data to estimate the cycles required for a crack to grow between two sizes.

Hybrid aluminum–steel structures combine aluminum’s low density with steel’s strength and cost efficiency. Joining is challenging because of different melting points, thermal expansion and brittle intermetallic compounds, so mechanical, adhesive and solid-state techniques are often considered.

Damage tolerance is a design philosophy that demonstrates a structure can sustain anticipated flaws safely until they are detected and repaired. It links crack-growth prediction, residual strength, inspection capability and maintenance intervals.

Formability is a material’s ability to undergo plastic deformation into a desired shape without necking, tearing, wrinkling or other failure. It depends on ductility, anisotropy, strain hardening, sheet thickness, friction and the forming path.