Stress Concentration

Stress concentration is a localized spike in internal stress that occurs when a structural component has a geometric irregularity, such as a hole, notch, or sharp corner.

How It Works#

Every aircraft structure carries load. That load travels through the material as internal stress. When the material's geometry is uniform, stress distributes evenly. When something interrupts that geometry, stress cannot flow smoothly and piles up at the disruption.

Think of stress like water flowing through a pipe. A sudden narrowing forces the flow to squeeze through a smaller space, increasing velocity. A geometric discontinuity does the same thing to stress. The local stress at the edge of a fastener hole, for example, can be two to three times higher than the average stress in the surrounding material.

Engineers quantify this effect with the stress concentration factor, written as $K_t$. It is a dimensionless ratio: the peak local stress divided by the nominal (average) stress in the section. A sharp, square notch produces a high $K_t$. A gently rounded fillet produces a much lower one. Designers reduce $K_t$ by eliminating sharp corners and smoothing transitions between sections.

Example in Aviation#

A technician drills a new fastener hole in an aluminum wing skin. The hole has a slight burr left on the edge from the drill bit. At that rough edge, $K_t$ rises sharply. Each flight cycle loads and unloads the wing, and the elevated local stress at the hole edge initiates a tiny fatigue crack. Over thousands of cycles, the crack grows. This is exactly how many real-world fatigue failures begin, and why aircraft maintenance manuals specify precise hole-finishing procedures.

Why It Matters#

Stress concentration is one of the leading contributors to fatigue cracking in metal airframes. Pilots and maintenance technicians who understand this concept recognize why seemingly minor surface defects, corrosion pits, and improper repairs deserve serious attention. A small scratch in the wrong location can reduce a component's fatigue life dramatically.

For students studying structures or preparing for maintenance certifications, stress concentration bridges the gap between textbook stress analysis and real inspection work. Recognizing a high-stress zone on a drawing, or understanding why a repair doubler needs rounded corners, depends directly on this concept.

Key Takeaways#

• Stress concentration is a local stress spike caused by geometric irregularities in a structure.
• Holes, notches, sharp corners, and corrosion pits all raise the stress concentration factor $K_t$.
• Higher $K_t$ means a greater risk of fatigue crack initiation at that location.
• Designers reduce $K_t$ with smooth fillets, rounded corners, and careful surface finishing.
• Maintenance procedures targeting hole quality and surface condition directly control stress concentration.