Which stress category occurs when a mass of flowing fluid induces a dynamic pressure on a conduit wall? Water hammer is an example.

• A. Fatigue stress
• B. Pressure stress
• C. Flow stress
• D. Thermal stress

Answer: (C)

Flow stress arises from the forces the moving fluid itself exerts on the wall, due to dynamic pressure associated with the flow and any changes in that flow. When fluid velocity is high or changes rapidly, it carries momentum that, upon interaction with a conduit wall, produces a pressure transient and a corresponding wall stress. Water hammer is a classic example because a sudden change in flow velocity creates a surge in pressure that acts on the pipe wall. This is different from static pressure acting on the wall (pressure stress), from temperature-driven stresses (thermal stress), or from repeated loading that causes fatigue stress. So the stress type described by a dynamic pressure from flowing fluid is flow stress.