Pluck a guitar string and its pitch depends on tension, length and mass. A structure is no different: its natural frequencies depend on mass and stiffness. Damage — a crack, a corroded section, a loosened connection — reduces stiffness locally, and that change ripples into how the whole structure vibrates. Vibration-based damage detection turns this physics into a monitoring method.
The core idea
Because natural frequencies and mode shapes are functions of stiffness and mass, a loss of stiffness shifts the frequencies down and subtly reshapes the modes. Measure those <strong>modal properties</strong> over time, compare them to a healthy baseline, and a persistent shift becomes evidence of damage.
Why vibration, and why global?
Vibration methods have a powerful advantage: a small number of sensors can sense the whole structure, because global modes involve the entire system. You do not need a sensor exactly on the flaw. Ambient excitation — traffic, wind, micro-tremors — is often enough, so the structure can be monitored in operation without any artificial shaker.
From frequencies to location
Natural frequencies tell you <em>that</em> something changed, but not <em>where</em> — two different defects can lower the same frequency by the same amount. Locating damage demands richer information:
- mode shapes and their curvature, which are locally sensitive to where stiffness was lost;
- model updating, where a numerical model is tuned until it reproduces the measured response, revealing which region softened;
- comparison of many modes at once, since damage affects each mode differently.
Across a daily cycle, a bridge's natural frequencies can move by several percent purely from temperature — often more than a real defect would cause. Credible vibration-based SHM must model and remove this swing before claiming damage.
Limits worth respecting
Global vibration is sensitive to global change but can be blind to small, local damage that barely moves the overall stiffness — an early fatigue crack, for instance. This is exactly why vibration methods are paired with local techniques like strain monitoring and acoustic emission near known critical details. The global view says the structure is healthy overall; the local view watches the spots most likely to fail first.
A mature, evolving method
Vibration-based detection is among the most studied approaches in SHM, with decades of theory and field experience behind it. Its frontier today is robustness: separating damage from environment reliably enough that a frequency shift triggers confident action rather than another investigation that finds nothing.