What are common signs of delamination in a laminate under load?

• A. Increases in stiffness under load.
• B. Strain localization without stiffness change.
• C. No changes in vibration or damping.
• D. Sudden reductions in stiffness, audible signs of debonding, changes in vibration/damping, and visible interlaminar cracks.

Answer: (D)

Delamination weakens a laminate by separating the layers, so the most telltale signs under load come from the sudden change in how the structure carries load and responds dynamically. As a crack grows between plies, load transfer is disrupted, causing a noticeable drop in stiffness. That stiffness loss often happens abruptly rather than gradually, because interlaminar failure reduces the laminate’s ability to stiffen with additional load. The crack’s propagation can produce audible signs of debonding as layers slip or separate. The dynamic response also changes: natural frequencies tend to shift lower and damping often increases because the cracked region dissipates energy differently than intact material. Visually, you can observe interlaminar cracks opening between layers. When you put all these together—stiffness reduction, possible audible cracking, changes in vibration and damping, and visible interlaminar cracks—they collectively indicate delamination under load. The other scenarios—stiffening under load, strain localization without a stiffness change, or no change in vibration or damping—don’t align with the behavior of a laminate that has started to delaminate.