Design Flaw Cracks (image #4)
What: Linear or chip-ridden cracks
Where: Transitions in plane or shape of a part
Why: Load paths interrupted
Severity: Medium to high
Fix: Generally not repairable without rebuilding
Parts with sharp corners or large shape transitions are more susceptible to cracking. Cracks are most common on decks and internal vessel structure, and the forces responsible for the crack development usually run 90 degrees to the crack direction. Remember that fiberglass composites are fairly damage-tolerant, so a crack doesn’t necessarily mean that a catastrophic failure is imminent.
If the parts were made of metal, on the other hand, once a crack appears, larger failure could occur fairly soon. This also means that you can monitor smaller cracks in a composite structure for growth. A simple pencil mark made at the ends of the crack will enable monitoring of its growth over time.
The cracks seen above (image #4) are located on the internal structure of a stringer/liner system. It’s not uncommon to find boats where compromises have been made while shaping the structure to include amenities. In other cases the design is just plain bad.
The forces traveling along this structural component take an abrupt turn around a corner. The radius on the left should have been larger, which would have helped allow the stresses to dissipate. The sharp corner on the right is just plain wrong: There is absolutely nowhere for the force on the structure to go.
Radiused, rounded or sloped inside corners generally result in higher structural integrity. These cracks grew on successive days of on-water testing, as the marks show.