Open-Hole Performance in Composite Laminates

Why holes are the weak point

Composites carry load through continuous fibers. A hole drilled through a cured laminate cuts those fibers. The load they were carrying has to divert around the opening, and it concentrates in the material at the hole boundary — which is precisely where the reinforcement has just been severed.

Drilling adds a second problem on top of the first. As the drill enters, it tends to lift and separate the surface plies; as it breaks through the far side, it pushes the last plies apart. Both mechanisms produce delamination around the hole, and delamination is a defect that grows under fatigue. Machining quality is sensitive to feed rate, spindle speed and how well the back face is supported, so the damage also varies from hole to hole and from operator to operator.

Designers deal with this the way they usually deal with an unavoidable knockdown: by adding material. The laminate is made thicker, or extra plies are added locally, so the reduced section still meets the load requirement with an acceptable margin. That works, but it spends weight and cost to recover strength the part originally had — which is an awkward trade in a structure chosen for being lightweight in the first place.

Forming the opening instead of cutting it

Tailored fiber placement offers a different answer: do not create the hole by removing material. Because the tow is steered along a programmed path, the reinforcement can be routed around the opening while the preform is being built. The fibers curve past the boundary and remain continuous, so the load path around the hole stays intact and there is no drilling operation to initiate delamination.

In our own work, replacing a drilled hole with a preformed opening surrounded by continuous curved reinforcement retains significantly more tensile strength than the drilled equivalent. The same principle extends to bearing surfaces and load-introduction features, where local reinforcement can be built into the preform rather than bonded or bolted on afterwards.

This is also why preforming tends to pay off most on parts with a lot of features. A flat panel with no penetrations has little to gain. A bracket, lug, or fitting with several load paths and several holes has a great deal to gain, because every one of those features is a place where a conventional laminate is being cut.

Further reading

Open-hole behaviour in notched laminates, and the specific case for tailored placement around the notch, have both been studied in published research. These are third-party works — we did not produce them, and they are linked here so you can read them at the source.