Predicting through-hole impact on boat structural girders

“Boat owners want an interior to be proud of,” says Susan Lake, senior R&D engineer at marine engineering firm High Modulus (Hamble, Hants, U.K., and Auckland, New Zealand), noting that that usually means concealing ventilation pipes, electrical wiring and other systems. Because these systems must share limited space, unanticipated access holes are often drilled through critical structural elements.

High Modulus set out to determine what impact these holes have on the strength of a hull/deck stiffener, built as a highly orthotropic laminate subjected to shear stress. Orthotropic refers to a laminate with perpendicular planes of elastic symmetry — in other words, having a balanced ply architecture. While there is much data on through-penetrations, Lake says conventional stress concentration models don’t always address this type of problem occurring in shear webs in marine structures.

High Modulus developed its own in-plane shear test method, which mimics real-world loading scenarios, and compared the test results with finite element analyses (FEA) performed with Hyperworks software developed by Altair Engineering (Troy, Mich.). The test samples were composite I-beams simulating shear webs, each 1,950 mm/6.4 ft long and 336 mm/1.2 ft deep. Each web featured two layers of 436 g/m2 E-glass double bias (±45°) stitched fabric at 53 wt-% on either side of a 15-mm/0.59-inch Airex 130 kg/m3 foam core, supplied by Alcan Airex AG (Sins, Switzerland). Holes ranging from 50 mm to 150 mm (2 inches to 6 inches) in diameter were drilled through test beams; slots up to 200 mm/8 inches long and diamond shapes also were created. A 50-ton hydraulic actuator applied pressure to the top flange of each beam to induce structural failure.

Not surprisingly, both the actual mechanical tests and the FEA models showed that through-holes caused stress concentrations that reduce the load-carrying capacity of the shear web. For example, a 336-mm test web with a single 50-mm diameter hole failed at a load only half of what was expected when compared to a solid, hole-free web 250 mm/10 inches deep.

“The lesson from this is that small holes have a big impact on the structural integrity of girders,” Lake notes, adding, “FEA can accurately predict this impact, providing engineers with a means of anticipating — rather than compensating for — problems during a vessel’s construction.”

A number of boats built by High Modulus customers already have benefited from this research, including the Humphreys 37m/121-ft motoryacht built by McMullen and Wing in New Zealand, which is due for launch in September, and the Tripp 125 lift keel sloop currently under construction at Vitters Shipyard in The Netherlands.