The Royal National Lifeboat Institution approached TWI with a request to conduct static load and deflection tests on composite hatches of original and modified design to ensure that a revised design would provide better in-service performance.
The project's objective was to conduct static load tests and deflection measurements on a series of hatches to establish the maximum load the hatches could withstand prior to cracking. To simulate actual conditions as closely as possible the hatches were delivered complete with rubber seals and one hatch frame.
The frame was supported horizontally on four 75mm by 75mm by 200mm steel blocks, one at each corner. A vertical load was transferred to the hatch, resting on the bed of the test machine, through a steel ball 50mm in diameter resting on a steel block, situated in the middle of the hatch panel.
Tests were conducted under ambient conditions in a servo-hydraulic test machine equipped with a 50kN calibrated load cell and a logger to record the applied load and displacement during testing. The test machine was operated in displacement control.
The first part of the test from zero load to 0.862kN load (equivalent to the minimum designed pressure of 86.2kN/m2) was conducted at a rate of 0.1mm/second.
After that point was reached the rate was increased to 0.4mm/second. Loading was ended when cracking was heard and visually detected. For a second specimen the test was terminated at the proof load of 0.862kN, which represents the design pressure of 86.2kN/m2.
TWI was able to tell the Institution the following:-
The original design endured a load of 23.6kN (equivalent to a pressure of 2360kN/m2). No cracks were visible, but there were audible cracks. The deflection was so great that the test machine ran out of stroke, so the test was stopped.
The new design with no bolts was tested to the minimum designed pressure load of 86.2kN/m2 (0.862kN applied load) and no cracking was heard or detected.
The new design including mechanical fasteners resisted a load of 35kN (equivalent to a pressure of 3500kN/m2) when multiple cracks were detected on the opposite side to the applied load.
Following these tests, during which static load and displacement measurements were recorded TWI was able to present the following conclusions to the RNLI.
The new design hatch panel in new condition is stiffer than the original and can withstand higher loads when in new condition.
The specimen manufactured to a new design resisted the design pressure load with no cracking and minimal deflection.
To learn more about TWI's testing capabilities contact Radu Dimitriu at TWI.