In view of the urgency of improving safety in Hong Kong's construction sites, a research project concentrating on the impact strength of hard hats, weathering effect on the helmet materials and analysis of head-neck injuries had been investigated for two years. This thesis summarizes the investigation results of the project....[ Read more ]
In view of the urgency of improving safety in Hong Kong's construction sites, a research project concentrating on the impact strength of hard hats, weathering effect on the helmet materials and analysis of head-neck injuries had been investigated for two years. This thesis summarizes the investigation results of the project.
First, the current situation of hard hat utilization in Hong Kong was reviewed and analyzed by a social survey performed at the various construction sites. The standards and literature for hard hats are also briefly reviewed as well.
More than 100 impact tests on new hard hats and used ones were performed under various conditions, i.e. standard, over-loading, off-central, pre-cracked, pre-warmed and pre-soaked etc. Although all the new hard hats of various brands in Hong Kong's market can successfully pass the standard tests, their safety factor is quite different. The dynamic performance of various hard hats depends on their material properties and structural configurations. Several failure modes of hard hats of different brands under various impact conditions were observed. In particular, the disconnection or fracture of suspension would result in a fatal injury to the user. Besides, the residual impact strength of hard hats significantly declines after certain years' service, and the used hats may not provide wearers with the expected protection.
The hard hat materials were also experimentally studied; uni-axial tensile tests, Charpy impact tests, stress relaxation tests and SHPB tests etc were performed. DTA and SEM studies were conducted to understand the aging mechanisms of hard hat materials. Accelerated weathering processes, i.e. high temperature, high humidity and high UV lighting, were made before conducting some of these tests. The weathering conditions have remarkable influence on the hard hats made of plastics. The impact strength of all the hard hat materials decreased evidently after the simulated weathering treatment Specially, the effect of UV lighting on ABS, i.e. material of hard shell of Super Quality™ brand, is most serious.
Neck injuries caused by impact on the hard hats were assessed for the first time in Hong Kong. Excessive compression and bending are two causes of neck-spine injuries, which may appear in central impact and off-center impact, respectively. The experimental results demonstrated that the impact energy, slightly higher than 50J, sometimes is already dangerous enough to cause a cervical spine injury for certain brands of hard hats, like JSP[registered trade mark][registered trade mark]
A simple mechanical model of a hard hat subjected to a striker impact has been proposed. The model can predict the dynamic performance of a hard hat under impact according to the material property and structural configuration. It may help to optimize the design of hard hat just by adjusting the model parameters. Also it is useful in predicting relevant neck injuries.
Finally, a safety assessment, including the performance comparison of hard hats of three popular brands used in Hong Kong, and some suggestions on hard hat selection and use are given.