Paul A. Brühwiler

Heated, perspiring manikin headform for the measurement of headgear ventilation characteristics

Details of a heated manikin headform with computer-controlled perspiration are presented. The heat exchange properties of the headform with and without perspiration are compared to those of human beings and other manikins, showing quite good agreement, and are then applied to measure the ventilation characteristics of motorcycle and bicycle helmets. Subtle differences between two helmets are observed in each case, illustrating the sensitivity of the headform.

Heat transfer variations of bicycle helmets

Abstract Bicycle helmets exhibit complex structures so as to combine impact protection with ventilation. A quantitative experimental measure of the state of the art and variations therein is a first step towards establishing principles of bicycle helmet ventilation. A thermal headform mounted in a climate-regulated wind tunnel was used to study the ventilation efficiency of 24 bicycle helmets at two wind speeds. Flow visualization in a water tunnel with a second headform demonstrated the flow patterns involved. The influence of design details such as channel length and vent placement was studied, as well as the impact of hair. Differences in heat transfer among the helmets of up to 30% (scalp) and 10% (face) were observed, with the nude headform showing the highest values. On occasion, a negative role of some vents for forced convection was demonstrated. A weak correlation was found between the projected vent cross-section and heat transfer variations when changing the head tilt angle. A simple analytical model is introduced that facilitates the understanding of forced convection phenomena. A weak correlation between exposed scalp area and heat transfer was deduced. Adding a wig reduces the heat transfer by approximately a factor of 8 in the scalp region and up to one-third for the rest of the head for a selection of the best ventilated helmets. The results suggest that there is significant optimization potential within the basic helmet structure represented in modern bicycle helmets.

Transplanar and in-plane wicking effects in sock materials under pressure:

The moisture transfer and absorption properties of fabrics play an important role in the evaluation of the overall wear comfort of the textile. The location of moisture in the textile influences the skin wetness as well as the skin/textile friction process. In this study, we used X-ray tomography to analyze the transplanar and in-plane water transport in different sock materials when two defined pressures were applied to the inner side by means of an adjustable screw. The materials used were polyamide, polypropylene and wool, and had very distinct hydrophilic/hydrophobic and hygroscopic properties. The in-plane wicking effect showed a clear time dependency for the polyamide and wool samples, while the spreading of the polypropylene samples was very scattered. This effect was generally larger in the outer side of the sock than in the inner side, showing a clear tendency of these socks to wick the moisture away from the skin. Applying a pressure generally increased the in-plane water transport, but it affected the water distribution throughout the thickness of the sock for the wool samples, as more water remained in the inner half. The transplanar wicking effect was the most efficient with the polypropylene sock under the high pressure condition, but with the low pressure, this sock was not able to absorb all the moisture and a small quantity of water remained at its inner surface. X-ray tomography was shown to be a powerful tool to analyze not only the water distribution in static conditions, but also the transient 3-dimensional water transport.

Heat loss variations of full-face motorcycle helmets.

Heat loss of 27 full-face motorcycle helmets was studied using a thermal manikin headform. The headform was electrically heated and positioned at the exit of a wind tunnel, so that the air stream flowed onto its front side. All helmets were measured in three sessions in which all the vents were opened or closed consecutively in random order. Average heat loss was calculated from a steady state period, under controlled environmental conditions of 22+/-0.05 degrees C, 50+/-1% RH and 50.4+/-1.1 km h(-1) (14.0+/-0.3 ms(-1)) wind speed. The results show large variations in heat loss among the different helmets, ranging from 0 to 4 W for the scalp section of the headform and 8 to 18 W for the face section of the headform. Opening all the vents showed an increase in heat loss of more than 1 W (2 W) for four (two) helmets in the scalp section and six (one) helmets in the face section. These levels of heat transfer have been shown to be the thresholds for human sensitivity in scalp and face sections. Furthermore, helmet construction features which could be identified as important for heat loss of motorcycle helmets were identified.

Are current back protectors suitable to prevent spinal injury in recreational snowboarders

Objective Back protectors for snowboarders were analysed with respect to their potential to prevent spinal injury. Design In 20 Swiss skiing resorts, athletes were interviewed on the slope. In addition, an online survey was conducted. The performance of 12 commercially available back protectors was investigated by means of mechanical testing. A currently used drop test according to standard EN1621 (motorcycle protectors), testing energy damping was supplemented by penetration tests according to standard EN1077, which reflects snowsport safety concerns. Results 6 out of 12 back protectors fulfilled the higher safety level defined in EN1621. Protectors making use of energy-absorbing layers performed particularly well. In contrast, hard shell protectors exhibited a higher potential to withstand the penetration test. The surveys confirmed that approximately 40–50% of snowboarders use a back protector. A large majority of users expect protection from severe spinal injury such as vertebral fractures or spinal cord injury. Conclusions The currently used test standards are fulfilled by many back protectors. Users, however, expect protectors to be efficient in impact scenarios that result in spinal injury, which are more severe than impacts as addressed in the current standards. This study highlights that there is a mismatch between the capabilities of current back protectors to prevent spinal injury in snowboarding and the expectations users have of these protectors.

Radiant heat transfer of bicycle helmets and visors

Abstract Twenty-six bicycle helmets and their associated visors were characterized for radiant heat transfer using a thermal manikin headform in a climate chamber to assess their ability to protect the wearer from heating by the sun. A single configuration for applied radiant flow of 9.3 W was used to assess the roles of the forward and upper vents and the visor. The helmets shielded 50–75% of the radiant heating without a visor and 65–85% with one. Twenty-three visors were shown to result in a relevant reduction of radiant heating of the face (>0.5 W), with 15 reaching approximately 1 W. Heating of the visor and/or helmet and subsequent heating of the air flowing into the helmet was nevertheless found to be a relevant effect in many cases, suggesting that simple measures like reflective upper surfaces could noticeably improve the radiant heat rejection without changing the helmet structure. The forward vents in the helmets that allow the transmission of radiant heat are often important for forced convection, so that minimizing radiant heating geneally reduces the maximization of forced convective heat loss for current helmets.

The effect of rowing headgear on forced convective heat loss and radiant heat gain on a thermal manikin headform

Abstract Both radiant and forced convective heat flow were measured for a prototype rowing headgear and white and black cotton caps. The measurements were performed on a thermal manikin headform at a wind speed of 4.0 m · s−1 (s = 0.1) in a climate chamber at 22.0°C (s = 0.05), with and without radiant heat flow from a heat lamp, coming from either directly above (90°) or from above at an angle of 55°. The effects of hair were studied by repeating selected measurements with a wig. All headgear reduced the radiant heat gain compared with the nude headform: about 80% for the caps and 95% for the prototype rowing headgear (P < 0.01). Forced convective heat loss was reduced more by the caps (36%) than by the prototype rowing headgear (9%) (P < 0.01). The radiant heat gain contributed maximally 13% to the net heat transfer, with or without headgear, showing that forced convective heat loss is the dominant heat transfer parameter under the chosen conditions. The results of the headgear – wig combinations were qualitatively similar, with lower absolute heat transfer.

Flexible and precise drop test system

A drop test system with flexibility in the choice of falling object has been constructed and characterized. Using the guided free fall principle, the system enables the study of impacts of a large range of objects on a wide selection of anvils, with high control of the position and orientation of the object. The latter is demonstrated with falls of a standard aluminium headform in mountaineering helmets on a kerbstone anvil, for which visual inspection with a high-speed camera confirms the desired accuracy. Impacts of a flat falling body on cylindrical polystyrene foam samples are used to derive stress–strain curves for materials of different density and for multilayer samples. In this case, the effects of striker orientation and placement on the resultant data are discussed, and the reproducibility of the data serves as an additional confirmation of the accuracy of the measurement apparatus and procedures. A check on the improvement in the level of positional and orientational striking precision achievable is obtained via an inter-laboratory comparison.