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Dr C.F. Lombard established the basic requirements of protective headgear in the 1950s; the protection system must stay in position during the accident event, the system must provide a barrier to prevent penetration and distribute impact force, and the impact energy must be absorbed to stop transmission of energy to the brain (Hurt and Thom 1994)
Modern motorcycle helmets are composed of the following basic components: an outer shell, an impact-absorbing inner liner, comfort padding, a retention system, a visor, and a ventilation system (Fernandes and Alves de Sousa 2013). Figure 1 shows these components on a full-face motorcycle helmet.
The hard outer shell is responsible for the distribution of impact force over a large area of the helmet, which therefore reduces the stress concentrations during an impact from reaching the head. This distribution of the impact force also increases the energy absorption capacity of the inner liner as a result of the larger effective area where the impact energy is present (Hurt and Thom 1994).
The shell absorbs the initial shock in an accident – although various studies have shown that only up to 34% of the total impact energy is absorbed by the shell (Di Landro et al. 2002). The hard outer shell prevents helmet penetration by a sharp or pointed object which may otherwise puncture the skull in an accident. The shell, typically 3-5 mm thick , also provides a structure to the inner liner so that abrasive contact with pavement or other impacting forces does not cause disintegration of the inner liner (Fernandes and Alves de Sousa 2013).
The inner liner foam absorbs the remaining force of the impact dispersed from the outer shell by crushing during impact and hence reduces the load transmitted to the head and brain (Hurt and Thom 1994). The softness and thickness of the liner are key factors so that the deceleration of the head occurs at a mild rate whilst it crushes the liner on impact (Fernandes and Alves de Sousa 2013).
Thick foam liners increase the volume and mass of the helmet, which affects the loading on the cervical spine.
Ideally, the stiffness of the liner is enough to decelerate the impact to the head in a smooth and consistent manner whilst completely crushing the liner. Optimal protective liner foam density depends on the site of impact; the foam density should be lower for the front and rear regions, and higher for the top region of impact (van den Bosch 2006).
When the helmet impacts with a sharp edge or point, shell strength and stiffness are key to prevent penetration to the head. When the helmet impacts with a flat surface, it is desirable to have a flexible shell as a shell with high strength and stiffness will distribute the force too widely in this case and will not allow yielding deformation of the liner (Hurt and Thom 1994). In this way, the efficiency of the liner is highly dependent on the properties of the shell.
The comfort padding ensures comfort to the motorcyclist and an adequate fit of the helmet by distributing the static contact forces. Due to its low stiffness, comfort foam has no injury reducing effect as it crushes completely without any significant absorption of impact energy (Fernandes and Alves de Sousa 2013).
The retention system consists of a chin strap which ensures that the helmet remains attached to the head at all times. The strap is bolted to each side of the outer shell. Chin straps and the foam inside the chin bar affect the rotation of the helmet on the head (Mills et al. 2009).
The forward rotation of the helmet or ‘forward roll-off’ is limited by proper location of the chin strap attachment to the shell, meaning that roll-off kinematics produce tightening of the chin strap rather than loosening (Hurt and Thom 1994).
The visor provides protection from objects which may impact the facial region and from weather conditions on ordinary rides (Fernandes and Alves de Sousa 2013). The visor is also equipped with water and scratch proof coating to provide clear vision to the motorcyclist & drivers.
The ventilation system conducts fresh air into the helmet and vents out exhaled air and humidity. This ensures that the temperature inside the helmet is reduced.
The hard outer shell is usually a thermoplastic material, such as polycarbonate (PC) or acrylonitrile-butadiene-styrene (ABS), or a composite material, such as fiber-reinforced plastics (FRP), i.e., glass-reinforced plastic (GRP) or carbon reinforced plastics (CRP), carbon fibre or Kevlar. ABS shells possess better impact performance than PC shells, but both materials show brittleness following long exposure to the environment (Gilchrist and Mills 1987).
GRP shells are favoured for their cost effectiveness; however, the more expensive Kevlar composite shells provide better performance for the application of racing motorcycle helmets (Shuaeib et al. 2002b). The thermoplastic shells are isotropic, meaning that the crystalline structure of the material is uniform in all directions and hence properties are uniform throughout the material.
The FRP shells exhibit anisotropic material behaviour in the plane of the shell, meaning that the properties of the material are affected by the different crystallographic orientations within the material structure (NDT Resource Center 2016).
The most common liner material is expanded polystyrene (EPS) foam. EPS is a synthetic cellular material with excellent shock absorbing properties and a good cost-benefit ratio. Its mass density in helmets varies from approximately 30-90 kg/m3 (Fernandes and Alves de Sousa 2013). EPS absorbs impact energy by permanent deformation (foam crushing or collapsing), providing protection to the motorcyclist.
Due to the permanent deformation of EPS foam, minimal protection is offered by the helmet if subsequent impacts occur in the same area as the first impact on the helmet (Gilchrist and Mills 1994; Shuaeib et al. 2002a, 2002b). EPS is also brittle in nature, and therefore placing ventilation channels in this foam is difficult (Fernandes and Alves de Sousa 2013).
Expanded polypropylene (EPP) foam is also used as liner material in helmets due to its resilient properties and allows for ventilation holes and channels to be included in the mould without foam breakage at the extraction stage.
Micro-agglomerate cork (MAC) is an alternative material to EPP which has good energy absorption capacity and high viscoelastic return. So, its ability to absorb energy is relatively unchanged after the first impact (Fernandes and Alves de Sousa 2013). However, MAC has a higher density than EPS which consequently increases the risk of injury.
The comfort padding involves a soft, flexible foam with low density, such as open-cell polyurethane (PU) or polyvinyl chloride (PVC), covered by a fabric layer which contacts and surrounds the head (Fernandes and Alves de Sousa 2013).
The chin strap is usually made of polyethylene terephthalate (PET) or nylon (Fernandes and Alves de Sousa 2013).
The visor is made of a strong and transparent material such as polycarbonate (PC) (Fernandes and Alves de Sousa 2013).
Due to the structural differences in the shell materials, the thermoplastic and composite shells possess different deformation mechanisms.
Thermoplastic shells can absorb energy by buckling and also permanent plastic deformation, while composite shells can absorb energy through fibre breakage, matrix cracking and delamination (Fernandes and Alves de Sousa 2013).
Damage mechanisms of ABS outer shell materials include crazing and shear deformation (Donald and Kramer 1982). The composite shells are capable of absorbing greater amounts of energy than thermoplastic isotropic materials due to their significant number of failure modes (Kostopoulos et al. 2002).
The three most common composite material types used for helmet shells; carbon fibre reinforced Polymer, glass reinforced polymer, and Kevlar fibre, are given in Figure 2 with their respective impact energy distribution from a 150 J blunt impact.
Note that the shell does exhibit some elastic deformation characteristics. However, liner materials, such as EPS, absorb impact energy through their ability to develop permanent deformation (Kostopoulos et al. 2002).
Injection moulding is the process generally used to manufacture thermoplastic shells (Hartung 1981). Molten polymer is injected into a die which contains a negative and positive mould at a pre-determined temperature and pressure.
The molten polymer is then allowed time to cure. The space between the positive and negative moulds forms the thickness of the shell (Shuaeib et al. 2002b). The advantages and disadvantages of manufacturing thermoplastic shells for motorcycle helmets are outlined in Table 1.
There are two possible methods generally used for the manufacture of composite shells.
The first, utilises a positive mould and a negative mould.
A releasing agent and a cover layer of resin, which contains the colour of the helmet and forms the outer thinnest layer of the helmet, is sprayed into the negative type mould. Thin strips of pre-cut glass fibre mats impregnated with highly viscous polyester resin and hardener are applied to the still-moist covering layer (Shuaeib et al. 2002b). A maximum of nine layers of polyester type glass fibres are manually applied and pressed into the negative type mould using special tools, to avoid the presence of bubbles (Hartung 1981).
The second manufacturing method, begins with the application of a thin layer of resin, generally epoxy, to the positive type mould (in this case, the foam inner liner of the helmet). An aramide, glass, or carbon fibre mat or tissue is then applied to the still-moist layer of resin. Referring to Figure 2, this fibre tissue (with dimensions approximately 100 cm by 45 cm) can be extended over the final positive type mould until the mould is covered up to the upper edge of the cut-out (Hartung 1981).
The laterally overhanging ends of the tissue are then tensioned appropriately until the entire surface of the positive type mould (foam inner liner) is covered uniformly up to a longitudinal chin protecting region of the helmet to be built-up without forcing the folds (Shuaeib et al. 2002b). The ends of the tissue are laterally folded over the longitudinal portions, forming the chin-protecting regions, which means that this relatively weak region is covered by two layers of tissue (Hartung 1981).
Following the first tissue application to the foam inner liner (positive type mould), it is carefully impregnated with artificial resin and then covered by the second tissue using the same technique. Hartung (1981) showed that a total of five tissues, applied over the foam inner liner, is sufficient to satisfy tough safety requirements. The advantages and disadvantages of manufacturing composite shells for motorcycle helmets are outlined in Table 2.
EPS foam liners are generally made by injection molding. The mould for an EPS helmet liner typically consists of a core and cavity with the gap between them defining the shape of the helmet. The core is hemispherical in shape and roughly modeled to match the shape of the top of the human head (Shuaeib et al. 2002b).
During the pre-expansion stage, raw polystyrene particles are inserted into a large cylindrical tank which is heated by a heat transfer fluid. The material in the tank is wiped by internal blades which contact with the heated tank walls. Hot air is used to pressurize the tank for a pre-determined time to cause the polystyrene to soften uniformly without expansion. A vacuum is then applied to the tank and the material is allowed to expand to the desired density (Shuaeib et al. 2002b). Pentane gas is also removed from the tank at this stage.
Following pre-expansion, the pressure inside the tank is returned to atmospheric, and the material is discharged to the holding bin. Figure 3 shows the typical pre-expansion equipment. While the material is still hot, it is fed directly from the holding bin into the mold. Feeding these foam beads, with a blowing agent such as hot air or superheated steam, into the mold and venting it causes additional expansion and forces the beads to conform to the shape of the mold and bonds the beads together. The mould is then cooled which allows the EPS to stabilise.
The core and cavity are then separated, leaving the EPS helmet liner attached to the core. The liner is then ejected from the core by a jet of air channeled into the core or by an ejecting pin that pushes the liner from the core. This ejection must be carefully executed to ensure the liner is not broken in the process. The core of the mold is usually coated with a release agent such as Teflon, to facilitate removal of the liner (Sibley and Ponzer 1994).
The advantages and disadvantages of manufacturing EPS foam liners for motorcycle helmets are outlined in Table 3.
Similar to the EPS foam liner, a conventional mould with a core and a cavity is used for the PP, PE, and Pb foam liners.
The core and the cavity are typically hemispherical in shape but extend beyond 180° of curvature to provide an undercut in the helmet. Due to the resilient nature of these foams, the undercut region of the helmet will bend to allow the helmet to be removed from the mould at the end of the process without causing any liner distortion (Shuaeib et al. 2002b).
Retention system attachment holes and ventilation holes are also able to be moulded into the liners at the same time. Figure 4 shows projections corresponding to the holes for ventilation and helmet chin straps in the rear of the helmet are configured on the cavity part of the mould and the holes for ventilation and helmet chin straps in the front of the helmet are configured on the core part of the mould (Gessalin 1984).
These projections are usually located perpendicular to the surface of the helmet and hence the minimum amount of helmet material is removed to accommodate the holes. This set-up of projections also allows the liner enough elasticity to be removed from the mould without fracturing. The advantages and disadvantages of manufacturing PP, PE, and Pb foam liners for motorcycle helmets are outlined in Table 4.
PU foam liners are manufactured by “pour in place” injection moulding.
The moulding tool consists of lower and upper mould cavities, with the upper mould cavity preheated to a pre-determined temperature and coated with a layer of releasing agent. The releasing agent layer is then coated with a layer of polyurethane lacquer, which forms a smooth film layer due to the residual heat of the upper mould cavity (Shuaeib et al. 2002b).
A foam material is injected into the plastic shell before the moulding tool is closed. Following the completion of the foaming process, the moulding tool is opened to remove a complete helmet settling from the shell, the foam liner formed of the foam material, and the smooth film layer. Figure 5 shows the moulding tool used to manufacture PU foam liner helmets.
The advantages and disadvantages of manufacturing PU foam liners for motorcycle helmets are outlined in Table 5.
Conventional helmets are assembled with the inner liner glued to the outer shell at small areas of the crown (Halldin et al. 2001). Other helmets utilise an interference fit between the liner and shell whereby the foam liner is expanded/moulded inside the outer shell during manufacturing. The chin guard liner foam is always glued to its corresponding section of the outer shell.
Comfort padding is included in the helmet via push buttons which attach various segments, usually three segments (2x cheek pads and 1x larger head pad), to the shell and/or liner of the helmet.
The retention system is generally fastened to the outer shell of a helmet via screws. The visor is attached to the shell of a helmet via screws and clip-lock mouldings in the shell.
Ventilation entry and exit holes at the chin guard and back of the helmet respectively contain small areas of metallic mesh glued to the interior side of the shell to filter smaller road debris from clogging the ventilation system of the helmet.
Four different NDT techniques were used in the correlation of physical damages present in the helmets; visual and correlation analysis, various x-ray techniques (fixed & mobile techniques), ultrasonic non-destructive testing (NDT), and microscopy analysis, before concluding the NDT technique of Holographic Interferometry was scientifically proven to overcome all of the above-mentioned shortfalls.
Ultrasonic non-destructive testing (NDT) techniques have had extensive success in the structural analysis of composite materials. The process involves an energy pulse which is transmitted from an ultrasonic transducer and travels through the thickness of a test material. This pulse will then be reflected from the back wall of the material and back to the transducer. Flaws within the material can then be detected and located if a subsequent signal pulse is reflected from the flaw rather than the back wall of the material.
At this stage, conventional pulse-echo ultrasonic NDT techniques have not been thoroughly investigated on motorcycle helmets. However, there is one important factor to consider before this technique is trialled on the sample helmets. Ultrasonic couplants are used in this NDT process to facilitate the unimpeded transmission of sound energy between the transducer and test surface (Olympus 2016). Careful attention must be payed to the interface couplant substance used on thermoplastic helmet shells as these substances, particularly the petroleum-based solutions, can degrade thermoplastic materials (Woishnis and Ebnesajjad 2012) – essentially making this method a destructive form of testing.
In conclusion, conventional pulse-echo ultrasonic NDT was found to be an unviable technique for the detection of ‘hairline’ fractures within the outer shell of a helmet. The thin and complex curvature of the outer shell, especially in composite shells, limits the through-thickness resolution and sensitivity of signal pulses and creates obstacles to good acoustic coupling of the transducer and the shell. This results in an extremely time-consuming and inefficient process for confirming the presence of stress fractures in the outer shell of helmets.
In 2009, an industrial X-ray computer tomography (CT) scanner was used to scan a full-face motorcycle helmet with a GRP shell and an EPS liner at EMPA Dübendorf for the purposes of creating a finite element analysis (FEA) model of the helmet. The chin bar was removed for the scan. The X ray source was set at 225 kV and 4 mA.
A total of 431 continuous slices were scanned with a pixel size of 0.40 by 0.40 mm in the horizontal plane with 0.60 mm between slices (Mills et al. 2009). Figure 7 shows small gaps between the liner and the shell near the hanger plates (steel components where the chin straps are attached). The liner is not bonded to the shell, it is an interference fit in the shell.
The metal components of the helmets, i.e., the hanger plates, result in artefacts in the images. These surrounding bright streaking artefacts are due to the high density of metals being outside the range of scanning, beam hardening, scatter effects, and Poisson noise (Boas and Fleischmann 2012). The image quality is inadequate for satisfactory extraction of geometry of the EPS liner as the differing densities did not contrast well and overlapped with the grey levels of other components.
Microscopy analysis allowed some potential fractures in the outer shell to be identified, but as most of these potential fractures did not match up on both sides of the samples cut by the waterjet they could not be confirmed as directly correlating to those potential stress fractures displayed in the x-ray images. Additional potential fractures were discovered in the microscopy analysis process, which were not visible in the x-ray images.
These differences were identified due to the fracture/defect may have been either in or out of plain of the x-ray imaging. Voids were also discovered in the shell sample cuts which were not present in the x-ray images.
The conclusions drawn from our initial R&D/investigation showed the current imaging techniques did not allow for a successful correlation of physical damages in the outer shell and inner liner of motorcycle helmets.
The Helmet Doctors identified from these shortfalls specialized areas were required in our R&D/study, including the development and optimization of technologies involved in lasers. Investigations into advancements in laser technologies & techniques allowed The Helmet Doctors reliable methods to ensure absolute confirmation of damages within the motorcycle helmets.
As a result, The Helmet Doctors were able to advance and settle their R&D & techniques in the NDT techniques of shearography and holographic interferometry.
Shearography is an NDT technology that is aimed at monitoring the derivative of the skin displacement of the sample. It is an optical interferometric technology and physically our system can be compared to a misaligned Michelson interferometer.
Working on displacement derivatives enables us to get rid of rigid body translation that disturbs other interferometric methods. Testing to detect internal defects inside the provided motorcycle helmet, Shearography aims to monitor the derivative of small skin displacements of samples along the Z axis.
To detect the internal defect, we induced a small thermal expansion of the motorcycle helmet samples. Internal stress due to the heterogeneities will be transferred to the surface and monitored by the advancements of the laser camera to identify, read, record & measure the defects down to as low as 10nm (nanometers).
Further details of our R&D and the evolution of our technology and techniques can be seen on this website under the tab of Technology > THD Know How
It is with much humbleness & gratitude from The Helmet Doctors, that we would like to formally recognize the collaboration and work of Professor Martin Veidt and the Research Team in the UQ Composite Group at The University of Queensland along with their EAIT Workshop for their guidance, skills, and time given to deconstructing all the motorcycle helmets supplied by The Helmet Doctors for our inspection and reporting requirements.
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