The preparation and characterisation of auxetic foams for the application of trauma attenuating backings

Abstract

Blunt body trauma, due to a heavy impact or blow, has the potential to cause catastrophic injury in a broad range of professions and workplace environments. At particular risk are those who work in areas such as the military, the engineering and construction industries, contact sports (such as American football or rugby) or indeed any other area where an individual has the potential to come into physical contact with other moving objects. Unlike other occupations, the primary cause of blunt trauma in the military is associated with high impact kinetic energies transferred onto the body through the deformation and displacement of body armour.

To date, the effects of behind armour blunt trauma (BABT) are counteracted by materials known as Trauma Attenuating Backings (TABs), which have the ability to dissipate and redistributed the impact energy. Unlike currently available TABs that utilise conventional materials, this thesis explores the feasibility of reducing the effects of BABT through the employment of auxetic foams. In contrast to conventional materials, auxetics exhibit a negative Poisson’s ratio when subjected to both tensile and compressive loading, whilst also exhibiting a range of enhanced mechanical properties.

Utilising the well-established three-step auxetic foam fabrication process, the influence of fabrication parameters with key focus on Poisson’s ratio and energy absorption have been explored. Auxetic foams were produced for a number of fabrication parameter combinations, where heating time and volumetric compression ratio were identified to be codependentin determining Poisson’s ratio and energy absorption behaviour. While many individual samples demonstrated a negative Poisson’s ratio with enhanced energy absorption, due to the presence of defective and/or damaged cellular structures throughout the foam structures, large variability and little sample repeatability were shown to exist between samples produced from the same combination of fabrication parameters.

A novel 3D printing approach was then developed to produce two types of repeatable pliable polymeric foam structures (re-entrant and honeycomb, auxetic and conventional, respectively) where the majority of geometric features have a dimensional error between 0.5% - 5.0% and exhibited negative Poisson’s ratios under both tensile and compressive loading.

Interestingly under direct impact conditions (7.2J of impact energy under freefall) no discernible difference regarding energy absorption was shown between the auxetic and conventional systems. However, when considered with respect to the peak force, the auxetic systems exhibited a lower overall mean peak force by a factor of two. Whilst the auxetic system significantly reduced the transmitted peak force but maintained similar energy absorption when compared to the conventional system, it was difficult to surmise whether or not the auxetic system performed better than the conventional system. However, with respect to the quasi-static testing (where the auxetics exhibited a greater energy absorption per unit volume due to both systems having an approximately same relative density), for a like-for-like weight basis the auxetic foams absorb more energy than their conventional counterparts due to their structure and not just because of the increase in relative density that the classic auxetic fabrication process instils.

Therefore, it may be concluded that auxetics are potentially suitable for the application as a trauma attenuating backing material as they demonstrated that they satisfy the two key requirements that TAB materials must possess: the ability to absorb energy and reduce the transferable force. However, due to a current lack of understanding on the effects governing BABT, particularly in the area of mechanical and biophysical interactions that take place, it is important that more rigorous testing is undertaken to explore other effects, including pressure wave propagation, ergonomics and replicating in-field conditions before committing auxetic foams to a TAB application.

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ORCID for Robert Wood: orcid.org/0000-0003-0681-9239

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