What it could feel like to press on an auxetic: effect of Poisson’s ratio on the indenter

Abstract

The behaviour of an indenter having a hard inner core and a softer outer shell, meant to emulate a human finger, pressing upon a sample covered with an isotropic negative/zero/positive Poisson’s top protective layer was investigated through static finite-element simulations. It was shown that if the material being indented had a comparable Young’s modulus to the outer shell of the indenter, a very negative Poisson’s ratio results in more pronounced deformations and stresses within the indenter due to the auxetic material offering a higher resistance to indentation. Pressing of an auxetic material with Poisson’s ratio of -0.90 having the same Young’s modulus as that of the softer outer core of the indenter (1MPa), was found to result in stresses in the indenter which were approximately double to those measured when pressing a conventional material with same Young’s modulus having Poisson’s ratio +0.45. This indicates that auxetics with a very high negative Poisson’s ratio may feel more uncomfortable when pressed with a finger compared to materials of the same Young’s modulus having a conventional positive Poisson’s ratio. However, it was also shown that in such scenarios, the auxetic protective layer being indented could be made much thinner whilst still retaining the same functionality. In fact, the simulations suggest that a conventional material with Poisson’s ratio +0.45 indents more than five times as much as an auxetic material with Poisson’s ratio of -0.90 having the same Young’s modulus of 1MPa. It was also shown that in situations where the material being indented had a much higher Young’s modulus to that of the indenter, the deformations and stresses within the indenter are practically identical irrespective of the Poisson’s ratio, i.e. a human finger is unlikely to be able to perceive the difference between an auxetic and a conventional positive Poisson’s ratio material.

On the effect of the poisson’s ratio on samples subjected to shearing

“…Thus, it is difficult to identify a specific range of Poisson's ratio that is likely to be optimal for specific sports applications without examining the specific requirements of the application and performing more detailed studies. As discussed elsewhere [58], here one should also consider aspects such inertness of the materials used, how they feel, their thermal and degradation properties, as well as manufacturing costs. In this respect, it should be mentioned that the search for the ideal system or Poisson's ratio value for use in specific sports applications should ideally commence by examining how existing auxetic 2D and 3D model structures, when manufactured at specific scales, specific shapes and dimensions and with specific materials behave under shear.…”

Auxetics and FEA: Modern Materials Driven by Modern Simulation Methods

“…Auxetics have associated with them a number of benefits and enhanced properties that are not that commonly encountered in most everyday materials [11]. They are highly desirable for their exceptional properties, including high indentation resistance [12][13][14][15][16][17][18][19][20], high fracture toughness [20][21][22][23], shear resistance [18,24,25], energy absorption [26,27] and other enhanced dynamic characteristics [28][29][30][31]. These unique attributes enable auxetics to find diverse applications across various fields such as in the medical field [32][33][34][35][36][37], sportswear applications [38][39][40][41][42], military defence equipment [43][44][45][46], as well as in the automotive [47][48][49] and aerospace [50][51][52] industries.…”

Focus on smart materials and structures for sporting protective equipment

“…A system of analytical models explained the response and could be used to predict required foam properties. Based on a similar trade-off between comfort and function, Attard et al studied contact between a finger and auxetic pad [11]. An auxetic system can offer similar function to its conventional counterpart over a lower space, which could allow streamlined padding systems.…”

“…The focus issue highlighted the strong interest in smart materials for sporting impact protection. Noted challenges include adapting between the wide range of requirements; changing user groups, body types, and different types of impacts [2][3][4][5][6][7][8][9][10][11]. The impact or loading conditions and response that should be targeted when selecting or optimising effective smart material properties are developing with knowledge of injury risks [2][3][4]11].…”

“…Noted challenges include adapting between the wide range of requirements; changing user groups, body types, and different types of impacts [2][3][4][5][6][7][8][9][10][11]. The impact or loading conditions and response that should be targeted when selecting or optimising effective smart material properties are developing with knowledge of injury risks [2][3][4]11]. Scaling smart-material manufacturing routes to achieve these, allowing transfer from laboratory to commercial products, is also noted as a challenge [2,7].…”