PSI - Issue 49

Teiba Ahmed et al. / Procedia Structural Integrity 49 (2023) 37–42 Teiba Ahmed et al. / Structural Integrity Procedia 00 (2023) 000–000

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1. Introduction As discomfort is a common issue with a range of orthoses, improving comfort can improve compliance of wear and medical outcomes. The effectiveness of an orthotic is often dependant on wear time and wear tightness (Ng et al., 2019). Physical discomfort of an orthotic are usually due to edges of the orthotic digging into the wearers skin (Xu et al., 2022). In additive manufacturing (AM), MEAM is attractive due to its low cost, high precision, quick manufacturing time and lightweight structure (Xu et al., 2022). However for orthotics MEAM is often limited by quality of the CAD slicer and in turn, mechanical strength. With the use of custom tool path designers, achieving specimens that are close to bulk properties is possible, making MEAM the preferred choice for price point (Moetazedian et al., 2021). The paper focuses on the application of living hinges to reduce edge stiffness for thin-walled 3D printed orthotics. The complaint mechanisms of the fundamental structures studied here are directly relevant to applications with living hinges and various other fields not limited to biomedical applications. Additive manufacturing provides the freedom to customise the stiffness for a specific application, patient, or position on a prosthesis. However, the findings are relevant to other manufacturing processes such as forming, as is the newly developed method of characterising and controlling complex geometrical living hinge structures. A new testing method, using cantilever testing principles, was used to evaluate the specimen’s bending stiffness. Specimens of a fixed length with sinusoidal waveforms of varying wavelengths but fixed amplitude at three different extrusion thicknesses were tested along with straight specimens (i.e. wavelength approaching infinity). To allow direct comparison between specimens of different thicknesses, the bending stiffness results for all specimens were normalised to the reference specimen for each given thickness.

2. Materials and Methods 2.1. Sine Wave Design Phase

The specimens were representative of a section of orthotic edge (Fig. 1). As the stiffness is not dependant on the length of the specimen (z-direction in Fig. 1), printing a representative section allowed for faster printing and testing, increasing the range of thin wall designs that could be considered.

Fig. 1. Representative specimen edge section from example orthotic showing layer deposition direction.