# The Additive Manufacturing of Glass: A Critical Review
## 4 Properties of AM Glass  
### Mechanical Properties  
To advance the field of glass AM technologies, a comprehensive characterisation and understanding of the mechanical properties of AM-prepared glass (using the many AM methods described herein) will be critical. Mechanical testing is fundamental, offering valuable information on load bearing capacity, fracture-related performance, and the overall reliability of manufactured components. This is essential for both ensuring product quality (of glass AM components) and meeting industry standards (in relation to the presently known performance of glasses in industrial use). Knowledge of mechanical properties enables informed material selection and design optimisation, which leads to waste reduction and enhanced overall performance. This can be achieved through assessing how AM glass behaves under various loading conditions, contributing to the evolution of glass AM technology, and realising the full potential of structural (or any new) applications. As with several facets of the domain of glass AM, there is a paucity of mechanical test data on glass AM parts. Early investigations, such as the original testing conducted by [PERSON] in 2015 [10], focused on semi-solid processed glass (using an approach that is also popularised as the MIT method). In the study, two sample types were compared, cut from a larger manufactured sample. One set was printed in a heated chamber set to 482 \({}^{\circ}\)C, and the other was printed with the chamber at room temperature (25 \({}^{\circ}\)C). Following an annealing cycle, both sample types exhibited negligible stress concentration along the layers. However, radial stresses were observed within the layers of the room-temperature samples. A three-point bending loading scenario was employed, applying a load along the glass layers (presented in Table 4). This loading orientation was selected to investigate interlayer adhesion. Of note is that samples printed in a chamber at 482 \({}^{\circ}\)C demonstrated a flexural strength five times greater than the room-temperature samples. This was attributed to the temperature differences during layer deposition. A large temperature difference between the deposited layer and previous layer appears more likely to impact (and potentially weaken) interlayer bonding. Additionally, the study noted that the fracture routes of 3D-printed samples prepared at 482 \({}^{\circ}\)C were observed to have cracks without a precise trajectory. Figure 28: A glass 3D printing process facilitating an overhang using support material. Pictured is (**a**) G-code preview, (**b**) 3D-printed component prior to support material removal, and (**c**) 3D-printed component with support removed. More rigorous versions of this study were performed by [PERSON] in 2018 [15; 64], where the peak stress in samples was quantified for tensile and compressive loading scenarios. Notable was the significant decrease in compressive strength from the small-scale rectangular samples to the complete large-scale prints. The cause of this was believed to be influences on the boundary conditions (i.e., in-plane friction) causing out-of-plane bending moments and compound stresses at the perimeters. This highlights that the characterisation of such influences is of great importance to the design of large-scale glass objects. Art and architecture necessitate complex forms which will experience loading scenarios much more complicated than those seen in a laboratory. In another study, [PERSON] (2018) [24] focused on AM glass structures that were deposited directly onto glass plates. In the study, 'joint strength' (akin to adhesion strength) was investigated, which is a fundamental building block for glass AM. Borosilicate glass rods were fused to borosilicate glass plates prior to annealing. Testing was then carried out in a cantilever bending orientation (Table 4). The results indicated that \(\sim\)66% of fractures occurred at the site of the joint.