# Preliminary Failure Analyses of Loaded Hot Water Bottles
## 4 Discussion
The nonlinear behaviour of rubber under compression observed in experiments might not have been perfectly replicated in the FE analysis, suggesting the need to refine material models accurately to capture stress concentration effects. In our test, the material modulus was computed from an experimental tensile test using the Ogden second-order hyperelastic material model [20; 42; 43]. The tensile test strips were cut from the central area hot water bottle to seek constant thickness; however, the strip thickness varied in range (from 2.37 mm to 2.26 mm), which might have affected the test results. Also, the material parameters calculation included a tensile data optimisation process, which means that the acquired parameter value (\(\upmu\) and \(\upalpha\)) could perform well under tensile conditions but might not fully represent the compression environment. Additionally, simplifying assumptions inherent in numerical simulations, such as the mesh resolution, element type, and modelling techniques, could contribute to deviations in stress concentration patterns, impacting the accuracy of stress distribution. The model used an idealised shape and even material distribution for all surfaces, but the products may have imperfections, which could influence the test results. In the FE analyses of the current study, a simplified assumption was made by modelling the effect of the water inside the bottle as an internal pressure rather than a fluid element to reduce the complexity of the model. The initial value of this pressure was empirically adjusted (via trial and error) to measure the experimental bottle volume and compensate for the lack of a fluid element simulation component. In addition, using contact pressure to validate the compressive stress on the hot water bottle has limitations. The contact pressure component is quite noisy unless a very narrow mesh grid is used, which surges the simulation computational time. According to the ABAQUS 6.14 FE analysis software manual, the software uses smoothing techniques for second-order surfaces to reduce this noise. Therefore, when the surface could not be well fitted to a second-order surface because of its complexity, as in the hot water bottle case, many scattered noisy disturbances were noted in the contact pressure distribution. Failures in hot water bottles can be attributed to various factors but are predominantly associated with manufacturing methods. A fundamental cause is inadequate heat sealing during the bottle body and stopper assembly. This can result in a weakened area that may fail when subjected to heat or pressure. The vulcanisation process results in thin bottle edges, which creates weak seams with a thinner cross-sectional area. These seams can collapse under the various stresses caused by filling the bottle with hot water. Due to the stress on polymer chains in the compression experiments, one primary reason for failure was believed to be the degeneration of rubber and additives mixing at material seams and discontinuities. This caused the bottle walls to lack durability and contributed to the bottle's failure under load. Additionally, due to the vulcanisation process, the thin wall thickness at the bottle side borders weakened these areas. Seams were prone to collapse under progressive stresses produced by compressing the bottle surfaces, as seen in the microscopic images in Figure 14, where the extent to which polymer chains split during cross-linking breakage from elevated stress levels can be observed. It is crucial to understand that the reliability of hot water bottles is highly dependent on the accuracy of their manufacturing and assembly processes. Any flaws in these areas can directly result in an increase in failure rates. Therefore, it is essential to ensure that hot water bottle manufacturing and assembly processes are carried out precisely to guarantee the highest level of reliability.