Question: What is the risk of a piece of medical silicone breaking off and becoming a choking hazard? How much force to tear?
Unfortunately like many problems, the answer to your question depends on the application, loads, and part size and geometry.
The risk of choking is difficult to quantify but a quick search suggests that thin sheets or hard round shapes like coins are significant choking hazards as we all know. The question becomes how to quantify risk of a deformable body with very low wet friction being released into the windpipe. It may be worthwhile to consult a physician specializing in respiratory to better understand the physiology and failure modes associated with choking.
To answer the second part of the question, factors affecting the tear strength may include temperature, degree of cross linking (how complete the curing reaction is), and sterilization but let’s assume a perfect material. If we choose a 70 durometer implantable material such as Nusil’s MED-4870 and we look up the tear strength of 1350psi on their website. The referenced standard uses the original cross sectional area. So using simple rod geometry in tension at 1mm (0.0394in) tearing occurs at 1.6lb while a 1/8” rod would tear at 16.5lb. As the complexity increases so does the challenge of estimating failure loads such as in balloons or composite products. This is where prototyping and early testing offers the ability to drive a product to failure without risk to life.