Full cocoon vs. cut and flattened walls: comparing stab testing methods as applied to Bombyx mori silk cocoons

The Bombyx mori silk cocoon serves as a protective covering around the pupa; its mechanical construction should therefore be resistant to predatorial attacks such as punctures, perforations and tears. The B. mori cocoon's resistance to such modes of damage is nevertheless still largely undocumented. The work presented here is part of a broad endeavour to describe the damage tolerance of B. mori silk cocoons. Here, we evaluate two different testing methods using an Instron-3369 testing apparatus to determine its resistance to stabbing by an HOSDB-standard knife. In the first method, the cocoon is stabbed through in its entirety (i.e. through an uncut and unimpaired structure), while in the second approach, the cocoon is cut open to form a rectangular quasi-flat sheet of cocoon wall which is tightly clamped prior to being punctured by the knife stab. The stabbing force was measured in both techniques. While the stabbing forces needed to puncture and perforate full cocoons and rectangular walls were approximately the same, there were noticeable differences in the force vs. extension curves in each stabbing method. The analysed results imply that it is preferable to directly test full cocoon walls rather than to artificially pre-prepare the cocoon wall into quasi-flat sheets and to then forcibly constrain them at specific edge locations of the quasi-flat sheet. This is because the artificial pre-preparation of cocoon walls forces them to adopt an unnatural geometrical form (quasi-flat) and, as a consequence, the wall deforms in an unnatural manner during the penetration stage of stabbing. This is an important finding since the vast majority of the mechanical testing research published utilises cut and quasi-flattened cocoon walls. There is currently no standardised test method for the puncture and perforation of material from B. mori cocoons. In conclusion, our research provides new insights into their preparation and testing.