Wong S.M*, Lim Chin Chin, Michael Tay Ming Kiong
17th Triennial meeting of IAFS, abstract no. P03-A0770. (poster presentation)
Background: This paper presents findings in the examination of damaged flexible compressed air tubings in a pharmaceutical manufacturing plant. Plant security personnel were anxious to determine whether the tubings had been deliberately damaged to sabotage the plant or had failed mechanically without human instrumentality. Fifteen blue plastic tubings, slightly discoloured in some areas, were submitted for the examination of damages at the ends. Control samples of similar tubings were submitted for the purpose of performing test cuts and other mechanical tests.
Methods: The following tests were performed on control samples and the cut, broken or torn ends obtained were compared microscopically with the ends of the questioned tubings:
- Cutting using three single-bladed cutting tools: box-cutter, knife with serrated cutting edge and knife with smooth cutting edge. Cutting for each tool was performed in three different modes: sawing, slicing and compression (pressing the blade down against the tubing till it cut right through).
- Cutting using three double-bladed cutting tools: scissors with smooth cutting edges, scissors with one smooth cutting edge and one serrated cutting edge, and diagonal pliers. Cutting was done by opening the cutting blades, placing a tubing near the pivot and then bringing the blades together.
- Notching (partial cut) with a box-cutter and then pulling with hands till severance.
- Clamping one end in a heavy vise and pulling the other end till the tubing reached its elongation limit and broke.
- Repeated severe flexing through an angle of approximately 300o at a small localised area.
Results: Test cuts with single-bladed and double-bladed cutting tools produced many characteristic striation marks on the cut ends. Double-bladed tools, in addition, produced characteristic marks diametrically opposite each other where the two blades pinched the flexible tubing before sliding past each other in a shearing action. Strong tensile forces resulted in severe elongation and thinning of the tubing, and formation of many cracks on the outside surface. Severe flexing resulted in surface marks but the tubing did not break.
The ends of questioned tubings did not have the striation or pinching marks observed in cuts made by double-bladed tools. The damages on the questioned tubings were also found to be different from those observed in tubings broken or torn by stretching – with or without notching. The questioned ends had a narrow raised ring at the inner diameter and small radial fissures, fibrils and surface roughness, indicating fairly uniform forces exerted against the circumference of the inner walls of the questioned tubings. These forces are consistent with bursting due to internal over-pressure.
Conclusion: The questioned tubings were not cut by a single-bladed or double-bladed tool, nor were they damaged by tensile forces or flexing. Human instrumentality was ruled out and the radially uniform features observed at the questioned ends are consistent with failure of the tubings at critically weakened areas due to sudden over-pressures of compressed air.