Bending Stress in Capillary

The bending stress on capillary is nearly identical to that of optical fiber, and as such, the same bending stress principles and test methods can be applied.  A lengthy discussion can be found in the Fiber Optics & Optical Fiber chapter under Mechanical Stress and Fiber Strength. Polymicro proof tests most products to a nominal bending stress level of 100kpsi. However, since the inner surface can be exposed to the elements, it is easy for users to affect the strength of capillary during routine handling. If chemical reactions or particulate debris cause stress cracks on the interior surface, they may reduce the resistance to bending stress damage. Proper storage of the capillary and simple laboratory practices such as filtering solvents and gases, using quality connectors, and insuring quality cleaves on capillary ends, can have direct impact on maintaining capillary strength. Storage, handling, and cleaving are discussed in the Appendix. Figure 3-1 shows the relationship of OD (including polyimide), bend radius, and applied bending stress for a number of standard polyimide coated capillary.