Stress Indicators, Inc., a Bethesda, Maryland company, has developed specialty mechanical bolts that feature built-in pressure sensors that display colors based on how well a bolt has been tightened. For now only indicated for the construction trade, we envision orthopedic and neuro surgeons may soon see these in the operating theater.
From the company’s technology page:
Operation of Stress Indicator’s DTI SmartBolts® is based upon the response of an optical microindicator element to the deflection of one internal portion of a fastener relative to another as the fastener elongates under tensile loading. This miniscule relative movement is transformed by patented DTI optical microindicators into a dramatic and reproducible color change.
The DTI MicroIndicator is an incredibly simple device that performs a rather sophisticated function. The microindicator is actually a miniaturized optical absorptance cell or variable-density filter that reproducibly changes its spectral transmittance as the cell thickness is varied. A brightly colored indicating area is placed at the rear wall of the cell and a fraction of a drop of an optically dense fluid, which strongly absorbs the spectral color of the indicator area, is contained in a hermetically sealed envelope between the indicator area and the clear window. Ambient light passing through the window is selectively absorbed by the filter cell, and is selectively reflected by the indicator area to pass once again through the cell and window to the observer.
Indicator Sensitivity may be precisely controlled by adjusting the composition of the indicator fluid and the mechanical arrangement of the different grades. Apart from sensitivity considerations, the operating concept is similar for all grades: when the tensile load is applied to the fastener, the relative deflection is mechanically coupled to the microindicator and the reflected brightness or color of the indicator will exhibit a dramatic and sensitive change as a direct function of bolt tension.