Stress Riser

So a while back I was taking the ferry and I walked past this car, also on the ferry. (Sorry these photos turned out so blurry -- kid slime all over my camera lens.)

In case you're wondering what you're looking at, this is a do-it-yourself luggage (or whatever) rack. It might also be a portable roof-top deck, but I'm not sure. I never got to talk to the people who made it.

As soon as I saw it I groaned, mostly because I know the side-effects -- specifically the stress risers -- that occur when retrofitting materials this way. A stress riser is an area in a material where stress is concentrated, or much higher than it is in the surrounding areas. The materials selected and the design utilized by car manufacturers are engineered to prevent one area of the car (like the roof) from wrecking another (like the windshield). (Unless you have a Volkswagon, in which case I'm pretty sure engineers were instructed to design the interior so that using the windshield wipers causes the trunk latch to pop open uncontrollably, or rolling down the window causes the horn to honk intermittently.)

When you drill a hole through the roof of your car and insert a bolt, the bolt now introduces a new vibration to the windshield, which in turn causes the glass to fracture. The movement of the bolt is almost invisible, but the accumulation of the vibration as the car drives down the road, the weight of the rack, and the lift of the rack in response to wind all introduce new loads that the window is not designed to take.

In case you are panicking that this blog is now going to be about cars, here's why I took and posted these pics for you to see. In biomechanics, we study stress risers to better understand injury and disease. Many of the "solutions" we have to injury, disease, and chronic pain -- joint replacements, spinal rods, pins, plates, screws, and mesh for hernias and the pelvic floor -- are examples of hardware that, while correcting the symptoms of one issue, go on to create new problems. (And P.S. it's not just foreign objects that create injury-making loads. Scar tissue as well as strengthening only some parts of your body can create relative material strength differences that behave as stress risers.)

The long-term effect of some interventions (like the re-fracturing of an area due to the stress risers created by screw holes original drilled to fix the fracture ) are now better researched, but many other interventions are not examined in this way. There is no data collection on the long-term result of a new set of forces.  It's also important to note that the statistics on "improvement" given to the patient do not include the statistics of separate issues arising from an intervention. It might even be impossible to know all of the different ways messing with one structure messes with all the others.  This parts vs whole perspective what what I was referring to in last year's Orange Autopsy post.

I like biomechanical science for both its simplicity and its complexity. This principle, that a change to one thing automatically changes everything else, is easy to grasp. The inability to quantify how it changes everything blows my mind. Yours too?