Today's Lesson: Interlocking Parts

What is it?

Every material has some inherent strength due to intermolecular bonds, microstructure, or interactions between different phases in a composite material. Simply stated, a material fails (deforms) when the stress in the material exceeds some critical stress value. The stress a material is under is expressed as a Force per Area. Thus a part with a larger cross section can withstand a larger force.

Buckling
Many parts used in structural applications have a large aspect ratio (ratio of length to width). If a long slender part is put under compression, it is likely to buckle. Buckling occurs when a part deforms long before the plastic limit is reached. This is easy to demonstrate with a flexible ruler, a straw, or a toothpick. If you try to compress it along the long axis it will bow out and fail to support the load.
Fcrit=(π2EI)/(KL)2

where E is the modulus of Elasticity
I is the area moment of inertia
L is the length of the column
K is a factor that depends on how the part is supported.

This equation reveals that buckling can be avoided by choosing a stiffer material, decreasing the length, or by increasing the area moment of inertia. Thus if you are designing a tube, try increasing the radius a little to increase the area moment. Buckling can also be avoided by using thinner parts to stiffen a long, flexible frame, as shown below:
Skyscraper_Challenge_Lesson_Plan.jpg
balljoint_display_t30415.jpg FlexiTubing_001-1Res_display_t17021.jpgBallSocket_preview_card.jpg
thing:1968 thing:17021 thing:5578