How holes affect your parts By Justin Swick

Flow of Stress Stress is just a force applied to an area. It flows like water in a river and it is advantageous for us to make it flow smoothly to avoid concentrations of it.

Stress Concentrations Stress is concentrated around features. We will refer to this as local stress. It can be many orders of magnitude greater than the stress in the rest of the part. high local stress

Local Stress For our previous example, a hole in a rectangular part will have a maximum stress at the edges of the hole. The stress concentration factor for this particular hole is 3, meaning that the local stress is 3 times more than the average stress across the part.

Stress Concentration Factor, K The stress concentration factor, represented by K, is related to the ratio of the diameter of the hole to the width of the part. In general: a smaller hole will have a smaller concentration of stress. R T

Stress Concentration Factor, K R T R T R T K=2.8 K=1.6 K=1.5 R/T = 0.9 R/T = 0.7 R/T = 0.5

What can we do to reduce K? bad betterbest Square corners are stress concentration heaven, plus they are hard to manufacture. This is easier to build and will last longer. All sheet metal shops cut square holes this way. This design provides for the least amount of stress, but may only be suitable for locating round parts like shafts.

What can we do to reduce K? bad betterbest Our part will break before it bends. Now we have two cracking locations, but we have reduced the stress concentration. The fillet gets rid of our corners (crack starters)

Which is better? Although we have removed more material for the second part, the stress concentration is lower, making it stronger.

Which is better? An oval will have a lower stress concentration than a circle, therefore, having two smaller holes inline with the stress allows it to flow away from the hole more slowly.

Which is better? In contrast, an oval facing the other direction would be worse. Higher K (worse) Lower K (better)

What does a crack look like? Stop drilled crack

Why do cracks propagate so fast? Under a microscope we can see that a crack has a tiny, needle like edge. We know that stress doesn’t like to flow around corners and will cause the part to rip. microscopic concrete cracks

What is actually happening at a crack? Short story: the atomic bonds are being broken. Metals are crystalline structures, meaning they have order. The boundary of each crystal grain is where cracks start. These crystals can slide against each other and re-form their bonds, unless they are held far apart.

Crystalline structure of metals

Crystalline Structure, Children’s Toy? Basically, metal behaves just like this magnetic children’s toy on a microscopic level. It can make and break bonds as it is moved a little bit, but when moved too much, it falls apart.

Microscopic Breaking

Microscopic Shearing new bond

Microscopic Bending new bonds

Microscopic Crack Forming 1/5

Microscopic Crack Forming 2/5

Microscopic Crack Forming 3/5

Microscopic Crack Forming 4/5

Microscopic Crack Forming 5/5

Microscopic Crack Stopper 1/5

Microscopic Crack Stopper 2/5

Microscopic Crack Stopper 3/5

Microscopic Crack Stopper 4/5

Microscopic Crack Stopper 5/5

CRACK STOPPED!

The problem with drilling crack stopping holes after the fact In aviation it is common to stop cracks that have occured in non-structural sheet metal parts by drilling a hole at the end of the crack and plugging the hole with a close fitting rivet. This will stop the crack temporarily, but it doesn’t solve the problem which is that the material isn’t strong enough to support the load it is subjected to. Drilling crack stoppers is a temporary fix!

Fatigue Here’s where the rubber hits the road. It is possible to design a part that performs excellent on the first use, but after repeated usage, breaks. This is frequently seen in aluminum car and airplane parts that have limited service life. Imagine bending a coat hanger… one bend does nothing, but bend it back and forth 100 times and it will break.

Fatigue The key to designing parts that will experience cyclical loading conditions (repeated load reversals) is to design them for infinite life. This is not possible for some materials, such as aluminum, which do not have a fatigue limit.

Fatigue Limit The S-N curve describes the relationship between the applied stress and cycles to failure. If you load something with a high load, it fails in fewer cycles. Going back to our coat hanger example, if you barely bend it, it will take many more cycles to break than if you really put a lot of force into it and bend it over on itself each direction.

S-N Curve Example The endurance limit or fatigue limit is the stress that can be applied to an object any number of times and it will not break Aluminum has no fatigue limit!

The Moral of the Story Don’t build springs or spring-like devices out of aluminum!!! Aluminum has no fatigue limit!

Why is my Motorcycle Frame Aluminum? Your motorcycle frame is designed to experience a very high number of cycles before it fails… It’s overbuilt, so it will last a long time, but eventually it will break! This guy had to ride halfway around the world before his frame cracked due to the extreme stress placed on it. luggage + bad roads + high miles = BROKEN! or high stress and high cycles Barney Says: “Surprise surprise surprise, it broke at a weld!”

Fatigue in action Fatigue failures are easy to spot. There will be a smooth region like a beach head, a crack origin, and a final overload region that is very rough for every fatigue failure. This is because small cracks develop at the surface and gradually grow until they arrive at the critical crack length, then the part breaks.

How do I design my parts to not break? 1.Figure out what the loads on your part will be. (use caution, this is tricky, you may want to guess high!) 2.Perform a stress analysis, taking care to compute it around notches, holes, etc. 3.Look up the endurance limit for the material and compare it to the expected highest local stress. If the local stress is lower, then you have infinite part life! (theoretically) 4.Double whatever you came up with!

Summary: What should I do? Don’t torch cut small holes Grind down everything with a rough edge Design all of your parts with chamfers, bevels, or fillets where they change shape De-burr your holes Torque Bolts, screws, and nuts to their proper values in a good sequence Don’t use chains with nicks in them!