Why do things break? And how to design them not to.

Why do things break… why do we care? Ask Bear Grylls, who spent minutes of television teaching us how to build a bridge only for it to dramatically explode as he hurled it to span a canyon valley. Ask the backyard DIY battler whose pagola took flight on a gusty day. Ask this guy. So read on, for a primer on why things break and how you can stop them.

What properties of a material can tell us if it is going to break?

Alright, here comes some material science knowledge so hold on to your brain. The world of engineering materials can be loosely split into to two categories. Brittle and Ductile. Brittle materials include glass, acrylic plastic, carbon fibre and concrete. These materials can require a lot of force to break, but do not deform much before breaking completely.

Ductile materials, such as steel, aluminum, rubber, ABS plastic, Nylon and many other plastics generally require less force to break them but deform a lot before breaking. This is useful because the deformation of the material under load absorbs energy and are often used in applications such as crumple zones in cars, or where deformation without breaking is better than breaking (bridges and cranes). In fact, brittle materials are often combined with ductile materials to provide a high strength and tough material. Think fibreglass, steel reinforced concrete and glass reinforced nylon.

Stiffness is different from strength, and relates the force applied to an object to how much that object deforms. Or, conversely, if an object is forced to deform, how much force (or in “stress”) is applied to the material of the object. To summarise.

Stress – The amount of force in a given section of an object.

Strength – The maximum stress that the object can withstand without breaking.

Toughness – The amount of energy an object absorbs while breaking.

Stiffness – How much an object deforms (stretches) under a given load.

How can we figure out if something is going to break?

Hand calculations – By making a few assumptions, researching the materials properties and applying some equations, one can predict what load or deflection will cause something to break. for example, bending stress can be used to predict the maximum load a beam can carry.

Build it and test it – This can be the fastest and cheapest, or the slowest and most expensive. But it is often the most accessible and understandable. If you test something and it breaks before you want it to then you know you have a problem. But what if is survives a simple test? Read on for when things get complicated.

FEA “Finite element analysis” – This is a type of computer simulation that breaks up the part being analysed into very small elements, then calculates the force in each of them. It is useful for complex shapes, complex forces or for highly optimised applications. This saves a real life prototype needing to be built and destructively tested for each new design iteration, although some real life testing is usually done as a final check or early on to calibrate the computer analysis.

Just over-design it – The method used for backyard patios without building consent and soviet era vehicles alike. If materials are cheap and space or weight isn’t an issue, this can be a genuinely good option. Planes are painstakingly designed to be 25% stronger than they have to be in the most extreme case, because keeping weight down is a much bigger benefit than the cost.  But why waste your time removing every unnecessary piece of material of your reclaimed wood bench-seat?

Boeing bend tests the wings of a 787
Boeing bend tests the wings of a 787. Source: Wired.com

What can make something break unexpectedly?

There are some extra factors that can cause things to break. If your building something, watch out for the sneaky bastards below.

Fatigue

Fatigue is the type of failure that occurs under cyclic loading, like that found in a car axle or a machine gun. Small cracks in the surface can grow with each cycle until the part fails, often at much lower forces than a constant, static load. A famous example of this is the “Boston Molasses Flood” where the collapse of a storage tank caused 2,000,000 gallons of molasses to be unleashed on residents, killing 21 and injuring 150. The cause of the failure of the tank was thought to be the cyclic filling and unfilling of the tank causing fatigue failure of a manhole at the base of the tank. Another factor was the stress concentration of the manhole cover. I love a good segway…

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Fracture of an aluminium crank arm. The dark wavy area represents cracks slowly growing with each cycle and light grainy area is the final failure. Source: Lokilech, via Wikimedia Commons

Stress Concentrators

Sharp internal corners can concentrate forces travelling through an object and cause them to break at under much lower forces, this is why crackers (almost) always break at the line formed into them. Check out the beautiful photoelastic visualisation of the stresses in a protractor below.

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The contours are very tight around the sharp internal corner of this protractor, indicating high stress. Source: Nevit Dilmen via Wikimedia Commons.

Temperature

First the easy thing one. If an polypropylene spatula gets too hot, it will soften or melt and you can no longer stir your stir fry. Great, now your spatula is broken and your bok choy tastes like plastic. But temperature goes beyond this. If something gets too cold, it can go from being ductile (read above: able to absorb a lot of energy before breaking) to being brittle. What should be called “the rubber ball in liquid nitrogen” effect, is more boringly known as the ductile-brittle transition temperature. This is a big reason behind why the Titanic failed so catastrophically. The relatively early steel hull plate used on the Titanic went from being ductile to brittle in the -2°C waters of the Arctic and the hull shattered, rather than denting. Hull plates in following years would be made to stay ductile and tough to much lower temperatures, despite having similar strength at room temperature. To feel like an olden day version of an air crash investigator, read more here.

leo_spinning
I think we all would have enjoyed a little less romance and a little more materials science in this movie.

Another factor of temperature that can cause failure is thermal expansion, especially when things are rigidly held like the railway tracks below. This also applies when there are large temperature gradients in an object, one side wants to shrink and the other wants to expand. Think a frozen beer bottle that shatters when dipped in boiling water.

heat expansion railways

Time

Time is also an important factor. “Creep” is when an object under load slowly deforms over weeks, months or years, at a force much lower than would break it quickly. An example of creep can be seen in sagging timber structures. The famous Pitch Drop Experiment is not an example of creep, pitch is technically a fluid not a solid, but it is awesome. Corrosion, e.g rust can also occur over time, and can lead to small cracks which act as stress concentrators (described

Resonance

Just like a tuning fork has a particular frequency, so does every solid object. Think of a car aerial being “twanged” by pulling or releasing it, or a guitar string being strummed. A suspension bridge is really just a big guitar string as far as physics is concerned. Resonance is the name given to when a force is applied at the same frequency as the object naturally vibrates. This creates an increased range of movement every time the force is applied, just like a person pushing a child (or a gleeful adult) on a swing. The movement is far higher than if the force was applied at a different frequency (picture a blindfolded person pushing the swing). In the case of the Tacoma Narrows Bridge, the input force was a strong wind. As you can see below, the results are spectacular. Another example is Londons Millennium Bridge which suffered from the cyclic loading of pedestrians steps.

tacoma_narrows
The Tacoma narrows bridge suffered from aeroelastic flutter, a phenomenon where the aerodynamic forces on an object occur at the same frequency as the object naturally vibrates (also called resonance).

Unexpected loads

Finally, there can be loads that you just never expected. Rapper Savage might perform on a stage designed for weedy alt university band members.

And so we learn. From the first hunter gatherer whose decoratively carved spear broke at the worst time, to the engineers who built the Titanic and the Tacoma Narrows bridge, to me trying to figure out why my stir fry tastes like a burning rubbish bin, we have slowly learned to stop things from breaking. I hope this starter guide helps your Pagola build or whatever you are planning to create. For a more in depth study on mechanical failure, check out the much-smarter-than-me people at MIT for a free online text.