Materials & Applications

Graceful Failures: Designing Sacrificial Components

Graceful Failures Designing Sacrificial Components

A sacrificial component is a part that is designed to fail in order to protect either a person or the rest of the devices. Sacrificial component implies that the part will take some kind of damage and get destroyed in order to prevent some more precious part of the circuit from taking damage. Usually, a sacrificial part is designed so that it’s easy to replace. Some examples of sacrificial components include:

  • Fuses – electrical protection
  • Shear bolts and shear pins – mechanical protection
  • Sacrificial anodes – corrosion protection

Selection of materials for sacrificial components offers an interesting challenge. A sacrificial component is meant to be the weakest link in the system, so if we make it too strong, some other more critical part could end up being damaged.

So how do we go about choosing materials for a part that is meant to fail? In this article, we will walk through material selection in a few examples sacrificial components.

Fuses (electrical)

Electrical fuses are probably the most familiar type of sacrificial component to most people. Because most electrical devices could be damaged or start a fire if too much current is allowed to flow through them, fuses are often used to prevent excessive current flow.

Installation of automatic fuses on a DIN rail.
Installation of automatic fuses on a DIN rail.

Traditional fuses are made from a metallic filament in a glass tube. As more and more current is driven through the filament, it heats up. If enough heat is generated to melt or vaporize the filament, then the fuse “blows”, destroying the filament and interrupting the electrical flow.

So if we were to select some possible fuse materials, we would consider these material properties:

  • High electrical conductivity
  • Low melting point
  • Easily shaped into filaments or sheets
electrical fuses - sacrificial components
Cartridge fuses are cylindrical in shape and have contact points at each end. They are rated for circuits over 30 amps and typically 240 volts.

Usually, metals offer the best combination of electrical conductivity and the ability to be shaped into filaments or sheets. However, different metals are suitable for making fuses with different current ratings. Generally, a higher-current fuse will be made from a higher-melting-point metal.

If we perform a Matmatch search for high conductivity and low melting point, we find some good candidate materials for lower-current fuses.

Material (source in link)Conductivity (S/m)Melting point (°C)
Zinc1.51 E+7 387
Aluminium1.40-1.80 E+6600
AuGe127.00 E+6356
AuSn205.00 E+6280

High-current rating fuses would require a higher melting point, so we can perform another Matmatch search to find materials with the highest electrical conductivity.

Material (source in link)Conductivity (S/m)Melting point (°C)
Silver6.00 E+7 961
Copper5.00 E+71084

Of these materials, zinc, aluminium, silver, and copper are all common fuse materials. (The gold alloys are too expensive). The metal filament in the fuse generates heat as current flows through it. Should the filament be subjected to current in excess of the fuse’s rating, it will heat up past the metal’s melting point, destroying the filament and breaking the circuit.

Shear bolts & shear pins

Shear bolts are another common type of sacrificial component, but instead of protecting a circuit from electrical overload, they protect mechanical devices from excessive force. Shear bolts are a type of fastener, and they protect the parts they hold together by fracturing at a lower force than would be required to damage the rest of the machine.

A shear pin (sometimes called a shear bolt) in a snow blower connects the auger to the shaft. It is designed to break away in case foreign objects come in contact with the moving parts.

One example of a shear bolt in action is in snowblowers (AKA snow throwers). Snowblowers often encounter unexpected obstacles hidden by the snow, such as a rock or a curb. When that happens, if the force of the auger striking the obstacle was transmitted to the machine’s engine, then serious damage could result. In order to prevent serious damage, shear bolts are used. Then, in the event the auger jams, the bolts fail rather than overloading and damaging the engine.

snow blower shear pin
If the pins are too brittle, then they will just keep breaking in low stress situations over and over. If the pins are too durable, then they put the snowblower's gears at risk by failing to break when they are most needed.

When selecting a material for a shear pin, we must take an unusual material selection approach:

  • Target a narrow range of strengths
  • Avoid materials that are too ductile

The importance of strength is intuitive: after all, we must know the strength of the bolt if we are to design it to fail at some desired level of force. However, rather than designing the strongest possible part, we need to choose a material that lets us target a particular strength. For example, we could carry out a Matmatch search to target materials with tensile strengths near 1000 MPa.

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Shear bolts and shear pins are intended to fail at a specific force level, relieving stress in a mechanical system. However, if the bolt bends or deforms before fracturing, then it could lead to unexpected failure modes or make replacement more difficult than intended. Therefore, we must avoid materials which are too ductile.

One method would be to choose yield strengths close to the target tensile strength. Alternatively, we can select materials with low elongation at failure. The result of either search should be viable. Keep in mind that a change in the shear bolt or shear pin diameter could also be made to allow for the selection of different materials.

Shear bolts and shear pins are not the only kind of sacrificial mechanical part. Sacrificial gears are sometimes used to protect geared drive systems. Crumple zones in cars are designed to absorb energy in the event of a crash, protecting the occupants.

All are good examples of cases where it may be desirable to select materials which are not the strongest available because a weaker material will lead to a more controlled and safer failure.

Sacrificial anodes

Sacrificial parts can also be made to protect metal parts from corrosion. Metal corrosion involves the metal acting as the anode of an electrochemical cell, or in other words, the metal gives up electrons when it corrodes. Another material has to act as the cathode and receive the electrons given up by the anode.

In the case of a simple metal part in contact with water, dissolved oxygen in the water usually plays the role of the cathode. This is why a piece of iron or steel left in a wet environment will rust.

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A metal can be protected by putting it into contact with another piece of metal that will act as the anode in its place. Not all metals are equally driven to form oxides. Metals can be ranked according to their oxidation potential, which measures an atom’s tendency to form an oxide. The table below shows some select oxidation potentials.

Ion formedOxidation potential (V w/ H reference electrode)
Fe3+0.04
Fe2+0.41
Zn2+0.76
Al3+1.66
Mg2+2.38

A metal part can be protected from corrosion by pairing it with another metal with a greater oxidation potential. The metal with the greater oxidation potential will act as the anode and suffer the effects of corrosion. The other part will act as the cathode, and it will be protected from corrosion so long as the two different metals are in electrical contact with one another.

sacrificial anode - sacrificial components
Vessel owners need to be aware of the location and condition of sacrificial anodes mounted on their boat. Many owners don’t realize the corrosion damage that can occur to stern drives and under water metal fittings if anodes are not regularly maintained or replaced.

For example, iron (Fe) is commonly paired with a sacrificial zinc (Zn) anode. The zinc will give up its electrons more easily than the iron, so the zinc will corrode while the iron component will remain intact. So long as some zinc remains in electrical contact with the iron, the iron will be protected.

Let's sum it up

There are cases where selecting the strongest, most durable material is not always best. Many systems can benefit from incorporating sacrificial components that protect the rest of the device when they fail. These include:

  • Electrical fuses that melt to prevent excessive current from harming a circuit
  • Shear bolts and pins that fracture to prevent mechanical damage to motors and other parts
  • Sacrificial anodes that corrode in order to protect another metal part

Do you have any comments? Share your opinion in the comment section below!

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