Welcome to Systems: Forces and Stresses!

Have you ever wondered why a bridge doesn't collapse under a heavy lorry, or why your TV remote doesn't snap in half when you change the batteries? It's all down to how designers manage forces and stresses. In the "Systems" category of D&T, we focus on how to make sure our electronic casings, mechanical frames, and structures are strong enough to do their job without breaking.

Don't worry if this seems a bit "sciencey" at first—we're going to break it down into simple movements that you see every day.


1. Understanding Forces and Stresses

A force is basically a push or a pull on an object. When a force acts on a material, it creates stress. In the Systems curriculum, you need to know three main types of stress:

A. Tension (The "Pulling" Force)

Tension occurs when a material is being pulled apart or stretched. Think of a game of tug-of-war—the rope is under tension.

  • Real-world example: The cables holding up a suspension bridge or the wires in a hanging electronic sign.
  • Memory Aid: Think of "Tension" as making something "Tense" and tight!

B. Compression (The "Squashing" Force)

Compression is the opposite of tension. It happens when a material is being pushed or squashed together.

  • Real-world example: The legs of a chair when you sit on it, or the pillars holding up a heavy control panel.
  • Memory Aid: "Compression" sounds like "Crushing."

C. Shear (The "Sliding" Force)

Shear forces act in opposite directions but not in the same line. It’s like two parts of a material trying to slide past each other. This is exactly how a pair of scissors works!

  • Real-world example: A bolt holding two moving metal arms together. If the arms pull in opposite directions, they might "shear" (snap) the bolt.
  • Memory Aid: Think of "Shears" (another word for large scissors).

Quick Review:
Tension = Pulling/Stretching
Compression = Squashing/Pushing
Shear = Sliding/Cutting


2. Reinforcement and Stiffening

Sometimes, the material we want to use (like a specific polymer for a casing) isn't strong enough on its own. Designers use clever tricks to reinforce (make it stronger) or stiffen (make it less likely to bend) the structure.

A. Triangulation

The triangle is the strongest shape in engineering. Unlike a square, which can be pushed over into a diamond shape (this is called racking), a triangle cannot be deformed without changing the length of its sides.

  • How it works: Adding diagonal supports to a square frame turns it into two triangles. This makes the structure rigid.
  • Systems context: You will see this in the metal frames of large machines or the lattice towers for communication systems.

B. Frame Structures

A frame structure is a skeleton of parts joined together to support a load. Instead of using a solid, heavy block of material, we use a "frame" to keep the product light but strong.

  • Example: The internal chassis of a computer or the frame of a drone.
  • Why use it? It saves money on materials and makes the system lighter.

C. Additives

In the "Systems" and "Polymers" world, we can change the properties of a material by adding things to it. These are called additives.

  • Reinforcing: Adding glass fibres or carbon fibres to a polymer (plastic) makes it much tougher and stiffer. This is a composite material.
  • Stiffening: Sometimes chemicals are added to make a plastic less flexible so it holds its shape better under heat or pressure.

3. Designing for Strength: Common Mistakes to Avoid

When you are answering exam questions about why a system failed, look for these issues:

  • Flexing: If a circuit board (PCB) is too thin and isn't supported, it will bend (flex). This can snap the soldered joints. Solution: Add stiffening ribs to the plastic casing.
  • Stress Points: Sharp corners are where materials usually snap. Solution: Use rounded corners (fillets) to spread the force.
  • Overloading: Using a material that is good in tension (like a wire) for a job that needs compression (like a pillar).

Key Takeaways for Your Revision

1. Identify the force: Is it pulling (tension), pushing (compression), or sliding (shear)?

2. Know your triangles: If a question asks how to stop a frame from wobbling, the answer is almost always triangulation.

3. Think about the "Skeleton": Frame structures are used to provide strength without adding too much weight.

4. Material help: Additives (like fibres) can be mixed into polymers to reinforce them from the inside.


Did you know? Many modern electronic devices use their outer shell as part of the structure to save space. This is why your smartphone feels so solid—the casing is designed to resist all three types of stress at once!