Design and Technology: Product Design (9252) | Forces and stresses

Hello Future Product Designers!

Welcome to one of the most important chapters in the technical principles section: Forces and Stresses. Don't worry if this sounds a bit like physics—we are going to focus purely on how these forces affect the materials you choose and the products you design.

Every single product, from a simple spoon to a massive skyscraper, is constantly dealing with invisible pushes and pulls. If you know how materials react to these forces, you can design products that are safe, reliable, and strong. Let’s get started!

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1. Understanding Forces and Stress: The Basics

When studying forces in design, we need to separate the action from the reaction.

1.1. Force (The Action)

A Force is the external action—it is the push or pull being applied to the product (e.g., you sitting on a chair, or the wind pushing against a sign).

1.2. Stress (The Material's Reaction)

Stress is the internal resistance that the material puts up against the external force. Think of it as the material fighting back to maintain its shape.

• If the force is too great, the material cannot resist the internal stress, and it fails (it breaks, bends, or snaps).

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2. The Five Main Types of Force (The Big Five)

Designers classify forces into five main types based on the direction they act upon a material. Memorizing these is essential!

2.1. Tension (The Pull)

Tension is the force that tries to pull a material apart or stretch it along its length. Materials that are strong under tension are called tensile.

• Real-World Analogy: A game of tug-of-war. The rope is under tension.
• Design Examples:
  • Suspension bridge cables (they pull the deck upwards).
  • The straps on a heavy backpack.
  • Ropes used to hoist a flag.

Key Takeaway: If a product is likely to be stretched or hung, it needs high tensile strength.

2.2. Compression (The Push or Squash)

Compression is the force that tries to squash, shorten, or squeeze a material together.

• Real-World Analogy: Squashing a can or stepping on a plastic bottle.
• Design Examples:
  • The legs of a chair or table supporting weight.
  • Bricks or concrete foundations holding up a wall.
  • Pillars or columns in a building.
• Designer Tip: Short, thick columns are excellent at resisting compression.

Key Takeaway: Materials like concrete and ceramics are extremely strong in compression.

2.3. Shear (The Slide or Slice)

Shear forces act parallel to the surface of the material and cause one section of the material to slide past another, usually resulting in a slicing or tearing action.

• Real-World Analogy: Using scissors to cut paper, or tearing a sheet of metal.
• Design Examples:
  • Rivets, bolts, or screws holding two overlapping plates of metal together (the force tries to cut the bolt in half).
  • The action of a punch or die when stamping a hole in metal.

Key Takeaway: Shear forces are usually tackled by strong fixing methods like rivets or thick bolts.

2.4. Torsion (The Twist)

Torsion is a twisting force applied to an object, usually causing it to rotate around its central axis.

• Memory Trick: Torsion = Twisting.
• Real-World Analogy: Wringing out a wet towel or turning a screwdriver.
• Design Examples:
  • Axles and drive shafts in vehicles (they transmit twisting power from the engine to the wheels).
  • Door handles or knobs.
  • Drill bits.

Key Takeaway: Products subjected to rotation (like shafts) need to be highly resistant to torsion.

2.5. Bending (The Combination Force)

Bending occurs when a material is curved, usually due to a load pushing perpendicular to the material’s length (like a beam). A bending force is a combination of tension and compression.

• The Two Sides of Bending:
  1. The outer side (the side that stretches) is under Tension.
  2. The inner side (the side that squashes) is under Compression.
• The Neutral Axis: There is a line running through the center of the beam that experiences neither tension nor compression. This is called the Neutral Axis.
• Design Consideration: Because bending creates both tension and compression, materials must be able to withstand both types of stress. This is why beams often use an ‘I’ or ‘H’ shape—to place the most material far away from the neutral axis, where the forces are strongest.

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3. How Materials Respond: Elasticity, Plasticity, and Failure

When a force creates stress within a material, the material reacts by changing shape. This change in shape is called Strain. We need to know when this change is temporary and when it's permanent.

3.1. Elasticity (The Bounce Back)

A material is Elastic if it returns exactly to its original size and shape once the stress (force) is removed.

• Example: A spring or a perfectly elastic rubber band. When you stop pulling, it snaps back.
• The Elastic Limit: This is the maximum force a material can handle before it starts to deform permanently. Designers always aim to keep the stress on a product well below its elastic limit.

3.2. Plasticity (The Permanent Dent)

If the force applied is greater than the elastic limit, the material experiences Plastic Deformation. This means the material is permanently changed and will not return to its original shape.

• Example: If you bend a paperclip too far, it stays bent. It has undergone plastic deformation.
• Ductile Materials: Materials that show a lot of plastic deformation before they finally break (like mild steel or soft metals) are called ductile. This is good because it gives a warning sign (a bend or a dent) before catastrophic failure.

3.3. Brittle Materials

Brittle materials (like glass, ceramics, and some plastics) show very little or no plastic deformation. They remain elastic up to a point, and then they suddenly snap or shatter without warning.

Designer’s Choice: In applications where sudden catastrophic failure is dangerous (like in a car chassis), ductile materials are safer. In applications needing stiffness (like a kitchen countertop), brittle materials might be acceptable.

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4. Designing for Safety: Factor of Safety (FoS)

In product design, knowing the absolute maximum stress a material can handle (its Ultimate Tensile Strength - UTS) is not enough. We must always plan for misuse, manufacturing defects, and unexpected high loads.

4.1. What is the Factor of Safety?

The Factor of Safety (FoS) is a design margin used to ensure that a product will always be much stronger than required for its normal function.

The Factor of Safety is a ratio:

\[ \text{Factor of Safety} = \frac{\text{Maximum Stress Material Can Withstand (UTS)}}{\text{Maximum Stress Expected in Use}} \]

4.2. Why Use a Safety Factor?

• To account for unknown or unpredictable forces.
• To account for tiny flaws in the material that occurred during manufacturing.
• To ensure longevity and durability (the product won't fail after repeated use).
• To prevent catastrophic failure, especially in products where failure could cause injury or death (e.g., aircraft, lifting equipment).

Example: If a cable is rated to snap at 1000kg (its UTS), but the designer knows it will only be used to lift a 250kg load, they have applied a Factor of Safety of 4 (1000 / 250 = 4). This means the product is four times stronger than it needs to be!

Remember: A higher Factor of Safety makes the product safer, but often heavier and more expensive. Finding the right balance is key to good product design.

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Well done! You have covered the essential principles of forces and stresses. Keep reviewing those 'Big Five' forces—they are the foundation for material choice and structural integrity in all product design!