Forces and Stresses

Forces and Stresses

Understanding how forces act on materials and structures is essential in AQA GCSE D&T. You must know the types of force, how materials respond to them, and how designers strengthen structures to resist failure.

Types of Force

Force	Description	Example
Tension	Pulling force that stretches a material	A rope in a tug of war, a guitar string
Compression	Pushing force that squashes or shortens a material	A column supporting a roof, a car spring under load
Shear	Opposing forces acting across a material, causing sliding	Scissors cutting paper, a rivet under sideways load
Bending	Combination of tension and compression — one side stretches, the other compresses	A shelf loaded in the middle, a diving board
Torsion	Twisting force applied along the length of a material	Turning a screwdriver, a drive shaft

Stress and Strain

• Stress = Force ÷ Cross-sectional area (measured in N/mm² or MPa)
• Strain = Extension ÷ Original length (no units — it is a ratio)

When a material is loaded, it experiences stress internally. The resulting deformation is strain.

Elastic deformation: The material returns to its original shape when the force is removed (like a spring). This occurs below the elastic limit.

Plastic deformation: The material is permanently deformed and does not return to its original shape. This occurs beyond the elastic limit.

Stress-Strain Behaviour

A stress-strain graph shows how a material responds to increasing load:

1. Elastic region — straight line, material returns to original shape (Hooke's Law applies)

2. Elastic limit / yield point — beyond this, deformation becomes permanent

3. Plastic region — material deforms permanently but continues to bear load

4. Ultimate tensile strength (UTS) — maximum stress the material can withstand

5. Fracture point — the material breaks

Brittle materials (cast iron, glass, acrylic) have little or no plastic region — they break suddenly without warning.

Ductile materials (mild steel, copper, aluminium) have a large plastic region — they deform significantly before breaking, giving warning of failure.

Strengthening Structures

Designers use several techniques to make structures stronger and more rigid without adding excessive weight:

Triangulation:

• Triangles are inherently rigid because their shape cannot be distorted without changing the length of a side
• Used extensively in bridges, roof trusses, crane jibs, and bicycle frames
• A rectangular frame can be made rigid by adding a diagonal brace (creating two triangles)

Webbing and ribbing:

• Ribs are raised sections on the surface of a component that increase stiffness without adding much weight
• Common in injection-moulded plastic products (e.g. underneath a plastic chair seat)
• Corrugated structures (e.g. corrugated cardboard, corrugated iron roofing) use the wave profile as continuous ribbing

Gussets:

• A gusset is a triangular or rectangular plate used to reinforce a joint, particularly at corners
• Added to shelving brackets, furniture frames, and metal structures
• Distributes force over a larger area, reducing stress concentration at the joint

Folding and bending:

• A flat sheet of material is weak in bending but becomes much stiffer when folded or formed into a channel, angle, or tube shape
• Example: an A4 sheet of paper is floppy, but folded into a concertina it can support a textbook
• I-beams and box sections exploit this principle — maximum stiffness with minimum material

Laminating:

• Gluing thin layers together (e.g. plywood, laminated timber beams)
• Cross-grain construction in plywood prevents warping and equalises strength in all directions
• Laminated timber beams (glulam) can span greater distances than solid timber

Beams and Structural Sections

Section	Description	Strength Characteristics
Solid rectangle	Simple, easy to produce	Moderate; material in the centre contributes little to bending resistance
I-beam (H-beam)	Flanges top and bottom, thin web	Very high bending resistance; most material where stress is greatest
Box section (hollow rectangle)	Closed hollow rectangle	Good in bending AND torsion; used in vehicle chassis
Tube (circular hollow)	Closed hollow circle	Excellent torsion resistance, good bending; scaffolding, bike frames
L-angle	L-shaped cross section	Good for bracket/corner reinforcement
T-section	T-shaped cross section	Used in concrete reinforcement and small structural frames

Modes of Failure

Products can fail in several ways:

• Fatigue failure — repeated cyclic loading causes cracks to grow over time until the material breaks, even below its UTS (e.g. a paperclip bent back and forth)
• Creep — gradual, permanent deformation under constant load over a long time, especially at high temperatures (e.g. lead roofing sagging over decades)
• Buckling — a slender column under compression bows outward and collapses sideways
• Stress concentration — sharp corners, holes, or notches create localised high-stress areas where cracks initiate; solved with fillets (rounded internal corners)

Exam Tips

• Be precise with force terminology — do NOT use "force" generically; name the specific type (tension, compression, shear, bending, torsion)
• When asked how to strengthen a structure, give a specific method and explain why it works (e.g. "add triangulation because triangles cannot be distorted without changing side lengths")
• Sketch diagrams showing where forces act — use arrows to indicate direction and label them
• Know the difference between elastic and plastic deformation and the significance of the elastic limit
• Stress concentration is a favourite topic — always suggest rounded corners (fillets) as a design solution
• If comparing materials under load, refer to whether they are brittle or ductile and explain the consequence for safety

Key Definitions

• Stress: The internal force per unit area within a material (N/mm² or MPa)
• Strain: The deformation of a material expressed as a ratio of change in length to original length
• Elastic limit: The maximum stress a material can withstand and still return to its original shape
• Triangulation: The use of triangular structures to create rigid, non-deformable frameworks
• Stress concentration: A localised area of high stress, typically at sharp corners, holes, or notches, where cracks are most likely to start