Forces and Stresses

Forces and Stresses

It is vital a designer takes forces and stress into account when designing, how something is stretched, bent, pulled and twisted will have an impact on its effectiveness, durability and quality.


Tensile strength is the how something withstands a pulling force. Suspension bridges rely on high tensile strength to stay up in the same way that a garden hammock does.


Is the opposite of tension, this is how well a material can with stand a force pressing down. A table has to withstand the compression pressing down on its legs. A heavy weight could squash on object and make it shorter.


When a long item is pushed down in the centre it is subjected to bending. A chair has to be able to withstand the bending caused by people sitting on it.


Torsion twists objects. An item which turns a lot needs to be able to withstand torsion to stop it snapping or bending such as a drill or propeller.


Shear force is when an object moves in opposite directions to create a force such as the cutting action of a pair of scissors or a guillotine.

Materials can be adapted to withstand forces and stresses better.

Forces and Stresses, figure 1


Materials can be laminated with a sandwich of something else to improve its strength. Paper can be made water resistant with a plasticised layer. Fabrics can be laminated to have layers with different properties for example, waterproof, breathability and warmth make up Goretex. Corrugated card is a zig zag of card laminated by thin card for strength. Plywood is layers of wood laminated together at right angles making it stronger.


The series of bends inside corrugated card makes it difficult to bend unless it’s scored.


When a material is folded it becomes harder to bend but the fold edge (crease) becomes more flexible.


Woven in several directions to increase its strength webbing is used for fabrics that must have a high tensile strength like seatbelts and climbing ropes.


Applying a heat set interfacing to the underside of fabrics keeps the softness of the outer fabric whilst stiffening and stabilising it from the inside. This is particularly useful with light fabrics with cuffs, collars and waistbands.