Forces, Stresses & Reinforcement
Forces, Stresses & Reinforcement
Every product in your material category experiences forces when it is used. A shelf bears downward weight, a phone case resists impact, and a garment stretches as the wearer moves. You need to identify five types of force, explain how each one acts on real products, and know the specific ways metals, papers and boards, polymers, systems, textiles and timbers can be reinforced and stiffened to resist them.
The five types of force and stress are tension (pulling), compression (pushing), shear (sliding), bending (a mix of tension and compression) and torsion (twisting).
The Five Types of Force
Tension stretches a material apart. A tow rope pulling a trailer, or the steel cable on a suspension bridge, is under tension. Materials that resist tension well, such as steel, are chosen for cables, chains and structural ties.
Compression is the opposite — a squeezing force. Table legs are compressed by the weight resting on the table top, and concrete foundations are compressed by the building above. Materials that are strong in compression, such as concrete and timber along the grain, are chosen for supporting structures.
Shear happens when two forces act in opposite directions along parallel planes. Scissors cut paper using shear force, and a rivet joining two metal sheets resists the sheets trying to slide past each other.
Bending combines tension and compression in one component. When a shelf sags under load, its top surface is in compression and its underside is in tension. If a material cannot resist both at once, it cracks on the tension side first.
Torsion is a twisting force. A screwdriver shaft is under torsion as it turns a screw, and drive shafts in machinery twist under torsional load. Hollow tube sections resist torsion efficiently because twisting (torsional) stress is highest at the outer surface and lowest at the centre, so material can be placed away from the centre where it does most work, saving weight.
Forces Across Material Categories
Metals mainly experience compression, tension and shear, plus electrical and magnetic forces where a metal case or component sits close to a circuit — a steel enclosure can accidentally shield or interfere with a signal.
Papers and boards are most affected by bending, torsion, shear and compression. A cereal box resists compression from stacking, while a greetings card must resist bending without cracking along its fold.
Polymers experience compression, tension and shear, and flexibility is also a key factor — a polypropylene hinge bends thousands of times without failing, while a rigid acrylic sheet can shatter under sudden impact.
Systems components and cases experience tension, compression, torsion and shear, particularly at connectors, switches and the joints of an enclosure that gets opened and closed repeatedly.
Textiles experience compression, tension and shear, along with natural forces built up within a fibre as it grows and the fabric’s own flexibility — a woven cotton seam under tension behaves differently from an elastic knitted cuff.
Timbers experience compression, tension, shear and natural forces built into the wood as it grew, such as the direction of the grain; engineers also use pre-stressed construction beams, deliberately loaded during manufacture so they resist bending better once installed.
Reinforcing & Stiffening Materials
Materials can be modified or shaped to resist force better without simply using more material. You need to know the specific reinforcement and stiffening technique used in your chosen material category, and be able to explain why each technique works.
Reinforcement techniques increase a material’s ability to resist force without adding much extra weight. Examples include lamination (timbers, papers/boards and textiles), work hardening and structural beams (metals), and triangulation (polymers).
Metals are reinforced through hardening and tempering, and by increasing carbon content, which raises hardness but can make the metal more brittle. Repeatedly bending or hammering a metal — work hardening — also stiffens it. Structural sections such as I, U, T and C beams concentrate material at the outer edges, where bending stress is greatest, so they resist bending while saving weight and material.
Papers and boards are stiffened using lamination, encapsulation (sealing a sheet inside a protective plastic film), corrugation, adding extra layers or ribs, and sandwich construction, which places two thin, rigid skins either side of a lightweight core.
Polymers gain rigidity from frame structures and triangulation — a triangle cannot distort without one of its sides changing length, so triangulated frames resist force efficiently. Suitable fabrication methods and additives, such as glass fibre, also add strength to a polymer product.
Systems products use composite materials in their casings and internal ribbing to strengthen plastic enclosures, adding stiffness around screw bosses and joints without needing to thicken the whole case.
Textiles are reinforced using ribs and boning to hold a shape, lamination to bond fabric layers together, embedded composite materials for extra strength, stay stitching to stop a seam stretching out of shape, and interfacing to give structure to panels such as collars and cuffs.
Timbers are reinforced through frame structures, lamination (plywood is layers of veneer glued with alternating grain), braces and tie bars, and by embedding composite materials within a timber structure to increase its resistance to force.
Beams, Triangulation & Redirecting Force
Structural shapes can redirect or share out force so no single point takes the full load. I, U, T and C beams place material where bending stress is highest — at the top and bottom — leaving less material in the middle, where stress is lowest. Triangulated frames, like a roof truss, spread load through several straight members instead of one, because a triangle’s fixed geometry resists distortion. Both principles let designers use less material while still resisting force safely.