Forces, Stresses and Reinforcement
Forces in Structures and Products
Every physical product experiences forces during its use — the weight of books compresses a shelf, a twisted door handle experiences torsion, and a loaded bridge beam bends under traffic. Designers must understand the nature of these forces so they can choose the right material, shape and reinforcement strategy to ensure the product survives its intended service life without failing. Exam questions on this topic expect you to identify the type of stress acting on a component and justify a reinforcement choice with material and structural reasoning.
The Five Types of Stress
Tension is a pulling or stretching force. When two forces act along the same axis in opposite directions, pulling the material apart, the material is in tension. Examples: a rope suspending a load, a chain on a bicycle, the bottom flange of a loaded beam.
Compression is a pushing or squeezing force. When two forces act along the same axis pushing towards each other, the material is compressed and tends to shorten or buckle. Examples: the legs of a chair supporting a seated person, the columns of a building, the top flange of a loaded beam.
Torsion is a twisting force. When one end of a component is held fixed and a torque is applied to the other end, the material twists. A screwdriver shaft, a car drive shaft and a door handle spindle all experience torsion in normal use.
Shear is a cutting or sliding force where adjacent sections of a material tend to slide past each other. Scissors, chisels and bolts connecting structural members all experience shear. Shear stress in a loaded beam is greatest close to its supports, where one section of the beam tends to slide past the next.
Bending occurs when a force acts perpendicular to the long axis of a component, causing it to curve. The top of a bent beam is in compression; the bottom is in tension. A shelf loaded at its centre, a diving board and a roof rafter all experience bending.
The five stresses are: tension (pulling apart), compression (pushing together), torsion (twisting), shear (sliding/cutting) and bending (top compressed, bottom stretched). A beam under a central load experiences both compression (top) and tension (bottom) simultaneously — this is a common exam question.
Reinforcement Methods by Material
Reinforcement increases a component’s resistance to failure without making it excessively heavy or expensive. Different material families use different structural techniques:
Papers and Boards
Corrugating paper creates a wavy internal layer that vastly increases bending stiffness. Corrugated cardboard packaging uses this principle: the fluted inner layer resists compression from the top surface, while the flat outer liners resist tension and provide a smooth printing surface. Without the corrugation, the same amount of paper would buckle under a fraction of the load.
Timbers
Laminating layers of wood (as in plywood) with the grain alternated at 90° distributes forces in multiple directions. A single piece of timber is strong along the grain but weak across it; plywood resists forces in both directions equally. Gussets — triangular corner reinforcements — are used at frame joints to resist racking (shear forces that push a rectangular frame into a parallelogram).
Metals
Metal sections can be formed into shapes that increase their second moment of area — effectively concentrating material away from the neutral axis to maximise resistance to bending. An I-beam (universal beam) is the classic example: the flanges are far from the centre, giving very high bending resistance with minimal material. Ribs on metal enclosures (such as those on the inside of cast aluminium gear housings) perform the same function, resisting bending while keeping the wall thin and light. Hollow box sections resist both bending and torsion more efficiently than solid sections of the same mass.
Polymers
Polymers are often reinforced at points of weakness using additional wall thickness, ribbing on the inside of moulded parts, or by embedding them in a matrix of fibres. Ribs on the inside of an injection-moulded plastic casing resist bending without increasing the outer wall thickness (which would cause sink marks and extend cooling time). Glass-reinforced polymer (GRP) uses glass fibres within a polyester resin matrix; the fibres resist tension that the resin alone could not withstand.
When a beam bends, stress is highest at the top (compression) and bottom (tension) surfaces and nearly zero at the neutral axis in the middle. An I-beam exploits this by placing the majority of its material at the flanges (top and bottom), where stress is highest, and using only a thin web in the middle where stress is low. The same mass of steel in a rectangular section would be far weaker in bending because much of the material sits near the neutral axis doing little structural work.
This principle also explains why hollow tubes resist bending and torsion better than solid rods of the same cross-sectional area: material near the centre contributes little to resisting these forces. Bicycle frames use aluminium or carbon fibre tubes for exactly this reason — light, stiff and strong because the material is distributed at the outer surface where stresses are highest.
Textiles and Fibres
Interfacing is a woven or non-woven fabric bonded to the reverse of a textile to add stiffness — used in shirt collars, waistbands and the structured panels of bags. Webbing — tightly woven strips of nylon, polyester or polypropylene — is used for straps and harnesses because it has very high tensile strength along its length. Quilting (sandwiching insulation between fabric layers, held by stitching) resists delamination and maintains the insulating layer’s thickness under repeated compression.
Reinforcement works by distributing stress over a larger area or redirecting it into a form the material can withstand. Corrugating (paper/boards), laminating (timber), ribbing and I-section profiles (metals), ribbing inside moulded parts (polymers) and interfacing/webbing (textiles) are all exam-required reinforcement techniques. Know at least one technique per material family.
The most common error in exam answers on this topic is confusing bending with compression. A shelf that sags in the middle is bending, not simply compressing — the top surface is in compression and the bottom is in tension. Stating only “the shelf is compressed by the books” misses the structure of the stress and will only score partial marks.
A second common error is describing reinforcement too vaguely. “Make it thicker” is not a reinforcement technique — it simply adds material without changing the distribution of stress. A specific answer names the structural form: “adding ribs to the inside of the moulded casing increases bending stiffness without increasing wall thickness, avoiding sink marks in the surface finish.” This level of specificity is required for the top mark band on applied questions.