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Glossary for Unit 4
Forces and Stresses CGP pg45
All materials structures and products have to withstand stress as certain forces are applied to them when
they are in use. The ability to withstand the stress is what allows them to perform their functions
successfully.
An applied force may be either static or dynamic:
A static load, such as a book resting on a table top, does not move.
A dynamic load, for example the same book dropped onto the table from height, does move, producing a
much greater force than static loads.
When new products are developed, designers must decide what type of loads they will have to cope with.
Products should be designed to cope easily with the maximum anticipated stress in order to be safe and
reliable.
Tension Compression
Tension occurs when a pulling force is Compression occurs when a pushing force is applied to either
applied to either end of a material. The end of a material. The object becomes compressed as it tries to
object becomes stretched as it tries to resist being squashed. For example, a spring in a suspension
resist being pulled apart. Tensile strength system or a clove of garlic in a garlic press, air in a balloon and
is the ability to resist being pulled apart. spray deodorant in a can are held under compression.
Holding liquid and gases under compression is an economical
way of storing them as they take us less room and their
releases can be easily controlled. Compressive strength is the
ability to resist being compressed or squashed.
Shear
A shear force is a force that acts on an object in a direction. Perpendicular to its length. For example, wind
pushing against a tree is a shear force. The object experiences shear stress, and may eventually snap or break.
Shear forces also occur when a material or a joint between materials is being pulled apart along different planes
or lines (tension is similar but in only one plane i.e. a straight line). The two forces will travel in opposite directions
from different planes or lines.
Bending Torsion
A bending force results in both tension and Torsion forces occur when a material is being
compression either side of its neutral axis. When twisted. The two ends of the material rotate in
two people are sitting at opposite ends of a opposite directions creating a twist, like wringing
seesaw, the beam will bend over the fulcrum. out a wet towel. Torsional strength is the ability
Stiffness is the ability of a material to resist being of a material to resist being twisted out of shape.
bent out of shape.
, Improving functionality CGP pg46
Many robust items are made from what we consider weak materials; they just need to be strengthened.
This can be done in a number of ways and the specific process required depends on the material used and
the final strength required.
Strengthening and enhancing materials à designers work out the direction and magnitude of forces that
will potentially be applied to all parts of a product. In this way, they do not waste materials, energy and tie
strengthening the while product if only one part needs to withstand a certain force.
- Reinforcing – this Is a way of strengthening a material or an object by adding material to it to
improve its ability to withstand force and stress.
e.g. Many buildings are made using concrete and steel; concrete can be brittle and can crack whereas steel
bends and flexes. Combining the two results in a very strong construction material called reinforced
concrete. combining tow or more materials permanently creates a composite material. It has more flex
than concrete alone, making it much safer and less prone to failure.
- Webbing – a strong fabric woven into strips from yarns, which are often made of synthetic fibres
such as nylon or polyester, or very high-strength materials such as Kevlar.
e.g. Webbing is used for belts of all kinds including car seatbelts, straps for securing loads on vehicles,
climbing equipment etc. because it Is very light, strong and flexible.
Stiffening materials àSome forms of reinforcement involve stiffening a material through manipulation.
Materials can be laminated to improve strength. Interfacing may be used to stiffen fabric. Folding and
bending techniques can be used to improve the mechanical and physical properties of a metal.
Laminating materials involves bonding two or more materials to improve a products strength, stability,
aesthetics and even its flexibility
When a piece of paper is laminated in a plastic pouch,
for example, its properties are changed. It becomes
water resistant, more durable and less prone to
tearing. It can retain its flexibility, but the thicker the
grade of plastic pouch used, the more rigid it will
become.
Fabric interfacing is used in textile garments as an additional layer in specific areas where extra structure,
shape or support is required. Areas that are commonly interfaced include collars, cuffs and waistbands.
There are two main types of interfacing: fusible or sew in.
Fusible (iron-on) interfacing uses heat to bond it to the
surface of the fabric. Sew-in is sandwiched between the
layers of the fabric and held in place with stitching.
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