Material Properties and Testing
FoundationSecondary 2 moves from knowing materials to testing them: hardness, tensile strength and conductivity tests measure wood, metal, plastic and fabric, and the results suggest suitable uses — evidence, not guesswork, behind every choice.
- Testing replaces feel with data when choosing materials
- Scratch tests compare hardness — the shallower the mark, the harder
- Tensile tests measure strength; a strip that stretches far before breaking is tough
文體小知識：科技知識說明文以客觀數據和清晰步驟講解測試方法與物料特性，令讀者學會以證據比較和選擇材料。
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1Secondary 1 taught us to recognise wood, metal and plastic; Secondary 2 asks us to verify their properties with numbers. Material testing means measuring hardness, tensile strength and conductivity under controlled conditions, so designers compare materials by data rather than by feel.
2The simplest hardness test is the scratch test: draw the same hard object across samples with similar force, and the shallower the mark the harder the material. In a tensile test a strip is clamped and pulled until it fails — a strip that stretches far before breaking is tough, one that snaps at once is brittle. A conductivity test needs only a battery and a bulb: light means conductor, darkness means insulator.
3Fabrics need tests too: abrasion resistance, absorbency and elasticity decide whether clothing lasts and feels comfortable. The key to every test is fairness — sample size, force and number of runs must be identical, or the results cannot be compared. To compare the impact resistance of two floor plastics, the same steel ball must fall from the same height.
4Test results exist to suggest suitable uses: choose plastic for insulation, metal for conducting heat, elastic material for shock absorption — or combine them, like a hair straightener with a heat-resistant plastic shell and metal plates, each doing what it does best. Mastering tests and reading data is where engineering thinking begins.
1Summary 重點摘要
Secondary 2 moves from knowing materials to testing them: hardness, tensile strength and conductivity tests measure wood, metal, plastic and fabric, and the results suggest suitable uses — evidence, not guesswork, behind every choice.
2Vocabulary cards Vocabulary
Measuring a material's properties under controlled conditions.
A material's resistance to scratching and denting.
The greatest pulling force a material survives before breaking.
A material's ability to absorb energy before it fractures.
How easily a material lets electric current pass.
A material that blocks current and heat, such as plastic and rubber.
Material made of woven or knitted fibres, such as cotton cloth.
Scraping samples with a hard object to compare hardness by mark depth.
A material's ability to resist wear from rubbing.
A material's ability to take in and hold water.
3Step by step Steps
- From knowing materials to testing themIn Secondary 1 we described materials by look and feel — wood is light, metal is heavy. In Secondary 2, designers let data speak: simple tests measure hardness, tensile strength and electrical conductivity, and the results decide what a material is fit for. Testing turns I think into I can prove, which is the heart of engineering thinking. For example, a phone maker never picks a casing material by feel; every candidate goes through drop, scratch and heat tests, and only the numbers decide — long before the product reaches customers.
- Hardness and the scratch testHardness is a material's resistance to scratching and denting, often compared with a simple scratch test: draw the same hard object across each sample with similar force — the shallower the mark, the harder the material. Among metals, steel is harder than aluminium; among plastics, acrylic is harder than soft rubber; fabrics depend on fibre and weave. For example, run the same steel key over aluminium, copper and plastic sheet: the aluminium shows the brightest mark, warning that it is the softest of the three and needs care where surfaces rub; a classroom desktop, scraped daily, needs a hard material to stay presentable.
- Tensile strength and toughnessTensile strength is the greatest pulling force a material survives before it breaks. In a simple test, clamp both ends of a strip, load it gradually, and record when it deforms and when it fails; a sample that stretches a long way before breaking is tough, one that snaps with little stretching is brittle. For example, pull equal-width strips of metal wire, plastic ruler stock and acrylic sheet: the wire stretches and thins only under great force, the plastic strip bends and springs back, while the acrylic snaps at once — a brittle material is wrong for straps and buckles that live under tension.
- Conductivity and heat testsElectrical conductivity is how easily a material lets current pass. Connect a sample into a simple circuit of battery and bulb: a lit bulb means conductor, a dark bulb means insulator. Metals conduct well; plastics, dry wood and rubber insulate, and heat conduction usually follows the same pattern. For example, a wire uses copper for its core so current flows freely, while a plastic sleeve wraps outside to protect the user; kitchen spatulas use plastic or wooden handles so the gripping end stays cool — designs that come straight from conductivity and heat tests.
- Testing fabric propertiesFor fabrics, tests reveal what the weave adds to the fibre: abrasion resistance is checked by rubbing sandpaper back and forth with a fixed force and counting how soon pills form; absorbency by watching a drop of water spread; elasticity by stretching, releasing and seeing whether the cloth recovers. For example, sportswear uses quick-drying, abrasion-resistant knit; school trousers use hard-wearing woven cloth; glove linings choose soft, stretchy fabric — one garment, different fabrics, each chosen by test results so the clothing is both durable and comfortable.
- Impact, bending and fair testingBeyond the three core properties, designers test to suit the product: impact tests drop a weight onto a sample to see whether it cracks; bending tests flex a strip repeatedly to count the cycles before failure; absorbency tests soak the sample and weigh the gain. Whatever the test, conditions must match for results to compare — same sample size, same force, same number of runs. For example, to judge which of two floor plastics resists impact, the same steel ball must fall from the same height; unequal drop heights make the conclusion worthless. This is the principle of fair testing.
- From results to suggested usesThe point of testing is matching data to needs: list the forces and conditions a product faces, compare each material's results line by line, choose the best fit — or combine materials so each does what it does best. For example, when a test report shows a plastic that resists heat and insulates and a metal that conducts heat well, a hair straightener gets a plastic body to shield the hand and metal plates to shape the hair, the two materials each playing their part. Reading and applying test results gives your design recommendations weight — and prepares you for material choice and sustainability in Secondary 3.
5Worked example Example
Example: you are choosing a phone case material and have three samples — acrylic sheet, silicone and thin aluminium. Design simple tests and recommend the best material.
- List the needs → resist impact, light, insulating, grippy, must not pass shocks straight to the phone
- Scratch test → the same key with the same force: acrylic marks clearly, aluminium dents, silicone barely marks
- Drop test → cushion a phone dummy with each sample and drop from the same height: silicone absorbs best, aluminium passes the shock through, acrylic may crack
- Conductivity and feel check → aluminium conducts and feels hot in summer; silicone insulates and grips
- Answer: Silicone is recommended. The scratch test shows it barely marks; in the drop test it deforms and absorbs the impact, protecting the phone best; it is light, insulating and non-slip. Aluminium is hard and sheds heat well, but it conducts, weighs more and passes impact straight to the phone; acrylic is hard but brittle and cracks under a blow. Comparing each test result with the needs turns the recommendation into evidence-based design rather than a hunch.
6Common mistakes Mistakes
7Quick check Quick check
Q1Which test compares the hardness of two materials? A. scrape both with the same steel key B. connect each into a circuit and watch the bulb C. soak both and weigh them
Q2Why does a wire have a copper core and a plastic sleeve?
Q3A plastic strip snaps suddenly with almost no stretching. What does this show, and what is it suited or unsuited for?
Q4What test conditions matter when comparing the abrasion resistance of two fabrics?
Q5How would heat-test results guide the choice of materials for a kettle's body and heating section?
8Recap Recap
- Testing replaces feel with data when choosing materials
- Scratch tests compare hardness — the shallower the mark, the harder
- Tensile tests measure strength; a strip that stretches far before breaking is tough
- Circuit tests sort conductors from insulators: metals conduct, plastics and dry wood insulate
- Keep test conditions identical so results can be compared and used to suggest uses