Inspecting mechanical properties
Mechanical properties tell how a material behaves under force. We measure them with standard tests on small samples called specimens.
- Tensile test: a bar is pulled slowly until it breaks. The machine records force and stretch. From the graph we read the yield strength (where it stops springing back), the ultimate tensile strength (UTS) (the highest stress) and elongation (how much it stretched before breaking, in %). A bar that stretches a lot is ductile; one that breaks with little stretch is brittle.
- Hardness test: a hard ball (Brinell) or a diamond (Vickers, Rockwell) is pressed into the surface. A smaller dent means a harder metal.
- Impact test: a swinging hammer breaks a notched bar (Charpy). The energy it loses is the toughness.
- Fatigue and bend tests: repeated small loads, or bending the sample, show if a part can last or if it cracks.
Elongation % = (final length − original length) ÷ original length × 100.
Microscopic examination of tissue (structure)
The "tissue" of a metal is its inside structure. To see it we make a flat, shiny, clean face.
- Cut a small piece.
- Mount it in resin so it is easy to hold.
- Grind it with finer and finer paper.
- Polish it to a mirror finish.
- Etch it with a weak acid. The acid eats grain edges and different parts at different speeds, so they show up.
- Look through a metallurgical microscope.
You can see the grains, their size, the different phases (for steel: soft ferrite and layered pearlite), and defects like holes or cracks. Small grains mean a stronger metal. The size you see is the size in the picture divided by the magnification.
Inspection with instruments
Measuring size: the ruler is not enough. A vernier calliper reads to 0.02 mm and a micrometer to 0.01 mm. A dial gauge shows a tiny change in size or flatness.
Finding hidden faults without damage (non-destructive testing, NDT):
- Ultrasonic test: a probe sends sound waves in. They bounce back from a crack or the far wall. Depth = speed × time ÷ 2.
- X-ray / radiography: rays pass through; a crack or hole shows dark on the film.
- Dye penetrant: red dye soaks into surface cracks and shows after cleaning.
- Magnetic particle: iron powder collects at cracks in iron and steel.
- Eddy current: a coil finds surface cracks and checks coatings.
Other instruments: a hardness meter, a thermocouple for temperature, a load cell for force. Good inspection records every reading so we can trace a problem later.
Try it: tap a coffee mug and a cracked mug. A crack changes the ring into a dull thud. That is the idea of acoustic testing.
Key formulas and definitions
- Stress σ = F ÷ A
- UTS = maximum force ÷ original area
- Elongation % = (L₁ − L₀) ÷ L₀ × 100
- Ultrasonic depth d = v × t ÷ 2 (t is the round-trip time)
- Real size = size in picture ÷ magnification
Worked examples
1. A test bar of area 100 mm² breaks at a maximum force of 30 kN. Find its UTS.
30 kN = 30 000 N. UTS = 30 000 ÷ 100 = 300 MPa.
2. A 50 mm gauge length becomes 60 mm at breaking. Find the elongation %.
(60 − 50) ÷ 50 × 100 = 20%. The bar is ductile.
3. A Charpy hammer has 300 J before the swing and 220 J after breaking the bar. How much energy did the bar absorb?
300 − 220 = 80 J. This is the toughness of the sample.
4. An ultrasound probe on a steel block hears an echo after 5 µs. The speed of sound in steel is 6000 m/s. Find the depth of the crack.
Round trip, so d = v × t ÷ 2 = 6000 × 5×10⁻⁶ ÷ 2 = 0.015 m = 15 mm.
5. A grain looks 0.5 mm wide in a ×100 photo. What is its real width?
0.5 ÷ 100 = 0.005 mm = 5 µm.
Common mistakes
- Thinking a bigger dent means a harder metal. The opposite is true.
- Forgetting to halve the time in an ultrasound problem. The pulse goes in and comes back.
- Mixing yield strength and UTS. Yield is where it stops springing back; UTS is the highest point of the graph.
- Skipping polishing and etching. Without them a microscope shows only scratches, not grains.