Five measurements describe how a solid responds to a load: stiffness (how far it stretches elastically), yield strength (where it starts to bend permanently), tensile strength (the highest stress it carries), ductility (how far it stretches before breaking) and toughness (the energy it absorbs before it fractures), with hardness as a separate surface measure. A material can score high on one and low on another, so no single number ranks materials. Alloying and heat treatment change strength a great deal and stiffness hardly at all.
Part of Materials Science Principles
Mechanical properties are the quantities that describe how a material deforms and fails under force: stiffness, yield strength, tensile strength, ductility, toughness, hardness and fatigue resistance.
Stress And Strain Put Every Test On The Same Scale
A force means little until it is divided by the area carrying it. Stress is force divided by cross-sectional area. One newton per square millimetre is one megapascal (MPa). Strain is the change in length divided by the original length, so it has no units.
Worked example: a 6 mm round rod has a cross-section of π × (6 / 2)² = 28.3 mm². A 5 kN pull (5,000 N) gives a stress of 5,000 / 28.3 = 177 MPa. The same pull on a 12 mm rod, with four times the area, gives a quarter of the stress, 44 MPa. This is why doubling a rod’s diameter does far more than doubling its strength.
Older articles in the corpus quote strength in pounds per square inch. One MPa is about 145 psi, so the table in Tensile Strength converts as follows:
| Material | Table value (psi) | Divided by 145 (MPa) |
|---|---|---|
| Wrought iron | 35,000-50,000 | 241-345 |
| Mild steel | 50,000-70,000 | 345-483 |
| Cast iron | 15,000-30,000 | 103-207 |
(For example, 50,000 / 145.04 = 344.7 and 70,000 / 145.04 = 482.6.) The Advanced Materials article quotes aluminium at 120 MPa for the pure metal rising to above 500 MPa for a strong alloy, which on this scale is about 17,400 psi rising to over 72,500 psi.
Elastic Means It Springs Back, Plastic Means It Stays Bent
Load a bar gently and it stretches in proportion to the load, then returns to its length when unloaded. That is elastic behaviour, and its slope on a stress-strain plot is the stiffness. Past the yield point it deforms permanently. The Advanced Materials article’s test for yield is exactly this: note where the sample stops springing back. The highest stress the bar reaches before it fails is the tensile strength.
Stiffness and strength are different things. A bar of aluminium, one third the density of steel according to Aluminum Smelting, deflects visibly more under the same load than a steel bar of the same shape, because aluminium is roughly three times less stiff. Heat treating an aluminium alloy to above 500 MPa does not close that gap. If a part must not sag, make the section deeper, not the alloy stronger.
Ductility And Toughness Decide Whether Failure Gives Warning
- Ductility is how much the material stretches before it breaks, usually quoted as the percentage elongation at fracture. A ductile bar necks down and gives visible warning.
- Toughness is the energy absorbed before fracture, the area under the whole stress-strain curve. The Advanced Materials article’s test is an impact test, a weight dropped on a notched sample. A material can be strong but have low toughness. A file is a good example: it is very hard and snaps if bent.
- Hardness is resistance to indentation. It is quick to measure and correlates loosely with strength, but it is not strength. See Hardness Testing.
- Fatigue resistance is the number of load cycles a part survives, covered in Failure Modes.
Tension and compression give different answers for the same material. Tensile Strength puts masonry at 5-15 times weaker in tension than in compression, and Annealing says glass is roughly 10 times weaker in tension. A material’s compressive strength is therefore no guide to using it in a chain or the bottom of a beam.
Structure Sets Each Property
Each property has a structural cause, which is why processing moves them. Strength comes from blocking slip, by grain boundaries, dissolved atoms, fine precipitates or trapped carbon. Ductility is slip being allowed to happen. Stiffness comes from the strength of the atomic bonds themselves, which alloying and heat treatment barely alter. That asymmetry explains why a hardened steel spring and a soft steel strip bend by the same amount under a small load but only one comes back.
Common Mistakes
- Using the tensile strength to design — a part that has yielded has already failed in service because it no longer fits. Design to yield, with a safety factor. Safety Factors defines allowable stress as failure stress divided by the factor.
- Mixing force units — a kilogram-force is 9.81 N. The Advanced Materials article’s Brinell loads of 3,000 kg and 500 kg are kilogram-force and must be converted before comparing with newtons.
- Treating hardness as a strength substitute — correlation holds only within one material family and needs your own calibration.
What’s Next
- Learn next: Tensile Testing — measuring yield and tensile strength on your own samples
- Combine with: Failure Modes — how loads become failures