What do hardness, tensile strength, creep, fatigue and impact resistance mean for polymers?

In one line

These mechanical properties describe how a polymer responds to force: resistance to indentation (hardness), maximum pulling stress (tensile strength), slow deformation under a constant load (creep), failure under repeated loading (fatigue), and ability to absorb a sudden blow (impact resistance).

Explained simply

Think of a plastic chair. Can a coin scratch it (hardness)? How much pull before it snaps (tensile strength)? Does the seat slowly sag after months of use (creep)? Does a hinge crack after being flexed thousands of times (fatigue)? Does it crack if dropped on a cold day (impact)? Engineers check each before choosing a polymer.

How it works

1. Hardness: resistance to indentation, scratching or abrasion. Measured by Shore durometer (A for soft rubbers, D for hard plastics) or Rockwell. Higher crystallinity and cross-linking increase hardness (Bakelite is harder than LDPE).
2. Tensile strength: maximum stress a material can withstand while being stretched before breaking. Increases with molecular weight, crystallinity, chain orientation, polar/H-bonding groups and cross-linking.
3. Creep: slow, time-dependent, permanent deformation under a constant load below the breaking stress. Polymers creep even at room temperature (especially above T_g) because chains slowly slide past each other. Cross-linking, crystallinity and fibre reinforcement reduce creep.
4. Fatigue: failure under repeated (cyclic) loading at stresses lower than the tensile strength. Tiny cracks grow with each cycle. Fatigue life = number of cycles to failure.
5. Impact resistance (toughness): energy absorbed when a sudden load (blow) is applied, measured by Izod or Charpy tests. Polymers above T_g or containing rubber particles (ABS, high-impact polystyrene) have high impact resistance; glassy polymers like general-purpose polystyrene are brittle.

Key formula

\( \text{Tensile stress } \sigma = \dfrac{F}{A} \)

• F = applied force (N), A = original cross-sectional area (m² or mm²)
• Unit: Pa or N/m²; commonly MPa (N/mm²)
• Impact strength is reported as energy per unit width/area, e.g., J/m (Izod) or kJ/m².

Quick example

A plastic strip of cross-section 10 mm² breaks at 500 N: tensile strength = 500/10 = 50 MPa. Uses: polycarbonate is chosen for helmets and safety shields because of very high impact resistance; nylon for gears because of good fatigue and wear resistance.

Common mistakes

• Confusing creep (constant load, slow) with fatigue (repeated load, cycles).
• Assuming high hardness means high impact resistance; hard thermosets are often brittle.
• Giving tensile strength in N instead of N/mm² or MPa.

How MU asks it

Syllabus says "introduction", so expect "Explain any two/three mechanical properties of polymers" (3–5 marks) or short notes. For full marks: definition of each property, how it is measured or expressed, the structural factors that improve it, and one application example.