How toughness different from strengh

1. Jan 23, 2008

shalu

hai everybody,
how is toughness different from strength??
from physics point of view, strength tells about how much force is needed to break a sample and toughness tells about how much energy is needed to break the sample
but that doesn't really tell what are the practical differences??
if the material is strong as well as it is tough also
what the difference

2. Jan 23, 2008

mgb_phys

Strength is usually the ultimate tensile stress - how much force per unit area it requires to break. Toughness is the resistance to starting to fracture.
Generally tough materials deform to reduce local stress and so don't break as easily.

3. Jan 24, 2008

Astronuc

Staff Emeritus
Supplementing mgb_phys's response -

Strength is related to the resistance to deformation. Stronger materials have lower strains for a given load/stress, or greater elastic modulus.

Toughness is the resistance to fracture or resistance to crack nucleation and propagation. In many applications, there is usually an assumption of a pre-existing flaw, e.g. a void or inclusion where stress accumulation could nucleate a crack.

Tough materials usually have some amount of plasticity ahead of crack tip, which "blunts" the crack tip and reduces the stress concentration.

4. Jan 27, 2008

rdt2

Strength is (loosely) the maximum force the component can take; toughness is the energy needed to make it fail.

Examples:

engineering ceramics are brittle, i.e they can't sustain much plastic deformation before they fracture. They have high strength but low toughness. They can take high static forces but are susceptible to, for example, impact loads.

structural steels are ductile, i.e they undergo considerable plastic deformation before they fracture. They have lower strength but higher toughness. They are then much more tolerant of impacts.

Ideally, we'd like high strength and high toughness but getting both in one material is difficult.

The third mechanical property is stiffness. That's independent of strength and toughness.