1. Strength of a mate­r­i­al

Strength is a mea­sure of the stress a mate­r­i­al can with­sand.

Two dif­fer­ent val­ues are typ­i­cal­ly used to define the strength of a mate­r­i­al, the ulti­mate strength and yield strength.

2.Material Stress and Straina

First, we need to explain some of the phys­i­cal con­cepts behind the mechan­i­cal prop­er­ties. The main one is stress. Stress tells you how big of a force applies to an area. In mechan­i­cal engi­neer­ing, it is most­ly expressed in MPa’s or N/mm2. Those two are inter­change­able. The for­mu­la for stress is:

σ=F/A, where F is force (N) and A is area (mm2).

The sec­ond impor­tant con­cept is strain. Strain has no unit as it is a ratio of lengths. It is cal­cu­lat­ed as fol­lows:

ε=(l‑l0)/l0, where l0 is start­ing or ini­tial length (mm) and l is stretched length (mm).

A.Young’s Mod­u­lus

From those two con­cepts we get to our first mechan­i­cal prop­er­ties – stiff­ness and elas­tic­i­ty as its oppo­site. It is an impor­tant fac­tor for engi­neers when solv­ing physics prob­lems (mate­r­i­al suit­abil­i­ty for a cer­tain appli­ca­tion).

Stiff­ness is expressed as Young’s mod­u­lus, also known as mod­u­lus of elas­tic­i­ty. As one of the pri­ma­ry mechan­i­cal prop­er­ties of mate­ri­als, it defines the rela­tion­ship between stress and strain – the big­ger its val­ue, the stiffer the mate­r­i­al.

This means that the same load would deform two equal­ly-sized parts dif­fer­ent­ly, if they have vary­ing Young’s mod­uli. At the same time, less­er val­ue means that the mate­r­i­al is more elas­tic.

B.Yield Strength

Yield stress or yield strength is the val­ue most often used in engi­neer­ing cal­cu­la­tions. It gives a mate­r­i­al a stress val­ue in MPa it can take before plas­tic defor­ma­tion. This place is called the yield point. Before it, a mate­r­i­al regains its for­mer shape when lift­ing the load. After exceed­ing the yield point, the defor­ma­tion is per­ma­nent.

There is a good rea­son for using yield stress as the most impor­tant fac­tor in mechan­i­cal engi­neer­ing. As can be seen from the stress-strain curve, when stress goes beyond the yield point, the dam­age is not yet cat­a­stroph­ic. That leaves a “cush­ion” before a con­struc­tion fails com­plete­ly to the point of break­ing.

C.Tensile Strength

Ulti­mate ten­sile strength, or just ten­sile strength, is the next step from yield strength. Also mea­sured in MPa’s, this val­ue indi­cates the max­i­mum stress a mate­r­i­al can with­stand before frac­tur­ing.

3,Toughness of mate­r­i­al

Tough­ness is the abil­i­ty of a mate­r­i­al to absorb ener­gy up to frac­ture. Mate­ri­als that can absorb a lot of ener­gy before frac­tur­ing have high tough­ness.

Tough­ness can be thought of as the area under the stress-strain curve. If the area is large, the mate­r­i­al will have high tough­ness and will be able to absorb a large amount of ener­gy before frac­tur­ing.