Difference B/W Hardness,Toughness,Ductility,Malleability

Good afternoon Seeker_Sas

I'll give you my 5 cent definitions. As a working metallurgist these definitions are the ones that lay people understood from me.

Hardness is resistance to penetration.(rockwell and Brinell tests typical) Some young women, we noted were harder than others...

Toughness is ability to withstand forces withstood upon impact (charpy or izod notched bar tests) A leather bag could catch a thrown bowling ball, a plastic bag could not, but it could carry it...A paper bag could too, until it got wet...

Ductility is ability to elongate or reduce in area prior to failure. Its opposite is brittleness. A glass rod is hard AND BRITTLE. A copper wire is soft and DUCTILE.(On a tensile test, % elongation or % reduction in area are indicators of ductility.)

Yield strength is the point in a tensile test (or application!) where the material "yields" or changes its length by deforming permanently. Having changed dimensions, the material is said to have yielded. (Think of a prisoner on a rack and you will fully understand that some will yield even before they are attached to the chains...)

On materials that do not have a defined yield point, a .2% offset line is used to establish the "yield point."

Modulus of elasticity is best visualized as the slope of the materials stress strain curve in the linear or elastic region. Slope is rise over run so stress divided by strain = elastic modulus. Steel guys (I'm a steel guy) use Young's modulus Which is tensilestress divided by tensile strain along that axis.

Malleability is ability to be cold worked (hammered). In my mind it seemed to follow ductility, but it was never really used in my practice as a "thing itself."

An example would be to take a piece of steel wire. bend it. now try to straighten it out again. the increased resistance to bending is a sign that the wire coldworked, it is less dutile, and you can see"feel" "less easier to work" Less malleable.

You didn't ask about UTS, but I'll throw that in so you have the whole suite.

Ultimate Tensile strength is the greatest force that can be withstood by the material while remaining in one piece.

I would advise you to buy a piece of licorice candy and a piece of hard stick candy and pull them in tension to better understand these terms. That's how I teach in my presentations. licorice is softer, more ductile and will elongate and you can see the yield point (it is akin to hot rolled steel.) The hard candy is harder, less ductile, more brittle, and has a higher Yield/tensile ratio. This is akin to coldrolled steel or cold drawn steel bars.

Look at the stress strain curve again with new eyes using these descriptions and see if the area under the curve gives you any indications of toughness, resiliency; and where (orwhether ) ther is a defined yield point to correlate to measures of ductility.

I regret that there are many professors called teachers, but few who actually teach.

However as a student, your curiosity and analysis can try to correlate these things by taking data that you know to look for correlating behaviors and asking if the curve says this, what about the points not on the curve? under? over?

Then Get any kind of material- string, fishing cord, wire, wood, candy, pretzels, bread, whatever, and perform similar tests.

I had a lot of fun teaching this using monfilament line, a bucket with water, and a freezer and hot water and a ruler.

milo's anti PH.D. disclaimer. These may not get you an "A " on the exam. But they will give you the mental pictures you need to think clearly about behavior of materials in systems under forces present. characterize the force's application and magnitude and "pulse" to choose which of these indicators become controlling for failure. Then do the math.

Its truly a wonderful field.

milo