# Can someone help explain the difference between strength and stiffness?

I just can't seem to get it through my head and I don't understand the difference. As far as I can understand, strength is a materials resistance to permanent fracture while stiffness would be resistance to temporary fracture. If possible could I also get a brief explanation of ductility and resistance to fracture, and the differences between all of these. I'm familiar with chemistry but new to engineering.

• A quick search for "define: strength of material" gives a lot of relevant results including youtube.com/watch?v=WSRqJdT2COE (which I haven't watched). Jan 9 at 2:17
• Video is really great! Thank you! It just doesn't talk about stiffness. Is stiffness yield strength? Jan 9 at 2:22
• stiffness would be resistance to deformation Jan 9 at 2:43
• so stiffness is just a more specific version of strength that only accounts for deformation and not fracture, and resistance to fracture is the opposite of this? And ductility is how partial a material is to one side or another? Jan 9 at 2:46
• Stiffness (or youngs modulus) tells you how much a material deforms under load. Strength (ultimate yield strenght) tells you when your material fails. They are not directly related. Less stiff materials deform more before they break, but that just means they require less force to deform. They might still break at same strength as a stiffer material. Jan 9 at 4:27

• Stiffness is the reverse of flexibility, the degree of deformation under stress, therefor a stiff material requires more stress or force to deform. eg, a cantilever beam under a normal load applied to its end will bend less if it is stiffer.
• Strength is the amount of load per square unit of the material that will deform the material irreversibly and permanently.
• Great! Very clear! Thank you so much! I will definitely remember this! If possible could i get a simple and easy to understand definition of ductility and resistance to fracture. Jan 9 at 17:05
• The strength definition here is technically "yield strength". There is also "ultimate tensile strength" which is the amount of load that will break the material. Jan 9 at 19:22
• @JohnBremer300, ductility is the capacity of a metal or a structure to deform under applied stress and absorb much energy before cracking or in a structure before collapsing. A metal could have very high yield but not be ductile, so it will have almost a sharp point in the stress vs strain curve past which it will break. But a very ductile rod can be pulled to a thin, long wire without breaking. The paradigm of push-over seismic design counts on the ductility of yielded members not to fail even after large plastic deformations at plastic hinges. Jan 9 at 21:35

a solid chunk of metal is springy. if you pull on it and let go, it will snap back to its original shape. how hard it pulls back when you pull on it with a given load is a measure of how stiff it is. Metals are very stiff in comparison to things like plastics and rubber. springy rubber can be stiffened by mixing clay or carbon black into it, or unstiffened (made softer) by mixing mineral oil into it.

Strength measures how hard you have to pull on something before it breaks in two. Most metals are very strong in comparison to plastics and rubber.

• Thanks! I would give you a point thingie but im new so sorry bro. Jan 9 at 5:21
• Stiff and strong: you put a load of one pound on it and it moves 0.000001", you have to put a load of 1000 pounds on it to break it.
• Springy* and strong: you put a load of one pound on it and it moves 1", you have to put a load of 1000 pounds on it** to break it.
• Springy and weak: you put a load of one pound on it and it moves 1", you put a load of two pounds on it and it breaks.
• Stiff and weak: you put a load of one pound on it and it moves 0.000001", you put a load of two pounds on it and it breaks.

* i.e., the opposite of stiff

** presumably in tension, in a very long room.