What’s the difference between stiff, strong, and tough?

academics engineering

Though the average person might think the words stiff, strong, and tough mean the same thing, engineers know that they in fact have very different meanings. Learning the difference between these terms will help you sound like a pro when discussing material properties. 

Understanding material properties through tensile testing

To understand these terms, we need to first understand a tensile test, which is used by engineers to evaluate the behavior of a material under stress. It is conducted by pulling on a piece of material and measuring how much stress is required to stretch a material until the material fails by fracturing. The stress is defined as the force over the cross-sectional area of the sample. The strain tells us how much the material stretches and is defined as the change in length over the initial length.

Screen Shot 2021-10-10 at 1.27.05 PM

From this test data, we plot stress on the y-axis and strain on the x-axis to get a stress-strain curve, which tells us how a material behaves under forces. Initially, the stress-strain curve is linear. This part of the curve is known as the elastic regime. A material deformed within this range of strain will return back to its original shape when the load is removed. Consider as an example a rubber band, which can return to its original shape after it is stretched. The slope of this linear portion is known as the elastic modulus, which is a measure of stiffness. A higher slope, or higher stiffness, means more force is required to deform the material.

Screen Shot 2021-10-10 at 1.29.12 PM

Yield strength: the point of no return

As you continue to load a material, eventually it yields. This means that if you remove the load, the material is permanently deformed, which is known as plastic deformation. Plastic deformation happens when enough stress is applied to irreversibly break the bonds between the atoms in a material.  The stress at which this transition from elastic to plastic deformation occurs is known as the yield strength. A material that is strong resists yielding, or permanently deforming. After the yield point, the stress continues to rise until it reaches its maximum, the ultimate tensile strength. This is the maximum stress that a material can withstand. Beyond this point, less stress is needed to further deform the material.

Screen Shot 2021-10-10 at 1.30.11 PMTough stuff: ductile vs. brittle

A tensile test comes to an end when a material fractures, or breaks apart. On the example graphs here, this is indicated by the x at the end of the curve. Some materials are ductile, which means they can undergo large plastic deformation without fracturing. An example is copper, which can be hammered and stretched and formed into complex shapes easily. The opposite of ductile is brittle, which means a material breaks under small deformation. A good example is glass. Though glass is a lot stiffer than copper (it takes a lot more force to stretch glass), it is also a lot more brittle, which means that it cannot deform very much at all before fracturing.

Screen Shot 2021-10-10 at 1.31.12 PM

Toughness is a measure of how much energy a material can absorb before fracturing. This is defined as the area under the stress-strain curve. An example of a tough and ductile material is spring steel, which is used in springs because it can undergo large deformations without fracturing. 

Screen Shot 2021-10-10 at 1.31.49 PMMaterials selection: stiff, strong, or tough?

So, do you want a material that is stiff, strong, or tough? This really depends on what applications you’re thinking about, which require different properties. Sometimes, you want something to be as strong as possible, to withstand as much force without deforming, like a steel structure. Sometimes we want something flexible that is tough, like rubber, which can easily deform but can withstand large deformation without fracture. Whatever the application, it’s important to think carefully about what these terms really mean when deciding what material to use.

Alice earned her PhD at MIT in Mechanical Engineering on the National Science Foundation Graduate Research Fellowship. She received her Bachelor's in Physics from University of Chicago. Currently, she teaches Engineering at MIT.


academics study skills MCAT medical school admissions SAT college admissions expository writing English MD/PhD admissions strategy writing LSAT GMAT physics GRE chemistry biology math graduate admissions academic advice ACT interview prep law school admissions test anxiety language learning career advice premed MBA admissions personal statements homework help AP exams creative writing MD study schedules test prep computer science Common Application summer activities history mathematics philosophy organic chemistry secondary applications economics supplements research 1L PSAT admissions coaching grammar law psychology statistics & probability legal studies ESL dental admissions CARS SSAT covid-19 logic games reading comprehension engineering USMLE calculus mentorship PhD admissions Spanish parents Latin biochemistry case coaching verbal reasoning DAT English literature STEM excel medical school political science skills AMCAS French Linguistics MBA coursework Tutoring Approaches academic integrity astrophysics chinese genetics letters of recommendation mechanical engineering Anki DO Social Advocacy admissions advice algebra art history artificial intelligence business careers cell biology classics dental school diversity statement gap year geometry kinematics linear algebra mental health presentations quantitative reasoning study abroad tech industry technical interviews time management work and activities 2L DMD IB exams ISEE MD/PhD programs Sentence Correction adjusting to college algorithms amino acids analysis essay athletics business skills cold emails data science finance first generation student functions graphing information sessions international students internships logic networking poetry resume revising science social sciences software engineering trigonometry units writer's block 3L AAMC Academic Interest EMT FlexMed Fourier Series Greek Health Professional Shortage Area Italian Lagrange multipliers London MD vs PhD MMI Montessori National Health Service Corps Pythagorean Theorem Python Shakespeare Step 2 TMDSAS Taylor Series Truss Analysis Zoom acids and bases active learning architecture argumentative writing art art and design schools art portfolios bacteriology bibliographies biomedicine brain teaser campus visits cantonese capacitors capital markets central limit theorem centrifugal force chemical engineering chess chromatography class participation climate change clinical experience community service constitutional law consulting cover letters curriculum dementia demonstrated interest dimensional analysis distance learning econometrics electric engineering electricity and magnetism escape velocity evolution executive function fellowships freewriting genomics harmonics health policy history of medicine history of science hybrid vehicles hydrophobic effect ideal gas law immunology induction infinite institutional actions integrated reasoning intermolecular forces intern investing investment banking lab reports linear maps mandarin chinese matrices mba medical physics meiosis microeconomics mitosis mnemonics music music theory nervous system neurology neuroscience object-oriented programming