Fracture Behavior With Cheese

Fracture Behavior withCheeseLearning Objectives1)2)Students will apply their knowledge of fractureto predict how different cheeses will fail.Students will discuss how geometry plays arole in fracture behavior.Tim e 60 minutesTopics1)2)3)Mechanical testing to characterize fracturepropertiesStress concentratorsSimulations of stress (computational materialsscience)Before the Lab:Supplies to buy every time you run theactivity:1)T w o d iffe re n t typ e s o f ch e e s e . C h o o s e c h e e s e w ithd iffe re n t typ e s o f fra c tu re .Cheese TypeFracture vageColby JackDuctile and/or1/6Prior KnowledgeRecommended for Instructor:1) Familiarity with basic mechanicalproperties-S tre ss vs. stra in g ra p h s fo r b rittle vs.d u ctile m a te ria ls-T h re e -p o in t b e n d te stin g2) Basic understanding of material fracture-U n d e rsta n d th e d iffe re n ce b e tw e e nd iffe re n t typ e s o f fra ctu re m e ch a n ism sIntergranularMuensterDuctileExtra Sharp CheddarCleavageSupplies to buy as needed:1)P a p e r p la te s fo r th e ch e e se2)G lo v e s fo r th o se w h o a re la c to s e in to le ra n t3)A kn ife a n d to o th p ick to m a ke th e n o tc h e s in th ech e e sePrior KnowledgeRecommended for Students:1) Types of bonding-C o va le n t, n o n -co va le n t, io n icThis work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/4.0/.

Fracture Behavior withCheese2/6Lab Set-up:1) Break students up into pairs. Half the groups will receive Extra Sharp Cheddar Cheese (oranother cheese that demonstrates failure via cleavage) and half the groups will receiveMuenster cheese (or another ductile cheese). Arrange the groups so that groups withdifferent cheeses are sitting close to each other to encourage sharing their results after theexperiment.2) Before the students arrive, make sure each group has:-O n e p la te w ith fiv e c h e e s e s a m p le s (s u g g e ste d m in im u m ).E a ch sa m p le sh o u ld b e a p p ro x im a te ly 2 in c h e s x 1 in ch .- T w o (m in .) c h e e s e s a m p le s w ith s h a rp n o tch--T w o (m in .) c h e e s e s a m p le s w ith b lu n t n o tchO n e (m in .) c h e e s e s a m p le w ith n o n o tchG lo ve s (if re q u e s te d b y th e p a rtic ip a n t)3) Pre-notch all of the cheese samples (see three-point bend test figure above). The sharpnotch is made using a knife to make a small slice. The blunt notch is made using a knife tocut a small wedge and then a toothpick to smooth out the edges. Video available onwebsite for sample prep and fracture testPre-lab Questions:The lecture before this lab should explain the basics of fracture and the types of fracturemechanisms.1) The cheese fracture activity fits into the structure and properties categories of thetetrahedron. Please explain why cheese fracture fits in these categories.Cheese fits into the structure and properties categoriesbecause the type of bonding in the cheese affectsthe way it fractures, which is a mechanical property.2) Match the type of fracture to its fracture surfaceIn te rg ra n u la rFracture Mechanics: Fundamentals and ApplicationsFIGUREIntergranular fracture in a steel ammonia tank. Photograph courtesy of W.L. Bradley.environmental cracking mechanisms are discussed in detail in Chapter 11. Figure 5.31 shows anintergranular fracture surface in a steel weld that was in contact with an ammonia environment.D u c tile F ra ctu reFracture Mechanisms in MetalsFIGUREHigh magnification fractograph of the steel ductile fracture surface. Note the spherical inclusionwhich nucleated a microvoid. Photograph courtesy of Mr. Sun Yongqi.Figure 5.6 illustrates the formation of the ‘‘cup and cone’’ fracture surface that is commonlyobserved in uniaxial tensile tests. The neck produces a triaxial stress state in the center of theFracture Mechanics: Fundamentals and ApplicationsFIGUREgrains.Formation of river patterns, as a result of a cleavage crack crossing a twist boundary betweensolid is approximately /P. Figure 5.14, however, indicates that the maximum stress achieved aheadof the crack tip is three to four times the yield strength. For a steel with S 400 MPa and210,000 MPa, the cohesive strength would be 50 times higher than the maximum stress achievedahead of the crack tip. Thus, a macroscopic crack provides insufficient stress concentration toexceed the bond strength.In order for cleavage to initiate, there must be a local discontinuity ahead of the macroscopiccrack that is sufficient to exceed the bond strength. A sharp microcrack is one way to providesufficient local stress concentration. Cottrell [24] postulated that microcracks form at intersectingC le a va g eFIGURERiver patterns in an A 508 Class 3 steel. Note the tearing (light areas) between parallel cleavageplanes. Photograph courtesy of Mr. Sun Yongqi.

Fracture Behavior withCheese3/6Running the Lab:1)H a v e th e s tu d e n ts sit in p a irs a t th e ir sta tio n s. E xp la in th e y w ill b e h a n d lin g ch e e se . O ffe r g lo ve s to th o se w h oa re la cto se in to le ra n t o r w o u ld p re fe r n o t to h a n d le ch e e se w ith th e ir b a re h a n d s.2)T h e y sh o u ld h a ve a lre a d y a n s w e re d th e p re -la b q u e stio n s. A sk th e stu d e n ts if th e y h a d a n y u n re so lve dq u e stio n s a b o u t th e p re -la b q u e s tio n s .3)In s tru ct th e stu d e n ts to re a d a ll o f th e d ire c tio n s .4)D o n o t g iv e th e m th e in stru c tio n s ve rb a lly . T h e g o a l is to g e t th e stu d e n ts to re a d e ve ryth in g a n d th in k a b o u tw h y th e y a re d o in g w h a t th e y a re d o in g . A t th is p o in t le t th e m ju m p in b u t e n co u ra g e th e m to ra ise th e ir h a n d sif th e y h a ve q u e stio n s .Expected Results from Three-point Bend Tests:W e p ro vid e stu d e n ts w ith m o re th a n o n e o f th e sa m e g e o m e try b e ca u se th e re a re o fte n d e fe cts th a t im p a ct th ew a y th e sp e c im e n s fa il. B y in sp e ctin g m o re th a n o n e sp e c im e n , stu d e n ts ca n a lso e xp lo re th e re a l-life va ria b ilityo f fa ilu re a n a lysis. T h e c h a rt b e lo w is co m p le te d to p ro vid e th e in stru cto rs w ith so m e in sig h t a b o u t b o th typ e s o fch e e se in e a ch g e o m e try; h o w e v e r, s tu d e n ts w o u ld o n ly b e w o rkin g w ith o n e typ e a t a tim e . T h e e xa m p le s b e lo wd o n o t in clu d e a n y d e scrip tio n s o f in te rg ra n u la r fra ctu re . W e h a ve fo u n d th a t th e se ch e e se s (e .g . fe ta o r C o lb yja c k) ca n b e a little fru stra tin g fo r stu d e n ts. In ste a d , w e su g g e st th a t yo u p ro vid e a fe w e xa m p le s to d e m o n stra tein te rg ra n u la r fra ctu re .S a m p leC ra c k n u c le a tio nS h a rp n o tc h # 1(for Extra SharpCheddar)S h a rp n o tc h # 2(for Muenster)C ra c k p ro p a g a tio n(S tra ig h t o r zig -za g ? )(D u c tile F ra c tu re , C le a v a g e o rIn te rg ra n u la r? )The crack shouldstart directly from theMost cracks will propagatedirectly across the specimen (i.e.Extra sharp cheddar should demonstratecleavage-like behavior with signaturesharp notch.straight)“river-pattern” markingsThe crack shouldstart directly from thesharp notch.Most cracks will propagate at anangle to the original notch ( 45º).Often this results in a morejagged or zig-zag propagationpath.B lu n t n o tch # 1(for Extra SharpCheddar)F ra c tu re s u rfa c eThe crack is stillexpected to startfrom the notch.While there may be morevariation than found with thesharp cracks, most will stillpropagate straight across theMuenster cheese typically displaysductile fracture behavior. The zig-zagcrack path and void formation ahead ofthe crack are signatures of this.The fracture surface should still havesignatures of brittle cleavage fracture.specimen.B lu n t n o tch # 2(for Muenster)The crack may startat the notch, but isjust as likely to startsomewhere else.This crack path is still likely to bezig-zaggy, but will also be themost unpredictable.N o n o tc hFor either type ofcheese, without anotch, the crack canstart anywhere.Without a pre-notch, the crack ismuch more difficult to predict andthe students will likely have toapply more force to initiate one.Still ductile fracture.This will depend on the type of cheese,but the signatures of fracture should bethe same as observed with the prenotched samples.Discussion Points:1) What does your fracture surface look like for each cheese? For each type of notch?2) Did all of your samples fail the same way? Can you rationalize why or why not?

Fracture Behavior withCheese4/6You Will Be Able To:1)2)Explain why geometry is important to fracture behaviorJustify you experimental observations of fractured cheeseSimulations of Stress in CheeseYou’ve seen how cheeses break when they have different initial crack geometries. Now you’lluse some computer software called OOF2 to get an idea for how the initial geometriesconcentrate stress in different locations and cause different kinds of fractures. Each group willbe assigned to simulate a different initial fracture geometry (due to time constraints), whichyou will present at the end. What you will see once you have finished the simulation is a visualrepresentation of where stress builds up in the cheese as it is initially strained (as you strainedit, using the three-point method.)Notes to the instructor: Nanohub, and OOF2 especially, can be kind of finicky.Try to use a browser with javascript support (Safari and Firefox are usually better thanChrome), and if the display cuts out, try refreshing the page before assuming theworst. Sometimes it really will just break for no adequately explained reason and thesimulation will have to be rerun (this is the case if there’s a connection error of some kind, inour experience.)If refreshing the page doesn’t work you can try going to the student’s “dashboard” andlaunching from currently-running sessions.One other thing to note is that some of these simulations could take 5-10 minutes to fully setup and solve, especially ones with more complex meshes (kinked crack in particular.)If the particular crack file will not load correctly just have them run a different crack.

Fracture Behavior withCheese5/6Example Solution Images:These won’t look identical from group to group because there are random stepsin the OOF2 setup, but they should be a solid guide for what to look at. Note thatif they don’t uncheck their original image file, their images may not appear tohave as much contrast as these do. Also, generally these can be most easilycreated by just screenshotting the Nanohub images. Note also that the scaleson the color bars are different, which may be relevant.Blunt CrackCenter CrackNarrow Crack