Matl Science and Engineering
Matl Science and Engineering MSE 250
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This 13 page Class Notes was uploaded by Jimmy Reichel on Saturday September 19, 2015. The Class Notes belongs to MSE 250 at Michigan State University taught by Andre Lee in Fall. Since its upload, it has received 48 views. For similar materials see /class/207341/mse-250-michigan-state-university in Material Science and Engineering at Michigan State University.
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Date Created: 09/19/15
A40247130 Tensile Testing Lab Jordan Bowman A40247130 MSE250004 Submission Date 041911 1Page A40247130 1 Abstract The purpose of this lab is to identify the many characteristics which can be measured and calculated using tensile testing It was assumed that by finding how an object deforms when a specified load is applied that many characteristics will be able to be accurately found about a specific material Included in these characteristics are strength ductility and toughness This experiment was performed using an Instron tensile testing machine according to ASTM American Society for Testing and Materials standards This machine then output data which can be made into a stress vs strain plot and further analyzed After the experiments were properly carried out the plot indicated areas of elastic deformation plastic deformation necking then finally fracture Using this detailed plot as well as previous knowledge about material science the hypothesis was verified graphically and numerically 2 Introduction The tensile test performed in this lab can be used to find many factors of strength of the materials being tested These factors are very critical in engineering when deciding what types of materials should be used for specific applications Each type of characteristic can be extremely beneficial in different uses depending on the desired qualities The characteristics which are commonly desired for observation when doing tensile testing include yield strength ultimate strength ductility toughness and hardness These qualities can be used to find when materials will fail or how much they will deform when a certain load is applied In engineering this information is vital to know what kind of material 2Page A40247130 to use in accordance with safety factors and application Elongation of materials can also be used to harden materials in a procedure called strain hardening One of the best ways to test the tensile strength of materials is by using an nstron testile testing machine This machine uses a constant rate of deformation to create a graphical representation of elongation vs applied force The elongation vs force plot can then be used to create a stress vs strain plot which is much more useful Critical points on the stress vs strain plot are used to find characteristics of the material being tested These tests are standardized by ASTMto provide methods for maximum accuracy in experimentation The slope of the linear portion of the graph is used to obtain Young s Modulus This material property is very important because it is tells how much stress can be applied to a material before it begins to deform plastically A material which has a high Young s Modulus represents qualities of high strength When using a material with a high Young s Modulus some ductility may be compromised During this experiment a variety of materials will be used in order to observe a broad spectrum of material characteristics and how they react to tensile testing Metals like galvanized steel and 70 30 Brass have very high yield strengths however they are not very ductile and begin to experience plastic deformation early on in elongation On the contrary a material such as High Density Polyethylene can withstand nearly twice as much elongation before deforming although it also maintains a much lower yield strengthMaterias which are ductile will generally have a much larger elongation period before fracturing This can be shown in a stress strain plot by the amount of strain at failure 3Page A40247130 A material which has both high strength and high ductility is considered tough Toughness is determined by the area under the stress strain curve from beginning until fracture An easy approximation of this value is found by multiplying the ultimate tensile strength by the strain at fracture It is for these various reasons that tensile strength testing is so important All of the values which can be found in tensile testing are crucial in almost all aspects of engineering Proper utilization of this data allows for optimized performance reduced risk and more accurate selection of materials 3 Background The stress strain plot is a vital piece of information which is obtained through tensile testing In a typical stress vs strain plot the initial section of the graph shows a linear slope during elastic deformation Elastic deformation refers to elongation in which a specimen will return to its original form once unloaded Once the specimen begins to deform plastically the graph begins to become nonlinear Plastic deformation refers to the region of elongation in which a material will not return to its original form once unloaded This point where the graph becomes nonlinear is called the yield point The specimen continues to deform plastically until the ultimate strength is reached Once the ultimate strength is reached necking will occur which means a large portion of the stress becomes localized at one small region This makes the specimen very susceptible to fracture in this region The Young s Modulus is one of the most important pieces of information that is taken from the stress strain plot This number gives insight into how much a material will deform 4Page A40247130 when a certain load is applied This is very important when designing structures and other applications where deformation is vital Once the Young s Modulus is found the yield point can also be found using the 002 offset method In this method a line is created parallel to the linear portion of the stress strain plot The point where this line crosses the stress strain curve is the yield point In almost all engineering applications materials are designed to operate inside the region of elastic deformation It is for this reason that the yield point is very important because this allows engineers to determine when an object will begin to plastically deform 4 Procedureamp Materials In order to make sure this lab can be repeated precisely and accurately ASTM has standardized the procedure from start to finish This includes how every specimen should be prepared the testing apparatus procedure and expected results Each of these specimens will have a similar testing procedure with small variations for each one In order to run the tensile test the specimen must be cut into a shape which will work with the testing apparatus being used In this case the nstron machine requires quotdog bone shaped testing specimen These specimens have a larger portion on each end which is used for gripping the specimen into the jaws of the nstron machine Figure 1 shows a classic example of what a dog bone specimen looks like The portion between the gripping ends is called the gage This is the portion ofthe sample which will deform 5Page 1440247130 4 m RA Figure 1 DogBone shaped specimen Once the dpgppne spedrnen has been rnadnined and prepped fur testing i39ts dirnensipns must be rneasured using Vernier Caiipers inis inciudes widin inidmess and gage ength inis wi39H be necessary iaier wnen preparing ine insirpn rnacnine fur testing 41 Straln Hardening We rst experiment wi39H be using 7030 Brass id ubserve siraininardening inis wi39H pe dpne py ipading and unipading a brass specirnen in prder L7 increase ine yieid strength in prderipipadm in in n n rnadnine and tighten the iaws arpund the specirnen wnen dping this make sure the specirnen is exaciiy verti39m m ensure uniaxiai ipading 0n the camputer pum39uri of the HH39HuL i u strai39riratei39s 39 i 39 p The rnacnine is rmw ready L7 run inrpugn a test in prder id ubserve the work nardening experienced in inis part pr the iap ine specirnen must be ipaded and ipaded severai iirnes Once the piasiic regipn pr derprrnaiipn is reacned an the eipngaiipn vs ipad pipi stop me iensiie ester and quotipg39 ine crpssnead dawn mi the ipad reacnes zerp Begin running the tester again wiinpui reseuing the ipad prgage iengin untH the ipad surpasses ine pripr tensHe ipading E Pega A40247130 o 42 7030 Brass vs Galvanized Steel For the second experiment use the same specimen as in Experiment 1 Jog the jaws again so the load is zero Reload the specimen this time at a higher strain rate about fifty percent per minute until fracture Now use a galvanized steel sample and repeat the experiment Now compare the Brass stress strain curve with that of galvanized steel You will be able to see their variance in material properties by the differences in the curves Measuring the cross sectional area near the failure point may be helpful in understanding why the specimen failed 0 43 PMMA Polymethyl Methacrylate The procedure for PMMA will be very similar to that used in Experiment 2 The strain rate for used in this material is 10 percent per minute Because PMMA is such a brittle plastic it will not deform very much and the fracture will be sudden with nearly no necking When the speed of the deformation is increased the strain to fracture decreases o 44 HDPE High Density Polyethylene Follow the same procedure for HDPE specimen For the HDPE specimen vary the strain rate from five percent to ten percent and lastly 100 percent per minute Compare the differences in the stress strain curves for each strain rate 0 45 2024 Aluminum The same procedure will be follow for the Aluminum specimen with a 5 in per minute strain rate This test should be done at least three times to ensure accuracy 7Page A4u24713u 5 Daxaat Data Analysis 51 Strain Hardening Tne stresssstrain plot for Experiment 1 is shown in Figure 2 Specimen 1 shows the Specimen Tneug39 39gi 3 shows the the linear portion of the graph which illustrates the elastic portion cf deformation You can see from the plot that each time the specimen is loaded and unloaded the yield strength increases greatly The point at which the graph begins to decrease is the necking point and the sharp decrease is the point where the fracture takes place When ng tne Young39s de the elongation by the org al length and take Modulus tnelinearpo n5imply the rise over mn approximated as 350 MPa 370 MPH and 400 MPH lspecimens 12 and 3 respectively and 450 MPH respectively Specimen 1 to 3 Extenzzan mm Figure 2 Experiment 1 Damn mac Brass slpage MUZA713U 527030 Blassvs Galvaniled Steel The experimentai data for Experiment 2 is given in Figures 3 and 4 beiow When comparing the two piots you can see based on the siope of the iinear portions the luuiig I iUuuiu the yieid point which is a good exampie of yieid point phenomena when iooking at the and 380 MPa respectiveiy and 300 MPa and SSDMPa forgaivanized steei Specimen 1 to 3 Tensiiesl ss iMPai We rm Figule 3 Expeviment 2 Data wao mass Specimen 1m 4 nSiie stress inpai TE 553 9V zuiiiumniamzi Exieniion min Figule A Expenment 2 Data Galvanixed steel PiPage A4024713D 53PMMA The expertmentat datath PMMA can be hmth betow tn thure 5 Thts brtttte ptasttc otsotays much dt erent oharaotertsttos than the ntetats tn Expertmems 1 and 2 You cart see hvtm man mt rather tow otttntate tenstte strength approxtntatety 90 mm The stratn rates tor one two and three respeotwety are 127 ntntnttn 535 ntntnttnano127 rrtrrt nttn VmA cart see thatthe curve changes outte a btt between the oth erent stratn rates Spettmen 1 to 3 s Exeneton tmm Figure 5 Experiment 3 Data PMMA 54HDPE The expertntentat data for expertmem 4 cart be found oetow tn thure 5 Thts ptot shows a great exampte of a oucttte ptasttc HDPE shows a very tow utttmate tenstte strength approxtntatety 25 MP3 and yteto strength approxtntatety 10 MP3 oetow but tt tn the PMMA The stratn ratesfor one two and three are 25 tn mtn 125 tn mtn and 125 tn mtn respecttvety You can see from the ptots that the faster the etongatton the tttgttt lutpage Amamu Spearmerr 1 to 3 E m E a m g r m la in m sa an m n Enemaan mm Figure 6 Experimenu Dates HDPK 552024 39um The Expenmenla data fur Expenmenl 5 can be lmmd m gure 7 mm As yuu can see lrumlhe stress vs Extensmn pm Ammmum 2024 rs a Very bnme mela It has a arrw strain at lraclure Spemmen 1 tn 3 2 tersugstress LMPa z 3 4 Enamn mm Figure 7 Experiment 5 Dates mu Aluminum mp A40247130 6 Discussion 0 61 Strain Hardening In Experiment 1 the specimen is loaded and unloaded in order to harden the specimen using strain hardening This phenomenon can be explained on the basis of dislocation As the metal specimen deforms there becomes more dislocations which causes the average distance of separation between dislocations to decrease As there becomes more and more dislocations they begin to repel each other thereby not allowing neighboring molecules to move as freely Since the atoms are not allowed to move as much the material becomes stronger and slightly more brittle o 62 7030 Brass vs Galvanized Steel In Experiment 2 70 30 Brass is compared to galvanized steel Both of these specimens are elongated until they reach failure using a strain rate of 05 in min The 70 30 Brass begins its plastic deformation much earlier in its elongation which means it has a much lower yield strength than the galvanized steel However both galvanized steel and 70 30 Brass have approximately the same ultimate tensile strength The difference between the two metals is caused by the arrangement of the cell structure The 70 30 Brass is a face centered cubic lattice whereas the galvanized steel is a body centered cubic lattice The face centered cubic lattice allows the atoms to begin slipping out of their equilibrium planes much earlier than the body centered arrangement of the steel lZlPage A40247130 o 63 PMMA In Experiment 3 PMMA was tested using three different strain rates The different strain rates cause the specimens to behave differently in multiple ways All three specimens have such a low yield strength it is hard to decipher which has the highest but the specimen which experienced a medium speed strain rate had the highest ultimate tensile strength This is due to the polymeric nature of PMMA being very brittle to increased strain rates at room temperature This plastic however can become extremely ductile if heated above 60 degrees Celsius Because the links in this polymer are so tightly bound once there is a tiny crack the specimen will fracture rapidly On the contrary if the polymer is at a higher temperature the links in the polymer will untangle and stretch out in plastic deformation 0 64 HDPE In Experiment 4 High Density Polyethylene was tested with three different strain rates This was very similar to Experiment 3 in which PMMA was tested however HDPE has a much lower glass transition temperature This means it stops acting brittle at a lower temperature unlike the PMMA At room temperature the HDPE will act very ductile which causes the long strands of polymers to untangle and stretch out when a force is applied instead of fracturing immediately like the PMMA o 65 2024 Aluminum In Experiment 5 Aluminum was tested using a relatively mild strain rate Aluminum shows characteristics of a brittle metal when compared to the Brass and steel tested earlier This is due to Aluminum being such a light metal The crystal structure of 2024 Aluminum is similar to that of Brass FCC however in the Aluminum specimen the low density of particles 13Page
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