Mechanics & Materials Lab
Mechanics & Materials Lab MEGR 3152
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This 9 page Class Notes was uploaded by Arlene Runolfsdottir III on Sunday October 25, 2015. The Class Notes belongs to MEGR 3152 at University of North Carolina - Charlotte taught by Staff in Fall. Since its upload, it has received 46 views. For similar materials see /class/228992/megr-3152-university-of-north-carolina-charlotte in Mechanical Engineering at University of North Carolina - Charlotte.
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Date Created: 10/25/15
Impact Toughness Measurement Experiment 02 Why measure impact toughness Structural materials such as many metals and alloys are used to build load bearing structures An engineer needs to know if the material will survive the conditions that the structure will experience in service In most applications catastrophic failure of the components of a structure should be avoided because it is unpredictable and therefore very dangerous To this end we would like to choose a material that will not fracture in a catastrophic manner Fracture of materials is a result of initiation and propagation of cracks Catastrophic fracture is usually brittle meaning that crack propagation experiences a small barrier and therefore is fast To quantify the materials resistance to catastrophic failure we use a quantity called fracture toughness This quantity re ects the amount of energy needed to sustain crack propagation The higher the fracture toughness the more the energy needed for a crack to grow Important factors which affect the fracture toughness of a structural material include test temperatures mode of mechanical loading and rate of mechanical loading or imposed strain rates and stress concentrations such as notches and cracks Such factors may arise due to wind or impacts etc These all tend to encourage fracture To some extent the complex interaction of these factors can be included in the design process by using fracture mechanics theory In circumstances where safety is extremely critical full scale engineering components may be tested in their worst possible service condition For example asks for the transportation of nuclear fuel were tested in a full scale crash with a train to demonstrate that they retained their structural integrity a 140 ton locomotive and three 35 ton coaches at 100 mph crashed into a spent fuel ask laid across the track with its lid facing the train The train was demolished but the ask remained sealed The peak impact force of the test train was greater than that of an InterCity 125 a highspeed UK passenger train traveling at 125 mph A theoretical fracture mechanics to evaluate the structural integrity of this situation would have been very difficult in reality Such full scale tests are extremely expensive and are very rarely conducted Fracture mechanics is also a fairly recent development in engineering design and measurement of the fracture toughness parameters that are required to perform a structural integrity assessment during the design process such as ch is quite timeconsuming and expensive Tests for the impact toughness such as the Charm Impact test were developed before fracture mechanics theory was available The impact test is a method for evaluating the relative toughness of engineering materials The Chaipy impact test continues to be used nowadays as an economical quality control method to assess the notch sensitivity and impact toughness of engineering materials It is usually used to test the toughness of metals Similar tests can be used for polymers ceramics and composites What is the Char Im act Te Figure 1 shows schematic of two impact toughness measure specimens and the test machine 8mm 032m Scab Chaim v stamng pasmon Hammcv zod Figure 1 Izod impact test specimen and Charpy impact test specimen a and an impact test system From WD Callister Chapter 8 Figure 2 displays a photo of a Charpy impact test system 2 The Charpy rrnpaettest systern 1n thrs s stern eonyertedrnto the energy absorbed dunng the fracture proeess ofthe speerrnen The speerrnens ean be eoo1ed or heatedpnor to rnountrng Frgure 3 A Charpy test speerrnen h mm r FHw Mr H h a xed dstanee constant patentml Energy to stnke the speerrnen at a xed yeloerty cansmnt Janette Energy Tan 1 rnatena1s absorb a lot of energy when fractured and Inimkmatenals absorb yery lmle energy What affects the Charpx Impact Energy To understand how the Charpy rrnpaet energy 15 affeeted by the properues of the rnatena1 we need to understandthe durerent eonmbuuons whreh make up the measured energy What is the Impact Energy The rrnpaet energy measured by the Charpy test rs the work done to fracture the speerrnen On impact the specimen deforms elastically until yielding takes place plastic deformation and a plastic zone develops at the notch As the test specimen continues to be deformed by the impact the plastic zone work hardens This increases the stress and strain in the plastic zone until the specimen fractures The Charpy impact energy therefore includes three major parts i the elastic strain energy ii the plastic work done during yielding and iii the work done to create the fracture surface surface energy For a brittle material the total fracture energy is dominated by the elastic energy and the surface energy However the elastic energy is usually not a significant fraction of the total energy for a ductile material The total energy of impact fracture in an impact fracture toughness test for a ductile material is dominated by the plastic work The total impact energy depends on the size of the test specimen and on the depth and root curvature of the notch Therefore a standard specimen size and notch geometry is used to allow comparison between different materials see Fig l The impact energy is affected by a number of factors such as Yield Strength and Ductility Notches depth of notch and root curvature Temperature and Strain Rate Fracture 39 39 brittle or ductile etc Yield Strength and Ductility Increasing the yield strength of a metal by processes such as cold work precipitation strengthening and substitutional or interstitial solution strengthening generally decreases the ductility This is the total plastic strain of the specimen to failure Usually percent elongation from a tensile test is used to assess the ductility of a material Figure 4 shows the schematic stressstrain curves of two materials or the same materials but with two different microstructure such as grain size phase size and distribution due to different heattreatment history different predeformation history etc The two stressstrain curves have different yield strengths the red curve has higher yield strength but smaller elongation to failure than the blue curve But if we examine the areas covered by the two curves it is apparent the one under the blue curve is much greater than that under the red curve It implies that even thought the material of red curve is stronger than that of the blue curve it costs much more energy to break or fracture a mechanical testing specimen made from the blue curve material We say that the material of the blue curve is a tougher material As we have discussed in Materials Science and Engineering lectures there are many way to increase the yield strength of a metal Such strengthening mechanisms include grain size reduction second phase strengthening precipitation dispersion etc solid solution strengthening cold working etc However the majority of these methods will lead to significantly reduced ductility and hence reduced fracture toughness The only exception is probably grain size reduction It has been found that metals with smaller grain size exhibit both increased yield strength and enhance fracture toughness Htgh yteid stiength inw duetmty Luw yteid stiength high duetmty Frgure 4 The yield strength tensile strength and duetrhty See text for dsseussron suess Strain Inereasrng the yield strength by these rneehanrsrns eneept for gram srze reduetron therefore deereases the Charpy rrnpaet energy smce 1ess p1astre work ean be d ne before the s am m the p1astre zone rs suffrerent to fraeture the test speerrnen An snerease m yield strength ean a1so affeet the rrnpaet energy by eausrng a ehange m the Mom mechanism Notches First the noteh aets as a stress eoneentrator m the speerrnen 1t greatly arnphfy the a um onn1 am tr esp equation 8 1 The stress eoneentratron ofthe noteh eauses yreidsng or p1astre deforrnatron to oeeur locally at the note Lit hinge ean develop at the noteh whreh reduees the totai amount of plastre deforrnatron m the test speerrnen Thrs reduees the ndly the eonstratnt of d work done by plastre deforrnatron ore fraeture eeo deforrnatron at the noteh snereases the tensile stress m the plastre zone The egree of The mcreased tensile stress eneourages fraeture and reduees the work done by p1astre deforrnatron before fraeture oeeurs Some rnat rais are more sensrtrye to notehes than others and a slandani noteh t dsus and noteh depth are therefore used to enable eornpanson between dsfferent enals The Charpy rrnpaettest therefore rndseates the notch sensitivin ofarnatenai Temperature and Strain Rate Since the Charpy impact energy comprises mostly of the local plastic work of the local yielding of the specimen it is affected by factors which change the yield behavior of the material such as temperature and strain rate It is through their effect that the motions of dislocations are in uenced Increasing the yield strength by lowering temperatures or raising the strain rates may impart effect on the ductility of the material and therefore in uences the Charpy impact energy The yield strength of body centred cubic BCC metals is more sensitive to strain rate and temperature than that of facecentred cubic FCC metals We use the term strain rate sensitivity to describe the dependence of a material s strength on the loading rate and in general practice it is defined as m6ln66ln where 6 is the yield or ow stress and is the strain rate s39l BCC metals have a value of m usually 10 times higher that FCC metals The Charpy impact energy of BCC metals such as ferritic carbon steel therefore has a stronger dependence on strain rate and temperature than that of FCC metals such as aluminum copper and austenitic stainless steel Fracture Mechanism The Charpy impact energy is affected by changes in the fracture mechanisms At ambient temperature most metals usually fracture by microvoid coalescence in which the plastic strain causes void nucleation around inclusions These microvoids grow and link up until nal failure occurs In BCC metals failure can also occur by cleavage along the 001 crystal planes at a critical tensile stress As the yield strength of the metal is increased the tensile stress in the plastic zone can become sufficiently high for cleavage to occur The fracture mechanism in a ferritic carbon steel therefore changes from microvoid coalescence to cleavage as the yield strength increases This can be caused by an increase in strain rate or a decrease in temperature The work of fracture of cleavage is much less than the work of fracture of microvoid coalescence since it involves much less plastic deformation The change in fracture mechanism therefore causes a sharp ductile t0 brittle transition in the Charpy impact energy The ductile to brittle transition DBTT is usually observed in most BCC metals and alloys But the exact temperature is a strong function of the type of material and for a given metal a strong function of the purity level Probably the most ductile BCC metal is tantalum since it has a DBTT temperature as low as 4K 269 C But for another BCC metal tungsten while ductile at even 77K or l96 C liquid nitrogen temperature in single crystal form its DBTT can be as high as 150 C in polycrystalline form Most FCC metals do now show the DBTT behavior On the contrary their fracture toughness may increase with decreased temperature an exception is iridium an FCC metal but very brittle For a tough metal or material its DBTT should be low vice versa Most BCC metals become brittle when we decrease the testing temperature or increase the load rate Figure 5 Two different fracture micromechanisms Le microvoid coalescence ductile and the nal fracture surface is identi ed with dimples and fracture consumes a large amount of energy Right cleavage brittle muchless energy consumed The Ductile t0 Brittle Tra tion nan A 39 L ofametal it terms of the Mum may A brittle fracture is a low energy fracture and a ductile fracture is a high energy fracture Some confusion often occurs because we can also use the terms Mile and mm to describe fracture mechanisms Microvoid coalescence is a mule fracture mechanism and cleavage is a bum fracture mechanism However it is possible for a law entry u w or more cleavage e femtie 52221 at low temperatures You should always be aware of both the toughness and the fracture mechanism Figure 6 Schematic ductile to brittle mm mm transition curves ForFCC metals such Inipanl Energy rem Bramre B mperature The ductile to brittle transition curve records the effect of temperature on the fracture energy The impact energy generally decreases with decreasing temperature as the M strength increases and the ductility decreases A sharp transition where the energy changes by a large amount for a small temperature changes can occur when there is a change in the fracture mechanism If the material has a sharp ductile to brittle transition then a transition temperature can be defined below which the material has poor toughness This can be used as a guideline for the minimum service temperature It is less easy to do this in materials with a smooth transition from ductile to brittle behavior The transition temperature may be defined using the mean impact energy between the highest and lowest values A transition temperature can also be defined using the lateral expansion of the specimen a measure of the amount of plastic deformation or changes in the fracture surface appearance Unfortunately these different measurements in the same material do not necessarily give the same transition temperature This problem was one of the factors which led to the development of fracture mechanics How is the Charpy Impact Energy used The Charpy impact test can be used to assess the relative toughness of different materials eg steel and aluminum as a tool for materials selection in design It may also be used for quality control to ensure that the material being produced reaches a minimum specified toughness level Difficulties arise when you attempt to answer questions such as quotWhat impact toughness must my steel have if I m to make an oil rig which will be subjected to wave impact in the North Sea at subzero temperaturesquot Design problems such as this can be tackled by the use of minimum impact energy for the service temperature which is based on previous experience For example it was found that fractures of the steel plate in Libergg ships in the 1940 s only occurred at sea temperatures for which the impact energy of the steel was 20 J This data was used to select steels for future ship designs This approach is often still used to specify minimum impact energy for material selection though the criteria are also based on correlations with fracture mechanics measurements and calculations It39s interesting to note that the impact energy of steel recovered from the Titanic was found to be very low brittle at l C This was the estimated sea temperature at the time of the iceberg impact Experimental Procedures How to do a Charpv impact test Heat or cool the specimen as required record the temperature of the specimen Mount the specimen Raise the hammer to a prescribed position Release the hammer Record the nal position of the hammer after impact Recover the specimen for further examination for fracture mechanisms etc 9959 Lab Report Reguirements 1 Format as always 2 Collect the data of impact energy from all the lab sessions at different temperatures 3 Process the data to get the impact energy as a function of specimen temperature for different materials tested 4 Present the data in both tables and plots 5 Discuss the results in terms of the factors that affect the fracture toughness of metals The major body of this chapter is adopted from the following internet source http www2 umi st ac uldmaterial researchintmi c fe atures charpy notes htm
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