www.cmmok.xinwen520.com                                                 Mechanical Testing 01B                            ...
www.cmmok.xinwen520.comREVISION HISTORY  01       10.05.2008       For Approval            CMM  Rev   Date (dd.mm.yyyy)   ...
www.cmmok.xinwen520.comMechanical testing - notched bar or impact testingNotched bar or impact testing. Part IBefore looki...
www.cmmok.xinwen520.comThe standard Charpy-V specimen, illustrated in Fig.1. Is 55mm long, 10mm square and has a 2mm deep ...
www.cmmok.xinwen520.com                                      A characteristic of carbon and low alloy steels is that they ...
www.cmmok.xinwen520.com                                    Austenitic stainless steels, nickel and aluminium alloys FCC, F...
www.cmmok.xinwen520.comNotched bar or impact testing. Part IIMany application standards therefore require impact testing t...
www.cmmok.xinwen520.comthe transition temperature also decreases, as does the upper shelf value, illustrated in Fig.3 and ...
www.cmmok.xinwen520.comOf the beneficial elements, manganese and nickel are possibly the two most significant, the nickel ...
www.cmmok.xinwen520.comDuctile vs. Brittle Behavior—Body-centered-cubic or ferriticalloys exhibit a significant transition...
www.cmmok.xinwen520.com                                 SEM image of brittle fracture in plain carbon steelA brittle failu...
www.cmmok.xinwen520.comDuctile tensile failure begins with uniform plastic deformation leading to localised microvoid coal...
www.cmmok.xinwen520.comExtract from ASTM 370 A5: A5. NOTES ON SIGNIFICANCE OF NOTCHED-BAR IMPACT TESTINGA5.1 Notch Behavio...
www.cmmok.xinwen520.comA5.2.5 Variations in notch dimensions will seriously affect       A5.4.3 A problem peculiar to Char...
www.cmmok.xinwen520.comTransitional temperature may be defined in the following ways:    Lowest temerature specimens exhib...
www.cmmok.xinwen520.comInspection & witnessingWitnessing of Charpy impact testing, following are to be considered;    Labo...
www.cmmok.xinwen520.com                 Crystalline, shining brittle fracture, absorbing little energy on impact.         ...
www.cmmok.xinwen520.com                            Charpy impact testing machine.                          Tested samples ...
www.cmmok.xinwen520.com                                 Sub-size 5 x 55mm sanples                    The lateral expansion...
www.cmmok.xinwen520.com        The ‘V’ notche of the samples were checked using calibrated shadow-graphic projection.     ...
www.cmmok.xinwen520.com                          Ductile fracture                          Ductile fracture               ...
www.cmmok.xinwen520.com                          Ductile and brittle boundary                                Brittle fract...
www.cmmok.xinwen520.comMicrostructural and Fractographic Characterization of a Thermally Embrittled Nuclear Grade         ...
www.cmmok.xinwen520.com                          Pg: 24/ 25
www.cmmok.xinwen520.com(Intergranular fracture in a nickel-chromium alloy, viewed under the scanning electron microscope. ...
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Charpy testing

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Charpy testing

  1. 1. www.cmmok.xinwen520.com Mechanical Testing 01B An Introduction of mechanical Testing Pictorial tutorial 机械性能试验 图文简易教材 Part 2:Charpy Impact Testing (Descriptive approach) 夏比冲击试验RIG/PLANTIn house training Radiographic, Dye Penetrant, Magnetic Particle, Eddy Current, Ultrasonic Testing. Post weld heat CMM NDT & Inspection ADDITIONAL CODE SDRL CODE TOTAL PGS treatment, infra red tomography. 21 Thickness gauging, Underwater inspection. FRPREMARKS Inspection.MAIN TAG NUMBER DISCIPLINE MechanicalCLIENT PO NUMBER DOCUMENT NUMBER REVCLIENT DOCUMENT NUMBER Mech-Course-001B 01 CMM NDT Services www.cmmok.xinwen520.com Pg: 1/ 25
  2. 2. www.cmmok.xinwen520.comREVISION HISTORY 01 10.05.2008 For Approval CMM Rev Date (dd.mm.yyyy) Reason for issue Prep Check ApprCHANGE DESCRIPTIONRevision Change description 01 For Approval Pg: 2/ 25
  3. 3. www.cmmok.xinwen520.comMechanical testing - notched bar or impact testingNotched bar or impact testing. Part IBefore looking at impact testing let us first define what is meant by toughness since the impact test is onlyone method by which this material property is measured.Toughness is, broadly, a measure of the amount of energy required to cause an item - a test piece or a bridgeor a pressure vessel - to fracture and fail. The more energy that is required then the tougher the material.The area beneath a stress/strain curve produced from a tensile test is a measure of the toughness of the testpiece under slow loading conditions. However, in the context of an impact test we are looking at notchtoughness, a measure of the metals resistance to brittle or fast fracture in the presence of a flaw or notch andfast loading conditions.It was during World War II that attention was focused on this property of notch toughness due to the brittlefracture of all-welded Liberty ships, then being built in the USA. From this work the science of fracturetoughness developed and gave rise to a range of tests used to characterise notch toughness of which theCharpy-V test described in this article is one.There are two main forms of impact test, the Izod and the Charpy test.Both involve striking a standard specimen with a controlled weight pendulum traveling at a set speed. Theamount of energy absorbed in fracturing the test piece is measured and this gives an indication of the notchtoughness of the test material.These tests show that metals can be classified as being either brittle or ductile. A brittle metal will absorb asmall amount of energy when impact tested, a tough ductile metal a large amount of energy.It should be emphasised that these tests are qualitative, the results can only be compared with each other orwith a requirement in a specification - they cannot be used to calculate the fracture toughness of a weld orparent metal. Tests that can be used in this way will be covered in future Job Knowledge articles. The Izod testis rarely used these days for weld testing having been replaced by the Charpy test and will not be discussedfurther in this article. The test is qualitative. The results can not be used to calculate the fracture toughness of materail.The Charpy specimen may be used with one of three different types of notch, a keyhole, a U and a V. Thekeyhole and U-notch are used for the testing of brittle materials such as cast iron and for the testing of plastics.The V-notch specimen is the specimen of choice for weld testing and is the one discussed here. Pg: 3/ 25
  4. 4. www.cmmok.xinwen520.comThe standard Charpy-V specimen, illustrated in Fig.1. Is 55mm long, 10mm square and has a 2mm deep notchwith a tip radius of 0.25mm machined on one face.Fig.1. Standard Charpy-V notch specimenTo carry out the test the standard specimen is supported at its two ends on an anvil and struck on the oppositeface to the notch by a pendulum as shown in Fig.2. The specimen is fractured and the pendulum swingsthrough, the height of the swing being a measure of the amount of energy absorbed in fracturing the specimen.Conventionally three specimens are tested at any one temperature, see Fig.3, and the results averaged. Fig.2. Charpy testing machine Fig.3. Schematic Charpy-V energy and % age crystallinity curves Increase the temperature increase the absorbed energy. Increase the temperature decrease the % of crystallinity. There is a transition temperature range during which there a distinct change from ductile to brittle or vise versa. Pg: 4/ 25
  5. 5. www.cmmok.xinwen520.com A characteristic of carbon and low alloy steels is that they exhibit a change in fracture behaviour as the temperature falls with the failure mode changing from ductile to brittle. This is a characteristic of BCC, Body center cubic ferritic structure. If impact testing is carried out over a range of temperatures the results of energy absorbed versus temperature can be plotted to give the S curve illustrated in Fig.3. This shows that the fracture of these types of steels changes from being ductile on the upper shelf to brittle on the lower shelf as the temperature falls, passing through a transition region where the fracture will be mixed. Fig.3a.The fracture surface of this Charpy impact specimen show that the fracture mode was a mixture of ductile (dull gray) and brittle (shiny, salt and peppery appearance). Many specifications talk of a transition temperature, a temperature at which the fracture behaviour changes from ductile to brittle. This temperature is often determined by selecting, quite arbitrarily, the temperature at which the metal achieves an impact value of 27 Joules - see, for example the impact test requirements of EN 10028 Part 2 Steel for Pressure Purposes. What the curve shows is that a ductile fracture absorbs a greater amount of energy than a brittle fracture in the same material. Knowing the temperature at which the fracture behaviour changes is therefore of crucial importance when the service temperature of a structure is considered - ideally in service a structure should operate at upper shelf temperatures. The shape of the S curve and the positions of the upper and lower shelves are all affected by composition, heat treatment condition, whether or not the steel has been welded, welding heat input, welding consumable and a number of additional factors. All the factors must be controlled if good notch toughness is required. This means that close control of the welding parameters is essential if impact testing is a specification requirement.Among many others, the factors that may affect Charpy impact values are: Composition. Heat treatment conditions. Welding. Welding consumables. Interpass temperature. Heat input. Welding processes. Works done on samples. Service history. Pg: 5/ 25
  6. 6. www.cmmok.xinwen520.com Austenitic stainless steels, nickel and aluminium alloys FCC, Face center cubic structure do not show this change in fracture behaviour, the fracture remaining ductile even to very low temperatures. This is one reason why these types of alloys are used in cryogenic applications. In addition to the impact energy there are two further features that can be measured and may be found as a requirement in some specifications. These are percentage crystallinity and lateral expansion. Assessment of Charpy notch toughness can be expessed in following ways: Impact energy % crstallinity. % lateral expansion. Determination of transition temperature.The appearance of a fracture surface gives information about the type of fracture that has occurred - a brittlefracture is bright and crystalline, a ductile fracture is dull and fibrous.Percentage crystallinity is therefore a measure of the amount of brittle fracture, determined by making ajudgement of the amount of crystalline or brittle fracture on the surface of the broken specimen.Lateral expansion is a measure of the ductility of the specimen. When a ductile metal is broken the test piecedeforms before breaking, a pair of ears being squeezed out on the side of the compression face of thespecimen, as illustrated in Fig 4. The amount by which the specimen deforms is measured and expressed asmillimeters of lateral expansion. ASME B31.3 for example requires a lateral expansion of 0.38mm for boltingmaterials and steels with a UTS exceeding 656N/mm 2, rather than specifying an impact value. Fig.4 Lateral expansionThe next article in this series will look at the testing of welds, how the impact strength can be affected bycomposition and microstructure and some of its limitations and disadvantages.The previous article looked at the method of Charpy-V impact testing and the results that can be determinedfrom carrying out a test. This next part looks at the impact testing of welds and some of the factors that affectthe transition temperature such as composition and microstructure. Within such a short article, however, it willonly be possible to talk in the most general of terms.Welding can have a profound effect on the properties of the parent metal and there may be many options onprocess selection, welding parameters and consumable choice that will affect impact strength. Charpy Impact: Full size 10mm x 10mm samples. Sample on left showing a brittle fracture surface with 8 ft-lbs of energy, < 10% Shear, 0.1mm lateral expansion. Sample on right showing a ductile fracture surface with 290 ft-lbs of energy, 100% Shear, 2.03mm lateral expansion. Pg: 6/ 25
  7. 7. www.cmmok.xinwen520.comNotched bar or impact testing. Part IIMany application standards therefore require impact testing to be carried out on the parent metal, the weldmetal and in the heat affected zone as illustrated in Fig.1 which is taken from BS PD 5500 Annex D. Thestandards generally specify a minimum impact energy to be achieved at the minimum design temperature andto identify from where the specimens are to be taken. This is done in order to quantify the impact energy ofthe different microstructures in the weld metal and the HAZs to ensure that, as far as possible, the equipmentwill be operating at upper shelf temperatures where brittle fracture is not a risk. Fig.1. PD5500 App D. location of Charpy specimens in weld HAZThese application standards may be supplemented by client specifications that impose additional and morestringent testing requirements, as shown in Fig.2 taken from an oil industry specification for offshore structures. Fig.2. Offshore client requirementsThe positioning of the specimens within a weld is extremely important both in terms of the specimen locationand the notch orientation. A specimen positioned across the width of a multi-pass arc weld will probably includemore than one weld pass and its associated HAZs. Quite a small movement in the position of the notch cantherefore have a significant effect on the impact values recorded during a test. Positioning a notch preciselydown the centre line of a single pass of a submerged arc weld can give extremely low impact values!Testing the heat affected zone also has problems of notch position since in a carbon or low alloy steel there willbe a range of microstructures from the fusion line to the unaffected parent metal. Many welds also use a Vpreparation as illustrated above and this, coupled with the narrow HAZ, means that a single notch may sampleall of these structures. If the impact properties of specific areas in the HAZ need to be determined then a K orsingle bevel preparation may be used.The standard specimen is 10mm x 10mm square - when a weld joint is thicker than 10mm the machining of astandard size specimen is possible. When the thickness is less than this and impact testing is required itbecomes necessary to use sub-size specimens.Many specifications permit the use of 10mm x 7.5mm, 5mm and 2.5mm thickness (notch length) specimens.There is not a simple relationship between a 10mm x 10mm specimen and the sub-size specimens - a 10mm x5mm specimen does not have half the notch toughness of the full size test piece. As the thickness decreases Pg: 7/ 25
  8. 8. www.cmmok.xinwen520.comthe transition temperature also decreases, as does the upper shelf value, illustrated in Fig.3 and this isrecognised in the application standards. Fig.3. Effect of size on transition temperature and upper shelf values Size decrease transitional temperature decrease. Size decrease impact energy decrease although not proportionally.In a carbon or low alloy steel the lowest impact values are generally to be found close to the fusion line wheregrain growth has taken place.Coarse grains generally have low notch toughness, one reason why heat input needs to be controlled to lowlevels if high notch toughness is required.For example, EN ISO 15614 Pt. 1 requires Charpy-V specimens to be taken from the high heat input area of aprocedure qualification test piece and places limits on any increase in heat input. Certain steels may also havean area some distance from the fusion line that may be embrittled so some specifications require impact testsat a distance of 5mm from the fusion line.Charpy-V tests carried out on rolled products show that there is a difference in impact values if the specimensare taken parallel or transverse to the rolling direction. Specimens taken parallel to the rolling direction test themetal across the grain of the steel and have higher notch toughness than the transverse specimens - onereason why pressure vessel plates are rolled into cylinders with the rolling direction oriented in the hoopdirection. Impact of specimen at rolling (parallel) direction has higher value than transverse direction.In a carbon or low alloy steel the element that causes the largest change in notch toughness is carbon with thetransition temperature being raised by around 14°C for every 0.1% increase in carbon content.An example of how this can affect properties is the root pass of a single sided weld. This often has lower notchtoughness than the bulk of the weld as it has a larger amount of parent metal melted into it - most parentmetals have higher carbon content than the filler metal and the root pass therefore has a higher carboncontent than the bulk of the weld.Sulphur and phosphorus are two other elements that both reduce notch toughness, one reason why steelproducers have been working hard to reduce these elements to as low a level as possible. It is not uncommonfor a good quality modern steel to have a sulphur content less than 0.005%. Pg: 8/ 25
  9. 9. www.cmmok.xinwen520.comOf the beneficial elements, manganese and nickel are possibly the two most significant, the nickel alloy steelsforming a family of cryogenic steels with the 9% nickel steel being capable of use at temperatures down to -196°C. Aluminium is also beneficial at around 0.02% where it has the optimum effect in providing a fine grainsize.Element that affect charpy impact energy: Negative effect: Carbon, Sulphur and phosphorous Positive effects: Nickel, Manganese, Nitrogen and Aluminium.Lastly, let us have a brief look at some of the other factors that can affect the impact values. These areconcerned with the quality of the specimen and how the test is conducted.It goes without saying that the specimens must be accurately machined, the shape of the tip of the notchbeing the most important feature. A blunted milling cutter or broach will give a rounded notch tip and this inturn will give a false, high impact value. Checking the tip radius on a shadowgraph is one simple way ofensuring the correct tip shape. Correct positioning of the specimen on the anvil is most important and this canbe done using a specially designed former.The last point concerns the testing of specimens at temperatures other than at room temperature. Whentesting at sub-zero temperatures the length of time taken to remove the specimen from the cooling bath,position it on the anvil and test it is most important. EN875 requires this to be done within five secondsotherwise the test piece temperature will rise making the test invalid - referring back to the impact energy vstemperature curve in the previous article will show why. Relevant SpecificationsBS 131 Part 4 Calibration of Impact Testing Machines for metals.BS 131 Part 5 Determination of CrystallinityBS 131 Part 6 Method for Precision Determination of Charpy-V Impact EnergyBS 131 Part 7 Specification for Verification of Precision Test MachinesEN 875 Destructive Tests on Welds in Metallic Materials - Impact TestsEN 10045 Part 1 Test MethodEN 10045 Part 2 Verification of Impact Testing MachinesASTM E23-O2A Standard Test Methods for Notched Bar Impact Testing of Metallic Materials.Toughness is an expression of measurement of energy to fracture a specimen.Tensile or yield is also an expression of toughness, toughness at slow loading condition of stress. Notchtoughness in the context of impact testing is the measurement of energy requiring to fracture a specimen with aflaw at fast loading. Pg: 9/ 25
  10. 10. www.cmmok.xinwen520.comDuctile vs. Brittle Behavior—Body-centered-cubic or ferriticalloys exhibit a significant transition in behavior when impacttested over a range of temperatures. At temperatures abovetransition, impact specimens fracture by a ductile (usuallymicrovoid coalescence) mechanism, absorbing relativelylarge amounts of energy. At lower temperatures, theyfracture in a brittle (usually cleavage) manner absorbing lessenergy. Within the transition range, the fracture willgenerallybe a mixture of areas of ductile fracture and brittle fracture.The temperature range of the transition from one typeof behavior to the other varies according to the material being tested. This transition behavior may be defined invarious ways for specification purposes.The specification may require a minimum test result for absorbed energy, fracture appearance, lateral expansion,or a combination thereof, at a specified test temperature.The specification may require the determination ofthe transition temperature at which either the absorbedenergy or fracture appearance attains a specified levelwhen testing is performed over a range of temperatures.Fracture MechanismsA fracture surface should be treated as a record of the history of a component failure. Detailed within the fracturesurface is evidence of loading history, environmental conditions and material quality.1 For the purposes of thisresource, fracture surfaces are classified as brittle, ductile and fatigue although some specimens may clearly fitinto a number of fracture categories. For example a fatigue failure fracture surface may exhibit a ductile finalfracture region. This will be taken into account and the fracture image will appear in all of the categories where itis relevant.Brittle FractureBrittle fractures have no plastic deformation and are usually characterised by a lack of necking with smooth/shinyfacets (as shown below), an appearance associated with fast crack growth.4 Pg: 10/ 25
  11. 11. www.cmmok.xinwen520.com SEM image of brittle fracture in plain carbon steelA brittle failure mode such as cleavage or intergranular (fracture along the grain boundaries) is seen. On amacroscopic level, chevron or radial markings may be observed as shown below. Radiating Ridge Fracture Surface (Callister, W.D., page 186)Ductile FractureConversely ductile fractures can be characterised by necking of the material due to plastic deformation. Afibrous/rough and dull fracture surface can be observed associated with slow crack growth.Plastic deformation is produced by a ductile failure mode such as microvoid coalescence leading to dimple rupturewhich can be seen below. Failure at the edges of the sample occurs at 45º to the loading direction due to themaximum shear stress being at 45º to the loading stress. SEM image of ductile fracture in plain carbon steel Pg: 11/ 25
  12. 12. www.cmmok.xinwen520.comDuctile tensile failure begins with uniform plastic deformation leading to localised microvoid coalescence and thendimple rupture in the necked region which experiences a tri-axial stress state on formation of the neck. Dimplerupture leaves pits and holes on the surface structure.Factors affecting the failure mode are: • Material Microstructure. • Temperature. • Strain Rate. • Environment (leading to corrosion). Pg: 12/ 25
  13. 13. www.cmmok.xinwen520.comExtract from ASTM 370 A5: A5. NOTES ON SIGNIFICANCE OF NOTCHED-BAR IMPACT TESTINGA5.1 Notch Behavior A5.2 Notch EffectA5.1.1 The Charpy and Izod type tests bring out notch A5.2.1 The notch results in a combination of multiaxialbehavior (brittleness versus ductility) by applying a single stresses associated with restraints to deformation in directionsoverload of stress. The energy values determined are perpendicular to the major stress, and a stress concentration atquantitative comparisons on a selected specimen but cannot the base of the notch. A severely notched condition isbe converted into energy values that would serve for generally not desirable, and it becomes of real concern inengineering design calculations. The notch behavior indicated those cases in which it initiates a sudden and complete failurein an individual test applies only to the specimen size, notch of the brittle type. Some metals can be deformed in a ductilegeometry, and testing conditions involved and cannot be manner even down to the low temperatures of liquid air,generalized to other sizes of specimens and conditions. while others may crack.A5.1.2 The notch behavior of the face-centered cubic metals This difference in behavior can be best understood byand alloys, a large group of nonferrous materials and the considering the cohesive strength of a material (or theaustenitic steels can be judged from their common tensile property that holds it together) and its relation to the yieldproperties. If they are brittle in tension they will be brittle point. In cases of brittle fracture, the cohesive strength iswhen notched, while if they are ductile in tension, they will exceeded before significant plastic deformation occurs andbe ductile when notched, except for unusually sharp or deep the fracture appears crystalline. In cases of the ductile ornotches (much more severe than the standard Charpy or Izod shear type of failure, considerable deformation precedes thespecimens). final fracture and the broken surface appears fibrous instead of crystalline. In intermediate cases the fracture comes after aEven low temperatures do not alter this characteristic of these moderate amount of deformation and is part crystalline andmaterials. In contrast, the behavior of the ferritic steels under part fibrous in appearance.notch conditions cannot be predicted from their properties asrevealed by the tension test. For the study of these materials A5.2.2 When a notched bar is loaded, there is a normalthe Charpy and Izod type tests are accordingly very useful. stress across the base of the notch which tends to initiateSome metals that display normal ductility in the tension test fracture. The property that keeps it from cleaving, or holds itmay nevertheless break in brittle fashion when tested or when together, is the “cohesive strength.” The bar fractures whenused in the notched condition. the normal stress exceeds the cohesive strength. When this occurs without the bar deforming it is the condition for brittleNotched conditions include restraints to deformation in fracture.directions perpendicular to the major stress, or multiaxialstresses, and stress concentrations. It is in this field that the A5.2.3 In testing, though not in service because of sideCharpy and Izod tests prove useful for determining the effects, it happens more commonly that plastic deformationsuceptibility of a steel to notchbrittle behavior though they precedes fracture. In addition to the normal stress, the appliedcannot be directly used to appraise the serviceability of a load also sets up shear stresses which are about 45° to thestructure. normal stress. The elastic behavior terminates as soon as the shear stress exceeds the shear strength of the material andA5.1.3 The testing machine itself must be sufficiently rigid deformation or plastic yielding sets in. This is the conditionor tests on high-strength low-energy materials will result in for ductile failure.excessive elastic energy losses either upward through thependulum shaft or downward through the base of the A5.2.4 This behavior, whether brittle or ductile, depends onmachine. whether the normal stress exceeds the cohesive strength before the shear stress exceeds the shear strength. SeveralIf the anvil supports, the pendulum striking edge, or the important facts of notch behavior follow from this. If themachine foundation bolts are not securely fastened, tests on notch is made sharper or more drastic, the normal stress at theductile materials in the range of 80 ft·lbf (108 J) may actually root of the notch will be increased in relation to the shearindicate values in excess of 90 to 100 ft·lbf (122 to 136 J). stress and the bar will be more prone to brittle fracture (see Table A5.1). Also, as the speed of deformation increases, the shear strength increases and the likelihood of brittle fracture increases. On the other hand, by raising the temperature, leaving the notch and the speed of deformation the same, the shear strength is lowered and ductile behavior is promoted, leading to shear failure. Pg: 13/ 25
  14. 14. www.cmmok.xinwen520.comA5.2.5 Variations in notch dimensions will seriously affect A5.4.3 A problem peculiar to Charpy-type tests occursthe results of the tests. Tests on E 4340 steel specimens9 have when high-strength, low-energy specimens are tested at lowshown the effect of dimensional variations on Charpy results temperatures. These specimens may not leave the machine in(see Table A5.1). the direction of the pendulum swing but rather in a sidewise direction.A5.3 Size EffectA5.3.1 Increasing either the width or the depth of the To ensure that the broken halves of the specimens do notspecimen tends to increase the volume of metal subject to rebound off some component of the machine and contact thedistortion, and by this factor tends to increase the energy pendulum before it completes its swing, modifications mayabsorption when breaking the specimen. However, any be necessary in older model machines. These modificationsincrease in size, particularly in width, also tends to increase differ with machine design. Nevertheless the basic problem isthe degree of restraint and by tending to induce brittle the same in that provisions must be made to preventfracture, may decrease the amount of energy absorbed. Where rebounding of the fractured specimens into any part of thea standard-size specimen is on the verge of brittle fracture, swinging pendulum.this is particularly true, and a double-width specimen mayactually require less energy for rupture than one of standard Where design permits, the broken specimens may bewidth. deflected out of the sides of the machine and yet in other designs it may be necessary to contain the broken specimensA5.3.2 In studies of such effects where the size of the within a certain area until the pendulum passes through thematerial precludes the use of the standard specimen, as for anvils. Some low-energy high-strength steel specimens leaveexample when the material is 1⁄4-in. plate, subsize specimens impact machines at speeds in excess of 50 ft (15.3 m)/sare necessarily used. Such specimens (see Fig. 6 of Test although they were struck by a pendulum traveling at speedsMethods E 23) are based on the Type A specimen of Fig. 4 of approximately 17 ft (5.2 m)/s. If the force exerted on theTest Methods E 23. A5.3.3 General correlation between the pendulum by the broken specimens is sufficient, theenergy values obtained with specimens of different size or pendulum will slow down and erroneously high energyshape is not feasible, but limited correlations may be values will be recorded.established for specification purposes on the basis of specialstudies of particular materials and particular specimens. This problem accounts for many of the inconsistencies in Charpy results reported by various investigators within the 10On the other hand, in a study of the relative effect of process to 25-ft·lbf (14 to 34 J) range. The Apparatus Section (thevariations, evaluation by use of some arbitrarily selected paragraph regarding Specimen Clearance) of Test Methods Especimen with some chosen notch will in most instances 23 discusses the two basic machine designs and aplace the methods in their proper order. modification found to be satisfactory in minimizing jamming.A5.4 Effects of Testing Conditions A5.5 Velocity of StrainingA5.4.1 The testing conditions also affect the notch behavior. A5.5.1 Velocity of straining is likewise a variable thatSo pronounced is the effect of temperature on the behavior of affects the notch behavior of steel. The impact test showssteel when notched that comparisons are frequently made by somewhat higher energy absorption values than the staticexamining specimen fractures and by plotting energy value tests above the transition temperature and yet, in someand fracture appearance versus temperature from tests of instances, the reverse is true below the transition temperature.notched bars at a series of temperatures. When the testtemperature has been carried low enough to start cleavage A5.6 Correlation with Servicefracture, there may be an extremely sharp drop in impact A5.6.1 While Charpy or Izod tests may not directly predictvalue or there may be a relatively gradual falling off toward the ductile or brittle behavior of steel as commonly used inthe lower temperatures. large masses or as components of large structures, these tests can be used as acceptance tests of identity for different lots ofThis drop in energy value starts when a specimen begins to the same steel or in choosing between different steels, whenexhibit some crystalline appearance in the fracture. The correlation with reliable service behavior has been stablished.transition temperature at which this embrittling effect takes It may be necessary to make the tests at properly chosenplace varies considerably with the size of the part or test temperatures other than room temperature. In this, the servicespecimen and with the notch geometry. temperature or the transition temperature of full-scale specimens does not give the desired transition temperaturesA5.4.2 Some of the many definitions of transition for Charpy or Izod tests since the size and notch geometrytemperature currently being used are: (1) the lowest may be so different.temperature at which the specimen exhibits 100 % fibrousfracture, ( 2) the temperature where the fracture shows a 50 % Chemical analysis, tension, and hardness tests may notcrystalline and a 50 % fibrous appearance, (3) the emperature indicate the influence of some of the important processingcorresponding to the energy value 50 % of the difference factors that affect susceptibility to brittle fracture nor do theybetween values obtained at 100 % and 0 % fibrous fracture, comprehend the effect of low temperatures in inducing brittleand ( 4) the temperature corresponding to a specific energy behavior.value. Pg: 14/ 25
  15. 15. www.cmmok.xinwen520.comTransitional temperature may be defined in the following ways: Lowest temerature specimens exhibit 100% fibrous. Lowest temerature specimens exhibit 50% fibrous Temperature correspond to mean of 100% fibrous and 100% crystalline. Temerature at specific impact energy.Points to ponders: On impacting there are 2 types of stresses induced; Normal and the 45 degree to the normal, Shear stress. Increase the severity of notches: The normal stress increase Increase the speed of deformation Shear strength increase Raising the temperature Lower the shear strength. Making it prone to shear failure. Shear failure lead to ductile failure absorbing more energy. Brittle failure occur when the normal stress exceed the cohesive force before the shear stress exceed the shear strength. For a sample showing ductile failure increase in size increase the amount of metal to be deform and therfore increase the amount of energy to fracture, however for a specimen on the verge of brittle fracture, increase in size may increase the restrain, by tending to induce brittle frature may decrease the amount of energy absorbed. Rationalizing with the normal stress and shear stress method. Increase the width, increase the degree of restraint and increase the shear strength, making it more prone to normal failure which is brittle. Pg: 15/ 25
  16. 16. www.cmmok.xinwen520.comInspection & witnessingWitnessing of Charpy impact testing, following are to be considered; Laboratory accreditation. Equipment calibration. Firmness or steadiness of equipment foundation. Temperature measurement calibration, Tong temperature. Time lapse between placement and striking by pendulum. Notch tip radii and notch configuration checks with shadow-graph. Specimen dimension mesurement. Code and standard reference. Sub size correlation factors Retest rules Acceptance criteria. Safety. Pg: 16/ 25
  17. 17. www.cmmok.xinwen520.com Crystalline, shining brittle fracture, absorbing little energy on impact. Fibrous, dull ductile fracture, absorbing relatively large amount of energy on impact. Pg: 17/ 25
  18. 18. www.cmmok.xinwen520.com Charpy impact testing machine. Tested samples fracture appearance. Pg: 18/ 25
  19. 19. www.cmmok.xinwen520.com Sub-size 5 x 55mm sanples The lateral expansion measurement using venier caliper Pg: 19/ 25
  20. 20. www.cmmok.xinwen520.com The ‘V’ notche of the samples were checked using calibrated shadow-graphic projection. Pg: 20/ 25
  21. 21. www.cmmok.xinwen520.com Ductile fracture Ductile fracture Pg: 21/ 25
  22. 22. www.cmmok.xinwen520.com Ductile and brittle boundary Brittle fractures Pg: 22/ 25
  23. 23. www.cmmok.xinwen520.comMicrostructural and Fractographic Characterization of a Thermally Embrittled Nuclear Grade Steel - Quenching and Tempering Pg: 23/ 25
  24. 24. www.cmmok.xinwen520.com Pg: 24/ 25
  25. 25. www.cmmok.xinwen520.com(Intergranular fracture in a nickel-chromium alloy, viewed under the scanning electron microscope. Note that the fracture takes place between the grains; and that fracture surface has a "rock candy" appearance which reveals the shapes of the individual grains.)Some Characteristics of Brittle Fracture • There is no gross, permanent deformation of the material. • The surface of the brittle fracture tends to be perpendicular to the principal tensile stress although other components of stress can be factors. • Characteristic crack advance markings frequently point to where the fracture originated. • The path the crack follows depends on the materials structure. In metals, transgranular and intergranular cleavage are important. Cleavage shows up clearly in the SEM.Some Characteristics of Ductile Fracture • There is permanent deformation at the tip of the advancing crack that leaves distinct patterns in SEM images. • As with brittle fractures, the surface of a ductile fracture tends to be perpendicular to the principal tensile stress, although other components of stress can be factors. • In ductile, crystalline metals and ceramics it is microscopically resolved shear stress that is operating to expand the tip of the crack. • The fracture surface is dull and fibrous. • There has to be a lot of energy available to extend the crack. Pg: 25/ 25

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