Charpy Testing

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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


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Mechanical testing - notched bar or impact testing Notched bar or impact testing. Part I Before looking at impact testing let us first define what is meant by 'toughness' since the impact test is only one 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 bridge or 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 test piece under slow loading conditions. However, in the context of an impact test we are looking at notch toughness, a measure of the metal's resistance to brittle or fast fracture in the presence of a flaw or notch and fast loading conditions.

It was during World War II that attention was focused on this property of 'notch toughness' due to the brittle fracture of all-welded Liberty ships, then being built in the USA. From this work the science of fracture toughness developed and gave rise to a range of tests used to characterise 'notch toughness' of which the Charpy-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. The amount of energy absorbed in fracturing the test piece is measured and this gives an indication of the notch toughness of the test material.

These tests show that metals can be classified as being either 'brittle' or 'ductile'. A brittle metal will absorb a small 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 or with a requirement in a specification - they cannot be used to calculate the fracture toughness of a weld or parent metal. Tests that can be used in this way will be covered in future Job Knowledge articles. The Izod test is rarely used these days for weld testing having been replaced by the Charpy test and will not be discussed further in this article.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'. The keyhole 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.


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The standard Charpy-V specimen, illustrated in Fig.1. Is 55mm long, 10mm square and has a 2mm deep notch with a tip radius of 0.25mm machined on one face.

Fig.1. Standard Charpy-V notch specimen

To carry out the test the standard specimen is supported at its two ends on an anvil and struck on the opposite face to the notch by a pendulum as shown in Fig.2. The specimen is fractured and the pendulum swings through, 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.


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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.


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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 brittle fracture 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 a judgement 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 piece deforms before breaking, a pair of 'ears' being squeezed out on the side of the compression face of the specimen, as illustrated in Fig 4. The amount by which the specimen deforms is measured and expressed as millimeters of lateral expansion. ASME B31.3 for example requires a lateral expansion of 0.38mm for bolting materials and steels with a UTS exceeding 656N/mm 2, rather than specifying an impact value.

Fig.4 Lateral expansion

The next article in this series will look at the testing of welds, how the impact strength can be affected by composition 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 determined from carrying out a test. This next part looks at the impact testing of welds and some of the factors that affect the transition temperature such as composition and microstructure. Within such a short article, however, it will only 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 on process 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.


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Notched bar or impact testing. Part II

Many application standards therefore require impact testing to be carried out on the parent metal, the weld metal and in the heat affected zone as illustrated in Fig.1 which is taken from BS PD 5500 Annex D. The standards generally specify a minimum impact energy to be achieved at the minimum design temperature and to identify from where the specimens are to be taken. This is done in order to quantify the impact energy of the different microstructures in the weld metal and the HAZs to ensure that, as far as possible, the equipment will be operating at upper shelf temperatures where brittle fracture is not a risk.

Fig.1. PD5500 App D. location of Charpy

specimens in weld HAZ

These application standards may be supplemented by client specifications that impose additional and more stringent testing requirements, as shown in Fig.2 taken from an oil industry specification for offshore structures.

Fig.2. Offshore client requirements

The positioning of the specimens within a weld is extremely important both in terms of the specimen location and the notch orientation. A specimen positioned across the width of a multi-pass arc weld will probably include more than one weld pass and its associated HAZs. Quite a small movement in the position of the notch can therefore have a significant effect on the impact values recorded during a test. Positioning a notch precisely down 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 will be a range of microstructures from the fusion line to the unaffected parent metal. Many welds also use a 'V' preparation as illustrated above and this, coupled with the narrow HAZ, means that a single notch may sample all of these structures. If the impact properties of specific areas in the HAZ need to be determined then a 'K' or single bevel preparation may be used.

The standard specimen is 10mm x 10mm square - when a weld joint is thicker than 10mm the machining of a standard size specimen is possible. When the thickness is less than this and impact testing is required it becomes 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 x 5mm specimen does not have half the notch toughness of the full size test piece. As the thickness decreases


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the transition temperature also decreases, as does the upper shelf value, illustrated in Fig.3 and this is recognised 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 where grain growth has taken place.

Coarse grains generally have low notch toughness, one reason why heat input needs to be controlled to low levels 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 a procedure qualification test piece and places limits on any increase in heat input. Certain steels may also have an area some distance from the fusion line that may be embrittled so some specifications require impact tests at 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 specimens are taken parallel or transverse to the rolling direction. Specimens taken parallel to the rolling direction test the metal across the 'grain' of the steel and have higher notch toughness than the transverse specimens - one reason why pressure vessel plates are rolled into cylinders with the rolling direction oriented in the hoop direction.

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 the transition 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 notch toughness than the bulk of the weld as it has a larger amount of parent metal melted into it - most parent metals have higher carbon content than the filler metal and the root pass therefore has a higher carbon content than the bulk of the weld.

Sulphur and phosphorus are two other elements that both reduce notch toughness, one reason why steel producers have been working hard to reduce these elements to as low a level as possible. It is not uncommon for a good quality modern steel to have a sulphur content less than 0.005%.


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Of the beneficial elements, manganese and nickel are possibly the two most significant, the nickel alloy steels forming 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 grain size.

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 are concerned 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 notch being the most important feature. A blunted milling cutter or broach will give a rounded notch tip and this in turn will give a false, high impact value. Checking the tip radius on a shadowgraph is one simple way of ensuring the correct tip shape. Correct positioning of the specimen on the anvil is most important and this can be done using a specially designed former.

The last point concerns the testing of specimens at temperatures other than at room temperature. When testing 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 seconds otherwise the test piece temperature will rise making the test invalid - referring back to the impact energy vs temperature curve in the previous article will show why.

Relevant Specifications

BS 131 Part 4 Calibration of Impact Testing Machines for metals. BS 131 Part 5 Determination of Crystallinity BS 131 Part 6 Method for Precision Determination of Charpy-V Impact Energy BS 131 Part 7 Specification for Verification of Precision Test Machines EN 875 Destructive Tests on Welds in Metallic Materials - Impact Tests EN 10045 Part 1 Test Method EN 10045 Part 2 Verification of Impact Testing Machines ASTM 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. Notch toughness in the context of impact testing is the measurement of energy requiring to fracture a specimen with a flaw at fast loading.


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Ductile vs. Brittle Behavior—Body-centered-cubic or ferritic

alloys exhibit a significant transition in behavior when impact

tested over a range of temperatures. At temperatures above

transition, impact specimens fracture by a ductile (usually

microvoid coalescence) mechanism, absorbing relatively

large amounts of energy. At lower temperatures, they

fracture in a brittle (usually cleavage) manner absorbing less

energy. Within the transition range, the fracture will

generally

be a mixture of areas of ductile fracture and brittle fracture.

The temperature range of the transition from one type

of behavior to the other varies according to the material being tested. This transition behavior may be defined in

various 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 of

the transition temperature at which either the absorbed

energy or fracture appearance attains a specified level

when testing is performed over a range of temperatures.

Fracture Mechanisms

A fracture surface should be treated as a record of the history of a component failure. Detailed within the fracture

surface is evidence of loading history, environmental conditions and material quality.1 For the purposes of this

resource, fracture surfaces are classified as brittle, ductile and fatigue although some specimens may clearly fit

into a number of fracture categories. For example a fatigue failure fracture surface may exhibit a ductile final

fracture region. This will be taken into account and the fracture image will appear in all of the categories where it

is relevant.

Brittle Fracture

Brittle fractures have no plastic deformation and are usually characterised by a lack of necking with smooth/shiny

facets (as shown below), an appearance associated with fast crack growth.4


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SEM image of brittle fracture in plain carbon steel

A brittle failure mode such as cleavage or intergranular (fracture along the grain boundaries) is seen. On a

macroscopic level, chevron or radial markings may be observed as shown below.

Radiating Ridge Fracture Surface (Callister, W.D., page 186)

Ductile Fracture

Conversely ductile fractures can be characterised by necking of the material due to plastic deformation. A

fibrous/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 rupture

which can be seen below. Failure at the edges of the sample occurs at 45º to the loading direction due to the

maximum shear stress being at 45º to the loading stress.

SEM image of ductile fracture in plain carbon steel


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Ductile tensile failure begins with uniform plastic deformation leading to localised microvoid coalescence and then

dimple rupture in the necked region which experiences a tri-axial stress state on formation of the neck. Dimple

rupture leaves pits and holes on the surface structure.

Factors affecting the failure mode are:

• Material Microstructure.

• Temperature.

• Strain Rate.

• Environment (leading to corrosion).


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Extract from ASTM 370 A5: A5. NOTES ON SIGNIFICANCE OF NOTCHED-BAR IMPACT TESTING A5.1 Notch Behavior A5.1.1 The Charpy and Izod type tests bring out notch behavior (brittleness versus ductility) by applying a single overload of stress. The energy values determined are quantitative comparisons on a selected specimen but cannot be converted into energy values that would serve for engineering design calculations. The notch behavior indicated in an individual test applies only to the specimen size, notch geometry, and testing conditions involved and cannot be generalized to other sizes of specimens and conditions. A5.1.2 The notch behavior of the face-centered cubic metals and alloys, a large group of nonferrous materials and the austenitic steels can be judged from their common tensile properties. If they are brittle in tension they will be brittle when notched, while if they are ductile in tension, they will be ductile when notched, except for unusually sharp or deep notches (much more severe than the standard Charpy or Izod specimens). Even low temperatures do not alter this characteristic of these materials. In contrast, the behavior of the ferritic steels under notch conditions cannot be predicted from their properties as revealed by the tension test. For the study of these materials the Charpy and Izod type tests are accordingly very useful. Some metals that display normal ductility in the tension test may nevertheless break in brittle fashion when tested or when used in the notched condition. Notched conditions include restraints to deformation in directions perpendicular to the major stress, or multiaxial stresses, and stress concentrations. It is in this field that the Charpy and Izod tests prove useful for determining the suceptibility of a steel to notchbrittle behavior though they cannot be directly used to appraise the serviceability of a structure. A5.1.3 The testing machine itself must be sufficiently rigid or tests on high-strength low-energy materials will result in excessive elastic energy losses either upward through the pendulum shaft or downward through the base of the machine. If the anvil supports, the pendulum striking edge, or the machine foundation bolts are not securely fastened, tests on ductile materials in the range of 80 ft·lbf (108 J) may actuallyindicate values in excess of 90 to 100 ft·lbf (122 to 136 J).

A5.2 Notch Effect A5.2.1 The notch results in a combination of multiaxial stresses associated with restraints to deformation in directionsperpendicular to the major stress, and a stress concentration atthe base of the notch. A severely notched condition is generally not desirable, and it becomes of real concern in those cases in which it initiates a sudden and complete failure of the brittle type. Some metals can be deformed in a ductile manner even down to the low temperatures of liquid air, while others may crack. This difference in behavior can be best understood by considering the cohesive strength of a material (or the property that holds it together) and its relation to the yield point. In cases of brittle fracture, the cohesive strength is exceeded before significant plastic deformation occurs and the fracture appears crystalline. In cases of the ductile or shear type of failure, considerable deformation precedes the final fracture and the broken surface appears fibrous instead of crystalline. In intermediate cases the fracture comes after a moderate amount of deformation and is part crystalline and part fibrous in appearance. A5.2.2 When a notched bar is loaded, there is a normal stress across the base of the notch which tends to initiate fracture. The property that keeps it from cleaving, or holds it together, is the “cohesive strength.” The bar fractures when the normal stress exceeds the cohesive strength. When this occurs without the bar deforming it is the condition for brittle fracture. A5.2.3 In testing, though not in service because of side effects, it happens more commonly that plastic deformation precedes fracture. In addition to the normal stress, the appliedload also sets up shear stresses which are about 45° to the normal stress. The elastic behavior terminates as soon as the shear stress exceeds the shear strength of the material and deformation or plastic yielding sets in. This is the condition for ductile failure. A5.2.4 This behavior, whether brittle or ductile, depends on whether the normal stress exceeds the cohesive strength before the shear stress exceeds the shear strength. Several important facts of notch behavior follow from this. If the notch is made sharper or more drastic, the normal stress at the root of the notch will be increased in relation to the shear 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.


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A5.2.5 Variations in notch dimensions will seriously affect the results of the tests. Tests on E 4340 steel specimens9 haveshown the effect of dimensional variations on Charpy results (see Table A5.1). A5.3 Size Effect A5.3.1 Increasing either the width or the depth of the specimen tends to increase the volume of metal subject to distortion, and by this factor tends to increase the energy absorption when breaking the specimen. However, any increase in size, particularly in width, also tends to increase the degree of restraint and by tending to induce brittle fracture, may decrease the amount of energy absorbed. Where a standard-size specimen is on the verge of brittle fracture, this is particularly true, and a double-width specimen may actually require less energy for rupture than one of standard width. A5.3.2 In studies of such effects where the size of the material precludes the use of the standard specimen, as for example when the material is 1⁄4-in. plate, subsize specimens are necessarily used. Such specimens (see Fig. 6 of Test Methods E 23) are based on the Type A specimen of Fig. 4 ofTest Methods E 23. A5.3.3 General correlation between the energy values obtained with specimens of different size or shape is not feasible, but limited correlations may be established for specification purposes on the basis of special studies of particular materials and particular specimens. On the other hand, in a study of the relative effect of process variations, evaluation by use of some arbitrarily selected specimen with some chosen notch will in most instances place the methods in their proper order. A5.4 Effects of Testing Conditions A5.4.1 The testing conditions also affect the notch behavior. So pronounced is the effect of temperature on the behavior ofsteel when notched that comparisons are frequently made by examining specimen fractures and by plotting energy value and fracture appearance versus temperature from tests of notched bars at a series of temperatures. When the test temperature has been carried low enough to start cleavage fracture, there may be an extremely sharp drop in impact value or there may be a relatively gradual falling off toward the lower temperatures. This drop in energy value starts when a specimen begins to exhibit some crystalline appearance in the fracture. The transition temperature at which this embrittling effect takes place varies considerably with the size of the part or test specimen and with the notch geometry. A5.4.2 Some of the many definitions of transition temperature currently being used are: (1) the lowest temperature at which the specimen exhibits 100 % fibrous fracture, ( 2) the temperature where the fracture shows a 50 % crystalline and a 50 % fibrous appearance, (3) the emperature corresponding to the energy value 50 % of the difference between values obtained at 100 % and 0 % fibrous fracture, and ( 4) the temperature corresponding to a specific energy value.

A5.4.3 A problem peculiar to Charpy-type tests occurs when high-strength, low-energy specimens are tested at low temperatures. These specimens may not leave the machine in the direction of the pendulum swing but rather in a sidewise direction. To ensure that the broken halves of the specimens do not rebound off some component of the machine and contact the pendulum before it completes its swing, modifications may be necessary in older model machines. These modifications differ with machine design. Nevertheless the basic problem is the same in that provisions must be made to prevent rebounding of the fractured specimens into any part of the swinging pendulum. Where design permits, the broken specimens may be deflected out of the sides of the machine and yet in other designs it may be necessary to contain the broken specimens within a certain area until the pendulum passes through the anvils. Some low-energy high-strength steel specimens leave impact machines at speeds in excess of 50 ft (15.3 m)/s although they were struck by a pendulum traveling at speeds approximately 17 ft (5.2 m)/s. If the force exerted on the pendulum by the broken specimens is sufficient, the pendulum will slow down and erroneously high energy values will be recorded. This problem accounts for many of the inconsistencies in Charpy results reported by various investigators within the 10 to 25-ft·lbf (14 to 34 J) range. The Apparatus Section (the paragraph regarding Specimen Clearance) of Test Methods E 23 discusses the two basic machine designs and a modification found to be satisfactory in minimizing jamming. A5.5 Velocity of Straining A5.5.1 Velocity of straining is likewise a variable that affects the notch behavior of steel. The impact test shows somewhat higher energy absorption values than the static tests above the transition temperature and yet, in some instances, the reverse is true below the transition temperature. A5.6 Correlation with Service A5.6.1 While Charpy or Izod tests may not directly predict the ductile or brittle behavior of steel as commonly used in large masses or as components of large structures, these tests can be used as acceptance tests of identity for different lots ofthe same steel or in choosing between different steels, when correlation with reliable service behavior has been stablished. It may be necessary to make the tests at properly chosen temperatures other than room temperature. In this, the service temperature or the transition temperature of full-scale specimens does not give the desired transition temperatures for Charpy or Izod tests since the size and notch geometry may be so different. Chemical analysis, tension, and hardness tests may not indicate the influence of some of the important processing factors that affect susceptibility to brittle fracture nor do they comprehend the effect of low temperatures in inducing brittle behavior.


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Transitional 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.


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Inspection & witnessing Witnessing 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.


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Crystalline, shining brittle fracture, absorbing little energy on impact.

Fibrous, dull ductile fracture, absorbing relatively large amount of energy on impact.


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Charpy impact testing machine.

Tested samples fracture appearance.


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Sub-size 5 x 55mm sanples

The lateral expansion measurement using venier caliper


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The ‘V’ notche of the samples were checked using calibrated shadow-graphic projection.


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Ductile fracture

Ductile fracture


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Ductile and brittle boundary

Brittle fractures


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Microstructural and Fractographic Characterization of a Thermally Embrittled Nuclear Grade

Steel - Quenching and Tempering


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(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 material's 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.

Charpy Testing

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