brittle fracture and the notched-bar impact test 1 · various types of notched-bar impact tests are...

Mechanical Engineering Department Materials Engineering 1 Laboratory

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Brittle Fracture and the Notched-Bar Impact Test 1

Content 1 Fracture .................................................................................................................................................... 2

1.1 The Brittle-failure Problem ............................................................................................................... 2

1.2 Types of Fracture in Metals .............................................................................................................. 3

2 The Notched-Bar Impact Test .................................................................................................................. 4

3 Impact Test Procedure ............................................................................................................................. 8

Appendix A: Theoretical Cohesive Strength of Metals ............................................................................ 10

Appendix B: Specimen Orientation for Laminated Materials ................................................................. 11

1 Adapted from: G.E. Dieter. Mechanical Metallurgy. 3rd ed., Mc Graw‐Hill Book Co., New York 1986; and other sources.


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1 Fracture

1.1 The Brittle Failure Problem

During World War II a great deal of attention was directed to the brittle failure of welded Liberty ships and T-2 tankers. Some of these ships broke completely in two, while, in other instances, the fracture did not completely disable the ship. Most of the failures occurred during the winter months. Failures occurred both when the ships were in heavy seas and when they were anchored at dock (figure 1 a) and b)). These calamities focused attention on the fact that normally ductile mild steel can become brittle under certain conditions. A broad research program was undertaken to find the causes of these failures and to prescribe the remedies for their future prevention. In addition to research designed to find answers to a pressing problem, other research was aimed at gaining a better understanding of the mechanism of brittle fracture and fracture in general. While the brittle failure of ships concentrated great attention on brittle failure in mild steel, it is important to understand that this is not the only application where brittle fracture is a problem (figure 2). Brittle failures in tanks, pressure vessels, pipelines, and bridges have been documented as far back as the year 1886.

Figure 1 a): The S.S. Schenectady broke in half Figure 1 b): Ship breaks in two in front while moored at the dock2. of the Yemen coast3.

Fracture is the separation, or fragmentation, of a solid body into two or more parts under the action of stress (see Appendix A). The process of fracture can be considered to be made up of two components, crack initiation and crack propagation. Fractures can be classified into two general categories: ductile fracture, and b) brittle fracture. A ductile fracture is characterized by appreciable plastic deformation prior to and during the propagation of the crack. An appreciable amount of gross deformation is usually present at the fracture

Figure 2: Brittle cracking surfaces. Brittle fracture in metals is characterized by a is of a wide flange beam4 rapid rate of crack propagation, with no gross deforma-

2 Off Beat Oregon, June 19, 2016; https://offbeatoregon.com/1606c.schenectady-cracked-ship-396.html 3 TomoNews US, July 19, 2013; https://www.youtube.com/watch?v=SR1xNHWlEeM 4 CivilDigital.com; https://civildigital.com/brittle-fracture-steel-brittle-fracture-ductile-material/


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tion and very little micro-deformation, it is similar to cleavage in ionic crystals (ceramics).

The tendency for brittle fracture is increased with decreasing temperature, increasing strain rate, and triaxial stress conditions (usually produced by a notch). Brittle fracture is to be avoided at all cost, because it occurs without warning and usually produces disastrous consequences.

Although fracture occurs in characteristic ways, depending on the state of stress, the rate of application of stress, and the temperature, it will be assumed, unless otherwise stated, that fracture is produced by a single application of a uniaxial tensile stress.

Since the ship failures occurred primarily in structures of welded construction, it was considered for a time that this method of fabrication was not suitable for service where brittle fracture might be encountered. A great deal of research has since demonstrated that welding, per se, is not inferior in this respect to other types of construction. However, strict quality control is needed to prevent weld defects which can act as stress raisers or notches. New electrodes have been developed that make it possible to make a weld with better properties than the mild-steel plate.

The design of a welded structure is more critical than the design of an equivalent bolted or riveted structure, and much effort has gone into the development of safe designs for welded structures. It is important to eliminate all stress raisers and to avoid making the structure too rigid. To this end, riveted sections, known as crack arresters, were incorporated in some of the wartime ships so that, if a brittle failure did occur, it would not propagate completely through the structure.

1.2 Types of Fracture in Metals

Metals can exhibit many different types of fracture, depending on the material, temperature, state of stress, and rate of loading. The two broad categories of ductile and brittle fracture have already been considered. Figure 3 schematically illustrates some of the types of tensile fractures which can occur in metals. A brittle fracture (figure 3-a) is characterized by separation normal to the tensile stress. Outwardly there is no evidence of deformation, although with X-ray diffraction analysis it is possible to detect a thin layer of deformed metal at the fracture surface. Brittle fractures have been observed in bcc and hcp metals, but not in fcc metals unless there are factors contributing to grain-boundary embrittlement.

Ductile fractures can take several forms. Single crystals of hcp metals may slip on successive basal planes until finally the crystal separates by shear (figure 3-b). Polycrystalline specimens of very ductile metals, like gold or lead, may actually be drawn down to a point before they rupture (figure 3-c). In the tensile fracture of moderately ductile metals the plastic deformation eventually produces a necked region (figure 3-d). Fracture begins at the center of the specimen and then extends by a shear separation along the dashed lines in figure 3-d. This results in the familiar "cup-and-cone" fracture.

Figure 3: Types of fractures observed in metals subjected to uniaxial tension,

(a) Brittle fracture of single crystals and polycrystals; (b) shearing fracture in ductile single crystals; (c) completely ductile fracture in polycrystals;

(d) ductile fracture in polycrystals. (a)

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Fractures are classified with respect to several characteristics, such as strain to fracture, crystallographic mode of fracture, and the appearance of the fracture. The terms commonly used to describe fracture in metals can be summarized as in table 1.

A shear fracture occurs due to extensive slip result of high resolved shear stresses on the active slip plane. The cleavage mode of fracture is controlled by tensile stresses acting normal to a crystallographic cleavage plane. A fracture surface which is caused by shear appears at low magnification to be gray and fibrous, while a cleavage fracture appears bright or granular, owing to reflection of light from the flat cleavage surfaces. Fracture surfaces frequently consist of a

mixture of fibrous and granular fracture, and it is customary to report the percentage of the surface area represented by one of these categories. Based on metallographic examination, fractures in polycrystalline samples are classified as either transgranular (the crack propagates through the grains) or intergranular (the crack propagates along the grain boundaries).

A ductile fracture is one which exhibits a considerable degree of deformation. The boundary between a ductile and brittle fracture is arbitrary and depends on the situation being considered. For example, nodular cast iron is ductile when compared with ordinary gray iron; yet it would be considered brittle when compared with mild steel. As a further example, a deeply notched tensile specimen will exhibit little gross deformation; yet the fracture could occur by a shear mode.

2 The Notched-Bar Impact Test Three basic factors contribute to a brittle-cleavage type of fracture. They are (1) a triaxial state of stress, (2) a low temperature, and (3) a high strain rate or rapid rate of loading. All three of these factors do not have to be present at the same time to produce brittle fracture. A triaxial state of stress, such as exists at a notch, and low temperature are responsible for most service failures of the brittle type. However, since these effects are accentuated at a high rate of loading, many types of impact tests have been used to determine the susceptibility of materials to brittle fracture. Steels which have identical properties when tested in tension or torsion at slow strain rates can show pronounced differences in their tendency for brittle fracture when tested in a notched-impact test.

However, there are certain disadvantages to this type of test, so that much work has been devoted to the development of additional tests for defining the tendency for brittle fracture, and much effort has been expended in correlating the results of different brittle-fracture tests.

Various types of notched-bar impact tests are used to determine the tendency of a material to behave in a brittle manner. This type of test will detect differences between materials which are not observable in a tension test. The results obtained from notched-bar tests are not readily expressed in terms of design requirements, since it is not possible to measure the components of the triaxial stress condition at the notch. Furthermore, there is no general agreement on the interpretation or significance of results obtained with this type of test.

A large number of notched-bar test specimens of different design have been used by investigators of the brittle fracture of metals. Two classes of specimens have been standardized for notched-impact testing. Charpy bar specimens are used most commonly in the United States, while the Izod specimen is favored in Great Britain (see figure 4).

Table 1: Commonly used terms to describe fracture in metals.

Behavior Terms Used

Strain to fracture Ductile Brittle

Mode Shear Cleavage

Appearance Fibrous Granular


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The Charpy specimen has a square cross section and contains a notch at the center of its length. Either a V notch or a keyhole notch is used. The Charpy specimen is supported as a beam in a horizontal position. The load is applied by the impact of a heavy swinging pendulum at approximately 16 ft/s (5 m/s) impact velocity applied at the mid-span of the beam on the side opposite from the notch. The specimen is forced to bend and fracture at a strain rate on the order of 103 1/s.

Figure 4: Sketch of the Charpy Figure 5: Stress distribution produced and Izod impact test configurations. in notched cylinder under uniaxial loading, σL = longitudinal stress; σT = transverse stress; σR = radial stress.

The Izod specimen is either circular or square in cross section and contains a V notch near one end. The specimen is clamped vertically at one end like a cantilever beam and is struck with the pendulum at the opposite end. Figure 4 illustrates the type of loading used with these tests. Note that in each case the notch is subjected to a tensile stress as the specimen is bent by the moving pendulum. Plastic constraint at the notch produces a triaxial state of stress similar to that shown in figure 5. The relative values of the three principal stresses depend strongly on the dimensions of the bar and the details of the notch. For this reason it is important to use standard specimens. The value of the transverse stress at the base of the notch depends chiefly on the relationship between the width of the bar and the notch radius. The wider the bar in relation to the radius of the notch, the greater the transverse stress.

The response of a specimen to the impact test is usually measured by the energy absorbed in fracturing the specimen. For metals this is usually expressed in foot-pounds (N-m) and is read directly from a calibrated dial on the impact tester. In Europe impact results are frequently expressed in energy absorbed per unit cross-sectional area of the specimen. Very often a measure of ductility, such as the per cent contraction at the notch, is used to supplement this information. It is also important to examine the fracture surface to determine whether it is fibrous (shear failure) or granular (cleavage fracture). Figure 6 illustrates the appearance of these two types of fractures. Note the gradual decrease in the granular region and increase of lateral contraction at the notch with increasing temperature.

The notched-bar impact test is most meaningful when conducted over a range of temperature so that the temperature at which the ductile-to brittle transition takes place can be determined. Figure 7 illustrates the type of curves which are obtained. Note that the energy absorbed decreases with decreasing temperature but that for most cases the decrease does not occur sharply at a certain temperature. This makes it difficult to determine accurately the transition temperature. In selecting a material from the standpoint of notch toughness or tendency for brittle failure the important factor is the transition temperature.

Figure 7 illustrates how reliance on impact resistance at only one temperature can be misleading. Steel A shows higher notch toughness at room temperature; yet its transition temperature is higher than that of steel B. Typically, the material with the lowest transition temperature is to be preferred.

Impact Load

Charpy V-Notched Sample

Impact Load

IzodV-Notched Sample

σRσTσL

σextσext


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Figure 6: Fracture surface of different Charpy Figure 7: schematic of the transition samples tested at different temperatures. temperature curves for 2 different mild steels.

Figure 8: Inherent variability of the impact energy as a function of sample configuration.

Notched-bar impact tests are subject to considerable scatter, particularly in the region of the transition temperature. Most of this scatter is due to local variations in the properties of the steel, while some is due to difficulties in preparing perfectly reproducible notches. Both notch shape and depth are critical variables, as is the proper placement of the specimen in the impact machine.

The shape of the transition curve depends on the type of test. As can be seen in figure 8, keyhole Charpy specimens usually give a sharper breaking curve than V-notch Charpy specimens. For a tough steel V-notch Charpy specimens generally give somewhat higher values than keyhole specimens. The transition temperature for a given steel will be different for different-shaped specimens and for different types of loading with different states of stress.

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Charpy Keyhole and V Notched SamplesAISI-SAE 1018 Low Carbon Steel

YellowBrass

Al 6061 T6

CR 1018 Steel

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CR 1045 Steel


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Because the transition temperature is not sharply defined, it is important to understand the criteria which have been adopted for its definition. The most suitable criterion for selecting the transition temperature is whether or not it correlates with service performance. In general, criteria for determining the transition temperature are based on a transition in energy absorbed, change in the appearance of the fracture, or a transition in the ductility, as measured by the contraction at the root of the notch. Figure 9 shows that the same type of curve is obtained for each criterion. The energy transition temperature for V-notch Charpy specimens is frequently set at a level of 10 or 15 ft-lb (14-20 N-m). Where the fracture appearance changes gradually from shear through mixtures of shear and cleavage to complete cleavage, with decreasing temperature, the transition temperature is frequently selected to correspond to a temperature where 50% fibrous (shear) fracture is obtained. Moreover, the ductility transition temperature is sometimes arbitrarily set at 1% lateral expansion at the notch. One characteristic of these criteria is that a transition temperature based on fracture appearance always occurs at a higher temperature than if based on a ductility or energy criterion.

Figure 9: Transition-temperature curves based on absorbed energy, fracture appearance,

and notch ductility for Charpy V Notched Samples.

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55⁰ C


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3 Impact Test Procedure The Charpy pendulum impact test is VERY DANGEROUS. The safety measures in the operation of the machine must be strictly adhered to. The following points should be observed:

• Always operate the machine between 2 persons (never alone). One person manipulates the specimens and the other operates the impact machine.

• Always place the protection guard in the correct position. • Always warn before releasing the pendulum with the established protocol; that is, the operator of

the pendulum says aloud: - all hands out!, and never operate the pendulum pin lever until all the others present clearly answer: - hands out!

• Always use gloves and tongs to manipulate the specimens; even those at room temperature. • Never touch cold media (dry ice: solid CO₂) or hot media (muffles and ovens). • Samples have to measured in advance, before placing them into the temperature conditioning

media.

1) Conditioning of the specimens at a temperature below ambient: a. Add enough alcohol to the conditioner container so that the test specimens are covered

with more than 25 mm of liquid. b. Place the specimens, in batches of at least 3, and the tongs in the container for cooling. c. Add ice (water ice or solid CO₂) to the external container to achieve the required

temperature. d. Wait for the specimens and the tongs to reach the test temperature, once this is

achieved, maintain the temperature for at least 5 minutes. Use agitation in the bath to keep the liquid temperature homogeneous.

e. Maintain the desired temperature in the bath with a tolerance of ± 1⁰C (± 2⁰F). 2) Conditioning of the specimens at a temperature above ambient:

a. Preheat a laboratory muffle to the desired temperature. b. Place the specimens, in batches of at least 3, and the tongs in the muffle for heating. c. Adjust the temperature of the oven to achieve the required temperature of the

specimens. d. Wait for the specimens and the tongs to reach the test temperature, once this is

achieved, maintain the temperature for at least 5 minutes. e. Maintain the desired temperature in the muffle with a tolerance of ± 1⁰C (± 2⁰F).

3) Test execution: a. The machine operator:

i. Verify that the anvil is free of debris and releases the machine break. ii. Rises the pendulum and secures it in its ratchet.

iii. Moves the energy pointer to its starting position (the maximum capacity of the machine).

iv. Positions himself behind the impact tester. b. The sample manipulator:

i. Take a sample from the temperature conditioning device using the tongs to maintain temperature as much as possible, and place it on the anvil of the machine.

ii. Stay positions in front of the impact tesyter. iii. It is important to monitor the following points:

a) The maximum time between the extraction and the correct placement of the specimen in the anvil must not exceed 5 s.


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b) The notch on the specimen should be centered and "look" in favor of the direction of the impact.

iv. Measure the sample temperature with the digital laser thermometer. c. The machine operator:

i. Following the established protocol, the operator of the pendulum, how is alredy positions behind the impact tester, says aloud: - all hands out!,

ii. Waits until all the others present clearly answer: - hands out! iii. Only then he release the pendulum using the ratchet lever iv. The pendulum drops and hits the specimen.

d. The sample manipulator: i. When the pendulum starts its descent after impacting the specimen, applies the

brake of the machine. ii. Takes the reading of the machine indicator.

iii. Records temperature and energy reading in the Charpy Impact Test Record. iv. Collects the sample pieces either from the cage or the anvil of the machine, and

places them on the measurement table. v. Takes the measurement of the flat shear area of the fracture surface by means

of a Vernier caliper. vi. Takes the measurement of the lateral expansion by means of a Vernier caliper.

vii. Records the readings in the Charpy Impact Test Record. viii. Identifies and sets the samples aside for later reference.


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Appendix A: Theoretical Cohesive Strength of Metals

Metals are of great technological value, primarily because of their high strength combined with a certain measure of plasticity. In the most basic terms the strength is due to the cohesive forces between atoms.

Figure A.1: Cohesive force as a function of the separation between atoms.

In general, high cohesive forces are related to large elastic constants, high melting points, and small coefficients of thermal expansion. Figure A.1 shows the variation of the cohesive force between two atoms as a function of the separation between them. This curve is the resultant of the attractive and repulsive forces between the atoms. The interatomic spacing of the atoms in the unstrained condition is indicated by a0. If the crystal is subjected to a tensile load, the separation between atoms will be increased. The repulsive force decreases more rapidly with increased separation than the attractive force, so that a net force between atoms balances the tensile load. As the tensile load is increased still further, the repulsive force continues to decrease. A point is reached where the repulsive force is negligible and the attractive force is decreasing5 because of the increased separation of the atoms. This corresponds to the maximum in the curve, which is equal to the theoretical cohesive strength of the material.

5 Attractive force between atoms is primarily electrostatic, hence it decreases with the square of the distance.

2

0amax

Co

hes

ive

Forc

e

Separation between atoms


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Appendix B: Specimen Orientation for Laminated Materials6

The designation of a specific sample consists of 2 elements: first the axis of the sample, and then the direction of the notch. These directions are determined according to the direction in which the crack that fractures the sample will propagate.

1. Designation of Specimen Axis: 1.1 The L-axis is coincident with the main direction of grain flow due to processing. This axis is

usually referred to as the longitudinal direction (see figures B.1, B.2, and B.3). 1.2 The S-axis is coincident with the direction of the main working force. This axis is usually

referred to as the short-transverse-direction. 1.3 The T-axis is normal to the L- and S-axis. This axis is usually referred to as the transverse

direction. 1.4 Specimens parallel to the surface of wrought products, processed with the same degree of

homogenous deformation along the L- and T axis may be called T specimens. 1.5 Specimens normal to the uniform grain flow of wrought products (or grain growth in cast

products), whose grain flow is exclusively in one direction, so that T- and S specimens are equivalent, may be called S specimens.

2. Designation of Notch Orientation: 2.1 The notch orientation is designated by the direction in which fracture propagates. This

letter is separated from the specimen-axis designation by a hyphen. In unique cases (figure B.3), when fracture propagates across 2 planes, 2 letters are required to designate notch orientation.

Radial Grain Flow Axial Grain Flow

Figure B.1: Fracture planes along Figure B.2: Fracture planes principal axes for prismatic sections. for cylindrical sections.

Figure B.3: Fracture Planes not Along Principal Axes for prismatic sections.

6 Adapted from ASTM E 23 – 07: Standard Test Methods for Notched Bar Impact Testing of Metallic Materials.

brittle fracture and the notched-bar impact test 1 · various types of notched-bar impact tests are...

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