Charpy Assay

V.E.F.

Francesc Crespo 1A2

Practice objectives

Determine:

The procedure to establish the corresponding Charpy test all specimens, trying to visualize at the time of testing physical structural changes that occur after the collision.

Calculate the values of the test. Graph Temperature/Resilience of different steels at different temperatures. Comparing the graphs and check resilience variation with temperature and electronic

structure. Comparing the results of the aluminum specimen and the aluminum with the steels. Prepare the metallographic specimen of the annealed steel in the rupture zone to

visualize the behavior of the grain shocking.

Theorical contents

This test was developed in 1900 by S.B. Russel (American) and G. Charpy (French) the name came because Georges Charpy’s efforts for standardization the test were more than his companion.This test is a destructive test because the specimen can’t be reusable.Now we go to explain the theory of this test.

This test is used for check the capacity of take energy before break of each material, normally metals, because each material can absorb certain energy. This capacity is called resilienceThis test has its theoretical basis in the potential energy and the gravity acceleration because this is the energy that the material takes, the impact energy; for this reason this test is also called impact test.

1. Charpy impact testing machine

This is the machine used for make the charpy test is a pendulum and the marked with the red label is the specimen used for make charpy test.The formulas to take the energy and the resilience are the following:

Resilience formula:

k=Ep0−EpfS

Where:k = Resilience.Ep0= is the initial potential energy.Epf= is the final potential energy.S= is the Section of the specimen (measured from the part serrated)

Potential energy formula:

Ep0=m∗g∗HEpf=m∗g∗h

Where:m = is the mass of the hammer.g = is the gravity.H= is the initial height (normally is the most height)h = is the final height (normally is less that H)

Data acquisition /Data and calculations

Previously to make the test we made the hardness test from the 6 specimens disaggregated in this way:2 from high alloy steel (one of them tempered).2 from low alloy steel (one of them tempered)2 from aluminum (one of them annealing)

The hardness test was made only on three specimens of the 6 original the data is the following:

Especimen Material Charge Hardness (HRc)

1 High alloy steel 150 Kp 95

2 Low alloy steel 150 Kp 27

3 Aluminum 60 (HRa) 33 (HRa)

1. Physical structural changes

The data extracted from this test are in the next table:

Material High alloy steel

High alloy steel (tempered)

Low alloy steel

Low alloy steel (tempered)

Aluminum

Aluminum (annealing)

Absorbed energy

27 10 2,5 9 2,9 6,5

Section 76,72 108,72 55,93

66,82 80,29 82,41

Aspect Tear, brightness low, elongation

Brightness more, less tear

Less tear, less bright

More tearmatt

tear, clear cut

More tear than the others

Resilience 0,35 0,09 0,04 0,13 0,03 0,07

2. Values of the test (data taken in class)

High al

loy Stee

l

High al

loy Stee

l (tem

pered)

Low al

loy Stee

l

Low al

loy Stee

l (tem

pered)

Aluminum

Aluminum (annea

ling)

0

20

40

60

80

100

120

HardnessSectionResilienceAbsorbed energy

1. Values of the test graph (data taken in class).

Fractures Charpy specimens tested

A steel has low carbon% ductile fracture when tested around room temperature, but it becomes brittle at low temperatures.The impact energy, also called the notch toughness, is used to quantitatively assess this ductile to brittle transition. The Charpy test is a method for measuring the energy of impact, their results are qualitative and design application no.The appearance of the fracture surface is indicative of the nature of the fracture. For ductile fracture, the fibrous surface is on the contrary fully brittle surface has a granular texture (or character cleavage). Fig.3.28 shows the fracture surface of two specimens tested Charpy and Figures 3.29 and 3.30 show in greater detail these fractures.

2. Aspect of the ductile fracture surface and a Charpy fragile.

3. Brittle fracture in Charpy specimen of low carbon steel at -190 ° C.

Brittle fracture

Brittle fractures appear bright and clear. Each crystal tends to fracture into a unique cleavage plane (which has low surface energy) level varies only slightly from one crystal to another, due to this brittle fracture generally a polycrystalline sample the light shine. The brittleness is a characteristic of the materials have a structure in the body centered cubic (BCC) and hexagonal type.

Brittle fracture is rapid and occurs without significant deformation due to rapid crack propagation. Normally occurs along specific crystallographic planes called fracture planes which are perpendicular to the applied voltage to produce a generally planar fracture surface.

4. Appearance of brittle fracture in a test tube Titanium Ti-6Al-4V.

In the case of titanium alloy specimen, it has a slot in the center as stress concentrator. This specimen was subjected to a Charpy test. She has a thin wall at one end that appears to be plastic deformation because it is not trying to join the two sides, the specimen returned to its original dimensions.

Brittle fracture has the following features:

• Very low plasticity, broken parts can come together again.• The crack is unstable, it spreads without a voltage increase, at very high (can reach

about 2000 m / s), which leads to catastrophic consequences in a piece that is in service.

• Generally the fracture surface is flat and normal to the maximum applied stress direction.

• The crack often progresses to "cleavage", breaking the links along well-defined crystallographic planes called "cleavage planes."

• The failure load is very small compared with the creep.• The fracture always starts in a stress concentration, such as a defect: porosity,

tears or cracks, corrosion damage, hydrogen embrittlement, etc.

Examples of materials that exhibit this type of fracture: low carbon steel, high-strength steels, aluminum and titanium alloys, ceramics, glass and concrete.

Macroscopic Aspects of Fracture Fragile

Most are transgranular brittle fractures, that is propagated through the grains, however, if grain boundaries are an area of weakness, it is possible that the fracture propagates as intergranular. Low temperatures and high deformations favor brittle fracture. The most important feature of the brittle fracture surface are radial marks (such as mentioned in ductile fracture); these markings extending across the surface to the vicinity of the free surfaces, which are formed tear zone (shear lips ) due to the relief of triaxial stress state.

When the piece has a dimension much smaller than the others, such as steel sheets and plates, plates, flat bars with hardened layers and regions, radial marks have a characteristic appearance in a "V" marks called "Chevron" (sergeant or gals) that point to the origin of the fracture.

A fracture may have radial markings from its inception, this happens when nucleation from a preexisting defect, such as a crack due to a thermal treatment, a lack of fusion welding, etc..

Determination of the origin of the crack

One of the most important points in the observation of the fracture surfaces is the determination of crack initiation and that when conditions allow, can be done by the following observations:

• The marks of chivaron radial or point to the origin of the crack.• When the fracture is initiated on the surface or very close to it, in the region of

fracture initiation no shear zone.• A piece fractured by impact, especially when it comes to high-alloy steels,

quenched and tempered, surface can occur in a number of brands rolling when the impact was not enough to complete the fracture, which must be reset. In the region of rolling marks appear reset converging in the direction of propagation.

Microscopic Aspects of Fracture Fragile

In most polycrystalline materials, the crack propagation corresponds to successive and repeated breakage of atomic bonds, along specific crystal planes. This process is known by the name of "cleavage". This type of fracture is said to be "transgranular" (or transcrystalline) as the grains cross fissures. This type of fracture can be seen microscopically in Fig. 3.26 b. In some alloys, the crack propagation occurs along the grain boundaries, fracture is called "intergranular" (or intercrystalline). The fig.3.27b is a photomicrograph obtained by scanning electron microscopy (SEM), which shows a typical intergranular fracture in which one can appreciate the three-dimensional nature of the grains. This type of fracture typically occurs after a process that weakens or region weakens grain boundaries. Brittle fracture has occurred in a large number of welded structures such as ships, bridges, pressure vessels and piping.

5. Characteristic appearance of brittle fracture surface, chevron lines and shear zones.

6. Examples of the appearance of a brittle fracture and a ductile.

• (a) fracture surface shows Chevron marks brittle fracture characteristics. The arrows indicate the origin of the crack.

• (b) ductile fracture. The fracture surface is rough, flat and perpendicular to the load direction, occurs in relatively thick sections of plane stress conditions. Fracture surfaces inclined at an angle of 45 ° with respect to the direction of the load. It occurs in thin sections in plane stress conditions.

7. Brittle fracture of structural steel, where you can see the origin indicated by the red arrow.

8. Brittle fracture of AISI 4340 steel screw.

Rolling marks on the fracture surface indicates that the crack was initiated at the upper edge. The shaded area containing a high concentration of calcium.

9. Intergranular brittle fracture of a low carbon steel.

Applications

10. Charpy specimen.

11. Charpy impact testing machine.

12. Toughness/Temperature graph.

13. Charpy specimen standardized dimensions.

Conclusions

This test measures the total energy absorbed during fracture of the material. The resilience is linked to the hardness property if the material is very hard the resilience of this material will be low. The conclusion is resilience is inversely to the hardness.

Bibliography

Tables

1. Physical structural changes. <Datos tomados en clase>2. Values of the test (data taken in class). <Datos tomados en clase>

Graphs

1. Values of the test graph (data taken in class). <Datos tomados en clase>

Images

1. Charpy impact testing machine. <http://eafpcisneros12.blogspot.com.es>2. Aspect of the ductile fracture surface and a Charpy fragile.

<http://www.analisisdefractura.com/fractura/fractura-fragil-2/>3. Brittle fracture in Charpy specimen of low carbon steel at -190 ° C.

<http://www.analisisdefractura.com/fractura/fractura-fragil-2/>4. Appearance of brittle fracture in a test tube Titanium Ti-6Al-4V.

<http://www.analisisdefractura.com/fractura/fractura-fragil-2/>5. Characteristic appearance of brittle fracture surface, chevron lines and shear zones.

<http://www.analisisdefractura.com/fractura/fractura-fragil-2/>6. Examples of the appearance of a brittle fracture and a ductile.

<http://www.analisisdefractura.com/fractura/fractura-fragil-2/>7. Brittle fracture of structural steel, where you can see the origin indicated by the red

arrow .<http://www.analisisdefractura.com/fractura/fractura-fragil-2/>8. Brittle fracture of AISI 4340 steel screw. <http://www.analisisdefractura.com/fractura/fractura-

fragil-2/>9. Intergranular brittle fracture of a low carbon Steel.

<http://www.analisisdefractura.com/fractura/fractura-fragil-2/>10. Charpy specimen. <Datos tomados en clase>11. Charpy impact testing machine. <http://www.finegrouptest.com/spanish/charpy-impact-testing-

machine.html/>12. Toughness/Temperature graph. <http://www.physicsforums.com/showthread.php?t=467408/>13. Charpy specimen standardized dimensions. <http://www.fundicionesgomez.com/Probeta-para-

Ensayo-de-Impacto-Charpy/>