Tensile testing

Tensile testing

Tensile testing on a coir composite. Specimen size is not to standard (Instron).

Tensile testing, also known as tension testing,[1] is a fundamental materials science test in which a sample is subjected to a controlled tension until failure. The results from the test are commonly used to select a material for an application, for quality control, and to predict how a material will react under normal forces. Properties that are directly measured via a tensile test are ultimate tensile strength, maximum elongation and reduction in area.[2] From these measurements the following properties can also be determined: Young's modulus, Poisson's ratio, yield strength, and strain-hardening characteristics.[3] Uniaxial tensile testing is the most commonly used for obtaining the mechanical characteristics of isotropic materials. For anisotropic materials, such as composite materials and textiles, biaxial tensile testing is required.

Tensile specimen

Tensile specimens made from an aluminum alloy. The left two specimens have a round cross-section and threaded shoulders. The right two are flat specimens designed to be used with serrated grips.

A tensile specimen is a standardized sample cross-section. It has two shoulders and a gage (section) in between. The shoulders are large so they can be readily gripped, whereas the gauge section has a smaller cross-section so that the deformation and failure can occur in this area.[2][4]

The shoulders of the test specimen can be manufactured in various ways to mate to various grips in the testing machine (see the image below). Each system has advantages and disadvantages; for example, shoulders designed for serrated grips are easy and cheap to manufacture, but the alignment of the specimen is dependent on the skill of the technician. On the other hand, a pinned grip assures good alignment. Threaded shoulders and grips also assure good alignment, but the technician must know to thread each shoulder into the grip at least one diameter's length, otherwise the threads can strip before the specimen fractures.[5]

In large castings and forgings it is common to add extra material, which is designed to be removed from the casting so that test specimens can be made from it. These specimens may not be exact representation of the whole workpiece because the grain structure may be different throughout. In smaller workpieces or when critical parts of the casting must be tested, a workpiece may be sacrificed to make the test specimens.[6] For workpieces that are machined from bar stock, the test specimen can be made from the same piece as the bar stock.

Various shoulder styles for tensile specimens. Keys A through C are for round specimens, whereas keys D and E are for flat specimens. Key:

A. A Threaded shoulder for use with a threaded grip
B. A round shoulder for use with serrated grips
C. A butt end shoulder for use with a split collar
D. A flat shoulder for used with serrated grips

E. A flat shoulder with a through hole for a pinned grip
Test specimen nomenclature

The repeatability of a testing machine can be found by using special test specimens meticulously made to be as similar as possible.[6]

A standard specimen is prepared in a round or a square section along the gauge length, depending on the standard used. Both ends of the specimens should have sufficient length and a surface condition such that they are firmly gripped during testing. The initial gauge length Lo is standardized (in several countries) and varies with the diameter (Do) or the cross-sectional area (Ao) of the specimen as listed

Type specimen United States(ASTM) Britain Germany
Sheet ( Lo / √Ao) 4.5 5.65 11.3
Rod ( Lo / Do) 4.0 5.00 10.0

The following tables gives examples of test specimen dimensions and tolerances per standard ASTM E8.

Flat test specimen[7]
All values in inches Plate type (1.5 in. wide) Sheet type (0.5 in. wide) Sub-size specimen (0.25 in. wide)
Gauge length 8.00±0.01 2.00±0.005 1.000±0.003
Width 1.5 +0.125–0.25 0.500±0.010 0.250±0.005
Thickness 0.188 ≤ T 0.005 ≤ T ≤ 0.75 0.005 ≤ T ≤ 0.25
Fillet radius (min.) 1 0.25 0.25
Overall length (min.) 18 8 4
Length of reduced section (min.) 9 2.25 1.25
Length of grip section (min.) 3 2 1.25
Width of grip section (approx.) 2 0.75 38
Round test specimen[7]
All values in inches Standard specimen at nominal diameter: Small specimen at nominal diameter:
0.500 0.350 0.25 0.160 0.113
Gauge length 2.00±0.005 1.400±0.005 1.000±0.005 0.640±0.005 0.450±0.005
Diameter tolerance ±0.010 ±0.007 ±0.005 ±0.003 ±0.002
Fillet radius (min.) 38 0.25 516 532 332
Length of reduced section (min.) 2.5 1.75 1.25 0.75 58

Equipment

A universal testing machine (Hegewald & Peschke)

The most common testing machine used in tensile testing is the universal testing machine. This type of machine has two crossheads; one is adjusted for the length of the specimen and the other is driven to apply tension to the test specimen. There are two types: hydraulic powered and electromagnetically powered machines.[4]

The machine must have the proper capabilities for the test specimen being tested. There are four main parameters: force capacity, speed, precision and accuracy. Force capacity refers to the fact that the machine must be able to generate enough force to fracture the specimen. The machine must be able to apply the force quickly or slowly enough to properly mimic the actual application. Finally, the machine must be able to accurately and precisely measure the gauge length and forces applied; for instance, a large machine that is designed to measure long elongations may not work with a brittle material that experiences short elongations prior to fracturing.[5]

Alignment of the test specimen in the testing machine is critical, because if the specimen is misaligned, either at an angle or offset to one side, the machine will exert a bending force on the specimen. This is especially bad for brittle materials, because it will dramatically skew the results. This situation can be minimized by using spherical seats or U-joints between the grips and the test machine.[5] If the initial portion of the stress–strain curve is curved and not linear, it indicates the specimen is misaligned in the testing machine.[8]

The strain measurements are most commonly measured with an extensometer, but strain gauges are also frequently used on small test specimen or when Poisson's ratio is being measured.[5] Newer test machines have digital time, force, and elongation measurement systems consisting of electronic sensors connected to a data collection device (often a computer) and software to manipulate and output the data. However, analog machines continue to meet and exceed ASTM, NIST, and ASM metal tensile testing accuracy requirements, continuing to be used today.[citation needed]

Process

The test process involves placing the test specimen in the testing machine and slowly extending it until it fractures. During this process, the elongation of the gauge section is recorded against the applied force. The data is manipulated so that it is not specific to the geometry of the test sample. The elongation measurement is used to calculate the engineering strain, ε, using the following equation:[4]

where ΔL is the change in gauge length, L0 is the initial gauge length, and L is the final length. The force measurement is used to calculate the engineering stress, σ, using the following equation:[4]

where F is the tensile force and A is the nominal cross-section of the specimen. The machine does these calculations as the force increases, so that the data points can be graphed into a stress–strain curve.[4]

Standards

Metals

  • ASTM E8/E8M-13: "Standard Test Methods for Tension Testing of Metallic Materials" (2013)
  • ISO 6892-1: "Metallic materials. Tensile testing. Method of test at ambient temperature" (2009)
  • ISO 6892-2: "Metallic materials. Tensile testing. Method of test at elevated temperature" (2011)
  • JIS Z2241 Method of tensile test for metallic materials

Flexible materials

  • ASTM D638 Standard Test Method for Tensile Properties of Plastics
  • ASTM D828 Standard test method for tensile properties of paper and paperboard using constant-rate-of-elongation apparatus
  • ASTM D882 Standard test method for tensile properties of thin plastic sheeting
  • ISO 37 rubber, vulcanized or thermoplastic—determination of tensile stress–strain properties

References

  1. ^ Czichos, Horst (2006). Springer Handbook of Materials Measurement Methods. Berlin: Springer. pp. 303–304. ISBN 978-3-540-20785-6. 
  2. ^ a b Davis, Joseph R. (2004). Tensile testing (2nd ed.). ASM International. ISBN 978-0-87170-806-9. 
  3. ^ Davis 2004, p. 33.
  4. ^ a b c d e Davis 2004, p. 2.
  5. ^ a b c d Davis 2004, p. 9.
  6. ^ a b Davis 2004, p. 8.
  7. ^ a b Davis 2004, p. 52.
  8. ^ Davis 2004, p. 11.

External links

Charpy impact test

Charpy impact test

The Charpy impact test, also known as the Charpy V-notch test, is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a given material's notch toughness and acts as a tool to study temperature-dependent ductile-brittle transition. It is widely applied in industry, since it is easy to prepare and conduct and results can be obtained quickly and cheaply. A disadvantage is that some results are only comparative.[1]

The test was developed around 1900 by S.B. Russell (1898, American) and Georges Charpy (1901, French).[2] The test became known as the Charpy test in the early 1900s due to the technical contributions and standardization efforts by Charpy. The test was pivotal in understanding the fracture problems of ships during World War II.[3][4]

Today it is utilized in many industries for testing materials, for example the construction of pressure vessels and bridges to determine how storms will affect the materials used.[3][5][6]

History

In 1896 S. B. Russell introduced the idea of residual fracture energy and devised a pendulum fracture test. Russell's initial tests measured un-notched samples. In 1897 Frémont introduced a test trying to measure the same phenomenon using a spring-loaded machine. In 1901 Georges Charpy proposed a standardized method improving Russell's by introducing a redesigned pendulum, notched sample and generally giving precise specifications.[7]

Definition

A vintage impact test machine. Yellow cage on the left is meant to prevent accidents during pendulum swing, pendulum is seen at rest at the bottom

The apparatus consists of a pendulum of known mass and length that is dropped from a known height to impact a notched specimen of material. The energy transferred to the material can be inferred by comparing the difference in the height of the hammer before and after the fracture (energy absorbed by the fracture event).

The notch in the sample affects the results of the impact test,[8] thus it is necessary for the notch to be of regular dimensions and geometry. The size of the sample can also affect results, since the dimensions determine whether or not the material is in plane strain. This difference can greatly affect conclusions made.[9]

The "Standard methods for Notched Bar Impact Testing of Metallic Materials" can be found in ASTM E23,[10] ISO 148-1[11] or EN 10045-1,[12] where all the aspects of the test and equipment used are described in detail.

Quantitative results

The quantitative result of the impact tests the energy needed to fracture a material and can be used to measure the toughness of the material. There is a connection to the yield strength but it cannot be expressed by a standard formula. Also, the strain rate may be studied and analyzed for its effect on fracture.

The ductile-brittle transition temperature (DBTT) may be derived from the temperature where the energy needed to fracture the material drastically changes. However, in practice there is no sharp transition and it is difficult to obtain a precise transition temperature (it is really a transition region). An exact DBTT may be empirically derived in many ways: a specific absorbed energy, change in aspect of fracture (such as 50% of the area is cleavage), etc.[1]

Qualitative results

The qualitative results of the impact test can be used to determine the ductility of a material.[13] If the material breaks on a flat plane, the fracture was brittle, and if the material breaks with jagged edges or shear lips, then the fracture was ductile. Usually a material does not break in just one way or the other, and thus comparing the jagged to flat surface areas of the fracture will give an estimate of the percentage of ductile and brittle fracture.[1]

Sample sizes

According to ASTM A370,[14] the standard specimen size for Charpy impact testing is 10 mm × 10mm × 55mm. Subsize specimen sizes are: 10 mm × 7.5 mm × 55mm, 10 mm × 6.7 mm × 55 mm, 10 mm × 5 mm × 55 mm, 10 mm × 3.3 mm × 55 mm, 10 mm × 2.5 mm × 55 mm. Details of specimens as per ASTM A370 (Standard Test Method and Definitions for Mechanical Testing of Steel Products).

According to EN 10045-1,[12] standard specimen sizes are 10 mm × 10 mm × 55 mm. Subsize specimens are: 10 mm × 7.5 mm × 55 mm and 10 mm × 5 mm × 55 mm.

According to ISO 148,[11] standard specimen sizes are 10 mm × 10 mm × 55 mm. Subsize specimens are: 10 mm × 7.5 mm × 55 mm, 10 mm × 5 mm × 55 mm and 10 mm × 2.5 mm × 55mm.

Impact test results on low- and high-strength materials

The impact energy of low-strength metals that do not show change of fracture mode with temperature is usually high and insensitive to temperature. For these reasons, impact tests are not widely used for assessing the fracture-resistance of low-strength materials whose fracture modes remain unchanged with temperature. Impact tests typically show a ductile-brittle transition for low-strength materials that do exhibit change in fracture mode with temperature such as body-centered cubic (BCC) transition metals.

Generally high-strength materials have low impact energies which attest to the fact that fractures easily initiate and propagate in high-strength materials. The impact energies of high-strength materials other than steels or BCC transition metals are usually insensitive to temperature. High-strength BCC steels display a wider variation of impact energy than high-strength metal that do not have a BCC structure because steels undergo microscopic ductile-brittle transition. Regardless, the maximum impact energy of high-strength steels is still low due to their brittleness. [15]

See also

Notes

  1. ^ a b c Meyers Marc A; Chawla Krishan Kumar (1998). Mechanical Behaviors of Materials. Prentice Hall. ISBN 978-0-13-262817-4. 
  2. ^ Siewert
  3. ^ a b The Design and Methods of Construction of Welded Steel Merchant Vessels: Final Report of a (U.S. Navy) Board of Investigation (July 1947). "Welding Journal". 26 (7,). Welding Journal: 569. 
  4. ^ Williams, M. L. & Ellinger, G. A (1948). Investigation of Fractured Steel Plates Removed from Welded Ships. National Bureau of Standards Rep. 
  5. ^ James A Jacobs & Thomas F Kilduff (2005). Engineering Materials Technology (5th ed.). Pearson Prentice Hall. pp. 153–155. ISBN 978-0-13-048185-6. 
  6. ^ Siewert, T. A.; Manahan, M. P.; McCowan, C. N.; Holt, J. M.; Marsh, F. J. & Ruth, E. A (1999). Pendulum Impact Testing: A Century of Progress, ASTM STP 1380. ASTM. 
  7. ^ Cedric W. Richards (1968). Engineering materials science. Wadsworth Publishing Company, Inc. 
  8. ^ Kurishita H, Kayano H, Narui M, Yamazaki M, Kano Y, Shibahara I (1993). "Effects of V-notch dimensions on Charpy impact test results for differently sized miniature specimens of ferritic steel". Materials Transactions - JIM. Japan Institute of Metals. 34 (11): 1042–52. ISSN 0916-1821. 
  9. ^ Mills NJ (February 1976). "The mechanism of brittle fracture in notched impact tests on polycarbonate". Journal of Materials Science. 11 (2): 363–75. Bibcode:1976JMatS..11..363M. doi:10.1007/BF00551448. 
  10. ^ ASTM E23 Standard Test Methods for Notched Bar Impact Testing of Metallic Materials
  11. ^ a b ISO 148-1 Metallic materials - Charpy pendulum impact test - Part 1: Test method
  12. ^ a b EN 10045-1 Charpy impact test on metallic materials. Test method (V- and U-notches)
  13. ^ Mathurt KK, Needleman A, Tvergaard V (May 1994). "3D analysis of failure modes in the Charpy impact test". Modeling and Simulation in Materials Science Engineering. 2 (3A): 617–35. Bibcode:1994MSMSE...2..617M. doi:10.1088/0965-0393/2/3A/014. 
  14. ^ ASTM A370 Standard Test Methods and Definitions for Mechanical Testing of Steel Products
  15. ^ Courtney, Thomas H. (2000). Mechanical Behavior of Materials. Waveland Press, Inc. ISBN 978-1-57766-425-3. 

External links

Brinnel Test

Brinell scale

Force diagram

The Brinell scale /brəˈnɛl/ characterizes the indentation hardness of materials through the scale of penetration of an indenter, loaded on a material test-piece. It is one of several definitions of hardness in materials science.

Proposed by Swedish engineer Johan August Brinell in 1900, it was the first widely used and standardised hardness test in engineering and metallurgy. The large size of indentation and possible damage to test-piece limits its usefulness. However it also had the useful feature that the hardness value divided by two gave the approximate UTS in ksi for steels. This feature contributed to its early adoption over competing hardness tests.

The typical test uses a 10 millimetres (0.39 in) diameter steel ball as an indenter with a 3,000 kgf (29.42 kN; 6,614 lbf) force. For softer materials, a smaller force is used; for harder materials, a tungsten carbide ball is substituted for the steel ball. The indentation is measured and hardness calculated as:

where:

BHN = Brinell Hardness Number (kgf/mm2)
P = applied load in kilogram-force (kgf)
D = diameter of indenter (mm)
d = diameter of indentation (mm)

Brinell hardness is sometimes quoted in megapascals, the Brinell hardness number is multiplied by the acceleration due to gravity, 9.80665 m/s2, to convert it to megapascals. The BHN can be converted into the ultimate tensile strength (UTS), although the relationship is dependent on the material, and therefore determined empirically. The relationship is based on Meyer's index (n) from Meyer's law. If Meyer's index is less than 2.2 then the ratio of UTS to BHN is 0.36. If Meyer's index is greater than 2.2, then the ratio increases.[1]

BHN is designated by the most commonly used test standards (ASTM E10-14[2] and ISO 6506–1:2005[3]) as HBW (H from hardness, B from brinell and W from the material of the indenter, tungsten (wolfram) carbide). In former standards HB or HBS were used to refer to measurements made with steel indenters.

HBW is calculated in both standards using the SI units as

where:

F = applied load (Newtons)
D = diameter of indenter (mm)
d = diameter of indentation (mm)

Common values

When quoting a Brinell hardness number (BHN or more commonly HB), the conditions of the test used to obtain the number must be specified. The standard format for specifying tests can be seen in the example "HBW 10/3000". "HBW" means that a tungsten carbide (from the chemical symbol for tungsten or from the Swedish/German name for tungsten, "Wolfram") ball indenter was used, as opposed to "HBS", which means a hardened steel ball. The "10" is the ball diameter in millimeters. The "3000" is the force in kilograms force.

The hardness may also be shown as XXX HB YYD2. The XXX is the force to apply (in kgf) on a material of type YY (5 for aluminum alloys, 10 for copper alloys, 30 for steels). Thus a typical steel hardness could be written: 250 HB 30D2. It could be a maximum or a minimum.

Brinell hardness numbers
Material Hardness
Softwood (e.g., pine) 1.6 HBS 10/100
Hardwood 2.6–7.0 HBS 1.6 10/100
Lead 5.0 HB (pure lead; alloyed lead typically can range from 5.0 HB to values in excess of 22.0 HB)
Pure Aluminium 15 HB
Copper 35 HB
Hardened AW-6060 Aluminium 75 HB
Mild steel 120 HB
18–8 (304) stainless steel annealed 200 HB[4]
Hardox wear plate 400-700 HB
Glass 1550 HB
Hardened tool steel 600–900 HB (HBW 10/3000)
Rhenium diboride 4600 HB
Note: Standard test conditions unless otherwise stated

Standards

See also

(Multi use Hardness Test)

References

Notes

  1. ^ Tabor, p. 17.
  2. ^ ASTM E10 – 14 Standard Test Method for Brinell Hardness of Metallic Materials
  3. ^ ISO 6506–1:2005 Metallic materials – Brinell hardness test – Part 1: Test method
  4. ^ 304: the place to start, retrieved 2009-03-31 .

Bibliography

External links

Vickers hardness test

Vickers hardness test

A Vickers hardness tester

The Vickers hardness test was developed in 1921 by Robert L. Smith and George E. Sandland at Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials.[1] The Vickers test is often easier to use than other hardness tests since the required calculations are independent of the size of the indenter, and the indenter can be used for all materials irrespective of hardness. The basic principle, as with all common measures of hardness, is to observe the questioned material's ability to resist plastic deformation from a standard source. The Vickers test can be used for all metals and has one of the widest scales among hardness tests. The unit of hardness given by the test is known as the Vickers Pyramid Number (HV) or Diamond Pyramid Hardness (DPH). The hardness number can be converted into units of pascals, but should not be confused with pressure, which uses the same units. The hardness number is determined by the load over the surface area of the indentation and not the area normal to the force, and is therefore not pressure.

Implementation

Vickers test scheme
The pyramidal diamond indenter of a Vickers hardness tester.
An indentation left in case-hardened steel after a Vickers hardness test. The difference in length of both diagonals and the illumination gradient, are both classic indications of an out-of-level sample. This is not a good indentation.

It was decided that the indenter shape should be capable of producing geometrically similar impressions, irrespective of size; the impression should have well-defined points of measurement; and the indenter should have high resistance to self-deformation. A diamond in the form of a square-based pyramid satisfied these conditions. It had been established that the ideal size of a Brinell impression was ⅜ of the ball diameter. As two tangents to the circle at the ends of a chord 3d/8 long intersect at 136°, it was decided to use this as the included angle of the indenter, giving an angle to the horizontal plane of 22° on each side. The angle was varied experimentally and it was found that the hardness value obtained on a homogeneous piece of material remained constant, irrespective of load.[2] Accordingly, loads of various magnitudes are applied to a flat surface, depending on the hardness of the material to be measured. The HV number is then determined by the ratio F/A, where F is the force applied to the diamond in kilograms-force and A is the surface area of the resulting indentation in square millimeters. A can be determined by the formula.

which can be approximated by evaluating the sine term to give,

where d is the average length of the diagonal left by the indenter in millimeters. Hence,[3]

,

where F is in kgf and d is in millimeters.

The corresponding units of HV are then kilograms-force per square millimeter (kgf/mm²). To calculate Vickers hardness number using SI units one needs to convert the force applied from newtons to kilogram-force by dividing by 9.806 65 (standard gravity). This leads to the following equation:[4]

where F is in N and d is in millimeters. A common error is that the above formula to calculate the HV number does not result in a number with the unit Newton per square millimeter (N/mm2), but results directly in the Vickers hardness number (usually given without units), which is in fact kilograms-force per square millimeter (kgf/mm²).

To convert the Vickers hardness number to SI units the hardness number in kilograms-force per square millimeter (kgf/mm²) has to be multiplied with the standard gravity (9.806 65) to get the hardness in MPa (N/mm²) and furthermore divided by 1000 to get the hardness in GPa.

Vickers hardness numbers are reported as xxxHVyy, e.g. 440HV30, or xxxHVyy/zz if duration of force differs from 10 s to 15 s, e.g. 440HV30/20, where:

  • 440 is the hardness number,
  • HV gives the hardness scale (Vickers),
  • 30 indicates the load used in kgf.
  • 20 indicates the loading time if it differs from 10 s to 15 s

Vickers values are generally independent of the test force: they will come out the same for 500 gf and 50 kgf, as long as the force is at least 200 gf.[5]

For thin samples indentation depth can be an issue due to substrate effects. As a rule of thumb the sample thickness should be kept greater than 2.5 times the indent diameter. Alternatively indent depth can be calculated according to:

Examples of HV values for various materials[6]
Material Value
316L stainless steel 140HV30
347L stainless steel 180HV30
Carbon steel 55–120HV5
Iron 30–80HV5
Martensite 1000HV
Diamond 10000HV

Precautions

When doing the hardness tests the minimum distance between indentations and the distance from the indentation to the edge of the specimen must be taken into account to avoid interaction between the work-hardened regions and effects of the edge. These minimum distances are different for ISO 6507-1 and ASTM E384 standards.

Standard Distance between indentations Distance from the center of the indentation to the edge of the specimen
ISO 6507-1 > 3·d for steel and copper alloys and > 6·d for light metals 2.5·d for steel and copper alloys and > 3·d for light metals
ASTM E384 2.5·d 2.5·d

Estimating tensile strength

If HV is expressed in kg/mm2 then the yield strength (in MPa) of the material can be approximated as σu ≈ HV×c ≈ HV/0.3, where c is a constant determined by geometrical factors usually ranging between 2 and 4.[7]

Application

The fin attachment pins and sleeves in the Convair 580 airliner were specified by the aircraft manufacturer to be hardened to a Vickers Hardness specification of 390HV5, the '5' meaning five kiloponds. However, on the aircraft flying Partnair Flight 394 the pins were later found to have been replaced with sub-standard parts, leading to rapid wear and finally loss of the aircraft. On examination, accident investigators found that the sub-standard pins had a hardness value of only some 200-230HV5.[8]

Currently some watch companies are improving the crystal watch glass by testing the strength using the Vickers hardness test. While they are progressively making their watches better, they are also using it as a marketing information for the consumer.

See also

References

  1. ^ R.L. Smith & G.E. Sandland, "An Accurate Method of Determining the Hardness of Metals, with Particular Reference to Those of a High Degree of Hardness," Proceedings of the Institution of Mechanical Engineers, Vol. I, 1922, p 623–641.
  2. ^ The Vickers Hardness Testing Machine. UKcalibrations.co.uk. Retrieved on 2016-06-03.
  3. ^ ASTM E384-10e2
  4. ^ ISO 6507-1:2005(E)
  5. ^ Vickers Test. Instron website.
  6. ^ Smithells Metals Reference Book, 8th Edition, ch. 22
  7. ^ "Hardness". matter.org.uk. 
  8. ^ Report on the Convair 340/580 LN-PAA aircraft accident North of Hirtshals, Denmark on September 8, 1989 | aibn. Aibn.no. Retrieved on 2016-06-03.

Further reading

  • Meyers and Chawla (1999). "Section 3.8". Mechanical Behavior of Materials. Prentice Hall, Inc. 
  • ASTM E92: Standard method for Vickers hardness of metallic materials (Withdrawn and replaced by E384-10e2)
  • ASTM E384: Standard Test Method for Knoop and Vickers Hardness of Materials
  • ISO 6507-1: Metallic materials – Vickers hardness test – Part 1: Test method
  • ISO 6507-2: Metallic materials – Vickers hardness test – Part 2: Verification and calibration of testing machines
  • ISO 6507-3: Metallic materials – Vickers hardness test – Part 3: Calibration of reference blocks
  • ISO 6507-4: Metallic materials – Vickers hardness test – Part 4: Tables of hardness values
  • ISO 18265: Metallic materials – Conversion of Hardness Values

External links

Hardness Test

Indentation hardness

Indentation hardness tests are used in mechanical engineering to determine the hardness of a material to deformation. Several such tests exist, wherein the examined material is indented until an impression is formed; these tests can be performed on a macroscopic or microscopic scale.

When testing metals, indentation hardness correlates roughly linearly with tensile strength.,[1] but it is an imperfect correlation often limited to small ranges of strength and hardness for each indentation geometry. This relation permits economically important nondestructive testing of bulk metal deliveries with lightweight, even portable equipment, such as hand-held Rockwell hardness testers.

Material hardness

As of the direction of materials science continues towards studying the basis of properties on smaller and smaller scales, different techniques are used to quantify material characteristics and tendencies. Measuring mechanical properties for materials on smaller scales, like thin films, can not be done using conventional uniaxial tensile testing. As a result, techniques testing material "hardness" by indenting a material with an impression have been developed to determine such properties.

Hardness measurements quantify the resistance of a material to plastic deformation. Indentation hardness tests compose the majority of processes used to determine material hardness, and can be divided into two classes: microindentation and macroindentation tests. Microindentation tests typically have forces less than 2 N (0.45 lbf). Hardness, however, cannot be considered to be a fundamental material property. Instead, it represents an arbitrary quantity used to provide a relative idea of material properties.[2] As such, hardness can only offer a comparative idea of the material's resistance to plastic deformation since different hardness techniques have different scales.

The main sources of error with indentation tests are poor technique, poor calibration of the equipment, and the strain hardening effect of the process. However, it has been experimentally determined through "strainless hardness tests" that the effect is minimal with smaller indentations.[3]

Surface finish of the part and the indenter do not have an effect on the hardness measurement, as long as the indentation is large compared to the surface roughness. This proves to be useful when measuring the hardness of practical surfaces. It also is helpful when leaving a shallow indentation, because a finely etched indenter leaves a much easier to read indentation than a smooth indenter.[4]

The indentation that is left after the indenter and load are removed is known to "recover", or spring back slightly. This effect is properly known as shallowing. For spherical indenters the indentation is known to stay symmetrical and spherical, but with a larger radius. For very hard materials the radius can be three times as large as the indenter's radius. This effect is attributed to the release of elastic stresses. Because of this effect the diameter and depth of the indentation do contain errors. The error from the change in diameter is known to be only a few percent, with the error for the depth being greater.[5]

Another effect the load has on the indentation is the piling-up or sinking-in of the surrounding material. If the metal is work hardened it has a tendency to pile up and form a "crater". If the metal is annealed it will sink in around the indentation. Both of these effects add to the error of the hardness measurement.[6]

The equation based definition of hardness is the pressure applied over the contact area between the indenter and the material being tested. As a result hardness values are typically reported in units of pressure, although this is only a "true" pressure if the indenter and surface interface is perfectly flat.[citation needed]

Macroindentation tests

The term "macroindentation" is applied to tests with a larger test load, such as 1 kgf or more. There are various macroindentation tests, including:

There is, in general, no simple relationship between the results of different hardness tests. Though there are practical conversion tables for hard steels, for example, some materials show qualitatively different behaviors under the various measurement methods. The Vickers and Brinell hardness scales correlate well over a wide range, however, with Brinell only producing overestimated values at high loads.

Microindentation tests

The term "microhardness" has been widely employed in the literature to describe the hardness testing of materials with low applied loads. A more precise term is "microindentation hardness testing." In microindentation hardness testing, a diamond indenter of specific geometry is impressed into the surface of the test specimen using a known applied force (commonly called a "load" or "test load") of 1 to 1000 gf. Microindentation tests typically have forces of 2 N (roughly 200 gf) and produce indentations of about 50 μm. Due to their specificity, microhardness testing can be used to observe changes in hardness on the microscopic scale. Unfortunately, it is difficult to standardize microhardness measurements; it has been found that the microhardness of almost any material is higher than its macrohardness. Additionally, microhardness values vary with load and work-hardening effects of materials.[2] The two most commonly used microhardness tests are tests that also can be applied with heavier loads as macroindentation tests:

In microindentation testing, the hardness number is based on measurements made of the indent formed in the surface of the test specimen. The hardness number is based on the applied force divided by the surface area of the indent itself, giving hardness units in kgf/mm². Microindentation hardness testing can be done using Vickers as well as Knoop indenters. For the Vickers test, both the diagonals are measured and the average value is used to compute the Vickers pyramid number. In the Knoop test, only the longer diagonal is measured, and the Knoop hardness is calculated based on the projected area of the indent divided by the applied force, also giving test units in kgf/mm².

The Vickers microindentation test is carried out in a similar manner welling to the Vickers macroindentation tests, using the same pyramid. The Knoop test uses an elongated pyramid to indent material samples. This elongated pyramid creates a shallow impression, which is beneficial for measuring the hardness of brittle materials or thin components. Both the Knoop and Vickers indenters require prepolishing of the surface to achieve accurate results.[citation needed]

Scratch tests at low loads, such as the , performed with either 3 gf or 9 gf loads, preceded the development of microhardness testers using traditional indenters. In 1925, Smith and Sandland of the UK developed an indentation test that employed a square-based pyramidal indenter made from diamond.[7] They chose the pyramidal shape with an angle of 136° between opposite faces in order to obtain hardness numbers that would be as close as possible to Brinell hardness numbers for the specimen. The Vickers test has a great advantage of using one hardness scale to test all materials.The first reference to the Vickers indenter with low loads was made in the annual report of the National Physical Laboratory in 1932. Lips and Sack describes the first Vickers tester using low loads in 1936.[citation needed]

There is some disagreement in the literature regarding the load range applicable to microhardness testing. ASTM Specification E384, for example, states that the load range for microhardness testing is 1 to 1000 gf. For loads of 1 kgf and below, the Vickers hardness (HV) is calculated with an equation, wherein load (L) is in grams force and the mean of two diagonals (d) is in millimeters:

For any given load, the hardness increases rapidly at low diagonal lengths, with the effect becoming more pronounced as the load decreases. Thus at low loads, small measurement errors will produce large hardness deviations. Thus one should always use the highest possible load in any test. Also, in the vertical portion of the curves, small measurement errors will produce large hardness deviations.

Nanoindentation tests

See also

References

Notes

  1. ^ Correlation of Yield Strength and Tensile Strength with Hardness for Steels , E.J. Pavlina and C.J. Van Tyne, Journal of Materials Engineering and Performance, Volume 17, Number 6 / December, 2008
  2. ^ a b Meyers and Chawla (1999): "Mechanical Behavior of Materials", 162–168.
  3. ^ Tabor, p. 16.
  4. ^ Tabor, p. 14.
  5. ^ Tabor, pp. 14-15.
  6. ^ Tabor, p. 15.
  7. ^ R.L. Smith and G.E. Sandland, "An Accurate Method of Determining the Hardness of Metals, with Particular Reference to Those of a High Degree of Hardness," Proceedings of the Institution of Mechanical Engineers, Vol. I, 1922, p 623–641.

External links

Bibliography

Mechanical Testing

Mechanical testing

Mechanical testing is an umbrella term that covers a wide range of tests, which can be divided broadly into two types:

  • those that aim to determine a material's mechanical properties, independent of geometry.
  • those that determine the response of a structure to a given action, e.g. testing of composite beams, aircraft structures to destruction, etc.

Mechanical testing of materials

Tensile test. A standard specimen is subjected to an gradually increasing load (force) until failure occurs. The resultant load-displacement behaviour is used to determine a stress–strain curve, from which a number of mechanical properties can be measured.

There exists a large number of tests, many of which are standardized, to determine the various mechanical properties of materials. In general, such tests set out to obtain geometry-independent properties; i.e. those intrinsic to the bulk material. In practice this is not always feasible, since even in tensile tests, certain properties can be influenced by specimen size and/or geometry. Here is a listing of some of the most common tests:[1]

References

  1. ^ Ed. Gale, W.F.; Totemeier, T.C. (2004), Smithells Metals Reference Book (8th Edition), Elsevier

General references

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