Mechanical Properties of Materials Under Static Loads

1. Compression test

1.1 Overview

The compression test is a test to determine the mechanical properties of materials under the action of axial static pressure, and is one of the basic methods of material mechanical property test.

It is mainly used to measure the yield point of metal materials under uniaxial compression at room temperature and the compressive strength of brittle materials.

Compressibility refers to the ability of materials to resist deformation and damage under compressive stress.

In engineering practice, there are many components bearing compression load, such as the column of large plant, the support of crane, the compression bolt of steel rolling mill, etc.

This requires the compression test evaluation of its raw materials.

1.2 Concept

Compressive yield strength: when the metal material presents a yield phenomenon, the compressive stress corresponds to the continuous deformation of the specimen without increasing the force during the test.

Upper compression yield strength: the highest compression stress before the first drop of the force due to the yield of the sample.

Lower compression yield strength: the lowest compression stress during the yield period without considering the instantaneous effect.

Tensile strength: for brittle materials, the maximum compressive stress of the sample during compression to failure.

Compressive elastic modulus: the ratio of axial compressive stress to axial strain within the range of linear proportional relationship between axial compressive stress and axial strain during the test.

1.3 Test equipment, instruments and samples

Equipment and instruments: 

(1) Universal testing machine for materials; (2) Vernier caliper.

Compression specimens are usually cylindrical, with circular and square cross sections.

When the specimen is compressed, the friction between the two ends and the indenter of the testing machine will constrain the lateral deformation of the specimen, and the shorter the specimen is, the greater the influence is;

However, if the sample is too long, it is easy to cause longitudinal bending and instability.

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1.4 Mechanical analysis of compression test

Mild steel

The low-carbon steel sample is installed on the testing machine, and under the axial pressure F, the deformation of the sample is △ l.

The relationship between the two is shown in the Fig below.

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Comparison of compression-stretch curves of low-carbon steel

Low carbon steel also has the elastic stage, yield stage and strengthening stage during compression.

The low-carbon steel will not break due to compression deformation, and will form a “drum” due to the influence of friction at the upper and lower ends.

When the specimen diameter is the same, the elastic stages of the compression curve and the tensile curve almost coincide, and the yield point is basically the same.

Low-carbon steel is a plastic material.

After the yield of the sample, the plastic deformation increases rapidly, and its cross-sectional area also increases.

The increased area can bear more load, so only the yield limit can be measured, but not the strength limit.

Cast iron

The cast iron sample is installed on the testing machine, and under the action of axial pressure F, the deformation of the sample is △ l.

The relationship between the two is shown in the figure.

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Mechanical Properties of Materials Under Static Loads 4

The compressive strength of gray cast iron is 3-4 times of its tensile strength.

The cast iron breaks under small deformation, becoming slightly “drum shaped”, and the normal of the section is 45-55 degrees from the axis;

When the sample diameters are the same, the compression curve and tensile curve of cast iron differ greatly, and their compressive strength is far greater than the tensile strength.

2. Bending test

2.1. Overview

Bending property refers to the mechanical property of materials under bending load.

Bending test is used to inspect the performance of materials under bending load.

Many machine parts (such as tools, beams, axles, etc. made of brittle materials) work under bending load, mainly used to measure the bending strength of brittle and low plastic materials (such as cast iron, high carbon steel, tool steel, etc.) and reflect the deflection of plasticity index;

The bending test can also be used to check the surface quality of materials.

The test is generally conducted at room temperature, so it is also called cold bending test.

2.2 Concept

Deflection: linear displacement of cross section centroid in the direction perpendicular to the axis during bending deformation;

Bending stress: the stress generated during bending;

Bending strain: slight change of unit length on the outer surface of the sample span center;

Bending modulus of elasticity: the ratio of bending stress to strain within the range of linear proportional relationship between bending stress and bending strain.

Bending strength: the bending stress when the load reaches the maximum value on or before reaching the specified deflection value.

2.3 Bending test principle

Place the specimen with a certain shape and size on the support with a certain span L, and apply a concentrated load to make the specimen produce bending stress and deformation.

The bending test is divided into three-point bending and four-point bending, and three-point bending is the most commonly used test method.

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2.4 Bending sample and test device

The cross-section shape of the bending test sample can be round, square, rectangle and polygon, but it should refer to the provisions of relevant product standards or technical agreements;

At room temperature, it can be cut by sawing, milling, planing and other processing methods. The tested part of the sample is not allowed to have any indentation and scar.

The edges must be filed round, and the radius should not be greater than 1/10 of the sample thickness;

The bending test is usually carried out on universal material testing machine or press brake machine;

The commonly used bending dies are roller type, V-type mold type, vise type, plate type, etc.

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2.5 Mechanical analysis of bending test

The bending curve is the relationship curve between the bending load obtained from the bending test and the bending deflection of the specimen.

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The maximum normal stress on the surface of the tensile side when the specimen is bent: σ= M/W.

M — maximum bending moment,

Three-point bending: M=FLs/4;

Four-point bending: M=Fa/2;

W – bending section coefficient,

For the round specimen with diameter d: W=π d3/32;

For the rectangular specimen with width b and height h: W=bh2/6.

2.6 Performance index

Bending strength – the maximum bending stress reached by the specimen before bending to fracture, calculated according to the elastic bending stress formula, expressed by the symbol σbb: σbb=Mb/W (Mb – bending moment at fracture)

The bending resistance of gray cast iron is superior to the tensile strength.

Fracture deflection fbb – place the specimen symmetrically on the bending test device, install the deflectometer at the measuring position in the middle of the specimen, continuously apply bending force to the specimen until the specimen breaks, and measure the deflection at the midpoint of the span at the moment of fracture.

3. Shear test

3.1 Overview

The shear test is used to test the shear strength of materials.

In fact, the shear test is to determine the maximum force of dislocation when the specimen fails in shear.

The engineering structures subject to shear force include bolts, pins, rivets, etc.

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The resultant force of the load acting on two sides of the sample is a pair of forces with equal size, opposite direction and close action lines, as shown in the figure:

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3.2 Classification of shear test

It is generally divided into the single shear test, double shear test, punching test, slit shear test and composite steel plate shear test.

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3.3 Sample and test device

Sample

The shear specimen is determined according to the shear test method and fixture.

Cylindrical sample: the diameter and length of the sample are determined according to the fixture, generally taking the diameter of 5, 10, 15 mm.

Punching plate sample: when the thin plate cannot be made into a cylindrical sample, punching shear sample can be used, and the thickness of the plate sample is generally less than 5mm.

Shear sample of actual parts: use actual parts such as rivets, bolts, etc.

Test setup:

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3.4 Determination of shear properties

The room temperature shear test shall be conducted at 10~35 ℃;

For different samples, select the appropriate device. When installing the device, it shall be consistent with the centerline of the indenter of the testing machine and shall not be eccentric; Shear test speed ≯ 15mm/min, high temperature ≯ 5mm/min;

High-temperature shear test: the test temperature rise time is ≯ 1h, and the heat preservation time is 15-30min.

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3.5 Processing of shear test data

After the specimen is cut, record the maximum test force F during the shear test.

Calculate the shear strength τ b, MPa according to the following formula.

Single shear strength: τb=F/S0 (S0 — original cross-sectional area of the sample, mm2)

Double shear strength: τb=F/2S0=2F/(πd2)(S0 — original cross-sectional area of the sample, mm2)

Double shear strength: τb=F/(πd0t)(d0 – punching diameter, mm2; t – sample thickness, mm)

4. Torsion test

4.1 General

Torsion test is a kind of test to determine the resistance of materials to torque, and is one of the basic test methods of material mechanical property test.

Torsion test is to apply torque T to the sample, measure the torque T and the corresponding torque angle φ, and draw a torsion curve diagram, which is generally twisted to fracture, so as to determine various torsional mechanical property indexes of metal materials.

In mechanical, petroleum, metallurgy and other projects, there are many instances of mechanical parts bearing torsional load, such as shafts, springs, etc.

Characteristic

The flexibility coefficient of stress state in torsion is large, so it can be used to measure the materials that are brittle in tension.

For example, the plasticity of quenched and low-temperature tempered tool steel.

In the torsion test, the plastic deformation of the cylinder is always uniform along the entire length.

The section and gauge length of the test piece are basically unchanged, and there is no necking phenomenon on the test piece during static tension.

Torsion test can clearly distinguish the fracture mode of materials, normal fracture or cutoff.

For plastic materials, the fracture surface is perpendicular to the axis of the test piece, and the fracture surface is flat with whirling plastic deformation traces.

The distribution of stress and strain on the cross section of the specimen during torsion test shows that the test is very sensitive to metal surface defects.

Therefore, torsion test can be used to study or inspect the surface quality of workpiece heat treatment and the effect of various surface strengthening processes.

In torsion test, the specimen is subjected to large shear stress, so it is also widely used to study the non simultaneity of initial plastic deformation.

Such as elastic aftereffects, elastic hysteresis and internal friction.

4.2 Application of torsion test

Torsion test can be used to measure all mechanical property indexes of shear deformation and fracture of plastic and brittle materials, and it has advantages that other mechanical property test methods cannot compare.

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Torsional fracture morphology

(a – cut-off fracture, b – normal fracture, c – layered fracture)

The fracture surface of the plastic material is perpendicular to the axis of the test piece, and the fracture surface is flat with whirling plastic deformation traces (Fig. a), which is caused by shear stress;

The fracture surface of brittle material is about 45 degrees to the axis of the test piece, forming a spiral shape (Fig. b);

If the axial cutting resistance of the material is lower than that of the transverse, layered or wood chip fracture may occur during torsional fracture (Fig. c).

According to the characteristics of fracture surface, the cause of fracture and the relative strength of torsional strength and tensile (compressive) strength of materials can be judged.

4.3 Principle of torsion test

During the test, with the increase of torque, the sections at both ends of the gauge length of the test piece continuously rotate relative to each other, which increases the torsional angle.

Use the drawing device of the test machine to draw a curve, namely Mn- φ Curve (also called torsion diagram).

According to the different properties of materials, the torsion curve can be divided into two typical types – low-carbon steel and cast iron.

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Torsion curve

The torsion diagram is similar to the stress-strain curve measured by the tensile test, because the shape of the specimen remains unchanged during torsion, and its deformation is always uniform.

Even when entering the plastic deformation stage, the torque still increases with the increase of deformation until the specimen breaks.

Torsion curve of mild steel

Elastic phase OA

When the curve reaches point D, in a weak part of the test piece (where the material is uneven or has defects), the deformation increases significantly, the effective cross section decreases sharply, and necking occurs.

Thereafter, the axial deformation of the specimen is mainly concentrated at the necking position, and the specimen is finally pulled off at the necking position.

When the applied torque does not exceed the elastic range, the deformation is elastic and the Mn-φ curve is a straight line.

When the shear stress at the edge reaches the shear yield limit, the corresponding torque is Mp.

The stress on the section is distributed linearly, and the shear stress on the surface is the largest. I.e τ max=Mn/Wn

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Torsion curve of mild steel
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Distribution of stress and strain during elastic deformation

Yield stage AB

When the elastic range is exceeded, the specimen begins to yield. The yield process is gradual from the surface to the center of the circle.

At this time, the Mn-φ curve begins to bend, the plastic zone of the cross section gradually expands to the center of the circle, and the stress on the cross section is no longer linear.

After the overall yield of the sample, a yield platform appears on the Mn-φ curve.

At this time, the minimum yield torque indicated by the active pointer is recorded as Ms.

When the applied torque does not exceed the elastic range, the deformation is elastic, Mn- φ curve is a straight line.

When the shear stress at the edge reaches the shear yield limit, the corresponding torque is Mp.

The stress on the section is distributed linearly, and the shear stress on the surface is the largest. I.e τ max= Mn/ Wn

yield strength τs=(3/4)(Ms/Wn)

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Strengthening stage BC

Mn after exceeding the yield stage- φ curve starts to rise again, indicating that the material has regained the ability to resist deformation, that is, the torque of the material must continue to increase if it wants to continue to deform.

Low carbon steel has a long strengthening stage, but there is no necking until fracture.

Torsional strength limit τb=(3/4)(Mb/Wn)

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Torsion curve of mild steel

Torsion curve of cast iron

When the Mn-φ curve of cast iron is loaded to a certain extent, it deviates from the straight line until it breaks.

It shows that the plastic deformation of cast iron before torsion is more obvious than that during tension.

The maximum shear stress at fracture of cast iron is defined as the strength limit τ b.

  τb=(3/4)(Mb/Wn)

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Cast iron torsion curve

4.4. Specimen for torsion test

According to the current standard, it can be divided into two types: cylindrical sample and tubular sample.

It is recommended to use cylindrical samples with a diameter of 10 mm, gauge distances of 50 mm and 10 mm, and parallel lengths of 70 mm and 120 mm respectively.

If specimens of other diameters are used, their parallel length shall be the gauge length plus twice the diameter.

The parallel length of the tubular specimen shall be the gauge length plus twice the outer diameter.

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4.5 Instrument and equipment for torsion test

Torsion testing machine

Different types of mechanical or electronic torsion testing machines are allowed.

The relative error of torque indication of the testing machine shall not be greater than ± 1%, which shall be verified regularly by the metrological department;

During the test, one of the two chucks of the testing machine shall be able to move freely along the axis, without additional axial force on the sample, and the two chucks shall remain coaxial;

The testing machine shall be able to continuously apply torque to the sample without impact and vibration, and keep the torque constant within 30s.

Torsion meter

It is allowed to use different types of torsion meters to measure the torsion angle, such as mirror type torsion meter, meter type torsion meter, electronic type torsion meter, etc.

It is recommended to use electronic type torsion meter.

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1-Sample; 2 – Fixed clamp block; 3 – Set nut; 4 – Rotating clamp block; 5 – Gauge length ruler; 6 – Digital dial indicator

4.6 Relevant mechanical properties

Test conditions: the test shall be conducted at room temperature of 10-35 ℃;

Torsion speed: it shall be within the range of 3 °~30 °/min before yielding, and not more than 720 °/min after yielding.

The change of speed shall have no impact.

(1) Determination of shear modulus

Graphic method

Record the torque torque angle curve with automatic recording method.

Read the torque increment and torque angle increment on the elastic linear segment of the curve.

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Torque-torsion angle curve

Step-by-step loading method

Within the range of elastic straight section, load the sample with no less than grade 5 equal torque.

Record the torque and corresponding torsional angle of each stage, calculate the average torsional angle increment of each stage, and calculate the shear modulus G according to the formula in the graphical method.

(2) Determination of specified non-proportional torsional strength

The torque-torsion angle curve is recorded by the automatic recording method.

Extend the cross twist angle axis of the elastic straight line segment on the curve to point O, intercept the OC segment, and make the parallel line CA of the elastic straight line segment through point C to point A, and the torque corresponding to point A is Tp.

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Specified non-proportional torsional strength: τb=Tp/W

(3) Determination of upper and lower yield strength

The diagram method or pointer method shall be used for measurement.

During the test, the automatic recording method shall be used to record the torsion curve, or the pointer of the torque dial of the testing machine shall be directly observed.

The maximum torque before the first drop is the upper yield torque, and the minimum torque in the yield stage excluding the initial instantaneous effect is the lower yield torque.

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Upper yield strength: τeH=TeH/W

Lower yield strength: τeL=TeL/W

(4) Determination of torsional strength

Apply torque to the specimen continuously until it breaks.

Read out the maximum torque of the sample before it is twisted from the recorded torsion curve or the torque dial of the testing machine, and calculate the torsional strength with the formula.

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Torsional strength:

τ m=Tm/W

( τ m – torsional strength;

Tm – maximum torque;

W – section coefficient)

4.7 Fracture analysis of the sample

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Carbon steel fracture shape: plane fracture

It shows that the fracture is caused by shear stress.

The trace of convoluted plastic deformation can be seen on the section, which is a typical ductile fracture.

Shear stress at fracture is defined as strength limit τ b.

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Cast iron fracture shape: 45 ° spiral fracture

It shows that the fracture is caused by the maximum tensile stress.

However, the maximum tensile stress breaks before the maximum shear stress reaches the strength limit, indicating that the tensile capacity of cast iron is weaker than its shear capacity.

During pure torsion, the surface of the round specimen is in the state of pure shear stress, and two principal stresses, σ1 and σ3, are respectively applied on the spiral surface with an angle of ± 45 º to the rod axis and equal to the absolute value of the maximum shear stress τmax.

Therefore, the fracture angle of the sample directly shows whether the material is tensile or shear, and the strength of the material’s own tensile and shear resistance.

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The surface of the round specimen is in the state of pure shear stress during pure torsion

5. Hardness test

5.1 General

Hardness characterizes the ability of solid materials to resist local deformation, especially plastic deformation, indentation or scratch, reflecting the hardness of materials.

Hardness is not a simple physical concept, but a comprehensive index of mechanical properties such as elasticity, plasticity, strength and toughness of materials.

For example, the scratch hardness test represents the ability of the metal to resist cracking, while the indentation hardness test represents the ability of metal to resist deformation.

There is a certain relationship between hardness data and other mechanical properties, such as tensile strength.

The reason is that both hardness and tensile strength are related to large plastic deformation tension.

5.2 Hardness test method and classification

The hardness test is the most widely used mechanical property test, which can be divided into the indentation method and the scratch method according to the stress mode.

In the press-in method, it can be divided into the static force test method and dynamic force test method according to different force application speeds.

The commonly used Brinell hardness, Rockwell hardness and Vickers hardness are static force test methods, while Shore hardness, Leeb hardness and hammering Brinell hardness are dynamic force test methods.

Scope of application of hardness measurement methods

Hardness measurement methodScope of application
Brinell hardness testParts with coarse grains and uneven structure shall not be used for finished products. In the hardness test of iron and steel parts, cemented carbide ball indenters have been gradually used to measure the hardness of annealed parts, normalized parts, quenched and tempered parts, castings and forgings.
Rockwell hardness testHardness inspection of batch, finished products and semi-finished products. Parts with coarse grains and uneven structure shall not be used. It is divided into three types: A, B and C.
Surface Rockwell hardness testTest the hardness of thin pieces, small pieces and the surface hardness of parts with thin or medium thickness hardening layer. N scale is generally used in hardness test of steel parts.
Vickers hardness testIt is mainly used to measure the hardness of small pieces and thin pieces, as well as the surface hardness of parts with shallow or medium thickness hardening layer.
Vickers hardness test under small loadTest the hardness of small pieces and thin pieces, as well as the surface hardness of parts with a shallow hardening layer. Measure the surface hardness gradient or hardened case depth of the case hardened parts.
Micro Vickers hardness testTest the hardness of micro parts, extremely thin parts or microstructures, and the surface hardness of parts with extreme or extremely hard hardening layers.
Shore hardness testIt is mainly used for on-site hardness inspection of large parts, such as rolls, machine tool surfaces, heavy construction, etc.
Hardness test of steel filesOn site hardness inspection for parts with complex shape and large pieces. 100% hardness inspection of batch parts. The hardness of the inspected surface shall not be lower than 40HRC.
Knoop hardness testTest the hardness of micro parts, extremely thin parts or microstructures, and the surface hardness of parts with extremely thin or extremely hard hardening layers.
Leeb hardness testOn site hardness inspection for large parts, assembly parts, parts with complex shape, etc.
Ultrasonic hardness testOn site hardness inspection for large parts, assembly parts, parts with complex shape, thin parts, nitrided parts, etc.
Hammer blow Brinell hardness testOn site hardness inspection of normalized, annealed or quenched and tempered large pieces and raw materials.

Shore hardness is also called the rebound method, so it can be divided into: indentation method, elastic rebound method and scratch method.

Hardness of the same type can be converted;

Different methods can only use the same material for calibration.

Hardness test characteristics

The experimental method is simple, without sample processing;

The surface damage caused is small and basically belongs to the scope of “non-destructive” or micro damage detection;

There is a certain relationship between it and other mechanical performance indexes under static load, for example, the strength value can be roughly inferred from the hardness;

The measurement range can be as large as multiple grains, as small as single grains, or even several atoms (NanoIndenter).

5.3 Concept

Brinell hardness (HB): the unit of measurement of the resistance of a material to permanent indentation deformation caused by the application of a test force through a cemented carbide ball indenter.

Knoop hardness (HK): The unit of measurement of the resistance of a material to permanent indentation deformation caused by the application of a test force through a diamond diamond cone indenter.

Shore hardness (HS): refers to the hardness expressed by the measured setback height of the striker pin when the striker pin (a small cone with a tip and a diamond drill embedded on the tip) falls onto the surface of the tested material from a certain height by using the elastic rebound method.

Rockwell hardness (HR): the unit of measurement of the resistance of a material to permanent indentation deformation caused by the application of a test force through a cemented carbide or a diamond cone indenter corresponding to a certain scale.

Vickers hardness (HV): the unit of measurement of the resistance of a material to permanent indentation deformation caused by the application of a test force through a diamond pyramid indenter.

Leeb hardness (HL): It refers to the hardness value calculated by the ratio of the rebound speed and impact speed of the punch at 1mm from the surface of the sample when the impact body with specified quality impacts the surface of the sample at a certain speed under the action of elastic force.

Standard block: It is used for indirect inspection of indentation hardness tester, and has standard block materials with qualified indentation values.

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5.4 Relationship between hardness and material tensile strength

The indentation hardness of metal is directly proportional to the tensile strength: σ b=kHB.

Where k is the proportional coefficient, and the k values of different metal materials are different.

After heat treatment, the hardness and strength of the same kind of metal change, but the k values remain basically unchanged;

After cold deformation, the k value of metal material is no longer a constant;

K of steel material is about 3.3;

Accurate strength data shall be obtained by direct measurement.

5.5 Brinell hardness

Principle: Use an indenter (ball) with a certain diameter to press into the surface to be measured with the corresponding test force.

After unloading for a specified time, measure the diameter of the indentation on the material surface, and then calculate the hardness value.

Indenter: hardened steel ball or hard alloy steel ball.

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Load, indenter diameter and holding time are the three elements of Brinell hardness test.

Representation of Brinell hardness: 120HBS10/1000/30

  • 120 – Indicates that the hardness value is 120
  • S – refers to quenched steel ball (W refers to cemented carbide steel ball)
  • 10 – indicates the diameter of the pressure head 10mm
  • 1000 – indicates that the load is 1000kgf
  • 30 – Indicates that the holding time is 30s (generally 10~15s is not marked)

Brinell hardness value is in kilogram force/mm2 (N/mm2);

The upper limit of Brinell hardness is HB650, which cannot be higher than this value.

Advantages:

The indentation area is large, reflecting the hardness performance of materials in a large range; The test data is stable, repeatable and widely used;

Suitable for materials with coarse grain, complex phase composition and large phase size.

Disadvantages:

It belongs to destructive testing, with large indentation, and cannot be tested on the surface of finished products;

The operation is complex, the efficiency is low, and continuous detection is not possible.

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Brinell hardness test equipment

5.6 Rockwell hardness

Principle: Use diamond cone or quenched steel ball indenter to press the indenter into the material surface under the action of test pressure F.

After holding for a specified time, remove the main test force and maintain the initial test force. Calculate the hardness value with the residual indentation depth increment.

During actual measurement, the Rockwell hardness value can be directly read out through the dial of the testing machine.

The Rockwell hardness load is large, so it is not suitable for measuring extremely thin samples and surface hardened layers. The surface Rockwell hardness measurement is adopted.

Advantage:

The operation is simple and fast, with high efficiency, and the hardness value can be read directly;

Small indentation, can measure finished products or thinner workpieces;

It can measure the hardness of materials with different hardness.

Disadvantages:

The indentation is small and the representativeness is poor;

The data repeatability is poor when the material has segregation or uneven structure;

Rockwell hardness data of different grades are not comparable.

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Indenter – diamond cone with a vertex angle of 120 ° or quenched steel ball with a diameter of 1.588mm;

1-1 – Position of indenter with initial load;

2-2 – Position of indenter after adding initial load+main load;

3-3 – Position of ram after removing main load;

he – elastic recovery after removing the main load

Hardness scales

ScaleIndenterLoad (kg)Purpose
Adiamond60HRA: less pressure and damage in hardness test of superhard alloy and thin steel plate penetrating the rigid surface layer
B1/16 foot ball100HRB: soft stainless steel, nonferrous metals
Cdiamond150HRC: Tungsten Carbide and Age Hardened Steel
Ddiamond100HRD: Surface hardened parts
E1/8 inch ball100HRE: cast iron, aluminum alloy, magnesium alloy, bearing and gold
F1/16 inch ball60HRF: Cold gift metal sheet steel, annealed steel, brass
G1/16 inch ball150HRG: phosphor bronze, copper plating, duralumin alloy
H1/8 inch ball60HRH: aluminum, zinc, lead
K1/8 inch ball150HRK: Bearing and gold

Representation of Rockwell hardness: 70HR30TW

  • 70 – Rockwell hardness number
  • HR – Rockwell hardness symbol
  • 30T – Rockwell scale symbol
  • W – Type of indenter: W represents cemented carbide; S for steel ball
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Rockwell hardness test equipment

5.7 Vickers hardness

Principle: press the indenter into the surface of the sample under a certain static testing force, and remove the testing force after holding it for a specified time, leaving a square and conical indentation on the surface of the sample.

Calculate the indentation area. Vickers hardness is the quotient of the detection force divided by the indentation surface area.

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Indenter – diamond material, square pyramid, face angle of 136 °

During Vickers hardness testing, the testing force can be selected arbitrarily for materials with uniform hardness, and the hardness value is unchanged, which is the biggest advantage of the Vickers hardness testing method.

The cone with a face angle of 136 ° is selected so that Vickers hardness and Brinell hardness have similar indications for comparison.

Representation method: The value in front of HV is the hardness value, and the value behind HV is the test force value.

The standard test holding time is 10~15s, and the holding time shall be marked if it is out of range.

600HV30 means that the hardness value obtained by using 30kgf test force and holding it for 10-15s is 600;

600HV30/20 means that the hardness value obtained by using 30kgf test force and holding for 20s is 600.

Scope of application: Vickers hardness, low load Vickers hardness and micro Vickers hardness according to the size of the test force.

Vickers hardness test: except for samples with very small and thin sample layers, the measuring range can cover all metals.

Vickers hardness test under low load: It is especially suitable for measuring the surface hardness of steel surface strengthening layer, chemical heat treatment surface layer, various infiltration layers, transition layers, etc.

Micro Vickers hardness test: In addition to the hardness test of products, it is also one of the most commonly used test methods in the research of metallography and metallography.

Advantages:

Wide range of applications, from very soft to very hard materials can be measured;

High measurement accuracy and comparability;

The hardness value is independent of the test force.

Disadvantages:

The measurement operation is troublesome and the measurement efficiency is low;

It is not suitable for mass production and measurement of inhomogeneous materials.

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Vickers hardness test equipment

5.8 Microhardness

Microhardness refers to the hardness test with loading less than 0.2kgf, which is divided into micro Vickers hardness and micro Knoop hardness.

The microhardness can measure the hardness of brittle materials such as ceramics, glass, agate, etc., with high sensitivity, and is suitable for evaluating the work hardening degree of fine wire.

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Vickers indenter indentation (left), Knoop indenter indentation (right)

Microhardness application

(1) Research on metal materials and metallography:

It is widely used to measure the hardness of various constituent phases in metals and alloys, analyze their contribution to alloy properties, and provide the basis for the correct design of alloys.

(2) Study on properties of metal surface layer:

Study on the properties of the diffusion layer, such as carburized layer, nitrided layer, metal diffusion layer, etc; (Research on the properties of the surface hardened layer.

For example, the metal surface is affected by mechanical processing and thermal processing.

(3) Study on the Inhomogeneity in Grain.

(4) Measurement of the hardness of very thin metal products.

5.9 Shore hardness

Principle:

Drop the diamond punch of the specified shape from the fixed height h0 on the surface of the sample, and the punch bounces up to a certain height h.

Calculate the Shore hardness value with the ratio of h to h0 (the hardness of the material is proportional to the callback height).

Unlike the previous three static indentation methods, Shore hardness is a dynamic force test method.

Mechanical Properties of Materials Under Static Loads 40
Shore hardness tester

Sample requirements:

The mass of the sample shall be at least 0.1kg, and the thickness shall be generally more than 10mm;

The test area of the sample shall be as large as possible;

The surface shall be free of oxide scale and foreign dirt, and shall not be magnetic.

Representation method:

The number before HS indicates the hardness value, and the number after HS indicates the type of hardness scale.

45HSC – hardness value measured by C-type hardness tester is 45;

45HSD means the hardness value measured by the D-type hardness tester is 45.

Advantages: simple operation and high efficiency; After the test, there is almost no indentation, and the test can be conducted on the finished product.

Disadvantages: low measurement accuracy, poor repeatability, suitable for testing with high accuracy requirements.

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