# Chapter 3 Mechanical Testing

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Chapter 3 Mechanical Testing
Mechanical Testing • Dynamic Mechanical Tests • Static Mechanical Tests • Hardness Testing

A mechanical force or load may be applied using five different methods.
Mechanical force is applied using five methods: tension, compression, shear, torsion, and flexure. See Figure 3-1. Tension is a force that occurs when the load is applied axially (parallel to the axis) on the test specimen in a stretching manner. Compression is a force that occurs when the load is applied axially on the test specimen in a squeezing manner. Shear is the application of two equal and parallel forces on an object from opposite directions. Shearing occurs when a force causes a material to separate along a plane parallel to the load. Pure shear is impossible to set up in a test without interference from other types of stresses in the devices that grip the test specimen. Torsion is the application of twisting force on an object. This causes internal shear, which varies from zero at the center of the test specimen to a maximum at the outside edge. Flexure is the application of a force that causes the bending of an object. Bending introduces tension on the area being stretched and compression on the area being squeezed on the opposite side. All five methods of applying mechanical force induce a stress in the test specimen material and cause an accompanying strain.

Longitudinal and transverse test specimens taken from cold-rolled plate material exhibit different mechanical properties. Selection of test specimens is a key consideration because metals often exhibit anisotropy (directional effects). This means mechanical properties vary in different directions (orientations). For example, a cold-rolled steel longitudinal test specimen is stronger than a transverse test specimen. The longitudinal test specimen is stronger because it is taken from the direction the steel is rolled. See Figure 3-2.

There are four classifications of cyclic stresses in fatigue
There are four classifications of cyclic stresses in fatigue. A cycle is each complete application of the stress. The stress ratio (R) of a cyclic stress is the minimum stress divided by the maximum stress (smin ÷ smax). The four classifications of cyclic stresses are low and high tensile stress (R > 0), zero stress and tensile stress (R = 0), unequal tensile stress and compression stress (R < 0), and equal tensile stress and compression stress (R = –1). See Figure 3-3. A cycle is one complete application of stress. With reciprocating or rotating components, the time and stress magnitude of each cycle is usually equal (uniform loading). This is not the case with other components such as automobiles, airplanes, and ships that are randomly loaded.

The magnitude of the fatigue limit depends on the stress repetition pattern, which is plotted on an S-N curve. From a series of fatigue tests an S-N curve is plotted. An S-N curve is a record of the amplitude of the cyclic stress (s or S) plotted against the number of cycles to failure (N). See Figure 3-4. High-cycle fatigue occurs at low stresses and millions of cycles (>104). Low-cycle fatigue involves fewer cycles (<104) and high stresses.

Metal tested at a low strain rate is ductile compared with the same metal tested at a high strain rate. Mechanical properties are strongly affected by the rate of straining. A metal tested at a low strain rate may fracture with a large amount of strain (elongation), but a metal at a high strain rate may break with little or no elongation. A metal is tough and ductile at the low strain rate and is brittle at the high strain rate. See Figure 3-5.

A universal pendulum impact tester can perform both the Charpy and Izod impact tests.
A notched bar impact test is a mechanical test that measures the force (produced by a dynamic load) needed to break a small machine-notched test specimen. The two principal impact tests, the Charpy and Izod, are performed on a universal pendulum impact tester. See Figure 3-6. In both tests, the energy required to break the test specimen is measured. The resulting measurement is an indication of toughness.

The V-notch is the most common Charpy and Izod impact test specimen.
The test specimen is a square-shaped bar containing a machined V-notch or keyhole notch (keyhole-shaped groove). A sawcut should never be used because it cannot repeatedly and precisely reproduce the same cut. See Figure 3-7. These notches are characteristic of both the Charpy and Izod tests. The only difference is the location of the notch on the test specimen. The purpose of the notch in the test specimen is to facilitate fracture in a controlled location.

The swing of the pendulum after it strikes the test specimen indicates the energy absorbed on impact. During a Charpy test, the test specimen is placed horizontally against the two supports at the bottom of the tester. The pendulum is raised to a standard height, giving it a potential energy of 240 ft·lb (325 J). The pendulum is released and the test specimen is struck and broken by the hammer as it swings through its arc. See Figure 3-8. The swing of the pendulum after it strikes the test specimen indicates the energy absorbed on impact and is measured in foot-pounds or joules. When struck by the pendulum, tough materials absorb a significant amount of energy and brittle materials fracture with relatively little energy absorbed. Tough materials cause the pendulum to travel shorter distances after striking the test specimen. With brittle materials, the pendulum travels longer distances after impact.

The main differences between the Charpy and Izod impact tests are the position of the notch and the method of support of the test specimen. The Izod impact test operates on a similar principle to the Charpy. The main differences are in the position of the notch on and the method of support of the test specimen. See Figure 3-9. The notch is located toward one end of the test specimen, which is gripped vertically, instead of horizontally, in a vise.

The sharper the inflection of the curve, the easier the estimation of the NDT temperature.
The Charpy test and drop weight test are used to determine the NDT temperature. The Charpy test does this by testing triplicate sets of test specimens over a range of temperatures. The results are plotted as impact strength against test temperature. See Figure There is a transition from low to high impact strength. Depending on the type of steel, this transition may be gradual or sharp. The sharper the inflection of the curve, the easier the estimation of the NDT temperature.

The drop weight test is more reliable than the Charpy when determining ductility.
The drop weight test is a more reliable method than the Charpy. The test specimen is a slab or plate that is up to 5/8″ thick. A weld bead made from a brittle alloy is laid down the center of the plate. The plate is brought to the test temperature and placed in the test fixture. It is supported along both ends parallel to the weld, with the weld side facing down. See Figure A weight located vertically above the center of the plate is allowed to drop on it, causing the plate to bend. Cracking of the weld bead is initiated at 3° of bend. After that point the weld bead continues to crack, which initiates a fracture. To ensure the strain induced in the plate is elastic, a stop is placed below the weld bead. The stop limits the amount of deflection of the plate to 5° of bend.

A universal testing machine can be mechanical or hydraulic.
A tensile test machine has two major components that are the means of applying the load to the test specimen and measuring the applied load. Some testing machines are designed for one type of test only, such as tension testing machines for testing chain and wire. Universal testing machines test specimens in tension or compression. See Figure 3-12.

A fillet is used on the tensile test specimen to minimize stress concentrations, and the gauge marks are always an equal distance from the center of the length of the reduced section. The transition from the ends of the tensile test specimen to the reduced section is shouldered, or made with a fillet. The shoulder minimizes stress concentrations. See Figure This is particularly important for brittle materials. The longitudinal axis of the test specimen should be symmetrical to avoid the introduction of bending loads during the test. The test specimen grips also must be symmetrical along the longitudinal axis of support to avoid the introduction of bending loads during the test. The shape of the ends of the test specimen is determined by the specimen gripping device that is used.

A variety of tensile test specimen ends are used to ensure secure and uniform gripping by the test machine. Tensile test specimens may be round or rectangular, depending on the stock from which they are obtained. See Figure For example, rectangular test specimens are obtained from plate or sheet, and round test specimens are taken from forgings or cast test bars.

An extensometer measures the extension of elongation of the tensile test specimen.
The tensile test procedure is conducted by fixing the test specimen firmly in the grips of the testing machine. An extensometer, a device for measuring the extension or elongation of the test specimen, is fitted to the specimen across its gauge length. See Figure An axial load is applied and the test specimen is stretched. As the test specimen is stretched, a load-extension (stress-strain) curve is plotted. The extensometer is removed before the test specimen breaks. The tensile test procedure is described in ASTM E8, Tensile Testing of Metallic Materials.

The load-extension curve shows load and extension limits for metals.
The shape of the load-extension curve varies according to the material, but tends to show common load and extension limits. See Figure The proportional limit is the maximum stress at which stress is directly proportional to strain. Beyond this point, stress is no longer proportional to strain. Up to the elastic limit, the tensile test specimen will return to its original length if the load is removed. The elastic limit is the maximum stress to which a material is subjected without any permanent strain remaining after stress is completely removed. Beyond this point, strain is permanent, or plastic.

Increased gauge length and reduced diameter at the narrowest point are measured and used to calculate the percent elongation and percent reduction in area. When the tensile test is completed, the broken test specimen is removed from the testing machine and fitted together. The new increased gauge length and the reduced diameter at the narrowest point are measured. This is usually at the break or immediately adjacent to it. See Figure These measurements allow the percent elongation and percent reduction in area to be calculated.

The yield strength, or 0.2% offset, is calculated by measuring the stress that causes a specific permanent strain (usually 0.2%). For metals without a yield point, a yield strength (artificial value) is obtained from the load-extension curve. The yield strength, or 0.2% offset, is calculated by measuring the stress that causes a specific amount of permanent strain (usually 0.2%). See Figure 3-18.

Percent elongation is calculated from the gauge length.
Percent elongation is calculated from the gauge length. The longer the gauge length, the less the effect necking down of the test specimen has on the final length. This results in lower percent elongation for a given metal. See Figure When the gauge length is made equal to 𝑘√𝐴, where k is a constant equal to 4.47 and A is equal to the cross-sectional area of the test specimen, the percent elongation value remains practically constant for different gauge lengths. The most common gauge length in tensile testing is 2″.

The guided bend test is an inexpensive and rapid method to check the quality of a weld.
The guided bend test consists of bending a rectangular piece of metal around a U-shaped die. This test is most commonly used to check the quality of welds. A welded test specimen is cut into a rectangular shape with the weld at the midpoint of the specimen. The weld is transverse to its axis. See Figure 3-20.

Cupping tests provide an indication of the formability of sheet metal.
Formability tests measure the ductility of sheet metal used for deep drawing or stretching. In cupping tests, a metal sheet test specimen is stretched over an advancing punch with a rounded head to determine the fracture point. See Figure Cupping tests are limited to predicting gross differences in formability and used as an inspection tool. For example, cupping tests provide a rapid indication of the ductility of a stock of sheet. Cupping tests are described in ASTM E643 Method for Conducting a Ball Punch Deformation Test for Metallic Sheet Materials.

A torsion testing machine is used for determining a metal’s resistance to shear.
A torsion test is a static test that measures a material’s resistance to shear. The test is performed by applying torque (twist) to a cylindrical bar or tube-shaped test specimen in a specially designed torsion testing machine. See Figure 3-22.

A tropometer is used to measure the degree of twist during a torsion test.
The amount of torque on the specimen is measured and recorded by a tropometer. A tropometer is a device used to measure the degree of twist. See Figure A tropometer is mounted on the test specimen while it is in the testing machine, similar to an extensometer.

The scleroscope hardness tester uses the height of rebound of a diamond-tipped hammer from the test specimen surface to determine hardness. A scleroscope hardness tester is an instrument that uses a test specimen that is freely supported horizontally and a glass tube that contains a diamond-tipped hammer positioned vertically over the specimen. The hammer is allowed to fall from a set height and the height of rebound is measured. See Figure The test shows that the higher the rebound, the harder the specimen. The scleroscope hardness test is described in ASTM E448, Scleroscope Hardness Testing of Metallic Materials.

The Equotip metal hardness tester can be used in five positions.
The Equotip hardness test is covered by ASTM A956, Standard Test Method for Equotip Hardness Testing of Products. A spring-loaded mechanism is triggered and propels the ball bearing toward the surface of the specimen. The rebound height is recorded electronically and displayed as a hardness number. See Figure 3-25.

The Brinell hardness tester applies a load for a specific period of time and causes an indentation that is used to calculate hardness. Brinell hardness tests use a machine to press a 10 mm diameter, hardened steel ball into the surface of the test specimen. See Figure The machine applies a load for a specific period of time and causes an indentation that is used to calculate hardness.

Soft or hard metals require careful measurement of their indentations in the Brinell test.
The load applied to the steel ball depends on the type of metal under test with the Brinell testing machine. With the Brinell testing machine, 500 kg is used for soft metals and thin stock, 1500 kg for aluminum castings, and 3000 kg for ferrous metals. The load is usually applied for 10 to 15 seconds. The diameter of the indentation is measured to ±0.05 mm using a low-magnification portable microscope. With soft or hard metals, care must be taken to measure the exact diameter of the indentation and not the apparent diameter caused by edge effects that result in a ridge or depression encircling the true indentation. See Figure 3-27.

The Rockwell test uses two loads, a minor and a major, that are applied sequentially to determine hardness. A 1/16″ diameter steel ball and a 120° diamond cone are the two types of indenters. The Rockwell hardness test uses two loads that are applied sequentially. See Figure A minor load of 10 kg is applied that helps seat the indenter and remove the effect of surface irregularities. A major load, which varies from 60 kg to 150 kg, is then applied.

The Rockwell designation system consists of the hardness number followed by HR, which is followed by the letter indicating the specific Rockwell scale. Several Rockwell hardness scales are used when measuring a variety of materials. See Figure The designation system has a hardness number that is followed by HR, which is followed by another letter that indicates the specific Rockwell scale. For example, a test specimen exhibiting a hardness reading of 40 HRA has a Rockwell hardness reading of 40 on the A scale, the indenter is a diamond cone, and the black numbers on the dial are used for reading the hardness.

The microhardness measured by a microhardness tester is always higher than the bulk surface hardness. During microhardness testing, the test specimen is placed under the microscope of the microhardness tester. See Figure The area of interest is focused at the intersection of the crosswires. The indenter is swung into place and the load applied for a set period of time. The load is then removed, the microscope swung back, and the length of the diagonals measured. From these measurements, the microhardness reading, either Vickers (HV) or Knoop (HK), is obtained from a chart. Microhardness testing is described in ASTM E384, Test Method for Microhardness of Metals.