Mechanical Testing

Mechanical testing reveals the elastic and inelastic behavior of a material when force is applied. A mechanical test

shows whether a material or part is suitable for its intended mechanical applications by measuring elasticity,

tensile strength, elongation, hardness, fracture toughness, impact resistance, stress rupture, and fatigue limit.

Various tests:

Tensile Test:

A tensile test, also known as a tension test, tests a material’s strength. It subjects the sample to uniaxial tension

until it fails. It’s is a common and important test that provides a variety of information about the material being

tested, including the elongation, yield point, tensile strength, and ultimate strength of the material. Tensile tests

are commonly performed on substances such as metals, plastics, wood, and ceramics.

Tensile test machine

Tensile specimen

Static compression test:

Static compression test is the capacity of a material or structure to withstand axially directed pushing forces.

When a specimen of material is loaded in such a way that it extends it is said to be in tension. On the other hand

if the material compresses and shortens it is said to be in compression.

Tensile machine can be used for the compression test it the direction of the traverse motion is inversed.

Specimen shapes are mostly cylinders or parallelepipeds.

Ductile materials behavior Brittle materials behavior

Failure modes of metals Tension test Compression test

Ductile material

(Mild steel)

Shear stress failure

(cone and cup shape)

End effects

(Pancake shape)

Brittle material

(Cast Iron)

Normal stress failure

(Flat facture surface)

Crack

(at 45 deg.)

Impact Testing: Toughness is a measure of the amount of energy a material can absorb before fracturing. It becomes of engineering importance when the ability of a material to withstand an impact load without fracturing is considered.

Two standardized tests, the Charpy and Izod, are commonly used to measure Impact Energy (sometimes referred to as Notch Toughness).

Creep Testing: High temperature progressive deformation of a material at constant stress is called creep. High temperature is a relative term that is dependent on the materials being evaluated. The stress that causes creep is usually less than the yield stress of a material. Creep is undesirable because it results in change of geometry of components and eventually in their failure.

Creep test Creep sample

Fatigue Testing: Fatigue testing is based on cyclic loads, a method used to determine the behavior of materials under fluctuating

loads. Specified mean loads (which may be zero) or alternating loads are applied to a specimen and the number

of cycles required to produce failure (fatigue life) is recorded. Generally, fatigue tests are repeated with identical

specimens and various fluctuating loads. Loads may be applied axially, in torsion, or in flexure.

Types of stress cycle

Fatigue testing Machine

Hardness Testing Hardness is a measure of a material’s resistance to localized plastic deformation (e.g., a small dent or scratch). Hardness testing involves a small indenter being forced into the surface of the material being tested under controlled conditions of load and rate of application. The depth or size of the resulting indentation is measured, which in turn is related to a hardness number; the softer the material, the larger and deeper the indentation, and the lower the hardness index number. Measured harnesses are only relative (rather than absolute) thus care must be taken when comparing values determined by different techniques.

Indentation in a workpiece made by application of (a) the minor load, and (b) the major load, on a diamond Brale indenter in Rockwell hardness testing.

Bend Testing:

Bend testing determines the ductility or the strength of a material by bending the material over a given radius.

Following the bend, the sample is inspected for cracks on the outer surface. Bend testing provides insight into the

modulus of elasticity and the bending strength or a material. Metallurgical offers three and four point bend

setups with interchangeable rollers for a variety of test configurations.

In this test a specimen with round, rectangular or flat cross-section is placed on two parallel supporting pins. The

loading force is applied in the middle by means loading pin.

Shear Testing:

Intertek provides a range of shear test methods, suitable for a wide variety of samples and materials. Shear tests

help measure the deformable mechanical properties of plastics, composites, fluids and other samples.

Shear stress applied along a parallel or tangential direction to a cross-section which differs from normal stress,

which is when stress is applied perpendicular to the face, are commonly found in bolts, pins and rivers used to

connect various structural members and machines components.

Torsion Tests:

Torsion test is not widely accepted as much as tensile test. Torsion tests are made on materials to determine such properties as the modulus of elasticity in shear, the torsion yield strength and the modulus of rupture. Often used for testing brittle materials and can be tested in full-sized parts, i.e., shafts, axles and twist drills which are subjected to torsional loading in service.

Torsion test machine

Shear (ductile) failure is along the maximum shear plane.

Tensile (brittle) failure is perpendicular to the maximum tensile stress (at 45o), resulting in a helical fracture.

Mechanical Properties

The mechanical properties of a material are those properties that involve a reaction to an applied load. The

mechanical properties of metals determine the range of usefulness of a material and establish the service life that

can be expected. Mechanical properties are also used to help classify and identify material.

Strength: It refers to the material's ability to resist an applied force. It is measured by the tensile strength. The

elastic limit is considered to guard material against permanent deformation. Rather than elastic limit, yield point

is used for ductile materials.

Yield point: If the stress is too large, the strain deviates from being proportional to the stress. The point at which

this happens is the yield point because there the material yields, deforming permanently (plastically).

Ductility: Extent of Plastic deformation material undergoes before Fracture. It is the opposite of brittleness. Ductility can be given either as percent maximum elongation emax or maximum area reduction.

Rigidity: It refers to inflexibility or resistance to change. It is important where deflections are limited by service

requirements. It is also known as the material property of shear modulus which is a measure of force per unit

area needed to change the shape of a material.

Resistance to Fatigue: It is measured by means of endurance limit and is important for members subjected to

cyclic loading. Harmful surface effects must be carefully controlled during heat treatment.

Damping Capacity: It refers to the material's capacity of absorbing vibrations. It is the energy which is dissipated

as heat by a unit volume of the material during a completely reversed cycle of stress. It is represented by a

mechanical-hysteresis diagram. It is related to internal friction in the material. High damping capacity is desirable

to decrease vibration, chatter and noise.

Toughness: Ability to absorb energy up to fracture. The energy per unit volume is the total area under the strain-

stress curve.

Resilience: It refers to the property of a material to absorb energy when it is deformed elastically and then, upon

unloading to have this energy recovered. In other words, it is the maximum energy per volume that can be

elastically stored.

Stiffness: Ability of material to resist deformation.

Hardness: It refers to various properties of matter in the solid phase which give it high resistance to various kinds

of shape change when force is applied. Ductility is desirable to relieve stress concentration in parts subjected to

static loading.

Corrosion Resistance: It is important in members subjected to corrosive environment. In the presence of stress

concentration its effect is particularly serious in cyclic loading. Other such considerations which are depending on

application are weight, electrical properties, thermal properties, resistance to wear, casting and forging

characteristics, machinability, low friction etc.

Standards

Tensile specimens are machined from the material to be tested in the desired orientation and according to the

standards. This way you insure that all other variables are minimized and you're really measuring the tensile

properties of the material. If you allow variation with respect to the test specimen, then results may vary greatly

because of the method of manufacturing, porosity, inclusions, etc. In this manner, you want to only compare

tensile properties and hopefully reduce variation caused by all of these other variables.

Metals

- ASTM E8 Standard Test Methods for Tension Testing of Metallic Materials

- ISO 6892 Metallic materials—Tensile testing at ambient temperature

- JIS Z2241 Method of tensile test for metallic materials

Flexible Materials

- 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

Tensile specimen

The specimen's shape is usually defined by the standard or specification being utilized, e.g., ASTM E8 or D638.

Its shape is important because you want to avoid having a break or fracture within the area being gripped. So,

standards have been developed to specify the shape of the specimen to ensure the break will occur in the

"gage length by reducing the cross sectional area or diameter of the specimen throughout the gage length. This

has the effect of increasing the stress in the gage length since stress is inversely proportional to the cross

sectional area under load.

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.

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

Equipment

Tensile test machines pull controlled loads on tensile or tension samples to measure material tensile properties

including uniaxial tension strength and elongation. Tensile test machines, tensile pull testers are special versions

of electromechanical universal testing machines and UTMs equipped with tensile testing accessories such as

tensile clamps, jaws, grips, fixtures for testing tensile bar, coupons, dog bone, and dumbbell sample geometries.

Other tensile tester accessories include axial extensometers, high temperature tensile furnaces, and ASTM ISO

DIN EN tensile fixtures made to tensile testing standard methods.

According to the loading type, there are two kinds of tensile testing machines; 1 – Screw Driven Testing Machine: During the experiment, elongation rate is kept constant. 2 – Hydraulic Testing Machine: Keeps the loading rate constant. The loading rate can be set depending on the desired time to fracture.

Types of fractures

A fracture is the (local) separation of an object or material into two, or more, pieces under the action of stress.

Figure Sequence of events in the necking and fracture of a tensile-test specimen: (a) early stage of necking; (b)

small voids begin to form within the necked region; (c) voids coalesce, producing an internal crack; (d) the rest of

the cross-section begins to fail at the periphery, by shearing; (e) the final fracture surfaces, known as cup- (top

fracture surface) and cone- (bottom surface) fracture.

Schematic illustration of the types of fracture in tension: (a) brittle fracture in polycrystalline metals; (b) shear

fracture in ductile single crystals ; (c) ductile cup-and-cone fracture in polycrystalline metals; (d) complete ductile

fracture in polycrystalline metals, with 100% reduction of area.

Brittle fracture

In brittle fracture, no apparent plastic deformation takes place before fracture. In brittle crystalline materials,

fracture can occur by cleavage as the result of tensile stress acting normal to crystallographic planes with low

bonding (cleavage planes). In amorphous solids, by contrast, the lack of a crystalline structure results in a

conchoidal fracture, with cracks proceeding normal to the applied tension.

Ductile fracture

In ductile fracture, extensive plastic deformation (necking) takes place before fracture. The terms rupture or

ductile rupture describes the ultimate failure of tough ductile materials loaded in tension. Rather than cracking,

the material "pulls apart," generally leaving a rough surface. In this case there is slow propagation and absorption

of large amount energy before fracture