UNIVERSAL TESTING MACHINEhttps://pursuitengineering.blogspot.com/2016/12/universal-testing-machine-compression.html

Introduction 

Mechanical testing plays an important role in evaluating fundamental properties of engineering materials as well as in developing new materials and in controlling the quality of materials for use in design and construction.

If a material is to be used as part of an engineering structure that will be subjected to a load, it is important to know that the material is strong enough and rigid enough to withstand the loads that it will experience in service.

As a result engineers have developed a number of experimental techniques for mechanical testing of engineering materials subjected to tension, compression, Fatigue, bending or torsion loading.

Universal Testing Machine

A universal testing machine (UTM), also known as a universal tester, materials testing machine or materials test frame, is used to test the tensile stress and compressive strength of materials.

It is named after the fact that it can perform many standard tensile and compression tests on materials, components, and structures

Universal Testing Machine

 Components

Load frame - usually consisting of two strong supports for the machine. Some small machines have a single support.

Load cell - A force transducer or other means of measuring the load is required. Periodic calibration is usually required by governing regulations or quality system.

Cross head - A movable cross head (crosshead) is controlled to move up or down.

Output device - A means of providing the test result is needed. Some older machines have dial or digital displays and chart recorders. Many newer machines have a computer interface for analysis and printing.

Test fixtures, specimen holding jaws, and related sample making equipment are called for in many test methods.

Tension test 

Tensile strength is defined as a stress, which is measured as force per unit area.

The most common type of test used to measure the mechanical properties of a material is the Tension Test. Tension test is widely used to provide a basic design information on the strength of materials.

The major parameters that describe the stress-strain curve obtained during the tension test are the tensile strength (UTS), yield strength or yield point (σy), elastic modulus (E), percent elongation (ΔL%) and the reduction in area (RA%).

Toughness, Resilience, Poisson’s ratio (v) can also be found by the use of this testing technique.

Tension test

Some materials will break sharply, without plastic deformation, in what is called a brittle failure. Others, which are more ductile, including most metals, will experience some plastic deformation and possibly necking before fracture.

The UTS is usually found by performing a tensile test and recording the engineering stress versus strain. The highest point of the stress-strain curve is the UTS. It is an intensive property.

Tensile strengths are rarely used in the design of ductile members, but they are important in brittle members. They are tabulated for common materials such as alloys, composite materials, ceramics, plastics, and wood. Tensile strength is defined as a stress, which is measured as force per unit area

Tension test

Concept

Many materials display linear elastic behavior, defined by a linear stress-strain relationship. For many applications, plastic deformation is unacceptable, and is used as the design limitation.

The UTS is not used in the design of ductile static members because design practices dictate the use of the yield stress. It is, however, used for quality control, because of the ease of testing. It is also used to roughly determine material types for unknown samples.

Objective 

Tension test is carried out; to obtain the stress-strain diagram, to determine the tensile properties and hence to get valuable information about the mechanical behavior and the engineering performance of the material.

Tension test

Testing System

The testing system consists of a tensile testing machine, a load cell, a power supply. Testing Machine is of hydraulic type (Universal Testing Machine). It is a load-

controlled machine. Load Cell provides an electrical circuit for measuring the instantaneous load along

the loading axis. Power Supply is connected to load cell. It feeds the load cell, amplifies the output

signal and displays the load.

Specimen

Tensile specimens are machined in the desired orientation and according to the standards. The central portion (gage portion) of the length is usually of smaller cross section than the end portions. This ensures the failure to occur at a section where the stresses are not affected by the gripping device.

Tension test

Procedure

Before the test1. Put gage marks on the specimen2. Measure the initial gage length and diameter3. Select a load scale to deform and fracture the specimen. Note that that tensile strength of the material type used has to be known approximately.

During the test1. Record the maximum load2. Conduct the test until fracture.

After the testMeasure the final gage length and diameter. The diameter should be measured from the neck.

Tension test

Tensile specimen after testing

Fatigue Test

A machine part or structure will fail, if improperly designed and subjected to a repeated reversal or removal of an applied load at a stress much lower than the ultimate strength of the material. This type of time-dependent failure is referred to as a cyclic fatigue failure such as suspended bridges, rails, or airplane wings

The failure is due primarily to repeated cyclic stress from a maximum to a minimum caused by a dynamic load.

The basic mechanism of a high-cycle fatigue failure is that of a slowly spreading crack that extends with each cycle of applied stress.

In order for a crack to propagate, the stress across it must be tension; a compression stress will simply close the crack and cause no damage.

Fatigue TestConcept

Though the fluctuating load is normally less than the yield strength of the materials, it results in fracture behaviour which is more severe than that achieved from static loading.

Fatigue failures are therefore unpredictable, and provide high-risk situations, if the operators are not aware of material behaviour when subjected to fatigue loading.

Most machinery and many structures do not operate under a constant load and stress. In fact, these loads and stresses are constantly changing.

A good example of this is a rotating shaft such as the axle on a railroad car. The bending stresses change from tension to compression as the axle rotates.

This constant change in stress can cause fatigue failure in which the material suddenly fractures. The process that leads to fatigue failure is the initiation and growth of cracks in the material.

Fracture occurs when the crack grows so large that the remaining uncracked material can no longer support the applied loads. The change in the loading with respect to time is more common from an engineering perspective and is generally considered to be mechanically induced.

Fatigue Test

Fatigue TestProcedure

The fatigue specimen is gripped on to a motor at one end to provide the rotational motion whereas the other end is attached to a bearing and also subjected to a load or stress.

When the specimen is rotated about the longitudinal axis, the upper and the lower parts of the specimen gauge length are subjected to tensile and compressive stresses respectively. Therefore, stress varies sinusoially at any point on the specimen surface.

The test proceeds until specimen failure takes place. The revolution counter is used to obtain the number of cycles to failures corresponding to the stress applied.

Increasing of the weight applied to the fatigue specimen results in a reduction in number of cycles to failure.

Fatigue Test

Torsion Test 

When a member of any cross sectional shape is subjected to a torque along its longitudinal axis, the torque tends to produce a rotation in the member with respect to its longitudinal axis.

This rotation causes twist the in member and this state is known as torsion.

Torsion testing is widely used for evaluating the elastic modulus, strength, shear modulus, shear strength and other properties of materials.

The main difference between torsion testing and tensile testing is that the stress is not uniform over the cross section of the test specimen. The only useful exception to that is a round thin-walled tube, when the wall is sufficiently thin.

Torsion Test

Concept Torsion theory was limited to the circular members and was developed on the following assumptions:

The cross section must be circular, without taper, no stress concentrations and the axis of rotation of the bar must be straight.

Torque must be applied by shear stresses that vary linearly with the same distance from the axis.

Angle of twist must be small and varies linearly along the longitudinal direction. Plane cross sections of the bar do not change after angular deformation and all radii must

remain straight: cross sections do not warp.

In many areas of engineering applications, materials are sometimes subjected to torsion in services, for example, drive shafts, axles and twisted drills. Moreover, structural applications such as bridges, springs, car bodies, airplane fuselages and boat hulls are randomly subjected to torsion.

Even though torsion test is not as universal as tension test and do not have any standardized testing procedure, the significance lies on particular engineering applications and for the study of plastic flow in materials. Torsion test is applicable for testing brittle materials such as tool steels.

Torsion Test

Torsion Test

The assumptions made in experiment include but are not limited to the following:

The torque is applied along the center of axis of the shaft.The material is tested at steady state (absence of strain rate effects).Plane sections remain plane after twisting (the circular section conforms to this condition).

 Bending Test

A bending test, also known as a bend test, is used to determine the strength of a material by applying force to the item in question and seeing how it reacts under pressure.

Typically the bend test measures ductility, the ability of a material to change form under pressure and keep that form permanently. In certain cases the bending test can determine tensile strength.

When using the bend test for this purpose, testers examine which side of the material breaks first to see what type of strength the material has. It also lets them know what kinds of pressure it holds up against and what kinds it doesn't.

To determine how ductile a material is, a bending test is used. Force is applied to a

piece of the material at a specific angle and for a specific amount of time.

The material is then bent to a certain diameter using force. After the bending test is over, the material is examined to see how well it held its shape once the pressure was removed, and whether or not the material cracked when pressure was applied.

Bending Test

Compression Test

Compression tests are used to determine how a product or material reacts when it is compressed, squashed, crushed or flattened by measuring fundamental parameters that determine the specimen behavior under a compressive load.

These include the elastic limit, which for "Hookean" materials is approximately equal to the proportional limit, and also known as yield point or yield strength, Young's Modulus (these, although mostly associated with tensile testing, may have compressive analogs) and compressive strength

Compression Test

Types of Compression Testing

Types of compression testing include Flexure/Bend Spring Testing Top-load/Crush

Compression Test

Benefits of Compression Testing

Compression testing provides data on the integrity and safety of materials, components and products, helping manufacturers ensure that their finished products are fit-for-purpose and manufactured to the highest quality.

The data produced in a compression test can be used in many ways including:

To determine batch quality To determine consistency in manufacture To aid in the design process To reduce material costs and achieve lean manufacturing goals To ensure compliance with international and industry standards 

Compression Test

Materials under Compression

Certain materials subjected to a compressive force show initially a linear relationship between stress and strain. This is the physical manifestation of Hooke's Law, which states

E = Stress (s) / Strain (e)

Where E is known as Young's Modulus for compression. This value represents how much the material will deform under applied compressive loading before plastic deformation occurs. A material's ability to return to its original shape after deformation has occurred is referred to as its elasticity.

Vulcanized rubber, for instance, is said to be very elastic, as it will revert back to its original shape after considerable compressive force has been applied.