Universal testing machine (UTM) is used to test the mechanical properties (tension, compression etc.) of a given test specimen by exerting tensile, compressive or transverse stresses. The machine has been named so because of the wide range of tests it can perform over different kind of materials. Different tests like peel test, flexural test, tension test, bend test, friction test, spring test etc. can be performed with the help of UTM.
The main features of the components and the functions of UTM are discussed in this article.
Components of Universal Testing Machine (UTM)
A universal testing machine consists of two main parts :
1. Loading Unit
2. Control Unit
The arrangement of the test specimen and the exertion of the load is held in the loading unit. The variations in the application of the load and the corresponding test result are obtained from the control unit.
In the image, the left unit with table and crosshead form the loading unit. The right unit with a load indicator arrangement is the control unit.
A. Loading Unit
The loading unit of a UTM consists of the following components :
>> Load Frame
>> Upper crosshead and Lower crosshead
>> Elongation Scale
1. Load Frame
The load frame of a universal testing machine can be made either by single support or by double support. The load Frame consists of a table (where the specimen is placed for the compression test), upper crosshead, and lower crosshead.
2. Upper Crosshead and Lower Crosshead
The upper crosshead is used to clamp one end of the test specimen. The lower crosshead in the load frame is the movable crosshead whose screws can be loosened for height adjustment and tightened. Both the crossheads have a tapered slot at the center. This slot has a pair of racked jaws that is intended to grip and hold the tensile test specimen.
3. Elongation Scale
The relative movement of the lower and upper table is measured by an elongation scale which is provided along with the loading unit.
B. Control Unit
The main components of the control unit in a universal testing machine are :
>> Hydraulic Power Unit
>> Load Measuring Unit
>> Control Devices
1. Hydraulic Power Unit
This unit consists of an oil pump that provides non-pulsating oil flow into the main cylinder of the load unit. This flow helps in the smooth application of load on the specimen. The oil pump in a hydraulic power unit is run by an electric motor and sump.
2. Load Measuring Unit
This unit has a pendulum dynamometer unit that has a small cylinder with a piston which moves with the non-pulsating oil flow. The pendulum is connected to the piston by pivot lever. The pivot lever deflects based on the load applied to the specimen. This deflection is converted to the load pointer and displays as the load on the dial.
The range of load application can be adjusted by means of a knob in the load measuring unit (0-100 kN; 0-250 kN; 0-500 kN and 0-1000 kN). The accuracy of measuring unit controls the overall accuracy of the machine.
3. Control Devices
The control devices can be electric or hydraulic. Electric control devices make use of switches to move the crossheads and switch on/off the unit. A hydraulic control device consists of two valves, Right Control Valve and Left Control Valve or Release Valve. A right control valve is used to apply load on the specimen. The left control valve is used to release the load application as shown in figure-1.
Functions of Universal Testing Machine
The main functions of UTM are to test the mechanical properties of materials. The standard tests performed by UTM are:
- Tensile Test
- Compression Test
- Adhesion Tests
- Pull-Out Tests
- Bending Test
- Hysteresis Test
UTM can be used to test a wide variety of materials like concrete, steel, cables, springs, steel wires and chains, slings, links, rope, winches, steel ropes, etc.
UTM provides the value of load application and the respective displacements. From the observed value, the load deflection graph is obtained. With the load value in the Y-axis and the displacement in the X-axis. The displacement is actually the movement of the crossheads during the load application.
Force v/s Displacement ( cross-head movement) Graph.
From the load deflection graph, the stress-strain analysis, modulus of elasticity, yield strength of the specimen tested can be determined.