Universal testing machine
A universal testing machine, 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.
- 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. Usually this is at a constant speed: sometimes called a constant rate of extension (CRE) machine. Some machines can program the crosshead speed or conduct cyclical testing, testing at constant force, testing at constant deformation, etc. Electromechanical, servo-hydraulic, linear drive, and resonance drive are used.
- Means of measuring extension or deformation- Many tests require a measure of the response of the test specimen to the movement of the cross head. Extensometers are sometimes used.
- 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.
- Conditioning - Many tests require controlled conditioning (temperature, humidity, pressure, etc.). The machine can be in a controlled room or a special environmental chamber can be placed around the test specimen for the test.
- Test fixtures, specimen holding jaws, and related sample making equipment are called for in many test methods.
The set-up and usage are detailed in a test method, often published by a standards organization. This specifies the sample preparation, fixturing, gauge length (the length which is under study or observation), analysis, etc.
The specimen is placed in the machine between the grips and an extensometer if required can automatically record the change in gauge length during the test. If an extensometer is not fitted, the machine itself can record the displacement between its cross heads on which the specimen is held. However, this method not only records the change in length of the specimen but also all other extending / elastic components of the testing machine and its drive systems including any slipping of the specimen in the grips.
Once the machine is started it begins to apply an increasing load on specimen. Throughout the tests the control system and its associated software record the load and extension or compression of the specimen.
- ASTM E4 - Practices for Force Verification of Testing Machines
- ASTM E74 - Practice for Calibration of Force Measuring Instruments for Verifying the Force Indication of Testing Machines
- ASTM E83 - Practice for Verification and Classification on Extensometer Systems
- ASTM E1012 - Practice for Verification of Test Frame and Specimen Alignment Under Tensile and Compressive Axial Force Application
- ASTM E1856 - Standard Guide for Evaluating Computerized Data Acquisition Systems Used to Acquire Data from Universal Testing Machines
- JIS K7171 - Standard for determine the flextural strength for plastic material & products
- ISO 5893 Rubber and plastics test equipment - Tensile, flexural and compression types (constant rate of traverse) - Specification
- EN ISO 6892-1: Metallic materials - Tensile testing - Part 1: Method of test at room temperature (replaces EN 10002)
- EN ISO 7500-1/-2 Metallic materials - Verification of static uniaxial testing machines - Part 1: Tension/compression testing machines - Verification and calibration of the force-measuring system; Part 2: Tension creep testing machines - Verification of the applied force
- Many other standard tests for specific materials and products. These usually call for the details of machine set-up, fixturing, and testing.
- Beach, Chandler B., ed. (1914). "Testing-Machine". The New Student's Reference Work. Chicago: F. E. Compton and Co.
- Video of a universal testing machine performing a tensile test