Universal testing machines (FSA, ZPM)

Universal testing machines, also known as UTMs, are sophisticated laboratory systems for carrying out complete materials testing: tensile, compression, bending, shear, interlaminar shear, static fatigue, creep and relaxation. The designations FSA (Floor-Standing Apparatus) and ZPM (Zero-Point Machine) identify high-end configurations used in R&D and in certified laboratories. They are characterised by the superior stiffness of the frame, the metrological accuracy of the load cell (classes 0.5 and 1 to ISO 7500-1) and the precision of the displacement measuring system (contact extensometers, video extensometers, DIC systems).

Configurations

UTMs are available in several configurations: single column (loads up to 5 kN); twin-column vertical (the most widespread, up to 100 kN); gantry (for higher loads, up to 600 kN or more); horizontal (for long specimens such as cables and ropes); biaxial (for combined tensile-torsion testing on tubes and components). A high-stiffness frame is essential to minimise the elastic deformation of the machine under load, which adds to that of the specimen and introduces errors in the force-displacement curve. High-end ZPMs achieve stiffness of hundreds of kN/mm, guaranteeing superior accuracy.

Measuring systems

The accuracy of a UTM depends on the load cell (strain gauge, class 0.5 or 1 to ISO 7500-1: ±0.5% or ±1% error on the force read, from 2% of full scale upwards) and on the strain measuring system. Contact (clip-on) extensometers measure the elongation of a portion of the specimen with accuracies of 0.5% F.S.; video extensometers track markers applied to the specimen with cameras and tracking algorithms; DIC systems (Digital Image Correlation) capture the entire 2D or 3D strain field by means of a stochastic pattern applied to the specimen surface, giving deformation maps, strain fields and local strains.

Materials testing

UTMs run tests to the main standards: tensile testing of metals (ISO 6892-1 at room temperature, ISO 6892-2 at high temperature, ISO 6892-3 at low temperature); tensile testing of plastics (ISO 527-1/2); tensile testing of composites (ISO 527-4/5); compression of metals (ASTM E9); bending of plastics (ISO 178); bending of metals; tensile testing of fabrics (ISO 13934); tensile testing of rubber (ISO 37); pull-off testing of adhesives and coatings; seal strength testing on packaging; bending strength testing on packaging; and crash testing on automotive components (at low to medium speeds).

Grips and special fixtures

The essential accessories are: mechanical grips for metal specimens (wedge, hydraulic, pneumatic), grips for plastics and fabrics (manual, pneumatic, screw-type), supports for bending tests (three or four point with adjustable span), compression platens for parallel plates, centreing systems, anti-rotation devices and special jaws. For advanced applications: climatic chambers for testing at controlled temperature (-70 / +250 °C), humidity chambers, liquid nitrogen cooling systems and water test tanks for environmental simulation.

Software and analysis

The software of a UTM manages: standardized test programs compliant with the standards, acquisition of force-displacement and force-strain curves, automatic calculation of parameters (maximum load, yield, elastic modulus, elongation at break, energy, toughness and so on), comparison with acceptance specifications, batch statistics, generation of formal test reports, traceable archiving and integration with LIMS. The advanced functions include fatigue analysis, cyclic testing, simulation of complex load profiles and synchronization with external DIC systems.

Standards and calibration

UTMs follow ISO 7500-1 for metrological verification (part 1 static machines, part 2 cyclic), with accuracy classes 0.5, 1, 2 and 3 according to the error on the measured force. Periodic calibration uses high-end master load cells calibrated to ISO 376 with traceability to national primary standards. Verification of axis squareness, grip alignment and machine stiffness is an integral part of qualification.