The Different Types of Universal Testing Machine

In addition to the climatic tests we commonly deal with, many materials must undergo mechanical tests to verify their resistance before being placed on the market.

The laboratory instruments that perform this type of test are called universal testing machines , and there are several types.

In the following article we will see what the different types of universal testing machines, what type of tests they perform and why they cannot do without climatic chambers.

What is the Universal Testing Machine?

A universal testing machine is a laboratory instrument capable of applying forces of various types and magnitudes to materials. It is called a universal machine due to its wide range of possible uses, but in the past it was more simply known as a "tensile machine" or "dynamometer."

As we'll see, the structure of these machines can vary, but they generally comprise a frame composed of one or two support columns. At the center of the frame is the moving crossbeam with an attached load cell that applies a controlled force to the sample being tested.

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How to Distinguish the Different Types of Universal Testing Machine

There are three parameters for classifying the different types of universal testing machines: operating principle, construction features and intensity of the force that can be developed.

Universal testing machines can have three different operating principles:

• Hydraulic
• Electromagnetic
• Tire

The most common are hydraulic, while the pneumatic ones are the rarest.

Regarding construction features, universal machines vary greatly depending on the model, and it is impossible to comprehensively summarize all the versions on the market. One of the fundamental differences, as mentioned, is the single- or double-column frame construction. Single-column universal machines are rarer and more limited in use. 

Finally, universal testing machines can be distinguished by the intensity of the force that the machine can develop, which can vary from a few hundred N up to 1000 or more kN.

Universal Testing Machine Tests

Materials passing through the universal testing machine must undergo two main categories of tests: the compressive strength test and the ultimate tensile strength test.

The compressive strength test is generally carried out in uniaxial conditions and verifies the resistance of materials to loads that tend to deform them and reduce their extension, or to compress them. 

It is mainly used on concrete and is regulated by this international standard.

The breaking load test, on the other hand, identifies the limit, in terms of applied external force or stress, beyond which a material becomes definitively unusable from the point of view of resistance.

It is further divided into 5 distinct tests, depending on the type of force applied:

• Tensile breaking load
• Compressive breaking load
• Breaking load in straight bending
• Torsional breaking load
• Breaking load at shear stress

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The FDM Environmental Chamber for Conditioning Testing in the Universal Machine

Some breaking load and compression tests are called “conditioning” because they stress the material to certain environmental conditions.

These tests must be carried out by placing the universal testing machine inside an environment where it is possible to control the humidity and temperature parameters, i.e. a climatic chamber.

The FDM environmental chamber is the ideal instrument for performing this type of combined testing and can be built in various sizes to accommodate all customer needs.

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Reference Standards

• ISO 7500-1 — Metallic materials – Calibration and verification of static uniaxial testing machines – Part 1: Tension/compression testing machines – Calibration and verification of the force-measuring system.
• ASTM E4 — Standard Practices for Force Verification of Testing Machines. American equivalent of ISO 7500-1 for calibration of universal testing machines.
• ISO 9513 — Metallic materials – Calibration of extensometer systems used in uniaxial testing.
• ASTM E83 — Standard Practice for Verification and Classification of Extensometer Systems.
• ISO 6892-1 — Metallic materials – Tensile testing – Part 1: Method of test at chamber temperature.
• ISO 6892-2 — Tensile testing at elevated temperature (applicable to thermal chamber tests).
• ASTM E8/E8M — Standard Test Methods for Tension Testing of Metallic Materials.

Authoritative Sources

• ISO – International Organization for Standardization. Maintains the ISO 7500 and ISO 6892 series for universal testing machines.
• ASTM International. North American reference standards ASTM E4, E83, E8.
• INRIM – National Institute for Metrological Research. Italian institute for force scale traceability (mass and gravitational acceleration age).
• ACCREDIA. Italian accreditation body for load cell and strain gauge calibration laboratories.

Frequently Asked Questions about the Universal Testing Machine

What accuracy classes does ISO 7500-1 provide?

ISO 7500-1 classifies the machine based on the relative error of the load cell. Class 0.5: relative error ≤ 0.5% of the indicated value, required for material characterization tests (R&D, aerospace batch qualification). Class 1: error ≤ 1%, industry standard for mechanical tests ISO 6892 and ASTM E8. Class 2: error ≤ 2%, less critical quality control applications. Class 3: error ≤ 3%, comparative tests only. Calibration is performed with sets of Class E2/F1 OIML R111 sample weights or with reference force transducers with an uncertainty of ≤ 0.1%. Typically, verification is performed at 20%, 40%, 60%, 80%, and 100% full scale with at least 5 rising and falling points to detect hysteresis. Recommended frequency: annually or every 10⁴-10⁵ load cycles.

How is a tensile test performed according to ISO 6892-1?

ISO 6892-1 defines method A and method B for uniaxial tensile testing at chamber temperature (10-35 °C) of metallic specimens. Method A (closed-loop strain rate): active strain rate control, 0.00025/s or 0.0067/s according to agreement; requires an ISO 9513 Class 1 extensometer (error ≤ 1%). Method B (crosshead speed): crosshead speed control, 5-30 mm/min for elastic, accelerating after yield. Main measurements: Rp0.2 (yield strength at 0.2% offset), Rm (ultimate tensile strength), A (elongation at break), Z (necking area). The standard specimen is type 3 with diameter d₀ = 10 mm and gauge length L₀ = 5d₀ = 50 mm according to ISO 6892 (proportional format). Acquisition requires frequency ≥ 10 Hz, accuracy F ± 1% and ε ± 1% according to classes.

When is a climate chamber integrated into a universal machine needed?

The integrated climatic chamber is required for non-ambient temperature testing according to ISO 6892-2 (high temperature, > 35 °C), ASTM E21 or ASTM E606 (fatigue). Typical ranges: -70 to +350 °C for structural metals, -80 to +200 °C for polymers, -196 °C (liquid nitrogen) for aerospace cryogenic steels. Integration requires: (1) high temperature extensometer (KM type strain gauge for < 350 °C, LVDT extensioneter for cryogenic applications) with ISO 9513 class 1; (2) Inconel 718 or Nimonic 80A superalloy grips for > 300 °C; (3) optical windows for non-contact video-extensometer (DIC system). The equilibrium soak time prior to testing is typically 30-60 minutes to ensure uniformity ± 3 °C over the gauge length according to Annex standards

How to calibrate a load cell and an extensometer?

The load cell is calibrated according to ISO 376 with a Class 00, 0.5 or 1 reference dynamometer (uncertainty 0.03%, 0.1%, 0.3%) applied to the mechanical chain. Procedure: 3 preload cycles at 100% FS, then 3 series of measurements with the dynamometer rotated 120° to compensate for misalignment effects. Typical combined uncertainty ≤ 0.5% for a Class 1 cell. Theextensometer is calibrated according to ISO 9513 with a certified displacement comparator (laser micrometer or interferometer) over the useful range, verified at 20%, 40%, 60%, 80%, 100% with forward-reverse scanning. Traceability goes back to the SI Units via ACCREDIA/INRIM. The calibration certificate documentation must include: uncertainty, correction tables, environmental conditions (20 ± 2 °C, 50 ± 10% RH), validity period typically 12 months.