Universality of universal testing machine

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release time : 2021-04-14 15:01:02

They are as common as dandelions in spring and as common as Swiss army knives. Sometimes they will accumulate dust and are destined to enter a dark corner, and sometimes they will be polished to a mirror surface due to heavy daily use. Of course, we are talking about Universal Test Machine (UTM).

Why is it universal? Historically, force testing machines have been called tensile testing machines because they can only pull force. Adapting them to compression requires the use of a mechanical cage, which converts tensile force into compressive force, leading to other measurement errors. Later pioneers developed UTM, which has the additional function of pulling and pushing without the use of adapters.

Today, the term UTM has been used interchangeably with "material tester" to reflect its main purpose. UTM can test various materials and components to provide valuable data for quality control and engineering personnel. Test results are mainly used to characterize materials, but can also be used for pass/fail testing of finished products and components.

Configure UTM

UTM includes the following main components:

Load cell

Test stand

Encoder / linear scale

Data collection software

Fixtures and fixtures

1. Load cell

Strain gauge-based load cells are the core of most measurement systems in UTM. A strain gauge applied to a deformable structure will stretch and compress when the structure is loaded. When strain gauges deform, their resistance will change, thus breaking the balance in the balanced circuit. This imbalance is measured, amplified and processed by the data acquisition hardware. The signal is converted into digital form, which can be used by indicator and PC. The data is expressed in optional units of measurement, such as lbF, kgF, and kN.

In some basic UTM, load cells and related electronic equipment can be replaced with dynamometers. The dynamometer integrates the load cell and indicator in a housing. The dynamometer can perform certain functions in a more efficient manner than a PC, such as peak force capture, high-speed data capture, and other real-time functions that are critical to speed and timing.

2. Test framework

UTM's wide range of endurance reflects a wide range of applications-from peeling a bag of potato chips to crushing concrete bridges. They can be single row (usually used for smaller forces) or double row (used for larger forces). Most UTMs are driven by one or two lead screws, while some machines use hydraulic systems, especially in cyclic testing applications.

The accuracy of force measurement is most affected by the range and accuracy of the load cell. UTM can be equipped with a variety of load cell forces. For example, a machine with a capacity of 10 kN can be used with a 500 N load cell to perform a wire crimp pull test, and then switch to a 10 kN load cell to perform a tensile test on the metal belt.

3. Encoder or linear scale

In addition to force, many applications also need to measure the deflection of the sample, such as spring and tensile testing. In this case, when the screw moves the crosshead up and down, the integrated encoder or linear grating ruler can measure the travel distance. Where small deformations may occur, such as metal tensile tests, extensometers can provide better resolution. The extensometer eliminates the error caused by the bending of the frame and load cell. The extensometer physically clamps two points on the sample and measures the tension or compression between them. Recent technologies include non-contact video and laser methods.

4. Data collection software

In some cases, raw force and distance data are sufficient, but in many applications, the functions of UTM will be demonstrated through data collection software, which calculates the results in a practical and useful way. For example, the elongation test of an elastic band can be represented by a curve of force versus distance or stress strain. When the sample reaches and exceeds its yield strength, the arc of the curve will change until it reaches its ultimate strength and finally drops to its breaking point. A lot of information can be obtained by analyzing the curve, including spring stiffness, Young's modulus and many other values. More advanced software packages can automatically calculate these values and overwrite previous test results for comparison.

5. Fixtures and fixtures

In order to deal with almost unlimited applications, many standard handles have been developed over the years. Common types include parallel jaw clamps for fabrics and films, wedge clamps for plastics and metals, compression plates for springs and foams, etc. Most standard handles use eye adapters to mount load cells, dynamometers and machine bases. This method uses pins and lock nuts to prevent the handle from loosening, and provides a way to quickly switch the handle for different tests.

UTM and force gauge

Although the huge gray area crosses the boundary between force testing and material testing, UTM can satisfy both applications. Its configuration, especially its distance measurement technology and data processing capabilities, determines whether it is a force tester or a material tester. The answers to the following questions help determine the level of complexity required to perform the test:

Do you need force or deflection data, or just force?

Is only a peak force required, or does the application require a force vs. time or force vs. deflection curve?

How much deflection measurement is required?

Do you need any specific calculations, such as spring stiffness, percent elongation, Young's modulus, etc.? Or, is the requirement simpler-for example, maximum force, breaking distance, or force at a given distance?

Is it overkill?

Modern dynamometers can perform many functions historically reserved for UTM. However, the important consideration to remember is that UTM can perform force testing, but force testers cannot perform material testing.

Compared with the material tester with the same load capacity, the cost of ownership of the force tester is lower. They are ideal for dedicated, well-defined applications that do not require complex data analysis or extensometers. However, the simplicity of the force gauge may limit its use in a diverse and constantly changing application environment. In this case, even if the full capabilities of UTM are not used every day, cost-benefit analysis may still make the balance in favor of UTM.

Maintain UTM

In addition to standard preventive maintenance procedures (such as lubrication of lead screws and bearings), calibration is also the most important service of UTM to ensure measurement accuracy. For some UTMs, calibration can only be performed on-site on the machine. Using other UTMs, the dynamometer or load cell and related electronic equipment can be removed and sent to the calibration laboratory. Either way, a regular calibration schedule is important to ensure that ongoing tests are accurate.

Through proper maintenance and regular calibration, UTM should be a loyal main force for many years, and hope that over time will develop a fine polishing agent, rather than a fine dust layer.

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