Material testing is that part of engineering design, development and research that relies on laboratory testing of one kind or another to answer questions. Testing is also required during manufacturing to ensure a material or product meets some predefined specification. A universal testing machine is used to measure the mechanical properties of materials in tension, compression, bending or torsion. This blog post goes over the two different types (electromechanical and hydraulic) of universal testing machines and why accuracy, repeatability, and resolution are important in obtaining correct test results.
Mechanical testing requires not only familiarity with measurement systems, but also some understanding of the planning, execution, and evaluation of experiments. On numerous occasions a company has contacted us because their test results did not match results from another lab or the results they were presently obtaining were different than historical values even though their manufacturing process had not changed.
There are two classes of testing machines, electromechanical and hydraulic, the principal difference is how the load is applied.
Electromechanical Testing Machines
The electromechanical machine employs a variable speed electric motor, gear reduction system and one, two or four screws to move the crosshead up or down. This motion loads the specimen in tension or compression. A range of crosshead speeds can be achieved by changing the speed of the motor. A microprocessor based closed-loop servo system can be implemented to accurately control the speed of the crosshead.
eXpert 2600 Universal Testing Machines
eXpert 2600 dual column electromechanical testing systems are available in table top or floor standing configurations and can go up to 600kN (137,000lbf) force capacity. The servo-control motor allows running tests at very slow net deflection rates.
eXpert 7600 Universal Testing Machines
eXpert 7600 single column electromechanical universal testing machines, are offered in three force capacities: 1kN (225lbf), 2.5kN (500lbf), and 5kN (1,000lbf). These units fit within a 431x520mm space and feature the industry’s largest vertical test space, making them ideal for testing high elongation materials such as rubber.
eXpert 5000 Universal Testing Machines
eXpert 5000 series electromechanical testing machines feature modular single and dual column frame components, base plates and detachable actuators. This set up provides the ability to configure your system in a variety of vertical or horizontal orientations for a wide range of applications. A popular configuration with the eXpert 5000 systems is the ASTM D3574 foam testing configuration for static and dynamic testing in tension and compression directions.
eXpert 4000 MicroTester
eXpert 4000 electromechanical micro testing systems are well suited for testing small sized samples of tissue, bone, biomaterials, fibers, threads, gels, thin films, metals, wire and more. With force capacities up to 5kN and a wide variety of grips, fixtures, heating and cooling chambers and fluid baths, the eXpert 4000 MicroTest systems are adaptable to a variety of high magnification imaging systems.
Hydraulic Testing Machines
A hydraulic testing machine employs either a single or dual acting piston to move the crosshead up or down. Most static hydraulic testing machines use a single acting piston or ram. In a manually operated machine, the operator adjusts the orifice of a pressure compensated needle valve to control the rate of loading. In a closed loop hydraulic servo system, the needle valve is replaced by an electrically operated servovalve for precise control.
All ADMET testing machines employ a closed loop system that continuously sends information from the closed-loop controller to the motor and also from the motor to the closed-loop controller. This constant feedback allows certain variables such as the load rate and the stress rate to remain as specified throughout the tests. Closed-loop systems provide higher accuracy due to the ability to react immediately to possible changes.
eXpert 1000 Universal Testing Machines
eXpert 1000 servo-hydraulic testing systems are ideal for testing metals, composites, medical devices and implants, concrete, webbing, and other materials at very high load capacities. Each frame employs strain gauge load cells for direct measurement of force. No need to compensate for piston friction and other non-linearities, these frames offer exceptional accuracy and precision while reducing long-term calibration and service costs. eXpert 1600 series are designed for static testing whereas the eXpert 1900 series are configured to meet the force-stroke-frequency requirements of fatigue testing applications.
Accuracy, Repeatability, and Resolution
The three basic definitions to remember with respect to how well a testing machine can measure stress and strain are accuracy, repeatibility, and resolution.
Accuracy is the ability to tell the true position of the crosshead. Accuracy is the maximum error between any two crosshead positions.
Repeatability (precision) is the ability of the crosshead to return to the same position over and over again. Repeatability is the error between a number of successive attempts to move the crosshead to the same position.
Resolution is the larger of the smallest programmable steps in crosshead position or the smallest mechanical step the crosshead can make.
Factors that Affect Accuracy, Repeatibility, and Resolution
A good test engineer must have an excellent understanding of the sources of errors that may be introduced during a test. Before commencing testing, the test engineer should review the choice of sensors and measurement instruments keeping in mind the suitability and accuracy of each. In order to make accurate measurements, in other words, one should know how to measure the errors in order to keep them from creeping into the results.
Sensors are at the heart of all mechanical testing measurements. The test frame, power transmission, grips and fixtures also affect the accuracy and repeatability of it’s sensors. Sensors that are mounted in the wrong position, are heated up, or are deformed by mounting bolts all introduce measurement errors.
The most important consideration in mounting a sensor is where to mount it in order to ensure that the desired quantity is accurately measured. One thing to consider is whether the sensor should be mounted on the input or output ends of a transmission. If the sensor is mounted on the input end of a transmission along with a motor, then the resolution of the system will be enhanced by a factor equal to the transmission ratio. However, backlash and compliance in the transmission, belts, ballscrews, test frame, grips and fixtures will also affect the output of the sensor. On the other hand, if the sensor is mounted on the output end of the transmission, it will more accurately measure the process but the resolution will be reduced.
Sensor Location in ASTM D790 Plastics Bend Testing
ASTM D790 governs the determination of the flexural modulus of unreinforced and reinforced plastics. The test setup requires a three point bend fixture with the loading nose midway between the supports, shown in the electromechanical testing system anatomy figure, above. The supports and loading nose are shown in light blue. The loading nose contacts the rectangular specimen at point 4 and is directly connected to the load cell. The test procedure involves deflecting the specimen until rupture occurs in the outer surface of the test specimen or until a maximum strain is reached.
Tangent modulus, secant modulus and chord modulus are three properties of interest. All three require accurate force and flexural strain measurements in order to obtain proper modulus readings. Assuming the load cell has been verified to meet ASTM E4 accuracy requirements, all force measurements should accurately represent the applied force. Flexural strain is directly related to the deflection of the specimen at the point midway between the supports.
Example 1 Using the Rotary Encoder Mounted on the Motor to Measure Flexural Strain
Most modern day electromechanical testing machines measure linear crosshead position with a rotary encoder mounted to the motor. The motor shaft, right angle transmission, synchronous belt, tapered roller bearings, ballscrew, ballnut, moving crosshead, load cell and loading nose are between the rotary encoder and the test specimen. When a force is applied to the specimen, strain measurement errors are introduced by the following:
– Torsional compliance in the motor shaft due to the applied torque. Because no machine component is truly rigid, one can think of the motor shaft as a torsion spring with a certain amount of torsional stiffness.
– Torsional compliance and mechanical backlash between mating gears in the right angle transmission. – Stretch in the synchronous drive belt.
– Compliance in the tapered roller bearings. Tapered roller bearings deform non-linearily, especially at loads which are a fraction of their rating. Preloading the bearings causes a proportionately smaller amount of deflection but may reduce the effective repeatability and resolution of the moving crosshead.
– Compliance and lead error in the ballscrews. A compressive load applied to the specimen will create a tensile load in the ballscrews which will cause them to stretch.
– Backlash in the ballnuts. When in the unloaded condition, gravity will cause the ball bearings in the ballnuts to be in contact with the upper bearing race. When the applied compressive load exceeds the weight of the moving crosshead, load cell and loading nose, the ball bearings will switch to contacting the lower bearing race.
– Compliance in the moving crosshead, load cell, loading nose, specimen supports and machine base. Again, no machine component is truly rigid, one can think of each component as a spring.
With this in mind, the important questions is: “How large is the total error compared to the strain I am trying to measure?”. There is no clear cut answer but if the overall stiffness of the machine is much greater than the stiffness of the specimen, one may be able to use this method for measuring flexural strain. A careful analysis of the test setup would be in order prior to measuring the flexural strain with the motor encoder.
Note: If one could replace the test specimen with a specimen that was infinitely rigid, the load vs. strain curve as measured by the rotary encoder would be non-linear. The non-linearities make it very difficult to map out the machine errors in software.
Example 2 Using a Linear Displacment Transducer between the Moving Crosshead and Top Bracket to Measure Flexural Strain
A second approach to measuring flexural strain might be to install a linear displacement transducer between the top bracket (in green) and the moving crosshead. In the figure above, it is shown as Displacement B. For this arrangement, when a force is applied to the specimen, strain measurement errors are introduced by the following:
– Compliance in the tapered roller bearings.
– Compliance in the ballscrews.
– Backlash in the ballnuts.
– Compliance in the moving crosshead, load cell, loading nose, specimen supports and machine base.
Because the Displacement B Transducer is closer than the rotary encoder to the specimen, there are fewer sources of error. Like example 1, however, there is no clear cut answer as to whether the errors introduced by measuring the relative movement between the moving crosshead and top bracket are small enough to be inconsequential. Again, a careful analysis of the test setup and results are in order.
Suggested Methods of Measuring Flexural Strain
When the methods used to measure flexural strain as outlined in example 1 and 2 are not sufficient, a device that measures the relative displacement between the underside of the specimen midway between the supports (point 3 in figure above) and the machine base (point 5) is commonly used. One such device is a deflectometer. Errors introduced by compliance in the supports and machine base are usually much smaller than the flexural strain in the specimen.
The method with the smallest measurement error involves attaching two bars on opposite sides of the specimen at points 1 and 2, on figure above. Points 1 and 2 are directly above the supports and reside on the neutral axis of the specimen. The bars only contact the specimen at points 1 and 2 and remain straight and unstressed when a load is applied to the specimen. A linear transducer is then affixed to the bar midway between the supports and measures the deflection of the specimen at point 4.
Click below for further reading on universal testing machine motor control and how ADMET controllers ensure that the specified test rate is accurately maintained throughout the test.
All experimental measurements include errors. Before commencing testing, a good test engineer will always ask the question: “Are my measurement errors small enough to not matter?” A thorough understanding of the sources and magnitudes of the errors is paramount to making accurate measurements.