Specimen alignment plays a critical role in material testing. If a specimen is not properly aligned prior to or during testing, the accuracy and the reliability of test results will be directly affected, and, depending on the method followed, results may not be acceptable. This blog post covers the effect of specimen misalignment on the test data, main reasons behind axial misalignment, and ways to improve alignment precision in mechanical test setups.
Ideal alignment occurs when the top and bottom test fixtures are in one straight line with the direction of force, the loading train, and with one another. In addition, the specimen must be clamped between the top and bottom grip jaws at the same centerline. Image below shows test setups with (1) an alignment tensile specimen, (2) specimen in skewed position, and (3) specimen that is off centered between the grips.
Tensile Specimen Alignment
How does specimen misalignment affect test results?
Specimen misalignment refers to misalignment in the direction of loading and in the axial direction of the test specimen. When test specimens are not properly aligned, bending moments occur between the applied force and the specimen axes during testing. Test data then include the additional bending strain superimposed on the axial strain. Since misalignment does not occur the same way in each test, results obtained will often vary from test to test.
Misalignment is recognized in a wide range of mechanical testing and fatigue testing activities. Many standard test methods accept results based on % bending or strain amplitude and specify the acceptable % bending. However, in certain test scenarios bending moments significantly influence the final test results and action must be taken to ensure precise alignment.
How does misalignment occur?
Testing machines may have deviations between the top and bottom grip centerline positions. ASTM E1012 notes a deviation between 0.03 to 3.18 mm (0.001 to 0.125 in) or more is within the standard range. In addition, it should be expected that test machine frames may become out of alignment as continuous load is applied over months and years.
Test fixtures and grips mounted on the testing frame using various couplings including threaded and unthreaded components, spherical seats and universal joints may also affect alignment precision.
Test specimens are often machined within the usual tolerance range of +0.05 to +0.25 mm (+0.002 to +0.010 in.). Although this is the standard tolerance range, in certain testing scenarios, these tolerances may contribute to poor alignment. In addition, the design of the specimen, such as length and length-to-diameter ratios, directly affect alignment precision.
Universal testing machines allow testing in various load directions and accommodate the use of a variety of test fixtures and grips. The test setup selected for a tensile test may include tensile wedge grips, hydraulic grips, manual vise, or pneumatic grips. In addition, if high or low temperature testing is performed, the system will include an environmental chamber.
Misalignment will occur if the test specimen slips from the grips. There are also instances where the grips clamp the specimen successfully in room temperature testing but when tested in elevated temperatures, slippage occurs and data is skewed due to bending strain occurring from misalignment.
Alignment Gripping Solutions
GW-XT Wedge Grips
Certain grip designs allow automatic sample alignment. ADMET high-capacity wedge grips are offered with a door and a crank handle to secure the sample in the grip jaws. The specimen is first inserted in between the grips. Before tightening the jaws, the door must be closed and the door latch must be secured with at least 75% of the specimen placed inside the grip jaw surface.
Other tensile grip designs come with the optional alignment fingers. Alignment fingers are metal inserts against which the sample can be placed to facilitate specimen alignment as well as quick specimen insertion and replacement.
Translation and Angle Alignment Coupling
Fatigue Grips with Alignment Fingers for Small Specimens
ADMET grips with the angle alignment coupling ensure that the specimen is perfectly vertical in the plane of the grip jaws. The incorporated alignment fingers are inserted, aligned, and tightened down to the grip faces, then removed once specimen is in exact alignment.
This design is now being commonly used to avoid misalignment in fatigue testing, in small specimen testing, and testing of specimens in environmental chambers. Designed in-house by ADMET Engineering, both the grips and the alignment coupling can be customized based on the dimensions of the samples.
How to detect misalignment?
Severely misaligned specimens will be visible to the eye after tightening the specimens in the grips.
If results are not as expected and misalignment is of concern, specimens can be marked after installation between the grip jaw faces to analyze if and where misalignment occurred.
Digital image processing systems may be used to observe specimen behavior in the testing setup and make corrections as needed.
Elastic strains at two or more positions about the perimeter of a specimen at constant stress loading can be measured to detect whether misalignment affects the results. If strain values are different, specimen alignment may be off.
ASTM International has developed ASTM E1012 to verify the alignment of testing machines, associated components and test specimens for practices that require the application of tensile or compressive forces under conditions where alignment is important.
ASTM E1012 Alignment Verification
ASTM E1012 has been developed to verify the specimen alignment under tensile and compressive loading. An alignment specimen and strain gauge sensors are required to verify the test equipment alignment. Strain gauges must be installed on the alignment specimen, which is then loaded multiple times to provide bending strain data for comparison.
ASTM E1012 is a complex standard that should be reviewed in detail.