Test Machine and Specimen Alignment
Gripping of Specimens
Strain and Displacement Measurement Techniques
Contact Extensometers
Non-Contact Extensometers
Strain Gauges
Crosshead Displacement
Linear Voltage Displacement Transducers (LVDTS)
Electronic Speckle Pattern Interferometry (ESPI)
References
This section considers the effect of test parameters (i.e. test machine and specimen alignment, load train stiffness, methods of gripping test specimens, accuracy of load and displacement transducer) on the accuracy and reliability of strength and long-term performance of bonded joints. Guidance is provided on the main factors that need to be controlled when carrying out mechanical testing. Consideration is given to cyclic fatigue and creep loading, and environmental conditions (i.e. elevated humidity and temperature).
The test machine should have high lateral rigidity and accurate alignment between the upper and lower gripping faces. The load train should be as short and as stiff as possible (i.e. no universal joints included). If the grips are articulated, as in the case of universal joints, then the specimen may be subjected to large bending and twisting loads, resulting in reduced joint strength. Avoid eccentric acting forces. Small lateral (1 to 2 mm) or angular (1 to 2 degrees) offsets in the loading train can lead to additional shear and bending stresses, resulting in premature joint failure. It is worth noting that the slope of the load-displacement response can be similar for poor and well-aligned specimens.
Figure 1: Alignment specimen with strain gauges
It is recommended that the alignment of the test machine and test specimen be checked at the centre of the gauge length using a strain gauged coupon specimen (Figure 1) [1-2]. Alignment specimens, which can be in the form of a rectangular or circular bar, need to be accurately machined to ensure errors in parallelism are < 0.2 mm/m and in concentricity (lateral offset) of 0.03 mm [1]. Strain gauges are bonded to the surface of the alignment specimen in order to monitor alignment and bending strains. Bending strains should be less than 3 to 5% of the average axial strain.
It is recommended that a device be used to ensure that the specimens are positioned in the grips in a repeatable manner (see Figure 1). An alignment fixture (or cell) can also be included in the loading train to minimise angular and lateral offset between the upper and lower machine grips or loading rods. The alignment cell is attached to the upper or lower crosshead of the test frame; whichever is the most convenient. Commercial alignment cells are available that allow lateral movement, tilt and rotation of the machine grip or loading rod. Strain gauges attached to the test piece can also be used to check alignment.
Figure 2: Bolted aluminium T-joint with strain gauges
Grips for holding test specimens to be loaded in tension should be attached to the test frame so that the major axis of the test specimen coincides with the direction of pull through the centreline of the gripping assembly (see Figure 2). The centre line of the specimen should be aligned with the axis of the loading fixtures to avoid bending and asymmetric loading. It is important than when loading test specimens in the grips that no lateral or angular offset is introduced to the specimen. Features, such as diagonal struts (Figure 3) provide additional rigidity, thus preventing side-ways movement of the test specimen.
Figure 3: Side loading fixture with bonded aluminium T-joint
Avoid rotating the grips during gripping operation. If one of the grips is articulated, this should be tightened first to prevent the specimen being subjected to large bending and twisting loads during tightening. Care should be taken to avoid axially stressing the specimen whilst the grips are being tightened. Any pre-stressing of the specimen should be kept to a minimum. Grips should be slowly tightened with any induced loads removed by progressively adjusting the crosshead position. The applied load on the specimen should be zero at the onset of testing. It may be necessary to use a device (i.e. metal spacer) during the test set-up to ensure good alignment and repeatable test results, as often the specimen width is less than the width of the mechanical grips.
Manual or servo-hydraulic grips can be used to hold specimens during testing. Wedge-action grips are recommended as the lateral force (i.e. pressure) applied to the test specimen in the gripping regions increases as the axial load applied to the specimen increases. Servo-hydraulic grips provide uniform pressure in a controllable manner. Gripping pressure should be sufficient to prevent specimen slippage throughout the duration of the test, but not excessive to initiate failure of the specimen at the grips. For cyclic loading, it is essential that fretting in the gripped region be prevented to avoid the possibility of premature failure.
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