Figure 4: TAST specimen with extensometers for measuring shear deformation
A number of contact and non-contact techniques are available for measuring strain and displacement. This section considers the use of contact extensometers, linear voltage displacement transducers (LVDTs), video extensometers, electronic speckle pattern interferometry (ESPI), strain gauges and crosshead movement for measuring strain and displacement under ambient and hostile environments, and static and cyclic fatigue loading conditions. With the exception of strain mapping techniques (e.g. ESPI and digital image correlation (DIC)), the measured strain will be an average strain in the bond-line. Also, strain gauges only measure strain at the location of the gauge.
Contact extensometers are the preferred method for measuring strain and displacement, and hence stiffness of bonded joints. It is recommended that two extensometers, attached to opposite faces of the specimen, be used to measure displacement [3]. Any bending of the specimen will be apparent from diverging displacement readings. It is recommended that the individual transducer readings be recorded so that the quality of the test data can be checked. Errors due to minor bending are minimised by taking the average measurement of the two displacement transducers. To minimise inclusion of adherend deflection in the measurement the contact points should be as close to the bond layer as possible. The deformation of the adherends needs to be accounted for when analysing the data, but where the stiffness of the adherend is very much greater than that of the adhesive layer then corrections may be minimal. Special transducers have been developed with three-point contact for measuring shear deformation (e.g. thick adherend shear test (TAST) shown in Figure 4 [3]). The three-point contact minimises rotation of the extensometers.
Where adherends are flexible, it is advisable to support the weight of the extensometer because allowing the extensometer to hang unsupported from the specimen may cause bending and introduce contact stresses. The contact forces should be sufficient to prevent slippage between the extensometer and the specimen, but not large enough to cut or nick the specimen surface causing the specimen to fail prematurely. It may be necessary to remove extensometers attached to a specimen prior to failure in order to prevent the possibility of the extensometer sustaining damage during failure. Failure can be a violent event, releasing considerable energy, thereby damaging or even destroying the extensometer.
An extensometer should be capable of measuring the change in gauge length with an accuracy of 1% of the applied displacement or better (i.e. equivalent to ± 0.5 mm for 10 % strain over a typical bond thickness of 0.5 mm). It is important that the extensometers are able to operate satisfactorily within the test environment (i.e. temperature and humidity), and that these devices are resistant to chemical attack when used in hostile environments. Precautions may need to be taken to insulate the leads to prevent moisture ingress.
Non-contact or optical extensometers (e.g. video extensometers) are available, which avoid contact damage and can be used up to failure, since there is no possibility of damage to the extensometer. Video extensometers are not particularly suited to measuring small strains (e.g. movements of a few mm), which limits their applicability to structural adhesive joints. There are however no restrictions on the upper limit. This is a convenient method for determination of strains above 10% (i.e. suitable for use with toughened epoxies or elastomers).
The technique relies on a remote camera monitoring the separation of two marks or lines inscribed on the test specimen, which define the gauge length. The change in separation of the two lines is recorded throughout the test. The gauge marks should be approximately equidistant from the mid-point, and the measured distance between the marks should be measured to an accuracy of 1%, or better. Gauge marks should not be scratched, punched or impressed on the specimen in any way that may cause damage to the specimen. It is advisable to ensure that there is a sharp contrast in colour between the specimen surface and the gauge marks. The lines should be as narrow as possible. There are no temperature restrictions as video extensometers can be located outside the test chamber. Some video extensometers are capable of measuring longitudinal and lateral strains (deformation) simultaneously.
Furthermore, measurement is normally only possible at one side of the joint so that bending cannot be evaluated. However, video extensometers have been used in tests on joints bonded with flexible adhesives where deflections are larger. Measurements of joint stress-strain curves have been in reasonably good agreement with contact extensometer results. Some modern systems provide capabilities for dot location measurements, which allows a limited strain mapping capability.
Next: Strain Gauges
