End notched flexureNo joint geometry has been standardised for Mode II testing. Problems do still exist. These problems include the plastic deformation of the substrate arms upon testing, the effect of friction on GIIc and the formation of micro-cracks ahead of the crack tip. There are several potential test methods including the end notched flexure (3-point and 4-point) and end loaded split methods.
End notched flexure
End-Notched Flexure (ENF): This test has emerged as the most commonly used test method for quantifying GIIc for metallic and laminated polymer composite adherends. The ENF test is essentially a DCB specimen loaded in three-point flexure. The dimensions are similar to those employed for the DCB specimen. The test is straightforward and can be conducted using standard mechanical test equipment. A flexure fixture is required, which can be purchased from a test machine manufacturer or produced in-house. Commercial fixtures are available.
Both static and fatigue loading can be used with these specimens. Tests may also be conducted under simulated service environments such as hot humid environments. Analysis is straightforward, although there is concern as to the method of pre-cracking specimens (see below).
The critical strain-energy release rate or fracture toughness GIIc is calculated as follows:
where P is applied load, E is the flexural modulus of adherend in the longitudinal direction, b is the specimen width, a is the crack length and h is the adherend thickness. The values of GIIc need to be corrected for transverse shear deformation and large displacement effects. Crack growth rate da/dN can be related to the total strain energy release rate GMAX using an empirical power-law relationship (i.e. Paris Law).
A 4-point version of the ENF method has been developed [64] for composite materials with the intention of overcoming the problems of unstable crack propagation associated with the 3-point test geometry. The specimen geometry is similar to the 3-point test. There has been limited success reported using this method. Specimens still tend to fail in unstable manner.
End-Loaded Split
End-Loaded Split: This test method employs a specimen similar to the DCB, which is held at one end in the support fixture and loaded through an end block bonded to the other end [65, 66]. The support fixture is large weighing approximately 25 kg so as to prevent vertical movement of the fixture. The fixture has been designed to slide on a linear bearing trolley so that load point is kept fixed.
The critical strain-energy release rate or fracture toughness GIIc is calculated as follows [66]:
where P is applied load, E is the flexural modulus of adherend in the longitudinal direction, b is the specimen width, ac is the effective crack length and h is the adherend thickness. F is a correction factor to account for large beam deflections. The value of F is determined using the calculated crack lengths rather than the measured values (see Appendix A in reference [66]).
| Advantages | Disadvantages |
|
Yields Mode II fracture toughness Compatible with metals and PMCs Straightforward/economic ASTM standard for composites Suitable for cyclic/environmental testing (QA only) |
Limited to rigid adherends and brittle adhesives Flexural test fixture required Unsuitable for generating design data Analysis required to account for Non-linear load-displacement Pre-cracking difficult Crack extension measurements difficult Moderate to large uncertainties in measurements No existing national or international standards |
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