Double Cantilever Beam Test Geometry Research Articles

The fracture behaviour of structural adhesives under high rates of testing

Structural adhesive joints were subjected to high loading rates in mode I and their resulting fracture behaviour was studied in detail. Joints were formed between unidirectional carbon-fibre epoxy composites and between aluminium alloy substrates bonded with a tough, single-part automotive adhesive (XD4600) from Dow Automotive. Double cantilever beam (DCB) and tapered double cantilever beam (TDCB) tests were performed, from quasi-static loading rates up to 15 m/s, and a test rig was developed incorporating high-speed video acquisition for the high-speed tests. A detailed data reduction strategy was developed to account for (i) the types of different fracture behaviour regimes encountered, (ii) the dynamic effects in the test data, and (iii) the contribution of kinetic energy in the specimen arms to the energy balance. Using the above data reduction strategy, increasing the test rate over six decades (from 10 −5 to 10 1 m/s) was found to lead to a reduction in the value of the adhesive fracture energy, G Ic , by about 40% of its quasi-static value, i.e. from 3.5 to about 2.2 kJ/m 2. Further, at quasi-static loading rates, the measured adhesive fracture energies were independent of substrate material and test geometry (i.e. DCB or TDCB). However, at faster loading rates, the TDCB tests induced higher crack velocities for a given loading rate compared with the DCB test geometry, and neither the test rate nor the crack velocity were found to be the parameter controlling the variation in G Ic with increased test rate. Thus, an isothermal–adiabatic model was developed and it was demonstrated that such a model could unify the DCB and TDCB test results. Indeed, when the G Ic values were plotted as a function of 1/√time, where the time was defined to be from the onset of loading the material to that required for the initiation of crack growth, the results collapsed onto a single master curve, in agreement with the isothermal–adiabatic model.

The effect of contact on the decohesion of laminated beams with multiple microcracks

The problem of interface decohesion in laminated beams is addressed with reference to the debonding double cantilever beam test geometry (DCB). The paper deals with the analysis of the influence of nonuniform bonding properties or interfacial defects on the crack propagation process and its stability. To this aim, the classical analytical approach based on the Euler–Bernoulli beam on an elastic foundation is extended to the presence of a general distribution of microcracks ahead of the macrocrack tip. The main features and limitations of this approach are carefully analyzed. In particular, it is shown that this simplified approach does not consider the unilateral contact condition along the interface, thus admitting a penetration between the two arms of the beam. A comparison with a finite element formulation is proposed to assess if this violation of the constraints inequalities, usually adopted in the case of uniform bonding, is still acceptable when interfacial defects are present. In order to fully describe the whole nonlinear behavior of the interface, a generalized interface constitutive law is used. The models comparison shows that, in the presence of interfacial defects, the effect of contact plays a crucial role in the description of the mechanical response of the joint.

A device for utilising the DCB test geometry at intermediate opening velocities

Interlaminar mode-I fracture characterisations of polymer matrix composites are generally realised statically using the Double Cantilever Beam (DCB) test geometry. For several applications, it is of interest to use this geometry at intermediate opening velocities (0.1 to 10 m/s) in order to measure evolutions of the fracture toughness with the loading rate. We show theoretically that a careful choice of the opening conditions is required for controlling the loading at the crack tip. In particular a symmetrical displacement imposed to the two arms of the DCB limits the vibrations of the sample. An experimental device is realised for reproducing experimentally these conditions on a glass/polyester composite. Up to the initiation of the crack propagation, the force and displacement signals are free of oscillations and the sample is mechanically equilibrated. It is then possible to interpret easily the data and measure the initiation value of G IC with the same methods that are used statically. The situation is different for the propagating value of G IC , since we observe that the sample is destabilised during the propagation of the crack.

The interfacial reaction of modified poly (arylether sulfone)s and polyaramides

It is shown that a fracture test using an asymmetric double cantilever beam test geometry is a powerful tool to study the effect of interfacial reactions on the improvement of the interfacial fracture toughness (G c ) of immiscible polymer systems. The G c values between a partially aromatic polyamide (PA) and a poly(arylether sulfone) (PSU) can be increased significantly when reactive PSUs are used which are obtained by grafting with maleic anhydride by introducing pyromellitic anhydride endgroups or by introducing carboxylic acid groups via copolymerization. Optical and atomic force microscopy investigations of the fracture surfaces show different failure mechanisms for weak and strong interfaces. For weak interfaces it was possible to determine the crack opening geometry using interference microscopy. For significantly reinforced interfaces rib marking lines on the PSU fracture surface can be observed. X-ray photoelectron spectroscopy (XPS) measurements reveal that with increasing toughness of the interface more and more cohesive failure of the PA occurs. This results in an increasing amount of nitrogen detected on the PSU fracture surface and simultaneously no sulfur is detected on the PA fracture surface.

In‐situ SEM observation of the fracture behaviour of titanium carbide/nickel aluminide composites

A range of experimental titanium carbide (TiC)/nickel aluminide (Ni3Al) composites have been developed. The Ni3Al content has been varied from 15 to 40 vol.%, for two different alloy compositions. The fracture behaviour of these composites was assessed in situ, within the chamber of a field emission gun scanning electron microscope. An applied moment double cantilever beam test geometry was employed, which allowed determination of the materials fracture resistance curve (or R‐curve), while simultaneously monitoring crack–microstructure interactions. All of the tested materials exhibited a pronounced R‐curve, with fracture resistance increasing with increasing crack length, up to a steady‐state plateau value. The highest steady‐state toughness values (up to 15 MPa m−1/2) were obtained for composites prepared with the highest Ni3Al binder content. In addition, these materials exhibited the highest strength and Weibull modulus. In‐situ examination of the fracture process demonstrated considerable crack wake bridging by ductile Ni3Al ligaments, with bridging zone length of >100 μm often observed. The use of a displacement mapping technique has demonstrated that Ni3Al plastic deformation is highly localized around the crack, typically within one TiC grain width (i.e. ≈3–5 μm) away from the crack.

Adhesion and welding in the system SAN/PMMA

Welding experiments using a double cantilever beam test geometry for a miscible poly(styrene-co- acrylonitrile) poly(methyl methacrylate) (SAN/PMMA) system showed the expected linear increase of the fracture toughness, G a, with the root of annealing time. For immiscible pairs of PMMA and SAN it was found that G a increases linearly with the equilibrium interfacial thickness or, equivalently, to χ − 1 2 , χ being the Flory interaction parameter.

Failure mechanisms of polymer interfaces reinforced with block copolymers

The fracture toughness (characterized by the critical energy release rate G c ) of interfaces between polystyren (PS) and poly(2-vinylpyridine) (PVP) reinforced with poly(styrene-b-2-vinylpyridine) was measured with a double cantilever beam test geometry. The effect of the PVP block degree of polymerization and the areal density of block copolymer chains at the interface on the measured G c and on the fracture mechanisms was investigated quantitatively.