Fracture toughness determination of adhesive and co-cured joints in natural fibre composites

Abstract

a b s t r a c tAdhesive bonding has become more efficient in the last few decades due to the adhesives developments, granting higher strength and ductility. On the other hand, natural fibre composites have recently gained interest due to the low cost and density. It is therefore essential to predict the fracture behavior of joints between these materials, to assess the feasibility of joining or repairing with adhesives. In this work, the tensile fracture toughness (G c n ) of adhesive joints between natural fibre composites is studied, by bonding with a ductile adhesive and co-curing. Conventional methods to obtain G c n are used for the co-cured specimens, while for the adhesive within the bonded joint, the J-integral is considered. For the J-integral calculation, an optical measurement method is developed for the evaluation of the crack tip opening and adherends rotation at the crack tip during the test, supported by a Matlab sub-routine for the automated extraction of these quantities. As output of this work, an optical method that allows an easier and quicker extraction of the parameters to obtain G c n than the available methods is proposed (by the J-integral technique), and the fracture behaviour in tension of bonded and co-cured joints in jute-reinforced natural fibre composites is also provided for the subsequent strength prediction. Additionally, for the adhesively-bonded joints, the tensile cohesive law of the adhesive is derived by the direct method.

Adherend thickness effect on the tensile fracture toughness of a structural adhesive using an optical data acquisition method

“…Fig. 6 also shows some deviation between specimens of identical conditions, but perfectly within reported deviations under identical testing conditions [40]. Actually, this scatter is related to experimental phenomena such as fabrication issues or small measurement errors or geometry deviations.…”

“…6. The deviation is larger, though, and whose justification lies on the experimental process to obtain GIC, which relies on a number of measured parameters and approximation functions, which are difficult to adjust to the experimental data [40]. While for the specimens with h ¼ 1 mm, a value of GIC ¼ 0.78170.146 N/mm was obtained, improvements of 12.6%, 37.7% and 40.2% were attained by increasing h up to 4 mm.…”

“…The process detailed here is an improvement of the work developed by the authors in reference [40]. For calculating δn and θo for a given image, the optical method requires the identification of eight points (Fig.…”

“…(5). in reference [40], where an average value of 1.182 N/mm was found in DCB specimens of natural fibre composites with h ¼ 5 mm. The tn-δn law for the test specimen followed in this section is presented in Fig.…”

Experimental estimation of the mechanical and fracture properties of a new epoxy adhesive

“…The CBBM is a relatively straightforward but robust method, based on an equivalent crack length (a eq ), and it only depends on the specimen's compliance during the test. Applied to the DCB test specimen, it gives Detailed explanations of the method can be found in the work of Campilho et al [26]. The value of a eq is estimated from the current specimen compliance and taking into (5) consideration the damage zone, E f is a corrected flexural modulus to account for stress concentrations at the crack tip and stiffness variability between specimens, and G AD is the shear modulus of the adherends.…”