J-integral analysis of the mixed-mode fracture behaviour of composite bonded joints

Abstract

The adhesive technology has been constantly growing and expanding into industrial environments, not only for traditional applications but also for high-end applications, where it has been competing fairly with the conventional connection technologies, such as welding, brazing, bolting and riveting. Its unique key features allow it to raise the type of technology to unreachable levels, for certain applications, by its competitors. Some of the advantages are the lightness of the adhesively-bonded joints, good behaviour under cycling and fatigue loading conditions, flexibility in bonding several types of materials and low stress concentrations. However, in order to design and develop efficient adhesively-bonded joints, the strength prediction must be accurate for the assessment of the fracture properties, mainly the critical energy release rate for tensile (JIC) and shear (JIIC), associated to the mode I and II, respectively. For most of the adhesively-bonded joints applications, the loading conditions under operational service feature a combination of different stresses, for instance tensile and shear stresses, from which the concept of mixed-mode came to exist. For this reason, the assessment of fracture properties under those conditions is essential, especially the energy release rates related to different mode-mixities. The fracture properties are related to Fracture Mechanics and are obtained through energetic analyses, from which three methods are often used: models based on the measurement of the crack length during the damage propagation, models based on an equivalent crack length and methods based on the Jintegral formulation. In the specific case of the J-integral it is furthermore possible to obtain the cohesive laws of the adhesive, which can be later used in the design of adhesively-bonded joints. This current work presents an experimental and numerical analysis of a Single-Leg Bending (SLB) adhesively-bonded joint where the specimens were bonded with three distinct adhesives, in order to assess and compare their behaviour under mixed-mode load conditions, fracture properties and cohesive laws. For that purpose, the J-integral formulation of Ji et al. [1] was considered to obtain the energy release rate for mode I and II, tensile (JI) and shear (JII), respectively, whereas the cohesive laws are attained through direct differential operation of the JI-w0 and JII-δ0 curves, where w0 and δ0 are the local normal separation and local tangential slip between the two adherends at the

Evaluation of the cracked lap shear test for mixed-mode fracture toughness estimation of adhesive joints

“…The data points of da Silva are offset to the other methods. Former works [36][37][38] using the SLB test resulted in α = 0.5 for the AV138 and α between 0.5 and 1 for the 2015, which was positively validated by CZM modelling. Thus, in this work, the numerical part that follows will clarify the found differences.…”

Numerical Modelling and Validation of Mixed-Mode Fracture Tests to Adhesive Joints Using J-Integral Concepts

“…Then, it was found that the chosen composite was a suitable alternative to conventional thermoset composites. Similarly, SLB for adhesive joints were modelled by Santos and Campilho [17] and Loureiro et al [18]. In both cases, the numerical models reproduced the experimental setup performed by the authors.…”

“…The procedure in this work involved using an optical method founded on taking high-resolution pictures during the tests (Figure 3), and then performing a vector and geometric analysis of the images captured during the tests by imaging software to produce a value of θ p , δ n , and δ s for each picture, which can be correlated with the testing machine data. More details about this procedure and geometrical extraction of the parameters from the pictures can be found in previous work [18].…”

“…Then, these results together with the results from G IC and G IIC lead to obtaining the fracture envelopes and the exponent values for the power laws. More recently, Loureiro et al [18] performed a similar experimental campaign testing seven specimens per adhesive type, for a total of three adhesive types. In this case, the J-integral method was used to calculate G I and G II .…”

“…These results agreed with previous research, indicating their validity. Furthermore, the parameters obtained in these works are necessary for the numerical modelling of bonded joints [17,18].…”