Micro failure analysis of adhesively bonded joints enhanced with natural cork particles: Impact of overlap length and particles volume fraction

Abstract

In this work, the effects of natural particles on the micro failure mechanisms and static strength of an epoxy are studied using scanning electron microscope (SEM) technique. To this end, bulk specimens and adhesively bonded single lap joints (SLJs) containing different amount of micro cork particles were fabricated and tested. To investigate the toughening mechanisms, the fracture surfaces of the specimens were analyzed using SEM and a magnifying glass. It was shown that the deviation of the crack path and formation of micro-cracks generate multi fracture surfaces which is one of the main sources of energy absorption and resulted in higher fracture energy of the enhanced adhesives. Corks also acts as a crack stopper, stabilizes the crack propagation and prevents unstable crack growth. However, the micro analysis showed that higher contents of cork lead to agglomeration of the particles which acts as a source of stress concentration, with a higher possibility of delamination between the matrix and the micro corks. Accordingly, there is an optimum amount of cork where the best adhesive fracture toughness is reached. The results showed that the optimum value of micro particles depends on the loading conditions.

Influence of microcork particles on the lap shear strength of an epoxy adhesive subjected to fatigue loading and different environmental conditions

“…Although the effect of cork particles on the mechanical behavior of adhesive bulk specimens was considered previously, the effect of cork particles on the strength of adhesive joints needs to be studied. The results recently published by the present authors 17 show that cork particles can significantly increase the shear strength of joints. Based on the obtained results, 5 vol.% of cork particles increase the failure load of the tested joints up to 70%.…”

“…For the static part, these mechanisms were already explained by the authors. 17 The formation of ripples (which are also called striations) is also one of the main microfailure mechanisms, which only occur in the cyclic loading condition. It also should be noted that the striation mechanism of metals is different from the striations of adhesives and there is only a similarity in appearance.…”

“…A typical tensile stress–strain curve of the adhesive is shown in Figure 1. The mechanical properties of the tested adhesive, already obtained in Akhavan-Safar et al., 17 are shown in Table 1. It was shown in previous studies 12 that cork particles of 125–250 μm size correspond to the best mechanical properties of the composite resin/cork.…”

“…This leads to more energy absorption which increases the fracture loads of the specimens. However, as mentioned in the literature, 17 a higher content of cork increases the possibility of agglomeration of the particles, acting as a source of stress concentration, with a higher possibility of lack of adhesion between the matrix and the microcorks, which results in a decrease of the static strength.…”

Cellular Auxetic Structures for Mechanical Metamaterials: A Review

“…Figure 4 shows examples of cellular microstructure in nature. Examples of cellular microstructure in nature; (a) micro cork particle [44], (b) wood [45], and (c) bone [46]. Figure 5 shows that cellular auxetic meta-materials can be categorized into re-entrant structures, chiral structures, rotational (semi-) rigid structures, crumpled and perforated sheet models, and other miscellaneous structures.…”