Issue 46

A. Akhavan-Safar et alii, Frattura ed Integrità Strutturale, 46 (2018) 266-274; DOI: 10.3221/IGF-ESIS.46.24 267 K EYWORDS . Epoxy; Micro cork particles; Micro Failure mechanisms; Fracture toughness; Volume fraction. I NTRODUCTION pplications of adhesives and specially epoxy resins, as an alternative to traditional fasteners, have considerably increased in different industries [1, 2]. Although epoxies show good mechanical properties, the structure of these thermoset polymers causes insufficient energy absorption with a low resistance to the propagation of cracks [1, 3, 4]. For this reason, improving their toughness has been extensively studied in the last decades [5-7]. Adding cork particles to epoxies as a toughening method has been considered in some works [8, 9]. Cork is elastic and light and consists of different layers with honeycomb shape which make it interesting for engineering applications [10, 11]. Cork’s low thermal conductivity combined with a reasonable compressive strength makes it an excellent material for thermal insulation purposes and when compressive loads are present [10, 12]. Also, different researchers have considered the cork as a sound insulator [13]. However, some limited studies have investigated structural aspects of these natural particles. Barbosa et al. [8] studied the effect of size and volume fraction of cork particles on the impact toughness of adhesive bonding. In their analysis, the cork particles ranging from 38 to 250 μm were considered. Also, the volume fraction of cork was changed from 1 to 5% by weight. According to their results, a small volume fraction of cork particles presents a better impact energy absorption than a large volume fraction of small particles. To investigate the effect of moisture on the degradation of an adhesive reinforced with micro cork particles, the same authors [9] studied the moisture effect on the adhesive enhanced with micro-cork particles. Their results showed that the cork particle does not have a great influence on the absorption/desorption of moisture. Barbosa et al. [14] also studied the effect of natural cork particles on the stress-strain behavior of an epoxy based adhesive. Their results showed that the cork particles cause higher ductility and improve the failure strain of the adhesive. In general, the cork particles have no direct influence on the curing process, although they slightly change the curing mechanism [15]. The current research deals with the assessment of the micro failure mechanisms of an epoxy enhanced with micro cork particles. The influence of the particle volume fraction on the strength of joints and bulk specimens was considered. To achieve this, single lap joints and dog-bone tensile specimens enhanced with various volume fractions of cork particles were manufactured and tested at room temperature. The fracture surfaces of the joints and bulk specimens were then analyzed using SEM technique and a magnifying glass. E XPERIMENTS Materials and properties raldite 2020, from Huntsman Advanced Materials (Pamplona, Spain) was used to bond the substrates. Tab. (1) gives the experimentally obtained mechanical properties of the tested epoxy. High strength steel was used as substrates to guarantee the elastic behavior of the adherends until joint failure. Cork particles of 125–250 μm size were also used to toughen the adhesive. It was shown in previous studies [8] that the mentioned size corresponds to the best mechanical properties of the composite resin/cork. The cork particles were supplied by Amorim Cork Composites (Mozelos, Portugal), without treatment. The particles used in the current paper are from a source similar to that of the previous study [16], and subsequently has the same size distribution shown in Fig. (1). Fig. (2) shows a typical structure of a micro cork particle. Young’s modulus (E) 2.7 GPa Poisson’s ratio (  ) 0.37 Maximum tensile strength (σ max ) 55 MPa Maximum tensile strain (ε max ) 3.5% Table 1 : Mechanical properties of the adhesive. A A