Damage adhesive composite joints

The paper is presented in three parts. First, the tests employed to determine the mixed mode fracture envelope of a glass fibre reinforced epoxy composite adhesively bonded with either a brittle or a ductile adhesive are briefly described. These include mode I (DCB), and mixed mode (MMB) with various mixed mode (I/II) ratios. In the second part of the paper different structural joints will be discussed. These include single and double lap shear and L-specimens. In a recent European thematic network lap shear and double lap shear composite joints were tested, and predictions of failure load were made by different academic and industrial partners [9,10]. It was apparent that considerable differences existed between different analytical predictions and FE analyses, and correlation with tests proved complex. In particular, the progressive damage development in assemblies bonded with a ductile adhesive was not treated adequately. A more detailed study of damage mechanisms was therefore undertaken, using image analysis combined with microscopy to examine the crack tip strain fields and measure adherend displacements. This is described below and correlation is made between predicted displacements and failure loads, based on the mixed mode envelope determined previously, and measured values. 

Bonded joints are extensively employed in the construction of composite structures in aerospace applications, maritime structures, lifting equipment, wind mills as well as automotive industries [3, 4, 5], Unlike the bolt hole in mechanical fastening that causes a stress concentration in the composite joint plates, adhesively bonded joints minimize the potential for stress concentration within the joint. Besides, applications where lower structural weight, improved damage tolerance design philosophy are required, adhesively bonded joints provides a potential solution. Bonded joints are an efficent fastening solution also for hybrid structures, i.e., structures where composite parts are connected to metal parts. 

Even though adhesively bonded composite joints have a number of advantages over conventional mechanical fastening, they have some drawbacks too. Adhesively bonded joint is a permanent joint and it can t be disassembled for maintenance and damage inspection. Therefore, it is highly essential to monitor the state of the joint throughout its service life in order to prevent drastic failures, to optimize use of the structure which also identifies the scope of future product development and improvements. 

Bowditch MR (1996) The durability of adhesive joints in the presence of water. Int J Adhes Adhes 16 73-79 Bowditch MR, Harper TI, Lane JA (2004) Feasibility study to compare steel and adhesive/composite-based emergency repair methods for damaged hulls, HSE report 293... 

Amanda et al., [15] established a technique which holds promise for detecting damage and evaluating different failure modes in adhesively-bonded composite-to-metal single-lap shear joints. They compared electrical resistance data using carbon nanotubes in adhesive with AE and found good agreement. 

Lim AS, Melrose ZR, Thostenson ET, Chou TW. Compos Sci Technol, 2011,71. 1183-1189. Damage sensing of adhesively-bonded hybrid composite/steel joints using carbon nanotubes. 

Structural timber-concrete composites (e.g., timber-concrete composite slabs and timber wall-concrete deck composite in which the connection between the two common constmction materials is made through a bonded joint). The aim is to replace traditional mechanical fasteners by an adhesive connection, which has several advantages in comparison with the former for instance, a bonded joint is able to distribute the applied load over the entire bonded joint area, resulting in a more uniform distribution of stress (compared to mechanical point connections), requires little or no damage to the adherends, adds very little weight to the structure, and has a superior stiffness and fatigue resistance. 

Andre, A., Linghoff, D., 2009. Damage modelling of adhesive joint in composite reinforced metallic beams. In NSCC, September. 

---