Study of Drilling-Induced Damage in Water-Degraded Natural Fibre Composites
Oliveira JC, Oliveira PD, Rocha MA and Loureiro NC*
ABSTRACT
Composite materials have experienced a growing adoption in high consumption industrial sectors such as transportation, aeronautics and aerospace, owing to their high specific strength, low density and design versatility [1,2]. However, their heterogeneous and anisotropic nature makes them particularly susceptible to several damage mechanisms, including matrix cracking, fibre breakage, delamination and interfacial degradation [3-5].
During drilling operations, many of these mechanisms occur simultaneously. Among the most common are fibre rupture, interlaminar delamination and matrix cracking, resulting from the thrust force and torque applied by the cutting tool [6,7]. Delamination, in particular, is widely recognised as the most critical failure mode, as it can significantly compromise the structural integrity of the component [8].
The objective of this study was to quantify the damage induced by drilling in natural fibre reinforced composites, assessing the influence of moisture induced degradation on their mechanical behaviour. For this purpose, specimens were prepared for tensile, flexural and bearing tests, enabling the evaluation of structural performance under different integrity conditions.
A portion of the laminated composites was submerged in water for a controlled period until saturation was reached, promoting degradation through moisture absorption.
This process is known to reduce fibre–matrix adhesion, induce matrix plasticization and accelerate delamination phenomena [9,10]. After saturation, the specimens were exposed to ambient air to allow natural evaporation, completing a hygroscopic ageing cycle that intensifies the deterioration of mechanical properties.
Subsequently, the specimens were drilled and subjected to radiographic inspection. The radiographic images were digitally processed using MATLAB®, enabling segmentation of the damaged region and quantification of the delaminated area. Based on these measurements, several delamination factors proposed in the literature were calculated, allowing a quantitative assessment of drilling induced damage [11-13].
Finally, bearing tests were conducted to evaluate the loss of mechanical strength associated with the combined effects of moisture degradation and drilling. The results were compared with previous studies to contextualize the impact of humidity on the structural integrity of the composites.
The results revealed a significant reduction in mechanical properties when the composite was first exposed to moisture and subsequently drilled. Therefore, the interaction between these two factors is critical for understanding the mechanical behaviour of natural fibre reinforced composites, corroborating findings reported in recent studies [7,9,14].
