Damage progression on fiber reinforced polymer (FRP) adhesively bonded single lap joints (ABSLJ) under quasi-static tension

Abstract

Adhesively bonded joints have proven to outperform their mechanically fastened joint counterparts, as they present a more structurally efficient method of load transfer, lower stress concentrations and better fatigue performance at reduced weight. In the specific case of the Adhesively Bonded Single Lap Joint (ABSLJ), bending-induced stresses that result from the load path eccentricity add up to the adherend inplane stresses. Moreover, significant peak peel and shear stresses develop at the lap ends of the adhesive and associated adherend interlaminar tensile stresses have a detrimental effect on the joint’s strength. Such joints made of Fiber Reinforced Polymer (FRP) adherends bonded with an epoxy adhesive layer sustain a substantial amount of damage, from failure onset to ultimate failure. With the purpose of design structurally efficient and damage tolerant composite joints, it is essential to understand the stress distribution and to accurately predict the damage initiation and propagation events in such joints made of composite materials.

A well-established set of Damage Progression Models (DPMs) in the framework on the Continuum Damage Models (CDMs) were developed as a tool to predict the global response,damage initiation load and ultimate load of the specimens. Hashin 3D, Puck and LaRC05 werethe implemented failure criteria to detect the initiation of damage in the adherends. After thispoint, the elastic properties of the detected damage elements were reduced according to sudden and gradual material degradation models. As for the adhesive, the von Mises criterion was used to detect the damage onset and a linear softening law modeled the material degradation. For the validation of the DPMs, the numerical results were compared against the data of an already published experimental study. Four different adherend layup sequences: [45/90/ − 45/0]2푠, [90/−45/0/45]2푠, [0/45/90/−45]2푠 and [45/0/−45/0]2푠 were studied based on data extracted
from the mechanical testing, Digital Image Correlation (DIC) and Acoustic Emission (AE).

Good correlations between numerical predictions and averaged experimental linear stiffnesses were found, particularly for the two configurations with the outmost ply at 45∘, for which the difference was lower than 5%. The initial non-linear stage of the global response seems to be governed by the longitudinal bending stiffness, while the subsequent linear behavior is controlled by the longitudinal membrane stiffness of the adherends. Regarding damage initiation, numerical predictions showed to be 11.5%, 7.5%, 29.9% and 6.1%, respectively, more conservative for the four analysed configurations, when compared to the AE results, whose established criterion should be further developed. With respect to the ultimate load, the relative differences between predictions and tests showed significant variability among the tested configurations; specifically the deviations were of: 33.2%, 37.4%, -0.4% and -13.71%.

Despite the encouraging results, an inherent shortcoming of CDMs is the representation of damage in a smeared manner due to the homogenization of the anisotropic material in the modeling process. A blended framework using CDMs to model intralaminar failure and discrete crack models to model interlaminar failure and matrix cracking might lead to more realistic damage patterns.