An infrared thermoelastic stress analysis investigation of single lap shear joints in continuous and woven carbon/fiber epoxy composites Rami Haj-Ali a , Rani Elhajjar b,n a School of Mechanical Engineering, Faculty of Engineering Tel-Aviv University, Ramat-Aviv, Israel b Department of Civil Engineering and Mechanics, University of Wisconsin-Milwaukee Milwaukee, WI, USA article info Article history: Accepted 30 June 2013 Available online 10 October 2013 Keywords: Bonded joints Composites Thermoelastic stress analysis Thermography Infrared abstract A full-field thermoelastic stress analysis infrared method is used to study the damage initiation and progression in prepreg uni-tape and woven carbon fiber/epoxy composite single lap joints. Two loading schemes are studied to detect the damage initiation in these joints. In the first scheme the loading is monotonically increased with cyclic loading performed at the holding times. In the second scheme, the loading is increased gradually and then decreased, followed by cyclic loading at the holding time. The thermoelastic stress analysis infrared measurements show that both methods are capable of predicting the onset of damage at the bonded joint. The observed measurements indicate non self-similar crack growth or non-uniform crack extension along the bondline. Microstructural analysis is performed at the locations where damage is believed to have occurred for specimens extracted before final failure. The investigation confirms the capability of this method to capture early stages of damage in bonded joints. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Engineering aircraft structures with composite materials require a detailed knowledge of durability and damage tolerance of individual structural components, especially in bonded joints due to the importance of initial manufacturing conditions. Issues such as the quality of the adhesive at the time of application, the surface preparation of the adherends, and the final void content are all known to affect the quality of the joint. Traditional mechanical testing methods using extensometers and strain gauges of composite joints may only measure linear load- deformation responses to failure, giving no indication of overload or failure initiation. They are also local measurements that are expensive to install and maintain over long periods of time. Non- destructive full-field real-time evaluation tools offer a significant refinement over traditional mechanical tests, such that failure initiation of critical components can be detected and identified early. Failure progression after initiation of damage in fiber- reinforced polymer joints is not well understood, especially for considerations of fatigue reliability after damage initiation. Differ- ent experimental methods and techniques, such as radue life of fiber-riography, photoelasticity, acoustic emission, and thermogra- phy, have been applied to investigate the fatigeinforced plastics (FRP). Bakis et al. [1] related the residual strength, stiffness, and fatigue life to corresponding damage states obtained from photo- elastic coating and thermal emission experiments for graphite/ epoxy laminates subjected to fully reversed fatigue loads. They observed the damage initiated around the hole for quasi-isotropic and orthotropic laminates; they also noted that matrix cracking and delamination patterns were different in both cases due to the interaction between adjacent plies. Compared with photoelastic data, the thermal emission was more sensitive to the minute deformations near the fracture paths in the surface plies. Swain et al. [2] investigated the effect of interleaves on the damage mechanisms and residual strength of notched composite lami- nates subjected to axial fatigue loading. They described the effect of interleaving in carbon epoxy laminates with normalized stiff- ness versus normalized life curves by examining residual strength and evaluated delamination by using X-ray radiography and dye- penetration. Although they were able to use traditional methods such as stiffness and strength in a quantitative measure of cumulative damage, the X-ray radiography results showed matrix cracking, delamination, and other damage mechanisms in a qualitative manner. Brien et al. [3,4] investigated damage and failure of angle ply laminated composites at or near the free edge by using X-ray radiography and optical methods. They investigated laminates using 3D Finite Element Analysis (FEA) for each config- uration, looking at in-plane shear and transverse normal stresses as indicators of matrix cracks in off-axis plies. Microscopy and X-ray radiography were applied on straight coupons of AS4/350-6 graphite epoxy laminates to qualitatively validate the 3D FEA Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ijadhadh International Journal of Adhesion & Adhesives 0143-7496/$ -see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijadhadh.2013.09.013 n Corresponding author. E-mail addresses: rami98@eng.tau.ac.il (R. Haj-Ali), elhajjar@uwm.edu (R. Elhajjar). International Journal of Adhesion & Adhesives 48 (2014) 210–216