Bondline Defects Research Articles

Effects of bondline defects on the bond behaviour of CFRP-steel double strap joints

Adhesive bonding is used frequently for the applications of carbon fiber-reinforced polymer (CFRP) composites in steel structures such as strengthening or joining. However, the bondline defects are inevitable and could negatively affect the bond behaviour between CFRP and steel. Thus, the bondline defects represent a critical hazard and require careful assessment. This paper presents an experimental and numerical study on the effects of bondline defects on the bond behaviour of CFRP-steel double strap joints. The variables include the defect size (up to 35% of the bonded area), number (up to 45) and locations in the bondline. Multiple defects with mixed sizes are considered in a numerical analysis to investigate the effect of the defect number in more detail. It is found that the bondline defects lead to a reduction in the joint capacity. The highest reduction in capacity (36%) is observed when multiple small-sized defects are presented in the bondline. With the increment of the defect area, the failure mode becomes a combination of steel-adhesive interface debonding and CFRP delamination. A design framework is proposed based on the experimental and numerical results for the conservative prediction of the joint capacity with consideration of bondline defects.

Fatigue Damage Tolerance of Adhesively Bonded Pultruded GFRP Double-Strap Joints with Adhesion Defects

This paper reports an investigation on the damage tolerance of adhesively bonded pultruded glass fiber–reinforced polymer (GFRP) joints under fatigue loading. Double-strap joints with and without bondline defects were subjected to constant amplitude fatigue loading. The study investigated the effects of the defect location, size, and number on the fatigue performance of the joints (i.e., failure mode, fatigue life). Results showed that the failure mode was a combination of delamination and adhesive fiber–reinforced polymer (FRP) interface debonding at the defect vicinities. The fatigue life of the joints was significantly reduced (up to 90%) when the defect was at the overlap end. A defect size larger than 10% of the total bonded area could yield a substantial reduction in fatigue life. Small defects (less than 10% of the total bonded area) could yield a reduction in fatigue life of up to 73% when their number exceeded four. The joint exhibited better damage tolerance under static than fatigue loading conditions.

Numerical investigations on the damage tolerance of adhesively bonded pultruded GFRP joints with adhesion defects

Adhesive bonding has gained increasing acceptance in joining pultruded glass fiber reinforced polymer (GFRP) sections. However, one of the major issues hindering the widespread application of adhesive bonding in building and construction industries is the difficulty in predicting the mechanical behavior of the joints in the presence of adhesion defects. This paper investigates the damage tolerance of the adhesively bonded pultruded GFRP double-strap joints under quasi-static tensile loading using finite element analysis. Defects of various locations (along the bond length), sizes, and numbers were embedded within the bondline. A two-dimensional (2D) stress analysis was conducted to assess the effects of the bondline defects on the out-of-plane displacement, stress distribution, and peak stresses of the joints. Three-dimensional (3D) progressive damage models were further developed to predict residual joint capacity and provide insights into the effects of the adhesion defects on the damage mechanism of the joints. The progressive damage model uses the cohesive zone model (CZM) to simulate progressive interlaminar damage within the adherends as well as adhesive-FRP interface debonding. The proposed FE model was validated with published experimental data, and an excellent correlation was found between experimental and numerical results. The validated model was then used in a parametric study to assess the effects of design parameters such as adhesive types (from highly brittle to highly ductile) and joint geometry (adhesive chamfering, adherend chamfering, and recessing) on the damage tolerance of adhesively bonded joints. This paper also reports a comparative study on the damage tolerance between GFRP double-strap, single-strap, and single-step joints. The stress analysis results revealed that the presence of the bondline defects could increase the out-of-plane displacement of the adherends and the peak shear and peel stresses at the overlap ends. High-stress concentrations were also observed in regions adjacent to the defect extremities. The double-strap joints exhibited better damage tolerance than the single-strap and single-step lap joints. The damage tolerance of the joints can be improved by using stress-reduction methods such as adhesive chamfering, adherend chamfering, and recessing.

Simultaneous tailoring of pulsed thermography experimental and processing parameters for enhanced defect detection and sizing in adhesive bonds in carbon fibre composites

Thermographic inspection provides opportunity to tailor non-destructive evaluation to specific applications. The paper discusses the opportunities this presents through consideration of adhesive bonds between composites, such as those joining structural members and outer skins, where access is restricted to a single side. To date, literature focusses on the development of either an experimental procedure or data processing approach. This research aims to demonstrate the importance of tailoring both of these aspects to an application to obtain improved defect detection and robust quantification. Firstly, the heating stimulus is optimised to maximise the thermal contrast created between defect and non-defect regions using a development panel. Traditional flash heating is compared to longer square pulse heating, using a developed shutter system, compromising between experimental duration and heat input. A pulse duration of 4 seconds using two 130 W halogen bulbs was found double the detection depth from 1 mm to 2 mm, revealing all defects in the development panel. Temporal processing was maintained for all data using thermal signal reconstruction. Spatial defect detection routines were then implemented to provide robust defect/feature detection. Spatial defect detection encompassed a combination of image enhancement and edge detection algorithms. A two-stage kernel filter/binary enhancement method followed by the use of Canny edge detection was found most robust, providing a sizing error of 1.8% on the development panel data. This process was then implemented on adhesive bonds with simulated bond line defects. The simulated defects are based on target detection threshold of 10 mm diameter void found at 1–2 mm depth. All simulated void defects were detected in the representative bonded joint down to the minimum diameter tested of 5 mm. By considering the tailoring of multiple aspects of the inspection routine independently, an overall optimised approach for the application of interest has been defined.

Damage tolerance of adhesively bonded pultruded GFRP double-strap joints

Compared with mechanical fastening, adhesive bonding offers numerous advantages in the joining of pultruded glass fiber reinforced polymer (GFRP) sections. However, one critical issue associated with bonded joints is the assessment of their mechanical behavior considering bondline defects. This paper presents the results of an experimental investigation of the damage tolerance of adhesively bonded pultruded GFRP joints considering bondline defects. Double-strap specimens with and without bondline defects were prepared and tested in tension until failure. The location (five different locations in the bond length and width directions), size (10%, 20%, and 30% of the total bonded area in both the bond length and width directions), shape (triangular, rectangular, square, circular, and elliptical), and number (between 1 and 8) of defects within the bondline were considered as variable parameters. This paper reports and discusses the observed failure modes, load–displacement curves, and joint capacities, considering the effects of bondline defects. Interface debonding was observed in regions neighboring the bondline defects, and delamination was observed in other bonded areas. All joints exhibited a linear elastic load–displacement response with sudden and brittle failure, regardless of the presence of bondline defects. The observed capacity reduction of up to 33% could be significant if the defects were located at the edges of the bonded area. Defects of different shapes resulted in similar reductions in joint capacity (21%). Furthermore, the joint was found to be more sensitive to irregularly shaped (e.g., triangular) defects, and reducing the number of defects was more effective in mitigating the reduction in joint capacity than reducing the total defect area was.

Damage Detection Using d15 Piezoelectric Sensors in a Laminate Beam Undergoing Three-Point Bending

A major inhibition to the widespread use of laminate structures is the inability of nondestructive testing techniques to effectively evaluate the bondline integrity. This work proposes and analyzes a bondline-integrity health monitoring approach utilizing shear-mode (d15) piezoelectric transducers. The d15 transducers were embedded in the bondlines of symmetric laminate structures to monitor and evaluate the bondline integrity using ultrasonic inspection. The d15 piezoelectric transducers made of lead zirconate titanate (PZT) enabled ultrasonic inspection of bonds by actuating and sensing antisymmetric waves in laminate structures. Design considerations, fabrication process, and experimental methods for testing a laminate specimen are presented. Designs included bondline-embedded d15 PZT piezoelectric transducers with surface-mounted transverse (d31) piezoelectric transducers for signal comparison. Defects in the bondline were created by a quasi-static three-point bending test, with results showing the ability of d15 piezoelectric transducers to detect bondline damage. Two damage indices based on Pearson correlation coefficient and normalized signal energy were implemented to evaluate the presence of damage and its severity. The experimental results demonstrate the ability of bondline-embedded d15 piezoelectric transducers to be used as actuators and sensors for ultrasonic health monitoring of bondline integrity. A comparison between surface-mounted d31 PZT and bondline-embedded d15 PZT sensors was also conducted. It was seen that signals sensed by bondline-embedded d15 PZTs showed higher distortion due to bondline defects compared with the sensed signals from the surface-mounted d31 PZT.

Process diagnostics for co-cure of sandwich structures using in situ visualization

Aerospace sandwich structures are often manufactured using an autoclave co-cure process, in which prepreg facesheets are cured and bonded to honeycomb core simultaneously. Co-cure involves coupled physical phenomena, including gas migration, prepreg consolidation, resin/adhesive flow and crosslinking, and potential mechanisms of defect formation. Due to the “black box” nature of the process, however, scientific understanding of the complex and interacting phenomena that cause defects remains incomplete. We address this challenge using an in situ visualization method that enables direct observation of the skin/core bond-line in realistic autoclave conditions. Five cases are presented, which span a range of process conditions leading to various defect-formation phenomena, and show the effectiveness of in-bag pressurization for preventing bond-line defects. We demonstrate that in situ diagnostics can eliminate much of the trial-and-error typically involved in process troubleshooting, by providing insights into the physics of co-cure that would otherwise be difficult to obtain.

Remote monitoring of bond line defects between a composite panel and a stiffener using distributed piezoelectric sensors

Structural health monitoring (SHM) using ultrasonic guided waves has proven to be attractive for the identification of damage in composite plate-like structures, due to its realization of both significant propagation distances and reasonable sensitivity to defects. However, topographical features such as bends, lap joints, and bonded stiffeners are often encountered in these structures, and they are susceptible to various types of defects as a consequence of stress concentration and cyclic loading during the service life. Therefore, the health condition of such features has to be assessed effectively to ensure the safe operation of the entire structure. This paper proposes a novel feature guided wave (FGW) based SHM strategy, in which proper FGWs are exploited as a screening tool to rapidly interrogate the representative stiffener-adhesive bond-composite skin assembly. An array of sensors permanently attached to the vicinity of the feature is used to capture scattered waves from the localized damage occurring in the bond line. This technique is combined with an imaging approach, and the damage reconstruction is achieved by the synthetic focusing algorithm using these scattered signals. The proposed SHM scheme is implemented in both the 3D finite element simulation and the experiment, and the results are in good agreement, demonstrating the feasibility of such SHM strategy.

Remote sparse distributed sensors to image bondline defects between a composite panel and a stiffener

Adhesive bonding is widely used to fix skins to reinforcing elements, like stiffeners, in aerospace composite structures. A well-cured bond offers uniform stresses, good joint strength, and improved fatigue and impact resistance, and is therefore crucial to the performance of the entire structure. In the previous work by the authors, ultrasonic feature guided wave (FGW) has been discussed as a screening tool for quick inspection of the bondline between a CFRP panel and stiffener. However, it was found that material damping and weak reflections by defects along the feature makes the use of a single transducer in pulse echo mode quite inefficient. In this study, a structural health monitoring (SHM) technique is proposed to inspect the bondline, based on the radiation of plate modes into the CFRP panel when an incident FGW interacts with a defect in that bondline. A sparse array of sensors organized on the CFRP panel away from the stiffener was used to capture the waves radiating into the panel. A synthetic focusing method was applied to process the recorded signals for imaging the damaged area in the bond. Adhesive defects with varying dimensions were successfully identified in 3D FE simulations, with supporting experimental results to validate this method.

Feature guided wave inspection of bond line defects between a stiffener and a composite plate

Adhesive bonding is widely used in aerospace composite structures. A continuous well-cured bond can offer good joint strength and improved fatigue and impact resistance, and is therefore crucial to the performance of the entire structure. This paper explores the feasibility of using feature guided waves (FGW) for rapid screening of the bond line between a stiffener and a carbon fiber reinforced polymer (CFRP) composite panel. Such FGWs are capable of focusing the wave energy along the stiffener and the bond layer, with limited radiation to the adjacent plate. The Semi-Analytical Finite Element (SAFE) approach is employed to understand the modal properties of FGWs that exist in the structure, and criteria are suggested to choose proper mode-frequency combination that is sensitive to adhesive defects. A shear horizontal type FGW mode is identified to be well suited, as it is easy to excite, and propagates with little dispersion and relatively low attenuation, while it retains sufficient energy around the bond layer. Both 3D Finite Element (FE) simulations and experiments are performed to study the interaction of the selected FGW mode with defects in the adhesive bond, and the results show excellent agreement. The reflection behavior and the wave-defect resonance phenomenon are investigated, which demonstrate the capability of the FGW for the bond line inspection.

Experimental and finite element studies of bolted, bonded and hybrid step lap joints of thick carbon fibre/epoxy panels used in aircraft structures

Abstract A prior study conducted by the authors had investigated the behaviour of thin composite double lap joint repairs. This investigation was an extension to the previous study and focused on analysing the behaviour of thick step lap joint repairs which are more complicated in nature. Finite element analysis (FEA) was performed to verify the static and fatigue strength of bolted, bonded and hybrid joint configurations. Thick carbon fibre/epoxy laminates and aerospace grade film adhesive and fasteners were selected. Several configurations were considered in this case. These include varying the number of fasteners in the joint region as well as inclusion of bondline defects. Adhesive non-linear material properties, fastener contacts and frictional forces were all included in the three dimensional (3D) finite element (FE) models. The progressive failure process was simulated and the Multicontinuum Theory (MCT) was used to determine the stress states in all specimens considered. The strain energy release rate (SERR) as a function of crack length was also utilised to gain further understanding into the fatigue behaviour. Overall the FE models are able to accurately predict the bonded, bolted and hybrid joint strengths. SERR results suggest it is vital to place fasteners closer to the ends of the overlap to suppress the peak peeling stresses and to delay the effects of early crack initiation. This overall improves the longevity of a conventional bonded joint design.

Experimental and finite element studies of thin bonded and hybrid carbon fibre double lap joints used in aircraft structures

Finite element analysis (FEA) is performed to verify the static and fatigue strength of mechanically fastened, bonded and hybrid double lap joints. These joints are made from thin carbon fibre/epoxy laminates applied in aircraft structures. Several configurations are considered, including variations in rivet array and the addition of bondline defects. Adhesive nonlinear material properties, rivet surface contacts and frictional forces were included in the three-dimensional (3D) Finite Element (FE) models. The Multicontinuum Theory (MCT) is used to simulate the progressive failure process and the stress state for all specimens, whilst the strain energy release rate (SERR) as a function of crack length for bonded and hybrid specimens are also compared. Results have shown the FE models are able to accurately predict the bonded, riveted and hybrid joint strengths. The position of the first row of fasteners is critical in determining the crack growth rate. As the crack enters the fasteners' clamping zone there is a significant drop in SERR resulting in a much slower crack growth rate, therefore increasing the fatigue resistance of the hybrid joint configuration.

Elastic Properties of Carbon Fibre-Reinforced Epoxy Composites

The aim of this work was to investigate the prediction of the elastic constants of carbon fibre-reinforced epoxy laminated composites using a simple mathematical approach. For this purpose, a Matlab Calculation Code (MCC) has been developed in MATLAB, which provides sufficient capabilities for analysis of the mechanical behaviour of composite materials. The MATLAB code enables a variety of fibre orientation angles to be generated and a wide variety of fibre volume fractions to be analyzed under mechanical loading. In order to validate the accuracy of prediction results, a commercial grade epoxy resin (Araldite LY-5052) and unidirectional carbon fibre (Torayca T700) were used to fabricate some samples of laminated composites with cross-ply configuration using the hand lay-up method. Experimental results such as elastic modulus, tensile strength as well as elongation at break were compared with the theoretical data extracted from MCC. The theoretical analysis were verified with experimental results for the same type of composites. Although the values were different, excellent agreement in the trends of the changes was found for the two approaches. Therefore, it was concluded that the MCC is capable of predicting the elastic constants of laminated composites with a high level of confidence for a wide range of fabricating conditions. The results also showed that the tensile properties of composites made with 3-layer were generally greater than those of the other evaluated composites. This kind of behaviour may be explained by bondline defects, which adversely influence the mechanical properties.

Mechanical performance of epoxy/carbon fiber laminated composites

The objective of this research work was to gain a better understanding of the mechanical properties of epoxy resin composites reinforced with carbon fiber. For this purpose, the effects of fiber orientations, resin types, and number of laminates on mechanical properties of laminated composites have been investigated. In the sample preparation, composites were manufactured by hand lay-up process, using a fiber-to-resin ratio of 40:60 (w:w). To investigate the effect of fiber orientation, angles of 0°, 35°, 45°, and 90° were selected. Results show that the mechanical properties, in terms of tensile, flexural and impact strengths, were mainly dependent on the fiber orientations followed by the number of laminates. At a similar fiber orientation, the composites made with EM500 epoxy resin showed the highest mechanical properties (such as tensile and flexural and impact strengths) compared to other evaluated composites. However, the differences were not highly significant. The results indicated that the mechanical properties of composites made with five-ply were generally slightly greater than three-ply composites. It may be due to the bondline defects, which adversely influence the mechanical properties. Scanning micrographs of the composites showed that the epoxy matrix material was fully adhered to the fibers, indicating a strong interface. It can be concluded that the order of increment parameters in the mechanical properties of the composites is fiber orientation > number of laminates > resin type. In addition, the tensile and flexural properties were superior in case of ±35° fiber orientations.

A fatigue assessment of dissimilar titanium alloy diffusion bonds

Diffusion bonds between the dissimilar titanium alloy pairing Ti 6/4 and Ti 550 were produced from laboratory scale plate stock materials. Metallographic sections, chemical composition and micro-hardness traverses across the bond line are all presented as evidence of the form and integrity of these joints. The mechanical performance of the bonds was assessed under both monotonic tensile and low cycle fatigue loading utilising specimens which contained bonds orientated either perpendicular or inclined to the principal stress axis. In general, near Ti 6/4 parent properties were measured under both loading conditions. However, performance was occasionally influenced by the presence of bond line defects, particularly under fatigue loading.

Non-destructive evaluation of impact damaged composite pipes and pipe couplings with bond line defects

An ultrasonic technique using the commercially available ANDSCAN system has been used to assess impact damage in filament wound glass reinforced epoxy pipe subjected to seawater at the rated internal pressure, 20 bar (2 MPa) at ambient temperature. The ultrasonic method developed has been used to quantitatively examine pipes impact damaged by three different nose-shaped indenters, both before and after exposure to seawater. Ultrasonic measurements of the extent of the damage have been compared with measurements taken from micrographs of sections of the damaged regions. A similar ultrasonic method has been used to detect and quantify lack-of-bond defects in adhesively bonded couplings. The test results show that impact damaged pipe exposed to seawater at the rated pressure for up to 6 months display no increase in the extent of the damaged region. Whilst in each test the pipes leaked from the damaged region initially, the leakage declined to almost zero over a maximum of about 20 weeks, depending on the nose-shape of the indenter. Ultrasonic measurements of the extent and distribution of the damage through the pipe wall agreed well with measurements taken from micrographs. Good agreement has also been achieved with the ultrasonic measurement of the lack-of-bond defects in the adhesively bonded couplings. The results have demonstrated that the ultrasonic method developed is an effective technique for the evaluation of impact damage in pipes and for the detection of lack-of-bond defects in couplings.

Observations in the structural response of adhesive bondline defects

In terms of adhesively bonded repairs there are two main concerns with regard to structural integrity. These are the effect of debonds and weak bondlines on the load transfer and durability of the joint. The influence of load transfer depends on the stiffness of the bondline. When this is degraded the out-of-plane deformation of the joint will be modified locally. Observation of the out-of-plane deformation is a key in the identification of weakened bondline and represents an indication that either poor surface preparation or aging effects have occurred. Holographic interferometry has been used to better understand the structural response of bondline defects, both debonds and weak bonds. The interferogram fringe patterns show the structural response and indicate whether the bondline has been broken or is weakly bonded. The significance of this observation is that weak bonds do affect the structural load response of the bondline in a number of ways. This effect is due to the reduction in bondline stiffness.

ATTENUATION OF ULTRASONIC INTERFACE WAVES ON METAL-POLYMER-METAL BOUNDARIES

The measured sensitivity of interface wave attenuation to defects near the bondline and to variations in the viscosity of the adhesive layer was compared with theoretical changes predicted by the Rokhin model. Differential interferometric optical measurements of interface wave attenuation due to defects near glass polymer metal boundaries are discussed. Pitch catch and pulse echo methods which use variable angle wedge transducers to generate and receive modified interface waves and to measure large bondline defects and adhesive viscosity are described.

Modeling Concepts for Studying Ultrasonic Wave Interaction with Adhesive Bonds

Abstract Recent research has shown that such adhesive bondline defects as chemical segregation, variation in cure, gas entrapment or inadequate surface preparation are responsible for many adhesively bonded structural failures. Analytical models have been developed in this work that can be used to relate these “flaws” to the manner in which they affect the reflection of an ultrasonic pulse from such a bondline. The results of this study provide a substantial resource base for extended research through which ultrasonic inspection can become a reliable NDT technique for bond strength determination.