Disbond Length Research Articles

Computational assessment of disbond growth behaviour in adhesively bonded wide joints or patch repairs for primary airframe structures

This paper evaluates the applicability of a damage slow growth management strategy to patch repairs or bonded joints of primary aircraft structures established earlier by the authors utilising wide bonded metal joints through a computational study using MSC Marc software. The adhesive element failure criteria was applied to establish the residual static strength of the joint as a function of disbond length. A cohesive zone element model implemented in a Ucohesive subroutine was used to evaluate the strain energy release rates (SERRs) as a function of disbond crack extent and predict the disbond growth in the joint. Similar to the 2D analysis conducted in the past, the results showed that for a wide joint with sufficient static strength safety margin under a typical fatigue loading that would propagate disbond, the disbond growth would be stable within a particular length range. Therefore, the slow growth approach would be viable when the patch is modelled to be large enough to allow expanded damage growth. Furthermore, numerical results on the load redistribution effect indicate an overall significantly slower disbond growth and longer fatigue life of the joint with part width disbond than that with full-width disbond.

Disbond contour estimation in aluminum/CFRP adhesive joint based on the phase velocity variation of Lamb waves

Adhesive lap joints between composite and metal plates have been widely used in industrial fields including the automotive industry, marine manufacturing and aerospace engineering. Low quality of operation, harsh environment, adhesive aging and other disadvantages may lead to disbonding. To assess the disbond contour at an adhesive interface, this study proposes a detection method based on the phase velocity variation of Lamb waves. First, the dispersion curves of Lamb waves in both single-layer and bonded multi-layer areas are acquired using the semi-analytical finite element (FE) method. Subsequently, numerical models of Lamb wave propagation in intact and disbonded joints are established. Due to the difference in phase velocity between relevant modes, the phase difference of Lamb wave between disbonded and intact joints is quantitatively linear with the disbond length under specific excitation, which is verified by the simulated signals based on FEs. Then, a probabilistic reconstruction algorithm based on phase delay is employed to localize the disbond center. On this basis, the edge points of the disbond are acquired, and the convex envelope of these points is sketched for disbond contour estimation. As a result, both the location and shape of the disbond can be obtained, thereby providing information for subsequent assessment. The experiment is carried out on an adhesive lap joint specimen composed of an aluminum plate and a quasi-isotropic carbon fiber reinforced plastic laminate, and the results demonstrate the effectiveness of the proposed method.

Experimental and computational assessment of disbond growth and fatigue life of bonded joints and patch repairs for primary airframe structures

The applicability of a damage slow growth management strategy to bonded joints/patch repairs of primary aircraft structures was evaluated through an experimental and computational study. Fatigue tests were conducted to investigate the entire process of disbond growth from initiation up to joint ultimate failure. The residual static strength of the joint as a function of disbond length was established using finite element modelling, in which the mesh size was calibrated using the static strength of the specimens measured in room temperature and dry (RD) and hot-wet (HW) conditions, based on the characteristic distance approach. A virtual crack close technique (VCCT) approach was utilised to assess the strain energy release rates (SERRs) as a function of disbond crack length. The measured disbond growth rates were correlated with the SERRs using a modified Paris law that enabled prediction of joint fatigue life. The fatigue test results indicated that for a joint having a sufficient static strength safety margin under a typical fatigue loading that would propagate disbond, the disbond growth would be stable in a particular length range. Thus, the slow growth approach would be feasible for a bonded joint/patch repairs if the patch is designed to be sufficiently large to allow extended damage propagation (whilst in the case when patch size must be limited, safe-life design for the patch termination region in critical repairs must be considered. Should disbond growth occur in this case, the joint must be repaired or replaced). The work presented in this paper validated the framework/procedure proposed previously by the authors (Tanulia et al., 2020) for managing damage slow growth in bonded joints/patch repairs. In the last part of this paper the planned follow-on research is briefly described.

A novel nonlinear Lamb wave based approach for detection of multiple disbonds in adhesive joints

This paper proposes a Lamb wave based technique for detecting multiple disbonds using novel damage features based on mode conversion and nonlinear breathing phenomena. An aluminum scarf joint with a top cap has been considered as the example structure. Generation and sensing of Lamb waves have been done through piezoelectric patches. Three possible cases of disbonds, i.e., horizontal disbond between the top cap and the scarf joint, inclined disbond at the scarf joint, and the simultaneous presence of both the horizontal and inclined disbonds have been considered. Numerical simulations and experiments have been run considering disbonds of different lengths at different locations. For extraction of damage features, the wave propagation response has been decomposed into symmetric and antisymmetric components. Next, damage features have been defined using the amplitudes of the superharmonics present in each of these components. Detection of disbond has been done in two steps – localization and quantification. The problem of disbond localization has been formulated as a classification problem and solved employing a Probabilistic Neural Network (PNN) using damage features extracted from numerical simulation data. Quantificationof disbond length has been done through Artificial Neural Networks (ANN) using the same damage features. Finally, this technique has been tested using experimental data.

A novel application of ultrasonic Lamb waves: studying adhesive effects on the inspection of coating debonding in a three-layer waveguide

ABSTRACT In this research, a thorough experimental and numerical study is conducted to investigate a novel application of the ultrasonic Lamb waves which is its capability to detect the coating disbond in a double-layer waveguide (aluminium-adhesive) and a triple-layer waveguide (aluminium-adhesive-coating). The experimental tests are set up and implemented to evaluate how the attenuation of Lamb waves is affected by the adhesive bonding beside the contribution of the coating layer to the behaviour of Lamb waves. In the presence of additive white Gaussian noise (AWGN), a range of important indices including energy index (EI), amplitude index (AI), pulse width index (PWI), and the time of flight index (ToFI) are worked out in association with the disbond length. It is inferred that a) even in the very noisy environment of 0 dB SNR, the Lamb waves are effectively able to identify debonding defects particularly using the EI, giving sensitivity rates 105% and 30% for triple-layer and double-layer, respectively; b) the contribution of adhesive to the absorption of the guided wave energy is 30%, 40%, and 50% in EI, AI, and PWI, respectively, corroborating the value of the features to distinguish the debonding between each layer of bonded structures.

Evaluation of mode II fatigue disbonding using Central Cut Plies specimen and distributed strain sensing technology

ABSTRACTThe lack of a widely-accepted test standard for characterizing the mode II fatigue disbond growth behavior of adhesively bonded interfaces is a challenge to the research community in terms of producing consistent and repeatable results. Typically, researchers apply the End Notch Flexure specimen, which is already used for static delamination studies. However, the needs for static and fatigue disbond growth characterization are not the same, resulting in some undesirable effects in such specimen. This study looks at a particular mode II test configuration known as the Central Cut Plies (CCP) specimen. A critical evaluation of the suitability of this specimen, including the influence of geometry, disbond measurement approaches and the stability of the disbond growth is carried out through a combination of numerical and experimental investigations. A distributed strain sensing system based on Rayleigh Backscattering provided a surface strain profile from which disbond growth rate data was obtained. A finite element model was used to verify the experimental results and determine the disbond length from the strain profiles. Results of this evaluation have shown that the CCP specimen is a promising specimen configuration for characterizing fatigue disbond growth; however, it also presents several challenges that require consideration in its application.

Using Carbon Nanofibre Sensors for In-situ Detection and Monitoring of Disbonds in Bonded Composite Joints

This paper focuses on the ability of carbon nanofibre (CNF) networks to in situ monitor fatigue induced disbond damage in adhesive bonded composite joints. The inclusion of CNFs in the epoxy adhesive increases its conductivity by five orders of magnitude. The improved electrical conductivity is utilized to evaluate the ability of the CNF network to monitor and detect the fatigue induced disbond damage by measuring the in-situ resistance changes using a four probe setup. The changes in total resistance was a function of the bulk electrical resistivity of the adhesive and the bond dimensions, which were related to the disbond length to model and determine the size of the disbond. The simple resistivity model was in good agreement with the resistance measured during fatigue testing. Good agreement was found between the optical disbond observations and the disbond length calculated using the proposed model. Finite element simulations were performed to ascertain the range of applicability of the proposed model. The simplicity of the disbond detection technique via direct current potential drop technique enables real time monitoring of crack growth in the composite structure.

A monitoring technique for disbond area in carbon fiber–reinforced polymer bonded joints using embedded fiber Bragg grating sensors: Development and experimental validation

This study evaluated fatigue-induced disbond areas in carbon fiber–reinforced polymer double-lap joints using embedded fiber Bragg grating sensors. When the disbond grew by cyclic loading, the embedded fiber Bragg grating sensors yielded reflection spectra having two peaks representing a step-like strain distribution generated by the disbond; the peak at the shorter wavelength corresponded to the unloaded disbond region. The ratio of the peak intensity at the shorter wavelength to that at the longer wavelength increased gradually with increasing disbond length. The relationship between the peak intensity ratio and the disbond length was analyzed by coupled structural–optical analysis and was validated by comparing analytical peak intensity ratio with the experiment results. The disbond length was then estimated from the measured spectra based on this analytical calibration relationship, but the estimated disbond area exceeded that observed using the ultrasonic C-scan technique. Additional experiments including destructive observation of the adhesive suggested that an embedded fiber Bragg grating sensor could detect a moving disbond tip earlier than conventional nondestructive techniques.

Performance of Vibration Based Damage Detection Algorithms for Detection of Disbond in Stiffened Metallic Plates

Though vibration based health monitoring has been the focus of attention for quite some time, there exist a strong requirement for an extensive and comprehensive study on the relative performance of different damage detection algorithms under different damage scenarios. To fulfill this objective, a stiffened aluminium plate has been selected to make a comparative study on the performance of several vibration based damage detection techniques, namely, Modal Curvature, Gapped Smoothing Method/Modal Curvature, Generalized Fractal Dimension and Uniform Load Surface Curvature based on four different criteria: variation of damage intensity (i.e., disbond length), position of stiffener and disbond, the effect of noise and capability to detect multiple damage. In addition to this, a new approach, based on the curvature of wavelet coefficients, has been presented. It is found that this novel approach is extremely effective in determining the presence and location of damage under different situations. The entire numerical modelling is done in ANSYS 14.0 and the damage detection algorithms written in MATLAB codes have been used to generate the required damage indices using the modal data retrieved from ANSYS. The study ultimately enlightens inherent characteristics of the various damage detection algorithms under different damage conditions.

Disbond monitoring at wing stringer tip based on built-in ultrasonic transducers and apulsed laser

An active ultrasonic diagnostic method for monitoring the occurrence and growth of adisbond at a wing stringer tip was developed and verified using built-in piezoelectriclead-zirconate-titanate transducers. The diagnosis was based on the ultrasonic arrival timedelay caused by an increase in the disbond-induced wave path and can provide quantitativeinformation on the disbond length. The change in the ultrasonic amplitude, which is aconventional measure for disbond detection, was also included in the diagnostics. Finally asingle transmission and multiple reception method was used to obtain the arrival time andamplitude distributions. Another active ultrasonic diagnostic tool based on a pulsed laserultrasonic generation was proposed for a nondestructive evaluation to obtain more reliableresults on the damage detected using the built-in transducers. The diagnosticswere verified using a metal skin-stringer structure as an experimental model, andthen applied to monitoring the disbond at a stringer tip in a composite wingbox.

11502 ラム波を用いたCFRP接着界面剥離の定量検査手法(機械材料)

This paper proposes an inspection technique using Lamb waves to detect a disbond in a CFRP skin-stringer structure. This technique employed the pitch-catch method using two piezoelectric elements especially designed for the low-frequency Lamb A_0 mode. We measured the arrival times of the transmitted A_0 mode as a function of disbond length. A remarkable delay in the arrival time due to the disbond was detected, and its length could be calculated based on a simple model for Lamb-wave propagation. We successfully detected and evaluated the disbond using the proposed technique and confirmed its ability to inspect the skin-stringer structure.

Strain-based health assessment of bonded composite repairs

Structural health monitoring (SHM) technology may be applied to composite bonded repairs to enable the continuous through-life assessment of the repair’s efficacy. This paper describes an SHM technique for the detection of debonding in composite bonded patches based on relative strain measurements. Two types of repairs known as the external doubler repair and the scarf repair are examined in this paper. A finite element study showed that the strain in the debonded region changes significantly compared to the undamaged state. An experimental investigation was conducted, using carbon/epoxy repairs to carbon/epoxy substrates and fibre Bragg grating sensors for the measurement of strain, to validate the theoretically predicted results for the external doubler repair. Debonds were detected by means of a differential strain approach, where two sensors were strategically positioned so that their strain differential increased with the disbond length. A threshold value may be set to indicate imminent critical repair failure. The experimental measurements agreed well with the numerical findings, indicating that the proposed scheme has great potential as a simple monitoring technique for composite bonded repairs.

Energy Release Rate of Double Beam-Plates with Repairing Elements

An engineering analysis for determining the strain energy release rate of symmetrically loaded double beam-plates with repairing or reinforcing elements in the transverse direction which are densely distributed across the width in the disbond regions is presented. The concept of strain energy release rate evaluated by computing the rate of energy released from the disbond front has been verified to be exactly the same and simpler to use than the most referenced work of Kanninen by differentiation of the total strain energy of the entire system with respect to the disbond length. Due to the simplicity of the new concept for determining the strain energy release rate, it allows one to derive the exact analytical solutions for double beam-plates containing repair or reinforce elements and satisfying continuity at the delamination front as well as boundary conditions at both ends of the system. Some comparisons of the strain energy release rate for double beam-plates without repair elements to other approximations are also made. Each side of a double beam-plate system in the perfect segment is modeled as a beam-plate on an elastic foundation, while disbond segments are treated as beam-plates under transverse loading and constrained by repairing or reinforcing elements as axially loaded members at discrete points. Some numerical results for a double cantilever beam-plate with one and two rows of reinforcing elements under end loads are presented for illustrating the procedure as well as for future reference. The general concept and analysis procedure should be applicable to other structural systems and for mixed-mode delaminations.

An innovative approach to fatigue disbond propagation in adhesive joints

An innovative approach to characterize the resistance of adhesively bonded joints to fatigue disbond propagation (FDP) is presented. A constitutive equation, known as the modified crack layer (MCL) model, is employed to extract parameters characteristic of the adhesive joint's resistance to FDP. These parameters are γ', the specific energy of damage, which reflects the fatigue disbond resistance of the adhesive joint and the dissipative characteristic of the joints, β'. Stress-controlled tension-tension fatigue experiments were conducted on lap joints fabricated from aircraft grade aluminum 2024-T3 and 3M structural adhesive. The disbond length was measured periodically along the edges of the bonded area at the four corners and the corresponding number of cycles was recorded. This is in order to calculate the disbond growth rate. The hysteresis loop was also recorded for each measurement from which both the energy release rate, J*, and the change in work, Wi, were determined. It was found that the proposed model describes the behavior of the adhesively bonded joints over the entire range of the energy release rate. Thus, the proposed model can provide a basis upon which the relationships between the microstructure and/or the processing conditions and the resistance of adhesively bonded joints to FDP can be constructed. Such relationships can guide the development of adhesively bonded joints with superior resistance to debonding and should aid in their lifetime assessment.

Fracture and fatigue behavior of scrim cloth adhesively bonded joints with and without rivet holes

—Tensile and fatigue disbond propagation studies on scrim cloth structural adhesive lap joints without and with rivet holes were performed. The geometry of the rivet holes is similar to that in a fuselage part of an aircraft. The joints were cycled in tension-tension fatigue at a frequency of 3 Hz and a maximum load, below the linear limit of the joint, which was obtained from the tensile tests of similar joints. The disbond length at each corner of the joint was viewed using a travelling optical microscope attached to a video camera. It was found that the static-tensile behavior of both types of joints (without and with rivet holes) consists of three stages: a linear stage followed by a region of increased non-linearity and then a 'yield' region. It is within this yield region that the rivet holes affect the strength of the joint. Stress analysis of the disbond problem under static loading revealed a strong mixed mode between the opening and shear mode stress intensity factors for both types of joints. T...

Theoretical considerations for the determination of adhesive-bond deterioration due to distributed disbonds

Theoretical considerations are presented for the determination of disbonds at an adherend/adhesive interface using ultrasonic techniques. Two simplified models are presented to relate the adherend/adhesive bond stiffness to the total disbond length at the adherend/adhesive interfaces. In addition, the analysis of Goland and Reissner for an adhesive lap joint has been extended to the case of a distribution of through-the-thickness disbonds in the adhesive layer. The shifts in the frequency minima of the reflection coefficient versus the total disbond length have been calculated. For the model in which the disbonds are located at arbitrary positions at the two adherend/adhesive interfaces, the theoretical results are shown to agree with the experimental results.