Small Delamination Research Articles

Characterization of Small Delamination Defects by Multilayer Flexible EMAT

Characterization of Small Delamination Defects by Multilayer Flexible EMAT

Interface Contact Thermal Resistance of Die Attach in High-Power Laser Diode Packages

The reliability of packaged laser diodes is heavily dependent on the quality of the die attach. Even a small void or delamination may result in a sudden increase in junction temperature, eventually leading to failure of the operation. The contact thermal resistance at the interface between the die attach and the heat sink plays a critical role in thermal management of high-power laser diode packages. This paper focuses on the investigation of interface contact thermal resistance of the die attach using thermal transient analysis. The structure function of the heat flow path in the T3ster thermal resistance testing experiment is utilized. By analyzing the structure function of the transient thermal characteristics, it was determined that interface thermal resistance between the chip and solder was 0.38 K/W, while the resistance between solder and heat sink was 0.36 K/W. The simulation and measurement results showed excellent agreement, indicating that it is possible to accurately predict the interface contact area of the die attach in the F-mount packaged single emitter laser diode. Additionally, the proportion of interface contact thermal resistance in the total package thermal resistance can be used to evaluate the quality of the die attach.

INVESTIGATION OF THE CORROSION BEHAVIOR OF ALUMINIUM 1980T1 IN SEAWATER AND DRILLING MUD BY ELECTROCHEMICAL METHODS

The main methods of corrosion protection of equipment and metal structures of offshore drilling platforms are considered. The corrosion resistance of the high-strength 1980T1 aluminium alloy used for the manufacture of the riser in seawater and drilling mud, the effectiveness of its protection with zinc-rich coatings using the methods of polarization resistance, potentiometry, as well as appearance assessment studied. In seawater at 20 and 40 °C, alloy 1980T1 refers to resistant materials - 3-4 (6 grade according to GOST 5272- 90), intense pitting formation is observed, in drilling mud the corrosion rate (Km) increases more than 10 times refer to seawater (low-resistance, 6 grade), which is due to the alkaline nature of the medium. There is intense continuous corrosion, large pitting, ulcers on the seam and its border. In seawater at 20 °C, the Km of aluminium and coatings are comparable. In the drilling mud at 20 °C, the coatings provide a 2,5…4-time reduction in Km. After 30 days of exposure in seawater and drilling mud at 20 °C, the zinc-polyurethane coating detached to40…70%, intense pitting of base observed. Bubbles all over the surface of zinc-epoxy coating observed. Zinc-silicate and zinc-polyurethanalkyde coating with a high concentration of zinc had only small delamination in the weld area. In seawater at 20…40 °С and drilling mud at 20 °C zinc-silicate coating has the greatest tread capacity, the zinc-polyurethane, zinc-polyurethanalkyde and zinc-epoxy coatings are insignificant. Additional protection of welds and pipe surfaces in seawater achieved using insulated, metallized, protective coatings and cathode protection, in drilling mud it is also necessary to study the possibility of pH reduction and the use of effective corrosion inhibitors.

A COMPARATIVE STUDY ON THE SINGLE AND DOUBLE-LAYERED DEPOSITION OF ALUMINUM 6063 OVER EN8 MEDIUM CARBON STEEL USING FRICTION SURFACING

This work estimates the optimum process parameters for the double-layer deposition of aluminum 6063 over EN8 medium carbon steel using friction surfacing. The control process parameters like axial force and feed rate of the consumable rod on two different deposition surface conditions, i.e. a good interface and a course interface during the process were investigated. The second layer deposition was performed over the roughness developed after the first layer deposition and contributed largely to the mechanical anchoring mechanism at the second coating interface. Assessment of the mechanical strength at the first coating interface (substrate/ coating 1) and second coating interface (coating 1/ coating 2) through bending and microhardness tests at both the interfaces was performed. The bending test results disclosed the existence of small delamination with no cracking at the second interface with a feed rate of 150[Formula: see text]mm/min at axial force of 5[Formula: see text]kN. An increase of 16.2% in the hardness value of consumable aluminum was seen at the first coating interface region. In contrast, the mean hardness at the second coating interface region was nearly the same as that of the consumable rod. Extensive studies of the surface coating and the microstructural features at both interfaces were performed through FESEM and EDAX analyses.

Early damage detection of fatigue failure for RC deck slabs under wheel load moving test using image analysis with artificial intelligence

Reinforced concrete (RC) bridge decks suffer from cracking damages caused by traffic loading and environmental-related defects, such as the alkali-silica reaction (ASR). These require inspections involving measuring crack width and density followed by essential maintenance and repair works, however, there are no signs for fatigue failure. In this study, the out-of-plane shear deformations which cause small delaminations (pits) along surface cracks are proposed as an early indicator for fatigue failure. Thus, un-damaged and ASR-damaged RC deck slabs are tested under moving wheel loading and, using image-recognition for surface cracks detection, the pits along surface cracks are captured using an artificial intelligence (AI) model. The results show that, while both crack and pit density increase over the fatigue life of un-damaged slabs, there is an earlier sudden increase in pit density. In the case of the ASR-damaged slab, surface cracking is almost constant over the fatigue life until a sudden increase just prior to failure. Pit density, however, increases over the fatigue life with an earlier rapid increase before failure. The density of pits along cracks would be, therefore, a significantly earlier indicator of fatigue failure than crack density, offering the potential for more efficient and automatable bridge inspections.

Wind Coefficient Distribution of Arranged Ground Photovoltaic Panels

Solar panels installed on the ground receive wind loads. A wind experiment was conducted to evaluate the wind force coefficient acting on a single solar panel and solar panels arranged in an array. The surface roughness did not have a significant effect on the change in vertical force, which is the wind force coefficient acting on the vertical surface of a single solar panel. An examination of the change in wind direction angle showed that the largest vertical force coefficient was distributed in the 0° forward wind direction on the front of the solar panel, the 345° reverse wind direction on the rear side, and the 135° and 225° diagonal directions on the rear panel. Furthermore, an examination of the change in wind force coefficient according to the change in solar panel inclination angle (β) showed that the drag coefficient was the highest at the 40° inclination angle of the panel (β), followed by the 30° and 20° inclination angles. However, the lift coefficient and vertical force coefficient were not significantly affected by the inclination angle of the panel. The wind force coefficient of the panels arranged in an array was influenced by the wind direction angle and panel position. With the exclusion of the nearest row at a wind direction angle of 0°, all the panels in the array showed lower coefficients than those in the single-panel experiment. In the case of the panels placed inside, the wind speed was decreased by the surrounding panels. As a result, the wind force coefficient was lower than that of the single-panel experiment. This outcome is attributed to the small delamination at the end of the panels by the surrounding array of panels compared with that of the single-panel experiment.

Effect of cooling strategies on performance and mechanism of helical milling of CFRP/Ti-6Al-4 V stacks

Hole-making for Carbon Fiber Reinforced Plastics (CFRP)/Ti-6Al-4 V stacks is crucial for the assembling strength of aircraft structure parts. This work carried out experimental work for helical milling (HM) of the stacks with sustainable cooling/lubrication (dry, MQL and cryogenic) conditions. Cutting forces and temperatures at the CFRP layer, Ti-6Al-4 V layer and the interface of stacks were obtained by a developed measuring system. The temperatures in CFRP machining at cryogenic condition varied from −167 °C to −94 °C, which were much lower than those at dry and MQL conditions. The maximum temperature near the interface of stacks for the ninth hole was higher than 240 °C due to heat conduction from Ti-6Al-4 V layer. The hole quality, hole diameter and tool wear mechanism at different cooling/lubrication conditions were presented and discussed. MQL condition generated mainly extrusion fracture for the fibers, due to the reduced friction effect compared with dry condition. MQL was helpful to reduce the feed mark at the hole surface of Ti-6Al-4 V alloy. The flank wear of cutting edge at MQL condition was better than those at dry and cryogenic conditions. Cryogenic cooling contributed to better CFRP surface with smaller delamination and hole entrance damage due to the increased resin strength and fiber brittleness. The damage near the entrance of CFRP were analyzed by the contact state of cutting edges and fibers. Additionally, hole diameters near the exit of CFPR layer were larger than other test positions. This work provided feasible processes for improving hole quality and tool life in hole-making of CFRP/Ti-6Al-4 V stacks.

Residual crashworthiness of braided composite tube with transverse multi-impact damages: Experimental and numerical study

In this study, damage behaviors and residual crashworthiness characteristics of braided composite tube with transverse pre-impact damages subjected to axial compression were investigated by experimental and numerical methods. The compression tests were conducted on intact, single impact and repeated impact tubes at identical, adjacent and opposite positions to analyze the effects of impact damages. A multi-step finite element model considering the progressive damage characteristics was developed in ABAQUS/Explicit to predict the failure behaviors of braided tube. It was found that the damage behavior and residual performance of braided tube subjected to axial compression were dominated by impact induced delamination distribution with respect to area and position. A relatively small delamination area induced by single impact resulted in similar progressive folding damage mode and energy absorption capacity as in intact tube. For repeated impact, delamination with increased area at identical position leaded to local buckling damage mode, while small delamination areas distributed at adjacent and opposite positions caused U-shape and W-shape fracturing damage modes, respectively. Consequently, the transformation of damage mode resulted in significantly reduced residual crashworthiness characteristics in repeated impact tubes compared to intact and single impact tubes.

Local Delaminations Induced by Interaction Between Intralaminar Cracking and Specimen Edge in Quasi-Isotropic CF/EP NCF Composites in Fatigue Loadings

Experimental results are presented on the onset and propagation of local delaminations caused by the interaction between specimen edges and intralaminar cracks in fiber bundles of 90° layers in quasi-isotropic [–45/90/45/0]s CF/EP noncrimp fabric (NCF) laminates subjected to tension-tension fatigue loadings. It is confirmed that the first damage mode is intralaminar cracking in 90° layers, which consists of intrabundle cracks and cracks in the matrix between bundles (often beginning from stitches). This damage mode triggers cracking in off-axis layers and local delaminations in positions where the 90° layer crack meets an adjacent layer. The process of local delamination is significantly enhanced at specimen edges, where the out-of-plane edge stresses contribute to the local delamination. During cyclic loadings, delaminations grow and coalesce along the edge and propagate towards the specimen center. These processes are quantified experimentally at different levels of cyclic load. In a low-stress fatigue, a very high number of cycles is required to detect small edge delaminations, and they stay at the edge. In high-stress cyclic tests, delaminations grow faster inside the composite: about 20% of the interface in the central zone can be delaminated. It is found that the reduction in the axial modulus is proportional to the relative delaminated area, proving that delamination is the major stiffness reduction factor in these laminates.

Static, free and forced vibration analysis of a delaminated microbeam-based MEMS subjected to the electrostatic force

In this paper, the delamination effect on the static and natural frequency response of a microbeam subjected to the nonlinear electrostatic force is studied using a semi-analytical approach for the first time. The Euler–Bernoulli beam assumption along with the non-classical modified couple stress theory is used to obtain the governing differential equation of motion and then a reduced-order model based on Galerkin’s decomposition method is obtained. At first the microbeam with very small delamination like an intact microbeam is solved and then the solution is compared with those reported in the literature and the solution obtained using 3D-coupled electromechanical software. After validation, the effects of delamination length and its locations in thickness and length directions on the microbeam behavior are investigated in details. It is shown that the delamination has remarkable effects on the characteristics of the microbeam, especially near the pull-in voltage. Also, the delaminated microbeam with various thicknesses is studied using both the classical and the non-classical theories. It is found that the difference between the two models is significant for the thin microbeam with a thickness near of below than its material length scale parameter. This investigation is helpful for the nondestructive detection of the delamination in the beams.

Ultrasonic and thermographic studies for CFRP inspections with real and simulated defects

This paper deals with an overview of improved UT control procedures, related to defect investigation on advanced CFRP composite elements for aeronautical structures; the inspection procedures were evaluated with different frequency probes, employing direct contact, water stream and immersion methods. In this work, ultrasonic and thermographic controls of CFRP components are presented for detection of different types of defects (real and artificial) in order to evaluate sensitivity and detectability techniques. This work presents a preliminary ultrasonic study on CFRP components employing different probes and tools.For these aims, inspections of CFRP plates with artificial defects are optimized and compared to thermographic acquisitions with active pulsed technique, in order to calibrate Ultrasonic Technique tools and verify procedures for suitable results. In a second stage, improved ultrasonic scanning investigations are applied on real parts, containing real production defects in terms of small delamination and diffused porosity of T-shape stringer and resin/matrix excesses between layers, localized in the thin cap and web of structural civil aircraft stringers.

A study on diagnosing both isotropic and orthotropic, intentionally damaged laminates

ABSTRACT Being the most detrimental mode in laminated structures, delamination initially degrades local structural strength, and lastly causes global structural failure, thus necessitating delamination identification at its incipient stages. For identification purposes, a comprehensive numerical and experimental study on delamination within both the laminated isotropic acrylic-glass and orthotropic epoxy-glass plates is presented. As the first step, frequency variations in the acrylic-glass plate induced by different-sized delamination are examined at varying the in-plane or out-of-plane location as well as boundary conditions of delamination. Tiny delamination is identified by the use of processed signals of Lamb wave simulated by spectral finite element (SFE) technique in the next step when noticing the delamination-induced changes of frequency to be inefficient for detection purposes. While utilising absorbing layers with the obtained mechanical properties along the axis of symmetry, a quarter of the plate in the SFE domain is modelled. The study is concluded with finding the accurate location and shape of small delamination using A0 or S0 modes. The preference of A0 mode for the delamination identification compared to S0 mode is indicated. Finally, for verification purposes of Lamb wave patterns in real-life applications, a composite epoxy-glass plate is experimentally investigated via laser Doppler vibrometer (LDV).

An experimental and numerical investigation into damage mechanisms in tapered laminates under tensile loading

Through-thickness thickness reductions in laminated composites are essential for weight and aerodynamic efficiency, but they can also be the site of damage initiation. Tensile failure mechanisms of modestly tapered laminates, loaded via gripping their thick and thin ends, and a severely tapered laminate, loaded by contact at its tapered section, were investigated via experiments and high-fidelity finite element modelling. The primary failure mode is by delamination, initiated from a terminated ply in the tapered region, which is quite sensitive to a small delamination defect at a ply drop location. Experimental measurements and FE predictions correlate very well in all cases for both location and load. In the severely tapered case, the contact stress influences the delamination and so a fibre failure criterion also needs to be considered, including both the fibre direction tensile stress and its interaction with through-thickness direct and shear stresses imposed by the contact.

Spectral finite element method for wave propagation analysis in smart composite beams containing delamination

PurposeThe purpose of the study is to present a frequency domain spectral finite element model (SFEM) based on fast Fourier transform (FFT) for wave propagation analysis of smart laminated composite beams with embedded delamination. For generating and sensing high-frequency elastic waves in composite beams, piezoelectric materials such as lead zirconate titanate (PZT) are used because they can act as both actuators and sensors. The present model is used to investigate the effects of parametric variation of delamination configuration on the propagation of fundamental anti-symmetric wave mode in piezoelectric composite beams.Design/methodology/approachThe spectral element is derived from the exact solution of the governing equation of motion in frequency domain, obtained through fast Fourier transformation of the time domain equation. The beam is divided into two sublaminates (delamination region) and two base laminates (integral regions). The delamination region is modeled by assuming constant and continuous cross-sectional rotation at the interfaces between the base laminate and sublaminates. The governing differential equation of motion for delaminated composite beam with piezoelectric lamina is obtained using Hamilton’s principle by introducing an electrical potential function.FindingsA detailed study of the wave response at the sensor shows that the A0 mode can be used for delamination detection in a wide region and is more suitable for detecting small delamination. It is observed that the amplitude and time of arrival of the reflected A0 wave from a delamination are strongly dependent on the size, position of the delamination and the stacking sequence. The degraded material properties because of the loss of stiffness and density in damaged area differently alter the S0 and A0 wave response and the group speed. The present method provides a potential technique for researchers to accurately model delaminations in piezoelectric composite beam structures. The delamination position can be identified if the time of flight of a reflected wave from delamination and the wave propagation speed of A0 (or S0) mode is known.Originality/valueSpectral finite element modeling of delaminated composite beams with piezoelectric layers has not been reported in the literature yet. The spectral element developed is validated by comparing the present results with those available in the literature. The spectral element developed is then used to investigate the wave propagation characteristics and interaction with delamination in the piezoelectric composite beam.

The influence of ambient environmental conditions in detecting bridge concrete deck delamination using infrared thermography (IRT)

Delamination is a serious form of deterioration in concrete bridge decks. Infrared thermography (IRT) is an advance non-destructive testing method for concrete bridge deck delamination detection by capturing the absolute thermal contrast (ΔT) on the concrete surface caused by the disruption in heat flow due to subsurface defects. However, as the ambient environmental conditions (e.g. wind velocity and solar radiation) of a bridge could significantly affect the measurement outcomes of IRT, the optimal times for infrared data collection are still unclear. In this paper, a series of experimental and numerical studies were carried out to investigate the effects of the rate of heat flux and wind velocity on ΔT on the surface of bridge decks with the aim of identifying the optimal inspection times for different geometry characteristics of delamination (i.e. size and depth). The developed model is firstly validated by the experimental data and then a series of parametric studies were carried out. The result shows that the heat flux rate plays an important role in the development of ΔT on concrete surface, especially for a relatively shallow and small size delamination. However, the influence of heat flux rate gradually diminishes with the increase in size and depth of delamination. In addition, it demonstrates that there is a positive linear correlation between the total heat energy (external irradiation) and square of the delamination depth. The current research represents an important step towards the development of an effective and efficient way for defect detection using IRT.

Small delamination detection in carbon fibre reinforced polymer composite beam by NDT and vibration analysis

Fibre reinforced polymer (FRP) composites have been prone to damages such as delamination, debonding, matrix cracking and fibre breakage. In this paper, detailed study has been carried out on determining the presence of small delamination in sub layers of carbon fibre composite beam using various non-destructive tests (NDT) and vibration signal analysis. We have concluded that ultrasonic and X-Ray Diffraction (XRD) tests have found to be more effective in detecting small delamination than X-Ray radiography and thermography tests. In the vibration test, we have observed that the natural frequency decreases and the rate of damping increases in delaminated carbon fibre composite beam as compared to composite beam without delamination. Besides, the fast fourier transform (FFT) results of vibration signal of delaminated beam have clearly shown higher harmonics and side bands. Finally, we have concluded that online vibration test can also be used effectively to detect presence of small delamination in FRP composites.

Detection of small delamination in mullite/Si/SiC model EBC system by pulse thermography

The pulse thermography (PT) technique was applied to the detection of the delamination of a multi-layered coating system composed of mullite/Si onto a reaction-bonded SiC substrate. The potential evaluation was carried out in order to detect internal delamination in multi-layered material system. Moreover, the observation of the cross sections and 3D views obtained by X-ray computed tomography (CT) indicated that the delamination occurred at the interface between the top coat and the bond coat layers. The changes in the temperature distribution obtained by PT indicated the existence of a delamination area in the top coat layer of the mullite. In particular, the lower temperature region corresponded to the delamination area. The experimental results confirmed that the PT technique is effective with respect to the internal delamination of multi-layered coating system.

Simulation Tools for a Fiber-Optic Based Structural Health Monitoring System

Probability of detection (POD) graphics allow for a change from qualitative to quantitative assessment for every damage detection system, and as such it is a main request for conventional non-destructive testing (NDT) techniques. Its availability can greatly help towards the industrialization of the corresponding Structural health monitoring (SHM) system. But having in mind that for SHM systems the sensors are at fixed positions, and the location of a potential damage would change its detectability. Consequently robust simulation tools are required to obtain the model assisted probability of detection (MAPOD) which is needed to validate the SHM system. This tool may also help for the optimization of the sensor distribution, and finally will allow a probabilistic risk management. INDEUS, simulation of ultrasonic waves SHM system, was a main milestone in this direction. This article deals with the simulation tools for a strain based SHM system, using fiber optic sensors (FOS). FOS are essentially strain/temperature sensors, either with multi-point or with distributed sensing. The simulation tool includes the finite element model (FEM) for the original and damaged structure, and algorithms to compare the strain data at the pre-established sensors locations, and from this comparison to extract information about damage occurrence and location. The study has been applied to the structure of an all-composite unmanned aircraft vehicle (UAV) now under construction, designed at Universidad Politecnica de Madrid for the inspection of electrical utilities networks. Distributed sensing optical fibers were internally bonded at the fuselage and wing. Routine inspection is planned to be done with the aircraft at the test bench by imposing known loads. From the acquired strain data, damage occurrence may be calculated as slight deviations from the baselines. This is a fast inspection procedure without requiring trained specialists, and it would allow for detection of hidden damages. Simulation indicates that stringer partial debondings are detected before they become critical, while small delaminations as those produced by barely visible impact damages would require a prohibited number of sensing lines. These simulation tools may easily be applied to any other complex structure, just by changing the FEM models. From these results it is shown how a fiber optic based SHM system may be used as a reliable damage detection procedure.

New Compound Drill Bit for Damage Reduction in Drilling CFRP

This paper focuses on finding a suitable drill bit for drilling Carbon Fiber Reinforced Plastic (CFRP). The drilling characteristics of the dagger drill, the double point angle drill, and the candle stick drill are studied. The results show that the side edge has poor ability to remove burrs, and the tip structure of the outer corner of the candle stick drill can greatly reduce the entry damage. However, the outer corner small tip structure of the candle stick drill cannot effectively remove the uncut fiber around θ = 0° of hole exit. The long secondary cutting edge of the dagger drill makes a small thrust force at the drilling exit stage and reduces the dropping speed of the thrust force, leading a lower impact on the laminate bottom. Then, a new tool is developed for drilling CFRP based on the advantages of the three kinds of drill bits. The drill has a long secondary cutting edge, a small tip diameter and a small tip structure of the outer corner. And its drilling characteristics are analyzed. The result shows that the new compound drill bit can effectively remove fibers and reduce thrust force at the drilling exit stage, form burr-free and small delamination hole. The drill reduces the waste of CFRP in manufacturing process since the exit damage is reduced substantially, which is in line with the concept of green manufacturing.

An estimate of the location of multiple delaminations on aeronautical CFRP plates using modal data inverse problem

With the increase in the use of composite materials, especially in the aeronautical industry, it is essential that a complete evaluation of the mechanical performance of such structures be undertaken, especially with regard to structural integrity. To assist in this task, structural health monitoring methodologies are employed in order to minimize time and maintenance costs, and errors arising principally from human factors, and which can occasionally result from the failure to properly inspect the aircrafts. This study addresses the use of an inverse method for delamination identification in carbon fiber reinforced polymers plates. First, the direct problem was modeled via a finite element method in order to obtain a faithful model that represented the real case studied. The inverse problem was solved by minimizing an objective function through genetic algorithms. Modal responses of delaminated plates are able to identify the possible location of multiple delaminations in laminated plates since the structural matrices are changed as a function of the induced damage. Numerical and experimental results showed excellent identification of small delaminations, reducing the initial search area by up to 96%, which can lead to savings in time and costs for the aeronautical industry.