Delamination In Composites Research Articles

A combination of failures of interfacial delamination and layer cracking in REBCO multilayered conductors

Rare-earth barium-copper-oxide (REBCO) coated conductors (CCs) are kinds of multiphase, multilayer, and composited superconducting tapes, which are becoming promising candidates for abundant high-energy and high-field applications owing to their outstanding superconducting properties. However, the mechanical failures of REBCO CCs composites, such as cracks in the brittle ceramic layers, and delamination on interfaces among weak multiple layers, have become the essential issues threatening the capability and stable operation of REBCO-based superconducting devices, especially under extreme low temperature and complex electromagnetic fields. In this study, a failure modeling in combination of the interfacial delamination and layer fracture in REBCO CCs composites was established and analyzed numerically by the finite element method. To effectively treat with the two mechanical failure modes, the interfacial delamination response and the fracture behavior were respectively captured using the cohesive zone model and the extended finite element method for the multilayered superconducting CCs. The failures initiation and evolution in REBCO multilayered structures were implemented based on the stress criteria. The characteristics of two failure modes of the multilayered superconducting CCs under uniaxial tension and bending were investigated, and those during cooling process from room temperature to 77 K were then simulated in consideration of residual thermal stresses accumulated. The results illustrated that the progressive mechanical failure behaviors were presented in combination of the interfacial delamination and fracture damages in the REBCO multilayered structure. The crack propagation in the superconducting layer can cause the delamination failure at the corresponding position on the interfaces between the superconducting layer and its adjacent layers, while the delamination damage on the interfaces releases the constraint between the layers further enhances the crack propagation. The residual thermal stress accumulated during cooling was found to significantly affect the mechanical behavior of the REBCO CCs, and the low-temperature makes a failure occurrence earlier compared to that at room temperature.

Multifunctional MEN-Doped Adhesives: Strengthening, Bond Quality Evaluation, and Variations in Magnetic Signal with Environmental Exposure

Adhesive bonding of polymer matrix composites offers various advantages over traditional fasteners, such as a uniform stress state, reduced weight, and delay of composite delamination. However, adhesive bonding has limited implementation due to challenges in the prediction of durability. This work introduces a new method to monitor an adhesively bonded composite joint by dispersing magneto-electric nanoparticles (MENs) into the polymer precursor and monitoring changes in their surface charge density by evaluating the output magnetic signal under an applied magnetic field. Real-time monitoring of the curing process of a polymer adhesive was performed and corroborated via thermal analysis and mechanical testing. Lap shear and end notch flexure testing showed that adding 1 vol% MENs led to a ~23% increase in shear strength and a ~12% increase in mode II critical energy release rates compared to the undoped adhesive. Adding 5 vol% MENs also increased the adhesive’s peak tensile stress by ~8%. Strengthening mechanisms of the doped adhesive were monitored using in situ electron microscopy. A correlation between water ingression and a change in the magnetic moment was observed. Results show the MENs’ potential as a structural health-monitoring tool for a wide range of materials and applications.

Tribological behavior of as-cast and wrought Al–Mg2Si hybrid composites reinforced by Ti-based intermetallics

In this study, the tribological properties of Al–15Mg2Si (AC), Al–15Mg2Si–3TiB2–TiAl3 (C31), and Al–15Mg2Si–5TiB2–4TiAl3 (C54) in-situ composites were evaluated. In the as-cast state, the optimal results were obtained for the C31 sample, where the wear rate and the average coefficient of friction (ACOF) under the load of 60 N decreased by ∼54% and ∼27% compared to those obtained for the AC sample, respectively. Morphological modification of primary and eutectic Mg2Si particles and increasing the volume fractions of TiB2 particles were characterized as key factors for improving wear resistance of C31 sample. On the other hand, the presence of needle-like TiAl3 particles resulted in severe delamination and separation of tribolayer and increased mass loss of C54 sample. Alternatively, the fragmentation and redistribution of eutectic/primary Mg2Si and TiB2/TiAl3 particles during elevated-temperature deofrmation, the reduction of casting defect by thermomechanical processing effects, as well as intense grain refinement by dynamic recrystallization (DRX) improved the mechanical properties of hot-extruded samples and led to a notable enhancement in the load bearing behavior and stability of the formed tribolayer even under high loads. Therefore, the wear mechanism changed from the combination of severe delamination, adhesive, and three-body abrasive wear in the as-cast composites to mild abrasive wear in the hybrid composites subjected to hot deformation by the extrusion process. Consequently, in the optimal condition and under load of 60 N, the wear rate decreased from 29.6 to less than 10 mg/km and ACOF decreased by about 24% in hot-extruded C54 sample compared to the AC sample.

Natural Frequency based delamination estimation in GFRP beams using RSM and ANN

The importance of delamination detection can be understood from aircraft components like Vertical Stabilizer, which is subjected to heavy vibration during the flight movement and it may lead to delamination and finally even flight crash can happen because of that. Any solid structure's vibration behaviour discloses specific dynamic characteristics and property parameters of that structure. This research investigates the detection of delamination in composites using a method based on vibration signals. The composite material's flexural stiffness and strength are reduced as a result of delaminations, and vibration properties such as natural frequency responses are altered. In inverse problems involving vibration response, the response signals such as natural frequencies are utilized to find the location and magnitude of delaminations. For different delaminated beams with varying position and size, inverse approaches such as Response Surface Methodology (RSM) and Artificial Neural Network (ANN) are utilized to address the inverse problem, which aids in the prediction of delamination size and location.

Effect of Graphene Nanoplatelets on Mechanical Performance of GFRP Composites

The carbonaceous nanofillers such as graphene nanoplatelets (GN) due to their unique properties have been increasingly used as nanofillers to improve the mechanical properties of FRP composites. In the present study, unidirectional glass fiber reinforced polymer (GFRP) composites and GFRP composites with graphene nanoplatelets (GN-GFRP) are fabricated using vacuum bag process and hand layup method. The percentage of GN added in GN-GFRP composites is 0.1% and 0.5% in case of hand layup method and 0.5% in case of vacuum bag process. The specimens are tested under uniaxial tension and three-point bending to study the effect of GN on the tensile and flexural performance of GFRP composites. In case of composites fabricated using hand layup method, the tensile strength of GFRP composites increased by 35.8% and 40.4% with the addition of 0.1% and 0.5% GN respectively. The tensile modulus of GN-GFRP composites with 0.5% GN decreased by 11.8% compared to GFRP composites. The flexural strength of GN-GFRP composites with 0.1% and 0.5% GN are found to have increased by 6.5% and 5.3% respectively compared to GFRP composites. The flexural modulus of GN-GFRP with 0.1% GN increased by 11% and the same for GN-GFRP with 0.5% decreased by 8.2% compared to GFRP composites. The tensile strength and modulus of GN-GFRP composites fabricated using vacuum bag process decreased by 24.4% and 7.7% respectively compared to GFRP composites. Scanning Electron Microscope (SEM) investigation reveals that poor adhesion of resin with the fibers caused delamination in GN-GFRP composites fabricated using vacuum bag process resulting in reduction of tensile properties.

Fracture toughness resistance curves for a delamination in CFRP MD laminate composites, Part I: Nearly mode II deformation

In this investigation nearly mode II initiation and resistance energy release rate values, required for delamination propagation, were determined based on quasi-static calibrated end loaded split (C-ELS) fracture tests. Two multi-directional (MD) carbon fiber reinforced polymer (CFRP) material systems were examined. The first was manufactured as a wet-layup, with an initial delamination between a unidirectional ply and a plain woven ply. The second was manufactured from a prepreg, with an initial delamination along an interface between two plain woven plies oriented differently. Two-dimensional finite element analyses (FEAs) of the tested specimens were performed. Based upon the FEA results, with use of the displacement extrapolation (DE) method, as well as the virtual crack closure technique (VCCT), stress intensity factors were calculated. The obtained values were used to determine the in-plane mixed-mode phase angle for each test, which indicated nearly mode II deformation. Fracture toughness resistance curves or R-curves were generated as a function of the delamination extension. The critical initiation and resistance energy release rate values were obtained from the stress intensity factors, as well as with the J-integral, which are local methods. In addition, the global experimental compliance method (ECM) was used. Small differences were observed between the results obtained by means of the two methods. From a comparison between the R-curves of the two material systems, it was seen that the initiation energy release rate values were higher for the prepreg by 25.5%. A greater difference was found in the increasing portion of the R-curves, as well as the steady state energy release rate values.

Z‐pin effect on interlaminar mechanical and ablation performance of quartz‐phenolic composites

AbstractWhen a spacecraft flies at a high speed through a high‐heat flow, the ablative composites suffer from severe delamination and bulging due to their weak interlaminar performance. This will interfere normal operations of spacecraft. To improve the interlaminar performance of ablative composites is of great significance to the ablative structures. This study investigated Z‐pin reinforced quartz‐phenolic composites and reinforcement mechanism. First, a thermogravimetry (TG) test was used to analyze the thermal decomposition process of the phenolic. Second, a series of tests, including a single Z‐pin pull‐out, a single‐lap joint tensile and an oxyacetylene ablation test, were conducted to investigate composites' ablation performance. We also deployed a 3D morphometer and an infrared thermography to reveal the function of Z‐pins. The results demonstrated that Z‐pins can improve composites' ablation performance in both structure and function. In the structure, Z‐pins suppress the interlaminar delamination of composites by enhancing the interlaminar strength. As the ablation time increases, interface bridging between the Z‐pin and laminates contributes increasingly to the interlaminar shear strength of laminates. After 60 s of ablation in a flame of 920°C, the interlaminar shear strength of Z‐pin specimens is 84.66% stronger than that of the normal composites. In the function, the Z‐pins suppress large areas of bulges by providing escape channels for pyrolysis gas, and reduce bulging areas by 60%. Z‐pin technology can effectively suppress mechanical erosion of the ablative surface due to the delamination and bulging. It is an important way to improve ablation performance of ablative structures.

Identifying delamination in carbon fiber composites based on defect modes in imperfect phononic crystals

Delamination is one of the most common types of damage in fiber-reinforced composites. In this paper, borrowing from the concept of defect modes in imperfect phononic crystals, we propose a method for identifying delamination damages in composite materials. Specifically, by periodically arranging concentrated masses on the surface of a finite carbon fiber beam, the carbon fiber beam becomes a phononic crystal, and defect modes in Bragg bandgaps can be observed when there is delamination. With the spectral element method formulation, we show that the frequencies of defect modes are directly related to the length and location of the delamination. By performing algebraic addition and subtraction on the generated defect mode frequencies when applying excitations at the two ends of a defected carbon fiber composite beam, an approximately linear correlation with the length and location of delamination can be obtained. Our analytical and experimental results indicate that, by introducing periodicity on the carbon fiber composite beam and applying appropriate post-processing algorithm to the generated defect modes, rapid and accurate identification of the delamination length and location can be achieved based on the bandgap characteristics of the imperfect phononic crystals.

Nondestructive damage evaluation of composites based on terahertz and X-ray image fusion

Inspection of thin and multi-depth delamination and foreign inclusion defects inside glass fiber reinforced polymer (GFRP) composites is a challenge for traditional non-destructive testing (NDT) techniques. Terahertz (THz) and X-ray computed tomography (CT) are two prevalent NDT methods, which also have their unique advantages and relative weakness individually. Specifically, in our previous study, it is found that THz NDT method has a higher contrast and X-ray CT NDT method has a higher resolution for detecting defects in GFRP composites. In this paper, a new fusion algorithm applied to the THz and X-ray CT NDT imaging data, based on the combination of saliency region analysis (SRA) and wavelet based multi-scale transforms (W-MST) is proposed to detect delamination and inclusion defects of GFRP composites. Moreover, the strategy of weighted least square optimization (WLSO) is adopted to eliminate the effects of unregistered images. An exhaustive number of combinations (36 in total) of the proposed fusion algorithm, which include the selection of fusion rules for saliency map, wavelet decomposition levels and wavelet basis functions, have been quantitively compared using objective evaluation indices such as standard deviation (SD) and spatial frequency (SF) to quantify the improvement of detection effect in terms of contrast and resolution. Among them, there are five optimal combinations applicable to five pairs of different source images, and the averages of the SD and SF indicators of fused images have increased by 126% and 190% compared with the source images, respectively. The inspection efficacy of defects using the proposed fusion method has been analyzed visually, showing that the new approach can effectively accentuate the complementary advantages of THz and X-ray NDT methods, resulting in quantifiably improved defect inspections.

Mode III dynamic fracture performance of carbon-epoxy laminates

The aim of this work is the development of a new test methodology for the characterization of delamination of composites under mode III of fracture and dynamic stress (fatigue). The phenomenon of delamination is one of the main failure mechanisms in composite materials configured by stacking several layers. The mode of fracture that has been studied is mode III to fatigue, with the difficulty that its simulation implies. The test device employed, which allows subjecting to the laminates to this loading mode, has already been previously tested in static tests. Characterization tests were performed on two different materials, both with different matrices and carbon fibers. An analytical model developed for the case of static fracture has been used and a statistical analysis of the results obtained has been carried out. A fractography of the fracture surface was also performed in order to determine the characteristics of this type of fracture in a dynamic regime.

Classification of Composite Delaminations via Pitch-Catch Phased Array Ultrasound

Classification of Composite Delaminations via Pitch-Catch Phased Array Ultrasound

Optimization of dry sliding wear characteristics of Al–25Zn/SiC hybrid composites by graphite reinforcement using artificial neural network and Taguchi’s method

ABSTRACT This paper proposes a unique Al25Zn/SiC/Graphite hybrid composite for plain bearing and other wear-resistant applications. Newly synthesized composites of Al–25Zn alloy reinforced with 3 wt-% graphite and 10, 15 and 20 wt-% SiC are tested using a pin on the disc tribometer following ASTM G-99 standard against EN24 disc. Sliding wear studies using the Taguchi L16 array are carried out for a set of parameters including pin temperature, speed, load and a constant sliding distance. The results reveal that adding 3 wt-% graphite into Al25Zn/SiC composites greatly enhances wear resistance, tensile strength, impact strength and ductility at the expense of hardness. The wear behaviour of the composites is predicted using an artificial neural network and a regression model. Adhesion is found the most common wear mechanism in matrix alloys, while abrasion and delamination in composites. Among the examined materials, the composite with 3 wt-% graphite and 15 wt-% SiC has the optimum combination of characteristics.

Analysis of Carbon Fiber-Reinforced Polymer Composites Delamination during Vibration Assisted Trimming using Historical Data Design

Delamination is one of the main issues during the CFRP cutting process. This problem attracts researchers to investigate to meet the stringent quality need. This paper evaluates the influence of Rotary Ultrasonic Assisted Trimming (RUAT) when slotting carbon fiber reinforced plastic (CFRP). The study investigates rotation speed, vibration amplitude, and frequency particularly. The correlation effect of these parameters is to be evaluated by response surface methodology (RSM) to identify the minimum delamination. Seventeen trials were conducted with 38 plies of multi-directional CFRP panel. The trimming quality was determined by the minimum delamination damage of the slotting area analyzed by the ImageJ software. The best slotting quality can be achieved by applying spindle speed, amplitude, and frequency of 5305 rpm, 2.75 µm, and 26.79 kHz.

On characterization of cohesive zone model (CZM) based upon digital image correlation (DIC) method

Cohesive zone model (CZM) has been extensively applied in numerical simulation of interfacial fracture behaviors such as composite delamination and fracture of adhesive joints. Accurate and efficient identification of CZM parameters signifies an important research topic to enable reliable analysis and design of these structures. This paper aims to derive the actual traction-separation law of CZM by means of experimental measurement with only digital image correlation (DIC) technique. A double cantilever beam (DCB) specimen was employed for experimental tests and a semi-empirical analytical model was developed to associate the traction-separation law with the global loading response. Explicit formulae of the initial stiffness, maximum traction stress and damage evolution were derived. In this study, the DCB specimens bonded with a ductile adhesive layer of Araldite® 2015 were fabricated and experimentally tested. The displacement distribution of the adhesive layer was quantified as the fundamental data for identification of CZM parameters. The initial stiffness, maximum traction stress and damage evolution of CZM were identified accurately. Based upon the extracted CZM parameters in the present work, a finite element (FE) model was developed and verified with the experimental results. Furthermore, a FE model with a traditional linear damage evolution was also established for comparison, thereby demonstrating the effectiveness of the proposed method for extracting the traction-separation law of CZM. By employing the present approach, the real traction-separation law can be better presented and accurate simulation of adhesive joint can be achieved.

A combined global-local approach for delamination assessment of composites using vibrational frequencies and FBGs

In this paper, a combined global-local strategy was developed for structural health monitoring (SHM) of fiber reinforced polymer (FRP) composites to assess the internal delamination damage, which has combined vibrational frequency shifts as the global index and the fiber Bragg grating (FBG) wavelength shifts as the local index. The delamination detection was carried out in two steps. The first step was based on the global damage index, which is the changes in multiple modes of frequencies measured by a non-contact scanning laser Doppler vibrometer (SLDV) system. Machine learning (ML) algorithms including support vector machine (SVM) and extreme learning machine (ELM) algorithms were used to initially predict the delamination interface, location, and size through frequency shifts. Numerical and experimental verification results show that SVM has a higher prediction accuracy than ELM when only a small number of samples exist, and the classification of SVM is better than its regression function to be used in the prediction of discrete delamination interfaces. To verify the damaged area predicted by frequency changes and further update the delamination edges with better accuracy, the second step has taken the changes in the wavelength of multiple FBG sensors as the local damage index. Multistage loads were applied at equidistant positions of the FRP specimens to induce deformations and the wavelength shifts in FBGs were used to determine the boundary of delamination. The results showed that delamination in FRP composites can be assessed more precisely using the global-local two-step SHM strategy, which can compensate for the shortcomings of only using vibration-based or FBG-based monitoring techniques. Besides, the SVM algorithm showed excellent predictive performance in both steps and had great potential in delamination damage monitoring.

Selection of an appropriate non-destructive testing method for evaluating drilling-induced delamination in natural fiber composites

Drilling is a principal machining process that is generally operated to make holes in composite parts to join and assemble into a complicated structures. Recently, the use of natural fiber composites (NFCs) has increased in engineering and industrial applications. Choosing an appropriate non-destructive technique to evaluate the drilling-induced delamination in NFCs is a vital and essential task. This paper investigates three different non-destructive techniques for evaluating and determining the size and shape of drilling-induced delamination in NFCs. According to the results from this research, it can be inferred that the image processing technique using a flatbed scanner can be utilized as an accessible, economical, and efficient method for assessment of drilling-induced delamination in NFCs.

Adhesive Boundary Element Method Using Virtual Crack Closure Technique

In this study, a new adhesive contact model is built upon a boundary element method (BEM) model developed by Pohrt and Popov (2015). The strain energy release rate (SERR) on the edge of the bonding interface is evaluated using Virtual Crack Closure Technique (VCCT) which is shown to have better accuracy and weaker mesh-size dependency than the closed-form SERR formula derived by Pohrt and Popov. A composite delamination criterion is proposed for crack nucleation and propagation. Numerical results predicted by the present model are in good agreement with the analytical solutions of two classic problems, namely, the axisymmetric parabolic contact and the sinusoidal waviness contact in the plane strain condition. The model of Pohrt and Popov can achieve a similar accuracy for the axisymmetric parabolic contact where the mesh grid is non-conforming to the crack front. Once the conforming mesh grid is used, the accuracy of their model is significantly deteriorated, especially at high work of adhesion and high mesh density. In both BEM models, however, the crack nucleation is found to be mesh-dependent which may be solved by introducing an upper limit for the tensile normal traction.

Sensing delamination in composites reinforced by ferromagnetic Z-pins via electromagnetic induction

This paper investigates a novel technique for sensing delamination in through-thickness reinforced composites based on electromagnetic induction. This sensing technique features ferromagnetic Z-pins and a pair of coils attached to a laminate; the first coil creates a magnetic field that is intensified by the ferromagnetic pins, whilst the second coil detects the magnetic flux change that is caused by the pin motion relative to the coil pair when delamination happens. This approach avoids potential interferences due to contact electrical resistances that exist in electrical-based sensing approaches. The viability of this sensing technique is demonstrated by monotonic and cyclic bridging tests, involving Nickel/Iron alloy Z-pins embedded in E-glass/913 laminates under controlled delamination. A simplified electromagnetic finite element analysis is presented to help interpret the experimental results. The sensitivity of the magnetic-based sensing technique increases with loading rate. Both mode I and mode II delamination events can be detected by a voltage signal from the sensing coil, albeit there exists an initial “blind spot” at low loading rates. This sensing technique also allows monitoring the pin bridging status, e.g. the switch of pin pull-out side, without modifications to the architecture of a Z-pinned composite regarding expected mechanical response.

Kriging and dimension reduction techniques for delamination detection in composites using electrical resistance tomography

The electrical resistance tomography (ERT) is a non-destructive evaluation (NDE) technique that uses the inherent conductivity variations of composites to characterize internal damage. This article presents a study on the use of Kriging surrogate models for inverse identification of delamination in composites using ERT. Kriging surrogates are used to predict the change in electrical response of composites with delamination and matrix cracking damage. The results show that Kriging surrogates can successfully identify delamination even in the presence of matrix cracking. The technique demonstrated here that combines model order reduction with Kriging surrogates provides a computationally efficient and accurate method for damage identification using ERT.

An enhanced greedy algorithm for failure resistant material design with application to composite delamination

In this work, an enhanced greedy algorithm is developed for failure resistant design of two-phase composites against delamination. Possessing the merits of a greedy algorithm, i.e. easy-to-implement and gradient-free, the computational efficiency is significantly improved. An objective functional based on interface elements is used to represent the failure resistance and evaluated by finite element computations. The search domain for each iteration is reduced by a heuristic strategy, thus leading to a reduced number of required finite element computations. Additionally, a criterion is introduced to adaptively determine the number of switching subvolumes for each correction step, leading to a reduced number of iterations. A numerical study shows that the computational complexity of the proposed algorithm is significantly reduced on the basis of an existing algorithm. Furthermore, we develop three different generation schemes for initial solutions and conduct a comparative study on them. A maximal speed-up factor of 12.5 compared to the existing algorithm is achieved.