Delamination In Laminates Research Articles

On the Cover: A Composite Rigid Double Cantilever Beam Specimen for Assessing the Traction–Separation Response of Mode I Delamination in Composite Laminates

On the Cover: A Composite Rigid Double Cantilever Beam Specimen for Assessing the Traction–Separation Response of Mode I Delamination in Composite Laminates

Experimental and numerical investigation on the mode I and mode II interlaminar fracture toughness of nitrogen-doped reduced graphene oxide reinforced composites

An experimentaland computational study is carried out to investigate the toughening effect of nitrogen-doped reduced graphene-oxide particles (ND-RGOP) in mode I and mode II delamination of carbon fiber/epoxy laminates. For mode I, both delamination initiation and propagation toughness are enhanced by 126.3% and 119.3% with 0.8 ND-RGOP addition. In addition to mode I toughness enhancement, ND-RGOP reinforcement also increases mode II toughness. The highest mode II delamination toughness value was obtained in 0.4ND-RGOP laminates with an increase of 45.6% compared to neat laminates. However, as seen in the bending tests, the maximum force of the end notch bending (ENF) specimens with 0.8 ND-RGOP added is not lower than that of neat laminates. The interlaminar fracture toughness is high due to the rough path of delamination with ND-RGOP addition, which is also observed by FESEM images of the delamination surfaces. In addition to the experiments, a cohesive zone model with parametric traction stress value was prepared in ABAQUS/CAE to obtain the critical traction stresses in the delamination of laminates. The critical normal traction stress of double cantilever beam (DCB) specimens is increased by over 20% with the addition of ND-RGOP. Besides, the critical interlaminar shear stress in mode II delamination for ENF samples is significantly improved.

Restoration of Compressive Strength of CFRP Plates with Edge Delamination through CNT Crack-bridging Deposited by Resin Pre-coating Solution

Edge delamination, with only micro-scale crack openings, in carbon-fiber-reinforced-polymer (CFRP) composites due to light impact, machining and structural assembly is challenging to repair. This study presents a simple pressureless method to repair edge delamination in CFRP laminates. The compressive strength of CFRP panels with around 5–10 mm deep edge delamination cracks have been fully restored without utilizing any special equipment. Acetone-diluted resin pre-coating (RPC) solution, consisting of 95 m/m% acetone and 5 m/m% resin (without hardener), is utilized to penetrate deeply into sharp delamination cracks through capillary action, and 1 m/m% CNT dispersed into the RPC solution (RPC + CNT) is deposited inside the delamination cracks forming CNT-toughened adhesive joints. The compressive strengths of long cure time (3 month) are compared with 2-week results, showing 2-weeks are not sufficient for complete curing. With the 3-month curing period, the compressive strength of CFRP with edge delamination is 100 % restored by the RPC + CNT technique.

A Composite Rigid Double Cantilever Beam Specimen for Assessing the Traction–Separation Response of Mode I Delamination in Composite Laminates

A Composite Rigid Double Cantilever Beam Specimen for Assessing the Traction–Separation Response of Mode I Delamination in Composite Laminates

PENGARUH SUDUT PAHAT PADA PENGGURDIAN KOMPOSIT SERAT DAUN NANAS / TERMOPLASTIK PADA PEMBENTUKAN DELAMINASI

This study aims to determine the effect of the Gurdi tool angle on delamination in composite laminates in the formation of delaminations. The spacing of the selected reinforcing fibers was 2.4 and 6 mm in a composite laminate of ethylene vinyl acetate (EVA) polymer matrix with reinforcing fibers from pineapple leaves. The method used in composite fabrication is hand lay-up, namely by direct casting on three layers of fiber that has been woven and tied to the sides of the mold. For cutting speed, Vf 72 mm / min, using a chisel with a diameter of 8 mm with a tool angle of 70o and 118o. Delamination was determined using the Feda method. The delamination sections were observed using a digital microscope and then processed using AutoCAD to measure Dmax and Ad. The delamination ratio in the top hole for the tool diameter of 8mm with a 70° angle at the variation of the distance of 2 mm, 4 mm and 6 mm were 1.268, 1.224 and 1.192 for the bottom hole were 1.078, 1.042 and 1.029, respectively. Meanwhile, the ratio of delamination in the upper hole for a tool diameter of 8mm with an angle of 118° at a distance variation of 2mm, 4mm and 6mm were 1.194, 1.194, 1.158 for the bottom hole, respectively, 1.046, 1.042 and 1.038.

Characterization on fatigue behavior and damage mechanism of C/BMI composites experienced thermal cycling

The multi-directional laminate CCF800H/AC631 bismaleimide composite material was exposed for a long time under the thermal-cycling environment (−60°C ∼+180°C), and the mass loss rate, FTIR spectra, DMA, tensile strength were tested. The fatigue stress level was determined according to the tensile strength and the fatigue performance of the before and after the thermal-cycling environment was tested. Macroscopic visual inspection and ultrasonic C-scan were used to characterize and analyze the fatigue damage of composite materials. The results show that with the increase in the number of thermal cycles, the mass loss of the composite material started with increased rapidly and then basically flat. The C/BMI composites underwent obvious thermal oxygen aging. After thermal-cycling, it would lead to changes in dynamic mechanical properties by a certain degree of post-curing, physical aging, and local interface debonding in composite materials. With the thermal cycles increased the composite material tensile strength first increased slightly and then decreased rapidly. After 300 thermal cycles, the composite materials occurred slightly damaged, and the fatigue life was apparently reduced compared with the original state. The fatigue failure modes of composite materials are mainly fiber fracture and multi-directional laminate delamination. At high stress levels, the stiffness of the specimen after thermal-cycling are lower decrease compared with original specimens, more stress levels would lead to more II stage rate of stiffness decline, and stiffness degradation curve and hysteretic energy recovery curve had enough effect to characterize damage effect of material environment induced by thermal-cycling environment factors.

Ultrasonic welded joint failure mode of glass fiber/polyetherimide composites

AbstractFailure modes of ultrasonic welding joints were investigated by using glass fiber/polyetherimide (PEI) composite material as a base material, and the mechanism of welding pressure with different failure modes and the effects of failure modes on joint strength were explored. The results demonstrate that ultrasonic welding joints have six failure scenarios. Peeling of PEI film and the base material surface on both sides, peeling of PEI film and the base material surface on one side, peeling of surface fiber and surface resin, laminate delamination, fiber breakage, ablation damage respectively. The welding pressure mainly affects the proportion of the first three failure modes. When the welding pressure increases, the proportion of the second failure mode is obvious, meantime the proportion of the first failure mode gradually decreases, the welding pressure mainly affects the proportion of the first three failure modes. The welding joint failure is often a mixed failure generated by the coupling of the abovementioned six failure modes rather than a single failure mode. The evolution of heat distribution with Pw was simulated using COMSOL. The simulation curves represent the evolution of the weld interface temperature at different position.

A spatiotemporal prediction model for rapid prediction of delamination growth in open-hole composite laminates

Delamination of laminates around the hole edge is one of the most common but most threatening forms of damage in composites. The numerical solutions based on the finite element method (FEM) are difficult to meet the requirements of online evaluation and decision-making of the structure operation in terms of prediction efficiency. In this study, an end-to-end spatiotemporal prediction model with Encoder-Translator-Decoder architecture is developed to rapidly predict the delamination growth of laminates under compressive loading. The prediction results of single-hole delamination growth at different locations show that the model has good accuracy and generalization. The trained model can complete an inference prediction at the millisecond level, which is hard to achieve with FEM. Furtherly, a transfer framework is designed to promote the forecasting of multi-hole delamination growth. The framework is designed with a coupling coding block for learning the complex relationship of delamination damage superposition, which can effectively accelerate the training and achieve better prediction accuracy and inference speed. The developed deep learning model has good potential to be extended and applied to predict the spatiotemporal evolution of other physical fields.

A numerical investigation of the interaction between interlaminar and intralaminar damages in a fatigued composite panel

AbstractThe fatigue behavior of composite materials is still a very challenging issue for the scientific community. So far, several numerical methodologies allow us to faithfully simulate the propagation of delamination due to fatigue. However, the reduction of material properties due to the applied cyclic load should be considered. The main objective of this work is to investigate the role of the material property degradation and the intralaminar damages on the propagation of delamination in composite laminates under fatigue loads. The numerical tool FT‐SMXB, based on the Paris Law and virtual crack closure technique, for the mimic of fatigue‐driven delamination, has been integrated with a user material subroutine, based on the generalized residual material property 7degradation model and the Hashin fatigue failure criteria. Typical aeronautical stiffened panel has been considered as test case. Matrix damages and strength degradation due to fatigue cycles have been found to influence the propagation of delamination.

Qualification of distributed optical fiber sensors using probability of detection curves for delamination in composite laminates

Despite the promising application of Distributed Optical Fiber Sensors (DOFS) in monitoring damage in composite structures, their implementation outside academia is still unsatisfactory due to the lack of a systematic approach to assessing their damage detection performance. The existing tool developed for non-destructive evaluation, Probability of Detection (POD) curves, needs to be adapted for structural health monitoring applications to account for spatial and temporal dependency. Damage detection performance with DOFS is deeply related to the inherent variability sources of the system, the strain transfer properties of the optical fiber, and the loading conditions, which determine the damage-induced strain on the structure. This paper establishes a systematic approach based on the Length at Detection (LaD) method to qualify DOFS for damage detection in composites under different scenarios. Specifically, this study considers two DOFS with different strain transfer properties for monitoring delamination in carbon fiber reinforced polymers double-cantilever beam specimens under mode I quasi-static and fatigue loading. The POD curves derived from the LaD method confirm that this methodology can quantify the change in the detection performance due to the DOFS type and the loading conditions. The study also proposes a practical solution to compare POD curves obtained with different sample sizes, by introducing the concept of virtual specimens to simulate the lower confidence bound convergence.

Lamb wave mode conversion and multiple-reflection mechanisms for simply and reliably evaluating delamination in composite laminates

Lamb wave propagation must be understood comprehensively for simply evaluating delamination during ultrasonic testing. However, the difference between wave propagation, visualized using laser Doppler vibrometer and pulsed-laser scanners, has not been sufficiently investigated, and knowledge of optimal conditions for evaluating delamination is limited. Thus, in this study, the mode conversion and multiple reflections of Lamb waves propagating in a delaminated cross-ply laminate were visualized using different laser scanners, delamination depths, and wave incident angles. Delamination was characterized using maximum-amplitude map postprocessing under specific conditions. Further numerical analysis revealed that owing to multiple reflections of the antisymmetric mode in incident and mode-converted waves, standing waves were generated in the delaminated sublaminate. Dispersion curve and flexural stiffness calculations confirmed the conditions required for high-amplitude standing waves, thereby providing guidelines for simply and reliably evaluating delamination during inspections.

Temperature effects on the mode II delamination propagation behavior of aerospace-grade CFRP multidirectional laminates

Composite laminates are prone to occur delamination. Currently, there is still lacking research on the temperature effect on mode II delamination of laminates, especially the multidirectional ones. Systematically experimental and numerical studies are conducted here. Delamination tests are performed at four temperatures (−50 °C, 23 °C, 80 °C and 130 °C) on aerospace-grade T800/epoxy multidirectional laminates using an end-notched flexure set-up. Pre-cracked and non pre-cracked fracture toughness, and R-curves are measured. The non pre-cracked fracture toughness is affected by temperature. Their values at −50 °C, 23 °C and 80 °C are relatively close to each other while the value at 130 °C is 34.14% lower than that at 23 °C. The pre-cracked fracture toughness exhibits negligible dependence on temperature and is significantly higher than the non pre-cracked one. R-curve phenomena are observed in specimens evaluated at different temperatures, which exhibit a good linear increase with the increase in delamination length. The R-curves measured at −50 °C∼80 °C are similar while significantly higher than that at 130 °C. In addition, the bridging stress is lower when the temperature is between −50 °C and 23 °C, compared to those at high temperatures of 80 °C and 130 °C. Numerical framework for simulating the mode II delamination growth behavior is established based on the cohesive zone model. Predicted results show that the reasonable interfacial strength of cohesive elements first increases and then decreases with the increase in temperature. Temperature effect on the damage area around crack tip is also revealed.

Non-Contact Detection of Delamination in Composite Laminates Coated with a Mechanoluminescent Sensor Using Convolutional AutoEncoder

Delamination is a typical defect of carbon fiber-reinforced composite laminates. Detecting delamination is very important in the performance of laminated composite structures. Structural Health Monitoring (SHM) methods using the latest sensors have been proposed to detect delamination that occurs during the operation of laminated composite structures. However, most sensors used in SHM methods measure data in the contact form and do not provide visual information about delamination. Research into mechanoluminescent sensors (ML) that can address the limitations of existing sensors has been actively conducted for decades. The ML sensor responds to mechanical deformation and emits light proportional to mechanical stimuli, thanks it can provide visual information about changes in the physical quantity of the entire structure. Many researchers focus on detecting cracks in structures and impact damage with the ML sensor. This paper presents a method of detecting the delamination of composites using ML sensors. A Convolutional AutoEncoder (CAE) was used to automatically extract the delamination positions from light emission images, which offers better performance compared to edge detection methods.

Delamination in GLARE laminates subjected to oblique low velocity impact considering friction

Study of oblique low velocity impact and the subsurface delamination caused in fibre metal laminates is not well reported in literatures. The present paper numerically investigates the oblique low velocity impact behaviour of clamped GLARE plates by a spherical impactor to study the influence of obliquity and coefficient of friction on the impact mechanism and on the extent of interfacial delamination. A three dimensional finite element formulation employing Newmark-β method has been developed for analyzing the oblique contact impact response of GLARE plate where Hertzian contact model is used for the normal component of contact and Mindlin and Deresiewicz theory is used for the tangential component. Results show that besides impactor velocity, obliquity of incidence and the coefficient of friction significantly influence the contact force history, contact duration and the delamination at the interfaces. In addition, the oblique angle also decides the instant and duration of reversal of tangential contact force where increasing obliquity leads to delayed instant and reduced duration of such reversal.

Lamb wavefield-based monogenic signal processing for quantifying delamination in composite laminates

Delamination is one of the common damages affecting the safety of composite structures. In this paper, a Lamb wavefield-based monogenic signal processing algorithm is proposed to quantify the delamination parameters in composite laminates, including location, size, shape, and depth. A quality-guided fast phase unwrapping algorithm is developed to solve the problem of phase wrapping after Riesz transform-based monogenic signal processing. Then, space distribution of the phase of Lamb wavefield can be extracted for calculating wavenumber distribution, which is related to the structural thickness or delamination depth and can be used for delamination imaging. Simulated Lamb wavefield signals calculated by finite element simulation are employed to evaluate the parameters of delamination in composite laminates. Compared with other traditional methods, the damage identification algorithm based on Riesz transform has excellent identification effect and shorter calculation time. The results show that the algorithm can be used not only for single delamination recognition but also for multi-delamination recognition with good accuracy. In particular, the interaction between incident waves along different ply directions and delamination is explored, and its influence on delamination quantification is studied, whose results are worthy of attention in engineering application. Finally, a completely non-contact laser ultrasonic system is established to obtain the Lamb wavefield with delamination. Experiments show that the algorithm can accurately quantify the location, size, shape, and depth of delamination.

Experimental analysis and prediction of CFRP delamination caused by ice impact

The delamination of Carbon Fiber Reinforced Polymer (CFRP) laminates under ice impact is a common damage feature, and it intensely threatens engineering application. However, the delamination mechanism and the prediction are extremely challenging to clarify, due to the complexity of the dynamic response of anisotropic CFRP laminates under ice impact. In this study, systematic experimental studies of CFRP laminates impacted by ice balls and C-scan non-destructive detection are performed to elucidate the delamination mechanism. To this end, multiple CFRP laminates of different thicknesses (2.4 mm and 3.6 mm) are manufactured, and then impacted by ice balls of different diameters (30 mm and 50 mm) using a gas gun system, where the ice balls are frozen with distilled water and the impact velocities vary from 46 m/s to 152 m/s. At last, a non-consistent deformation mechanism of CFRP delamination is proposed to explain the mesoscopic delamination behaviors via dynamic deformation analysis of CFRP laminates, and it is validated by the experimental results. Furthermore, the delamination prediction of CFRP laminates is obtained using the non-consistent deformation mechanism and a corresponding prediction model. This study can contribute to understand and efficiently predict the delamination of CFRP laminates under low-energy ice impact.

Prediction of transverse crack multiplication of CFRP cross-ply laminates under tension-tension fatigue load

This study aims to predict the transverse crack accumulation behavior of CFRP cross-ply laminates for evaluation of their remaining life. The tension-tension fatigue tests of CFRP cross-ply laminates were used to investigate the relationship between transverse crack accumulation behavior and the number of cycles of the fatigue tests. We also studied the damage accumulation behavior, including delamination of cross-ply laminates under fatigue loading, and it was indicated that the damage accumulation behavior depends on the maximum load of the fatigue tests. We proposed a method that can predict transverse crack accumulation behavior using a power-law between transverse crack density growth rate and the energy release rate associated with single transverse crack formation. As a result, the proposed method can estimate the applied number of cycles on CFRP cross-ply laminates.

An experimentally validated 3D progressive fatigue damage model for fatigue life prediction of Flax-epoxy laminates

A finite element model that incorporates a 3D progressive fatigue damage model (3D PFDM) is developed to simulate the fatigue behavior of Flax-epoxy (FE) composites. Input parameters of the model are determined from testing the material properties in all directions. A double-notched shear test is used to characterize the out-of-plane shear properties of the laminate under both static and fatigue loading conditions. The 3D PFDM, which employs a stochastic distribution of the material properties, is implemented thorough a user-defined subroutine in ABAQUS finite element software. Results of the finite element analysis of voids-containing laminates are in good agreement with experimental results. The predicted fatigue life of all layups is more influenced by voids at higher loading levels compared to laminates without voids. A further verification of the 3D PFDM model was performed using the cross ply and quasi-isotropic configurations and comparing the predicted free-edge delamination at the interface of the 90° ply and adjacent layers to the SEM micrographs of free edges of specimens. From a practical perspective, the proposed 3D PFDM could be applied to a wide range of composite materials to predict edge delamination in laminates with different fiber orientations.

Effects of residual stresses on interlaminar radial strength of Glass-Epoxy L-bend composite laminates

The built-in heterogeneity of the composite laminates has been exploited to tailor the stiffness and strength requirements of modern structures to meet the specific functional demands. However, the non-homogeneity in these composites is the root cause for most of their failures. One of the undesirable consequences of the inherited heterogeneity is the development of cure-induced stresses during composite manufacturing. This work aims to investigate the influence of process-induced stresses on interlaminar radial strength in curved composite laminates. Glass-Epoxy (GE) laminates of two different thicknesses were prepared by hand lamination technique using V-shaped tooling and cured under room temperature. The state of residual stresses in GE laminates is varied by post-curing these laminates at different temperatures. Curved bending strength (CBS) and corresponding interlaminar radial stress for delamination of L-bend laminates were evaluated experimentally using four points bending test. The residual stress profile in each GE laminate is experimentally characterized by employing the Slitting method. The results indicate that the residual stresses have a negligible effect on the critical stress for initial delamination in GE laminates. But, the critical stress for delamination was found to be independent of the laminate thickness and increased with higher curing temperatures. The delaminated surfaces of L-bend laminates were studied using a scanning electronic microscope (SEM). The enhancement in the critical stress due to post-curing can be attributed to the improved fiber-matrix interfacial bonding with higher curing temperature.

Experimental Identification of Frictional Effects on Interlaminar Toughness of Composite Laminates in 4ENF Test

BackgroundUnderstanding and characterizing delamination in composite laminates is fundamental for analysing their structural integrity, since operational loads may promote the propagation of interlaminar defects. Propagation often occurs in mode II and is driven by shear stress. However, the methods used to characterise this propagation mode are affected by frictional effects between crack surfaces.ObjectiveThis work aims to build up an experimental method to identify the effect of friction in a 4 points End Notched Flexure test, which does not require the use of analytical or numerical models of the specimen.MethodsThis goal is achieved by performing a series of loading–unloading cycles before the delamination test, which helps to calibrate an analytical expression that estimates the energy dissipated by friction, and the analytical expression is then inserted into the formulation of the Irwin-Kies equation. Experimental validation is carried out considering tests on different materials and different friction conditions between the crack surfaces, as well as validation by means of a virtual experiment being performed for comparison with an analytical model presented in literature. The novelty of this method lies in the fact that it does not require the development of any analytical or numerical model of the specimen and consequently no calibration between models and the experiment is required.ResultsTests on composite specimens show good results, the friction contribution estimated by the method is comparable with those presented in the literature. Moreover, the virtual experiment shows that there is a good match between the results obtained using this method and those obtained using an analytical model presented in the literature.ConclusionsThis method seems to provide satisfactory results for both real and virtual experiments, moreover, the procedure is relatively simple, making it a suitable method for the evaluation of frictional effects in the 4 point End Notched Flexure test.