Delamination Patterns Research Articles

Machine learning based segmentation of delamination patterns from sparse ultrasound data of barely visible impact damage in composites

Composite laminates, known for their strength and flexibility, are widely used but can delaminate under certain barely visible impact. Ultrasound Transmission (UT) scans can detect such failures, but the noise in the scans makes automation of delamination morphology challenging. Machine learning could solve this, but it requires substantial training data. Furthermore, given the considerable time and cost of conducting impact tests on composite laminates, such data remains sparse. This study overcomes data sparsity by enlarging UT scan dataset using image augmentation techniques and synthetic data generation methods. A combination of rotation and elastic deformation of real UT images produced augmented data. Synthetic data was generated by mimicking the statistical variations in real data, including delamination length and rotation per ply, and adding gradients to match the original image noise. A U-Net based machine learning segmentation approach was utilized to capture local and global information through a series of convolutions to apply per-pixel classification. Systematic experimentation was conducted to study the influence of adding augmented and synthetic data on segmentation accuracy. The results show that the use of augmented data increases accuracy significantly. With the addition of synthetic data to the augmented data, there is a marginal improvement in accuracy. However, feature edge accuracy is significantly improved. The preliminary results presented here show the promising use of machine learning techniques for complex, non-destructive inspection methods.

Carbon Fiber Reinforced Polymer (CFRP) Laminates: Flexural Strengthening Method for Prestressed Concrete Beams with Anchorage Loss

Abstract Post-tensioning (PT), a method of pre-stressing, involves the use of high-strength steel strands/ tendons to reinforce concrete or other materials. On the contrary, Carbon Fiber-Reinforced Polymers (CFRP) are lightweight, high-strength materials with used to strengthen concrete structures by adhering the polymer to the concrete element. Challenges with post-tensioned elements include reverse curvature of the post-tensioning strands, tendon misplacement, and frequent damage in the anchorage and dead-end zones. These difficulties frequently cause bulging of the surrounding concrete, even at lower stress levels, and can lead to concrete bursting when tension exceeds certain threshold. This study investigates into the potential of CFRP strengthening technique to improve the flexural capacity of post-tensioned concrete beams with anchorage loss. Through an experimental program, the study compares the performance of control beams to those reinforced with different layers of CFRP. The results of this study demonstrated that there was a significant increase in flexural capacity, ranging from 45.31% to 78.62% for single layers and 87.17% to 153% for double layers of CFRP sheet. Additionally, the research examines how different levels of prestressing and CFRP wraps influence crack formation and delamination patterns of carbon fiber, with promising results. It was also noted that optimal usage of CFRP fibers and tendons is found to be critical. The study suggests exploring alternative fiber types and orientations for future study.

Symmetry breaking and high-order instability of telephone cord buckles triggered by in-plane gradients of thin films

Buckle delaminations are ubiquitous in natural and artificial systems including tectonic plates, biological tissues, film structures, and two-dimensional materials. Although the formation and manipulation of various buckle delaminations have been extensively investigated in the past few decades, understanding complex buckle delamination patterns is still a challenge. Here we report on the symmetry breaking and high-order instability of telephone cord (TC) buckles triggered by in-plane gradients of thin films, including strain gradient and thickness gradient. Our experimental observations reveal the unique morphological profile of the asymmetric TC buckle due to the strain gradient. Impact of in-plane strain gradient on morphological changes of TC buckles is elucidated by simulations of buckling and delamination of the thin film. Phase diagrams for five buckle delamination morphologies, i.e., symmetric TC buckle, asymmetric TC buckle, symmetric bilateral branched buckle, asymmetric bilateral branched buckle, and unilateral branched buckle with respect to average strain and strain gradient (or thickness gradient) have been numerically established. This work could promote a better understanding of complex buckle delamination patterns with symmetry breaking and high-order instability and controllable fabrication of ordered buckle delamination patterns by regulating in-plane gradients of thin films.

Curved grid‐scored foam core sandwich composites under low‐velocity impact loads: Effects of curvature, stacking sequence, and core finishing

AbstractThe objective of this paper is to experimentally investigate how material and geometric parameters, including face sheet layup, curvature, and core finishing, affect the contact force‐displacement behavior, absorbed energy, and damage modes of curved sandwich composites with grid‐scored foam under low‐velocity impact loads. An instrumented drop‐weight testing machine with a hemispherical impactor was used to perform low‐velocity impact tests at different energy levels. Angled‐ply stacking had the maximum contact force and impact resistance for low curvature radii. In the event of penetration, the impact energy absorption of all curved sandwich panels increased as the curvature increased. The core finishing influenced the contact forces of the curved and flat sandwich panels but not the penetration energy. The curvature and face sheet layups affected the delamination size and pattern. Angle‐ply and cross‐ply face sheets with the smallest and maximum radius of curvature, respectively, decreased penetration depth. The resin‐filled channels reduced foam cell impact damage and face‐core debonding. LVI penetration resistance was found to be highest for angle‐ply stacking and knife‐cut foam at small curvature radii. The proposed research will also provide naval designers with valuable insight into the impact response of sandwich structures used in curved parts of the hull and deck.Highlights The effect of curvature on the LVI responses varied due to face sheet layups. Core finishing affected contact forces, but not the absorbed energy. Curvature and face sheet layups affected the damage modes. Resin‐filled channels limited damage area and acted as stoppers for debonding.

Damage Prediction of Buckled Composite Single Stiffened Panel Subject to Static Compression

The initial flaws in laminated composite structures are susceptible to inter-/intraply damage. Progressive damage analysis tools help the understanding of the mechanisms for damage initiation and growth while assisting in the design, analysis, and sustainment methods. The global–local modeling approach for the single-stringer postbuckled panel evaluated in this effort used Teflon inserts to provide initial “cracks” for model validation. The effort aimed to develop, evaluate, and enhance methods to predict damage initiation, progression, and failure of postbuckled composite hat-stiffened panels using the commercially available software Abaqus. Three methods were considered: the virtual crack closure technique (VCCT), the cohesive zone model (CZM), and enhanced Schapery theory (EST). The VCCT and CZM successfully predicted multiple delaminations. EST works with the CZM to capture delamination and facilitate interactions between interlaminar and intralaminar failures. When comparing the damage morphology against the experimental data produced through the NASA Advanced Composites Project, the present models captured the delamination, matrix, and fiber damage patterns. The VCCT and EST predictions were within 2% of the experimental peak load.

Structural Health Monitoring of Repairs in Carbon-Fiber-Reinforced Polymer Composites by MWCNT-Based Multiscale Sensors

The precision maintenance of delaminated carbon-fiber-reinforced polymer composites calls for the high demand of continuous, in situ monitoring of the damage-repair process along with the in-service status of the repaired region. Moreover, the repaired region faces a high risk of re-damage; therefore, in-service monitoring is highly desired. However, the current repair process lacks the in situ monitoring function, leading to the mechanism and evaluation of the repair approach being unclear. Here, we implanted multi-wall carbon nanotubes (MWCNTs) at the interface between the carbon fiber and resin matrix of the damaged region to achieve in situ monitoring of the repair, compression, and seawater-immersion processes. By depositing both the coupling agent and MWCNTs at the interfaces, a high recovery efficiency of 85% was achieved, which was independent of the delamination pattern shapes. The electric resistance changes of MWCNT-modified panels could effectively identify the resin permeation and solidification processes and could be used to in situ monitor the structural health of the repair region when it is subjected to the compression and seawater immersion tests. This strategy, combining high-efficient repair and precision maintenance, demonstrates potential in the structural applications of carbon-fiber-reinforced polymer composites.

Chemo-mechanical model for degradation of oil paintings by amorphous and crystalline metal soaps

Metal soap formation is recognised as a critical degradation mechanism in historical oil paintings, which threatens the preservation of museum collections worldwide. Metal soaps form via a complex sequence of chemical reactions between metal ions released by the pigments and saturated fatty acids originating from the drying oil. The latest advances in chemistry research suggest that metal ions and saturated fatty acids may initially react by means of a reversible reaction, which leads to the formation of metal soaps in an amorphous state. Metal soaps may subsequently crystallise via an irreversible reaction into large aggregates that deform the paint layers, potentially triggering delamination, cracking, and ultimately flaking of the paint. This paper proposes a chemo-mechanical model to predict metal soap formation and the consequent mechanical damage in historical oil paintings. The chemical process is described in terms of a set of diffusion–reaction equations, which account for both the reversible reaction between free saturated fatty acids and metal ions forming amorphous metal soap, and the subsequent irreversible reaction to crystalline metal soap. The chemical model is two-way coupled with a mechanical model that effectively describes the cracking processes caused by metal soap formation and growth. The coupling is generated from the mechanical model by accounting for the development of a chemically-induced growth strain in the crystalline metal soap. In addition, the presence of cracks locally hampers the diffusion of chemical species, which is taken into account in the chemical model through a dependency of the diffusion parameter at the crack faces on the amount of mechanical damage generated. The spatial development of the crystalline metal soap phase is simulated by using a tailor-made scanning algorithm that identifies the reaction zone in which metal soap formation takes place. The proposed model is calibrated on experimental data presented in the literature. The model is subsequently applied to analyse two numerical examples that are representative of typical metal soap-related degradation processes observed in historical oil paintings, revealing that the growth process of crystalline metal soap, the deformation of the paint surface, and the consequent cracking and delamination patterns are predicted in a realistic fashion.

Crack propagation in the Double Cantilever Beam using Peridynamic theory

In this study, Peridynamic (PD) theory is used to model mode-I delamination in unidirectional and multidirectional laminated composites. Experiments are conducted to determine mode-I fracture toughness in a unidirectional carbon-epoxy Double Cantilever Beam (DCB) specimen where the crack propagation remains on the original notch plane. The PD model of the DCB geometry is generated using an in-house pre-processor code in MATLAB and implemented in ABAQUS software. The brittle damage law in the original PD model is modified to a bilinear law to capture progressive softening. PD results are found to be in good agreement with the experimental results in terms of force–displacement curves and crack length. Next, this PD approach is applied to a multidirectional angle-ply DCB specimen. The PD model shows that delamination path jumps between the layers as the delamination grows. Force–displacement behaviour and delamination patterns obtained using PD model are compared with the corresponding experimental results from Gong et al. (2018). As a result, PD theory with bilinear softening law is found to successfully capture force–displacement relations and delamination migration in multidirectional laminated composites under mode-I loading conditions.

Experimental and numerical analysis of wrinkles influence on damage mechanisms and strength of L-Shape cross-ply composite beams

In this work, experimental and finite element (FE) analysis of L-shape cross-ply fiber-reinforced composite beams are carried out to investigate the effect of out-of-plane wrinkles on the interlaminar strength and competing damage mechanisms. L-shape laminates are fabricated with manufacturing and geometrical variabilities, cured in a temperature and pressure-controlled autoclave process, and mechanically tested under four-point-bending tests. An in-situ high-fidelity FE models are constructed based on in-plane micrograph images of beam cross sections. Explicit FE failure simulations are performed to study the effect of wrinkles and to differentiate among different failure modes. Simulation results show a reasonable agreement with the test data in terms of delamination onsets, matrix cracking and through thick strength. Combined delamination, matrix cracking and kinking pattern is significant in the specimens with visible out-of-plane wrinkle. While, in the specimen with no noticeable wrinkle, a major single delamination is observable leading to final failure. The competing damage mechanisms is highly dependent on the assumed fracture properties and in-situ-condition of finite element model, regardless of good agreement between simulated global mechanical response and test data.

Numerical investigation on fracture behavior in novel material structure inspired from first hierarchy of human bone

In this work, at first, we modeled the bone first hierarchy structure made up of collagen fibril, and soft protein to evaluate the strength, and simulated the delamination pattern at the interface of the HAp, protein. We also analyzed a novel material structure inspired by the staggered structure of the bone at the first hierarchy level for its strength, and fracture behavior such as delamination at the interface. The soft matter considered is PLA polymer while the hard constituent is HAp. A commercial finite element analysis tool, ABAQUS 6.14 was used for the modeling purpose. The models were subjected to displacement controlled uniaxial tensile loading in a plane strain condition. The interfaces of the soft and hard constituents were modelled using cohesive surface modelling. Porous as well as non-porous models both were analyzed. The stress concentration was found to be higher in case of the porous model which is quite natural as the pores can be the points of stress concentration in comparison to the non-porous model. The delamination at the interface was correctly simulated by the use of cohesive surface elements. This study can be the first step towards novel material structure design mimicked from bone hierarchy.

Surface orange patinas on the limestone of the Batalha Monastery (Portugal): characterization and decay patterns

Samples of orange patinas found on a limestone window tracery and an ornament of the Batalha Monastery have been investigated by X-ray micro-diffractometry (μ-XRD) and low-vacuum scanning electron microscopy coupled with energy dispersive spectrometry (LV-SEM + EDS). The aim of the study was to determine the composition of the layered patinas, assess whether they have been intentionally applied or naturally formed, and study their degradation patterns. Preliminary results revealed that the orange patinas on the window tracery and the ornament showed different compositions and appearance, suggesting distinct formation pathways. Orange patinas on the ornament, which are now showing decay and delamination patterns, mainly consisted of gypsum with hematite as a minor component, implying the possibility of an intentional application of a mixture of ochre and lime as tint plaster. Orange patinas on the window tracery show, instead, the presence of Ca-oxalates, abundant weddellite, and minor whewellite, with minor hematite suggesting the yellowish/orange color as being due to Ca-oxalate patinas imbedding soil dust airborne particles. Such patina was possibly formed naturally either by the chemical attack due to atmospheric air pollutants from traffic exhausts emissions or by bacterial activity. No delamination was observed on the window tracery sample with granular decohesion as the major decay phenomenon. A comparison was made between this patina and the so-called scialbatura, a surface yellowish coating often found by conservators on limestone and marble in ancient monuments in the Mediterranean region.

Strain-rate-dependent progressive damage modelling of UHMWPE composite laminate subjected to impact loading

A comprehensive strain-rate-dependent (SRD) finite element modeling procedure, based on continuum damage mechanics has been developed to predict the behavior of ultra-high molecular weight polyethylene (UHMWPE) fiber composite laminates under impact loading. A user-defined material subroutine implemented into ABAQUS/Explicit is used to define SRD constitutive damage model of UHMWPE composite. The effect of strain rate on the material properties is adapted to the experimental data by introducing a new hyperbolic function and the strain-rate effect factor (SEF) for use in the modeling. The range of [Formula: see text] to [Formula: see text] is considered for strain rate, which includes both high velocity and low velocity impact regimes. A homogenized sub-laminate approach is employed to more accurately capture the out-of-plane failure mechanisms. Cohesive Zone Model (CZM) with the constitutive model based on bilinear traction-separation is implemented to simulate the inter-laminar delamination between the sub-laminates. High velocity ballistic impact as well as low velocity drop-weight impact are simulated, and the results are validated with experimental observations from the literature. Results show that the presented SRD model, offers more accurate prediction of the projectile residual velocity compared to the SRD model using logarithmic function or without considering the strain rate effect. Moreover, detailed views of failure modes such as tension/shear fiber and matrix shear in layers, and the delamination patterns are obtained and investigated.

Curvature-controlled delamination patterns of thin films on spherical substrates

SummaryPeriodic delamination patterns in multilayer structures have exhibited extensive applications in microelectronics and optics devices. However, delamination behaviors of a closed thin shell on spherical substrates are still elusive. Herein, a unique instability mechanism of buckle delamination in a closed thin film weakly bonded to spherical substrates is studied by experiments, simulations, and theoretical analyses. The system of an Al film depositing on polystyrene spheres subjected to thermal mismatch strain is used for demonstration. Unlike traditional phenomena of wrinkling and wrinkle-induced delamination under increasing misfit strain, the weak adhesion between the core and shell results in a periodic pattern of delaminated hexagonal dimples that emerges directly from the smooth sphere configuration, before which no wrinkling occurs. Both substrate curvature and interfacial adhesion are revealed to control the dimple size and delamination width. These findings open a new venue for manifesting new controllable features for surface microfabrication.

Analysis of delamination and heat conductivity of epoxy impregnated pancake coils using a cohesive zone model

Epoxy-impregnated pancake coil is observed to delaminate locally when its temperature is cooled from room temperature to 77 K. The delaminations cause the degradation of the critical current of the coil. Besides, the delamination may degrade the thermal conductivity and affect the thermal conductive path. Thus, in this study, the delamination behavior of epoxy-impregnated pancake coil during the cooling process is analyzed through a coupled thermal–mechanical cohesive zone model. The measured transverse tensile strength of coated conductors has shown considerable scatter and Weibull distribution has been adopted to fit the transverse tensile strength. The simulations are able to capture the main characterizations of the observed delaminations with the cohesive strength of the cohesive element following a Weibull distribution. After the cooling process, a heat spot is applied to the degraded positions to analyze the temperature field and the thermal conductive path in the damaged coil. Compared to the original undamaged coil, the delaminations indeed degrade the thermal conductivity and increase the thermal resistance. What’s more, the effects of the variation of the thickness of the epoxy and different interfacial cohesive strength distributions of the coated conductor are considered. The results show that reduction of the thickness of the epoxy can lower the radial stress and release the damage, and the random distribution of the cohesive strength inside the coated conductor dominantly determines the delamination pattern of the coil.

In situ capture of impact-induced progressive damage and delamination in fiberglass composite laminate with a high-speed optical imaging method

Delamination is one of the severest failure modes for laminated composite materials during applications, especially when the composites are subjected to dynamic loading like impact. The impact-induced delamination patterns in laminates have been studied before but the progressive delamination evolution during the impact process has rarely been revealed through experimental testing. Capturing the real-time delamination progression is of great significance for understanding the fundamentals of the composite performance and damage resistance during the dynamic loading process. This study investigates the feasibility of using a high-speed optical imaging method for in-situ capturing the impact-induced progressive damage and delamination evolution in fiberglass composite laminate [0/90/0/…]13. The progressive delamination areas are further quantified through a sequential series of captured images using a custom algorithm. The relations between the progressive delamination evolution and the real-time absorbed energy, as well as the impact force and deflection, have been investigated. This study provides insights into the experimental testing method of dynamic damage and failure process in materials and structures.

Influence of different pretreatments on Ti-6Al-4V surface integrity and scratch-resistance of epoxy coating: Analysis of topography, microstructure, chemistry and wettability

The improved scratch resistance of epoxy-based coatings on titanium alloy is greatly needed in the current biomedical device sector. Due to commercial factors, this improvement must be either non-consumptive or inherited from the antecedent machining steps. This study contributes an extensive understanding of Ti-6Al-4 V surface topography, microstructure, chemistry and wettability generated with various processes, such as milling, polishing, hydrofluoric acid-etching and micro-blasting with different process parameters by utilizing optical and electron microscopy, energy-dispersive X-ray spectroscopy, X-Ray diffraction and surface tension. The phenolic resin is then spin-coated, cured and scratched with a conical indenter in a constant loading mode. The influence of roughness parameters, microstructural and chemical changes on wettability and delamination size, pattern and mode are analyzed. The root mean square gradient of surface slope and the developed interfacial area ratio are found to be in good correlation with the delamination factor. The delamination factor and specific traction force after micro-blasting are improved by a factor of two compared to that achieved by polishing or milling and is superior to that obtained by acid-etching. This study, therefore, clearly demonstrates the strong potential of micro-blasting as a pretreatment technique to enhance the adhesion strength and scratch-resistance of epoxy coating on titanium implants.

Delamination behavior of L-shaped composite beam with manufacturing defects

Composite structures are more susceptible to manufacturing defects than conventional materials. Fiber misalignment, cracks, and voids are typical types of manufacturing defects in laminated composites. Defects can greatly reduce structural integrity and load carrying capacity, so their effects on material and structural strengths must be understood. In this paper, the effect of manufacturing defects on the progressive delamination behavior was studied for carbon/epoxy composite laminated L-beam under four-point bending test conditions. The defects of wavy plies and pure resin that had flowed out were characterized from surface photography and then transformed into finite element modeling using semi-automatic approach to which pre-delamination and void were included. Next, progressive failure analyses were performed with the interface delamination and matrix direction failure accounted for by cohesive zone modeling. Numerical results were examined focusing on the effects of defects on the peak load reduction and the variation of delamination pattern. The stacking sequence effect was also investigated.

Effect of tow gaps on impact strength of thin composite laminates made by Automated Fiber Placement: Experimental and semi-analytical approaches

Automated Fiber Placement is currently being used to manufacture large and complex composite structures. However, the final composite products may include manufacturing defects such as gaps and overlaps, which may reduce the mechanical performance of the structure. The effect of these defects on the compression strength and also medium velocity impact loading with the impact energies of 15 J–50 J have been experimentally investigated earlier. However, there is still a lack of knowledge in the study of impact response of and damage propagation in composite plates at low-velocity impact loading in the presence of the manufacturing defects. In this paper, the effect of periodically induced gaps on the low-velocity impact response of the thin composite plates has been experimentally investigated. For this purpose, quasi-isotropic Carbon/Epoxy polymer composite plates have been manufactured with AFP process, including periodical patterns of gaps, and the obtained impact responses of the plates have been compared with the results of the baseline samples. The baseline sample is a similar sample that has been manufactured by hand layup technique. Furthermore, a two degree of freedom mass-spring model is also proposed to account for the effect of the manufacturing defect on the impact response of the laminates with induced defects. The model includes a non-linear damage model to account the delamination propagation during the impact process. Ultrasonic C-Scan analysis has also been performed to capture the projected delamination pattern. Results indicate that the AFP manufacturing defects can reduce the impact resistance of the composite plates by about 17% and also has an effect on the delamination area of the samples for low levels of impact energy. Microscopic observation is further performed to investigate the interaction of manufacturing defects and damage caused by impact. It is shown that delamination initiation likely occurs in the gap area.

Statistical analysis of favorable conditions for thermographic inspection of concrete slabs

Infrared thermography has been successfully applied as a remote non-destructive testing approach for access to the internal conditions of reinforced concrete bridge slabs. Due to the different thermal properties of concrete and air or water present in the subsurface defects, damaged areas can be detected by the thermal contrast recorded on the inspected concrete surface. Thus, inspections should be conducted at favorable conditions, where the highest thermal gradients facilitate damage detection. In this study, experimental models were created to simulate bridge slabs with different delamination patterns. A multivariate regression analysis was used to explain and predict the thermal behavior of the test samples using data collected at different weather conditions and different periods of the day and the year. Based on experimental and regression analysis, it was observed that environmental conditions have an important influence on the concrete surface temperature variation and, therefore, on thermal contrast. Within the scope of this study, time frames between 12:00 noon and 3:00 pm, with temperatures between 27.5 and 37.9 °C, a high level of direct solar radiation and low atmospheric pressure values set an ideal scenario for thermographic inspections in reinforced concrete bridge slabs under passive heating.

Geometrically nonlinear regularized extended finite element analysis of compression after impact in composite laminates

The geometrically nonlinear Rx-FEM formulation was applied to compression after impact (CAI) strength prediction. The impact-induced damage including permanent out-of-plane dent, delaminations, and matrix cracks were included into the analysis domain prior to application of compressive loading. While the inclusion of the dent and delaminations was based directly on provided information, impact-induced matrix cracks were incorporated into the analysis configuration by application of a pseudo-impact load prior to CAI. The resulting matrix cracks matched the accompanying delamination patterns in a consistent manner. Influence of several parameters and their combinations on CAI strength prediction was performed. It was found that accounting for the thermal residual stresses introduced additional buckling instability similar to that of the dent and allowed accurate prediction without simulating the dent. Two approximate delamination patterns representative of time-of-flight C-scan and tap-test inspection results were examined and found to be reasonably conservative with 9% and 14% margins, respectively.