DCB Test Research Articles

Progressive damage analysis of green composite laminates subjected to in-plane crashworthiness

In this paper the initiation and propagation of delamination in thin-walled green composite structures were studied. In particular, laminates of flax and hemp fibers with epoxy resin were investigated from experimental and numerical points of view, using the finite element code LS-DYNA. First, an experimental campaign was conducted using DCB and 4ENF tests to obtain the values of the interlaminar fracture modes, required for the cohesive fracture modeling. In addition, in-plane crashworthiness tests were performed on composite plates to analyze the damage of the specimens, under both quasi-static and dynamic conditions. A trigger mechanism was considered to help delamination to start and to ensure the most progressive crushing possible. The predominant damage mode was splaying, but some ply fragmentation in the middle layers and some buckling phenomena were also observed. In general, more stable progressive crushing was observed under dynamic conditions than under quasi-static ones, even if a reduction in terms of absorbed energy was obtained by moving to the dynamic condition. The numerical predictions showed a good agreement with the experimental reference results, validating the current framework for crashworthiness analysis of green composite structures.

Damage characterisation of GFRP composites based on clustering acoustic emission events utilizing single-failure-cause tests as reference

A new method to identify causes of fracture in composites based on acoustic emission (AE) and clusterization of AE data based on reference datasets is presented within the manuscript. Acoustic Emission (AE) is a widely used non-destructive method for fracture analysis, but data due to their multidimensionality are not easy to analyze especially if the acoustic events appear simultaneously and have similar parameters even if they are an effect of different failure mechanisms. In this research, we utilize an unsupervised learning algorithm besides the simplest K-means, through fuzzy c-means to Gaussian Mixture Model (GMM) and spectral clustering to investigate the dataset obtained from the three-point bending test manufactured by us composite. The analysis is preceded by data curation, feature determination (Laplacian score) and the best number of cluster investigations (DB index, Caliński-Harabasz score, and Silhouette method) To enable interpretation of the clustering we run an additional three groups of tests covering fibre breakage (two methods), resin fracture (in tension and in compression) and delamination (DCB test) creating reference datasets. These datasets were statistically analyzed and kernel density estimators were generated for each AE feature as well as amplitude-frequency characteristics. Clusters obtained for the main dataset were then assigned to particular causes of failure by comparing them with the reference dataset. It was found that clusters generated using spectral clustering were the most realistic ones, as it was possible to assign the cause of failure to them.

Experimental study on toughening UV pre-curing adhesives for electrically conductive applications

Epoxy adhesives are not conductive materials, usually requiring the use of fillers to create electrical bridging between the joint's substrates. This work aims at the implementation of UV fixation, instead of clamping jigs, on doped joints to ensure enough contact bridging pressure during curing. As such, an UV pre-curable adhesive was mechanically characterised in terms of strength – bulk tensile test and TAST (shear) –, and fracture toughness – DCB (mode I) and ENF (mode II) tests –, in the neat state (reference). Afterwards, for the doped configurations – 5%v/v conductive spheres (Sph), 15%v/v aluminium (Al) and 10%v/v graphite (C) – only mode I and II tests were considered accessing the filler's toughening effect. The electrical conductivity of the real joints was assessed through electric discharge machining (EDM). In the end, the Sph's were deemed necessary for bond thickness control. And the Al (mode I +50 % and II +61 %) and C (mode I +25 % and II +42 %) powders presented improvements in toughening the adhesive. However, UV-fixation failed to provide enough clamping pressure, during curing, to maintain the electrical bridging necessary for EDM.

A method for modelling arbitrarily shaped delamination fronts with large and distorted elements

The simulation of delamination in layered composites is currently limited for large structures. Typically, the use of Cohesive Zone Modelling leads to a requirement of keeping the elements smaller than 1.0 mm. As an alternative, this article presents an Energy Release Rate-based cohesive method enabling the use of elements significantly larger (up to 5 mm). A novel algorithm is presented to use the Virtual Crack Closure Technique with distorted elements not aligned with the delamination front. When the propagation criterion is met, a cohesive law is introduced to model the progressive crack growth along the newly created crack surface, ensuring to dissipate the correct amount of energy. The method is validated for different propagation tests. Notably, Double Cantilever Beam and End-Notched Flexure tests are accurately modelled with large and distorted elements. Finally, a partially reinforced DCB test demonstrates the ability of the method in representing an evolving delamination front.

Steady-state crack propagation during DCB test with viscoelastic-viscoplastic interface – Correlation between rheological behavior and effect of crack propagation rate

Adhesively bonded joints are known to suffer from creep and relaxation phenomena due to the viscous nature of the adhesive. These effects are detrimental to the durability of structural joints as they can lead to progressive and slow crack propagation rate phenomena and possible delayed failure. Master curve approaches are often used to assess the durability of structural assemblies, but the physical justification for such approaches requires additional investigation to more accurately assess the coupling between structural effects and the possible complex long-term rheological behaviour of the adhesive layer. Following similar approaches developed to study the steady-state crack propagation regime in bulk materials, an Eulerian description of crack propagation along a viscous adhesive layer during a double cantilever beam test is implemented. Taking into account viscoelastic-viscoplastic and various hardening rules, some master curves can be obtained describing the evolution of the crack propagation rate as a function of the stationary loading conditions. With such a model, the role of adhesive layer behaviour and joint geometry on crack propagation conditions can be discussed, as well as the applicability of such a master curve approach.

A Numerical Method for Unstable Propagation of Damage in Fiber-Reinforced Plastics with an Implicit Static FE Solver

Finite element analyses of the propagation of damage such as fiber compressive failure and delamination have greatly contributed to the understanding of failure mechanisms of fiber-reinforced plastics owing to extensive studies on methodologies using Continuum Damage Mechanics and Fracture Mechanics. Problems without the need for consideration of inertia, such as Double-Cantilever Beam tests, are usually solved by implicit FE solvers, and explicit FE solvers are appropriate for phenomena that progress with very high velocity such as impact problems. However, quasi-static problems with unstable damage propagation observed in experiments such as Open-Hole Compression tests are still not easy to solve for both types of solvers. We propose a method to enable the static FE solver to solve problems with unstable propagation of damage. In the present method, an additional process of convergence checks on the averaged energy release rate of damaged elements is incorporated in a conventional Newton–Raphson scheme. The feasibility of the present method was validated by two numerical examples consisting of analyses of Open-Hole Compression tests and Double-Cantilever Beam tests. The results of the analyses of OHC tests showed that the present method was applicable to problems with unstable damage propagation. In addition, the results from the analyses of DCB tests with the present method indicated that mesh density and loading history are not significantly influential to the solution.

Fracture toughness and fiber bridging mechanism for mode-I interlaminar failure of spread-tow woven composites

Given the current lack of research data on the delamination behavior of spread-tow woven composites, this study aims to systematically characterize the mode-I interlaminar fracture behavior of plain weave spread-tow fabrics (STFs) composites. DCB tests were conducted to investigate the influence of different ply thicknesses, unit cell sizes, and interface angles on delamination behavior and fiber bridging. The experimental results revealed a stick–slip crack propagation behavior in mode-I delamination of studied material, and a detailed analysis was performed on the influence mechanism of unit cell size on this behavior. Fiber bridging was found to be the primary mechanism responsible for the increased interlaminar fracture toughness in the plain weave STFs composites. Fractography analysis indicated that regions rich in fibers and resin in thicker-ply laminates promote fiber bridging, while an interface angle inconsistent with the direction of crack propagation inhibits fiber bridging. The concept of average steady-state fracture toughness (average Gs) was introduced as a means to assess propagation fracture toughness and two methods were proposed to obtain bridging tractions for characterizing the fiber bridging phenomenon. The bridging tractions and average Gs were used as key parameters to establish a trilinear cohesive zone model (CZM) for simulating mode-I delamination. The numerical results aligned well with the experimental results, demonstrating the applicability of the parameter determination strategy based on the average Gs and bridging tractions for the selection of parameters to use in the CZM.

Analytical evaluation of laminated composite DCB test data for achieving validated modelling analysis

Analytical evaluation of laminated composite DCB test data for achieving validated modelling analysis

Acoustic emission with machine learning in fracture of composites: preliminary study

In this paper, preliminary studies on the failure analysis of hybrid composite materials utilizing acoustic emission and machine learning are presented. The main purpose of this study was to analyze the possibilities of using machine learning techniques as a way to better cluster the data obtained from acoustic emission. In this paper, we focus on data preparation, feature extraction (Laplacian score), determination of cluster number (Caliński–Harabasz, Silhouette, and Davies–Bouldin), and testing three clustering techniques, namely K-means, fuzzy C-means, and spectral clustering. The dataset was obtained by testing fiber metal laminates—composites consisting of metal and composite layers. Two experimental tests were realized on pre-cracked rectangular specimens—one with loading in mode I and one with loading in mode II (DCB—double cantilever beam and ENF—end-notch flexural test). Elastic waves were recorded during these tests via an acoustic emission system. Preliminary studies show that the proposed method can be used successfully to cluster data obtained in this way. The obtained dataset was split into 3 clusters (for the ENF test) and 5 clusters (DCB test). In the next stages of the research campaign, based on the presented results, we intend to change the approach to semi-supervised by running additional single-cause damage tests to enhance the achieved results and enable easier damage recognition.

Experimental and numerical investigations on the mode I delamination growth behavior of reinforced laminates via a ZPI (Z-pin pre-hole insertion) technique

This paper proposes a low-in-plane damage ZPI (Z-pin pre-hole insertion) process. The delamination growth behavior and the influence of Z-pin insertion density on the mode I interlaminar fracture toughness of laminate was explored experimentally and numerically. Meanwhile, Z-pin pull-out and shear tests were conducted in order to provide accurate data for the DCB simulation test. What’s more, an infinite-focus microscope was used to scan the specimen’s defect caused by the ZPI process. It can be found that the ZPI process can lead to less Z-pin inclination, fibre breaking, and crack initiation at the bonded interface. The DCB test results show that the load-displacement curve can be distinguished into four stages for the pinned specimen. In addition, a specimen pinned with a higher Z-pin volume fraction to more suitable for increasing the G IC. The simulation results show that the simulated maximum load and the G IC of the specimen are 286.63 N and 940.06 J·m−2, with a 0.56% and 10.10% error from the experimental result, indicating that the proposed FE model is validated by comparing the predictions with the experimental results. These results can provide some guidance for fabricating the reinforced laminates via the ZPI process.

A comparative numerical-experimental investigation on the tensile behaviour of bonded, rivetted and hybrid composite joints configurations

This paper deals with the development of accurate numerical models to predict the tensile mechanical behaviour of composite joints. The problems of damage initiation and propagation in composite joints for an economic solution with minimized oversizing is addressed. Different types of joints (Single Lap Joint, Single Strap Joint, Joggle Joint) and couplings (bonded, rivetted, hybrid) have been combined to find out the most suitable and reliable joining technique depending on the design constraints. On the basis of preliminary DCB and ENF tests, the characteristics of the adhesive material (stress limits and fracture energies) have been determined. These data have been employed for the numerical simulations on bonded and hybrid (bonded/rivetted) joints. The introduced numerical models, taking into account geometrical non-linearity and progression of damage in composite plies, have been preliminary validated by ad-hoc experimental tests to demonstrate their effectiveness in simulating the mechanical behaviour of general composite joints.

Mode I and mode II fracture behavior in nano‐engineered long fiber reinforced composites

AbstractDelamination still remains one of the common concerns in continuous fibers reinforced polymer composites. In the literature, the incorporation of nanoconstituents into constitutive composites plies has been suggested with the aim of improving interface toughness. In this study, vertically aligned carbon nanotubes forests (VACNTs) are transferred at composites interfaces, and mode I (DCB tests) and mode II (ENF tests) fracture tests carried out on composite samples. Microscopic analysis of reference and nano‐engineered composites help to understand fracture behavior and toughness. While the crack path in ENF samples is observed at interlaminar VACNTs‐resin interfaces, the crack path in DCB samples is intralaminar, located within the unidirectional carbon fiber plies. These microscopic observations give explanation for unstable crack propagation in nano‐engineered ENF samples and the unchanged toughness from the nano‐engineered to the reference zone in DCB samples.

Fracture mechanical stability of DCB and 4ENF tests complemented with linear springs

In this work the stability of the DCB and 4ENF fracture specimens is analyzed using some beam theories and the compliance calibration method. The main goal is that the systems are complemented with linear springs and it is investigated how the springs influence the stability of crack propagation from the theoretical standpoint. Large number of experiments were carried out including crack initiation tests providing the experimental load displacement responses, the compliance datasets and the qualitative observation of crack initiation. Apart from the analytical method the compliance calibration method was employed to evaluate the experimental data and to determine the fit curves of the compliance datasets, the R-curves and finally the bifurcation curves or in other words the critical displacement and crack length relationships including several additional spring stiffnesses for both test configurations. The results indicate that - as expected - the smaller the spring stiffness is the worst the stability of the DCB specimen will be, leading to the possibility of destabilization. On the contrary the analysis and experiments confirm that the 4ENF test cannot be destabilized independently of the stiffness of additional springs.

Effect of Flame Treatment on Bonding Performance of GF/EP Pultrusion Sheets Used for VARI Process

This paper presents an easy and low-cost flame treatment method to improve the bonding performance of GF/EP (Glass Fiber-Reinforced Epoxy) pultrusion plates, which are using widely for large size wind blades. In order to explore the effect of flame treatment on the bonding performance of the precast GF/EP pultruded sheet vs. the infusion plate, the GF/EP pultruded sheets were treated with different flame treatment cycles and were embedded in the fiber fabrics during the vacuum-assisted resin infusion process (VARI). The bonding shear strengths were measured by tensile shear tests. It is found that after 1, 3, 5, and 7 flame treatments, the tensile shear strength between the GF/EP pultrusion plate and infusion plate increased by 8.0%, 13.3%, 22.44%, and -2.1%, respectively. This indicates that the maximum tensile shear strength can be obtained after five times of flame treatment. In addition, DCB and ENF tests were also adopted to characterize the fracture toughness of the bonding interface with the optimal flame treatment. It is found that the optimal treatment gives increments of 21.84% and 78.36% for G I C and G II C, respectively. Finally, the surficial topography of the flame-treated GF/EP pultruded sheets were characterized by optical microscopy, SEM, contact angle test, FTIR, and XPS. The results show that flame treatment plays an impact on the interfacial performance through the combination of physical meshing locking and chemical bonding mechanism. Proper flame treatment would remove the weak boundary layer and mold release agent on the surface of the GF/EP pultruded sheet, etch the bonding surface and improve the oxygen-containing polar groups, such as C-O and O-C=O, to improve the surface roughness and surface tension coefficient of pultruded sheet to enhance the bonding performance. Excessive flame treatment destroys the integrity of epoxy matrix on bonding surface which results into the exposure of the glass fiber, and the carbonization of release agent and resin on the surface loosen the surficial structure, which reduces the bonding properties.

NEW SOLUTIONS IN THE PRODUCTION OF COMPOSITES - MECHANICAL PROPERTIES OF COMPOSITES REINFORCED WITH TECHNICAL EMBROIDERY AND WOVEN FABRIC MADE OF FLAX FIBERS

The main purpose of the article is to present the new possibilities of producing natural fiber composite reinforcement. In this case, a computer embroidery machine by ZSK type JCZA 0109-550 was used. A technical embroidery with a stitch length of 2 mm was made on the machine. The embroidery was made of flax roving with a linear density of 400 tex. The woven fabric was made of the same flax roving as the embroidery, with a surface mass of 400 g/m2. Composites were then produced from the technical embroidery and woven fabric using the infusion method with epoxy resin. The individual configurations differed from each other in the orientation of the roving in the embroidery samples. Samples for tensile strength and tensile elongation tests consisted of 4 layers, while samples for the DCB test consisted of 6 layers, with the addition of a separating foil between the 3rd and 4th layer. Composites were then subjected to strength tests - tensile strength, tensile elongation and DCB test (Double Cantilever Beam test), on the INSTRON machine. During the action of force along the direction of the fibers, composites containing technical embroidery as reinforcement were characterized by higher strength than composites containing woven fabric as reinforcement. Additionally, embroidery is a barrier to the formation of interlayer cracks. Technical embroidery is made on the basis of Tailored Fiber Placement (TFP) technology. This technology allows optimizing the mechanical values of the composite reinforcement.

EXPERIMENTAL AND NUMERICAL STUDY OF THE INFLUENCE OF MANUFACTURING PARAMETERS ON THE INTERLAYER BEHAVIOR OF 3D PRINTING MATERIALS WITH AND WITHOUT REINFORCEMENT FIBERS

This work carries out an in-depth analysis of the influence of the manufacturing parameters on the behavior of a material with better mechanical properties than the usual ones, PLA and ABS, such as PETG (Polyethylene terephthalate with glycol), and the carbon fiber reinforced version. This study performs the experi-mental characterization on interlaminar fracture of this material, with the aim of acquiring a better knowledge of it, but also generates a model of material behavior that takes into account the effects on the properties of the printing profile, in other words, of the selected manufacturing parameters. In order to achieve such objective, a finite element numerical model has been developed, using cohesive elements to simulate the contact zones between layers. Also, failure and degradation modes of the material would be enclosed in the model behavior. A bilinear behavior model is used to simulate the response of the interlaminar zones. The properties of the model, as well as its subsequent adjustment, are supported from the results obtained in the experimental phase. As a final result of the work, the authors propose a series of graphs that allow the selection of these properties to be used in the model, depending on the manufacturing parameters selected for a future application. Keywords: FDM; PETG; DCB test; cohesive modeling

Strength of dissimilar adhesively bonded DCB joints and its connection with the failure pressure of composite repair systems

The application of adhesively bonded joints in petroleum industry over other traditional joining methods has increased significantly in recent years mainly because of the potential for light weight structures, fast repairing systems and low temperature applications. The principal benefits are design flexibility and joining of dissimilar materials. Therefore, accurate analysis and study about the mechanical behavior of dissimilar materials joints are fundamentally required since composite materials are widely used as repair systems for metallic pipelines with localized flaws or damage that prejudice the serviceability. Experimental analyses were performed on dissimilar adhesively bonded DCB joints, similar to a composite repair applied to a pipeline, to investigate the design factors that affect the joint behavior. This study shows that the critical energy release rate obtained in dissimilar DCB joints tests can be correlated with the one obtained in hydrostatic tests performed in pipelines with though-wall defects repaired with composite sleeves. It is proposed an approximated theoretical framework that allows obtaining an estimate of critical fracture parameters that arise in composite repair standards only using simpler and less expensive DCB tests in the place o hydrostatic test.

Loading Rate and Temperature Interaction Effects on the Mode I Fracture Response of a Ductile Polyurethane Adhesive Used in the Automotive Industry.

Due to their high elongation at failure and damping capacity, polyurethanes are one of the main types of adhesives used in automotive structures. However, despite the wide range of applications of adhesives, their fracture mechanics behavior is still poorly studied in the literature, especially when both the loading rate and ambient temperature change. Accordingly, the main aim of the current work is to deal with the research gap. In the current research, mode I fracture energy of a ductile polyurethane adhesive with adaptive properties for its industrial application is determined at different test speeds and temperatures. Tests were done at quasi-static, intermediate, and high-speed levels and each at three different temperatures, including low, high, and room temperature. Mode I fracture toughness was determined using DCB tests. Increasing the loading rate from quasi-static to 6000 mm/min was found to significantly increase the maximum strength of the tested DCBs (from 1770 N to about 4180 N). The greatest sensitivity to the loading rate was observed for the DCBs tested at room temperature, where the fracture energy increased by a factor of 3.5 from quasi-static (0.2 mm/min) to a high loading rate (6000 mm/min). The stiffness analysis of the DCB samples showed that the transition from below the Tg to room temperature decreases the bond stiffness by about 60%, while a further temperature increase (from 23 °C to 60 °C) has no significant effect on this parameter. Since polyurethane-bonded joints often experience a wide range of temperatures and loading rates in service, the obtained results can be used to design these joints more securely against such loading/environmental conditions.

Novel mode III DCB test setups and related evaluation methods to investigate the fracture behaviour of adhesive joints

In this work, two test setups in proximity to the familiar Double Cantilever Beam and Tapered Double Cantilever Beam tests are proposed which allow for determining the critical mode III energy release rate (ERR) of adhesive joints. The experiments are performed on an elastic–plastic adhesive (SikaPower® 498) and evaluated using data reduction schemes based on nonlinear elastic fracture mechanics (J-integral) and linear elastic fracture mechanics (Irwin-Kies equation). Next to the data reduction schemes based on linear elastic fracture mechanics in accordance with ISO 25217, crack length independent and corrected beam theory approaches are presented which incorporate measurements of the rotational angles of the load introduction points. Crack length and process zone length are measured during the experiments using backface strain measurements. Furthermore, a multi-specimen compliance calibration is employed to allow simplified testing in future studies. The obtained critical ERRs and cohesive laws are compared and discussed thoroughly to gain insight into the advantages and disadvantages of the different data reduction schemes. The results implicate that the J-integral evaluation method is the most reliable for evaluation of the mode III fracture energy, as no assumptions need to be made about the behaviour of the adherends when determining the mode III ERR.

Evaluation of the interlaminar fracture toughness on composite materials using DCB test on symmetric and unsymmetric configurations

A procedure to measure the interlaminar fracture toughness of a composite material is proposed in this work. This property is evaluated using the DCB test, the new formulation developed being applicable to small thicknesses and nonsymmetric configurations. The GC evaluation proposed in this paper does not require to know the evolution of the crack length with the load. This fact facilitates the performance of the test and allows complex situations (crack jumps, tests at high temperature,…) to be taken under consideration. An experimental campaign has been carried out on four different configurations, three symmetric (2 + 2, 4 + 4 and 8 + 8 layers) and one nonsymmetric (4 + 8 layers). The results show that, when they are appropriately treated, the values obtained are independent of the number of layers of the adherents and even of the symmetry of the joint, validating the use of the DCB test on nonstandardized coupons.