Delamination Tests Research Articles

Assessment of bonding durability of CLT and glulam made from oak and mixed poplar-oak according to bonding pressure and glue type

The objective of this study is to evaluate the influence of manufacturing parameters on the bonding quality of CLT and glulam made from oak and mixed poplar and oak species. The bonding parameters studied were bonding pressure and glue type. To test the bonding durability, the specimens were subjected to a vacuum pressure cycle followed by drying to the initial weight. The bonding quality was then measured from delamination, residual shear strength, and wood failure percentage (WFP). Bonding pressure and type of glue appeared to have a significant influence on the bonding quality of CLT or glulam, with also significant interactions with the kind of species bonded. If a 0.8 N/mm2 bonding pressure seemed to provide better results, vacuum-pressed (0.085 N/mm2 bonding pressure) mixed poplar-oak CLT or glulam glued with PUR were very close to reaching the requirements of the standards. Generally, CLT or glulam made from pure oak delaminated more than the mixed specimens. However, their residual shear strength can be comparable or even superior to what was obtained with mixed poplar-oak specimens. As a result, residual shear strength after delamination test may be interesting to consider as an additional criterion to assess the glue line integrity of hardwood CLT or glulam products.

Continuum damage analysis of delamination in composite laminates using a stress-based layerwise plate model

This contribution addresses the modelling of delamination in a stress-based layerwise plate model. Such models benefit from a good representation of the local 3D stress field, especially inter-laminar stresses which are natural generalized stress unknowns arising in their derivation. We extend their construction to situations involving onset and propagation of interfacial delamination. We propose a variational formulation relying on a continuous damage description of the opening and sliding interfacial degradation processes using two damage variables. A mixed-mode delamination propagation criterion can be obtained through proper coupling between both variables. Finally, the model predictions are validated against classical delamination tests in pure and mixed modes.

Basalt Fiber Hybridization Effects on High-Performance Sisal-Reinforced Biocomposites.

The increasing attention given to environmental protection, largely through specific regulations on environmental impact and the recycling of materials, has led to a considerable interest of researchers in biocomposites, materials consisting of bio-based or green polymer matrixes reinforced by natural fibers. Among the various reinforcing natural fibers, sisal fibers are particularly promising for their good mechanical properties, low specific weight and wide availability on the current market. As proven in literature by various authors, the hybridization of biocomposites by synthetical fibers or different natural fibers can lead to an interesting improvement of the mechanical properties or, in turn, of the strength against environmental agents. Consequently, this can lead to a significant enlargement of their practical applications, in particular from quite common non-structural applications (dashboards, fillings, soundproofing, etc.) towards semi-structural (panels, etc.) and structural applications (structural elements of civil construction and/or machine components). Hybridizations with natural fibers or with ecofriendly basalt fibers are the most interesting ones, since they permit the improvement of the biocomposite’s performance without an appreciable increment on environmental impact, as occurs instead for synthetic fiber hybridizations that are also widely proposed in the literature. In order to further increase the mechanical performance and, above all, to reduce the aging effects on high-performance sisal-reinforced biocomposites due to environmental agents, the hybridization of such biocomposites with basalt fibers are studied with tensile, compression and delamination tests performed by varying the exposition to environmental agents. In brief, the experimental analysis has shown that hybridization can lead to further enhancements of mechanical performance (strength and stiffness) that increase with basalt volume fraction and can lead to appreciable reductions in the aging effects on mechanical performance by simple hybridization of the surface laminae. Therefore, such a hybridization can be advantageously used in all practical outdoor applications in which high-performance sisal biocomposites can be exposed to significant environmental agents (temperature, humidity, UV).

Hydroxymethylated resorcinol (HMR) primer to improve the performance of wood-adhesive bonds – A review

A hydroxymethylated resorcinol (HMR) primer was developed in the 1990s to improve the performance of epoxy-bonded wood lamellas for use in moist conditions. Its chemical composition resembles a strongly diluted resorcinol-formaldehyde adhesive, which is applied before the actual adhesive. The effectiveness of HMR priming is proven for different wood species and adhesive types based on delamination tests and shear strength measurements. However, the primer is not used in the industrial mass production of glued laminated timber. The reason can be found in additional process steps, therefore costs, and the fact that certain combinations of currently broadly available wood species and adhesives yield proper bonds without the additional use of a primer. Nevertheless, due to ongoing changes in the availability of wood species, it becomes increasingly important to consider the use of currently less-used wood species, including the corresponding adaptations in the production process. Since it is known that the HMR primer improves the bond performance, but is not used due to necessary process changes, it would be interesting to integrate the primer's functionality into existing adhesive systems. This review summarizes and structures the available information on the HMR primer from previous research. Furthermore, that information is critically discussed and a model of functionality is introduced. Finally, it is evaluated if the existing knowledge about the HMR primer is sufficient to transfer its mode of action into existing adhesive systems or which questions need to be addressed to do so.

Timber-concrete composite structural flooring system

An integrated solution is presented for the execution of building structures using timber-concrete composite (TCC) sections that make efficient use of the mechanical properties of both materials. The system integrates flooring and shaped prefabricated beams composed of a lower flange of glued laminated timber (GLT) glued to one or more plywood or laminated veneer lumber (LVL) ribs and linked to an upper concrete slab poured in situ. The parts may be prefabricated in T shape (only one rib), in π shape (two ribs), or with multiple ribs to create wider pieces, thereby reducing installation operations.The basis of the system is the timber-concrete shear connection in the form of holes through the ribs, which are filled by the in situ-poured concrete. The connection is complemented with the arrangement of reinforcement bars through the holes.Three test campaigns were undertaken. Shear tests of the timber-concrete connection in 12 test pieces. Shear test along the wood-wood glue line (72 planes tested) and wood -plywood (24 planes tested). Delamination test of the glued planes (24 wood-wood planes and 8 wood-plywood planes). The results indicate a high strength joint, with ductile failure and high composite effect. Likewise, the shear test results along the glue line and the delamination tests show section integrity under demanding hygrothermal conditions.Preliminary sizing curves were developed considering the Gamma Method to evaluate the performance of the system. The results show the possibilities of the system, as pouring the upper slab concrete in situ makes it possible to create continuous semi-rigid joints between the elements. This gives rise to slender flooring structures, light and with high stiffness plane against horizontal forces.

An efficient semi-analytical method to extract the mode II bridging-traction law in ENF tests directly from the experimental load displacement data

Large scale fiber bridging ahead of the crack tip of ENF specimen is one of the most common toughening mechnisms, leading to a significant resistance phenomenon on the fracture toughness. In order to account for the toughening effect of the fiber bridging to accurately model the delamination of composite laminates, a physically semi-analytical method is presented to determine the mode II bridging law with the assumption of linear form directly from the experimental load displacement curve. The method includes the deflection analysis of the ENF specimen based on the beam theory considering the closing force introduced by fiber bridging and the inverse method iteratively determining the bridging law parameters. To validate the proposed method, the identified bridging tractions by the current method are integrated in the traditional cohesive law to simulate the whole delamination propagation process of undirectional laminates with different materials and multidirectional laminates. Good agreements between the numerical results and experimental ones indicate the accuracy and the applicability of the current method. The major advantage of the proposed method is that it reduces the instructmentation needed in the delamination test and it can be adopted as an efficient method to investigate the delamination behaviour of specimens.

Application of a novel testing device to characterize the layer adhesion of co-extruded polymer sheets

Co-extrusion is a common and efficient way to produce polymer products out of various layers with different materials tailoring their properties. Adhesion between the individual layers is crucial for the overall performance of the multi-layer product. The existing methods to characterize layer adhesion, such as delamination tests, peel-off test, and tensile-shear tests, were originally developed for testing adhesive joints, composites, and laminates and have limited applicability to co-extruded polymer products. In this study, first the existing methods for a two-layer polymer sheet using various compatible materials are evaluated. These combinations formed strong interfaces which could not be characterized with the existing methods. Next, a novel layer adhesion testing device which is capable to apply pure shear loads at the interface is presented. Using this testing device enables characterization of the strong interfaces of the co-extruded sheets with adhesive failure observed. Finally, tests with various interfacial overlapping lengths are performed and yield strength, failure strength, and the energy absorbed are evaluated to proof the suitability of the novel testing device to characterize layer adhesion in multi-layer polymer sheets.

Configuration of the ELS test for the determination of the mode II fracture toughness in adhesive joints

In order to obtain Mode II fracture toughness of unidirectional fiber composite materials, the End-Loaded Split (ELS) has enjoyed great success because of its higher stability. As a result, crack growth initiation and propagation values are obtained, the latter being especially used in interlaminar progressive damage models. The propagation values must be obtained when the Failure Process Zone (FPZ) has completely developed. In delamination tests of unidirectional composites, such as those contemplated in ISO 15114, the dimension of the FPZ can be neglected against the rest of the dimensions of the specimen, whereby the self-similar crack growth is reached just after the initiation of the crack propagation. However, in adhesive joints, the dimension of the FPZ increases significantly due to the plasticity of the adhesives. And even it can be larger than the entire length reserved for crack propagation. When this occurs, only initiation values are obtained, always inferior to the values of propagation, being these values too conservative to reproduce the behaviour of the joint. In this paper we present a methodology to define the configuration of the ELS test and the specimen dimensions required to obtain propagation results in adhesive joints. The methodology defined is based on two criteria that take into account the length of the FPZ and the stability of the test. As a result, a domain of validity of the test is obtained, which, for a certain type of adhesive and specimen configuration, it indicated is there will be propagation or not.

Material Parameters Identification for Modelling of Carbon Rod Structures Delamination

The aim of the present work is to compare the interlaminar shear strength and fracture toughness of glued carbon fiber rods obtained using different experimental approaches and provide the effective way to characterise the interlaminar properties for reliable simulation of the delamination. Five different test methods (tension, single shear test, and double shear test, mode I and mode II delamination tests) were performed. Using the explicit LS-DYNA code the finite element model capable of simulating the damage process of bonded connection was developed. The interlaminar connection and delamination criteria were calibrated using the parameter identification methodology implemented in LS-OPT optimization tool.

Delamination Testing of AlSi10Mg Sandwich Structures with Pyramidal Lattice Truss Core made by Laser Powder Bed Fusion

Sandwich structures possess a high bending stiffness compared to monolithic structures with a similar weight. This makes them very suitable for lightweight applications, where high stiffness to weight ratios are needed. Most common manufacturing methods of sandwich structures involve adhesive bonding of the core material with the sheets. However, adhesive bonding is prone to delamination, a failure mode that is often difficult to detect. This paper presents the results of delamination testing of fully additive manufactured (AM) AlSi10Mg sandwich structures with pyramidal lattice truss core using Laser Powder Bed Fusion (LPBF). The faces and struts are 0.5 mm thick, while the core is 2 mm thick. The inclination of the struts is 45°. To characterise the bonding strength, climbing drum peel tests and out-of-plane tensile tests are performed. Analytical formulas are derived to predict the expected failure loads and modes. The analytics and tests are supported by finite element (FE) calculations. From the analytic approach, design guidelines to avoid delamination in AM sandwich structures are derived. The study presents a critical face sheet thickness to strut diameter ratio for which the structure can delaminate. This ratio is mainly influenced by the inclination of the struts. The peel tests resulted in face yielding, which can also be inferred from the analytics and numerics. The out-of-plane tensile tests didn’t damage the structure.

Fatigue delamination behaviour of carbon fibre/epoxy composites interleaved with thermoplastic veils

Interleaving thermoplastic veils has proved to enhance the interlaminar fracture toughness of carbon fibre/epoxy composites under static loading conditions. However, the fatigue delamination behaviour has yet to be investigated. Herein, meltable Polyamide-12 (PA) veils and non-meltable Polyphenylene-sulphide (PPS) veils were used for interlay toughening of unidirectional (UD) and non-crimp fabric (NCF) laminates that were manufactured using a prepreg process and resin transfer moulding process, respectively. The results of Mode-I fatigue delamination tests demonstrated a significant improvement in the fatigue life of the laminates due to interleaving. Additionally, the fatigue resistance energy has been maximumly increased by 143% and 190% for the UD and NCF laminates, respectively. The microscopy analysis revealed that the toughening mechanisms of thermoplastic veils were affected by the form of the thermoplastic veils in the laminates (melted or non-melted), the fracture mechanisms of the reference laminates and the adhesion/miscibility between the thermoplastic veils and the epoxy.

Simulating delamination in composite laminates involving large scale fiber bridging based on the mixed-mode three-linear cohesive zone model

A three-linear cohesive zone model (CZM) superposed by two bilinear laws for modelling mixed-mode delamination with the effect of large scale fiber bridging is proposed in this study. The parameters in the formulation can be obtained from pure mode delamination tests or selected by reliable guidelines, making the CZM easy to be implemented in the finite element model. A novel method to determine the bridging strength by applying varied values in the different regions of the prearranged delamination path is developed. The division of the region is based on the concept of the length of the fracture process zone (FPZ). Simulations of mode I delamination in unidirectional and multidirectional composite laminates and mixed-mode I/II delamination in multidirectional laminate are performed in the finite element analysis software. Good agreement between the predicted load–displacement responses and the corresponding experimental results are achieved by adopting the established CZM and the bridging strength determination method, validating their efficiency in predicting the delamination behaviors under both pure mode and mixed-mode loadings.

Delamination analysis of 3D-printed nylon reinforced with continuous carbon fibers

The inconsistent interfacial bonding of 3D-printed materials is accompanied by increasing scattering of the measured fracture toughness in delamination tests. This article focuses on the analysis of the delamination tests using theories developed for conventional composites. To this end, nylon 3D-printed specimens reinforced with carbon fibers are subjected to the mode I Double Cantilever Beam (DCB) and the mode II End-Loaded Split (ELS) tests. Several analytical methods and numerical models are employed to measure the fracture toughness and simulate the behavior of the specimen during delamination. The uncertainty of the data is considered by expanding the deterministic character of the models utilizing the non-intrusive stochastic pseudo-spectral projection method. The results of the simulations are well in agreement with the experimental results and support the use of stochastic techniques in combination with linear fracture mechanics for the delamination analysis of 3D-printed composites.

Causes of differential behavior of extractives on the natural cold water durability of the welded joints of three tropical woods

This study aims to investigate the causes of the differential behavior of extractives on the water resistance of the welded joints of three tropical woods. The welded joints of these chemically characterized woods were obtained by rotational dowel welding. The respective extractives contents obtained during the successive extractions were 12.98 ± 0.90% for Iroko, 17.36 ± 0.40% for African pearwood and 18.16 ± 1.00% for Padauk. Phytochemical screening revealed a high concentration of polyphenols in the extractives of Padauk and Iroko compared to African pearwood. The highest delamination rate (76.66%) was obtained with African pearwood. The highest tensile strength before (4.09 MPa) and after (1.94 MPa) the delamination test was obtained with Padauk. MALDI-TOF mass spectrometry analysis revealed non-leaching after 30 days in cold water of lignin and hemicelluloses fragments from Padauk at 536 Da and 698 Da, but just for hemicelluloses fragments as shown by a glucose pentamer at 837 Da, a glucomannan fragment, and of a glucose pentamer with a xylose residue attached at 970 Da that disappeared completely after 30 days of African pearwood water immersion. However, cold water immersion of Iroko appears to remove resveratrol at 250 Da, a stilbene, as well as multiprotonated stilbene at 256 Da while other stilbene class polyphenols such as geranyl trihydroxy stilbene at 364 Da and chlorophorin at 379 Da seem unaffected by water. SEM analysis confirmed perfect adhesion of Padauk and Iroko joints which indicates a strong correlation between the water resistance of the welded joint and the type of extractives.

In vitro evaluation of delamination resistance of PEEK and CFR-PEEK.

Poly-ether-ether-ketone (PEEK) and carbon fiber reinforced PEEK as orthopedic implant materials exhibit excellent material properties. Although delamination of PEEK materials has been reported in knee joint wear research, the delamination resistance behavior remains unclear. In this study, the delamination resistance of PEEK materials was investigated; these materials were compared to ultra-high molecular weight polyethylene (UHMWPE). The results of a ball-on-flat type delamination test indicated that the PEEK materials underwent delamination considerably earlier than UHMWPE, and the contact area of the PEEK materials was smaller than that of UHMWPE. Moreover, the indentation modulus, hardness, and coefficient of friction were higher for PEEK materials than for UHMWPE. The reduced tendency of PEEK materials to undergo deformation to mitigate stress concentration at low conformity contact conditions contributed to their inferior delamination resistance compared to that of UHMWPE. The delamination resistance of the PEEK materials was equivalent to that of degraded UHMWPE, which highlights the risk of delamination of PEEK implants in a clinical context. Consequently, when using PEEK materials as an implant component loaded at a low conformity contact condition, the material selection and component design must be carefully considered. Overall, the results of this study can help guide the future development of PEEK-based implants.

Methodology to Calibrate the Dissection Properties of Aorta Layers from Two Sets of Experimental Measurements

Aortic dissection is a prevalent cardiovascular pathology that can have a fatal outcome. However, the mechanisms that trigger this disease and the mechanics of its progression are not fully understood. Computational models can help understand these issues, but they need a proper characterisation of the tissues. Therefore, we propose a methodology to obtain the dissection parameters of all layers in aortic tissue via the computational modelling of two different delamination tests: the peel and mixed tests. Both experimental tests have been performed in specimens of porcine aorta, where the intima-media and media-adventitia interfaces, as well as the medial layer, were dissected. These two tests have been modelled using a cohesive zone formulation for the separating interface and a hyperelastic anisotropic material model via an implicit static analysis. The dissection properties of each interface have been calibrated by reproducing the force-displacement curves obtained in the experimental tests. The values of peak and mean force of the experiments were fitted with an error below 10%. With this methodology, we intend to contribute to the development of reliable numerical tools for simulating aortic dissection and aortic aneurysm rupture.

Study of the adhesion of metal coatings on synthetic textile materials

This paper is devoted to the study of the adhesion of metal coatings on synthetic textile materials based on polyester, polyamide and polypropylene. The metal coating was applied to the surface of textile materials by magnetron sputtering. To assess the adhesion of the metal coating to the substrate, the method of delamination testing using a tape with constant stickiness was used.

Comparative Investigation of the Mechanical and Wear Properties of Multilayer Si-DLC/DLC Films

In this study, multilayer diamond-like carbon films 1 µm thick with different numbers of multilayer repetitions were deposited onto austenitic stainless steel SUS304 substrates using different source gases. The influence of the gas and the difference in the number of multilayer repetitions on the adhesion strength and wear resistance of the films was subsequently investigated. The samples were subjected to cross-sectional microstructure observations, elemental analysis by glow-discharge optical emission spectroscopy, nano-indentation tests, Rockwell indentation tests, friction and wear tests, and delamination tests. The nano-indentation tests showed that the films prepared using C2H2 gas were harder than those prepared using CH4 gas. In addition, irrespective of the gas used, the film hardness was improved when four layers rather than two layers were deposited. However, the hardness of the eight layers film decreased. The Rockwell indentation tests showed no improvement in adhesiveness when the gas or the number of multilayer repetitions was varied. The wear tests revealed that the friction coefficient of the films prepared using C2H2 gas was smaller than that of films prepared using CH4 gas. No difference was observed with increasing number of multilayer repetitions. The delamination tests showed that the distance until delamination of the films prepared using C2H2 gas was longer than that of the films prepared using CH4 gas. In addition, the distance until delamination was improved by changing the number of multilayer repetitions from two layers to four layers for both gases but decreased when the number of repetitions was extended to eight layers. In this study, the value of H/E (hardness/Young’s modulus) increased and various characteristics improved with increasing number of multilayer repetitions.

Use of the fiberglass reinforcement method in thermoplastic mouthguard materials to improve flexural properties for enhancement of functionality.

The purpose of this study was to evaluate the application of fiberglass reinforcement method in thermoplastic mouthguard materials to improve flexural properties and adhesive strength. Commonly used two types of commercial mouth guard materials (ethylene-vinyl acetate copolymer-based and polyolefin-based) were reinforced with glass fiber clothes by two-step hot press. Flexural strength and adhesive strength with each base material were examine via three-point bending test and delamination test, respectively. Ethylene-vinyl acetate copolymer-based fiberglass-reinforced material has significantly greater adhesive strength with base material and improvement of flexural properties compared with polyolefin-based material. These results suggest that flexural properties of both conventional commercial mouthguard materials were improved when the glass-fiber-reinforced method was applied to reinforce mouthguard materials, and more, ethylene-vinyl acetate copolymer was more desirable for the base material.

A new model characterizing the fatigue delamination growth in DCB laminates with combined effects of fiber bridging and stress ratio

For composite multidirectional laminates widely used in engineering, there is still no effective model to characterize the fatigue delamination behavior with the combined effects of fiber bridging and stress ratio. A new fatigue delamination model was thus proposed in this study to deal with this problem. The model was established based on the fatigue driving force and fatigue delamination resistance, which characterized the effects of stress ratio and fiber bridging, respectively. A series of mode I fatigue delamination tests on composite multidirectional laminates were conducted under various stress ratios (0.1, 0.3 and 0.5) and different pre-crack lengths. Both the experimental results of this study and public fatigue data from laminates with other materials and various interfaces showed that a single master curve with a narrow band can be obtained by using the new model for all fatigue data under different stress ratios and fiber bridging effects. In addition, the determined model parameters obtained from fatigue data under specific stress ratios were used for predicting the fatigue delamination behavior under other stress ratios. Good agreements between the predictions and experimental results can be achieved, which further indicated the model’s applicability.