Delamination Tests Research Articles

A re-evaluation of mixed-mode cohesive zone modeling based on strength concepts instead of traction laws

New methods for implementing cohesive zones in computational mechanics were derived by re-interpreting cohesive-zone “traction laws” as “strength models” and using concepts from damage mechanics. When applied to pure mode I or mode II loading, this approach is identical to prior methods. But, when applied to mixed-mode loading, it defines a new approach called strength cohesive zone modeling (or “strength CZM”). Compared to previous methods, strength CZM provides improved modeling for problems with changing mode mixity, allows independent selection of normal and tangential strength models, and reveals limitations in any method based on effective displacements. New modeling shows that mode mixity, which changes during crack growth, depends on cohesive zone properties and on which end of the cohesive process zone is interpreted as the crack tip. These observations suggest many prior experimental results need reanalysis. By considering a range of independent strength models, strength CZM predicts that all GI-GII failure envelopes are convex. Precracking steps recommended in delamination testing protocols are considered. Strength CZM can realistically model precracking while prior methods generate unrealistic predictions.

On the robustness of the stabilized finite element method for delamination analysis of composites using cohesive elements

We demonstrate the ability of a stabilized finite element method, inspired by the weighted Nitsche approach, to alleviate spurious traction oscillations at interlaminar interfaces during composite delamination. The method allows for the use of any value for the cohesive stiffness and obviates the need for ad hoc approaches to estimate the minimum penalty stiffness for accurate analysis. Simulation studies involving constant strain patch tests and benchmark delamination tests under mode-I, mode-II, and mixed-mode loadings show evidence of traction oscillations with the standard method. In contrast, the stabilized method is able to alleviate traction oscillations, which illustrates its efficacy and robustness.

Analytical model for the estimation of the hygrothermal residual stresses in generally layered laminates

Delamination tests are commonly used for the evaluation of the fracture toughness. The analytical formulation used for the tests is derived for homogeneous symmetric laminates with a crack propagating in the middle plane. The difference between manufacturing and operating temperature and moisture conditions is neglected. The present paper considers geometries with asymmetric generic stacking (asymmetric and unbalanced) that are under the influence of hygrothermal effects. Explicit formulas are derived for the parameters associated with delamination tests that are of practical interest for in-situ monitoring. The results depict a good agreement with Finite Element analyses.

High-performance methylammonium-free ideal-band-gap perovskite solar cells

Summary The development of mixed tin-lead (Sn-Pb)-based perovskite solar cells (PSCs) with low band gap (1.2–1.4 eV) has become critical not only for pushing single-junction devices toward the maximum efficiency given by the Shockley-Queisser limit, but also for enabling all-perovskite tandem devices beyond this limit. However, achieving high power-conversion efficiency (PCE) and long-term device operation stability simultaneously remains a significant challenge for Sn-Pb-based PSCs. Here, we demonstrate near ideal-band-gap (∼1.34 eV) methylammonium-free Sn-Pb-based PSCs with high efficiency (∼20%) and promising operational stability of maintaining >80% of initial PCE over 750 h under maximum-power-point tracking. The key to this success is the use of a SnCl2⋅3FACl complex additive that improves the microstructure and reduces the development of residual stress in the Sn-Pb perovskite thin films, which in turn enhances the efficiency and stability of the Sn-Pb-based ideal-band-gap PSCs.

Loading rate dependency of strain energy release rate in mode I delamination of composite laminates

This work aims at studying the loading rate dependency of mode I delamination growth in CFRPs, using typical fracture toughness analysis through both the R-curve and the crack tip opening rate. The average SERR is a method of data reduction based on energy balance which has been previously introduced to characterize delamination growth under different types of loading conditions in a similar manner. In the present research, the application of this method was extended to further analyze the results of delamination experiments at different loading rates. Mode I delamination tests on double cantilever beam specimens were performed at displacement rates varying from standard quasi-static testing up to 400 mm/s. A clear decrease in the propagation fracture toughness as well as in the average SERR was observed at high loading rates. The reduced fracture resistance at elevated rates was physically explained in correlation with fiber bridging, fiber breakage, and matrix cleavage observed in fracture surfaces via scanning electron microscopy.

Adhesion Properties of Thin Film Multilayers: Comparison of Nanoindentation and Four-Point-Bending Techniques

In this study, we investigated the adhesion strength of SiO2/SiN/TiW/Cu film stacks on silicon by the use of cross-sectional nanoindentation (CSN) technique. The delamination occurred along the SiN/TiW interface as determined by means of SEM and EDX analysis. The critical energy release rate was determined as a quantitative measure of the adhesion strength by application of analytical models as well as Finite Element Method (FEM). Comparative measurements on samples of the same layer composition using the well-established four-point bending (4PB) technique were performed to validate the results of the CSN measurements. FEM was performed to calculate the loading conditions and stress distribution in the samples. The calculations also allowed separating the contribution of plastic and elastic energy in the metallization layers during delamination testing and thereby estimating the value of the interfacial adhesion energy. The experimental results show the good applicability of both the 4PB and CSN method for determining quantitative values of the fracture toughness of thin-film interfaces found in microelectronic components and indicate a good agreement between the two methods.

Damage Mechanism Based Approach to the Structural Health Monitoring of Wind Turbine Blades

A damage mechanism based approach to the structural health monitoring of wind turbine blades is formulated. Typical physical mechanisms of wind turbine blade degradation, including surface erosion, adhesive fatigue, laminate cracking and in some cases compressive kinking and failure are reviewed. Examples of a local, damage mechanism based approach to the structural health monitoring of wind turbine blades are demonstrated, including the monitoring of leading edge erosion of wind turbine blades, adhesive bond failure, plydrop delamination, static and dynamic plydrop tests, and bolt and laminate fatigue. The examples demonstrate the possibilities of monitoring specific damage mechanisms, and specific localizations of wind turbine blades.

Improved strength properties of LVL glued using PVAc adhesives with physical treatment-based Rubberwood (Hevea brasiliensis)

Abstract The properties of the laminated veneer lumber (LVL) composed of the boiled veneer of Rubberwood (Hevea brasiliensis) using polyvinyl acetate (PVAc) adhesives in various cold-pressing time and various conditioned time with loaded and unloaded were studied. Five-ply LVL was produced by boiling veneer at 100°C for 90 min as pretreatment and cold-pressing time at 12 kgf cm−2 for 1.5, 6, 18, and 24 h then conditioned at 20°C and 65% relative humidity (RH) with loaded (12 kgf cm−2) and unloaded for 7 days as physical treatment. Especially for the delamination test, the specimens were immersed at 70 ± 3°C for 2 h and dried in the oven at 60 ± 3°C for 24 h; then, the specimens were solidified at room temperature (20°C and 65% RH) with loaded (12 kgf cm−2) and unloaded for 7, 10, 12, and 14 days. To determine the performance of LVL, the density, moisture content (MC), delamination, modulus of elasticity (MOE), modulus of rupture (MOR), horizontal shear strength, and formaldehyde emission tests were conducted according to the Japanese Agricultural Standard (JAS 2008) for structural LVL. The MOE and MOR values were significantly influenced by the physical treatment, however, neither to horizontal shear strength nor to formaldehyde emission. The best performance of LVL has resulted from unloaded LVL with cold-pressed time for 18 h; the MOE and MOR values were 9,345.05 ± 141.61 N mm−2 and 80.67 ± 1.77 N mm−2, respectively. The best value of the horizontal shear strength was obtained from the LVL with 18 h cold-pressing time and conditioned with loaded (13.10 ± 1.47 N mm−2) and unloaded (12.23 ± 1.36 N mm−2). The percentage of delamination values decreased with an increase in the cold-pressing time and conditioning time. The lowest value of delamination (19.06%) was obtained from the LVL with 24 h cold-pressing time and conditioned with loaded for 14 days. Except the delamination test, all other properties fulfilled the JAS.

In-vivo blood flow parameters can predict at-risk aortic aneurysms and dissection: a comprehensive biomechanics model

Abstract Background Abnormal blood flow patterns can alter the material properties of the thoracic aorta via altered vascular biology and tissue biomechanics. In-vivo haemodynamic assessment of the aorta is yet to penetrate clinical practice due to our limited understanding of its effect on aortic wall properties. The decision for surgical treatment is based on size thresholds, limited to a single measurement of aortic diameter from routine imaging, although many aortic dissections (40–60%) occur below these size thresholds. This multi-centre study aims to assess the clinical utility of biomechanics principles in thoracic aortic aneurysm (TAA) risk rupture prediction using a substantial sample size. Methods Fifty-five patients undergoing surgery for root or ascending TAA were recruited from five cardiac centres. Bicuspid aortic valves and connective tissue disease were excluded from this study.Haemodynamic assessment Pre-operative 4-dimensional flow magnetic resonance imaging (4D-MRI) were conducted. Direct 4D-flow analysis and computational fluid dynamics (CFD) were performed creating detailed wall shear stress (WSS) maps across the whole aneurysms. Aortic wall assessment The aneurysmal aortic sample was obtained from surgery and subjected to region specific uniaxial failure tests in the circumferential and longitudinal directions, as well as delamination testing within the aortic media. Whole aneurysm histological characterisation was also conducted using computational pathology techniques. Blood flow, tissue mechanics and microstructural properties were used to develop a risk prediction model with assessment of elastin, collagen and smooth muscle cell composition, as well as failure strain assessment and dissection energy function. Results Outcomes of mechanical properties were: Young's Elastic Modulus as a measure of aortic stiffness (0.85 MPa ±0.69), as well as maximal tensile strength (0.49 MPa ± 0.36), which demonstrated reduced aortic wall strength in the outer curvature. This correlated with increased wall shear stress (WSS) (up to 10 Pa) and flow velocity (up to 43 l/min). Regions of abnormal flow and tissue mechanics correlated significantly with degraded medial microstructure (elastin abundance: 34 vs 66%; collagen abundance 26 vs 57%, p<0.05). Conclusions CFD modelling has the potential to provide a risk prediction of acute events in TAA beyond the current size classification, as validated by altered aortic tissue properties. Future longitudinal studies are warranted to validate this methods in moderately enlarging thoracic aortas. Flow, mechanical, histology properties Funding Acknowledgement Type of funding source: Foundation. Main funding source(s): NIHR Imperial College BRC

FEA simulation and experimental validation of mode I and II delamination at the carbon/glass/epoxy hybrid interface: Physical-based interpretation

The aim of this study was to carry out simulations and perform experimental quasi-static delamination tests in modes I and II to characterize the mechanical behavior at a hybrid interface. For that purpose, contact angle, infrared spectroscopy, and energy balance model results were obtained to characterize the physical interfacial energy behavior. The simulations and experimental tests presented similar values and trends, indicating that this is a viable method for predicting the critical fracture toughness of hybrid laminated composites. The low interfacial energy of the stitching (PS) and the epoxy matrix showed a decrease in the experimental strain energy release. The hybrid interface (carbon/glass/epoxy) showed an improvement in fracture toughness, which was physically elucidated through the synergy of high CF/epoxy interfacial energy strain combined with the toughness interaction via organosilane in GF/epoxy interface. In addition, the directional change in the micro-cracks generated between the two interfaces (rough fracture) requires an increase in energy to propagate the delamination as a result of the synergy between the CF and GF stiffness, also confirmed by the physical-based model.

Effect of ACQ treatment on surface quality and bonding performance of four Malaysian hardwoods and cross laminated timber (CLT)

The objective of this study is to determine the effects of alkaline copper quaternary (ACQ) treatment on the surface quality and bonding performance of four Malaysian hardwood species, namely batai, sesenduk, rubberwood and kedondong. The samples were impregnated with 2% ACQ preservatives and bonded with phenol-resorcinol–formaldehyde resin for cross laminated timber (CLT) fabrication. The changes in density and the retention of both ACQ and copper after the treatment were recorded. Surface roughness and wettability of both treated and untreated samples were measured. Block shear and delamination tests were performed to evaluate the bond-line strength of CLT. The study revealed that the average surface roughness (Ra) of each species increased significantly. Wettability of batai, sesenduk, rubberwood and kedondong was significantly higher than that of the untreated samples suggesting an improvement in surface wettability. For single-species CLT, treated rubberwood has the highest shear strength followed by kedondong, sesenduk and batai with values of 9.53 N/mm2, 6.00 N/mm2, 5.68 N/mm2 and 4.19 N/mm2, respectively. While for mixed-species CLT, the combination of ACQ-treated rubberwood-sesenduk-rubberwood has the highest block shear strength with a value of 8.05 N/mm2. No delamination was observed from all samples. ACQ treatment was found to not affect the block shear strength significantly. Therefore, ACQ preservatives can be used to produce CLT with good bonding performance.

The unusual delamination phenomenon of three kinds of lithium‐ion battery separators

AbstractAn unusual delamination phenomenon on three kinds of lithium‐ion battery separators was investigated in this study, which is closely associated with the compression resistance of the separator. The delamination trends of three kinds of polymer separators were analyzed systematically. Scanning electron microscopy, two‐dimensional wide‐angle X‐ray diffraction, compression and delamination tests show that a separator prepared by the dry process with uniaxial stretching delaminate hardest due to abundant connection or support structure along the thickness direction. However, the multilayer structure stacked along the thickness direction can be seen clearly for separators experiencing biaxial stretching, both for the dry process and wet process. In particular, a separator prepared by the wet process has the poorest connection or support structure along the thickness direction, leading to the strongest delamination trend and the worst compression resistance. Based on this, we control the delamination trend of the separator by adjusting the condensed structure of the precursor film. Besides, the delamination test can quantify the delamination trend of different kinds of separators accurately and further supplement the evaluation criteria of lithium‐ion battery separators, which can help us to better understand and design a separator with optimal microporous structure. © 2020 Society of Industrial Chemistry

Mechanical and thermal properties of PVC and polyurethane foam hybrid composites

Abstract Sandwich foam composites are mostly used today in structural applications in aerospace and automobile industries due to light weight and high stiffness of the sandwich composites. In this research work, the composites were fabricated by using polyurethane and PVC foam and carbon fiber as skin. Sandwich composites were fabricated by the hand lay-up method. Mechanical tests such as tensile, flexural, impact, double shear, inter delamination and hardness tests were conducted. A thermo gravimetric analysis (TGA) test was also performed for the analysis of the temperature characteristics of the sandwich composites. The internal structures of tested samples were analyzed through a scanning electron microscope (SEM). The results show that the tensile, flexural, impact, thermal and inter delamination strengths of PVC foam composites are superior to those of other composites.

Experimental assessment of Fully-Uncoupled Multi-Directional specimens for mode I delamination tests

This paper proposes an experimental study whose goal is to validate the concept of Fully-Uncoupled Multi-Directional (FUMD) specimens for delamination tests. In order to reduce the likelihood of delamination jump, a glass/epoxy UD-fabric composite was used to fabricate double cantilever beam specimens. Mode I tests were performed with standard UD specimens and with FUMD specimens having the following delamination interfaces: 0 °//0 °, 0°// 15°, 0 °//30 °, 0 °//45 ° and −45 °//45 °. Delamination fronts after the tests were observed by means of ultrasonic C-scans. By comparison with the UD specimens, it is shown that FUMD sequences are able to promote a fairly symmetric and flat delamination front. Analysis of the rotations of such specimens during the test confirms that they do exhibit an uncoupled mechanical behaviour. Critical energy release rate is found to be dependent on interface plies mismatch angle, but not on global stiffness of the specimens.

Characterization of natural – Synthetic fiber reinforced epoxy based composite – Hybridization of kenaf fiber and kevlar fiber

The work deals with the mechanical characterization of Kenaf and Kevlar fiber reinforced epoxy-based composites. The tensile, flexural, double shear, impact, hardness and de-lamination tests were conducted on composite laminates developed through hand lay-up process. Morphological analysis was conducted to examine the split surface of the tested specimen. The experimental investigation reveals that the mechanical properties of epoxy composites were enhanced by the hybridization of Kenaf and Kevlar fiber.

Comparison of different bonding parameters in the production of beech and combined beech-spruce CLT by standard and optimized tests methods

Interest in the use of beech wood in construction is growing steadily. Considering its high mechanical performance, it has a large potential to be used for the production of glued timber structural products. However, the issue of structural bonding remains to be solved before effective production and safe use of beech wood in these products can be achieved. Three adhesives (one-component polyurethane – PUR, PUR + primer and melamine-urea–formaldehyde – MUF) and two press systems (hydraulic and vacuum press) were explored in the production of pure beech and combined beech-spruce-beech cross laminated timber (CLT). An evaluation of the bonding quality was performed with both standardized and optimized (the specimen layers were oriented with the wood grain forming an angle of 45° with respect to the load application) delamination and shear test methods.None of the adhesives that were tested met the requirements for delamination tests provided by the current standard for softwood. Specifically, PUR yielded the poorest performance in the production of the CLT panels entirely made of beech; the addition of a primer improved the bonding, achieving results comparable to those observed for MUF. On the contrary, in the production of beech-spruce panels, the three adhesives showed similar results. The press system was not a relevant factor in terms of bonding quality.In regard to testing, a size effect was noticed in the delamination test: the larger the specimen the greater the delaminations that were observed. The shear tests on dry specimens proved to be less efficient in determining the influence of the several bonding parameters, even if a 45° grain orientation seemed to reduce the rolling shear during the shear test and to improve the response of the test in this regard. Combining delamination pretreatment and shear made the test more sensitive and objective.

Effect of Silane Coupling Treatment on the Adhesion between Polyamide and Epoxy Based Composites Reinforced with Carbon Fibers

The increasing efforts aimed to design structures with reduced weight and better mechanical performances has led in recent years to a growing use of fiber reinforced polymer materials in several fields such as marine. However, these materials can be composed of chemically very different elements and, hence, may be difficult to joint. This research aims to improve the adhesion between a thermoplastic matrix of polyamide reinforced with short carbon fibers (PA12-CR) and a carbon fiber reinforced epoxy matrix (CFRP). Two different silane coupling agents, (3-Aminopropyl)trimethoxysilane (AM) and (3-Glycidyloxypropyl)trimethoxysilane (EP), were applied, through the spray deposition method, on the PA12-CR substrate to create a reactive layer between the adherents. Different deposition methods and coupling agents curing conditions were also investigated. The wettability of the PA12-CR surface as well as the chemical modifications induced by silane treatments were investigated through contact angle and Fourier Transform Infrared spectroscopy (FTIR) analyses. Furthermore, the interfacial adhesion between PA12-CR and CFRP substrates was evaluated through Mode I delamination tests (DCB). The effectiveness of the most promising treatment was finally verified on sandwich structures, having PA12-CR printed as internal core and CFRP laminates as external skins, through quasi-static three-point bending mechanical tests. Overall, the epoxy-based silane (EP) allowed significantly better resistance to the delamination up until the tensile failure of the CFRP substrate.

Blue gum: assessment of its potential for glued laminated timber beams

Portuguese forests are mainly composed of hardwoods, which represent 69% of the forest area in 2010, being Blue gum (Eucalyptus globulus Labill.) the most abundant species (26%). The suitability of Blue gum for structural applications was demonstrated in previous studies. However, the most common uses are still related to the pulp and paper industry as well as energy production, mainly due to sawing and drying issues. The present paper describes a preliminary study on the potential application of non-destructive tests and analytical methods to predict the most important mechanical properties of glued laminated timber (glulam) beams made of Blue gum. The potential of mixed beams made of Blue gum and poplar was also analysed. The longitudinal vibration method and the transformed section method were considered. A total of 11 non-finger jointed full-scale glulam beams (7 of Blue gum and 4 mixed) were manufactured in the laboratory and tested for modulus of elasticity and bending strength assessment. After the bending tests, the density and moisture content of the beams were determined, and the bonding performance was checked by delamination and shear strength tests. Correlation coefficients were established between the predicted values and the experimentally assessed mechanical properties, indicating a huge potential of the predictive methods used. The mechanical properties of non-finger jointed beams tested were above the typical values found in the literature for the most common hardwoods available in European forest.

The coating delamination mitigation of epoxy coatings by inhibiting the hydrogen evolution reaction

The mitigation of coating delamination is one of the important challenges for epoxy coatings under the cathodic protection. Herein we report the possibility to mitigate the coating delamination of epoxy coatings by inhibiting the hydrogen evolution reaction in the presence of thioureas. The wire beam electrode method, electrochemical impedance spectroscopy and traditional delamination test were employed to evaluate the mitigation efficiency in this work. Results show that in the absence of thioureas, the delamination rate increases significantly once the cathodic protection potential is more negative than hydrogen evolution potential, suggesting that hydrogen evolution reaction plays an important role in the coating delamination. Fortunately, the coating delamination is remarkably mitigated by adding thioureas since the hydrogen evolution reaction is suppressed. WBE results show that when delamination commences, the electrodes at the defect region act as anodes, while the electrodes around the defect region act as cathodes. In the presence of thioureas the delamination area is much less than that in the absence of thioureas. Therefore, the coating delamination of epoxy coatings under the cathodic protection could be successfully suppressed through inhibiting the hydrogen evolution reaction. The finding of this work could offer a new insight to reduce the cathodic delamination rate in both theoretical research and practical application engineering applications.

Explorations of the adhesion and anti-corrosion properties of the epoxy coating on the carbon steel surface modified by Eu2O3 nanostructured film

In the present study, the epoxy coating adhesion and adsorption on the carbon steel surface treated by a high free energy nano-porous film based on europium (Eu) oxide were estimated using different analyses. Furthermore, the Eu2O3 film impact on the steel@Eu-oxide/epoxy anti-corrosion properties, performance, and cathodic disbondment resistance was investigated. By atomic force microscopy (AFM) analysis, the steel@Eu-oxide roughness, and through the FE-SEM method, the deposited film homogeneity/uniformity degrees were examined. The uniform coverage of the steel surface by a nanostructured Eu-oxide film led to the free energy (γs) improvement up to 30%. The EIS outcomes indicated a significant increase (about 57%) in the scratched coating corrosion resistance against saline attack in the case of the Eu-treated sample. The cathodic delamination test results displayed about 58% improvement of the epoxy resistance against cathodic disbondment. The simulation outcomes strongly affirmed the strengthened coating dry/wet adsorption on the europium oxide adsorbents compared with iron oxides.