Double Cantilever Beam Fracture Test Research Articles

Multiscale experimental characterisation of mode-I interfacial fracture behaviour of vitrimer composites

Vitrimer composites with bond- exchange reactions offer significant advantages in terms of repair and recyclability. The mechanical properties of such composites can be designed by modifying the epoxy/anhydride ratio, thus allowing their interface properties to be customised. In this study, the mode-I interfacial fracture properties of vitrimer carbon fibre composites with different epoxy/anhydride ratios are investigated experimentally and numerically. The results show that composites with an epoxy/anhydride group ratio of 1:0.5 exhibit the highest fracture toughness, with a maximum value of 1.6 mJ/mm2. Meanwhile, the analysis of the composites’ microscopic fracture morphology indicates their good bonding performance with carbon fibres. Subsequently, a multiscale numerical approach is developed to simulate the mode-I fracture response of composite laminates and establish a trilinear cohesive model to simulate the macroscopic interfacial fracture behaviour. In this approach, a matrix elastic–plastic constitutive model and the J-integral method are applied to obtain the stress distribution in the fibre-bridging phase via a double cantilever beam fracture test. The macroscopic fracture behaviour obtained via simulation is consistent with the experimental results. Therefore, the proposed multiscale numerical simulation method, which correlates the matrix properties with macroscopic fracture parameters, allows the interlaminar properties of composites with novel matrices to be evaluated effectively.

Interlayer fusion bonding of semi-crystalline polymer composites in extrusion deposition additive manufacturing

This work focuses on the evolution of interlayer fracture toughness properties of fiber-reinforced, semi-crystalline polymers in the extrusion deposition additive manufacturing (EDAM) process. Further, this work bridges the gap between the additive process conditions (time-temperature history) and the effective layer-to-layer fracture properties developed within a printed component. This is the first step to predict delamination that can occur during printing, during cooling to room temperature after printing, and during service performance of an additively manufactured geometry. A phenomenological model is developed for fusion bonding of semi-crystalline polymer matrix composites by coupling the interdiffusion of polymer chains with the evolution of polymer crystallinity. While the interdiffusion is captured by reptation theory of polymer dynamics, the evolution of crystallinity is modeled by phenomenological crystallization kinetics and crystal melting dynamics. Further, a methodology is developed to determine the critical strain energy release rate, GICof the interlayer interface and experiments are conducted utilizing the double cantilever beam fracture test geometry. Predictions of GIC as a function of thermal history are compared with experiments.

Temperature and contact pressure effects on the adhesion development of high performance thermoplastic composites

Thermoplastic composites offer new manufacturing prospects, thanks to the ability to melt the matrix. Welding, tape placement, 3D printing, overmoulding, or even stamping involve adhesion of the thermoplastic polymer at high temperature. First, under heat and pressure, contact at the microscopic scale is ensured by the deformation of surface roughness, this is the intimate contact step. Then, the development of the mechanical strength of the assembly is controlled by the diffusion of macromolecules at the interface which is defined as the healing step. Nowadays, continuous manufacturing processes tend to be faster and present very short residence time which could limit the adhesion development. A good understanding of these mechanisms is therefore very important to control and predict such industrial processes. Welding tests at different temperatures and contact pressures were carried out over a large range of residence times using a controlled welding bench enabling very short welding times (down to 1 second). The mechanical adhesion between PEKK-carbon composite samples was characterized using double cantilever beam fracture tests. Adhesion was found to develop in two steps which could be described as an intimate contact-healing coupled step and a pure healing step. From this, the healing kinetics was identified and an empirical model was developed to account for the effect of pressure on adhesion build-up. This model could then be compared with existing models to describe the establishment of intimate contact between the coupons.

Using an embedded Buckypaper to monitor mode I crack growth in bonded joints

ABSTRACT Structural Health Monitoring (SHM) systems are commonly integrated in bonded joints for different applications in order to detect and predict potential fatigue and damages caused by internal and external agents they are subjected to during their operation life. However, each of the current SHM systems presents some drawbacks, as the reduction of structural properties or the difficulty in its implementation. This work deals with the use of a Carbon Nanotube film (Bukypaper) as a sensor in double cantilever beam (DCB) bonded joint specimens for crack growth monitoring purposes. Buckypaper (BP) sheets were manufactured and integrated in adhesive film layers and its integration quality was studied by Scanning Electron Microscopy (SEM). The effect of the integration of different thickness BPs in the mechanical performance of the bonded joints was also assessed performing DCB fracture tests. Also, electro-mechanical tests were conducted using the BP as the sensor, monitoring the output in the electrical resistance during the crack propagation. It was observed the capability of BP to detect and locate the damage during the test with a linear dependence between the electrical resistance measured in the BP and the deduced crack growth, proving the potential of the BP to monitor self-sensing bonded joints.

Adhesion of High Temperature Thermoplastic Composites

Thermoplastic composites offer new manufacturing prospects, thanks to the ability to melt the matrix. Welding, tape placement, 3D printing, overmoulding, or even stamping involve adhesion of the thermoplastic polymer at high temperature. One of the leading mechanism that enable to build up strength of adherent over the substrate is called healing. It is classically described as the diffusion of the macromolecular chains across the interface that enable bridging. In continuous processes, such as some which are used in advanced manufacturing of aerospace structures, cycle times are very short. Thus, it is unclear how suitable these materials are for fast continuous processes – the limiting factor being the time required to gain adhesion. To model and predict such processes, a controlled welding bench was designed enabling very short welding times (~5s). The mechanical adhesion between coupons was assessed by fracture of the welded interface via double cantilever beam fracture test. The welding kinetics were observed for the initial stages of healing with a behavior that should fit the t1/2trend. From this, it was possible to compare the accuracy and repeatability of interface healing at short time scales to that of samples which were closer to fully welded. Concurrently, the reptation of the polymer melt was assessed on a commercial rheometer, by computing relaxation time from the viscoelastic response. The correlation confirms that the mechanical quality is associated with the macromolecule diffusion. It was also observed that for high temperature thermoplastics, reticulation occurs, which inhibits molecular diffusion and increases the relaxation time. The major implication is that the preprocessing of the polymer will impact the subsequent adhesion step. For example, plates processed with an autoclave for a couple of hours would require a welding time an order of magnitude longer than the adhesion of a virgin prepreg in the tape placement process.

Surface Structure and Stability of Partially Hydroxylated Silica Surfaces.

Surface energies of silicates influence crack propagation during brittle fracture and decrease with surface relaxation caused by annealing and hydroxylation. Molecular-level simulations are particularly suited for the investigation of surface processes. In this work, classical MD simulations of silica surfaces are performed with two force fields (ClayFF and ReaxFF) to investigate the effect of force field reactivity on surface structure and energy as a function of surface hydroxylation. An unhydroxylated fracture surface energy of 5.1 J/m2 is calculated with the ClayFF force field, and 2.0 J/m2 is calculated for the ReaxFF force field. The ClayFF surface energies are consistent with the experimental results from double cantilever beam fracture tests (4.5 J/m2), whereas ReaxFF underestimated these surface energies. Surface relaxation via annealing and hydroxylation was performed by creating a low-energy equilibrium surface. Annealing condensed neighboring siloxane bonds increased the surface connectivity, and decreased the surface energies by 0.2 J/m2 for ClayFF and 0.8 J/m2 for ReaxFF. Posthydroxylation surface energies decreased further to 4.6 J/m2 with the ClayFF force field and to 0.2 J/m2 with the ReaxFF force field. Experimental equilibrium surface energies are ∼0.35 J/m2, consistent with the ReaxFF force field. Although neither force field was capable of replicating both the fracture and equilibrium surface energies reported from experiment, each was consistent with one of these conditions. Therefore, future computational investigations that rely on accurate surface energy values should consider the surface state of the system and select the appropriate force field.

Thermoplastic/thermoset multilayer composites: A way to improve the impact damage tolerance of thermosetting resin matrix composites

We present a method to improve the impact damage tolerance of thermoset resin matrix composites by exploiting thermoplastic material impact resistance properties. Two configurations of thermoplastic/thermoset multilayer composites are considered: (a) thermoplastic material is used on sample surface as shock absorber and (b) thermoplastic film is inserted in between thermoset layers to act as interlayer. We aim at increasing adhesion strength between thermoplastic and thermoset resin matrix. We show that mode I critical strain energy release rate, determined by the wedge double cantilever beam fracture test, increases significantly when a third amorphous polymer interlayer compatible with both thermoplastic and thermoset resin is inserted. The analysis of scanning electron microscope fracture surfaces helps us to clarify the adhesion mechanism involved. The high adhesion between thermoplastic and thermoset resins obtained with this method, together with the ability of thermoplastics to dissipate the impact damage in the first plies ensure the reliability of this stacked configuration. Finally, we show that these multilayer composite significantly increases the impact resistance of composite materials.

Interface debonding characterization by image correlation integrated with Double Cantilever Beam kinematics

A procedure is proposed for the identification of spatial interfacial traction profiles of peel loaded Double Cantilever Beam (DCB) samples, from which the corresponding traction–separation relation is extracted. The procedure draws upon recent developments in the area of non-contact optical techniques and makes use of so-called Integrated Digital Image Correlation (I-DIC) concepts. The distinctive feature of the I-DIC approach proposed herein is that the unknown degrees of freedom are not displacements or rotations, but the set of interfacial fracture properties describing the traction profile. A closed-form theoretical model is developed to reconstruct a mechanically admissible displacement field representing the deformation of the adhering layers during debonding in the DCB fracture test. The proposed modeling accounts for the spatial traction profile along the interface between the adherends using few degrees of freedom, i.e. crack tip position, maximum stress and size of the process zone. By minimizing the correlation residual with respect to the degrees of freedom, the full set of interfacial fracture properties is obtained through a one-step algorithm, revealing a substantial gain in terms of computational efficiency and robustness. It is shown that the identified traction profile can be effectively combined with the crack opening displacement to extract the corresponding traction–separation relation, i.e. the key input data for any cohesive zone model (CZM). The proposed procedure is validated by post-processing virtually deformed images generated through the finite element method. The robustness with respect to noisy data, as well as the low sensitivity to the initial guess, are demonstrated.

Multiscale toughness amplification in natural composites

ABSTRACTHard biological materials such as bone and nacre exhibit remarkable mechanical performance, particularly in terms of fracture toughness, despite the weakness of their constituents. Mechanical performance of nacre and bone can largely be explained through their staggered microstructure where stiff inclusions of high aspect ratio are embedded in a softer matrix. The mineral inclusions provide hardness and stiffness while the organic matrix introduces ductility. The high performance of these natural structures is unmatched by any synthetic ceramic, which therefore makes them a substantial source of inspiration for development of new artificial materials. While the modulus and strength of these structures are well understood, fracture toughness remains unclear and controversial. In this work, chevron double cantilever beam fracture tests show that the interfaces in nacre have a low toughness, comparable to that of the tablets (in J terms). This highlights the important role of structural design on fracture toughness. At the next step, a fracture model is presented to explain the toughness amplification observed in natural staggered structures based on two essential extrinsic toughening mechanisms: crack bridging and process zone. The modeling results show that toughness can be further amplified by incorporating high concentrations of small inclusions with high aspect ratio. This conclusion is applicable to construction and optimization of natural and biomimetic composites.

Fracture test for double cantilever beam of honeycomb sandwich panels

A modified double cantilever beam (DCB) test geometry is designed to investigate the fracture behavior of honeycomb sandwich panels containing embedded artificial pre-crack, and measure the strain energy release rate of the laminate facesheet/honeycomb cores interface. However, in terms of our DCB fracture test, owing to that the pre-crack does not propagate expectedly along the interface of facesheet/honeycomb core, a new fracture mode, namely IKP (initiation of interlaminar delamination, kinking into facesheet and propagation of interlaminar delamination), has been found.

Mode I fracture toughness estimation of wood by DCB test

Abstract The mode I fracture toughness of wood obtained by the double cantilever beam (DCB) test was analyzed by six conventional analysis methods: two methods based on elementary beam theory, Williams’s correction method, modified beam theory method, and two compliance calibration methods. In addition to these methods, the compliance combination method, which was originally proposed by the author for obtaining the mode II fracture toughness by the end notched flexure (ENF) test, was applied to the DCB test. In the compliance combination method, the strain at a certain point of the cracked portion of the specimen was measured, as was the loading-line displacement and critical load for crack propagation. Similarly to the mode II fracture toughness analysis for wood by the ENF test, the compliance combination method is more advantageous in practice than the other ones examined here in that the fracture toughness can be determined by the DCB fracture test alone without requiring separate tests, which should be conducted in the modified beam theory and compliance calibration methods.

Changes in the interfacial properties of PC/SAN blends with compatibilizer

AbstractBlock copolymers of polycarbonate‐b‐poly(methyl methacrylate) (PC‐b‐PMMA) and tetramethyl poly(carbonate)‐b‐poly(methyl methacrylate) (TMPC‐b‐PMMA) were examined as compatibilizers for blends of polycarbonate (PC) with styrene‐co‐acrylonitrile (SAN) copolymer. To explore the effects of block copolymers on the compatibility of PC/SAN blends, the average diameter of the dispersed particles in the blend was measured with an image analyzer, and the interfacial properties of the blends were analyzed with an imbedded fiber retraction (IFR) technique and an asymmetric double cantilever beam fracture test. The average diameter of dispersed particles and interfacial tension of the PC/SAN blends were reduced by adding compatibilizer to the PC/SAN blends. Fracture toughness of the blends was also improved by enhancing interfacial adhesion with compatibilizer. TMPC‐b‐PMMA copolymer was more effective than PC‐b‐PMMA copolymer as a compatibilizer for the PC/SAN blends. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2649–2656, 2003

The effects of PC‐PMMA block copolymer on the compatibility and interfacial properties of PC/SAN blends

AbstractBlock copolymers of polycarbonate (PC) and polymethylmethacrylate (PMMA), PCb‐PMMA, were examined as compatibilizers for blends of PC with styrene‐co‐acrylonitrile (SAN) copolymer. PC‐b‐PMMA was added to blends of PC with SAN containing various amounts of AN. The average diameter of the dispersed particles was measured with an image analyzer, and the interfacial properties of the blends were analyzed with an imbedded fiber retraction (IFR) test and an asymmetric double cantilever beam fracture test. The average particle size and interfacial tension of the PC/SAN blends reached a minimum value when the SAN copolymer contained about 24 wt% AN. A maximum in the adhesion energy was also observed at the same AN content. Interfacial tension and particle size were further reduced by adding PC‐b‐PMMA to the PC/SAN blends. Fracture toughness of the blends was also improved by enhancing the interfacial adhesion by the addition of PC‐b‐PMMA. The addition of PC‐b‐PMMA copolymer was more effective at improving the interfacial properties of PC/SAN blends than was varying the AN content of the SAN copolymers. The interfacial properties of the PC/SAN blends were optimized by adding a block copolymer and using an SAN copolymer that had minimum interaction energy with PC.

Effects of reactive reinforced interface on the morphology and tensile properties of amorphous polyamide-SAN blends

The effects of reactive reinforced interface on the morphology and tensile properties of amorphous polyamide (a-PA) and styrene-acrylonitrile (SAN) copolymer blend have been investigated using styrene maleic anhydride (SMA) copolymer as a reactive compatibilizer. The anhydride groups of SMA copolymer can react with the amine groups of polyamide and form in situ graft copolymers at the a-PA–SAN interfaces during the blend preparation. The interfacial adhesion strength of the reactive reinforced interface was evaluated quantitatively using an asymmetric double cantilever beam fracture test as a function of SMA copolymer content using a model adhesive joint. The interfacial adhesion strength was found to increase with the content of SMA copolymer and then level off. The morphological observations of a-PA–SAN (80/20 w/w) blends showed that the finer dispersion of the SAN domains with rather narrow distribution was obtained by the addition of SMA copolymer into the blends. The trend of morphology change was not in accord with that of the interfacial adhesion strength with respect to the content of SMA copolymer. However, the results of tensile properties showed very similar behavior to the case of the interfacial adhesion strength with respect to SMA content; that is, there was an optimum level of the reactive compatibilizer beyond which the interfacial adhesion strength and tensile strength did not change significantly. These results clearly reveal that tensile properties of polymer blend are highly dependent on the interfacial adhesion strength. Furthermore, it is suggested that the asymmetric double cantilever beam fracture test using a model interface is a useful method to quantify the adhesion strength between the phases in real polymer blends. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1925–1933, 1998

Mechanical effects according to the type of poly(styrene- co-methyl methacrylate) copolymers at polystyrene/poly(methyl methacrylate) interfaces

The effects of incorporating a thin layer of various types of polystyrene-poly(methyl methacrylate) copolymers, i.e. block, random, and graft copolymer on the interfacial adhesion between PS and PMMA have been investigated. The fracture toughness of the interface was measured using an asymmetric double cantilever beam fracture test and the enhanced toughness effect of copolymers was compared. The fracture toughness of the interface increased with increasing layer thickness of the copolymer and each copolymer had the layer thickness of saturated fracture toughness. The fracture toughness increased in the order of block > graft > random copolymer.

Effect of ABC Triblock Copolymers with an Elastomeric Midblock on the Adhesion between Immiscible Polymers

The effect of polystyrene-block-poly(1,2-butadiene)-block-poly(methyl methacrylate) triblock copolymers (SBM) and their hydrogenated analogs polystyrene-block-poly(ethylene-co-butylene)-block-poly(methyl methacrylate) (SEBM) on the fracture toughness of the interface between immiscible polymers has been studied by using an asymmetric double cantilever beam fracture test. The fracture energy GI has been investigated as a function of the copolymer interfacial areal density Σ and elastomer content as well as the temperature at which the samples were fractured. No reinforcement of the interface was found for a wide range of Σ for all investigated triblock systems at room temperature. In fact, for a range of Σ values the triblock was found to weaken, rather than strengthen, the interface. In addition, it was found that mixing just 25% of triblock into polystyrene-block-poly(methyl methacrylate) diblock copolymer was sufficient to remove the interfacial toughening effect of the diblock. The triblock copolymers were found to toughen, rather than weaken, the interface when the sample was fractured at a temperature below the glass transition temperature of the elastomer. These results are in agreement with a model that is based on the assumption that it is impossible to transfer enough stress across a plane glass−rubber interface to initiate crazing in the glassy polymers.

Effect of block copolymer dopants and alkylsilane monolayers on the fracture toughness of polystyrene-glass interfaces

The asymmetric double cantilever beam fracture test has been used to study the fracture toughness of polystyrene (PS)-glass interfaces reinforced with poly(deuterostyrene-b-2 vinyl pyridine) (dPS-PVP) as a function of degree of polymerization of the blocks. The effect of modifying the glass substrate with various selfassembled monolayers is also described. For the block copolymer with degrees of polymerization,NdPS=656 andNPVP=46 (referred to asNdPS —NPVP or 656-46), located at the interface between glass and PS, the interface fails by chain scission at areal chain densities, Σ, of the block copolymer below a critical value, Σ*. Above this value, e.g. Σ > Σ*, the interface fails by crazing followed by chain scission. For the 656-46 diblock copolymer, the transition is located at Σ*=0.03 chains nm−2, which results in a calculated force to break a C-C bond along the polymer backbone of approximately 2 × 10−9N. For the 800–870 diblock copolymer at the interface between glass and PS, failure occurs due to chain scission. Fracture of both the 656-46 and the 800–870 block copolymers at the interface between (OTS) octadecyltrichlorosilane monolayer coated glass and PS is due to chain pulloff of the block copolymer from the OTS coated glass. Very little additional stress was transferred across the interface, resulting in fracture toughnesses comparable to that of a PS-glass interface with no block copolymer added.

Effect of End-Anchored Chains on the Adhesion at a Thermoset-Thermoplastic Interface

The asymmetric double cantilever beam fracture test was used to measure the critical energy release rate, or fracture toughness, G, of an epoxy-polystyrene (PS) interface as a function of the grafting density, S, and degree of polymerization, N, of carboxylic acid terminated deuterated polystyrene chains (dPS-COOH), The chain ends anchor to the epoxy, and their tails penetrate into the PS homopolymer. Forward recoil spectrometry on the fracture surfaces provided a method to determine the total S as well as the mechanism of interface failure. For N = 159 the grafted chains were too short to entangle and pull out of the PS, leading to no enhancement in G over that of a bare interface. When the chain length was increased to N = 412 and 535, there was sufficient stress transfer to initiate crazes in the PS that break down by either disentanglement or scission of DPS-COOH chains in the craze fibrils. For chains of N = 688, 838, and 1478 the chains broke near the epoxy-PS interface at low S before significant crazing of the PS occurs. A transition from chain scission to crazing occurred at S{sub c} = 0.03 chains/nm{sup 2} for all three chain lengths, agreeing withmore » experiments on diblock copolymer modified thermoplastic-thermoplastic interfaces and with the prediction from the fracture mechanism map that the transition is independent of chain length. The authors observed a nearly linear decrease in the maximum achievable S as N increased that can be explained in part by an entropic barrier that opposes the addition of new chains to the grafted brush. The toughest interfaces occurred with intermediate length grafted chains, N = 838, when the chains were well entangled and the grafted brush was dense enough, S > S{sub c}, to cause energy dissipation through craze formation.« less

Graphite/polyimide composites with improved toughness

AbstractStudies were performed to determine the toughness characteristics of composites prepared from modified addition‐type polyimides, using Celion 6000 graphite fiber as the reinforcement. The polyimides were prepared from aromatic diamines containing flexibilizing ether connecting groups. The composite flexural and short beam shear strengths were determined at room temperature and elevated temperatures. Composite toughness was evaluated using 10° off axis tensile tests and double cantilever beam fracture tests at room temperature. The effects of the flexibilized resin structure on composite mechanical properties, toughness characteristics, and thermo‐oxidative stability are discussed.

A Fracture Mechanics Method for Determining the Crack Propagation Resistance of Embrittled Aluminum Bicrystals

Abstract A fracture mechanics testing method has been adapted to the study of the crack propagation resistance of high purity aluminum bicrystals which fail under liquid metal embrittlement conditions. The tapered double cantilever beam fracture test makes it possible to determine the threshold crack extension force for a propagating crack and the crack velocity as a function of driving force above this threshold. Modifications of this test have been made so that it can be used to determine GILME and crack velocity for 99.99+% pure aluminum bicrystals in a Hg-3 a/o Ga atmosphere. The test is shown to be reproducible, and variations in GILME due to differences in crystallographic orientation of the bicrystals are attainable.