Crack Initiation Energy Research Articles

The mix-mode fracture behaviour of aerospace composite joints co-cured by different forms of advanced thermoplastics

The co-cure joining of thermoset composites by thermoplastics is an advanced approach to develop robust composite joints. Herein, Polyethylenimine (PEI) and Polyetheretherketone (PEEK) polymers in a form of film, woven mesh, hollow mesh and carbon fibre reinforced plastic were utilised as interlayers for the co-cure joining of an aerospace composite. A cracked-lap shear test was used to investigate the fracture toughness and mechanisms of the co-cured composite joints under a mixed mode-I/II loading condition. The experimental results revealed that the insert of PEI and PEEK interlayers significantly enhanced the mix-mode crack resistance of the composite joints by different levels. The most remarkable increase in the crack initiation energy and crack propagation energy was 161% and 613%, respectively, that were obtained by adding the PEEK film interlayers. The main toughening mechanisms of the PEI film joined composites were crack-pining and matrix toughening of the PEI particles, which were formed by the diffusion of the PEI resins into the composite epoxy matrix during the curing process. Plastic deformation and fracture of the PEEK resins were the main toughening mechanisms of the composites that were co-cure joined by the PEEK films, woven meshes, hollow meshes and carbon fibre reinforced tapes. The crack resistance of the composite joints correlated well with the level of the PEEK fracture phenomena during the mix-mode fracture process.

Fracture Behavior of AlMg4.5Mn Weld Metal at Different Temperatures under Impact Loading

This paper deals with a three-component aluminum alloy AlMg4.5Mn that was welded using a GTAW process in the shielded atmosphere of Ar+70%He+0.015%N2. The weld-metal toughness was evaluated at three different temperatures using instrumental Charpy pendulum impact testing to measure not only the total energy, but also the crack initiation energy and the crack growth energy. Fractographic analysis of the fracture surfaces and EDS analysis of large second-phase particles on fractured surfaces at each temperature were also carried out. Fractographic analysis at different temperatures indicated a clearly distinguishable fracture mechanism. It was inferred that the absorbed energy was closely correlated with the fracturing of surfaces. Moreover, it was concluded that with decrease in the amount of microscopic voids and dimples, the total energy absorbed also decreased.

Swelling induced debonding of thin hydrogel films grafted on silicon substrates.

We report on the delamination of thin (≈μm) hydrogel films grafted to silicon substrates under the action of swelling stresses. Poly(dimetylacrylamide) (PDMA) films are synthesized by simultaneously cross-linking and grafting preformed polymer chains onto the silicon substrate using a thiol-ene reaction. The grafting density at the film/substrate interface is tuned by varying the surface density of reactive thiol-silane groups on the silicon substrate. Delamination of the films from well controlled line defects with low adhesion is monitored under a humid water vapor flow ensuring full saturation of the polymer network. A propagating delamination of the film is observed under the action of differential swelling stresses at the debonding front. A threshold thickness for the onset of this delamination is evidenced which is increasing with grafting density while the debonding velocity is also observed to decrease with an increase in grafting density. These observations are discussed within the framework of a nonlinear fracture mechanics model which assumes that the driving force for crack propagation is the difference between the swelling state of the bonded and delaminated parts of the film. Using this model, the threshold energy for crack initiation was determined from the measured threshold thickness and discussed in relation to the surface density of reactive thiol groups on the substrate.

Twinning-induced high impact toughness of titanium alloy at cryogenic temperature

The impact toughness of titanium alloys is one of the key factors determining their safe service in cryogenic temperature environments. In this work, the temperature-related impact properties of grade 2 pure titanium and Ti-2.5Al–3Zr–1Mo alloys are investigated. It is found that the impact properties and deformation mechanisms of the above two materials are different with decreasing temperature (20 °C, 0 °C, −50 °C, −100 °C and −196 °C). Speaking of pure titanium: the impact toughness doesn't change obviously, and crack initiation energy (Wi) gradually increases while crack propagation energy (Wp) decreases (Wp of −196 °C is 40% lower than that of 20 °C while Wi is 90% higher). The slip-dominated deformation mechanism at 20 °C gradually transforms into twinning-dominated deformation mechanism under cryogenic temperature. High-density twins formed at −100 °C and −196 °C can effectively relieve stress concentration and lead to better cryogenic impact toughness. By contrast, Wi and Wp of Ti-2.5Al–3Zr–1Mo alloy decrease with decreasing temperature. The impact toughness of Ti-2.5Al–3Zr–1Mo alloy is higher than that of pure titanium when the temperature is above −100 °C. At 20 °C, the synergistic effect of the tortuous crack path and deformation twins of Ti-2.5Al–3Zr–1Mo alloy promotes its impact toughness higher than that of pure titanium. However, the twinning density of Ti-2.5Al–3Zr–1Mo at −196 °C is much lower than that of pure titanium, which cannot release stress effectively and cause a brittle fracture. The research provides theoretical and experimental basis for the application of titanium alloys in cryogenic environment, and provides guidance for the development of titanium alloys with better cryogenic impact toughness.

Influence of microstructure on crack initiation and propagation behavior in swaged tungsten heavy alloy during Charpy impact process

In this study, microstructure evolution and mechanical properties of 90W–7Ni–3Fe alloy under different swaging levels were systematically examined, with an emphasis on the relationship between the deformed microstructure and impact failure behavior. The deformed microstructure was characterized by scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM), and the impact toughness of specimens were tested using an instrumental Charpy impact machine. Microstructure observations showed that swaging significantly changed shape of W grains and increased the W contiguity. With increasing of deformation levels, microstructure changed from high density dislocations to subgrain structures. Furthermore, pronounced <101> texture is also found in the W phase at the highest deformation level. With increasing of deformation level, the tensile strength sharply increased from 950 to 1313 MPa and the tensile strain decreased from 27% to 9%. The impact test results indicate that the total impact absorbed energy consists of crack initiation energy and crack propagation energy. As deformation level increases, crack initiation energy decreased from 81 J to 23 J, while crack propagation energy first increased to 23 J and then decreased to 10 J. The decrease in crack initiation energy is mainly caused by the increase in dislocation density and W contiguity. The variation of crack propagation energy is relatively complex and influenced by several factors, such as W contiguity, the ability of the matrix phase to resist crack extension, the increased W cleavage event, and so on. Our study provides a deep insight into the relationships between microstructure and impact toughness of swaged tungsten heavy alloys.

Prediction of the peak load and crack initiation energy of dynamic brittle fracture in X70 steel pipes using an improved artificial neural network and extended Finite Element Method

In this paper, a robust technique is presented to predict the peak load and crack initiation energy of dynamic brittle fracture in X70 steel pipes using an improved artificial neural network (IANN). The main objective is to investigate the behaviour of API X70 steel based on two experimental tests, namely Drop Weight Tear Test (DWTT) and the Charpy V-notch impact (CVN), for steel pipe specimens. The mechanical properties in the brittle fracture behaviour of API X70 steel pipes are predicted utilizing numerical approaches with different crack lengths. Next, to simulate the impact of API X70 steel pipes at lower temperatures through a numerical approach, a cohesive approach using the extended Finite Element Method (XFEM) is used. The data obtained are used as input for the proposed IANN using Balancing Composite Motion Optimization (BCMO), Particle Swarm Optimization (PSO) and Jaya optimization algorithms, to predict the peak load values and crack initiation energy of dynamic brittle fractures in API X70 steel with different crack lengths. The results show the effectiveness of ANN-PSO and ANN-BCMO based on the convergence of the results and the accuracy of the prediction of the peak load and crack initiation energy. Note that, the source codes are publicly available at https://github.com/Samir-Khatir/JAYA-ANN.git.

Revealing impact failure behavior of tungsten composite using instrumented Charpy impact testing

The impact toughness of W-Ni-Fe alloys with different W content (90, 93 and 95 wt%) were studied by means of instrumented Charpy impact testing, and the relationship between impact toughness and microstructural parameters was discussed, with an emphasis on impact failure behavior. With increasing of W content, the elongation decreases from 31% for 90 W to 21% for 95 W, but the tensile strength slightly increases from 940 to 974 MPa and the yield strength increases from 655 to 688 MPa. The impact load-displacement curves reveal that the impact toughness is predominantly controlled by the crack initiation energy and decreased with increasing of W content, whereas the crack propagation energy is relatively low and remains constant. The decrease in the crack initiation energy attributed to a decrease in matrix volume fraction and an increase in W contiguity. 90 W shows the highest crack initiation energy due to the severe plastic deformation during crack initiation process, which contains multiple deformation characteristics such as dislocation tangles, dislocation forest, stacking faults, nano-scaled twins and subgrains. The formation of those features is discussed, and it is found that they are expected to provide a toughening effect. From the perspective of crack nucleation and growth, it is also found that the high crack initiation energy of 90 W profited from the high energy-consuming processes of W cleavage events, micro-crack arrest, blunting and bridging mechanisms. Although these three alloys display different failure features of crack propagation paths, they present similar crack propagation energies. The current study provides a deep insight into the impact failure behavior of WHAs.

Investigation on microstructure and impact toughness of simulated heat affected zone of high strength low alloy steels by laser-arc hybrid welding

The microstructure and impact toughness of the simulated heat affected zone (HAZ) of a high strength low alloy steels by laser-arc hybrid welding (LAHW) were investigated in this paper. Homogeneous simulated HAZ specimens with different grain sizes were prepared using the welding thermal simulating technique. Instrumented impact test was conducted to investigate the toughness of the simulated HAZ. Multi-scale sub-structure characterization of the simulated HAZ specimens was implemented by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD). The instrumented impact test found that the peak temperature T M mainly affected the crack propagation process, the crack propagation energy E p decreased as the increase of T M , while the crack initiation energy E i barely changed, the decrease of crack stable energy E a led to the change of E p . The multi-scale sub-structure characterization showed that the prior austenite grain size (PAGS), packet width, block width both increased with the rise of T M . The block width was similar to the facet size in the crack unstable propagation zone of the simulated HAZ specimens. The block was the microstructure unit controlling the crack propagation process of the LAHW simulated HAZ specimens.

Effect of Cu Content on Microstructure and Mechanical Properties for High-Strength Deposited Metals Strengthened by Nano-Precipitation

With the rapid development of low-carbon high strength steel, higher requirements are put forward for the matching welding consumables. The deposited metals with 0.62–2.32% Cu addition was prepared by tungsten inert gas welding via metal cored wire. The effect of Cu element on microstructure and mechanical properties of deposited metals were investigated. The multiphase microstructure of deposited metals consists of bainite, martensite, residual austenite, and martensite-austenite constituents. It is found that Cu decreases the start temperature of martensite (Ms) and enlarges the temperature range of bainite from 372 K to 416 K, improving the formation of bainite. With the increase of Cu content, the fraction of martensite decreases and the shape of M-A constituents changes from strip into granular. There are BCC and FCC Cu precipitates in deposited metals. The diameter of Cu precipitates is 14–28 nm, and the volume fraction of it increases with the increase of Cu content. Meanwhile, the deposited metals with 1.79% Cu can achieve a 10% enhancement in strength (yield strength, 873–961 MPa, ultimate tensile strength, 1173–1286 MPa) at little expense of impact toughness (64.56–56.39 J at −20 °C). Cu precipitation can effectively improve the strength of the deposited metals, but it degrades toughness because of lower crack initiation energy. The deposited metal with 1.79% Cu addition shows an excellent strength-toughness balance.

Scale-up fabrication of gradient AGS in austenitic stainless steels achieves a simultaneous increase in strength and toughness

Simultaneous strengthening and toughening of austenitic stainless steel are of great importance in safety-critical applications. Gradient structured steels have thus attracted significant interest as they can achieve unprecedented strength and toughness synergy. However, the traditional fabrication process of gradient metals is relatively complex and limited to small depths of gradient. In this contribution, we have developed a novel processing route consisting of inhomogeneous rolling and annealing to fabricate a large gradient profile (∼5 mm) in the austenite grain size. The fabricated bulk gradient austenitic steel exhibits higher tensile strength (∼690 MPa) and Charpy impact energy (11.8 J) compared with the uniform-grain-size structural steel (∼650 MPa and 10.9 J, and ∼580 MPa and 11.3 J for fine grained (FG) and coarse grained (CG) steels). The deformation mechanisms of the bulk gradient austenitic steel were further explored. The combined contribution of the hetero-deformation induced (HDI) strengthening and transformation-induced plasticity (TRIP) effect results in the ideal work-hardening capability and tensile strength of the gradient austenitic steel. Thanks to the deep gradient, the fraction of deformation-induced martensite decreases along the fabricated gradient. The pronounced TRIP effect near the coarse-grained V-notch improves the crack initiation energy (Ei), while the less martensite formed in the fined-grained interior raises the crack propagation energy (Ep). This study provides new insights into the circumvention of the strength-toughness trade-off and the fabrication of gradient structured steels with a large gradient profile.

Influence of strength and notch shape on crack initiation and propagation behavior of advanced high strength steel sheets

Advanced high-strength steels (AHSSs) are widely adopted in car body structures to achieve weight reduction for energy efficiency and crash safety. This study investigates the effects of material strength and notch shapes on crack initiation and propagation behaviors in AHSS sheets used in car body structures. Tensile tests were conducted for three types of Japanese grade AHSS (JSC590, JSC980, and JSC1180) specimens and two notch root radii (R0.5 and R5). The crack initiation and propagation behaviors were found to strongly depend on the material strength and notch root radius. The crack generated from the notch root for the R0.5 specimens, whereas it generated from 1 to 2 mm inside the notch root for the R5 specimens. Unstable crack propagation occurred beyond the maximum load for the R5 and JSC1180 specimens, whereas only stable crack propagation occurred for the R0.5 specimens. From the digital image correlation analyses, we found that the specimen notch radius affects the stress triaxiality and equivalent plastic strain around the notch root. The JSC980 material had the highest crack initiation energy, which correlates with the plastic dissipation energy utilized for thickness reduction. The JSC590 material exhibited the highest crack propagation energy, which correlates with the true fracture strain.

Investigating dynamic fracture in marble-mortar interface under impact loading

• Compared with the M-S-750 series specimens, the transgranular phenomenon is more common in the M-R-750 series specimens. • The crack propagation velocity in the BSCT specimen is lower than the speed of Rayleigh wave in the M40 mortar. • With the increase of loading rate, the crack propagation speed between CPG adjacent wires does not necessarily increase. • The discrepancy of wave impedance between marble and mortar results in the existence of shear stress in the interface zone of the BSCT specimen. • Compared with the BSCT specimens with smooth interfaces, the fracture properties ( K 1 and G ) of the BSCT specimens with rough interfaces are improved significantly. The mechanical property of the mortar-rock interface is a key factor in dominating the effectiveness of many reinforcement measures in rock mass engineering. Investigating the mortar-rock interface is of important practical consequences. In this paper, a bi-material single cleavage triangle (BSCT) configuration, composed of marble and mortar, was employed in a split Hopkinson pressure bar (SHPB) experiment with interface roughness and loading rates considered. The experimental–numerical method was used to obtain the crack initiation energy release rate G and the complex stress intensity factor ( K = K 1 + i K 2 ) of the onset interface crack. Besides, AUTODYN software was applied for analyzing the propagation law of stress waves in the BSCT specimen, and the failure mechanism of the mortar-rock interface was revealed. The results show that the interface crack growth rate in the BSCT specimen is lower than the P-wave, S-wave, and Rayleigh wave in the M40 mortar. The discrepancy of wave impedance between marble and mortar results in the existence of shear stress in the interface zone of the BSCT specimen. As the loading rate increases to 920 GPa/s, the fracture properties ( K 1 and G ) of all BSCT specimens are significantly improved. The improvements in the fracture properties are more significant for the BSCT specimens with rough interfaces compared to the BSCT specimens with smooth interfaces.

Study on Intrinsic Influence Law of Specimen Size and Loading Speed on Charpy Impact Test.

Charpy impact energy/impact toughness is closely related to external factors such as specimen size. However, when the sample size is small, the linear conversion relationship between the Charpy impact energy of the sub-size and full-size Charpy specimens does not hold; the Charpy impact toughness varies with the size of the specimen and other factors. This indicates that studying the internal influence of external factors on impact energy or impact toughness is the key to accurately understanding and evaluating the toughness and brittleness of materials. In this paper, the effects of strain rate on the flow behavior and the effects of stress triaxiality on the fracture behavior of 30CrMnSiNi2A high-strength steel were investigated using quasi-static smooth bar and notched bar uniaxial tensile tests and Split Hopkinson Tensile Bar (SHTP). Based on the flow behavior and strain rate dependences of the yield behavior, a modified JC model was established to describe the flow behavior and strain rate behavior. Charpy impact tests were simulated using the modified JC model and JC failure model with the determined parameters. Reasonable agreements between the simulation and experimental results have been achieved, and the validity of the model was proved. According to the simulation results, the impact energy was divided into crack initiation energy, crack stability propagation energy and crack instability propagation energy. On this basis, the effects of striker velocity and specimen width on the energy and characteristic load of each part were studied. The results show that each part of the impact energy has a negligible dependence on the hammer velocity, but there is a significantly different positive linear relationship with the width of the sample. The energy increment of each part also showed an inverse correlation with the increase in the sample width. The findings reveal that the internal mechanism of Charpy impact toughness decreases with the increase in sample width; to a certain extent, it also reveals the internal reason why the linear transformation relationship of Charpy impact energy between sub-size specimens and standard specimens is not established when the specimens are small. The analytical method and results presented in this paper can provide a reference for the study of the dynamic behavior of high-strength steel, the relationship between material properties and sample size, and the elastic–plastic impact dynamic design.

Comparison on Impact Toughness of High-Strength Metastable β Titanium Alloy with Bimodal and Lamellar Microstructures

The impact toughness of a high-strength metastable β titanium alloy (Ti-5Cr-4Al-4Zr-3Mo-2W-0.8Fe) with two typical microstructures is studied by Charpy impact tests. The bimodal microstructure (BM) and the lamellar microstructure (LM) are obtained by the solution and aging treatments and the β annealing, slow cooling and aging treatments, respectively. In the impact crack initiation process, the deformation capacities of the primary α (αp) phase, secondary α (αs) phase and transformed β (βt) matrix in the BM are very different, and the stress gradient at the interface of the three causes the crack initiation. The lamellar α (αl) phase and βt in the LM satisfy the BOR relationship, and the effective slip transfer between α and β phases slows down the crack initiation. Meanwhile, the appearance of deformation twins in the LM improves the crack initiation energy. In the crack propagation process, the lack of coordinated deformation between the α and β phases in the BM leads to rapid crack propagation. In the LM, the deformation of αl and βt is relatively more coordinated, so the severe plastic deformation is only concentrated near the crack and at the interface. The secondary crack initiation and the crack propagation along the twin boundary reduce the stress concentration at the crack tip. The deformation twins and zigzag propagation path can improve the crack propagation energy. To summarize, the alloy with LM exhibits better impact toughness than the alloy with BM.

Significant improvement in CGHAZ toughness of HSLA steel via welding with trailing mechanical treatment

The Coarse-grained heat-affected zone (CGHAZ) is well known as the local brittle zone in the welding joints of HSLA steels. In the present work, compressive strain is applied to the specimen during the CGHAZ welding thermal simulation process to achieve microstructure refinement and modification for toughness improvement. The strain is applied to the specimen when the temperature of the simulated thermal cycle decreased to 1100 °C, with the purpose of achieving dynamic recrystallization in the CGHAZ. Parallel experiments without the compressive strain are also conducted as a reference. The microstructure of the CGHAZ with and without the compressive strain was observed by optical microscopy (OM) and scanning electron microscopy (SEM). Transmission electron microscopy (TEM) was used for observation of the matrix and microstructure of martensite-austenite (M-A) constituents. Electron back-scattered diffraction (EBSD) techniques were used to get the information of boundary misorientation distribution. The toughness of the specimens under each condition was assessed through instrumented Charpy impact test at −20 °C. Results showed that the compressive strain refined prior austenite grain (PAG) size and packet size, while the block width and lath width were almost unaffected. Refinement of the PAG also modified the microstructure of M-A constituents. The microstructure refinement and modification of M-A constituents improved crack initiation energy and crack propagation energy, resulting in significant promotion of toughness from 60.20 J to 163.10 J.

Effect of temperature and specimen orientation on Charpy impact toughness

Steel grade 14MoV6 3 is used for manufacturing of boilers and steam pipelines designed for steam temperatures up to 560°C. This paper presents the experimental analysis which included impact toughness test. The analysis resulted in impact energy values, and their respective components, crack initiation and crack propagation energy, and represents the initial stage of extensive research involving the behavior of 14MoV6 3 steel. The aim was to determine the effects of temperature (room vs. operated), as well as the effects of specimen orientation with regards to the rolling direction, on the impact toughness and the relation between its components.

Effect of tetragonal zirconia nanoparticle content on enamel strength

The fracture failure of enamel is caused by hole-type flaws and cracks generally present in enamel. In this study, the effect of tetragonal zirconia nanoparticles (nano-t-ZrO2) on enamel strength was investigated. The addition of nano-t-ZrO2 improved the surface morphology and reduced the porosity of the enamel as determined by morphological and porosity distribution analyses. The strain on the enamel surface was recorded using a strain gauge attached to the enamel surface. The crack initiation force and strain were measured using the strain gauge, and the crack propagation diagrams at various stages of the tensile process were recorded. The relationship between the crack initiation energy and nano-t-ZrO2 content was found to be non-linear. Nano-t-ZrO2 affected the strength of the enamel. Moreover, the tensile force required for the appearance of cracks first increased and then decreased with an increase in the nano-t-ZrO2 content. The addition of nano-t-ZrO2 to the enamel matrix suppressed the formation of stress concentration regions and decreased the brittleness of the enamel. At a nano-t-ZrO2 content of more than 5 wt%, the ZrO2 particles agglomerated.

Ultra-high-strength multi-alloyed steel with enhanced cryogenic toughness using thermally stable retained austenite

In low-carbon steel, obtaining ultra-high-strength inevitably deteriorates ductility and cryogenic toughness. Therefore, in this study, we investigated the role of “lamellarization” on the fraction and distribution of retained austenite on a low-carbon multi-alloyed steel subjected to a novel quench-lamellarization-intercritical tempering (QLIT) treatment and elucidated its effects on mechanical properties and fracture mechanisms. The performance of the proposed method was compared with that of quench-intercritical tempering (QIT) treatment. The results showed that compared with QIT (800 °C + 620 °C) treatment, adding an L-step in QL680IT and QL710IT significantly increased the fraction of retained austenite and stimulated VC precipitates inside tempered martensite. Lamellarization temperature modified the alloying element enrichment and the first martensitic formation, which further influenced the nucleation and kinetics of reverse austenitic transformation during the subsequent IT process. Ni-, Mn-, and Cr-enriched retained austenite had high thermal stability after holding at −196 °C. Nearly all the retained austenite transformed to martensite under constant tensile and fast impact loads. The transformation-induced plasticity of the high fraction retained austenite significantly improved the strain hardening rate and delayed necking, increasing the elongation, and maintaining tensile strength of 1 GPa. Moreover, the transformation-induced toughness of retained austenite with homogeneous distribution increased the impact energy from 27 to 84 J at −196 °C. The filmy retained austenite at the tempered martensite packet and block boundaries delayed the crack initiation and arrested its propagation via crack deflections, crack-tip passivation, and crack closure, which improved the crack initiation and propagation energies and changed the impact fracture from cleavage facets to large dimples. In addition, the dispersion of nanoscale vanadium carbides in multi-step heat treatment effectively improved the strength without reducing the low-temperature toughness.

The evolution of dynamic energy during drop hammer testing of Brazilian disk with non-persistent joints: An extensive experimental investigation

Rock mass is well known as a discontinuous, heterogeneous, and anisotropic material. The behavior and strength of rock mass is heavily controlled by the condition and orientation of discontinuities (faults, joints, bedding planes) and discontinuity sets. Under dynamic loading conditions, rock bridges along non-persistent discontinuity planes may crack, and a fully persistent discontinuity may form, potentially affecting the stability of a rock structure. The study of the dynamic behavior of rock discontinuities has critical implications for civil engineering, the mining industry, and any other areas where rock mass is utilized as a structural foundation in areas prone to dynamic loading conditions, such as those formed during earthquake events. In this paper, cement-mortar-based Brazilian disks containing open, non-persistent joints were constructed and subjected to impact loading to investigate their impact energy behavior. The effect of some parameters, such as joint continuity factor (the relationship between joint length and rock bridge length), bridge angle, joint spacing, joint orientation, and impact angle were investigated to estimate the required Dynamic Energy for Crack Initiation (DECI), Dynamic Energy for Crack Coalescence (DECC) and failure pattern of specimens. The results of the experiments revealed an increasingly continuous joint reduces the DECI and DECC, while larger joint spacings past the middle value of those experimented increase the DECI and DECC. The bridge angle and loading direction do not affect DECI, but by increasing bridge angle DECC decreases, and it increases by increasing loading direction angle. Finally, an optimization analysis was conducted which showed that joint spacing and joint continuity factors significantly affects DECI, and joint continuity factor and loading direction have significant effect on DECC.

Effects of graphene network formation on microstructure and mechanical properties of flax/epoxy nanocomposites

Various contents of graphene nanoplatelets (GNPs), ranging from 0 to 0.7 wt%, with an efficient solvent method were used to reinforce flax/epoxy composites and thereby correlate GNPs network formation with the degree of reinforcement in resultant nanocomposites. The microstructural features characterised by wide X-ray diffraction (WAX), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) determined a correlation between GNPs reinforcing efficiency, nanolayers network formation and the number of layers. TEM analysis clarified that with a change in GNPs content, the distribution of nanolayers transformed from a zonal distribution to efficiently bridged networks contributing to maximum improvement in tensile and flexural properties at 0.5 wt% of GNPs. This was supported by morphological observations developed by SEM confirming various reinforcing mechanisms thanks to GNPs networks leading to enhanced interfacial adhesion between fibre and matrix. Likewise, with formation of networks, crack length at the interface of fibre and matrix and crack density in nanocomposites, determined from optical microscopy images of tensile-tested samples, received a significant decrease. TEM observations also proved that plane-to-plane and edge-to-edge contact areas of GNPs had paramount influences on reinforcement efficiency in nanocomposites. Exceeding the optimum content (0.5 wt%), the load transfer capability induced by GNPs network formation was adversely affected by plane-to-plane contacts of GNPs eventually leading to substantial reductions in mechanical properties. In drop weight impact analysis, GNPs networks also enhanced crack initiation energy and consequently reduced crack propagation energy and damage area in nanocomposites.