Multi-walled carbon nanotubes (MWCNTs) were added to an epoxy resin in an effort to improve the fracture toughness of bulk epoxy and also when used as matrix for carbon fibre reinforced epoxy ...composites (CFRPs). The incorporation of MWCNTs to bulk epoxy and CFRPs moderately increased the mode-I fracture energy, and significantly increased the mode-II fracture energy, i.e. the average mode-II fracture energy of CFRPs increased from 2026 J/m2 to 3406 J/m2 due to the addition of 0.5 wt% MWCNTs, and further to 5491 J/m2 due to the addition of 1 wt% MWCNTs. The superior toughening performance of MWCNTs in mode-II fracture is attributed to two reasons: 1) increased MWCNT breaking and crack deflection mechanisms under shear load; and 2) large fracture process zone accompanied with extensive hackle markings and micro-cracks ahead of the mode-II crack tip of CFRPs, which resulted in significant number of MWCNTs contributing to toughening mechanisms.
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•MWCNTs were used to enhance epoxy and carbon fibre composites.•Mode-I and Mode-II fracture behaviour was studied.•The addition of MWCNTs moderately increased Mode-I fracture toughness.•The addition of MWCNTs significantly increased Mode-II fracture toughness.
•Time-temperature superposition approach is utilized to investigate PMMA mode-II fracture.•Critical brittle-ductile transition (BDT) rates/temperatures are predicted by KJC/KIIC master curves.•The ...BDT state of shear fracture under complex condition can be predicted from limited test conditions.
The rate-temperature dependent brittle-ductile transition (BDT) behavior of PMMA (polymethyl methacrylate) mode-II fracture was investigated through tensile-loading shear fracture (TLSF) tests. The dimensionless ratio KJC/KIIC was chosen as the index to identify the brittle/ductile state. Assuming the fracture property to be viscosity-dominated, the time-temperature superposition principle based on the Arrhenius equation was adopted. The master curves of KJC/KIIC in a wide range of loading rate and temperature were constructed with the experimental data obtained under limited test conditions. With the derived critical BDT band from the master curve, the BDT state of PMMA’s fracture under complex loading conditions was successfully predicted and experimentally validated. The results are helpful to describe the BDT behavior and provide an equivalent approach to investigate the mode-II fracture which can substantially save the experimental cost and time.
•Mode-I + III and mode-II + III fracture behavior of the aerospace grade alloy AA7085 is studied.•A new loading device has been designed to induce out-of-plane mode in addition to mode-I and ...mode-II.•Specimen level boundary conditions were derived for fracture parameter determination by simulation.•A script in Ansys to determine fracture parameters using XFEM under Mode-I + III and II + III loadings.•The crack deflection angle and onset of fracture observed in the experiment were compared to the standard criterion.
This study experimentally examines the onset of fast fracture and crack deflection in AA 7085 for both mode-I + III and mode-II + III fracture modes. A new loading device has been developed by modifying the Richards loading device to induce out-of-plane mode in addition to mode-I and mode-II loading. The specimens assembled with the loading device were loaded using a uniaxial testing machine. Specimen level boundary conditions were derived to determine the fracture parameters at the onset of fracture using XFEM. The deflection angle of the crack and the fracture envelope were compared against standard criteria to predict failure.
•The mode II dynamic fracture processes of rock were observed by the 3D high-speed digital image correlation technique.•The dynamic fracture and damage process as well as deformation characteristic ...of granite are analyzed in detail.•The crack propagation path of granite is obtained and is found to be nearly independent of the loading rate.•The roughness and curvature of shear fracture surface reduce with the growth in loading rate based on fractal theory.
The influence of the loading rate on the mode I fracture property of rocks has been extensively investigated through experiments. However, the rate-effect of the dynamic mode II fracture has rarely been reported. This study employed the modified split Hopkinson pressure bar (SHPB) system to conduct a series of dynamic mode II fracture tests under a wide range of loading rates using the modified short-core-in-compression (SCC) method. The complete dynamic fracture processes of rock were observed using the three-dimensional high-speed digital image correlation (3D HS DIC) technique. The results reveal that the dynamic mode II fracture toughness increases with the loading rate at a power relationship. By virtue of the evolution of the surface principal strain and displacement, the dynamic fracture and damage process and the deformation characteristic of the SCC specimen were analyzed in detail. The dynamic fracture processes are divided into the elastic stage, plastic deformation stage, unstable crack growth stage, and post fracture stage. The crack propagation path of the SCC specimen was obtained under impact loading. It was found that new cracks first initiate from two notch tips and then propagate toward the central area in the expected shear fracture band (ESFB); the path is nearly independent of the loading rate. Additionally, the macro–micro fracture surface morphology of the SCC specimen was measured using a 3D optical scanner as well as scanning electron microscope (SEM) respectively. It was found that the roughness and curvature in the fracture surface created by shearing generally decrease as the loading rate increases, in accordance with fractal theory. The SEM result indicates intergranular fracture occurs more frequently on the shear fracture surface under static loading, while transgranular fracture is more common on the shear fracture surface at high loading rates.
•Surface roughness profile characteristics that affect adhesion and corrosion creep.•Why adhesion test results do not correlate with corrosion protection.•Surface profile factors affecting rate of ...corrosion creep.•Disciplines outside polymer coatings included in review.
Specifications for coating many forms of infrastructure and equipment include abrasive cleaning, then measurement of the surface profile and adhesion on the assumption that they are linked to long term prevention of corrosion undercutting the coating. Studies find no quantitative connection between adhesion and corrosion protection, but many believe in a link. Although corrosion is a molecular phenomenon that starts at the interface, adhesion values are measured by a device attached to the coating at some distance from the interface, so it is difficult to directly connect corrosion with adhesion. Understanding how adhesion and the spread of corrosion under the coating are influenced by surface roughness entails surface metrology, fracture mechanics, surface energy and viscoelasticity. The impact of surface roughness cannot be not determined by a simple, or single, statistical parameter for variation in substrate height variation. Conventional “pull-off” adhesion testing does not characterize coating-metal interactions that might prevent water and electrolyte causing corrosion, they must be determined otherwise. The additional surface area created by the abrasion increases the number of adhesive interactions and the local slope of the surface engages Mode II loading that increases the force that the interface can support. Both these surface attributes may also slow the spread of corrosion across the interface. In order to prevent the diffusion of water and electrolyte across the interface, a coating should form well at the interface, be tough and as hydrophobic as realistically possible.
In this work an analytical solution for the calculation of the Energy Release Rate (ERR) under high-speed mode II loading conditions for an End-Loaded Split specimen, at crack initiation, is ...obtained. The analytical formulation is based on the Timoshenko beam theory and on a simple contact condition for the unbonded region of the specimen. The contribution of the shear effects added to the analytical model through the Timoshenko beam theory and the existence or not of pure mode II loading during a high-speed loading are studied. The proposed model is validated against numerical results obtained from finite element analysis. Also, a comparison against a previous analytical model based on the Euler theory is performed. Finally, a methodology is proposed in order to estimate a lower boundary limit for the crack initiation time after which the loading conditions can be assumed as pure mode II for testing using the standard End-Loaded Split configuration. The results show the dependency of the dynamic ERR on the vibrational characteristics of the specimen and also that accounting for the shear effects in the analytical model produces results closer to the numerical ones. It is also proven that in order to load in pure mode II an ELS specimen under high-speed loading the displacement must be applied on both adherents. This is to avoid the complex contact conditions and the asymmetry of the boundary conditions that result in a mode I component for the ERR.
•Analytical modelling of the Strain Energy Release Rate for high-speed mode II loading.•Timoshenko theory better predicts the vibrational characteristics of ELS specimens.•At high-speed an ELS specimen loaded at one adherent results in mixed mode loading.
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.
In this paper, we propose a strain gage technique that allows an accurate measurement of the mode II stress intensity factors (SIFs). In order to ensure a good KII measurement, the size of the ...correct radial location of the strain gages is identified by using a methodology based on finite element analysis (FEA) and consolidated by solid theoretical foundations. The extent r1,max and r2,max of the validity of the five- and six-term representation of generalized Westergaard stress functions, which requires the use of two and three strain gages, respectively, is determined using the proposed method, and its dependence on the crack length to width ratio is also investigated. Numerical results obtained are in good agreement with the theoretical predictions, and confirm that the present method makes it possible to determine the KII with high precision when two (or three) strain gages are well placed within r1,max (or r2,max). Also, the numerical experiments show that the use of three strain gages is necessary when the size of the linear behavior region r1,max is very small. In addition, the effect of friction between the crack lips was checked and shown to have no effect on KII in pure mode II.
•A new strain gages technique for the determination of mode II SIF is proposed.•Valid regions theory for the locations of strain gages is demonstrated.•Computation of the upper bound of the valid regions is also substantiated.•The proposed technique gives accurate values of KII.•Locations of the strain gages outside the valid regions are also discussed.
