Impact damage resistance of novel adhesively bonded natural fibre composite:Steel hybrid laminates

Synthetic fibre reinforcements are increasingly replaced with plant fibres but an improvement in the mechanical performance of biocomposites is required. Flax composite exhibits fibre failure and perforation even at low impact energies. This paper investigates the viability of improving the impact resistance of flax-epoxy biocomposite by hybridisation with a thin metal layer. High-speed cameras and optical microscopy were used to measure the dissipated energy and to identify the different damage modes. The impact response of hybrid biocomposites was compared to a reference GFRP composite and monolithic biocomposites and it was shown that the deformation and damage is significantly reduced in the hybrid configuration. Additionally, a numerical model was developed in Abaqus/Explicit and validated in terms of the displacement history and damage modes. The study reveals the effect of various material configurations and thicknesses on impact damage resistance and proves that the penetration resistance of biocomposites is improved by hybrid construction.publishedVersionPeer reviewe

High strain rate testing of ultra fine grained aluminium at micro and macro length scales112

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Failure prediction for high-strain rate and out-of-plane compression of fibrous composites

This work presents a detailed analysis of failure prediction for a glass fiber reinforced plastic (GFRP) composite under out-of-plane compression at quasi-static (10−3 and 1 s−1) and high strain rates (103 s−1). The simulations were compared with the experiments of a recent study (Pournoori. et al. Int. J. Impact Eng., 147 (2021)). The failure at different strain rates was predicted using the three-dimensional (3D) Hashin failure criterion implemented into the finite element analysis by the Abaqus user-defined subroutines UMAT and VUMAT. According to the results, the criterion predicted failure onset well in terms of force level, location, and failure mode, without any fitting parameters. The inter-fiber failure was the dominant failure mode at all studied strain rates in simulations. The 3D Hashin criterion predicted that the failure onset occurred at a low strain level close to the experimental nonlinearity point with a ±7% difference between them while the coefficient of variation of related strains at nonlinearity point was 15.4% at low and intermediate rate tests. The virtual crack closure technique simulations of fracture for low and high rate tests indicated that the GFRP deformation involves some dissipation, which causes a type of nonlinear response prior to the peak force.publishedVersionPeer reviewe

Dynamic Mode Ⅱ fracture behavior of rocks under hydrostatic pressure using the short core in compression (SCC) method

The shear failure of rocks under both a static triaxial stress and a dynamic disturbance is common in deep underground engineering and it is therefore essential for the design of underground engineering to quantitively estimate the dynamic Mode Ⅱ fracture toughness KⅡC of rocks under a triaxial stress state. However, the method for determining the dynamic KⅡC of rocks under a triaxial stress has not been developed yet. With an optimal sample preparation, the short core in compression (SCC) method was designed and verified in this study to measure the dynamic KⅡC of Fangshan marble (FM) subjected to different hydrostatic pressures through a triaxial dynamic testing system. The formula for calculating the dynamic KⅡC of the rock SCC specimen under hydrostatic pressures was obtained by using the finite element method in combination with secondary cracks. The experimental results indicate that the failure mode of the rock SCC specimen under a hydrostatic pressure is the shear fracture and the KⅡC of FM increases as the loading rate. In addition, at a given loading rate the dynamic rock KⅡC is barely affected by hydrostatic pressures. Another important observation is that the dynamic fracture energy of FM enhances with loading rates and hydrostatic pressures.publishedVersionPeer reviewe

A numerical analysis of weakening of a granitic rock by piezoelectric excitation of quartz

This work presents a numerical model to simulate intergranular damage in a granitic rock by oscillating piezoelectric excitation of quartz dispersed in the structure. The damage evolution at grain boundaries was assumed to be related to fatigue and it was modelled using cohesive elements and a damage evolution model formulated in terms of discipation of mechanical work. An explicit representation of the granular mesostructure was built, and it was subjected to high-voltage alternating-current exitation. The effect of the fatigue damage on the mechanical properties was quantified by simulated tension and compression tests. The numerical results show that the electrical treatment can potentially cause rock weakening due to fatigue, but the model needs to be calibrated with experimental data for a quantitative analysis.Peer reviewe

Weakening of tensile strength of granitic rock by HV-HF-AC actuation of piezoelectric properties of Quartz : a 3D numerical study

High-voltage and high-frequency alternating current (HV-HF-AC) excitation of piezoelectric properties of Quartz is a potential method to induce cracks in granite. This was recently shown in a numerical feasibility study [6], where cracking was induced on cylindrical rock samples made of granite by sinusoidal AC excitation at the frequency of ~100 kHz and the amplitude of ~10 kV. However, this study did not investigate the weakening effect due to this cracking on the tensile strength of the sample. The present study addresses this topic numerically. For this end, a numerical method based on 3D embedded discontinuity finite elements for rock fracture and an explicit time stepping scheme to solve the coupled piezoelectro-mechanical problem is adopted. Rock heterogeneity and anisotropy are accounted for at the mineral mesotructure level. A preliminary numerical simulation demonstrates that the HV-HF-AC treatment reduces the tensile strength of a cylindrical granite sample by 12 %, making it thus a potential non-conventional pre-treatment method in comminution and excavation of Quartz bearing rocks and ores.publishedVersionPeer reviewe

Microscale Strain Localizations and Strain-Induced Martensitic Phase Transformation in Austenitic Steel 301LN at Different Strain Rates

Microscopic strain and strain-induced phase transformation during plastic deformation in metastable austenitic steel were investigated at different strain rates. Quasi in-situ tension tests were performed sequentially with well-defined elongation intervals at room temperature at strain rates of 10−3 s−1 and 10−1 s−1. The tests were monitored by high-resolution optical imaging with a microscopic lens at a resolution of 0.23 µm/pixel. The macroscopic temperature was also measured with an infrared (IR) camera. The microstructure-level strain localizations were observed on the surface of the etched specimens by means of microscale digital image correlation (µDIC). Additionally, the microstructure was characterized by electron backscatter diffraction (EBSD) at the same location before and after deformation. The results of the study indicated that microscopic strain localizations favored the formation of α′-martensite particles. At the lower strain rate, high local strain concentrations formed at several locations in the microstructure, correlating with the areas where the formation of large martensite islands occurred. Martensite particles of various sizes formed nearby each other at the lower strain rate, whereas at the higher strain rate, martensite islands remained small and isolated. Although the macroscopic increase in temperature at both the studied strain rates was very low, at the higher strain rate, local heating on the microscopic scale could take place at the newly nucleated martensite embryos. This inhibited the further growth of the martensite particles, and local strain distribution also remained more homogeneous than at the lower strain rate.publishedVersionPeer reviewe

Effects of strain rate and adiabatic heating on mechanical behavior of medium manganese Q&P steels

In this work, the mechanical behavior and properties of four different multiphase steels was studied in tension at strain rates of 10−4, 10−2, 0.5 and 800 s−1. The four materials include a medium manganese (3%) steel grade overcritically and intercritically annealed and Q&P heat treated and two industrially produced TRIP-assisted steels, DH800 and TRIP700 steels, which have different retained austenite morphology. The temperature and strain of the specimens were studied using high speed infrared thermography (IRT) and digital image correlation (DIC). The mechanical response of the Q&P steels had considerably higher tensile strength than the two industrially produced steels. The Q&P steel with a higher austenite volume fraction strain hardened significantly more than the other steels. The DH800 steel and the intercritically annealed Q&P steel heated less with ΔT of 25 °C during uniform deformation than the TRIP700 steel and the overcritically annealed Q&P steel with ΔT of 35 °C. However, the industrially produced steels DH800 and TRIP700 had higher uniform elongation of 0.12 mm/mm and 0.14 mm/mm whereas the Q&P steels reached only 0.09 mm/mm, meaning that the heating rate of the Q&P steels was considerably steeper. In addition, the stronger necking of the DH800 and TRIP700 steels led to much higher maximum temperatures before failure (max. 260 °C) than those observed for the Q&P steels (max. 140 °C). The Taylor-Quinney coefficients of the Q&P steels were large in the beginning of the plastic deformation (0.65–0.95) but decreased as a function of plastic deformation, whereas the Taylor–Quinney Coefficients of the DH800 and TRIP700 steels were lower (0.5–0.6) but increased gradually as a function of plastic deformation.publishedVersionPeer reviewe

In situ damage characterization of CFRP under compression using high-speed optical, infrared and synchrotron X-ray phase-contrast imaging

The strain rate dependency and failure modes of carbon fiber reinforced plastic (CFRP) laminate were investigated under out-of-plane compressive loading. Simultaneous high-speed optical and infrared imaging were used to measure full-field deformation and temperature in the dynamically loaded specimens. The damage initiation and propagation inside the CFRP laminates at high strain rates were characterized using in-situ ultra-fast synchrotron X-ray phase contrast imaging (XPCI). The visually observed damage onset occurs at the strain value of 4.2 ± 0.6% as a transverse shear fracture at the free edge of specimens. The local temperature increases significantly to 185 °C due to damage initiation at high strain rates, while at low strain rates the temperature rise occurs after the final shear band forms. The XPCI and post-failure analysis provide an integrated perspective on the formation of a diagonal shear crack and disintegration of the specimen into two pieces with the fracture of plies in the in-plane transverse direction. Scanning electron microscopic (SEM) study was integrated with XPCI results to append the time scale for the post-mortem failure pattern as well as the length scale for microcracks and filament-level failure.Peer reviewe

Finite-element simulations of split Hopkinson test of Ti-based alloy

Ti-based alloys are extensively used in aerospace and other advanced engineering fields due to their high strength and toughness, light weight, excellent corrosion resistance and ability to withstand extreme temperatures. Since these alloys are hard to machine, there is an obvious demand to develop simulation tools in order to analyse the material's behaviour during machining and thus optimise the entire machining process. The deformation processes in machining of Ti-alloys are typically characterized by high strains and temperatures. Split Hopkinson Pressure Bar (SHPB) technique is a commonly used experimental method to characterize the material behaviour at high strain rates; the stress-strain relation of the material is derived from the obtained experimental data. A computational study on a three-dimensional finite element model of the SHPB experiment is performed to assess various features of the underlying mechanics of deformation processes at highstrain and -strain-rate regimes. In the numerical analysis, an inhomogeneous deformation behaviour is observed in the workpiece at the initial stages of compression contrary to a standard assumption of stress and strain homogeneity in the specimen