Micromechanical Tests on Natural Fibre Composites with Enzymatically Enhanced Fibre–Matrix Adhesion

Natural fibre–reinforced composites are more sustainable than other composites with respect to the raw materials. Their properties are attractive due to high specific properties, and especially so wherever high damping is valued. As the interphase between fibre and matrix is the region of highest stresses, a strong bond between fibre and matrix is essential for any composites’ properties. The present study compares two methods of determining the interfacial shear stress in natural fibre–reinforced composites: the single fibre fragmentation test and the single fibre pullout test. The studied composites are flax fibre reinforced epoxy. For a variety of fibre–matrix interaction, the fibres are treated with a laccase enzyme and dopamine, which is known to improve the fibre–matrix shear strength. In the observed samples, single fibre fragmentation test data, i.e. of fracture mode and fragment length, scatter when compared to pullout data. In single fibre pullout tests, the local interfacial shear strength showed a 30% increase in the laccase-treated samples, compared to the control samples. The method also permitted an evaluation of the frictional stress occurring after surface failure.

Experimental Research On The Technology of Two-Pass Different Temperature Rolling For Thick Steel/Aluminum/Aluminum-Alloy Composite Plate

Thick steel/aluminum/aluminum-alloy composite plate is one of the key materials connecting steel structures and aluminum alloy structures, and has been widely used in shipbuilding industry and other fields. However, steel/aluminum/aluminum-alloy composite plates with a total thickness of more than 10 mm and a steel layer thickness of more than 5 mm are prone to problems such as inconsistent deformation of component metals and low bonding strength during the rolling process, and cannot be continuously prepared. In order to solve this problem, this article proposes a two-pass different temperature rolling process for thick steel/aluminum/aluminum-alloy composite plates, and conducts research on Q235B steel, 1060 aluminum and 5083 aluminum alloy as component metals. The results show that the process is reliable. It can prepare Q235B/1060/5083 composite plates with a thickness of 15.65 mm without oxygen protection measures. Meanwhile, the interfacial shear and pull-off strength of the composite plates obtained under different experimental conditions in this article are higher than the requirements of the US military standards MIL-J-24445A and Chinese ship standard CB20091-2012. And the composite plates showed good performance in 90° and 137° bending tests without obvious defects. Under the best condition of them, a 1.48 μm interlocking diffusion layer was formed at the steel/aluminum interface of the composite plates, and the interfacial shear strength exceeded 70 MPa, and the interfacial pull-off strength exceeded 110 MPa. Finally, according to the experimental results, the reasons for the feasibility of the two-pass different rolling of thick steel/aluminum/aluminum-alloy composite plates are given.

Interfacial Shear at the Atomic Scale

Understanding the interfacial properties between an atomic layer and its substrate is of key interest at both the fundamental and technological level. From Fermi level pinning to strain engineering and superlubricity, the interaction between a single atomic layer and its substrate governs electronic, mechanical, and chemical properties of the layer-substrate system. Here, we measure the hardly accessible interfacial transverse shear modulus of an atomic layer on a substrate. We show that this key interfacial property is critically controlled by the chemistry, order, and structure of the atomic layer-substrate interface. In particular, the experiments demonstrate that the interfacial shear modulus of epitaxial graphene on SiC increases for bilayer films compared to monolayer films, and augments when hydrogen is intercalated between graphene and SiC. The increase in shear modulus for two layers compared to one layer is explained in terms of layer-layer and layer-substrate stacking order, whereas the increase with H-intercalation is correlated with the pinning induced by the H-atoms at the interface. Importantly, we also demonstrate that this modulus is a pivotal measurable property to control and predict sliding friction in supported two-dimensional materials. Indeed, we observe an inverse relationship between friction and interfacial shear modulus, which naturally emerges from simple friction models based on a point mass driven over a periodic potential. This inverse relation originates from a decreased dissipation in presence of large shear stiffness, which reduces the energy barrier for sliding.

Design, Analysis and Experiment of the Fiber Push-Out Device Based on Piezoelectric Actuator

In this study, a fiber push-out device based on a piezoelectric actuator was designed, analyzed and tested, and its experimental environment was designed. The piezoelectric actuator includes a flexible beam. By using response surface analysis (RSM), taking the large displacement as the objective function and on the premise of meeting the strength requirements, the structural parameters of the flexible beam were analyzed. In the process of fiber push-out, the interfacial shear stress was estimated by establishing the system integrating the fiber-matrix-composite three-phase model and the piezoelectric actuator model using the analytic method, and the theoretical analysis results of the fiber interface mechanical properties were given. A prototype of the system was made, and the performance tests of the piezoelectric actuator and the fiber push-out device were carried out. The test results showed that the designed piezoelectric actuator can achieve a stepping resolution of 6.67 μm and a maximum displacement of about 100 μm at the input voltage of 150 V, which is consistent with the design results. The extrusion test of a single fiber was carried out using a piezoelectric actuator. The mechanical properties of the interfacial layer during the push-out process were measured and the interfacial shear strength was calculated, which is consistent with the theoretical analysis results. Finally, based on the mechanical properties obtained from the test, the loading failure process of the fiber was simulated by finite element analysis, which well explained the failure process of the fiber, thus verifying the feasibility of the designed fiber push-out device.

A novel thermal-mechanical model and the characteristics of interfacial stress in the laminated structure for flexible electronics

Flexible electronics have attracted rapidly growing interest owing to their great potential utility in numerous fundamental and emerging fields. However, there are urgent issues that remain as pending challenges in the interfacial stress and resulting failures of flexible electronics, especially for heterogeneous laminates of hard films adhered to soft polymer substrates under thermal and mechanical loads. This study focuses on the interfacial stress of a representative laminated structure, that is, the Si film is adhesively bonded to soft polydimethylsiloxane with a plastic polyethylene terephthalate substrate. An novel thermal-mechanical coupling model for this flexible structure is established in this paper, which presents the essential characteristics of interfacial shear stress. In addition, under thermal and mechanical loads, a typical case is investigated by combining an analytical solution with numerical results using the differential quadrature method. Furthermore, thermal and mechanical loads, material and geometry parameters are quantitatively explored for their influences on the interfacial shear stress. Targeted strategies for decreasing stress are also suggested. In conclusion, the thermal-mechanical model and application case analyses contribute to enhancing the design of interfacial reliability for flexible laminated structures.

Predicting interlaminar damage behaviour of fibre-metal laminates containing adhesive joints under bending loads

This study includes experimental and numerical investigations on fibre-metal laminate structures containing adhesive joints under static bending loads. Experimental tests were carried out on Glare® 4B specimens manufactured in-house and containing doubler joint features. Numerical analyses were performed using Abaqus software including damage in the glass fibre reinforced polymer layers, ductile damage in the resin pockets (FM94 epoxy) and plasticity in the metal layers. A new cohesive zone model coupling friction and interfacial shear under through-thickness compressive stress has been developed to simulate delamination initiation and growth at the metal/fibre interfaces with the adhesive joint under flexural loading. This model is implemented through a user-defined VUMAT subroutine in the Abaqus/Explicit software and includes two main approaches, firstly, combining friction and interfacial shear stresses created in the interlaminar layers of the fibre-metal laminate as a result of through-thickness stresses and secondly, considering elastic-plastic damage behaviour using a new cohesive zone model based on the trapezoidal law (which provides more accurate results for the simulation of toughened epoxy matrices than the commonly used bilinear cohesive zone model). Numerical results have been validated against experimental data from 4-point bending tests and a good correlation observed with respect to both crack initiation and evolution. Delamination and shear failure were noted to be the predominant failure modes under bending stresses as expected. This is due to the higher mode-II stresses introduced during bending which cause different damage evolution behaviour to that seen for axial stresses. Finite element results revealed that both friction and shear strength parameters generated from through-thickness compression stresses have a significant effect in predicting damage in fibre-metal laminate structures under this type of loading.