Proposal of bond criterion for hot roll bonding and its application

The aim of this work was to develop a bond criterion for laminated composites prepared by hot rolling. 7075 Al/AZ31B Mg/7075 Al laminated composites were fabricated by hot rolling at different reduction ratios and temperatures, and the hot rolling process was also simulated by finite element methods (FEM). The FEM results show that two stages existed for an option position of the interface during hot rolling, viz. the bonded interface forming period and the post-bonded period. Bonded interface would be damaged during the latter due to second tensile stress and tear stress (due to the sticking friction between the Al plates and the rollers during the rolling). A bond criterion for laminated composites fabricated by hot rolling was proposed, which includes a strain threshold and a critical bonding strength. The predicted bond results of the 7075 Al/AZ31B Mg/7075 Al laminated composite fabricated by hot rolling from the proposed bond criterion agreed with the experimental data.     

Effect of elastic–plastic matrix on thermo-mechanical response of continuous anisotropic fiber-reinforced composites

Based on a concentric cylinder model, the analytical elastic–plastic solution of deformations and stresses for the composites reinforced with transversely isotropic, circumferentially orthotropic and radially orthotropic fibers subjected to axisymmetric thermo-mechanical loading is developed. How the plasticity, volume fraction, physical and mechanical properties of the matrix affect the elastic–plastic thermo-mechanical response of the composites is investigated. The plasticity of the matrix decreases greatly the axial compressive stress in the matrix, but more noticeably increases the axial compressive stress in the fiber. For the composites reinforced with transversely isotropic, circumferentially orthotropic and radially orthotropic fibers, decreasing the volume fraction, thermal expansion coefficient and Young's modulus, and increasing the yield stress and hardening parameter of the matrix can lower the maximum equivalent stress of the fiber. However, increasing the yield stress and hardening parameter of the matrix raises the maximum equivalent stress of the matrix.     

Fatigue life prediction of carbon fibre reinforced laminates by using cycle-dependent classical laminate theory

In this work a study about the adaption of the classical laminate theory for fatigue loads is presented. Cycle dependent stiffnesses of single UD 0°, UD 45° and UD 90° plies are implemented in order to calculate the fatigue-induced stiffness decrease of a multidirectional lay-up with the stacking sequence [0°/+45°/−45°/90°/90°/−45°/+45°/0°]. As second input alternative, UD 0°, UD 90° and ±45° plies are used. The calculated cycle-dependent stiffness parameters are compared to experimentally measured fatigue data of the multidirectional lay-up. The experimental test procedure used for the measurement of cycle-dependent stiffness parameters has been published previously. Results show that the experimentally measured stiffness decreases of the multidirectional lay-up can be estimated accurately based on the cyclic unidirectional input parameters.     

Influence of titanium carbide on the interlaminar shear strength of carbon fibre laminate composites

The potential use of carbon fibre laminate composites is limited by the weak out-of-plane properties, especially delamination resistance. The effect of incorporating titanium carbide to the mesophase pitch matrix precursor of carbon fibre laminate composites on interlaminar shear strength is studied both on carbonised and graphitised composites. The presence of titanium carbide modifies the optical texture of the matrix from domains to mosaics in those parts with higher concentrations and it contributes to an increase of fibre/matrix bonding. This fact produces an increase of the interlaminar shear strength of the material and changes the fracture mode.     

Validation of full 3D and equivalent tape laminate modeling of plasticity induced non-linearity in 2 × 2 braided composites

Fiber reinforced polymer matrix braided composites exhibit considerable non-linear response. Plasticity induced non-linear behavior of 2 × 2 braided composites was investigated using a two scale finite element modeling approach based on Hill’s yield function for orthotropic materials. The analysis was validated by comparing the macroscopic stress–strain predictions for a variety of 2 × 2 biaxial braids with experimental data. Experimental results were also compared with equivalent tape laminate analyses, which required only three elements and much less computational time. All the braids show considerable plasticity induced non-linearity. Predictions of both full 3D and equivalent tape laminate plastic analyses agree reasonably well with the experiments. Performance of braided composites was compared with tape laminates and it was seen that equivalent tape laminates have a better performance than their braid counterparts in terms of longitudinal modulus, but in terms of percentage moduli degradation due to plasticity, both braids and tapes have similar performance.     

Ductility of polylactide composites reinforced with poly(butylene succinate) nanofibers

Sustainable “green nanocomposites” of polylactide (PLA) and poly(1,4-butylene succinate) (PBS) were obtained by slit die extrusion at low temperature. Dispersed PBS inclusions were sheared and longitudinally deformed with simultaneous cooling in a slot capillary and PBS nanofibers were formed. Shearing of PBS increases nonisothermal crystallization temperature by 30 °C. Tensile deformation was investigated by in-situ experiments in SEM chamber. Dominant deformation mechanism of PLA is crazing, however, there are dormant shear bands formed during slit die extrusion. Pre-existing shear bands are inactive in tensile deformation but contribute to ductility by blocking, initiating and diffusing typical craze growth. PBS nanofibers are spanning PLA craze surfaces and bridging craze gaps when PLA nanofibrils broke at large strain. Straight crazes become undulated because either dormant or new shear bands become activated between crazes. Due to interaction of crazes and shear bands the ductility increases while high strength and stiffness are retained.     

Hierarchical reinforcement of polyurethane-based composites with inorganic micro- and nanoplatelets

Hierarchically reinforced structures are widespread in nature but less common among man-made materials. In this paper, we show that polyurethane-based thermoplastic polymers can be hierarchically reinforced with laponite nanoplatelets and alumina microplatelets to reach strength and elastic modulus that are, respectively, 7- and 29-fold higher than that of the pure polymer matrix (91.7 MPa and 6.97 GPa, respectively). We find that the selective reinforcement of the polyurethane hard domains with laponite nanoplatelets is key to keep the polymer matrix sufficiently ductile for the incorporation of high concentrations of alumina microplatelets. Effective reinforcement of the polymer with microplatelets of different surface chemistries was only possible after annealing the composite at 130 °C to promote strong bonding at the oxide/polymer interface. Large-area composite films and bulk parts exhibiting good alignment of alumina microplatelets were obtained through conventional tape-casting. The concept of hierarchical reinforcement demonstrated here can be explored to obtain composite materials covering a wide range of mechanical properties using only a few reinforcing building blocks within the same polymer matrix.     

Tensile behaviour of stainless steel fibre/epoxy composites with modified adhesion

In this study we investigate the tensile behaviour of unidirectional and cross-ply composites reinforced with ductile stainless steel fibres and modified adhesion to the epoxy matrix. Results show that annealed stainless steel fibres have a potential in designing tough polymer composites for structural applications. The stiffness of the UD composites made from these fibres is 77GPa combined with the strain-to-failure between 15% and 18% depending on the level of adhesion. Silane treatments were used to modify the adhesion. By treating the stainless steel fibres with different silane coupling agents, an increase of 50% in the transverse 3-point-bending strength was realised. Increasing the adhesion by 50% leads to a higher tensile strength and strain-to-failure in both UD and cross-ply laminates and a higher in-situ strength of the 90° plies. It also delays formation of matrix cracks and hinders growth of debonding.     

Imaging microstructure and stress fields within a cross-ply composite laminate

Microfocus X-ray diffraction can be used in a scanning acquisition mode to image a fibre reinforced composite material in terms of a range of different parameters. This includes microstructural information, as well as detail of emerging stress fields around open hole geometries. In this study, the in situ deformation of a cross-ply laminate specimen is investigated. The results show a number of interesting phenomena related to stress transfer in such systems. This includes stress concentrations in fibres adjacent to the open hole and local shear forces acting upon fibres perpendicular to the deformation axis. In addition to imaging the sample, the technique also allows a number of mechanical parameters to be estimated. This includes transverse strain and longitudinal stiffness. Using isotropic elasticity theory, the stress concentration tangential to the open hole can be determined analytically.     

Micromodel-based simulations for laminated composites

We develop a calculation strategy for the simulation of a complete microscopic model. This strategy enables one to account for damage mechanisms in laminated composites. The model mixes discrete and continuous approaches by introducing potential rupture surfaces and a damageable continuous medium. This approach requires suitable calculation tools unavailable in industrial analysis codes. The strategy presented is multiscale in space and is based on a decomposition of the domain into substructures and interfaces. This strategy enables one to simulate complex problems with multiple cracks. In practice, to use such a model, the strategy must be improved in order to handle very large numbers of substructures and interfaces and to estimate the rupture criteria for the surfaces introduced into the model. We provide simple examples which demonstrate the capabilities of the microscopic model.     

Hygromechanical coupling in laminate composites and its effect on interlaminar failure

Graphite-Phenolic (GPh) laminates are used as structural parts under mechanical and hygrothermal loadings. High shrinkage strains during drying may cause premature interlaminar failure which originates from the hygromechanical coupling. In this study, modified coupled hygromechanical governing equations which include non-constant coefficient of moisture expansion (CME), were derived. The model was experimentally validated by measuring transient weight and strain between various equilibrium states of humidity, obtaining relevant material parameters. Finally, a set of three point bend specimens was used to obtain the strength profile during drying, which revealed a minimal value at a characteristic time. Using a non-local failure criterion it was possible to associate the temporal strength profile with the build-up of hygromechanical stresses.     

Localised blast loading of fibre–metal laminates with a polyamide matrix

This paper presents the results of localised blast tests on fully clamped square fibre–metal laminate panels, manufactured using sheets of 2024-O aluminium alloy, woven glass–fibre reinforced polyamide and a polypropylene adhesive. The fracture properties of the composite–metal interface were determined using the single cantilever beam geometry and the measured interfacial fracture toughness was between values in the literature for thermosetting composites and aluminium/glass fibre polypropylene. Observations from blast experiments performed on panels with different stacking configurations are reported. Diamond and circular back face damage were observed, along with pitting, global displacement and tearing of the front face. Examinations of sectioned panels are presented and multiple debonding, plastic deformation and fibre fracture were identified within the panels.     

Compression creep rupture behavior of a glass/vinyl ester composite laminate subject to fire loading conditions

The growing use of polymer matrix composites in civil infrastructure, marine and military applications provides the impetus for developing mechanical models to describe their response under combined mechanical and fire loading. A viscoelastic stress analysis using classical lamination theory is conducted on an E-glass/vinyl ester composite. The model includes a characterization of the non-linear thermo-viscoelasticity and its inclusion into a compression strength failure criterion for the prediction of laminate failure under combined compressive load and temperature profile simulating fire exposure. By accounting for the viscoelastic non-linearity at T_g, the proposed model yields good predictions for lifetimes of the studied composite ([0/+45/90/−45/0]_S).     

Stiffness control in adaptive thin-walled laminate composite beams

The aim of this paper is to verify the control of the stiffness that is feasible to achieve in a thin-walled box-beam made from a laminate by including an adaptive material with variable stiffness. In this work, a material having a strongly varying Young Modulus under minor temperature changes was included in the cross-section. An analytical model was used to estimate the position of shear centre and the axial, bending, torsional, and shear stiffnesses of the cross-section. Two cross-sections were analysed, one with an adaptive wall and another with two adaptive walls. In both sections, the torsional stiffness could be strongly altered with minor temperature variations. In the section with one adaptive wall, the shear centre and thus the bending–twist coupling was also strongly modified. A study was made of the influence on the control of stiffnesses exerted by the overall cross-section thickness and the thickness of the adaptive walls.     

Hierarchical lattice composites for electromagnetic and mechanical energy absorptions

A new hierarchical radar absorbing structure with multifunction of ultra-light weight, anti-crushing and radar absorption was designed and made by glass fiber reinforced lattice composites filled with radar absorbing foams. Experiments were performed to reveal the electromagnetic absorption and anti-crushing behaviors. Mechanisms of the composite in electromagnetic absorption and anti-crushing were analyzed. The newly designed composite lattice displays excellent performances in absorbing both microwave and mechanical energies at ultra-light weight. To balance the anti-crushing and radar absorption behaviors, key factors of the composite lattices, including the panel thickness, the relative density of the lattice, the cell dimension and the geometry, were revealed based on the analysis and experiments.     

Grain size effect on the strengthening behavior of aluminum-based composites containing multi-walled carbon nanotubes

Strengthening efficiency of multi-walled carbon nanotubes (MWCNTs) is investigated for aluminum-based composites with grain sizes ranging from ∼250 to ∼65 nm. The strength of composites is significantly enhanced proportional to an increase of the MWCNT volume. However, the increment differs depending on deformation mode of the matrix. The strengthening efficiency of MWCNTs in ultrafine-grained composites is comparable with that predicted by the discontinuous fiber model, whereas the efficiency becomes half of the theoretical prediction as grain size is reduced below ∼70 nm. For nano-grained aluminum, activities of forest dislocations diminish and dislocations emitted from grain boundaries are dynamically annihilated during the recovery process, providing a weak plastic strain field around MWCNTs. The observation may provide a basic understanding of the strengthening behavior of nano-grained metal matrix composites.     

Effective moduli of multi-scale composites

The effective moduli of a multi-scale composite are evaluated by a bottom-up (hierarchical) modeling approach. We focus on a two-scale structure in which the small scale includes a platelet array inside a matrix, and the large scale contains fibers inside a composite matrix. We demonstrate that the principal moduli of the multi-scale composite can be fine-tuned by the platelet arrangement and orientation. As a case study, we consider the phenomenon of fiber micro-buckling within the multi-scale composite. It is found that the compressive micro-buckling strength can be considerably increased for specific platelet orientations. The multi-scale design approach presented here can be used to generate novel families of composite materials with tunable mechanical properties.     

Indentation of a laminated composite plate with an interlayer rectangular void

We employ the Eshelby–Stroh formalism to study generalized plane strain infinitesimal deformations caused due to the indentation by a rigid circular cylinder of an elastic laminated plate with a through-the-width rectangular void between two adjoining layers. Assuming that the void does not close during the indentation process, we find the indentation modulus (i.e., the slope of the indentation load vs. the indentation depth curve) as a function of the void size, the void position, elastic moduli of the layers, and boundary conditions at the edges. The change in the indentation modulus caused by an interlayer void parallel to the major surfaces of an anisotropic plate can potentially be used to estimate the void size and location.     

High-strength and highly-uniform composite produced by anodizing and accumulative roll bonding processes

The anodizing and accumulative roll bonding (ARB) processes are used in this paper as a new, effective alternative for manufacturing high-strength and highly-uniform aluminum/alumina composites. Four different thicknesses of alumina layers are grown on the substrate using an anodizing process and the microstructural evolution and mechanical properties of the resulting aluminum/alumina composite are investigated. Microscopic investigations of the composite show a uniform distribution of alumina particles in the matrix. It is found that alumina layers produced by the anodizing process neck, fracture, and depart as the number of accumulative roll bonding passes increases. During ARB, it is observed that as strain increases with the number of passes, the strength and elongation of the produced composites correspondingly increase. Also, by increasing alumina quantity, tensile strength improves so that the tensile strength of the Al/3.55 vol.% Al₂O₃ composite becomes ∼3.5 times greater than that of the annealed aluminum used as raw material.