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This article proposes a new algorithm for computing dispersion curves of ultrasonic guided waves in multi-layered composites with large number of layers. The algorithm is based on balancing the eigenvalue problem of the Spectral Collocation Method (SCM) formulation. The SCM has proven effective in analyzing single-layer and simple waveguides. However, it struggles with large multi-layer structures due to numerical instability caused by irregular and sparse matrices in the eigenvalue problem. The proposed algorithm for balancing has significantly reduced both the conditioning measure and the matrix norm in the matrix system. The optimization of spectral formulation enables accurate calculation of dispersion curves and characterization of displacement/stress profiles using Matlab software. This precise characterization and mode separation are essential for selecting ultrasonic sensors for damage detection. A comparison was made with Dispersion Calculator (DC) software. The algorithm was first validated using a hybrid multi-layered composite [CFRP-Al-CFRP-Al], which was successful. The validated algorithm was then quantitatively evaluated using a cross-play laminate T800M913 [0/90/0/90] in terms of three parameters: Number of collocation points, wave propagation direction and thickness. The algorithm is then applied to a large number of layers using three configurations: Symmetric layup T80M913[0/90]10s, hybrid layup [CFRP-Al]50 and challenging layup T800M913[0/90]100. The study found that the balancing algorithm, when combined with the SCM, is effective for structures with a large number of layers. Finally, optimizing the SCM will enhance its competitiveness as an effective tool in ultrasonic non-destructive testing for the studied structures of great industrial interest.

This article proposes a new algorithm for computing dispersion curves of ultrasonic guided waves in multi-layered composites with large number of layers. The algorithm is based on balancing the eigenvalue problem of the Spectral Collocation Method (SCM) formulation. The SCM has proven effective in analyzing single-layer and simple waveguides. However, it struggles with large multi-layer structures due to numerical instability caused by irregular and sparse matrices in the eigenvalue problem. The proposed algorithm for balancing has significantly reduced both the conditioning measure and the matrix norm in the matrix system. The optimization of spectral formulation enables accurate calculation of dispersion curves and characterization of displacement/stress profiles using Matlab software. This precise characterization and mode separation are essential for selecting ultrasonic sensors for damage detection. A comparison was made with Dispersion Calculator (DC) software. The algorithm was first validated using a hybrid multi-layered composite [CFRP-Al-CFRP-Al], which was successful. The validated algorithm was then quantitatively evaluated using a cross-play laminate T800M913 [0/90/0/90] in terms of three parameters: Number of collocation points, wave propagation direction and thickness. The algorithm is then applied to a large number of layers using three configurations: Symmetric layup T80M913[0/90]10s, hybrid layup [CFRP-Al]50 and challenging layup T800M913[0/90]100. The study found that the balancing algorithm, when combined with the SCM, is effective for structures with a large number of layers. Finally, optimizing the SCM will enhance its competitiveness as an effective tool in ultrasonic non-destructive testing for the studied structures of great industrial interest. [ABSTRACT FROM AUTHOR]

In recent years, explosion shock wave has been considered as a signature injury of the current military conflicts. Although strong shock wave is lethal to the human body, weak shock wave can cause many more lasting consequences. To investigate the protection ability and characteristics of flexible materials and structures under weak shock wave loading, the blast wave produced by TNT explosive is loaded on the polyurethane foam with the density of 200.0 kg/m3 (F-200) and 400.0 kg/m3 (F-400), polyurea with the density of 1100.0 kg/m3 (P-1100) and structures composed of the two materials, which are intended for individual protection. Experimental results indicate that the shock wave is attenuated to weak pressure disturbance after interacting with the flexible materials which are not damaged. The shock wave protective capability of single-layer materials is dependent on their thickness, density and microscopic characteristics. The overpressure, maximum pressure rise rate and impulse of transmitted wave decrease exponentially with increase in sample thickness. For the same thickness, F-400 provides better protective capability than F-200 while P-1100 shows the best protective capability among the three materials. In this study, as the materials are not destroyed, F-200 with a thickness more than 10.0 mm, F-400 with a thickness more than 4.0 mm, and P-1100 with a thickness more than 1.0 mm can attenuate the overpressure amplitude more than 90.0%. Further, multi-layer flexible composites are designed. Different layer layouts of designed structures and layer thickness of the single-layer materials can affect the protective performance. Within the research range, the structure in which polyurea is placed on the impact side shows the optimal shock wave protective performance, and the thicknesses of polyurea and polyurethane foam are 1.0 mm and 4.0 mm respectively. The overpressure attenuation rate reached maximum value of 93.3% and impulse attenuation capacity of this structure are better than those of single-layer polyurea and polyurethane foam with higher areal density.

Lightweight and high-toughness carbon fiber/phenolic ablator (CFPA) is required as the Thermal Protection System (TPS) material of aerospace vehicles for next-generation space missions. To improve the ablative properties, silica sol with good particle size distribution prepared using tetramethoxysilane (TMOS) was blended with natural rubber latex and deposited onto carbon fiber felt, which was then integrated with phenolic aerogel matrix, introducing nano-silica into the framework of CFPA. The modified CFPA with a low density of 0.28e0.31 g/cm³ exhibits strain-in-fracture as high as 31.2% and thermal conductivity as low as 0.054 W/(m.K). Furthermore, a trace amount of nano-silica could effectively protect CFPA from erosion of oxidizing atmosphere in different high-temperature environments. The oxyacetylene ablation test of 3000 °C for 20 s shows a mass ablation rate of 0.0225 g/s, a linear ablation rate of 0.209 mm/s for the modified CFPA, which are 9.64% and 24.82% lower than the unmodified one. Besides, the long-time butane ablation test of 1200 °C for 200 s shows an insignificant recession with mass and linear ablation rate of 0.079 g/s and 0.039 mm/s, 16.84% and 13.33% lower than the unmodified one. Meanwhile, the fixed thermocouple in the test also demonstrates a good thermal insulation performance with a low peak back-face temperature of 207.7.C, 12.25% lower than the unmodified one. Therefore, the nano-silica modified CFPA with excellent overall performance presents promising prospects in high-temperature aerospace applications. [ABSTRACT FROM AUTHOR]

The relative potency of common toughening mechanisms is explored for layered solids and particulate solids, with an emphasis on crack multiplication and plasticity. First, the enhancement in toughness due to a parallel array of cracks in an elastic solid is explored, and the stability of co-operative cracking is quantified. Second, the degree of synergistic toughening is determined for combined crack penetration and crack kinking at the tip of a macroscopic, mode I crack; specifically, the asymptotic problem of self-similar crack advance (penetration mode) versus $$90^{\circ }$$ symmetric kinking is considered for an isotropic, homogeneous solid with weak interfaces. Each interface is treated as a cohesive zone of finite strength and toughness. Third, the degree of toughening associated with crack multiplication is assessed for a particulate solid comprising isotropic elastic grains of hexagonal shape, bonded by cohesive zones of finite strength and toughness. The study concludes with the prediction of R-curves for a mode I crack in a multi-layer stack of elastic and elastic-plastic solids. A detailed comparison of the potency of the above mechanisms and their practical application are given. In broad terms, crack tip kinking can be highly potent, whereas multiple cracking is difficult to activate under quasi-static conditions. Plastic dissipation can give a significant toughening in multi-layers especially at the nanoscale. [ABSTRACT FROM AUTHOR]

In this study, Al–Cu-based three-layered metal matrix composites reinforced with TiC particles were produced successfully by a conventional hot pressing method under argon (Ar) atmosphere. To evaluate the effect of heat treatment, the composites were treated for 24 h at 530°C and then aged in an oil bath at 180°C for various aging periods. The multi-layered composite was characterized by SEM, EDS, XRD, and the Vickers microhardness test. It was concluded that the peak hardness for the multi-layered composite was observed, when the aging period was extended up to approximately 12 h. The hardness of the middle layer increased from approximately 140 to 291 HV within 12 h. The corrosion resistance of the composites decreased with increasing aging time and some preferential corrosion attack was detected in the composite layers. In addition, it was found that the pitting susceptibility of unaged composites was also higher than that of all the aged specimens. [ABSTRACT FROM AUTHOR]

• Development of a new procedure for the simple determination of the stress intensity factor in multilayered cantilevers. • Procedure is applicable for multilayered cantilevers and three-point bend specimens with variations of the Young's modulus. • Introduced procedure is applicable for the smart design of multilayered components with highly improved fracture toughness. • The procedure is verified for different multilayer cases by finite element computations. • Validation of procedure by analyzing experiments conducted on nanoscale structures made of multilayered TiN/SiO x thin films. In modern engineering applications, multilayered structures are extensively used. The material property variations between the layers affect the fracture behavior of the multilayer by the reduction or enhancement of the crack driving force. This "material inhomogeneity effect" complicates the determination of the stress intensity factor of cracks in multilayered components, and a combination of experimental measurements and numerical computations was necessary to determine correctly the fracture toughness. A procedure is presented in this paper, which enables the simple determination of the stress intensity factor from analytic relations, without numerical analysis. These relations are valid for multilayered cantilevers and multilayered three-point bend specimens with variations of the Young's modulus. The procedure can be also used for the design of tough multilayered composites. The procedure is verified for different multilayer configurations by finite element computations and validated with the help of experimental results from literature. [ABSTRACT FROM AUTHOR]

This work presents a computational approach for the design of functionally graded composite with the combination of topology optimization. It is clear that the shell stacking sequence in most exoskeleton sustains their body weight and defends most of the environmental implications. This feature effectively improves the structural robustness in supporting impact waves or defending the failure extensions. Imitate the achievements from bio-inspired laminating, shell is laminated in a coat-base structure and assigned as orthotropic with optimal orientation. Without introducing any new variables, the interpolation model extended from the two-step filter and the multi-gradient topology optimization process precisely defines the location of candidate materials. Besides, another smoothing and projection in the Gradient Norm after the two-step filtration is applied to eliminate the interference of adjacent boundary. Four typical materials are introduced in each layer, and their orientations are optimized by the energy approach at the Base and the gradient approach at the border. We discusses two benchmark problems and an application case in result to demonstrate the effectiveness and orientation independence of the procedure. The optimized structures are evaluated compared to the single-coated result through stress and buckling analysis. The proposed multi-layer composites can be fabricated by filament winding process or hot forming. [ABSTRACT FROM AUTHOR]

Abstract: In this investigation, aluminum layers are bonded together using epoxy adhesive. The adhesive is modified using different additives and the influence of adhesive composition on interfacial fracture energy is measured via double cantilever beam (DCB) test. To characterize the mechanical behavior of the adhesive, compression and impact tests were incorporated. The results of compression and impact tests show that compressive and impact properties of adhesives are functions of type and content of modifier. In the DCB test, it was observed that while addition of rubber particles increase interfacial fracture energy of epoxy, incorporating SiC particles decrease this parameter. Also, the results of present study illustrate that there is an optimum for the interfacial fracture energy where beyond that the impact energy of the multi-layered composite drops sharply. [Copyright &y& Elsevier]

Abstract: This study presents characterization of thermo-mechanical viscoelastic behaviors of thick-section multi-layered fiber reinforced polymer (FRP) composite materials. The studied multi-layered systems follow viscoelastic behaviors of thermo-rheologically complex materials (TCM), which allow for stress and temperature variation with time. The multi-layered composites consist of alternating layers of unidirectional fiber (roving) and randomly oriented continuous filament mat (CFM). Isothermal creep-recovery tests at various stresses and temperatures are performed on axial, transverse, and 45° off-axis specimens having E-glass/vinylester and E-glass/polyester systems. Both tension and compression creep tests are conducted for 30min followed by 10min recovery. Analytical representation of a nonlinear single integral equation is applied to model the thermo-mechanical viscoelastic responses. Long-term material behaviors are then obtained through the vertical and horizontal shifting using analytical and graphical shifting procedures. The vertical and horizontal shift factors are associated with nonlinear stress and temperature dependent parameters. Linear extrapolation of transient creep compliance can be used to extend the material responses for longer times. The extended long-term (8–16 months) creep strains of the uniaxial specimens are verified with the long-term experimental data of Scott and Zureick [Compression creep of a pultruded E-glass/vinylester composite, Compos Sci Technol, 58 (1998) 1361–1369]. Finally, sensitivity analyses are conducted to examine the impact of error in material parameter characterizations to the overall long-term material behaviors. [Copyright &y& Elsevier]

Abstract: Previous work by the authors investigated cold roll bonding followed by annealing as a simplistic method for processing Ti/TiAl3 multi-layered composites from elemental titanium and aluminum foils. In this study, the Ti/TiAl3 multi-layered composites were subjected to further processing at higher temperatures and increased cyclic cold rolling to promote the formation of gamma-based titanium aluminide (γ-TiAl). The resulting γ-TiAl phase was characterized using light microscopy equipped with digital image analysis, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and Vickers microhardness testing. The thermodynamics and kinetics of the diffusion reaction were investigated using differential scanning calorimetry (DSC) and enthalpy (H), entropy (S), and capacity (C) thermodynamic calculations. The physical, chemical, mechanical, and microstructure changes that accompanied the phase transformation to γ-TiAl was discussed. [Copyright &y& Elsevier]

In this study, multi-layered Ti/Al composites were prepared using a cyclic cold roll bonding/annealing process. The effect of initial thickness reduction by cold roll bonding and cyclic cold roll bonding/annealing on the formation and development of the titanium aluminide phase has been investigated in detail. The phases formed were characterized using light microscopy equipped with digital image analysis system and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The thermodynamics and kinetics of the diffusion reaction were investigated using differential scanning calorimetry (DSC) and enthalpy (H), entropy (S) and capacity (C) thermodynamic calculations. The diffusion reaction mechanism at this stage was discussed. [Copyright &y& Elsevier]

The fabrication of Ti-aluminide multi-layered composites have attracted great attention for their excellent mechanical properties, such as high specific strength, high specific stiffness, tolerable toughness, and low density. The preparation of the composite produced by a hybrid procedure composed of Vacuum Hot Pressing (VHP) and Hot Isostatic Pressing (HIP) using Ti foils and Al foils has been performed. Further, X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray Spectrometry (EDXS) were carried out to identify the microstructure and phase formation of the composite. In addition, an in-situ three-point bending test was conducted on the notched specimen to observe the crack propagation behavior carefully. The results indicate that the densified composite was obtained without any apparent voids and pores which could undesirably develop into the source of cracks. Furthermore, all the pure Al foils were totally consumed to form a series of the Ti-Al compounds through the diffusive reaction between the adjacent Ti and Al foils. Moreover, the in-situ observation demonstrates the initiation and propagation of cracks in the intermetallic layers and the role of residual Ti layers to blunt and bridge the cracks by their plastic deformation. This study provides a new strategy for fabricating the Ti-aluminide multi-layered composites. [ABSTRACT FROM AUTHOR]

In this work, a novel Ti/Al/Al‐Cf multi‐layered composite consisted of periodically alternating brittle and ductile layers has been successfully fabricated through the biological shell structure consideration as the prototype. Pure titanium foils (Ti), aluminum foils (Al), and woven C fibers (Cf) with Ni coating are adopted and stacked in a specific order, namely the Ti, Al, Cf, and Al order. The results demonstrated that all layers are well bonded. The Al3Ti and Al3Ni phases are formed at the Ti/Al and Cf/Al interface, respectively. Compression and three‐point bending tests are conducted to investigate the mechanical properties of the bio‐inspired composites. Due to the special layered structure and reinforcing effect of the Cf, the novel material has high compressive strength (565 MPa), as well as high bending strength (413 MPa). It is discovered that the hierarchical structure can effectively delay crack propagation and absorb energy during fracture. The authors' work provided new sight for the design of a new bio‐inspired composite and better understanding the relationship of mechanical performance and hierarchical structure. A novel biomimetic Ti/Al/Al‐Cf multilayered composite is designed and fabricated. Compression and flexural strength reach 565 MPa and 413 MPa, respectively. Compression testing reveals the structure can delay crack propagation. Composite reinforcement behavior achieve through interfacial effects and Cf. [ABSTRACT FROM AUTHOR]