Bottom Face Sheets Research Articles

Response of the orthogonal corrugated sandwich panel under low-velocity impact with different shaped impactors

This study aims at investigating the low-velocity impact resistance of the composite orthogonal corrugated sandwich panel through experimental and simulation methods, considering different shaped impactors. To that scope, the progressive damage model with the sandwich panel is developed and numerical investigations are carried out in ABAQUS / Explicit. The results show that as the impactor changes from flat to hemispherical and then to conical shape, the first peak load gradually decreases, and the maximum displacement of the impactor and total energy absorption gradually increase. The high impact energy makes the sandwich panel more compliant. Furthermore, the expansion area of fiber damage, matrix damage and delamination in the top face sheet, core and bottom face sheet depends on the impactor shape and impact energy. Under high impact energy, the damage under the conical impactor mainly expands along the thickness direction of the sandwich panel rather than in the in-plane direction, while the flat impactor causes large damage area in the in-plane direction as well.

Vibration Analysis of Damaged Viscoelastic Composite Sandwich Plate

This paper presents a study of free and forced vibration of composite sandwich plate with and without damage using the finite element analysis developed under ANSYS APDL software. This modeling uses the 8-node shell 281 element to modelized the composite laminates of the top and bottom face sheets of the sandwich plate and the 20-node higher order solid 186 element to modelized the viscoelastic core. Two sets of boundaries are considered; Simply supported and clamped boundary conditions at all edges. The effect of damaged face sheets layers on natural frequencies, mode shapes, the frequency and transient responses of the sandwich plate is investigated. Numerical results show remarkable shifts of the natural frequencies, frequency and transient responses due to the presence of damage. It is concluded that the natural frequencies decreasing is due to the loss of structural stiffness. Natural frequencies of the sandwich plates are close to those issued from numerical results available in the literature.

Cryogenic insulation performance of polyimide foam-filled fluted-core composite sandwich panels

Foam-filled composite sandwich structures have demonstrated superior thermal insulation performance, making them highly suitable for cryogenic tanks in launch vehicles. This study focused on the cryogenic insulation characteristics of carbon fiber-reinforced plastic (CFRP) fluted-core sandwich panels filled with polyimide foam. The fluted-core sandwich panel was simplified into a homogeneous multilayer plate to establish a one-dimensional heat transfer theoretical model. This model was used to predict steady-state temperature approximate solutions by linearizing the temperature-dependent thermal conductivity. The sandwich specimens were fabricated using an integrated forming co-cured method. Self-developed cryogenic insulation experiment and finite element simulations were conducted to acquire temperature distributions and validated the precision of the theoretical model. Results showed that the specimens achieved insulation temperature difference exceeding 143.05 °C, with effective thermal conductivity reaching 0.0326 W/(m·°C), significantly lower than that of CFRP. The temperature distributions along the thickness direction displayed a nonlinear increasing trend. The cryogenic thermal short effect was noticeable at the junction of the bottom facesheet and web. An analysis of geometric parameter influences revealed that increasing the short span, core height, or reducing web thickness could enhance thermal insulation characteristics with minimum effective thermal conductivity of 0.0276 W/(m·°C) and lead to a more uniform distribution of bottom temperatures with minimum temperature difference of 3.30 °C. These findings provide a foundational analysis for designing cryogenic insulation structures.

Mechanical performance of stitched foam-filled honeycomb sandwich panels

Foam-filled Honeycomb Sandwich panels (FHS) are widely used in various industrial applications due to their superior multifunctional properties. However, these sandwich panels fail in some extreme conditions due to interfacial failure between the face sheet and core. To overcome this issue, novel stitched foam-filled honeycomb sandwich panels (SFHS) were fabricated by integrating the top face sheet and bottom face sheet by the stitching technique. Stitching through the face sheet and core is to improve the interfacial strength and increases the mechanical properties of the sandwich panel. FHS and SFHS were prepared by the Vacuum Assisted Resin Transfer molding (VARTM). Flexural, flatwise, and edgewise compression analysis values for SFHS panels are 2.02, 1.28, and 1.25 times higher than for FHS panels. SFHS panel weight increased slightly but had superior mechanical properties than FHS. These innovative SFHS panels can be broadly used for applications including structural, flooring, and body panels.

Study on low-velocity impact response of kevlar/epoxy-polyurethane sandwich panels

This study aims to investigate the maximum energy absorption of sandwich panels featuring composite facesheets and a polyurethane foam core under low-velocity impact. The research explores various impactor head geometries, fiber orientations, and the number of composite layers on the panel facesheets. Three different impactor heads with flat, hemispherical, and conical shapes were used for experimental impacts. Numerical simulations were performed using Abaqus/Explicit finite element software, with damage initiation in the composite layers determined by the three-dimensional Hashin criterion. The results revealed that the conical-head impactor caused the highest energy absorption, accompanied by the greatest displacement and velocity changes. Among specimens with different fiber orientations, the 60° fiber layers exhibited a 9.41% and 8.45% higher maximum force compared to the 30° and 45° fiber layers, respectively. Furthermore, the study investigated the influence of the number of composite layers in the facesheets. It was found that panels with more layers in the bottom facesheet demonstrated a 4.94% increase in energy absorption compared to panels with more layers in the top facesheet. This research provides valuable insights into optimizing sandwich panel designs for enhanced energy absorption during low-velocity impact scenarios.

Analytical study on the dynamic mechanical behaviours of foam-core sandwich plate under repeated impacts

The foam-core sandwich structures are probably subjected to repeated impact loadings in engineering applications, resulting in structural plastic deformation and accumulative damage. A valuable analytical approach is needed to predict the accumulative plastic deformation of foam-core sandwich structures under repeated impacts. In this paper, an analytical model of the dynamic responses of foam-core sandwich plates under repeated impacts is proposed using the circumscribing yield locus and the inscribing yield locus based on the rigid-perfectly plastic theory. By employing the principle of energy dissipation rate balance, the analytical expressions of structure permanent deformation and peak force are obtained from the iteration of initial conditions. In addition, polyvinyl chloride (PVC) foam has steady chemical properties and corrosion resistance, which can better adapt to the practical environment and meet the needs of structural protection. To validate the proposed analytical model, the repeated impact dynamic mechanical behaviours of PVC foam-core sandwich plates were experimentally and numerically studied. Results show that theoretical results such as the permanent deformation, peak force, and deformation modes of foam-core sandwich plates under repeated impacts are in good agreement with experimental and numerical results. With the increase in impact numbers, the global bending deformation and local indentation deformation of facesheets increase, the PVC foam core layer is constantly being compressed, and the bottom facesheet plays an increasingly important part in the repeated impact energy absorption with a maximum value of 29.20 %.

The relationship between the impact position interference and CAI strength of composite sandwich structures under double impacts

Sandwich structures are susceptible to multiple-impacts, and such multiple-impacts will typically occur at certain distances from each other. Therefore, the influence of five different distances between impact positions (DBIP) and three impact energies on compression after impact (CAI) strength are considered and discussed for honeycomb sandwich structure with composite face sheets in this paper. By comparing two typical impact characteristics, i.e., maximum impact force and displacement, the trend of CAI strength shows a turning-point, which is closely related to the size of the impactor. However, high impact energy (15 J) leads to damage in the bottom face sheet during the second impact, and the presence of damage in both top and bottom face sheet reduces CAI strength, resulting in the turning point of CAI strength not exist. It was observed that there is obvious buckling failure, and some cracks along the width direction from the impact point to the edge after CAI. To provide a more comprehensive understanding of double impacts and CAI behavior, a finite element (FE) model considering different DBIP is developed. In addition, the predicted fraction of energy absorbed by top face sheet, honeycomb core, and bottom face sheet is also obtained to reveal the relationship between the impact position interference and CAI strength.

Energy dissipation mechanism and ballistic characteristic optimization in foam sandwich panels against spherical projectile impact

This study systematically examines the energy dissipation mechanisms and ballistic characteristics of foam sandwich panels (FSP) under high-velocity impact using the explicit non-linear finite element method. Based on the geometric topology of the FSP system, three FSP configurations with the same areal density are derived, namely multi-layer, gradient core and asymmetric face sheet, and three key structural parameters are identified: core thickness (tc), face sheet thickness (tf) and overlap face/core number (no). The ballistic performance of the FSP system is comprehensively evaluated in terms of the ballistic limit velocity (BLV), deformation modes, energy dissipation mechanism, and specific penetration energy (SPE). The results show that the FSP system exhibits a significant configuration dependence, whose ballistic performance ranking is: asymmetric face sheet > gradient core > multi-layer. The mass distribution of the top and bottom face sheets plays a critical role in the ballistic resistance of the FSP system. Both BLV and SPE increase with tf, while the raising tc or no leads to an increase in BLV but a decrease in SPE. Further, a face-core synchronous enhancement mechanism is discovered by the energy dissipation analysis, based on which the ballistic optimization procedure is also conducted and a design chart is established. This study shed light on the anti-penetration mechanism of the FSP system and might provide a theoretical basis for its engineering application.

Bending performance and failure mechanisms of hybrid and regular sandwich composite structures with 3D printed corrugated cores

The effect of core geometry and hybridization on the bending performance and failure mechanisms of carbon fibre-reinforced polymer (CFRP) and glass fibre-reinforced polymer (GFRP) corrugated sandwich composite structures (SCS) were experimentally investigated using a three-point bend test. The CFRP and GFRP corrugated cores and facesheets were produced using Fused Filament Fabrication (FFF) and vacuum-assisted infusion processes, respectively. Three types of corrugated SCSs were built: SCSs with different core geometries (circular, square, trapezoidal, sinusoidal, and triangular), hybrid SCSs with different CFRP and GFRP cores and facesheets, and fully 3D-printed CFRP and GFRP SCSs. The corrugated SCS with square core geometry outperformed due to the presence of vertical walls and a large bonding area. The hybrid SCSs with a CFRP core showed a significant load drop due to shear failure in the 3D-printed core caused by weak inter-layer bonding. In contrast, the hybrid SCS with a GFRP core deformed plastically and absorbed more energy without failure due to strong inter-layer bonding. In fully 3D printed SCSs, the GFRP specimen failed catastrophically due to higher bending stress at the bottom facesheet, while the CFRP undergoes plastic deformation without failure. Results elucidate that the hybrid corrugated SCS with a GFRP top facesheet and 3D-printed core is an appropriate configuration for superior bending performance. The proposed 3D-printed SCS enables optimising complex shapes and material distribution in the core, resulting in improved strength-weight ratios. These findings will make an important contribution to the design and development of fibre-reinforced 3D-printed SCS for lightweight and high-performance applications.

Bilayer lattice structure integrated with phase change material for innovative thermal protection system design

In recent years, ceramic matrix composite (CMC) lattice structure has become a promising candidate for a new generation of integrated thermal protection systems (ITPSs) used in the ultrahigh-temperature environment. However, the thermal short circuit effect caused by lattice cores limits their application. In this work, we propose a bilayer CMC lattice structure filled with phase change material (PCM) and thermal insulation material for integrated thermal protection systems (ITPSs) to mitigate the thermal short circuit effect. The numerical simulation method is used to verify the effectiveness of the bilayer lattice structure in mitigating the thermal short circuit effect compared with one-layer ITPS. By inserting the middle face sheet, the thermal insulation effect enhances about 16%. Furthermore, the effects of thermal insulation materials, thermal conductivity and thickness of paraffin/expanded graphite composite PCM, and the diameter of the lattice core strut on the thermal insulation performance are discussed. The results show that the thickness of PCM has an optimal solution. Optimizing results show that when the aerogel insulation fiber is used as the thermal insulation material, the thermal conductivity k=0.82 W/(m⋅K) and the thickness t3=10 mm of PCM, the diameter of the core strut d=1.5 mm, the bilayer PCM-ITPS has the best thermal insulation performance. When the ultrahigh-temperature load is continuously loaded, the highest temperature of Top Face Sheet is more than 2300°C, but the highest temperature of Bottom Face Sheet is only 30.12°C, which thermal insulation effect is far better than the traditional ITPS. This work gives a novel scheme to solve the thermal short circuit effect, improving the thermal protection performance and accelerate the application of lattice structure in ITPS.

Modeling and Re‐Examination of Nonlinear Vibration and Nonlinear Bending of Sandwich Plates with Porous FG‐GPLRC Core

In this article, an inhomogeneous model is proposed to predict the material properties of porous GPL‐reinforced composite (GPLRC) and to predict the mechanical responses of a sandwich plate which consists of top and bottom metal face sheets and a porous GPLRC core. The GPLRC core is assumed to be multilayers, and each layer may have different values of porosity coefficient to achieve a piece‐wise functionally graded pattern. Young's moduli along with shear modulus of porous GPLRC core are predicted by a generic Halpin‐Tsai model in which the porosity is included. Thermo‐mechanical properties of both metal face sheets and porous GPLRC core are assumed to be temperature‐dependent. The governing equations of motion for porous sandwich plates are solved by applying a two‐step perturbation approach to obtain the analytical solutions for the two cases of nonlinear vibration and nonlinear bending of porous sandwich plates. Numerical studies are performed to compare the results obtained from the present model and the equivalent isotropic model (EIM). The results reveal that, for most cases, the difference of natural frequencies between two models is over 30%, and the vibration frequency–amplitude curves and the bending load–deflection curves are underestimated by using the EIM.

Flexural behaviors of sandwich panels with AR-glass textile reinforced concrete under low-velocity impact

Textile reinforced concrete（TRC）can be used as the face-sheet for sandwich panels due to its lightweight, excellent tensile strength, high energy absorption, and ductility. This study investigates the flexural behavior of alkali-resistant glass-textile reinforced concrete (ARG-TRC) sandwich panels with different textile layers, core thicknesses, and interface forms under various impact velocities. A new manufacturing technique for ARG-TRC sandwich panels is proposed. The sandwich panels with four textile layers have superior load-carrying capability and impact resistance than those with two layers. The impact load can be transferred more effectively for panels with a core thickness of 40mm, allowing the top and bottom face-sheets to bear the load simultaneously for better impact resistance. Ribbed panels can reduce the possibility of side core tensile fracture and core crushing while improving flexural stiffness. It was also found that TRC sandwich panels typically exhibit four failure modes under the impact load, including face-sheet fracture, core shear fracture, side core tensile fracture, and core crushing fracture. The findings of this study can provide some insights into promoting the design and application of TRC sandwich panels in residential and industrial structures.

Ice impact response and energy dissipation characteristics of PVC foam core sandwich plates: Experimental and numerical study

In this paper, the dynamic responses and energy dissipation characteristics of polyvinyl chloride (PVC) foam core sandwich plates under ice impact are investigated. The ice impact tests of PVC foam core sandwich plates were conducted by employing the horizontal impact experimental apparatus. The finite element simulations were conducted to analyze the dynamic response of PVC foam core sandwich plates based on soil and concrete material model for ice impactor. It was demonstrated that numerical results were in good agreement with experimental results. The deformation modes of the top facesheets were coupling of local indentation with global bending deformation, while the deformation modes of bottom facesheets were overall bending deformation. The permanent deformation of face sheets show that the impact resistance of sandwich plate is better than that of equivalent weight hull plate (EWHP). In addition, based on the actual navigation environment of ship, the effect of impact angle and ice floe shape on dynamic response and energy dissipation are analyzed.

Dynamic Responses of Sandwich Beams with Polymethacrylimide (PMI) Foam Cores When Subjected to Impact Loading.

This paper focusses on the load-sustaining and transfer mechanisms of sandwich beams with various types of PMI foam cores under low-velocity impact loading. In the case of quasi-static loading, the different failure modes, failure loads, and deflections were obtained, which agreed well with the results predicted by the theory of sandwich structure. In the case of impact loading, the clamped sandwich beams were subjected to the impact of a striker bar with a momentum of 10 kg∙m/s to 20 kg∙m/s. The de-acceleration of the strike bar was measured to analyze the impact force and energy absorption, and the corresponding failure modes were also obtained. The results showed that the impact force and the corresponding duration time increases with the increases in the thickness of the face sheet and the density of the core. In addition, the failure modes of the sandwich beams transferred from the shear failure mode to the tensile failure mode, which was attributed to the strength ratio between the bottom face sheet and the core. In combination with the experimental results and the plastic hinge theory, the deformation mechanisms of the different sandwich beams are also discussed.

Buckling and free vibration analysis of bio-inspired laminated sandwich plates with helicoidal/Bouligand face sheets containing softcore

Helicoidal laminates inspired by mantis shrimp crustacean can sustain higher loads compared to conventional laminated structures. However, there is still a lack of bending studies on the plates inspired by these helicoidal structures. The buckling and free vibration studies have been undertaken in the present work on the laminated composite and sandwich plates inspired by the helicoidal structures of the biological characters. Investigations are carried out using higher-order zigzag theory. For sandwich plates, the top and bottom face sheets are assumed to be made up of helicoidal layup schemes. Different kinds of helicoidal schemes, namely, recursive, exponential, semicircular, Fibonacci, and linear helicoidal are considered during the analysis. The buckling and free vibration behavior of helicoidal-based laminated sandwich plates are compared with the conventional laminated sandwich plates such as uniform distribution and cross-ply configurations. The influence of the number of layers, helicoidal schemes, geometric properties, and end conditions on the buckling and free vibration analysis of bio-inspired helicoidal laminated composite and sandwich plates is carried out in detail.

Effects of core air gaps and steel inserts on thermal response of GFRP-balsa sandwich panels subjected to fire

Sandwich systems composed of combustible glass fiber-reinforced polymer (GFRP) face sheets and balsa cores must fulfill fire resistance requirements if used, for instance, in bridge or building construction. Air gaps and steel inserts in the balsa core, the latter to suspend external pipes below bridge decks for example, may therefore present critical zones where heat ingress is facilitated, and fire resistance is therefore reduced. The thermal responses of such GFRP-balsa sandwich panels comprising through-thickness air gaps or steel inserts, and subjected to a one-sided external fire, were thus investigated by furnace experiments and fluid-structure interaction simulations. Through-thickness air gaps and steel inserts between the face sheets cooled the surrounding zones close to the hot face and heated those located towards the cold face. In T-shape steel inserts penetrating the bottom face sheet, the fire-exposed flange collected the heat and the whole section around the insert was significantly heated. The results revealed that the remaining cross section might be considerably reduced by char formation around the air gaps and steel inserts. Although the effect is quite local, it may be significant if slabs are unidirectional or if it occurs at critical locations, e.g., in the vicinity of supports.

Failure mechanisms of fluted-core sandwich composite panels under uniaxial compression

Fluted-core sandwich composite structures are major load-bearing components used in launch vehicles. Herein, analytical models were proposed to investigate the failure mechanisms of fluted-core sandwich composite panels, including global buckling, local buckling, and material failure. Failure mode maps of fluted-core sandwich panels were constructed to analyze the intrinsic impact mechanism of geometric parameters on failure behavior. Integrated fluted-core sandwich composite panels were then fabricated by adopting the integrated forming process and co-cured method. Uniaxial compression tests and finite element simulation were also conducted to verify the analytical predictions with good consistency. The effects of the geometric variable on both load-bearing capacity and failure modes of the structures were explored. The results indicated the occurrence of global buckling at small distances between the top and bottom face sheets, resulting in undesired load-carrying capacities. The decrease in width of short span or increase in thickness of face sheets prevented such local-buckling behavior, resulting in material failure. The triangular-shaped core with zero short spans was identified as a better option for material failure as dominant failure mode for designing load-bearing sandwich panels. These findings look promising for lightweight and load-carrying applications, providing a reliable reference for the initial design of such sandwich structures.

Structural response of sandwich structures with CFRP face sheets under quasi-static indentation and high velocity impact: An experimental and numerical study

Honeycomb sandwich structures (HSSs) with carbon fibre reinforced polymer (CFRP) composite face sheets are extensively used as light-weight structures in aerospace engineering due to their high strength-to-weight ratio and energy absorption properties. However, the composite face sheets are highly vulnerable to impact loads and cause damage to the structure based on the impact energy. This study investigates the structural response of HSS with CFRP face sheets under quasi-static indentation and a wide range of impact energy: low to high velocity impact. A finite-element model was developed and numerical simulations were carried out at various impact energies, thereby providing deeper insights into the impact dynamics and understanding various damage states such as dent, front face sheet perforation, core damage and rear face sheet penetration. The numerical simulation result was compared with the experimentally tested HSS using a single-stage gas gun under 53.6 J to validate the finite-element model in terms of deformation and damage status. A quasi-static indentation test was conducted and numerically predicted force data under impact test for the complete perforation case was compared to address the dependency of rate of loading. The carbon nanotube (CNT) with various weight percentages (wt%), such as 0.2, 0.4 and 0.6, was added to the matrix system through a vacuum assisted resin transfer technique and experiments were conducted at 79, 107 and 135 J. The impact resistance increases with CNT addition and hence no perforation was recorded for all the test cases of 0.6 wt% CNT addition. The influence of CNT addition on the damage area is more on the bottom face sheet and a 57% reduction in damage area was recorded for the case of 0.6 wt% CNT addition at 135 J impact energy when compared to the neat carbon/epoxy composite.

Multi‐walled carbon nanotube grafted 3D spacer multi‐scale composites for electromagnetic interference shielding

AbstractThe development of structural fiber reinforced polymer composites with various additional functionalities is becoming a hot research area to achieve the application of multi‐functional composites in the aerospace and automotive industries. An innovative material solution is 3D spacer composites with distinctive anisotropic structural characteristics. Herein, we report the manufacturing of multi‐walled carbon nanotubes (MWCNTs) grafted of 3D spacer glass/epoxy multi‐scale composites and their electromagnetic interference shielding efficiencies (EMSE). To manufacture multi‐scale composites, we utilized dip coating, vacuum filtering, and vacuum infusion methods to introduce MWCNTs of the woven fabric, while we also modified the epoxy resin with MWCNTs to increase electrical conductivity of intrinsic insulator epoxy resin. Owing to the rectangular‐shaped channel structure, which is beneficial for multiple reflection and scattering between top and bottom face sheets, the resultant 3D spacer multi‐scale composite represented a good EMSE performance of −18.3 dB in the frequency range of 8.2–12.4 GHz with an increase of 107% comparing the corresponding neat composite counterpart. Moreover, we measured the in‐plane conductivity as 1.89E‐2 S/m after MWCNTs grafting, while the out‐of‐plane conductivity remained three times lower than the in‐plane conductivity. Dynamic mechanical analysis revealed that the storage modulus increased almost three times with the MWCNTs grafting, while glass transition temperature shifted to higher temperatures (from 77.5 to 89.7°C). Therefore, we anticipate that our study will expand the use of 3D spacer composites in the aviation and automotive industries.

Study on CNT reinforced functionally graded sandwich conical shell subjected to low-velocity impact under thermal environment

In the present work, low-velocity impact behavior of pre-twisted carbon nanotubes (CNTs) reinforced functionally graded (FG) sandwich conical shell panels is studied using finite element method. The top and bottom face sheets are reinforced with single-walled CNTs reinforced FG materials with various patterns. The temperature-dependent material properties of the CNTs reinforced functionally graded facings are evaluated using micromechanical models. The dynamic equilibrium equation of the impacted sandwich panel is formulated using Lagrange’s equation. A modified Hertzian contact law is used to evaluate contact force. The solutions of the resulting equations are obtained using Newmark’s time integration method. After validation study of the present method, the effects of the CNTs grading pattern, velocity of the impactor, size of the impactor, CNTs volume fraction, operating temperature, angle of twist, and core-to-facing thickness ratio on the low-velocity impact response of the CNTs reinforced functionally graded sandwich conical shell panel are studied in detail. Numerical results show that increasing the volume fraction of CNTs increases the contact force while a lower contact force is predicted at elevated temperatures.