Compression strength of sandwich panels with sub-interface damage in the foam core

This paper addresses the effect of a local quasi-static indentation or a low-velocity impact on the residual strength of foam core sandwich panels subjected to edgewise compression. The damage is characterized by a local zone of crushed core accompanied by a residual dent in the face sheet. Experimental studies show that such damage can significantly alter the compressive strength. Theoretical analysis of the face sheet local bending is performed for two typical damage modes (with or without a face–core debonding). The solutions allow estimation of the onset of (a) an unstable dent growth (local buckling) or (b) a compressive failure in the face sheet. The theoretical results are in agreement with the test data for two considered sandwich configurations.     

Low-velocity heavy-mass impact response of slender metal foam core sandwich beam

The objective of this work is to investigate the dynamic large deflection response of fully clamped metal foam core sandwich beam struck by a low-velocity heavy mass. Analytical solution and ‘bounds’ of dynamic solutions are derived, respectively. Also, finite element analysis is carried out to obtain the numerical solution of the problem. Comparisons of the dynamic, the quasi-static and numerical solutions for the non-dimensional maximum deflection of the sandwich beam with non-dimensional initial kinetic energy of the striker are presented for different cases of mass ratio, impact velocity and location. It is seen that the dynamic solution approaches the quasi-static one as the mass ratio of the striker to the beam is large enough, the quasi-static solution is in good agreement with the numerical results and both solutions lie in the ‘bounds’ of dynamic solutions. The quasi-static and numerical results for the impact force against the maximum deflection of the sandwich beam are obtained. It shows that the quasi-static solution can offer adequate accuracy to predict the low-velocity heavy-mass impact response of fully clamped sandwich beam.     

Static and low-velocity impact behavior of sandwich beams with closed-cell aluminum-foam core in three-point bending

In this paper, the response and failure of sandwich beams with aluminum-foam core are investigated. Quasi-static and low-velocity impact bending tests are carried out for sandwich beams with aluminum-foam core. The deformation and failure behavior is explored. It is found that the failure mode and the load history predicted by a modified Gibson's model agree well with the quasi-static experimental data. The failure modes and crash processes of beams under impact loading are similar to those under quasi-static loading, but the force-displacement history is very different. Hence the quasi-static model can also predict the initial dynamic failure modes of sandwich beams when the impact velocity is lower than 5 m/s.     

Z 向增强泡沫夹芯复合材料冲击损伤及冲击后压缩性能

基于热压罐成型工艺, 选择了树脂柱Z向增强泡沫芯材、碳纤维Z向增强泡沫芯材、Kevlar纤维缝纫增强泡沫芯材3种Z向增强复合材料结构, 对夹芯结构进行了低速冲击损伤和冲击后压缩(CAI)性能研究, 考察了不同Z向增强方式对冲击损伤面积和破坏模式的影响。结果表明, Z向增强对泡沫芯材产生了初始损伤, 其冲击后损伤面积大于未增强泡沫夹芯结构; 但Z向增强改变了夹芯结构的压缩破坏机制。通过选用合适的Z向增强材料和Z向增强参数, 能够提高夹芯结构的压缩强度和CAI强度。其中当增强材料为碳纤维, 增强参数为10 mm×10 mm时, 压缩强度提高了13%, CAI强度提高超过40%。     

Modelling of low-energy/low-velocity impact on Nomex honeycomb sandwich structures with metallic skins

In the aircraft industry, manufacturers have to decide quickly whether an impacted sandwich needs repairing or not. Certain computation tools exist at present but they are very time-consuming and they also fail to perfectly model the physical phenomena involved in an impact. In a previous publication, the authors demonstrated the possibility of representing the Nomex™ honeycomb core by a grid of nonlinear springs and have pointed out both the structural behaviour of the honeycomb and the influence of core-skin boundary conditions. This discrete approach accurately predicts the static indentation on honeycomb core alone and the indentation on sandwich structure with metal skins supported on rigid flat support. In this study, the domain of validity of this approach is investigated. It is found that the approach is not valid for sharp projectiles on thin skins. In any case, the spring elements used to model the honeycomb cannot take into account the transverse shear that occurs in the core during the bending of a sandwich. To overcome this strong limitation, a multi-level approach is proposed in the present article. In this approach, the sandwich structure is modelled by Mindlin plate elements and the computed static contact law is implemented in a nonlinear spring located between the impactor and the structure. Thus, it is possible to predict the dynamic structural response in the case of low-velocity/low-energy impact on metal-skinned sandwich structures. A good correlation with dynamic experimental tests is achieved.     

Experimental study on fatigue failure and damage of sandwich structure with PMI foam core

Sandwich structures consisting of aluminium skin sheets and polymethacrylimide foam core have been gradually used in the high‐speed trains. The static mechanical properties and fatigue damage of the sandwich structures with polymethacrylimide foam core were experimented in three‐point bending and were discussed. The failure mode is identified as local indentation. The static strength was obtained, and it showed good consistency with the forecasting formula. The fatigue property and damage evolution were also researched under cyclic loading. The fatigue life curve and the fitting formula were submitted. The fatigue damage evolution started from the skin sheet fracture and then the foam core indentation. The displacement at the midpoint as the damage parameter was discussed, and the evolution prediction formula was submitted, which showed great agreement with the experimental results.     

Free vibration analysis of simply supported sandwich beams with lattice truss core

Free vibration of AISI 304 stainless steel sandwich beams with pyramidal truss core is investigated in the present paper. The lattice truss core is transformed to a continuous homogeneous material. Considering the deformation characteristics of the sandwich beam, the following assumptions are made: (1) the thickness of the sandwich beam remains constant during deformation; (2) for the thin face sheets, only bending deformation is considered, neglecting the effect of transverse shear deformation; (3) for the core, only shear deformation is considered as the core is too weak to provide a significant contribution to the bending stiffness of the sandwich beam. The shear stress is assumed to be constant along the thickness of the core. The governing equation of free vibration is derived from Hamilton's principle, and the natural frequencies are calculated under simply supported boundary conditions. Finally, numerical simulation is carried out to get the mode shapes and natural frequencies. Our results show that the theoretical solutions agree well with the numerical results. It indicates the present method would be useful for free vibration analysis of sandwich beams with lattice truss core.     

An analytical solution for the large deflections of a slender sandwich beam with a metallic foam core under transverse loading by a flat punch

The large deflections of slender ultralight sandwich beams with a metallic foam core are studied under transverse loading by a flat punch, in which interaction of bending and stretching induced by large deflections is considered. Firstly, a unified yield criterion for metallic sandwich structures considering the effect of core strength is proposed, which is valid for metallic sandwich cross-sections with various core strengths and geometries. This can reduce to the yield criterion for a solid monolithic cross-section and the classical yield criterion for sandwich cross-sections with a weak core, respectively. Then, analytical solutions for the large deflections of fully clamped and simply supported metallic foam core sandwich beams are derived under transverse loading by a flat punch, respectively. Comparisons of the present solutions with experimental results are presented and good agreements are found. The effects of the core strength, the size of loading punch and the boundary conditions on the structural response of sandwich beams are discussed in detail. It is shown that the axial stretching induced by large deflections has significant effect on the load-carrying and energy absorption capacities of sandwich structures in the post-yield regime, and the load-carrying and plastic energy absorption capacities of metallic foam core sandwich beams may be underestimated as the core strength is neglected in analysis, especially for the sandwich beams with a strong core.     

Modelling of composite sandwich structures with honeycomb core subjected to high-velocity impact

In this study the perforation of composite sandwich structures subjected to high-velocity impact was analysed. Sandwich panels with carbon/epoxy skins and an aluminium honeycomb core were modelled by a three-dimensional finite element model implemented in ABAQUS/Explicit. The model was validated with experimental tests by comparing numerical and experimental residual velocity, ballistic limit, and contact time. By this model the influence of the components on the behaviour of the sandwich panel under impact load was evaluated; also, the contribution of the failure mechanisms to the energy-absorption of the projectile kinetic energy was determined.     

Finite element modelling of core thickness effects in aluminium foam/composite sandwich structures under flexural loading

This paper models the flexural behaviour of a composite sandwich structure with an aluminium foam core using the finite element (FE) code LS-DYNA. Two core thicknesses, 5 and 20 mm, were investigated. The FE results were compared with results from previous experimental work that measured full-field strain directly from the sample during testing. The deformation and failure behaviour predicted by the FE model compared well with the behaviour observed experimentally. The strain predicted by the FE model also agreed reasonably well with the distribution and magnitude of strain obtained experimentally. However, the FE model predicted lower peak load, which is most likely due to a size effect exhibited by aluminium foam. A simple modification of the FE model input parameters for the foam core subsequently produced good agreement between the model and experimental results.     

球形孔泡沫铝合金三明治梁的三点弯曲变形

研究了球形孔泡沫铝合金的单轴压缩性能,得到了抗压强度与相对密度的关系;与多边形孔泡沫铝合金和泡沫纯铝作了对比,发现球形孔使力学性能有了较大的提高.根据球形孔泡沫铝合金三明治梁三点弯曲的载荷(P)位移(δ)曲线研究了四种常见破坏模式并建立了破坏模式图.用极限载荷公式得到的计算值与极限载荷的实验值吻合良好.球形孔泡沫铝合金力学性能高于多边形孔泡沫铝合金及泡沫纯铝,因而其三明治梁的力学性能最好.     

Enhanced mechanical performance of foam core sandwich composites with through the thickness reinforced core

The purpose of this study is to improve the mechanical performance of the foam core sandwich composites with a rather simpler method of core reinforcement. With this aim; sandwich composite panels are manufactured using only-perforated foam and perforated-stitched foam as the core with multi-axial glass fabrics as the facesheet materials by vacuum infusion method using epoxy resin. Sandwich composites with perforated core, stitched core and plain core have been compared in terms of compressive, bending, shear and impact performances. It was seen that newly proposed perforated core specimens and stitched core specimens with relatively insignificant weight increase have superior mechanical performances than plain core specimens. Thus reinforcing foam core with perforation and stitching is proposed as simpler but very effective method in performance improvement for the sandwich composites.     

A smart multifunctional polymer nanocomposites layer for the estimation of low-velocity impact damage in composite structures

A new smart polymer nanocomposite layer, based on semiconductor Cu₂S nanoparticles embedded into an amorphous polystyrene matrix, was developed for the detection of low-velocity impact damage on composite structures. This material exhibits a combination of optical and electro-magnetic properties, in way that it can be employed to visualize in quick and effective manner barely visible impact damage (BVID). In particular, based on the phenomenon of the photoluminescence, this layer showed specific light emission in the visible range of the electromagnetic spectrum under UV-light excitation at different wavelength. The results of a series of low-velocity impact tests illustrated a visible contrast colour on the damaged area, representing the flaw. In fact, the study of the optical characteristics of this smart material is of particular benefit for fast, cost-effective and in situ damage identification. Hence, once the impact location has been determined, the detailed information of the damage can be further investigated by others non-destructive techniques.     

On the free vibration of sandwich panels with a transversely flexible and temperature dependent core material – Part II: Numerical study

This paper, the second of two, presents a numerical study of a simply-supported sandwich panel that is based on the mathematical formulation that appears in part I. The solution of the unknowns in the case of a simply-supported panel is based on a trigonometric series solution and it converts the set of PDE’s into an algebraic set of equations that are described by stiffness and mass matrices. In addition, it studies numerically, for a specific sandwich panel construction, the effects of the degradation of the mechanical properties of the core as a result of the thermal field on the free vibration response of the two computational models. The results of the mixed formulation model, denoted by MF, and the displacement formulation, denoted by DF, reveal a significant reduction of the eigenfrequencies as well as a shifting of the eigen-modes from higher modes to lower ones with increasing temperature.