The honeycomb (HC) core of sandwich structures undergoes flexural loading and carries the normal compression and shear. The mechanical properties and deformation response of the core need to be established for the design requirements. In this respect, this article describes the development of the smallest possible representative cell (RC) models for quantifying the deformation and failure process of the Nomex polymer-based hexagonal HC core structure under the out-of-plane quasi-static loadings. While the hexagonal single and multi-cell models are suitable for the tension and compression, a six-cell model is the simplest RC model developed for shear in the transverse and ribbon direction. Hashin’s matrix and fiber damage equations are employed in simulating the failure process of the orthotropic cell walls, using the finite element (FE) analysis. The FE-calculated load–displacement curves are validated with the comparable measured responses throughout the loading to failure. The location of the fracture plane of the critical cell wall in the out-of-plane tension case is well predicted. The wrinkling of the cell walls, leading to the structural buckling of the HC core specimen in the compression test, compares well with the observed failure mechanisms. In addition, the observed localized buckling of the cell wall by the induced compressive stress during the out-of-plane shear in both the transverse and ribbon direction is explained. The mesoscale RC models of the polymer hexagonal HC core structure have adequately demonstrated the ability to predict the mechanics of deformation and the mechanisms of failure.

The extensive applications of honeycomb (HC) core in sandwich structures necessitates the influence of the cellular geometry and cell wall base material on the mechanical response to be quantified. In this respect, this paper establishes the mechanics of the deformation and the failure processes of the HC core under the out-of-plane compressive, tensile, and shear loading. The corresponding mechanical properties are determined and the mechanisms of failure of the HC core structure are identified. The influence of the relative density (ρ * /ρ s ) and the cell aspect ratio (H/c) of the hexagonal HC core on the compressive deformation response, the out-of-plane properties and the characteristic dissipation energy density (DED) of the structure is established. Results show that the compressive strength increases exponentially from 1.5 to 10.6 MPa over the relative density range of 0.028(ρ * /ρ s ) 0.125. The out-of-plane shear modulus, G 13 and G 23 are 33.9 and 58.2 MPa, while the shear strength, τ 13 and τ 23 are 1.07 and 2.03 MPa, respectively. The HC core with a low aspect ratio (H/c<2.64) failed due to the early debonding of the double-wall hexagonal cells, while at H/c 2.64, by elastic buckling of the cells. A phenomenological model is formulated to highlight the combined effects of both parameters on the compressive strength (σ c ) of the HC cores, covering the range of 0.028(ρ * /ρ s )0.056 and 2.5(H/c)5.62. Furthermore, the characteristic dissipation energy density (DED) under the out-of-plane compression varies linearly within the range of 2.5<(H/c)<5.62 for the HC core with ρ * /ρ s =0.056. The HC core with H/c=3.96, but with twice higher ρ * /ρ s exhibits about twice larger DED. These resulting properties and failure mechanisms of the anisotropic paper-based HC core are useful for the validation of the predictive computational models. Nomenclature ρ * density of the HC core σ c axial compressive strength of HC core ρ s density of the bulk material A c cross-sectional area of HC core sample c hexagonal cell size E 3 Young's Modulus under compression loading E T Young's Modulus under tensile loading G 13 shear modulus in transverse direction G 23 shear modulus in ribbon direction H core height OPEN ACCESS RECEIVED

This paper investigates the damage development ahead of mode I delamination tips in carbon /epoxy laminates using scanning electron microscope (SEM). Two techniques were adopted for the investigation; the first technique consisted of the application of stepwise load increments on DCB (double cantilever beam) specimens inside the SEM, while images were recorded until delamination onset. For the second technique, the DCB specimens were fatigue tested under different combinations of monotonic and cyclic loading. After the fatigue tests, the specimens were kept open in the microscope by insertion of steel wedges allowing the inspection of the delamination tips. The investigation revealed that multiple micro-cracks are formed parallel to the delamination growth direction ahead of the tip that coalesces. Micro-cracks that were formed 2 or 3 plies away from the delamination plane were observed to cause fibre bridging.

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The tribological enhancement of nano-lubricant that consist of formulated Eichhornia Crassipes carbon nanotubes (EC-CNTs) dispersed in base rapeseed oil using sodium dodecyl sulfate (SDS) and oleic acid (OA) as surfactants was conducted in this study. The experiment was done using high frequency reciprocating rig, applying 0.5 mass%, 1 mass% and 1.5 mass% EC-CNTs alongside 2% and 5% of the various surfactants and sonicated for 25 minutes and 50 minutes respectively. The use of surfactants and sonication was to reduce agglomeration. The outcome showed that SDS yielded better results than OA, further revealed that the use of 2% SDS under sonication time of 50 minutes reduced the solution agglomeration compared to 5% SDS and sonication time of 25 minutes. The tribological test conducted on EC-CNTs concentration effects was based on friction and wear reduction, load carrying ability, lubricant film stability, wear scar micrograph and mechanism of particles. The results indicated that inclusion of nanoparticles substantially enhanced the tribological characteristics. However, pronounced enhancement was recorded with 1 mass% EC-CNT compared to counterpart lubricants. Friction and wear reduction by 1 mass% EC-CNT were 65.4% and 63.6% respectively against base oil. The nanoparticles exhibited excellent mechanism for which the tribological enhancement was achieved.