The review outlines modern trends in theoretical developments, novel designs and modern applications of sandwich structures. The most recent work published at the time of writing of this review is considered, older sources are listed only on as-needed basis. The review begins with the discussion on the analytical models and methods of analysis of sandwich structures as well as representative problems utilizing or comparing these models. Novel designs of sandwich structures is further elucidated concentrating on miscellaneous cores, introduction of nanotubes and smart materials in the elements of a sandwich structure as well as using functionally graded designs. Examples of problems experienced by developers and designers of sandwich structures, including typical damage, response under miscellaneous loads, environmental effects and fire are considered. Sample applications of sandwich structures included in the review concentrate on aerospace, civil and marine engineering, electronics and biomedical areas. Finally, the authors suggest a list of areas where they envision a pressing need in further research.

Review of current trends in research and applications of sandwich structures

An experimental and numerical study has been carried out to understand and predict the compressive failure performance of quasi-isotropic carbon–epoxy laminates with out-of-plane wrinkle defects. Test coupons with artificially induced fibre-wrinkling of varied severity were manufactured and tested. The wrinkles were seen to significantly reduce the pristine compressive strength of the laminates. High-speed video of the gauge section was taken during the test, which showed extensive damage localisation in the wrinkle region. 3D finite element (FE) simulations were carried out in Abaqus/Explicit with continuum damage and cohesive zone models incorporated to predict failure. The FE analyses captured the locations of damage and failure stress levels very well for a range of different wrinkle configurations. At lower wrinkle severities, the analyses predicted a failure mode of compressive fibre-failure, which changed to delamination at higher wrinkle angles. This was confirmed by the tests.

/pdf/compressive-failure-of-laminates-containing-an-embedded-4dhjj616fl.pdf

Compressive failure of laminates containing an embedded wrinkle; experimental and numerical study

This paper presents an experimental study on the effect of localised wrinkling defects on the strength of glass reinforced epoxy laminates. A method for generating wrinkles based on the ‘natural’ mechanism of formation was developed and used to make larger scale laminates, with the aim of being representative of industrial components. Wrinkle severity was characterised using optical microscopy and correlated to the results obtained from tensile testing, which showed that the most serious misalignment can cause a 40% strength knock-down. Two optical systems – Digital Image Correlation and Video Extensometry were used to visualise the strain distribution, which showed highly localised concentrations surrounding the wrinkle. The role of the strain distribution in causing the initiation and progression of damage is explored, and finally some implications for the design of composite components are considered.

Damage progression and defect sensitivity: An experimental study of representative wrinkles in tension

The formation of out-of-plane ply deformation causes significant reductions in the mechanical properties of composites. In-plane fibre misalignments also cause reductions in the compressive strength, yet the origins of these defects are misunderstood. This paper presents a new mechanism for the formation of wrinkles, which is based upon the shear forces generated as a result of mismatches in the coefficient of thermal expansion of composite and tool, as well as the process of ply slippage that occurs during consolidation into radii. Using a U-shaped tool, defects in composite spars have been characterised using light microscopy, showing that the tool geometry and prepreg bridging leads to “instability sites,” which lead to wrinkles up to 750 μm in height as well as in-plane misalignment of 0° plies of up to 50°. Increasing the frictional shear stress through omission of release film prevents the formation of wrinkles, supporting the mechanism presented in this paper.

A new mechanism for the formation of ply wrinkles due to shear between plies

The increasing demand for wind power has led to a significant increase in the number and size of wind turbine blades manufactured globally. As the number and physical size of turbines deployed grow...

Damage detection and full surface characterization of a wind turbine blade using three-dimensional digital image correlation

Investigation of failure mechanisms in GFRP sandwich structures with face sheet wrinkle defects used for wind turbine blades

Investigation of Failure Mechanisms in GFRP Sandwich Structures with face Sheet Wrinkle Defects used for Wind Turbine Blades

Fatigue failure of sandwich beams with face sheet wrinkle defects

A unidirectional glass fiber/epoxy composite was characterized under multi-axial loading by testing off-axis specimens under uniaxial tension and compression at various angles relative to the fiber direction. Iosipescu shear tests were performed with both symmetric and asymmetric specimens. Tests were performed on both 1-2 and 1-3 material coordinate planes. Strain gauges and Digital Image Correlation were used to record the stress–strain responses. A new approach was used to define a ‘failure initiation strength’ by analyzing the recorded stress–strain curves. The experimentally determined failure stresses were compared with the predictions of the maximum stress, Tsai-Wu and Northwestern University failure criteria. It was found that using the approach of analyzing the stress–strain curve to define a point of material failure initiation, it was possible to obtain good correlation between the experimental data and predictions by both the Tsai-Wu and the NU failure criteria.

Martin Klitgaard Leong

Papers

Investigation of failure mechanisms in GFRP sandwich structures with face sheet wrinkle defects used for wind turbine blades

Investigation of Failure Mechanisms in GFRP Sandwich Structures with face Sheet Wrinkle Defects used for Wind Turbine Blades

Fatigue failure of sandwich beams with face sheet wrinkle defects

Interlaminar/interfiber failure of unidirectional glass fiber reinforced composites used for wind turbine blades

Proceedings ECCM15 - 15Th European Conference On Composite Materials, Venice, Italy, 24-28 June 2012