The recent development in the marine industry with largerships built in sandwich construction and also the use of moreadvanced materials has enforced improvements of design criteriaregarding wrinkling. The commonly used Hoff’s formula isnot suited for the highly anisotropic fibre reinforced sandwichface sheets of today. The work presented herein investigates the wrinklingphenomenon. A solution to wrinkling of anisotropic sandwichplates subjected to multi-axial loading is presented. Thesolution includes the possibility of skew wrinkling where thewrinkling waves are not perpendicular to the principal loaddirection. The wrinkling angle is obtained from the solutiontogether with the maximum wrinkling load. This method has beensupported with tests of anisotropic plates subjected touni-axial and bi-axial loading. The effect of the face sheet local bending stiffness showsthe importance of including the face sheet stacking sequence inthe wrinkling analysis. The work points out the influence ofthe face sheet local bending stiffness on wrinkling. Threedifferent means of improving the wrinkling load except changingcore material is evaluated. The effect of the differentapproaches is evaluated theoretically and also throughcomparative testing. The transition between wrinkling and pureface sheet compression failure is investigated. Theoreticaldiscussions are compared with compressive test results of twodifferent face sheet types on seven different core densities.The failure modes are investigated using fractography. Theresults clearly show how the actual sandwich compressionfailure mode is influenced by the choice of core material,changing from wrinkling failure to face sheet micro bucklingfailure as the modulus density increases. Finally, a new approach is presented where the wrinklingproblem is transferred from a pure stability problem to amaterial strength criterion. The developed theory providesmeans on how to decide which sandwich constituent will failfirst and at which load it will fail. The method give insightto and develop the overall understanding of the wrinklingphenomenon. A very good correlation is found when the developedtheory is compared with both finite element calculations and toexperimental tests. <b>Keywords:</b>wrinkling, local buckling, imperfection,stability, anisotropy, sandwich

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<p>The recent development in the marine industry with largerships built in sandwich construction and also the use of moreadvanced materials has enforced improvements of design criteriaregarding wrinkling. The commonly used Hoff’s formula isnot suited for the highly anisotropic fibre reinforced sandwichface sheets of today.</p><p>The work presented herein investigates the wrinklingphenomenon. A solution to wrinkling of anisotropic sandwichplates subjected to multi-axial loading is presented. Thesolution includes the possibility of skew wrinkling where thewrinkling waves are not perpendicular to the principal loaddirection. The wrinkling angle is obtained from the solutiontogether with the maximum wrinkling load. This method has beensupported with tests of anisotropic plates subjected touni-axial and bi-axial loading.</p><p>The effect of the face sheet local bending stiffness showsthe importance of including the face sheet stacking sequence inthe wrinkling analysis. The work points out the influence ofthe face sheet local bending stiffness on wrinkling. Threedifferent means of improving the wrinkling load except changingcore material is evaluated. The effect of the differentapproaches is evaluated theoretically and also throughcomparative testing. The transition between wrinkling and pureface sheet compression failure is investigated. Theoreticaldiscussions are compared with compressive test results of twodifferent face sheet types on seven different core densities.The failure modes are investigated using fractography. Theresults clearly show how the actual sandwich compressionfailure mode is influenced by the choice of core material,changing from wrinkling failure to face sheet micro bucklingfailure as the modulus density increases.</p><p>Finally, a new approach is presented where the wrinklingproblem is transferred from a pure stability problem to amaterial strength criterion. The developed theory providesmeans on how to decide which sandwich constituent will failfirst and at which load it will fail. The method give insightto and develop the overall understanding of the wrinklingphenomenon. A very good correlation is found when the developedtheory is compared with both finite element calculations and toexperimental tests.</p><p><b>Keywords:</b>wrinkling, local buckling, imperfection,stability, anisotropy, sandwich</p>

Thesis (M.S.)--California Polytechnic State University, 2009. / Mode of access: Internet. Title from PDF title page; viewed on Jan. 21, 2010. Major professor: Dr. Faysal Kolkailah. "Presented to the faculty of California Polytechnic State University, San Luis Obispo." "In partial fulfillment of the requirements for the degree [of] Master of Science in Aerospace Engineering." "December 2009." Includes bibliographical references (p. 92-93).

Thesis--University of Florida, 1970. / Manuscript copy. Vita. Description based on print version record. Bibliography: leaves 171-183.

Thesis (M.C.E.)--University of Delaware, 2009. / Principal faculty advisor: Jennifer Righman McConnell, Dept. of Civil & Environmental Engineering. Includes bibliographical references.

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This thesis focuses on closed-cell aluminium foams (ALPORAS and ALULIGHT) and on sandwich panels comprising these foams laminated with 2/2 twill carbon fibre (MTM56/0300) skins. The thesis experimentally and analytically investigates the response of foam-only panels (ALPORAS) to indentation with various indenter sizes and shapes; and also studies the behaviour of sandwich panels to contact damage caused quasi-statically or by impact. Quasi??static uniaxial compression testing is used to determine the mechanical properties of the foams (ALPORAS and ALULIGHT). It is revealed that the plastic collapse strength (σ* pl) obtained from the stress??strain curves is lower than the values predicted by the Gibson-Ashby theoretical model. This phenomenon is explained by the fact that the aluminium foams tested are imperfect, non-homogeneous and non-isotropic, and show a distinct cell elongation. Whereas, the Gibson-Ashby theoretical model was based on the finite element method applied to the response of a unit tetrakaidecahedral closed cell having flat faces. The experimental work shows that the deformation of the foam-only panels to indentation is caused by progressive crushing of the cell bands and by shearing and tearing of the cell walls. This thesis presents new analytical models for the response of the foam-only panels and estimates the applied deformation load in all types of indentation. By fitting the experimental load-displacement curves, the shear strength (τ* pl) and the tear energy (γ) are deduced. Compared to the literature, more consistent results are obtained for the shear strength (τ * pl) and the tear energy (γ) from all types of indentation. It is also suggested to determine (τ * pl) and (γ) through indentations with long punches (FEP and LCP), instead of hemi-spherical or cylindrical indenters, because indentation on enclosed areas shows some indenter size dependence. It is concluded that thinner panels are not suitable for the determination of the tear energy (γ) since the densification of the foam is achieved before the tear resistance is fully engaged. Another objective of this thesis is to study the response of sandwich panels comprising a closed??cell aluminium foam core and laminated with carbon fibre skin to quasi-static and impact local damage. Special attention is paid to the residual (remnant) strength in bending of the already indented sandwich panels (quasi-statically or by impact) up to the failure point. The remnant strength in bending is determined by carrying out four point bending strength tests. The local damage is located on either the compressive or on the tensile side of the sandwich panels. Thus, the capacity of the panels to resist transverse loads after they have been locally damaged at contact is investigated. The contact damage on the sandwich panels is experimentally simulated using spherical indenters. The quasi-static indentation is carried out at a low constant velocity (0.5mm/min) ?? the induced contact damage is found to be independent on the sample thickness but dependent on the indenter diameter. On the contrary, the impact test indicates velocity-dependence of the failure mode of the sandwich panel (i.e. skin breakage or punch through) which is found from the load-displacement curves. The results reveal that there is a correlation between the area of the contact damage and the remnant strength, and that the use of metal foam cores leads to high contact damage resilience of composite structures.