Experimental Research On The Failure Modes And Fatigue Behavior Of Honeycomb Sandwich Structures

Sandwich structures have been widely employed as the major components in fields such as aviation, aerospace. Yet there is still little use of them in merchant shipbuilding industry as a matter of costs and technology immaturity. Amendments adopted in 2011 by the IMO set mandatory measures to reduce emissions of greenhouse gases from international shipping, with the Energy Efficiency Design Index (EEDI) made mandatory for new ships. EEDI of new ships may be achieved mainly by adoption of new equipment and energy-saving technologies. Due to the characters of high strength-to-weight ratio and stiffness-to-weight ratio, sandwich structures are promised to play an important role in fulfilling the lightweight of the ships. Further researches need to be made to push on the utilization given the condition that few researches have been made about metal honeycomb sandwich structures.In this dissertation, experimental method is used as the primary method to investigate failure modes of alloy honeycomb sandwich structures in three-point bending in combination with ordinary beam theory and the fatigue life of them from a phenomenological point of view:There are several failure modes for sandwich structures and the occurrence of them relates to the materials and experimental conditions. Not all of them will occur in one condition. Analytical predictions are derived for the three-point bending strength due to face yield, indention and core shear. Firstly, ordinary beam theory is employed to get the stress distribution in the beam sections and simplified equations are presented to calculate the normal stresses in the faces and the shear stresses in the core. Critical load of face yield failure mode is derived from face yield under bending. Two models are used in indention failure:one treats both the face sheets and the core as rigid, ideally plastic solids, with the core undergoing compressive yield and the face sheets forming plastic hinges. The other one assumes that indention collapse is a local instability of the compressive face sheet and there is only elastic deformation in the face. For core shear failure mode, two kinematically admissible displacement fields, which are decided by the geometry and material properties, are considered.Specimens with different spans are tested using three-point bending tests in accord with some rules and thus we can get the failure load and load-displacement curve of each span. Failure modes and mechanisms are viewed by analyzing the loads, displacements, strains and other parameters and by the videos. Two failure modes appear in the experiments:indention of small span specimens and face yield of long span specimens. When indention happens, collapses begin from the middle of the height in the core. The moment-displacement curves show that shear load has more effects on the deformation of the short span specimens. Failure mode maps are generated and compared with the experiments. Results show that indention model with elastic faces is more suitable for the specimens. The analytical predictions meet the experiments well.In the three-point bending fatigue tests, effects of different load ratios and cell orientations on the fatigue life are investigated. The S-N curves of two cell orientations in one load ratio are got. For alloy honeycomb sandwich structures, the L configuration appears to be more suitable in high load level while specimens in W configuration are better choices in low load level. The dispersion of fatigue life in low load level is high. Load ratios have influences on the fatigue life in the condition that a lower load ratio leads to a shorter life. Fatigue life is fitted using a three-parameter power law and the prediction of life is made on the basis of stiffness degradation.