Elsevier

Composite Structures

Volume 125, July 2015, Pages 287-294
Composite Structures

Stiffness and strength degradation of damaged truss core composites

https://doi.org/10.1016/j.compstruct.2015.02.019Get rights and content

Abstract

Truss core laminates display stiffness and strength/density ratios superior to those seen in foam cored laminates. However, this superiority is held only for ideal shaped struts. If the truss core is damaged, its performance rapidly decreases towards that of a foam. The present study investigates the stiffness and strength degradation with imposed core deformation/damage. This is done for a pyramidal core structure made by electro-discharge machining from AA5083 alloy. The experiments are compared with finite element predictions. The effect of the strain rate sensitivity is studied by performing the tests at different temperatures and by FE simulations with different material data sets. The results show reasonable agreement between experiments and modeling. The stiffness of a damaged truss core rapidly degrades and reaches the performance levels seen in foams after ≈8% of deformation. The results show that a high strain rate sensitivity significantly influences post-buckling core behavior and is able to decrease the stiffness and strength degradation rate.

Introduction

An important avenue of materials research is developing materials which are able to fill the gaps in materials property space. Ashby and Bréchet [1] provide an overview of different design strategies for achieving this. Particularly successful solutions are materials with an internal architecture which provides an additional degree of freedom for tuning materials properties. Structural variants of these types of materials provide high stiffness/density or strength/density ratio compared to bulk materials (see also Fleck et al. [2]). There are two distinct categories of these architectured materials. They are characterized by the type of deformation that prevails at their structural elements. It can be bending dominated or stretch dominated [3]. A typical representative of the first category is a foam structure. Its structural elements are randomly organized and oriented which results in domination of bending deformation [4], [5]. Stretch dominated architectures are represented by periodic truss structures which are characterized by an organized pattern of identical structural members. The dominating loading is tension–compression [6]. Structures are generally more resistant to stretching than to bending and therefore the structural performance of periodic truss structures is typically superior to foams.

Truss structures exceed the performance of foams by factor of 0.3/ρ¯ in compressive stiffness and 1/ρ¯ in compressive strength where ρ¯ is relative density of the structure [2], [7], [8]. However, this superiority is valid only for a perfect undamaged truss core. If the struts are damaged i.e. bent, the performance of the structure tends to degrade towards the properties of foams. Studies published to date mostly focus on two aspects: the optimization of the structure geometry to achieve the highest ratio of stiffness/strength to density [9], [10], and failure mechanism characterization e.g. yielding, buckling, cracking [11], [12]. The present study focuses on the performance of a periodic core damaged by previous compressive loading over its plastic buckling limit. Particular attention is paid to the degradation of the core stiffness.

The present study is inspired by the work of Queheillalt et al. [13] who produced truss structures using electro-discharge machining (EDM). In this approach, the strut-face sheet joints have the same properties as the base material so the effects of welded/brazed joints are eliminated. We investigate the stiffness of the damaged core using a compression test with controlled unloading sequences. The experiments are performed at room and elevated temperature. The primary reason for testing at elevated temperature is that it is an effective way to change the material constitutive behavior and thereby to reveal influence of material parameters on truss core behavior. Elevated temperature behavior also has practical application for storage of hot liquids or as insulation layers in engines. The experiments are evaluated with help of a finite element modeling (FEM). The numerical study is then extended to different materials in order to investigate the influence of strain rate and strain hardening on the performance of a damaged truss core. This represents a kind of computational experiment where the mechanical response of the structure is investigated as a function of different material parameters. A similar approach is very often used in problems of performance optimization of complex structures which combine geometry or topology with material properties [14], [15], [16]. Complexities arise in the present case, especially during the post-buckling phase where the struts are bent and, the distribution of strain and strain rate along the struts is non-uniform. The material is sensitive to strains so these variations can significantly change the material properties along the struts which will in return influence the strut geometry and consequently the mechanical response of whole structure. The paper is organized as follows: after describing the experiments, the FE model and materials are outlined. Then the experimental and simulation results are presented, followed by analysis and discussion.

Section snippets

Samples preparation

Truss core structure samples were prepared from a bulk piece of AA5083 aluminum alloy by EDM cutting method. The electrodes were applied in two perpendicular directions as shown in Fig. 1a to produce specimens with 4 pyramidal cores. An advantage of this production method is that the struts are perfectly bonded to the face sheets and this bond has the same material properties as the rest of the sample. The geometrical parameters of the specimen are described in Fig. 1b. Important notice should

Finite element model

The compression tests are modeled using a FE model. There are two aims of the simulations. The first one is to match the compression response of the truss structure obtained in experiments and the second one is to investigate the influence of material properties on the performance of truss structures.

Experimental results

The first part of the results is dedicated to the experiments. The fundamental outputs used in the structural stiffness and strength analysis are the force–displacement curves shown in Fig. 6a–c. These follow quite well the behavior reported by Queheillalt et al. [13]. As compressive strain is applied, the load climbs steeply to a peak, which is characterized by the onset of buckling. The peak load is seen to drop with increasing test temperature in the present case. With increasing strain, the

Discussion

A rapid degradation of truss core stiffness is seen during compression of the structure. This phenomenon maps out the transition from a stretch to a bending dominated structure. However, despite being rapid, the transition is not instantaneous so there remains a region of deformation (damage) that may be tolerated while still retaining performance superior to a bending dominated structure, such as a foam. After the application of a compressive strain of 5% to the structure, the stiffness is

Conclusions

The compression experiments of the pyramidal core truss structures made of the AA5083 aluminum alloy were performed at three different temperatures. An evolution of the stiffness of the damaged core was evaluated. The FEM study was conducted upon these results to investigate the influence of material strain hardening and strain rate hardening on stiffness and strength of damaged truss core structure. The results of this study can be summarized in the following points:

  • The stiffness and strength

Acknowledgments

Work on this project has been supported by ARC Centre of Excellence for Design in Light Metals and by the Education for Competitiveness Operational Programme, project CZ.1.07./2.3.00/20.0197 Multidisciplinary research team in design of materials and its involvement in international cooperation.

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