The primary aim of this research work is to examine the mechanical properties per weight density of novel core materials for use in sandwich panels. Composite lattice core sandwich structures with relative densities in the range of 3% to 35% were manufactured and tested under quasi-static compression loading conditions. Collapse strength, failure mechanisms and energy absorption characteristics of the lattice structures have been evaluated. Since these core material shapes are unique, research involved developing suitable manufacturing methods. The study started by looking at introducing simple through thickness lattice structure into PET foam cores. This was achieved by drilling the foam material, glass fibers were then inserted into the perforations. The panel was then infused with resin using the vacuum assisted resin transfer molding process. This was then extended to look at the possibility of removing the core by adopting a-lost mold manufacturing procedure that would leave a free-standing lattice structure. This involved inserting reinforcing fiber tows through holes in wax blocks. Following infusion with an epoxy resin and subsequent post curing, the preforms were heated to a temperature above that required to melt the wax, leaving well-defined lattice structures based on vertical, pyramidal, modified-pyramidal, octet configurations and others based on what are termed BCC, BCCz, FCC and F2BCC designs. Compression tests showed that the strength of individual struts and the corresponding cores increases with strut diameter and fiber volume fraction. Smaller diameter struts failed in buckling, whereas the larger diameter columns failed in a crushing mode involving high levels of energy absorption. Truss core structures with 4 mm diameter columns, based on 28% fiber volume fractions offered specific energy absorption values above 70 kJ/kg. Compression tests on the four lattice structures based on BCC, BCCz, FCC and F2BCC designs indicated that the F2BCC lattice offered the highest compression strength of approximately 12 MPa. Although, when normalized by relative density, the BCCz lattice structure out-performed the three remaining structures. The specific energy absorption values of the lattices were relatively high, ranging from 44 kJ/kg for the BCC lattice to 80 kJ/kg for the BCCz structure. Similarly, the specific compression strengths of some of the lattices have been shown to be superior to those of more traditional core materials. [...]
Table of Contents
Chapter 1: Introduction
1.1 Overview
1.2 Thesis Objective
1.3 Thesis Outline
1.4 Sandwich Panel Design Concept
1.5 Cellular materials
1.5.1 Stochastic cellular materials
1.5.2 Periodic cellular materials
1.6 Manufacture
1.6.1 Autoclave molding
1.6.2 Filament winding
1.6.3 Resin transfer molding
1.7 Vacuum Assisted Resin Transfer Molding (VARTM) process
1.7.1 Variations of the VARTM process
1.7.2 VARTM process and quality of composite
1.8 Elastic and strength properties of composite materials
1.8.1 Elastic properties of composite materials
1.8.2 Strength properties of composite materials
Chapter 2: Experimental procedures
2.1 Introduction
2.2 Lattice fabrication
2.2.1 Consumable materials
2.2.2 Constituent materials
2.2.3 Preparation Process
2.2.4 Infusion Process
2.2.5 Post infusion process
2.3 Hybrid core sandwich panel studies
2.3.1 Overview
2.3.2 Materials and Manufacturing
2.3.3 Compression tests
2.3.4 Interfacial fracture tests
2.4 Vertical, pyramidal, and octet lattice studies
2.4.1 Overview
2.4.2 Materials and fabrication
2.4.3 Compression tests
2.5 BCC, BCCz, FCC and F2BCC lattice studies
2.5.1 Overview
2.5.2 Materials and fabrication
2.5.3 Compression tests
2.6 Other lattice structures
Chapter 3: Analytical Modeling
3.1 Introduction
3.2 Elastic properties of parent material
3.3 Elastic values
3.4 Strength values
3.5 Analytical model of the compressive response
3.5.1 Analytical predictions for the response of composite pyramidal truss core
3.5.2 Analytical predictions of the vertical column core response
3.5.3 Analytical predictions for the response of the modified pyramidal truss core
3.5.4 Summary
3.5.5 Analytical predictions of the response of the octahedral lattice core
3.5.6 Analytical predictions of the response of the BCC core
3.5.7 Analytical predictions of the response of the FCC core
3.5.8 Analytical predictions of the response of the BCCz core
3.5.9 Analytical predictions of the response of the F2BCC core
Chapter 4: Finite Element Analysis
4.1 Introduction
4.2 ANSYS FE package
4.3 Constitutive models for the composite material
4.3.1 Elastic response
4.3.2 Damage initiation & progression model for the fiber reinforced composites
4.4 Quasi-static Finite element modelling
4.4.1 Modelling of lattice core sandwich structures
4.5 Numerical analysis results
4.5.1 Vertical lattice
4.5.2 Pyramidal lattice
4.5.3 Modified pyramidal lattice
4.5.4 BCC lattice
4.5.5 BCCz lattice
4.5.6 FCC lattice
4.5.7 F2BCC lattice
4.6 Conclusions
Chapter 5: Results and Discussion
5.1 Introduction
5.2 Resin flow and post-manufacture visual assessment
5.2.1 Hybrid GFRP/PET core
5.2.2 Vertical, pyramidal and octet lattice
5.2.3 BCC, BCCz, FCC and F2BCC lattice
5.3 Compression tests
5.3.1 Hybrid GFRP/PET core
5.3.2 Vertical, pyramidal and octet lattice
5.3.3 BCC, BCCz, FCC and F2BCC lattice
5.4 Skin-core Interfacial fracture toughness
5.4.1 Hybrid GFRP/PET core
Chapter 6: Conclusions and Future Work
6.1 Introduction
6.2 Conclusions
6.3 Recommended future work
Research Objective & Topics
The primary aim of this research is to investigate the fabrication and mechanical performance of all-composite lattice core sandwich panels. The research focuses on developing a viable lost-mold manufacturing technique to produce these structures, characterizing their mechanical behavior under quasi-static compression, and establishing analytical and finite element models to predict their stiffness and strength.
- Fabrication of novel composite lattice core sandwich structures.
- Investigation of mechanical properties, specifically compression strength and failure mechanisms.
- Development of analytical and finite element models for performance prediction.
- Optimization of structural efficiency and fiber volume fractions.
- Characterization of skin-core interfacial fracture toughness.
Excerpt from the Book
3.5 Analytical model of the compressive response
The goal of this section is to obtain the elastic properties and collapse strength of the various truss core configurations as a function of the apparent composite material properties and the core geometry. Extensive work has been carried out in developing relations between the parent material properties and the elastic response and the strength prediction of the various lattice core structures [5–11]. The pyramidal core equations included in this chapter serve as an introduction to the topic of deriving elastic and strength response of lattice structures in terms of the parent material properties and geometric properties of the core. This work is further extended to derive similar relations for the vertical column core and the modified pyramidal core.
Summary of Chapters
Chapter 1: Introduction: Discusses the aerospace industry's need for lightweight, high-performance sandwich structures and reviews existing cellular materials and manufacturing methods.
Chapter 2: Experimental procedures: Details the materials, the lost-mold manufacturing process, and the specific test procedures used to measure mechanical properties.
Chapter 3: Analytical Modeling: Derives the mathematical frameworks for predicting the elastic modulus and collapse strength of the various lattice truss core geometries.
Chapter 4: Finite Element Analysis: Provides the methodology and results of numerical simulations performed in ANSYS to validate the analytical models developed in Chapter 3.
Chapter 5: Results and Discussion: Presents the experimental data, including visual assessments, compression tests, and interfacial fracture testing, comparing these results to the analytical and FE predictions.
Chapter 6: Conclusions and Future Work: Summarizes the findings regarding the fabricated lattice structures and suggests potential avenues for future research.
Keywords
Lattice structures, Sandwich structures, Mechanical properties, Resin infusion, Finite Element, Composites, VARTM, Unidirectional fiber, Compression strength, Micromechanics, Core material, Lost mold technique, Failure mechanisms, Structural efficiency, Energy absorption.
Frequently Asked Questions
What is the primary focus of this thesis?
This thesis examines the fabrication, mechanical properties, and predictive modeling of novel all-composite lattice core sandwich panels for aerospace applications.
Which materials were used for the lattice structures?
The study utilizes carbon fiber reinforced plastic (CFRP), glass fiber reinforced plastic (GFRP), and natural jute fibers within epoxy resin matrices.
What is the main objective of the proposed manufacturing method?
The goal is to develop a "lost-mold" technique that allows for the creation of complex, free-standing lattice cores that improve mechanical performance and enable better resin infusion.
Which modeling approaches are applied to predict performance?
The research uses both micromechanical analytical models to derive equations for elastic properties and finite element analysis (FEA) via the ANSYS software package for numerical validation.
What are the key mechanical characteristics evaluated?
The research evaluates specific stiffness, compression strength, failure mechanisms, and skin-core interfacial fracture toughness under quasi-static loading conditions.
How are fiber volume fractions controlled?
The fiber volume fraction within the lattice struts is precisely controlled by varying the number of fiber tows inserted during the threading process of the lost-mold technique.
How does the BCCz lattice performance compare to other designs?
The BCCz lattice is shown to offer superior strength properties and specific energy absorption compared to the standard BCC and octet structures, making it highly effective for energy-absorbing systems.
What role does the "lost-mold" material play?
The wax or salt mold serves as a temporary internal structure that is removed post-curing (by melting or dissolution), resulting in a precise, free-standing composite lattice core.
- Quote paper
- Hassan Ziad Jishi (Author), 2016, The Fabrication and Mechanical Properties of Continuous Fiber Composite Lattice Structures, Munich, GRIN Verlag, https://www.grin.com/document/345395
