DIPLOMA

"Experimental and theoretical investigations
of stiffened and unstiffened composite panels
under uniform transversal loading."

WS 2002/2003 University of Rostock
Jan Broekel

 

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Australian Maritime College
Certificate

This is to certify that the thesis entitled "Experimental and theoretical investigations of stiffened and unstiffened composite panels under uniform transversal loading" being submitted to the Department of Lightweight Construction and Construction Design at the University of Rostock is an experimental and theoretical research work carried out by Mr. J. Broekel under my supervision and guidance at the Australian Maritime College. The results embodied in this thesis have not been submitted to any other University or Institute for the award of any degree or diploma.

All data sources and references are stated properly.

Launceston, 01.02.2003

(G. Prusty)

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Table of Content

CERTIFICATE 2
TABLE OF CONTENT 3
LIST OF FIGURES 5
LIST OF TABLES 7
1. INTRODUCTION 8
2. BASICS OF COMPOSITE STRUCTURES 8
2.1 COMMON FIBRES 9
2.2 GLASS FIBRE 9
2.3 COMMON MATRICES 10
2.4 EPOXY MATRIX 10
2.5 FIBRE ­ MATRIX INTERACTION 11
2.6 FAILURE TYPES OF COMPOSITE MATERIALS 12
2.6.1 Micro Failure Mechanism 12
2.6.2 Macro Failure Mechanism 13
3. EXPERIMENTAL INVESTIGATION OF A CANTILEVER BEAM 14
3.1 SETUP AND PROCEDURE 14
3.2 RESULTS AND DISCUSSION 16
3.3 E-MODULUS 17
3.4 CONCLUSION 19
4. EXPERIMENTAL INVESTIGATION OF COMPOSITE PANELS 22
4.1 PROBLEM STATEMENT 22
4.1.1 Commonly Used Methods and Former Experiments 22
4.1.2 Method Adopted 24
4.2 SPECIMEN PROPERTIES 24
4.2.1 Sample Preparation 26
4.2.2 Stiffeners 27
4.2.3 Sensor Setup 30
4.3 EXPERIMENTAL PROGRAM AND INSTRUMENTATION 32

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4.4 TEST RESULTS AND ANALYSIS 33

4.4.1 Panel-1, no Stiffener 34

4.4.2 Panel-2, one Stiffener 36

4.4.3 Panel-3, two Stiffeners 39

4.5 DISCUSSION 41

4.5.1 Panel-1, no Stiffener 42

4.5.2 Panel-2, one Stiffener 44

4.5.3 Panel-3, two Stiffeners 48

4.6 DISCUSSION AND ERRORS 50

5. LAMINATE THEORY FOR THE UNSTIFFENED PANEL 54

5.1 ELASTIC PROPERTIES 54

5.2 ANALYTIC APPROACH WITH KNOWN FORMULAS 55

5.3 LAMINA STRENGTH AND FAILURE THEORIES 55

5.3.1 Lamina Strength and Failure Mechanism 55

5.3.2 Failure Theories 56

5.4 LAMINATE STRENGTH AND FAILURE 58

5.4.1 First Ply Failure 61

5.4.2 Ultimate Laminate Failure 62

6. FINITE ELEMENT ANALYSIS 63

6.1 SETUP 63

6.2 RESULTS 65

6.2.1 Panel-1, no Stiffener 65

6.2.2 Panel-2, one Stiffener 68

6.2.3 Panel-3, two Stiffeners 70

6.3 DISCUSSION AND ERRORS 73

7. COMPARISON 75

7.1 DEFLECTIONS 75

7.2 STRAINS 77

7.3 FAILURES AND STRESSES 78

8. CONCLUSION 83

REFERENCES 84

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APPENDIX A: NOMENCLATURE 87

APPENDIX B: CALCULATIONS 88

List of Figures

Figure 2-1: Chemical Structure of an Epoxy Matrix [14] 11
Figure 2-2: Failure Appearance in a Bended Cross Ply Composite [15] 12
Figure 3-1: Cross Section of Laminate Including Strain Gauges 14
Figure 3-2: Top and Side View of Cantilever Beam with Strain Gauges 15
Figure 3-3: Specimen before Cutting 15
Figure 3-4: Structure Failure and Delamination of Layer 1, 2 and 3 16
Figure 3-5: Load ­ Strain Plot for all six Gauges 17
Figure 3-6: Stress ­ Strain Plot of all six Gauges 17
Figure 3-7: Plots of all E-moduli 19
Figure 3-8: Compression Failure at the Bottom of the Specimen 21
Figure 4-1: Specimen Layout and Strain Gauge Positions 25
Figure 4-2: Hand Lay-Up Fabrication Process [4] 26
Figure 4-3: Hand Lay-Up Production of Panel 27
Figure 4-4: Nominal Stiffener Cross-Sectional Shapes [22] 28
Figure 4-5: Stiffener Fabrication, (a) Mould, (b) Half, (c) not glued Stiffeners 28
Figure 4-6: Glued Stiffeners with Clamps 29
Figure 4-7: Stiffener Geometry according to C. S. Smith [19] 29
Figure 4-8: Stiffener Position on the Panels 30
Figure 4-9: Three Wire Cabling of a Quarter Bridge [23] 31
Figure 4-10: General Stiffener Design and Strain Gauge Positions 31
Figure 4-11: Test Rig with Clamping, Cabling and Dial Gauges on Panel-1 32
Figure 4-12: Spots and Numbers of single and double stiffened Panels 33
Figure 4-13: Top (left) and Bottom (right) of Panel with Hole Enlargement 34
Figure 4-14: Load - Deflection Plot, Panel 1 35
Figure 4-15: Demounted unstiffened Panel with Cracked Edges 35
Figure 4-16: Test Rig with Clamping, Cabling and Gauges on stiffened Panel 36
Figure 4-17: Stages of Stiffener Failure 36
Figure 4-18: Stages of Panel Failure 37
Figure 4-19: Load ­ Deflection Plot, Panel 2 38

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Figure 4-20: Demounted single stiffened Panel with Cracked Edges 38

Figure 4-21: Stiffener Failure, Load: 140 KPa 39

Figure 4-22: Different Stiffener Failure Types 39

Figure 4-23: Load ­ Deflection Plot, Panel 3 40

Figure 4-24: Failure of double stiffened Panel, Load: 220 KPa 40

Figure 4-25: Demounted double stiffened Panel with Cracked Edges 41

Figure 4-26: Layer ­ Strain and Load ­ Strain, Panel 1, SPOT 1 42

Figure 4-27: Layer ­ Strain Plots, Panel-1, SPOT 2 and 4 43

Figure 4-28: Layer ­ Strain Plots, Panel 1, SPOT 3 and 5 44

Figure 4-29: Layer ­ Strain Plots, Panel-2, SPOT 2 and 4 45

Figure 4-30: Layer ­ Strain and Load ­ Strain, Panel-2, SPOT 1 46

Figure 4-31: Layer ­ Strain Plots, Panel-2, SPOT 3 and 5 46

Figure 4-32: Load ­ Strain Plots, Panel-2, Stiffener 47

Figure 4-33: Layer ­ Strain Plots, Panel-3, SPOT 2 and 4 48

Figure 4-34: Layer ­ Strain and Load ­ Strain, Panel-3, SPOT 1 49

Figure 4-35: Layer ­ Strain Plots, Panel-3, SPOT 3 and 5 49

Figure 4-36: Load ­ Strain Plot, Panel-3, Stiffeners 50

Figure 4-37: Maximum Deflection Comparison 51

Figure 4-38: Load ­ Deflection at the Centre of the Panels 52

Figure 4-39: Load ­ Strain, Centre of Stiffeners 52

Figure 4-40: Strain Comparison of the SPOT 5 and 3 in Layer-C 53

Figure 5-1: Off-Axis Loading of a Unidirectional Lamina [22] 57

Figure 5-2: Reaction Forces along the edges at a load of 200 kPa 60

Figure 5-3: Reaction Moments along the edges at a load of 200 kPa 61

Figure 6-1: Stacking Sequence simulated in ANSYS 64

Figure 6-2: Deflection of Panel-1, in m 65

Figure 6-3: Strains in 0° direction, Layer A and Layer C, Panel-1 66

Figure 6-4: Strains in 90° direction, Layer B and Layer D, Panel-1 66

Figure 6-5: von Mises Stress, Layer A and Layer C, Panel-1 67

Figure 6-6: von Mises Stress, Layer B and Layer D, Panel-1 67

Figure 6-7: Shear Stress in Layer D, XY and XZ Direction, Panel-1 68

Figure 6-8: Deflection of Panel-2, in m 68

Figure 6-9: Strains in 0° direction, Layer A and Layer C, Panel-2 69

Figure 6-10: Strains in 90° direction, Layer B and Layer D, Panel-2 69

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Figure 6-11: von Mises Stress, Layer A and Layer C, Panel-2 69

Figure 6-12: von Mises Stress, Layer B and Layer D, Panel-2 70

Figure 6-13: Shear Stress in Layer D, XY and XZ Direction, Panel-2 70

Figure 6-14: Deflection of Panel-3, in m 71

Figure 6-15: Strains in 0° direction, Layer A and Layer C, Panel-3 71

Figure 6-16: Strains in 90° direction, Layer B and Layer D, Panel-2 72

Figure 6-17: von Mises Stress, Layer A and Layer C, Panel-3 72

Figure 6-18: von Mises Stress, Layer B and Layer D, Panel-3 72

Figure 6-19: Shear Stress in Layer D, XY and XZ Direction, Panel-3 73

Figure 6-20: FEA ­ Deflection Comparison at Failure Loads 74

Figure 7-1: Deflection Comparison FEA ­ Experiment 76

Figure 7-2: Bending ­ and Membrane Strain Relationship [25] 77

Figure 7-3: von Mises Layer stresses in Panel-1, load 200 kPa 80

Figure 7-4: von Mises Layer stresses in Panel-2, load 130 kPa 80

Figure 7-5: von Mises Layer stresses in Panel-3, load 190 kPa 81

List of Tables

Table 3-1: Cantilever Beam Characteristics 15
Table 3-2: E-Modulus from different Data Sources 19
Table 4-1: Common Strain Measuring Methods 23
Table 4-2: Basic Panel Characteristics 24
Table 4-3: Mechanical Properties of Glass Fibres used for the Panels 25
Table 4-4: Mechanical Properties of Epoxy Resin used for the Panels 26
Table 5-1: Lamina Material Properties 55
Table 6-1: Results of FEA 74
Table 7-1: Maximum Strain and Deflection Comparison 78
Table 7-2: von Mises stresses FEA and Experiment, Panel-1, 200 kPa SPOT 1 79
Table 7-3: von Mises Stresses and Failure Criteria for Panel-1 81

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1. Introduction

With the increased use of composite materials in various structural applications, the subject of analysing the properties of composite unstiffened and stiffened panels has received widespread attention. Laminated panels are considered as the basic modules of high performance boats, aircraft and many other complex structures, which require less specific weight, better durability, and excellent damage tolerance and are often subject to air-blast loading or under water shock. The failure mode of fibre reinforced composite materials is rather more complex than that of isotropic material. This is because of the different properties of fibres and matrix the composite is made of. There has been a number of failure theories developed since the first industrial usage of composites in the early 1980s. Those theories are based on different failure criteria and often just cover some special set up. In the last years computer simulations based on FEA were set up to predict the failure of composites. Because there is only few experimental data available for comparison, the need for experimental investigations of composites is big.

2. Basics of Composite Structures

The laminated panels are made of fibres reinforced composite material. This means while using two different materials with specific properties a new one is created with the combination and partly improvement of the former separated material properties. When building the composite material normally the final row product is build at the same time. Therefore, the material can be designed in the best way to fulfil the needs of the product. A matrix that is temperature and acid resistant (ceramic or glass matrices) or special fibres with extremely high tensile modulus (boron: 379-414 Gpa) [4] can be chosen. Besides that, the fibre direction can be arranged to take the maximum tensile load.

In a composite, the fibre carries the most of the tensile load of a composite structure and has to take the tensile stress out of the matrix. The Matrix has the job to transfer stresses between and to the fibres, to provide a barrier against an adverse environment and to protect the surface of the fibres from mechanical damage. The matrix plays a minor role in the tensile load carrying of a composite structure. On the

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other hand the matrix supports the fibre against buckling and provides most of the interlaminar shear strength. In addition, though fibres are strong, they can be brittle.

The matrix can absorb energy by deforming under stress. This is to say, the matrix adds toughness to the composite.

2.1 Common Fibres

The most commonly used reinforcing fibre is the glass fibre. Due to the low cost and the easy usage of glass fibres, it has been considered for building the specimens and is described separately. However, more expensive fibres available on market are as following.

The Aramid Fibre, precisely Para-aramid, has an excellent stability against temperature changes and tensile strength up to seven times that of steel wire. That is why this fibre is often used in the car tire industry and for high performance ropes. The most known aramid fibre is Kevlar, which has been used reinforcement in many marine and aerospace applications where high tensile strength and impact damage resistance is needed. The major disadvantages of aramid composites are the low compressive strength and the difficulty in machining and cutting them.

The Graphite Fibre, also called carbon fibre, can be purchased in a wide range of tensile module (from 207 GPa up to 1035 GPa). Their advantages are the high fatigue strength and the exceptional high tensile strength-to-weight ratio. On the other side are the high costs, the low impact resistance and the high electric conductivity. The Boron Fibre is very expensive but has because of it relatively large diameter a good resistance to buckling which gives boron fibre-reinforced composites a high compressive strength. The tensile modulus ranges from 379 Gpa to 414 Gpa. Ceramic Fibres, like SiC and AL2O3, have the special advantage of being resistant against high temperatures. Both fibres can be used to reinforce metal matrices [4].

2.2 Glass Fibre

The glass fibre is an inorganic, synthetic, multifilament material. Glass fibres are advantageous because of the low cost, non-flammable, electrically nonconductive, corrosion-resistant and strength characteristics. The major disadvantages are the low

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tensile modulus, the relatively high specific gravity, the sensitivity to abrasion with handling, the relatively low fatigue resistance and the high hardness.

Melting the raw material in a high temperature furnace and then drawing the molten material into filaments makes Glass fibres. Glass fibres are amorphous and isotropic and are long, three-dimensional networks of silicon, oxygen, and other atoms arranged in a random fashion. Glass Fibres Forms can be Fibres, Roving, Chopped Strands, Yarns, Fabrics, Mats and Fillers.

2.3 Common Matrices

The primary consideration in the selection of a matrix is its basic mechanical properties that include tensile modulus, tensile strength, and fracture toughness. A Polymeric Matrix is made of a polymeric material, which is a collection of large numbers of polymer molecules of similar chemical structure. In solid state, the molecules are frozen in space. In a thermoplastic polymer, the linear molecules have no chemical linking between them and the thermoplastic polymer can be softened, melted and reshaped as often as desired by applying heat and pressure to weaken the intermolecular forces. In a thermoset polymer, the molecules are chemical linked and once these joints are formed the polymer cannot be melted and reshaped again.

Metal Matrix Composites are based most commonly on aluminium and titanium. They are mostly reinforced with continuous fibres, particulates, or whiskers. Metal matrix composites have superior mechanical properties and often-unique physical characteristics. Because of the costs, these materials are rarely used. Applications can be found in aerospace engine components or high-end automobile engine cylinders.

Ceramic Matrix Composites are made of glass or ceramic matrices reinforced with fibres, whiskers or particulates. Currently they have limited high-temperature applications, but because of their low density, high modulus, strength and toughness they have a big potential as a class of advanced engineering structural material [4].

2.4 Epoxy Matrix

Epoxy resin is defined as a molecule containing more than one epoxide groups. The epoxide group also termed as, oxirane or ethoxyline group, is shown in Figure 2-1.

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