Wood Fracture Characterization

150 Pages·2018·17.941 MB·English

by  Marcelo F S F de Moura, Nuno Dourado

Wood Fracture
Characterisation

Wood Fracture
Characterisation
Marcelo F. S. F. de Moura and Nuno Dourado
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Contents
Preface ............................................................................................................................................vii
Acknowledgements .......................................................................................................................ix
Authors ............................................................................................................................................xi
1 Introduction .............................................................................................................................1
1.1 Global Overview and Applications ............................................................................1
1.2 Wood – How Does It Form? .........................................................................................5
1.3 Wood Constituents and Micro-Structure ..................................................................7
1.4 Wood at the Mesoscale ...............................................................................................10
References ...............................................................................................................................14
2 Wood Mechanical Behaviour .............................................................................................15
2.1 Elastic and Strength Properties ................................................................................15
2.1.1 Young’s Moduli and Normal Strengths .....................................................16
2.1.2 Poisson’s Ratios ..............................................................................................20
2.1.3 Shear Moduli and Strengths ........................................................................20
2.2 Strength Failure Criteria ............................................................................................26
2.3 Fracture Mechanics Based Approaches ...................................................................29
2.3.1 Linear Elastic Fracture Mechanics .............................................................29
2.3.2 Cohesive Zone Models .................................................................................32
References ...............................................................................................................................35
3 Mode I Fracture Characterisation .....................................................................................37
3.1 Double Cantilever Beam ............................................................................................37
3.1.1 Test Description..............................................................................................37
3.1.2 Classical Data Reduction Schemes ..............................................................37
3.1.3 Modified Experimental Compliance Method (MECM) ...........................39
3.1.4 Compliance-Based Beam Method (CBBM) ................................................40
3.1.5 Numerical Validation ....................................................................................42
3.1.6 Experimental and Numerical Results .........................................................45
3.2 Single-Edge-Notched Beam Loaded in Three-Point-Bending .............................47
3.2.1 Test Description..............................................................................................47
3.2.2 Data Reduction Scheme Based on Equivalent LEFM ...............................49
3.2.3 Compliance-Based Beam Method ...............................................................51
3.2.4 Numerical Validation of the Compliance-Based Beam Method .............53
3.2.5 Experimental and Numerical Results .........................................................55
3.3 Tapered Double Cantilever Beam ............................................................................55
3.3.1 Test Description..............................................................................................55
3.3.2 Data Reduction Scheme ................................................................................55
3.3.3 Compliance-Based Beam Method ..............................................................58
3.3.4 Numerical Validation ....................................................................................60
3.3.5 Experimental and Numerical Results .........................................................61
3.4 Compact Tension Test ................................................................................................62
3.5 Conclusions of Mode I Fracture Tests ......................................................................64
References ...............................................................................................................................64
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vi Contents
4 Mode II Fracture Characterisation ....................................................................................67
4.1 End-Notched Flexure Test ........................................................................................68
4.1.1 Test Description..............................................................................................68
4.1.2 Classical Data Reduction Schemes ..............................................................69
4.1.3 Compliance-Based Beam Method ..............................................................70
4.1.4 Experimental and Numerical Results .........................................................72
4.2 End-Loaded Split Test (ELS) ......................................................................................74
4.2.1 Test Description..............................................................................................75
4.2.2 Classical Data Reduction Schemes ..............................................................76
4.2.3 Compliance-Based Beam Method ...............................................................77
4.2.4 Experimental and Numerical Results .........................................................78
4.3 Four End-Notched Flexure Test ...............................................................................79
4.3.1 Test Description..............................................................................................81
4.3.2 Compliance Calibration Method ................................................................81
4.3.3 Compliance-Based Beam Method ..............................................................82
4.3.4 Experimental and Numerical Results .........................................................84
4.4 Conclusions of Mode II Fracture Tests .....................................................................87
References ...............................................................................................................................87
5 Mixed-Mode I + II Fracture Characterisation .................................................................89
5.1 Single-Leg Bending Test ............................................................................................90
5.1.1 Test Description..............................................................................................90
5.1.2 Compliance-Based Beam Method ...............................................................91
5.1.3 Numerical Analysis .......................................................................................91
5.1.4 Experimental Results ....................................................................................93
5.2 End Load Shear-Mixed Mode Test ..........................................................................93
5.2.1 Test Description..............................................................................................93
5.2.2 Compliance-Based Beam Method ..............................................................94
5.2.3 Numerical Analysis .......................................................................................95
5.2.4 Experimental Results ....................................................................................96
5.3 Mixed-Mode Bending Test ........................................................................................97
5.3.1 Test Description..............................................................................................97
5.3.2 Experimental Analysis and Results ............................................................99
5.3.3 Numerical Validation ..................................................................................102
5.4 Conclusions of Mixed-Mode I + II Fracture Tests ................................................103
References .............................................................................................................................103
6 Structural Applications – Case Studies .........................................................................105
6.1 Wood Bonded Joints .................................................................................................105
6.1.1 Repaired Beam under Tensile Loading ....................................................105
6.1.2 Repaired Beam under Bending Loading ..................................................109
6.1.3 Reinforcement of Wood Structures ...........................................................113
6.2 Wood Dowel Joints ...................................................................................................117
6.2.1 Steel–Wood–Steel Connection ...................................................................117
6.2.2 Wood–Wood Joint ........................................................................................122
6.3 Conclusions of Structural Applications .................................................................129
References .............................................................................................................................130
Index .............................................................................................................................................133
Preface
Wood is increasingly used in structural applications. In fact, the interest in applying
renewable resources in structural design is growing due to ecological and environmental
reasons, and energy shortages. Solid wood is generally used in frames, buildings, truss
roof structures in buildings, bridges, towers, railroad infrastructures and in many other
applications. Damage and failure behaviour of wood members in tensile, compressive or
shear loading are extremely important to account in wooden structures subjected to high
working stresses. Structural details involving wood member’s connections also require
special attention for a safe design. Effectively, damage under tensile, compressive or shear
loading can occur at the joints or within the lumber of many members. The criteria cur-
rently used in wood structural design are based on stress or strain analyses. However,
many structural applications of wood involve discontinuities and singularities such as
notches or holes, which lead to important stress concentration effects. Additionally, wood
as a natural and biological material presents drastic variations in its inner structure as a
result of internal defects such as knots, variation of grain orientation, reaction wood and
others that contribute to a considerable source of variability at several levels. The conse-
quence of these aspects is the consideration in the actual design codes of several and high
safety factors in the design of wood structures.
To overcome these drawbacks and limitations, a promising line of research consists in
employing fracture mechanics concepts to wood design. Such methodology can contrib-
ute significantly to a better understanding and more reliable design methods concern-
ing the project of wood structural applications. Therefore, a comprehensive description
of the fundaments on fracture mechanics is presented in this book, as well as the neces-
sary extensions that account for wood specificities. In this context, a wide description and
development of several fracture tests appropriate for wood fracture characterisation under
different loading modes is the main focus of this book. The described work includes new
fracture tests applied to wood, new data reduction schemes and numerical models based
on cohesive zone analysis that are frequently used to validate the proposed experimental-
based methodologies. In addition, several structural details involving connections of wood
members have been analysed for safe design in the last chapter, using the experimental
and numerical tools described throughout the book.
In summary, this book addresses the following main targets:
• Proposal of accurate methods for the design of wood members in structures;
• Development of approaches that allow decreasing the material consumption in
structural applications;
• Enhancement of the attractiveness of wood for structural application purposes;
• Stimulation of the building sector to employ wood as a competitive construction
material, that allows boosting the forestry and woodworking industry sectors;
• Valuable contribution towards ecological and environmental issues.
The main goal of this book is to stimulate the readers to use the theories and methods
described herein as tools for safe, ecological and efficient wood structural design.
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viii Preface
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Acknowledgements
The authors thank the Portuguese Foundation for Science and Technology for supporting
the work here presented through the projects ‘Design of wood-bonded joints’ (POCTI/
EME/45573/2002), ‘Design of wood and wood-bonded joints under mixed-mode load-
ing’ (POCI/EME/56567/2004) and ‘Repair of wood structures using artificial composites’
(PTDC/EME-PME/64839/2006).
The authors also thank Professor José Joaquim Lopes Morais from the University of
Trás-os-Montes e Alto Douro for his advices, ideas and participation in the generality of
the topics. All the students participating in the projects are also recognised. Professors
Stéphane Morel and Gérard Valentin from the University of Bordeaux are also acknowl-
edged. The authors would like to highlight and express their gratitude to the students
Manuel António Lima da Silva, Jorge Marcelo Quintas de Oliveira and João Pedro da Costa
Reis for their good spirit and valuable experimental and numerical work that contributed
decisively to the presented work. The authors are also grateful to Professor José Lousada
for his courtesy in providing the wood histological images.
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