project-work-e2015

Institute for Structural Analysis

Project Work 2015

Prof. Dr.-Ing. habil. Michael Kaliske

Supervisor: Dipl.–Ing. Christian Jenkel Dresden, 29/04/2015

by Mohhammad Afsar Sujon

Analysis of failure in timber boards under tensile loading initiated by knots- a study of basic failure mechanism

Institute for Structural Analysis 2

Contents

Models and Methods

Analysis and

Results

Introduction

Conclusions and

Outlook

3

1 Introduction

2 Models and Methods

3 Analysis and Results

4 Conclusions and Outlook



Objectives

Identify basic failure mechanism for longitudinal tensile failure

Checking applicability of existing cohesive material model

Identify basic failure mechanism for timber boards containing knot

A method to identify possible crack paths can be developed

5

Reasons For Selecting This Topic

Finite element method(deeper knowledge)

Fracture mechanics

Macrostructure and microstructure of wood

Failure and fracture morphology

Fracture mechanics models

Effect of knots

Cohesive zone model ABAQUS



Figure: After load effect on timber beam

Figure : Close up of failed section of bottom chord. Note: significant knot sections weakened the chord’s ability to properly transfer tension

forces

Figure : Typical failure of a tension member (Bottom Chord) on a modified bow string truss.

Fracture is seen through the mid section as well as through the bottom face of the chord

Necessity of Research

7

Introduction

Traditional construction material Wood is preferable

Good material and mechanical properties

Fabricated to a variety of shapes and sizes

Economically available Renewable and biodegradable

Main drawbacks Anisotropic material Irregular grains and knots Decay if not kept dry Flammable

Motivation for research on wood


8

1 Introduction





9

Failure types


Figure : Typical tension failure patterns observed: [a] splinter, and [b] shear and tension failure. [Gibson and Ashby, 1988]

Figure : Other typical failure patterns observed in tensile tests: [a]shear failure, and [b] pure tension failure. [Gibson and Ashby, 1988]

Figure: tension specimens failed in (a)split, (b) shear and (c)

tension.[Bartůňková, 2013]


Figure : Theoretically possible failure patterns: [a] splinter,

[b]shear and tension failure, [c] shear failure; and [d] pure

tension failure. [Gibson and Ashby, 1988]

Failure types [Macroscopic and Microscopic view]

Figure : Crack propagation for opening mode [I] loading:

cell-wall breaking [a], cell-wall peeling [b]. [Gibson and

Ashby, 1988]


Figure: Typical load-displacement curves obtained by the wedge splitting test in the RL [a]

and TL [b] systems. [Reiterer, 2002]

Figure : Typical stress-strain curves for wood loaded in

compression in L, R and T direction and

for tension in L direction. [Holmberg,

1998]

Typical Relationships curves


Figure : Cell structure deformations at failure under various loading conditions. [a] Compression, [b]

tension, [c]shear; and [d]combined shear and compression. [Holmberg, 1998]

Figure : Tensile failure in spruce [Picae abies] showing mainly transverse cross-wall failure of the earlywood [left] and longitudinal intra-wall

shear failure of the latewood cells [right] [magnification× 200, polarized light].[Peter, 2010]

Failure types [Microscopic view]

13Institute for Structural Analysis

Knots

Introduction

Figure : Intergrown knot [a], encased knot [b] [Kretschmann, 2010]


Figure: Notation of the knots, in accordance to DIN 4047-1 [12]

Knots

15

1 Introduction





16

Intergrown knot under tension

Figure : Strain distribution around an intergrown knot under a tensile load of 55kN [9.92MPa] [Gerhard, Jochen &Andrea, 2012]



Figure : Strain peaks within an intergrown knot under a tensile

load of 55kN [9.92MPa]. (Gerhard,

Jochen &Andrea,

2012)

Figure : Strain peaks within a dead knot

under a tensile load of 66kN [11.9MPa]. (Gerhard, Jochen &Andrea, 2012)

Strain peaks within a knot


Figure : Strain distribution around a narrow side knot under a load of 55kN [9.92MPa]. Top: upper side. Bottom: lower side. The dashed line illustrates the knot located on the opposite side of the board.

(Gerhard, Jochen &Andrea, 2012)

Strain distribution around a narrow side knot


Figure : Strain distribution around two knots arranged abreast under a tensile load of 55kN [9.92MPa]. Top: upper side. Bottom: lower side. The dashed line illustrates the knot located on the opposite side of

the board. (Gerhard, Jochen &Andrea, 2012)

Strain distribution around two knots


Figure : Strain distribution around two knots arranged diagonal shifted and the fracture pattern. Top: upper side, tensile load: 141kN [25.4MPa]. Bottom: lower side, tensile load: 55kN [9.92MPa]. The dashed line illustrates the knot located on

the opposite side of the board. The dash-dotted line shows the fracture pattern. (Gerhard, Jochen &Andrea, 2012)

Strain distribution around two knots


Figure : Strain distribution within a knot cluster containing three knots and the fracture pattern. Top: upper side, tensile load: 66kN [11.9MPa]. Bottom: lower side, tensile load: 45kN [8.12MPa]. The dashed line illustrates the knot located on the

opposite side of the board. The dash-dotted line shows the fracture pattern. (Gerhard, Jochen &Andrea, 2012)

Strain distribution within a knot cluster


Figure : Cohesive model: representation of the physical damage process by separation function within numerical interfaces of zero height—the cohesive

elements. [Schwalbe, Scheider, Cornec, 2012]

Cohesive zone model


Figure : Form of the TSL: [a] [Needleman, 1987] ,[b] [Needleman, 1990] ,[c] Hillerborg [1976],[d][ Bazant ,2002], [e] [Scheider, 2001] ,[f] [Tvergaard, 1990] .

[Schwalbe, Scheider, Cornec, 2012]

TSL [Traction Separation Law]


Figure : Geometry, nodes and local coordinate system of 16-node interface

element. [Schmidt, 2008]

Figure : Schematic traction-separation behavior of the material model.

[Schmidt, 2008]

Cohesive zone model, Schmidt, 2008


Figure : Brittle behavior

in tension and shear

.[FHWA]

Figure : Load vs. (clip gauge) displacement for the specimen Ac1.

Tensile strength parallel to grain

[Bartůňková, 2013]

Applicability of model of Schmidt, 2008


Using ABAQUS for FEM

Without Knot

Figure : Model for simulation in ABAQUS

Figure : Energy vs. time

curve after giving load

27Institute for Structural Analysis

Figure : Stress intensity after giving load of 50 kN



Figure : Model for simulation in ABAQUS




Figure : Longitudinal surface strains () for

maximum load 30 kN, recorded for Load tests

no. A1-A4. Top row: Surface photos. Middle

row: contour plots. Bottom row: section

diagrams for the sections (dashed lines) shown in the middle row. [Jan, Anders,

Bertil, 2010]

Longitudinal surface strains


Do objectives are fulfilled

Identify basic failure mechanism for longitudinal

tensile failure Checking applicability of

existing cohesive material model

Identify basic failure mechanism for timber boards

containing knot A method to identify possible crack paths can be developed

Fulfilled

Fulfilled

Fulfilled

Partly fulfille

d

32

1 Introduction





Content


Figure : Streamline-mesh: (a) determination of crack path, (b) simulation results for timber board. [Jenkel, 2014]

Figure : Stream line approach: (a) stream lines around knots, velocity vectors in (b) regular mesh and (c) direct meshing. [Jenkel, 2014]

Stream line approach

34

An appropriate element configuration should be chosen to analyze The Stream Line Approach could be used as an alternative method with

less drawbacks For further studies, an experimental test should be performed in order to

compare the results The meshing and element defined in ABAQUS can be more defined to

make better simulation A better simulation software can be used in future to get better results The design of the knot and defined cohesive property should be more

precise. Because there are differences between knot types and they act differently .


Outlook

Conclusion and Suggestion


References

[Bartůňková, 2013]. Eliška Bartůňková, [2013], Method for determination of the softening behavior of wood and the applicability of a nonlinear fracture mechanics model, CTU in Prague.

[FHWA].Federal Highway Administration (FHWA).United States Department of Transportation. Publication Number: FHWA-HRT-04-096Date: August 2005.

[Gerhard, Jochen &Andrea, 2012]. Gerhard Fink, Jochen Kohler, Andrea Frangi, [2012], Experimental analysis of the deformation and failure behavior of significant knot clusters.

[Gibson and Ashby, 1988]. Gibson, L. J. Ashby, M, F, [1988]. Cellular solids, Structure and Properties, Oxford: Pergamoon.[Jan, Anders, Bertil, 2010] Jan Oscarsson, Anders Olsson, Bertil Inquest. [2010]. Strain fields around a traversing edge knot in a spruce specimen exposed to tensile forces.

[Holmberg, 1998]. Holmberg, S. Persson, K. Petersson, H. [1998], Nonlinear mechanical behavior and analysis of wood and fiber materials, Division of Structural Mechanics, Lund University, Lund, Sweden.

[Jenkel, 2014].Christian Jenkel, Michael Kaliske. [2014].Finite element analysis of timber containing branches – An approach to model the grain course and the influence on the structural behavior.

[Kretschmann, 2010]. Kretschmann, D. E. [2010], Mechanical Properties of Wood, Chapter 5, Wood handbook - Wood as an engineering material, General Technical Report FPL-GTR-190, Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, 508 p.

[Peter, 2010]. Peter Domone, John Illston, [2010]. Construction Materials: Their Nature and Behavior, Fourth Edition, Chapter 54: Strength and failure in timber.

[Reiterer, 2002]. Reiterer, A. Sinn, G. Stanzl-Tschegg, S. E. [2002], Fracture characteristics of different wood species under mode I loading perpendicular to the grain. Mater Sci Eng A332:29–36.

[Schmidt, 2008].Jörg Schmidt, Michael Kaliske. [2008]. Models for numerical failure analysis of wooden structures, Technische Universität Dresden, Institute for Structural Analysis, D-01062 Dresden, Germany.

[Schwalbe, Scheider, Cornec, 2012]. Karl-Heinz Schwalbe, Ingo Scheider, Alfred Cornec.[2012] Guidelines for Applying Cohesive Models to the Damage Behaviour of Engineering Materials and Structures, ISBN 978 3 642 29493 8.


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