CHARACTERIZATION OF THE SHEAR BEHAVIOUR OF WOOD USING THE IOSIPESCU TEST J. C. Xavier Master of...

CHARACTERIZATION OF THE SHEAR BEHAVIOUR OF WOOD USING THE

IOSIPESCU TEST

J. C. Xavier

Master of Science Thesis

LMPF seminary - 11/12/2003

University of Trás-os-Montes e Alto DouroVila Real, Portugal

Plan

Introduction

The Iosipescu test

Numerical simulation of the Variable Span Method

Numerical simulation of the Iosipescu test

Experimental work

Presentation and discussion of the experimental results

General conclusions and future work

Introduction Wood modelling at the macroscopic level:

Shear properties:

─ Shear moduli : GLR , GLT , GRT .

─ Shear strengths : SLR , SLT , SRT .

fff

f

f

f

fff

fff

fff

ffff

66

55

44

333231

232221

131211L

R

T

RT

LT

LR

L

R

T

RT

LT

LR

f11 f12 f13

f21 f22 f23

f31 f32 f33

f44

f55

f66

[5] prEN 408. European Committee for Standardization, 2000.[6] ASTM D198-94. American Society for Testing and Materials, 1994.

[7] ASTM D143-94. American Society for Testing and Materials, 1994.[8] NP 623. Portuguese Standard, 1973.

Drawbacks of the standardized tests for the identification of the shear properties of wood [3,4]:

(i) Give only the shear properties parallel to the fibres

(shear moduli : GLR, GLT and shear strengths : SLR e SLT).

(ii) The variable span method [5,6] proposed for the determination of EL and GLR (or GLT), is not a

fundamental test.

(iii) The failure of the specimen of the shear block test [7,8] proposed for the identification of SLR and SLT,

occurs under stress concentrations.

[3] Yoshihara et all.. Journal of Wood Science, 44:15-20, 1998.[4] Rammer D.R. e L.A. Soltis. Res. Pap. FPL-RP-527, FPL, 1994.

L [0,0025; 0,035] cL

F/2

F

h

b

F/2

Nominal distribution of the shear stress Real distribution

of the shear stress

Increase of the shear stress

Aim of this work:

Investigation of the applicability of the Iosipescu shear test for characterizing the shear behaviour of wood Pinus Pinaster Ait.

(i) simultaneous identification of the shear modulus and the shear strength, in a particular symmetry plane.

Justifications for the choice of the Iosipescu test:

(ii) possible application of this test method for all the symmetry planes of wood thanks to the small size of the specimen.

Among different shear tests for orthotropic materials : Iosipescu test

(standard test for synthetic composite materials [9]).

[9] ASTM D 5379-93. American Society for Testing and Materials, 1993.

The Iosipescu test Iosipescu specimen [9]:


General view of the Iosipescu fixture [9]:


Wedge adjusting screw

Specimen

Stationary part of fixture

Movable part

of the fixture

Fixture linear guide rod

Base

Adjustable wedges to tighten the specimen

Attachment to the test machine

Data processing [9]:

Experimental information:

+45º , –45º , P

Engineering shear strain:

Nominal shear stress:

6=P/A

Apparent shear modulus:

Apparent shear strength:

G12= 6 / 6

a

S12= P / Aa ult


6=+45º – – 45º

For an orthotropic material the distributions of 6 and 6

are not homogeneous [10,11].

The correction factors C e S are calculated through finite element analyses.

G12 = CSG12

a

[10] Pierron F. e A. Vautrin. Composite Science and Technology, 5:61-72, 1994.[11] Pierron F. Journal of Composite Materials, 32(22):1986-2015, 1998.

where C = 6 / (P/A) and S = 6 / 6oo ros

G12

a

Aspects about the identification of G12:

[10] Pierron F. e A. Vautrin. Composite Science and Technology, 5:61-72, 1994.[11] Pierron F. Journal of Composite Materials, 32(22):1986-2015, 1998.

The distribution of 6 through

the thickness of the specimen

can be heterogeneous due to

geometrical imperfections of its

loading surfaces [10,11].

This effect is eliminated by considering 6 as the average of the shear

strains measured over both lateral faces of the specimen [10,11].

Grandes deformaçõesPequenos módulos

Pequenas deformaçõesGrandes módulos

Face frontal do provete

Indeformado

Deformado

P

Large deformationsSmall modulus

Small deformationsLarge modulus

Front face of the specimen

Undeformed

Deformed

Aspects about the identification of S12:

[12] Pierron F. e A. Vautrin. Composite Science and Technology, 57(12):1653-1660, 1997.[13] Pierron F. e A. Vautrin. Journal of Composite Materials, 31(9):889-895, 1997.[14] Odegard G. e M. Kumosa. Journal of Composite Materials, 33(21):1981-2001, 1999.

The failure of the specimens occurs under a homogeneous stress state

although both 6 and 2 components exist [12-14].

S12 should be determined through a failure criterion.

S12 = P / A represents an overestimated value.a ult

2 component should be calculated from finite element analysis,

introducing in the model an suitable shear constitutive law.

Numerical simulation of the variable span method

Aim:

Investigating the applicability of the variable span method [5,6] for validating the Iosipescu test.

3D models of the three-point-bending test developed in ABAQUS 6.2-1®.

Finite element models:

Wood was modelled as:

– continuous;

– homogeneous;

– orthotropic;

– linear elastic.

[5] prEN 408. European Committee for Standardization, 2000.[6] ASTM D198-94. American Society for Testing and Materials, 1994.

F

F/2F/2L

500

20

20R

L

L = 120, 135, 160, 200 and 400 mm

Configuration of the specimens in the three-point-bending tests:

Geometrical model used in the finite element analyses:

20

L/2

D E,I

F,J

B

C,H

A,G

10

x,L

y,R

A,E,D

B

C,F

G,I

H,J

Elastic properties used in the numerical models:

EL(1) ER

(1) ET(1) LR

(1) LT(1) RT

(1) GLR(2) GLT

(2) GRT(2)

(GPa) (GPa) (GPa) (GPa) (GPa) (GPa)

15,13 1,91 1,01 0,47 0,05 0,59 1,11 1,10 0,18(1) Pinus Pinaster Ait. [15].(2) Pinus Tarda L. [16].

Calibration of the friction coefficient:

Element C3D8

Mesh and boundary

conditions of the model :

[15] Pereira J.L. MSc Thesis, UTAD (in progress).[16] FPL. FPL-GTR-113, 1999.

Euler-Bernoulli beam theory:

EL =L3 F

4h4 f

a f1 = uy

f2 = uy

f3 = uy

f4 = uy – uy

A

B

C

C D

D A

B

C x,L

y,RA

B

C

Numerical results:

Timoshenko beam theory:

f1 f2 f3 f4

EL (GPa) 16,63 (9,9%) 16,05 (6,1%) 16,01 (5,8%) 15,57 (2,9%)

GLR (GPa) k = 1,2 0,74 (33,6%) 1,12 (0,6%) 1,22 (9,6%) 1,94 (74,6%)

k = 1,5 0,92 (17,0%) 1,39 (25,8%) 1,52 (37,0%) 2,42 (118,3%)

Lh

EL a

1 =

EL

2 1

+GLR

k

This assumption is not verified at midspan (AC ):

Kinematical assumption of the Timoshenko beam theory :

─ The deflection is the same for each point belonging to the same

vertical cross section, initially perpendicular to the neutral axis.

A

C

B

Numerical simulation of the Iosipescu test

Aims:

Determination of the stress and strain fields in the central region of the Iosipescu

specimen of Pinus Pinaster Ait.

Computation of the correction factors C and S.

Finite element models:

2D models developed in ANSYS 7.0® and ABAQUS 6.2-1®.

The hypothesis and elastic properties are the same as the ones used

in the numerical simulation of the variable span method.

Nominal dimensions of the Iosipescu specimen:

Mesh of the finite elements models:

5577 nodes and 1800 elements.

Elements: PLANE82 (ANSYS)CPS8 (ABAQUS)

L

R

L

T

R

T

Boundary conditions [17-19]:

Flexão no plano

i.base:

ii.iterative(LR plane):

iii. with contact:

AN

SYS

AB

AQ

US

[17] Pierron F. PhD, University of Lyon I, 1994.[18] Ho H. et al. Composite Science and Technology, 46:115-128, 1993.[19] Ho H. et al. Composite Science and Technology, 50:355-365, 1994.

Average engineering shear strain ()

Ave

rage

she

ar s

tres

s (M

Pa)

Base BC

Iterative BC

Contact BC

Reference


Ave

rage

she

ar s

tres

s (M

Pa)

Base BC

Iterative BC

Contact BC

Reference

Comparaison and validation of the boundary conditions:

Numerical results:

(LR plane)

Stress and strain fields for the LR specimen:

LR/|P/A|

─ Stress field in the central region of the specimen:

─ Stress distribution along the vertical line between notches:

RR/|P/A|LR/|P/A|

RR/|P/A|R

L

─ Strain field over the strain gauge area :

LR/|LR| RR/|LR|

Stress and strain fields for the LT specimen:

LT/|P/A|

─ Stress field in the central region of the specimen:

─ Stress distribution along the vertical line between notches:

TT/|P/A|

LT/|P/A|

TT/|P/A|T

L


LT/|LT| TT/|LT|

Stress and strain fields for the RT specimen:

RT/|P/A|

─ Stress field in the central region of the specimen: ─ Stress distribution along the vertical line between notches :

TT/|P/A|

RT/|P/A|

TT/|P/A|

T

R

RR/|P/A|


RT/|RT| TT/|RT|

Calculation of the correction factors C and S:

AC =6O

FYi=1

m

S=+45º – –45º)i=1

n

ii

n6

1

Symmetry planes

Correction factors

for wood C S CS

LR 0,97 0,99 0,95 (4,8%)

LT 0,92 0,99 0,91 (8,6%)

RT 1,04 0,97 1,01 (0,6%)

Experimental work Preparation of the specimens:

Material: wood of Pinus Pinaster

Ait. (maritime pine), 74-year-old, from Viseu (Portugal).

Iosipescu specimen:

RT specimen

LT specimen

LR specimen

─ Moisture content: 9,5% – 12,1%;

─ Density: 0,537 – 0,623;

─ 0/90 strain gauge (CEA-06-125WT-350), bonded on both faces of the

specimens, with the M-Bond AE-10 adhesive.

EMSE fixture [20]:

[20] Pierron F. Ecole des Mines de Saint-Etienne, No. 940125, 1994.

Tightness of the wedges with a dynamometrical key : 1 Nm

Experimental procedure:

INSTRON 1125 Universal test machine with the capacity of 100 kN

Data acquisition system : HBM SPIDER 8

Temperature of 23ºC (1ºC) and relative humidity of 45% (5%)

Controlled displacement rate

of 1 mm/min

Experimental equipment :

5 kN Load cell

Presentation and discussion of the experimental results

LR specimens:

Typical experimental data measured for the LR specimens:

(A)

(A)

(B)

(B)

Linear deformation measured with strain gauges ()

Load (N)

Front face (A)

Front face (A)

Back face (B)

Back face (B)

Apparent average LR – LR curves :

─ The response of the specimens contain some variability.

─ The curves are nonlinear; the source of such nonlinearity can be attributed to [19,21]:

(1) the nonlinear behaviour of the material;

(2) the geometric nonlinearity;

(3) the nonlinearity due to the contact conditions specimen/fixture.

[21] Kumosa M. e Y. Han. Composite Science and Technology, 59:561-573, 1999.


Ave

rage

she

ar s

tres

s (M

Pa)

Dispersion of the shear moduli values (GLR , GLR and GLR ):a, A a, B a

GLR GLR GLR

Mean (GPa) 1,41 ± 0,151 1,54 ± 0,181 1,48 ± 0,121

C.V.2 (%) 14,1 15,0 10,3

a, A a, B a

─ Reduction of the dispersion of GLR when the average of 6 , between the

measurements on both faces of the specimen, is considered.

a

(1) Confidence intervals at 95% confidence level;(2) Coefficient of variation (C.V.).

Specimens

She

ar m

odul

i (G

Pa)

Moisture content (u), density (d) and shear moduli (GLR, GLR) :c

Specimens u (%) d GLR (GPa) GLR (GPa)

1 11,9 0,561 1,33 1,27

2 11,8 0,607 1,59 1,52

3 12,1 0,612 1,57 1,50

4 11,8 0,605 1,62 1,54

5 12,1 0,615 1,48 1,42

6 10,3 0,538 1,32 1,23

7 10,0 0,537 1,22 1,16

8 10,4 0,609 1,53 1,46

9 9,4 0,614 1,65 1,58

Mean 11,1 0,589 1,48 ± 0,121 1,41 ± 0,111

C.V.2 (%) 9,1 5,6 10,3 10,3

ca

a

─ Applying the t test for equality of means between two samples it is concluded

that GLR and GLR belongs to the same population, at a 95% confidence level.a c


GLR shear modulus identified by the Iosipescu and off–axis [22] tests:

Test method

Iosipescu Off-axis

d GLR (GPa) d GLR (GPa)

Mean 0,589 1,41 ± 0,111 0,582 1,11 ± 0,041

C.V.2 (%) 5,6 10,3 4,0 7,0

─ The dispersion of the GLR values are of the same order of magnitude.

─ Applying the t test of equality of means, it is concluded that the GLR values

from both tests lead to different proprieties, at 95% confidence level.

[22] Garrido N. MSc thesis, UTAD (in progress).

─ The average of GLR is great than GLR in 26%.

Iosipescu Off-axis


LR – time curves :

Initial cracks

Geometric nonlinearity due to the rotation of the fibres

Nonlinearity due to the specimen/fixture contact

Crushing of the loading surfaces of the specimen

Time (s)

She

ar s

tres

s (M

Pa)

Shear stresses identified in the LR specimens :

Specimens LR LR

1 14,4 14,9

2 12,6 16,3

3 17,2 18,6

4 16,2 17,9

5 19,1 19,1

6 13,2 13,8

7 14,9 15,0

8 15,9 16,8

9 19,5 19,5

Mean (MPa) 15,9 ± 1,91 16,9 ± 1,61

C.V.2 (%) 15,2 12,1

1f ult

─ It is not possible to identify SLR using a suitable failure criterion, since the nonlinear shear constitutive law of

wood Pinus Pinaster Ait. is not known.


Shear stresses identified by the Iosipescu and off–axis [22] tests :

Test method

Iosipescu Off-axis

LR LR LR SLR1

Mean (MPa) 15,9 ± 1,91 16,9 ± 1,61 14,1 ± 0,91 16,5 ± 1,51

C.V.3 (%) 15,2 12,1 12,1 16,7

1f ult ult

(1) Shear strength determined using the Tsai – Hill failure criterion;(2) Confidence intervals at 95% confidence level;(3) Coefficient of variation (C.V.).

─ The Iosipescu test gives a good estimation of SLR for wood Pinus Pinaster Ait.:

LR < sLR < LR

1f ult


LT specimen:

Typical experimental data measured for the LT specimens:


Load (N)

Front face (A)

Front face (A)

Back face (B)

Back face (B)

Apparent average LT – LT curves :

─ The response of the specimens contain same variability.

─ The curves are nonlinear; the source of the nonlinearity can be attributed to [19,21]:

(1) the nonlinear behaviour of the material;

(2) the geometric nonlinearity;

(3) the nonlinearity due to the contact conditions specimen/fixture.


Ave

rage

she

ar s

tres

s (M

Pa)

Dispersion of the shear moduli values (GLT , GLT and GLT ):a, A a, B a

GLT GLT GLT

Mean (GPa) 1,33 ± 0,121 1,34 ± 0,101 1,34 ± 0,081

C.V.2 (%) 12,2 10,3 8,5

a, A a, B a

─ Reduction of the dispersion of GLT when the average of 6 , between the


a


Specimens

She

ar m

odul

i (G

Pa)


that GLT e GLT belongs to the same population, at a 99% confidence level.

Moisture content (u), density (d) and shear moduli (GLT, GLT) :c

Provetes u (%) d GLT (GPa) GLT (GPa)

1 11,7 0,603 1,38 1,26

2 11,7 0,595 1,41 1,29

3 11,7 0,590 1,43 1,31

4 11,5 0,599 1,55 1,42

5 11,4 0,592 1,29 1,17

6 10,8 0,581 1,19 1,09

7 10,6 0,606 1,38 1,26

8 11,3 0,556 1,27 1,16

9 10,8 0,574 1,21 1,11

10 10,5 0,593 1,25 1,15

Média 11,2 0,589 1,34 ± 0,081 1,22 ± 0,071

C.V.2 (%) 4,5 2,6 8,5 8,5

ca

a

a c


GLT shear modulus identified in the Iosipescu and off–axis [22] tests :

Test method

Iosipescu Off-axis

d GLT (GPa) d GLT (GPa)

Mean 0,589 1,22 ± 0,071 0,538 1,04 ± 0,051

C.V.2 (%) 2,6 8,5 4,0 8,1


─ The dispersion of the GLT values are of the same order of magnitude.

─ Applying the t test of equality of means, it is concluded that the GLT values


─ The average of GLT is great than GLT in 17%.

Iosipescu Off-axis


LT – time curves :

Initial cracks

Geometric nonlinearity due to the rotation of the fibres

Nonlinearity due to the specimen/fixture contact

Crushing of the loading surfaces of the specimen

Time (s)

She

ar s

tres

s (M

Pa)

Shear stresses identified in the LT specimens :

(1) These values does not follow a Normal distribution (Shapiro-Wilk test);

(2) Confidence intervals at 95% confidence level;

(3) Coefficient of variation (C.V.).

Specimens LT LT

1 15,4 16,6

2 14,7 19,0

3 15,5 17,3

4 14,5 18,6

5 16,1 17,3

6 16,5 18,1

7 19,1 20,6

8 16,0 17,5

9 14,7 18,1

10 16,1 18,1

Média (MPa) 15,9 1 18,1 ± 0,82

C.V.3 (%) 8,4 6,1

1f ult

─ It is not possible to identify SLT using a suitable failure criterion, since the nonlinear shear constitutive law of

wood Pinus Pinaster Ait. is not known.

Shear stresses identified in the Iosipescu and off–axis [22] tests :

Test method

Iosipescu Off-axis

LT LT LT SLT1

Mean (MPa) 15,9 18,1 ± 0,81 14,0 ± 0,81 16,6 ± 1,01

C.V.3 (%) 8,4 6,1 9,5 10,9

[22] Garrido N. MSc Thesis, UTAD (in progress).

1f ult ult

(1) Shear strength determined using the Tsai – Hill failure criterion;(2) Confidence intervals at 95% confidence level;(3) Coefficient of variation (C.V.).

LT < sLT < LT

1f ult

─ The Iosipescu test gives a good estimation of SLT for wood Pinus Pinaster Ait.:

RT specimen:

Typical experimental data measured for the RT specimens:


Load (N)

Front face (A)

Front face (A)

Back face (B)

Back face (B)

─ The response of the specimens contains some variability.

─ The curves are nonlinear.

Apparent average RT – RT curves :


Ave

rage

she

ar s

tres

s (M

Pa)

Dispersion of the shear moduli values (GRT , GRT and GRT ):a, A a, B a

GRT GRT GRT

Mean (GPa) 0,278 ± 0,0631 0,286 ± 0,0301 0,282 ± 0,0381

C.V.2 (%) 27,2 12,4 16,2

a, A a, B a


─ Reduction of the dispersion of GRT when the average of 6 , between the


a

Specimens

She

ar m

odul

i (G

Pa)


that GRT e GRT belongs to the same population, at a 95% confidence level.

Moisture content (u), density (d) and shear moduli (GRT, GRT) :c

Specimens u (%) d GRT (GPa) GRT (GPa)

1 11,3 0,542 0,221 0,216

2 11,6 0,551 0,254 0,259

3 11,7 0,559 0,341 0,348

4 11,7 0,556 0,271 0,276

5 12,1 0,548 0,249 0,254

6 10,2 0,622 0,338 0,345

7 9,8 0,622 0,311 0,318

8 11,4 0,623 0,280 0,285

Média 11,2 0,578 0,282 ± 0,0381 0,288 ± 0,0391

C.V.2 (%) 7,2 6,5 16,2 16,2

ca

a

a c


GRT shear modulus identified by the Iosipescu and Arcan [23] tests:

Ensaio de corte

Iosipescu Arcan

d GRT (GPa) d GRT (GPa)

Média 0,578 0,288 ± 0,0391 0,650 0,229 ± 0,0351

C.V.2 (%) 6,5 16,2 5,9 24,0

[23] Oliveira M. MSc thesis, UTAD (in progress).


─ The dispersion of the GRT values is slightly greater in the Arcan tests.

─ Applying the t test of equality of means, it is concluded that the GRT values


─ The average of GRT is great than GRT in 20%.

Iosipescu Arcan

RT – time curves :

Cracks Cracks

Time (s)

She

ar s

tres

s (M

Pa)

Shear stresses identified in the RT specimens :

Specimens RT RT

1 2,38 3,29

2 2,76 3,88

3 0,97 4,16

4 2,86 3,36

5 4,65 4,65

6 1,01 4,62

7 1,16 5,63

8 3,27 5,18

Mean (MPa) 2,38 ± 1,08 2 4,35 ± 0,702

C.V.2 (%) 54,3 19,2

1f ult


Shear stresses identified by the Iosipescu and Arcan [23] tests :

Test method

Iosipescu Arcan

RT RT

Mean (MPa) 4,35 ± 0,701 4,54 ± 0,311

C.V.2 (%) 19,2 12,1

ult ult

[23] Oliveira M. MSc Thesis, UTAD (in progress).

─ It was found a good agreement between RT values identified in both tests.

However, as the failure of the Iosipescu RT specimens does not correspond to

shear, it is not possible to say that the Iosipescu test gives a good estimation

for sRT to wood Pinus Pinaster Ait.

ult


Comparison between the LR and LT specimens:

─ Applying the t test of equality of means between two

samples it is concluded, for a 95% confidence level, that :

(1) GLR and GLT are different properties, with GLR > GLT .

(2) LR and LT are equal properties, suggesting that for

wood Pinus Pinaster Ait.: SLR = SLT .

ult ult

General conclusions and future work

The GLR, GLT and GRT shear moduli identified by the Iosipescu test are

greater than the ones obtained by the off-axis and Arcan tests by

26%, 17% e 20%, respectively, leading to different properties at a

95% confidence level.

Although it is not possible to directly identify the shear strengths SLR,

SLT and SRT using the Iosipescu test, it was proved that this test gives a

good estimation of these properties, at least for the LR and LT planes.

Perpectives :

─ The use of identification technics, based on optical measurements

and heterogenous fields, in order to identify several mechanical

properties from only one test method.

─ The use of a micro/macro approach that allows the estimation of

the macroscopic behaviour of wood through the characterization

of its micro-structure.

CHARACTERIZATION OF THE SHEAR BEHAVIOUR OF WOOD USING THE IOSIPESCU TEST J. C. Xavier Master of...

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