Quality control of glulam: Shear tests of glue lines Ren Steiger - - PowerPoint PPT Presentation

Materials Science & Technology

Quality control of glulam:

Shear tests of glue lines

COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 René Steiger EMPA, Wood Laboratory Dübendorf, Switzerland Ernst Gehri

• Prof. emeritus ETHZ

Rüschlikon, Switzerland

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

Block shear tests of glue lines are the most commonly used test methods to assess glue line integrity in continuous factory quality control.



There is a wide variety of test methods to determine shear strength of wood and wood-based composites.



Test specimen / test procedure shall allow to build up a uniform state of shear stress in a well defined shear zone. However, with most of the established test devices this is difficult to achieve under the premises of efficiency, practicability and economy.



In order to make the quality control process more reliable and to allow better comparability of test results produced by different companies and institutes, shortcomings of existing test standards and test equipments need to be analysed and screened for improvements.

Motivation

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Quality control of glulam

 Test of bonding strength of glue lines: EN 386

 Delamination tests according to EN 391  Block shear tests according to EN 392

 Test piece  Loading scheme

Sizes: length  width b: 40 to 50 mm thickness t: 40 to 50 mm

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Different materialisations of the loading scheme

 Test results depend on:

 Construction of equipment  Procedure of testing

 wedge-shaped punch  carves  stresses perpendicular to the bond line statically indeterminate loading  forces and stress distribution acting on glue line depend on test device (geometry, friction, stiffness) and variation of specimen geometry

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Examples of test devices (1)

Block shear test device used for quality control by Neue Holzbau, Lungern, Switzerland

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Examples of test devices (2)

Block shear strength test device for adhesive bonds (ASTM D 905) by Wyoming Test Fixtures, Inc, USA Shear grip for wood testing by Thuemler GmbH, Nürnberg, Germany

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Examples of test devices (3)

Devices used by the Swiss Federal Laboratories for Materials Testing and Research, Empa

Double-sided test Roš, 1924/25 EN 392 block shear test

indented steel

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Analysis of static equilibrium

 Eccentricity e  M = e  Av , compensated by M = h  Ah  Both e and Ah are indeterminate (depending on the test device used)  Actually there is a state of compression at an angle  = arctan(Ah/Av)  Deformation of the compressed zone  uplift of test bar  Uplift prevented: significant bending stresses are added!

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Assessment of the current test method

 Advantages:

 Simple (test equipment, specimen preparation, procedure, analysis)

 Shortcomings:

 Non uniform shear stress distribution  Results depend on equipment  Results can be influenced by the tester  Results cannot be compared directly  Shear + normal stress

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Approach for an optimized test method

 Perform compression tests with an inclination of 14°

(similar to panel shear tests to derive shear strength parallel to the grain according to EN 408)

 Omit tedious sample preparation by constructing a new

test device

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Strain element with stress-fibre-angle  0

  

 2

cos     

 2 90

sin      



sin cos

90 ,

  



  

1 2 



 



sin cos

, 90 , ,

  

c v

f f

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Prototype of new test device

 loading scheme

A F f

u a v

 cos

, 

 materialisation

  = 14° = 1:4  do not hinder occurring shear strain

 pivot bearings

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Prototype of new test device

 Complete picture  Loaded specimen  Self-adjusting pivot

bearings Hydraulic jack, 120 kN

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Preparation of test pieces



Width b = 50 mm Thickness t = 50 mm Length  = 50 mm



Cutting of test pieces

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Evaluation of the new device

 Glulam GL 24  Norway spruce  MUF, PUR, RF, UF, EPI  Supplier: 8 GL companies  5 month’s survey  nglulam = 32

nspecimen = 128 nshear tests = 1280

 Pairs of bars  Comparison new/old device

50 50 50 50 Bond line 1 Bond line 10 Edge bar (E) Centre bar (C) Slice A, tested with the established device [mm] Slice B, tested with the new device

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Requirements according to EN 386

10 20 30 40 50 60 70 80 90 100 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Shear strength f v,a [N/mm2] Minimum wood failure percentage [%]

Average values Individual values

Table 1: Minimum wood failure percentages relating to the shear strength (EN 386:2001) Average values Individual values Shear strength fv,a [N/mm2] 6 8 fv,a  11 4  fv,a < 6 6 fv,a  10 Minimum wood failure percentage 1) 90 % 72 % 45 % 100 % 74 % 20 % For values in between, linear interpolation shall be used.

1) For average values the minimum wood failure percentage is: 144 – 9fv,a

For the individual values the minimum wood failure percentage for shear strengths fv,a  6 N/mm2 is: 153,3 – 13,3fv,a.

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Comparison: Shear strength by type of adhesive

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 C e s t C n e w F e s t F n e w G e s t G n e w B e s t B n e w D e s t D n e w E e s t E n e w A e s t ( R F ) A n e w ( R F ) A e s t ( U F ) A n e w ( U F ) H e s t ( E P I ) H n e w ( E P I ) Glulam plants, grouped acc. to type of adhesive used in glulam production Shear strength fv,a [N/mm2]

RF, UF, EPI PUR MUF

90% 10% mean

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Comparison: Wood failure by type of adhesive

10 20 30 40 50 60 70 80 90 100 C e s t C n e w F e s t F n e w G e s t G n e w B e s t B n e w D e s t D n e w E e s t E n e w A e s t ( R F ) A n e w ( R F ) A e s t ( U F ) A n e w ( U F ) H e s t ( E P I ) H n e w ( E P I ) Glulam plants, grouped acc. to type of adhesive used in glulam production Percentage of wood failure [%]

PUR MUF RF, UF, EPI

90% 10% mean

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Correlation of shear strengths

y = 0.3522x + 4.162 R 2 = 0.3017 n = 269 1 2 3 4 5 6 7 8 9 10 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Shear strength f v,a from established device [N/mm2] Shear strength fv,a from new device [N/mm2]

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Cumulative frequency of wood failure of block shear specimens: MUF

10 20 30 40 50 60 70 80 90 100 10 20 30 40 50 60 70 80 90 100

Frequency Percentage of wood failure [%]

Established device New device

MUF-type adhesives

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Cumulative frequency of wood failure of block shear specimens: PUR

10 20 30 40 50 60 70 80 90 100 10 20 30 40 50 60 70 80 90 100

Frequency Percentage of wood failure [%]

Established device New device

PUR-type adhesives

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0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 10 20 30 40 50 60 70 80 90 100

Percentage of wood failure Shearing strength fv,a [N/mm2]

Minimal values acc. to EN 386 Producer A, RF, establ. device Producer A, UF, establ. device Producer B, establ. device Producer C, establ. device Producer D, establ. device Producer E, establ. device Producer F, establ. device Producer G, establ. device Prodcucer H, establ. device Producer A, RF, new device Producer A, UF, new device Producer B, new device Prodcuer C, new device Producer D, new device Producer E, new device Producer F, new device Producer G, new device Producer H, new device

Comparison of test results to requests of EN 386: Mean values

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Comparison of test results to requests of EN 386: Individual values UF, RF, EPI

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 10 20 30 40 50 60 70 80 90 100

Percentage of wood failure Shearing strength fv,a [N/mm 2]

Minimal values acc. to EN 386 MUF, existing test device MUF, new device 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 10 20 30 40 50 60 70 80 90 100

Percentage of wood failure Shearing strength fv,a [N/mm 2]

Minimal values acc. to EN 386 PUR, existing test device PUR, new device 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 10 20 30 40 50 60 70 80 90 100

Percentage of wood failure Shearing strength fv,a [N/mm 2]

Existing shearing test device New shearing test device Minimal values acc. to EN 386

UF, RF, EPI specimens

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 Using established shearing tools many factors (strength of wood,

shape and size of specimen, design of the shearing tool, friction conditions at the support, test person, rate of loading, etc.) interfere or bias the measurements.

 It is not possible to directly compare test results derived by either

different persons or using different block type shearing tools.

 Axial compression tests at an oblique angle of 14° to the grain are a

good alternative to established shearing tests according to EN 392 and similar standards.

 The prototype of a new shearing tool is capable of producing reliable

shear strength values not being influenced by the person who carries

• ut the tests.

 Shear test results exhibit lower values and lower variation when being

carried out with the new shearing tool. With respect to wood failure percentages no differences were found.

Findings

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Acknowledgement

 Financial Support

 Swiss Federal Office for the Environment FOEN

(Project WHFF 2007.04)

 Test specimens supplied by Swiss Glulam Industry

 www.glulam.ch

 Production / Co-Engineering of new test device

 Prof. emeritus E. Gehri, ETH Zurich  Zum Wald Maschinen- und Apparatebau, Erlenbach, Switzerland

www.zu-wald.ch

 Experimentals

 Mr. Walter Risi and Mr. Michael Strässle

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Discussion