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

Motivation 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 . COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 2 Materials Science & Technology

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 Sizes: length  width b : 40 to 50 mm thickness t : 40 to 50 mm  Loading scheme COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 3 Materials Science & Technology

Different materialisations of the loading scheme  Test results depend on:  Construction of equipment  Procedure of testing  wedge-shaped punch  carves statically indeterminate loading  forces and stress distribution acting on glue line depend on test device  stresses perpendicular to the bond line (geometry, friction, stiffness) and variation of specimen geometry COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 4 Materials Science & Technology

Examples of test devices (1) Block shear test device used for quality control by Neue Holzbau, Lungern, Switzerland COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 5 Materials Science & Technology

Examples of test devices (2) Block shear strength test device for Shear grip for wood testing adhesive bonds (ASTM D 905) by by Thuemler GmbH, Nürnberg, Germany Wyoming Test Fixtures, Inc, USA COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 6 Materials Science & Technology

Examples of test devices (3) indented steel Double-sided test EN 392 block shear test Roš, 1924/25 Devices used by the Swiss Federal Laboratories for Materials Testing and Research, Empa COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 7 Materials Science & Technology

Analysis of static equilibrium  Eccentricity e  M = e  A v , compensated by M = h  A h  Both e and A h are indeterminate (depending on the test device used)  Actually there is a state of compression at an angle  = arctan(A h /A v )  Deformation of the compressed zone  uplift of test bar  Uplift prevented: significant bending stresses are added! COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 8 Materials Science & Technology

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 COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 9 Materials Science & Technology

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 COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 10 Materials Science & Technology

Strain element with stress-fibre-angle  0     2  0  1      cos sin f f  , 0 , 90 , v c           2 2 sin cos   90 0        cos sin  0 , 90 COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 11 Materials Science & Technology

Prototype of new test device  F cos ,  u f v a A  materialisation  loading scheme   = 14° = 1:4  do not hinder occurring shear strain  pivot bearings COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 12 Materials Science & Technology

Prototype of new test device Hydraulic jack, 120 kN  Complete picture  Loaded specimen  Self-adjusting pivot bearings COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 13 Materials Science & Technology

Preparation of test pieces Width b = 50 mm  Thickness t = 50 mm Length  = 50 mm Cutting of test pieces  COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 14 Materials Science & Technology

Evaluation of the new device  Glulam GL 24 Centre bar (C) Edge bar (E)  Norway spruce  MUF, PUR, RF, UF, EPI Bond line 10  Supplier: 8 GL companies  5 month’s survey 50  n glulam = 32 Slice B, tested with the new device n specimen = 128 Bond line 1 n shear tests = 1280 50 50 50 Slice A, tested with  Pairs of bars the established device [mm]  Comparison new/old device COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 15 Materials Science & Technology

Requirements according to EN 386 Table 1: Minimum wood failure percentages relating to the shear strength (EN 386:2001) Average values Individual values f v,a  11 4  f v,a < 6 f v,a  10 Shear strength f v,a [N/mm 2 ] 6 8 6 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  f v,a For the individual values the minimum wood failure percentage for shear strengths f v,a  6 N/mm 2 is: 153,3 – 13,3  f v,a . 100 Minimum wood failure percentage [%] 90 80 70 60 50 40 30 20 Average values Individual values 10 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Shear strength f v,a [N/mm 2 ] COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 16 Materials Science & Technology

Comparison: Shear strength by type of adhesive MUF PUR RF, UF, EPI 14.0 12.0 Shear strength f v,a [N/mm 2 ] 10.0 8.0 6.0 4.0 2.0 0.0 ) w ) ) ) t w t w t t w t w t w ) ) I s F F I s s s s s F F P P e e e e e e e e e e e e R U R U E E n n n n n n C F G D B E ( ( ( ( C F G D ( ( B E t w t w w t s s s e e e e e e n n A A n H A A H 90% Glulam plants, grouped acc. to type of adhesive used in glulam production mean 10% COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 17 Materials Science & Technology

Comparison: Wood failure by type of adhesive RF, UF, EPI MUF PUR 100 90 Percentage of wood failure [%] 80 70 60 50 40 30 20 10 0 ) w w ) ) ) t w t t t w t w t w ) ) I I s s F F s s s s F F P P e e e e e e e e e e e e R U R U E E n n G n n n n C F B D E ( ( G ( ( ( C F B D E ( w w t t w s s t s e e e e e e n n A A n H A A H 90% Glulam plants, grouped acc. to type of adhesive used in glulam production mean 10% COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 18 Materials Science & Technology

Correlation of shear strengths 11 Shear strength f v,a from new device [N/mm 2 ] 10 9 8 7 6 5 4 3 y = 0.3522 x + 4.162 R 2 = 0.3017 2 n = 269 1 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Shear strength f v,a from established device [N/mm 2 ] COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 19 Materials Science & Technology

Cumulative frequency of wood failure of block shear specimens: MUF 100 90 80 Percentage of wood failure [%] 70 MUF-type adhesives 60 50 40 30 20 10 Established device New device 0 0 10 20 30 40 50 60 70 80 90 100 Frequency COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 20 Materials Science & Technology

Cumulative frequency of wood failure of block shear specimens: PUR 100 90 80 Percentage of wood failure [%] PUR-type adhesives 70 60 50 40 30 20 Established device 10 New device 0 0 10 20 30 40 50 60 70 80 90 100 Frequency COST E53-Final Conference, Edinburgh, UK, 4-7th May 2010 21 Materials Science & Technology