Temperature Dependent Strength of Bonded Timber Joints

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Information about Temperature Dependent Strength of Bonded Timber Joints
Technology

Published on November 9, 2009

Author: till.vallee

Source: slideshare.net

Temperature Dependent Strength of Adhesively Bonded Timber Joints Thomas Tannert, Research Fellow Till Vallée , Professor Simon Hehl, Research Assistant Timber & Composites Laboratory Bern University of Applied Sciences

Outline Introduction Experimental investigations Numerical modeling Discussion Conclusions

Introduction

Experimental investigations

Numerical modeling

Discussion

Conclusions

Introduction Adhesively bonding almost always outperforms mechanical fastening when it comes to stiffness and strength of timber joints One critical issue related to structural application of adhesively bonded joints is their thermo-mechanical behavior It is tempting to see the adhesive as the critical element in the compound

Adhesively bonding almost always outperforms mechanical fastening when it comes to stiffness and strength of timber joints

One critical issue related to structural application of adhesively bonded joints is their thermo-mechanical behavior

It is tempting to see the adhesive as the critical element in the compound

Caveat In the following, investigations related to the effect of elevated temperatures are presented which relay the concerns related by practitioners about temperature a structure is likely to encounter during its service life, specifically outside extraordinary events like fires Thus, the investigations focus on a temperature range effectively encountered in middle-Europe – 25 °C to +75 °C (–13 F to 167 F) extentend herein to +100 °C

In the following, investigations related to the effect of elevated temperatures are presented

which relay the concerns related by practitioners about temperature a structure is likely to encounter during its service life, specifically outside extraordinary events like fires

Thus, the investigations focus on a temperature range effectively encountered in middle-Europe

– 25 °C to +75 °C (–13 F to 167 F)

extentend herein to +100 °C

Introduction: timber vs. temperature from –200 °C to +200 °C, strength is almost linearly dependent on temperature, with strength decreasing towards higher temperatures As a rule of thumb, strength of timber decreases by around 1% per °C Below temperatures of around +100 °C, the reduction in strength is reversible Strong interaction between strength at different temperatures and moisture content

from –200 °C to +200 °C, strength is almost linearly dependent on temperature, with strength decreasing towards higher temperatures

As a rule of thumb, strength of timber decreases by around 1% per °C

Below temperatures of around +100 °C, the reduction in strength is reversible

Strong interaction between strength at different temperatures and moisture content

Introduction: adhesive vs. temperature Both strength and stiffness are affected by temperature … A key element in the quantification of the latter is being the glass transition temperature, T g For temperatures below T g , both strength and stiffness are roughly independent of T When T reaches T g , significant drop of both strength and stiffness, residual around 20–25% Further degradation at much higher temperatures, characterized by the degradation temperature T d … which lies outside the scope of this study

Both strength and stiffness are affected by temperature

… A key element in the quantification of the latter is being the glass transition temperature, T g

For temperatures below T g , both strength and stiffness are roughly independent of T

When T reaches T g , significant drop of both strength and stiffness, residual around 20–25%

Further degradation at much higher temperatures, characterized by the degradation temperature T d

… which lies outside the scope of this study

Experimental investigations Adhesively bonded double lap joints (DLJ) Timber: spruce Adhesives a “classical” epoxy, SikaDr330 a “classical” polyurethane, SikaForce7851 All specimens thermally conditioned For at least 8h, prior to testing in a climatic chamber

Adhesively bonded double lap joints (DLJ)

Timber: spruce

Adhesives

a “classical” epoxy, SikaDr330

a “classical” polyurethane, SikaForce7851

All specimens thermally conditioned

For at least 8h, prior to testing in a climatic chamber

Experimental investigations: Mechanical characterization Timber Failure criterion full set for stiffness and strength in all directions Including statistical props Characterization at +25 °C and +75 °C Adhesives Tensile/shear at room temp DMA analysis

Timber

Failure criterion

full set for stiffness and strength in all directions

Including statistical props

Characterization at +25 °C and +75 °C

Adhesives

Tensile/shear at room temp

DMA analysis

Experimental investigations: Statistical characterization

Experimental investigations: Mechanical characterization No significant differences for timber, between room temperature (~25 °C) and 75 °C, not meas. beyond However, significant decrease in MC: 50°C: ~9%, 75°C: ~5%, 100°C: ≤ 1% (almost oven dry) DMA indicates that T g lies ~ 58 °C for SikaDur330, and ~47 °C for SikaForce7851 Stiffness drops to around 25–30% of original value No data for strength at different T for adhesive

No significant differences for timber, between room temperature (~25 °C) and 75 °C, not meas. beyond

However, significant decrease in MC:

50°C: ~9%, 75°C: ~5%, 100°C: ≤ 1% (almost oven dry)

DMA indicates that T g lies

~ 58 °C for SikaDur330, and ~47 °C for SikaForce7851

Stiffness drops to around 25–30% of original value

No data for strength at different T for adhesive

Experimental results

Experimental results Failure mode is, at all investigated temperatures and for both adhesives, characterized by a brittle failure inside the adherend, i.e. timber Strength is independent on the adhesive type used Although significant difference in stiffness between both (around 10:1) Strength is clearly not dependent on the temperature under which they were tested at least if T ≤ 75 °C, and at least only significantly dependant for T = +100 °C

Failure mode is, at all investigated temperatures and for both adhesives, characterized by a brittle failure inside the adherend, i.e. timber

Strength is independent on the adhesive type used

Although significant difference in stiffness between both (around 10:1)

Strength is clearly not dependent on the temperature under which they were tested

at least if T ≤ 75 °C, and at least only significantly dependant for T = +100 °C

Numerical modeling All configurations were numerically modeled Using Ansys v11 Material props gathered for this purpose including the orthotropic properties of timber Including the plastic properties of the adhesives Including the stiffness reduction due to temp. Including strength increase due to MC Determination of all stress components that trigger failure: σ x , σ y , τ xy

All configurations were numerically modeled

Using Ansys v11

Material props gathered for this purpose

including the orthotropic properties of timber

Including the plastic properties of the adhesives

Including the stiffness reduction due to temp.

Including strength increase due to MC

Determination of all stress components that trigger failure: σ x , σ y , τ xy

Discussion Temperature significantly reduces stiffness of the adhesives …according to DMA Lower stiffness of the adhesives affects the stress distribution along the bonded splice … according to FEA, stresses are reduced However, failure remains triggered by the strength of timber … which is not affected, at least for T ≤ +75 °C Indicating that adhesive remains stronger than timber

Temperature significantly reduces stiffness of the adhesives …according to DMA

Lower stiffness of the adhesives affects the stress distribution along the bonded splice

… according to FEA, stresses are reduced

However, failure remains triggered by the strength of timber

… which is not affected, at least for T ≤ +75 °C

Indicating that adhesive remains stronger than timber

Numerical modeling Size effects Weibull probabilistic  y  y  xy  xy

Numerical modeling Stress-based strength prediction Following the most stressed element Probabilistic strength prediction Including size-effects and statistical information Predictions for epoxy

Stress-based strength prediction

Following the most stressed element

Probabilistic strength prediction

Including size-effects and statistical information

Conclusions Adhesively bonded joints exposed to commonly encountered elevated temperatures must not systematically be as problematic as first thought Joint strength is not exclusively driven by a stress based approach Strength prediction methods should not only process stress magnitudes; probabilistic methods are such methods, which show to be very effective … the interested listener is kindly invited to listen to my talk on “probabilistic strength…” at this conference

Adhesively bonded joints exposed to commonly encountered elevated temperatures must not systematically be as problematic as first thought

Joint strength is not exclusively driven by a stress based approach

Strength prediction methods should not only process stress magnitudes; probabilistic methods are such methods, which show to be very effective

… the interested listener is kindly invited to listen to my talk on “probabilistic strength…” at this conference

Thank you for your attention I’m happy to answer your questions [email_address]

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