01 Dec 2022

Robustness of hardwood bonding

WHFF Project 2019.02   

Project management: Martin Arnold, Walter Risi 

 

The most important facts in brief  

  • In the project, the inclusion of robustness criteria for quality assessment in wood bonding was investigated  
  • The following objectives were pursued by the project: Definition of robustness in the context of wood bonding; Development of a concept to quantify robustness with respect to the bonding process and long-term behavior; Identification and quantification of relevant influencing factors; Demonstration of the concept with respect to bonding quality by means of pilot investigations.  
  • Robustness in the context of wood bonding was defined as stable bonding quality as a result of the lowest possible sensitivity of the adhesives and the bonded products to deviating or changing production and usage conditions.   
  • Depending on the type of adhesive or specific product, other influencing factors can be decisive for the robustness. In the case of hardwood bonding, the type of wood, the annual ring position of the bonded parts, the wood moisture content and the pressing pressure are particularly important.  
  • The inclusion of ‘robustness’ as a quality criterion in the development and selection of adhesives could contribute to an improved understanding of hardwood bonding and a reliable, consistent bonding quality.  

  

Project description 

Bonding is a key technology in modern wood construction. For successful practical use, however, not only the (mechanical) performance of the bonded joints is crucial, but also their reliability, durability and economic efficiency. Particularly in the case of hardwood bonding, however, there has been a lack of comprehensive studies and long-term experience with regard to reliability and durability. The associated concept of robustness is an important basis for planning and decision-making in engineered timber construction, which is also to be introduced in wood bonding with this project.  

Robustness is generally understood as the relative insensitivity of materials or systems to changing production and usage conditions. However, robustness usually has a specific meaning in different fields of expertise. In the context of wood bonding, the term has hardly been used. Robustness concepts seem to be particularly applicable here in the areas of bonding quality and long-term behavior.   

With regard to bonding quality, this can be used to describe the insensitivity of the bonding process to deviations from standard conditions, with the aim of minimizing incorrect bonding. In the case of long-term behavior, the focus is on sensitivity to load and climatic fluctuations, which manifests itself as creep deformation or delamination of bonded joints.   

The project investigated the inclusion of robustness criteria for quality assessment in wood bonding, as an alternative to tightly controlling the bonding process or restricting the conditions of use of bonded products.   

The starting point for the project work was a specific definition of robustness in the context of wood bonding, combined with the development of a corresponding evaluation concept. However, a comprehensive quantification of the robustness of the bonding process and the long-term behavior required very extensive test data, which went far beyond the scope of the present project. Therefore, the project aimed at proposing and testing a possible investigation concept on the basis of selected pilot tests. This concept was also applied to previously published results from other research projects. The focus was on examples of the bonding process. Questions concerning long-term behavior could be investigated using analogous approaches.   

 

Conclusions 

The most important results are summarized below.   

 Robustness in the context of wood bonding was defined as stable bonding quality as a result of the lowest possible sensitivity of the adhesives and the bonded products to deviating or changing production and usage conditions.   

 Bonding quality during production is primarily the result of well-matched individual components and process parameters. In the case of long-term behavior, load and climate fluctuations as well as aging phenomena must also be taken into account.   

 The established quality tests and characteristics specified in the adhesive and product standards for load-bearing timber construction are well suited for quantification. Depending on the issue, additional metrics can also be included.   

Adhesives vary in terms of their robustness. The bonding quality always depends on the specific material combinations. The basic properties of the different adhesive classes are naturally reflected in the robustness. However, the examples given in the report also show major differences in bonding quality between different products of the same adhesive type. Finally, adhesives can also vary in their robustness to different quality characteristics.   

Depending on the type of adhesive or specific product, other influencing factors can be decisive for the robustness. In the case of hardwood bonding, the type of wood, the annual ring position of the parts to be bonded, the wood moisture content and the pressing pressure are particularly important.   

The question of whether adhesives can be simultaneously robust in terms of bonding quality and long-term behavior could not be investigated in detail within the scope of this project. However, it was assumed that only an initially perfect bonding quality leads to an expected and reliable long-term behavior.   

Effects of influencing factors are always dependent on the specific material combinations and therefore generalizations should only be made with caution. In order to correctly interpret differences in the proposed robustness parameters, the effects investigated should always be classified on the basis of the absolute measured data. This avoids overestimating effects that are small in absolute terms but show differences in robustness.   

In conclusion, the authors suggested that the robustness approach should be used as a supplement or alternative to tight control of the bonding process. This would allow more targeted formulation of constraints on the manufacturing and use conditions of bonded products and the development of new approaches to optimization (e.g., through improvements in critical bonding parameters).   

 

The project was supported by the Swiss Forest and Wood Research Promotion WHFF-CH of the Federal Office for the Environment BAFU. 

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