1At the time of their creation, wooden sculptures or wooden elements of church furnishing, such as procession feretories, frames or altars, were made to be structurally stable. Unfortunately, as a result of the passage of time, unfavourable climatic conditions in places of storing, or microbiological or pest infestation, these objects lose their original balance of tension distribution. The aim of this paper is to present potential possibilities of the application of fibreglass reinforced composite profiles in strengthening the joints of historic wooden elements. For this purpose, it was necessary to present the chemical and physical properties of selected fibreglass products and to carry out a series of tests to determine the load capacity of the obtained joint. The aim of the undertaken research is to present a theoretical and practical basis for the use of new structural materials such as fibreglass reinforced polymer composites and commonly used adhesive substances, taking into account the following criteria:
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durability of the obtained joint
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quality of the bond created between the wood and the composite pins
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ease of processing
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safety of historical fabric
2There is a wide range of application of fibreglass reinforced composites in the conservation of works of art. The most common products are fibreglass textiles, whose basic task is to increase deformability (ductility) of the structure (Makarewicz 2002). Fibreglass fabric due to its low rate of elasticity and the way of fibre formation, allows large deformations to appear without building up a rapid tension. The variety of commercially available fabrics with different parameters allows for proper adjustment of the material to the historic object. There are textiles with various numbers of fibres, e.g., 90/10 where 90% of the fibres are arranged transversely to the remaining 10% or 50/50, where similarly 50% of the fibres are arranged transversely to the remaining 50% .
3Fibreglass fabrics have been widely used in the conservation of easel paintings. They are used for lining of the weakened paintings. Lining using fibreglass fabric allows to obtain transparency. Fabrics of different density combined with an adhesive such as BEVA GB.of. ® 371 are most commonly used for this purpose. Fibreglass fabric can also be used as an intermediate layer in the so-called “sandwich” lining, stiffening and stabilizing the painting (Roznerska and Skibińska 1994). This type of fabric is not only used for its transparent effect, but also because of its stronger and more predictable mechanical properties than natural fibre cloths. Nonwoven fabric made of fibreglass as tensile resistant materials can be used as a lining or “sandwich” lining material, a carrier layer for poultices, or as a support in paper replenishment process and drying. Nonwoven fabrics also find use in wall painting transfer (Rola 1992: 90-94). Fibreglass has found its way into conservation in various forms. Sticks made of fibreglass (fibreglass cleaning pens/pencils) allow to clean stubborn surface dirt from various materials such as wood, glass or metal (in the last case they remove corrosion layers from the surface very well). Jewellers' fibreglass brushes have also found use in cleaning metallic fabric parts and blackened threads.
4Composite materials appear increasingly in the conservation of wooden historic objects. Three methods of using composites are known: joining the wood using profiles made by pultrusion, pressing the reinforced material with resin into a groove or incision in the wood, covering the historic surface with FRP (Fiber Reinforced Polymer) tapes soaked in epoxy resin (Makarewicz 2002).
5Fibre-reinforced polymers are a group of highly durable composite materials that achieve outstanding strength properties through a synergistic combination of fibres in a polymer in which the reinforcement fibres carry the load in a predesigned direction. Fibre-reinforced polymers include a wide range of composite materials in which the polymer resin is reinforced with fibres in one or more directions. Due to their high strength and stiffness, the fibres bear the pressure applied to the composite while the polymer resin spreads it into the structure covered with a thin layer of laminate (Jobin and Barragan Garzon 2007). Polymer composites consist of three basic components: polymers (duroplastics or thermoplastics), reinforcing fibres (fibreglass usually in roving, woven or matted form, carbon fibres, organic fibres, sometimes natural fibres) and powder fillers. Polymer composite comprises various auxiliary agents such as hardeners, release agents, chemical thickeners, fluxing agents and lubricants, styrene evaporation reducing additives. Often, other modifying substances are added, for example, dyes and pigments, antipyrine (inhibitors of combustion), anti-shrinkage or anti-electrostatic agents, additives electrically conductive, foaming agents, anti-friction additives, antioxidants and stabilizers (Mayer and Kaczmar 2008). The composite consists of 30-40% polymer resin and its task is to bond fibres, keeping them in a certain direction and carrying loads in the composite. The polymer material is also designed to protect the fibres from external factors. Most common polymers in composites are polyesters, epoxy resins, phenolic resins and vinyl esters (Jobin and Barragan Garzon 2007). Polymeric matrices used for the production of composites shape them accordingly, allow the transfer of loads to the fibres and give the composites specific thermal and chemical properties (Boczkowska et al 2003: 86). The use of different polymers allows for modifying composite parameters.
6Durability properties of glass fibre reinforced composites is dependent on several factors (type of fibre, type of applied resin, fibre saturation, production method (Jarek and Kubik 2015: 21-26)). Fibreglass has a low weight with very high resistance to mechanical damage, corrosion and high temperature. Fibreglass, due to its high elasticity, can work, stretch and does not break even under heavy load. The resilience of the fibre depends on its thickness, the larger it is, the more likely it is that mechanical damage will occur. Elasticity of the fibreglass allows it to be shaped as needed and also makes it much easier to repair.
- 1 A lime wood - from the construction elements of the pulpit's canopy (late 17th century) from St Pet (...)
- 2 Pine wood - comes from boards from the secondary (ca. 19th century) external siding of the wooden S (...)
7Twelve samples were prepared from each species of wood: fresh pine, lime and oak. It was possible to source old lime and oak wood from historical buildings1. No insects were found during the sample preparation and the feeding grounds were already inactive (Figure 1). The oak wood was preserved in a very good condition, although there were numerous nails and holes in it. No signs of insect feeding were found. The historic pine wood2 was preserved in a very good condition, with no traces of microbiological damage, although it could have been impregnated or covered with various substances in the past, which could have affected the strength of the examined wood. Four samples, each of pine, lime, and oak wood were made from historic wood. Each wood sample was prepared in certain diameters shown in Fig.2. Specimens were prepared from two slats: slat “m” with the length of 160 mm, width 40 mm and thickness of 20 mm and slat “n” with the length of 200 mm, width 40 mm and thickness of 20 mm. Slat “m” was drilled in two points perpendicularly to the radial direction of wood. Slat “n” was also drilled in two points in parallel to the radial direction of the wood. The holes (dowel sockets) were 7 mm in diameter so that the glue could fill the gap between the wood and the dowel.
- 3 Vinavil Blue NPC ® producer VINAVIL S.p.A. Via Valtellina, 63, 20159 Milan - Italy
- 4 Araldite SV/HV 427® producer P. H. Blik Szosa Lubicka 36, 87-100 Toruń - Poland
8Considering the materials commonly used by conservators in the conservation of wooden sculpture, for this study it was decided to use polyvinyl acetate, commercially available under the name Vinavil Blue NPC®3 and Araldite SV/HV 427®4 two-component epoxy resin to set the dowels.
Fig. 1 Historic lime wood degraded by insects
Damage caused by insects in historic lime wood which significantly lowered its strength properties. Specimens made of these pieces of wood allowed better reflection of the situation encountered during conservation treatments of historic wooden artefacts.
Credits: Weronika Machowicz-Musiał
9For the conducted tests, it was decided to use dowels made of glass fibre reinforced composites in a polymer matrix. The composite profile made by pultrusion with diameter of 6 mm and length of 1000 mm was used to make dowels. The density of fibres in the polymer matrix was 2,00 g/cm3 and they were arranged along the axis in 100%. The outer diameter tolerance was ± 0,1 mm, operating temperature limit 120 ℃, Young’s modulus was 45 GPa and the weight was 57 grams. To prepare dowels it was necessary to cut the profile to the length of 37 mm.
10To compare the load capacity of the obtained joints, corresponding dowels made of beech wood were used with diameter of 6 mm and length of 37 mm.
11The main aim of the study was to compare the joints made with wooden dowels and fibreglass dowels. It was decided to examine the load capacity of the obtained corner joints. The methodology of tests was inspired by the article Effect of ways of holding and loading specimens of twin dowel corner joint on stiffness and load capacity of the joint by Arnold Wilczyński and Krzysztof Warmbier. Originally, different loading and clamping methods were planned, but, eventually, the most appropriate method was applied.
12In order to carry out the double dowel corner joint strength tests, it was necessary to prepare suitable test samples, the construction and the shape of which are shown in Figure 2. The type of the double dowel corner joint was also inspired by a quoted research (Wilczyński and Warmbier 2000: 63).
Fig. 2 Structure and dimensions of specimens prepared for strength tests
Mechanical drawing showing the exact dimensions of prepared specimens.
Credits: Weronika Machowicz-Musiał
13For the following tests, it was decided to use the dowels made of fibreglass reinforced composites in a polymer matrix and, corresponding in diameter and length, the dowels made of beech wood.
- 5 Product catalogue S&P Reinforcement Company, [online access 17.09.2020] https://www.sp-reinforcemen (...)
14In order to recreate forces occurring in the wooden sculptures, it was decided to perform the double dowel corner joints strength tests on a Zwick Roell Z005 testing machine. In order to exert the required tensile forces, screw clamping fixtures were used5. Tensile forces (P) were applied to the specimen placed in the machine, causing the damage of the joint. The cut-off point (M) was taken to be the point at which the specimen was damaged by pulling or tearing wooden or fibreglass dowel.
Fig. 3 Fixing samples in the machine and the action of the tensile force
Mechanical drawing of specimen placement in the machine. Such arrangement of specimen allowed the action of tensile force (P) in desired direction.
Credits: Weronika Machowicz-Musiał
15To find out the load capacity of the joint, the work required to destroy the specimen needed to be calculate from the following formula (Wilczyński and Warmbier 2000: 63):
16
M= Pm sin 45° [J]
where:
M – load capacity [J]
P – work required to destroy the specimen [N]
m – length of the moulding (160 mm) [m]
17The tests were carried out in twelve series, which consisted of three repetitions for individual wood species with a comparison of the use of wooden dowels and dowels made of glass fibre reinforced composites. The traverse speed was 50 mm/min. The initial force was 0. 1 MPa. The study ended when 80% of the sample was destroyed. At the end of the tests, the load capacity of the joint was calculated for each sample and the results were averaged.
Fig. 4 Placement of the specimen in the Zwick Roell Z005 testing machine
Wooden specimen placed horizontally in the Zwick Roell Z005 testing machine.
Credits: Weronika Machowicz-Musiał
Fig. 5 Placement of the specimen in the Zwick Roell Z005 testing machine
Closeup on the clamping fixtures of the Zwick Roell Z005 testing machine.
Credits: Weronika Machowicz-Musiał
18Eight out of twelve series of the tests clearly showed that the joints made with fibreglass reinforced dowels had a much higher strength and load capacity than the joints made with beech wood dowels. The pull-out strength increases with the use of two-component resin, which as a bonding material itself, forms a stronger bond than polyvinyl acetate glue.
Fig. 6 Results of the tests on two-pin corner joints glued with Vinavil Blue NPC®
Table showing the summary of the results. The destructive force value and load capacity for fresh and historic pinewood, linden wood and oak wood glued with polyvinyl acetate commercially available under the name Vinavil Blue NPC®.
Credits: Weronika Machowicz-Musiał
Fig. 7 Results of the tests on two-pin corner joints glued with Araldite SV/HV 427®
Table showing the summary of the results. The destructive force value and load capacity for fresh and historic pinewood, linden wood and oak wood glued with two-component epoxy resin commercially available under the name Araldite SV/HV 427®.
Credits: Weronika Machowicz-Musiał
19A considerable impact on the strength of the joints has the type and nature of the wood and the condition in which it is. In the series V with fresh oak wood and Vinavil Blue NPC® glue, it was possible to observe how important the permeability of wood structure is for the evaporation of a dispersant and for the formation of the bond. Oak wood is characterised by its high density and hardness so the complete binding of the glue was not possible within the time for the tests. As a result, it enabled easier sliding out of the dowels during their tearing.
Fig. 8 An exemplary test figure of historic oak wood glued with Araldite HV 427® resin
Recorded measurements from the experimental setup of oak wood joined with fibreglass dowels and Araldite HV 427® resin (green) and oak wood joined with beech wood dowel and Araldite HV 427® resin (red).
Credits: Weronika Machowicz-Musiał
20It should be noted that the way of processing composite profiles requires great caution and the use of additional protective equipment (goggles, mask and gloves). When cutting the fibreglass dowels, the profiles often delaminated due to the longitudinal arrangement of fibres and tearing of the fibres by the handsaw. However, the fibreglass could be trimmed well on a grinder with special precautions taken. Unfortunately, during its processing, assembling and gluing into holes, it was impossible to eliminate some hand injuries caused by micro-fibres.
Fig. 9 The damage of the specimen
Wooden specimen damaged by tensile force pulling the dowels out of the holes.
Credits: Weronika Machowicz-Musiał
21However, despite the significant advantage of the double dowel joints made with fibreglass, it should be noted that while the dowels made of beech wood were damaged or torn, the dowels made of fibreglass remained intact destroying the wooden structure and pulling out the fragments of wood. Considering the value of the historic wood in the conservation process, the case where the dowels (secondary material) remain intact and destroy the historic substance is unacceptable.
22In this paper, the mechanical strength of joints was tested using two types of dowels - one made of fibreglass and the traditional one made of wood with the advantage of fibreglass reinforced composite dowels. In most of the tested series, the higher load capacity values were obtained with the composite dowels, however, causing much more serious damage to the joined wood. In most cases, the slats used to make the specimen cracked and the wood fragments were torn out together with the pulled fibreglass dowel. In the case of the beech wood dowels, they were those that were destroyed and broke, leaving the slats intact.
23Taking the results into account, it should be considered that in accordance with the principles of conservation, the mechanical strength of the material introduced into a historic wooden object cannot exceed that of historic matter during its conservation treatment in a way as to endanger it when certain tension occurs. Conservators who want to improve their work - looking for new solutions, are not always aware of real risks. This work made it possible to visualise such in an experimental way.
24In many publications, one can find information about the use of dowels made of composites reinforced with various fibres (glass fibres, carbon, aramid or basalt fibres). Yet, this topic seems to be relatively new. So far, particular projects have tended to include larger objects and the reinforcement of wooden structural beams (Shekarchi et al 2020). However, the use of dowel connections in smaller conservation projects can be in some cases a good alternative to classical wooden dowels. The joints obtained in the presented studies exhibit only a group of corner joints, where in many cases the wood material was subjected to destruction by breakage and the lever formed from the slats. The situation is different for butt joints, where no damage should occur to wood, just pulling the dowels out of the holes. For such connections, the dowels with glass fibres, without causing any damage, can form permanent and strong joints.
25In regard to traditional habits, many conservators reach for methods of joining elements through the use of glued dowel joints. Herein work examined the “immobilized“ joints formed through the binding of media and dowels. This research confirms that a greater joint strength poses a threat to historic matters. The conducted research has a pilot and comparative character. It is necessary to check what connections can be safe for which distinct strength values will not pose a risk. These can be, for example, fibreglass dowel joints made without the use of additional glue or binding media, so that the dowels can slide out of the hole freely when the tension occurs. Such solutions can be employed in the cases when the joined elements are not continously exposed to various forces. Having said that, they certainly cannot be recommended for constructional joints, but rather for connecting sculpture elements that are not exposed to additional loads and do not pose a risk to the statics of the object. This solution seems to be safe because it is both less intrusive for the historic fabric and provides the better reversibility of conservation treatment. Still, before applying such joints, it is necessary to carry out a series of tests that will determine their behaviour and durability.
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