Abstract
Wooden composites reinforced with advanced engineering materials are promising as building materials. The use of these materials has been increasing in recent years. It is important to understand the behaviour of their deformation under load for optimum design of composite materials. There is limited information about the deformation behaviour of wooden composites under different loads. In this study, strain and displacement distributions were measured for wood structural materials made with glass fibre, carbon fibre, oak (Quercus robur), and polyurethane resin. The digital image correlation method (DIC) was used for this purpose. Deformation behaviours were determined from the images recorded under specific loads in the bending test. There was an increase of 17.3% in bending strength of wood composites with the addition of glass fiber. The cracking process was visualized for different advanced engineering materials. The imagery clearly showed the development of the strain and displacement field. The deformation behaviours of reinforced and unreinforced wood composites were different. The strain distribution of wood composites significantly affected the strength properties.
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Effects of Different Advanced Engineering Materials on Deformation Behaviour of Wood Structural Materials
Timucin Bardak *
Wooden composites reinforced with advanced engineering materials are promising as building materials. The use of these materials has been increasing in recent years. It is important to understand the behaviour of their deformation under load for optimum design of composite materials. There is limited information about the deformation behaviour of wooden composites under different loads. In this study, strain and displacement distributions were measured for wood structural materials made with glass fibre, carbon fibre, oak (Quercus robur), and polyurethane resin. The digital image correlation method (DIC) was used for this purpose. Deformation behaviours were determined from the images recorded under specific loads in the bending test. There was an increase of 17.3% in bending strength of wood composites with the addition of glass fiber. The cracking process was visualized for different advanced engineering materials. The imagery clearly showed the development of the strain and displacement field. The deformation behaviours of reinforced and unreinforced wood composites were different. The strain distribution of wood composites significantly affected the strength properties.
Keywords: Wooden composites; Digital images; Structural materials; Deformation; Strain measurement
Contact information: Furniture and Decoration Program, Bartin Vocational School, Bartin University, 74000, Bartin, Turkey; *Corresponding author: timucinb@bartin.edu.tr
INTRODUCTION
Wood is a strong, enduring, and renewable raw material (Hu et al. 2015). However, wood composites have poor load-carrying capacity compared with concrete and steel (Dunlop and Fratzl 2013; Zhou et al. 2015). In recent years, fibre-reinforced elements have been widely used to strengthen wood structural materials, as they present a significant strength increase with minimal encumbrance and mass increase (Ianasi 2015; Tekieli et al. 2017). Additionally, the costs of synthetic fibres are rapidly decreasing (Wei et al. 2013). Evaluation of the mechanical behaviour of wooden structures is important for both design and durability. Strain and displacement parameters should be properly addressed in construction projects.
Optical methods are frequently used to analyze engineering problems (Valle et al. 2018). In the field of solid mechanics, it is important to know the deformation behaviours that occur on the surface of inspected materials (Yoneyama et al. 2016). Digital image correlation (DIC) is a method to characterize various aspects of wood displacement, crack tips, and strain fields (Ling et al. 2009). Digital image correlation is based on the correlation of grey values of successive digital images of the undeformed and the deformed specimen (Sutton et al. 1988; Shen et al. 2015). It is widely used in mechanical engineering and provides advanced information (Pan et al. 2009; Caminero et al. 2014; Tekieli et al. 2017). This method allows measurements over the entire surface of the examined specimen, far beyond a limited number of distinct points (Mahal et al. 2015). Furthermore, the cumulative crack openings of microcracks present in the material can be clearly defined (Luo et al. 1993; Alam et al. 2014). Digital image correlation has been applied to measure the mechanical features of wood (Serrano and Enquist 2005; Ljungdahl et al. 2006; Sjödin et al. 2006; Jeong and Park 2016; Bardak et al. 2017). However, there has been limited study to assess deformation behaviours of wood composite materials based on DIC.
This study investigated the effects of different advanced engineering materials on strain and displacement distributions of wooden composites. Moreover, cracking profiles were determined. Open source two-dimensional (2D) DIC software (Ncorr) was employed to determine the whole field displacements under specific loads in the bending tests (Blaber et al. 2015). The DIC method appears to be a suitable method for analyzing and describing deformations in wood composites reinforced with advanced engineering materials. The results showed that deformation behaviours of carbon- and glass-reinforced wood composites were different from those of wood composites without reinforcement. Moreover, the impact on bending strength of advanced engineering materials was determined. The highest bending strength values were found in glass fibre-reinforced composites.
EXPERIMENTAL
The research consisted of two main stages: The experimental bending test of the samples, followed by the analysis of the displacement and strain of the samples from the images obtained by the digital camera.
Material and Testing
Two different reinforced (carbon fibre, glass fibre) wooden composites and unreinforced wooden composite were tested in the bending test. We used carbon and glass fibres, having bidirectional fibres network in study. Carbon and glass fibres had a thickness of 0.25 mm. Table 1 presents the mechanical properties of the glass fibre and carbon fibre used in experiment.
Table 1. Mechanical Properties of the Glass and Carbon Fibres
Before the test, wood (Quercus robur) samples were prepared in a climate-controlled room. Temperature and relative humidity were adjusted to 20 °C and 50%, respectively. The specific weight of the wood used in the study was 0.67 g/cm3. In the bending test, modulus of rupture (MOR) and modulus of elasticity (MOE) of the wood material were 85 MPa and 11950 MPa, respectively. The composites were produced separately using polyurethane adhesive with carbon and glass fibres. A pressure of 0.8 N/mm2 was applied to the composites for 4 h. The wooden composites were 330 mm long with rectangular cross sections of 50 mm × 40 mm. Figure 1 illustrates the wood composites used in the experiment. All the composites were used with a layer of reinforcing material. The weight ratio between adhesive and carbon fibre was 0.84. And the weight ratio between adhesive and glass fibre was 0.60. The adhesive layer had a mean thicknesses of approximately 0.15 mm in all composites. Table 2 shows the properties of the wood composites.
Table 2. Properties of Wood Composites
Bending strengths of the wooden composites were determined according to the TS 2474 standard (1976). The distance between the centers of the two supports was 300 mm free span. The wood composites were aimed to achieve failure in 300 ± 50 s. For each wooden composite, ten tests were made in the bending test. Statistical analysis of the results was made using ANOVA of SPSS software (IBM Corporation, New York, USA).
Fig. 1. The wood composite used in the experiment
Digital Image Correlation
Digital image correlation is a non-contact method popularly used for measuring deformation based on speckle patterns (Pan et al. 2009; Vora et al. 2018). It gives important information about deformations of materials. To examine displacement, strain, and fracture distribution on the reinforced (carbon fibre, glass fibre) and unreinforced wooden composites, bending tests were performed using a universal testing machine equipped with LabVIEW (National Instruments, Austin, TX, USA), Ncorr software, and a digital camera measurement system (Nguyen et al. 2017). Figure 2 shows the experimental setup for the 2D DIC method.
Fig. 2. The experimental setup for the 2D DIC method
LabVIEW software was used to acquire and prepare images (Hryniewicz et al. 2015). This software is a visual programming language. It is popular and easy to use, compared to other programming languages (Mahmoodi et al. 2018).
The camera (Basler ace acA1600-20gc) had a resolution equal to 1626 pixels × 1236 pixels. The camera was used to record the entire surface image of the composites in 24-bit RGB colour and 1624 pixel × 1234 pixel resolution JPEG format (.jpg). Deformation was determined using open source code and MATLAB-based Ncoor software (Harilal et al. 2015).
RESULTS AND DISCUSSION
Static Bending Test
The main purpose of this study was to analyze strain and displacement distributions caused by fracture in the wood composite samples with DIC. Table 3 shows measured values of force for non-reinforced wood composites (UC), the wood composites reinforced with carbon fibre (CC), and the wood composites reinforced with glass fibre (GC).
Table 3. Bending Strengths of the Composites
* Duncan’s multiple range test was performed.
** Rows with the same letter do not differ statistically (p < 0.05).
The bending strength of the wood composites was increased with the addition of glass fibre by 17.6% compared with the unreinforced composites. The results were consistent with previous studies (Thorhallsson et al. 2017). However, carbon fibre did not increase the bending strength.
The lack of significant bending strength increase in carbon fibre-reinforced composites can be explained by the low strength of the interface between wood and carbon fibre (Tavakkolizadeh and Saadatmanesh 2001).
Measurement of Displacement and Strain
The DIC method is an effective means of measuring deformation behaviour (Li et al. 2013). The fields of displacement and strain give a general overview of the efforts that occur in the structure (Bigaud et al. 2018).
Bending tests were carried out on different types of unreinforced wood composites (UC), wood composites reinforced with carbon fibre (CC), and wood composites reinforced with glass fibre (GC). Their displacement/strain maps were compared. Figure 3 shows the load-displacement curves of all wood composites based on the results obtained from the experimental and DIC method.
Fig. 3. The load-displacement curves of all wood composites based on the results obtained from the experimental and DIC method.
Table 4 shows deformation and strain values for the wood composites.
Table 4. Deformation and Strain Values for Wood Composites
Max = maximum, min = minimum
Fig. 4. The Ɛxx (strain along the x-direction) contours of UC, CC, and GC at different load levels
As can be seen from the curves, the DIC method and experimental results were compatible with each other. In Fig. 3 and Table 4, it was observed that glass reinforced composites decreased the displacement at different load levels (20%, 40%, 60% and 80% of FMAX).
Fig. 5. Ɛxy (shear strain) contours of UC, CC, and GC at different load levels
In the literature, it has been emphasized that the highest Ɛxx (strain along the x-direction) contours of composites can be used for tracking crack development at various stages of loading (Suryanto et al. 2017). The size and length of the strain contours indicate the progression of existing cracks. Based on the DIC images, cracks begin at the bottom of all composites. After 60% of the maximum load, reinforced and unreinforced composites showed differences. Strain contours in non-reinforced composites concentrated at one point while reinforced composites concentrated at several points. Figure 4 shows Ɛxx (strain along the x-direction) contours of UC, CC, and GC at different load levels. When examining DIC images in ultimate load, the differences between crack modes were clearly seen in wood composites. The maximum strain contours (Ɛxx) in the unreinforced composites were concentrated in the bottom part. This shows that the failure was in the wood. The maximum strain contours (Ɛxx) in carbon and glass reinforced composites were concentrated on the bottom and left sides. These contours indicate that the failure was caused by the adhesive line and wood. The results indicate that, in the case of improved bonding performance with the reinforcing material, higher strength can obtain in carbon and glass reinforced composites.
Figure 5 shows Ɛxy (shear strain) contours of UC, CC, and GC at different load levels. The shear stress of carbon and glass reinforced wood composites under maximum load was concentrated on the left side. However, the strain in the unreinforced wood composites was concentrated in the bottom.
The shear strain along adhesive layer was examined under a certain load (60% of the maximum load) to obtain more details on wood composites. Figure 6 shows the shear strain values at 60% load level throughout the adhesive layer of reinforced and unreinforced wood composites.
Fig. 6. The shear strain of values at 60% load level in different wood composites
Fig. 7. Ɛyy (strain along the y-direction) contours of UC, CC, and GC at different load levels
When the adhesive layer was examined, the lowest shear strain values were observed in the glass reinforced wood composites. The results showed that glass fibre has a positive effect on the adhesive line. It has been reported that the smaller the strain value in the adhesive layer means better strength (Guan et al. 2014). This is the cause of the highest bending strength of glass-reinforced wood composites. When Table 4 and DIC images are examined, the lowest shear strain values were observed in glass reinforced composites supports this view. The highest strain values were seen in carbon reinforced wood composites.
Figure 7 shows Ɛyy (strain along the y-direction) contours of UC, CC, and GC at different load levels. In general, it appeared that the stress was distributed more homogeneously in these composites. The strain field along the y-direction was different in the carbon fibre-reinforced composites. Glass reinforced and unreinforced composites showed strain concentrated in the bottom.
The strain field along the y-direction was different in the carbon fibre reinforced composites. In general, the maximum Ɛyy fields of the carbon fiber reinforced composites were concentrated on the left side while the unreinforced and glass fibre reinforced composites were concentrated in the bottom. The maximum Ɛyy fields in the glue line of carbon fiber reinforced composites indicate that the carbon fiber and adhesive were a mismatch with each other. When the DIC images were examined, it was found that a large strain area was formed on the left side of the carbon reinforced composites under the ultimate load. This indicates delamination in the adhesive area. The delamination caused by mismatch was thought to be the main reason for the lowest bending strength in carbon fiber reinforced composites.
It is difficult to determine the critical deformations of composite materials under different loads. Conventional methods of deformation measurement are inadequate in some cases. Further information on materials can be achieved through the DIC method. In this study, DIC was successfully used to determine deformation behaviours of different composites. Differences in strain and displacement fields were measured in three different composite materials. The interaction between glue and fibre material could be attributed to the difference in carbon fibre and glass fibre reinforced composites.
CONCLUSIONS
- The results showed that polyurethane adhesive was not suitable for carbon fibre reinforced wood composites.
- In carbon fibre reinforced composites, it was found that there is more Ɛxy (shear strain) in the adhesive line than in other composites.
- It was determined that the strength of bending was improved significantly in wood composites prepared with glass fiber.
- DIC method results showed that the type of reinforcement material used in the wood composites produced with polyurethane affects the crack mode.
- As a result, it was determined that the performance should be improved by paying attention to adhesive compatibility and strain dispersion. It is not sufficient just to add reinforcements to reinforce the wood composite
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Article submitted: July 5, 2018; Peer review completed: October 11, 2018; Revised version received: November 6, 2018; Accepted: November 7, 2018; Published: November 14, 2018.
DOI: 10.15376/biores.14.1.180-192