Research into Corner L Separable Assemblies in Storage Furniture

• This research is based on the testing of the impact of the board (particleboard and medium density ﬁ berboard (MDF)) and hardware type used to connect the sides and the bottom or top of the storage furniture on the joint strength. The hardwares of the following trade mark were used: the con ﬁ rmat screw, Mini ﬁ x, Maxi ﬁ x, Stablo ﬁ x, RV and Solo 32. Such hardwares make it possible to disassemble the furniture as well as to assemble it (when moving the furniture) without reducing joint strength when reassembling it due to wear and tear on the material (with the exception of the con ﬁ rmant screw). Research results showed that assemblies with the Maxi ﬁ x hardware installed in the MDF base exhibited the highest values of bending moment (the average bending moment of 85.88 Nm). On the other hand, assemblies with the Mini ﬁ x hardware installed in the particleboard exhibited the lowest values of bending moment (over three times lower than the Maxi ﬁ x) (the average bending moment of 24.50 Nm). Assemblies with the con ﬁ rmat screw exhibited satisfactory results, although, aesthetic requirements seem to be the main problem here (the average bending moment of 40.38 Nm for the particleboard and of 64.24 Nm for the MDF board).


INTRODUCTION 1. UVOD
Storage furniture is everywhere around us and there is hardly a living quarter or business premises without it. Such furniture is usually of considerable dimensions so that it is desirable for such furniture to be reassemblable so that it may be easily transported and delivered to its user. It is designed for the storage of objects. It may be classifi ed according to its place of use, dimensions, purpose, constituent material and technology, construction, historic style, etc. Storage furniture is most commonly constructed as cabinets of larger dimensions, so that the desirable construction is the one that makes it possible to assemble and disassemble the cabinet corpus at its end user's location. Therefore, this type of furniture should be connected with fi ttings. Assembling principles for the cabinet corpus are determined by corner plane L assemblies and the mutual position of the side and the middle side in relation to the bottom and the top. In case of larger cabinets, its back is an important part of its construction. The cabinet back is a plate element closing the corpus from the back side and maintaining the position of the corpus sides at right angles in relation to the bottom and the top. It has a big impact on construction strength and rigidity.
Yerlikayae (2012) tested the corner L joint for strength on specimens made of particleboard. The testing was done on fi ve specimen types connected with fi ve angled plane assembly types: butt joint reinforced with fi ber-glass fabric, dowel joint, dowel joint reinforced with fi ber-glass fabric, dowel joint with the Minifi x hardware, dowel joint and the Minifi x hardware reinforced with fi ber-glass fabric. The results showed that the glass-fi ber composite layer considerably enhances the strength of a joint, and that the increase is signifi cantly higher under compressive load than under tensile load. Both types of test recorded the highest joint strength, i.e. for the dowel joint and for the Minifi x hardware reinforced with glass-fi ber fabric. Atar and Ozcifci (2007) analyzed the impact of screws and dowels on the strength of the angled plane assembly. Materials used for the research were: solid beechwood, particleboard, medium density fi berboard (MDF) and wood core plywood. Elements used for assembling corner joints were dowels, screws and PVAc adhesive. The highest value of bending moment of 133.7 Nm was measured on the specimen made of MDF, and the lowest of 111.0 Nm on the specimen made of wood core plywood.
Vassiliou and Barboutis (2009) analyzed the static bending strength of the joint with hardwares of four manufacturers: Hettich, Germany; Häfelle, Germany; Lama, Slovenia and of an unknown manufacturer. As their results showed, static bending strength depends on the manufacturer, hardware type and the type of the material on which the hardware is applied. The results also showed that certain metal hardwares exhibit higher static bending strength values than the plastic ones. Furthermore, the research revealed that the hardware exhibits higher static bending strength values when ap-plied in the MDF (plastic hardware by 32.58 %, metal hardware by 35.72 %) than when applied in the particleboard.
Kurelia and Altinoka (2011) researched into mechanical properties of corner joints (in relation to pressure and tension) made of particleboard, fi berboard and particleboard reinforced with synthetic resins. Three testing types were carried out: testing the corner L joint for compressive strength, testing the corner L joint for tensile strength and testing the corner T joint for tensile strength. The Minifi x system was used as the hardware in two versions, with plastic and metal housing. The results showed that the strongest joint was the one in the MDF connected with the metal or plastic Minifi x system or in the improved MDF. This also leads to the conclusion that the board and hardware type infl uence joint strength and that, when making the construction joint, attention should be paid to the choice of the board and hardwares.
Jivkov and Grbac (2011) examined the impact of static and dynamic loading on the bending moment in the angled plane assembly. The specimens were made of 18 mm thick MDF and connected with the dowel, screw, confi rmat screw and bolt with the turning pin (two types). The joints with the screw and the confi rmat screw exhibited the best results. The joint with the dowel exhibited a 40 % lower bending moment than the assembly with the screw. According to the authors, the assembly with the bolt with the turning pin exhibited the lowest bending moment as expected (80 % lower than the screw).
Ayrilmis and Akbulut (2018) analyzed the screw withdrawal resistance and the interlayer strength in fi berboards. The screw withdrawal resistance increases with the increase in the fi ber length in the middle layer and the increase in the resin portion in the middle layer, with the increase in the surface/middle layer ratio and the increase in the board density. Smardzewski and Ożarska (2005) examined the mathematical model of the rigid composition with the confi rmat screw and the numeric model of offi ce furniture assembled with the confi rmat screw. It turned out that corner assembly models with the confi rmat screw produce fi rmly fastened assemblies. The deformation of the assembly, as well as of elements to be assembled, represents the rigidity of the construction, which depends on the element geometry after deformation and material properties. The force applied on the screw contributes to its shear loading. When designing and constructing furniture with corner joints, it is important to take into account that material rigidity properties and the fl exibility coeffi cient are characteristic of the angled plane assembly.
Smardzewski and Klos (2011) determined the values of the substitute linear elasticity module in corner joints with dowels subjected to pressure and tension testing. The joint defl ection was also tested, and alternative models were made. The research was conducted on the angled plane assemblies with dowels made of solid beech wood with the dimensions of Ø 8 mm × 32 mm. The specimens were made of 16 mm particleboard. The values of numeric calculations obtained from the models truly represent the shape of the tested joints, which are signifi cantly lower than laboratory measurement results. On the other hand, the comparison of the calculation results obtained from models containing nods with substitute linear elasticity modules and the empirical results shows that they are mostly somewhat higher.
Eren and Eckelman (1998) fi gured out that joint strength and the number of joint components had a correlation. They explained that there are signifi cant differences in strength from board to board. Zhang and Eckelman (1993) analyzed the rational design of multidowel corner joints in 19 mm-thick particleboard case construction. Their results indicated that maximum strength was obtained when the distance between the dowels was at least 76 mm. Žulj et al. (2015) researched into the angled plane L assembly made of MDF with the density of 0.826 g/ m 3 , 18 mm thick and melamine faced. The boards were assembled with dowels, wood biscuits, the Minifi x hardware and Tofi x hardware. The bending moment was highest in the dowel joint (34.87 Nm), then in the wood biscuit joint (32.34 Nm) and the Minifi x hardware (20.35 Nm). The lowest value was recorded in connections with the Tofi x hardware (14.85 Nm). Tofi x is also the weakest joint, whose average bending moment is 2.4 times lower than the one of the dowel joint. Župčić et al. (2012) researched into the strength of angled plane L assemblies made of particleboard (with the average density of 0.63 g/m 3 ) connected with the bolt with the turning pin, wood biscuit dowel, screw and confi rmat screw. As research results show, the highest bending moment values are those in assem-blies with the confi rmat and screw, and the lowest in assemblies with the bolt with the turning pin. The glued joint with dowels and biscuits exhibits higher bending moment values in relation to the decomposable joint with the bolt with the turning pin because of the adhesive, which reduces the delamination of the particleboard, increasing thus joint strength.
The type of the selected assembly or connecting element signifi cantly impacts bending moment values and therefore the strength of storage furniture.
The board type, the storage furniture is made of, is not only an important strength factor of such furniture, but also an important factor regarding the price of the future product. Due to larger overall dimensions of such furniture, construction rigidity and decomposable assembly are required. Therefore, the study aimed at researching into the impact of the board type (particleboard and MDF) and the hardware that connects elements on assembly strength.

Making specimen samples 2.1. Izrada uzoraka
The research was conducted at the Faculty of Forestry, University of Zagreb. The tested specimens had the dimensions of 100 mm × 100 mm × 100 mm (angled plane assembly, L assemblies) and the specimens consisted of two elements of different dimensions, one of which had the dimensions of 100 mm × 100 mm × 18 mm and the other of 100 mm × 82 mm × 18 mm, connected into the L assembly. The specimens The boards were conditioned at (23 ± 2) °C and (50 ± 5) % relative air humidity for twenty days in the climatic chamber in the Furniture Testing Laboratory, and then the required dimensions were cut. After the cutting, holes were drilled on the elements depending on the hardware applied ( Figure 1) and L specimens were assembled in the workshop of the Faculty of Forestry.
The average moisture content was determined by HRN EN 322:2003 and it was 7.50 % for the particleboard and 7.41 % for the MDF board. Density was also determined on the same probes. The average density, determined by HRN EN 323:2008, of the improved particleboard was 0.69 g/cm 3 and 0.86 g/cm 3 of the improved MDF.

Testing method 2.2. Metoda ispitivanja
The assembled specimens were conditioned for 25 days (the samples were not assembled under laboratory conditions and were again conditioned) and then tested on the universal testing machine. These assembled specimens were tested on a computer-aided Shimadzu AG-X universal testing machine. The displacement speed during the testing was 5 mm/min. The specimens were tested with the articulation gripping jaws, which enabled their precise positioning ( Figure  2). A total of 120 specimens were used for the testing, all of them properly assembled with no visible defects or other damage on the board.

REZULTATI I RASPRAVA
The research results (Table 2, 3, 4 and Figure 3) show that the board and hardware type have an impact on both the bending moment of the assembly and its strength (as the parameters are the same for all specimens and no adhesive was applied when assembling). The choice of board type has a statistically signifi cant impact on the joint strength in all hardware types except for S32 specimens. As a result, joints made of MDF exhibit statistically higher bending moment values in all hardware types (except for S32 specimens)  than the joint made of particleboard. Such results were to be expected as the MDF is of a more homogeneous structure as compared to the particleboard and exhibits different mechanical properties. The hardware type also has a considerable impact on bending moment or joint strength results in specimens made of particleboard and MDF. The highest bending moment value was recorded in MAF-MDF specimens (Maxifi x) made of MDF with the average value of 85.88 Nm, while the lowest bending moment value of only 25.40 Nm was recorded in MF-IT specimens (Minifi x) made of particleboard. For purposes of the research, 18 mm thick boards were used as they have adequate thickness for all hardware types. The Maxifi x hardware is of a robust construction so that it takes a large portion of the loading in the joint. Besides that, this hardware type may be applied on thicker boards so that it can be assumed that, under such conditions, joint strength would be higher. During the testing, the delamination of the board occurred in all MAF specimens. This points to the fact that wood core plywood, veneered wood panel or solid wood panel (solid across its width and length) would reach better joint strength values with this hardware as they are more resistant to delamination. According to the research of Jivkov and Grbac (2011), samples jointed with a confi rmation screw have the highest values of bending moment (33.42 Nm), while samples jointed with Minifi x and Rafi x have the lowest values of bending moment, 9.75 Nm and 6.02 Nm, respectively. The distribution is similar as in this research.
Different hardware types installed in the MDF exhibit a statistically signifi cant difference in the bending moment except for RV and CO specimens and STF and MF specimens. MAF specimens (MDF) have 2.4 higher values as compared to STF specimens, indicating that the choice of the hardware type has a large impact on the assembly strength when assembling storage furniture. IT specimens show a somewhat smaller ratio of 2.1 times (between MAF-IT and MF-IT).
In case of L assemblies, the aesthetic of the assembly matters as well, besides its strength. All hardware types used for this research are on the inner side of the joint except for the confi rmat screw. The confi rmat screw is therefore the least acceptable aesthetically, as it is visible on the furniture lateral side. Maxifi x is a robust and big system that may be hidden beneath cover caps and shows the best results for the boards tested. It may be installed in boards of larger dimensions and, therefore, proves to be the most acceptable. The strength of the same assembly on the various plate materials may deviate signifi cantly. These deviations are even greater with different structural assemblies on the same material (Tkalec and Prekrat 2000).
Besides the strength and aesthetic of the joint, there are other factors that need to be considered such as hardware complexity for installation, its price, market availability, etc. All these factors infl uence the functionality, safety, reliability and aesthetic of furniture and its technological and fi nancial availability to the customer. Figure 4 shows load-displacement curve. Force increases in accordance with displacement up to its maximum value and starts decreasing thereafter. The increase in force to the maximum value (for all assemblies) is similar. However, its decrease differs and mostly depends on the hardware type. In MF, MAF and STF specimens, due to force, the following phenomena occur: board delamination in the hardware installation zone, screw head passing through the board in CO specimens, screw withdrawal from the board in RV hardwares, hardware bending in S32 specimens. In case of S32 hardware, the board type has no statistically signifi cant impact on joint strength due to the specifi c nature of such hardware and hardware bending when tested. Screws that fasten the hardware to the boards and the fi tting itself take joint strain (in CO, RV and S32). Therefore, no joint fracture occurs, but rather an easy screw withdrawal from the board and hardware bending.  M=43.042 (7) M=27.294 It may be assumed that edge processing (with a mini ABS tape for example) would increase the strength of the assembly in MAF, MF and STF. The ABS tape would strengthen the board edge increasing the bending moment or the strength of the joint. The visible edge of the particleboard is always coated so that attention should be given to the hardware orientation with regard to the processed edge (the side or the top or the bottom). It can therefore be assumed that an installed hardware in a specifi c piece of furniture would exhibit even higher values (except for furniture made of lacquered MDF).

CONCLUSIONS 4. ZAKLJUČAK
The board type has a statistically signifi cant impact on the strength of assemblies (except for S32 specimens). The hardware installed in the improved MDF has statistically higher bending moment values as compared to the hardware installed in the improved particleboard (except for S32 specimens).
The hardware type has a signifi cant impact on the maximum bending moment or assembly strength. In specimens made of particleboard and MDF, there are statistically signifi cant differences as shown in Table 4. Depending on the hardware type, the average maximum bending moment increases by over two times independent of the board type, which constitutes a signifi cant difference as a furniture strength indicator. Therefore, a proper hardware choice plays an important role in determining the decomposable construction of future storage furniture.
Joint strength in S32 specimens depends on the force required for hardware bending so that the board type has no statistically signifi cant impact.
When testing, the assembly fracture also depends on the hardware type. Board delamination is characteristic for most hardwares except for assemblies with the confi rmat screw and the RV hardware fastened by screws.
The Maxifi x hardware exhibited the best results of all tested hardware with regard to the bending moment independent of the board type, it is acceptable aesthetically as it is installed from the inner side of the assembly and enables the application on boards with the thickness of over 18 mm.