Abstract
The lateral load-slip behavior of a single-shear metal-to-particleboard single-screw connection (SMPSC) was investigated. The connection consisted of a layered particleboard main member fastened to a metal plate as a side member using a 4.8-mm diameter sheet metal screw. A mechanics-based approach was used to evaluate critical factors on the lateral load resistance performance of SMPSCs. Experimental results indicated that ultimate screw-bearing strengths in face and core layers of evaluated particleboard materials were 100.0 and 29.9 MPa, respectively. This significant difference of screw-bearing strength in material layers significantly affected the lateral resistance load capacity of SMPSCs. The proposed mechanical models considering material layer effects on screw-bearing strengths were verified experimentally as a valid means for deriving estimation equations of lateral resistance loads of SMPSCs evaluated in this study.
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Contribution of Face and Core Layers to Lateral Load Resistance of Single-Shear Metal-to-Particleboard Single-Screw Connections
Yan Wang a and Jilei Zhang b,*
The lateral load-slip behavior of a single-shear metal-to-particleboard single-screw connection (SMPSC) was investigated. The connection consisted of a layered particleboard main member fastened to a metal plate as a side member using a 4.8-mm diameter sheet metal screw. A mechanics-based approach was used to evaluate critical factors on the lateral load resistance performance of SMPSCs. Experimental results indicated that ultimate screw-bearing strengths in face and core layers of evaluated particleboard materials were 100.0 and 29.9 MPa, respectively. This significant difference of screw-bearing strength in material layers significantly affected the lateral resistance load capacity of SMPSCs. The proposed mechanical models considering material layer effects on screw-bearing strengths were verified experimentally as a valid means for deriving estimation equations of lateral resistance loads of SMPSCs evaluated in this study.
Keywords: Single shear screwed connections; Lateral load resistance; Particleboard; Screw bending moment; Screw-bearing strength; Yield theory
Contact information: a: Academy of Design & Art, Hunan Institute of Engineering, Xiangtan 411104, China. Former PhD Student, Department of Sustainable Bioproducts, Mississippi State University; b: Department of Sustainable Bioproducts, Mississippi State University, P. O. Box 9680, MS 39762-9820;
* Corresponding author: jz27@msstate.edu
INTRODUCTION
Particleboard (PB) is commonly produced from various sizes of wood particles through applying thermosetting adhesive to the particles and pressing a loose mat of the particles with heat and pressure (Williamson 2002). The consumption of PB is mainly in furniture manufacturing (UNECE 2016), especially in the cabinet industry because of its good strength properties, smooth surfaces, and low price. A single-shear metal-to-particleboard single-screw connection (SMPSC) is commonly seen in jointing door hinges to PB structural components in cabinet construction. Therefore, knowing the lateral load resistance capacity of a SMPSC can provide fundamental information in assisting the strength design of cabinets constructed of PB.
Previous studies related to lateral resistance load capacities of single-shear mechanical fastener wood-to-wood connections (Blaß and Bejtka 2002; Hansen 2002; Taj et al. 2009;) and metal-to-wood (Aune and Patton-Mallory 1986; Hunt and Bryant 1990; Chui et al. 2006) or wood-based composite connections (Karacabeyli et al. 1998; Sinha and Byrne 2013; Kuang et al. 2017; Yu et al. 2017) considered a main member as a homogenous material, i.e., assuming that the dowel-bearing strength of the material underneath the dowel is the same (Soltis et al. 1986).
PB is typically made in layers. The faces of PB usually consist of fine particles, while the core is made up of coarser ones. Normally, a symmetrical “U”-shape density profile across the panel thickness, i.e., face layers have higher density than core material, will be developed during the mat forming and hot processing process. This density profile across the PB panel thickness can lead to the face layer material having higher mechanical properties, such as dowel-bearing strength, than the core. Karacabeyli et al. (1998) indicated that glulam rivet connections constructed of low density spruce-pine-fir and hemlock-fir were 5% and 25% lower in lateral capacity than high density Douglas-fir-larch.
The main objective of this study was to develop prediction equations for lateral resistance loads of SMPSCs. The specific objectives were to 1) evaluate screw-bearing strengths of PB materials; 2) characterize the lateral load-slip behavior and failure mode of SMPSCs; 3) propose mechanical models based on the screw and PB member failure modes for describing the internal force distribution in the connection at different loading stages; 4) derive equations based on proposed mechanical models for predicting lateral resistance loads of SMPSCs at different loading stages; and 5) validate derived prediction equations experimentally.
EXPERIMENTAL
Materials
In this study, full-sized four-layers M-2 Grade (ANSI A208.1 2016.) PB panels provided by Roseburg Particleboard Company (Taylorsville, MS, USA), measured 2,464-mm long × 1,245-mm wide × 28.6-mm thick, were used. The grade #4140, 3.3-mm thick alloy steel metal plate was purchased from McMaster-Carr Company (Douglasville, GA, USA). The #10 Phillips flat head sheet metal screws (Table 1 and Fig. 1) were purchased from the Hillman Group Company (Cincinnati, OH, USA).
Table 1. Sheet Metal Screw Critical Dimensions
Note: Values in parentheses are coefficients of variation in percentage.
Fig. 1. The general configuration of a sheet metal screw with critical dimensions.
Experimental Design
Screw connections
The general configuration of a SMPSC used in this study is shown in Fig. 2. The connection consisted of a PB main member attached to a metal-plate side member through a single screw. The PB main member measured 101.6-mm long × 254.0-mm wide × 28.6-mm thick according to ASTM D1761 (2012). The metal-plate side member measured 228.6-mm long × 88.9-mm wide × 3.3-mm thick. 15 replicates of SMPSCs were tested. In addition, three replicates of SMPSCs were loaded to each of five averaged deformation levels (0.76, 1.27, 2.54, 3.81, and 5.08 mm) to investigate how the joint failure progressed in terms of when the PB material started being crushed and when the screw started being bent.
Fig. 2. The general configuration of a SMPSC: front view, side view, and 3D view.
Basic material properties
Figure 3a shows the general configuration of a half-hole specimen used for evaluating screw-bearing strengths in PB materials. The specimen measured 76.2-mm long × 76.2-mm wide with a 3.3-mm diameter half-hole drilled through board thickness ASTM D5764 (2018). Figure 3b shows three specific types of specimens cut for evaluating screw-bearing strengths in full thickness PB, face layers, and core layer. Cutting to prepare layers from which to assemble the core and face specimens was done with a band saw. The replicates were 27, 36, and 36 for full thickness PB, core layer, and face layer specimens, respectively. Bending properties of 20 randomly selected screws were tested according to ASTM F1575 (2017). 15 replicates specific gravity (SG) and moisture content (MC) of PB materials measured 50.8-mm long × 50.8-mm wide × 28.6-mm thick were tested according to ASTM D2395 (2017) and ASTM D4442 (2013), respectively.
Fig. 3. The general configuration of (a) a half-hole specimen used for evaluating screw-bearing strengths in particleboard and (b) three specific types of specimens cut for evaluating screw-bearing strengths in full thickness PB, face layer, and core layer.
Specimen Preparation and Testing
Figure 4 shows the cutting pattern for preparing the screw-bearing strength specimens. Particularly, the core layer material was dyed with a green color during the manufacturing process. Four sliced face layers of PB materials, each measuring 4.57 mm, were clamped together as one face layer specimen measured 18.28 mm in its thickness. The core layer specimen thickness measured 20 mm. All PB main members were randomly cut from the rest of leftover sections after the preparation of screw-bearing strength specimens. Prior to testing, all specimens were conditioned in an environmental humidity chamber controlled at 20 ± 2 °C and at 50 ± 5% relative humidity for 40 h. The SG and MC samples were cut from each tested main member.
Fig. 4. Cutting pattern used for preparing screw-bearing strength specimens.
All screw-bearing, screw-bending, and connection tests were performed on a hydraulic SATEC universal testing machine purchased from INSTRON company, (Norwood, MA, USA). Figure 5a shows the setup for evaluating the lateral resistance load-slip behavior of a SMPSC. The screw-driving torque was set to 2.8 N-m (Tor et al. 2015). A linear variable differential transformer (LVDT) electromagnetic device was attached on the PB main member to measure the connection slip. The loading speed was 1.0 mm/min ASTM D1761 (2012). Figure 5b shows the details of how the two metal plates were attached to the PB main member for specimens tested for the investigation of connection failure progress. Two metal pieces were attached to the main member; by such means these two metal plates could be removed after the tested connection reached the desired slip. The bent shape of a tested screw was examined using an INSPEX X-ray inspection system purchased from KODEX company (Nutley, NJ, USA).
Fig. 5. Test setups for evaluating the lateral load-slip behavior of a single-shear metal-to-particleboard single-screw connection using (a) one metal piece and (b) two removable metal pieces as the side member, respectively
Figure 6 shows the setup for evaluating half-hole screw-bearing strength properties in PB materials.
Fig. 6. Test setup for evaluating half-hole screw-bearing strength properties in particleboard
The screw was compressed into a half-hole PB specimen with a constant rate of 1 mm/min ASTM D5764 (2018). The critical screw-bearing strength values at proportional limit (Fe,o,pl), 5% offset yield (Fe,o,y), and ultimate (Fe,o,u) for PB in board thickness; screw-bearing strength values at proportional limit (Fe,c,pl), 5% offset yield ( Fe,c,y), and ultimate (Fe,c,u) for PB core layer; and screw-bearing strength values at proportional limit (Fe,f,pl), 5% offset yield (Fe,f,y), and ultimate (Fe,f,u) for PB face layer materials, were calculated using Eq. 1,
Fe = P/dt (1)
where Fe is the characteristic screw-bearing strength (MPa); P is the compressive load (N); d is the screw thread diameter (mm); and t is the thickness of a PB specimen (mm).
Figure 7 shows the setup for evaluating the bending moment of screws used in this study. The center-loading bending test at a constant displacement rate of 6.35 mm/min was implemented with a span of 22.9 mm, ASTM F1575 (2017). The critical bending moments of Mpl at proportional limit, My at yield point, and Mu at ultimate point (N-mm) were calculated using Eq. 2,
M = PbSbp/4 (2)
where Pb is the test bending load at each critical point as determined from load-displacement curves (N); and Sbp is the span between two supports (mm).
Fig. 7. Setup for evaluating the bending moment capacity of screws
Load-slip curves and failure modes of all tested connections were recorded. The yield load of a tested connection was determined through fitting a straight line to the initial linear portion of the load-slip curve recorded, offsetting this line by a slip equal to 5% of the screw major diameter.
RESULTS AND DISCUSSION
Basic Physical and Mechanical Properties
PB SG averaged 0.7 with a coefficient of variation (COV) of 5.0 % and PB MC averaged 7.0% with a COV) of 4.1% based on 15 replicates. Figure 8 shows a typical bending moment-displacement curve of screw bending strength tests. The mean values with their COVs of screw bending moments at proportional limit, Mpl; at yield point, My; at ultimate point, Mu; were 11,190 (2.1%), 12,659 (1.9%), and 13,338 (2.8%) N-mm, respectively.
Figure 9 shows typical load-embedment curves of screw-bearing strength tests performed in face layer, core layer, face and full thickness PB, respectively. Table 2 summarizes mean values of screw-bearing strengths in different layers of PB materials, including their values at proportional limit, yield point, and ultimate point for face layer, core layer, and full thickness PB, respectively. Mean comparisons among three values within each strength value column were performed at the 5% significance level using the protected least significant difference (LSD) multiple comparison procedure, i.e., LSD values were 3.0, 2.9, and 2.88 MPa for proportional limit, yield, and ultimate strengths, respectively.
Mean comparison results indicated that in general, there were significant differences in screw-bearing strengths among three different layered materials evaluated. In other words, screw-bearing strengths in face layer materials were significantly higher than those in full thickness PB, followed by those in core materials.
Table 2. Mean Value of Screw-Bearing Strength Properties in Particleboards
Note: Values in parentheses are coefficients of variation in percentage. Means in each column not followed by a common letter are significantly different from one another at the 5% significance level.
Fig. 8. A typical bending moment-displacement curve of tested screws
Fig. 9. Typical bearing strength-imbedding displacement curves of screws tested in face layer, core layer, and full thickness PB of evaluated particleboard materials
Screwed Connections
Load-slip curves and failure modes
Figure 10 is a typical load-slip curve of SMPSCs when subjected to a lateral load, having three different stages. Stage 1 started from the first linear-elastic line to the first offset yield point. The linear portion of Stage 1 was because of PB material underneath the screw was compressed in its elastic region and there was no sign of the screw being bent (Fig. 11a) as the lateral load increased from 0 to 1,560 N. There was an initial linear-yield portion up to a specific load level (1,870 N in this curve) where the yield portion was mainly because of the screw having a slight one-point bent (Fig. 11b) at the PB-to-metal contact surface. In other words, one plastic hinge was developed, rather than fractured PB materials because there was not any obvious compressive fracture having occurred at PB material underneath the screw (Fig. 11a and b).
Stage 2 is the second linear-yield portion (Fig. 10), which started from the first offset yield point to the second offset yield point. In this stage, the PB material moved its deformation transition from elastic to plastic, i.e., the linear portion up to a lateral load level (3,110 N) means that the PB material underneath the screw was still in its elastic region, and further increasing the load fractured the PB material underneath the screw (Fig. 11c and d). Meanwhile, the screw started its two-point bending process (Fig. 11c) when the lateral load reached a level (3,110 N), i.e., the second screw plastic hinge appeared in the inner section of the PB, and two plastic hinges (Fig. 11d) were developed as the load increased to the second yield loading point (3,780 N).
In Stage 3 the lateral load started from the second offset yield point, reached its maximum value (Fig. 11e) at a slip level (5 mm), and then dropped gradually. In this stage, the screw continued its two plastic hinge bending process (Fig. 11e), started its pulling-out process, and ended with screw head broken off (Fig. 11f), while the PB material underneath the screw continued its compressed-yielding process (Fig. 10e). The screw started its pulling-out when the lateral load reached its maximum (Fig. 11e and f).
Fig. 10. A typical lateral load-slip curve of single-shear metal-to-particleboard single-screw connections