Abstract
The influences of various factors on the tensile strength loading caused by steel bolt-nut connections on Cupressus funebris wood components in directions perpendicular and parallel to the grain were studied. The focus was the interference fit parameter between the nut and the pre-drilling hole, the nut embedded depth, and the embedded nut diameter using the single factor method. The optimum parameters for bolt-nut connections with C. funebris wood components both perpendicular and parallel to the grain component structures were obtained via a single factor experiment and verified by simulation using ANSYS finite element analysis software. The experimental and simulation-based results revealed that the optimum parameters with maximum tensile strength for the bolt-nut connections with C. funebris wood components were an interference fit of -0.80 mm, a nut embedded depth of 25 mm, and an embedded nut diameter of 15.5 mm. Considering the needs of practical applications, the optimum interference fit parameter between the nut and pre-drilling hole was -0.80 mm, increasing the length of the nut within the allowable range of part size was beneficial, and it was not suitable to increase the diameter of the embedded nut to improve the part’s performance.
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Effects of Selected Joint Parameters on Tensile Strength of Steel Bolt-nut Connections in Cupressus funebris Wood
Ming Chen,a,b Shangguan Li,a,b and Jianhua Lyu a,b,*
The influences of various factors on the tensile strength loading caused by steel bolt-nut connections on Cupressus funebris wood components in directions perpendicular and parallel to the grain were studied. The focus was the interference fit parameter between the nut and the pre-drilling hole, the nut embedded depth, and the embedded nut diameter using the single factor method. The optimum parameters for bolt-nut connections with C. funebris wood components both perpendicular and parallel to the grain component structures were obtained via a single factor experiment and verified by simulation using ANSYS finite element analysis software. The experimental and simulation-based results revealed that the optimum parameters with maximum tensile strength for the bolt-nut connections with C. funebris wood components were an interference fit of -0.80 mm, a nut embedded depth of 25 mm, and an embedded nut diameter of 15.5 mm. Considering the needs of practical applications, the optimum interference fit parameter between the nut and pre-drilling hole was -0.80 mm, increasing the length of the nut within the allowable range of part size was beneficial, and it was not suitable to increase the diameter of the embedded nut to improve the part’s performance.
Keywords: Tensile strength; Cupressus funebris wood; Bolt-nut connection; Optimum parameter
Contact information: a: College of Forestry, Sichuan Agricultural University, Chengdu 611130, China; b: Sichuan Provincial Colleges and Universities Wood Industry and Furniture Engineering Key Laboratory, Sichuan Agricultural University, Chengdu 611130, China; *Corresponding author: ljh@sicau.edu.cn
INTRODUCTION
Cupressus funebris is one of the four endemic species of the genus Cupressus to China, which is its country of origin (Fu et al. 1999). Cupressus funebris wood is a good material for construction of buildings, ships, and furniture, and it has been used since ancient times.
Most of the research literature focuses on forest resource management, biodiversity, and essential oils of Cupressus funebris (Zheng et al. 2011; Hou et al. 2013; Li et al. 2015; Lyu et al. 2018). Moreover, only a few studies have been conducted on the physical and mechanical properties of the joint performance of wood-based panel parts connected by hardware connectors; furthermore, studies on solid wood parts connected by hardware connectors or tenon and mortise joints are even rarer (Ozkaya et al. 2010; Zhang et al. 2015; Chen et al. 2016, 2018; Chen and Lyu 2018; Demirel et al. 2018). Therefore, there are almost no correlative studies on the joint performance of C. funebris wood parts connected by hardware connectors or tenon and mortise joints.
A metal insert nut is a part of a special structural connector that can be embedded in advance. It works with other parts, such as bolts, to produce a strong bonding force. Its connecting principle is to increase the contact area with the base material through wide, continuous, or discontinuous threads, and to increase bonding strength with the help of surface friction. It is widely used in the wood product industry because of its simple structure, mature technology, low cost, and strong bonding properties (Li 2013; Hao et al. 2015).
This study had three objectives. The first was to investigate the optimum parameters for steel bolt-nut connections with C. funebris wood components. The second was to determine the effects of the bolt-nut connections that were parallel and perpendicular to the grain of the Cupressus funebris wood parts on the joint performance of wood-based steel bolt-nut connections. The third objective was to fill the gap in the research field and to improve the level of standardization and the industrial utilization rate of C. funebris wood.
EXPERIMENTAL
Materials
The Cupressus funebris wood was selected as the experimental material and was purchased from a local timber enterprise (Yunsheng Wood Industry Co., Ltd., Chengdu, China). The tree was 45 to 50 years old. The values of hardness in the end, tangential, and radial faces of the wood were 5333 N, 4178 N, and 4066 N, respectively. Moreover, the modulus of elasticity, bending strength, compression strength parallel to grain, radial compression (entire) strength perpendicular to grain, and tangential compression strength of the wood were 8.2 GPa, 89.8 MPa, 45.8 MPa, 9.4 MPa, and 8.9 MPa, respectively.
The metric bolts and nuts were selected as the hardware connectors, since they are usually used in Cupressus funebris wood furniture. They were purchased from Guangdong Hui Cheng Feng Hardware Co., Ltd. (Shenzhen, China). The nut was a hexagonal socket nut with tooth threads both on the inside and outside surfaces of the nut and with zinc plating and coloring. The bolt was manufactured using grade 4.8 carbon steel, whose circular diameter matched the inside tooth of the nut, and it was 100 mm in length with zinc plating and coloring. The insert nut and the corresponding fitting bolt are illustrated in Fig. 1.
Fig. 1. The insert nut (left) and fitting bolt (right)
Methods
The tensile strength was determined in accordance with the Chinese nail-holding test method (GB/T 14018 2009). The tests were conducted using a Reger microcomputer-controlled electronic universal testing machine (Model RGM-2100; Shenzhen Reger Instrument Co., Ltd., Shenzhen, China). The rate of loading was 10 mm/min, and the loading continued until a non-recoverable drop-off in load occurred. The load and displacement values were expressed in newtons and millimeters, respectively. The load and displacement values were obtained from a load-displacement diagram. A linear regression analysis was applied between the displacement and the corresponding loading to calculate the relationship between deflection and loading. Subsequently, the wood material was machined to a size of 50 mm × 50 mm × 50 mm (length × height × thickness) using a flat plane and sliding table saw, consecutively. All of the components were sequentially numbered. The test setup is shown in Fig. 2.
The influencing factors, including the interference fit parameters, nut embedded depth, and insert nut diameter, were investigated through the single factor experiments. Both the variance analysis and regression analysis were applied using IBM SPSS Statistics software (Version 19, IBM, Armonk, NY, USA).
Fig. 2. Setup of tensile strength loading in steel bolt-nut connection tests
Interference fit parameters experiment
The steel insert nut used in this test satisfied the M6 specification, as shown in Fig. 3. An interference fit, also called a press fit or shrink fit, is a type of connection method used to join parts together in products or structures. In this type of joint, the two parts being connected are of slightly different sizes, and this discrepancy holds the pieces together. Making the insert nut maximum external diameter slightly larger than diameter of pre-drilling hole creates an interference fit joining the two parts. Changes in the amount of interference are accomplished by making the dimensions where the two parts are joined larger or smaller. In this part of the study, the influence of the pre-drilling interference fit parameters and maximum outer diameter of the nut on the tensile strength were studied using the single factor experiment for the samples that were perpendicular to the grain and parallel to the grain. These influences were studied using seven gradient tests and six repeated tests, as listed in Table 1.
After the experiment, the damage forms were examined, and the experimental data were analyzed via variance analysis and range analysis. Meanwhile, the results of equivalent stress and equivalent strain were compared with the results of the ANSYS workbench software (Version 15.0, ANSYS, Canonsburg, PA, USA).
Fig. 3. Cross-section of steel insert nut in interference fit parameters experiment (unit: mm)
Table 1. Interference Fit Parameter of Pre-drilling Diameter in Interference Fit Parameters Experiment
Nut embedded depth experiment
The same type of steel insert nut that satisfied the M6 specification was used in this experiment, as shown in Fig. 4.
Fig. 4. Cross-section of steel insert nut in nut embedded depth experiment (unit: mm)
The influence of the nut length on the tensile strength was studied by the single factor experiment for the samples that were perpendicular and parallel to the grain. The samples were studied for six different gradients at 10 mm, 13 mm, 15 mm, 17 mm, 20 mm, and 25 mm nut lengths and with six repeated tests. After the experiment, the failure forms were examined, and the experimental data were analyzed via variance analysis and range analysis. Meanwhile, the results of equivalent stress and equivalent strain were compared with the results of the ANSYS workbench software.
Diameter of insert nut experiment
Three different kinds of steel insert nuts (M6, M8, and M10) were used in this experiment, which were classified into three categories based on the diameter of the inner teeth of the nuts, i.e., 6 mm, 8 mm, and 10 mm. The influence of the insert nut diameter, i.e., the maximum external diameters of the threads (10.5 mm, 12.5 mm, and 15.5 mm), on the tensile strength were studied by the single factor experiment for test pieces that were perpendicular to the grain and parallel to the grain. The test pieces were studied with six repeated tests, as shown in Fig. 5. An optimum interference fit parameter value of -0.8 mm and nut embedded depth of 25 mm were chosen from the results of previous experiments.
After the experiment, the failure forms were examined, and the experimental data were analyzed using variance analysis and range analysis. Meanwhile, the results of equivalent stress and equivalent strain were compared with the results of the ANSYS workbench software.
Fig. 5. Cross-section of steel insert nut in diameter of insert nut experiment (unit: mm)
RESULTS AND DISCUSSION
Interference Fit Parameters Experiment
Failure mode analysis
The failure characteristics and simulation verification in tensile strength tests of perpendicular-to-grain test pieces are shown in Table 2.
When the interference fit parameter was -0.2 mm, the depth of thread embedded in the specimen was very shallow, and there were minor scratches on the inner wall of the aperture, which indicated that the force on the inner wall of the aperture was minor. Moreover, there was no obvious peeling off of the wood fiber at the end face of the specimen.
Table 2. Failure Characteristics and Simulation Verification of Tensile Strength Tests on Perpendicular-to-Grain Test Pieces in Interference Fit Parameters Experiment
