深水网箱护栏力学性能分析及优化

摘要

护栏是深水网箱浮架系统不可或缺的重要构件,对支撑网衣、保持养殖容积、协同网箱浮架抵抗波浪流冲击起到重要作用.该文分别对中国当前应用的圆型深水网箱和六三型深水网箱护栏开展了弹塑性阶段的力学试验和有限元分析.结果表明:1)波流和气流的共同频率达到深水网箱护栏固有频率14.25 Hz时更易产生共振疲劳破坏;2)冲角为0°的集中载荷使护栏变形最大,此时扶手管中心为最易变形区域,立柱下端为最易破坏区域;3)同规格同材料的圆形扶手管抗形变能力显著大于六边形扶手管;4)扶手管的载荷-位移方程服从对数函数分布.护栏结构优化结果显示将管材的圆形截面处理为椭圆,护栏的水平抗弯性能增加6.58%.%Guardrail is an indispensable component of the deep-water net cage floating system, and it plays an important role to maintain culture volume of net cage and to resist the impacts of wave and current with net cage floating system. But there are few researches on guardrail mechanical properties of deep-water net cage. In order to improve the safety of deep-water net cage structure, three-dimensional solid models of guardrail were established by software Unigraphics NX, these models were calculated by finite element method of the software ANSYS Workbench. Firstly, mechanics properties of hexagon-triangle and circle type guardrails were investigated; Secondly, the collected data in the experiment and numerical simulation were analyzed by using linear and nonlinear regression methods to understand the mechanical performance of guardrails, and finally the design of guardrail was optimized with consideration of experiment and simulation results. The results of statics test showed that for high density polyethylene (HDPE) guardrail, tensile yield strength was 23.8 MPa, Young's modulus was 1240 MPa, and flexural strength was 25.2 MPa. Results of finite element modal analysis indicate that natural frequency range of guardrail was 14.25-33.59 Hz; stress range was 6.370-21.658 MPa. Fatigue vibration experiment was conducted by using hydraulic servo fatigue testing machine. Vibration frequency was set as 5 Hz, and vibration amplitude was 80% maximum compressive intensity of stress value (19.6 MPa). Results of fatigue test showed that fatigue fracture time was 23.5 h. According to finite element statics analysis, the biggest, medium and smaller displacement, stress and strain of guardrail was occurred in attack angle of 0°, 45°, and 90°. The displacement, stress and strain of concentrated load was higher than those of distributed load. The guardrail failed when the concentrated load was 4 kN in attack angle of 0°, and displacement, stress and strain was 70.47 mm, 28.922 MPa, 0.026434, respectively. The maximum displacement area was in the center of guardrail-handrail; the maximum stress area was in the bottom of guardrail-column. When attack angle to guardrail is 0°, the column was simplified as cantilever beam model. When attack angle is 90°, handrail was simplified as the form of tension and compression bar model. Based on finite element linear analysis of circular and hexagonal handrails, handrails of circle and hexagon failed when distributed loads were 6 kN and 16 kN, respectively, and stiffness of circular handrail was greater than that of hexagon significantly. Based on finite element nonlinear analysis of circular and hexagonal handrails, HDPE material model was set up by bilinear isotropic hardening model for isotropic materials (BISO). Deviations between simulation value and experimental value of guardrail and handrail at elastic stage were 8.67% and 0.27%, respectively. Deviation between simulation data and experimental data of handrails at plastic stage was analyzed by using method of Kolmogorov–Smirnov (K-S) two - factor fitting goodness examination. And the examined result of hexagonal handrail was p=0.8613>0.05, namely, there is no significant difference between finite element simulation and mechanics experiment in linear and nonlinear process. The sectional rigidity of guardrail was improved by designing the cross-sectional shape of guardrail-pipe. Compared with hollow-circle section of guardrail, flexural property of hollow-oval section and hollow- square section was increased by 6.58%, 6.83% respectively, in same materials. This study provides theoretical basis for the structural and mechanical properties for guardrail system.