Damage characteristics of glass fiber reinforced polymer-concrete composite beams based on fractal theory
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摘要: 工程实际中，玻璃纤维增强树脂复合材料(GFRP)-混凝土组合梁往往出现较大的损伤而破坏，有必要对其破坏过程进行研究。为了研究此类构件的损伤特性，通过声发射仪器对1片GFRP工字梁和6片不同螺栓连接的GFRP-混凝土组合梁的四点弯曲试验加载的全过程进行了监测。并基于分形理论对试验梁加载过程的声发射能量信号进行了相空间重构，计算了试验梁各加载阶段的声发射能量信号分形维数值。研究表明，试验梁的声发射能量时间序列具有分形特征，且关联维数可很好地描述GFRP-混凝土组合梁损伤破坏的整个阶段；归纳各试验梁的声发射能量时间序列分维曲线的演化模式：纯弯段区域的损伤演化模式为“早期峰”至“低幅波动”；剪跨区的损伤演化模式为“低幅波动”至“持续高幅波动”；将试验梁加载过程的试验现象与分维值的演化相对照，则可将分维值的“持续高幅波动”作为试验梁失稳的前兆。根据关联维数，提出“损伤预警”的可能。其相应的分维值出现“持续高幅波动”，则损伤“预警点”出现，结构的承载力已达到其极限的70%左右，需要加强监测。
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关键词:
- 桥梁工程 /
- 损伤预警 /
- GFRP-混凝土组合梁 /
- 声发射(AE) /
- 分形理论
Abstract: In engineering practice, glass fiber reinforced polymer (GFRP)-concrete composite beams usually fail as a result of larger damages, and hence it is of great significance to study the damage process. In order to study the damage characteristics of such components, four-point bending loading tests were carried out for a GFRP I-beam and six GFRP-concrete composite beams connected by different bolts, and the whole process was monitored by an acoustic emission (AE) device. On the basis of the phase-space reconstruction of the AE energy signals, the fractal dimension of AE energy signals were calculated in each loading stage. The results show that the time serials of AE energy is of fractal characteristics. The dimension can well describe the complete damage stages; The evolution of the fractal dimension curve of AE energy time series is summarized: the mode in pure bending section is “early peak” to “low amplitude fluctuation”; the mode in shear span area is “low amplitude fluctuation” to “continuous high amplitude fluctuation”; by comparing the phenomenon of loading process of test beam with the evolution of fractal dimension value, the “sustained high amplitude fluctuation” of the fractal dimension can be regarded as the precursor of the instability of the test beam. According to the dimension, the possibility of “early warning” was proposed. Early warning points can be identified when there is enduring high-amplitude fluctuation for corresponding fractal dimension. The bearing capacity of the structure has reached about 70% of the allowable extreme, suggesting necessary intensified monitor. -
图 1 GFRP-混凝土试验梁加载及横断面示意图
Figure 1. Loading and cross section diagram of GFRP-concrete test beam
图 6 GFRP-混凝土试验梁延迟时间确定
Figure 6. Determination of delay time of GFRP-concrete test beams
图 7 GFRP-混凝土试验梁关联维数随嵌入维数变化曲线
Figure 7. Correlation dimension curves with embedding dimension of GFRP-concrete test beams
表 1 玻璃纤维增强树脂复合材料(GFRP)-混凝土试验梁基本参数
Table 1. Basic parameters of glass fiber reinforced polymer (GFRP)-concrete testing beams
Specimen
numberBolt d*h/
mmBeam height
H/mmBolt layout L0 — 120 — L-190-1 M12*50 190 Longitudinal hole spacing of the beam is 120 mm, and the transverse hole spacing is 50 mm L-190-2 M18*50 190 L-190-3 M24*50 190 L-240-1 M12*90 240 L-240-2 M18*90 240 L-240-3 M24*90 240 
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表 2 声发射(AE)硬件设置
Table 2. Hardware settings of acoustic emission (AE)
Threshold/dB Gain/dB Analog filter Fixed threshold Floating threshold Internal gain Preamplifier gain Lower/kHz Upper/kHz 40 6 0 40 20 200
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表 3 GFRP-混凝土试验梁试验现象汇总
Table 3. Summary of test phenomena of GFRP-concrete test beams
Specimen number Experimental phenomena Failure mode Bearing capacity/kN L0 — Vertical shear occurred in the web near the loading point, and then
shear failure occurred along the beam lengthWeb shear failure 46 L190 L190-1 Slight crackling-intensive crackling-concrete crushing in the middle of
the beam top span-Web shear failure-concrete fracture at the endBending failure (failure of concrete due to ultimate compressive strain) 67 L190-2 66 L190-3 61 L240 L240-1 Crackling-cracks appear at the loading point and pure bending section,
and expand with the increase of load-crackles are dense-Web shear
failure along the beam length, and the concrete is crushedWeb shear failure 82 L240-2 86 L240-3 91
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表 4 各GFRP-混凝土试验梁“预警荷载”及关联维数最大值
Table 4. Warning load and maximum of correlation dimension of GFRP-concrete test beams
Specimen number L0 L-190-1 L-190-2 L-190-3 L-240-1 L240-2 L-240-3 ω/% 0 0.90 2.04 3.61 1.03 2.32 4.13 Pu/kN 46 67 66 61 82 86 91 Warning load 0.56Pu 0.74Pu 0.73Pu 0.68Pu 0.78Pu — 0.78Pu 1# dimension 0.50 0.60 0.46 2.82 0.58 — 0.70 3# dimension 1.92 0.65 0.51 0.29 1.33 — 0.53 Notes: ω—Percentage of bolt in concrete beam; Pu—Ultimate load value.
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