摘要
采用随机生成算法投放钢纤维,建立了随机乱向、定向钢纤维增强水泥基复合材料(SFRC、ASFRC)三点弯曲梁细观有限元数值模型,计算了不同纤维掺量下SFRC试件和ASFRC试件加载断裂的全过程,分析了三点弯曲梁开裂截面处的纤维应力,研究了定向钢纤维的细观增强机理. 结果表明:SFRC试件和ASFRC试件荷载-裂缝张开口位移全曲线的模拟值与试验值符合较好,峰值荷载的误差在10%以内;SFRC试件和ASFRC试件的峰值荷载与纤维合力的最大值均随着纤维掺量的增加而增大,当纤维掺量为0.8%、1.2%、2.0%时,ASFRC试件的峰值荷载较SFRC试件提高了75%、111%、141%,纤维合力的最大值较SFRC试件增大了202%、144%、127%;定向钢纤维可以有效改善水泥基复合材料的断裂性能,显著提高钢纤维的利用率,延缓裂缝的扩展.
在水泥基体中加入适量的钢纤维可以有效抑制裂缝的扩展,改善水泥基复合材料的断裂性
采用磁场法可以实现钢纤维的定向分布. 慕儒
细观数值模拟便于分析基体和钢纤维在断裂过程中任意时刻的受力状态,有利于深入研究钢纤维的增强机理. Wang
本文采用随机生成算法投放钢纤维,建立了SFRC和ASFRC三点弯曲梁细观有限元数值模型,计算了不同钢纤维掺量(体积分数)下SFRC试件和ASFRC试件加载断裂的全过程,结合断裂试验验证了数值模型的有效性,分析了开裂截面处的纤维应力,研究了定向钢纤维的细观增强机理.
在440 mm × 100 mm × 100 mm区域内投放直径(Df)为0.5 mm,长度(Lf)为30 mm的圆直型钢纤维,钢纤维总数量(N)通过以下方法确
(1) |
式中:为投放区域的体积,m
在SFRC试件中,钢纤维呈随机分布.本文参考文献[

图1 不同角度范围内的随机乱向钢纤维细观数值模型
Fig.1 Meso‑numerical model of random steel fibers in different angle ranges
在ASFRC试件中,钢纤维分布方向与主拉应力方向一致,基于随机乱向钢纤维的投放算法,确定钢纤维掺量为0.8%,试件尺寸为440 mm×100 mm×100 mm,控制纤维投影与X轴夹角为0°,建立定向钢纤维细观数值模型,如

图2 定向钢纤维细观数值模型
Fig.2 Meso‑numerical model of aligned steel fibers
基体选用三维八节点实体单元建模,钢纤维选用三维二节点桁架单元建模.采用黏聚裂纹模

图3 三点弯曲梁模型跨中处钢纤维的分布
Fig.3 Distribution of steel fibers in the middle of the three point bending beam model
在建立钢纤维增强水泥基复合材料细观有限元数值模型时,通常认为钢纤维和基体完全黏结,无相对滑动,因此将单根钢纤维与水泥砂浆基体拉拔试验所得拉拔力-位移曲线等效为钢纤维的拉伸应力-应变关
拉拔试验使用P·O 42.5普通硅酸盐水泥、细度模数为2.6的天然河砂和自来水来制作水泥砂浆基体,钢纤维为弹性模量210 GPa的圆直型钢纤维. 钢纤维埋深(Hf)为20 mm,埋入角度为0°、30°、45°、60°,每种角度制备10个试件,试件尺寸为100 mm×100 mm×100 mm.拉拔试验在SUNS20kN型万能试验机上进行,加载速度为0.4 mm/min,当钢纤维被完全拔出或被拔断时结束试验.

图4 倾斜纤维加载示意图
Fig.4 Schematic diagram of inclined steel fiber loading(size: mm)
钢纤维的拉拔力-位移曲线如

图5 钢纤维的拉拔力-位移曲线
Fig.5 Curves of pull‑out force‑displacement of steel fiber
(1)峰值荷载的理论计算结果与试验结果相差不大,位移存在一定误差. 这是钢纤维倾斜角度增大导致的测量误差,总体上理论计算结果与试验结果吻合.
(2)当埋入角度小于45°时,钢纤维拉拔力-位移曲线的峰值荷载随着埋入角度的增加逐渐增大且向右偏移;当埋入角度大于45°时,峰值荷载有减小的趋势.这是因为当纤维倾斜角度过大时,纤维埋入端处的基体应力集中程度加重,水泥砂浆破坏速度加快,使钢纤维被拔出所需要的拉拔力变小.
为计算简便,在开展有限元模拟分析时,将理论计算的拉拔力-位移曲线等效为钢纤维的拉伸应力-应变关系,如

图6 基于拉拔力-位移曲线确定的钢纤维等效应力-应变关系曲线
Fig.6 Determination of steel fiber stress‑strain curves base on pull‑out force‑displacement curves
(2) |
(3) |
(4) |
式中:为钢纤维拉拔荷载,kN;为钢纤维滑移位移,mm;为钢纤维与拉拔力之间的初始夹角,(°).
根据纤维投影与X轴的夹角范围,分别指定相应角度的等效应力-应变关系. 基于扩展有限元法,采用位移加载的方式模拟荷载作用下SFRC和ASFRC三点弯曲梁断裂破坏的全过程.
采用P·O 42.5普通硅酸盐水泥、细度模数为2.6的天然河砂、钢纤维、自来水和聚羧酸型高效减水剂来制作钢纤维增强水泥砂浆. 调试后确定基体水灰比(质量比)为0.36,设计纤维掺量为0.8%、1.2%、2.0%,具体的材料配合比见
Vf/% | Mix proportion/(kg· | |||
---|---|---|---|---|
Water | Cement | Sand | Superplasticizer | |
0.8 | 235.00 | 651.00 | 1 302.00 | 2.56 |
1.2 | 235.00 | 651.00 | 1 302.00 | 2.64 |
2.0 | 235.00 | 651.00 | 1 302.00 | 3.28 |
Steel fiber type | Density/(kg· | Lf/mm | Df/mm | Lf/Df | E/GPa | ft/MPa |
---|---|---|---|---|---|---|
Round straight | 7 850 | 30 | 0.5 | 60 | 210 | 1 150 |
每种钢纤维掺量下均制备SFRC试件和ASFRC试件,试件尺寸为440 mm×100 mm×100 mm,跨高比为4,跨中设置宽2 mm的预制裂缝,缝高比为0.4,标准养护28 d. 制备SFRC试件时采用常规方式振捣,制备ASFRC试件时需将盛有钢纤维增强水泥砂浆的试模置于振动台上的通电线圈内进行振捣,如

图7 ASFRC试件的制备装置
Fig.7 Preparation device of ASFRC specime
断裂试验在伺服万能试验机上进行,采用位移加载方式,加载速率为0.15 mm/min. 选用量程为20 kN的外接荷载传感器测量荷载(P),在试件裂缝跨中底部设置YYJ‑10/10型夹式引伸计,用于测量裂缝张开位移(CMOD). 试验加载示意图如

图8 断裂试验加载示意图
Fig.8 Loading schematic diagram of fracture test(size: mm)
ft/MPa | E/GPa | μ | GF/(N·m |
---|---|---|---|
3.2 | 30 | 0.2 | 0.148 |
SFRC试件和ASFRC试件的荷载-裂缝张开口位移(P‑CMOD)曲线如

图9 SFRC试件和ASFRC试件的荷载-裂缝张开口位移曲线
Fig.9 P‑CMOD curves of SFRC and ASFRC specimens
(1)当钢纤维掺量为0.8%时,SFRC试件的P‑CMOD曲线有明显的陡降,随着钢纤维掺量的增加,陡降程度有所缓解,而ASFRC试件的P‑CMOD曲线在达到峰值荷载后均下降缓慢. 这是因为钢纤维定向后,其分布方向与试件所受主拉应力方向一致,桥接裂缝的钢纤维得到了充分利用,阻裂增韧效率提高.
(2)当钢纤维掺量为0.8%和1.2%时,SFRC试件模拟曲线的下降段略低于试验曲线,但曲线趋势大致相同,当钢纤维掺量为2.0%时,曲线吻合较好;ASFRC试件的模拟曲线与试验曲线均吻合较好. 因此,模拟结果可以较好地反映2种试件断裂的全过程,表明建立的细观有限元数值模型具有一定的有效性.
试验与模拟所得SFRC试件和ASFRC试件的峰值荷载如
Specimen No. | Specimen type | Vf/% | Tested peak load/kN | Mean value/kN | Simulated peak load/kN |
---|---|---|---|---|---|
SFRC‑0.8‑1 | SFRC | 0.8 | 3.09 | 2.70 | 2.53 |
SFRC‑0.8‑2 | 2.56 | ||||
SFRC‑0.8‑3 | 2.44 | ||||
ASFRC‑0.8‑1 | ASFRC | 0.8 | 4.40 | 4.72 | 4.49 |
ASFRC‑0.8‑2 | 4.72 | ||||
ASFRC‑0.8‑3 | 5.04 | ||||
SFRC‑1.2‑1 | SFRC | 1.2 | 2.76 | 2.79 | 2.97 |
SFRC‑1.2‑2 | 2.84 | ||||
SFRC‑1.2‑3 | 2.76 | ||||
ASFRC‑1.2‑1 | ASFRC | 1.2 | 6.48 | 5.88 | 5.35 |
ASFRC‑1.2‑2 | 5.65 | ||||
ASFRC‑1.2‑3 | 5.50 | ||||
SFRC‑2.0‑1 | SFRC | 2.0 | 2.77 | 3.69 | 3.84 |
SFRC‑2.0‑2 | 3.88 | ||||
SFRC‑2.0‑3 | 4.43 | ||||
ASFRC‑2.0‑1 | ASFRC | 2.0 | 8.42 | 8.90 | 8.97 |
ASFRC‑2.0‑2 | 8.51 | ||||
ASFRC‑2.0‑3 | 9.76 |

图10 试件开裂破坏图及跨中开裂截面处的钢纤维分布
Fig.10 Cracking failure diagram of the specimen and distribution of steel fibers at the cracked section in the middle
不同加载时刻ASFRC试件主拉应力方向()的应力云图如

图11 不同加载时刻ASFRC试件的主拉应力方向应力云图
Fig.11 stress cloud diagram of ASFRC specimen at different loading moments

图12 试件跨中开裂截面处纤维的应力分布图
Fig.12 Fiber stress distribution diagram on cracked surface in mid‑span of specimens

图13 试件跨中开裂截面处的纤维合力-荷载曲线图
Fig.13 F‑P curves of fibers on cracked surface in mid‑span of specimens
(1)当钢纤维掺量为0.8%、1.2%、2.0%时,SFRC试件的钢纤维合力最大值分别为3.985、7.082、8.970 kN,ASFRC试件的钢纤维合力最大值分别为12.047、17.283、20.361 kN, ASFRC试件钢纤维合力的最大值较SFRC试件分别增大了约202%、144%、127%,且ASFRC试件的增幅随着钢纤维掺量的增加而减小.
(2)2种试件的钢纤维合力均先增大后减小,且当钢纤维掺量相同时,在整个加载过程中ASFRC试件的钢纤维合力始终大于SFRC试件. 在加载初期,钢纤维合力近似为线性增长,钢纤维处于弹性阶段,此时的伸长量为自身弹性变形;随着荷载的增加,裂缝开始向上扩展,钢纤维合力呈非线性增长,当达到峰值荷载时,开裂面上钢纤维的端部开始与基体脱黏,由钢纤维侧面与基体的黏结力承载主要荷载,基体开裂后的延性增强;荷载减小,钢纤维合力继续增大,当钢纤维合力增至最大值时,钢纤维与基体完全脱黏;钢纤维合力减小表明纤维被拔出,此时由钢纤维未拔出部分与基体之间的摩擦力共同抵抗荷载作用,继续加载,试件逐渐丧失承载能力,沿裂缝破坏. 当钢纤维掺量为2.0%时,F‑P曲线上的Pmax点和Fmax点较钢纤维掺量为0.8%和1.2%时更接近,这是由于随着钢纤维纤维掺量的增加,峰值荷载逐渐增大,同时钢纤维掺量的增加使钢纤维间距减小,钢纤维与基体之间的黏结作用减小,更易滑动被拔出,钢纤维合力增大的幅度减小,由此Pmax点和Fmax点较为接近.
(1)钢纤维拉拔力的峰值随着钢纤维倾角的增加呈现先增大后减小的趋势.
(2)随机乱向、定向钢纤维增强水泥基复合材料(SFRC、ASFRC)试件的峰值荷载均随着钢纤维掺量的增加而增大,相同钢纤维掺量下ASFRC试件的峰值荷载明显大于SFRC试件. 当钢纤维掺量为0.8%、1.2%、2.0%时,ASFRC试件的峰值荷载较SFRC试件分别提高了75%、111%、141%,表明钢纤维定向可以有效改善钢纤维增强水泥基复合材料的断裂性能.
(3)SFRC和ASFRC三点弯曲梁断裂全过程荷载-裂缝张开口位移全曲线的计算值与试验值符合较好,峰值荷载误差在10%以内,验证了有限元模型的有效性.
(4)当钢纤维掺量相同时,ASFRC试件跨中开裂面上的钢纤维数量大于SFRC试件;当钢纤维掺量为0.8%、1.2%、2.0%时,ASFRC试件纤维合力的最大值较SFRC分别提高了202%、144%、127%. 表明钢纤维定向后可以显著提高钢纤维的利用率,延缓裂缝的扩展.
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