A Novel Method for Measuring Flexure Strength of Brittle Materials

A novel test method to measure flexure strength for silicon slices and other brittle materials was suggested by using simple supported circular slice samples and steel ball impact loading. The maximal tensile stress expression under concentrating impact load for center of simple supported circular plate was derived based on kinetic energy theorem and small deflection theory of thin plate. The instrument used in the method is simple, the test results are accurate, and the test does not need large scale loading apparatus. An experimental correct method was presented for test results deviated from small deflection limit when the slices were thinner or the strength of slice was higher.

Effect of layer thickness on the flexural property and microstructure of 3D-printed rhomboid polymer-reinforced cemented tailing composites

For mines with poor ore bodies and surrounding rocks,the general mining method does not allow the ore to be extracted from underground safely and efficiently.For these mines,the downward layered filling mining technique is undoubtedly the most suitable mining method.The downward filling mining technique may eliminate the troubles relating to poor ore deposit conditions,such as production safety,ore loss rate,and depletion rate.However,in this technique,the safety of the artificial roof of the next stratum is of paramount importance.Cementitious tailings backfilling(CTB)that is not sufficiently cemented and causes collapses could threaten ore production.This paper explores a diamond-shaped composite structure to mimic the stability of a glued false roof in an actual infill mine based on the recently emerged three-dimensional(3D)printing technology.Experimental means such as three-point bending and digital image correlation(DIC)techniques were used to explore the flexural characteristics of 3D construction specimens and CTB combinations with different cement/tailings weight ratios at diverse layer heights.The results show that the 3D structure with a 14-mm ply height and CTB has strong flexural characteristics,with a maximum deflection value of 30.1 mm,while the 3D-printed rhomboid polymer(3D-PRP)structure with a 26-mm ply height is slightly worse in terms of flexural strength characteristics,but it has a higher maximum flexural strength of 2.83 MPa.A combination of 3D structure and CTB has more unique mechanical properties than CTB itself.This research work offers practical knowledge on the artificial roof performance of the downward layered filling mining technique and builds a scientific knowledge base regarding the successful application of CTB material in mines.

Flexural Fatigue Behavior of Layered Hybrid Fiber Reinforced Concrete

In order to obtain the fatigue life of layered hybrid fiber reinforced concrete （LHFRC） at different stress levels, flexural fatigue tests were carried out on specimens. The relation between fatigue lives and stress levels was simulated using the two-parameter Weibull distribution. Furthermore, both single- logarithmic and double-logarithmic regressive equations of various reliabilities were derived. It is evident that LHFRC gets the advantage of longer fatigue life over common concrete.

Determination of work-hardening exponent for sheet metal using three-point method

Three characteristic points in the deformation history of a fractured tensile specimen are selected tocalculate two values of n( n1 and n2 ) to represent the hardening ability of material during the homogeneous plas-tic deformation and the following large plastic deformation. Experimental results obtained with mild streel andred copper show that n determined using the three-point method proposed is better to describe the hardening a-bility of material. It is therefore concluded that three-point method can be used to describe the hardening prop-erty of material during both homogeneous deformation and large plastic deformation.

Rapid repair of severely earthquake-damaged bridge piers with flexural-shear failure mode

An experimental study was conducted to investigate the feasibility of a proposed rapid repair technique for severely earthquake-damaged bridge piers with flexural-shear failure mode. Six circular pier specimens were first tested to severe damage in flexural-shear mode and repaired using early-strength concrete with high-fluidity and carbon fiber reinforced polymers （CFRP）. After about four days, the repaired specimens were tested to failure again. The seismic behavior of the repaired specimens was evaluated and compared to the original specimens. Test results indicate that the proposed repair technique is highly effective. Both shear strength and lateral displacement of the repaired piers increased when compared to the original specimens, and the failure mechanism of the piers shifted from flexural-shear failure to ductile flexural failure. Finally, a simple design model based on the Seible formulation for post-earthquake repair design was compared to the experimental results. It is concluded that the design equation for bridge pier strengthening before an earthquake could be applicable to seismic repairs after an earthquake if the shear strength contribution of the spiral bars in the repaired piers is disregarded and 1.5 times more FRP sheets is provided.

Damage detection method in complicated beams with varying flexural stiffness

A damage detection method for complicated beam-like structures is proposed based on the subsection strain energy method （SSEM）, and its applicability condition is introduced. For a beam with the continuously varying fiexural stiffness and an edge crack, the SSEM is used to detect the crack location effectively by numerical modal shapes. As a complicated beam, the glass fiber-reinforced composite model of a wind turbine blade is studied based on an experimental modal analysis. The SSEM is used to calculate the damage index from the measured modal parameters and locate the damage position in the blade model successfully. The results indicate that the SSEM based on the modal shapes can be used to detect the damages in complicated beams or beam-like structures for engineering applications.

Analysis of Crack Expansion and Morphology of Cross-Laminated Timber Planar Shear Test

To describe the dynamic cracking process of the CLT vertical layer,the correlation between a load-displacement curve,specimen cracking,and planar shear failure mechanism of the CLT were explored.A three-point bending test and an improved planar shear test are used to evaluate the shear performance of the CLT.In this study,the load-displacement curve is recorded,the experimental part is synchronized with the video,the dynamic process of cracking of the vertical layer is observed and analyzed throughout the test.From the load-displacement curve,the image characteristics of the initial cracking and the sudden increase of the cracking of the specimen are summar-ized.The description results of the whole dynamic process of the CLT vertical layer cracking are analyzed by pla-nar shear strength value,cracking phenomenon,and azimuth angle of cracking surface.The main conclusions show that the three-point bending test and the improved plain shear test can be used to test the plain shear strength of the CLT,with a difference of only 5.7%.The original crack and the new crack expansion account for 18.9%and 81.1%of the main cracking surface,respectively.And the vertical layer of the CLT specimen under three-point bending has three cracking morphologies,such as radial shake,ring shake,neither along with the radial shake nor along with the ring shake.The azimuth angle of the cracking surface of the CLT vertical layer under planar shear is quite consistent with the first main plane azimuth of the vertical layer of the CLT specimens under the three-point bending test and the shearing test.The shape in the cracking direction of the left half-span or the right half-span of the vertical layer of the specimen is similar to the Chinese character eight.

Quasi-Static Flexural Properties of a Pultruded Glass Fiber/Unsaturated Polyester Square Pipe

This paper explores quasi-static flexural properties and fracture behavior of a pultruded glass fiber/unsaturated polyester square pipe for automotive structural applications. Three-point flexural testing is performed in an Instron Universal Testing Machine with steel jigs supporting the top and bottom surfaces of the pipe. Acoustic emission (AE) measurements are recorded during flexural testing to evaluate initial fracture in the pipe structure. After final fracture, five cross-sections of the pipe are cut at 50-mm intervals along the longitudinal axis, with the first cut located at the mid-span of the pipe. Cross-sections of a pipe from an interrupted test where initial fracture is detected from the AE method are also prepared. Damage locations and behavior on each cross-section are observed. The flexural testing results show that the cumulative AE counts increase rapidly from 2.5 kN, that final failure occurs at a maximum load of approximately 13 kN, and that corresponding initial and final failure occurs in the two corner regions on the compressive side of flexural loading. Failure initiates by stress concentrations due to the upper jig on the top surface during bending. The cross-sectional observations also reveal clear deformation behavior of the pipe where failure is present, marked by inward bending of the top surface and upper corners located on the compressive side, near the jig. The locations of maximum stresses and deformations obtained from finite element analysis of this pipe structure are in very good agreement with the experimental observations.

Flexural Bond Behavior of Rebar in Ultra-High Performance Concrete Beams Considering Lap-Splice Length and Cover Depth

This study intends to find out the correlation between the cover depth and the bond characteristics of UHPC through pull-out tests of UHPC specimens with different cover depths and bond tests of rebar using flexural members. In this experimental study, specimens are fabricated with the lap-splice length as test variable in relation with the calculation of the lap-splice length for 180- MPa UHPC. Moreover, specimens are also fabricated with the cover depth as test variable to evaluate the effect of the cover depth on the UHPC flexural members. The load-displacement curves are analyzed for each of these test variables to compute the lap-splice length proposed in the K-UHPC structural design guideline and to evaluate the influence of the cover depth on the flexural members. As a result, the stability of the structural behavior can be significantly enhanced by increasing slightly the cover depth specification of the current UHPC Structure Design Guideline from the maximum value between 1.5 times of rebar diameter and 20 mm to the maximum value between 1.5 times of rebar diameter and 25 mm.

玄武岩筋混凝土桥面铺装试验研究

目的研究玄武岩筋替换桥面铺装层中传统钢筋的抗弯性能,为该类加铺玄武岩筋混凝土铺装层提供参考。方法笔者制作了7根玄武岩筋混凝土桥面铺装试验梁和3根普通钢筋混凝土梁对其进行四点加载试验,研究钢筋直径、数量以及铺装层厚度等对试验梁抗弯性能的影响。结果只改变铺装层纵筋类型,铺装层纵筋为玄武岩筋的矩形梁屈服荷载提高了1.23%~8.96%,抗弯承载力提高了0.6%~2.65%;相同筋材类型,随着钢筋直径增大到10 mm,普通钢筋混凝土梁的抗弯承载力提高了13.86%,玄武岩筋混凝土梁的抗弯承载力提高了16.17%;铺装层纵筋数量的改变,对梁的开裂荷载、抗弯承载力的作用较小;只增加铺装层厚度,玄武岩筋混凝土桥面铺装试验梁开裂荷载最大提高了50%,屈服荷载最大提高了14.47%,极限荷载最大提高了11.43%。结论在一定的铺装层厚度和纵筋根数条件下,将铺装层中玄武岩筋代替普通钢筋,不仅可以发挥玄武岩筋抗腐蚀的特点,还能提高抗弯承载力。

薄壁型钢UHPC组合箱梁受弯性能试验研究

提出一种新型薄壁型钢UHPC组合箱梁,开展了6根薄壁型钢UHPC组合箱梁与1根薄壁型钢箱梁对比梁的四点弯曲静载试验,利用三维数字图像相关测量技术对箱梁的应力-应变与变形及裂缝性能进行测试分析。研究结果表明:外包UHPC能使薄壁型钢箱梁的破坏形态由局部压屈破坏转变为底部薄壁型钢下翼缘及纵筋先后达到屈服后顶部UHPC压碎的正截面受弯破坏;外包50 mm厚UHPC的未配纵筋与箍筋的组合箱梁试件XL2的正截面受弯承载力比薄壁型钢箱梁试件XL1的对应值提升77.5%;薄壁型钢表面焊接了纵筋与箍筋的组合箱梁试件XL3~XL7的整体协同工作性能得到了大幅度提升,其中,XL4试件的正截面受弯承载力比未配纵筋与箍筋的XL2试件的对应值提升了178.8%;增大薄壁型钢壁厚与外包UHPC层厚度也能在一定程度上提高薄壁型钢UHPC组合箱梁的正截面受弯承载力与抗裂能力。

钢-PVA混杂纤维高性能混凝土断裂性能与弯曲韧性试验

为了研究钢-聚乙烯醇(polyvinyl alcohol,PVA)混杂纤维对高性能混凝土(high performance concrete,HPC)的增强增韧效果,以钢纤维体积分数、PVA纤维体积分数和矿粉掺量为三因素,采用正交试验,设计并制作16组试件,进行四点加载和三点加载弯曲试验,获取了荷载(P)-跨中挠度(δ)曲线和荷载(P)-裂缝张口位移(crack mouth opening displacement,CMOD)曲线,测定了28 d龄期后的弯曲韧性和断裂性能等。研究表明:混凝土中掺入钢纤维或PVA纤维,P-δ曲线和P-CMOD曲线都变得更加饱满,对其有较为明显的增强增韧效果;特别是当两种纤维和一定量的矿粉掺入混凝土中,曲线下降段趋势变得更加平缓,混掺纤维高性能混凝土试件比单掺纤维混凝土试件具有更好的变形能力,掺入1.5%的钢纤维、0.2%的PVA纤维和20%的矿粉的效果最佳,对弯曲韧性和断裂性能的提升最为显著,具有不错的正混杂效应。

ECC-混凝土叠合梁弯曲试验及裂缝发展的分形表征

为明确ECC(工程纤维增强型水泥基复合材料)-混凝土叠合梁抗弯性能及影响参数,并定性表征裂缝发展情况,设计、制作11根不同材料强度等级、配筋率、ECC层厚度、PVA纤维体积掺量的ECC-混凝土叠合梁及3根不同混凝土强度等级的混凝土梁进行四点弯曲试验,研究承载力、延性及裂缝发展等情况,并基于分形理论及数字图像处理技术,采用分形维数对不同影响参数下的叠合梁裂缝发展规律进行表征。结果表明:ECC-混凝土叠合梁较相同材料强度和配筋率的混凝土梁极限承载力和抗裂性能明显更优;材料强度等级提高,模型梁承载力及延性均提高;适筋范围内,配筋率越小模型梁延性越好;ECC层厚度对模型梁的承载力及延性影响不大;PVA纤维体积掺量为2.0%时,裂缝整体分布更均衡。计算分形维数得到的叠合梁表面裂缝分布具有自相似性;加载过程中叠合梁的分形维数小于混凝土梁,与试验结果相符,梁表面裂缝分形维数在一定范围内随着材料强度等级、配筋率、ECC层厚度、PVA纤维体积掺量的增加而降低,分形理论结合数字图像技术求得的裂缝分形维数可作为叠合梁裂缝分布及受弯损伤程度的定性表征指标。

液压支架结构的可靠性设计研究

针对国内液压支架在结构件可靠性方面与国外产品的差距,依托2套出口支架产品,阐述了应用于中厚煤层高工作阻力液压支架的顶梁底座柱窝、掩护梁、连杆的抗弯、抗扭性结构设计,对这些结构的技术特征、创新性、应用范围进行了分析研究。2套出口支架顺利通过了按美国CONSOL标准进行的60 000次型式试验,试验结果表明,所述结构切实提高了支架的抗弯、抗扭性能,提高了液压支架的可靠性。

30 m预应力UHPC箱梁抗弯性能试验及结构优化研究

为了解大尺寸预应力UHPC箱梁的抗弯性能,以预应力UHPC公路桥——河北石磁跨线桥为背景,设计、制作1片长30 m的足尺预应力UHPC箱梁模型进行四点弯曲试验,结合有限元计算结果,分析试验现象及裂缝分布、跨中荷载~位移曲线、受拉区UHPC及筋材应力~应变曲线。在此基础上分别考虑普通钢筋配筋情况、预应力筋数、腹板厚度和底板厚度等参数对预应力UHPC箱梁进行结构优化设计研究。结果表明:箱梁破坏时受压区UHPC未压溃、受拉区钢纤维被拔出,裂缝贯通底板;根据荷载~位移曲线,箱梁受力可分为弹性阶段、裂缝发展阶段、裂缝快速发展阶段和破坏阶段;根据应力~应变曲线,受拉区UHPC在纵筋屈服时仍能贡献抗拉能力;试验现象及力学指标实测值均说明了预应力UHPC箱梁抗弯能力还有很大富裕空间,可进一步优化。完全不配置普通钢筋的预应力UHPC箱梁仍能满足抗弯承载能力极限状态设计要求;底部预应力筋数增多可有效提高受压区UHPC的抗压强度利用率;腹板厚度过薄会造成腹板主拉应力过大问题,不宜薄于8 cm;采用体外预应力的方法可有效减薄底板所需厚度。

基于数据库的玻璃纤维增强复合材料箍筋弯拉强度计算方法

玻璃纤维增强复合材料箍筋弯拉强度试验主要研究弯折半径与直径之比对弯拉强度的影响,直线段抗拉强度对弯拉强度的影响研究较少。本文基于ACI 440.3R-12中B.5试验方法,开展3组(9个弯拉试件)不同直线段抗拉强度的玻璃纤维增强复合材料箍筋弯拉强度试验研究。3组试件直线段抗拉强度分别为530(A组)、850(B组)和1100 MPa(C组),弯折半径与箍筋直径之比均为3。试验结果表明:所有试件均发生玻璃纤维增强复合材料箍筋弯折段被拉断的破坏模式;A、B、C组试件弯拉强度分别为325、445和530 MPa。本文系统收集了国内外已有93个弯拉试件,并结合本文9个弯拉试件建立了玻璃纤维增强复合材料箍筋弯拉强度试验数据库。基于数据库统计分析,提出了保证率不小于95%的玻璃纤维增强复合材料箍筋弯拉强度计算方法。

洞口对轻钢骨架轻混凝土组合墙平面外抗弯性能的影响

对2个足尺轻钢骨架轻混凝土组合墙板试件进行平面外抗弯性能试验研究,分析设置窗洞口对轻钢骨架轻混凝土组合墙平面外破坏特征、荷载⁃挠度曲线、刚度及其退化的影响。试验表明,轻钢骨架轻混凝土组合墙平面外受弯状态下,墙板受拉侧裂缝沿龙骨发展;开设窗洞口后,窗洞口角部出现应力集中现象,裂缝从窗角向四周延伸。轻钢骨架轻混凝土组合墙设置窗洞口后,极限承载力降低26.22%,平面外抗弯刚度降低48.01%。基于试验建立轻钢骨架轻混凝土组合墙精细化有限元模型,并进行平面外受力性能数值模拟和参数分析,分析轻钢龙骨厚度和墙板厚度等变量对墙板平面外抗弯性能的影响。

全铁尾矿混凝土梁受弯性能试验研究

为了充分利用铁尾矿,对铁尾矿混凝土结构的力学性能进行探究,以铁尾矿粉为掺合料,铁尾矿碎石和铁尾矿砂分别作为粗、细骨料,制备成全铁尾矿混凝土(FITC)梁,并与以粉煤灰作掺合料,普通碎石及河砂作粗、细骨料的常规混凝土(CC)梁进行弯曲试验对比。首先,测试铁尾矿的各项参数,验证其作为混凝土原材料的可行性;其次,基于现行规范公式计算梁各阶段的承载力和挠度;最后,将FITC梁和CC梁的实测数值和计算数值对比分析。结果表明:FITC的强度比CC略低,弹性模量显著降低;FITC梁的开裂弯矩和极限弯矩与CC梁相当,梁的抗裂能力与承载力并未因FITC的强度和弹性模量比CC低而明显降低;在裂缝出现之前,FITC梁的混凝土应变与平截面假定基本一致;在荷载作用下,FITC梁受拉区混凝土应变比CC梁的大;FITC梁的延性、裂缝开裂间距均与CC梁接近,FITC梁弯剪区斜裂缝比CC梁更接近梁的顶部;在同等弯矩下,FITC梁的挠度大于CC梁的挠度,FITC梁的受拉纵筋应变也大于CC梁;中国现行设计规范对FITC梁的承载力计算是安全的,但挠度计算公式需要修正。

小剪跨比铁路圆端桥墩拟静力试验及易损性分析

为厘清小剪跨比圆端铁路桥墩横桥向的地震损伤机理,以国内某铁路沿线桥梁为参考开展3组缩尺比1∶8的拟静力试验,对滞回曲线的延性、残余位移、滞回耗能和刚度退化4个方面进行分析,以试验结果为参考使用OpenSees建立纤维截面有限元模型并进行耐震时程动力计算,以桥墩的位移延性比为损伤指标,基于试验中观察得到的4个关键节点定义无、轻微、中度、严重和完全损伤5种损伤状态之间的位移延性比临界值,通过易损性分析桥墩的损伤发展历程。研究结果表明:1)配筋率从0.3%提升至0.7%,桥墩的承载能力提高至1.37倍,极限位移提高至1.22倍,最终残余位移提高至1.55倍,耗散能量增加至1.80倍,初始刚度提升至1.08倍,最终刚度提升至1.12倍。提高配筋率可以提升桥梁抗震性能,也能增加桥梁抗震韧性。2)当桥墩配筋率由0.3%提至0.7%时,在多遇地震下,轻微损伤概率从87.2%降为29.8%;在设计地震下,中度损伤的概率从63.5%降为30.0%;在罕遇地震下,严重损伤的概率从61.7%降为16.4%。纵筋配筋率可以大幅降低桥墩的损伤概率,提升桥墩的抗震性能。3)在现行铁路规范允许的延性系数为4.8时,3个桥墩均处于严重损伤的阶段。当发生罕遇地震时,3个桥墩出现完全破坏的概率为6.4%、0和1.4%,小剪跨比圆端铁路桥墩在强震作用下具有一定的抗震安全储备。

带预应力UHPC华夫桥面板抗弯性能试验研究

为研究钢纤维类型、预应力筋等因素对UHPC华夫桥面板力学性能的影响,以北京亦庄文昌桥为背景进行抗弯试验研究。设计制作了4组板肋梁试件,对比分析钢纤维类型(直线型、端钩型)、预应力筋等因素对试件破坏模式、裂缝发展以及抗弯承载力等性能的影响,并基于美国《UHPC华夫板设计指南》,提出考虑预应力筋的华夫桥面板抗弯承载力计算公式。结果表明:底部纵向受拉筋的屈服是试件进入破坏阶段的特征之一,试件最终为适筋梁受弯破坏;设置预应力筋能一定程度提升试件的强度和延性,并能有效增加初裂缝附近UHPC的应变;设置预应力筋能明显改善试件的耐久性,而且钢纤维为直线型时提升效果比端钩型更显著;提出的抗弯承载力计算值与试验值吻合较好,能准确计算UHPC华夫桥面板的抗弯承载力。