Fatigue behavior of a multi-scale cement composite

To conceive structural elements with a cement composite and without any reinforcement except steel fibers is a very exciting and difficult challenge, which can change the building construction fields. To achieve this objective the Laboratoire Central des Ponts et Chaussées (LCPC) has developed a new cement composite, CEMTEC_multiscale®, which is strain hardening in tension and has a very high uniaxial tensile strength, more than 20 MPa.The present paper is on an experimental research related to this composite fatigue behavior. The principal results obtained are the following:

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A strong correlation exists between the initial static damage and the fatigue endurance limit.

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Below a loading ratio R = 0.65 (ratio between the applied stress and the characteristic static stress), failure during bending fatigue tests never appears with CEMTEC_multiscale® specimen.

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An 8% gain is observed between the bending static behavior of the specimens being previously loaded in fatigue and those not being loaded in fatigue.

     

Flexural performance of ring-type steel fiber-reinforced concrete

Steel fiber-reinforced concrete (SFRC) with randomly dispersed, short straight steel fibers hardly fails by fiber yielding, and the postpeak behavior is governed by mechanisms related to fiber pullout. It would be more desirable if more fracture energy could be consumed by fiber yielding at failure. It has been experimentally demonstrated in this research that SFRC with the ring-type steel fibers failed by more energy consuming mechanisms other than fiber pullout. Consequently, significant improvements in flexural toughness were obtained as compared to that of SFRC with conventional straight steel fibers.     

Research on increasing effect of solution polymerization for cement-based composite

Cement-based materials are brittle and low in flexural strength, which can be greatly improved by adding organic monomers to cement-based matrixes due to the polymerization effect of added the organics. The solution polymerization behaviors of organic monomer in cement paste and its effect on cement properties have been studied in this paper. The results show that the organic monomers used in the experiment can well polymerize themselves in cement pastes by solution polymerization effect under common conditions. As results, the cement composites have a flexural strength between 17.8 and 33.4 MPa, greatly dependent on the amount of organic monomers added and is stable when it is immersed in water, acid, alkali and salt solutions. Besides, the setting time of the composites can be controlled and regulated.     

Notch fatigue behavior of needled C/SiC composite under random vibration loading

In order to study the properties of needled C/SiC composite under random vibration loading, a kind of plate specimen with arc notch was designed. Random vibration tests were carried out on a vibration table, the vibration fatigue life and vibration response signal at the dangerous position of each specimen were obtained. The strain amplitude probability distribution of vibration response signal was obtained by rainflow counting method, which can be approximately described by the Dirlik distribution. The attenuation history curve of natural frequency for the specimen presented obvious nonlinearity. In addition, the microscopic fracture morphology of failed specimens showed that the major failure mode of fibers in axial and transverse direction are pull-out and splitting, respectively. And the fiber bundles adhering strongly to the matrix can form a neat fracture surface.     

Effect of preform and carbon matrix on bending strength of Cf/Cu/C composites

Aiming to obtain composites with appropriate mechanical properties for pantograph sliders, copper mesh modified carbon/carbon (C_f/Cu/C) composites were prepared by chemical vapor infiltration (CVI) in C₃H₆ +?N₂ atmosphere and impregnation-carbonization (I-C) with furan resin. In this paper, C_f/Cu/C composites with two kinds of preforms and carbon matrixes were obtained. The effect of preforms and carbon matrixes on bending strength was investigated. The results indicated that the bending strength of carbon fiber/copper mesh reinforced pyrolytic carbon matrix composites was about 181.39–195.43?MPa, while that reinforced resin carbon matrix composites had the worst bending strength around 54.45–57.04?MPa, in terms of the same preform. The bending strength of C_f/Cu/C composites in the parallel orientation and vertical orientation were also similar. As for C_f/Cu/C composites with the same carbon matrix, the bending strength of C_f/Cu/C composites with non-woven fiber/fiber web/copper mesh type preform was higher than that with fiber web/copper mesh type preform. However, the bending strength of carbon fiber/copper mesh reinforced resin carbon matrix composites showed the opposite trend, and its reasons were analyzed and discussed taking advantage of the fracture mechanisms.     

Bending and compressive behaviours of a new cement composite

The Laboratoire Central des Ponts et Chaussées (LCPC) has recently developed and patented a new cement composite, the CEMTEC_multiscale, which is stress hardening in tension and has a very high uniaxial tensile strength, more than 20 MPa. This paper is about the determination of the compressive and bending behaviors of the CEMTEC_multiscale used in the frame of ribbed slabs.The principal results obtained are the following:

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the characteristic modulus of rupture is equal to 42 MPa for the “slab” function;

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the characteristic modulus of rupture is equal to 48 MPa for the “rib” function;

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the ultimate tensile strain is around 5 10⁻³;

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the characteristic strength and ultimate strain in compression are equal to 205 MPa and 4 10⁻³, respectively; and

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the Young modulus is equal to 55 GPa and the Poisson coefficient is equal to .21.

     

Enhanced bending strength and microwave dielectric properties of Li2MgTi3O8 ceramics by adding Si3N4 reinforcing phase

In the 5G era, the dielectric materials used in microwave electronic components must have not only have good microwave dielectric characteristics but also excellent structural characteristics. Li₂MgTi₃O₈ (LMT) ceramics have excellent microwave dielectric properties; however, their low bending strength limits their further applications in the 5G era. In this work, the dielectric properties and bending strength of LMT ceramics were optimized by the addition of Si₃N₄ reinforcing phase using a solid-phase method, and the effects of Si₃N₄ addition on the sintering properties, microscopic structure, crystalline phase, dielectric properties and bending strength of ceramics were investigated. The X-ray diffraction pattern indicates that all ceramics exhibit spinel structure. Combined with the phenomenon of grain reduction in the SEM graph, it indicates that the addition of Si₃N₄ can inhibit the grain growth and achieve the purpose of fine-grain strengthening. The dispersion enhancement of second phase particles is also one of the reasons for the increase of bending strength. LMT ceramics doped with 0.5 wt% Si₃N₄ exhibited the maximum bending strength after sintering at 1050 °C for 4 h, which was 76.97% higher than that of pure LMT ceramics. In addition, the ceramics exhibited outstanding dielectric properties: a dielectric constant of 23.20, quality factor of 49344 GHz, and temperature coefficient of ?5.90 ppm/°C. The high bending strength and good microwave dielectric properties indicate that Si₃N₄-added LMT ceramics can be effectively applied in the 5G era.     

Effects of flocculants and sizing agents on bending strength of fiber cement composites

Wood fiber is used to replace asbestos in the manufacture of fiber cement due to its high availability, low cost and good reinforcement properties. The different chemical composition of the cellulose fibers makes its compatibility with the cement much more complex than that of asbestos fibers. In the Hatschek process a suitable flocculant is needed when using cellulose fibers. The right selection of the flocculant is crucial due to its effect on mineral fines retention, dewatering and formation and, as a consequence, on the overall efficiency of the machine. This paper shows how anionic poly-acryl-amides (A-PAM), the most common flocculants used in Hatschek machines, have a negative effect on the bending strength properties of fiber cement sheets. In order to overcome this problem fiber surface treatment, with sizing agents, is proposed in this paper. Sizing with styrene-acrylate copolymers and alkyl ketene dimer produces an increase in bending strength properties.     

氮化硅铁对炮泥高温抗折强度及抗渣性的影响

以棕刚玉、碳化硅、沥青及焦炭为主要原料,研究了氮化硅铁加入量为0、4%、8%、12%、16%、20%、24%、28%时对炮泥高温抗折强度的影响及加入量为6%、12%、18%、24%时对炮泥抗高炉炉渣侵蚀的影响。结果表明:氮化硅铁加入量在24%以内时,不能提高炮泥800℃的高温抗折强度,而当氮化硅铁加入量≥12%时,对1000℃以上高温抗折强度的提高效果即已显现,尤其对1400℃的高温抗折强度的提高非常明显;氮化硅铁的加入对炮泥高温抗渣侵蚀性影响不大。     

Dynamic behaviour and visco-elastic damage model of ultra-high performance cementitious composite

The dynamic behaviour of ultra-high performance cementitious composite (UHPCC) with compressive strength of 200 MPa with different steel fiber volume fractions was studied under impact using the split Hopkinson pressure bar. Three aspects of the testing: a gimbal device, wave shaping and direct strain measurement, were used to increase experimental accuracy. Results indicate that UHPCC has obvious strain rate effects. The peak stress, peak strain, elastic modulus and the area under the stress–strain curve increase with increasing strain rate. When the strain rate exceeds a threshold value, specimens with and without fibers begin to fracture. At high strain rate the unreinforced specimens fracture into small parts while fiber reinforced ones only have fine cracks on the edges. A visco-elastic damage model of UHPCC is proposed based on a nonlinear visco-elastic model (the ZWT model) and the material damage measured by the ultrasonic wave velocity method.     

A comparison of bending strength between adhesive and steel reinforced concrete with steel only reinforced concrete

Cracking of brittle cementitious composites subjected to excessive loading causes a potential reduction in material performance. Steel bars or metal fibers typically act as tensile reinforcing in concrete composites to increase the material's structural capacity in bending and to delay or prevent matrix cracking.The goal of this research is to determine whether the performance in bending strength and material integrity of a typically reinforced cementitious composite may be improved through the release of “healing” chemicals, such as adhesives, from hollow fibers into cracks induced by loading in addition to the metal reinforcing. Adhesive-filled repair fibers are intended to break immediately upon cracking in the concrete thereby activating the healing process with the release of a sealing or adhering substance. This self-repair occurs whenever and wherever cracks are generated.     

Relationship between bending strength of bulk porous silicon carbide ceramics and grain boundary strength measured using microcantilever beam specimens

The relationship between the bending strength of bulk porous SiC ceramics and the grain boundary strength measured using microcantilever beam specimens of SiC bicrystals was investigated. The average value of the grain boundary strength was 39.2 GPa, and its higher value was roughly equal to that derived using an ab-initio calculation. The strengths of the specimens having only one neck were estimated by analyzing the effect of the specimen size on the strength of bulk porous SiC ceramics and by also analyzing the grain boundary strength and the stress concentration at the neck surface. The estimated strengths were generally consistent of the order of several hundred MPa, meaning that the strength of porous SiC ceramics should be dependent on the stress concentration at the neck and the grain boundary strength. Furthermore, they were in a better agreement using smaller neck curvature, smaller neck diameter, and lower grain boundary strength.     

Modeling and optimization of the sandwich beam specimen in three-point bending for adhesive bond characterization

Standard tests for adhesive bond characterization suffer for several deficiencies. The simplest specimens to make and test are lap joint geometries (e.g. single, double, symmetric, etc.) that generate complex stress distributions with irregularities and even singularities of the stress state. Those with the stress state closer to pure shear (e.g. napkin ring or Arcan) are difficult to make and require special test fixtures. This paper examines the stress state in the adhesive of a simple beam specimen obtained by bonding two flat plates one upon the other and loading the final sandwich in three-point bending. An elementary theory is used to optimize the specimen for in-situ measurements of either shear strength or shear modulus of the adhesive. The accuracy of the model is validated with finite element analyses, showing good agreement between the analytical and finite element model and also providing suggestions for the best geometry to be adopted for practical implementation of the test.     

磷石膏基复合胶凝材料的性能优化及机理研究

采用水泥、矿渣粉、粉煤灰和减水剂对磷石膏进行改性。最终得到的磷石膏基复合胶凝材料的强度为原状磷石膏的2倍,软化系数从0.5提高至0.8。磷石膏基复合胶凝材料的比强度和孔隙率之间存在明显的线性关系,随着孔隙率的减小比强度增加。通过扫描电镜（SEM）对磷石膏基复合胶凝材料微观形貌的演变过程进行表征,发现随着矿渣粉、水泥、粉煤灰和减水剂的掺加,基体由疏松转变为致密;主要的水化产物二水石膏从针状转变为棒状或片状,并且出现了水化硅酸钙（C-S-H）凝胶,其填充于体系内部的孔隙并将二水石膏连成整体。利用X射线衍射（XRD）分析和傅里叶变换红外吸收光谱（FT-IR）测试对水化产物的微观结构进行研究。结果表明,复合体系中的主要产物为二水石膏,但是由于可用水量的减少,体系中仍剩余少量磷石膏未水化。     

Evaluation of fresh state,rheological properties,and compressive strength performance of cementitious system with grinding aids

Grinding aids (GA) affect the fresh and hardened state properties of cement systems in addition to the economic and ecological advantages they provide. Nevertheless, there is not enough information in the literature regarding the effect of GA utilization on the rheological properties of cementitious systems. In this study, the effect of different types of GA utilization on the grinding efficiency, setting time, and physical properties of cement and rheological properties of cement paste was investigated. Besides, adsorption of the water-reducing admixture (polycarboxylate ether [PCE]) to the cement containing GA was investigated by total organic carbon analysis. According to the results, regardless of the type of GA, while the grinding efficiency was positively affected by GA utilization in cement, the adsorption degree of PCE, some fresh and rheological properties of the mixtures were negatively affected. With the use of GA, an increase was observed in the early age compressive strength of mortar mixtures. Consequently, the properties of cementitious systems containing GA should be taken into consideration to assess GA performance in addition to the grinding efficiency.     

石膏品种对硅酸盐--硫铝酸盐复合体系水泥性能的影响

在试验研究不同石膏品种对硅酸盐-硫铝酸盐复合体系水泥凝结时间、标准稠度需水量、强度等性能影响的基础上,探讨了石膏品种对硅酸盐-硫铝酸盐复合体系水泥性能的影响机理。结果表明：二水石膏对该种复合体系水泥的缓凝作用比硬石膏明显,硬石膏易引起复合体系水泥急凝和需水量增大。石膏品种对硅酸盐一硫铝酸盐复合体系水泥强度的影响较复杂,与水泥体系中含铝矿物及其水化溶液中SO4^2-离子浓度有关;在蒸馏水和饱和石灰水中,二水石膏的溶解速度比硬石膏快,溶解度比硬石膏低。推导证实,石膏的溶解速度和溶解度是决定硅酸盐-硫铝酸盐复合体系水泥性能的主要因素。     

Fractography of 3YPSZ/316L functionally graded composite subjected to indentation and flexural bending tests

In this paper, stainless-steel (AISI316L) and 3 mol% Y₂O₃-partially stabilized ZrO₂ were used for fabrication of functionally graded material by spark plasma sintering. To make a temperature gradient in FGMs, a specific die configuration was utilized, which led to higher densification. In order to study the crack initiation and propagation behavior in the layered composites during bending and indentation tests, the section morphology of the layers was analyzed by optical microscopy and scanning electron microscopy. The results showed that cracks propagate through the PSZ matrix when there is no trace of steel. However, when cracks reach steel particles they grow around the particles and choose the interface of steel and matrix as a preferable route for propagation. Furthermore, the presence of small amounts of steel particles remained on the fracture surface, demonstrated that the steel particles can improve the fracture toughness of the composite.     

高镁镍渣-磷石膏基复合胶凝材料的制备与性能评价

采用机械研磨的方式将高镁镍渣（HMNS）和磷石膏（PG）筛分至不同粒度,评价了掺不同粒度高镁镍渣和磷石膏复合胶凝材料的力学性能,并对强度形成机制进行了分析。评价了各种配比下浆体的抗压强度和体积稳定性,并分析了其作用机制。试验表明：以HMNS∶PG∶富钙硅质材料ZL=5.4∶3.6∶1配比制得试样的28 d抗压强度为4.43 MPa;室温环境下养护56 d,SP6线性收缩为1.02×10-3 mm/mm,体积稳定性优良;水化产物Ca(OH)2更少,Ca（OH）2与HMNS-PG体系反应生成了CSH凝胶和AFt,结构更为稳定。     

The effect of TiO2 additive on sinterability and properties of SiC-Al2O3-Y2O3 composite system

In the current research, the effects of TiO₂ additive on mechanical and physical properties of SiC bodies, sintered by liquid phase methods were investigated. Al₂O₃ and Y₂O₃ were used as sintering-aids (10?wt% in total) with an Al₂O₃/Y₂O₃ ratio of 43/57 to provide liquid phase during Sintering. TiO₂ was also used as the oxide additive with an amount ranging from 0 to 10?wt%. After scaling and mixing the starting materials by a planetary mill, the obtained slurry was dried at 100?℃ for four hours. The derived powders were finally pressed under a pressure of 90?MPa. The samples were then pyrolyzed and sintered at 600?℃ and 1900?℃, respectively under argon atmosphere for 1.5?h. Phase analysis showed no trace of TiO₂ after the sintering process, demonstrating the complete TiO₂ to TiC transformation. The results showed that an increase in TiO₂ content up to 5?wt%, led an improvement in all the measured properties including the relative density, hardness, Young's modulus, bending strength, indentation fracture resistance and the brittleness factor, reaching to 96.2%, 24.4?GPa, 395.8?GPa, 521?MPa, 5.8?MPa?m^1/2 and 286.5?×?10^?6 m^?1, respectively. However more than 5?wt% additive resulted in a decrease in all the above-mentioned properties. Microstructural studies demonstrated that crack deflection and crack bridging were the major mechanisms responsible for an increase in the indentation fracture resistance.     

Study on performance of composite polymer films doped with modified molecular sieve for lithium-ion batteries

To improve the tensile strength and ionic conductivity of composite polymer films for lithium-ion batteries, molecular sieves of MCM-41 modified with sulfated zirconia (SO₄²⁻/ZrO₂, SZ), denoted as MCM-41/SZ, were doped into a poly(vinylidene fluoride) (PVdF) matrix to fabricate MCM-41/SZ composite polymer films, denoted as MCM-41/SZ films. Examination by transmission electron microscope (TEM) shows that modified molecular sieves have lower aggregation and a more porous structure. Tensile strength tests were carried out to investigate the mechanical performance of MCM-41/SZ films, and then the electrochemical performance of batteries with MCM-41/SZ films as separators was tested. The results show that the tensile strength (σ_t) of MCM-41/SZ film was up to 7.8 MPa; the ionic conductivity of MCM-41/SZ film was close to 10⁻³ S cm⁻¹ at room temperature; and the coulombic efficiency of the assembled lithium-ion battery was 92% at the first cycle and reached as high as 99.99% after the 20th cycle. Meanwhile, the charge-discharge voltage plateau of the lithium-ion battery presented a stable state. Therefore, MCM-41/SZ films are a good choice as separators for lithium-ion batteries due to their high tensile strength and ionic conductivity.