Fatigue behavior and failure mechanism of basalt FRP composites under long-term cyclic loads

This paper studies the fatigue behavior of basalt fiber reinforced epoxy polymer (BFRP) composites and reveals the degradation mechanism of BFRP under different stress levels of cyclic loadings. The BFRP composites were tested under tension–tension fatigue load with different stress levels by an advanced fatigue loading equipment combined with in-situ scanning electron microscopy (SEM). The specimens were under long-term cyclic loads up to 1 × 10⁷ cycles. The stiffness degradation, S–N curves and the residual strength of run-out specimens were recorded during the test. The fatigue strength was predicted with the testing results using reliability methods. Meanwhile, the damage propagation and fracture surface of all specimens were observed and tracked during fatigue loading by an in-situ SEM, based on which damage mechanism under different stress levels was studied. The results show the prediction of fatigue strength by fitting S–N data up to 2 × 10⁶ cycles is lower than that of the data by 1 × 10⁷ cycles. It reveals the fatigue strength perdition is highly associated with the long-term run-out cycles and traditional two million run-out cycles cannot accurately predict fatigue behavior. The SEM images reveal that under high level of stress, the critical fiber breaking failure is the dominant damage, while the matrix cracking and interfacial debonding are main damage patterns at the low and middle fatigue stress level for BFRP. Based on the above fatigue behavior and damage pattern, a three stage fracture mechanism model under fatigue loading is developed.     

含醚间位芳香族聚酰胺共聚物的制备与表征

通过在芳香族聚酰胺化合物中引入4,4'-二氨基二苯醚制备含醚间位芳香族聚酰胺(PMIA)共聚物,并对其相关性能进行了检测。试验结果表明:通过反应成功将4,4'-二氨基二苯醚引入了共聚物,含醚PMIA共聚物溶液的表观黏度随剪切速率的升高而降低;制备的含醚PMIA共聚物起始分解温度低于PMIA;随着4,4'-二氨基二苯醚加入量的增加,含醚PMIA共聚物成纤能力降低;纤维断裂强度以及弹性模量降低,断裂伸长率升高,耐热持久性下降。     

不同软硬段含量的聚醚酯纤维结构与性能的关系

为了说明聚醚酯纤维的形态结构与性能的对应关系,选择不同软硬段含量的聚醚酯切片进行熔融纺丝试验。结果发现:随着硬段含量的增加,纤维的断裂强度增大,但回弹性降低;当硬段质量分数为65%时,纤维的断裂强度为1.12 cN/dtex,而回弹性降至50%左右。通过广角X射线散射、小角X射线散射以及双折射、声速取向等试验进一步分析了不同软硬段含量的聚醚酯纤维的形态结构变化,发现在硬段含量高的聚醚酯纤维内部有利于形成较高的取向结构,有助于纤维强度的提升;然而,此时纤维内形成较小尺寸的结晶结构分散于无定形区内,相分离程度低,致使其回弹性降低。     

Mechanical behavior of hybrid steel-fiber self-consolidating concrete: Materials and structural aspects

This work presents the preliminary results of an experimental investigation on the mechanical behavior of self-consolidating concrete reinforced with hybrid steel fibers in the material and structural scale. Straight and hooked end steel fibers with different lengths and diameters were used as reinforcement in fiber volume fractions of 1.0 and 1.5%. In the fresh state the concrete was characterized using the slump flow, L-box and V-funnel tests. To determine the effect of the hybrid reinforcement on the plastic viscosity and shear yield stress a parallel plate rheometer was used. Following, the mechanical response was measured under tension and bending tests. In the flexural test, the movement of the neutral axis was experimentally determined by strain-gages attached to compression and tensile surfaces. Furthermore, the mechanical response of the material under bi-axial bending was addressed using the round panel test. During the test the crack opening was measured using three linear variable differential transformers (LVDT’s). The cracking mechanisms were discussed and compared to that obtained under four point bending and direct tension. The obtained results indicated that the fiber hybridization improved the behavior of the composites for low strain and displacement levels increasing the serviceability limit state of the same through the control of the crack width. For large displacement levels the use of the longer fibers led to a higher toughness but with an expressive crack opening. Due to its structural redundancy the round panel test allowed the formation of a multiple cracking pattern which was not observed in the four point beam tests. Finally, the obtained material’s properties were used in a nonlinear finite element model to simulate the round panel test. The simulation reasonably agreed with the experimental test data.     

Deep‐black‐coloring effect of fabrics made of noncircular cross‐section polyester filaments

The effects of noncircular cross‐section (NCCS) poly ethylene terephthalate (PET) filaments and its shape factor on deep‐black‐coloring of dyed fabrics were investigated by comparing to that of the circular cross‐section PET ones. Indexes such as K/S, L* and Integ values were used for characterizing the deep‐black‐coloring effect on fabrics. The results indicated that fabrics made with NCCS PET filaments exhibited good deep‐black‐coloring effects. The calculated shape factor of the NCCS PET fiber had a significant correlation with the degree of deep‐black‐coloring exhibited by the fabric made from the fibers. A qualitative optical analysis of the NCCS PET fibers was carried out to explain the causes of the deep‐coloring of the NCCS fibers. This analysis implies that the contours of the NCCS fiber composed of surfaces with varied curvature increase the scattering of light by lowering specular reflection and increasing interior reflected and refracted light. This, in turn, strengthens the deep‐coloring effect. © 2013 Wiley Periodicals, Inc. Col Res Appl, 39, 511–518, 2014     

碳纤维布加固RC梁后的荷载-挠度全曲线的简化计算

为了得到碳纤维布加固RC梁后的荷载-挠度全曲线,许多文献的分析过程都需要借助计算机程序进行迭代计算,以确定每一个时段的中和轴高度,由此可得到相应的荷载-挠度关系。为了能在实际的工程设计中进行简化计算,本文通过求出各个界限荷载值以及与之对应的位移,利用各个界限点之间线性插值来得到荷载-挠度全曲线关系。给出了钢筋混凝土结构的分阶段平截面弹塑性分析法。计算结果与试验结果、有限元分析结果吻合良好,表明了该方法具有较高的精度,为碳纤维布加固梁提供了参考依据。     

表层内嵌碳纤维筋喷浆法在桥梁加固中的应用

主要研究了内嵌碳纤维筋喷浆加固法在板桥加固领域中的应用,从技术要点、施工方法和质量控制三方面对加固桥梁进行研究,通过对大游底桥加固前与加固后的动载试验对比分析,表明桥梁的抗弯和抗剪均有较大提高,达到了较好的加固效果,具有很好的推广价值。     

Evaluation of a large dish‐type concentrator solar lighting system for underground car park

To utilize solar energy more efficiently and reduce lighting power consumption in underground public spaces such as car park, a large dish‐type concentrator solar lighting system is put forward along with its evaluation, which is a unique design to apply a laminated layer of beam split thin‐film coating and thin‐film solar cells onto the dish reflector. The collected sunlight is split into 2 parts, one being reflected into a fiber optical bundle and transmitted for daylighting, while the rest being absorbed by solar cells for electricity generation as the other way to replenish daylighting. A set of 4 solar lighting systems using 3.28‐m diameter dish are designed to meet the lighting requirement in a 1771‐m² underground car park. A mathematical model is adopted to calculate the output power and conversion efficiency of solar cells distributed on the parabolic dish surface. The indoor illuminance distribution is given by lighting simulation. The results indicate that the average daylight illuminance in the car park can vary between 62.7 and 284 lx on February 25, 2016 and between 62.7 and 353 lx on August 17, 2016 for 2 chosen days, respectively. For the presented design, the electricity produced by solar cells is just enough to power light‐emitting diodes for lighting meeting a criterion at night. Considering about 19% conversion efficiency of solar cells and the efficacy of 129.5 lm/W of light‐emitting diodes, the hybrid solar lighting system can have about 40% utilization ratio of solar energy, so it can be concluded that a sufficient lighting provision can be provided by the proposed large dish‐type concentrator solar lighting system for applications in underground car park.     

V2O5 nanosheets assembled on 3D carbon fiber felt as a free-standing electrode for flexible asymmetric supercapacitor with remarkable energy density

As an emerging energy storage device, supercapacitor is widely investigated owing to its excellent capability, quick charge-discharge and tremendous cycle life. The operation potential window, energy density and mass loading of supercapacitor must be taken into deep consideration for its practical application. In this work, an outstanding electrode based on CFF@V₂O₅ nanosheets was prepared. Then a free-standing asymmetric supercapacitor with CFF@V₂O₅ composite as positive electrode and CFF@AC as negative electrode was assembled. Owing to the functional groups produced on CFF after the activation, V₂O₅ nanosheets was immobilized. The composite exhibits remarkable specific capabilities of 1465 mF cm^?2 (492 F g^?1). The energy density of the assembled free-standing asymmetric supercapacitor achieves 0.928 mWh cm^?3 when the power density is 17.5 mW cm^?3. After 6000 charging-discharging cycles as under normal, bended and anti-bended conditions for respective 2000 cycles, the device retains 89.7% of the initial capacitance, exhibiting fascinating cycle stabilization. Finally, two devices linked series can lighten a LED of 1.8 V for 2 min after charging for 2.5 min, which is inspiring for the practical application and production of self-supporting asymmetric supercapacitors.