Toughness enhancement and equivalent initial fracture toughness of cementitious composite reinforced with aligned steel fibres

Based on theoretical analysis and numerical simulation, the impact of steel fibres on the stress intensity factor (SIF) at the crack tip for cementitious composite was studied. The enhanced toughness of steel fibre reinforced cementitious composite (SFRC) in resisting cracks was explained by the decrement of SIF caused by steel fibre inclusions at the crack tip of the composite. The equivalent initial fracture toughness was used to characterize the crack initiation of SFRC. A simplified method for determining the of SFRC was proposed based on a linear regression method. Fracture tests were conducted on three‐point bending notched beams with different steel fibre volume fractions and specimen sizes to study the crack initiation behaviour of aligned steel fibre reinforced cementitious composite (ASFRC). of ASFRC was calculated, and the size effect of was analysed. The results showed that slightly increased with the steel fibre volume fraction and gradually became stable. For the tested specimens, whose heights varied between 40 and 100 mm, the specimen size had little impact on the .     

Interfacial damage in fibre‐reinforced composites subjected to tension fatigue loading

ABSTRACT The interfacial behaviour of fibre‐reinforced composites subjected to tension fatigue loading is studied based on the shear‐lag model. The governing equations of this problem are obtained and solved. In order to describe the interfacial debonding, the Paris fatigue crack growth formula as well as a modified degradation model for the coefficient of friction is adopted. Finally some important values related to interfacial debonding are obtained. In the present investigation, Poisson's contraction is considered.     

Fracture mechanics‐based progressive damage modelling of adhesively bonded fibre‐reinforced polymer joints

A quasi‐static progressive damage model for prediction of the fracture behaviour and strength of adhesively bonded fibre‐reinforced polymer joints is introduced in this paper. The model is based on the development of a mixed‐mode failure criterion as a function of a master R‐curve derived from the experimental results obtained from standard fracture mechanics joints. Consequently, the developed failure criterion is crack‐length and mode‐mixity dependent, and it takes into account the contribution of the fibre‐bridging effect. Energy release rate values for adhesively bonded double‐lap joints are obtained by using the virtual crack closure technique method in a finite element model, and the numerically obtained strain energy release rate is compared to the critical strain energy release rate given by the mixed‐mode failure criterion. The entire procedure is implemented in a numerical algorithm, which was successfully used for predicting the strength and R‐curve response of adhesively bonded double‐lap structural joints made of pultruded glass fibre‐reinforced polymers and epoxy adhesives.     

Unified characterisation of in‐plane and out‐of‐plane constraint based on crack‐tip equivalent plastic strain

Constraint can be divided into two conditions of in‐plane and out‐of‐plane, and each of them has its own parameter to characterize. However, in most cases, there exists a compound change of both in‐plane and out‐of‐plane constraint in structures, a unified measure that can reflect both of them is needed. In this paper, the finite element method (FEM) was used to calculate the equivalent plastic strain (ɛ_p) distribution ahead of crack tips for specimens with different in‐plane and out‐of‐plane constraints, and the FEM simulations based on Gurson–Tvergaard–Needleman (GTN) damage model and a small number of tests were used to obtain fracture toughness for the specimens with different constraints. Unified measure and characterisation parameter of in‐plane and out‐of‐plane constraints based on crack‐tip equivalent plastic strain has been investigated. The results show that the area A_PEEQ surrounded by the ɛ_p isoline ahead of crack tips can characterize both in‐plane and out‐of‐plane constraints. Based on the area A_PEEQ, a unified constraint characterisation parameter A_p was defined. It was found that there exists a sole linear relation between the normalised fracture toughness J_IC/J_ref and regardless of the in‐plane constraint, out‐of‐plane constraint and the selection of the ɛ_p isolines. The unified J_IC/J_ref−reference line can be used to determine constraint‐dependent fracture toughness of materials. The FEM simulations with the GTN damage model (local approach) can be used in obtaining the unified J_IC/J_ref−reference line for materials with ductile fracture.     

玻璃钢管的长期刚度测量及预测

为建立玻璃钢管长期刚度理论模型并对其长期刚度进行预测。首先通过自主设计的恒位移加载试验装置,在不同初始挠度的恒位移条件下对纯环向缠绕和纯交叉缠绕铺层的玻璃钢管开展了长期刚度试验研究;然后,在试验基础上建立了不同初始挠度下玻璃钢管刚度与时间的折线双对数回归模型、50年后刚度降幅与初始挠度的线性回归模型,进而提出了玻璃钢管刚度降幅关于时间与初始挠度的二次曲面预测模型;最后,预测了玻璃钢管50年后的剩余刚度,研究了时间和初始挠度对玻璃钢管刚度的影响。结果表明:纯环向缠绕铺层的玻璃钢管抵抗刚度衰减的能力明显优于纯交叉缠绕铺层的玻璃钢管,纯环向缠绕铺层能有效提高玻璃钢管的刚度及其抵抗径向变形的能力,纯环向缠绕铺层的玻璃钢管有较好的长期力学性能。时间为8 313.2 h的测试数据表明提出的玻璃钢管刚度降幅预测模型具有较高的精度和较强的实用性。      

Perspective Measurement of the Out‐of‐plane Displacement and Normal Strain Field Distributions Inside Glass Fibre‐reinforced Resin Matrix Composite

A depth‐resolved wavenumber‐scanning interferometer (DRWSI) was built up to measure the out‐of‐plane displacement and normal strain field distributions on the front surface, rear surface and internal glass fibres of a glass fibre‐reinforced resin matrix composite before and after loading. Series of the fringe patterns were recorded, while the wavenumber of the laser, monitored online by an optical wedge, was scanned by tuning the temperature. Random sampling Fourier transform is used to overcome the non‐linearity of the wavenumber series. In the end, the distributions of the out‐of‐plane displacements and normal strain field are presented as the applied loads were 10 µm, 20 µm and 30 µm, respectively. In conclusion, DRWSI is a suitable method to measure the mechanical properties inside resin composite non‐destructively.     

Mode I fracture toughness of the thermally pretreated red Verona marble by means of the two‐parameter model

The paper aims to analyse the effects of pretreatment thermal cycles on both mechanical and fracture parameters of the red Verona marble, which is a natural stone of sedimentary formation. The effects of the thermal pretreatment, consisting of freeze/thaw cycles and simulating the atmospheric ageing on the material, are evaluated in terms of changes of the aforementioned parameters. Note that a wide variety of both specimen types and methods to determine mode I plain strain fracture toughness of rocks are available in the literature. The two‐parameter model originally proposed for plain concrete is herein adopted. Such a method, based on the experimental data obtained from three‐point bending tests on single edge‐notched specimens, is able to take into account the slow nonlinear crack growth occurring before the peak load, typical of quasibrittle materials, and presents the advantages of easy specimens preparation and simple test configuration.     

Interlaminar Fracture Toughness of Carbon Fibre‐Reinforced Polymer Laminates With Nano‐ and Micro‐Fillers

Abstract: Vapour growth carbon nanofibres (CNF) and lead zirconate titanate (PZT) piezoelectric particles were added in the matrix of carbon fibre‐reinforced polymer laminates. The fracture toughness of the modified composites was measured under mode I and mode II loading and compared with plain epoxy carbon fibre‐reinforced composites. The mode I fracture toughness of the composites improved with the incorporation of the carbon nanofibres and deteriorated with the incorporation of PZT piezoelectric particles. When both fillers were added in the composite matrix, the mode I fracture toughness improved but to a lesser extend. The mode II fracture toughness of the modified composites was improved in all three cases. The aforementioned behaviour was attributed to competing fracture mechanisms instigated by the different fillers, and backed by fractographic evidence from the failed composite coupons; during the tests, the acoustic emission activity of the coupons was monitored and classified in three major energy absorbing mechanisms which were attributed to the failure of distinct composite phases.     

玻璃钢约束阻尼复合结构的阻尼特性

以玻璃纤维增强树脂作为约束层主要材料、丁腈橡胶为阻尼层、钢板为基板制备约束阻尼复合结构, 运用动态黏弹谱仪和悬臂梁共振法, 研究温度、约束层刚度和阻尼层结构对约束阻尼复合结构减振效果的影响。结果表明:自由阻尼复合板的最大阻尼范围落在阻尼层的玻璃化转变区;玻璃钢约束层能将复合结构的阻尼拓展至阻尼层的高弹态区域, 增加阻尼层厚度可以提高约束复合板的阻尼性能;提高孔隙率同样有利于约束复合板阻尼性能提升;铝板约束层提升作用尤为显著, 然而在海洋环境、干湿交替等强腐蚀场合中, 铝板极易腐蚀而丧失约束功能, 因此在这类特殊场合下耐腐蚀的玻璃钢具有优势。      

Fatigue behaviour of the bond interface between carbon fibre‐reinforced polymer sheets and concrete

Externally bonded carbon fibre‐reinforced polymers (CFRPs) have been applied to retrofit and strengthen civil structures. In this study, four‐point bending beams were manufactured and tested to examine the fatigue behaviour of the CFRP–concrete interface. The results indicated that the specimens exhibited debonding failure in the concrete beneath the adhesive layer under static loading. However, when cyclic loads were imposed on the small beams, debonding failure may occur in the adhesive layer. Moreover, fitting expressions were proposed to predict the shear stress–slip relationship between the CFRP sheets and concrete and the flexural strength of the CFRP‐strengthened beams under static loads, and good agreement with the test data was obtained. Finally, a fatigue life prediction model was also presented to capture the fatigue life of the CFRP–concrete interface under cyclic loads. The calculation results showed that the fatigue strength of the CFRP–concrete bond interface was approximately 65% of the ultimate load capacity.     

不同纤维掺量下聚乙烯醇纤维增强工程水泥复合材料梁剪切韧性试验

为研究聚乙烯醇纤维增强工程水泥复合材料（PVA/ECC）无腹筋梁的剪切韧性,基于5组PVA/ECC梁受剪破坏试验结果,以剪切韧性指数和斜裂缝综合指数为指标,对不同纤维掺量下PVA/ECC梁的斜截面剪切韧性进行了研究与评价。结果表明:PVA纤维的掺入能改善梁的开裂性能,明显提高梁受荷全过程的变形能力及斜截面承载力,从而提高构件的剪切韧性;PVA纤维体积分数在0~2vol%范围内时,其值越大,加载过程中消耗的能量越多,斜截面抗剪承载力越高,破坏之前的总变形越大,梁的剪切韧性越好。      

Unified correlation of in‐plane and out‐of‐plane constraints with fracture toughness

In this paper, the specimens with different geometries and loading configurations were used to study the unified correlation of in‐plane and out‐of‐plane constraints with fracture toughness by using numerical simulation method. The results show that the unified constraint parameter A_p which was defined on the basis of the areas surrounded by the equivalent plastic strain isolines ahead of crack tip can characterise both in‐plane and out‐of‐plane constraints induced by different specimen geometries and loading configurations. A sole linear relation between the normalised fracture toughness J_IC/J_ref and was obtained. The J_IC/J_ref ‐ line is a unified correlation line of in‐plane and out‐of‐plane constraints with fracture toughness of a material, and the constraint dependent fracture toughness of a material can be determined from the unified correlation line. The results also demonstrate that the out‐of‐plane constraint effect is related to the in‐plane constraint effect, and there exists interaction between them. The higher in‐plane constraint strengthens the out‐of‐plane constraint effect, whereas the lower in‐plane constraint is not sensitive to the out‐of‐plane constraint effect.     

Synthesis and property of short tungsten fibre/Zr-based metallic glass composite

The short tungsten fibre reinforced Zr_41.2Ti_13.8Ni_10.0Cu_12.5Be_22.5 bulk metallic glass composites with macroscopic isotropic mechanical properties were prepared by infiltration and rapid solidification. The diameters of the tungsten fibres are 300, 500 and 700?µm with a length of 1000?µm and the fibre volume fraction in three kinds of composites is between 60 and 65%. The room temperature compressive deformation behaviours of these composites were investigated systematically. The results show that the strength and plasticity of the composites increase with the decrease in the tungsten fibre diameter. The maximum compressive strength and plastic strain of the composite with 300?µm fibre, respectively, reach 3079?MPa and 37%. The fracture modes of all the composites are shear fracture. The superior compressive property of the composites with short tungsten fibre is due to the competition among different fracture modes and the inhibition effect of the interface on the shear band extension.     

Behavior of a flat internal delamination within a fiber reinforced cross-ply composite

A penny-shaped delamination is modeled as a flat octahedral shaped crack between layers of a cross-ply laminate. The fibers of the laminate intersect the edges of the delamination at angles of 0°/90°, +45°/−45°, 90°/0° and −45°/+45° as one proceeds along the delamination edge. Two lay-ups are considered, a cross-ply consisting of two layers, and a symmetric composite, consisting of three layers. The delamination is always between two of the layers. Both tension and shear are applied to the outer boundary of the body. Stress intensity factors about the delamination edge are calculated by means of a conservative M-integral. These are employed to calculate the interface energy release rate and corresponding phase angles. Use is made of existing experimental results to predict the location of propagation along the edge of the delamination. It was found that the most dangerous regions along the delamination front occurred for the 0°/90° or 90°/0° interfaces.     

连续玻纤/PE增强复合带弹性参数计算方法

为研究连续玻纤/PE增强复合带的弹性性能,建立了连续玻纤/PE增强复合带模型,并对其材料及结构提出了基本假设,在此基础上采用理论公式法求解了复合带的弹性参数.建立了复合带在单向拉伸状态下的有限元模型,对玻纤与PE进行了位移耦合处理,求得了复合带沿玻纤长度方向的弹性模量,该结果与理论结果吻合很好.通过拉伸实验获得了复合带玻纤长度方向与带宽方向的弹性模量,带宽方向的弹性模量与理论结果吻合;而在玻纤长度方向上,由于加工工艺、材料状态等与假设条件之间的偏差,以及实验测试过程中存在的误差,其弹性模量与理论解、数值解相比偏低.     

玻璃钢纤维增强塑料薄壁管抗冲击性能的实验研究

本文采用分离式Hopkinson压杆（SHPB）实验系统,研究了玻璃钢纤维增强塑料（GFRP）薄壁管在低速冲击载荷作用下的抗冲击性能,探讨了薄壁管的截面形状和壁厚对其冲击破坏模式、动态应力-应变曲线和比吸能值（S_EA）的影响。实验结果表明：GFRP圆管的动态切线模量较方管的大,同壁厚的圆管的抗冲击性能较方管好;方管随壁厚的适当增加,抗冲击性能也增加。通过综合分析抗冲击性能评价参数,发现GFRP方管的吸能性能较圆管的好,且随壁厚的略微增加,吸能性能增强。与铝合金圆管相比,在相同实验条件下,GFRP圆管的动态压缩模量和冲击应力峰值较铝合金圆管大,峰值应变值较铝合金圆管小,比吸能值较铝合金圆管的大,GFRP管的抗冲击性能也较铝合金圆管好。其结果可为GFRP管类结构的优化设计及工程应用提供基础实验数据和给予理论指导。     

Mechanical behaviour and microstructural characterization of 3D four‐directional braided SiO2f/SiO2 composites

In this paper, SiO_2f/SiO₂ composites reinforced by 3D four‐directional braided quartz preform were prepared by the silica sol‐infiltration‐sintering method in a relatively low sintering temperature (450 °C). To characterize the mechanical properties of the composites, mechanical testing was carried out under various loading conditions, including tensile, flexural and shear loading. The microstructure and the fracture behaviour of the 3D four‐directional braided SiO_2f/SiO₂ composites were studied. The tensile strength, flexural strength and the in‐plane shear strength were 30.8 MPa, 64.0 MPa and 22.0 MPa, respectively. The as‐fabricated composite exhibited highly nonlinear stress–strain behaviour under all the three types of loading. The tensile and flexural fracture mechanisms were fully discussed. The fracture mode of the 3D four‐directional braided SiO_2f/SiO₂ composite in the Iosipescu shear testing was based on a mixed mechanism because of the multi‐directivity of the composite. Owing to low sintered temperature, the fibre/matrix interfacial strength was weak. The SiO_2f/SiO₂ composites showed non‐catastrophic behaviour resulting from extensive fibre pull‐out during the failure process.     

Effects of divider orientation split‐out on fracture toughness

The Split‐out phenomenon is a sudden instability, which takes place near the crack tip, and is generated by a rapid growth and arrest of a crack at the divider orientation. This phenomenon is related to stress triaxiality in the front of the crack tip when the structure is under plane strain conditions, plus the existence of weak interfaces oriented normal to the thickness direction. The weak interfaces can be generated as a result of some metallurgical process such as hot‐rolling lamination in materials with a high level of impurities, as well as steels where the structure results in a strong banding of ferrite and pearlite. When a cracked structure is loaded, the high σ_z stresses related to the plain strain state can lead to a split‐out brought about by delamination of the weak interfaces. During a fracture toughness test, this is noticed as a drop in the load‐displacement record, which is similar to that produced by the well‐known pop‐in instability in welded joints. Although the load drops are very similar in both phenomena, their etiologies are completely different, since the pop‐in is produced by an unstable crack growth and its ulterior arrest, due to local brittle zones located generally at the heat affected zone (HAZ) at or near a welded joint. As the split‐out has been less studied than the pop‐in, and due to the similarities between both load‐displacement records, it is very common to consider both instabilities under the same failure acceptance criteria. In this way, the split‐out is usually treated as a critical event, and therefore, many materials are rejected when split‐outs occur in fracture testings. However, when a split‐out occurs, the σ_z stress is drastically reduced, leading to a condition close to a plane stress situation (in fact, the σ_z stress reduces to 0 at the crack surfaces of the split‐out). The aim of this work was to study the effect of the split‐out in the fracture toughness of hot laminated steels working in the upper‐shelf region. In order to achieve this, the fracture toughness was evaluated by means of the resistance curve J‐R of the material, using a single specimen test method. The occurrence of load drop and displacement increase in the test record are explained based on a constant main plane crack length hypothesis.     

Delamination resistance of GFRP‐epoxy rods with nanoparticle‐ and microparticle‐modified matrix and its correlation with the fracture properties of epoxy nanocomposites

Pultruded glass fiber–reinforced plastic (GFRP) rods are industrially produced for various applications. The mechanical properties of GFRP rods can be tailored, eg, with modified epoxy matrix by adding microsized and nanosized fillers. In this work, we investigated the transferability of the fracture properties of epoxy nanocomposites to GFRP epoxy rods. For this purpose, nanosilica particles, micron‐sized rubber, and micron‐sized calcium carbonate were used as fillers. Firstly, epoxy plates with different combinations of nanosized and micron‐sized fillers were produced and their fracture toughness evaluated. Secondly, using binary and ternary epoxy filler systems as matrices, GFRP rods were produced, and their fracture toughness was determined using a modified testing procedure derived from standard composite double cantilever beam specimens. Lastly, the correlation between fracture properties of epoxy nanocomposites and GFRP rods was analysed. Besides that, the fractographic investigations of both epoxy nanocomposites and GFRP rods were carried out and discussed.     

玻璃纤维增强树脂基复合材料夹芯板-钢板连接接头的弯曲性能

以PVC泡沫或Balsa轻木为芯材的玻璃纤维增强树脂基复合材料（GRP）夹芯板目前广泛应用于船舶与海洋工程结构中。论文设计不同参数的GRP夹芯板-钢板混合接头模型,进行四点弯曲加载下的静力及疲劳试验研究,同时运用ABAQUS软件结合MSC.fatigue软件对接头的静态及疲劳弯曲失效进行数值模拟,分析了接头的弯曲强度、刚度和失效模式,并研究了接头填充区材料及长度、钢板嵌入填充区长度等参数对接头弯曲性能的影响。结果表明：弯曲载荷作用下接头破坏发生在连接结合部,失效模式则因填充区的不同设计而不同;对提高接头的弯曲性能较为明显的设计参数包括将钢板延伸到接头填充区或者选择Balsa轻木替代PVC泡沫芯材;对于受到疲劳弯曲载荷的接头模型,在较大疲劳载荷水平下,所有试件在未达到10⁶次循环时均发生了疲劳破坏;而在相对较小的疲劳载荷水平下,经过10⁶次循环后所有试件全部完好,并且接头的剩余强度与疲劳试验前的静强度相近,表明小载荷水平下接头的疲劳次数对其承载能力无影响。