Modeling formability of multiaxial warp knitted fabrics on a hemisphere

The aim of this paper is to describe a model for the prediction of the formability of a multiaxial warp knitted (MWK) fabric to a 3D surface. For this purpose, we first characterized in detail the forming behavior of MWK fabrics containing two bias inserting yarns (TBMWK fabric). Through experimental observation, it was found that the two bias inserting yarns always tend to gather along the weft direction. The angle between the two bias yarns has a linear relationship with the perpendicular distance from the measured points to the longitudinal axis of the hemisphere during forming process. The slope of this linear relationship is also linear with the magnitude of radius of the pressing hemisphere provided that the radius is larger than 7 cm.Based on the above finding, a mathematical model is established for predicting the deformations of TBMWK fabrics during the hemisphere-forming process. The shape of flat TBMWK fabric that can yield the corresponding hemisphere during the forming process as well as local deformations can be calculated through this model. The hemisphere-forming experiments show that the present model is workable and accurate. The results from both the model and experiments suggest that the shape of flat TBMWK fabric that can yield the corresponding hemisphere which is close to a rectangular, not to a square as presented by woven fabric.The method developed in the paper has laid a foundation for further modeling of the forming behavior of MWK fabrics onto other 3D surfaces. More importantly, it is of great value to find that the two bias inserting yarns always tend to gather along the weft direction of the fabric which is a starting point for modeling of the forming behavior of MWK fabrics.     

经编间隔织物增强柔性复合材料冲击性能

针对传统个体防护材料刚硬、限制人体活动的缺点,设计了2种柔性复合材料: 剪切增稠液(STF)和硅橡胶填充经编间隔织物(WKSF)柔性复合材料,并对其冲击性能进行研究。WKSF具有上、下两个表层和间隔丝构成的间隔层,在其间隔层中加入STF和硅橡胶2种柔性材料。STF采用将纳米SiO₂分散于聚乙二醇(PEG)中制成,硅橡胶采用硅胶和固化剂混合而成。采用流变仪对STF的流变性能进行测试,采用Instron落锤冲击仪对WKSF及其复合材料的冲击性能进行测试。实验表明: STF在达到临界剪切速率后出现剪切增稠现象,纯织物的冲击过程可分为弹性、平台和压实3个阶段,且具有明显的平台阶段;经填充后所制成的2种复合材料的冲击过程与纯织物明显不同,其载荷-位移曲线呈线性;加入硅橡胶的复合材料刚度较大,没有应变率效应;加入STF的复合材料具有较好的能量吸收性能和明显的应变率效应。     

Mechanically pulped sisal as reinforcement in cementitious matrices

The performance as reinforcement of fibres obtained from commercial and by-product sisal (Agave sisalana) by thermomechanical pulping and chemi-thermomechanical pulping (CTMP) processes was investigated. Ordinary Portland cement (OPC) and chemically activated blast furnace slag (BFS) were examined as binders. The flexural strengths of OPC- and BFS-based composites incorporating 8% fibre reinforcement by mass were similar at 28 days and ranged from 18 to 22 MPa. Corresponding modulus of elasticity values were in the region of 11 GPa for the OPC-based composites and 7 GPa for the BFS-based composites. Water absorption values at 8% fibre content lay in the range of 21–31% by mass and density values in the region of 1.5 g/cm³. Fracture toughness increased with fibre content, reaching a value of 1.6 kJ/m² at a content of 12% in the case of by-product sisal CTMP in the BFS matrix. Scanning electron microscopy provided interfacial bonding information that can be related to the mechanical performance of these fibre-reinforced pastes.     

Fabrication and mechanical behaviors of cementitious composites reinforced by 3D woven lattice sandwich fabrics

Glass fibers were firstly woven to form three-dimensional (3D) woven lattice sandwich fabrics (WLSFs) which then were applied to reinforce cementitious foams and mortars to fabricate novel ductile cementitious composites. Failure behaviors of WLSF reinforced cementitious composite structures were studied through compression and three-point bending experiments. The WLSF greatly enhances the strength of cementitious foams at a level of four times. For cementitious mortars, compression strength of WLSF reinforced blocks is a little greater for the fraction of the textile is small as well as the compression strength of the textile pillars is not strong. But in flexure, excellent stretching ability of the glass fiber textiles greatly improves the flexural behavior of WLSF reinforced cementitious composite panels. Load capacity and ultimate deflection of these composite panels were greatly enhanced. Flexural capacity of the WLSF reinforced beam is four times greater. Reinforced by WLSF, failure of the cementitious composite is ductile.     

Split Hopkinson pressure bar testing of 3D multi-axial warp knitted carbon/epoxy composites

High-strain-rate compression experiments were performed on 3D MWK carbon/epoxy composites with different fiber architectures at room and elevated temperature using an SHPB apparatus. Macro-fracture and SEM micrographs were examined to understand the failure mechanism. The results show the dynamic properties increase with the strain rate and show a high-strain-rate sensitivity. Meanwhile, composites with [0°/0°/0°/0°] have higher properties. Moreover, composites show temperature sensitivity and the properties decrease significantly, especially for composites with [0°/90°/+45°/−45°]. The results also indicate composites take on more serious damage and failure with the strain rate. The failure of composites with [0°/0°/0°/0°] behaves in multiple delaminating, overall expansion and 0° fibers tearing. While that of composites with [0°/90°/+45°/−45°] is mainly interlaminar delaminating, local fibers tearing and fracture on different fiber layers. In addition, with increasing the temperature, the composite shows less fracture and becomes more plastic. The damage of matrix yielding, interface debonding and twisting of fibers increase significantly.     

Design carboxymethyl cotton knitted fabrics for wound dressing applications: Solvent effects

Carboxymethyl cotton knitted fabrics (CM-CKFs) for wound dressings were fabricated with different solvents: water, ethanol–water, and isopropanol–water. The FTIR analysis showed that carboxymethylation reaction in the CM-CKFs occurred with the different solvents. With the water, ethanol–water and isopropanol–water order and the decrease of water ratio in the mixed solution, the degree of substitution (DS) increased, the crystallinity index of the CM-CKFs decreased, crystal structure of cellulose changed from type I to II, and appreciable increased in fiber diameter and gel formation. The water absorbency and water retention of the CM-CKFs increased with DS. The water absorbency of the CM-CKFs treated in water, ethanol–water (v/v = 3/1), and isopropanol–water (v/v = 3/1) was 1.86, 6.95, and 14.86 g/g, respectively. The breaking force of the CM-CKFs was lower than that of cotton knitted fabrics, but the breaking force increased with the DS, so did the breaking elongation. The water vapor permeability and water diffusibility decreased with the DS. The results demonstrated that the carboxymethylation of CM-CKFs occurring in isopropanol–water was most effective and uniform. The findings have theoretical and practical significance for the industrial uses of carboxymethyl cellulose dressing.     

The effects of deforming knitted glass fabrics on the basic composite mechanical properties

The effects of deforming knitted fabrics on the tensile and compressive properties of their composites have been investigated for the weft-knit Milano rib fibre architecture. The properties have been studied for both the course and wale directions for composites with fabrics deformed in either of the two directions. It was found that any change in the mechanical properties of the deformed composites with respect to their undeformed counterpart is strongly related to the changes in the knit structure brought about by the induced deformation to the knitted fabric. Deformation in the knitted fabric also affects the tensile fracture mode whereby increased deformation, be it wale- or course-wise, transforms transverse fracture to shear fracture in either loading axis. On the contrary, the compressive fracture mode is insensitive to fabric deformation. Fractographic studies using stereo-optical and scanning electron microscopy have further revealed that tensile failure is caused by fibre breakages occurring at two locations of the knitted loops—one, at the leg components and, two, at fibre crossover points, whilst compression failure is controlled by Euler buckling of the looped fibres of the knitted composite. All these characteristics were revealed to be related to the microstructure of the knitted composite laminates.     

Accelerated ageing characteristics of glass-fibre reinforced cement made with new cementitious matrices

Glass-fibre reinforced cement composites have been made using new types of cement matrices and subjected to hot water immersion ageing regimes. The matrices were characterised by pore solution expression and analysis, XRD and DTA/TGA. Composite specimens were subjected to direct tensile tests. The new matrices displayed substantially reduced pore solution alkalinity and calcium hydroxide precipitation when compared with ordinary Portland cement (OPC), thus providing a less aggressive environment for the fibres. Mechanical tests showed that the rate of degradation of composites made from the new matrices, when aged at 65°C in water, was an order of magnitude lower than that for OPC matrix composites. The validity of this type of accelerated test is discussed.     

Effect of CF/GF fibre hybrid on impact properties of multi-axial warp knitted fabric composite materials

The multi-axial warp knitted fabric (MWK) is a useful reinforcement for composite. Higher mechanical performance resulted from no crimp of the fibre bundle is achieved compared with the general textile composites. For the fibre bundle of MWK, only one type of reinforcement fibre among glass, carbon fibre bundle, and so on has been selected. The mechanical properties and the cost of MWK composite depend on the feature of the fibre bundle. In order to extend the applicability of composite, the concept of “fibre hybrid composite” was applied into the MWK composite. Two kinds of fibre bundle; carbon and glass, were used in the 0/90 multi-axial warp knitted fabric. As the fibre hybrid composite; the inter-layer hybrid composite in which each layer was fabricated by carbon and glass fibre bundle respectively was investigated. Impact properties of composite were investigated by using drop weight impact tests. In case of unsaturated polyester, total energy and progressive energy of inter-layer fibre hybrid composite realized the highest value in all specimens. However, in case of epoxy resin, inter-layer hybrid composite didn’t realize the highest value in all specimens. The difference in energy absorption capability could be described with the fracture mechanism.     

Experimental study on the tension fatigue behavior and failure mechanism of 3D multi-axial warp knitted composites

An experimental study is described in this paper dealing with the tension–tension fatigue and failure mechanism of 3D MWK composites with different fiber architectures and material sizes. Macroscopic fracture morphology and SEM micrographs are examined to understand the fatigue damage and failure mechanism. The results show the fatigue properties and failure mechanism of composites can be affected significantly by the fiber architecture and material size. The fatigue life of material A(0°/0°/0°/0°) with small fiber orientation angle is significantly longer than that of material B(+45°/−45°/+45°/−45°). For material A, the fatigue properties of the long composite are better than that of the short one. It is 0° fiber bundles fracture under fatigue stress which cause the material failure and the long composite provides more space for the formation and propagation of local fatigue micro-cracks. However, for material B, the short composites have better fatigue properties. Moreover, the materials show typical ±45° zigzag fatigue fracture and obvious shear behavior. The fatigue cracks for the long composite can be spread more quickly along the fiber/matrix interface due to the fiber bundles realignment.     

Development of durable cementitious composites using sisal and flax fabrics for reinforcement of masonry structures

Natural fibres are one of the most studied materials. However, the use of these fibres as reinforcements in composite materials for structural applications, especially for existing or historical masonry structures, remains a challenge. In this study, efforts were made to develop sustainable composites using cementitious matrices reinforced with untreated bi-directional fabrics of natural fibres, namely, flax and sisal fibres. The fibres were mechanically characterised by tensile tests performed on both single yarns and fabric strips. Ageing effects due to fibre mineralisation in alkaline cement paste environments may cause a reduction in the tensile strength of natural fibres. The matrices used to study fibre durability were a natural hydraulic lime-based mortar (NLM) mix with a low content of water-soluble salts and a lime-based grouting (NLG) mix containing natural pozzolans and carbonated filler. Tensile tests on impregnated single yarns subjected to wetting and drying cycles by exposure to external weathering were conducted at different ages to quantify these problems. Composite specimens were manufactured by the hand lay-up moulding technique using untreated fibre strips and an NLG matrix. The mechanical response of natural fibre reinforced cementitious (NFRC) composites was measured under tension, and the effect of the matrix thickness was also addressed. Both sisal and flax fibres showed good adhesion with the NLG matrix, making them capable of producing composites with ductile behaviour and suitable mechanical performance for strengthening applications in masonry structures.     

Mechanical properties of PVC coated bi-axial warp knitted fabric with and without initial cracks under multi-axial tensile loads

The paper presents a study on the mechanical properties of PVC coated bi-axial warp knitted fabric with and without initial cracks under multi-axial tensile loads. The mechanical behaviors of the same coated fabric under mono-axial and multi-axial tensile testing conditions are compared. The influences of the initial crack length and orientation on the mechanical performance are analyzed. It is found that while the coated fabric demonstrates anisotropic properties under mono-axial tensile condition, the same fabric rather behaves more isotropic under multi-axial tensile loads. It has also found that the mechanical performance decreases with increase of the initial crack length for a given crack orientation, and that the initial crack orientation perpendicular to a tensile direction has a maximal effect in the reduction of the mechanical performance in this direction. The crack propagation is always progressively along the weft direction of the fabric under multi-axial tensile condition no matter how the initial cracks are orientated.     

Optimization of the solidification/stabilization process of MSW fly ash in cementitious matrices

The solidification/stabilization (S/S) process of municipal solid waste (MSW) fly ash in cementitious matrices was investigated in order to ascertain the feasibility of a washing pretreatment of fly ash with water as a means of maximizing the ash content of cementitious mixtures. Four types of fly ash resulting from different Italian MSW incineration plants and ASTM Type III Portland cement were used in this study. Ash-cement mixtures with different fly ash/cement (FA/C) ratios were made using untreated and washed fly ash. Washing of fly ash with water was realized by a two-stage treatment (liquid/solid=25; mixing time=15 min for each stage). The cementitious mixtures were characterized for water demand, setting time, mechanical strength, and heavy metals leachability. Comparison between the above properties of mixtures incorporating untreated and washed fly ash (particularly, setting characteristics), coupled with economical evaluation of the S/S process when applied to untreated and washed fly ash, proved the feasibility of washing pretreatment as a means of maximizing the incorporation of MSW fly ash in cementitious matrices (ash content up to 75%-90% by weight of total solid).     

Influence of properties at micro- and meso-scopic levels on macroscopic level for weft knitted fabrics

Knitted fabrics and particularly weft knitted fabrics are used as composite material reinforcements due to their ability to be draped and to give three-dimensional shape by molding or by knitting. This paper presents the strong connection of all the scales of the knitted fabric (fiber, yarn and fabric) on the final knitted fabrics and its mechanical and physical properties. For this purpose, only one polymer material is used, made of two different fibers in terms of length and fineness. These fibers are used to make different yarns with two structures then three plain-weft-knitted-fabrics are considered in terms of the loop length. The fibers have not the same bending rigidity because fiber cross-section areas are different. This has an influence on the three-dimensional loop shape and on the roughness, thickness and real area of contact of fabrics. This phenomenon is the same with the two yarn structures. The results presented here bring into light that the loop length does not influence the fabric thickness.     

Early-age autogenous cracking of cementitious matrices: physico-chemical analysis and micro/macro investigations

High-performance cement-based materials, characterized by low water-to-cement (W/C) ratio and high cement content, are sensitive to early-age cracking because their autogenous shrinkage rate and magnitude are particularly high during this period. This article firstly presents experimental tools especially designed for the measurement of free and restrained autogenous shrinkage at early-age. Then, the results of a multi-parameter experimental study conducted on three different types of binder are analyzed. The physico-chemical deformations of cement pastes and mortars were measured from the very early-age up to several days in saturated and autogenous conditions to investigate the effects of binder, water-to-binder ratio, presence of aggregates and temperature on the driving-mechanisms leading to early-age autogenous cracking. Complementary tests such as hydration rate measurement and microscopic observations were also performed. Among the three binders used, the blast furnace slag cement shows higher chemical strain, for a given quantity of chemically-bound water, and higher early-age autogenous shrinkage. The presence of aggregates generates a local restraining effect of cement paste deformations, leading to the formation of microcracks in the surrounding cement paste. Ring test results reveal that the first through crack of cement pastes systematically appears for maximal internal stress values lower than the material tensile strength, estimated with three-point flexural tests. This phenomenon may be due to diffuse damage of the cementitious matrix, whose deformations are partially restrained.     

Novel cement matrices by accelerated hydration of the ferrite phase in portland cement via chemical activation: kinetics and cementitious properties

The effects of the admixture of potassium citrate and potassium carbonate to portland cement with and without gypsum on the kinetics of the dissolution of the clinker phases, formation of hydration products, and cementitous properties in ISO-mortar and concrete were investigated. Quantitative X-ray diffraction (QXRD) data indicate that the admixture of citrate to portland cements increases the dissolution rate of the ferrite phase. In the presence of 1 to 3% potassium citrate, the ferrite phases dissolve nearly completely within 6 hours. Citrate is assumed to act via surface complexation and ligand-promoted dissolution. Set and hardening coincides with the formation of AFm phases. Semiquantitative background evaluation of XRD spectra indicates that citrate promotes the formation of noncrystalline phases, in which iron(III)hydroxide presumably forms the polymeric backbone whereas carbonate stabilizes crystalline AFm areas. Optimum early strength in ISO-mortar and concrete is correlated with the formation of crystalline and noncrystalline phases.     

The effect of high salt concentration on the integrity of silica-fume blended cementitious matrices for waste immobilization applications

Silica Fume is a commonly used pozzolanic additive for cementitious matrices used for immobilization of Low Level Waste (LLW). Cementitious systems containing silica-fume are used to reduce the leachability of various hazardous species. However, during the last years several publications have shown that commercially available densified silica-fume (DSF) does not fully disperse within cementitious pastes and concrete mixes, but rather tends to form agglomerated particles which range in size from tens to hundreds of microns. Cementitious matrices containing such agglomerates are prone to the alkali-silica reaction (ASR). As radioactive waste streams often contain high alkali salt concentrations, the occurrence of ASR, deleterious osmotic pressure or other degradation mechanisms in cementitious waste matrices must be considered. The aim of this research was to study the effect of high salt content in DSF bearing pastes on the integrity of the immobilized waste form and its efficiency to immobilize low level radioactive waste. The dependence of matrix integrity on both salt and silica fume concentration is presented.     

Reinforcement effects of polyvinyl alcohol and polypropylene fibers on flexural behaviors of sulfoaluminate cement matrices

The fracture behavior of unoiled/uncoated polyvinyl alcohol (PVA) fiber reinforced sulphoaluminate cement (SAC) matrices was experimentally investigated and compared with those of polypropylene (PP) fiber reinforced SAC and PVA fiber reinforced Portland cement (PC) matrices in this study. In the experimental investigation, three-point bending tests were carried out for notched fiber reinforced cement beams. Special attentions were paid on their deflection-hardening and multiple crack patterns. The different flexural behaviors between the plain SAC and PC matrices were evaluated using the double-K fracture model. The results indicate that the PVA fiber reinforced SAC matrices exhibited better flexural behaviors when compared with the PVA fiber reinforced PC matrix having comparable matrix strength. The bond strength between SAC matrix and PVA fiber are relatively better than that between the counterpart PC matrix and PVA fiber, while the bond strength between SAC matrix and PVA fiber is obviously stronger than that between the SAC and PP fibers.