Investigation of microstructure and mechanical properties of steel fibre–cast iron composites

Abstract

The aim of the present experimental study was to investigate improvement of the toughness and strength of grey cast iron by reinforcing with steel fibres. The carbon content of the steel fibres was chosen to be sufficiently low that graphite flakes behaving as cracks were removed by carbon diffusion from the cast iron to the steel fibres during the solidification and cooling stages. To produce a graphite free matrix, steel fibres with optimum carbon content were used and the reinforced composite structure was cast under controlled casting conditions and fibre orientation. Three point bend test specimens were manufactured from steel fibre reinforced and unreinforced flake graphite cast iron and then normalising heat treatments were applied to the specimens at temperatures of 800 and 850°C. The fracture toughness and strength properties of the steel fibre reinforced material were found to be much better than those of unreinforced cast iron. The microstructures of the composite at the fibre–matrix transition zone were examined.     

Effect of fibre curl on the properties of wood pulp fibre-cement and silica sheets

Curl has been induced in unbleached softwood kraft pulp fibres by treatment in the laboratory at 20% consistency in a planetary mixer. Steam treatment of the fibres to set the curl more strongly was found to be detrimental to fibre properties as deduced from handsheet properties. A means of producing two-ply specimens in the laboratory was devised and a tensile test for interlaminar bond strength was developed. The use of curly fibres in reinforced cement and silica sheets gave sheets with improved wet interlaminar bond strengths, relative to sheets prepared from conventionally treated fibres but had little effect on the values of modulus of rupture and fracture toughness.     

Flexural toughness characteristics of steel–polypropylene hybrid fibre-reinforced oil palm shell concrete

Hybridization of steel–polypropylene leads to improvements of both the mechanical and ductility characteristics of concrete. In this investigation, the effect of steel, polypropylene (PP) and steel-PP hybrid fibres on the compressive strength, tensile strength, flexural toughness and ductility of oil palm shell fibre reinforced concrete (OPSFRC) was studied. The comparison on the above said properties between the specimens prepared with crushed and uncrushed oil palm shell (OPS) as lightweight coarse aggregate was also carried out. The experimental results showed that the highest compressive strength of about 50 MPa was produced by the mix with 0.9% steel and 0.1% PP hybrid fibres. The highest increments in the splitting tensile and the flexural strengths of the OPSFRC were found up to 83% and 34%, respectively. However, the mixes with 1% PP fibres produced negative effects on both the compressive and tensile strengths. The results on the toughness indices showed that the OPSC possess no post-cracking flexural toughness. Though, the flexural deflection and toughness of the OPSC was significantly enhanced by the addition of fibres; the dominance of the steel fibre on the first crack flexural deflection and toughness of OPSFRC was evident. The mixes with 0.9% steel and 0.1% PP hybrid fibres reported the highest improvement in toughness index and residual strength factor.     

The effect of basalt fibres on fracture toughness of asphalt mixture

This paper deals with the effect of basalt fibres on fracture toughness of asphalt mixture. For this purpose, basalt fibres with three different contents (i.e., 0.1%, 0.2%, and 0.3% by weight of asphalt mixture) and lengths (ie, 4, 8, and 12 mm) are incorporated into asphalt mixture to prepare fibre‐reinforced asphalt mixtures. Fracture tests are then carried out on these mixtures under four different modes of loading (i.e., pure mode I, pure mode II, and two mixed modes of I/II) using semicircular bend (SCB) specimens. The results exhibit that the fracture toughness increases with the enhancement of the fibre content. In addition, increase in the length of basalt fibre results in reduction of the fracture toughness of asphalt mixture. However, the asphalt mixture containing 0.3% of basalt fibres with the length of 4 mm shows the highest fracture toughness compared with other mixtures. It is also found that the basalt fibre improves mode I fracture toughness of asphalt mixtures more significantly than mode II one. Statistical analysis is also performed on the experimental data. Analysis of ANOVA demonstrates that all the three factors investigated in this study (i.e., length of basalt fibre, content of basalt fibre, and mode of loading) have significant influence on the fracture toughness of asphalt mixtures.     

Mechanical properties of high strength concrete reinforced with metallic and non-metallic fibres

This paper focuses on the experimental investigation carried out on high strength concrete reinforced with hybrid fibres (combination of hooked steel and a non-metallic fibre) up to a volume fraction of 0.5%. The mechanical properties, namely, compressive strength, split tensile strength, flexural strength and flexural toughness were studied for concrete prepared using different hybrid fibre combinations – steel–polypropylene, steel–polyester and steel–glass. The flexural properties were studied using four point bending tests on beam specimens as per Japanese Concrete Institute (JCI) recommendations. Fibre addition was seen to enhance the pre-peak as well as post-peak region of the load–deflection curve, causing an increase in flexural strength and toughness, respectively. Addition of steel fibres generally contributed towards the energy absorbing mechanism (bridging action) whereas, the non-metallic fibres resulted in delaying the formation of micro-cracks. Compared to other hybrid fibre reinforced concretes, the flexural toughness of steel–polypropylene hybrid fibre concretes was comparable to steel fibre concrete. Increased fibre availability in the hybrid fibre systems (due to the lower densities of non-metallic fibres), in addition to the ability of non-metallic fibres to bridge smaller micro cracks, are suggested as the reasons for the enhancement in mechanical properties.     

Effect of variation in fibre volume fraction on modes I and II delamination behaviour of 5HS woven composites manufactured by RTM

Composites produced by resin infusion techniques will inevitably suffer from variation in resin distribution due to imprecise fibre placement and distortion of the preform during mould closure and infusion. This paper describes an investigation into the effect of variations in fibre volume fraction (FVF) on mode I and mode II delamination behaviour for 5 harness satin (5HS) woven carbon–fibre/epoxy resin composites manufactured by resin transfer moulding (RTM). Additionally, the effect of satin face tow orientation on interlaminar toughness was investigated. In mode I, it was found that toughness increased with increasing FVF and that a strong correlation between fracture surface damage and measured interlaminar fracture toughness was observed. In mode II, measured toughness values were higher than expected and tests were repeated using a mixed-mode rig with 5% mode I. It was found that fracture toughness measurements in pure mode II are significantly affected by friction or mechanical interlocking between the delamination surfaces.     

Round-robin analysis of the RILEM TC 162-TDF beam-bending test: Part 3—Fibre distribution

A round robin test programme was executed amongst several laboratories on the beam test recommended by the RILEM TC 162-TDF [1]. In the proposed test method, the mid-span deflection is to be measured on both sides of the beam (referred to as δ₁ and δ₂). A systematic fibre counting exercise was carried out on several beam specimens to investigate whether there is a correlation between differences between δ₁ and δ₂ and the fibre distribution. The findings of the investigation suggest that differences between δ₁ and δ₂ are not strongly linked with the fibre distribution regardless of concrete strength. It is likely that this phenomenon arises because the supports and loading points have enough degrees of freedom to accommodate any unevenness on the specimen surface. This reflects well on the robustness of the proposed test method as it means that the proposed boundary conditions are able to adapt and tolerate (to a certain degree) surface non-uniformity. However, it is also suggested that significant differences between δ₁ and δ₂ may be brought about by experimental errors. The fibre count also reveals that toughness increases with the number of fibres across the critical section.     

Strength and fracture properties of industrially prepared steel fibre reinforced concrete

The paper reports on a study of steel fibre reinforced concrete (SFRC) which was prepared using normal industrial mixing, compaction and curing conditions. Both strength (compressive and tensile) and fracture (toughness measurements) characteristics have been investigated with test specimens prepared from 5 m long SFRC piles. The piles contained only steel fibre reinforcement and were manufactured in exactly the same way as ordinary piles.Slight differences in the tensile strengths (determined via torsion tests) were observed due to the existence of preferential fibre orientation. Flexural tests on notched beams (to evaluate fracture characteristics) produced a much more stable, reproducible, test than that observed for un-notched beams. Hence, it is concluded that the notched beam is a better geometry in terms of test stability and reliability. The results showed that tests specimens taken from industrially prepared SFRC displayed similar characteristics compared to that observed with test specimens prepared under laboratory conditions, with regards to the strength, fracture characteristics and, in particular, the variation observed.     

Distribution of steel fibres in rectangular sections

In this paper a calculation method is explained to predict the total number of fibres crossing a rectangular section. The largest part of the paper deals with the theoretical calculation of an orientation factor. The orientation factor is defined here as the average length of the projection on the longitudinal axis of all fibres crossing a section, divided by the fibre length. Once the orientation factor is found, a simple calculation gives the number of fibres crossing a crack. Since the proposed approach is to a large extent new, there is a need for verification with test results. For this reason a fibre counting was done on 107 Rilem beam specimens, involving different fibre types. The comparison with the calculated number of fibres shows that the model provides good predictions of the number of fibres crossing a section.     

Comparison of Interlaminar Fracture Toughness in Unidirectional and Woven Roving Marine Composites

This paper investigates the effect of fibre lay-up and matrix toughness on mode I and mode II interlaminar fracture toughness (G_Ic and G_IIc) of marine composites. Unidirectional and woven roving fibres were used as reinforcements. Two vinyl ester resins with different toughness were used as matrices. Results from both modes showed toughness variation that is consistent with matrix toughness. Values of G_Ic were not significantly influenced by fibre lay-up except at peak load points in the woven roving/brittle-matrix composite. Each peak load point, caused by interlocked bridging fibres, signified the onset of unstable crack growth. For unidirectional specimens, crack growth was stable and G_Ic statistically more reliable than woven roving specimens, which gave fewer G_Ic values due to frequent unstable crack growth. Mode II tests revealed that, except for crack initiation, G_IIc was higher in woven roving composites. This was due to fibre bridging, perpendicular to the crack growth direction, which encouraged stable crack growth and increased energy absorption. Mode II R-curves were obtained for the woven roving specimens. These R-curves provide additional information useful for characterising delamination resistance. The paper concludes that composites with woven roving fibres show similar mode I delamination characteristics to the unidirectional composites; but their mode II delamination characteristics, after crack initiation, are quite different.     

Effects of hybrid composition of LCP and glass fibres on abrasive wear of reinforced LLDPE

The hybrid of liquid crystalline polymer (LCP) fibres and glass fibres (GF) provide a combination of modulus and toughness to semi-crystalline linear-low-density-polyethylene (LLDPE). LCP and GF fibres reinforced composites were studied using two-body abrasion tester under different applied loads. Two sets of fibre reinforced LLDPE, 10 and 20 vol%, were investigated. The contents of LCP and glass fibres were varied as 25, 50, 75 and 100 vol% of overall volume of fibres in LLDPE. The effect of replacing glass fibre with LCP fibre on wear is reported. Wear loss increased with the applied loads and glass fibre contents in LLDPE. The replacements of glass fibres with LCP fibres improved abrasive wear resistance of composite. The composite containing 20 vol% of glass fibres in LLDPE showed the specific wear rate nearly double to that of LCP fibre reinforced LLDPE. Incorporation of LCP fibre improved wear resistance of glass fibre reinforced LLDPE. Worn surfaces were studied using SEM. Glass fibres were broken in small debris and removed easily whereas LCP fibres yielded to fibrillation during abrasive action. The overall wear rate was governed by the composition and test conditions.     

Plasma-treated ultra-high-strength polyethylene fibres improved fracture toughness of poly(methyl methacrylate)

Use of ultra-high-strength polyethylene (UHSPE) fibres in composites has been limited by problems with adhesion to the matrix. The present study presents a gas-plasma treatment of UHSPE staple fibres (Spectra 900 and 1000) to improve adhesion to poly (methyl methacrylate). The gases used were nitrogen, argon and carbon dioxide for treatment times of 0 and 1 min. No significant differences were observed in flexural stresses, and only modest improvements were seen in the flexural modulus and the stress-intensity factor by reinforcing the cements with surface-modified UHSPE fibres. At least a sixfold improvement in the toughness index (fracture energy) was observed by reinforcing the cements with carbon dioxide treated fibres with a fibre index, l/d, of 776 at 1 wt %. The fibres added reinforcement by a crack-bridging effect that significantly improved the toughness index. The plasma treatment altered the fibre surface sufficiently to cause significant differences in the susceptibility to plastic flow during the fibre pull-out process.     

Pre- and post-peak toughening behaviours of fibre-reinforced hot-mix asphalt mixtures

Recycled plastic fibre-reinforced hot-mix asphalt (HMA) mixtures have better fatigue resistance than plain HMA. The toughening effects of recycled plastic fibre-reinforced HMA were characterised using direct tensile loading tests. Adding a small quantity of recycled plastic fibres to HMA was found to significantly increase the mixture's fracture energy and toughness, which were calculated using the pre- and post-peak stages of tensile force–displacement curves. A theoretical model representing the pre-peak behaviour of fibre-reinforced HMA with direct tension-softening curves for various fibre contents is presented here. The enhanced toughness through post-peak analysis was also observed using toughness indices associated with fibre-bridging effect after the pre-peak composite stress. The pre-peak fracture energy model and post-peak toughness indices appeared to be governed by the direct tensile toughening of fibre-reinforced HMA's enhanced fibre-bridging effects. The pre-peak fracture energy model demonstrates the effect of fibre content on the strain energy density during the pull-out process within the pre-peak composite stress region. The maximum pre-peak fracture energy of a coarse-graded HMA mixed with recycled plastic fibres is achieved at a fibre content of 0.4% of the total weight of the HMA. The increases in the toughness indices within the post-peak composite stress region indicate that the fatigue resistance of fibre-reinforced HMA is at least 30% greater than that of control HMA.     

Assessment of fibre orientation in ultra high performance fibre reinforced concrete and its effect on flexural strength

Fibre distribution and orientation in a series of round panel specimens of ultra high performance fibre reinforced concrete (UHPFRC) was investigated using electrical resistivity measurements and confirmed by X-ray CT imaging. By pouring specimens in different ways, the orientation of steel fibres was influenced and the sensitivity of the electrical resistivity technique was investigated. The round panels were tested in flexure and the results are discussed in relation to the observed orientation of fibres in the panels. It was found that the fibres tended to align perpendicular to the direction of flow. As a result, panels poured from the centre were significantly stronger than panels poured by other methods because the alignment of fibres led to more fibres bridging the radial cracks formed during mechanical testing.     

Single fibre pullout tests on auxetic polymeric fibres

A novel route for production of auxetic fibres has been adapted from conventional melt extrusion techniques. These fibres were reproduced, characterised and tested, for the first time, to assess the potential of auxetic fibres as reinforcements in composite materials. Initial experimental work has included the embedding of single fibres in modified epoxy resin. Auxetic fibre specimens were then compared with conventional fibre specimens through a specially designed fibre pullout testing procedure. The auxetic specimens displayed superior anchoring properties. The average maximum force at de-bonding of the auxetic fibres (0.95 N) was observed to be over 100% higher than that for conventional ones (0.44 N) and the average energy required to fully extract the auxetic fibre from the modified resin was 8.3 mJ while the conventional fibre required only 2.5 mJ on average. The results indicate that composites employing auxetic fibres as the reinforcement phase will exhibit enhanced resistance to failure due to fibre pullout.     

The influence of fibre sizing on the strength and fracture toughness of glass fibre composites

The influence of the fibre/matrix interface strength on fibre cross-over bridging in a crack along fibres is investigated. Four different composite systems (commercial glass fibre with two different sizings and two matrix resins) resulting in strong and weak interfaces were manufactured. Their crack growth resistance during crack propagation with fibre bridging in a double cantilever beam specimen loaded with end moments was measured. Bridging laws were derived from the experimental results and correlated with the chemical interface characteristics and a micromechanical model. It was found that a strong interface provided higher transverse strength and crack initiation loads, while the weak interface exhibited higher toughness due to enhanced fibre bridging. Composites with different matrix resins showed large variations in bridging behaviour even if their transverse strength was similar.     

Processing property characteristics for long glass fibre reinforced polyamide

The influence of microstructure on the stiffness and toughness of ‘long’ glass fibre reinforced nylon 66 mouldings was studied. Using the results of previous work which established the influence of processing conditions on moulding microstructure, the effects of processing on resulting mechanical properties are inferred. Whilst impact toughness is shown to be insensitive to processing conditions, slow crack propagation toughness is greatest in situations where the processing produces a pronounced core region of long fibres orthogonally aligned to the direction of crack propagation. Model studies of the influence of microstructure on stiffness suggests little sensitivity of modulus to likely variations in the processing parameters.     

Work of fracture of fibre-reinforced polymers

The work of fracture has been measured by bending tests on notched specimens of graphite and glass fibre reinforced polyester resins. Fibre bundles were used to increase the effective fibre diameter and improve the uniformity of the fibre strength.The results indicated that very tough specimens could be produced by these means (fracture surface energies of up to 11 kg/mm) and that toughness was determined by the strength, modulus and diameter of fibre bundles, as well as the volume fraction of fibre bundles. Failure occurred by fibre fracture close to the matrix fracture surface, and the fracture-surface energy appeared to result from the relative movement between fibre bundles and matrix as the fibres bridging the crack were stretched within the matrix. The work of fracture correlated well with the fibre-matrix interfacial stress, calculated from the observed stress transfer length.     

Effect of polypropylene fibre reinforced mortars on steel reinforcement corrosion induced by carbonation

Low amounts of polypropylene fibres are added to concrete as a secondary reinforcement to control cracking. Whether this addition might improve the corrosion resistance of the concrete reinforcement by increasing the resistance to carbonation, via reducing penetrating paths, is the subject addressed in the present paper. Crack control by the fibres in plastic state mortars and crack evolution with time have been studied. Furthermore, the influence of crack width on the steel bar corrosion induced by carbonation has also been monitored. Circular specimens made of mortar have been employed in the experimental phase of the study, using a water/cement ratio of 0.50 and cement/sand ratio of 2/1. The polypropylene fibre content was 0% and 0.5% by volume. Low modulus polypropylene fibres may control the crack width in specimens subjected to inadequate curing conditions. No relationship between crack width and time for corrosion initiation has been observed. However, a beneficial effect of fibre addition on the corrosion rate was found.     

Effects of Fibre Geometry and Volume Fraction on the Flexural Behaviour of Steel‐Fibre Reinforced Concrete

Abstract: This work aims in studying the mechanical behaviour of concrete, reinforced with steel fibres of different geometry and volume fraction. Experiments include compression tests and four‐point bending tests. Slump and air content tests were performed on fresh concrete. The flexural toughness, flexural strength and residual strength factors of the beam specimens were evaluated in accordance with ASTM C1609/C1609M‐05 standard. Improvement in the mechanical properties, in particular the toughness, was observed with the increase of the volume fraction of steel‐fibres in the concrete. The fibre geometry was found to be a key factor affecting the mechanical performance of the material.