1068-1302/12/0102-0049 2012 Springer Science+Business Media, Inc. 49

Powder Metallurgy and Metal Ceramics, Vol. 51, Nos. 1-2, May, 2012 (Russian Original Vol. 51, Jan.-Feb., 2012)

NEW STRUCTURAL UNIDIRECTIONAL-FIBER MATERIAL

V. G. Borovik,1,2 O. N. Grigorev,1 and V. N. Subbotin1

UDC 666.193

Structural basaltic materials are analyzed to establish the relationship of their structure to strength and fracture resistance. It is shown that their strength is determined by the size of initial crack-like defects, and fracture resistance by the amount and extent of weak boundaries that are perpendicular to the growing main crack. Samples of one-component unidirectional basalt fiber material of channel structure were produced by hot pressing. It is shown how temperature and time under pressure influence the structure and fracture behavior of the material.

Keywords: basalt, hot pressing, structural material, fibrous structure, bending strength, fracture resistance.

INTRODUCTION

Basalts are among the most abundant minerals. The production of materials from various basalts is attractive because raw materials are readily available and there is no need for far-distance transportation, enrichment, and special preparation, etc. Moreover, basalts are one-component raw materials that require one-stage no-waste processing to become finished products or structural members [1]. Various basaltic materials are characterized by wide ranges of tensile/compressive strength.

Among the basaltic materials currently in use, those produced by casting followed by crystallization [1–5] have minimum strength (40–50 MPa). Cast basalt has preferentially crystalline structure. Rapid controlled-cooling crystallization leads to a fine-grained randomly oriented microstructure. In zones of slow crystallization, nonequiaxed grains extended along the temperature gradient form a preferentially oriented structure. These grains are elongated and approximately perpendicular to the surface. The boundaries of these grains may be equivalent to the initial cracks perpendicular to the surface of a casting.

Despite low fracture toughness (KIc) and specific fracture energy (as those of other glass and glass-

ceramics), cast basalts are used to fabricate pipelines for conveying loose abrasive compounds, special facing tiles, etc. Cast basalt products are serviceable at temperatures ranging of 200 to 600°C. These products have high corrosion and wear resistance, compressive strength, and hardness. It is clear that the bending and tensile strengths are determined by grain size and imperfection in the surface layer of a casting. Defects in this layer are caused by the residual compressive stresses induced near the surface of a casting cooled at highly different surface and bulk temperatures.

1Frantsevich Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kiev, Ukraine.

2To whom correspondence should be addressed; e-mail: v_borovik@inbox.ru.

Translated from Poroshkovaya Metallurgiya, Vol. 51, No. 1–2 (483), pp. 65–74, 2012. Original article submitted July 26, 2010.

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Thin amorphous basalt fibers [6] are known to have maximum strength (to 2 GPa). They are produced by high-speed drawing of basalt melt through bushing nozzles. Fibers have almost no initial crack-like defects due primarily to the absence of crystalline structure and weak grain boundaries that are not parallel to the fiber axis. This is why fibers have so high strength despite the low crack resistance of basalt, as well as other glasses. Basalt fibers are used in composition with epoxy and other binders to fabricate various basalt plastic products such as sheets, coats, gas cylinders, reinforcements for concrete, etc. The volume fraction of fibers in unidirectional composites with maximum strength is from 40 to 60%. Therefore, the tensile strength of unidirectional basalt fiber reinforcement reaches 800–900 MPa. As other fibrous composites, basalt plastics have high fracture toughness and specific fracture energy due to the presence of many weak grain boundaries perpendicular to the growing main crack.

When used in structures that are not required to be resistant to high temperature (road, bridges, coastal protection, etc.), glass- and basalt-plastic reinforcement considerably surpasses ordinary steel reinforcement in corrosion resistance, strength, and cost per unit weight. The fire-resistance requirement for industrial, office, and residential buildings, hospitals, and garages, however, rules out the use of polymer-containing materials as concrete reinforcement. The standards [7, 8] require that any replacement of steel reinforcement do not reduce the fire-resistance of structures. It is also required that reinforcement do not lose its load-bearing capacity below 550–600°C. To meet this requirement, the temperature of steel reinforcement must not exceed 593°C in ordinary concrete and 426°C in prestressed concrete [9]. This means that any other reinforcement must remain strong at the same temperatures.

Note that the heat resistance of cast one-component basalt materials is consistent with the requirements for concrete reinforcement. However, the brittleness and low strength of these materials do not allow extending their field of application, including as reinforcement.

Comparing basaltic materials with different structures shows that their strength can be increased by providing unidirectional fiber structure without the need to use other components (binder, matrix, etc.).

The objective of this paper is to demonstrate the possibility of producing one-component structural basaltic material with unidirectional fiber structure and high fracture resistance and bending (tension) strength and to study the influence of various technology factors on its structure and fracture behavior.

MATERIAL AND PRODUCTION TECHNOLOGY

The material was produced as follows. An RB-12 basalt fiber roving (made by the BAVOMA Scientific and Technological Center of the Institute for Problems of Materials Science) was used to form a tow 10 mm in diameter. It was made highly unidirectional by combing along the entire length. The fibers were tied together with a fine cotton thread every 5 mm along the tow length. Then the tow was cut into fragments, each 45 mm long, that were laid into the impressions of a graphite mould. The hot-pressing direction was perpendicular to the tow.

The material was produced by hot pressing in CO2 atmosphere at temperature 680–800°C and pressure 25–

30 MPa for different periods of time (to 40 min). The temperature of the mould chase was monitored with an optical pyrometer and chromel–alumel thermocouple.

RESULTS AND DISCUSSION

The hot pressing of the fiber tow restrained against elongation causes the viscoplastic deformation of the fibers in the cross-section of the tow, which is plane strain conditions. Modeling the distortion of the fiber cross-sections due to plastic deformation [10–12] shows that the material flows from the near-surface contact areas between fibers to near-surface areas forming pore channels. Figure 1 shows plastic-strain intensity (PSI) fields of fiber cross-section for two cases of position relative to the pressing direction and different values of relative density () and maximum PSI (max) reached during the process. Note that the same color in different PSI fields

corresponds to different PSI levels.

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a b c d

e f g h

Fig. 1. Plastic-strain intensity fields (max) in a fiber tow pressed to different values of relative density

(): max = 0.137 (a), 0.237 (b), 0.404 (c), 0.740 (d), 0.263 (e), 0.373 (f), 0.832 (g), and 2.656 (h); =

= 0.915 (a, e), 0.933 (b, f), 0.970 (c, g), and 1.0 (d, h)

The outer rectangular boundary (Fig. 1a–h) indicates the region occupied by a unit cell of the undeformed (before pressing) material. Each boundary surrounds a PSI field whose relative size is commensurable with the unit cell after pressing. Figure 1a–d shows the unit cell oriented so that it is uniaxially compressed along its long side (external pressure is applied to the short side). This orientation of the cell models the formation of contact areas whose normals make angles of 30 and 90° to the pressing direction. Figure 1d–h shows the unit cell oriented so that it is uniaxially compressed along its short side (external pressure is applied to the long side). This orientation of the cell models the formation of contact areas whose normals make angles 0 and 60° to the pressing direction.

Thus, the PSI fields in Fig. 1a–d show the development of plastic zones near the contact areas oriented at angles 30 and 90° to the pressing direction. The PSI fields in Fig. 1d–h show the pattern of plastic strains near the contact areas oriented at 0 and 60° to the pressing direction.

The width of the contact areas between fibers is bounded on two sides by pore channels. The cross sections of the channels have the form of a hypocycloid (Fig. 2).

The curved surfaces (of the first order) on which the shear stresses in fiber cross-section are maximum (slip lines [10]) shift during densification. After the relative density becomes higher than 0.95, the entire surface of the fiber undergoes plastic deformation. After that, new shear planes in fiber cross-section intersect those occurred earlier in the surface layer, thus promoting mixing (fragmentation) of the surface layers of the fiber. Due to the plane strain along the fiber length, this mixing (fragmentation) occurs uniformly and simultaneously (in modeling) along the whole length of the fiber, in a plane perpendicular to its axis. This effect has a potential to form a substructure located in the surface layers of each fiber and consisting of essentially nonequiaxed elements similar to fibrils. This substructure is parallel to the fiber and increases its fracture resistance if the bond among its elements is sufficiently weak.

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Fig. 2. Pore channel with a cross-section similar to the three-cusp hypocycloid on the fracture of the sample

In [13], it was shown how the effective elastic characteristics of materials produced by plastic pressing a fiber tow are related to the geometrical parameters of its microstructure in the case of perfect contact between fibers.

Figure 3 shows the variation in temperature during pressing. The samples are numbered in such a way that the process number is indicated before the decimal point and the sample number in the mould after the point. To assess the sensitivity of acoustic methods to the parameters of hot pressing, we measured the speeds of sound in fiber cross-section in the direction of pressing (ch) and in the transverse direction (cb). The values of sound speed

for each sample are collected in tables in Fig. 3. It can be seen that the speed of sound in both directions increases as the temperature increases from 700 to 800°C and the pressing duration increases from 15 to 45 min. Moreover, there is anisotropy in the speed of sound in the pressing direction and in its plane in some samples.

Figure 4 shows load–deflection curves in three-point bending tests of samples without notch. Note that all samples were not parted in the tests. This is seen from the points on the load–deflection curves that correspond to unloading. The values of strength indicated near the peaks were calculated on the assumption that the samples are

Fig. 3. Variation in temperature during pressing of samples 2, 3, 4, and 7

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isotropic and linear elastic, i.e., these values are estimated. Noteworthy is similar maximum loads on the left- and right-hand sides of Fig. 4. This suggests that minor changes in pressing conditions (temperature and duration) within the indicated ranges do not cause noticeable reduction in the strength of fibers.

Quasiductile fracture behavior (Fig. 4, samples 2 and 4.2) may be attributed to the transformation of the main crack perpendicular to the fiber into a set of longitudinal and transverse shear cracks along channels. Calculating the stress intensity factors (SIFs) for cracks in a material with parallel pore channels with cusped cross-sections (Fig. 2) shows that as the main crack approaches the channels, the SIF at the fronts of channel cracks reaches levels comparable with the SIF at the front of main cracks [14]. This causes fragmentation of the front of the main crack into longitudinal and transverse shear cracks along the fibers. When displacing relative to each other, the edges of these cracks do much work against friction, which is responsible for the quasiductile fracture behavior of the material (Fig. 4, samples 2 and 4.2).

Comparing the curves (Fig. 4) and the speeds of sound in the respective samples (tables in Fig. 3) shows that the fracture energy (area under the load–deflection curve) correlates with the speed of sound in the cross-section of the fibers. Recall that a decrease in the speed of sound is equivalent to a decrease in the elastic modulus in the same direction, which is because of the degradation of mechanical contact (smaller contact areas and worse contact quality) between fibers.

Figure 5a, b presents photos of the lateral surfaces of middle parts of samples 2 and 3 after three-point bending tests. They demonstrate microstructural signs and causes of quasiductile fracture behavior. No traces of the main crack are observed on the lateral surface of sample 2 whose load–deflection curve is shown in Fig. 4. In sample 3, the main crack transformed to longitudinal and transverse shear cracks along the fiber boundaries only for a very short (finite) period of its growth. Therefore, its load–deflection curve is typical of brittle materials everywhere, except for the section where the crack stopped. Note that the transformation of the main crack to longitudinal and transverse shear cracks along the boundaries of fibers is responsible for their pulling out.

The fracture fragments of samples 4.2 and 7.3 (Fig. 5c, d) indicate that the pressing duration does not affect the morphology of the fiber cross-section. However, having almost identical microstructural morphology, these samples considerably differ in fracture energy (the area under the load–deflection curve in Fig. 4).

Noteworthy is the good agreement between the microstructural morphology of the material (Fig. 5c, d) and the results of modeling the distortion of unidirectional fibers during hot pressing involving plastic deformation [11,

Fig. 4. Load versus deflection in three-point bending tests

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a b

c d

Fig. 5. Lateral surfaces of samples 2 (left) and 3 (right) after tests and fragments of the fracture of samples 4.2 and 7.3

12]. Moreover, Tyrannohex silicon carbide material [15], whose production method involves hot pressing of piled sheets of Si–Al–C–O fibers, has a similar morphology.

CONCLUSIONS

It has been shown that hot pressing of a tow of unidirectional basalt fibers at the softening temperature of basalt (about 700°C) produces a material consisting of densely packed faceted fibers and pore channels with hypocycloidal cross-sections. The structural morphology of the material is in good agreement with the results of modeling the plastic pressing of a fiber tow.

It has been established that an increase in pressing temperature from 700 to 800°C and in the time under pressure to 40 min leads to an increase in the elastic moduli in the cross-section of the fibers and to a decrease in the fracture energy along the fibers, the bending strength of the samples remaining almost the same.

We have obtained a one-component unidirectional basalt fiber material whose strength is twice as high as the strength of cast basalt. This material allows extending the application of basalt. It can be used as a structural material for making parts that function under tensile stresses that are higher than those typical of cast basalt.

ACKNOWLEDGEMENTS.

The authors thank N. P. Brodnikovskii for help in mechanical tests, A. Yu. Koval’ and V. B. Sobolev for help in electron microscopy.

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