Basic Properties of Glass Fiber

This article studies the basic properties of glass fiber, including appearance characteristics, density, tensile strength, the reason for the high strength of glass fiber, and the factors affecting the its strength.

(1) Appearance characteristics

Generally natural or artificial organic fibers have deep wrinkles on their surface. The surface of the glass fiber is a smooth cylinder, and its cross section is almost a complete circle, from a macro point of view, the surface is smooth, so the holding force between the fibers is very small, which is not conducive to bonding with the resin. Because it is a cylinder, the gap is more dense when the glass fibers are close to each other. It is beneficial to increase the glass content of FRP products.

(2) Density

The density of glass fiber is larger than that of other organic fibers, but it is lower than that of general metals, almost the same as aluminum. Therefore, it is possible to replace aluminum and titanium alloys with glass fiber reinforced plastic in the aviation industry. The density of glass fiber is closely related to the composition, generally about 2.5-2.7g/cm3, but the dense density of high-elastic glass fiber containing a large number of heavy metals can reach 2.9g/cm3, in general, the density of non-alkali fiber is larger than that of alkali fiber, see the following table

FiberWool silk cotton rayon nylon carbon fiberglass fiberglass fiber
1.28~1.331.3~1.451.50~1.601.50~1.601.141.4with Alkaliwithout Alkali


(3) Tensile strength
The tensile strength of glass fiber is dozens of times higher than that of glass with the same composition, for example, the tensile strength of alkali glass is only 40-100MPa, and the strength of glass fiber drawn with it can reach 2000MPa, and its strength is 20-50 times higher, as can be seen from the following table, the tensile strength of glass fiber is higher than that of high-strength alloy steel.

Wool flaxcotton silk nylon Alloy Steelaluminium alloyglassglass fiber 
fiber diamete
( μm)
Tensile strength(Mpa)160~300350300~700440300~600160040~46040~1201000~3000

1, the reason for the high strength of glass fiber
Many experts and scholars have put forward various hypotheses on the reasons for the high strength of glass fiber.
(1) Microcrack hypothesis
The microcrack hypothesis holds that the theoretical strength of glass depends on the gravitational attraction between molecules or atoms, and its theoretical strength is very high, up to 2000-12000MPa. However, the measured strength is very low, because there are micro-cracks of different numbers and sizes in the glass or glass fiber, which greatly reduces the strength. Microcracks are distributed in the entire volume of glass or glass fiber, but the surface microcracks are the most harmful. Due to the existence of micro-cracks, the glass is stressed unevenly under the action of external forces, and the stress concentration is generated at the most harmful micro-cracks, so that the strength is reduced. The strength of glass fiber is much higher than that of glass, because the high temperature molding of glass fiber reduces the heterogeneity of glass solution, so that the chance of micro-cracks is reduced. In addition, the cross section of the glass fiber is small, and with the decrease of the surface area, the probability of the existence of micro-cracks is also reduced, so that the fiber strength is increased. It has been explicitly proposed that the reason why the strength of glass fibers with fine diameters is higher than that of fibers with coarse diameters is that the size and number of microcracks on the surface are smaller, which reduces the stress concentration and makes the fibers have higher strength. (2) Molecular orientation hypothesis
The molecular orientation hypothesis holds that in the glass fiber molding process, the glass fiber molecules are oriented because of the traction effect of the wire drawing machine, thus improving the strength of the glass fiber.
2. Factors affecting the strength of glass fiber
(1) The influence of fiber diameter and length on tensile strength
In general, the finer the diameter of the glass fiber, the higher the tensile strength, see the table below, but the strength of the same diameter fiber drawn at different drawing temperatures may also be different.
(2) The influence of the quality of glass liquid on the strength of glass fiber

Tensile trength(MPa)3000~38002400~29001750~21501250~17001050~1250

The tensile strength of glass fiber is related to its length. With the increase of fiber length, the tensile strength decreases significantly, as shown in the following table:

Glass fiber length(μm)fibre diameter(μm)Mean tensile strength(MPa)

The influence of diameter and length on tensile strength of glass fiber can be explained by microcrack hypothesis. Because with the decrease of the diameter and length of the fiber, the micro-cracks in the fiber will be correspondingly reduced, thus improving the fiber strength

(3) The influence of chemical composition on strength

Generally, the higher the alkali content, the lower the strength. The tensile strength of non-alkali fiber is 20% higher than that of alkali fiber, as shown in the following table:

Glass fiber length(μm)fibre diameter(μm)Mean tensile strength(MPa)

1, the influence of crystalline impurities: when the glass composition fluctuates or the temperature of the leakage plate fluctuates or decreases, it may lead to the appearance of crystallization in the fiber. It has been proved that the strength of crystallized fibers is lower than that of non-crystallized fibers.
2, the small bubbles in the glass liquid will also reduce the strength of the fiber. It has been tested that the strength of the glass fiber with a diameter of 5.7μm ‘pulled by the glass liquid containing small bubbles is 20% lower than that of the fiber drawn by the pure glass liquid. (4) The influence of molding conditions on the glass fiber
It has been proved by practice that the strength of the glass fiber drawn by the leakage plate is higher than that of the fiber drawn by the glass rod. In the glass rod method, the fiber produced by gas heating is stronger than that produced by electric heating wire. For example, the strength of 10μm glass fiber made with a leakage plate is 1700MPa, while the strength of glass fiber with the same diameter is only 1100MPa. This is because the glass rod is only heated to soften, the viscosity is still very large, and the fiber is subjected to great stress when drawing. In addition, the glass rod method is drawn at a lower temperature, and its cooling rate is lower than the leakage plate method.
(5) The influence of surface treatment on strength
In continuous drawing, it is necessary to apply an infiltrating agent on a single fiber or fiber bundle, which forms a protective film on the surface of the fiber to prevent friction between the fibers during textile processing, and damage the fiber to reduce strength. After heat treatment to remove the infiltrating agent, the strength of the glass cloth decreases a lot, but after treatment with an intermediate binder, the strength can generally recover, because the intermediate binder coating on the one hand protects the fiber, and on the other hand makes up for the surface defects of the fiber.
(6) Influence of storage time on strength
Glass fiber storage for a period of time after its strength will be reduced, this phenomenon is called fiber aging. It is mainly the result of the erosion of fibers by moisture in the air. Therefore, the strength of the fiber with high chemical stability is small, such as the strength of the alkali fiber that is also stored for 2 years is reduced by 33%, and the non-alkali fiber is reduced very little. (7) Influence of load time on strength
The strength of glass fiber decreases with the increase of load time. This is especially true when the ambient temperature is high. It may be that the moisture adsorbed in the microcrack, under the action of external force, speeds up the propagation of the microcrack.
(4) The elastic elongation of glass fiber
The elongation of the fiber refers to the percentage of elongation of the fiber under the action of external forces until it breaks. The elongation of the glass fiber is lower than that of other organic fibers, generally ‘(about), and the degree of elongation is proportional to the applied force until the fiber breaks, and there is no yield point. After the load is removed, the original length can be restored, so the glass fiber is completely elastic.
(5) Wear resistance and folding resistance of glass fiber
The wear resistance of glass fiber refers to the ability of fiber to resist friction; The folding resistance of glass fiber refers to the ability of the fiber to resist breaking. Fiberglass has poor performance in both cases. When the surface of the fiber absorbs water, the microcrack propagation can be accelerated, and the wear resistance and folding resistance of the fiber can be reduced. In order to improve the flexibility of glass fiber to meet the requirements of textile process, appropriate surface treatment can be used. For example, after being treated with 0.2% cationic surfactant aqueous solution, the wear resistance of glass fiber is 200 times higher than that of untreated, and the flexibility of the fiber is generally expressed by the size of the bending radius before breaking. The smaller the bending radius, the better the flexibility. For example, when the glass fiber diameter is 9μm, its bending radius is 0.094mm, and when the microfiber diameter is 3.6μm, its bending radius is 0.038mm.
(6) Electrical properties of glass fiber
Because the glass fiber has good dielectric property, good heat resistance, small moisture absorption, and does not burn, alkali-free glass fiber products have been widely and effectively used in the electrical and motor industry.
(7) Thermal properties of glass fiber
The thermal conductivity of glass fiber is low, especially the density of glass wool products is small, long life and high temperature resistance, widely used in building and industrial thermal insulation, heat insulation and cold insulation, is an excellent thermal insulation material.
The thermal conductivity of glass (that is, the heat passed through the unit heat transfer area of 13 with a temperature gradient of 1℃/m and a time of 1h) is 0.7-1.28W(m.K), but after drawing the glass fiber, the thermal conductivity is only 0.035W(m.K., the main reason for this phenomenon is that the gap between the fibers is large and the density is small. The smaller the density, the smaller the thermal conductivity, mainly because of the low thermal conductivity of the air. The smaller the thermal conductivity, the better the thermal insulation performance. When the glass fiber is wet, the thermal conductivity increases and the heat insulation performance decreases.
(8) Sound absorption performance
Glass fiber also has excellent sound absorption and sound insulation properties, which are widely used in construction, machinery and transportation. The sound absorption coefficient is the ratio of the sound energy absorbed by the surface of the object when the sound wave reaches the surface to the total sound energy falling on the surface. The sound absorption coefficient of general materials is related to the vibration frequency of the sound source object. For example, the sound insulation material made of cotton, when the audio is changed from 200HZ to 1200HZ, the sound absorption coefficient can be changed from 0.09 to 0.92, so the sound absorption coefficient of various materials has certain audio characteristics.
The sound absorption coefficient and frequency characteristics of glass wool are closely related to the volume density, thickness and diameter of glass fiber. The general rule is: with the increase of density, the absorption coefficient continues to increase.

Key words: carbon fiber composite material, CFRP, composite material processing, delamination, splintering, fraying, drilling tool
Article source: Modern Machine Shop
By Peter Zelinski
Published: August 12, 2008
Drilling into composites is a real challenge. Drilling a metal workpiece simply removes the material and emptying the drilled holes, while drilling the laminate workpiece, the woven layers are pushed forward, resulting in delamination at the hole outlet, or splintering leads to a defect workpiece, as shown in Figure 1.


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