Proceedings of the 3rd International Conference on Maritime and Naval Science and Engineering
The Impact Behaviour of Composite Materials CHIRCOR MIHAEL1, DUMITRACHE RAMONA2, DUMITRACHE COSMIN LAURENTIU3 Navigation and Maritime Transport Constanta Maritime University Constanta, 104 Mircea cel Batran ROMANIA 1
[email protected]; 2
[email protected]; 3
[email protected] Abstract: Compared to traditional materials (steel and its alloys), composite materials have unique advantages of stating: high strength and stiffness - including the request variables and shock, good resistance to the environment (temperatures, rain, UV, humidity, temperature variations, etc.) and good mechanical stability, low density, and insulating properties and high fire, high resistance to wear and corrosion.
Key-Words: composite, test, impact, resistance, energy, strength
of the fibers from de matrix and to a considerable absorption energy. When designing the composite materials, it is necessary to make sure that the links between the fibers and the matrix aren’t too weak, because a low shear resistance also influences in a negative way the impact behavior. A simple analysis method of impact properties is the measurement of the stiffness expressed in the necessary energy for breaking a random geometry bar. For homogeneous and isotropic materials, Izod (Figure 2) or Charpy conventional tests (Figure 1) and notched bars are being used – which lead to great tension concentrations, thus minimizing the energy necessary for breaking. These tests may be also successfully used in the case of the composite materials, for comparative studies.
1 Introduction In terms of structure, materials can be divided into four basic categories: metals, polymers, ceramics, composite materials. A composite structure is a material composed of two or more phases combined in a macroscopic scale, whose properties are superior constituent materials, acting in an independent manner. In other words, a composite combining at least two different materials both chemically and geometrically.
2 The Impact Behaviour of Composite Materials
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Hammer
t
Specimen 40 mm Anvil
Fig. 1. Schematic representation of Charpy testing stand
45
22 mm
Missile
2.1 Impact Testing Devices Unlike metals, fiber reinforced composite materials don’t undergo plastic deformations after the impact. Near the impact area may appear elastic deformations (in the case of a low intensity impact) or deteriorations of the material (the separation of the fibers from the matrix, matrix cracking, fiber breaking). The absorbed energy consequent to the impact depends, among others parameters, on the fiber – resins link resistance. If this link is strong, a continuous crack may spread along the material. In the case of a weak link, the generated crack may have an irregular form, leading to a rapid separation
Specimen t
Grip
Fig. 2 Schematic representation of Izod testing stand
ISBN: 978-960-474-222-6
Proceedings of the 3rd International Conference on Maritime and Naval Science and Engineering
testing the unidirectional or canvas fiber reinforced multilayer composite materials on low velocity impact. These tests can be run on various energy levels, thus obtaining: the delaminated surface area; the depth of the missile mark; the residual traction resistance after the impact.
Although the deterioration mechanisms (matrix crack, fiber separation or yield) may occur in an independent manner, the interaction of these factors and the fiber type, the matrix type/state and environment state and matrix – fiber links define the impact and the possible yielding of the material as a very complex phenomenon. There are many methods for identifying the performances of the composite materials on impact. Figures 3 and 4 show two devices.
Semi-spherical missile Kerfs
Guiding column Supporting elements
Bracing columns Bracing frames
Fig. 3 Low velocity impact device used for pre-stressed composite materials
Base plate
Fig. 5 Impact stress device
The device has a compact structure consisting in a OL37 rectangular stand plate having the dimensions of 500 x 300 x 20 [mm]. The plate is provided with a window of 350 x 200 [mm] in dimensions. The bar – plate, with dimensions of 300 x 150 [mm] is provided with four holes of 7Ф in the corners, which insure the grip on the base plate through a frame. The impact is obtained using a missile with a mass varying from 3 to 6 kg, which is lead in a 1,65 m long cylindrical column. In order to obtain various energy levels, the leading column is provided with 10 equidistant holes for retaining the missile at different heights. When the retainer is removed, the missile slides and hits the bar which is stuck on the plate. The missile has the length of 195 mm and is made of a cylindrical body with a 50 mm diameter which continues with a semispherical area with the radius of 8 mm. The obtained energy levels vary from 5
Fig. 4 Blast gun for vmed of pre-stressed composite materials
The various ways of breaking lead to various unconventional types of mechanisms related to the absorbed energy during material yielding on impact. The breaking manners and accordingly the absorbed energies are influenced by factors, such as: fiber orientation, bar geometry, impact velocity, etc. A widely used test in the case of impact behavior study of composite plates is the dropweight test, where the bar placed on a rigid stand is hit by a body with known weight which falls down from a certain height. This height may vary in order to obtain de desired impact velocity. In Figure 5 is showed an impact stress device for fiber reinforced multilayer composite materials, in accordance to the 04.26.383 IGC standard for
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ISBN: 978-960-474-222-6
Proceedings of the 3rd International Conference on Maritime and Naval Science and Engineering
comparable as size, knowing the total energy is not enough for explaining material behavior during breaking. In order to establish the E energy absorbed by the bar, based on impact experimental data, the following formula is often used:
to 95 J. In order to obtain lower energies (1…15 J), a smaller size missile could be used (1 kg mass and 10 mm semispherical radius). The device prevents the multiple impact.
Load P
Phase of breakage initiation
Ei
Phase of brakage spreading
E = Ea(1- Ea/4E0),
(1)
where Ea is the energy registered by the oscilloscope on the energy – time diagram and Eo is defined by the following formula:
Ep Time [t]
Eo = mv2/2, Fig. 6 The phenomenon which appears during the impact
where vo is the initial impact rate. 2.3 The Influence of Various Parameters on Impact Properties Using the devices showed in Figures 3 and 4, Jain L.K and Yiu-Wing Mai have tested the impact behaviour of the panels made from a small number of fiber glass or phenolic resins layers at firs not subjected to any traction and afterwards subjected to traction and compression forces with imposed deformations of 0,5%, using low speeds (a few m/s), medium speeds (3-200 m/s) and high speeds (over 900 m/s, energy impact higher than 2,69 kJ). Before being tested the panels were analyzed using an undestructive control method (ultrasound scanning) in order to track down delamination or the presence of some faults. After impact, the assays were visually analyzed and part of them were analyzed by ultrasounds in order to track down the delamination spreading. Comparative measurements were made at various positive and negative pre-deformation grades.
2.2 The Impact Energy The phenomenon which occurs during the impact, according to the various loads, is showed by Figure 6. The force – time diagram can be split in two phases: one, when the breakage begins and the second when the breakage is spreading. As the load increases during phase one, the elastic deformation energy is gathered in the bar, subsequently leading to a microscopic scale breakage. At the same time, yielding mechanisms at a microscopic level can occur (micro-buckling of the fiber on the compressed side, link breaking in the fiber – matrix interface, etc.) When the critical load is reached at the end of the first phase, the composite material bar may yield due to the traction or shear breakage, depending on the relative values of the inter-laminar traction or the shear resistance. Once reached this point, the spreading of the breakage may take place in a catastrophic way – as in the case of fragile materials (at high loads) or in a progressive way – by keeping on absorbing the energy (at low loads). The total impact energy Et – registered on the testing device or by an oscilloscope (on the energy – time variation diagram) during loading – consists of Ei initiating energy and Ep spreading energy. The fragile materials are defined by high values of the breakage initiation energy and by low values of the breakage spreading energy, while tough materials are defined by a low breakage initiation energy and by a high spreading energy. Giving the fact that after summarization fragile and tough materials may have total impact energies
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(2)
Generally, the impact with or without imposed deformations leads to the appearance of some delamination, fiber fractures and, sometimes, material perforations caused by the bullet. The deteriorations are analyzed by the width of the delaminated area, indicated after the ultrasound scanning. These attempts show that on impact stresses simultaneously made with a bar predeformation, smaller breakage deformations were obtained in comparison to the impact followed by a traction/compression stress (Figure 7.).
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ISBN: 978-960-474-222-6
Proceedings of the 3rd International Conference on Maritime and Naval Science and Engineering
Medium velocity impact resistance. Measurements obtained by C ultrasound scanning showed that row materials don’t change after traction prestressing, while Kevlar and carbon fiber composite materials are considerably changed after compression pre-stressing. Also, carbon fiber composite materials considerably depend on resin’s stiffness: the matrix gets stiffer as the delamination rate decreases. In the case of an impact with an 2.8 J and 80 m/s energy, after a traction pre-stressing, the performance of carbon fiber composite material considerably depends on fiber resistance because, if the fiber resistance gets higher, the material will have a better behaviour. Also, IMS and HTA carbon-epoxy fibers have an increased sensibility to compression pre-stress.
Fig. 7 Delamination area depending on extension / compression pre-deformation
High velocity impact resistance. The impact with high speeds has been tested on a small number of test-pieces. The high speeds tests show that the fiberglass multilayered materials undergo a much bigger delamination than carbon or Kevlar fibers but, when they are subjected to an impact with a 5,56 mm perforating bullets/ missiles, they absorb twice the energy. The impact with a speed of 1,4 km/s on a prestressed bar showed major composite deterioration leading to the perforation of the entire plate.
Fig. 8 A few impact resistances
Low velocity impact resistance. Measurements made with the C scanner indicate a slight increase in traction pre-stressed row material deteriorations and a considerably increase in compression stressed carbon fiber reinforced composite materials deteriorations (Figure 7). For all carbon fiber types, the obtained deteriorations decrease as the resins’ stiffness increases. For a 1,75 ms-1 strain rate and a 5J energy, residual deformation of XAS carbon fiber composite materials decreases as the pre-tension increases, the same happening to a composite material made of fiberglass canvas enclosed in a phenolic resin or to a Kevlar composite bar, though glass-epoxy bars have a high deformability before they break. On compression, in the case of carbon fiber composite materials, is shown a decrease of the properties comparing to the others two (R glass and XAS) where no change was observed after the compression pre-stressing. Fiberglass enclosed ceramic matrix composite material also undergoes changes on compression pre-stressing.
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When using thin multilayered materials with high rigidity fibers, the deteriorations occurring after the impact can be very serious. Although some factors (fiber orientation, type of canvas, etc.) may seriously affect the impact resistance, the carbon is often used in combinations with one or more types of fiber in engineering structures stressed on severe impact. For example, the hybrid canvas where at least two types of fibers are bonded in the same matrix. The Figure 1.8. shows comparative impact resistance of some fiber reinforced multilayered composite materials, often used in the industry field. The impact behaviour of fiber reinforced composite materials had been studied using a standard Charpy impact device, which provided the first very useful results. For a fiber reinforced polymeric composite material it had been shown that its impact behavior depends on time, which is the speed that the hammer has when it hits the bar. Rotem and Lifschitz proved that the tensile
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Proceedings of the 3rd International Conference on Maritime and Naval Science and Engineering
of Mallick and Broutman obtained on cross linked fiber composite materials are showed in Figure 10. On the line which indicates the influence of fiber orientation on the absorbed energy, a symmetry can be seen with respect to the 45° direction, where the chart reaches a minimum value.
strength of a fiber reinforced composite material increases as the strain rate increases, on fiber direction. Later, Broutman made some attempts regarding the impact on fiberglass and polymeric resins or epoxy reinforced composite materials or on graphite, glass and Kevlar fiber hybrid composite materials using a device endowed with an impactor by varying some parameters. The plates on which the attempts were made are unidirectional fiber composite materials or cross linked fiber oriented at 0° and 90°. During these attempts the following parameters have been modified: fiber orientation, impact rate, the impact velocity, missile’s weight, the bar’s dimensions, height of fall. One of the important parameters which affect the composite materials behavior on impact is the fiber orientation. The effect of the fiber’s orientation angle on the impact of composite materials has been investigated by Mallick and Broutman, on Eglass epoxy multilayered materials. The exact configuration of the composite materials was [0/90/04]s, respectively [(0/90)3/0]s. Each of the two configurations was made of 13 layers of 0,25 mm. The rectangular bars were cut so that the fibers from the external layers formed 0°, 15°, 45°, 75° and 90° angles, in the longitudinal direction (Fig. 9). In all these cases, the loading has been made perpendicularly on the multilayered material’s plan. The absorbed energy related to multilayered material width dependent on the fiber orientation direction is showed in Figure 10. P
θ y
b
l θ
h
x L/2
L/2
z
Fig. 9 Impact test schematization Fig. 10 The influence of fiber orientation on the absorbed energy by an unidirectional glass-epoxy, respectively a cross linked composite material [0,90]
It’s been showed that the lowest value appears when the fiber orientation direction is 60°. However, the results obtained by Agarwal and Narang’s on a Charpy impact device showed that in the case of composite materials with unidirectional fibers, the impact energy decreases as the fiber orientation direction increases. The minimum impact energy appeared at 90° angles. The results
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Another important parameter is the interface strength between fiber and matrix, which intensively affects the breakage of the composite materials breakage.
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Proceedings of the 3rd International Conference on Maritime and Naval Science and Engineering
strength (approximately 5 ksi) delamination is proved to be the principal way of subsidence of multilayered materials. So, in the case of polyester multilayered materials, the total impact strength may be increased by decreasing the inter-phase links. It can be observed the fact that the highest value of the impact strength is reached when the shear strength is minimum. The breakage initiation needs a much lower energy when the inter-phase link is weak, the maximum value of this energy being reached during delamination, which appears after the breakage initiation in the multilayered materials. The bar supports a much lower capacity during spread but all this time it absorbs a much bigger amount of energy because of the great distortions which may appear. In the case of the epoxy multilayered materials the inter-phase link is not so decreased as to induce a drastic delamination in the composite material. So, in the case of polyester matrix multilayered materials, at higher values than the critical shear strength necessary for reaching the minimum value of the impact energy, this energy increases as the interlaminar shear strength increases. The way that the epoxy matrix breaks is mainly the result of the fibers fracture.
Yeung and Broutman varied the interface conditions modifying the surface treatment of the fiberglass canvas. For the matrix, polyester or epoxy resins have been chosen. In order to determine the fiber – matrix interface strength, its apparent shear strength was measured. In the case of the composite material with polyester matrix it had been shown that the inter-phase strength may have various values depending on the chemical bounding agent between fiber and matrix, applied on fibers’ surface. On epoxy matrix composite materials the inter-phase strength can’t vary because the epoxy resins are capable to create a strong link between fiber and matrix, even if the bounding agent is missing. The results of impact tests made on Charpy device for epoxy and polyester composite materials are showed in Figure 11, where the impact energies values (initial, spreading, total) on surface unit, U1i = Ei/bh , U1p = Ep/bh , U1t = Et/bh ,
(3)
are calculated according to the apparent shear resistance. It can be easily noticed that the specific energy increases as the shear resistance of the polyester and epoxy multilayer materials increases. At the same time, the bending resistance of the multilayer materials increases, thus showing a very good inter-phase bond as well as high values for the inter-laminar tensions. The impact energies necessary for breakage have higher values for the epoxy matrix multi-layered materials as compared to the ones with polyester matrix.
3 Conclusion In many cases, using composites is more efficient. For example, in the highly competitive market, one is continuously looking for ways to lower the overall mass of the craft without decreasing the stiffness and strength of its components. This is possible by replacing conventional metal alloys with composite materials. Even if the composite material costs may be higher, the reduction in the number of parts in an assembly and the savings in fuel costs make them more profitable. These may include improved strength, stiffness, fatigue and impact resistance, thermal conductivity, corrosion resistance, etc. References: 1. A.F. Avila et al., International Journal of Impact Engineering 34 40, 2007, pp. 28–41 2. Agarwal, B.D., Broutman, L.J., Analysis and performance of Fiber Composites, New York, Wiley, 1990 3. Aymerich F, Onnis R, Priolo P., Analysis of the fracture behaviour of a stitched single-lap joint composites, 2005;36(5):603 - 14
Fig. 11 The influence of fiber – matrix interface strength on impact energy of glass-polyester composite materials.
In the case of polymer multilayered materials, the spreading and total impact energy lines show a minimum. At lower values of apparent shear
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