- Email: [email protected]
Experimental investigation of low-velocity impact characteristics of woven glass fiber epoxy matrix composite laminates of EP3 grade
Materials and Design 31 (2010) 4553–4560
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Technical Report
Experimental investigation of low-velocity impact characteristics of woven glass fiber epoxy matrix composite laminates of EP3 grade N. Rajesh Mathivanan *, J. Jerald Dept. of Production Engg., National Institute of Technology, Tiruchirappalli 620 015, India
a r t i c l e
i n f o
Article history: Received 6 January 2010 Accepted 27 March 2010 Available online 1 April 2010
a b s t r a c t This work presents the results of an experimental investigation concerning the low-velocity impact behavior of woven glass fiber epoxy matrix composite laminates. Experimental test were performed according to ASTM standards using an instrumented falling weight impact testing machine. Impact test were conducted to characterize the type and extent of the damage observed in laminate for range of thickness subjected to different impact velocities. Correlation of the residual indentation of the impacted specimens provides a criterion for the extent of the damage. As the impact energy was increased, the samples experienced one of two types of damages: a crack from the center of the laminate to the edge, or significant damage consisting of a dent localized in the region of impact. The history of relevant kinematical, dynamic and energetic quantities, both to synthesize the dependency of the energy parameters and force threshold values on the impact velocity are discussed. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Composite laminate is a combination of fiber and resin mixed in proper form. These fibers can be woven in certain pattern to obtain desire material properties as per design requirements. Properties of the composite material can be tailored as per design specifications. One of the unique properties of composite is that it has high specific strength. Composites are being utilized as viable alternatives to metallic materials in structures where weight is a major consideration, e.g., aerospace structures, high-speed boats and trains. However, their behavior under impact loading is one of the major concerns [1], since impacts do occur during manufacture, normal operations, maintenance and so on. A list of the studies on the impact response of composite materials and structures can be found in review papers [2–6]. The resulting damage due to impacts, often in the form of delaminations, matrix cracking and fiber failures may severely reduce the structural strength and stability. Matrix deformation and micro-cracking, interfacial debonding, lamina splitting, delamination, fiber breakage and fiber pull-out are the possible modes of failure in composites subjected to impact loading. Even though fiber breakage is the ultimate failure mode, the damage would initiate in the form of matrix cracking/lamina splitting and would lead to delamination. Damage-free composites are necessary for their effective use [7].
* Corresponding author. Tel.: +91 9894502459. E-mail address: [email protected] (N.R. Mathivanan). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.03.051
Impact damage is generally not considered to be a threat in metal structures because, owing to the ductile nature of the material, large amounts of energy may be absorbed. At yield stress the material may flow for very large strains (up to 20%) at constant yield before work hardening. In contrast, composites can fail in a wide variety of modes and contain barely visible impact damage (BVID) which nevertheless severely reduces the structural integrity of the component. Most composites are brittle and so can only absorb energy in elastic deformation and through damage mechanisms, and not by plastic deformation. Zhang and Richardson [8] revealed that there was a significant reduction in flexural properties due to the impact-induced damage and that the residual flexural strength is more susceptible to damage than residual modulus. A number of studies are available on the impact behavior of unidirectional laminated composites. Even though there are some studies on the impact behavior of woven fabric composites [8–17], further studies are necessary for their effective use in structural applications. The objective of this work is to present and discuss some experimental results obtained during a low-velocity impact testing conducted on glass fiber epoxy matrix laminate plates. The impact behavior of this particular class of composite material is then analyzed from an energy viewpoint, by means of two parameters [18]: the saturation impact energy and damage degree of a plate specimen subjected to a drop-dart test according to the ASTM D 3029 standard. The drop-dart tests have been conducted selecting different levels of the dart kinetic energy at impact by modification of the drop height. Thus, impact velocities are different in one test from the other and, consequently, the plates are submitted to different deformation rate.
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2. Experimental procedure 2.1. Fabrication of composite laminates The tested material was woven glass fiber epoxy matrix composite laminates of EP3 grade produced by ICP (India) Pvt. Ltd., Bangalore, in three different thicknesses; namely 2, 4 and 6 mm. Fiber reinforcement was kept constant for each batch of plates. Woven ‘E’ glass fabric, type C of IS: 11,273 were used. An epoxy matrix based on Lapox L-12 resin and K-5 hardener was selected for making composite panels. The volume fraction of glass fibers was 63%. Identical woven fabric layers were selected depending on the thickness of the composite laminates and fabricated by hand lay-up process. The composite panels were first cured at room temperature for 12 h under a pressure of 0.2 MPa using a hydraulic press. The post-curing was carried out at 120 °C for 4 h and then cooled to room temperature.
2.2. Impact test methods The impact tests on composite laminates could be performed in a variety of methods. Several different types of impact testing equipment in use today are capable of measuring the force during an impact. However, the most commonly employed methods are: a. Drop/falling weight test method. b. Penetrating impact test method using pneumatic guns. The drop weight test method is employed for low velocity ranges and is primarily used to investigate the impact behavior under lower acceleration. This type of impact test helps to understand the behavior of materials when they are subjected to impact loads. Falling weight impact testing unit enables to simulate a wide variety of real-world impact conditions and collect detailed performance data. The penetrating test involves higher impact velocities. These involve damages which are more severe and which could lead to immediate failure of the material as in case of a bird strike on an aeroplane where the relative velocities between the two objects are so high that the material of the airplane could suffer instant fracture. Such impact scenarios are simulated using pneumatic air guns which can potentially provide velocities up to a 100 m/s; to help study the behavior of materials subjected to such high impact loads. The tests were performed using an instrumented falling weight testing machine with no energy storage device: the maximum impact energy is limited by the adjustable falling height (up to a maximum of about 1500 mm) and the fixed mass, 10 kg, of the impactor. This gives up to a peak of about 150 J of energy with an impact velocity of 6.25 m/s completely supplied by the gravitational field. The impactor mass together with the height of drop determines the energy of impact. With an increase in mass and height the potential energy of the dart will increase and thus on releasing the tool holding assembly the potential energy is converted to kinetic energy. Falling weight impact test equipment setup is shown in Fig. 1. The dart material used was steel. Standard equipment is used in order to acquire, sample, collect and store the signal from a load cell positioned in the proximity of the head of the dart. In accordance with ASTM D 3029 standard [19–22] a batch of square, thin (150 mm side with varying thickness; 2, 4 and 8 mm) specimens was clamped on a fixture with a rectangular slot (sq. 100 mm). The dart had a hemispherical head of 10 mm radius; the piezoelectric load cell is placed at the other extremity of the calibrated cylindrical rod that constitutes the dart, at which the pushing mass was connected. Fig. 2 shows the specimen clamping
Fig. 1. Instrumented falling weight impact test equipment.
Fig. 2. Specimen clamping apparatus.
apparatus, specifically designed in order to assure the constancy of the clamping force, through the pre-loading of four helical springs. Fig. 3 illustrates the schematic diagram of the dart and the specimen mounted on the base plate. Specimens of three thicknesses were used in this investigation; 2, 4 and 8 mm. A fixed impactor mass of 15.69 N with the dart was released from varying heights up to 1.5 m. The vertical guides of the impact tower were lubricated frequently to minimize any friction generated during the descent of the impactor. 3. Results and discussion There has been a growing interest to use composite materials in structural applications on account of their outstanding properties
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Fig. 3. Schematic diagram of specimen being impacted.
compared to conventional materials. Hence it gains high importance to predict or determine their response to an impact loading. In this work, variation of impact parameters such as contact force, displacement, impact velocity, and absorbed energy versus time or deflection is examined in order to figure out damage process of woven fabric composites in an impact event. The study is expected to be helpful in understanding the overall response of woven fabric composite plates under impact loading. For this purpose, a number of tests were performed under varied impact energies ranging from approximately 7.85–35.23 J. Therefore, in the following, energy profile of the woven composite, the load–deflection curves and the images of some damaged specimens are discussed. The mechanical system consists of the free falling dart and the constrained specimen. The energy balance equation of the system consists of six terms: three of them refer to the falling dart (namely one kinetic, one elastic and one gravitational term) and other three refer to the specimen and its constraint device (namely one kinetic, one elastic and one gravitational term) [23].
Moreover, we can define the work done by the external actions on the system and the energy dissipated by the system, internal or external, which irreversibly transform kinetic or potential energies into irrecoverable energy, mainly due to the fragmentation of the material and too little extent due to the friction between the specimen and the dart. In order to deal with this energy balance equation, the following approximation can be made: (1) the dart can be considered as a rigid body, so its elastic energy is set to zero; (2) the specimen mass is negligible, if compared with the impactor mass, so its gravitational and kinetic energy variation can be considered negligible. 3.1. Free fall, stop, rebound If the energy absorbed by the specimen is not too high a rebound occurs. Fig. 4 illustrates four different graphs for a case of re-
Fig. 4. Typical graphs for the rebounding case.
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Fig. 5. Typical graphs for the dart stop case.
Fig. 6. Typical graphs for the perforation case.
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bound. Fig. 4a and b, indicates the time histories of the dart displacement and velocity. Fig. 4c and d, indicates the histories of the force and deformation energy. Measurable ‘‘end of contact” time can be detected when the force between specimen and dart returns to null (Fig. 4d). At this time the energy balance can be formulated and thus obtain the whole energy dissipated by the specimen in order to initialize and propagate fractures inside the material. Another measurable characteristic time can be defined when the dart velocity becomes zero, at this time the dart has reached its ‘‘maximum penetration’’ Df into the specimen (Fig. 4a and b). There are two quantities involved in this situation: the first one, Dh; is controlled by the experimenter, the other one, Df; can be evaluated only a posteriori but nevertheless known once the test is actually performed.
It is worth noting that the maximum deflection, in the case here examined, is about two times the plate thickness. Therefore, membrane stiffening occurs and laminate membrane strength is mainly involved. The force history (Fig. 4c and d) shows two thresholds: the first one is at 2250 N, where the curve sharply changes its look and a deviation is visible, the second one is at 5620 N, where the curve sharply drops down and then takes again to grow but with a slope lower than the previous one. The first threshold can be interpreted as the indication of the first material damage. The second threshold occurs at the first lamina failure. The force versus displacement graph (Fig. 4c) shows a closed loop. The area under the curve is the absorbed energy that is initially progressively transferred from the dart to the plate and then
Fig. 7. Superposition of force–displacement curves for different impact velocity.
Front
Front
Specimen 1
Specimen 2
Back
Specimen 1
Back
Specimen 2
Fig. 8. Images of damaged specimens 1, 2, 3, 10, 11 and 19.
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Front
Specimen 3
Back
Specimen 3
Front
Specimen 10
Back
Specimen 10
Front
Specimen 11
Back
Specimen 11
Specimen 19
Back
Specimen 19
Front
Fig. 8 (continued)
given back from the plate to the rebounding dart, the area included inside the loop refers to the energy absorbed during the impact. Fig. 4d shows the deformation energy history: energy progressively grows until the maximum displacement is reached (the
maximum energy level is equal to the initial kinetic energy of the dart) then energy progressively decreases until the dart detaches from the plate. It is reasonable to suppose that from this point to the end of the test the specimen does not dissipate energy
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any further, only releasing the residual part of the elastic energy stored during the penetration of the dart. At final instant the energy value shown in the graph is equal to the dissipated energy. 3.2. Free fall and stop This limit situation occurs when the dart stops without rebounding. Fig. 5 indicates four different graphs for a case of dart stop. Fig. 5a and b, indicates the time histories of the dart displacement and velocity. Fig. 5c and d, indicates the histories of the force and deformation energy. If the dart does not rebound the specimen does not have any residual internal energy to exchange with the dart. At the instant when the dart stops, it has reached the maximum displacement: its ‘‘maximum penetration’’ into the specimen, from Fig. 5a it can be read as value equal to 15.6 mm. At this instant the specimen has dissipated the whole amount of energy, no further energy increment could be dissipated by mean of ‘‘internal’’ fragmentation. Perforation, which is the other dissipative mechanism, is incipient but does not take place. In some sense the material is ‘‘saturated’’. The force history (Fig. 5c and d) shows two thresholds: the first one is at 870 N, where the curve sharply changes its look and deviates, the second one is at 2250 N, where the curve suddenly drops down to 2200 N and than maintains its value (the curve is approximately flat). The first threshold can again be interpreted as the indication of the first material damage. The force versus displacement graph (Fig. 5c) shows a closed loop. The area under the curve is the deformation energy that is progressively transferred from the dart to the plate and, in this case of absence of either a rebound or a perforation, is also the energy absorbed during the impact. 3.3. Free fall and perforation The penetration threshold is defined as the impact energy, when the impactor does not rebound from the surface of the specimen for the first time whereas; the perforation threshold is defined as the absorbed energy, when the tip of the impactor pierces the surface of the specimen. A small increment of energy beyond ‘‘saturation’’ means perforation. Fig. 6 indicates four different graphs for a case of perforation. Fig. 6a and b, indicates the time histories of the dart displacement and velocity, Fig. 6c and d, indicates the histories of the force and deformation energy. The force versus displacement graph (Fig. 6c) does not show a closed loop. The area under the curve is the deformation energy that is progressively transferred from the dart to the plate, when the saturation of the load carrying capacity of the plate is reached, perforation takes place. At this instant the maximum energy absorbed by the material damage mechanisms is read. The energy appears to grow further; this is due to the friction of the edges of the perforation hole against the lateral surface of the dart. In Fig. 6c and d it is well visible that, after perforation, the force remains nearly constant and the energy grows with a constant slope. 3.4. Energy curves The experimental tests were performed on three specimens for every thickness; 2, 4 and 8 mm. Tests were performed two times at three energy levels determined by the falling height, namely 500, 1000 and 1500 mm. The corresponding values of the nominal impact velocities are 3.132, 4.429 and 5.425 m/s. From an energy point of view, Fig. 7 shows eight curves of force versus displacement of the dart at various levels of impact energy. This figure allows us to clearly detect the transition from rebound to perforation: in the perforation cases the curve ‘‘folds’’ onto itself toward decreasing displacement, while in the rebound cases the displacement is monotonically increasing. The transition case is
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shown by the fifth curve (corresponding to 1000 mm of falling height and 5.125 m/s of nominal impact velocity) that evidences very little rebound of the dart after the maximum penetration has been reached. Furthermore it must be noted that the curves are well superimposed upon each other up to the point where the energy initially supplied to the dart is completely transferred to the plate (rebound cases) or the perforation takes place. Specimens have been examined after the impact test with the aim of establishing a correlation between the test conditions and the plate damage. For comparison, images of some damaged specimens are given in Fig. 8. Damage extent at both front (impacted) and back side of the specimens 1, 2, 3, 10, 11 and 19 are given. For a low energy level (falling height of 500 mm, velocity at the impact 3.132 m/s), this fiber layout shows good impact resistance characteristics: almost all the energy is released back to the rebounding dart. The maximum energy is reached at about 7.696 J. Specimens 2 and 3 exhibit complete perforation, when compared with other specimens. 4. Conclusions The behavior of woven fabric composite laminates of EP3 grade subjected to low-velocity impact loading has been examined, submitting to the standard instrumented falling weight test a number of plates made by glass fiber epoxy matrix layers with woven reinforcement and with three different thicknesses. The impactor exhibited three conditions; rebound, stop and puncture on the specimens, when subjected to impact at different energy levels. The experimental setup allows acquiring the impacting dart force versus time signals. The tests have three different results; namely rebound, stop or perforation – on the basis of the potential energy initially supplied to the dart by fixing its dropping height (the mass attached to the dart is fixed). The force versus displacement and the energy versus displacement curves for each type of composite laminate plates have been drawn on the same graph showing an impressive superimposition. These results permit to state that, at least for the considered material, the considered type of loading and in the considered range of impact speed. The glass fiber epoxy matrix has no sensitivity to its mechanical characteristic to the strain rate effect, because the composite sensitivity to strain rate is mostly driven by the resin behavior. The examination of the energy values allows us to define two parameters of interest for the designer: the saturation impact energy (i.e. the maximum energy bearable by the material without perforation) and the damage degree (i.e. the ratio between the total energy transformed (stored and dissipated) and the dissipated part of it). Acknowledgements The authors thank the Department of Production Engineering, National Institute of Technology, Tiruchirappalli, for their support and encouragement for carrying out this work. The authors also thankfully acknowledge the Management, PES Institute of Technology, Bangalore for providing facilities in carrying out these tests and for the technical support during the realization of this work. References [1] Abrate S. Impact on laminated composite materials. Appl Mech Rev 1991;44(4):155–89. [2] Abrate S. Impact on laminated composite materials. Appl Mech Rev 1991;44:155–90. [3] Cantwell WJ, Morton J. Impact resistance of composite materials – a review. Composites Part A 1991;22:347–62.
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[4] Abrate S. Impact on laminated composites – recent advances. Appl Mech Rev 1994;47:517–44. [5] Richardson MOW, Wisheart MJ. Review of low-velocity impact properties of composite materials. Composites Part A 1996;27:1123–31. [6] Bibo GA, Hogg PJ. Review – the role of reinforcement architecture on impact damage mechanisms & post-impact compression behavior. J Mater Sci 1996;31:1115–37. [7] Naik NK et al. Damage in woven–fabric composites subjected to low-velocity impact. Compos Sci Technol 2000;60:731–44. [8] Ramakrishnaiah KN, Naik NK. Impact response of polymer matrix woven fabric composites. In: Proceedings of 8th ISME conference emerging trends in mechanical engineering. New Delhi: Tata McGraw-Hill Publishing Co., Ltd.; 1993. p. 319–24. [9] Davies GAO, Hitchings D, Zhou G. Impact damage and residual strengths of woven fabric glass/polyester laminates. Composites Part A 1996;27A:1147–56. [10] Lifshitz JM, Gandelsman M. Effect of near-surface impact-induced damage on the residual strength of woven glass/epoxy composite beams. Compos Sci Technol 1997;57:205–16. [11] Karaoglan L, Noor AK. Frictional contact impact response of textile composite structures. Compos Struct 1997;37:269–80. [12] Ebeling T, Hiltner A, et al. Delamination failure of a woven glass fiber composite. J Compos Mater 1997;31:1318–33. [13] Naik NK, Chandra Sekher Y. Damage in laminated composites due to low velocity impact. J Reinf Plast Compos 1998;17:1232–63.
[14] Hirai Y, Hamada H, Kim JK. Impact response of woven glass fabric composites – I. Effect of fiber surface treatment. Compos Sci Technol 1998;58:91–104. [15] Hirai Y, Hamada H, Kim JK. Impact response of woven glass fabric composites – II. Effect of temperature. Compos Sci Technol 1998;58:119–28. [16] Wu E, Chang LC. Loading rate effect on woven glass laminated plates by penetration force. J Compos Mater 1998;32:702–21. [17] Siow YP, Shim VPW. An experimental study of low velocity impact damage in woven fiber composites. J Compos Mater 1998;32:1178–202. [18] Zhang ZY, Richardson MOW. Low velocity impact induced damage evaluation and its effect on the residual flexural properties of pultruded GRP composites. Compos Struct 2007;81:195–201. [19] ASTM D 3029. Standard test method for impact resistance of rigid plastic sheeting or parts by means of a tup (Falling Weight). Philadelphia: American Society for Testing and Materials; 1982. [20] Choi HY, Downs RJ, Chang FK. A new approach toward understanding damage mechanism and mechanics of laminated composites due to low-velocity impact. J Compos Mater 1991;25:992–1011. [21] Lagace PA et al. A preliminary proposition for a test method to measure (impact) damage resistance. J Reinf Plast Compos 1993;12(5):584–601. [22] Sjoblom P, Hartness JT, Cordell TM. On low-velocity impact testing of composite materials. J Compos Mater 1980;22:30–52. [23] Belingardi G, Grasso F, Vadori R. Energy absorption and damage degree in impact testing of composite materials. In: Proceedings of the 11th international conference on experimental mechanics, Oxford (UK); 1998.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Technical Report
Experimental investigation of low-velocity impact characteristics of woven glass fiber epoxy matrix composite laminates of EP3 grade N. Rajesh Mathivanan *, J. Jerald Dept. of Production Engg., National Institute of Technology, Tiruchirappalli 620 015, India
a r t i c l e
i n f o
Article history: Received 6 January 2010 Accepted 27 March 2010 Available online 1 April 2010
a b s t r a c t This work presents the results of an experimental investigation concerning the low-velocity impact behavior of woven glass fiber epoxy matrix composite laminates. Experimental test were performed according to ASTM standards using an instrumented falling weight impact testing machine. Impact test were conducted to characterize the type and extent of the damage observed in laminate for range of thickness subjected to different impact velocities. Correlation of the residual indentation of the impacted specimens provides a criterion for the extent of the damage. As the impact energy was increased, the samples experienced one of two types of damages: a crack from the center of the laminate to the edge, or significant damage consisting of a dent localized in the region of impact. The history of relevant kinematical, dynamic and energetic quantities, both to synthesize the dependency of the energy parameters and force threshold values on the impact velocity are discussed. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Composite laminate is a combination of fiber and resin mixed in proper form. These fibers can be woven in certain pattern to obtain desire material properties as per design requirements. Properties of the composite material can be tailored as per design specifications. One of the unique properties of composite is that it has high specific strength. Composites are being utilized as viable alternatives to metallic materials in structures where weight is a major consideration, e.g., aerospace structures, high-speed boats and trains. However, their behavior under impact loading is one of the major concerns [1], since impacts do occur during manufacture, normal operations, maintenance and so on. A list of the studies on the impact response of composite materials and structures can be found in review papers [2–6]. The resulting damage due to impacts, often in the form of delaminations, matrix cracking and fiber failures may severely reduce the structural strength and stability. Matrix deformation and micro-cracking, interfacial debonding, lamina splitting, delamination, fiber breakage and fiber pull-out are the possible modes of failure in composites subjected to impact loading. Even though fiber breakage is the ultimate failure mode, the damage would initiate in the form of matrix cracking/lamina splitting and would lead to delamination. Damage-free composites are necessary for their effective use [7].
* Corresponding author. Tel.: +91 9894502459. E-mail address: [email protected] (N.R. Mathivanan). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.03.051
Impact damage is generally not considered to be a threat in metal structures because, owing to the ductile nature of the material, large amounts of energy may be absorbed. At yield stress the material may flow for very large strains (up to 20%) at constant yield before work hardening. In contrast, composites can fail in a wide variety of modes and contain barely visible impact damage (BVID) which nevertheless severely reduces the structural integrity of the component. Most composites are brittle and so can only absorb energy in elastic deformation and through damage mechanisms, and not by plastic deformation. Zhang and Richardson [8] revealed that there was a significant reduction in flexural properties due to the impact-induced damage and that the residual flexural strength is more susceptible to damage than residual modulus. A number of studies are available on the impact behavior of unidirectional laminated composites. Even though there are some studies on the impact behavior of woven fabric composites [8–17], further studies are necessary for their effective use in structural applications. The objective of this work is to present and discuss some experimental results obtained during a low-velocity impact testing conducted on glass fiber epoxy matrix laminate plates. The impact behavior of this particular class of composite material is then analyzed from an energy viewpoint, by means of two parameters [18]: the saturation impact energy and damage degree of a plate specimen subjected to a drop-dart test according to the ASTM D 3029 standard. The drop-dart tests have been conducted selecting different levels of the dart kinetic energy at impact by modification of the drop height. Thus, impact velocities are different in one test from the other and, consequently, the plates are submitted to different deformation rate.
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2. Experimental procedure 2.1. Fabrication of composite laminates The tested material was woven glass fiber epoxy matrix composite laminates of EP3 grade produced by ICP (India) Pvt. Ltd., Bangalore, in three different thicknesses; namely 2, 4 and 6 mm. Fiber reinforcement was kept constant for each batch of plates. Woven ‘E’ glass fabric, type C of IS: 11,273 were used. An epoxy matrix based on Lapox L-12 resin and K-5 hardener was selected for making composite panels. The volume fraction of glass fibers was 63%. Identical woven fabric layers were selected depending on the thickness of the composite laminates and fabricated by hand lay-up process. The composite panels were first cured at room temperature for 12 h under a pressure of 0.2 MPa using a hydraulic press. The post-curing was carried out at 120 °C for 4 h and then cooled to room temperature.
2.2. Impact test methods The impact tests on composite laminates could be performed in a variety of methods. Several different types of impact testing equipment in use today are capable of measuring the force during an impact. However, the most commonly employed methods are: a. Drop/falling weight test method. b. Penetrating impact test method using pneumatic guns. The drop weight test method is employed for low velocity ranges and is primarily used to investigate the impact behavior under lower acceleration. This type of impact test helps to understand the behavior of materials when they are subjected to impact loads. Falling weight impact testing unit enables to simulate a wide variety of real-world impact conditions and collect detailed performance data. The penetrating test involves higher impact velocities. These involve damages which are more severe and which could lead to immediate failure of the material as in case of a bird strike on an aeroplane where the relative velocities between the two objects are so high that the material of the airplane could suffer instant fracture. Such impact scenarios are simulated using pneumatic air guns which can potentially provide velocities up to a 100 m/s; to help study the behavior of materials subjected to such high impact loads. The tests were performed using an instrumented falling weight testing machine with no energy storage device: the maximum impact energy is limited by the adjustable falling height (up to a maximum of about 1500 mm) and the fixed mass, 10 kg, of the impactor. This gives up to a peak of about 150 J of energy with an impact velocity of 6.25 m/s completely supplied by the gravitational field. The impactor mass together with the height of drop determines the energy of impact. With an increase in mass and height the potential energy of the dart will increase and thus on releasing the tool holding assembly the potential energy is converted to kinetic energy. Falling weight impact test equipment setup is shown in Fig. 1. The dart material used was steel. Standard equipment is used in order to acquire, sample, collect and store the signal from a load cell positioned in the proximity of the head of the dart. In accordance with ASTM D 3029 standard [19–22] a batch of square, thin (150 mm side with varying thickness; 2, 4 and 8 mm) specimens was clamped on a fixture with a rectangular slot (sq. 100 mm). The dart had a hemispherical head of 10 mm radius; the piezoelectric load cell is placed at the other extremity of the calibrated cylindrical rod that constitutes the dart, at which the pushing mass was connected. Fig. 2 shows the specimen clamping
Fig. 1. Instrumented falling weight impact test equipment.
Fig. 2. Specimen clamping apparatus.
apparatus, specifically designed in order to assure the constancy of the clamping force, through the pre-loading of four helical springs. Fig. 3 illustrates the schematic diagram of the dart and the specimen mounted on the base plate. Specimens of three thicknesses were used in this investigation; 2, 4 and 8 mm. A fixed impactor mass of 15.69 N with the dart was released from varying heights up to 1.5 m. The vertical guides of the impact tower were lubricated frequently to minimize any friction generated during the descent of the impactor. 3. Results and discussion There has been a growing interest to use composite materials in structural applications on account of their outstanding properties
N.R. Mathivanan, J. Jerald / Materials and Design 31 (2010) 4553–4560
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Fig. 3. Schematic diagram of specimen being impacted.
compared to conventional materials. Hence it gains high importance to predict or determine their response to an impact loading. In this work, variation of impact parameters such as contact force, displacement, impact velocity, and absorbed energy versus time or deflection is examined in order to figure out damage process of woven fabric composites in an impact event. The study is expected to be helpful in understanding the overall response of woven fabric composite plates under impact loading. For this purpose, a number of tests were performed under varied impact energies ranging from approximately 7.85–35.23 J. Therefore, in the following, energy profile of the woven composite, the load–deflection curves and the images of some damaged specimens are discussed. The mechanical system consists of the free falling dart and the constrained specimen. The energy balance equation of the system consists of six terms: three of them refer to the falling dart (namely one kinetic, one elastic and one gravitational term) and other three refer to the specimen and its constraint device (namely one kinetic, one elastic and one gravitational term) [23].
Moreover, we can define the work done by the external actions on the system and the energy dissipated by the system, internal or external, which irreversibly transform kinetic or potential energies into irrecoverable energy, mainly due to the fragmentation of the material and too little extent due to the friction between the specimen and the dart. In order to deal with this energy balance equation, the following approximation can be made: (1) the dart can be considered as a rigid body, so its elastic energy is set to zero; (2) the specimen mass is negligible, if compared with the impactor mass, so its gravitational and kinetic energy variation can be considered negligible. 3.1. Free fall, stop, rebound If the energy absorbed by the specimen is not too high a rebound occurs. Fig. 4 illustrates four different graphs for a case of re-
Fig. 4. Typical graphs for the rebounding case.
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Fig. 5. Typical graphs for the dart stop case.
Fig. 6. Typical graphs for the perforation case.
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bound. Fig. 4a and b, indicates the time histories of the dart displacement and velocity. Fig. 4c and d, indicates the histories of the force and deformation energy. Measurable ‘‘end of contact” time can be detected when the force between specimen and dart returns to null (Fig. 4d). At this time the energy balance can be formulated and thus obtain the whole energy dissipated by the specimen in order to initialize and propagate fractures inside the material. Another measurable characteristic time can be defined when the dart velocity becomes zero, at this time the dart has reached its ‘‘maximum penetration’’ Df into the specimen (Fig. 4a and b). There are two quantities involved in this situation: the first one, Dh; is controlled by the experimenter, the other one, Df; can be evaluated only a posteriori but nevertheless known once the test is actually performed.
It is worth noting that the maximum deflection, in the case here examined, is about two times the plate thickness. Therefore, membrane stiffening occurs and laminate membrane strength is mainly involved. The force history (Fig. 4c and d) shows two thresholds: the first one is at 2250 N, where the curve sharply changes its look and a deviation is visible, the second one is at 5620 N, where the curve sharply drops down and then takes again to grow but with a slope lower than the previous one. The first threshold can be interpreted as the indication of the first material damage. The second threshold occurs at the first lamina failure. The force versus displacement graph (Fig. 4c) shows a closed loop. The area under the curve is the absorbed energy that is initially progressively transferred from the dart to the plate and then
Fig. 7. Superposition of force–displacement curves for different impact velocity.
Front
Front
Specimen 1
Specimen 2
Back
Specimen 1
Back
Specimen 2
Fig. 8. Images of damaged specimens 1, 2, 3, 10, 11 and 19.
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Front
Specimen 3
Back
Specimen 3
Front
Specimen 10
Back
Specimen 10
Front
Specimen 11
Back
Specimen 11
Specimen 19
Back
Specimen 19
Front
Fig. 8 (continued)
given back from the plate to the rebounding dart, the area included inside the loop refers to the energy absorbed during the impact. Fig. 4d shows the deformation energy history: energy progressively grows until the maximum displacement is reached (the
maximum energy level is equal to the initial kinetic energy of the dart) then energy progressively decreases until the dart detaches from the plate. It is reasonable to suppose that from this point to the end of the test the specimen does not dissipate energy
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any further, only releasing the residual part of the elastic energy stored during the penetration of the dart. At final instant the energy value shown in the graph is equal to the dissipated energy. 3.2. Free fall and stop This limit situation occurs when the dart stops without rebounding. Fig. 5 indicates four different graphs for a case of dart stop. Fig. 5a and b, indicates the time histories of the dart displacement and velocity. Fig. 5c and d, indicates the histories of the force and deformation energy. If the dart does not rebound the specimen does not have any residual internal energy to exchange with the dart. At the instant when the dart stops, it has reached the maximum displacement: its ‘‘maximum penetration’’ into the specimen, from Fig. 5a it can be read as value equal to 15.6 mm. At this instant the specimen has dissipated the whole amount of energy, no further energy increment could be dissipated by mean of ‘‘internal’’ fragmentation. Perforation, which is the other dissipative mechanism, is incipient but does not take place. In some sense the material is ‘‘saturated’’. The force history (Fig. 5c and d) shows two thresholds: the first one is at 870 N, where the curve sharply changes its look and deviates, the second one is at 2250 N, where the curve suddenly drops down to 2200 N and than maintains its value (the curve is approximately flat). The first threshold can again be interpreted as the indication of the first material damage. The force versus displacement graph (Fig. 5c) shows a closed loop. The area under the curve is the deformation energy that is progressively transferred from the dart to the plate and, in this case of absence of either a rebound or a perforation, is also the energy absorbed during the impact. 3.3. Free fall and perforation The penetration threshold is defined as the impact energy, when the impactor does not rebound from the surface of the specimen for the first time whereas; the perforation threshold is defined as the absorbed energy, when the tip of the impactor pierces the surface of the specimen. A small increment of energy beyond ‘‘saturation’’ means perforation. Fig. 6 indicates four different graphs for a case of perforation. Fig. 6a and b, indicates the time histories of the dart displacement and velocity, Fig. 6c and d, indicates the histories of the force and deformation energy. The force versus displacement graph (Fig. 6c) does not show a closed loop. The area under the curve is the deformation energy that is progressively transferred from the dart to the plate, when the saturation of the load carrying capacity of the plate is reached, perforation takes place. At this instant the maximum energy absorbed by the material damage mechanisms is read. The energy appears to grow further; this is due to the friction of the edges of the perforation hole against the lateral surface of the dart. In Fig. 6c and d it is well visible that, after perforation, the force remains nearly constant and the energy grows with a constant slope. 3.4. Energy curves The experimental tests were performed on three specimens for every thickness; 2, 4 and 8 mm. Tests were performed two times at three energy levels determined by the falling height, namely 500, 1000 and 1500 mm. The corresponding values of the nominal impact velocities are 3.132, 4.429 and 5.425 m/s. From an energy point of view, Fig. 7 shows eight curves of force versus displacement of the dart at various levels of impact energy. This figure allows us to clearly detect the transition from rebound to perforation: in the perforation cases the curve ‘‘folds’’ onto itself toward decreasing displacement, while in the rebound cases the displacement is monotonically increasing. The transition case is
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shown by the fifth curve (corresponding to 1000 mm of falling height and 5.125 m/s of nominal impact velocity) that evidences very little rebound of the dart after the maximum penetration has been reached. Furthermore it must be noted that the curves are well superimposed upon each other up to the point where the energy initially supplied to the dart is completely transferred to the plate (rebound cases) or the perforation takes place. Specimens have been examined after the impact test with the aim of establishing a correlation between the test conditions and the plate damage. For comparison, images of some damaged specimens are given in Fig. 8. Damage extent at both front (impacted) and back side of the specimens 1, 2, 3, 10, 11 and 19 are given. For a low energy level (falling height of 500 mm, velocity at the impact 3.132 m/s), this fiber layout shows good impact resistance characteristics: almost all the energy is released back to the rebounding dart. The maximum energy is reached at about 7.696 J. Specimens 2 and 3 exhibit complete perforation, when compared with other specimens. 4. Conclusions The behavior of woven fabric composite laminates of EP3 grade subjected to low-velocity impact loading has been examined, submitting to the standard instrumented falling weight test a number of plates made by glass fiber epoxy matrix layers with woven reinforcement and with three different thicknesses. The impactor exhibited three conditions; rebound, stop and puncture on the specimens, when subjected to impact at different energy levels. The experimental setup allows acquiring the impacting dart force versus time signals. The tests have three different results; namely rebound, stop or perforation – on the basis of the potential energy initially supplied to the dart by fixing its dropping height (the mass attached to the dart is fixed). The force versus displacement and the energy versus displacement curves for each type of composite laminate plates have been drawn on the same graph showing an impressive superimposition. These results permit to state that, at least for the considered material, the considered type of loading and in the considered range of impact speed. The glass fiber epoxy matrix has no sensitivity to its mechanical characteristic to the strain rate effect, because the composite sensitivity to strain rate is mostly driven by the resin behavior. The examination of the energy values allows us to define two parameters of interest for the designer: the saturation impact energy (i.e. the maximum energy bearable by the material without perforation) and the damage degree (i.e. the ratio between the total energy transformed (stored and dissipated) and the dissipated part of it). Acknowledgements The authors thank the Department of Production Engineering, National Institute of Technology, Tiruchirappalli, for their support and encouragement for carrying out this work. The authors also thankfully acknowledge the Management, PES Institute of Technology, Bangalore for providing facilities in carrying out these tests and for the technical support during the realization of this work. References [1] Abrate S. Impact on laminated composite materials. Appl Mech Rev 1991;44(4):155–89. [2] Abrate S. Impact on laminated composite materials. Appl Mech Rev 1991;44:155–90. [3] Cantwell WJ, Morton J. Impact resistance of composite materials – a review. Composites Part A 1991;22:347–62.
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