Impact Testing of Advanced Composites

Transcription

1 Impact Testing of Advanced Composites by Joshua M. Duell The advantages of composite materials are numerous and well documented. Composite materials are often used in environments in which they will suffer from impact damage. For example, damage can occur from a hammer being dropped on a composite pipe or from a bullet striking composite armor. Since impact damage resistance is such an important property for composite materials, this chapter will be devoted to the theory behind impact testing, and the procedures used to perform impact testing. The chapter will describe in detail the best way to perform impact tests. Different ways to evaluate impact data will be examined, as well as ways to characterize the impact induced damage. Impact testing fits into two main categories: (a.) low velocity impact, and (b.) high velocity impact [1]. These two main categories lead to three main types of impact testing. Charpy impact testing and drop weight impact testing fall into the category of low velocity impact testing (here it should be noted that an impact test machine can be used for high velocity impact also; for reference see ASTM D 3763 [2]). Ballistics impact testing falls into the category of high velocity impact testing. Technology has increased to the point that there are now sophisticated measuring devices for instrumented impact testing. For all low velocity instrumented impact test devices there are three major components: the dynamic load cell (or tup), the data display system, and the signal conditioning unit [3]. The tup is placed on the impactor used to strike the specimen. Within the tup is a strain gage that measures the change in strain vs. time as the impactor strikes the specimen. The signal conditioning unit removes the noise associated with the signal, and the data display system plots the measured data. An in-depth exploration on the benefits and methods of each type of impact test will be explored more in the following sections. 6.1 Charpy Impact Testing Charpy impact testing has been used for many years to test the impact toughness of various metals. The advent of modern composites brought 97

2 98 Advanced Topics in Characterization of Composites about materials with properties that depend on their orientation. Consequently, new test methods had to be found to accurately test the directionally dependent impact resistance of composite materials. Since Charpy impact testing is both cheap and fast, its use was extended to composites. A Charpy impact test machine is shown in the following picture. Figure 6.1 Charpy impact tester Specimen Preparation The specimen that fits into the Charpy impact tester is rectangular with a notch cut in one side. The notch allows for a predetermined crack initiation location. Many composite Charpy impact tests are performed without the notch cut into the specimen. In these cases it should be noted in the experimental procedure if the notch is not present. A typical Charpy impact specimen is shown in the following figure. Figure 6.2 Charpy impact specimen

3 Impact Testing of Advanced Composites 99 For a typical fiber reinforced polymer Charpy specimen, L = 126 ± 1 mm, D = 12.7 ± 0.15 mm, and 3.00 mm < w < 12.7 mm [4]. A typical composite Charpy specimen is first made by creating a large panel of laminae oriented in the direction that is dictated by the problem definition. When creating a panel using a wet lay-up or prepreg technique, the wet-out reinforcement plies are stacked in the desired configuration to get a panel that is several layers thick. The specimen is then placed in a vacuum to remove excess resin, and allowed to cure (with or without external pressure and heat). A figure of the final panel lay-up is displayed in the following figure. Figure 6.3 Plate lay-up Once the composite specimen has finished curing, the composite specimen is removed from the vacuum bag setup. (For more information on composite lay-up preparation, refer to the work by Adams, Carlsson, and Pipes [5].) The resultant plate can then be cut into small rectangles which will be used as Charpy impact specimens. The final step is to cut the notch into the specimen Test Setup and Procedure The Charpy impact test method works by placing a notched specimen (with the notch facing away from the point of contact) into a large machine with a pendulum of a known weight. The pendulum is raised to a known height and allowed to fall. As the pendulum swings, it impacts and breaks the specimen, rising to a measured height. A figure displaying the process is shown below.

4 100 Advanced Topics in Characterization of Composites h f h i Figure 6.4 Charpy impact procedure The difference in the initial and final heights is directly proportional to the amount of energy lost due to fracturing the specimen. The total energy of fracture is determined by Γ = mg h h ) (6.1) total ( o f where total is the total energy, m is the mass, g is gravitational acceleration, h o is the original height, and h f is the final height. The failure types for composite Charpy impact tests depend on the specimen orientation. Often, specimens exhibit fiber fracture and fiber pull-out, while other times delamination failure is the primary failure mode. Examples of post fracture Charpy impact specimens with different failure modes are shown below. Figure 6.5 Picture of failed composite Charpy impact specimens [6].

5 Impact Testing of Advanced Composites 101 Specimens were tested with lay-up angles of 0, 10, 22.5, 30, 45, 67.5, and 90 degrees (from left to right in Figure 6.5). It is possible to see that specimen 1 failed from fiber breakage and pull-out. Specimen 2 failed from a combination of fiber pull-out and fiber-matrix separation, and specimens 3-7 failed at the fiber-matrix interface. Composites therefore may need to be tested in different fiber directions due to the anisotropy of the material. The failure type is important when characterizing composites Instrumented Charpy Impact Machines Advances in technology have led to instrumented Charpy impact testers. A Charpy impact testing machine like the one shown in Figure 6.1 can be used with high speed photography to yield data that shows the progression of the crack growth in accordance with time [7]. Other types of instrumented Charpy impact testers use an electronic tup on the pendulum to measure strain rate as the pendulum passes through the specimen. The strain data can be converted to force and recorded as a function of time. Data can be gathered to show the change in the toughness with respect to the velocity of the pendulum. A graph comparing three different types of composite materials is shown in the following figure where C v is called the Charpy energy: Figure 6.6 A toughness comparison of three different composite materials [8]. From Figure 6.6 it can be seen that the S-glass composite performs much better than that of the other two composites. Charpy impact testing can also be used to compare different types of composite layups, including woven and unidirectional laminates. In composite materials, unidirectional prepreg materials are the most tested of any configuration of composite materials. In the following table, a comparison is shown for the test results of both woven and unidirectional carbon fiber composites.

6 102 Advanced Topics in Characterization of Composites Table 6.1 Impact Energy of Carbon Fiber Composites [9] Unidirectional Woven Impact Strength 450 kj/m kj/m 2 Absorbed Energy 4.48 J 2.36 J Charpy impact testing has been expanded to devices that are capable of measuring impact strength, impact force, and impact deflection [9]. 6.2 Drop Weight Impact Testing Drop weight impact testing is another type of low velocity testing, and it is the most common test for composite materials. Drop weight impact tests are done to test the impact behavior on composite plates, which most closely resemble impact damage in the field. When using a drop weight impact tester, two categories of damage can occur. The first is clearly visible impact damage (CVID), which can easily be seen by the naked eye. The second type of damage is barely visible impact damage (BVID), which can seldom be seen by the naked eye. Evaluation of both types of damage can be enhanced through the use of post-impact testing (discussed in Section 6.5). In drop weight impact testing, a mass is raised to a known height and released, impacting the specimen. The choice can be made between either an instrumented or non-instrumented test machine. A figure displaying how an instrumented impact machine works is shown below. W Flag Instrumented Tup Velocity gate H v 0 x(t) Support Specimen Figure 6.7 Example of the operation of a drop weight impact test machine

7 Impact Testing of Advanced Composites 103 In Figure 6.7, x(t) is the coordinate system, H is the initial drop height, W is the impact weight, and v o is the impact velocity. The tup is a hemispherical impactor that measures the strain during impact [10]. An example of an instrumented impact test fixture is shown in the following figure. Figure 6.8 Drop weight test device (This photo has been provided by Instron Corporation) Specimen Preparation and Test Setup The specimens used for drop weight testing are flat panel composite specimens. The lay-up and material used is dependent on the desired results of the testing. The flat panel specimen preparation is similar to that of the panels used in the Charpy impact testing shown in Figure 6.3. Following the creation of the panel, specimens are cut to the desired size. This flat specimen is then inserted into the test machine and clamped along its edges. The clamps can be placed in a circumferential configuration or in a rectangular configuration, based on the design and test specifications (a common standard is ASTM D 5628 [10]). Once the test specimen is clamped down, the mass is raised to the desired height, and the mass is locked into place.

8 104 Advanced Topics in Characterization of Composites Test Procedure and Data Evaluation Once the machine is set to its correct configuration and all data acquisition software is running, the mass is released and allowed to impact the plate. A picture of an impacted test specimen is shown in the following figure. Figure 6.9 Post impact drop weight test specimen [11]. The strains measured by the tup are loaded into a software program, and the data obtained from the test can be examined to see the impact resistance of the plate. The data is plotted as force, energy, or displacement vs. time. The data, however, is not always conclusive and often post-impact analysis is required [11]. A graph typical of an instrumented impact test is shown below. Load(Nt) Energy(N-M) 1200 Specimen FF Damage Initiation Load (N) Energy (J) Time (msec) Figure 6.10 Example plot of force vs. time curve for drop weight impact test.

9 Impact Testing of Advanced Composites 105 The graph shown in Figure 6.10 gives information about how the material behaves during the impact process. The load drops upon fracture initiation. In order to try to gain more information, the impacted sample is often analyzed using post-impact analysis techniques. Post-impact testing can determine the amount of strength left before ultimate failure, or examine the failure mode of the specimen after it has been impacted Plate Impact Theory In order to characterize composites during impact, theory has been developed by Jang, Huang, Hsieh, Kowbel, and Jang [12]. This theory consists of treating the specimen like a beam in bending. In this treatment, the plate is clamped along two sides only. This theory is different from a more classic approach in which the plate is constrained on all sides. In order to begin, Newton s second law is used and the solution for acceleration, a(t), is given as P( t) a( t) = g (6.2) M where g is the gravitational acceleration constant, P(t) is the load with respect to time, and M is the mass of the impactor. t = 0 at the time when impact has just begun, and, knowing the initial conditions, v ( t) = V, at t = 0 x( t) = 0, at t = 0 (6.3) where V is the velocity just prior to impact. Equation 6.2 can be integrated to obtain an expression for v(t), and this can be integrated to obtain a solution for x(t). v( t) = V x( t) = 0 t + g t P( t )' dt' M v( t )' dt' (6.4) It is important to remember that equations 6.4 work as long as the composite laminate is not punctured. Once the previous quantities are known, it is now possible to solve for the energy absorbed as a function of time.

10 106 Advanced Topics in Characterization of Composites t = Γ( t) P( t )' v( t )' dt' (6.5) Integrating equation 6.5 from 0 until the time t that the impactor is no longer touching the specimen will yield the total energy total. This is a simplified method used to determine the energy absorbed by the specimen Ballistics Testing Ballistics testing is a form of high speed testing that is used to test the ultimate impact strength of composites. High velocity testing is characterized by an impactor traveling in the range of m/s [13]. For high velocity impact conditions, structural response is less important than in a low velocity case, and the damage area is more localized; therefore the geometrical considerations are less important [14]. Ballistics testing consists of firing a high speed projectile at an object and determining after the impact how localized the damage is. This is a good method for testing impact resistance of composites, and has been used for testing products such as composite armor Test Setup and Procedure Ballistics testing is complicated, and care has to be taken during the setup. A typical setup of a conventional ballistics test apparatus is shown in the following figure. Gas Diaphragm Sabot Pellet Velocity Sabot Strain gages Magnet Sabot pin Specimen Figure 6.11 Apparatus for ballistic impact testing

11 Impact Testing of Advanced Composites 107 The test apparatus shown in Figure 6.11 is typically known as a gas-gun impact test machine. The gas-gun apparatus works by forcing gas into the back of the diaphragm which expands and applies pressure to the sabot. The sabot pin is released, and the sabot is forced along the barrel. The velocity pins measure the time it takes for the front of the sabot to travel the distance between the two pins, and outputs a velocity. The sabot is then blocked by the sabot stops, and the pellet is allowed to proceed to impact the test specimen. The strains can then be measured using strain gages applied to the specimen. The pellet used for testing is usually one with a high hardness, and will be made of a hard steel, or zirconia (for the setup shown in Figure 6.11 the pellet would have to be magnetic, as it is held in place by a magnet). An inert gas is used to fill the chamber to cut down on possible accidents Data Evaluation When performing ballistics testing, data evaluation often has to be accompanied by a different method for defining the damage done. There are different ways to characterize that damage, and those pertaining to postimpact damage analysis will be discussed in the following section. A specimen with ballistics impact damage is shown, Figure 6.12 Example of Ballistics Impact Damage [15] where Ra is a radial crack propagating outwards from the impact center, and Ri is a ring crack propagating around the impact center. When doing

12 108 Advanced Topics in Characterization of Composites calculations for ballistics testing, some assumptions must be made. One assumption is that the velocity of the pellet remains constant from the time that it is measured with the velocity pins until the time that it contacts the specimen. The second is that the energy lost from the pellet is proportional to the energy absorbed by the specimen. The remaining energy of the pellet can be measured using sensors to detect its rebound velocity (or in the case of a through laminar failure its remaining velocity). When calculating the necessary unknowns that come with ballistics testing, the equations can become rather complex. For those higher order calculations, refer to the results by Fujii, Yasuda, and Tanabe [13]. 6.4 Comparison of Test Methods A comparison of the different types of test methods can help in making a decision regarding the correct test to use. Charpy impact testing is easy and fast, which allows the researcher to generate large amounts of data. Charpy impact testing is simple in its scope, and therefore its results are not indepth, nor do they reveal a great amount about the material. The Charpy impact test itself is easily set up, and therefore a great number of specimens and a large amount of data can be collected in a very small amount of time. With the advent of high speed photography, Charpy impact tests are now able to produce results that help to show the propagation of the crack, and instrumentation allows the user to measure the force more accurately. The data, however, may not be suitable for some composite materials due to the anisotropy of the material [16]. For these composites, little value can be gained from the results as to the overall behavior of the material. Due to the simplistic model presented by Charpy impact testing, very few types of post impact test methods are effective. Drop weight impact testing is a more common real world scenario than that of the Charpy impact test, and consequently greater amounts of information can be gathered as to the behavior of the laminate. Drop weight impact testing also allows for different configurations to be used. Therefore, drop weight testing tends to be the preferred method when using low velocity impact testing. Drop weight testing results may also be enhanced from post impact testing. Ballistics testing cannot be directly compared to the previous two testing techniques, because it is a high velocity test. Ballistics testing can be more complicated. However, often high velocity impact testing is needed, and cannot be avoided. Ballistics testing is highly effective at replicating the behavior of the composite at high speed impact and allows the user to characterize the material by analyzing measured data. It should be noted

13 Impact Testing of Advanced Composites 109 that it is best to perform a low velocity test when possible, due to simplicity. As is the case with drop weight machine testing, ballistics test results can be enhanced greatly by post-impact testing. 6.5 Test Methods for Post Impact Damage Testing Impact testing of composites may not be all that is needed for characterization. In order to see the damage mechanism or the failure type, sometimes a post-impact analysis is needed for the damaged specimen. There are several ways in which to test a damaged composite specimen Ultrasound Ultrasonic methods are commonly used to analyze composites. The ultrasonic method works by sending continuous waves of sound into the object and capturing the reflected sounds. The reflected sounds paint an image of the object, and in this manner determine the size and shape of the defects. One main disadvantage of the conventional ultrasound test is that in order to gain a good picture of the material layers, the measurement device has to be in contact with the material in order to minimize impedance. Newer technology can enable non-contact ultrasonic testing techniques to be used [17]. Different types of ultrasonic testers currently exist. There are simple devices that give only a digital readout of the thickness to complex systems that show the size and shape of the defects Microwave Testing Microwave testing is another way to analyze impact damage in composite materials. Microwave testing works by sending microwaves through a material, and the defect causes the waves to change. These waves are then absorbed into a thin film, which can be enhanced to display defects in the specimen. The microwave testing techniques are beneficial in that the air surrounding the composite gives very little impedance to the microwaves. Microwave testing can be hazardous, as microwaves can cause health concerns. As with ultrasonic testing, there are different types of microwave testing. One technique of interest is the microwave NDI technique [18] Compression Testing Compression testing after impact is a way to determine how much strength the impacted specimen still contains after initial testing. Compression testing is different from the previous two techniques, in that it is a destructive test. Compression after impact can be used to determine the

14 110 Advanced Topics in Characterization of Composites ultimate stress and strain of a specimen in compression after impact. The data is useful to see if the damaged specimen is still strong enough to perform its designated function. There are many different types of compression testing, and therefore many different types of compression fixtures. There are several different ASTM standards that deal with the different types of compression testing (however, they do not mention compression after impact, and care needs to be taken if using these standards). A good outline of the mathematical relationships, and test procedures can be found in SACMA SRM 2R [19]. The SACMA standard is one of two standards that exist on post impact compression testing of composite materials, the other being a standard created at Boeing Other Types of Post Impact Testing There are also several other types of post-impact damage testing, most of which are non-destructive (ND) test methods. Some of the other types of testing are three and four-point bending, x-ray, optical, scanning electron microscopy (SEM), and thermographic testing. Three and four-point bending works by performing the bending test on the impacted specimen. However, there are no standards to govern performing this test on postimpact specimens. X-ray testing works by blasting the sample with x-rays and measuring the difference in the reflected x-rays. Optical testing works by placing the impacted specimen under a microscope to analyze the fracture surface. The SEM technique works by sending electrons to the specimen and measuring the returning electrons to gain a visual image of the fracture surface. Both optical and SEM allow the user to see only the surface of the specimen, instead of being able to see the inner layers of the lamina. Thermographic testing works by flash-heating the material and measuring the heat as it is dissipated from the sample using an infrared camera. Thermographic testing is gaining popularity due to its accuracy and effectiveness Deciding on a test technique The costs and benefits of the various methods presented here are diverse, and in order to decide on what type of post-impact test to do, time, cost, availability of a test method, and the information that is needed has to be considered. For Charpy impact testing, only the fracture surface would need to be viewed, and optical techniques would be advantageous (both inexpensive and fast). However, for ballistics and impact testing, the layers beneath the surface would need to be seen to determine if any residual damage has been caused. In the case of a ballistics and drop weight impact

15 Impact Testing of Advanced Composites 111 test specimen, one may want to characterize the strength left in the composite. Therefore, a compression test may need to be done to the specimen. The post-impact test technique for a laminated plate may be be either qualitative or quantitative. Non-destructive testing is qualitative and shows the damage caused by impact testing. Destructive testing is quantitative and will give the material properties (such as modulus, stress, strain, etc.) of the composite after impact. If necessary, both types of testing can be done, provided the ND technique is done first. References 1. R. Mantena, P. (Dept. of Mechanical Engineering, The University of Mississippi); Mann, R.; Nori, C. Low-velocity impact response and dynamic characteristics of glass-resin composites; Journal of Reinforced Plastics and Composites, 20, p , ASTM D 3763, High Speed Puncture Properties of Plastics Using Load and Displacement Sensors, W.R. Hoover, Effect of Test System Response Time on Instrumented Charpy Impact Data; Instrumented Impact Testing, ASTM STP 563, p , ASTM D 6110, Determining the Charpy Impact Resistance of Notched Specimens of Plastics; D.F. Adams, L.A. Carlsson, R.B. Pipes, Experimental Characterization of Advanced Composite Materials, CRC Press, 3 rd ed Tomita, Yoshiyuki (Osaka Prefecture Univ); Tamaki, Toru; Morioka, Kojiro, Effect of fiber strength on notch bending fracture of unidirectional long carbon fiber-reinforced epoxy composites; Materials Characterization, 41, p , Oct, C.B. Bucknall (Cranfield Inst of Technology), Relevance of impact testing in the materials science of polymers; Plastics, Rubber and Composites Processing and Applications, 17, p , R.H. Toland, Impact Testing of Carbon-Epoxy Composite Materials; Instrumented Impact Testing, ASTM STP 563, p , M. Nagai and H. Miyairi, The Study on Charpy impact testing method of CFRP; Advanced Composite Materials: The Official Journal of the Japan Society of Composite Materials, 3, p , ASTM D 5628 Standard Test Method for Impact Resistance of Flat, Rigid Plastic Specimens by Means of a Falling Dart (Tup or Falling Mass), S. Ujihashi, (Tokyo Inst of Technology); Intelligent method to determine the mechanical properties of composites under impact loading; Composite Structures, 23, p , 1993.

16 112 Advanced Topics in Characterization of Composites 12. B.P. Jang, C.T. Huang, C.Y. Hsieh, Kowbel, W.; Jang, B.Z., Repeated impact failure of continuous fiber reinforced thermoplastic and thermoset composites; Journal of Composite Materials, 25, p Sep, K. Fujii, (Materials and Structures Laboratory, Tokyo Institute of Technology); Yasuda, E.; Tanabe, Y., Dynamic mechanical properties of polycrystalline graphites and a 2D-C/C composite by plate impact; International Journal of Impact Engineering, 25, p , May, C. Navarro (Carlos III Univ), J. Rodriguez, and R. Cortes, Analytical modelling of composite panels subjected to impact loading; Journal De Physique. IV : JP, 4, p C8-515-C8-520, Sept, Y. Akimune, Influence of Powder Characteristics on Impact Damage in SiC- Whsker/Si 3 N 4 Composites; Journal of the European Ceramic Society, 6, p , S. Ujihashi (Tokyo Inst of Technology), Intelligent method to determine the mechanical properties of composites under impact loading; Composite Structures, 23, p , M.C. Bhardwaj, G.F. Stead, Non-Contact Ultrasound: A New Dimension for in and Post-Process Analysis of Materials; 33 rd International SAMPE Technical Conference, p 46-57, R. Zoughi, C. Lebowitz, S. Lukes, Preliminary Evaluations of the Potential and Limitations of Microwave NDI Methods for Inspecting Graphite Composites; Materials Science Forum, , p , SACMA SRM 2R-94 Compression After Impact Properties of Oriented Fiber- Resin Composites