G Ic G IIc. Figure 1: The FML concept: alternating metal and composite layers.

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1 Fracture toughness testing in FML V.P. Brügemann, MSc, Delft University of Technology, Kluyverweg 1, 69 HS, Delft, The Netherlands J. Sinke, MSc, Delft University of Technology, Kluyverweg 1, 69 HS, Delft, The Netherlands H. de Boer, PhD, Advanced Lightweight Engineering, Rotterdamseweg 145, 68 AL, Delft, The Netherlands Nomenclature A: [J] Fracture energy a: [mm] The propagated crack length a o [mm] Initial crack length c [mm] Length variable [J/mm ] Mode I fracture toughness energy G Ic G IIc [J/mm ] Mode II fracture toughness energy G 13 [MPa] Interlaminar shear modulus L [mm] span length P [N] Critical load to start the crack w: [mm] Width of the specimen δ [mm] Crosshead displacement at the moment of crack delamination onset ABSTRACT Fiber Metal Laminates (FML) is hybrid laminates made of alternating layers of thin metal sheets and composite layers. GLARE is based on aluminum alloys, glass fibers and epoxy adhesive/resin, is currently the most advanced laminate of the FML-family. Since GLARE is to be used on a number of aircraft, a good understanding of the parameters that determine the material behavior is necessary. One of these parameters is the fracture toughness: A measure for the energy required to fracture a material. Fracture toughness testing of composite materials without metal layers is relatively straight forward, as plasticity will not occur. However, plasticity will occur in the metal layers during testing of the fracture toughness of Glare and this will obscure the measured fracture energy. The focus of this paper is the development of proper testing techniques for hybrid materials, for fracture energy testing in different fracture modes: mode I, mode II and mixed mode (combinations of mode I and II). The results of these tests can be verified with finite element simulation models. This paper focuses on the development of proper test specimen, the testing and the data processing. An introduction to Fiber Metal Laminates (FML). Fiber Metal Laminates are a family of hybrid materials, based on one concept: Each laminate is made of alternating layers of thin metal sheets and thin composite layers (see figure 1). The composite layers consist of, 3 or 4 unidirectional prepreg layers, oriented in one (UniDirectional) or orthogonal directions (Cross-Ply). Figure 1: The FML concept: alternating metal and composite layers.

2 The alloy for the metal layers can be selected from a wide range of different metal alloys, although for aerospace applications usually aluminum alloys are used. A similar freedom of choice is available for the prepreg materials: a number of fiber and matrix systems can be applied in FML. Of course, compatibility is important and the hybrid laminate should offer special features when compared to existing materials (metal alloys and full composites). FML can be made in many different lay-ups: starting at a /1-lay-up ( layers of metal alloy and one composite layer), the lay-up can be increased by adding multiples of a metal and a composite layer: 3/-, 4/3-lay-up, etc. A brief history of FML [1] The first FML have been developed in the 1980s as a result of research for aircraft materials with increased Damage Tolerance. The first FML type was ARALL (ARamid ALuminum Laminates), laminates consisting aluminum alloys and composite layers made of aramid fibers and a metal adhesive (AF-16 by 3M). During the late 1980s better FML were developed: GLARE, FML made of aluminum alloys and glass fiber epoxy composite. FML show, compared to common aluminum alloys: increased fatigue resistance, high impact resistance, fire resistance, durability, and other favorable Damage Tolerant properties. Compared to full composites the benefits are related to its plastic behavior like formability, ductility, toughness, and good bearing properties. In the mid 1990s Airbus adopted GLARE laminates to be used on the Airbus A380 in large sections of the fuselage skin, dominated by fatigue loads. Glare was brought to the required level of readiness by extensive research, performed at Airbus, Stork Fokker, the NLR and the TU Delft. Mechanical testing of FML In the 1990s and early 000s a large number of tests have been performed on FML. Many tests were to improve the laminates or to qualify and certify GLARE for its application on the A380-aircraft. Most tests were coupon tests: tests to investigate a wide range of material properties under different environments. But also tests at large scale, like skin panels with bonded-on or riveted stringers have been tested. The largest test piece has been a full-scale barrel test (see figure ), in which a number of different materials, including GLARE, have been tested. Figure : The Mega-Liner Barrel Interfaces in FML An important topic of research is the failure of a material or structure. In case of hybrid materials like FML, more than one failure scenario is possible. Each of the ingredients may fail: the metal layer by cracking, like metal sheet, the fibers may break, or the matrix may crack. When fibers fail, a sudden and violent failure of the laminate is the result. In case of matrix cracking, usually the overall laminate will not fail. Another type of failure is the consistency failure, when the coherence between some elements of the laminate disappears. The most important example of this is the delamination of layers. Within a FML two different interfaces can be distinguished (see figure 3): The interface between the adhesive and the metal (A) The interface between the adhesive and the fibers (C) In case of delamination one of the two interfaces fail or the matrix / adhesive fails (B) in a cohesive manner.

3 A Figure 3: Different locations for a delamination. B C Failure of interfaces In the research of the interface strength the laminate can be regarded as a chain, and the weakest link fails first. The interface between different materials, in our case the metal layer and the composite layer, can be loaded by a combination of loads. All loads can be decomposed in peel stresses, perpendicular to the interface, and two inplane shear stresses. This leads to three basic failure modes: Fracture mode I, II and III (see figure 4). Mode I Mode II Mode III Figure 4: Three different fracture Modes Mode I is the failure by peeling, when a load is applied perpendicular to the surface of the interface. During this failure the crack propagates in a stable manner. The mode II failure: failure by shear in the plane of the interface. In mode II, the fracture starts at an edge with displacements due to failure perpendicular to that edge. In fracture mode III, which is also an in-plane shear failure, the displacements associated to this failure are parallel to the free edge. For wide laminates the latter failure mode is uncommon, and therefore not discussed in this paper. The strength of the interface has a significant influence on mechanical properties like: Fatigue resistance. During the crack propagation in the metal layers the fibers in the composite layers bridge the crack: stresses in the metal layer near the crack are bypassed via the fiber layers. So the stresses (and strains) in the fibers increase locally. Due to local delamination of the metal and the fiber layers, the fibers are able to take up these stresses and bypass them over the crack. If local delamination would not exist, also the fibers would fail and FML would not have its excellent fatigue resistance. Bearing: Aircraft structures are assembled using a large number of rivets that are installed in the laminates, transferring loads from one element to the other. The rivets apply a bearing load to the edges of the holes. These bearing loads result in deformation (ovalisation) of the holes and in local delaminations. In particular these delaminations, depending of the strength of the interfaces, are important for the value of bearing failure. If the interface strength can be measured and modeled accurately, more reliable results will be obtained for the simulation of material and structural problems of FML structures. The development of these models is based on a combination of testing and numerical modeling. Test methods for interface strength [] For the experimental part of this research several test methods could be used. In this section four categories of tests will be described briefly, without being complete or comprehensive. Three categories are related to testing of adhesives; the last one is related to testing of composites: 1. The first category of tests is the peel tests. These tests are qualitative in nature and are designed to test the maximum peel stress of the interface. The interface is subjected to a vertical load via a flexible adherent. A typical test of this kind is the Roller Drum Peel Test or Bell Peel Test (see figure 5, left) as described in ASTM test specification D 3167 (Note: all mentioned ASTM Test standards can be found in the Annual Book of ASTM Standards [3]). During this test the Force and displacement of the clamping devices are recorded. The maximum peel force is determined per unit width of the specimen. Since the local stresses and strains at the crack front are very complex, and the result is strongly dependent on the used materials and dimensions, this kind of test is used only for comparative studies. For a good

4 comparison one should test different interfaces using the same set-up: (preferably) adherents, dimensions, test conditions. Other tests for the testing of the peel strength are Stripping Strength Test (ASTM D 903), the T-peel test (ASTM D 1876), and the Climbing Drum Peel Test (ASTM D 1781). Figure 5: Left and middle: The Roller Drum Peel Test or Bell Peel Test. (ASTM D 3167) Right: Tensile testing of the adhesive/interface (ASTM D 897).. The second category to be mentioned is the shear tests. In these tests the interface is loaded in shear. A number of these tests are based on lap joints. Of course, the single lap joints do not result in a pure shear mode; for the double lap shear specimen the peel stresses are limited (though not absent). If the thickness of the adherents in the specimen increase, the peel stresses in symmetrically loaded specimen, like in double lap joint tests (ASTM D 358) and in thick adherent specimen (similar to ASTM D 3165) diminish and can be neglected. In these tests shear strength of the adhesive or the interface can be determined. Another type of specimen that can be used for the shear strength of an interface is the Inter Laminar Shear Strength (ILSS) or short Beam Shear test (ASTM D 344). In this test a small coupon specimen is tested in three point bending until failure. The test is an unstable one. The maximum load of the test is used to determine the shear strength of the interface. 3. The third category is the tensile and cleavage tests. In these tests the tensile strength of the adhesive/interface is determined, which is an average value of a particular surface. Most of these specimens have a butt type adhesive joint, which requires very accurate alignment during testing. A typical example is the tensile test according to ASTM D 897 (see figure 5, right). Another test method is the test for the cleavage strength of bond lines/interfaces. In these tests the interface is loaded by peel forces again, but in this case the adherents are rigid. Typical test methods are Double Cantilever Beam tests (ASTM D 3433), having adherents with increasing bending stiffness, and Wedge edge tests as proposed by Boeing (ASTM D 376). 4. The last category to be mentioned here are the fracture energy tests as used for composites. The Double Cantilever Beam specimen is used to test Mode I, Mode II and Mixed mode failures. For each Mode of fracture a different set-up has been specified. During testing the fracture energy or fracture load is determined, which is used to calculate the fracture energy of the interface under that particular loading. The composite specimen used in these tests show elastic deformations and therefore, upon unloading, the stored elastic energy can be subtracted from the total applied energy. This type of specimen is used for the testing of FML and will be further discussed in the next paragraph. Fracture energy testing - The selected test methods The fracture energy tests that have been mentioned in the previous section are among the few test types that can be used for the generation of quantitative values of interface strength: values that can be used as input for numerical calculations and simulations. Most of the other methods result in qualitative data that is unsuitable for input in numerical models. Therefore fracture toughness tests were used. The test methods of mode I, mode II and mixed mode are described below.

5 Mode I [ref 4] (see figure 6) In Mode I, peel forces load the test specimen, and the fracture is a tensile fracture of the interface. For determination of a quantitative value of the fracture energy of the interface, the load and displacement are recorded during the test. From the Load-displacement plot the fracture toughness energy of the interface (G Ic ) is calculated (see below). Figure 6: Mode I Test setup (left) and specimen dimensions (right) The specimen is shown in figure 6, and has a length of about 300 mm and a width of about 5 mm. The precrack in the specimen is prepared using a release film with a predefined length (a 0 ). At one end of the specimen, the pre-crack, hinges are mounted to the specimen in order to be able to apply the peel load. The test procedure is: 1) Application of a thin layer of correction fluid to the edge of the specimen to visualize crack propagation ) Secure the hinges to the specimen 3) Preloading the specimen to obtain a sharp notch/crack: Apply a load to the specimen, at a constant cross head speed, until the crack has propagated 10-15mm, mark crack end location, and unload specimen. 4) Testing of the specimen: Reapply, at a constant cross head speed, the load and let the crack propagate about 100 mm starting at the pre-path; mark final location of crack. 5) Calculate the fracture toughness (G IIC ) using the following formula: G IC = A a w J / mm (1) With: A: [J] Fracture energy, the green area indicated in Figure 7 a: [mm] the propagated crack length w: [mm] width of the specimen Figure 7: Typical force displacement curve of mode I testing

6 Mode II [ref 5] (see figure 8) In Mode II the test specimen is loaded by shear stresses, introduced by a bending moment. The fracture of the interface occurring during this test is an in-plane shear fracture. For the determination of a quantitative value of the fracture energy of the interface, the load and displacement during the test are recorded. Using the maximum load and displacement from the plot the fracture toughness energy of the interface (G IIc ) is calculated. Note: The specimen is either the second or remaining part of a specimen tested in mode I, or a new specimen containing a release film with a predefined length (a 0 ) in case of fiber bridging during mode I testing. As fiber bridging occurred during mode I testing (see figure 9), a new specimen was used for mode II testing. P Figure 8: Mode II test setup (left) and specimen dimensions (right) a o P/ P/ L The test procedure is as follows: 1) The specimen is positioned on the supports ) Specimen is loaded until crack growth occurs. 3) Calculate the fracture toughness (G IC ) using the following formula: G IIc = 9 a o 3 4 w L P ( C C SH 3 [ ( a / L) ] o ) J / mm () Where the specimen compliance C is given by: C = δ / P and C SH 3 L 6L + 3a0 a0 = mm / N (3) 0whG 13 With: G IIc [J/mm ] Mode II fracture toughness energy G 13 [MPa] Interlaminar shear modulus a o [mm] Initial crack length see figure 8 P [N] Critical load to start the crack δ [mm] Crosshead displacement at the moment of crack delamination onset w [mm] Width of the specimen L [mm] span length see figure 8 Figure 9: Fiber-bridging in a Mode I test specimen

7 Mixed Mode [ref ] (see figure 10) The third type of tests is the Mixed Mode test; a test in which the interface is loaded with peel and (in-plane) shear forces (a combination of mode I and mode II testing). For the test series different ratios between the Mode II and Mode I can be selected. This ratio is determined by the values of the span length (L) and the variable (c) (see figure 10) according to equation 4 (for c L 3 ). Mode II Mode I = 3 4 ( c + L) ( 3c L) (4) The test is recorded with a Force-displacement curve, like the Mode II test. Again, depending on the Mixed Mode ratio, instability of the fracture may occur. However, the focus of this paper is on finding a specimen for testing mode I and mode II. For the time being, it is assumed that the same geometry that is used for the two other tests, can be used in mixed mode testing. Figure 10: Mixed mode test setup (left) and specimen dimensions (right) Modifications to the test method Several modifications to the original test specifications were made [ref 4 and 5]. These modifications were largely needed because these tests were designed for composites like Carbon Fiber Reinforced Plastics instead of FML. An important difference between the two material types is the occurrence of plastic deformation of the thin metal layers in FML during mode I and mode II tests. As this plastic deformation consumes energy and cannot be distinguished from the energy required for delamination, modifications have to be made to obtain reliable values for the fracture toughness energy. A method to overcome the plasticity during the mode I, mode II and mixed-mode tests is the application of additional doublers on the FML to increase the thickness of the cantilever beams. In this way, stresses in the thin aluminum layers will be reduced and kept in the elastic regime. Three possible material candidates for the doublers are investigated: Carbon fiber reinforced Steel Aluminum Carbon reinforced doublers are not favorable due to the high material and labor costs involved. Moreover, their high stiffness might result in accumulating energy in the specimen before delamination starts, which could result in unstable crack growth. Steel was considered a suitable candidate due to the relative high stiffness and high yield strength. Therefore, it was decided to bond mm steel doublers (yield strength 400 MPa) with cold-bonded structural adhesive (3M 933A). The piano hinges were bolted to the specimen, as bonding them resulted in de-bonding of the hinges during the test. During testing the bond between the doublers and the steel plates proved to be insufficient and

8 delamination occurred. Some trials were made to improve the surface treatment of the steel plates, but even when the doublers remained attached, still some plasticity occurred. During these test trials with steel doublers a FE model was created to find the optimum thickness and material type for the doublers in order to prevent plasticity during testing and prohibiting cleavage at the same time. It was found that Aluminum 7075-T6 doublers with a thickness of 4.5mm could be used. Two available thicknesses were chosen that approximate this optimum. These were of 0.16 (4.1 mm) and 0.5 (6.4 mm) thickness. With the 6.4 mm doublers no plasticity was observed resulting in mode I tests. However, during pre-path loading, unstable crack propagation occurred (cleavage effect). Also plasticity occurred during mode II testing. With the 4.1 mm doublers, no plasticity occurred in mode I, and stable crack growth was found. The mode II tests also showed no plasticity. The final specimen (see figure 11) that was used for the tests was therefore a set of Aluminum 7075-T6 doublers on both sides of the specimens with a thickness of 4.1mm. In between, it was chosen to test a GLARE /1 lay-up in which the aluminum layers had a thickness of 0.4mm. Figure 11: Final test specimen geometry Relevant parameters Parameter study In order to verify whether the modified specimen resulted in proper test data a few additional experiments were performed, which are described in this section. Final crack length mode I In the ideal case of mode I testing the fracture energy G IC is constant over the entire crack length. To verify this, one of the Mode I specimen has been tested incrementally. At a number of locations (see figure 1), the correlation is made between the force and the location of the crack tip Force [N] Croshead displacement [mm] 8 Figure 1: Indications on specimen (left) and force displacement curve (right) The fracture toughness is then calculated using two methods (results in figure 13): 1. From the start of the crack until the different data points (start to point 1, start to point etcetera). At the start of the test, a relative low value of the fracture energy is found that is increasing with increasing cumulative crack length. Comparing the values to the value of the specified crack length of the test (100 mm), variations of about 10% are found for the crack lengths between 40 and 140mm.. At each increment during the test (start to point 1, point 1 to point etcetera). With this method, more variation of the fracture toughness is found (maximum 17%). Possible causes why the fracture toughness is not constant may be:

9 Due to horizontal forces introduced by the clamping. This horizontal force that may occur is not constant during the test, and may influence the measured fracture toughness. When the distance between the crack tip and the hinges increases the applied force decreases and the radius of the curved cantilevers increase. This may have an effect on the stress distribution at the crack tip, which affects the fracture energy value of the test GIC Fracture toughness 0-1, 0-, 0-3,... Fracture toughness 0-1, 1-, -3, Delamination length [mm] Figure 13: Relation between fracture toughness and Delamination length: Absolute (left) and relative (right) Precrack length mode II During testing of Mode II an unstable crack propagates in the specimen, with a pre-crack length of 30mm. When, for a pre-crack of 30 mm, comparing the tests results to the theoretical ones at the onset of unstable crack growth, it was found that the experimental fracture toughness was lower than the theoretical value. To tackle this problem, some tests were prepared for increasing initial crack lengths (a) of 30, 50 and 70mm (see figure 14). It was found that for increasing initial crack length, the experimental fracture toughness values of the tests are closer to the theoretical values. The reason for the found difference between theoretical and tested value is: With increasing unstable crack propagation, an increasingly high load has to be applied to the specimen. Small imperfections that might be present in the specimen may lead to premature failure at a lower load than required to find the actual fracture toughness. By increasing the initial crack length, the required force applied to the specimen becomes lower and more stable crack growth occurs. As a result, the tested fracture toughness is more similar to the theoretical value. From numerical modeling it was found that no complete failure of the interface occurred at the time that the crack starts to propagate. In other words: the fracture energy calculated from the experimental data is not the real energy that is required to let the crack propagate over a certain length, but the amount of energy that is required to let the crack propagate without increasing the force. As a result, the fracture toughness that is found during tests is lower than the theoretical value a=30mm, L=95mm, Test, GIIC=1.59N/mm a=50mm, L=95mm, Test, GIIC=1.86N/mm a=70mm, L=95mm, Test, GIIC=.08N/mm a=30mm, L=95mm, Theoretical, GIIC=.3 N/mm a=50mm, L=95mm, Theoretical, GIIC=.3 N/mm a=70mm, L=95mm, Theoretical, GIIC=.3 N/mm Force [N] Displacement [mm] Figure 14: Comparing tested force displacement curves theoretical ones

10 Is the mode I specimen actually tested in mode I? To ensure that the test really measures only Mode I fracture energy, a picture of the consumed Mode II fracture energy is given in Figure 15. Figure 15 shows the top view of the specimen with the location of the release film on the left hand side. The maximum value for the consumed Mode II fracture energy is only N/mm and this value is only reached at the free edges of the specimen. Hence, it can be concluded that the amount of Mode II is negligible e e+00 Damaged part of specimen Undamaged After test Release film Figure 15: Consumed mode II fracture energy during mode I testing (top view of specimen) Conclusions / Recommendations Due to plasticity occurring in FML, a number of modifications were made to the test specifications. It was proven that additional layers of 7075 Aluminum with (thickness 4.1mm) prevent plasticity occurring during both mode I and mode II tests. The parameter studies have shown that the proper specimen was found. Therefore, the test results can be used as input for numerical calculations. In this paper, no verification was made to the quality of mixed mode testing. This should be the focus of future work. Acknowledgements This paper is based on the work of the FIMELAS project. The authors wish to acknowledge the scientific cooperation with the FIMELAS partners: Airbus and Advanced Lightweight Engineering. References 1 Vlot, A., GLARE History of the development of a new aircraft material, Kluwer Academic Publishers, The Netherlands, 001 ASM Handbook, Volume 8 Mechanical Testing and Evaluation, 000, ASM International 3 Annual Book of ASTM Standards, Airbus industry test method, Carbon fiber reinforced plastics, Determination of interlaminar fracture toughness energy, Mode I, AITM , Issue, June Airbus industry test method, Carbon fiber reinforced plastics, Determination of interlaminar fracture toughness energy, Mode II, AITM , Issue, June 1994