Acoustic Emission testing to evaluate a new parameter in composite materials: Delay cracking Load

Transcription

1 More info about this article: 8 th International Symposium on NDT in Aerospace, November 3-5, 2016 Acoustic Emission testing to evaluate a new parameter in composite materials: Delay cracking Load Giuseppe NARDONI 1, Nasim FALLAHI 1, Pietro NARDONI 1, Mario TURCONI 2, Franco MONTI 2 1 I&T NARDONI INSTITUTE, Brescia, 25124, Italy Phone: , Nardoni.campus@gmail.com 2 Center of integrated training systems, LEONARDO Aircraft, Vengono Superiore (VA), 21040, Italy mario.turconi@leonardocompany.com Abstract The aim of this paper is to investigate a new parameter in the behavior of carbon-epoxy composite materials: Delay Cracking Load (DCL). The delay cracking load is a constant load that applied at a three point flexural bending test give up to a delay cracking process up to the complete fracture in composite specimens. Only Acoustic Emission technique is able to predict the delay cracking in constant load. DCL is an important parameter in the characterization of the composite materials. The interval between the load steps based on this research shall not be less than 2-5 days. Keywords: delay crack load, Acoustic emissions, Carbon-epoxy composite, fracture mechanisms. 1. Introduction Carbon fiber composites [1, 2] in the early 60s were used because of high strength and stiffness coupled with low weight and high resistance to corrosion and fatigue compared with traditional materials such as metals (see figure 1). Carbon fibers are a new generation of high strength fibers that used in composites with light weight matrix such resins. For this reason Fiber composites are suitable to make aircraft parts (Fig 2). Till today, knowledge about plastic materials failure limit is more complex than for linear elastic materials like metals. Plastics can be dimensioned by means of critical strain as failure limit. So, acoustic emission method is a valuable analysis in a tensile test, to get quick value of critical strain of a new material [3]. Interface fracture is one of the critical failure modes for composite structures, which may lead to the separation of plies and eventually to the fault of the component: it is therefore necessary to strengthen the interlaminar fracture toughness for highly reliable composite materials and structures. Knowledge about the fracture mechanisms is vitally important in different kinds of loading test. As some of the authors have already demonstrated, interleaving small diameter

2 fibers between one or more interfaces, improves strength, toughness and delamination resistance of composites without reducing in the in-plane properties or adding weight [4-9]. For composite materials design, knowledge of fracture mechanisms is vitally important, so AE is one of the suitable techniques to investigate the online fracture mechanisms [5, 10-14]. The most important advantages of AE is on-line monitoring for fracture or friction between the layers or fibers and the capability to detection of the damage mechanisms [15]. Most common analyzing method for AE, uses certain features extracted from signals such as amplitude, energy, count, rise time [16]. Ratio of strength/stiffness to weight is high Stiff Strong Light weight Corrosion Figure 1. Benefits of using composite materials Figure 2. Different kinds of composite materials in aerospace field The successful development and design of continues fiber reinforced carbon epoxy composites are dependent on a thorough understanding of basic properties such as fracture and delay failure, slow crack growth or damage accumulation[17]. 2. Materials and Methods 2.1. Composite fabrications Panels were fabricated by hand lay-up process, after the lay-up, panels were cured by vacuum

3 bag in an autoclave in 185 C (365 F) according to process specification provided by supplier. Carbon unidirectional preprag fiber-epoxy composite (table 1) were used in constant load to investigate the delay crack in carbon epoxy composite in sequence loading (Fig 3). Table 1: unidirectional 0/90 preprag of carbon epoxy composite Properties Value Percentage of resin 33% Percentage of fibers 67% Preprage (weight per unit area) 134 g/m 2 Fiber type Unidirectional carbon fibers 0 / Mechanical flexural bending testing The three point flexural bending test has appointed to perform flexural test for the characterization of composite materials and two AE sensors have been applied on the specimens (Fig 4). Mechanical hydraulic test has been set up to minimize the noise level on the acoustic emission system during the step loading on the specimens. The flexural test has been calculated according to the ASTM D790[18]: AE features such as count, rise time, energy and amplitude, = 2 (1) : Stress in the outer fibers at midpoint, MPa (psi). P: load at a given point on the load-deflection curve, N (Newton). L: support spans (mm). b: width of beam tested (mm) and, d: depth of beam tested (mm). (A) (B) Fig 4: (A) Three-Point bending flexural test by AE, (B) Three-points bending test on carbon epoxy composite through silent hydraulic pressurized

4 2.3. Acoustic emission equipment The AMSY-6 acoustic emission system 1 was used for damage monitoring during monotonic 3-point bending examination of composites in mechanical hydraulic test. The AMSY-6 was equipped with Vallen software for data acquisition and analysis. Loading data were transferred directly to AMSY-6 system allowing for correlation with acoustic signals obtained from examined GFRP and CFRP specimens. The AE signals were registered using two broad-band sensors attached to specimen s surface with rubber band and vacuum grease as a coupling agent (Fig 4B). The Vallen AEP4 preamplifiers had the gain set to 34. System threshold setting varied from 34 to 46. Sensitivity of the sensors and placement of the sensors was calibrated by Hsu-Nielsen source preceded the actual measurements. After the calibration step, AE signals were recorded during the mechanical testing. Signal features such as count, rise time, energy and amplitude were used for investigating of the failure mechanisms (Fig 5). Figure. 5 Amplitude vs. time presentation of AE signals during the delay cracking load. The first and second load applied before in the period of 7 days does not provoke any delay cracking. 3. Results and Discussion In the Carbon epoxy specimens, the same procedure has been applied as for the glass-epoxy composites. The delay cracking load has been identified at the third applied load. This load has been kept constant for all the test duration. After 184 hours the first high amplitude clusters of 1 Manufactured by VALLEN System GmbH, Icking, Germany.

5 AE signals representing the micro cracks in fiber and matrix (see Fig 5). From 222 hours a scattered high amplitude signals ranging from (60-95 db) appear in the monitor of AE. The pattern distribution of these signals was different from the glass epoxy composites, not in periodical cluster but in the sky stars shape signals. At the end time (264 hours) through the thickness surface breaking crack emerged in the specimens (see figure 5). The crack was along the axis of the high stressed area (figure 6). At the end stage SEM picture captured from the surface of fracture (Fig 7). Figure. 6 Main surface breaking fracture appeared after continues process of delay cracking Figure. 7 SEM from the fracture in carbon epoxy composite: (A),(C): matrix cracking and fiber breakage (B) fiber breakage (D) matrix cracking[19, 20] 4. Future work The important of the phenomena of delay cracking load in the project and manufacturing composite materials speed up a research to correlate the final ration of strength data with the delay cracking load data. Most probably this will have as a result more reliable composite material for the construction which are more and more complex and extended in the application. The recent introduction of nanofibers (Fig 8) in composite material manufacture will give

6 ongoing research may give a contribution improving the mechanical properties of the composite materials [19, 20]. 5. Conclusion Delay cracking load is a new investigation in composite materials. In 3-point bending by apply the constant load for a long time, fracture mechanisms occur. Knowledge of the critical load is an important parameter to prevent delay crack loading. Acoustic Emission technique is a suitable method to detect the delay cracking load in composite materials. For the better indication of delay cracking load, attention to the amplitude of the signals, strongly provides. so the knowledge of critical constant load could help to prevent the failure mechanisms by the time in each composite material. Figure. 14 (a) nanofibers of nylon 6,6 (b) fracture mechanisms in Nano nylon 6,6 in carbon epoxy composite Reference [1] Carboni M, Gianneo A, Giglio M. A Lamb waves based statistical approach to structural health monitoring of carbon fibre reinforced polymer composites. Ultrasonics. 2015;60: [2] Carboni M, Gianneo A, Giglio M. A LOW FREQUENCY LAMB-WAVES BASED STRUCTURAL HEALTH MONITORING OF AN AERONAUTICAL CARBON FIBRE REINFORCED POLYMER COMPOSITE. [3] F. Willems JB, Ch. Bonten. Detection the critical strain of fiber reinforced plastics by means of acoustic emission analysis. 32th European Conference on Acoustic Emission Testing. Czech Czech Society for Nondestructive Testing; p [4] Bacon R. Growth, Structure, and Properties of Graphite Whiskers. Journal of Applied Physics. 1960;31: [5] Dzenis YA, Reneker DH. Delamination resistant composites prepared by small diameter fiber reinforcement at ply interfaces. Google Patents; [6] Kim J-s, Reneker DH. Mechanical properties of composites using ultrafine electrospun fibers. Polymer Composites. 1999;20: [7] Refahi Oskouei A, Zucchelli A, Ahmadi M, Minak G. An integrated approach based on acoustic emission and mechanical information to evaluate the delamination fracture toughness at mode I in composite laminate. Materials & Design. 2011;32: [8] Arumugam V, Sajith S, Stanley AJ. Acoustic Emission Characterization of Failure Modes in GFRP Laminates Under Mode I Delamination. Journal of Nondestructive Evaluation. 2011;30:213-9.

7 [9] Wevers M. Listening to the sound of materials: Acoustic emission for the analysis of material behaviour. NDT & E International. 1997;30: [10] Philippidis TP, Nikolaidis VN, Anastassopoulos AA. Damage characterization of carbon/carbon laminates using neural network techniques on AE signals. NDT & E International. 1998;31: [11] Marec A, Thomas JH, El Guerjouma R. Damage characterization of polymer-based composite materials: Multivariable analysis and wavelet transform for clustering acoustic emission data. Mechanical Systems and Signal Processing. 2008;22: [12] Bar HN, Bhat MR, Murthy CRL. Parametric Analysis of Acoustic Emission Signals for Evaluating Damage in Composites Using a PVDF Film Sensor. Journal of Nondestructive Evaluation. 2005;24: [13] Ramesh C, Ragesh H, Arumugam V, Stanley AJ. Effect of Hydrolytic Ageing on Kevlar/Polyester Using Acoustic Emission Monitoring. Journal of Nondestructive Evaluation. 2012;31: [14] Yan J, Heng-hu Y, Hong Y, Feng Z, Zhen L, Ping W, et al. Nondestructive Detection of Valves Using Acoustic Emission Technique. Advances in Materials Science and Engineering. 2015;2015:9. [15] Inasaki I. Application of acoustic emission sensor for monitoring machining processes. Ultrasonics. 1998;36: [16] Lu C, Ding P, Chen Z. Time-frequency Analysis of Acoustic Emission Signals Generated by Tension Damage in CFRP. Procedia Engineering. 2011;23: [17] Knauss WG. Delayed failure the Griffith problem for linearly viscoelastic materials. International Journal of Fracture Mechanics. 1970;6:7-20. [18] ASTM. Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials. ASTM: astm; [19] Palazzetti R, Zucchelli A, Gualandi C, Focarete ML, Donati L, Minak G, et al. Influence of electrospun Nylon 6,6 nanofibrous mats on the interlaminar properties of Gr epoxy composite laminates. Composite Structures. 2012;94: [20] N. Fallahi G. Nardoni, H. Heidary, R. Palazzetti, X.T. Yan, A. Zucchelli. Supervised and Nonsupervised AE Data Classification of Nanomodified CFRP During DCB Tests. FME TRANSACTIONS. 2016;44.