Thermography- A Tool for Understanding Dynamics of Destructive Testing in Composites

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1 SINCE2013 Singapore International NDT Conference & Exhibition 2013, July 2013 Thermography- A Tool for Understanding Dynamics of Destructive Testing in Composites Khee Aik Christopher LEE, Teik Sheng NG, Lun Siang TAN, Wern Sze TEO, Dmitry Vladimirovitch ISAKOV Precision Measurements Group, Singapore Institute of Manufacturing Technology 71 Nanyang Drive, Singapore , Singapore kalee@simtech.a-star.edu.sg Abstract. We have explored 18 plies of balanced and symmetrical quasi-isotropic CFRP laminate (±45PW/ [45/0/-45/90]2s /±45PW). Based on the Short-Beam Strength (SBS) Standard test method, a simple analysis show that plies parallel to the beam will be the strongest in the laminate due to high support strength of the fibers resisting the load. Plies that are orthogonal to the beam will be the weakest in the lamina because the fibers play no part in resisting the load. As a result of this orientation the weak matrix has to bare the whole load. It is clear that the laminate stays symmetrical if we cut it along 0 o or 90 o plies. However the analysis above indicates that the response of the beams cut along these orientations cannot be the same. In order to understand this issue we combine SBS test with thermography. The assumption is that the maximum load experienced by the sample corresponds to the catastrophic failure of the composite. At this failure the potential energy from the load will be transferred into other forms of energy with considerable amount converted into thermal energy at the location of the failure. The observations have confirmed this assumption. Heat release was observed with local temperatures rising by several degrees. The hot spots mainly appear at orthogonal layers, confirming our analysis from above. The results have also shown that the failure at maximum load is not the first failure that the beam is undergoing. We have recorded the dynamics of the SBS test at frame rate of 200Hz (5ms per frame) using thermography. This allows us to link the first failure in the laminate with the inflection point in SBS curve. This point indicates the end of elastic deformation and should be considered as the actual degradation criteria with the initiation of plastic deformation. Catastrophic failure manifests itself either as fiber-matrix debonding or distributed micro-cracking, with the failure mode determined by the ply orientation in relation to the beam. Detailed analysis and evaluation have reviewed that the failure is mainly related to the fibermatrix debonding at layers orthogonal to SBS beam. It is clear that the on-set of plastic deformation affects the CFRP integrity and properties considerably, thus the maximum load value for SBS beam strength estimation might not be adequate. Based on these results we would like to recommend that destructive tests for composites, like SBS, are always accompanied with thermography imaging.

2 1. Introduction The introduction of composite in aerospace components has seen an increasing trend over the past decade. For example, Boeing 787 Dreamliner consists of 50% composite materials [1] in the air frame composition. Most of these composites are Carbon Fiber Reinforced Plastic (CFRP) laminates, promising ultimate strength to weight value [2]. However the question for strength estimation remains critical with most methods based on destructive testing. One of the standard methods widely accepted globally is the Short Beam Shear Strength (SBS) test [3]. Non-destructive testing (NDT) technique like thermal imaging of structures has shown promising result in various applications [4]. The proposed idea of a combination of Destructive Testing (DT) with thermography presents several advantages, allowing better control during experimentation, simpler data interpretation, and extracts detailed information on sample properties based on collected data [5]. The potential energy released during SBS indentation converts mainly into thermal energy, which can be related to the SBS curve presented in ASTM standard. This allows research exploration of mechanical observation with theoretical prediction. The ASTM International standard is acceptable for both balanced and symmetric composites for the simulation of failure modes related to interlaminar shear. However, the internal stresses and dynamics are very complex and thus, other failure modes can contribute to the alteration of composite properties. At the end of the test it is impossible to distinguish the exact failure mode dominating, initiating or affecting the outcome. In order to overcome this issue we combine SBS with thermography and observe the failure modes occurring in real time. We focus on balanced quasi-isotropic CFRP structures in particular, balanced on both ends of the mid-plane. However there are no indications in the standard stating the cutting direction of the SBS beam. It is only required to ensure that the beams under comparison be cut along the same direction. This resulted in the ambiguity of the short beam strength of composites, since the orientation of the plies contribute significantly to the mechanical behaviour of the beam. Additionally it is difficult to estimate strength degradation in composites using this method. For example during one-sided heat damage different plies will be exposed to different temperatures. The orientations corresponding to outer layers might have predominant impact on the overall degradation of composite. Through combining thermography imaging with SBS test we target to reveal the dependence of failure mode on ply orientation.

1. Short Beam Shear (SBS) Test The short beam strength evaluation of the untreated samples was conducted in accordance with ASTM D2344-00 (Standard Test Method for Short-Beam Strength of Polymer

The beam then is loaded in three point bending using support anvils of 0.125 inch diameter and loading nose of 0.25 in diameter, as shown in Figure 1. Specimen width, b =3.6mm and thickness, h =7.

3 1. Short Beam Shear (SBS) Test The short beam strength evaluation of the untreated samples was conducted in accordance with ASTM D (Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates). The beams were cut from the same sample to maintain similar properties in composite. The beam then is loaded in three point bending using support anvils of inch diameter and loading nose of 0.25 in diameter, as shown in Figure 1. Specimen width, b =3.6mm and thickness, h =7.2mm measurements were taken at the mid-section of the test specimens using a micrometer. The span length was set to 4 times of thickness ±0.3mm. Loading was conducted at a constant rate of 1mm/min and is terminated when one of the following conditions occur: (i) a load drop off of more than 30% was attained, (ii) two-piece specimen failure and (iii) the displacement exceeding the thickness of the specimen. The short beam strength was calculated from the maximum load, Pmax, and the specimen dimensions using the equation below: ℎ ℎ = 0.75 ℎ Figure 1. Short beam strength test set up (ASTM D ) The typical SBS curve is shown in figure 2. According to the ASTM standard [3] only the maximum value of the load is considered. However, there are other features of the curve that can describe the dynamic response of the sample. First, of all the slope of the linear region represents the elastic deformation of the beam. The deviation from linearity indicates the on-set of plastic deformations. The catastrophic failure manifests itself as a sharp drop in load value. For different sample it can be one or several drops depending on the ply orientation and the material properties..

Figure 2. Typical SBS curve of composite material 3. Sample Description We have used a set of 18 plies CFRP laminates with layout (±45PW/ [45/0/-45/90]2s /±45PW).

4 Figure 2. Typical SBS curve of composite material 3. Sample Description We have used a set of 18 plies CFRP laminates with layout (±45PW/ [45/0/-45/90]2s /±45PW). This layout is balanced and symmetrical to the standard requirement. The composite short beams are cut in two directions relatively to the beam at 0 and 90 orientation, as simulated in Figure 3. In the SBS set-up the load is mainly absorbed by plies with fibers oriented along the beam, while in orthogonal plies load is absorbed solely by matrix. Hence, such orthogonal plies will be the weakest in the layout. In our experiments the position of such orthogonal plies changes when we cut the short beam along different directions. For example, for 0 o beams there are two weak layers in the middle of the beam, and for the 90 o beams the same layers are the strongest in the lay-out. Just from this analysis it is clear that performance of the sample in the SBS test strongly depends on the orientation along which the sample was cut. Hence the response of the beam cannot be the same for both orientation and a proposed solution is required to enhance our drawn conclusion. This prediction is confirmed by SBS measurements, as shown in Table 1. The SBS value is higher for beams cut along 90 o ply. From the SBS curves we have also noticed that the slope of the elastic region is 11% lower for 90 o beams. This indicates that the two central plies with fibers along the beam (Fig.3b) will make the beam more flexible in comparison with 0 o beams (Fig.3a). Table 1. Short beam strength of untreated samples at 0 o and 90 o orientation Orientation 0 o 90 o SBS, MPa ± ± 0.68 Elastic slope ratio, %

5 Figure 3. Comparison of ply orientation for short beams cut along difference plies: (a) along 0 ply; (b) along 90 ply. 4. SBS combined with Thermography Thermography was used concurrently with the SBS test for both 0 and 90 beam at the initiation of the experiment. Prior to the experiment, both samples contain similar signals on the ply orientation. Orthogonal layers to the beam were seen having lower thermal distribution and thermal contrast than parallel layers, but display distinguishable characteristics before and during data extraction which will be highlighted. Both 0 and 90 beams display notable SBS characteristics based on the typical SBS curve. In figure 4 the destructive test results for 0 beam were plotted over time with thermal signal overlapping the information. Elastic deformation could be distinguished due to linearity of SBS curve at the initial stage. This phenomenon is similar to the 90 beam shown in figure 5 prior to the initiation of plastic deformation.

6 Plastic deformation for both orientations produces similar non-linear behaviour through SBS testing. However the differences become apparent through comparison of thermal signal. A spark at the orthogonal plies in the middle of 0 beam was discovered prior at the on-set of plastic deformation before dissipating into the beam. In 90 beam no spike was recorded at the plastic deformation, but a global increase in thermal signal is noticeable before the catastrophic failure. A sharp and distinct spike in the signal aligns directly at the first breakpoint of 90 beam before cooling down gradually due to heat dissipation, while the spike was notable at the second breakpoint in the 0 beam. The 0 beam at the first breakpoint received a smaller yet distinctive spike due to spark ignition at the same orthogonal layers. Figure 4. SBS result combined with thermal imaging for 0 beam

7 Figure 5. SBS result combined with thermal imaging for 90 beam In order to understand the behaviour in Figure 4 and 5 we need to look at the possible failure patterns at microscopic level in CFRP subjected to SBS test. The possible failures are: 1) fiber cracking; 2) matrix cracking and 3) fiber-matrix debonding. Fiber cracking can occur only if the stress is perpendicular to the fiber direction. In Figure 7 (b) and (c) we observed that the heat generation is concentrated at the layers orthogonal to the beam direction. In these layers fibers are parallel to the stress. Hence, fiber cracking is not representing the failure. If matrix cracking takes place the damage is expected to be distributed within the matrix and due to low thermal conductivity the released heat will take a considerable time to dissipate. This kind of behaviour is actually observed in figure 6 for the region between onset of plastic deformation and catastrophic failure. Hence, we can conclude that the failure in 90 o beams starts with matrix cracking. However, for 0 o beams the behaviour is different.

Figure 6. Modes of failure in composite structure The last possible failure is related to fiber-matrix debonding. If such debonding occurs it will be strictly localized at the fiber surface.

8 Figure 6. Modes of failure in composite structure The last possible failure is related to fiber-matrix debonding. If such debonding occurs it will be strictly localized at the fiber surface. Due to high thermal conductivity of the fiber, the heat produced anywhere along the fiber will quickly reach the surface under observation and we should expect sharp increase of surface temperature. At the same time the high surface to volume ratio of the fiber will ensure that the heat is quickly transferred to the surroundings. Hence, the temperature will drop very fast. This is exactly what we observed for all catastrophic failures: 1) very localized heat generation (down to a single fiber), 2) sharp increase in temperature, and 3) similarly sharp drop in temperature. Hence, we can conclude that catastrophic failure is mainly manifested by fiber-matrix debonding. At the same time the on-set of plastic deformation for 0 o beams demonstrates temperature response with similar behaviour and thus, the debonding is responsible for it. This shows a peculiar fact that plastic deformation in CFRP cut along different ply is initiated by different failure patterns. In reference to figure 6, matrix cracking usually occurs when the mechanical compression reaches the threshold of the matrix bonding strength. This is known to occur during the plastic deformation stage, and this exothermic reaction produces heat. However the thermal conductivity of the matrix prevents the dissipation of heat, storing the energy within the short beam affected region. Therefore we can expect a thermal gradient from the onset of plastic deformation until energy is released. In fiber cracking, energy is created and released instantaneously. The fibers in the composite act as the supporting structure, hence it is usually the toughest section to break. In the event of fiber cracking, energy is produced almost instantaneously in the form of a huge spark, resulting in the release of thermal energy. This outpour of energy dissipates almost immediately due to the high thermal conductivity of the fiber, as seen from the first and second breakpoint on the SBS curve. The spark can be noted on the maximum point with a steep declining slope, signifying the release of thermal energy. These reactions are mostly undetectable by the naked eye and rarely occur, but when it does thermography allows the possibility of detecting these events. In fiber-matrix debonding, the release of energy occurs when the bonds between the fiber and matrix are compromised. Unlike previous modes, the energy is released at the interconnecting bond. In the event of a debonding, one could expect both the carbon fiber and the matrix contributing to the heat dissipation rate in the short beam. The path of most heat

dissipation is expected through the fiber due to higher thermal conductivity, with the matrix affecting the total rate of heat dissipation. (a) (b) (c) Figure 7.

In order to confirm the prediction about position of the weakest ply, the SBS test was combined with thermography imaging as shown in Figure 7.

In figure 7(b) and 7(c) this phenomenon is detected, representing the 0 and 90 ply respectively.

This signifies that the first failure of cracking occurs at one of the central plies that are orthogonal to the beam orientation.

9 dissipation is expected through the fiber due to higher thermal conductivity, with the matrix affecting the total rate of heat dissipation. (a) (b) (c) Figure 7. SBS test combined with thermography: (a) set-up; (b) IR images for 0 o orientation beams; (c) IR images for 90 o orientation beams. In order to confirm the prediction about position of the weakest ply, the SBS test was combined with thermography imaging as shown in Figure 7. In this set-up the cracking of the sample during SBS test will result in energy release that produces hot spots at the cracking location. In figure 7(b) and 7(c) this phenomenon is detected, representing the 0 and 90 ply respectively. It could be seen in figure 7(b) that the initial spark occurs in the central region. This signifies that the first failure of cracking occurs at one of the central plies that are orthogonal to the beam orientation. A simple comparison with the 90 ply in figure 7(c) to the 0 ply in figure 7(b) shows that the spark recorded from 0 ply is larger. This matches the drawings from figure 3 with the 0 ply spark recorded matching the two orthogonal layers, confirming our analysis in section 3. The catastrophic failure that represents the sharp drop in the load value in SBS curves (See Figure 3) also occur at the orthogonal layers. Moreover, the thermal

10 dissipation images can be seen following only the orthogonal layers for both 0 and 90 orientation, while the parallel fibers did not shown any abnormalities. Hence it can be concluded that SBS test is mainly measuring the load capacity of orthogonal layers and not the whole CFRP. Conclusion In this paper we have shown that by combining destructive testing with thermography a dynamics of failure in composites can be observed in real time. Such observations allow us to directly pin-point the origins of plastic deformation and catastrophic failures. We were also able to clearly distinguish that the origin of plastic deformation depends on the direction of the stress in relation to ply orientations. It is clear that the on-set of the plastic deformation represents a considerable change in the CFRP integrity and thus, the use of the maximum load value for strength estimation might not be adequate. Based on these results we would like to recommend that destructive tests for composites, like SBS, are always accompanied with thermography imaging. References [1] Hale, J. (2006) Boeing 787 from the Ground Up. Boeing Edge, Iss. QTR_04 p [2] McGRAW-HILL ENCYCLOPEDIA OF Science & Technology, 8th Edition, (c)1997, vol. 1 p 375 [3] ASTM International, Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminate, Designation: D 2344/D 2344M, Reapproved 2006 [4] D.J. Titman, Applications of thermography in non-destructive testing of structures, NDT&E International 34 (2001), pg [5] DHV Industries, Inc. Destructive Testing. Quality Inspection Laboratory. Retrieved April 9, 2013, from

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