Damage Assessment in Aerospace Grade Carbon Fiber Composites subjected to Drop Weight Mechanical Impact

Transcription

1 Journal of Engineering (JOE) ISSN: Vol. 3, No. 1, 2014, Pages: Copyright World Science Publisher, United States Damage Assessment in Aerospace Grade Carbon Fiber Composites subjected to Drop Weight Mechanical Impact Rabia Abid Cranfield University Defence Academy of the United Kingdon Shirvenham, Swindon, SN6 8LA Abstract In this work, experiments have been performed on carbon fiber composite flat panels to assess damage to these composites subjected to different mechanical impact energy levels. In the low range samples were tested from 0.5 J to 5J, and for high impact energies samples were tested from 20J to 68J. In the low range up to 5J the sample layups used were cross ply with 16plies and quasi-isotropic with 20 plies. For high impact energies above 20 J only quasiisotropic samples were used. Mechanical Impact set up with an impactor size of 16 mm has been used. Residual Strength has been measured before and after the mechanical impact testing using three point bend tests. Ply level damage and surface delamination has been measured using scanned electron microscopy for samples impacted under low impact range up till 5J. Samples enduring higher impact energies were C scanned for damage visualization, no microscopy was performed for samples impacted at higher impact energy. Key Words carbon composites, mechanical impact, residual strength 1. INTRODUCTION Today Carbon Composites have revolutionized the concept of light weight aircraft. These materials comprise fibers reinforced in a resin matrix, oriented at different angles to add mechanical strength to the aircraft. It is therefore important to assess the mechanical properties of these materials having wide use in industry. Drop Weight Impact is one test which provides sufficient information about the material, especially it is relevant to bird strikes experienced by commercial aircrafts. The amount of bird strikes experienced by commercial aircraft can be countless. Carbon composites comprise fiber layers oriented at different angles to add mechanical strength to the material on the whole. However damage due to bird strikes may lead to expensive repair costs and redesigning parts. It is therefore important to assess the impact threat levels due to bird strikes. Drop Weight Impact Test can be used to replicate a bird strike event, by striking a component with an impactor at a particular velocity. 2. BACKGROUND Barely visible impact damage (BVID) is defined as the minimum damage that is reliably detected during scheduled inspections [1]. For composites, this is most often defined according to a dent depth criterion but additional criteria can also be used to establish the limit of detectability within appropriate testing limits. The limit corresponds to a probability of detection of 90% within a confidence interval of 95% and provides a reasonable level of robustness for the structure design. The goal is to determine the ultimate load leading to BVID. Two values for the BVID criterion are typically used based on the type of visual inspection performed, namely DET and GVI. Note that the visibility of large impact damage varies between materials and structures. Relaxation, as defined by is the phenomenon whereby damage becomes less detectable over time. Damage that is detectable at the time of impact can become undetectable after an inspection due to mechanical and thermal cycling, wet and ambient ageing and temperature changes [1]. Table 1 lists the typical impact threat levels for general aircraft based. Typical impact threats range from 35 J to 90 J. For fuselage skin, the threat reaches 140 J, while for doorway zones, the threat can be as high as 238 J. Table 1. Typical impact threats for different regions on an aircraft [1]. Typical impact threat 35 J, 10 5 /FH (static cut-off) 90 J 10 9 /FH (damage tolerance cut-off) HTP root/rear fuselage skin 140 J, 10 5 /FH (static cut-off) Doorway zones J, 10 5 /FH (static cut-off)

2 J, 10 9 /FH (damage tolerance cut-off) Cantwell and Morton [2] have investigated impact damage in carbon fibre-reinforced composite. The authors used a nitrogen-operated gas gun to conduct high-velocity impact tests on a variety of stacking configurations. Postimpact damage analysis was performed using several methods, including X-radiography, ultrasonic C-scanning, optical microscopy, and the deply technique. Delfosse and Poursatip [3] concluded that there is a complete energy balance for impacts on CFRP laminates that includes three major energy terms: the energy stored elastically, the energy absorbed in creating matrix damage, and the energy absorbed in creating fibre damage. Their analysis also contained two smaller terms: the energy for permanent indentation and a system loss term. The energy absorbed by matrix and fibre damage, i.e., Emo and EFD, respectively, can be calculated from the total damage area, A, and an energy release rate, G, which is the energy required to create one unit area of damage [3]. The authors adopted an approach based on the maximum load, which proved inadequate in their study because the loads levelled off once a certain displacement or energy (approximately 25 J) had been reached. They found that damage increased without affecting the maximum measured force. They concluded that the force-based approach works well for the onset of damage, whereas an energy-based approach is suitable for determining the extent of damage. Another conclusion in their work was that for full perforation, approximately 30% of the energy is consumed by matrix damage, while approximately 70% of the energy is consumed by fibre damage. 3. PROGRAM OF WORK FOR IMPACT TESTS Based on the discussion above and considering Table 1 threat levels, in this work, experiments were conducted in two ranges on three different material lay ups as in Table 2. All samples were then studied for damage through microscopy/c scanning and then tested for mechanical parameters. Flexural Strength was measured post impact test. The material used for mid-range cross ply and quasi isotropic samples was different and is being referred to as Material A and B (material description has not been revealed in this paper). Table 2: Samples tested in the testing regime Material Sample Ply Lay Up No. of Dimensions (LxBxW) Impact Test Range Plies A [(0,90)4]s mm x 50 mm x 1 mm 1-5J B [0/90/90/0/+45/ 45/0/90/90/0]s mm x 50 mm x 1.5 mm 0.5-5J A (45,135,0,0,90,0,135,0) s mm x 100 mm x t *mm 20-68J *t=4, 5, 6 mm A. Low Impact testing on Cross Ply Samples /Quasi Isotropic Samples In this range samples were of two ply layups, cross ply and quasi isotropic, 16 and 20 plies respectively. A total of 5 samples were impacted from 1 to 5J, all being identical 50 mm x 50 mm x 1 mm in size. For quasi isotropic plates the dimensions are 50mmx50mmx1.5 mm. The testing for these plates started at 0.5 J and then in interval of 0.5 J the impact energy was increased per sample to document the damage evolution in more detail. Microscopy was performed for all the samples, a total of 10 samples were used each impacted at a different impact energy and 0.5 J greater than the last impacted sample. B. High Impact Energy Testing on Quasi Isotropic Samples In this range samples were cut to large sizes of 100 mm square panels with 3 different thicknesses, from 4 mm to 6 mm, being tested, three samples in each thickness were tested and the same ply lay ups, this means that the a total of three different ply layups were tested. C. Mechanical Strength Experiments on Samples Impacted Mechanical Strength tests were performed on all samples from all testing ranges. 3-point bend tests were chosen as the measure of residual strength of the material after the mechanical impact. 4. EXPERIMENTAL SET UP Drop-weight impact experiments were conducted on all samples using an instrumented drop-weight impact test machine. The impact conditions were designed in accordance with the American Standard for Impact Behaviour of Rigid Plastics. The specimens were maintained between four large clamps that restrained movement of the entire specimen apart from a circular unsupported area at the centre. The impactor was a polished hemispherical tup (or striker) with a diameter of 16 mm. The tup was attached to an inertial-mass that accelerated down a guided channel under gravity. Each sample was impacted once only within the impact energy ranges defined in Table 2. The impacts were controlled by

3 164 the release height of the impactor, that is, by varying its potential energy. The rebounded impactor was restrained by a rebound capture mechanism to avoid a second impact with the specimen. The impactor velocity immediately prior to impact and just after rebound was measured with a pair of light gates that allowed the kinetic energy to be calculated before and after impact. This allowed the energy absorbed by the specimen as damage to be estimated. 5. RESULTS AND DISCUSSION A. Cross Ply Samples 1-5J Cross ply samples subjected to 1-5 J of the same size and same material A were studied for delamination on the surface and measured using photography as shown in Figure 1. As can be seen from the Figure for all samples the damage penetrated the lowest layer, straight on. However the surface delamination size at 1 J is 4 cm the largest which also indicates that depth of damage may be less as damage spreads on the surface more than in the depth, however looking at the microscopic images the depth of damage is the same for all energy levels whereas the surface delamination size decreased with increasing impact energy. From 1 J to 5 J the surface delamination length decreased from 4 cm to 2 cm, i.e. 50%. Figure 1: Carbon Composite Samples impacted from 1J -5J surface delamination (left) and ply level damage (right) B. Quasi Isotropic Samples 1-5J In this range samples were all 20 plies, only selected samples were tested for residual strength as there was no significant damage found upon microscopy. The damage visualization using microscopy clearly showed that damage patterns under 0.5 J to 5 J reflect less damage, therefore up to 5 J the damage endured by an aircraft made of carbon composites will be insignificant.

4 165 Figure 2: Ply level damage studied in the impact range from 0.5 J to 5J in intervals 0.5J, showing minimum damage in 20 layered quasi isotropic samples C. Quasi Isotropic Samples 20-68J Delaminated area was captured using C scanning. Coupons of 40 mm x 100 mm were extracted from the damaged sites and then C scanned to evaluate the damage at the same scale. The 4 mm sample had the largest damaged area as can be seen from the image (a) in Figure 3. The C scanning may provide useful information about the damage extent however it is not always the case the mechanical strength also decreases with increased surface damaged size. (a) (b) (c) Figure 3: (a) C scans of samples impacted with thickness 4 mm (b) C scans of samples impacted with thickness 5 mm (c) C scans of samples impacted with thickness 6 mm 6. RESIDUAL STRENGTH ANALYSIS The residual strength of samples in both ranges were tested. 1) Mechanical tests on samples impacted with 0.5-5J Table 2 below shows the flexural characteristics of 50 mm by 50 mm test coupons, each such sample was divided into 4 smaller samples for the bend tests. As seen clearly the piece 3 for cross ply sample under 1 and 3 J showed strength values of 797 and 629 MPa roughly. Looking at the results the undamaged areas of the samples showed similar values of MPa. Near the impacted site the values drop to MPa and then at the impact point a clear decrease is seen and values recorded range from MPa. For quasi-isotropic samples, all samples could not be used for mechanical tests, the 50 mm by 50 mm sample on the whole was not divided into equal 4 portions. Table 2 therefore shows the samples that were extracted. As can be seen for undamaged areas the values range from MPa whereas near the impact point

5 Flexural Strength (MPa) 166 these values drop to MPa range and then at the impact sites these values are MPa. The percentage decrease roughly at 3 J for cross ply sample is 57 %, whereas for quasi isotropic sample at 3 J a similar trend is seen, i.e. a decrease in mechanical strength of 57%. However if we closely analyse then at 4 J, a decrease of 85% is also seen with the largest value of 2113 MPa and the lowest value being 311MPa. The data recorded therefore under this section is very significant to assess the damage inflicted due to drop weight mechanical tests to carbon composites. Drop-weight Energy (J) Table 2: Residual Strength of Samples impacted under mid-range mechanical impact Ply lay-up Flexural Strength (MPa) of 50 mm 50 mm pieces [0/90/90/0/+45/-45/0/90]s /90/90/0/+45/-45/0/90]s /90/90/0/+45/-45/0/90]s [0/90/90/0/+45/-45/0/90]s [(0,90)s] [(0,90)s] [(0,90)s] [(0,90)s] [(0,90)s] ) Mechanical tests on samples impacted with 20-68J The flexural tests performed for these samples showed a higher mechanical strength away from the impact point, (please see Figure 4), note that samples used for the bend tests were sub-samples from the entire sample, therefore the sample was divided into testing coupons near and away from the impact point. Sample coupons with 6 mm thickness had the lowest flexural strength reaching below 100 MPa at 68 J, whereas at the same impact energy, strength recorded for 4 mm sample was 200 MPa. This indicated that the damage on a thinner sample was less compared to the thicker sample for the same impact energy. The damage for the thinner samples 4 mm maybe more at the ply level than the surface delamination, whereas for a thicker sample the surface spread will be more and ply penetration of damage may be less, however for this range of testing ply level damage analysis was not performed, instead C Scans were obtained as can be seen in the previous section. The percentage decrease in flexural strength, for a 4 mm sample at 68 J is about 76%, whereas for 5 mm sample at the same impact energy the strength drop to 83%, and for a 6 mm sample it drops about 78%. Looking at impact energy of 20J, though the 6 mm sample mechanical strength drop to 56 %, whereas for 5 mm the decrease is 31%. For impact energy 40J the sample with 4 mm thickness recorded a decrease approximately of 50%, whereas 5 mm recorded 62%, and 6 mm had a decrease of 77% mm 5 mm 6 mm Impact Energy (J) Figure 4: Flexural Strength parameters for samples impacted under 4mm, 5 mm and 6 mm. 7. CONCLUSIONS The conclusions for the experimental investigation performed in this paper are summarized below.

6 The damage to quasi isotropic samples till 5 J in carbon composites is not significant, and it does not affect the mechanical strength of the carbon composites particular to a 20 ply sample as in this study. 2. The damage to cross ply laminates however was greater at all impact energy levels including 1J. As shown in microscopy the damage penetrated to the bottom ply at all impact energies. This also suggests that a quasi-isotropic sample having diagonal layers is perhaps stronger mechanically than a cross ply sample whereas for a cross ply layup the damage will be greater. A typical strength value for an undamaged cross ply layup can be approximated as 1456MPa and that for a quasi-isotropic sample it is roughly 2300MPa. 3. The mechanical strength decrease allowed the trends to be observed between impact energies and mechanical strength. At higher energies of 20J to 68J the damage for three different thicknesses for a quasi-isotropic layup was assessed, the percentage decrease showed that under 20 J the sample that had the largest decrease in mechanical strength was 5 mm, and under 40J, the sample with thickness 6 mm has the largest decrease and under 68J, 5 mm sample had the largest decrease. 4. The results overall provide an insight into typical values of mechanical strength post mechanical drop weight impact of low and high energies. REFERENCES [1] E. Morteau and C. Fualdes, "Composites at Airbus Damage Tolerance Methodology " in FAA Workshop for Composite Damage Tolerance and Maintenance ed, 2006 [2] W. J. Cantwell and J. Morton, "Detection of impact damage in CFRP laminates," Composite Structures, vol. 3, pp , // [3] D. Delfosse and A. Poursartip, "Energy-based approach to impact damage in CFRP laminates," Composites Part A: Applied Science and Manufacturing, vol. 28, pp , // 1997.