Darbe yükü etkisindeki sandviç plağın dinamik davranışının sayısal ve deneysel olarak incelenmesi
Investigation of dynamic response of the sandwich plate subjected to the impact load experimantally and numerically
Yazar: NAMIK KEMAL ALPAYDIN
Danışman: DOÇ. DR. HALİT S. TÜRKMEN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Uçak ve Uzay Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2012
Konu: Uçak Mühendisliği = Aircraft Engineering
Dizin: Darbe = Impact ; Darbe dayanımı = Impact strength ; Sandviç yapı elemanları = Sandwich structure elements ; Sonlu elemanlar = Finite elements

Özet:
Sandviç yapılar, yüksek mukavemet/ağırlık oranı, yüksek enerji sönünmleme ve yüksek eğilme momenti gibi özelliklere sahiptir. Bu avantajları nedeniyle sandviç malzemelerin kullanımı, özellikle havacılık sanayinde yaygın olarak tercih edilmektedir. Hava araçlarının bakımı ve operasyonu sırasında yabancı cisimlerin çarpması sonucu oluşabilecek hasarlar yüksek maddi zararlara ve hatta ölümlere neden olabilirler. Bu yüzden sandviç yapıların darbe yükü altında davranışının incelenmesi önem teşkil eder.Bu çalışmada, yüzeyleri aramid elyaf çekirdek malzemesi balpeteği olan sandviç plakanın düşük hızlı darbe yükü etkisinde oluşan dinamik cevabı sayısal ve deneysel olarak incelenmiştir. Deneyler ve analizler öncelikle malzeme özellikleri bilinen alüminyum plaka ile gerçekleştirilmiştir. Burada amaç, mekanik özellikleri bilinen izotropik malzeme ile yapılan analizlerin doğrulanmasıdır. Ayrıca elde edilen sonuçlar sandviç yapının sonuçları ile karşılaştırılmıştır.Deneylerde kullanılan sandviç plaka el yatırması tekniği ile üretilmiştir. Bu yöntemde elyaflar düz bir zemine konularak epoksi fırça yardımıyla her katmana aynı miktarda ve homojen olarak sürülmüştür. Plaka üretimi İTÜ-UUBF Kompozit Yapı Laboratuarı'nda yapılmıştır.Plaka tüm kenarlarında metal bir çerçeve ile civatalarla sabitlenmiş ve hareket etmeyecek şekilde düz bir zemin üzerine konulmuştur. Burada kullanılan metal çerçevenin plakaya göre çok daha rijit olmasına dikkat edilmiştir. Böylece darbe sırasında çerçevenin davranışının plakaya etkisi ihmal edilmiştir. Plakanın alt yüzey merkezine üç kanallı gerinim(strain) ölçer rozeti yapıştırılmış ve üst yüzey merkezine belirli yüksekliklerden serbest düşürülen çelik bilyenin çarpması sonucu oluşan birim uzama değerleri yüksek hızlı veri toplama sistemiyle dinamik olarak ölçülmüştür. Bu deney düzeneği ABAQUS yazılımıyla modellenmiş ve analizleri explicit(açık) çözücü kullanılarak Sonlu Elemanlar(SE) Metodu ile gerçekleştirilmiştir. Plaka kabuki elemanlarla modellenmiştir. Serbest düşürülen bilyenin elastik davranışı ihmal edilecek düzeyde olduğundan kaskatı(rigit) modellenmiştir. Elde edilen sonuçlar deney sonuçları ile uyumlu bulunmuştur. Ayrıca analizler farklı çarpma hızlarında gerçekleştirmiş ve bu durumun birim uzama ve yerdeğiştirme üzerindeki etkisi incelenmiştir.Ayrıca yapıya verilecek eğriliğin etkisini incelemek amacı ile aynı malzemeden yapılmış farklı eğrilik yarıçaplarına sahip sandviç kabuk yapılar için aynı analizler tekrarlanmıştır. Böylece aynı yükleme durumu için kabuk ve plakaların dinamik cevapları karşılaştırılmıştır.

Abstract:
The sandwich structures are widely used in aerospace industry. This is because that they can resist against high bending moments. Also, these structures have several advantages as high strength/weight ratio, high energy absorbtion etc. Sandwich structures provide an efficient solution to increase bending stiffness without significant increase in structural weight.Honeycomb composite structures are used in constructions that require high mechanical endurance, especially where the energy resulting from an impact is absorbed. In order to build considerably light systems, these honeycomb structures are placed between the inner and outer layers of sandwich structures. Putting a honeycomb between the layers increases the moment of inertia and bending strength of the structure.Honeycombs are generally used as filling element in sandwich structures. A sandwich panel is made by covering both sides of the honeycomb structure with surface sheets using an adhesive. The main function of the surface sheets in sandwich panels is to bear the axial and shear loads. On the other hand, honeycomb augments the bending strength of the structure by increasing the sectional thickness and enables the transfer of loads along the thickness. These very light structures allow construction of systems that have the same strength as metals but are much lighter than them.The aircraft control surfaces, interior sections, wing skins are made of sandwich structures. . Also, these structures have a broad area of use in other aircrafts such as unmanned aerial vehicles (UAV) and helicopters. Especially in UAVs, which are used frequently for military and civil purposes in recent years, sandwich composite materials are preferred. The flight time of small powered vehicles is much less than that of larger ones. So, decrease in weight becomes even more critical especially if payloads such as cameras or weapons are added in.However, these structures are usually weak in the thickness direction. In service, the loads acting in the thickness direction can seriously harm the sandwich structures.Particularly, the impact loads such as tool drops, bird strikes, and runway debris may cause delamination of these structures. An impact load might inflict more damage on composite and sandwich structures than on metals. These damages usually occur in the inner regions of the structure and cannot be seen by visual inspection. Due to these damages, the structural strength of the system decreases substantially. This could cause accidents resulting with loss of property and even life.For these reasons it is important to investigate the sandwich structures subjected to the impact load.xxThis study aims to demonstrate that the impact response of a composite plate to a low speed impact effect can be accurately projected with a numerical model that has been made for analyzing this case. Thus, by developing analysis methods it would be possible to examine the structural behavior of these materials with lower costs and less time.In this study, the dynamic behavior of the isotropic aluminum and sandwich panels subjected to the impact load are investigated experimentally and numerically. The aluminum panel results are used for validation due to well-known mechanical properties.The sandwich panel is manufactured using the honeycomb core material and laminated composite face sheets. The face sheet materials used in this study are manufactured using an aramid fabric and epoxy resin.The wet hand lay-up technique is used to produce the sandwich panel. The curing is achieved by using a heated vacuum table. The panel is cured under a vacuum of about 720 mBar and a temperature of 50oC for 5-6 hours.The panels are fixed at all edges and the impact load is applied on the panel. The dynamic response of the panel is measured using the strain gauges.The panels are also modelled using Abaqus finite element software. The analysis of the impact test is achieved. The numerical and experimental results are compared. The results are found in an agreement. The final results are concluded.The sandwich panel is manufactured using wet hand lay-up technique. Aramid fabric and aramid honeycomb materials are used for the face sheets and core, respectively. The aramid fabric used for the face sheet has a ±45o fiber orientation. Two layers are used for each face and the thickness of the one layer is 0.55 mm resulting in a face sheet thickness, tf=1.1 mm.The mechanical properties of the honeycomb used for the panel are given as E3=25 MPa, ?=29 kg/m3, ? =0.3 and the thickness, tc=5mm.The panels are fixed to the frame from all sides using ten bolts and a strain rosette is glued to the center of the panel at the back side.The metal frame is more rigid than the panels so the behavior of frame may be neglected during the impact. The strain rosette is connected to a high speed data acquisition system.A metal pipe, which is 0.50 m in length, is fixed to a table over the panels using a clamp. This pipe is used as a guide to be able to drop the steel ball on the center of the panel. So, leaving the steel ball from the upper end or lower end of the pipe resulted in a 1.0 or 0.5 m drop height.A spherical steel ball is dropped from certain heights on the front side of the sandwich panel. Considering the natural frequency of the sandwich panel is high, the data collection frequency is selected as 10000 Hz.The steel ball is dropped three times for each drop height (0.5 m and 1.0 m). The strain is digitized and transferred to a computer by using a data acquisition system.The panels and steel ball are modeled using Abaqus finite element software. The panels and steel ball are discretized using S4R and R3D4 elements, respectively. The steel ball elastic behavior is negligible so it is modeled rigid elements. The S4Rxxielement has 4 nodes and each node has 6 degrees of freedom, which are translation and rotation in x, y and z directions.Impact analysis are performed by using Abaqus/Explicit solver. The explicit dynamics analysis procedure is based upon the implementation of an explicit integration rule together with the use of diagonal element mass matrices. The equations of motion for the body are integrated using the explicit central-difference integration rule.The analysis is started near the impact time to shorten the CPU time. For this purpose, the velocity corresponding to the related drop height is calculated and is applied as an initial velocity to the steel ball. The analysis are performed for ten different drop height from 0.1m to 1.0m.The strain is measured and calculated at the center on the back side of the sandwich panel. The peak strain is predicted well for both `drop height? cases. It is shown that a vibration occurs as a result of the impact load.The test side of this study is used for validation of FE analysis. Good agreement is obtained between numerical and experimental results.The strain increased as the drop height is increased as expected. It is shown that the increase in the strain is a linear function of the drop height. However, it is necessary to perform experiments for more than two `drop height? cases to give a solid decision about relation between `drop height? and peak strain.The experimental results show that there is a discrepancy between strain values in 0o and 90o channel directions. Some faults during production of the panel and drop test related to the symmetry may cause this discrepancy between strains measured in 0o and 90o channel directions.In addition, in order to examine the effect of curvature, the sandwich shells with different curvatures are modeled and re-perform the analysis in same load conditions. The obtained results are compared with the panel results. The increase in curvature provides decrease in peak displacements and peak strains. Under the same loading condtions, use of sandwich structures are provide significant decrease in weight by comparison with metals. Also analysis show that correct numerical model can provide good prediction of low velocity impact response.In this study, the tests and analyses are conducted on an aluminum plate with known material properties and the consistency of the results showed that the developed analysis method is correct. Therefore, the analyses made with sandwich plates and shells are considered as accurate. With a bowing, a sandwich structure could be more impact resistant compared to a flat sandwich structure which is made with same materials and has a same mass. Furthermore, the analyses showed that compared to sandwich plate, impact causes higher amounts of unit elongation on aluminum plate which has the same mass as the sandwich plate but thinner than it. By using sandwich structures, lighter and high strength systems can be made. Impact tests are made by free falling steel shots from defined heights on to the plates. Two plates are used in the tests which are aluminum plate that have isotropic material properties and composite sandwich plate. Impact tests are first conducted on the aluminum plate. Data obtained by measurements are compared to finite element analysis results. This aims to verify the developed analysis methodology with a material that has more easily determinable mechanical properties.