Darbe hasarına maruz kalan sandviç kompozitlerin statik mukavemetinin incelenmesi
Investigation of static strength of sandwich composites subjected to impact damage
Yazar: ABDULLAH İKİZ
Danışman: PROF. DR. ZAHİT MECİTOĞLU
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Uçak ve Uzay Mühendisliği Ana Bilim Dalı / Uçak ve Uzay Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Havacılık Mühendisliği = Aeronautical Engineering ; Mühendislik Bilimleri = Engineering Sciences ; Uçak Mühendisliği = Aircraft Engineering

Özet:
Havacılık sektöründe oldukça sık tercih edilen sandviç yapılı kompozit malzemeler, hava araçları üzerinde darbeye maruz kalma ihtimali yüksek bölgelerde kullanıldıkları için darbe davranışları açısından inceleme konusu olmuştur. Köpük ve petek formunda iki farklı nüve malzemesi ve tabakalı kompozit yüzey levhalarından oluşan sandviç yapıların darbe sonrası kalan dayanımlarının analiz edildiği bu çalışmada öncelikle sandviç yapıların genel tanımı verilmiş ve hasar modlarına giriş yapılmıştır. Daha sonra darbe mekaniğini oluşturan unsurlar açıklanmış, darbe sınıflandırılması ve darbe için kullanılan test yöntemleri anlatılmıştır. Tezin esas hedefini oluşturan üçüncü bölümde darbeye maruz kalmış sandviç yapıların kalan dayanımlarını tespit edebilmek amacıyla sonlu elemanlar yöntemi kullanılarak bir sayısal analiz modeli geliştirilmiştir. Eş zamanlı olarak deneysel bir çalışma yürütülmüş, elde edilen sonuçlar sayısal analiz modeli çıktıları ile karşılaştırılarak modelin doğruluğu tespit edilmeye çalışılmıştır.
Sayısal analiz için en başta sandviç yapıyı oluşturan bileşenlerin malzeme ve hasar modelleri hazırlanmış, yardımcı elemanlar rijit unsurlar olarak tanımlanmıştır. Kompozit yüzey levhaları, sürekli ortam mekaniği temelinde hasar başlangıcını ve ilerlemesini ayrı ayrı ele alan bir kurguda modellenmiştir. Aynı zamanda tabakalar arası ve yüzey levhası-nüve arası ayrılmayı tespit edebilmek amacıyla kohezif bölge modellemesi tekniği kullanılmıştır. Nüve yapı için plastisite tabanlı ezilebilir köpük modeli tercih edilmiştir. Tüm bileşenler için belli bir kalınlık tanımlanmış ve her birine özgü malzeme modeli atanmıştır. Daha doğru ve az çabayla çözüm elde edebilmek amacıyla sonlu elemanlara ayırma aşamasında özel tedbirler uygulanmıştır. Kalan dayanımı tespit edebilmek maksadıyla iki aşamalı simülasyon hazırlanmıştır. İlkinde sandviç yapı darbeye maruz bırakılmış, ikinci aşamada ise darbe almış haldeki yapıya uzunlamasına eksenden basma yükü uygulanmıştır. Darbeye maruz kalmış tabakalı yapılarda en çok boyuna basma direnci azaldığı için bu yöndeki kalan basma mukavemetinin tespitine odaklanılmıştır. Sayısal analiz modeli ticari bir sonlu eleman yazılımı olan Abaqus/Explicit programıyla oluşturulmuş ve yüksek performanslı bilgisayar sistemleri ile çözdürülmüştür. 
Sonraki aşamada sayısal modeli doğrulamak amacıyla deneysel çalışma yürütülmüştür. TUSAŞ - Türk Havacılık ve Uzay Sanayii A.Ş.'nin katkılarıyla iki farklı nüve malzesine sahip test numunesi imal edilmiş ve iki farklı deney düzeneğinde testler gerçekleştirilmiştir. İTÜ Uçak ve Uzay Bilimleri Fakültesi Kompozit Yapı Laboratuvarı imkanları ile darbe aşaması için parçacık fırlatmalı gaz basınçlı test düzeneği kullanılırken darbe sonrası mukavemetinin tespiti için üniversal basma test düzeneği kullanılmıştır. Ek olarak darbe nedeniyle yaşanan mukavemet düşüşünü tespit edebilmek için darbe öncesi hasarsız parçalara da basma testi uygulanmıştır. Literatürdeki çalışmaların aksine görece düşük kütleli ve orta hızlı darbeler incelemeye tabi tutulmuştur. 
Son kısımda ise geliştirilen sayısal modelden elde edilen sonuçlar hem literatürdeki benzer çalışmalarla hem de deneysel çalışmada elde edilen çıktılar ile karşılaştırılarak doğrulanmaya çalışılmıştır. Deneysel doğrulama için belli bir enerji seviyesi ele alınmış, daha sonra sayısal model yardımıyla farklı enerji seviyelerindeki darbe davranışı ve kalan basma mukavemeti incelenmiştir.

Abstract:
Sandwich structured composites are freuquently preffered materials in the aviation industry as in other working areas such as automotive, marine and sporting goods industry. Their advantageous properties of low-weight but higher flexural stffness and ease of manufacturing are main reasons of their widely usage which is expanding day by day. On the other hand, sandwich materials have some drawbacks due to their layered structure that makes them vulnerable to out of plane loads like foreign object impact.
The outer skin of an aircraft is generally made from sandwich panels to resist the compression and buckling damage but these exterior regions are more likely to be exposed to impact events like runway debris, hail and bird strike in service. Some of them creates internal cracks and debonding faces that can not be detected by visiual inspection. Therefore, it is critical to design skin panels safely in terms of impact damage.  
For that purpose, residual strength properties of sandwich structures subjected to impact were examined in this thesis. Firstly, an introduction was given about constituents forming sandwich structures and different material combinations used commonly in the aviation industry. Laminated composite facesheet and foam or honeycomb cores together with epoxy-based film adhesive as interface are generally used combinations among various material choices. Secondly, damage modes related to impact event of sandwich materials were defined to understand their damage mechanisms and damage behavior for out of plane loads. Most effective damage modes during an impact are matrix cracking in facesheet, interlaminar delamination, facesheet-core debonding and core crushing. 
Last introductory matter handled about the topic of sandwich impact was the damage classification and characteristics. Although many different categorizing techniques were asserted severally by researches, an inclusive grouping method according to damage velocity was suggested by the author. Additionaly, other parameters affecting impact mechanics were listed to recognize the importance of getting proper test environment. Mass, shape and impact angle of the impactor are main parameters related to foreign object causing damage. On the other hand, facesheet and core thickness and facesheet stacking sequence are other examined variables in terms of structure subjected to damage. 
Numerous experimental studies have been conducted by investigators to explore the effects of these mentioned factors. Two generic types of test method are available in order to characterize impact and post-impact behavior. While damage event is succeeded by a drop weight test machine or a gas gun test machine in the first step, a compression after impact test is perfomed to damaged part as a following process for obtaining residual properties. Because the compression strength decreases more than any other strength values for sandwich composites, compression test is preffered after impact.
Main focus of this study is to develop a numerical model based on the finite element method which estimates residual compression strength of a sandwich panel exposed to impact damage by using one of available tools named Abaqus/Explicit. It is a convenient tool for modelling a dynamic event like impact. The designing of indicated numerical model has three main stages; creation of material constituve relations for sandwich structure, impactor and other auxiliary objects, determination of finite element modelling strategies and construction of impact and CAI test simulations.
Sandwich material and damage model require three different material definitions which refer three components forming sandwich namely facesehet, interface and core. Due to the fact that examined facesheet was chosen to be a laminated composite, two different materials are needed to design the facesheet: one for reinforcement fiber and one for bonding matrix material. In other words, composite laminate should contain both intralaminar and interlaminar scope of modelling. 
Continuum damage mechanics (CDM) approach was used to represent mentioned in-plane behavior of composite. Damage consists of two main phases as initiation and evolution in this method. Elastic properties and damage iniatiation criteria were used to define the first stage. As a result of the oriented nature of fiber reinforced layers, ortotropic elastic properties were adopted into elastic stage. Moreover, Hashin damage criteria was selected to implement for detection of damage initation level. Damage evolution phase of intralaminar model uses fracture-energy based linear softening law. In order to create interlaminar model of laminate, cohesive zone model (CZM) method is the most suitable option. It has two main stages, iniatiation and evolution, as well and utilizes bilinear traction-seperarion law. As a third material constitutive model, adhesive between facesheet and core was modelled in terms of CZM as well. Lastly, crushable foam plasticity model available in Abaqus/Explicit was used to identify the core behavior. In addition to sandwich, other objects like impactor, fixing clamps and supporting plate were defined as rigid bodies because their deformation is not of interest. 
FEM modeling strategies were analysed in terms of modelling scale, discretization and mesh element types. Laminated composites comprise of fiber and matrix in microlevel and oriented plies in macrolevel. Therefore, it should be determined at which level it will be studied. As a means of obtaining delamination damage, a meso-scale level was used to model laminated facesheet. A rectangular shape sample was designed to simulate sandwich plate and the center part of this rectangular plate was discretized with finer mesh because of the fact that impact causes very localized effect on structure. Mesh element dimensions increase also gradually towards edges of plate. Element type selection is another important step to get meaningful results. Sections using CZM material model should have cohesive elements. While core of the sandwich was represented by 3D solid elements, 3D continuum shell elements were assigned to intralaminar section.
Two stage simulation was developed in order to reflect of impact and CAI test. An assembly with seven objects which are sandwich, impactor, supporting plate and four clamps was created in the impact test simulation. Three different interactions were defined between these objects. One of them is between impactor and sandwich, the other one is between clamps and sandwich and the last one is between sandwich and supporting plate. After running of impact simulation, the damaged sandwich part was put in second simulation referring CAI test by using "restart" option of Abaqus/Explicit. New BCs were determined to avoid buckling deformation during compression.
An experimetanl study was carried out in order to verify improved numerical model. For this purpose, two different types of sandwich plate having foam and honeycomb cores were manufactured as test samples according to proper ASTM test standards with the support of Turkish Aerospace company. Both upper and lower facesheets have four carbon fiber reinforced unidirectional prepreg plies and they are stacked in orientation angle from outer to inner as [+45/0/-45/90]. The foam material is Rohacell 51 WF and the honeycomb material is HRH-36-4.8-32 Kevlar. FM-300K is the film adhesive between facesheet and core. All sandwich samples have a rectangular shape with dimensions of 150 x 100 mm according to test standards and they were manufactured using autoclave machine.
A gas gun-based impact test mechanism and a universal test machine of Composite Structure Laboratuvary of Faculty of Aeronautics and Astronautics from Istanbul Technical University were utilized for impact and compression after impact testing, respectively. The gas gun test machine consists of a gas tank, a 40-bar compressor, a 5-liter pressure vessel, a solenid valf in capacity of maximum 100 bar, a horizontal barrel of 25 mm diameter, a velocity meter and a skeletal structure that serves for locating plate. A steel ball with diameter of 18 mm was used as impactor and it was located in a sabot made of plastic which enables to center the ball properly. Four horizontal toggle clamps together with a steel supporting plate were attached to skeletal structure in order to fix sandwich sample. The velocity of impactor was kept as 40 m/s by arranging proper pressure level. 3-axis strain gauge and an accelerometer were bonded back surface of sample plate to obtain related datas during impact. 
The universal test machine was used to get compressive strength of both damaged and undamaged samples. With the aim of preventing of buckling deformation, sample sandwich plates were inserted in a 12-pieces test fixture made of steel machined parts. Two unidirectional strain-gauges were bonded both front and back surface of samples to get strain values during compression. Three samples for foam-cored sandwich and three samples for honeycomb-cored sandwich were utilized in each test event. 
In the last part of the thesis, the verification of numerical model was succeded by comparison with results of both similar studies from literature and experiments conducted by author. Simulation result accuracy was investigated in terms of compression strength before impact, impact mechanics and compression strength after impact. Force-time history, strain-time curves and impact damage form were used as indicators for evaluation of impact mechanics. For one certain level of energy, test results were utilized to verify numerical analysis model developed in the scope of this thesis. After that, numerical model was used for further investigations of impact events occurred in different energy levels. While mass was kept constant, velocity of impactor was changed from 40 m/s to 10 m/s to get proper energy level.
The results revealed that developed numerical model produces quite accurate solutions to predict impact damage and post-impact compressive strength of sandwich structure. Compression strength after 40 m/s velocity impact was decreased %66 for foam cored sandwich structures and %68 for honeycomb cored sandwich structures. Both sandwich structures have much more capacity to carry compressive loads as the impact energy level drops. After 10 m/s velocity impact, the foam cored sandwich structure can still protect the half of its undamaged compressive strength.