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Title:	Analysis of elastic interacting cracks in finite body
Authors:	Ruslizam Daud (P49892)
Supervisor:	Prof. Dr. Ahmad Kamal Ariffin Mohd Ihsan
Keywords:	Cracked Structural failure Fracture mechanics
Issue Date:	31-Dec-2012
Description:	The analysis, prediction and prevention of structural failure based on single cracked body are no longer adequate to support the existing fracture mechanics approaches for multiple cracked body. This is due to the occurrence of elastic crack interaction in manufactured and assembled thin-walled structures which consist of multiple stress concentration features such as notches, corners, bends and holes. The in-service structural assessment guideline for cracked body, known as fitness-for-service (FFS) codes by the ASME boiler and pressure vessel code (Section XI, Articles IWA-3330), JSME FFS codes and BSI (PD6493 and BS7910) codes still consider multiple cracks as one larger crack or combined cracks by conforming to certain rules and conditions. For certain technical parameter of cracked body geometry, those codes are reported to be inaccurate. The present analytical and numerical solutions encountered many limitations with assumptions and unable to represent the real physical and mechanics behavior of elastic cracks interaction. Due to this, the study on providing better solution for fracture and failure prediction dealing with multiple cracked bodies is critical. This thesis presents a mathematical model called stress shielding damage (SSD) and new numerical models as the alternative solutions to assess the elastic crack interaction based on shielding interaction effect. The aims of this research is to develop the elastic crack interaction model based on the theory of Griffith strain energy release rate and regularized energy fracture model which is based on damage phase field model. In order to assess the FFS codes on interacting cracks, the model development is limited to brittle fracture and failure mechanics in the phase of crack interaction before crack propagation (BCP). The FFS codes assumption on two parallel cracks as single crack basis is translated into crack interaction limit (CIL) and crack unification limit (CUL). Finite element (FE) method is employed to develop two numerical models which based on energy release rate and displacement technique by modification and enhancement of existing J-integral and displacement extrapolation method. Both developed solution models are based on the variation of shielding effect from CIL to CIL in the transition of no interaction to weak and strong interaction phase. Over one hundred two-dimensional FE models were executed in this study based on variation of crack interval ratio and crack-to-width ratio. Stress shielding fracture parameter results are gathered in a form of J-integral value, stress intensity factor (SIF), damage shielding factor, crack interaction factor for mode I and II fracture. From the results, it can be seen that the strain energy release value based on J-integral trend satisfies the energy theoretical model of Griffith and phase field model, thus corrected the CIL and CUL assumption. The convergence and intersection point of SIF and crack interaction factor justified the point of CIL and CUL in interacting cracks. The measured damage shielding factor is also satisfied with damage energy parameter which well agreed with assumption of single independent and combined crack in FFS codes. The trend of mode I and II SIF provides significant understanding on crack interaction particularly at closed space cracks. For validation, the results were compared with analytical, numerical and experimental data from literature and have provided the excellent agreement to proposed numerical models. Finally, further works on interacting cracks in the phase of during crack propagation (DCP) are proposed to extent the assessment of CIL and CUL.,Analisis, ramalan dan pengelakkan kegagalan bagi struktur berasaskan jasad mengandungi satu rekah tidak mampu untuk menyokong kaedah mekanik patah sedia ada bagi menyelesaikan jasad mengandungi berbilang rekah. Ianya disebabkan berlakunya interaksi elastik rekah dalam struktur dinding-nipis buatan dan terpasang yang mengandungi takukkan, sudutan, bengkokkan dan lubangan. Panduan penilaian struktur bagi jasad rekahan dikenali sebagai kesesuaian-untuk-servis (FFS) kod iaitu kod dandang dan bejana tekanan ASME (Sekyen XI, Artikel IWA-3330), kod kesesuaian-untuk-servis JSME dan kod BSI (PD6493 dan BS7910) masih mengandaikan rekahan berbilang sebagai satu rekahan besar atau gabungan rekah bersasaskan peraturan dan keadaan tertentu. Kod-kod tersebut dilaporkan tidak tepat pada parameter teknikal geometri rekahan jasad tertentu. Penyelesaian analitikal dan berangka yang telah dibangunkan masih terhad dengan pelbagai andaian serta masih tidak mampu mewakili gelagat fizikal dan mekanik bagi interaksi elastik rekah. Oleh yang demikian, satu kajian bagi menyediakan penyelesaian terbaik bagi ramalan patah dan gagal merujuk kepada jasad berbilang rekah adalah kritikal. Tesis ini mempersembahkan satu model matematik bernama model rosak perisaian tegasan (SSD) dan model berangka baru sebagai penyelesaian alternatif bagi menilai interaksi elastik rekah berdasarkan kesan interaksi perisaian. Tujuan penyelidikan ini adalah untuk membangunkan model interaksi rekah elastik berasaskan teori kadar pembebasan tenaga terikan Griffith dan model patah yang berasaskan tenaga teratur dan model medan fasa rosak. Untuk menilai kod FFS bagi rekahan interaksi, pembangunan model dihadkan kepada mekanik patah dan gagal rapuh dalam fasa interaksi rekah sebelum rekahan merambat (BCP). Andaian kod FFS pada dua rekahan selari diterjemahkan kepada had interaksi rekah (CIL) dan had kesatuan rekah (CUL). Kaedah unsur terhingga (FEM) dilaksanakan bagi membangunkan dua model berangka berasaskan kadar pembebasan tenaga terikan dan teknik anjakan melalui pengubahsuaian dan penambahbaikan kaedah asal kamiran-J dan kaedah penentuluaran anjakan. Kedua-dua model adalah berasakan variasi kesan perisaian daripada CIL kepada CUL dalam transisi fasa tanpa interaksi kepada fasa interaksi lemah dan kuat. Lebih seratus model unsur terhingga dua dimensi dilaksanakan berdasarkan kepada kadar jarak rekah dan kadar rekah kepada lebar. Keputusan parameter perisaian tegasan adalah dalam bentuk kamiran-J, faktor keamatan tegasan (SIF), faktor rosak perisaian, faktor interaksi rekah bagi mod I dan II patah. Merujuk kepada keputusan, didapati haluan nilai pembebasan tenaga berasakan kamiran-J seerti dengan model teori Griffith dan model medan fasa dipatuhi, jadi mengesahkan andaian CIL and CUL. Penumpuan SIF dan faktor keamatan tegasan membenarkan titik had CIL and CUL. Pengukuran faktor kesan perisaian juga menepati parameter tenaga rosak dan andaian satu rekah bebas dan kesatuan rekah dalam kod FFS. Haluan mod I dan II SIF menyediakan kefahaman penting interaksi rekah terutamanya pada rekah-rekah berjarak dekat. Untuk pengesahan, data model berangka yang dibangunkan dibandingkan dengan data daripada kaedah analitikal, berangka dan eksperimen dan satu keseertian yang baik diperolehi. Akhirnya, cadangan kerja-kerja penyelidikan dalam interaksi rekah diketengahkan terutamanya dalam fasa interaksi rekah semasa rekah merambat (DCP) bagi menentukan CIL and CUL,Ph.D
Pages:	202
Call Number:	TA409 .R847 2012 3
Publisher:	UKM, Bangi
Appears in Collections:	Faculty of Engineering and Built Environment / Fakulti Kejuruteraan dan Alam Bina

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