PSI - Issue 14

Volume 14 • 201 9

ISSN 2452-3216

ELSEVIER

SICE 2018 (2nd International Conference on Structural Integrity and Exhibition 2018)

Guest Editors: K Bhanu Sankara R ao R Sunder Vikram J a y aram S Go p alakrishnan K Go p inath Kartik P rasad

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XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal Thermo-mechanical modeling of a high pressure turbine blade of an airplane gas turbine engine P. Brandão a , V. Infante b , A.M. Deus c * a Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal c CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal Abstract During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data. 2nd International Conference on Structural Integrity and Exhibition 2018 Editorial R, Sunder* BISS (P) Ltd, Bangalore, India This special issue contains select papers from the 2 nd International Conference on Structural Integrity and Exhibition (SICE 2018) that was held in Hyderabad, India, July 25-27, 2018. The SICE series of biennial conferences are an initiative of the Indian Structural Integrity Society (INSIS) to bring together practicing specialists from industry, national laboratories and the academia. Given the sustained and rapid growth of the country’s industrial economy and considering the variety of strategic stake holders in energy, transportation, civil infrastructure as well as defence, the enthusiasm and energy to participate in SICE should not come as a surprise. Thus, SICE 2016 co-hosted by the Indian Institute of Science, Bangalore and Indian Institute of Technology, Madras under the Chairmanship of Prof. Raghu Prakash attracted over 150 participants. At SICE 2018, we had over 250 contributions and over 350 delegates. SICE 2020 will be hosted by the Indian Institute of Technology, Bombay (Mumbai), where even greater national and international participation is expected. The SICE meetings underscore the attractiveness of India as a conference destination. They offer 5-star hospitality and a single platform that unites hundreds of largely young stake holders from industry, R&D and academia, who carry the responsibility for structural integrity across the energy, transportation, civil construction and defence sectors of a rapidly growing industrial economy that is India. Thanks to government sponsorship and generous material co tributions from interested industrial p rtners, org nizers of SICE meetings have been able to keep Registration Fe aff rdable so that financial constraints don’t pose a challenge to participation. This s also made pos ible by the wholly voluntary effort of the entire organizing team. As a conseque ce, SICE2018 attracted several hundre abstract submissions, of which 190 were shortlisted for oral presentation and another 70 for the poster sessions. This is apart from the plenary and invited presentations by several scientists of global repute from Australia, Canada, France, Germany, Italy, Japan, Russian Federation, Sweden and the United States. The Conference enjoyed the active support of our outgoing Founder-President of INSIS, Prof. Ashok Saxena, who was a constant source of advice and useful suggestions to the Organization Committee. SICE2018 enjoyed financial support and encouragement from the Gover ment of India and from as many as a dozen industrial sponsors. The efforts © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation.

* Corresponding author. Tel.: +91-9880-432322 E-mail address: rs@biss.in

2452-3216 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers.

* Corresponding author. Tel.: +351 218419991. E-mail address: amd@tecnico.ulisboa.pt

2452-3216 © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216  2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers. 10.1016/j.prostr.2019.05.001

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of SICE2018 Organizing Committee Chairman, Dr. Vikas Saxena, Director, Defence Metallurgical Research Laboratory (DMRL) and his team headed by the young and dynamic Conference Convener, Dr. Kartik Prasad deserve special mention. Kartik orchestrated the concerted effort of over a dozen Sub-Committees entrusted with the responsibility of different activities associated with putting together this major event. The resounding success of SICE 2018 was largely due to the highly structured and tireless ‘behind the scenes’ effort of dozens of DMRL scientists over the preceding year. This involved interaction with over 500 interested participants of the two events, with potential sources of financial support and sponsorship and with providers of logistical and other services, filtering and sorting the hundreds of abstract submissions. planning the sequence of technical presentations across several parallel sessions, handling the entire logistics of the events over an entire week (this included the 2-day Workshop that preceded the Conference) and also including an attractive cultural programme that enthralled local and overseas delegates alike. The INSIS Executive Committee instituted the honorary status of Founding Fellows of INSIS to recognize the lifetime achievements of ten active researchers of global repute. Prof. Krishnan Balasubramanian (IIT, Madras), Prof. Alberto Carpinteri (Politecnico di Torino), Dr. Ravi Chona (Air Force Research Laboratory, USA), Prof. B. Dattaguru (Jain University), Prof. Masahiro Endo (Fukuoka University), Prof. R. Narasimhan (Indian Institute of Science, Bangalore), Dr. Sergey Panin (Institute of Physics of Strength of Materials, Tomsk), Dr. N. Eswara Prasad (DMSRDE, DRDO, Kanpur), Prof. K. Ravi-Chandar (University of Texas) and Prof. Ashok Saxena (University of Arkansas) were formally inducted at SICE 2018 as Founder Fellows of INSIS. Most of them were present at SICE2018 to receive formal citations of their contributions. Many of them delivered plenary lectures. INSIS Young Scientist’s award for special achievements was awarded to Prof. Suhasini Gururaja of the Indian Institute of Science (academia) and Dr. S. Ammanagi from BISS (industry). The venue for SICE2018 was Courtyard by Marriott, just across from the picturesque Hussainsagar Lake, a pleasant tourist landmark that stands between the twin cities of Hyderabad and Secunderabad. For three days, the venue saw the busy movement of conference delegates moving across parallel sessions and through the SICE exhibition stalls manned by industrial sponsors of the event that included leading test and analytical equipment manufacturers and their representatives. The Session chairpersons and Co-Chairs did a commendable job of ensuring timely closure of presentations with adequate time for discussion. A team of knowledgeable volunteers scouted around all the parallel sessions and across the poster stands to identify outstanding contributions that were recognized by a Certificate and token cash reward at the SICE2018 valedictory meeting that concluded the event. The awards were segregated to identify outstanding contributions from post-graduate students, from research scholars and from professional researchers. The ‘Conference hangover’ came in the way of the stupendous effort required to have all the written submissions reviewed and revised as required to conform to the demanding criteria of this journal. The guest-editors of this special issue would like to express their appreciation to the reviewers as well as authors for having exercised their effort in a timely manner. On behalf of the almost 200 members of INSIS, I would like to express my sincere appreciation to the DMRL team, to the SICE2018 sponsors and to all the delegates for an eventful and inspiring meeting of the Structural Integrity community. We look forward to the opportunity of meeting up with all of you at SICE2020 to be organized conjointly with the Indian Institute of Technology, Bombay at the financial capital and megapolis of Mumbai in December 2020.

R. Sunder, President, Indian Structural Integrity Society (INSIS) On behalf of the SICE2018 Editorial Team

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2 nd International Conference on Structural Integrity and Exhibition 2018 A Comparative Study of the Behavior of CFRP and GFRP Laminates in a Plate Specimen Using Modified Virtual Crack Closure Technique (MVCCT) Shubha Javagal a* , Raju J. b , Katta Venkataramana a a Department of Civil Engineering, National Institute of Technology Karnataka, Surathkal, 573201, India b Principal Scientist, STTD, CSIR- National Aerospace Laboratories, Bangalore 560008, India. 2 nd International Conference on Structural Integrity and Exhibition 2018 A Comparative Study of the Beh vior of CFRP a d GFRP Laminates in a Plate Specimen Using Modified Virtual Crack Closure Technique (MVCCT) Shubha Javagal a* , Raju J. b , Katta Venkataramana a a Department of Civil Engineering, National Institute of Technology Karnataka, Surathkal, 573201, India b Principal Scientist, STTD, CSIR- National Aerospace Laboratories, Bangalore 560008, India. The applications of polymer composites in aircraft industry have exponentially increased in the recent years due to their high strength to weight ratio. Presence of delaminations in composites is inevitable which affects the structural stability due to reduction in structural stiffness and strength. The degradation of a structural component depends on the geometric characteristics of delam n , nature of loading and material characteristics. Damage tolerance study is thus ss ntial to determine extent of de radation of he structure du t th presence of dela ination. The present paper brings about a comparision be ween the b hav ur of a s andard plate specim made up of C rbon Fibre Reinforced Polymer (CFRP) and Glass Fibre Reinforced Polymer (GFRP) l minates with circular delaminations of varying diam ters and subject to compre sive load. A constan compressive load in terms of initial displace en s was ap lied on a quasi-i otropic square plate sp cime of dimensions 200 mm x 200 mm with a thickn ss of 2.88 mm of oth CFRP an GFRP configuratio s. A circular delamination was int oduced at the centre of the plate and ts diameter and position along the thi kness dir ction were varied and studied. Using ABAQUS codes of pr ct ce, Strain Energy Release Rat (SERR) was computed. The princi les of Modified Virtual Crack Closure Technique (MVCCT) was used to compute SERR. Delamination propagates whe the computed mixed mode e ergy release ate exceeds the critical value, G C . Depending on the external l ading and material properties, Total str in energy release rate, G T (G T =G I +G II +G III ) w s used to predict growth of delamination. The onset of delaminat on growth was determined by pl tting th values of (G T /G C ) acro s various delamination sizes along the thickness of the plate and re or . © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers. Abstract The applications of polymer composites in aircraft industry have exponentially increased in the recent years due to their high strength to weight ratio. Presence of delaminations in composites is inevitable which affects the structural stability due to reduction in structural stiffness and strength. The degradation of a structural component depends on the geometric characteristics of delamination, nature of loading and material characteristics. Damage tolerance study is thus essential to determine the extent of degradation of the structure due to the presence of delamination. The present paper brings about a comparision between the behaviour of a standard plate specimen made up of Carbon Fibre Reinforced Polymer (CFRP) and Glass Fibre Reinforced Polymer (GFRP) laminates with circular delaminations of varying diameters and subject to compressive load. A constant compressive load in terms of initial displacements was applied on a quasi-isotropic square plate specimen of dimensions 200 mm x 200 mm with a thickness of 2.88 mm of both CFRP and GFRP configurations. A circular delamination was introduced at the centre of the plate and its diameter and position along the thickness direction were varied and studied. Using ABAQUS codes of practice, Strain Energy Release Rate (SERR) was computed. The principles of Modified Virtual Crack Closure Technique (MVCCT) was used to compute SERR. Delamination propagates when the computed mixed mode energy release rate exceeds the critical value, G C . Depending on the external loading and material properties, Total strain energy release rate, G T (G T =G I +G II +G III ) was used to predict growth of delamination. The onset of delamination growth was determined by plotting the values of (G T /G C ) across various delamination sizes along the thickness of the plate and reported. Keywords: Virtual Crack Closure Technique, Strain Energy Release R te, CFRP, GFRP, Delamination. Abstract

Keywords: Virtual Crack Closure Technique, Strain Energy Release Rate, CFRP, GFRP, Delamination.

*Corresponding author. Tel.: +91-8105176460 E-mail address: shubha.javagal@gmail.com *Corresponding author. Tel.: +91-8105176460 E-mail address: shubha.javagal@gmail.com

2452-3216  2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers. 10.1016/j.prostr.2019.07.070 2452-3216 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers. 2452-3216 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers.

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1. Introduction Composite material is a combination of two or more materials mixed together in a suitable proportion to obtain the desired structural properties. Usually, the components in composites can be identified physically since they interface with one another. The properties of composite materials are superior to that of the individual materials used in their composition. A composite material is generally made up of a fibrous material oriented in alternating directions which are embedded in a resin matrix. This arrangement is responsible for the high strength to weight ratio in composites. Composites are friendly and flawless but damages are inevitable, especially the presence of delamination, which can be defined as an interlaminar disbond. Presence of delamination between various layers that are oriented in different directions is one of the prevalent outcomes of low velocity impact in composites. Low velocity impact events can take place during working of the material or during maintenance and can be considered as one of the major threats on composite laminates. The presence of delamination reduces residual strength and stiffness of the structure causing structural degradation. This calls for the designers and researchers to study the significance of delamination, especially in laminate structures. The damage tolerance study may be useful to understand the structural behaviour of the delaminated structure. The pioneer works in Fracture Mechanics were carried out by Rybicki and Kanninen taking into consideration, the crack tip forces and relative displacement of the cracks in order to calculate SERR [Kanninen, (1973)]. This was further extended to 3D specimens, thus the 3D Virtual Crack Closure Technique was developed [Shivakumar et. al., (1998)]. An overview to the history, approach and applications of VCCT was presented by Krueger [Krueger, (2004)]. The concepts of VCCT were later applied to study delamination growth and its behaviour in Carbon Fibre Composite Laminate when subjected to spectrum fatigue loads [Raju et. al., (2014)]. In the present study, principles of Finite elemental Analysis are applied on square plates made up of Carbon Fibre Reinforced Polymers (CFRP) and Glass fibre Reinforced Polymers (GFRP). Critical Energy Release rates in the three modes of fracture; G I, G II and G III are determined. It is assumed that these values of energy release rates are material specific properties and are independent of the stacking sequence and geometry of the specimen. Total Energy Release rate G T is computed and considered as the major criterion for the growth of delamination in composites. 2. Finite Element Modelling of the Specimen A standard square plate, 200 mm x 200 mm in dimension with a thickness of 2.88 mm was considered for analysis. A quasi-isotropic lay-up in the sequence [(+45/-45/0/90) 2 ] s made up of unidirectional composites was modelled with individual layer thickness of 0.18 mm. The geometric details of the plate are tabulated in Table 1. The material properties of the CFRP and GFRP used was obtained from existing literature [Raju et. al., (2013) and Amaro et. al., (2013) respectively] and are populated in Table 2 and Table 3. Using ABAQUS standard platform, 3 D finite element model of the plate specimen was generated and is shown in Figure 1. The meshing of the specimen was carried out in a way to ensure a smooth propagation of delamination by introducing fine mesh sizes ahead of the delamination front. Regions lying out of the area of interest were coarsely meshed to reduce the number of elements and thus the analysis time. Analysis was carried out by introducing a circular delamination at the centre of the plate by varying the delamination sizes from 10 mm to 100 mm along the laminate thickness. Contact was simulated by introducing BNODES (Bond nodes) throughout the layer except for the considered delaminated area. This absence of BNODES simulated required delamination. The various diameters of delamination considered are shown in Figure 2 and the different cases analysed by introducing the delaminations at different layers are tabulated in Table 4. Table 1. Geometrical Properties of the plate Dimensional Properties Dimensions of plate : 200 mm x 200 mm Layer Thickness : 0.18 mm each Thickness of plate : 2.88 mm Layup Sequence : [(+45/-45/0/90) 2 ] s

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Figure 1. Meshing pattern of the plate specimen

Table 2. Material Properties of CFRP [Raju et. al., (2013)] Young’s Modulus (GPa) Fracture Toughness (J/m 2 )

Poisson's ratio

Shear Modulus (GPa)

E 11 =139.4 E 22 =10.16 E 33 =10.16

G IC =0.3

ν 12 =0.3 ν 13 =0.3 ν 23 =0.43

G 12 =4.6 G 13 =4.6 G 23 =3.5

G IIC =0.48 G IIIC =0.48

Table 3. Material Properties of GFRP [Amaro et. al., (2013)] Young’s Modulus (GPa) Fracture Toughness (J/m 2 )

Poisson's ratio

Shear Modulus (GPa)

E 11 =50

G IC =0.15 G IIC =0.3 G IIIC =0.3

ν 12 =0.34 ν 13 =0.34 ν 23 =0.38

G 12 =3 G 13 =3

E 22 =10.16 E 33 =10.16

G 23 =2.79

Figure 2. Delamination sizes considered for analysis.

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Table 4. Various cases considered for analysis .

Thickness of Top plate (mm)

Thickness of Bottom plate (mm)

Figure

Cases of study

1.44

1.44

Case 1 (between 90° and 90° plies)

(8 plies)

(8 plies)

1.62

1.26

Case 2 (between 90° and 0° plies)

(9 plies)

(7 plies)

1.8

1.08

Case 3 (between 0° and -45° plies)

(10 plies)

(6 plies)

1.98

0.90

Case 4 (between - 45° and +45° plies)

(11 plies)

(5 plies)

2.16

0.72

Case 5 (between +45° and 90° plies)

(12 plies)

(4 plies)

2.34

0.54

Case 6 (between 90° and 0° plies)

(13 plies)

(3 plies)

2.52

0.36

Case 7 (between 0° and -45° plies)

(14 plies)

(2 plies)

2.7

0.18

Case 8 (between - 45° and +45° plies)

(15 plies)

(1 plies)

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2.1. Loads and Boundary Conditions

All the degrees of freedoms were held on one of the edges of the bottom plate and a compressive load in terms of initial displacement was applied on to the opposite edge. The remaining free edges were also constrained [Raju et. al., (2014)] such that delamination growth shall initiate from the centre of the plate. A non-linear static analysis was carried out to compute SERR.

2.2. Computation Of Strain Energy Release Rate

The major assumption of VCCT is that the strain energy released during the propagation of crack is always equal to the energy required to close the crack [Krueger, (2004)]. The total value of SERR obtained can be divided into the three corresponding loading modes, opening, sliding shear and tearing shear similar to the Benzeggagh and Kenane criterion [Benzeggagh and Kenane, (1996)] which is used to compute the SERR contributing to the propagation of delamination in the Finite element analysis. The equation for the Critical energy release rate, G c computed using B K law. � = �� + ( ��� − �� )[ � � ⁄ ] � (1) Where, G IC = Critical Energy Release rate in mode I. G IIC = Critical Energy Release rate in mode II. G IIIC = Critical Energy Release rate in mode III. G S = G II + G III G T = G I + G II + G III η = 1.75 , an experimental value obtained from existing literature [Raju et. al., (2014)]. Delamination is expected to grow when, for a given mixed-mode ratio, the Total Strain Energy Release Rate (G T ) crosses the value of Critical Strain Energy Release Rate (G C ), which is given by B-K law. � � ≥ 1 (2) ⁄ 3. Results and Discussion B-K law was used to compute SERR in the finite element analysis using ABAQUS codes. In order to compute the SERR along the delamination front the compressive load was kept constant and the delamination sizes were varied from 10 mm to 100 mm. SERR values in the corresponding three modes of failure were determined from the analysis. Critical Strain Energy Release Rate, G C and Total Strain Energy Release Rate, G T were also computed. For Case 1, it was found that the values of G I , G II, G III and G eff were 0.010 N/mm, 0.421 N/mm, 0.275 N/mm and 1.255 N/mm respectively for a delamination size of 90mm. From these obtained values, G T and G T / G C were computed to be 0.706 N/mm and 1.313 N/mm respectively. It was found that the propagation of delamination initiated at 88 mm in the CFRP specimen whereas, at 92 mm for the GFRP specimen for Case 1. To better understand the behavior of the crack front in between plies, the values of Strain Energy Release Rates responsible for the propagation of delamination for both CFRP and GFRP specimen were plotted against the various delamination sizes as shown in Figure 3. It was observed that at the surface level (Case 7 and Case 8), the specimen failed even when a delamination of 10 mm diameter was considered for both CFRP and GFRP configurations. As the thickness of the plate reduces, its ability to take load reduces and thus these specimen failed when a delamination of 10 mm was introduced. Failure of the specimen also depends on the orientation of fibres of the layers where delamination is present. In these cases, further analyses were not necessary. The bonding state of the plate specimen after the crack propagation is shown in Figure 4.

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Figure 3. Comparision of G T /G C for CFRP and GFRP.

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Figure 4. Delamination Growth Pattern.

4. Conclusion From the results obtained from the analysis of CFRP plate specimen, the values of SERR obtained in both shearing mode and tearing mode (Mode II and Mode III) during the analyses are very close to each another and quite dominant when compared to Mode I. From the results obtained, it is also inferred that smaller delamination sizes at the surface level is sufficient to initiate delamination growth. This implies that surface level delamination causes more damage to the stability of the structure and thus should be avoided. In the next phase of analysis, GFRP plate specimen was studied and the results obtained are compared with the data from CFRP analysis. From the plotted graph it is inferred that crack growth is comparable with both materials and crack propagation is delayed in GFRP laminate as compared to the CFRP laminate. Acknowledgements Authors wish to thank and acknowledge the authorities of CSIR-National Aerospace Laboratories, Bangalore and National Institute of Technology Karnataka, Surathkal for their kind and continuous support during this research work. References Amaro, A.M., Reis, P.N.B., de Moura, M.F.S.F., Neto, M.A., 2013. Influence of multi-impacts on GFRP composites laminates, Composites: Part B 52, ISSN 1359-8368, 93-99. Benzeggagh,M. and M.Kenane, 1996. Measurement of mixed mode delamination fracture toughness of unidirectional glass/epoxy composites with mixed-mode bending apparatus, Composite Science and Technogly 56, 439-449. Kanninen M.F., 1973. An augmented double cantilever beam model for stud crack propagation and arrest, International Journal of Fracture 9, The Netherlands. Krueger, 2004. Virtual crack closure technique: History, approach, and applications. American Society of Mechanical Engineers, Appl. Mech. Rev. 57 No.2. Raju J., Duragkar M.S., Jagannathan N., Manjunatha C.M., 2013. Finite Element Analysis and Prediction of Onset of Mode I Delamination Growth under a Tensile Spectrum Load Sequence, ISAMPE National Conference on Composites INCCOM-12. Raju J., Sreedhar D.S., Majunatha C.S., 2014. Prediction of Delamination Growth and Behaviour in a Carbon fibre composite Laminate subjected to Constant amplitude Compression-Compression Fatigue Laods, Global Journal of Engineering Design and Technology 3(4), 19-23.

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Shivakumar K.N., Tan P.W, Newman J.C. Jr., 1988. A Virtual Crack-Closure Technique for Calculating Stress Intensity Factors for Cracked Three Dimensional Bodies, International Journal of Fracture 36, The Netherlands, 43-50.

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Procedia Structural Integrity 14 (2019) 806–819

2nd International Conference on Structural Integrity and Exhibition 2018 Analysis of Irregular Structures under Earthquake Loads Siva Naveen E a,d , Nimmy Mariam Abraham b,d,* , Anitha Kumari S D c,d 2nd International Conference on Structural Integrity and Exhibition 2018 Analysis of Irregular Structures under Earthquake Loads Siva Naveen E a,d , Nimmy Mariam Abraham b,d,* , Anitha Kumari S D c,d a Former Post-graduate Student, b Assistant Professor, c Associate Professor d M. S. Ramaiah University of Applied Sciences, Bangalore, 560058, India a Former Post-graduat Student, b Assistant Professor, c Associate Professor d M. S. Ramaiah University of Applied Sciences, Bangalore, 560058, India

Abstract Behavior of a multi-storey building during strong earthquake motion depends on structural configuration. Irregular configuration either in plan or in elevation is recognized as one of the major causes of failure during earthquakes. Thus irregular structures, especially the ones located in seismic zones are a matter of concern. Structures generally possess combination of irregularities and consideration of a single irregularity may not result in accurate prediction of seismic response. The choice of type, degree and location of irregularities in the design of structures is important as it helps in improving the utility as well as aesthetics of structures. Hence, the present study addresses the seismic response of reinforced concrete structures possessing various combinations of irregularities. A nine-storeyed regular frame is modified by incorporating irregularities in various forms in both plan and elevation to form 34 configurations with single irregularity and 20 cases with combinations of irregularities. Along with the regular configuration, 54 irregular configurations are analyzed and compared. All the frames are subjected to seismic loads and the response of the structures is computed numerically. It is observed that irregularity considerably affects the seismic response. Out of various types of single irregularities analyzed, stiffness irregularity is found to have maximum influence on the response. Among the cases having combinations of irregularities, the configuration with mass, stiffness and vertical geometric irregularities has shown maximum response. The results of this study would aid in designing of irregular structures judiciously without compromising their performance. © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers. Abstract Be avior of a multi-storey building dur ng strong arthquake motion depends on str ctural configuration. Irregular configuration ith r in plan r in elevation i recognized as one of the maj r auses of failure during earthquakes. Thus rregula str ctures, espe ially the ones located in seismic zones are a matter of oncern. Stru tures generally posses combination f irregulariti s c nsideration of a single irregularity may not result in accur te prediction of seismic response. The choice of type, degree and location of irregulariti in the esign of tructure important as it helps in improvi g the utility as well as aesthetics of structures. Hence, the present study addresses the seis ic respons of reinforced concrete structures p ssessing vari us combi ations of irregularities. A nine-s oreyed regular frame is modified by incorporating irregularities in various forms in bo plan and elevation to f rm 3 configu ations with single irregularity and 20 cas s with combinations of irregulari ie . Along with the regular c figuration, 54 irregular nfigurations a e nalyzed and compared. All the frames are subjected to eismic loads and the response of the structures is computed numerically. It is ob erv d that irregularity considerably affects the seismic Out of various types of single irregularities analyzed, stiffn ss irre l rity is found to have maximum influence on the response. Among the cases having combinations of irregularitie , the configuration with mass, stiffness and vertical geometric irregularities has shown maximum response. The results of this study would aid in designing of irregular structures judiciously without compromising their performance. © 2018 The Authors. Published by Elsevier B.V. This is a open access article under the CC BY-NC-ND lic nse (https://creat vecommons.org/licenses/by- c-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers. © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers.

Keywords: Configuration, horizontal irregularity, vertical irregularity, combination, response, displacement, storey drift Keywords: Configuration, horizontal irregularity, vertical irregularity, combination, response, displacement, storey drift

*Corresponding author. Tel.: +91 9448234542 E-mail address: nimmy555@gmail.com *Corresponding author. Tel.: +91 9448234542 E-mail address: nimmy555@gmail.com

2452-3216© 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers. 2452-3216© 2018 Th Authors. Published by Elsevier B.V. This is a open access article und r the CC BY-NC-ND lic nse (https://creat vecommons.org/licenses/by- c-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers.

2452-3216  2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of Peer-review under responsibility of the SICE 2018 organizers. 10.1016/j.prostr.2019.07.059

Siva Naveen E et al. / Procedia Structural Integrity 14 (2019) 806–819 Siva Naveen E, Nimmy Mariam Abraham, Anitha Kumari S D / Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction The behavior of any building depends on the arrangement of structural elements present in it. The important aspects on which the structural configuration depends are geometry, shape and size of the building. When a building is subjected to dynamic loads, inertia forces are developed and gets concentrated at the center of mass of the structure. Usually, the vertical members such as columns and shear walls resist the horizontal inertia forces and the resultant of these forces gets concentrated at a point called center of stiffness. When the center of mass doesn’t coincide with the center of stiffness, eccentricity develops in the structure. Eccentricity occurs due to the irregular arrangement of structural configuration which in turn induces torsion in the structure. Location and size of structural elements have significant effect on torsional coupling which results in damage of structures. Regular structures have no significant discontinuities in plan or in vertical configurations. Irregular structures have certain physical discontinuities either in plan or in elevation or both which affect the performance of the structure subjected to lateral loads. Irregularities in the distribution of mass, stiffness and geometry along the height of any building are grouped as vertical irregularities. Horizontal irregularities can be attributed to the presence of discontinuities in plan. Different structural irregularities affect the seismic response of structures in different ways. Irregularities are introduced in real structures for both aesthetics and utility. The magnitude of variation in response depends on the type, degree and location of irregularities present. The judicious choice of these parameters in the design of structures improves performance of the structure. So far, many researchers have investigated the effect of seismic response on structures having vertical and horizontal irregularities. Valmundsson and Nauhave (1997) studied the seismic behavior of multistoried buildings having vertical structural irregularities and concluded that 30% decrease in stiffness have increased the storey drift in the range of 20-40%. Guevara et al. (1992) focused on the effect of floor plan on the seismic behavior of structures. Study includes the dynamic analysis of H and L shaped buildings. The paper suggests that buildings having H and L shaped plan should be divided into rectangular blocks separated by seismic joints. Wood (1992) investigated the effect of seismic response of setback structures and concluded that the presence of setbacks did not affect the seismic behavior. The behavior was found to be similar to that of regular structure having no setback. Khoure et al. (2005) studied the response of nine-storey steel frames with setback irregularities and observed that the higher torsion response at the upper portion of the setbacks. Tremblay and Poncethave (2005) studied the dynamic behavior of building frames with irregular distribution of mass. They have concluded that both static and dynamic analysis methods are ineffective in predicting the response of the frames having mass irregularity. Gokdemi et al. (2013) studied the effect of torsional irregularity on structures. According to the authors, torsion is caused due to the eccentricity between center of mass and center of stiffness. The intensity of moment due to torsion was found to be a function of eccentricity ratio. Ozmen et al. (2014) performed parametric studies on six buildings with varying shear wall positions. Based on the floor rotations, a torsional irregularity coefficient was proposed. According to their findings, as the number of storey decreases, the torsional irregularity coefficient increases and the maximum storey rotations occur for the top storeys. Ahmed et al. (2016) have studied the effect of seismic response of L shaped buildings. Equivalent static and response spectrum methods were performed using ETABS software. They observed that the response of L shaped building is higher than that of the regular frame due to torsion. Patil et al. (2017) studied the dynamic response of multi-storey buildings with plan asymmetry. They have numerically analyzed multi storyed frames having different plan shapes. They have reported that the increase in height of T and L shaped buildings increases the displacement response and stress at the re-entrant corners. As per the study conducted on existing literature, researchers have mainly focused on single irregularity. The works reported are mainly related to mass, stiffness or geometry. Real structures contain multiple irregularities in various combinations. However, the studies on the effect of combination of irregularities are scarce. Hence, the present research addresses the behavior of structures having combination of irregularities, subjected to ground motion. The study includes the analysis of both regular and irregular multistoried reinforced concrete (RC) frames. Irregularities in both elevation and plan are studied. The vertical irregularities considered for the study include mass, stiffness and vertical geometric irregularities. The horizontal irregularities considered are torsion and re-entrant corner irregularities. A total of 54 irregular frames are analyzed. Out of the cases studied, 34 configurations have single irregularity and 20 possess combination of irregularities. The main purpose of the study is to identify critical combinations of irregularities.

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2. Methodology In the present work, seismic response of frames having different configurations are obtained numerically using a finite element based software, ETABS. The major inputs are geometry of the frame including dimensions of storeys and columns, total mass of each floor, modulus of elasticity, damping ratio and earthquake data. The modulus of elasticity for the material is taken as 20000 MPa. Rayleigh damping is assumed with a damping ratio of 4%. It is also assumed that the structure starts from rest on load application. The results are structural response in the form of storey displacement, storey drift, base shear and overturning moment. In order to validate the results, response obtained for the regular configurations is compared with the results reported in literature. For both validation and subsequent analysis, same methodology is adopted. 3. Validation Moehle (1984) has conducted experiments on a small scale nine-storyed test structure subjected to El-Centro North-South 1940 ground motion (scaled). The results reported are used to validate the model developed for the present study. Two frames having nine storeys and three bays were placed opposite and parallel to each other. The frames together carry a total weight of 460 kg at each floor level. The typical storey height is 0.229 m. The frame has three bays in the direction of length and one bay in the direction of width. The dimension of each bay in the direction of length and width are 0.305 m and 0.914 m respectively. At the base, the frames were subjected to simulated earthquake base motion in horizontal direction parallel to the plane of the structure. The same test structure is modeled and analyzed numerically using ETABS. The first three natural frequencies and the top storey displacement of the structure obtained numerically are compared with the experimental results reported by Moehle (1984). The same is tabulated in Table 1.

Table 1. Validation of the model. Particulars

Literature (Moehle,1984)

Obtained

1 st natural frequency (Hz) 2 nd natural frequency (Hz) 3 rd natural frequency (Hz) Top storey displacement (mm)

4.5

2.4

14.4 28.3 16.1

15.3 29.5 16.9

4. Analysis of frames with regular and irregular configurations A structure is said to be irregular, when certain structural parameters exceed the limits specified by standards. Table 2 shows the limits for mass (M), stiffness (S), vertical geometric (VG), re-entrant corner (REC) and torsional (T) irregularities prescribed by IS1893:2016 (Part I).

Table 2. Irregularity limits prescribed by IS 1893:2016 (Part I) (i = storey number, a = adjacent storey number, Δ max = maximum deformation and Δ avg = average deformation). Type of irregularity Classification Limits Mass (M) Vertical irregularity M i < 1.5M a Stiffness (S) Vertical irregularity S i < S i+1

Vertical geometry (VG) Re-entrant Corner (R) Torsion (T)

Vertical irregularity Horizontal irregularity Horizontal irregularity

VG< 1.25 VG a R i <= 15% Δ max <= 1.5Δ avg

A nine-storey scaled frame with a storey height of 0.229 m is considered for the study. The frame has six bays in the direction of length and three bays in the direction of width. The dimension of each bay in the direction of length and width are 0.305 m and 0.914 m respectively. Each floor carries a lumped mass of 2760 kg. The irregularities are

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incorporated by changing the vertical and horizontal configurations of the regular frame. Apart from the regular case, 54 irregular configurations are analyzed, out of which, 34 cases possess single irregularity and 20 possess combination of irregularities.

4.1. Configurations having single irregularity

Vertical irregularities include mass, stiffness and vertical geometric irregularities whereas horizontal irregularities include re-entrant corner and torsional irregularities. The 24 frame configurations with single vertical irregularity along with the regular configuration and the 10 configurations having single plan irregularity are shown in Figures 1a and 1b respectively. For all the cases analyzed, bay length and number of storeys are kept constant. The details of various irregularities incorporated in each case are as follows:  Mass irregularity (MI) Four different cases of mass irregularity (MI-1 to MI-4) are adopted for the analysis. Irregularity is introduced by increasing the mass of a particular storey. For MI-1 and MI-2, the mass is increased at 4th storey by 1.5 and 2 times respectively than the regular frame. For MI-3 and MI-4 the mass is increased at second and seventh storeys by 1.5 and 2 times respectively.  Stiffness Irregularity (SI) Ten different cases of stiffness irregularities (SI-1 to SI-10) are considered for the analysis. Irregularity is introduced by reducing the number of columns, increasing the length of the columns or changing the cross section area of the columns. For the cases SI-1 and SI-9, number of columns is reduced from 28 to 12 with respect to the regular frame. For SI-2, SI-3, SI-4 and SI-5 cases, the length of the columns is increased. For SI-6, rectangular columns are replaced with circular columns for the first two floors, without changing the total cross sectional area of the columns. For SI-7, rectangular columns are replaced with circular columns for the first two floors, with increased total cross sectional area of the columns. For SI-8, the cross sectional area of the columns for first two floors is increased to 0.056m×0.042m from 0.051m×0.038m. For the first two floors, the cross sectional area is more compared to the remaining floors. For SI-10, the number of columns was reduced to 12 and the length of the columns was increased to 0.458 m from 0.229 m. Table 3 shows the percentage reduction in storey stiffness with respect to adjacent storey, at the location of irregularity.  Vertical Geometric Irregularity (VGI) Ten different cases of vertical geometric irregularity (VGI-1 to VGI-10) are considered for the analysis. Irregularity is introduced by varying vertical configuration along the height. For the cases VGI-1, VGI-2, VGI-3, VGI-7, VGI-9 and VGI-10, the horizontal dimension of the lateral resisting force system is reduced as shown in Figure 1a. For the cases VGI-4, VGI-5, VGI-6 and VGI-8 lateral dimension is increased.  Re-entrant Corner Irregularity (REC) Re-entrant corner irregularity is introduced by varying plan configuration of the frames. Seven different cases (REC-1 to REC-7) are considered for analysis.  Torsional Irregularity (TI) Torsional irregularity is introduced by varying vertical structural elements. Three different cases (TI-1 to TI-3) are selected for the analysis. For case TI-1, irregularity is incorporated by introducing circular columns of size 0.06m. For case TI-2, irregularity is incorporated by introducing two shear walls of thickness 0.042m at the corner of the building. For the case TI-3, irregularity is incorporated by introducing columns of different types of cross sections, two columns of dimensions 0.06m×0.04m, one column of dimensions 0.055 m×0.04m, sixteen columns of dimensions 0.051m×0.038m and two columns of dimension 0.05m×0.035m along with two shear walls of thickness 0.042m at the corner.