PSI - Issue 33

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Procedia Structural Integrity 33 (2021) 1–2

IGF26 - 26th International Conference on Fracture and Structural Integrity Preface Vittorio Di Cocco a  , Paolo Ferro b , Carmine Maletta c , Luciana Restuccia d , Giacomo Risitano e , Andrea Spagnoli f Università di Cassino e del Lazio Meridionale, Italy a* Università di Padova, Italy b Università della Calabria, Italy c I 26 - 26th International onference on racture and tructural Integrity r f itt ri i cc a  , a l err b , ar i e aletta c , cia a est ccia d , iac isita e , rea a li f Università di Cassino e del Lazio eridionale, Italy a* Università di Padova, Italy b Università della Calabria, Italy c

Politecnico di Torino, Italy d Università di Messina, Italy e Università di Parma, Italy f Politecnico di Torino, Italy d Università di essina, Italy e Università di Parma, Italy f

© 2021 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) Peer-review under responsibility of the scientific committee of the IGF ExCo 2021 The Authors. Published by ELSEVIER B.V. This is an open access article under the -NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-revie Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo © 2021 The uthors. Published by ELSE IER B. . This is an open access article under the CC BY- C- license (https://creativeco ons.org/licenses/by-nc-nd/4.0) Peer-review Statement: Peer-review under responsibility of the scientific co ittee of the I F ExCo

Keywords: Preface, Fracture, Structural Integrity. Keywords: Preface, Fracture, Structural Integrity.

1. Preface The Italian Group of Fracture (IGF) is a cultural association devoted to: (i) spreading and promoting works and researches about fracture phenomena, even forming workgroups; (ii) promoting all the activities concerning the development of materials and structure testing standards; (iii) cooperating with foreign associations with the same intents; (iv) organizing meetings, workshops, conferences, debates and courses about fracture phenomena and (v) publishing meetings proceedings, news, journals. Despite the worldwide pandemic crisis of this time, IGF managed to organize in Turin its biannual meeting. The IGF26 meeting took place in the wonderful venue of the Castello del Valentino in May 26-31, 2021 in a hybrid form. All the delegates were allowed to participate to the conference either in remote or in presence and the brave (and vaccinated) participants in presence were able to taste the delicious Piedmont cuisine and to enjoy the beaty of the city of Turin! It is possible to state that it was a successful event: during the conference 163 presentations coming from 28 different countries were showed, and 3 plenary lectures were carried out by José Antonio Correia (Universidade do Porto, Portugal), Liviu Marsavina (Politehnica University of Timisoara, Romania) and Donato Firrao (Politecnico di Torino, Italy). The presentations were scheduled in 16 sessions, covering a wide range of topics related to theory, modelling and experiments of fracture and structural integrity phenomena. Research works ranged from new materials development and design strategies aimed at improving their integrity to advanced production or joining technologies focused on topology optimization or side effects mitigation (residual stress, distortions, defects, etc.). All these issues are in line with the most urgent today challenges of the European agenda such as climate change, lightweight design and resource exploitation, among the others. The constructive and vibrant discussions taken at the end of each presentation are a further confirmation of the high scientific quality of the event as well as of the significant level of interactions among the participants. 1. Preface The Italian roup of Fracture (I F) is a cultural association devoted to: (i) spreading and pro oting orks and researches about fracture pheno ena, even for ing orkgroups; (ii) pro oting all the activities concerning the develop ent of aterials and structure testing standards; (iii) cooperating ith foreign associations ith the sa e intents; (iv) organizing eetings, orkshops, conferences, debates and courses about fracture pheno ena and (v) publishing eetings proceedings, ne s, journals. espite the orld ide pande ic crisis of this ti e, I F anaged to organize in Turin its biannual eeting. The I F26 eeting took place in the onderful venue of the astello del alentino in ay 26-31, 2021 in a hybrid for . ll the delegates ere allo ed to participate to the conference either in re ote or in presence and the brave (and vaccinated) participants in presence ere able to taste the delicious Pied ont cuisine and to enjoy the beaty of the city of Turin! It is possible to state that it as a successful event: during the conference 163 presentations co ing fro 28 different countries ere sho ed, and 3 plenary lectures ere carried out by José ntonio orreia ( niversidade do Porto, Portugal), Liviu arsavina (Politehnica niversity of Ti isoara, o ania) and onato Firrao (Politecnico di Torino, Italy). The presentations ere scheduled in 16 sessions, covering a ide range of topics related to theory, odelling and experi ents of fracture and structural integrity pheno ena. esearch orks ranged fro ne aterials develop ent and design strategies ai ed at i proving their integrity to advanced production or joining technologies focused on topology opti ization or side effects itigation (residual stress, distortions, defects, etc.). ll these issues are in line ith the ost urgent today challenges of the European agenda such as cli ate change, light eight design and resource exploitation, a ong the others. The constructive and vibrant discussions taken at the end of each presentation are a further confir ation of the high scientific quality of the event as ell as of the significant level of interactions a ong the participants.

* Corresponding author. Tel.: +39.07762994334 E-mail address: v.dicocco@unicas.it * Corresponding author. Tel.: +39.07762994334 E-mail address: v.dicocco unicas.it

2452-3216 © 2021 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) Peer-review under responsibility of the scientific committee of the IGF ExCo 10.1016/j.prostr.2021.10.001 2452-3216 © 2021 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) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo 2452-3216 © 2021 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) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo

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According to the IGF tradition, all the presentations were video-recorded and they are now available in the IGF YouTube channel (https://www.youtube.com/playlist?list=PLT1-2PyZ6QrI8Cl6fsogBttlTvnkoXTHt). During IGF members meeting, the new ExCo was elected and prof. Filippo Berto (NTNU, Norway) was appointed IGF President. In addition, prof. Liviu Marsavina was awarded with the Paolo Lazzarin IGF medal and José A.F.O. Correia with the Manson-Coffin IGF medal; finally, prof. Giuseppe Ferro (Politecnico di Torino, Italy) was awarded with the IGF Honorary Membership. Congratulations to all the winners! This special issue of Procedia Structural Integrity collects more than one hundred thirty papers related to the presentations given during the IGF26 conference. The number and the quality of the papers is an important sign of the of the good health of the fracture and structural integrity community. We hope to meet you soon in presence in the upcoming IGF events!

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Procedia Structural Integrity 33 (2021) 652–657

© 2021 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) Peer-review under responsibility of the scientific committee of the IGF ExCo Click here and insert your abstract text. © 2021 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) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo Keywords: lattice-based structure, construction plate; damage; failure; weight optimization; civil engineering 1. Introduction The mechanical performance of metallic sandwich panel with lattice-based core superiors the stiffened foam based concept under major loading conditions [1-2]. Moreover, the advanced concepts were investigated, e.g., fiber reinforced composites for sandwich panels [3]. However, such lattice-based structures are still unconventional for analys . The ultimate compressive load is estimated by he simulati n rocedure using Abaqus FEA software. The sult is that proposed tructure shows 1.4 higher ultimate load in comparison with conventional concrete plate reinforced by steel bars, while the metal consumption is identical. Click here and inse t yo r abstract text. © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article und r the CC BY-NC-ND licens (https:// reativecommons.org/licenses/by-nc-nd/4.0) Peer-review Statem nt: Peer-review nder responsibility of the scientific committee of the IGF ExCo Keywords: lattice-based structure, construction plate; damage; failure; weight optimization; civil engineering 1. Introduction The mechanical performance of metallic andwich panel wit l ttice-based ore superiors the s iffened foam based concept under maj r loa ing conditions [1-2]. Moreover, the advance concepts we investigated, e.g., fiber reinforced composites for sandwich panels [3]. However, such lattice-based structures are still unconventional for IGF26 - 26th International Conference on Fracture and Structural Integrity 3D lattice-based structural elements for industrial application Alexey Fedorenko a *,Boris Fedulov b , SergeyJurgenson c , Evgeny Lomakin b a Center for Design, Manufacturing and Materials, Skolkovo Institute of Science and Technology, Bolshoy Boulevard 30, bld. 1, 121205, Moscow, Russia b Department of Mechanics and Mathematics, Lomonosov Moscow State University, GSP-1, Leninskiye Gory 1, 119991, Moscow, Russia c Moscow Aviation Institute (National Research University), Volokolamskoe shosse, 4, 125993, Moscow, Russia Abstract A concept of construction plate based on steel sandwich panel with pyramidal lattice core is proposed. A space between the outer sheets of the sandwich is filled with concrete. The considered plate has large dimensions within standard range in order to provide meaningful data for industrial case and capture geometry nonlinearity in stress analysis. The ultimate compressive load is estimated by the simulation procedure using Abaqus FEA software. The result is that proposed structure shows 1.4 higher ultimate load in comparison with conventional concrete plate reinforced by steel bars, while the metal consumption is identical. IGF26 - 26th International Conference on Fracture and Structural Integrity 3D lattice-based structural elements for industrial application Alexey Fedorenko a *,Boris Fedulov b , SergeyJurgenson c , Evgeny Lomakin b a Center for Design, Manufacturing and Materials, Skolkovo Institute of Science and Technology, Bolshoy Boulevard 30, bld. 1, 121205, Moscow, Russia b Department of Mechanics a d Mathematics, Lomonos v Moscow State University, GSP-1, Leninskiye Gory 1, 119991, M scow, Russia c Moscow Aviation Institute (National Research University), Volokolamskoe shosse, 4, 125993, Moscow, Russia Abstract A concep of construction plat b sed on steel sandwich pan l wit pyram al lattice core is propos d. A space between the outer she ts f the sandwich is i led with concrete. The co sidered plate has large d mensions within standard range in ord r t provide me ningful data for industrial c se and capt geometry nonlinearity in stress

* Corresponding author. E-mail address: a.fedorenko@skoltech.ru * Correspon ing author. E-mail address: a.fedorenko@skoltech.ru

2452-3216 © 2021 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) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo 2452-3216 © 2021 The Authors. Published by ELSEVIER B.V. This is an open access article und r the CC BY-NC-ND licens (https:// reativecommons.org/licenses/by-nc-nd/4.0) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo

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industry. One of the reasons is the complexity of manufacturing due to the need of stamping and welding processes [4], which are frequently considered problematic for strength prediction and quality control. The modelling of sandwich properties can be done by the modern computation methods [5], while the manufacturing is effectively resolved by robotics and nondestructive control. This work presents the construction panel, composed by two outer steel sheets and pyramidal core between, as presented in Fig.1.

Fig. 1. Sandwich panel with lattice pyramidal core (the space between outer sheets is filled with concrete)

The space between the sheets is filled with the concrete, so lattice structure plays a role of reinforcement similar to bars in conventional construction plate. In previous study [6] authors show that it is possible to achieve the stiffness and strength characteristics of the proposed structure under three-point bending at least two times higher in comparison with conventional plate in condition of equal metal consumption. In addition, the compressive load is also should be considered since compression prevails in many elements of the building. Intuitively, the considered conventional plate has no reinforcements in out of plane direction and therefore has significantly lower buckling load in comparison with proposed structure. Nevertheless, it is important to analyze the failure modes of both plates under compression, since it is governed by the complex response of the concrete and reinforcement.

2. Modelling of plate compression 2.1. Plate structure and dimensions

The total plate dimensions are shown in Fig.2, and the cases of conventional bars or pyramidal lattice are considered. The cell size of reinforcement using conventional bars with the radius of 7.7 mm is 150 mm. The unit rib of lattice has dimensions of 100x300 mm and a thickness of 2 mm. The total mass of steel reinforcements is 560 kg in both cases.

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Fig. 2. Dimensions of the plate and two variants of the reinforcement

2.2. Materials Concrete and steel properties used for simulation are presented in Table 1.

Table 1. Mechanical properties.

Concrete

Metal 1000 1000

Tension plasticity limit (N/mm 2 ) Compression yield limit (N/mm 2 ) Failure strain at tension (%) Failure strain at compression (%)

5

500

0.2

7 9

2

Modulus (N/mm 2 )

35,700

200,000

Poison ratio

0.2

0.3

Elastoplastic material model was chosen for modelling of concrete with pressure dependable Drucker-Prager plasticity criterion [7]. Flow theory with Kolmogorov failure criterion [8] was chosen for nonlinear part of constitutive relations: 0 (1 ) ( ) pl eq C k      , (1) 1 ( ) pl eq D d      , where 0 3 / 2 ij ij S S   , ij ij ij S      , / 3 ii    , 1( ), 0( ) ij ij i j i j       , 0 /     ; , , D C k  - parameters that have to be determined experimentally. For the modelling of reinforcement conventional plasticity model von Mises criterion was used. 2.3. Finite element formulation Abaqus Explicit FEA software [9] was used for the simulation of plate compression with the boundary conditions, as shown in Fig.3. An explicit time integration scheme was used due to the convergency issues in static formulation. A comparison between dynamic and static assumptions was conducted for linear portion of load to avoid dynamic effects. The plate is composed by 867000 C3D8R elements corresponding to concrete material, and

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24600 B31 beam elements are embedded in solid domain via special mesh constraint (Abaqus embedded element) in case of conventional plate. In case of proposed lattice structure 139000 S4R shell elements are embedded in solid domain. An “all with self” general contact concept is used to capture fragments interaction during fracture process.

Fig. 3. Boundary conditions of the problem

3. Simulation results and discussion A comparison of failure modes is presented in Fig.4 for the conventional plate and in Fig.5 for the proposed one through the ductile damage criterion value representation (DUCTCRT) for concrete and steel elements. An important similarity is that the damage initiation occurs in concrete on free edge of the plate, and this location can be reinforced by additional steel cells or sheets. The damage growth in concrete leads to the subsequent global buckling of the steel grid in case on conventional plate. For the proposed plate damage on the edge results multiple buckling of pyramidal lattice and outer sheets in the vicinity of free edge.

Fig. 4. Failure mode of conventional plate under compression: (a) damage on the free edge initiation; (b) final failure mode

Fig. 5. Failure mode of the proposed lattice-based plate under compression: (a) damage initiation on the free edge; (b) final failure mode

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The loading diagrams are shown in Fig.6. for both concepts with representation of the ultimate force of pure concrete plate. The proposed concept shows the ultimate force of 74,000 kN, while the conventional one only 52,000 kN, so the increase factor is 1.4. The slope of loading curves in the elastic range is similar for the both concepts, so no enhancement in stiffness for the proposed sandwich, and one can conclude in this case stiffness is governed by the concrete. The ultimate force of pure concrete is 32,000 kN. Notice pure concrete strength in Fig.6 was also estimated by simulation, but it is close to the simple calculation as a product of a compression yield limit (Table 1) and a cross-section area.

Fig. 6. Loading diagrams of proposed, conventional and a pure concrete plate without reinforcement

Conclusion The compressive loading case was considered for the comparison of strength properties of the conventional concrete plate with bar reinforcement, and the proposed concept of the plate based on sandwich with pyramidal core. The ultimate force is extremely large for the both concepts, but it is increased up to 1.4 times for the proposed concept. Together with promising characteristics of the proposed structure under bending, presented in previous work [6], it seems effective for custom industrial applications, where enhanced requirements to stiffness and strength are hardly satisfied by conventional plates. Acknowledgements This research was supported by the Russian Science Foundation (grant No. 20-11-20230). References [1] A.G. Evans, J.W. Hutchinson, N.A. Fleck, M.F. Ashby, H.N.G. Wadley, The topological design of multifunctional cellular metals, Prog. Mater. Sci. 46 (2001) 309–327. https://doi.org/10.1016/S0079-6425(00)00016-5. [2] G.W. Kooistra, V. Deshpande, H.N.G. Wadley, Hierarchical corrugated core sandwich panel concepts, J. Appl. Mech. Trans. ASME. 74 (2007) 259–268. https://doi.org/10.1115/1.2198243. [3] B. Wang, L. Wu, L. Ma, Y. Sun, S. Du, Mechanical behavior of the sandwich structures with carbon fiber-reinforced pyramidal lattice truss core, Mater. Des. 31 (2010) 2659–2663. https://doi.org/10.1016/j.matdes.2009.11.061. [4] H.N.G. Wadley, N.A. Fleck, A.G. Evans, Fabrication and structural performance of periodic cellular metal sandwich structures, Compos. Sci. Technol. 63 (2003) 2331–2343. https://doi.org/10.1016/S0266-3538(03)00266-5. [5] Y. Zhu, Q. Qin, J. Zhang, On effective mechanical properties of an orthogonal corrugated sandwich structure, Mater. Des. 201 (2021) 109491. https://doi.org/10.1016/j.matdes.2021.109491.

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[6] B.N. Fedulov, A.N. Fedorenko, S.A. Jurgenson, M.M. Kantor, E.V. Lomakin, Construction plate enforced by metamaterial elements, in: Procedia Struct. Integr., 2020. https://doi.org/10.1016/j.prostr.2020.10.020. [7] Drucker, D.C. and Prager, W., 1952. Soil mechanics and plastic analysis or limit design. Quarterly of applied mathematics, 10(2), pp.157-165. [8] Kolmogorov, W.L., 1970. Spannungen deformationen bruch. Metallurgija, p.230. [9] Abaqus manual, Providence, USA, https://www.3ds.com/

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Procedia Structural Integrity 33 (2021) 443–455

© 2021 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) Peer-review under responsibility of the scientific committee of the IGF ExCo Abstract Since its first meeting in 1985, ESIS TC4 has held regular semiannual meetings with between 15 and 35 participants, has organized a series of conferences (the first in 1994, then triennial since 1999) and has developed six ISO test standards on the fracture of polymers, polymer composites and adhesives with another two currently going through ISO standardization and ballots, and several more under development. The activities have also resulted in publications, including two books and two review papers. Initial activities focused on round robins providing test methods for determination of fracture properties for, e.g., technical data sheets, quality assurance, materials selection, or materials development and optimization and materials modelling. These procedures defined standard specimens, test rigs and test conditions. For polymers, standards for specific ranges of loading rate and for composites and adhesively bonded joints, procedures for different loading modes and mode mixes were developed. Recently, standard composite specimens with unidirectional fiber orientation were shown to overestimate the delamination resistance of multidirectional laminates under cyclic fatigue loading. First round robin data from the environmental stress cracking tests show the potential for discriminating between the different susceptibilities of polymers to environmentally induced fracture. Future activities will include elastomeric materials, simulation and modelling in combination with experiments or prediction of fracture behavior. Another topic of recent interest concerns digital tools, e.g., image analysis, automated data acquisition, data fitting and analysis. Guidelines on how to best reduce extrinsic scatter and eliminate human errors will improve the data quality. IGF26 - 26th International Conference on Fracture and Structural Integrity 35 years of standardization and research on fracture of polymers, polymer c mposites and adhesives in ESIS TC4: Past achievements an futur directions Andreas J. Brunner a* , Laurent Warnet b** , Bamber R.K. Blackman c* a Retired Scientist, P.O. Box 645, CH-8052 Zürich, Switzerland b University of Twente, Faculty of Engineering Technology, Horst Complex N208, P.O. Box 217, 7500 AE Enschede, The Netherlands c Department of Mechanical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK * ESIS TC4 Co-chair, ** ESIS TC4 Secretary Abstract Since its first meeting in 1985, ESIS TC4 has held regular semiannual meetings with between 15 and 35 participants, has organized a series of conferences (the first in 1994, then triennial since 1999) and has developed six ISO test standards on the fracture of polymers, polym r composites and adhesive with another two currently go through ISO stand r ization and ballots, and several more under dev lopmen . The activities have also resulted in public tions, including two books and two eview papers. Initial activiti focused on round robi s providing test methods for determinatio of fracture properties for, e.g., technical data he ts, quality assuranc , materials sele tion, or materials development and optim zation and materials modelling. These p ocedures def ned tandar specimens, test rigs and test conditi n . For polymers, standards for specif c rang s of loading rate and for composite and adhesively bonded joints, procedures for diff rent loading modes a od mixes w re develop d. Recently, standard composite specimens with unidirectional f ber orientati n w e hown to overestimate the delamin tion resist ce f multidirectio al laminat s under cyclic fatigue loading. First round r bin data from the environ ental st ss cracking tests show the potential f r discrim nating between the different susc p ib lities of polymers t nvironm ntally induced fracture. Future activ t es will include ela tom ri materi ls, simulation and m delling combination with experiment or predi t on of fracture behavior. Another top c of recent inter st concerns digital tools, e.g., image analysis, automated da acquisition, d ta fitting and nalys s. Guidelines on how to best reduce extrinsic scatter and elimi ate hu an err rs ll im rove the data quali y. Keywords: Fracture testing of polymers, polymer composites and adhesives; standardizaton; fracture mechanics design; digital tools; IGF26 - 26th International Conference on Fracture and Structural Integrity 35 years of standardization and research on fracture of polymers, polymer composites and adhesives in ESIS TC4: Past achievements and future directions Andreas J. Brunner a* , Laurent Warnet b** , Bamber R.K. Blackman c* a Retired Scientist, P.O. Box 645, CH-8052 Zürich, Switzerland b University of Twente, Faculty of Engineering Technology, Horst Complex N208, P.O. Box 217, 7500 AE Enschede, The Netherlands c Department of Mechanical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK * ESIS TC4 Co-chair, ** ESIS TC4 Secretary

Keywords: Fracture testing of polymers, polymer composites and adhesives; standardizaton; fracture mechanics design; digital tools;

* Corresponding author E-mail address: andreas.brunner@empa.ch

* Corresponding author E-mail address: andreas.brunner@empa.ch

2452-3216 © 2021 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) Peer-review under responsibility of the scientific committee of the IGF ExCo 10.1016/j.prostr.2021.10.051 2452-3216 © 2021 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) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo 2452-3216 © 2021 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) Peer-review Statement: Peer-review under responsibility of the scientific committee of the IGF ExCo

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1. Introduction This contribution presents the past, current, and envisaged future activities of ESIS TC4 on Fracture of Polymers, Polymer Composites and Adhesives; originally founded as "Task Group on Polymers and Composites" within the European Group of Fracture (EGF). The activities of the committee include the development of fracture test procedures for submission as international standards, and the related fracture research and dissemination of results. The present paper firstly summarizes the achievements of ESIS TC4over the past 35 years, then secondly it focusses on open issues in fracture testing that currently are or will soon become activities within the committee. Some of these questions are for example how fracture data from material tests on composites can be applied to structural design (Jones et al. 2017); how the effects from environmental exposure can be assessed (Kamaludin et al. 2017, Bredács et al. 2018, Contino et al. 2021); and how recent developments in digital technology may contribute to minimizing scatter in the experimental data (Brunner 2020) or speed up design and development of components and structures (Chisholm et al. 2019). Another issue relates to the development of new types of engineering materials based on polymers. Materials for which the applicability of the standard tests has to be assessed are, e.g., foams or aerogels (see, e.g., Banerjee and Sinha Ray 2020, Liu et al. 2020, Lee and Park 2020), self-healing polymers or composites (see, e.g., Wang and Urban 2020), nano-modified polymers (see, e.g., Sankasubramanian et al. 2019), or polymer-based metamaterials (see, e.g., Askari et al. 2020). This question also has relevance for new processing and production methods, such as the additive manufacturing of polymers or of polymer composites, see, e.g., Dev Nath and Nilufar (2020) and Jafferson et al. (2021) that may exhibit new types of defects or defect distributions differing from those from established methods. However, there are also "conventional" engineering materials, such as elastomers, for which no standard fracture test methods yet exist.

Nomenclature AM

Additive Manufacturing ASTM American Society for Testing and Materials, International BS British Standard CD Committee Draft ( standard) CEN Comité Européen de Normalisation C-ELS Calibrated End-Loaded Split ( test-rig ) DCB Double Cantilever Beam ( specimen ) G IC

Critical Mode I Energy Release Rate [ or ] Mode I Interlaminar Fracture Toughness

ECF EGF ENF ESIS ISO

European Conference on Fracture

European Group on Fracture ( renamed ESIS in 1990 ) End-Notched Flexure ( specimen ) European Structural Integrity Society International Organisation for Standardisation

J C Critical Fracture Energy [ or ] Critical Fracture Toughness ( from path-independent contour integral ) JSA Japanese Standards Association JIS Japanese Industrial Standard K IC Critical Mode I Stress Intensity LEFM Linear Elastic Fracture Mechanics NWI New Work Item ( for standardisation ) PVC Poly-Vinyl-Chloride RR Round Robin ( test involving several participating laboratories ) TC Technical Committee 3-ENF three-point-ENF ( test or specimen )

Standardization of fracture tests for polymers, polymer composites and polymer-based adhesives requires development of test procedures and their validation in RRs for assessing in-laboratory and inter-laboratory scatter and

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their respective accuracy and precision data. Such activities often take place in technical committees, e.g., ASTM D20 or D30, ESIS TC4, or JSA. Test procedures are then submitted to standards organizations, e.g., ISO, JSA, ASTM, or CEN. These documents are typically handled in a step-wise balloting process with the aim of them becoming accepted by majority votes both in respect of technical content in addition to formal requirements and language. 2. Overview of ESIS TC4 activities to date 2.1. Development of ISO standards within ESIS TC4 Since 1985, ESIS TC4 has developed six ISO standards (three on plastics, two on fiber-reinforced composites, and one on adhesives), another two standards are currently in the ISO balloting process (one is a peel test for flexible laminates, e.g., used in packaging, and the other a plane stress fracture resistance test for thin plastic films and sheets), see Table 1 for details. Several test procedures are in preparation (see Table 2), and proposals for new test developments are discussed or under consideration.

Table 1. ISO test standards developed by ESIS TC4 (active documents and in ballot). Standard title Designation Active since Plastics — Determination of fracture toughness (G IC and K IC ) — Linear elastic fracture mechanics (LEFM) approach ISO 13586 Issued 2000; revised 2018 Fibre-reinforced plastic composites - ISO 15024 Issued 2001

Determination of mode I interlaminar fracture toughness, G IC , for unidirectionally reinforced materials Plastics - Test method for Tension-Tension Fatigue Crack Propagation Plastics — Determination of fracture toughness (G IC and K IC ) at moderately high loading rates (1 m/s) Plastics — Determination of fracture toughness (G IC and K IC ) — Linear elastic fracture mechanics (LEFM) approach — Amendment 1: Guidelines for the testing of injection-moulded plastics containing discontinuous reinforcing fibres Adhesives — Determination of the mode 1 adhesive fracture energy of structural adhesive joints using double cantilever beam and tapered double cantilever beam specimens Fibre-reinforced plastic composites — Determination of the mode II fracture resistance for unidirectionally reinforced materials using the calibrated end-loaded split (C-ELS) test and an effective crack length approach Peel test for the determination of interlaminar fracture energy of flexible packaging laminates Plastics — Determination of fracture toughness of films and thin sheets: the essential work of fracture

ISO 15850

Issued 2002; revised 2016

ISO 17281

Issued 2002; revised 2018

ISO 13586/Amd 1

Amendment issued 2003, withdrawn 2018, replaced by “Testing of plastics containing short fibres” as Informative Annex B in ISO 13856:2018 and Informative Annex C ISO 17281:2018

ISO 25217

Issued 2009; based on BS 7991 (issued 2001)

ISO 15114

Issued 2014

ISO/CD 18485

Standardization presently on hold, pending RR for precision statement

ISO/CD 23524.2

In ballot (Spring 2021)

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Table 2. Fracture test procedures under development by ESIS TC4.

Document working title

Current status and remarks

Development started in

Plastics - High loading rates >1 m/s

1992

One RR completed, not submitted to ISO, data published by Leevers et al. (2014) Several RR on J testing multi-specimen procedure since 1988, data published by Hale and Ramsteiner (2001), three RR on J testing with load separation criterion since 2008, not submitted to ISO yet, RR data published by Agnelli et al. (2015) RR completed, summarized in TC4 Annex to J testing protocol, not submitted to ISO, pending completion of J testing protocol, data published by MacGillivray (2001) One RR completed, a second RR is in preparation, no RR data published yet Preliminary tests, no RR yet, data published by Blackman et al. (2016) Preliminary tests, no RR yet, test details are published by Williams & Patel (2016) and data by Patel et al. (2009) Two RR completed, third RR on alternative procedure and data analysis taking fiber bridging into account is in progress, see Yao et al. (2017) for details Preliminary tests performed at selected laboratories, RR in preparation One RR completed, ISO draft document in preparation, fixed mode ratio I:II is 4:3 One RR completed (C-ELS and 3-ENF), draft in preparation, not submitted to ISO yet, selected data published by Brunner (2015) Three RR completed, intended informative annex to ISO 13856 and ISO 17281 First RR with two materials in progress, test based on ISO/CD 18485 mandrel peel test for flexible laminates First RR with two materials in progress, test based on ISO/CD 18485 mandrel peel test for flexible laminates RR still in progress, the procedure is based on ISO 15114 for fiber reinforced composites, no RR data published yet

Plastics – Determination of fracture initiation by J-testing of ductile polymers at slow speeds using the load separation criterion

1992/2008

Plastics - Impact J testing

1993

Fibre-reinforced plastic composites – Determi nation of mode I interlaminar fracture toughness, GIC, for unidirectionally reinforced materials at moderately high loading rates (~1 m/s) Adhesives — Determination of the mode II adhesive fracture energy of structural adhesive joints using the calibrated end-loaded split (C ELS) test and an effective crack length approach

2004

2005

Polymer films and coatings - Scratching

2006

Polymers - Cutting

2006

2008

Fibre-reinforced plastic composites - Determination of mode I interlaminar fracture toughness, G IC , for unidirectionally reinforced materials under cyclic fatigue fracture loading

Environmental Stress Cracking

2008

2011

Fibre-reinforced plastic composites — Determination of fixed-ratio mixed mode I/II fracture resistance for unidirectionally reinforced materials using the inverted calibrated end loaded split (C-ELS) test and an effective crack length approach Fibre-reinforced plastic composites — Determination of mode II interlaminar fracture toughness, G IIC , for unidirectionally reinforced materials under cyclic fatigue fracture loading

2011

Plastics - Notching of polymers

2013

2015

Fibre-reinforced plastic composites - Laminate mandrel peel test for thin composites

2015

Fibre-reinforced plastic composites - Laminate mandrel peel test for thin composites

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Several procedures "under development" listed in Table 2 indicate rather long time-periods since the development started. There are various reasons for this, e.g., the limited availability of suitable materials to perform RR, the limited availability of participating laboratories with suitable test equipment and operators (for some activities involving novel apparatus, e.g., the laminate peel test, the limited availability of the apparatus has been overcome by creating a “travelling rig” whereby the test fixture is sent from one RR participant to the next). Frequently the data analysis for a test has posed problems, e.g., the need to obtain acceptable in-laboratory and inter-laboratory scatter has been challenging with sometimes for former achieved only. The standardization process, at least in ISO, now follows a rather strict timeline that allows for the development of a new standard in about three years from the time of acceptance as a NWI to an active standard. ASTM subcommittee D30.06 also organized a RR on mode I fatigue fracture of polymer composites with two carbon fiber epoxy composites (IM7/977-3, and G40-800/5267-1), and one glass fiber epoxy composite (S2/5216) in 2009-2010 with one laboratory from ESIS TC4 participating. Partial results were published by Stelzer et al. (2012) and by Murri (2014) and Brunner (2015). However, there is no active ASTM standard published and the topic is not included among the current ASTM work items. In April 2021, the ASTM D30.06 website lists two work items related to activities of ESIS TC4. The first is WK67477 "Standard Test Method for Determination of the Mode II Interlaminar Fatigue Crack Growth Rate and Onset of Unidirectional Fiber Reinforced Polymer Matrix Composites Using the End Notched Flexure (ENF) Test" and the second is WK74182 "Characterizing Mode-I Interfacial Fracture Toughness of Adhesives with Composite Adherends". ESIS TC4 had performed preliminary tests with a few selected laboratories on mode II interlaminar fatigue fracture comparing the C-ELS with a 3-ENF test-rig, as well as fatigue fracture under fixed ratio mixed mode I/II with the inverted C-ELS test-rig in one laboratory in 2012. These tests indicated the basic feasibility; selected data of both RR have been published by Brunner (2015). Development of standard test procedures will continue within ESIS TC4, and potential topics for new activities are discussed below. Table 3. Fracture Mechanics Test Methods for Polymers, Composites and Adhesives, ESIS Publication 28 (2001). Chapter title Pages Author(s) Introduction to linear elastic fracture mechanics 3-10 J.G. Williams K c and G c at slow speeds for polymers 11-26 J.G. Williams Determination of fracture toughness (G IC and K IC ) at moderately high loading rates 27-58 A. Pavan The measurement of Kc and Gc at slow speeds for discontinuous fibre composites 59-72 D.R. Moore

73-89

W. Böhme

Determination of the impact fracture toughness K Id of plastics at high rates of loading “>1m/s”

Fatigue crack growth of polymers Introduction to elastic-plastic fracture mechanics

91-116 119-122

L. Castellani, M. Rink

J.G. Williams

J-Fracture toughness of polymers at slow speed J-Fracture toughness of polymers at impact speed Introduction to adhesion and adhesives Peel testing of flexible laminates Fracture tests on structural adhesive joints Introduction to delamination fracture of continuous fibre composites Essential Work of Fracture

123-157 159-175

G.E. Hale, F. Ramsteiner

H. MacGillivray

177-195 199-202 203-223 225-267 271-275 277-305 307-333 335-359

E.Q. Clutton A. Kinloch

D.R. Moore, J.G. Williams B. Blackman, A. Kinloch

P. Davies

Mode I delamination Mode II delamination

A.J. Brunner, B.R.K. Blackman, P. Davies P. Davies, B.R.K. Blackman, A.J. Brunner B.R.K. Blackman, A.J. Brunner, P. Davies

Delamination fracture of continuous fibre composites: Mixed-mode fracture

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2.2. Dissemination of fracture test development and research Standards or guidelines have not been the only product or output of ESIS TC4. The committee has organized a series of eight conferences on the fracture of polymers, composites and adhesives to date (the first in 1994, then tri annual from 1999 on) and currently is preparing two more (a virtual conference replacing that planned for 2020 in September 2021 and a regular conference in September 2023). In addition to these, ESIS TC4 in 2016 also organized a symposium on "Advanced Fracture Mechanics Testing of Polymers, Adhesives and Composites" within ECF-21. Elsevier (2001) has published selected ESIS TC4 test procedures for polymers, polymer composites and adhesives in a book as ESIS Publication 28. Table 3 shows the titles of the papers and the authors. A second book, ESIS Publication 33 by Elsevier (2004), presented applications of fracture mechanics to polymers composites and adhesives. Selected contributions from the first three conferences have been published as books, namely ESIS Publication 19 by Mechanical Engineering Publications, Ltd. (1995) and as ESIS Publication 27 (2002) and ESIS Publication 32 (2003) by Elsevier. Since 2006, conference contributions are published in special issues of Engineering Fracture Mechanics. Selected papers from the TC4 symposium at ECF-21 in 2016 have been published in Volume 2 of Procedia Structural Integrity in 2016. Two ESIS TC4 status reviews for delamination resistance of fiber-reinforced polymer-matrix composites have been published by Davies et al. (1998) and Brunner et al. (2008). Recent meetings of ESIS TC4 since 2018, in addition to discussing the test procedures under development and the associated RR data review, have also included technology discussions on state-of-the-art issues in fracture and open questions. The topics covered so far have included "Composites fatigue fracture" (by Prof. René Alderliesten, TU Delft), the "Status of modelling and simulation of fracture mechanics" (by Prof. Jordi Renart, Universitat Girona), "Environmental effects on fracture" (by Prof. Jörg Fischer, Johannes Kepler Universität Linz) and on "Smarter testing" (by Prof. John-Alan Pascoe, TU Delft). It is likely that future fracture testing and test developments will benefit from suitable modeling or simulations. On one hand, this will assist the optimization of test set-ups and identify critical parts as well as support RR data analysis and interpretation, and on the other, it will advance the use of fracture mechanics based design of components or structures by implementing so-called "smarter" testing techniques. This term refers to a combination of composite material testing with extensive modelling and simulation, replacing some of the testing in the so-called building block approach for validating structural designs, see, e.g., Chisholm (2019) for details. 2.3. Fracture test data publications by ESIS TC4 Members of ESIS TC4 have also published several research papers reporting and discussing RR data and analysis, and selected examples are noted here. The precision statement in ISO 13586:2018 contains RR results on a polyamide material and that in ISO 17281:2018 results on a PVC material from the RR performed by ESIS TC4. The RR data for the fracture of polymers at loading rates around 1 m/s were published by Pavan and Williams (2000). ISO 25217 on quasi-static mode I fracture of structural adhesives does not provide a precision statement, but the RR data from the development of the test procedure were published by Blackman et al. (2003). ISO/CD 23524.2 on essential work of fracture of thin polymer films has a precision statement with RR data from five laboratories, and data have also been published by Williams and Rink (2007). For composites, selected RR data of quasi-static delamination resistance tests were published by Davies et al. (1990) for an unspecified unidirectional carbon fiber epoxy laminate, for a CF/epoxy and a CF/PEEK by Davies et al. (1992), for quasi-static mode II of a CF/epoxy (HTA-12000 carbon fibres in Toho 113 epoxy resin, produced from Toho Q-1113-1450 prepreg) again by Davies et al. (1999), for quasi-static mode I testing of unidirectional and cross ply laminates made of T300 fibers and 970 epoxy and IM7 fibers and 977-2 epoxy in a book chapter by Brunner (2008) and a research paper by de Morais et al. (2002). RR data from quasi-static and cyclic fatigue fracture RR under mode I and mode II loading with unidirectional CF/epoxy and a CF/PEEK (type IM7/977-1, IM7/977-2, IM7/977-3, G30-500 12k/Rigidite 5276 and AS4/PEEK, respectively) were published by Brunner et al. (2009), Brunner et al. (2013), Stelzer et al. (2014), and in a book chapter by Brunner (2015). Selected ESIS TC4 members also participated in the joint RR on mode I delamination resistance of polymer composites organized by ASTM, EGF and JIS in the early 1990ies. The data from this joint RR have been published by O'Brien and Martin (1993). JSA performed a RR on mode I delamination resistance without international participation, the results were published by Hojo et al. (1995).

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3. Discussion of open issues in fracture test standardization and approaches 3.1. Fiber bridging in polymer composites

The so-called delamination resistance curve from quasi-static mode I tests (G IC plotted versus delamination length) of polymer composites reflects fiber bridging between the two beams of the standard unidirectionally fiber-reinforced DCB specimens. The amount of large-scale fiber bridging and of delamination resistance typically increases with increasing delamination length after delamination initiation, until reaching saturation at which the delamination resistance remains roughly constant (with some variation about an average propagation value). The difference between initiation and average propagation values depends on the type of composite and may range from about 100 J/m 2 to several hundred J/m 2 or more, see, e.g., Sørensen and Jacobsen (2000) or Brunner (2015). For cyclic fatigue fracture, fiber bridging also affects the data, in that case, the curves describing average delamination rate per load cycle for a range of G IC values are shifted to higher values of G IC for higher amounts of fiber bridging. An important issue is the use of the materials' fracture test data in structural design with polymer composites. In composite parts or structures, the fiber orientation is often not unidirectional. Hence, delamination resistance in most composite structures does not show significant fiber bridging effects and is lower than the standard test values. One exception to that, however, are wind rotor blades were the fiber-bridging is considered in the design, see, e.g., Sørensen (2020). Of course, delamination propagation in components and structures is affected by more than just the fiber orientation and the resulting fiber bridging, Other factors include, e.g., shape of the part and ply drop-offs, residual stresses from manufacturing, or defects from processing, see, e.g., Sørensen (2020). Hojo and Aoki (2015) had proposed a procedure for the determination of Mode I fatigue fracture yielding data that allow extrapolation to a curve without fiber bridging. The procedure was a so-called "constant-G" test, but this was rarely used, due to the required machine control that was not available on all test machines at that time. Recently, Yao et al. (2017) developed a multi-step Mode I fatigue fracture test with alternating quasi-static and cyclic fatigue loads applied with increasing load and displacement levels, respectively. This yielded a set of curves that eventually produced a steady state for which subsequent curves overlapped. A back-extrapolation procedure then resulted in a curve without fiber-bridging effects that proved more conservative than the data from fatigue fracture cycles run at a given load or displacement level that were dominated by fiber-bridging. Using a modified Hartman-Schijve equation to plot the data as discussed by Jones et al. (2012, 2014) provides explicit values of delamination thresholds as well as quantitative scatter estimates for that, see, e.g., Mujtaba et al. (2017) or Jones and Kinloch (2020). Depending on the laminate lay-up and the fiber orientation in the different plies, a propagating delamination may branch into two or more delaminations, e.g., in multidirectional laminates as observed by Choi et al. (1999). This may be beneficial for increasing the delamination resistance of the material by the different plies or fiber bundles bridging the delaminations, but it is difficult to quantify the delamination resistance in terms of critical energy release rate G and R-curves (G versus delamination length a). Khudiakova et al. (2021a, 2021b) discuss approaches of how laminates with multiple delaminations might quantitatively be characterized for delamination resistance under quasi-static and cyclic mode I fatigue fracture loadings, respectively. Since only a limited amount of data has been analyzed this approach will require additional investigations for validation. 3.2. Environmental effects on fracture and fatigue fracture of polymers, polymer composites and adhesives Environmental exposure may come from many different sources and may induce a wide range of degradation mechanisms in polymers, including several that affect their fracture toughness as discussed by, e.g., Hinkley and Connell (2012). For assessing the long-term durability of a polymer or polymer composite part or structure, these effects require quantification. ESIS TC4 is preparing a RR on environmental stress cracking of polymers based on the preliminary investigations by, e.g., Kamaludin et al. (2017), Bredács et al. (2019) and Contino et al. (2021). For polymer composites, effects of environmental exposure have been discussed by, e.g., Broughton (2012) or Davies et al. (2012), the latter specifically focusing on marine environment. Environmental effects on adhesives joints are discussed by, e.g., Dillard (2010) or Costa et al. (2017). Of course, the variety of environmental conditions in the different service environments, typically comprising an ambient medium (e.g., air, humidity, service fluids) often combined with temperature variations requires an extensive experimental effort. Such effects will have to be