PSI - Issue 7

Volume 7 • 2017

ISSN 2452-3216

ELSEVIER

3rd International Symposium on Fatigue Design and Material Defects, FDMD 2017

Guest Editors: Stefano Beretta Stefano Foletti Gianni N icoletto R o b erto T ovo

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ScienceDirect Available online at www.sciencedirect.com Av ilable o line at ww.sciencedire t.com cienceDirect Structural Integrity Procedia 00 (2016) 000 – 000 Procedia S ructural Int grity 7 ( 7) 1–2 Available online at www.sciencedirect.com ScienceDirect Structural Integrity Procedia 00 (2017) 000–000 Structural Integrity Procedia 00 (2017) 000–000

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www.elsevier.com/locate/procedia 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. Copyright © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Sci ntific Committee of the 3rd I ternational Symposium on Fatigue Design nd Material Defects. 3rd International Symposi m on Fatigue Design and Material Defects, FDMD 2017, 19-22 September 2017, Lecco, Italy Editorial S. Beretta a *, S. Foletti a , G. Nicoletto b , R. Tovo c a Politecnico di Milano, Department Mechanical Engineering, Via la Masa 1, Milano 20156, Italy b Università di Parma, Dipartimento di Ingegneria e Architettura, Parco Area delle Scienze 181/A, 43125 Parma, Italy c Università di Ferrara, Dipartimento di Ingegneria, Via Saragat 1, 44122, Ferrara, Italy © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects. The actual fatigue strength and service life of structural components is typically controlled by the defect population due to the manufacturing process. These defects accelerate the initial phase of fatigue damage accumulation into a physical micro crack whose dimensions involve a few microstructural units. The concepts of defect-tolerant design, developed more than 20 years ago, aim to cover the gaps among stress based design approaches with generous safety factors, fracture-mechanics-based residual life assessments and NDE requirements. Metal Additive Manufacturing (AM) exemplifies an emerging field where the capability to predict fatigue properties and service life of components using the defect-tolerant design approach has great potential. After t previous successful events of Trondheim i 2011 and of Paris in 2014, the third editi n of the Fatigue D ign nd Material Defec s symposium held in Le co, Italy, in September 2017 attracted searchers and ex rts from all over the world (European Union, Japan, China, India, Russia, Algeria, Canada, United States, Argentina). During the symposium, the more than 100 presentations provided an update of the ongoing research on the key connections between manufacturing processes, fatigue properties and component design in industry. The complete program of the FDMD3 Symposium and the Book of Abstracts can be found at: http://www.fdmd3.polimi.it/programme/. Thematic sessions on specific topics (i.e. experimental techniques, fatigue thresholds, VHCF, additive manufacturing, multiaxial fatigue, advanced materials) and on industrial applications (defect assessment methods, 3rd International Symposium on Fatigue Design and Material Defects, FDMD 2017, 19-22 September 2017, Lecco, Italy Editorial S. Beretta a *, S. Foletti a , G. Nicoletto b , R. Tovo c a Politecnico di Milano, Department Mechanical Engineering, Via la Masa 1, Milano 20156, Italy b Università di Parma, Dipartimento di Ingegneria e Architettura, Parco Area delle Scienze 181/A, 43125 Parma, Italy c Università di Ferrara, Dipartimento di Ingegneria, Via Saragat 1, 44122, Ferrara, Italy © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects. The actual fatigue strength and service life of structural components is typically controlled by the defect population due to the manufacturing process. These defects accelerate the initial phase of fatigue damage accumulation into a physical micro crack whose dimensions involve a few microstructural units. The concepts of defect-tolerant design, developed more than 20 years ago, aim to cover the gaps among stress based design approaches with generous safety factors, fracture-mechanics-based residual life assessments and NDE requirements. Metal Additive Manufacturing (AM) exemplifies an emerging field where the capability to predict fatigue properties and service life of components using the defect-tolerant design approach has great potential. After the previous successful events of Trondheim in 2011 and of Paris in 2014, the third edition of the Fatigue Design and Material Defects symposium held in Lecco, Italy, i September 2017 attracted researchers and experts from all over the world (European Union, Japan, China, India, Russia, Alger a, Canada, United Sta es, Argentina). During the symposium, the more than 100 presentations provided an update of the ongoing research on the key connections between manufacturing processes, fatigue properties and component design in industry. The complete program of the FDMD3 Symposium and the Book of Abstracts can be found at: http://www.fdmd3.polimi.it/programme/. Thematic sessions on specific topics (i.e. experimental techniques, fatigue thresholds, VHCF, additive manufacturing, multiaxial fatigue, advanced materials) and on industrial applications (defect assessment methods, © 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.: +39-0223998246; fax: +39-02-23998202. E-mail address: stefano.beretta@polimi.it 2452-3216 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects. 2452-3216 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects. * Correspondi g author. Tel.: +39-0223998246; fax: +39-02-23998202. E-mail address: stefano.beretta@polimi.it

* 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 Copyright  2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects. 10.1016/j.prostr.2017.11.052

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welds, power generation) promoted lively discussions among participants. This special issue of the Procedia-Structural Integrity includes all the contributions submitted in final form after the Symposium for review to the Scientific Committee. The effort of the authors and of the colleagues that volunteered for the revision process is therefore acknowledged. The organization of such an important event was made possible by the generous support of invited lectures and dedicated events by important companies and technical partners and by the tireless assistance to organizers and participants provided by the FDMD3 Team and staff of Polo di Lecco (Politecnico di Milano). S. Beretta, S. Foletti, G. Nicoletto and R. Tovo

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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. Copyright © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects. 3rd International Symposium on Fatigue Design and Material Defects, FDMD 2017, 19-22 September 2017, Lecco, Italy Airbus approach for F&DT stress justification of Additive Manufacturing parts Jon Mardaras * , Philippe Emile, Alain Santgerma Airbus Operations SAS, 316 Route de Bayonne, 31060 Toulouse Cedex 09, France Abstract Additive Manufacturing (AM) is rapidly expanding in aviation due to the advantages it offers compared to conventional manufacturing routes. It allows the production of geometry optimized complex parts with an efficient use of material. However, in order to design eliable parts using this novel manufacturing oute, the changes it involves in mater al properties, defects and shape of parts need to be understood. This paper presents an indus rial structural analysis approach applied by AIRBUS to justify newly introduced AM parts on aircraft. Some areas for future development and improvement are also presented. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects. Keywords: Additive Manufacturing; Powder Bed; Fatigue; Defects; 1. Introduction Additive Manufacturing (AM) is ra idly xpanding in aviation due to the advantages it offers compa d to conventional manufacturing routes. It allows the production of geometry optimized complex parts with an efficient use of material and in a relatively simple and quick manner. However, in order to design reliable parts using this 3rd International Symposium on Fatigue Design and Material Defects, FDMD 2017, 19-22 September 2017, Lecco, Italy Airbus approach for F&DT st ess justification of Ad itive Manufacturing parts Jon Mardaras * , Philippe Emile, Alain Santgerma Airbus Operations SAS, 316 Route de Bayonne, 31060 Toulouse Cedex 09, France bstract Additive Manufacturing (AM) is rapidly expanding in aviation due to the advantages it offers com ared to conventional manufacturing routes. It allows the production of geometry optimized complex parts with an efficient use of material. However, in order to esign reliable parts using this novel manufacturing route, the changes it involves in material properties, defects and shape of parts need to be understood. This paper presents an industrial structural analysis approach applied by AIRBUS to justify newly introduced AM parts on aircraft. Some areas for future development and improvement are also presented. © 2017 The Authors. Publishe by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material D fects. Keywords: Additive Manufacturing; Powder Bed; Fatigue; Defects; 1. Introduction Additive Manufacturing (AM) is rapidly expanding in aviation due to the advantages it offers compared to conventional manufact ring routes. It allows the production of geometry optimized complex parts with an efficient use of material and in a relatively simple and quick manner. However, in order to design reliable parts using this © 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. E-mail address: jon.mardaras@airbus.com * Corresponding author. E-mail address: jon.mardaras@airbus.com

2452-3216 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects. 2452-3216 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects.

* 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 Copyright  2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects. 10.1016/j.prostr.2017.11.067

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novel manufacturing route, the changes it involves in material properties, defects and shape of parts need to be understood. This paper presents an industrial structural analysis approach applied by AIRBUS to justify newly introduced AM parts on aircraft. It shows that the Fatigue & Damage Tolerance (F&DT) stress justification approach for answering the certification requirements is consistent with the conventional process already applied on plates and forgings. The paper initially discusses the certification requirements to be met by aircraft parts. The types of defects generated by AM, and the associated inspection techniques are presented. Stress methods and design values are explained after. Finally, areas for future development and improvement are highlighted before reaching the conclusions. This document focuses on Titanium 6Al-4V material with Powder Bed technology. 2. Certification requirements AIRBUS determined that the current certification standard is fully applicable for AM parts. The AIRBUS processes for material qualification, design value definition and stress analysis approach for AM have been agreed with the European Aviation Safety Agency (EASA). Although current regulation is found to be appropriate, Airworthiness Authorities and OEMs are working to set an industrial standard specific to AM technology. The main applicable certification standards chapters are discussed below in the context of AM application: • Materials - 25.603 (a)(b)(c) : Approved technical and material specifications are in place specific to the AM technology. These specifications have been established on the basis of experience and tests. Suitability and durability of materials are accounting for the environmental conditions expected in service. • Fabrication methods - 25.605 (a)(b) : The manufacturing processes used are qualified according to approved process specifications. Close control of all the produced parts and a consistent sound structure is ensured through the qualification test programme and the inspection procedures defined in these specifications. • Material design values - 25.613 (a)(b)(c) : The material strength properties are defined from tests with material produced following the approved specifications for AM. Results obtained from different machines, powders and material directions are considered and design values derived following statistical treatments. Approved material design values are available for static, fatigue and damage tolerance evaluations. Comparisons with data from same material but conventional technologies (plates, forgings, castings) are also performed. • Damage tolerance and fatigue evaluation of structure - 25.571 : Fatigue & Damage Tolerance approaches applied on conventional technologies remain applicable. The only differences being the inclusion of fatigue knock-down factors related to the surface finish process of the AM part. For damage tolerance based inspection thresholds, the initial flaw size currently used on conventional technologies is found to remain valid for AM Powder Bed technology. 3. Type of defects Understanding the type of defects created by AM technology is a key point in order to properly address the certification requirements and implementing appropriate F&DT justification approaches. In the frame of effect of defects, some work was conducted to characterize the influence of various parameters. As part of the research phase, a catalogue of typical defects was built so that the right technique to detect them could be down-selected. Defects can be classified in two categories: internal defects and surface defects. 3.1. External: possible defects and inspection means External defects can be classified in two main categories: the ones inducing cracks or voids and the ones inducing roughness or printing artefacts. In the defect category inducing cracks or voids, the defects described in Table 1 may exist (non-exhaustive). This first category of defects can be detected quite easily by classical liquid penetrant testing inspection. The challenge is to have an appropriate surface preparation allowing good interpretation of the results.

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In the defect category inducing roughness/printing artefacts, the defects described in Table 2 may exist (non exhaustive). This second category of defects is quite easy to detect by a detailed visual inspection. These defects are independent from the roughness level inherent to the AM process. The amount of roughness can be measured in order to quantify it.

Table 1. External defects inducing cracks or voids. Recoating/Delamination (bad recoating of the powder due to damage on the recoater)

Part lifting (from supporting structures or built plate) creating local delamination due to high thermal stresses

Table 2. External defects inducing roughness/printing artefacts.

Swelling (surface distortion due to poor thermal conductivity)

Bobbling (excessive downfacing roughness)

Quilting (raised ridges on supported down surfaces)

Slippage defects (stepwise positioning error between layers)

In summary, most external defects other than the intrinsic surface roughness can be detected easily by means of quality control procedures. Additionally, these defects can be minimized through selection and understanding of correct process parameters. For this reason, only the influence of the intrinsic roughness and potential increased local roughness are considered in the calculation method. These are the only external defects that will be present on final production parts.

3.2. Internal: possible defects and inspection means Internal defects can be classified in two main categories: cracks or voids and inclusions. Regarding cracks or voids, the following may exist (non-exhaustive): • Gas porosity (entrapped gas in the material) • Lack of fusion (material not fused during the process)

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Internal cracking (crack from residual stress)

•

• Undercuts (voids created between contour melt and core melt)

Figure 1. Internal defects.

This first category of defects can be detected by X-ray or Computed Tomography (CT) means. Both techniques have limitations. CT is able to detect defects of a very small size but it is very time consuming and expensive. The X-Ray technique is cheaper but has a limited resolution versus CT. It also implies to know the printing direction when performing the inspection particularly to detect linear defects. The effect of these defects can be reduced significantly by applying a high pressure at an elevated temperature (Hot Isostatic Pressing or HIP). Studies have demonstrated the efficiency of such technique to eliminate a majority of the defects and improve material isotropy. Regarding inclusions, these defects are linked to the inclusion of foreign material in the base material coming from various source of contamination during the whole process. These inclusion defects can be very easily detected by X-ray or CT means. In terms of calculation, the small pores or inclusions still present after HIP need to be accounted for. In any case, this kind of defect is quite similar to the ones that can be found on traditional technologies. 4. Stress calculation Fatigue design values are obtained from testing fatigue coupons of various stress concentration levels. The reference material properties are defined based on coupons produced under qualified procedure with HIP treatment and machined surfaces. This is taken as reference because it is deemed to be defect free. In these conditions, AM Powder Bed Ti64 material can be considered almost equivalent to conventional plate material in terms of fatigue performance. Figure 2 shows fatigue tests on AM reference condition in comparison with plate material data on same fatigue coupon. Crack propagation tests are also done at various R-ratios and including overload effects. Results are found to be in line with those from conventionally produced material as reported by other authors such as Leuders et al (2013). Fracture toughness results exceed those obtained on plates. Damage tolerance threshold inspection justification of parts can still be done using a conventional approach, i.e. assuming the presence of a manufacturing Rogue Flaw of 1.27mm at the most loaded area. This approach is proven to be conservative by Equivalent Initial Flaw Size (EIFS) studies conducted to characterize the surface defects and by internal defect pore size measurements. Internal pore type defects are kept below 0.5mm through process parameter control and quality control. EIFS characterization of surface defects rarely exceeds 0.5mm for as-built surfaces on Powder Bed technology.

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Figure 2. Fatigue stress allowable for various configurations.

The following sub-chapters provide further details on the consideration of the effect of defects on the stress method and design values. 4.1. External External defects, namely surface roughness, cause the fatigue initiation to happen much earlier from near surface. This is evident in Figure 2, which shows that the fatigue stress allowable for an as-built surface is approximately a third of the machined coupon one. It also shows that surface defects take control over internal defects. This is evidenced by the impact of the as-built surface on the fatigue stress allowable being greater than the absence of HIP on machined coupons, and also by the as-built fatigue allowable being independent of performing or not the HIP. This is in line with findings by Greitemeier et al (2017) and similar trends are reported by Li et al (2016) and Lewandowski et al (2016). Surface machining remains the most viable solution for fatigue critical applications but at the cost of limiting the geometrical complexity of the part in order to make the machining feasible. The availability of efficient surface finishing techniques is of prime interest to restore an acceptable fatigue performance associated to the external surfaces. In order to account for the surface roughness, the surface states obtained through various finishing techniques are characterized and correlations with surface parameters such as Ra are pursued. However, such correlations are not straightforward. Although improvements of Ra generally lead to a fatigue improvement, discrepancies are found in fatigue life for processes resulting on the same Ra. Currently, the influence of non-machined surface condition on fatigue life is quantified by coupon fatigue tests representative of surface finishing processes used on the AM parts. Based on these coupon test results, a data bank of fatigue Knock Down Factor (KDF) versus surface improvement process and roughness is built. The KDF obtained are applied to the fatigue design values determined for the reference material (machined surface conditions + HIP). Figure 2 provides a few examples on how the fatigue stress allowable can vary depending on the surface finishing technique used. It can be seen that a sand blasting and chemical milling process can improve significantly the fatigue stress allowable compared to an as-built surface, although not enough to bring it to the level of machined coupons. Figure 3 provides details on surface finish and fracture surface differences between the above mentioned coupon conditions. As-built coupons generally show a high Ra around 15-25µm and multiple crack initiation sites,

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whereas the coupon with the sand blasting and chemical milling shows an improved surface with Ra below 6.3µm and fewer initiation sites.

As-built coupon

Coupon with surface improvement applied

Figure 3. Surface roughness and fracture surface differences between as-built and surface improvement coupon.

4.2. Internal Internal defects on final parts are currently limited to small pores or inclusions thanks to the HIP and the quality control. These remaining small pores and inclusions are considered to have a minor effect on the general fatigue and it is considered inherently through the fatigue test data obtained from coupons produced with machined surfaces. The use of HIP to minimise internal defects plays a key role in having a good fatigue performance as shown in Figure 2, which shows a significant reduction on fatigue stress allowable on machined coupons if HIP is not performed. 5. Future developments The current knowledge of AM technology can already allow the replacement of parts currently produced following conventional production routes where a favorable business case in terms of production cost can be justified. This means keeping standard designs which can be machined and hence having a similar performance. However, the main benefit of additive manufacturing for the future is the wide range of opportunities it offers in designing topology optimized complex geometry parts as the example shown in Figure 4.

Figure 4. Example of topology optimised lug.

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In order to take full credit of this technology for use on fatigue loaded structure, developments in the following areas are needed: • Surface improvement techniques: as mentioned in the surface defects chapter, surface roughness characterization versus fatigue and associated surface finishing processes is a key topic to fully enable topology optimized parts. On topology optimized parts, the machining of the surface will not be feasible and surface finishing techniques which restore good fatigue performance are needed to fully exploit the AM potential. • Refined damage tolerance approach: the application of the deterministic approach currently used becomes increasingly difficult and conservative as geometrical complexity of parts produced by AM increases. More refined probabilistic methods and tools considering defect distribution and size are investigated and followed with interest. This could also be an enabler for the removal of HIP requirement in a future by considering the influence of the increased pore size population. 6. Conclusion In conclusion, justification means are in place for AM parts by following conventional approaches. Improvement areas are identified to efficiently develop the full potential of AM technology, especially moving towards complex geometry and fatigue sized parts. Improvements are expected on the short to mid-term for surface defect characterization, surface improvement techniques, and refined damage tolerance methodologies. References Leuders, S., Thone, M., Riemer, A., Niendorf, T., Troster, T., Richard, H.A., Maier, H.J., 2013. On the mechanical behaviour of titanium alloy Ti6Al4V manufactured by selective laser melting: Fatigue resistance and crack growth performance, International Journal of Fatigue, Volume 48, Pages 300-307 Li, P., Warner, D.H., Fatemi, A., Phan, N., 2016. On the fatigue performance of additively manufactured Ti-6Al-4V to enable rapid qualification for aerospace applications, 57th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference. Lewandowski, J., Seifi, M., 2016. Metal additive manufacturing: a review of mechanical properties, Annual Review of Materials Research, Volume 46, Pages 151-186 Greitemeier, D., Palm, F., Syassen, F., Melz, T., 2017. Fatigue performance of additive manufactured Ti6Al4V using electron and laser beam melting, International Journal of Fatigue, Volume 94, Part 2, Pages 211-217.

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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. Copyright © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects. 3rd International Symposium on Fatigue Design and Material Defects, FDMD 2017, 19-22 September 2017, Lecco, Italy Analysis of Crack Extension Mechanism in the Near-Threshold Regime in an Aluminum Alloy M.Wicke a *, A. Brueckner-Foit a , T. Kirsten b , M. Zimmermann b , F. BuelBuel c , H.-J. Christ c a Institue for Materials Engineering, University of Kassel, D-34125 Kassel, Germany b Institue for Materials Engineering, Technical University of Dresden, D-01069 Dresden, Germany c Institue for Materials Engineering, University of Siegen, D-57068 Siegen, Germany Abstract Short cracks initiated from pre-existing flaws are known to propagate in an intermittent way below threshold as the crack tip field interacts with the microstructure. If the stress amplitude is very low, similar effects as observed for short cracks may happen to long cracks leading to unexpected crack extension. This phenomenon is studied in this paper using flat dogbone specimens of a commercial aluminum alloy in two heat treatment states. Compression pre-cracked specimens were used to determine the threshold by continuous load increase according to a procedure proposed by Pippan et al. [Pippan et al. (1994)]. On this basis, crack growth experiments with an approximately constant ΔK -value propagation rate were performed at a stress ratio of R = 0.1. Results indicate that primary precipitates act as microstructural barriers causing crack deflection and crack branching. If the stress amplitude is low enough, shear-dominated crack extension has been found to be possible even for long cracks in the near-threshold regime. Both mechanisms keep the crack from extending continuously. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd I ternational Symposium on Fatigue Design and Material Defects. 3rd International Symposium on Fatigue Design and Material Defects, FDMD 2017, 19-22 September 2017, Lecco, Italy Analysis of Crack Extension Mechanis in the Near-Threshold Regime in an Aluminum Alloy M.Wicke a *, A. Brueckner-Foit a , T. Kirsten b , M. Zimmermann b , F. BuelBuel c , H.-J. Christ c a s Kassel 34125 Kassel b Institue for Materials Engineering, Technical University of Dresden, D-01069 Dresden, Germany c Institue for Materials Engineering, University of Siegen, D-57068 Siegen, Germany Abstract Short cracks initiated from pr -existing flaws are known to propagate in an int rmittent w y below threshold as the crack tip field interacts with the microstructure. If the stress amplitude is very low, similar effects as observ d for short cracks may happen to long cracks leading to unexpected crack extension. This henomenon is studied in this paper using flat dogbone pecimens of a commercial aluminum all y in two heat treatment states. Compression pre-cracked specimens were used to determine the threshold by continuous load increase ccording to a procedure proposed by Pippan et al. [Pippan et al. (1994)]. On this basis, crack growth xperiments with an approximat ly constant ΔK -value propagation rate were performed at stress ratio of R = 0.1. Results indicate that primary precipitates act as icrostructural barriers causing crack deflection and crack branching. If the stress amplitude is low enough, shear-dominated crack extension has been found to be possible even for long cracks in the near-threshold regime. Both mechanisms keep the crack from extending continuously. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material D fects. © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of PCF 2016. Keywords: Aluminum alloys; compression pre-cracking; threshold; fatigue crack growth Keywords: Aluminum alloys; compression pre-cracking; threshold; fatigue crack growth

Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation.

* Corresponding author. Tel.: +49 561-804-3656 E-mail address: marcel.wicke@uni-kassel.de * Corresponding author. Tel.: +49 561-804-3656 E-mail address: marcel.wicke@uni-kassel.de

2452-3216 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects. 2452-3216 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects.

* 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 Copyright  2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects. 10.1016/j.prostr.2017.11.083

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1. Introduction Very low stress amplitudes occur under very high cycle fatigue (VHCF) loading or if the crack extension of long cracks in the near-threshold regime is studied. Both scenarios come together, if interest is focused on the components containing pre-existing flaws which are subjected to vibrations of very small amplitude. In that case, similar effects as observed for short cracks may occur with long cracks resulting in unexpected crack extension mechanisms. Thus, it may be worthwhile to study the effect of very small cyclic loads on the crack-extension behavior of long cracks. The determination of the long crack threshold has been standardized by ASTM E-647 and relies on the so-called load-shedding procedure, in which the range of the stress intensity factor is decreased in steps until the crack propagation rate falls below a value of typically 10 -10 m/cycle. However, crack closure effects have been reported to shift the threshold to higher values leading to numerous discussions concerning the meaning of the threshold value determined by this procedure in the past years (e.g. [Forth et al. (2003), Newman et al. (2005), Newman and Yamada (2010)]). Non-conservative estimates of the threshold value can be circumvented by an alternative approach developed by, among others, Pippan et al. [Pippan (1987), Pippan et al. (1994)]. After compression pre-cracking of specimens containing a sharp notch, the load amplitude is increased in steps until the crack propagates in a stable manner. The threshold is thus approached from below solving the problems encountered with the load-shedding procedure mentioned above. In a previous study [Stein et al. (2017)] analyzing the near-threshold behavior of long cracks in a commercial aluminum alloy we have shown that the combination of crack generation by compression pre-cracking with a subsequent crack propagation at a constant amplitude is an ideal procedure for investigating the crack extension at very low stress amplitudes. Results pointed out that there are two major mechanisms keeping the crack from continuous extension at a stress ratio of R = -1. First the crack front was pinned by primary precipitates with the amount of pinning found to be dependent on the spatial distribution of the primary precipitates. It was shown that local pinning of the crack front led to significant kinking of the fracture surface with white lines, representing ridges of ductile fracture which bridged the crack faces, extending into the crack surface. The second mechanism was shear controlled crack extension of very long cracks with plastic zones ahead of the crack tip very similar to stage-I small cracks. Misorientation fields derived from EBSD data revealed that such cracks extending in a shear-dominated mode can be deterred by primary precipitates, but remain in the shear-dominated mode afterwards. Shear-dominated crack extension as observed in [Stein et al. (2017)] can occur at a stress ratio of R = -1. This effect is well-known but rarely reported for higher R-ratios. Consequently, it may be worthwhile to study the near-threshold crack extension behavior of long cracks at R ≠ -1. To this end, fatigue crack growth tests at a stress ratio of R = 0.1 were performed in this study using the aluminum alloy investigated in [Stein et al. (2017)] in two heat treatment states with the aim of analyzing the influence of the microstructure and precipitates on crack growth. The paper is organized as follows: In section 2 the material and specimens are described, whereas the experimental methods are presented in section 3. An overview of the fracture mechanic tests is given in section 4. Crack growth of long cracks in the near-threshold regime and crack advance of small cracks are related in section 5, which is followed

by some concluding remarks. 2. Material and Specimens

The material analyzed in this study is the aluminum alloy EN-AW 6082 in peak-aged (6PA) and overaged conditions (6OA). R p0.2 and R m for the material 6PA were determined in a tensile test as 336 MPa and 344 MPa, and 262 MPa and 294 MPa for material 6OA, respectively. A rolled sheet with a thickness of 20 mm and the microstructure shown in Fig. 1a taken in rolling direction (RD) served as base material. The grains are elongated with a maximum length of up to 2000 – 3000 µm in rolling direction and 100 – 200 µm in transverse direction (TD). Two types of primary precipitates can be detected in the microstructure: Mg-based ones (dark spots in Fig. 1a) and Fe based ones (bright spots in Fig. 1a), which are arranged in a line-like fashion in rolling direction.

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Fig. 1 a) EN-AW 6082 SEM-micrograph of grain structure taken in rolling direction; b) Specimen geometry and c) Part-through notch

Flat dog-bone specimens with a total length of 40 mm and the geometry depicted in Fig. 1b were machined out of the sheet material both in rolling (-LS) and transverse direction (-TS). After mechanical and electrolytic polishing, a part-through notch with a depth of 50 - 200 µm depending on the experiment performed and a notch radius smaller than 20 µm was cut in the specimen radius using a razor blade polishing technique similar to that originally proposed by Nishida et al. [Nishida et al. (1996)]. Although this technique is common practice in ceramics, it can also be used for introducing sharp notches exhibiting only little plastic deformation in metals if the load applied to the razor blade is small enough. 3. Experimental Methods Fatigue crack growth tests were performed at a stress ratio of R = 0.1 on a Rumul Mikrotron resonance machine equipped with a 20 kN load cell and a sinusoidal force with a frequency of ca. 150 Hz in laboratory atmosphere. A long-distance microscope was used to monitor the crack growth on the specimen surface, enabling the documentation of the crack propagation when the cyclic load is interrupted by stopping the testing machine. The crack growth rate was calculated using the measured crack length and the number of test cycles between the scans. The specimens were pre-fatigued in compression to introduce a pre-crack, which is open when unloaded. After applying up to 500.000 cycles at a stress ratio of R = 20 and a stress level of σ min = -290 MPa for material 6PA and σ min = -208 MPa for material 6OA, respectively, the experiment was started. The threshold of the stress intensity range ΔK was determined using the stepwise increasing load amplitude crack growth test described in [Pippan et al. (1994)]. After pre-cracking in compression, testing was changed to a pull-pull load (R = 0.1) at a low stress intensity range of about Δ K = 0.6 MPa√m in order to let the crack propagate. If no crack growth was detected after 500.000 cycles, the stress amplitude was increased by 5 - 10 %. This procedure was repeated until the crack propagated in a stable manner. The threshold was then defined using the stress amplitude which led to continuous crack growth. Once the threshold was known, specific values of the stress amplitude were selected corresponding to ranges to the stress intensity factor close to the threshold value for the starting crack length of 250 µm. The crack was then propagated at a constant stress amplitude according to the procedure proposed in [Stein et al. (2017)] until the crack advance, visible on the surface, amounted to 50 µm. The stress amplitude was then decreased such that the initial value of the range of the SIF was recovered. This procedure, allowing a crack propagation at a nearly constant stress intensity range with a maximum variability of ΔK of 10 %, was repeated until the crack growth rate fell down to 10 −11 m/cycle, which was defined as the criterion for a crack stop, or no further decrease of the stress amplitude was possible due to limitations of the testing machine.

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4. Experimental Results 4.1. Long Crack Threshold

In order to measure the long-crack threshold, three experiments on specimens of both materials in LS- and TS orientation were performed according to the method described previously. The resulting da/dN vs. Δ K-curves of two experiments is shown in Fig. 2. Left to the dashed lines, representing the threshold values of Δ K th = 1.34 MPa√m for material 6PA-LS (Fig. 2a) and of Δ K th = 1.30 MPa√m for material 6PA-TS (Fig. 2b), respectively, the cracks stopped repeatedly as schematically illustrated by the circles on the x-axis, so that the stress amplitude had to be increased by 5 - 10 %. After reaching the threshold value, the cracks grew stably at a constant stress amplitude and showed long crack behavior.

Fig. 2. da/dN vs. Δ K-curve with threshold Δ K th determination:a) 6PA-LS; b) 6PA-TS.

Reaching the threshold further leads to a change of the crack growth rate of at least one order of magnitude. While the crack growth rate is in a range between 10 -11 and 10 -10 m/cycle below the threshold, it increases to values of 10 -10 to 10 -9 m/cycle after passing the threshold. This notable transition of the crack growth rate, which already indicates that there is a certain minimum crack growth rate required for continuous crack extension as discussed in [Stein et al. (2017)] in detail, was also observed in the additional experiments. An overview of the determined threshold values is given in table 1.

Table 1. Threshold data for material 6PA and 6OA. 6PA-LS 6PA-TS

Δ K Δ K Δ K Δ K ����

6OA-LS

6OA-TS

th , 1 th , 2 th , 3 th

1.34 1.13 1.38 1.28

1.30 1.51 1.32 1.38

1.10 1.17 1.20 1.16

1.13 1.33 1.14 1.20

The variance in the threshold values already points out the dependency of this material parameter on both, the measurement method and microstructure. Results indicate that the peak-aged material has superior near-threshold fatigue crack propagation resistance compared to the overaged material, which is seen in terms of higher values of ΔK th . The shift of ΔK th towards lower values with increasing aging is in agreement with the results presented in the literature for various aluminum alloys (e.g. [Vasudevan and Suresh (1982), Lafarie and Gasc (1983), Suresh et al.