Scholarly article on topic 'Design for Additive Manufacturing – Supporting the Substitution of Components in Series Products'

Design for Additive Manufacturing – Supporting the Substitution of Components in Series Products Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Christoph Klahn, Bastian Leutenecker, Mirko Meboldt

Abstract The Additive Manufacturing technologies Selective Laser Sintering (SLS) and Selective Laser Melting (SLM) are capable to produce thermoplastic or metal parts, which are fit for end-user products. Both technologies create three-dimensional objects directly from a 3D CAD model with little restrictions regarding the shape of the object. This geometrical freedom in design can be utilized to largely improve the functionality of series products by substituting conventional parts with additive manufactured ones. Four criteria are presented here to identify components of a product for a re-design. A successful re-design has to meet the needs of the producer and his customers. The selection criteria and success factors for a re-design are demonstrated in four cases.

Academic research paper on topic "Design for Additive Manufacturing – Supporting the Substitution of Components in Series Products"

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Procedía CIRP 21 (2014) 138 - 143

24th CIRP Design Conference

Design for Additive Manufacturing - Supporting the Substitution of

Components in Series Products

Christoph Klahn*, Bastian Leutenecker, Mirko Meboldt

ETH Zurich, Tannenstrasse 3, 8092 Zurich, Switzerland

Corresponding author. Tel.: +41-44-632 74 87. E-mail address: Klahn@inspire.ethz.ch

Abstract

The Additive Manufacturing technologies Selective Laser Sintering (SLS) and Selective Laser Melting (SLM) are capable to produce thermoplastic or metal parts, which are fit for end-user products. Both technologies create three-dimensional objects directly from a 3D CAD model with little restrictions regarding the shape of the object. This geometrical freedom in design can be utilized to largely improve the functionality of series products by substituting conventional parts with additive manufactured ones. Four criteria are presented here to identify components of a product for a re-design. A successful re-design has to meet the needs of the producer and his customers. The selection criteria and success factors for a re-design are demonstrated in four cases. © 2014 ElsevierB.V. Thisisan open accessarticle under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Selectionandpeer-reviewunderresponsibilityofthe International Scientific Committee of "24th CIRP Design Conference" in the person of the Conference Chairs Giovanni Moroni and Tullio Tolio

Keywords: Additive Manufacturing; Rapid Manufacturing; Design for X;

1. Additive Manufacturing

Additive Manufacturing (AM) refers to a variety of different manufacturing processes to produce three-dimensional objects based on a 3D CAD model by adding layers of material [1]. No individual tooling is required. This makes Additive Manufacturing an ideal technology for rapid prototyping and rapid manufacturing [2]. Rapid prototyping has been used in development since the nineteeneighties to generate visualizations of products early in development. In this application, fast production is more important than the durability of fabricated objects. Nowadays, Fused Deposition Modeling and Stereolithography are well known Rapid Prototyping technologies.

For end-user products, individual parts as well as series products, higher durability and long term stability are required. Additive Manufacturing of individual parts or small lot sizes for end-users and industrial applications is commonly referred to as rapid manufacturing. Two major technologies have emerged in this field during the last decade. Selective

Laser Sintering (SLS) for processing thermoplastics and Selective Laser Melting (SLM) for metals.

Both technologies are based on powder bed fusion and use a laser to selectively solidify the powder. Fig. 1 shows the manufacturing process. The 3D CAD model is sliced into layers and transferred to the AM machine. The parts are produced in a cyclic process. In the first step a layer of powder is applied. Next, a laser beam is directed on the powder bed and solidifies the powder based on the shape of the current layer. In the last step of the cycle the building platform is lowered by the layer thickness. In this cycle of powder layer application, exposure and lowering of the platform the part is build up layer by layer. The part is surrounded by the powder bed until the last layer is finished. [3]

Despite the similar process cycles of Selective Laser Sintering and Selective Laser Melting, there are differences that justify the different process names.

In Selective Laser Sintering the building chamber and the thermoplastic powder bed are heated to a temperature just below the glass transition temperature and only little laser

2212-8271 © 2014 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of the International Scientific Committee of "24th CIRP Design Conference" in the person of the

Conference Chairs Giovanni Moroni and Tullio Tolio

doi:10.1016/j.procir.2014.03.145

Fig. 1: Process of Additive Manufacturing by powder bed fusion [3]

energy is required to melt the powder. This leads to low thermal stresses, but also ages the unsolidified powder [2,4].

Selective Laser Melting processes conventional engineering metal alloys with high melting temperatures. The energy to melt the powder is delivered by the laser beam. The focused energy input causes thermal stresses. Supports connect the parts to the building platform to prevent deformations. They are removed after a heat treatment eliminated thermal stresses [3]. Little damage is done to the powder surrounding the part and thus, it can be reused in many building jobs [5]. The mechanical properties of metals processed by SLM exceed those of investment casting and almost reach those of wrought materials [2,6].

SLS and SLM are both industrially mature manufacturing processes. They provide the opportunity to manufacture individual parts with a very high degree of design freedom. It is no longer necessary for the designer to think about Design for Manufacturing, since almost any shape can be manufactured. The design process can focus on a more important issue: to improve the function of the system.

This is a challenge for designers, who are not familiar with these new manufacturing technologies. Current research addresses two issues: the development of design rules by identifying and quantifying remaining restrictions in manufacturability [7,8], and assessing the impact on the design process [9,10,11]. Both research questions are important and require significant efforts. Equally important is the question of which parts of a product to choose for a redesign. A selection criteria based approach is presented here.

2. Selection Criteria

Not all parts of a system are equally suited for Additive Manufacturing or bear the same potential for an improvement of the overall system. An analysis of the predecessor or an initial design identifies those parts where a change in manufacturing technology provides the biggest benefit to the system's performance. The following selection criteria can be applied to select components for a re-designed to fully exploit the geometric freedom of AM.

2.1. Integrated Design

An integrated design includes various functions in one part. The benefits of such integration result from the reduced number of parts. There are fewer interfaces between parts, the components are more compact and less assembly is required.

The objective of the selection criterion integrated design is to identify assemblies or groups of parts, which can be redesigned into one single part. It is obvious that selected parts should not move relative to each other. Suitable assemblies have either only one function, but are split in many parts due to manufacturing constraints, or different functions have been separated in different parts to reduce the complexity of each part. Both steps are not necessary with AM.

2.2. Individualization

Individualization is driven by the wish to meet different customers' needs and to gain an advantage over competitors by this. Consequences of individualization are more variations and smaller lot sizes. For an economic production the product has to be separated in standard components and customized components. The assembly of standard parts and customized parts leads to an individualized product at reasonable costs. The standard parts are mass produced by conventional manufacturing technologies whereas the customized parts are manufactured in small lot sizes. Since AM requires no tools and fixtures, an economic production of individual parts is feasible.

Complex parts with a high variability meet the individualization criterion. Those parts are often found at interfaces to humans or surrounding structures.

2.3. Lightweight Design

Lightweight designs are found in mobile and dynamic applications. The reduced weight improves the performance of the product.

AM's geometrical freedom of design allows placing material only in locations where it is needed for the function of the part. This increases the complexity, but reduces material and weight. Less material is melted to produce the lightweight part and by this, manufacturing time and costs are saved. This is a contrast to conventional manufacturing, where increased complexity does lead to higher costs.

A weight optimization requires knowledge on the applied load cases. The most weight can be saved on complex load bearing parts. Those are the ones selected for a lightweight redesign for Additive Manufacturing.

2.4. Efficient Design

The objective of the selection criteria efficient design is to improve the efficiency of the product in operation. This can be achieved by reducing the losses in the product during operation or by an increased performance of the component, respectively. The biggest effects in this context have parts, which are involved in the transportation of mass or energy or in the conversion of energy. These are the ones that should be selected and analyzed for a re-design.

These four criteria help to identify parts and assemblies for a re-design for AM. The parts are not necessarily assigned exclusively to one criterion. Due to this, it is possible to identify the same parts by applying different criteria.

3. Examples for the Substitution of Components

The best way to demonstrate the potential of Additive Manufacturing is by successful examples. Four examples from different industries and manufactured with different materials are presented here. Each example can be identified by one or more of the described selection criteria. The components were chosen based on the criteria, but for a re-design a broader view on the function and the objective of the parts was necessary.

3.1. Integrated Design of a Medical Device

The first part is from a medical device developed and manufactured by MTS to treat orthopedic problems, like osteoarthritis, with shockwaves. The handheld device is pressed on the skin of the patient by the physician and releases a shockwave into the tissue. The metabolism is locally increased and supports the healing process. [12]

The waves are generated inside the part depicted in Fig. 2 by high voltage discharges between two adjustable electrodes surrounded by water. The shockwaves are guided and focused into the tissue by the reflector at the end of the part. The discharge creates gases and vapors. These fumes need to be extracted and fresh water has to be refilled continuously. [12]

In the development process, it became clear that the desired handling properties could not be achieved by a design for

A-A .¿a

Connectors Internal Channels

Fig. 2: Additive Manufactured part of a medical device

conventional manufacturing. Conventional manufacturing would have required splitting the reflector into many single parts. Since it is part of a handheld device, a compact and lightweight design was required. The assembly of the reflector was identified as the component, where an integrated design yields the most benefit to the usability of the device.

This demonstrates the non-exclusiveness of the criteria. The reflector can also be identified by applying the criteria for lightweight design and the improved handling to increase the efficient use by the medical practitioner.

The integrated design criterion was chosen, because a reduced number of parts had several beneficial side effects besides less weight and better handling. To avoid any risk of electric shocks resulting from the use of high voltage and water, the part has to be watertight and non-conductive. By a reduction of parts fewer interfaces have to be sealed. This reduces the risk of leaking and consequently, reduces the risk of electric shocks. The reflector is made from a non-conductive thermoplastic by SLS and is postprocessed to ensure the water impermeability. In addition to improved handling and product safety, the assembly time and costs are reduced. [12]

The re-design was done in two iterations by MTS and Inspire. At first the water and vapor channels were integrated to reduce installation space and weight. The second design loop unlocked the full potential of AM to include further functions into one highly integrated design. With this iteration, the part was further optimized and the costs were reduced. The final design fulfilled the requirements regarding product safety, handling and production costs. [12]

3.2. Individualized Phased Array Inspection Tool

Pipes are an important element in industrial plants. The pipes in a power plant need to be inspected regularly to ensure safety and reliability. Especially weldings have to be checked after installation and during operation. Usually ultrasonic inspection is used, which requires considerable space for probe handling. In confined places the inspection is done by x-rays, which has disadvantages. Work has to be stopped and the surrounding area is evacuated during x-ray imaging. The evaluation of the x-ray images is done offline and the results are not immediately available for rework of defects. A longer downtime of the power plant during installations and maintenance is the result. [13]

To increase the welding productivity Alstom Inspection Robotics developed a handheld phased array inspection tool for the ultrasonic inspection of butt welds of small diameter tubes up from 1" and a clearance as narrow as 12mm. With the device no evacuations are necessary and the results can be immediately evaluated. [13,14]

To inspect confined places, the ultrasonic probe and position sensors are integrated into the device in Fig. 3. Due to the compact design, the parts of the inspection robot are very complex.

Fig. 3: Phased Array Inspection Tool [13]

The piping in the power plants of Alstom's customers varies in diameter, thickness, material and available space for inspection. The inspection tool is customized to fit into the individual plant layout. The sensor array and the electronics remain unchanged in most cases and the design variations are limited to the body of the phased array inspection tool. The metal parts of the body are the ones that have to fit the customer's application. The re-design of these parts aimed for a compact system, a good protection of the sensors and a design that is easy to adapt to the customers' applications.

3.3. Efficient Injection Molding Tool

For mass production of thermoplastic parts, injection molding is still the manufacturing technology of choice. In a cyclic process molten thermoplastic is injected into a mold, cooled until it solidifies and then the finished part is ejected. The faster the cycle of injecting, cooling and ejecting is performed the more parts can be manufactured. Therefore, the cycle time is the indicator for productivity of the process. It is mainly determined by the efficiency of the heat transfer from the plastic into a cooling system of the mold. [15]

An improvement of the thermal efficiency of the mold in a re-design has the highest impact on productivity. The outside surface of the injection molding tool is determined by the shape of the plastic part. Only the space inside the tool can be modified. For an efficient heat transfer, cooling channels need to be placed close to the surface. To ensure a good quality of parts, with low warping and homogenous surfaces, only little variations in surface temperature are allowed. This requires a contour following path of the cooling channels. [15]

In conventionally manufactured molds, drilled holes form the cooling channels. Since the distance between a straight bore and a curved mold surface varies, the temperature at the mold surface is not constant. To overcome this AM mold inserts with conformal cooling channels are used. The channels inside the insert maintain a constant distance to the surface and can branch and converge when needed. [16]

The best temperature control is achieved with a conformal cooling grid like the one depicted in Fig. 4. It was developed by Hofmann Innovation Group. Instead of a set of individual channels a network extends right underneath the surface. This design can only be manufactured with Additive Manufacturing. The inserts are made from a high quality tooling steel. They are as durable as conventional mold inserts and reduce the cycle time significantly. [16]

Fig. 4: Injection molding tool insert with conformai cooling grid [16]

3.4. Lightweight Aircraft Bracket

The aircraft industry has a strong need to save weight. The weight of the aircraft is strongly linked to the fuel consumption and therefore, to the operating costs and the flight performance. The introduction of new materials like titanium alloys, composites and honeycomb sandwich structures were driven by the higher strength to weight ratio at a given level of safety [17,18].

The freedom of design in AM has been identified as an opportunity to maximize weight saving. For a lightweight redesign the impact of parts on the weight of the aircraft was assessed. Possible results are a big saving on a single part or

optimization of a part that is used in many places throughout the aircraft.

Fig. 5(a) shows a conventional bracket to connect cabin monuments or other assemblies to the fuselage. To mount the bracket to the sandwich structure of the monument the top layer and part of the honeycomb inside are removed. The void is filled with filler and the top layer is reinstalled. The bracket is fixed on this solid fiber mount with inserts and fasteners. A tie-rod connects the primary structure to a swivel eye in the bracket. All load cases introduce tensile forces on the tie-rod with a dimensioning load case of 35kN. [19]

Designs like the one presented in Fig. 5(a) are used in all sizes throughout series aircrafts. To demonstrate the weight saving potential of a Design for AM to Airbus a topology optimization was performed on a bracket at TU HamburgHarburg. The objective was to identify the geometry with the highest stiffness and the least weight. The optimized bracket design in Fig. 5(b) is made from Titanium Alloy TiAl6V4 and can only be manufactured by AM. The weight was reduced by 41% from an original 330g to 195g. [19]

Fig. 5: (a) original design and (b) re-design of an aircraft bracket [19]

This seems to be little compared to an aircraft's empty weight of over 100t, but since theses brackets are used throughout the aircraft the savings cumulate to a significant weight saving. To maximize the outcome it is beneficial to take surrounding structures into account. This might require a new approach to the overall design of a system. Breaking with traditional concepts is difficult for designers, who applied them for most of their career. It can be seen from the design in Fig. 6 that the challenging of traditional design paradigms is worth the effort. [19]

The whole attachment point was taken in the scope of a redesign. Fig. 6 shows the assembly and the inside of the redesigned attachment point. The hollow cylinder is glued into the sandwich plate in place of the fiber mount. A modified interface between the tie-rod and the cylinder compensates angular variations and makes the swivel eye obsolete. The part count is reduced to the pre-assembled cylinder and a modified tie-rod. This reduces assembly lead time and costs.

More than 80% of the weight of the original attachment point was saved. A secondary weight saving effect might result from the better load transfer into the sandwich structure. In Fig. 5 the force has an off-set to the neutral axis of the sandwich structure creating an additional momentum. In the integrated design the load is transferred directly into the neutral axis. Without the additional momentum the strength of the surrounding structure can be reduced. [19]

4. Success of a re-design for Additive Manufacturing

The presented designs from different applications show the use of the four selection criteria to identify components for a re-design and demonstrate the potential of a re-design. Each example helps to gain an understanding for a general criteria-based guideline to identify promising parts and assemblies.

Additive Manufacturing is usually more expensive than conventional manufacturing and needs to justify higher costs with benefits in performance. A successful AM product meets performance and economic requirements. The presented cases are successful because they addressed both.

4.1. Technical success through performance

From the presented cases it is clear that after the selection of a part the re-design is not limited to the applied criteria. The view has to be broadened and all functions and properties have to be taken into account. From this set of features the design objectives can be derived. Not all objectives are equally important to the overall performance of the component. The overview in table 1 shows primary and secondary objectives of the presented re-designs. No objective is assigned to all parts. In each application the freedom of design was used to improve the performance according to the individual relevant factors.

Although the objectives in table 1 are very different, one thing can be seen from the examples: With the capabilities of Additive Manufacturing a significant increase in performance was realized. Not one single performance indicator was improved, but a combination of different, sometimes unrelated, indicators. In the presented cases this was not possible with conventional manufacturing technologies.

Table 1: Objectives of the re-design for Additive Manufacturing

Medical device Inspection tool Mold insert Aircraft bracket

Primary Design Design Constant Weight

objective space space Temperature

Secondary Weight Protection Efficient Assembly

objectives of sensors cooling time

Assembly Easily

costs customizable

Sealing

Lot size

It is not possible to compile a fixed set of objectives suitable to be addressed with Additive Manufacturing, due to the variety of application of industrial and end-user serial

products. Instead a general guideline is introduced: The more different requirements are on a part or assembly, the more likely it is that a further optimization is prohibited by the capabilities of conventional manufacturing technologies. The theoretical optimum solution often involves complex structures beyond the limits of conventional manufacturing. This is especially true for concurring requirements, like high strength and low weight in lightweight applications. To overcome this deadlock a change in manufacturing technology is required. The freedom of design in Additive Manufacturing enables the designers to leave the beaten tracks and to find new solutions.

4.2. Economic success through cost benefits

Additive Manufacturing is always in a competition to conventional manufacturing technologies. This is even the case, if a conventional product is inferior in performance, but cheaper than the additive manufactured one. From a manufacturer's point of view, a successful re-design either costs less in production than a conventional one or it creates more revenue.

Additive Manufacturing does not require an individual tooling. Therefore, small lot sizes can be manufactured with little investment costs. This is demonstrated by the medical device in Fig. 2, which is produced in a lot size of 150 parts per year [12]. Compared to the costs of an injection molding tool, it is economically reasonable to additive manufacture the parts, even if the sales argument of the improved handling is not considered. The reduction of overall manufacturing costs can be seen in cases where the conventional manufacturing process for small and medium lot sizes requires significant investments in tools and fixtures.

The improved performance of re-designed parts can also justify higher production costs of AM. The conformal cooling of the injection molding tool in Fig. 4 improves the quality, reduces the scrap rate and shortens the cycle time significantly. The increased efficiency of injection molding pays for the higher costs of the Additive Manufactured insert. The lightweight aircraft bracket might generate more revenue to the aircraft operator by allowing him to transport more payload over longer distances. More significantly the reduced fuel consumption of the aircraft will lower the operating cost by 20.000€ over the aircraft's lifetime [20].

Lower operating costs and higher productivity both have a beneficial effect over the lifetime of the product. A small improvement of a few percent cumulates over many years and economically justifies the investment in Additive Manufacturing. This leverage explains why the mold insert and the aircraft bracket are economically reasonable although only few objectives were addressed in the re-design.

The third way of creating an economically successful Additive Manufactured product is by creating a product with a strong unique selling point. This was archived by all presented products. They offer excellent properties and customers are expected to accept the extra costs for this excellence. This is especially true for the inspection tool. The compact design allows the user to inspect places no competing ultrasonic product can reach. In these situations the

customer has the choice between using a customized, compact ultrasonic inspection tool and performing x-ray inspections accepting all its disadvantages.

5. Conclusions

Additive Manufacturing is an emerging manufacturing technology with a high degree of freedom in design. It provides the opportunity to further improve products beyond the limits of conventional manufacturing. To enable this potential it is necessary to identify components, which can be designed for Additive Manufacturing. An approach for the systematic search for components and factors for a successful additive manufactured product are presented here.

The systematic search can be performed either on an existing product or during the development of a new product. The motivations to use Additive Manufacturing are grouped into four criteria: integrated design, individualization, lightweight design and efficiency. Each criterion can be applied on the components of products to identify promising candidates for a design for Additive Manufacturing. The criteria are not exclusive. A component, which fits in more than one group, can be an even more promising candidate for a re-design for Additive Manufacturing.

Once a component is selected the design for Additive Manufacturing should not be limited on the requirements of a single criterion. A detailed analysis of the parts will reveal a set of different and possibly concurring objectives. A successful design addresses all of the objectives and improves the product in a multitude of directions.

The presented cases illustrate the use of clusters in a systematic search and how the re-design for Additive Manufacturing contributed to the success of the product. It was shown that a product, to be successful, needs to be improved in both a technological and economic direction.

On the economic side the investment in the change of design and process has to pay off either by lower manufacturing costs or by benefits during the lifetime of the product. Production costs are usually reduced by simplifying the assembly or when Additive Manufacturing replaces a manufacturing technology that requires a significant investment in individual tools.

To have a return on investment during the lifetime of a product, the user has to experience a benefit during operation. Possible benefits are lower operating costs, a higher productivity and a product that has a unique performance. To have access to this improved functionality, the user is willing to pay the manufacturer for his investment in AM.

The presented cases underline the maturity of Additive Manufacturing to substitute conventional components in series products. By a systematic search for suitable components and by fully utilizing the geometric freedom in the re-design impressive increases in performance can be realized. This opens new perspectives in product development.

References

[1] ASTM International. Standard Terminology for Additive Manufacturing Technologies(F2792 - 12a). West Conshohocken, PA; 2012.

[2] Terry Wohlers (ed.). Wohlers Report 2013 - Additive Manufacturing and 3D Printing State of the Industry - Annual Worldwide Progress Report. 18th ed. Fort Collins, CO: Wohlers Associates; 2013.

[3] Meiners W. Direktes Selektives Laser Sintern einkomponentiger metallischer Werkstoffe.

[4] Caulfield B, McHugh PE, Lohfeld S. Dependence of mechanical properties of polyamide components on build parameters in the SLS process. J Mater Process Technol 2007;182(1-3):477-88.

[5] Seyda V, Kaufmann N, Emmelmann C. Investigation of aging processes of Ti-6Al-4V powder material in laser melting. Phys Procedia 2012;39:425-31.

[6] Spierings AB, Starr TL, Wegener K. Fatigue performance of additive manufactured metallic parts. Rapid Prototyping J 2013;19(2):88-94.

[7] Thomas D. The Development of Design Rules for Selective Laser Melting. PhD-Thesis. University of Wales Institute; 2009.

[8] Adam GA, Zimmer D. Design for Additive Manufacturing - Element transitions and aggregated structures. CIRP J Manuf Sci Technol 2014;7(1):20-8.

[9] Campbell T, Williams C, Ivanova O, Garrett B. Could 3D Printing Change the World? Technologies, Potential, and Implications of Additive Manufacturing. Strategic Forsight Report 2011.

[10] Hopkinson N, Hague RJ, Dickens PM (eds.). Rapid Manufacturing: An Industrial Revolution for the Digital Age. Chichester: John Wiley & Sons; 2006.

[11] Campbell RI, Jee H, Kim YS. Adding Product Value through Additive Manufacturing. In: International Conference on Engineering Design, ICED13; 2013.

[12] Leutenecker B, Lohmeyer Q, Meboldt M. Konstruieren mit generativen Fertigungsverfahren - Gestalterische Lösungen für die Substitution von Serienbauteilen. In: Krause D, Paetzhold K, Wartzack S, editors. Design for X: Beiträge zum 24. DfX-Symposium Oktober 2013. Hamburg: TuTech Innovation; 2013.

[13] Alstom Inspection Robotics. Business Case - Inspection of Butt Welds. Zurich; 2013.

[14] Alstom Inspection Robotics. Palm Scanner - Phased Array Inspection Tool. Zurich; 2013.

[15] Pötsch G, Michaeli W. Injection molding: An introduction. 2nd ed. Munich, Cincinnati: Hanser Gardner Publications; 2008.

[16] Gebhardt A. Understanding Additive Manufacturing - Rapid Prototyping - Rapid Tooling - Rapid Manufacturing. 1st ed. München: Carl Hanser; 2011.

[17] Ekvall JC, Rhodes JE, Wald G. G. Methodology for Evaluating Weight Savings from Basic Material Properties. In: Abelkis PR, Hudson C, editors. Design of Fatigue and Fractur Resistant StructuresDesign of Fatigue and Fractur Resistant Structures: ASTM International; 1982, p. 328-41.

[18] Herrmann AS, Zahlen PC, Zuardy I. Sandwich Structures Technology in Commercial Aviation - Present Applications and Future Trends. In: Thomsen OT, Bozhevolnaya E, Lyckegaard A, editors. Sandwich Structures 7: Advancing with Sandwich Structures and Materials. Dordrecht: Springer; 2005, p. 13-26.

[19] Emmelmann C, Petersen M, Kranz J, Wycisk E. Bionic Lightweight Design by Laser Additive Manufacturing (LAM) for Aircraft Industry. In: Ambs P, Emmelmann C, Curticapean D, Meyrueis PP, Knapp W, Kuznicki ZT, editors. SPIE Eco-Photonics: Sustainable Design, Manufacturing, and Engineering Workforce Education for a Green Future: SPIE; 2011, p. 80650L.

[20] Emmelmann C, Sander P, Kranz J, Wycisk E. Laser Additive Manufacturing and Bionics: Redefining Lightweight Design. Phys Procedia 2011;12:364-8.