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Advanced design rules for optimMAl Dynamic properties of Additive Manufacturing products

Periodic Reporting for period 1 - A_MADAM (Advanced design rules for optimMAl Dynamic properties of Additive Manufacturing products)

Reporting period: 2017-01-01 to 2018-12-31

The A_MADAM project has the mission to enhance application of an exciting new technology, the additive manufacturing (AM), in mechanical engineering. Our objective is to discover proper way to design mechanical components and structures to be able to withstand dynamic loads that arise in exploitation.

Additive manufacturing (AM) belongs to key enabling technologies where Europe has the leading research role. The AM technologies put considerably fewer limits on shape of the manufactured objects than the conventional technologies, and offer unparalleled freedom of design to industrial and mechanical designers. Since the AM technologies are insensitive to production scale, they put in focus knowledge and creativity of designers instead of low prices of mass production, paving the way to development of new business models. The AM technologies, therefore, represent a technology platform that may best serve Europe in its intent to develop knowledge-based economy, driven by innovation.

However, the industrial deployment of AM technologies is hindered by a gap that exists between the excellent research and its exploitation in industry.
The research knowledge about the AM technologies is published in scientific journals and in conference proceedings, in a form suitable for researchers. On the other hand, industrial and mechanical designers in their work use sets of design rules, which represent condensed and comprehensive form of the research findings that are easy to follow. The lack of sets of design rules for AM technologies is one of important reasons why engineers often prefer conventional technologies to AM.

The A_MADAM project intends to use the research capacities and partnerships developed in previous EU funded projects to carry out systematic studies of dynamic mechanical properties (fatigue, fracture mechanics and impact resistance) of products manufactured by AM with the goal to establish a proper rule set for design of products of AM technologies. Since the aim of the project is to “translate” the research findings into engineering rules, the consortium consists of two universities and three SMEs that use AM technologies for rapid prototyping, rapid manufacturing and rapid tooling applications.
The objective of the WP1-DEFINE is to define and update the research program of the project. The desired outcomes are defined in terms of required limiting values, functional dependencies and calculation procedures that will enable mechanical designers to predict dynamic behaviour of products manufactures by AM. The initial research program describes 1) the experimental and theoretical methodologies and 2) the test samples that should be used to obtain the results that will lead to definition of the design rules, which are the ultimate outcome of the project. The designed initial research program of the project A_MADAM will study the dynamic behaviour of AM products by investigations of their
• fatigue behaviour and
• fracture mechanics
The objective of the WP2-PRODUCE is actual production of the 882 samples defined by the research program of the A_MADAM project, which is devised within the scope of the WP1-DEFINE.
The project activities that are aimed at characterization of samples, performing of initial series of tests and optional performing of additional series of tests are gathered in the work package WP3 TEST. The experiments are performed at University of Bologna and in the company “Topomatika” in Zagreb. The project activities that are aimed at analysis of results of initial tests and optional additional tests, as well as on definition of the design rules according to the research results, are gathered in the work package WP4 ANALYZE.
Both workpackages have the following task breakdown:
-Fatigue behaviour studies
---studies of the standardized test samples
------bending tests
------tension tests
---studies of the AM-specific structures
------damage-tolerant structures
------lightweight structures
-Fracture mechanics studies
---development of the optical deformation measurement methodology
---3-point bending tests
Each of the three research directions of the project defined by the research program developed within the WP1-DEFINE brought a progress beyond the state-of-the-art:
---Influence of the material properties to fatigue behaviour of AM products
The study is performed on two materials, maraging and stainless steel, using two opposite principles for manufacturing of the samples:
o manufacturing of the samples using only the AM technologies, when the samples are manufactured by DMLS technology with the final dimensions („as-built“ samples);
o manufacturing of the samples using a combination of the additive and subtractive technologies, when the samples are manufactured by DMLS technology with increased dimensions (“allowance”), and the manufacturing to the final dimensions is performed by machining (“machined samples”);
The following conclusions may be derived from the present studies that included both machined and as-built maraging steel samples, as well as machined stainless steel samples:
1. The optimal properties of maraging steel AM products are achieved by the sequence of the heat treatment followed by shot-peening on “as-built” samples; a fatigue limit in the range 25-30% of the ultimate tensile strength of the material may be expected (Figure 1);
2. The optimal properties of stainless steel AM products are achieved by the sequence of the heat treatment followed by shot-peening on “hybrid” samples, where the allowance increases the fatigue resistance up to the thicknesses around 3 mm; due to the combination of two factors, a fatigue limit in the range 35-40% of the ultimate tensile strength of the material may be expected for crack propagation in the building direction and in the directions of layers, while the fatigue limit may increase up to 50% of the ultimate tensile strength of the material for crack propagation in the directions of 45 degrees with respect to the building direction (Figure 2);
--- Influence of advanced shapes to fatigue behaviour of AM products
The initial results arise from an extensive numerical test campaign that was performed on different samples with innovative design based on the AM capabilities show that it is theoretically possible to design structures with an extrinsic toughening mechanism against crack propagation using a relatively simple geometrical shape that may be manufactured using the AM technologies (Figure 3). The experimental test campaign should give the final word on the interesting idea.
--- Fracture mechanics AM products
The initial results of a test campaign aimed to find the fracture mechanics properties of polyamide PA2200 samples manufactured by SLS. The results show that the linear elastic fracture mechanics concepts can be used only as a very rough approximation (Figure 4), so the research direction, as far as plastics samples are concerned, will be the computation of the J-integrals. The difference between KIC of the samples with manually created cracks and samples with cracks built by SLS is around 10%.
Comparison of theoretical (blue) and experimental (red) values of stress factors
Fatigue limit of stainless steel samples (D,a) denotes the orientation and allowance (mm)
Fatigue limit of maraging steel samples (D,a) denotes the orientation and allowance (mm)
Stress intensity factor as a function of the crack size for the different specimen types