Modeling 3D Printed Cellular Structures: Challenges

In this post, I discuss six challenges that make the modeling of 3D printed cellular structures (such as honeycombs and lattices) a non-trivial matter. In a following post, I will present how some of these problems have been addressed with different approaches.

At the outset, I need to clarify that by modeling I mean the analytical representation of material behavior, primarily for use in predictive analysis (simulation). Here are some reasons why this is a challenging endeavor for 3D printed cellular solids – some of these reasons are unique to 3D printing, others are a result of aspects that are specific to cellular solids, independent of how they are manufactured. I show examples with honeycombs since that is the majority of the work we have data for, but I expect that these ideas apply to foams and lattices as well, just with varying degrees of sensitivity.

1. Complex Geometry with Non-Uniform Local Conditions

I state the most well-appreciated challenge with cellular structures first: they are NOT fully-dense solid materials that have relatively predictable responses governed by straightforward analytical expressions. Consider a dogbone-shaped specimen of solid material under tension: it’s stress-strain response can be described fairly well using continuum expressions that do not account for geometrical features beyond the size of the dogbone (area and length for stress and strain computations respectively). However, as shown in Figure 1, such is not the case for cellular structures, where local stress and strain distributions are non-uniform. Further, they may have variable distributions of bending, stretching and shear in the connecting members that constitute the structure. So the first question becomes: how does one represent such complex geometry – both analytically and numerically?

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Fig 1. Honeycomb structure under compression showing non-uniform local elastic strains [Le & Bhate, under preparation]

2. Size Effects

A size effect is said to be significant when an observed behavior varies as a function of the size of the sample whose response is being characterized even after normalization (dividing force by area to get stress, for example). Here I limit myself to size effects that are purely a mathematical artifact of the cellular geometry itself, independent of the manufacturing process used to make them – in other words this effect would persist even if the material in the cellular structure was a mathematically precise, homogeneous and isotropic material.

It is common in the field of cellular structure modeling to extract an “effective” property – a property that represents a homogenized behavior without explicitly modeling the cellular detail. This is an elegant concept but introduces some practical challenges in implementation – inherent in the assumption is that this property, modulus for example, is equivalent to a continuum property valid at every material point. The reality is the extraction of this property is strongly dependent on the number of cells involved in the experimental characterization process. Consider experimental work done by us at PADT, and shown in Figure 2 below, where we varied both the number of axial and longitudinal cells (see inset for definition) when testing hexagonal honeycomb samples made of ULTEM-9085 with FDM. The predicted effective modulus increases with increasing number of cells in the axial direction, but reduces (at a lower rate) for increasing number of cells in the longitudinal direction.

This is a significant challenge and deserves a full form post to do justice (and is forthcoming), but the key to remember is that testing a particular cellular structure does not suffice in the extraction of effective properties. So the second question here becomes: what is the correct specimen design for characterizing cellular properties?

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Fig 2. Effective modulus under compression showing a strong dependence on the number of cells in the structure [Le & Bhate, under preparation]

3. Contact Effects

In the compression test shown in the inset in Figure 2, there is physical contact between the platen and the specimen that creates a local effect at the top and bottom that is different from the experience of the cells closer the center. This is tied to the size effect discussed above – if you have large enough cells in the axial direction, the contribution of this effect should reduce – but I have called it out as a separate effect here for two reasons: Firstly, it raises the question of how best to design the interface for the specimen: should the top and bottom cells terminate in a flat plate, or should the cells extend to the surface of contact (the latter is the case in the above image). Secondly, it raises the question of how best to model the interface, especially if one is seeking to match simulation results to experimentally observed behavior. Both these ideas are shown in Figure 3 below. This also has implications for product design – how do we characterize and model the lattice-skin interface? As such, independent of addressing size effects, there is a need to account for contact behavior in characterization, modeling and analysis.

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Fig 3. Two (of many possible) contact conditions for cellular structure compression – both in terms of specimen design as well as in terms of the nature of contact specified in the simulation (frictionless vs frictional, for example)

4. Macrostructure Effects

Another consideration related to specimen design is demonstrated in an exaggerated manner in the slowed down video below, showing a specimen flying off the platens under compression – the point being that for certain dimensions of the specimen being characterized (typically very tall aspect ratios), deformation in the macrostructure can influence what is perceived as cellular behavior. In the video below, there is some induced bending on a macro-level.

5. Dimensional Errors

While all manufacturing processes introduce some error in dimensional tolerances, the error can have a very significant effect for cellular structures – a typical industrial 3D printing process has tolerances within 75 microns (0.003″) – cellular structures (micro-lattices in particular) very often are 250-750 microns in thickness, meaning the tolerances on dimensional error can be in the 10% and higher error range for thickness of these members. This was our finding when working with Fused Deposition Modeling (FDM), where on a 0.006″ thick wall we saw about a 10% larger true measurement when we scanned the samples optically, as shown in Figure 4. Such large errors in thickness can yield a significant error in measured behavior such as elastic modulus, which often goes by some power to the thickness, amplifying the error. This drives the need for some independent measurement of the manufactured cellular structure – made challenging itself by the need to penetrate the structure for internal measurements. X-ray scanning is a popular, if expensive approach. But the modeler than has the challenge of devising an average thickness for analytical calculations and furthermore, the challenge of representation of geometry in simulation software for efficient analysis.

Fig 4. (Clockwise from top left): FDM ULTEM 9085 honeycomb sample, optical scan image, 12-sample data showing a mean of 0.064″ against a designed value of 0.060″ – a 7% error in thickness

6. Mesostructural Effects

The layerwise nature of Additive Manufacturing introduces a set of challenges that are somewhat unique to 3D Printed parts. Chief among these is the resulting sensitivity to orientation, as shown for the laser-based powder bed fusion process in Figure 5 with standard materials and parameter sets. Overhang surfaces (unsupported) tend to have down-facing surfaces with different morphology compared to up-facing ones. In the context of cellular structures, this is likely to result in different thickness effects depending on direction measured.

Fig 5. 3D Printed Stainless Steel Honeycomb structures showing orientation dependent morphology [PADT, 2016]
For the FDM process, in addition to orientation, the toolpaths that effectively determine the internal meso-structure of the part (discussed in a previous blog post in greater detail) have a very strong influence on observed stiffness behavior, as shown in Figure 6. Thus orientation and process parameters are variables that need to be comprehended in the modeling of cellular structures – or set as constants for the range of applicability of the model parameters that are derived from a certain set of process conditions.

Figure
Fig 6. Effects of different toolpath selections in Fused Deposition Modeling (FDM) for honeycomb structure tensile testing  [Bhate et al., RAPID 2016]

Summary

Modeling cellular structures has the above mentioned challenges – most have practical implications in determining what is the correct specimen design – it is our mission over the next 18 months to address some of these challenges to a satisfactory level through an America Makes grant we have been awarded. While these ideas have been explored in other manufacturing contexts,  much remains to be done for the AM community, where cellular structures have a singular potential in application.

In future posts, I will discuss some of these challenges in detail and also discuss different approaches to modeling 3D printed cellular structures – they do not always address all the challenges here satisfactorily but each has its pros and cons. Until then, feel free to send us an email at info@padtinc.com citing this blog post, or connect with me on LinkedIn so you get notified whenever I write a post on this, or similar subjects in Additive Manufacturing (1-2 times/month).

See how the Experts Pitch a Startup Company – And Win the Unicorn Cup

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What if a group of tech startup experts all pitched the same company, the goal being to show the best way to do a pitch? That is what PADT’s Pitch Perfect is about.   We are asking some of Arizona’s most talented investors, entrepreneurs, and mentors to volunteert where they can show the right way to pitch a technology startup. And the best presenters will receive the incredible “Unicorn Cup” award – a fine work of craftsmanship handcrafted in a foreign land.

This event is part of our Nerdtoberfest celebration on October 27th, 2016 and will be from 4:30-6:00, right before our open house. So far CEI has stepped up as a competitor, and we are looking for 3 more experienced people to take the challenge.

shutterstock_startups1We will have four teams have 10 minutes each to present the company, followed by 5 minutes of questions from the judges.  Each company gets the same Angel Funding application for the company: Barqk! – Let your dog speak.

Anyone can attend and we will be streaming the presentations live.  Just register for our Nerdtoberfest, www.padtinc.com/nerdtoberfest, if you would like to attend.

We are still looking for Judges and presenters, so email Eric Miller (eric.miller@padtinc.com) if you are interested in volunteering for either.

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Phoenix Business Journal: The Cloud, the other enabler for what is next

Just-Published-PBJ-1If you work in tech you probably hear the phrase “The Cloud” almost every day. It is a useful buzzword because it shortly and concisely creates an image that represents a critical enabler for much of the breakthrough and disruptive technology that is headed our way. In “The Cloud, the other enabler for what is next” I take a look at what is going on and how it is impacting technology across the board.

Advanced Electrification – Industry Application

As it progresses, we here at PADT would like to share some examples of companies working within the five topics that this campaign focuses on (Advanced Electrification, Machine & Fuel Efficiency, Effective Lightweighting, Thermal Optimization, and Aerodynamic Design) in order to give you a better idea as to how they can be applied within the industry.

Advanced Electrification – Additional components of vehicle electrification

Manufacturers of electric vehicles are finding additional ways to reduce electric loads by any means necessary, often in ways you wouldn’t think of at first.

Engineers on the eBooster® team in Kirchheimbolanden, Germany (Image credit: BorgWarner)

Want to learn more? Click Here for more information on how ANSYS simulation software can benefit companies working in the field of Advanced Electrification.

Join PADT in exploring the impact of breakthrough energy innovation as well as how ANSYS simulation solutions can be used to help combat the challenges that this area presents. Fill out the registration form to receive additional information on each topic, along with updates regarding the release of various webinars as the campaign progresses.

Invitation: Annual Open House is Coming – Nerdtoberfest

nerdtoberfest-logo-1d-400wThe best party in town for engineering and manufacturing is PADT’s annual open house, and this year we are making it better with Nerdtoberfest.  We are inviting more partners and customers to join us to celebrate all that the state has to offer for those of us who make our living making things. Take a tour, meet new people, learn about what is new. Just join us! We expect between 200 and 300 people.

Read on to learn more or register here.

The Basic Details

  • When:
    • October 27th, 2016
    • Metal 3D Printing Seminar: 3:00 – 4:00 pm
    • Perfect Pitch Competition – Watch some of the best tech startup mentors in town show how to pitch a company, and win the “Unicorn Cup”: 4:30 – 6:00 pm
    • Open House: 6:00 – 9:00 pm
  • Where:
    • 7755 S Research Dr, Suite 110, Tempe, AZ 85284
  • What:
    • Come see PADT’s new metal 3D Printer – it is very cool
    • Attend a special pitch event where the valley’s best startup mentors pitch the same company to a group of judges
    • Marvel at the latest structural, fluid, and electromagnetic simulation software
    • See our lab and manufacturing areas
    • There will be beer, there will be pizza. We will have water and soda as well.
    • Stand in awe as you gaze upon our 1000+ core compute cluster
    • Meet the who’s who of Arizona technology people
  • Who:
    • Engineers, manufacturers, makers, teachers, innovators, inventors, suppliers
    • Those who are interested in and of the above or just like being around smart people
    • Our Sponsors include: the Center for Entrepreneurial Innovation (CEI), Arizona Technology Council, RevAZ, Arizona Commerce Authority, Mesa Community College and AzAMI, and the Greater Phoenix Economic Council (GPEC)

Enough info? Register here or keep reading.

What’s Going On?

If you have never been to one of our open houses before, they are simply a gathering of the local tech community to learn more about how things are designed and made. We have stations set up that show the software and hardware we use to improve product development. A big draw is our extensive 3D Printing capabilities as well as our leading edge virtual testing software.  It is a great way to expose your family, friends, kids, and coworkers to what engineering and manufacturing are all about.

Something new, a Pre Show

Before the open house we will have a talk on metal 3D Printing from 3 to 4 pm. Dr. Dhruv Bhate will go over how metal 3D Printing works, talk about why it is such a breakthrough, and share some examples of how to use it effectively.

Then we have a new event from 4:30-6:00: Watch the Experts in our “Perfect Pitch” event. We have asked some of the area’s best mentors to startups to come and pitch the same company. See how a pitch should be done and share more than a few laughs along the way.

Meet others who make stuff

The best part of this event is getting to meet other people who work in engineering and manufacturing.  You can go to events all year, but they tend to be focused on an industry, startups, or a certain part of the state. Everyone comes to PADT’s open houses and it is a great chance to meet and share ideas.

So, just come.  It is fun and we promise two things:  You will learn something and you will meet someone new.  Bring the family, bring your neighbors.

Let’s celebrate what we do!

Register at bit.ly/nerdtoberfest so we know how much beer, water, soda, and pizza to get.

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Phoenix Business Journal: ​Are we going too far to achieve maximum productivity?

Just-Published-PBJ-1You can have too much of a good thing.  If we step back and look at what technology businesses are doing today to push productivity to extremes, it may be time to ask “Are we going too far to achieve maximum productivity?

My thought is that we have in two areas  – we have stretched employees to the breaking point and we don’t set aside time for innovation.  In this blog post I share why this hurts productivity in the long run and what we can do about it.

Machine & Fuel Efficiency – Industry Application

As it progresses, we here at PADT would like to share some examples of companies working within the five topics that this campaign focuses on (Advanced Electrification, Machine & Fuel Efficiency, Effective Lightweighting, Thermal Optimization, and Aerodynamic Design) in order to give you a better idea as to how they can be applied within the industry.

Machine & Fuel Efficiency – Infiniti unveils its Twin-Turbo VC-T Engine at the PMS 2016

Infiniti Motor Company gave a tech briefing on a new remarkable engine at the Paris Motor Show 2016. The VC-T Engine is the world’s first production-ready compression ratio machine. It will be available from

All you need to know about Infiniti's Twin Turbo V6 Engine

Want to learn more? Click Here for more information on how ANSYS simulation software can benefit companies working in the field of Machine & Fuel Efficiency.

Join PADT in exploring the impact of breakthrough energy innovation as well as how ANSYS simulation solutions can be used to help combat the challenges that this area presents. Fill out the registration form to receive additional information on each topic, along with updates regarding the release of various webinars as the campaign progresses.

Advanced Electrification – Industry Application

As it progresses, we here at PADT would like to share some examples of companies working within the five topics that this campaign focuses on (Advanced Electrification, Machine & Fuel Efficiency, Effective Lightweighting, Thermal Optimization, and Aerodynamic Design) in order to give you a better idea as to how they can be applied within the industry.

Advanced Electrification – Regenerative energy harvesting in 48V mild hybrids

48V mild hybrids are incremental improvements to conventional internal combustion engine vehicles so they can handle four times the current and store four times as much electricity.

Image result for Regenerative energy harvesting in 48V mild hybrids Read more at: http://www.energyharvestingjournal.com/articles/9959/regenerative-energy-harvesting-in-48v-mild-hybrids

Want to learn more? Click Here for more information on how ANSYS simulation software can benefit companies working in the field of Advanced Electrification.

Join PADT in exploring the impact of breakthrough energy innovation as well as how ANSYS simulation solutions can be used to help combat the challenges that this area presents. Fill out the registration form to receive additional information on each topic, along with updates regarding the release of various webinars as the campaign progresses.

Digital Engineering: Fast-Forwarding Next-Generation Product Development

digital_engineering_logoNothing beats seeing a product we were part of hit the shelves, except seeing that product become a success.  The Globalstar Spot3 project was even better because we were able to apply the full range of PADT’s capabilities to contribute to this success: Product Development, Simulation, and 3D Printing.

In “Fast-Forwarding Next-Generation Product Development” PADT’s Mike Landis outlines how we applied leading edge technology and a proven process to quickly develop Globalstar’s next-generation design, not just for performance but also for manufactuing.  The article is a great overview of the service PADT has to offer and how we partner with customers to make their innovation work.

If you have a new generation of an existing product line, or a brand new product under development and want a better product to market faster, PADT is here to help with our design, simulation, 3D Printing, test, and manufacturing expertise.  Just give us a call at 1-800-293-PADT or email info@paditnc.com.

 

PADT Events – October 2016

PADT-Events-LogoWelcome to October, fall, and the last three months of the year. This month we have our normal collection of events plus a very special open house at our Tempe Office to celebrate engineering and manufacturing in Arizona.


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This very special event introduces a new name to our annual open house in Tempe, AZ.  This year we are partnering with many different local groups to make the event even bigger and better.  We usually get between 200 and 300 people for these fun and informative evenings where you can not only learn more about PADT and what we do, but also about the Arizona engineering and manufacturing community.

Check your email for more information, visit www.padtinc.com/nerdtoberfest for updates, or just go ahead and register here.


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October 13: Tucson, AZ
SOUTHERN ARIZONA TECH + BUSINESS EXPO

This is the event for technology companies in Southern Arizona.  With networking, great keynote speakers, and presentations you will not want to miss this opportunity to dig deep into the fantastic tech community in and around Tucson. PADT will be there as will many of our customers.

See a great video, the agenda, and register here.


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October 18-19: Salt Lake City, UT
AmCon Salt Lake, 2016

The annual AmCon show in Salt Lake is one of our favorite Utah events.  This is where our customers and others involved in design and manufacturing gather together to learn and network. With free seminars on a variety of products and most of the key vendors that support the state, it is time well spent. We will be there again this year and hope to see you there.

Get the details and register here.


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October 21: Golden, CO
AIAA Rocky Mountain Section’s 5th Annual Technical Symposium

The AIAA Rocky Mountain Section’s 5th Annual Technical Symposium aims to bring together the Rocky Mountain area’s innovative, enthusiastic, and technical minds for a one-day event to collaborate, network, and share ideas about new technologies and implementation strategies. The event includes a main hall for keynote speakers, food & networking, sponsor booths, and human relations tables from local companies as well as panel sessions on the Direction of the Industry, STEM Education and Outreach, Job Market/Career Advancement, and Colorado Aerospace Initiatives in the Rocky Mountain region.

PADT will be speaking and will have a booth at this great event, held at the C. Learn more here.


isbf-biogabrication-logoOctober 29-31: Winston-Salem, NC
Biofabrication 2016

One of the most exciting areas of new research that bridges PADT’s capabilities is Biofabrication – manufacturing biological materials and structures. PADT will be in attendance and will also be presenting.

Register here.


PADT-Webinar-LogoWe are still scheduling this month’s webinars, check back for more info as we set them up.

ANSYS How To: Result Legend Customization and Reuse

ansys-mechanical-custom-legend-0A user was asking how to modify the result legend in ANSYS Mechanical R17 so Ted Harris put together this little How To in PowerPoint:

padt_mechanical_custom_legend_r17.pdf

It shows how to modify the legend to get just what you want, how to save the settings to a file, and then how to use those seettings again on a different model.  Very simple and Powerful.

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Advanced Electrification – Industry Application

As it progresses, we here at PADT would like to share some examples of companies working within the five topics that this campaign focuses on (Advanced Electrification, Machine & Fuel Efficiency, Effective Lightweighting, Thermal Optimization, and Aerodynamic Design) in order to give you a better idea as to how they can be applied within the industry.

Advanced Electrification – Hybrid Cost Advantage

One of the world’s leading experts in the electrification of cars says that hybrid technology has already reached price parity with diesel, and that this trend is likely to continue as the cost of diesel cars goes up due to tougher regulations.

Satoshi Ogiso at his office in Kariya, Jpana (Photo; Bertel Schmitt)

Want to learn more? Click Here for more information on how ANSYS simulation software can benefit companies working in the field of Advanced Electrification.

Join PADT in exploring the impact of breakthrough energy innovation as well as how ANSYS simulation solutions can be used to help combat the challenges that this area presents. Fill out the registration form to receive additional information on each topic, along with updates regarding the release of various webinars as the campaign progresses.

 

Phoenix Business Journal: ​3D printing takes a giant step forward toward production manufacturing

Just-Published-PBJ-1This year’s IMTS show in Chicago saw the introduction of some great new 3D Printing technology that makes the creation of end-use parts from additive manufacturing even more feasible. “3D printing takes a giant step forward toward production manufacturing” shares my observations on the subject.

Phoenix Business Journal: ​Arizona solidifies position as a leader in space technology

Just-Published-PBJ-1NASA launched the OSIRIS-REx mission on September 8, 2016. Not only is this a cool mission to explore our solar system, but it’s a big deal because Arizona has a ton at stake in its success.  “Arizona solidifies position as a leader in space technology” goes over what ASU, UofA and KinetX, Inc. contributed to this great project.

Prototyping Today: Five Unique Considerations for 3D Printing Production Parts

prototype_today_logoAs 3D Printing makes it’s long awaited move from being dominated by prototyping to manufacturing production parts, companies need to consider a few key issues.  In “Five Unique Considerations for 3D Printing Production Parts” I share what we have learned at PADT as we have helped customers make this transition.