Manufacturers in almost every industry are looking to design lighter parts, whether for quicker cars, cheaper air travel, or providing that extra edge on the competition in a sporting event. But it's often difficult to know where to begin, with the endless combinations of materials, processes, and design tools available today. Whether you are interested in putting existing parts on a diet, or starting from a blank sheet, the following five approaches to light-weighting may help identify strategies that can work for you.

First, let's clarify our objective in light-weighting: the word optimize is sometimes used loosely and can lead us astray. It's better to think of these approaches as steps towards an optimum and recognize that achieving total optimality is a climb up an asymptotic mountain. How far you choose to climb depends on how much a pound costs to your design. Extra baggage on the way to orbit costs a lot more than on your average road trip.

1 Materials Selection

To get started, you need to select the right material. Why use steel if you can get away with aluminum? There's a weight reduction of approximately two thirds on the table, albeit with compromises: reduced stiffness, higher cost, and more difficulty in welding. A great way of visualizing these tradeoffs and rankings of mechanical properties is with Ashby Charts, which essentially represent the menu of materials that an engineer can select from (more on that later).

Figure 1. Ashby Chart for evaluating stiffness-to-weight ratio of conventional materials. This essentially represents the menu of (conventional) materials that engineers have to choose from. (Chart created using CES EduPack 2018, Granta Design Ltd.)

These charts can compare any combination of properties; the one shown in Figure 1 being useful for evaluating stiffness-to-weight ratio. Note the logarithmic scale. Serious lightweighting can occur just through proper material selection. Our menu of (conventional) materials varies over factors of millions in modulus, and thousands in density. Surely, some opportunity for light-weighting lies somewhere in between.

2 Part or Function Consolidation

Material on its own is just feedstock. Only with shape comes a structure or a part. But even using the best material can result in an overweight part if the form doesn't fit the function. Many assemblies are designed as such due to manufacturing rules, fastened together with nuts and bolts. These alone can add up significantly in weight, not to mention assembly time.

Figure 2. A pipe assembly lightweighted through part consolidation and conformal ribbing. (Image Credit: nTopology)

Approaching such problems from a function-first mindset, without the shackles of traditional manufacturing rules, can trim a lot of the excess that is not directly contributing to the overall function. The bulky flanges and bolts in the pipe system in Figure 2, for example, don't help us at all with controlling fluid flow (i.e. why we're even building a part). Why try optimizing something that shouldn't even exist in the first place?

3 Conformal Ribbing

Figure 3. Conformal ribbing on a turbopump casing, produced with electron beam melting (EBM) by Zenith Tecnica. (Image Credit: nTopology)

Conformal ribbing is one of the more popular methods on this list and is very prevalent in aerospace. Whether built from polymers, metals, or composites, conformal ribbing aims to enable thinner walls while improving buckling resistance (otherwise you're left with a crumpled soda can).

For polymers, ribbing can be very affordable and easily mass-produced. Look underneath a plastic shipping palette or fold-out table, for example, or even the side of a milk crate. You will find conformal ribbing (Figure 3), which engineers use to lightweight (and reduce material usage) in these relatively simple structures.

Figure 4. Machined metal isogrid on a spacecraft capsule. The high cost to get vehicles to orbit motivates advanced lightweighting techniques. (Image Credit: NASA)

But what can we do with stiffer and stronger materials? Isogrid, shown in Figure 4, has long been popular in the space industry, with design handbooks dating back to the 1970s. This is very expensive to produce via conventional machining — the cost-per-pound for machined metal isogrid is likely only justifiable for high-end aerospace applications — but additive manufacturing is now making this technique much more accessible.

4 Lattices and Architected Materials

Lattices, cellular materials, periodic structures, and architected materials — these are as diverse as the assortment of names used to describe them. Fundamentally, they are hybrids of solid material and empty space (or gas) with the precise arrangement dictating the effective mechanical properties. The amount of material is, of course, important as well and is referred to as relative density: the mass of the lattice divided by the mass of a solid block of its parent material.

Some typical suspects in the cellular world include honeycombs, foams, lattices, and shell-like continuous structures such as gyroids, each having their own pros and cons. These can be employed in a variety of ways: uniformly as shown in Figure 5, as an infill of a shelled part, conformally over a surface, or spatially varying within a complex domain.

Figure 5. An assortment of the many topologies available for lightweighting, with brief descriptions. (Image Credit: nTopology)
  • Honeycombs are hard to beat in stiffness-to-weight ratio in their extruded direction but are much softer in the other two orthogonal directions (by around an order of magnitude).

  • Octet lattices are stiff in all three principal directions but more difficult to manufacture.

  • Stochastic foams are much more compliant but the easiest to finely tailor to a design's functional requirements through cell size and relative density.

  • Auxetic honeycombs can be designed to exhibit special properties like a negative Poisson's ratio (contracts inwards when compressed, instead of barreling outwards).

  • The gyroid and broader family of triply periodic structures inherently have two sides or fluid domains to them, making them natural candidates for heat-exchanger applications. They can be easily tailored or biased in different directions to alter stiffness or flow properties as needed.

Cellular materials are perhaps most applicable to engineering in the form of sandwich structures. These are prevalent throughout all levels of the lightweighting value spectrum, from cardboard packaging to commercial aircraft to interplanetary spacecraft. The effect is all the same: separate two thin sheets of material (the bread) by a lightweight core (the filling, available in the flavors shown above), and you can attain tremendous strength and stiffness increases for a minor increase in mass. Turn this around by keeping strength and stiffness constant, and you have significant lightweighting potential.

5 Topology Optimization

Figure 6. A titanium spacecraft bracket, lightweighted with topology optimization and built with electron beam melting (EBM). (Image Credit: nTopology)

Last stop: topology optimization (Figure 6). Some refer to this as generative design but we'll stick to the engineering term here. Several methods and algorithms exist but the general idea is to apply loads and restraints to a design region and then whittle away material from this region until some constraints and objectives are met. At the bare-bones level, this is usually minimizing compliance (or maximizing stiffness) subject to a volume target. Many cases require additional constraints like maximum stress or displacement, or some restriction on manufacturing technique. The result is usually something exotic and organic-looking, which perhaps adds to its allure.

While seemingly hands-off, this approach is far from it. Some steps of it deserve automation, like reconstructing native geometry from the resultant density field or mesh. But users must take extra care in setting up their problems appropriately and understand the assumptions behind the algorithms that they wield. One common oversight is that many simple codes neglect buckling. Consider the very long, thin, slender ligaments seen in many generative design examples and how these would behave when they run over a bump on the road — likely suboptimally.

What if those loads are applied off-angle? Uncertainty quantification and design for robustness techniques are being developed for this now. This is less specific to topology optimization methods but relevant to lightweight design in general: as you get closer and closer to the optimum, the result is only as appropriate as the design assumptions.

Conclusion

Hopefully, these examples have you thinking about where you can cut weight from your parts through one or several of the approaches described above: care in material selection, consolidating parts by their functions to eliminate fasteners and redundancy, conformal ribbing for stiffening, multifunctional design with lattices and architected materials, or topology optimization to make it lightweight and look great simultaneously. The value of a pound here and there can add up significantly — the thousands of dollars per pound for spacecraft applications makes the combination of all of these methods often justifiable.

Lightweighting is clearly an important strategy across multiple industries. Whatever the goals of a particular design engineering team or OEM, the compounding effects that the methodologies described here can have on one another are motivating engineers to take their light-weighting efforts to new creative heights.

This article was written by Jonathan Harris, Ph.D., Lead Application Engineer at nTopology, New York, NY. For more information, visit here .


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This article first appeared in the January, 2020 issue of Tech Briefs Magazine.

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