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New manufacturing methods such as 3D printing have enabled an unprecedented degree of control over the properties of materials. This has the potential to revolutionize materials properties and to thereby greatly expand the design space available to engineers. This could have a major impact in industries where system design and performance is constrained by materials properties, including transportation, robotics, aerospace, and medicine. However, many challenges stand in the way, including limited design tools, a limited materials palette, and an inability of classic analytical approaches to describe the complex, heterogeneous materials produced by multimaterial additive manufacturing techniques.

At the Architected Materials Laboratory our work investigates the above challenges, with current projects falling into three main categories:

(1) GEOMETRIC CONTROL OF MECHANICAL PROPERTIES

Traditional manufacturing is primarily concerned with the external shape of parts (as controlled via casting, machining, etc.). 3D printers allow construction of parts with the same outer shapes as traditional manufacturing, but also with nearly arbitrary internal structure. This new capability opens up vast new design spaces. How can we use internal geometry to improve mechanical properties, such as toughness, or to impart new functionality, such as the control of how stress waves propagate through the material during impact?

(1.1) Geometry and fracture

3D printing and other advanced manufacturing techniques have enabled unprecedented control of geometry. Origami, beams, and systems of connected polygons are all examples of structures for which the mechanical response can be drastically changed by even small changes to geometry. In recent work we have shown how subtle variations to the internal geometry of 3D printed structures can lead to drastic changes to the fracture behavior. In (a) we mimic the example of natural materials, in which careful use of defects allows control of how cracks propagate through a structure during failure. In (b) we study how variations in lattice geometry allows different failure characteristics in lightweight cellular materials. Ultimately, work in this area will enable tougher lightweight materials without sacrificing other essential mechanical properties.

Our publications related to this topic:

  • C. Mo, J. R. Raney, “Spatial programming of defect distributions to enhance material failure characteristics,” Extreme Mechanics Letters 2019;100598.

 

(1.2) Geometry and wave propagation

High-speed camera video of a metamaterial being impacted by a hammer. The medium supports propagation of elastic vector solitons, with properties that depend strongly on the direction of propagation.

 

Minor variations in geometry are known to have a large influence on dynamic properties such as wave propagation. Many previous studies have explored how this affects propagation of small-amplitude linear elastic waves, e.g., leading to band gaps. In our group we are interested in high-amplitude loading, such as that experienced during impact. The internal geometry of the medium has an extremely large effect on the response during such nonlinear loading events, leading to a variety of rich phenomena, such as the propagation of elastic vector solitons, controlled focusing or spreading of impact energy, the propagation of transition waves, and conversion of impactor kinetic energy into stored elastic energy (and “phase changes”) within the medium.

In another study, we explored the use of pre-stored elastic energy to allow nonlinear transition waves to propagate through a structure, despite the fact that the material itself (a soft, 3D printable elastomer) dissipates energy rapidly.  The movement of the transition waves can be highly tuned by locally varying the geometry of the beams, which can be easily accomplished due to the soft nature of the architecture.  We made use of this to design soft mechanical logic devices, allowing a single system to switch between OR and AND gate behavior.

transitionwaves

This work is supported by the Army Research Office.

Our publications related to this topic:

 

(2) ADVANCED MANUFACTURING AND MATERIALS

Custom multimaterial 3D printer. Pictured: Printing of soft sensors (Sole3D, senior design team).

 

While 3D printers are useful for demonstrating new ideas, their utility as practical manufacturing platforms has been minimal due to their limited materials palette (typically brittle thermoplastics or photocrosslinked polymers) and poorly-understood microstructures (particularly in metal 3D printing). We develop new functional materials and hardware to allow more useful multimaterial 3D printing, and seek to leverage the unique aspects of additive processes to impart multiscale structure, similar to that observed in natural materials.

Natural lightweight materials such as wood, bone, and shells consist of complicated multiscale features, such as heterogeneous compositions and spatially-varying fiber alignments.  By careful material design and simultaneous hardware development, we have developed 3D printable fiber-reinforced composite inks.  By designing novel 3D printing nozzles we are able to control fiber alignment and compositional heterogeneity to achieve enhanced mechanical performance, following approaches and designs inspired by nature.

Since fiber alignment is controlled via the shear field in the nozzle, fiber alignment can be controlled independently of the composition of the composite. In the above image we demonstrate control of fiber alignment in (a) carbon fiber-epoxy; (b-c) “sticky rice” (amylopectin)-cotton fiber composites, which comprise a non-toxic, renewable composite made entirely from natural materials; and (d-g) highly-stretchable silicone-glass fiber composites. Because of the spatially-varying fiber alignment and mechanical properties, new experimental methods are also required. Here (d-g) we show in situ multiscale experimental characterization with a fiber alignment map and local stress-stretch field (the latter obtained via digital image correlation). We use these tools to understand the mechanisms leading to failure in complex composites.

Our publications related to this topic:

 

(3) INTELLIGENT MATERIALS

 

Is it possible to imagine intelligence as a material property, just as stiffness or strength is?

For example, consider the Venus flytrap, which, upon partially closing around potential prey, waits to determine whether or not to begin digestion based on the size and movement of the object. Obtaining this level of complexity in synthetic systems usually requires familiar combinations of microprocessors, sensors, actuators, etc. In this work, we seek to “embody” some degree of decision-making ability and responsiveness (i.e., logic and actuation) into materials themselves rather than relying on mechatronic systems. Our recent work in this area has made use of 3D printable composites which swell anisotropically upon exposure to defined environmental stimuli. This in conjunction with geometric instabilities can result in materials that behave like programmable logic gates in response to their environment.

Most interestingly, this work demonstrates the idea of embodying logic in a structure which can operate on multiple distinct environmental stimuli. For example, one can envision a deployable structure that only opens if precise conditions are met (e.g., “deploy structure if and only if the temperature is above freezing and the structure has been exposed to humidity above 80% for a period of at least 3 minutes). Behaviors such as this can be programmed solely as a function of material composition and local geometry, enabling some degree of autonomous function without resorting to traditional mechatronics.

This work has been supported by the Army Research Office and the Air Force Office of Scientific Research.

Our publications related to this topic: