Precision Engineering 42 (2015) 346–351 Contents lists available at ScienceDirect Precision Engineering jo ur nal homep age: www.elsevier.com/locate/precision Technical note Entangled structures as high cycle compression springs Folkers E. Rojas ∗ , Alexander H. Slocum Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA a r t i c l e i n f o Article history: Received 2 February 2015 Accepted 20 April 2015 Available online 27 May 2015 Keywords: Entangled structures Foams Fibrous metals Springs a b s t r a c t Entangled structures, such as steel wool, can be used as inexpensive, high cycle, low stiffness, thin profile compressive springs where uniform pressure on a surface is required particularly in elevated temperature and/or harsh environments. Mechanical compression tests were performed on a variety of steel wool sam- ples to determine the stress–strain curve behavior over high cycles. After initial conditioning cycles, good repeatability can be obtained with hysteresis dependent on strain. The results show a nonlinear behavior over large strains (>10%) and reasonable linear behavior for strains less than 10%. The properties of an entangled structure spring can be selected to achieve the desired stiffness for a particular application. © 2015 Elsevier Inc. All rights reserved. 1. Introduction The mechanics of a random array of interwoven fibers, i.e. entan- gled wires such as found in steel wool, have been studied for applications such as damping, isolating, or filtering. In semiconduc- tor test applications “Fuzz Buttons,” a compacted random array of wire, have been used as temporary electrical contacts for “low sig- nal distortion, high frequency, and low insertion force, planarity, and shock/vibration resistance” [1]. Contact testers made from micro entangled structures have been used by the semi-conductor industry to test Through Silicon Via (TSV) and micro-bumps arrays [2,3]. The characterization of entangled wires is also very important for micro-electronics, micro fluidics, micro sensors and actuators that are based on entangled silica nano-wire structures [4]. The generation of the silica nanowire entangled arrays are grown, and not created via a mechanical means and they often appear as entan- gled [4,5]. Entangled structures are also used by the space industry to isolate instruments or to damp vibrations [6,7]. Entangled structures have also been evaluated as compati- ble replacements for porous aluminum as they have been shown to have good toughness and strength [8,9]. Entangled titanium wire materials (ETWM) and porous titanium foams have been considered for tissue reconstruction, orthopedic implants, and bone repairs [10–14]. The challenge for the medical industry has been to match the mechanical properties of entangled wire structures to similar mechanical properties of bone which has been attempted by altering the effective porosity of the structure ∗ Corresponding author. Tel.: +1 603 513 8997; fax: +1 603 225 3102. E-mail address: Folkers.Rojas@gmail.com (F.E. Rojas). created by the entangled wires [11,14]. While compressive properties and pseudo-elastic hysteresis behaviors have been char- acterized for ETWMs, a high cycle analysis of the nonlinear elastic region of ETWMs is needed to make the design process more deter- ministic. Prior work has shown that entangled structures have a three-stage stress–strain behavior under compressive loads: (1) nonlinear elastic deformation, (2) strain-hardening, and (3) den- sifying [10]. Here we build on this work by studying the higher cycle compliance of entangled structures under uniaxial compres- sion for the use as an inexpensive and repeatable low-moderate stiffness thin profile large contact area spring. This type of spring has wide application in presses and rectilinear format batteries. This work also examines the effect of densification on the appar- ent Young’s modulus for entangled structures for comparison with existing analytical models. Analogies are made with low density materials such as foams, composites, and elastomers in order to help develop modeling tools. 2. Experimental procedures 2.1. Sample characterization One method for generating an entangled metal wire structure involves wrapping the wire around a rod to create a loose steel wire bundle. The unconsolidated deformed wire is placed into a set of molds where it is compressed and sintered to remove internal stresses [8,10,15]. Common steel wool, which is much more compli- ant and inexpensive, is made by shaving small strands from straight steel wire and consolidating the strands into an entangled struc- ture using a needle punch machine to generate a mat of wool. The http://dx.doi.org/10.1016/j.precisioneng.2015.04.013 0141-6359/© 2015 Elsevier Inc. All rights reserved.