The Turner Group is investigating approaches to use geometry and structuring to improve the performance of interfaces and bonded joints. We have recently investigated the mechanical and fracture behavior of these interfaces, as described in the papers below.
Disorder enhances the fracture toughness of 2D mechanical metamaterials
S Fulco, MK Budzik, H Xiao, DJ Durian, KT Turner
PNAS nexus 4 (2), pgaf023 (2025)
https://doi.org/10.1093/pnasnexus/pgaf023
Mechanical metamaterials with engineered failure properties typically rely on periodic unit cell geometries or bespoke microstructures to achieve their unique properties. We demonstrate that intelligent use of disorder in metamaterials leads to distributed damage during failure, resulting in enhanced fracture toughness with minimal losses of strength. Toughness depends on the level of disorder, not a specific geometry, and the confined lattices studied exhibit a maximum toughness enhancement at an optimal level of disorder. A mechanics model that relates disorder to toughness without knowledge of the crack path is presented. The model is verified through finite element simulations and experiments utilizing photoelasticity to visualize damage during failure. At the optimal level of disorder, the toughness is more than 2.6x of an ordered lattice of equivalent density.
Enhancing toughness through geometric control of the process zone
S Fulco, MK Budzik, KT Turner
Journal of the Mechanics and Physics of Solids 184, 105548 (2024)
https://doi.org/10.1016/j.jmps.2024.105548
Material architecture provides an opportunity to alter and control the fracture process zone shape and volume by redistributing the local stresses at a crack tip. Properly designed structures can enlarge the plastic zone and enhance the effective toughness. Here, we use a pillar array as a model structure to demonstrate how variations in geometry at a crack tip control the size and shape of the plastic zone and can be used to engineer the effective toughness. Elastic–plastic finite element simulations are used to quantify how the pillar width, spacing, and height can be varied to tailor the size and shape of the plastic zone. A set of analytical mechanics models that accurately estimate the shape, volume, and resulting toughness as a function of the base material properties and geometry are also presented. A case study extends the analysis to sets of non-regular pillar arrays to illustrate how architecture can be used to alter toughness along the crack path.
Mechanics-Based Optimization of Shear Lap Joints for Enhanced Force Capacity
SS Pande, S Fulco, KT Turner
ACS Applied Engineering Materials 2 (3), 574-581 (2024)
https://doi.org/10.1021/acsaenm.3c00568
Bonded single lap joints are used to join structural components in automotive, aerospace, and other engineered systems. The stresses in the joint are nonuniform, and high stresses near the edge typically limit the force capacity of the joint. Here, we optimize the design of the adherend geometry to improve the stress uniformity at the interface and enhance the force capacity. Through the use of machine learning and mechanics-based finite element analysis, we quantify the functional relationship between the geometry of the adherend and the variance of the interface stress distribution. A neural network is used to identify an optimized adherend geometry with improved stress uniformity for cases with different constraints on joint stiffness and manufacturability. A fracture mechanics analysis is used to predict the force capacity enhancement of the optimized designs, and the results are verified through experiments on joints consisting of aluminum adherends bonded with a cyanoacrylate adhesive. The experimentally measured force-capacity enhancement is 2.4×, which closely agrees with model predictions.
Architected adhesive joints with improved fracture toughness
T Pardoen, KT Turner, MK Budzik
Advances in Structural Adhesive Bonding, 1105-1122 (2023)
https://doi.org/10.1016/B978-0-323-91214-3.00012-0
In this chapter, strategies for improving the properties of adhesive joints through geometry and internal structuring are presented, with a focus on recent developments. In the past several years, multiple approaches have been explored to use architecture, patterning, and material heterogeneity to enhance the toughness and damage tolerance of adhesive joints. Many of these strategies exploit well-known phenomena such as crack front deflection, crack blunting, and the introduction of dissipative and/or bridging layers. The objective of this chapter is to review several principles that can be used to enhance toughness as well as show several embodiments of these principles through architecting of joints.
Decoupling toughness and strength through architected plasticity
S Fulco, MK Budzik, ED Bain, KT Turner
Extreme Mechanics Letters 57, 101912 (2022)
https://doi.org/10.1016/j.eml.2022.101912
We investigate the elastic–plastic fracture of architected materials through experiments and theory, with a focus on understanding the combined effects of material length scale and geometry, using a pillar array as a model structure. We show that load sharing across the pillars, and hence toughness, can be controlled by changing the spatial distribution and height of the pillars. A simple relation is presented to relate the extent of the plastic fracture process zone to the pillar array structure and the resulting toughness. This relation allows for quantitative prediction of failure loads of specimens with plasticity localized to a structured region and reveals that strength and toughness can be decoupled through architecture. Our findings establish a foundation for design of architected materials with enhanced fracture toughness.
Adhesion of beams with subsurface elastic heterogeneity
A Luo, KT Turner
Journal of the Mechanics and Physics of Solids 159, 104713 (2022)
https://doi.org/10.1016/j.jmps.2021.104713
Beam-like structures with enhanced and selective adhesion as well as low stiffness are of interest for applications such as microtransfer printing processes and soft robotic grasping devices. The stiffness and adhesion of compliant beams can be altered through the inclusion of structured stiff insets that alter the mechanical response and stress distribution at the adhered interface. Here, the adhesion behavior of compliant beams with embedded stiff insets below the surface are investigated. The length of inset and the spacing of the insets relative to the beam height are key parameters that control the adhesion behavior. The inclusion of a stiff inset increases the effective adhesion of the beam, but the maximum adhesion enhancement is only realized when the inset exceeds a critical inset length. The spacing between the insets plays a key role as well and inset spacings <∼1.5 times height of the beam result in interactions between insets that can be exploited to increase the effective adhesion. Overall, this work provides an understanding of how subsurface elastic heterogeneity affects delamination behavior of beams and provides guidance for the design of heterogeneous beams containing insets to realize controlled adhesion and bending rigidity.
Mechanics and fracture of structured pillar interfaces
S Heide-Jørgensen, MK Budzik, KT Turner
Journal of the Mechanics and Physics of Solids 137, 103825 (2020)
https://doi.org/10.1016/j.jmps.2019.103825
Material architecture and geometry provide an opportunity to alter the fracture response of materials without changing the composition or bonding. Here, concepts for using geometry to enhance fracture resistance are established through experiments and analysis of the fracture of elastic-brittle, polymer specimens with pillar-structures along the fracture plane. Specifically, we investigate the fracture response of double cantilever beam specimens with an array of pillars between the upper and lower beams. In the absence of pillars, unstable crack growth and rapid catastrophic failure occur in the double cantilever specimens tested in displacement control. Introducing pillars at the interface by removing material via laser cutting yields a discontinuous interface and leads to a more gradual fracture process and an increase in the work of fracture. The pillar geometry affects the failure load and, notably, increasing the slenderness of the pillars leads to higher critical failure loads due to greater load sharing. The effect of pillar geometry on fracture is established through experiments and analysis, including analytical modeling and finite element simulations. An analytical model that includes the macro-scale response of the beam and the micro-scale response of the pillars is presented and describes the key effects of pillar geometry on fracture response.