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One-dimensional slender bodies can be deformed or shaped into spatially complex curves relatively easily due to their inherent compliance. However, traditional methods of fabricating complex spatial shapes are cumbersome, prone to error accumulation and not amenable to elegant programmability. In this letter, we introduce a one-dimensional origami based on attaching Miura-ori that can fold into various programmed two- or three-dimensional shapes. We study the out-of-plane displacement characteristics of this origami and demonstrate with examples, design of slender bodies that conform to programmed complex spatial curves. Our study provides a new, accurate, and single actuation solution of shape programmability.

Embodiments of the present invention provide programmable materials capable of real-time, significant adjustment in their mechanical response. When combined with autonomous sensing and control strategies, these materials can be used in a new series of structural components with enhanced static and dynamic efficiency. An embodiment of the present invention provides an apparatus comprising an array of one or more unit cells, formed from a material, each cell defining a shape; and links coupled to the unit cells, at least a subset of the links enabling changing of an elasticity of at least a subset of the unit cells or at least a sub- array of the unit cells as a function of a state of the at least a subset of the links, the state including ON and OFF states.

Experiments were performed to examine the efficiency of surfactants to remove multi-walled carbon nanotubes (MWCNTs) from silicon substrates with nano and microscaled features. In the first set of experiments, nanoscale features were induced on silicon wafers using SF6 and O2 plasma. In the second set, well-defined microscale topological features were induced on silicon wafers using photo lithography and plasma etching. The etching time was varied to create semi-ellipsoidal pits with average diameter and height of 7–9 mm, and 1–3 mm, respectively. For the cleaning process, the MWCNTs were wiped off using a simple wiping mechanism by two different surfactants and distilled water. The areal density of the MWCNTs was quantified prior to and after the removal using scanning electron microscopy (SEM) and post-image processing. For a surface featured with nanoscale asperities, the removal efficiency was measured to be in the range 83–99% based on substrate type and surface roughness. No evident relationship was observed between the etching time and the removal efficiency. For surfaces with microscale features, increasing the etching time results in appearance of larger pits and significant decrease in the removal efficiency. VC 2015 American Institute of Chemical Engineers Environ Prog, 00: 000–000, 2015

Topological management of materials at a Micro-scale is one of the fundamental building principles of nature. This combination of material and structural properties results in marked changes in the properties of solids. Nowadays physicists, chemists, materials scientists and engineers explore those effects by synthesizing, characterizing, and modeling Micro-lattice materials from all material classes. Applications have been identified in the fields of ultra-lightweight structures, thermal equipment, electrochemical devices, high absorption capacity and bio-repair materials. This article aims to review recent progress in the development of such advanced Micro-lattice materials.

This paper highlights the effects of cell wall material strain hardening and density functional gradation (FG) on in-plane constant-velocity dynamic crushing response and impact behavior of hexagonal honeycombs. Results show that cell wall material strain hardening influences the distinct deformation modes induced by crushing velocity generally observed in regular hexagonal honeycombs. This is seen by a delay in the onset of localized deformation up until intermediate crushing velocities after which localization becomes dominant smearing out differences brought about by cell wall material strain hardening (plasticity convergence). In addition, during the impact loading on regular honeycombs, it was found that increasing the cell wall material strain hardening decreases the rate of gain of maximum crushing strain with increments in initial kinetic energy of impact. On the other hand, introducing FG brings about new deformation patterns due to changes in material distribution and preferential cell wall collapse of the weaker members. Interestingly, although the dynamic localization effect at higher crushing velocities observed earlier was not found to be particularly affected by FG, gradient convergence (i.e. smearing out the effects of FG due to higher velocities analogous to plasticity convergence) was not observed. On the contrary, gradient convergence emerged at higher impacting velocities primarily brought about by a combination of initial deformation localization and its subsequent advancement into FG region ahead. The kinetic energy threshold for the emergence of this gradient convergence effect was found to be considerably delayed by cell wall material strain hardening.

Origami, widely known as the ancient Japanese art of paper folding, has recently inspired a new paradigm of design for mechanical metamaterials and deployable structural systems. However, lack of rationalized design guidelines and scalable manufacturing methods has hindered their applications. To address this limitation, we present analytical methods for designing origami-based closed-loop units with inherent foldability, and for predicting their folding response (e.g., folding force, bistability, and area and volume change by folding). These units can be employed as building blocks for application-driven design and modular construction of foldable structures with desired performance and manufacturing scalability.

Displacement and stress fields in a functionally graded (FG) fiber-reinforced rotating disk of nonuniform thickness subjected to angular deceleration are obtained. The disk has a central hole, which is assumed to be mounted on a rotating shaft. Unidirectional fibers are considered to be circumferentially distributed within the disk with a variable volume fraction along the radius. The governing equations for displacement and stress fields are derived and solved using finite difference method. The results show that for disks with fiber rich at the outer radius, the displacement field is lower in radial direction but higher in circumferential direction compared to the disk with the fiber rich at the inner radius. The circumferential stress value at the outer radius is substantially higher for disk with fiber rich at the outer radius compared to the disk with the fiber rich at the inner radius. It is also observed a considerable amount of compressive stress developed in the radial direction in a region close to the outer radius. These compressive stresses may prevent any crack growth in the circumferential direction of such disks. For disks with fiber rich at the inner radius, the presence of fibers results in minimal changes in the displacement and stress fields when compared to a homogenous disk made from the matrix material. In addition, we concluded that disk deceleration has no effect on the radial and hoop stresses. However, deceleration will affect the shear stress. Tsai–Wu failure criterion is evaluated for decelerating disks. For disks with fiber rich at the inner radius, the failure is initiated between inner and outer radii. However, for disks with fiber rich at the outer radius, the failure location depends on the fiber distribution.

We present a novel cellular metamaterial constructed from Origami building blocks based on Miura-ori fold. The proposed cellular metamaterial exhibits unusual properties some of which stemming from the inherent properties of its Origami building blocks, and others manifesting due to its unique geometrical construction and architecture. These properties include foldability with two fully-folded configurations, auxeticity (i.e., negative Poisson’s ratio), bistability, and self-locking of Origami building blocks to construct load-bearing cellular metamaterials. The kinematics and force response of the cellular metamaterial during folding were studied to investigate the underlying mechanisms resulting in its unique properties using analytical modeling and experiments.

Biomimetic scales are known to substantially alter the mechanics response of the underlying substrate engendering complex nonlinearities that can manifest even in small deformations due to scales interaction. This interaction is typically modeled using a-priori homogenization with an enforced periodicity of engagement. Such a framework is fairly useful especially when dealing with the structural length scale which is at least one order of magnitude greater than the scales themselves since individual tracking of a large number of scales become insurmountable. On the other hand, this scheme makes several assumptions whose validity has not yet been investigated including infinite length of the substrate and rigidity of the scales. The validity of these assumptions and the accuracy and limitations of associated analytical models are investigated. Finite element based numerical studies were carried out to identify the critical role of edge effects and other non-ideal behavior such as violation of periodicity and nonlinear constitutive response on scale rotation. Our investigation shows that several important quantities show a strong saturation characteristic which justify many of the simplifying assumptions whereas others need much greater care.

We report the deformation behavior and mechanical properties of a polymeric micropillar, which measures approximately 10 μm by 30 μm in size by measuring the loading/unloading response using an in situ force measurement system. When the single poly(dimethylsiloxane) (PDMS) micropillar was subjected to compression, we observed a periodic wrinkle and global (Euler) buckling at the sidewall. During unloading, we found the pull-off force (adhesion force) to increase for higher values of preloading and also for lower loading/unloading rates. From the slope of the load–displacement curves measured in situ, we calculated the effective elastic stiffness of the PDMS micropillar to be about 2.03 MPa. In addition to the current work, we report that this method can be used more broadly for in situ measurement of the intrinsic mechanical and adhesion properties of polymers and other relatively soft materials.

A study on the bending response of a composite curved panel with pyramidal metallic truss cores suitable for functional applications is presented using a combination of analytical modeling, three-point bending experiments and finite element (FE) based simulations. The aluminum pyramidal cores were constructed using an interlocking method before curing with composite face sheets to fabricate the final structure. A theoretical model was developed to analyze the experiments and develop failure criteria. Three failure modes: (i) face wrinkling, (ii) face crushing, and (iii) debonding between face sheet and truss cores, were considered and theoretical relationships for predicting the collapse load associated with each mode were developed. The experiments were carried out on two sets of specimens with differing face sheet thickness which clearly indicated the important role played by core debonding in determining the peak load of the structure. Localized buckling instabilities were also reported for samples with thinner face sheets. The role of debonding in determining strength was further highlighted by a comparison with FE simulations with suppressed debonding.

This paper presents a simple method to calculate fire duration and flue gas temperatures for reinforced concrete (RC) chimneys with fiberglass reinforced plastic (FRP) liners based on experimentally determined burning characteristics of the liner material. Implementation of the method to calculate fire durations and the transient heat transfer conditions is demonstrated for single- and four-liner chimneys. A parametric study is carried out for chimney designs and geometries ranging from 100 m to 300 m in height and 7 m to 40 m in diameter, with 1–4 liners and varying opening configurations. The results are used to identify a limited number of cases for which the RC chimney undergoes the most extreme reduction in its post-fire residual strength. Analytical estimations of the chimney residual strength after the fire are obtained using a method established based on the procedure outlined in the American Concrete Institute (ACI) 307 Standard for chimney strength calculations. Calculations for a series of critical configurations of RC chimneys, with FRP liner geometries within the practical design limits detailed in this paper, show that the post-fire structural capacity of the chimneys would not lead to catastrophic failures especially because the chimney is not expected to see other high design lateral loads such as wind or earthquake simultaneously.

Anisotropic hierarchical honeycombs of uniform wall-thickness are constructed by repeatedly replacing each three-edge vertex of a base hexagonal network with a similar but smaller hexagon of the same orientation, and stretching the resulting structure in horizontal or vertical directions to break the isotropy. The uniform overall thickness is then adjusted to maintain the constant average density. The resulting fractal-appearing hierarchical structure is defined by the ratios of replacement edge lengths to the underlying network edge length and also the cell wall angle. The effective elastic modulus, Poisson׳s ratio and plastic collapse strength in the principal directions of hierarchical honeycombs were obtained analytically as well as by finite element analyses. The results show that anisotropic hierarchical honeycombs of first to fourth order can be 2.0–8.0 times stiffer and at the same time up to 2.0 times stronger than regular honeycomb at the same wall angle and the same overall average density. Plastic collapse analysis showed that anisotropic hierarchical honeycomb has the larger plastic collapse strength compared to regular hierarchical honeycomb of the same order at certain oblique wall angles. The current work provides insight into how incorporating anisotropy into the structural organization can play a significant role in improving the mechanics of the materials structure such as regular or hierarchical honeycombs, and introduces new opportunities for development of novel materials and structures with desirable and actively tailorable properties.

We present a numerical minimization procedure to determine the macroscopic ‘plastic collapse strength’ of a tessellated cellular structure under a general stress state. The method is illustrated with sample cellular structures of regular and hierarchical honeycombs. Based on the deformation modes found by minimization of plastic dissipation, closed-form expressions for strength are derived. The current work generalizes previous studies on plastic collapse analysis of lattice structures, which are limited to very simple loading conditions.

The role of adhesive–adherend interface morphology (through intentional deviation from a flat joint plane) on the mechanical behavior of adhesively bonded lap joints is studied. Two mirror-image types of joints with a zigzag interface containing ‘positive and negative’ interlocking teeth were fabricated and their tensile behavior was measured and compared to the response of a standard flat joint. Numerical simulations were used to explore the role of tooth height and width on the stress distribution in the adhesive, and on crack propagation and arrest after initial fracture. The data suggest that stress distribution along the bond line – and thus, the initial fracture load of the joint – is altered considerably by the positive and negative interlocking teeth. The tendency of a crack to either propagate along the bond or to arrest also depends strongly on morphological details. When crack arrests, the bonded joint can sustain a higher load and thus benefits from some of the intrinsic properties of the adherends (e.g. the plasticity of metal adherends) to enhance energy absorption and toughness. Our findings provide insight for the development of robust multi-material and multi-component structural systems with tailorable properties, and for understanding the role of interface morphology in some biological systems.

We have created a multifunctional dry adhesive film with transferred vertically aligned carbon nanotubes (VA-CNTs). This unique VA-CNT film was fabricated by a multistep transfer process, converting the flat and uniform bottom of VA-CNTs grown on atomically flat silicon wafer substrates into the top surface of an adhesive layer. Unlike as-grown VA-CNTs, which have a nonuniform surface, randomly entangled CNT arrays, and a weak interface between the CNTs and substrates, this transferred VA-CNT film shows an extremely high coefficient of static friction (COF) of up to 60 and a shear adhesion force 30 times higher (12 N/cm2) than that of the as-grown VA-CNTs under a very small preloading of 0.2 N/cm2. Moreover, a near-zero normal adhesion force was observed with 20 mN/cm2 preloading and a maximum 100-μm displacement in a piezo scanner, demonstrating ideal properties for an artificial gecko foot. Using this unique structural feature and anisotropic adhesion properties, we also demonstrate effective removal an...

Stress field in functionally graded (FG) rotating disks with non-uniform thickness and variable angular velocity is studied numerically. The elastic modulus and mass density of the disks are assumed to be varying along the radius as a power-law function of the radial coordinate, while the Poisson's ratio is kept constant. The governing equations for the stress field is derived and numerically solved using the finite difference method for the case of fixed-free boundary conditions. Additionally, the effect of material gradient index (i.e., the level of material gradation) on the stress field is evaluated. Our results show that the optimum stress field is achieved by having a thickness profile in the form of a rational function of the radial coordinate. Moreover, a smaller stress field can be developed by having greater mass density and elastic modulus at the outer radius of the disk (i.e., ceramic-rich composites at the outer radius). The numerical results additionally reveal that deceleration results in shear-stress development within the disks where a greater deceleration leads to greater shear stress; however this has almost no effect on the radial and circumferential stresses. Furthermore, the shear stress can cause a shift in the location of the maximum Von Mises stress, where for small deceleration, maximum Von Mises stress is located somewhere between the inner and outer radii, while for large deceleration it is located at the inner radius. This report was made possible by a NPRP award [NPRP 5–068-2–024] from the Qatar National Research Fund (a member of the Qatar Foundation).

Background Changes to the integrity of the acromioclavicular (AC) joint impact scapulothoracic and clavicular kinematics. AC ligaments provide anterior-posterior stability, while the coracoclavicular (CC) ligaments provide superior-inferior stability and a restraint to scapular internal rotation. The purpose of this cadaveric study was to describe the effect of sequential AC and CC sectioning on glenohumeral (GH) kinematics during abduction (ABD) of the arm. We hypothesized that complete AC ligament insult would result in altered GH translation in the anterior-posterior plane during abduction, while subsequent sectioning of both CC ligaments would result in an increasing inferior shift in GH translation. Methods Six cadaveric shoulders were studied to evaluate the impact of sequential sectioning of AC and CC ligaments on GH kinematics throughout an abduction motion in the coronal plane. Following an examination of the baseline, uninjured kinematics, the AC ligaments were then sectioned sequentially: (1) Anterior, (2) Inferior, (3) Posterior, and (4) Superior. Continued sectioning of CC ligamentous structures followed: the (5) trapezoid and then the (6) conoid ligaments. For each group, the GH translation and the area under the curve (AUC) were measured during abduction using an intact cadaveric shoulder. Total translation was calculated for each condition between ABD 30° and ABD 150° using the distance formula, and a univariate analysis was used to compare total translation for each axis during the different conditions. Results GH kinematics were not altered following sequential resection of the AC ligaments. Disruption of the trapezoid resulted in significant anterior and lateral displacement of the center of GH rotation. Sectioning the conoid ligament further increased the inferior shift in GH displacement. Conclusion A combined injury of the AC and CC ligaments significantly alters GH kinematics during abduction. Type III AC separations, result in a significant change in the shoulder’s motion and may warrant surgical reconstruction to restore normal function.

In this paper, we mimic the venous morphology of a typical plant leaf into a fiber composite structure where the veins are replaced by stiff fibers and the rest of the leaf is idealized as an elastic perfectly plastic polymeric matrix. The variegated venations found in nature are idealized into three principal fibers — the central mid-fiber corresponding to the mid-rib, straight parallel secondary fibers attached to the mid-fiber representing the secondary veins and then another set of parallel fibers emanating from the secondary fibers mimicking the tertiary veins of a typical leaf. The tertiary fibers do not interconnect the secondary fibers in our present study. We carry out finite element (FE) based computational investigation of the mechanical properties such as Young’s moduli, Poisson’s ratio and yield stress under uniaxial loading of the resultant composite structures and study the effect of different fiber architectures. To this end, we use two broad types of architectures both having similar central main fiber but differing in either having only secondary fibers or additional tertiary fibers. The fiber and matrix volume fractions are kept constant and a comparative parametric study is carried out by varying the inclination of the secondary fibers. We find significant effect of fiber inclination on the overall mechanical properties of the composites with higher fiber angles transitioning the composite increasingly into a matrix-dominated response. We also find that in general, composites with only secondary fibers are stiffer with closed cell architecture of the secondary fibers. The closed cell architecture also arrested the yield stress decrease and Poisson’s ratio increase at higher fiber angles thereby mitigating the transition into the matrix dominated mode. The addition of tertiary fibers also had a pronounced effect in arresting this transition into the matrix dominated mode. However, it was found that indiscriminate addition of tertiary fibers may not provide desired additional stiffness for fixed volume fraction of constituents. In conclusion, introducing a leaf-mimicking topology in fiber architecture can provide significant additional degrees of tunability in design of these composite structures.

Intercondylar notch impingement is detrimental to the anterior cruciate ligament (ACL). Notchplasty is a preventative remodeling procedure performed on the intercondylar notch during ACL reconstruction (ACLR). This study investigates how ACL graft geometry and both tibial and femoral insertion site location affect ACL-intercondylar notch interactions post ACLR. A range of ACL graft sizes are reported during ACLR, from 6mm–11mm in diameter. Minor variability of up to 3mm in ACL insertion site locations is reported during ACLR. Several 3D finite element (FE) knee joint models were constructed using three ACL graft sizes and polar arrays of tibial and femoral insertion site locations. Each knee model was subjected to flexion, tibial external rotation, and valgus motion. Impingement force and contact area between the ACL and the intercondylar notch compared well with published cadaver study results. A 3mm shift in the antero-lateral direction of the tibial insertion site of the average and maximum size ACL increased impingement force by 155.4% and 242.9% respectively. A 3mm shift in the anterior-proximal direction of the femoral insertion site of the average and maximum size ACL increased impingement by 292.6%, and 346.2% respectively. Simulated notchplasties of 4mm and 5mm reduced graft impingement force by 89.4% and 100% respectively for the simulations with greatest impingement. For the kinematics applied, the results show that small differences in graft size and insertion site location may lead to large increases in impingement force and contact area. The study aims to improve ACLR success rates by understanding how minor variations in graft size and insertion site location affect intercondylar notch impingement. Because minor variations in insertion site location during ACLR are a known occurrence, the results of this study may support the argument for performing notchplasty during ACLR.Copyright © 2016 by ASME

Origami structures have gained tremendous attention due to their extreme kinematic performance. However, typical origami structures suffer from poor load-bearing capacity due to extreme slenderness of facets. In this letter, we introduce a technique to design an origami-equivalent compliant mechanism which preserves the origami kinetics and kinematics while offering higher load-bearing capacity compared to the original origami structure. In this technique, we offer an energy equivalence principle between the origami and the compliant mechanism. We validate the principle using experimental investigation for a square-twist origami pattern. This principle thus opens up a significant avenue for designing deployable and programmable structures.

This paper introduces the date palm leaf fiber (DPLF) as a possible natural fiber candidate to the automotive applications. An extensive experimental program has been carried out to examine the energy absorption capability of rectangular tubes made of DPLF. The test rig has been designed to characterize different types of date palm leaf species. The species with the highest mechanical properties has been employed as a reinforcement to make the tested composite rectangular tubes. The wet wrapping manufacturing technique has been used to fabricate the composite structure. The fabricated tubes have been tested axially and laterally to examine the effect of loading directions into their crushing behavior and energy absorption capability. The crushed morphology, fracture surfaces, and sections through the specimen were studied by means of visual, optical microscopy and scanning electron microscopy (SEM). The results showed that the axially tested tubes have higher energy-absorption capability compared to the laterally crushed tubes. - 2019 The authors would like to acknowledge the financial support of the Qatar National Research Fund (a part of the Qatar Foundation) through the National Priorities Research Program NPRP 05-068-2-024. Appendix A Scopus

We demonstrated the feasibility of harvesting mechanical energy through the proper design and installation of a lattice structure which undergoes snap-through deformation under applied mechanical loading. First, the theoretical formulations for both symmetric and asymmetric modes of the snap-through deformation in a 2D lattice structure were derived. Then, experiments were conducted on the prototype to measure the energy harvesting ability at different frequencies and to investigate the capability of charging a capacitor connected to the lattice prototype. Finally, the effects of the defect in the lattice on energy harvesting were discussed. Our results showed that the average generated voltage across a 25 kΩ resistor increased by increasing the frequency of loading. However, energy stored in a capacitor was independent of loading frequency. For a defective structure with a fixed vertex, the generated voltage is lower yet increasing with the frequency of loading. The designed structure is robust and provides sustainable energy output under cyclic loading even with the presence of defects and imperfections.We demonstrated the feasibility of harvesting mechanical energy through the proper design and installation of a lattice structure which undergoes snap-through deformation under applied mechanical loading. First, the theoretical formulations for both symmetric and asymmetric modes of the snap-through deformation in a 2D lattice structure were derived. Then, experiments were conducted on the prototype to measure the energy harvesting ability at different frequencies and to investigate the capability of charging a capacitor connected to the lattice prototype. Finally, the effects of the defect in the lattice on energy harvesting were discussed. Our results showed that the average generated voltage across a 25 kΩ resistor increased by increasing the frequency of loading. However, energy stored in a capacitor was independent of loading frequency. For a defective structure with a fixed vertex, the generated voltage is lower yet increasing with the frequency of loading. The designed structure is robust and provi...

We present a novel method for fabricating carbon fiber composite sandwich panels with lattice core construction by means of electrical discharge machining (EDM). First, flat-top corrugated carbon fiber composite cores were fabricated by a hot press molding method. Then, two composite face sheets were bonded to each corrugated core to create precursor sandwich panels. These panels were transformed into sandwich panels with near-pyramidal truss cores by EDM plunge-cutting the corrugated core between the face sheets with a shaped cuprite electrode. The flat top corrugation permits adhesive to be applied consistently, and the selected dimensions leave a substantial bond area after cutting, resulting in a strong core-to-sheet bond. The crushing behavior of this novel construction was investigated in flatwise compression, and the results were compared to analytical expressions for strength and stiffness.

Elastic liners are used for in situ repair and retrofitting of pipes as a cost effective alternative to the replacement of damaged parts and sections. In this paper, we studied the role of an elastic liner on the buckling behavior of a cracked cylindrical shell using finite element method. A special meshing scheme that could mimic the stress singularity at the crack tip was employed to model the cracked shells. Linear eigenvalue analysis was carried out to study the effect of crack geometry (length and orientation) as well as the material properties and thickness of the elastic liner on the buckling load and buckling shape of the cylindrical shell. We considered a combination of axial compression and internal pressure which is a typical loading for pipelines and pressurized liquid-retaining structures. Our results show that cracked shell's strength and mode of buckling for different crack length and orientations can be largely influenced by thickness and relative stiffness of the liner layer. In particular we report a gradual transition from local to global instability due to these size and orientation effects. Finally, the role of internal pressure on structural stability and local buckling of cracked shells, which strongly depends on the crack orientation and liner thickness, is discussed.

In this paper, a CT based structural rigidity analysis (CTRA) method that incorporates bone intrinsic local curvature is introduced to assess the compressive failure load of human femur with simulated lytic defects. The proposed CTRA is based on a three dimensional curved beam theory to obtain critical stresses within the human femur model. To test the proposed method, ten human cadaveric femurs with and without simulated defects were mechanically tested under axial compression to failure. Quantitative computed tomography images were acquired from the samples, and CTRA and finite element analysis were performed to obtain the failure load as well as rigidities in both straight and curved cross sections. Experimental results were compared to the results obtained from FEA and CTRA. The failure loads predicated by curved beam CTRA and FEA are in agreement with experimental results. The results also show that the proposed method is an efficient and reliable method to find both the location and magnitude of failure load. Moreover, the results show that the proposed curved CTRA outperforms the regular straight beam CTRA, which ignores the bone intrinsic curvature and can be used as a useful tool in clinical practices. Beth Israel Deaconess Medical Center Department of Orthopaedic Surgery, the NIH LRP (L30 AR056606) (A.N.), and in part by a NPRP award (NPRP 5-086-2- 031) from the Qatar National Research Fund (a member of the Qatar Foundation)

International audience; Pressurized elastic capsules arise at scales ranging from the 10 m diameter pressure vessels used to store propane at oil refineries to the microscopic polymeric capsules that may be used in drug delivery. Nature also makes extensive use of pressurized elastic capsules: plant cells, bacteria and fungi have stiff walls, which are subject to an internal turgor pressure. Here, we present theoretical, numerical and experimental investigations of the indentation of a linearly elastic shell subject to a constant internal pressure. We show that, unlike unpressurized shells, the relationship between force and displacement demonstrates two linear regimes. We determine analytical expressions for the effective stiffness in each of these regimes in terms of the material properties of the shell and the pressure difference. As a consequence, a single indentation experiment over a range of displacements may be used as a simple assay to determine both the internal pressure and elastic properties of capsules. Our results are relevant for determining the internal pressure in bacterial, fungal or plant cells. As an illustration of this, we apply our results to recent measurements of the stiffness of baker's yeast and infer from these experiments that the internal osmotic pressure of yeast cells may be regulated in response to changes in the osmotic pressure of the external medium.

International audience; Thin shells are found in nature at scales ranging from viruses to hens' eggs; the stiffness of such shells is essential for their function. We present the results of numerical simulations and theoretical analyses for the indentation of ellipsoidal and cylindrical elastic shells, considering both pressurized and unpressurized shells. We provide a theoretical foundation for the experimental findings of Lazarus et al. [following paper, Phys. Rev. Lett. 109, 144301 (2012)] and for previous work inferring the turgor pressure of bacteria from measurements of their indentation stiffness; we also identify a new regime at large indentation. We show that the indentation stiffness of convex shells is dominated by either the mean or Gaussian curvature of the shell depending on the pressurization and indentation depth. Our results reveal how geometry rules the rigidity of shells.

BACKGROUNDThe purpose of this biomechanical study was to determine the effect of shortened clavicle malunion on the center of rotation of the glenohumeral (GH) joint, and the capacity of repair to restore baseline kinematics.METHODSSix shoulders underwent automated abduction (ABD) and abbreviated throwing motion (ATM) using a 7-DoF automated upper extremity testing system in combination with an infrared motion capture system to measure the center of rotation of the GH joint. ATM was defined as pure lateral abduction and late cocking phase to the end of acceleration. Torsos with intact clavicle underwent testing to establish baseline kinematics. Then, the clavicles were subjected to midshaft fracture followed by kinematics testing. The fractured clavicles underwent repairs first by clavicle length restoration with plate fixation, and then by wiring of fragments with a 2-cm overlap to simulate shortened malunion. Kinematic testing was conducted after each repair technique. Center of rotation of the GH joint was plotted across all axes to outline 3D motion trajectory and area under the curve.RESULTSThroughout ABD, malunion resulted in increased posterior and superior translation compared to baseline. Plate fixation restored posterior and superior translations at lower abduction angles but resulted in excess anterior and inferior translation at overhead angles. Throughout ATM, all conditions were significantly anterior and superior to baseline. Translation with malunion was situated anterior to the fractured and ORIF conditions at lower angles of external rotation. Plate fixation did not restore baseline anteroposterior or superoinferior translation at any angle measured.CONCLUSIONSThis study illustrates the complex interplay of the clavicle and the GH joint. While abnormal clavicle alignment alters shoulder motion, restoration of clavicle length does not necessarily restore GH kinematics to baseline. Rehabilitation of the injured shoulder must address the osseous injury and the dynamic forces of the shoulder girdle.

Background The rotator interval (RI) has been exploited as a potentially benign point of entry into the glenohumeral (GH) joint. Bounded by the supraspinatus, subscapularis and coracoid process of the scapula, the RI is believed to be important in the shoulder’s soft tissue balancing and function. However, the role of the RI in shoulder kinematics is not fully understood. The purpose of this study is to describe the effect of the RI on GH motion during abduction of the arm. Methods Six shoulders from three cadaveric torsos were studied to assess the impact of changes in the RI during abduction under four conditions: Intact (Baseline), Opened, Repaired (repaired with side-to-side tissue approximation, no overlap) and Tightened (repaired with 1 cm overlap). For each group, the GH translation and area under the Curve (AUC) were measured during abduction using an intact cadaveric shoulder (intact torso). Results GH kinematics varied in response to each intervention and throughout the entire abduction arc. Opening the RI caused a significant change in GH translation. The Repair and Tightened groups behaved similarly along all axes of GH motion. Conclusions The RI is central to normal GH kinematics. Any insult to the tissue’s integrity alters the shoulder’s motion throughout abduction. In this model, closing the RI side-to-side has the same effect as tightening the RI. Since suture closure may offer the same benefit as tightening the RI, clinicians should consider this effect when treating patients with shoulder laxity. This investigation provides an improved perspective on the role of the RI on GH kinematics during abduction. When managing shoulder pathology, surgeons should consider how these different methods of RI closure affect the joint’s motion. In different circumstances, the surgical approach to the RI can be tailored to address each patient’s specific needs.

This paper presents a novel method for rapid fabrication and prototyping of low-density carbon fiber sandwich panel cores based on laser beam cutting (LBC). Using LCB, sandwich panels with lattice core constructions with oblique and vertical strut morphologies were fabricated from two fiber-orientation architectures. Scanning electron microscopy images illustrate the relatively small extent of damage from laser cutting. The shear strength of the lattice cores was improved by eliminating core-to-face sheet bond failures. Crushing and shear responses of fabricated truss cores were measured, and analytical models are presented to predict the stiffness, strength and dominant failure modes under each loading condition. The sandwich-panel cores investigated appear to be promising candidates for lightweight systems and multifunctional applications.

Linear eigenvalue buckling analysis was carried out for singly and doubly cracked cylindrical thin shells under axial compression using the finite element method. First, the effect of crack size and orientation on the buckling behavior of an axially loaded shell with a single crack was studied. Then, the buckling behavior of a cylinder with two parallel longitudinal cracks was investigated. Two different buckling shapes with cross-sectional deformation profiles that resemble letters M (symmetric) and N (anti-symmetric) were identified as the first buckling modes of the cylinder. The exchange between these local buckling modes due to variation of crack size and spacing was illustrated. The transition between these two buckling shapes can be used to estimate the ‘maximum interaction distance’ of the cylinder cracks—the separation distance beyond which the two cracks do not interact in affecting the buckling load of the cylindrical shell. The influence of shell thickness and crack length on the maximum interaction distance was quantified for cylinders with two co-centered (i.e., parallel offset) or collinear longitudinal cracks. Additional simulations were carried out for cylinders with multiple symmetrically spaced longitudinal cracks to show how the behavior of single and double cracks can give the buckling load and mode shape of cylinders with multiple cracks.

The process of adaptive bone remodeling can be simulated with a self-optimizing finite element method. The basic remodeling rules attempt to obtain a constant value for the strain energy per unit bone mass, by adapting density. The precise solution is dependent on the loads, initial conditions, and the parameters of the remodeling rule. While there are several investigations on developing algorithms leading to the bone density distribution in the proximal femur, these algorithms often require a large number of iterations. The aim of this study was to develop a more efficient adaptive bone remodeling algorithm, and to identify how the bone density distribution of the proximal femur was affected by parameters that govern the remodeling process. The forces at different phases of the gait cycle were applied as boundary conditions. The bone density distributions from these forces were averaged to estimate the density distribution in the proximal femur. The effect of varying the initial bone density, spatial influence function, non-linear order of the adaptive algorithm, and the influence range on the converged solution were investigated. The proposed procedure was shown to converge in a fewer number of iterations and requiring less computational time, while still generating a realistic bone density distribution. It was also shown that varying the identified parameters within reasonable upper and lower bounds had very little impact on the qualitative form of the converged solution. In contrast, the convergence rate was affected to a greater degree by variation of these parameters. In all cases, the solutions obtained are comparable with the actual density in the proximal femur, as measured by Dual-energy X-ray absorptiometry (DEXA) scans.

A series of analytical relationships is presented to predict the mechanical properties and response of open three-dimensional Voronoi tessellation of face-centered cubic structures called rhombic dodecahedrons. The cell edge material was assumed to be elastic–perfectly plastic, and the effective mechanical properties of the cellular structure were related to the cell edge material properties and the relative density of the cellular structure. Detailed finite element models were carried out to establish the validity of the analytical models. In the elastic regime, the monodisperse cellular structure is orthotropic and near-incompressible in all loading directions, and its response is governed by bending deformation of the cell edges. The yield strength of the cellular structure in all loading directions is equal. We also studied the role of irregularity in the organization of the cellular structure on its mechanical properties. The irregularity in the cellular structure organization was introduced by moving the vertices of a regular cellular structure in three orthogonal directions by a random value within a predefined range called the “irregularity index”. At a constant overall relative density, increasing the level of irregularity increases the effective elastic modulus and significantly decreases the effective yield strength of the cellular structure. We also studied the mechanical properties of the cellular structure tied to rigid plates, in view of the application of cellular structure as the core construction of sandwich panels. In this case, the cellular structure is significantly stiffer and its mechanical response is dominated by cell wall stretching.

Structural performance in direct (pure) shear and three-point bending was investigated for sandwich panels with a carbon fiber pyramidal truss core. Analytical estimates for sandwich panel strength for each loading condition were presented for possible competing failure modes. In the experimental part of the study, pyramidal truss cores were made using the hot press molding technique and then attached to flat carbon fiber composite face sheets to build all-composite sandwich panels. Panels with different configurations (e.g., core relative density and face sheet thickness) were tested to probe different failure modes and investigate the mechanical properties. In general, measured failure loads showed good agreement with the analytical predictions. Failure mechanism maps illustrate the controlling failure mechanisms in various regions of parameter space.

Quasi-static uniform compression tests and low-velocity concentrated impact tests were conducted to reveal the failure mechanisms and energy absorption capacity of two-layer carbon fiber composite sandwich panels with pyramidal truss cores. Three different volume-fraction cores (i.e., with different relative densities) were fabricated: 1.25%, 1.81%, and 2.27%. Two-layer sandwich panels with identical volume-fraction cores (either 1.25% or 2.27%), and also stepwise graded panels consisting of one light and one heavy core, were investigated under uniform quasi-static compression. Under quasi-static compression, load peaks were identified with complete failure of individual truss layers due to strut buckling or strut crushing, and specific energy absorption was estimated for different core configurations. In the impact test, the damage resulting from low-velocity concentrated impact was investigated. Our results show that compared with glass fiber woven textile truss cores, two-layer carbon fiber composite pyramidal truss cores have comparable specific energy absorptions, and thus could be used in the development of novel light-weight multifunctional structures.

A series of experiments and detailed finite element simulations were carried out to study the performance of single lap joints with non-flat interfaces loaded at a small angle to the mean bond plane. In the experimental part, fiber reinforced epoxy composite adherends with sinusoidal bonding surfaces (but no change to the exterior sample dimensions) were fabricated in a purpose-built mold. This construction method allowed the fibers to follow the sinusoidal profile for maximum strength. Then the sinusoidal surfaces were joined by a controlled-thickness layer of epoxy. Single lap joints with a flat interface were also fabricated using the same composite material and with the same overall shape. The experiments showed that the interface non-flatness has significant effect on the mechanical behavior and strength of the bonded joints. Finite element simulations of a simplified two-dimensional model with isotropic adherends were also carried out to estimate the distribution of shear and peeling stresses along the bonded joints, and the results were related to the experimental observations. Parametric variations were studied via FEA to highlight the role of interface shape, adhesive elastic modulus and a central void size on the distribution of stresses, and inherently the overall strength and behavior of the bonded joints.

Most conventional materials expand in transverse directions when they are compressed uniaxially resulting in the familiar positive Poisson’s ratio. Here we develop a new class of two dimensional (2D) metamaterials with negative Poisson’s ratio that contract in transverse directions under uniaxial compressive loads leading to auxeticity. This is achieved through mechanical instabilities (i.e., buckling) introduced by structural hierarchy and retained over a wide range of applied compression. This unusual behavior is demonstrated experimentally and analyzed computationally. The work provides new insights into the role of structural organization and hierarchy in designing 2D auxetic metamaterials, and new opportunities for developing energy absorbing materials, tunable membrane filters, and acoustic dampeners. NPRP award [NPRP 7-882-2-326] from the Qatar National Research Fund (a member of the Qatar Foundation).

A simple and effective in situ method for strengthening or healing thin-wall structures is presented. In this method, a liquid-state gap-sealing foam is injected within the enclosed spaces of a structure. After injection it expands to fill and pressurize the cavities, then solidifies in few hours. The stiff pressurized foam enhances load carrying capacity both by supporting part of the load, and by retarding the buckling of thin-wall structural components. A simple demonstration of the proposed technique is provided by load-testing thin-wall beverage cans, and also both intact and damaged aluminum honeycomb, filled with commercially available gap-sealing polyurethane foam. By adding foam, the structures' peak load and energy absorption were significantly enhanced. The injected foam partially restored the original undeformed shape during unloading, highlighting the potential advantage to apply this method for multiple-use energy absorbing components. Qatar National Research Foundation (QNRF) Award Number NPRP 09-145-2-061 Scopus

Wood plastic composite (WPC) specimens were fabricated in shapes relevant to flute (musical instrument) production using African blackwood powder and phenolic resin in a hot compression molding setup. The roles of composition and processing parameters on the mechanical properties (flexural strength, elastic modulus, and impact failure energy) were systematically investigated. Cracks were observed in composites with more than 70% wood particles due to the formation of gas in the system during manufacturing, and lack of fluidity in the system to flush out entrapped air and the produced gases. Based on these results, the optimum temperature, pressure, and wood volume fraction for manufacturing WPC in the form of a flute is developed. A further series of experimental procedures were performed to improve the mechanical properties of WPC samples by studying the addition of short glass fibers to the molding compound prior to hot pressing. The results showed that the addition of short fiber did not improve the strength of WPC but rather reduced its strength compared to unreinforced composite. This was attributed to lack of bonding between the short fibers and composite matrix.

We studied the behavior and residual structural capacity of reinforced concrete chimneys subjected to an uncontrolled fire. We used a combination of a heat transfer finite element model–to obtain the temporal distributions of temperature during the fire event–and the structural model of concrete chimney design provided by the American Concrete Institute (ACI 307–08). This approach allows estimating the reduction in the vertical (axial) strength and moment strength of the chimney both during a fire and post-fire, and gives a direct estimate of the reduction in the safety factors of the concrete chimney. Using this method, we examined the impact of various design parameters on the residual structural capacity of a concrete chimney subjected to an internal fire. An iterative finite element method was also presented as an alternative to the ACI 307 calculations. Moreover, finite element calculations were used to study the role of thermal stresses on the axial strength of the chimney during fire. Our study provides insight into possible failure mechanisms of concrete chimneys damaged due to fire and could suggest possible approaches for minimizing the risk of chimney failure due to an uncontrolled fire.

A series of analytical and experimental investigations is presented to study the response and failure of pyramidal truss core sandwich panels made of carbon fiber composite under axial compression. In the analytical part of the study, three failure modes: (i) Euler or core shear macro-buckling, (ii) face wrinkling, and (iii) face sheet crushing, were considered and theoretical relationships for predicting the failure load associated with each mode were presented. In the experimental part of the study, three different sets of specimens were manufactured to probe the three failure modes mentioned above. Pyramidal truss cores were fabricated using a hot-press molding technique and were bonded to composite face sheets made of fiber reinforced composite. The response of the sandwich panels under axial compression was measured up to failure. The measured peak loads obtained in the experiments showed good agreement with the analytical predictions. The experiments also provide insight into the post-failure response of the sandwich panels.

The in-plane dynamic crushing of two dimensional honeycombs with both regular hexagonal and irregular arrangements was investigated using detailed finite element models. The energy absorption of honeycombs made of a linear elastic-perfectly plastic material with constant and functionally graded density were estimated up to large crushing strains. Our numerical simulations showed three distinct crushing modes for honeycombs with a constant relative density: quasi-static, transition and dynamic. Moreover, irregular cellular structures showed to have energy absorption similar to their counterpart regular honeycombs of same relative density and mass. To study the dynamic crushing of functionally graded cellular structures, a density gradient in the direction of crushing was introduced in the computational models by a gradual change of the cell wall thickness. Decreasing the relative density in the direction of crushing was shown to enhance the energy absorption of honeycombs at early stages of crushing. The study provides new insight into the behavior of engineered and biological cellular materials, and could be used to develop novel energy absorbent structures.

Instability of a thin film attached to a compliant substrate often leads to emergence of exquisite wrinkle patterns with length scales that depend on the system geometry and applied stresses. These patterns have potential applications in many areas including tissue engineering, flexible electronics and semiconductor industry. In this talk, we will present several novel techniques for polymer surface engineering, including creation of controlled hierarchical patterns and wrinkles with high amplitude/wavelength aspect ratio. The applications of these techniques in microfluidics, optics, and also in development of biomimetic surfaces will be discussed.

A new method for fabricating carbon fiber composite pyramidal truss cores was developed based on the molding hot-press technique. In this method, all the continuous fibers of composite are aligned in the direction of struts and thus, the truss structure can fully exploit the intrinsic strength of the fiber reinforced composite. The microstructure and organizations of fibers of fabricated composite structures were examined using scanning electron microscope. The crushing response of the truss cores was also investigated and the corresponding failure modes were studied and complemented with an analytic model of the core crushing response. Our results show that the fabricated low-density truss cores have superior compressive strength and thus, could be used in development of novel lightweight multifunctional structures.

The residual compressive stresses induced by the autofrettage process in a metal vessel are limited by metal plasticity. Here we showed that the autofrettage of layered metal–ceramic composite vessels leads to considerably higher residual compressive stresses compared to the counterpart metal vessel. To calculate the residual stresses in a composite vessel, an extension of the Variable Material Properties (X-VMP) method for materials with varying elastic and plastic properties was employed. We also investigated the autofrettage of composite vessels made of functionally graded material (FGM). The significant advantage of this configuration is in avoiding the negative effects of abrupt changes in material properties in a layered vessel – and thus, inherently, in the stress and strain distributions induced by the autofrettage process. A parametric study was carried out to obtain near-optimized distribution of ceramic particles through the vessel thickness that results in maximum residual stresses in an autofrettaged functionally graded composite vessel. Selected finite element results were also presented to establish the validity of the X-VMP method.

Subject-specific three-dimensional finite element models of the knee joint were created and used to study the effect of the frontal plane tibiofemoral angle on the stress and strain distribution in the knee cartilage during the stance phase of the gait cycle. Knee models of three subjects with different tibiofemoral angle and body weight were created based on magnetic resonance imaging of the knee. Loading and boundary conditions were determined from motion analysis and force platform data, in conjunction with the muscle-force reduction method. During the stance phase of walking, all subjects exhibited a valgus-varus-valgus knee moment pattern with the maximum compressive load and varus knee moment occurring at approximately 25% of the stance phase of the gait cycle. Our results demonstrated that the subject with varus alignment had the largest stresses at the medial compartment of the knee compared to the subjects with normal alignment and valgus alignment, suggesting that this subject might be most susceptible to developing medial compartment osteoarthritis (OA). In addition, the magnitude of stress and strain on the lateral cartilage of the subject with valgus alignment were found to be larger compared to subjects with normal alignment and varus alignment, suggesting that this subject might be most susceptible to developing lateral compartment knee OA.

Slope stability is a critical safety and production issue for mining. Major wall failure can occur seemingly without any visual warning, causing loss of lives, damage to equipment, and disruption to the mining process. Monitoring systems, ranging from simple piezometers and extensometers to highly sophisticated radars and global navigation satellite systems, are employed to predict impending instabilities and failure. Here, we provide a review of the available monitoring systems used in slope management and highlight their major advantages and shortcomings. We propose a simple method for evaluating the effectiveness and reliability of monitoring systems to warn of pending slope failures. The method is based on constructing monitoring reliability maps for the slope by evaluating two slope parameters: Expected deformation to failure and critical reading frequency, which depend on the slope characteristics (e.g., geology and design), service condition (e.g., rainfall, blast), and the economic impact of the failure. The reliability of a deformation monitoring system can be subsequently assessed by identifying three parameters of the system: Coverage area (large or discrete), Deformation monitoring precision, and Measurement frequency. The application of the method to most commonly used deformation monitoring systems is demonstrated. The advantages and implications of the proposed method are highlighted.