Simultaneous Increase Of Stiffness Research Articles

Similar biomechanical properties of four tripled tendon graft models for ACL reconstruction

PurposeThe present study tested and compared the biomechanical properties of four different triplicate graft tendon techniques.Methods32 tripled tendons from the common extensor muscle of bovine fingers were tested on a material testing machine, passing the end loop over a metal rod of a clevis connected to the load cell on the upper side, and fixing the lower end to a clamp. The samples were divided into four groups: (A) tripled with a free end sutured only to one of the two fixed bundles (B) tripled with a free end positioned between the two fixed strands and sutured to both (C) tripled with an S-shape and all the three strands sutured together at the upper and lower extremities of the graft (D) partially quadrupled with the free end sutured together with the other three bundles at the upper extremity. Each sample was pretensioned at 50 N for 10 min and then subjected to 1000 load control cycles between 50 and 250 N. Finally, each sample was subjected to a load to failure test.Authors also present some preliminary results on the feasibility of a non-contact and full-field Thermoelastic Stress Analysis technique, based on Infrared Thermography, to evaluate the level of stress on the whole graft, and hence on each strand, during fatigue loading.ResultsEighty five percent of the samples failed at the level of the clamp. The cyclical elongation progressively decreased in all the samples and there was a simultaneous increase in stiffness. An increased stiffness was noted between Group 2 vs Group 3 and Group 2 vs Group 4 at the 500th and 1000th cycle. The failure loads were as follows: (a) 569.10 N, (b) 632.28 N, (c) 571.68 N, (d) 616.95 N. None of the parameters showed a statistically significant difference between the four groups.ConclusionThis study reported similar biomechanical behavior of four different models of tripled grafts suitable for ACL reconstruction. In addition, the biomechanics of overall tripled tendon grafts seems more affected by the viscoelastic property of the tendon itself rather than the preparation method.

Selective Laser Sintering of Lignin-Based Composites

Lignin is introduced as a suitable component for selective laser sintering (SLS) of polyamide (PA12) to reduce costs while maintaining or improving processability and performance. Alkali lignin (sourced as a polydisperse, amorphous powder) was used at a volume concentration of up to 60 vol % for three-dimensional (3D) printing of complex, layered structures. The latter were obtained as high axial aspect objects, produced in flat, flipped (90°), and vertical directions, which were further examined to elucidate the effect of lignin as a suitable component in SLS. The composite withstood heating during SLS, and sintered PA/lignin showed 30% less degradation at elevated temperatures compared to pure PA. The morphological, wetting, mechanical, and thermal characteristics associated with the 3D-printed structures were compared. For instance, the strength and wettability were highly dependent on processing orientation. Compared to objects produced from neat PA, those that included lignin presented a higher porosity (∼10%) with a simultaneous increase in stiffness (increased Young modulus, by ∼16%, and reduced tensile strength, by ∼7%). Owing to differences in surface roughness and composition, an important difference in the water contact angle (CA) of the samples printed in the flipped and flat orientations was observed (55 and 126°, respectively). Overall, SLS is shown as a developmental step toward lignin valorization in composites while allowing reduced cost, scalability, and facile processing.

Flexible Bionanocomposites from Epoxidized Hemp Seed Oil Thermosetting Resin Reinforced with Halloysite Nanotubes

Hemp seed (Cannabis sativa L.) oil comprises a variety of beneficial unsaturated triglycerides with well-documented nutritional and health benefits. However, it can become rancid over a relatively short time period, leading to increased industrial costs and waste of a valuable product. The development of sustainable polymers is presented as a strategy, where both the presence of unsaturation and peroxide content could be effectively used to alleviate both the waste and financial burden. After the reaction with peroxyacetic acid, the incorporation of halloysite nanotubes (HNTs), and the subsequent thermal curing, without the need for organic solvents or interfacial modifiers, flexible transparent materials with a low glass-transition temperature were developed. The improvement in the thermal stability and both the static and dynamic mechanical properties of the bionanocomposites were significantly enhanced with the well-dispersed HNT filler. At an optimum concentration of 0.5 wt % HNTs, a simultaneous increase in stiffness, strength, ductility, and toughness was observed in comparison to the unfilled cured resin. These sustainable food-waste-derived bionanocomposites may provide an interesting alternative to petroleum-based materials, particularly for low-load-bearing applications, such as packaging.

A proposed approach to mitigate the torsional amplifications of asymmetric base-isolated buildings during earthquakes

In this paper, the rotational behavior of asymmetric base-isolated buildings is compared with the similar asymmetric fixed-base buildings. A vast range of lead rubber bearings with different periods is considered to evaluate the effect of the isolation degree on seismic responses of the structures. The simulation results confirm that the base isolators are able to reduce rotation of stories. However, this reduction is negligible in large eccentricities. The numerical simulations show that increasing period of isolators results in large displacement at bearings located on the flexible edge of the isolation system. This paper proposes practical solutions to reduce torsional responses of the base-isolated structures. In order to investigate the effectiveness of the proposed solutions, four structural models are defined. Considering three earthquake excitations, the simulation results of these models indicate that increase in stiffness of flexible edge of isolation system can reduce torsional responses of asymmetric 3-story base isolated structure. Furthermore, the simultaneous increase in stiffness of flexible edge of isolation system and superstructure lead to a suitable reduction of torsional responses in asymmetric 8-story base-isolated structure.

Aligned bioinspired cellulose nanocrystal-based nanocomposites with synergetic mechanical properties and improved hygromechanical performance.

Natural high-performance materials inspire the pursuit of ordered hard/soft nanocomposite structures at high fractions of reinforcements and with balanced supramolecular interactions. Such biomimetic design principles remain difficult to realize for bulk nanocomposites. Herein, we establish an effective drawing procedure that induces a high orientation of crystalline cellulose nanocrystals (CNCs) in a matrix of carboxymethylcellulose (CMC) at high level of reinforcements (50 vol %). We show such alignment in rather thick bulk films and report synergetic improvement with a simultaneous increase of stiffness, strength, and work-to-fracture as a function of the degree of alignment. Scanning electron microscopy and two-dimensional X-ray diffraction quantify the alignment of the cylindrical nanoparticles and link it to the extent of drawing and improvements in mechanical properties. We further show that the decline in mechanical properties of such waterborne all biobased nanocomposites at high relative humidity can be balanced using supramolecular modulation of the ionic interactions by exchanging the monovalent Na(+) counterion, present in CMC and CNC with di- or trivalent Cu(2+) and Fe(3+). This contribution demonstrates the importance of aligning one-dimensional reinforcements to achieve synergetic improvement in mechanical properties in sustainable bioinspired nanocomposites and suggests pathways to prepare water-stable materials based on a waterborne processing route.

Innovative direct nanoparticle dispersion injection into injection molding processing

ABSTRACTNanoparticles are applied in polymer matrices because of building up an inorganic network within an organic network resulting in a simultaneous increase of stiffness and toughness provided that a good degree of dispersion with a very low amount of agglomerates can be realized. To prevent these kinds of agglomerates, the incorporation of appropriate nanoparticle dispersions is promising. Using nanoparticle containing dispersions, we can produce nanocomposites via a direct injection molding process with excellent nanoparticle dispersion in the resulting nanocomposite. TiO2‐ (15 nm and 300 nm) and Al2O3‐ (13 nm) particle containing dispersions were manufactured and incorporated in polyamide 6 (PA6) via the addition of a nanoparticle containing dispersion within an injection molding process (IM). Agglomeration has to be suppressed during injection molding process and the nanoparticles have to cross over from the dispersing agent to the polymer matrix melt in which they have to be well distributed. The resulting mechanical (tensile and Charpy impact) and morphological (SEM and TEM) properties of those nanocomposites are discussed. The impact toughness and Young's modulus could be enhanced with slightly decreasing tensile strength. The nanoparticles could be well distributed in the polymer matrix. The achievable distribution quality is equal to the distribution achieved in a melt kneading process. Thus, materials with enhanced elongation at break with a very good dispersion quality were produced in a single production process. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40641.

The strengthening effect caused by an elastic contrast—part II: stratification by a thin stiff layer

To quantify the gain in strength of a layered heterogeneous structure caused by the elastic contrast between the layers, especially if no crack deflection is observed at the interface, two original mechanisms baptized “step-over” and “jump-through” were proposed in Part I. They addressed the ability of a crack to pass through an interface and were applied to a bimaterial structure; whereas part II is dedicated to a homogeneous beam embedding a thin stiff film. The asymptotic expansions differ significantly since the small parameter is now the layer thickness. Unlike the first part where it was difficult to evidence the gain in toughness due to the superposition of two effects: a simultaneous increase in stiffness and in toughness, here it is possible to characterize the strength enhancement in using a single parameter. A discussion of the possibility to repeat the effect by multiplying the thin films is presented at the end.

Poly(trimethylene terephthalate)/silica nanocomposites prepared by dual in situ polymerization: synthesis, morphology, crystallization behavior and mechanical properties

AbstractThe sol–gel technique has provided promising opportunities for the preparation of polymer/inorganic hybrid materials at the molecular level, which ensures the inorganic particles are well dispersed in the organic matrix. In this work, poly(trimethylene terephthalate) (PTT)/silica nanocomposites were fabricated via the sol–gel technique and in situ polymerization. Fourier transform infrared and nuclear magnetic resonance analyses confirmed that some PTT molecular chains were grafted to the surface of silica. Unlike pure PTT, the grafted PTT was insoluble in a mixed solvent of chloroform and hexafluoro‐2‐propanol. Both transmission electron microscopy and scanning electron microscopy showed that the silica particles, with a size of 40–50 nm, were homogeneously dispersed in the PTT matrix with no preferential accumulation in any region. Differential scanning calorimetry revealed that the glass transition temperature and cold‐crystallization peak of the composites gradually increased with increasing silica loading. A simultaneous increase of stiffness and toughness was observed for the nanocomposites. Moreover, polarized optical microscopy showed that the nanocomposites exhibited interesting banded spherulites, which were different from banded spherulites of PTT previously reported. The nanocomposites with covalent bonding between silica and PTT gave a series of interesting results, such as elevated glass transition temperature, simultaneous increase of stiffness and toughness as well as surprising banded spherulites. Copyright © 2009 Society of Chemical Industry

Effect of sub-micron silica fillers on the mechanical performances of epoxy-based composites

Solid state thermo-mechanical properties, as well as low and large strain mechanical behaviour, of epoxy composites filled with sub-micron pyrogenic silica are discussed in this paper. The reinforcement mechanisms involved are investigated. Two distinct series of pyrogenic silica were used: hydrophilic silica with various specific surface areas and silica grafted with various organo-modifications. Furthermore, two series of networks, having either a high or low crosslink density, and resulting thus either in glassy or rubbery materials at room temperature, were considered. Dynamic mechanical analysis, uniaxial tensile tests and fracture mechanic tests were performed. All our results showed that pyrogenic silica leads to an improvement of network mechanical properties both in the glassy and rubbery states. The simultaneous increase of stiffness and toughness was observed, demonstrating the great potential of pyrogenic silica for the reinforcement of thermosetting systems. This exceptional behaviour has been interpreted in terms of the interactions and morphology developed.

Investigation on flip chip solder joint fatigue with cure-dependent underfill properties

A cure-dependent viscoelastic constitutive relation is applied to describe the curing process of epoxy underfill in flip chip on board (FCOB). The chemical shrinkage of the epoxy underfill during the curing process is applied via incremental initial strains. Thus, the stress and strain build-up, caused by the simultaneous increase in stiffness and shrinkage during the curing process, are simulated. Accelerated fatigue experiments with thermal cycles from -55/spl deg/C to 80/spl deg/C are carried out for a specially designed flip chip configuration. Based on the obtained curing induced initial stress and strain fields, thermo-mechanical predictions are presented for the test carriers. The solder bumps are modeled with temperature dependent visco-plastic properties. A combination of a Coffin-Manson based fatigue relation and a creep fatigue model is used as fatigue failure criterion. The results show that the finite element method (FEM)-based fatigue life predictions match better with the experimental results, if the curing induced initial stress state is taken into account. The effect of cure-induced hydrostatic stress is qualitatively investigated by using a modified energy partitioning damage model with a correction factor in the creep damage formulation to take into account the effect of the hydrostatic stress.