Low Structural Strength Research Articles

Multifunctional ANF/ CNT/FCIP aerogels with superior sound and electromagnetic wave absorption and mechanical properties

Aerogels exhibit bright prospect for multifunctional applications in noise reduction, radar stealth, load bearing, etc. Existing aerogels however suffer from poor sound absorption at low frequencies, narrow-band electromagnetic wave absorption and low structural strength. Here, we propose a novel lightweight aerogel composed of aromatic polyamide nanofiber (ANF)/ carbon nanotubes (CNT)/ flake carbonyl iron powder (FCIP) to boost acoustic/electromagnetic absorption and mechanical performance simultaneously. The FCIP additives create surface roughness and enlarge the pore size of the aerogel. As a result of the enhanced viscous energy dissipation by surface roughness and better impedance match achieved due to the enlarged pores, sound absorption at low frequency is significantly improved. Besides, the electrically and magnetically conductive network generated by embedded FCIP brings about effective electromagnetic absorption covering the whole X-band. Moreover, the aerogels undergo an increase in skeleton thickness and pore size by FCIP, resulting in maximum compressive stress increment. Our study provides clear guidance for the performance enhancement of aerogels that advances the development of aerogels for multifunctional applications.

Kinematics and dynamics characteristics of a double-ring truss deployable antenna mechanism based on triangular prism deployable unit

In order to effectively improve the single-ring truss deployable antenna mechanism due to the large aperture caused by the problem with low structural strength and low-profile accuracy, a series of double-ring truss deployable antenna mechanisms (DRTDAM) are proposed with constant height during folding and deployment process. First, a variety of DRTDAMs are proposed based on tetrahedral units and their topologies are analyzed. Secondly, degree-of-freedom (DOF) characteristics of DRTDAM proposed in this paper are analyzed based on the screw theory and screw-constrained topological graphs and based on this, the kinematic characteristics of DRTDAM is investigated. Thirdly, the dynamics of the whole DRTDAM is built with Newton-Euler equations of multi-rigid body system. Finally, the correctness of above analysis is verified through dynamics analysis software ADAMS and numerical analysis software MATLAB, and the principle prototype is produced to verify the correctness of DOF analysis. The mechanism proposed in this paper enriches the configuration of DRTDAM, and the process of kinematic characterization method is clear and simple, which is meaningful for the research in space complex mechanism.

Experimental study and infrared detector application research of micro miniature refrigerators

The MMR（Micro Miniature Refrigerator）is a novel Joule-Thomson cryocooler manufactured by micro-machining technologies, its axial length is significantly shorter than traditional Joule-Thomson cryocoolers commonly employed in infrared detectors. MMRs can greatly reduce the size of infrared detectors upon successful implementation. However, contemporary MMR products encounter challenges such as relatively low cool-down rates, cooling power, and structural strength. To address these issues and enhance the cool-down performance of the MMR for effective application in infrared detectors, a calculation model describing flow and heat transfer besides working characteristics of the MMR is proposed and verified. MMR prototypes are fabricated and experimentally studied. Building upon theoretical analysis and experimental findings cooling performance enhancement methods including transitioning the MMR material from glass to metal and modifying the structure and channel patterns of the MMR are introduced, the cooling performance of the MMR is thus greatly improved. Furthermore, an integrated design incorporating an MMR into an infrared detector is proposed, the axial length of this infrared detector is reduced by 65.3 % compared to conventional infrared detectors. And the MMR is further enhanced to adapt the working conditions in infrared detectors, 60.9 MPa working pressure and 38 s cool-down time is achieved, the cool-down requirements of infrared detectors is satisfied. Notably, the proposed MMR and infrared detector design in this paper exhibit technical advantages over similar products reported in the literature.

Structure, rheology and stability of walnut oleogels with different carboxylation degree of cellulose nanofiber

The effects of carboxylation degree (0.3–2.4 mmol/g) of cellulose nanofiber (CNF) on the microstructure and mechanical properties of edible walnut oleogels were comprehensively examined. The oleogels were well prepared by emulsion-templated approach for potential substitute of conventional saturated or trans-fats in food products. The results demonstrated that the oil-binding capacity (OBC) and textural strength of oleogels enhanced with the increase of CNF carboxyl content, while the structural strength (G' in rheological measurement) and the resistance to shear thinning was first decreased and then increased. It possibly reflected the competition on the dominant structuring mechanism by hydrogen bonding from cellulose hydroxyl groups and electrostatic interactions from –COONa function. With the combined mechanism, oleogel with low structural strength and relatively high OBC (CNF carboxyl content of 1.2 mmol/g, OBC >83 %, G′ ≈ 7 × 104 Pa and firmness of 0.30 N) and oleogel with enough structural rigidity and high OBC (CNF carboxyl content of 1.8 mmol/g, OBC >89 %, G′ of up to 1.7 × 105 Pa, and firmness of up to 0.66 N) were both fabricated. This reveals the feasibility of regulating oleogel structure and textual properties by using CNF as the unique oleogelator and simply changing its surface carboxyl function.

Effect of microwave curing on the properties of aggregate-mastic interfacial transition zones in cement emulsified bitumen mixture containing steel slag

The poor properties of the aggregate-mastic interfacial transition zones (ITZ) are the core factor for the low structural strength of cement emulsified bitumen mixture (CEBM), while incorporating steel slag with high microwave responsiveness into CEBM and followed by microwave curing may be an effective method to enhance ITZ properties. To verify this hypothesis, the ITZ properties of CEBM containing steel slag were characterised and analysed by a series of techniques such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and nanoindentation. The results showed that the ITZ properties increased continuously with microwave curing time. Specifically, the thickness of ITZ continuously decreased, while the elastic modulus and hardness continuously increased. When the microwave curing time was less than 7 min, the growth in cement hydrates and temperature simultaneously enhanced the ITZ properties. When the microwave curing time exceeded 7 min, the cement hydrates gradually decreased due to the water content falling below the minimum required for cement hydration, and the improvement of ITZ properties mainly depended on the temperature rise. Remarkably, the microwave heating characteristics of steel slag resulted in a tighter bond with bitumen, which led to consistently better surface ITZ properties than limestone. However, it should be avoided that the microwave curing time is too long, as this would result in the steel slag temperature being too high and aging its surface bitumen. Additionally, the indirect tensile strength of CEBM affected by the ITZ properties was still increased even after the decrease in cement hydrates.

Magnesium Test: Universal and Ultra‐Sensitive Method for Measuring Reliability of Thin‐Film‐Encapsulated Bioelectronic Implants in Physiological Environment

AbstractNext‐generation bioelectronic implants require miniaturization, durability, and long‐term functionality. Thin film encapsulations, prepared with inorganic or hybrid organic/inorganic designs, are essential for ensuring protection, low water permeation, adaptability, and structural strength. It is equally important to precisely measure their barrier performance, especially for in vivo use, to ensure the manufacture of reliable bioelectronics. Current monitoring solutions are not adequate: they are bulky, lack sensitivity, and are incompatible with microfabricated devices. Here, a comprehensive method is introduced to quantify the permeability of thin‐film encapsulation for bioelectronic implants both in situ and in real time. This method relies on monitoring the electrical resistance of the Mg film, which experiences corrosion due to water permeation, leading to Mg hydrolysis. An analytical model is proposed that predicts and quantifies this permeation, and is adaptable for various types of encapsulations, including hybrid multilayers. An unprecedented ultra‐low detection limit of 3 × 10−8 g m−2 d−1 at room temperature is demonstrated and the monitoring approach is validated in vivo using polyimide and poly(dimethylsiloxane)‐coated bioelectronics.

Investigation of sisal fiber incorporation on engineering properties and sustainability of lightweight aggregates produced from municipal solid waste incinerated bottom ash

The large-scale disposal of bottom ash resulting from municipal solid waste incineration poses a significant challenge in terms of effective management. While Lightweight aggregates made of municipal solid waste incinerated bottom ash, referred to as MSWI BA, show promise, their practical application is hindered by low structural strength and the presence of certain contaminants. To address this, a novel and green solution is proposed in this study, which involves utilizing sisal fibers (SF) to enhance the solidification of the synthesized aggregates. In this study, the synthesized lightweight aggregates (SLWAs) consisting of cement, different replacement levels of ground MSWI BA (60%, 70%, and 80%), and/or the cut SF were manufactured by a disc pelletizer. The influence of incorporating SF on the SLWAs was investigated across various aspects, encompassing the physical characteristics, strength performance, microscopic properties, and sustainability. The results showed that when SF was incorporated, the production efficiency of coarse SLWAs was considerably improved, especially up to a 66.5% increase with under the 70% replacement level of MSWI BA. Moreover, the surface roughness and strength were improved due to the SF addition. In the system of the CEM-MSWI BA matrix, the fiber incorporation not only accelerates the cement hydration reaction but also fills the pore generated from the swelling of metallic aluminum in MSWI BA, achieving the strength enhancement of SLWAs. More importantly, some contaminant ions including chloride, chromium, and copper from MSWI BA, could be effectively absorbed and solidified by the SF. Furthermore, the utilization of SF in SLWAs brings about great sustainability concerning greenhouse gas emissions and production costs. Consequently, incorporating SF proves to be an effective and green solution, thus promoting the practical implementation of the SLWAs.

Research on structural strengthening technology for regenerative denitration catalysts.

The cost of replacing failed selective catalytic reduction (SCR) catalysts and their disposal as hazardous solid waste is high. If failed catalysts are recovered and regenerated into new SCR denitration catalysts, the cost of flue gas denitration can be effectively reduced. However, regenerated SCR catalysts have relatively low structural strength and activity and cannot yet form an effective replacement. In this study, aluminum dihydrogen phosphate, aluminum nitrate, and aluminum sulfate were used as structural strengthening agents in the regeneration of SCR catalysts, and over-impregnation, drumming-assisted impregnation, and ultrasonic-assisted preparation techniques were compared. The corresponding regenerated SCR catalysts were then prepared and analyzed for compressive strength, wear strength, H2-TPR, NH3-TPD, and in situ IR. Factors influencing the structural strength, physical properties, and catalytic activity of the regenerated catalysts were investigated. The best results were obtained as follows: compressive strength of 4.57 MPa, wear rate of 0.088% kg-1, and denitration of 58% after 10 min of drumming-assisted impregnation in an aluminum sulfate solution with a concentration of 16%. Based on this, a synergistic method for catalyst activity and structural strengthening was explored to support the design of better SCR catalysts for regeneration.

Thermal performance of Parabolic Trough Solar Collector

The research progress of numerical simulation of parabolic trough solar collector was introduced. Especially the numerical simulation studies of the internal flow field in the collector tube and the external flow field of the concentrator were reviewed. In the study of the internal flow field in the collector tube, the type of heat transfer fluids and the characteristics of optical concentration have a significant effect on the heat collection performance and thermal stress distribution, especially in the water/steam medium heat collection loop. The unique gas-liquid two phase flow superposition of the non-uniform heat flux distribution outside the tube may lead to serious thermal stress bending deformation of the collector tube. In the study of the external flow field of the concentrator, due to the high actual wind speed and the low structural strength of the ultra-thin lens, the concentrator is subjected to wind load deformation, resulting in the loss of optical efficiency, and even leading to the failure of the parabolic trough collector structure, which directly affects the normal operation of the whole collector field.

A Topological Valley Stamping Plate for Low-Frequency-Vibration Isolation and Wave Manipulation in Helicopters

Vibration isolation and wave manipulation play a significant role in helicopter applications, from small elastic devices for information processing to large solid structures for vibration control. The emergence of topological elastic metamaterials has opened up new possibilities for modulating vibrations and waves in solids. However, most designs are limited to laboratory settings due to complex manufacturing or low structural strength. In this study, we propose a topological elastic metamaterial with a stamping triangle whose orientation θ can be rotated to achieve topological phase transitions between −30° and 30°. Unlike traditional manufacturing methods, the utilization of stamping technology provides a cost-effective means of production, allowing for wider application across diverse industries. Moreover, the metamaterial has the capability to manipulate elastic waves along specific trajectories and effectively decrease common vibrations (more than 50 dB), typically occurring at a frequency of 24 Hz in helicopters. Our approach also enables the control of elastic wave propagation along interfaces through the emergence of topological edge states. The practical implementation of this design will enhance the confidence in using topological elastic metamaterials for vibration isolation and wave manipulation in helicopters.

Synergistic Effects between Multiphase Thermal Insulation Functional Phases on the Mechanical and Heat Insulation Properties of Silicone Rubber Composites.

This work utilizes the synergistic effect between three different functional phases of thermal insulation, i.e., hollow ceramic microspheres (HCMs), hollow silica microspheres (HSMs), and hydroxy silicone oil blowing agent, to prepare a flexible and efficient thermal insulation composite with low thermal conductivity and high structural strength. First, the effects of the three phases on the mechanical and thermomechanical properties of silicon rubber (SR) were analyzed using a scanning electron microscope (SEM), a thermogravimetric (TG) analyzer, a thermal conductivity meter, and a universal testing machine, respectively. Then, the thermal insulation mechanism of multiphase thermal insulation composite materials was analyzed. The results show that the thermal stability and mechanical performance of composites were significantly enhanced, particularly for sample 18H, whose Tmax and char yield reached 621.3 °C and 77.5%, respectively, representing a respective increase of 12.1 and 35.3% compared to those of pure SR. After heat treatment at 1000 °C, the linear shrinkage of the sample diameter was about 9.4%, while the thermal conductivity was as low as 0.064 W/(m·K), which was 53.2% lower than that of the pure matrix SR. We believe that this work can serve as a reference for the preparation of heat insulation and protection materials with low thermal conductivity and high structural strength.

Measuring the Vulnerability of the Urban Fabric in the Face of Land Subsidence (Case Study: District 17 of Tehran)

Urban vulnerability is the probability of damage to cities and urban infrastructures against natural and unnatural events. In this regard, in the last half century, land subsidence has involved many countries as a potential risk for urban, industrial and agricultural areas. With the occurrence of land subsidence, the physical and structural characteristics of the structures built in the city are changed and show different behavior. For this reason, it is important to consider the effect of land subsidence on the urban fabric in which there are exposed and buried structures. The involvement of the 17th district of Tehran with the issue of land subsidence was the initial idea of the current research. Currently, the maximum land subsidence rate in this area is 25 cm per year. The present research has been carried out in the form of spatial studies and by implementing the analytical model in four steps. The results of the model implementation showed that the urban fabric of the 17th district is prone to high vulnerability due to features such as high population and building density, the presence of worn-out fabric, buildings with low structural strength and very low construction quality of buildings. In total, about 657 hectares (80 percent) of the built-up lands in the 17th district of Tehran are under the influence of land subsidence.

Influence of reversible swelling and preload force on the failure behavior of a lithium-ion pouch cell tested under realistic boundary conditions

Safety of lithium-ion batteries plays an important role in the context of advancing electrification for vehicles. Pouch cells suffer from low structural strength and are often constrained within a battery module to guarantee mechanical integrity. The effect of constraints and SOC-dependent changes on the mechanical abuse behavior was not sufficiently investigated.A total number of 36 pouch cells were indented with a flat-end cylinder under different boundary conditions until mechanical failure and thermal runaway occurred. The pouch cells were constrained at 30 % SOC with a preload force of 0, 300 or 4000 N and charged to 0 %, 30 %, 60 % or 100 % SOC before indentation.The maximum indentation force, corresponding indentation, initial stiffness and failure behavior indicated a dependency on the preload force. The stiffness at greater indentation was similar for all boundary conditions indicating a pre-compression and flattening of unevenness. Internal stress within the separator resulted in earlier short circuit and mechanical failure for increasing preload force. The mechanical constraint led to increased gas pressure during thermal runaway.The results in this publication give rise to an additional consideration of preload force and boundary conditions imposed by a battery module in abuse testing and simulation approaches in the future.

Experimental study of micromilling process and deburring electropolishing process on FeCo-based soft magnetic alloys

FeCo-based soft magnetic alloys are commonly used in macroscale devices to improve its electromagnetic performance, whereas they have been barely used in the microscale. Current FeCo alloy micromanufacturing processes present some difficulties like low structural strength, oxidation at high temperature processes, stoichiometry mismatches in deposition processes and tough workability. In this work, a microcutting of FeCo-2 V-based soft magnetic alloys process is presented and described as an alternative method to obtain microparts with high magnetic properties and good geometrical finish. The results of the machining process are analysed by varying the machining parameters such as depth of cut, tool diameter, rotation speed and feed speed on simple machining operations. The study has been done for 50-μm-diameter endmill and 250-μm-diameter endmill tools. It concludes that the roughness is minimized when machining parameter combination is 0.24 mm/min of feed rate and 8 μm of depth of cut size for a 250-μm-diameter tool, while for a 50-μm-diameter tool, the selected feed rate is 0.24 mm/min, for a depth of cut between 2 and 4 μm. An automated precision 3-axis CNC station is used. Shapes needed for actuators such as angular slots, disks, or slender square geometries are shown in this work with excellent magnetic and mechanical properties. Additionally, a complementary electropolishing process is described. This process helps to eliminate burr in edges and residuals of the milling operation. This study demonstrates that micromilling can be a good alternative for microfabrication of FeCo-2 V components, suitable for precision microassemblies on MEMS.

Biochar-mediated Anammox granular sludge: Formation/characteristics of pitaya type granular sludge and enhanced performance of Anammox process

Traditional Anammox granular sludge (AnGS) has disadvantages such as long formation time and low structural strength. Biochars have the potential for particle formation and structural stability of AnGS. Herein, different biochars (peach, coconut, jujube, and litchi biochars) were added to four expanded granular sludge bed (EGSB) reactors to investigate changes in sludge characteristics during Anammox start-up and stable Anammox granulation. Results showed that reactors with peach and coconut biochars achieved rapid start-up of Anammox, which were 15 days faster than that of the control reactor. Scanning electron microscopy (SEM) results showed the formation of the pitaya type AnGS with biochar as the crystal nucleus. Compared with the control, pitaya type AnGS particularly enriched the large-sized particle (1.0–3.0 mm), and had a higher specific Anammox activity (SAA) at the same particle size, along with an increase in mechanical strength (by ∼13%) and the relative abundance of functional bacteria (by 28–109%).

Preferred injection method and curing mechanism analysis for the curing of loose Pisha sandstone based on microbially induced calcite precipitation.

As a loose rock formation with low lithogenic property, low structural strength, and poor intersand cementation, Pisha sandstone is susceptible to chemical weathering and extreme soil erosion and has become an important source of sediment for the Yellow River. There is limited information available on the conditions of microbial distribution homogeneity under grain-mediated conditions in Pisha sandstones, as well as on the influence of dissolved minerals on calcium carbonate morphological mechanisms. In this paper, microbially induced calcium carbonate deposition was used to reinforce and improve the loose Pisha sandstone. First, the influence laws of the single-phase/self-absorption two-phase injection method and added solvent on the curing indexes such as curing volume, curing depth, calcium carbonate yield, and unconfined compressive strength of the specimens were discussed. Field emission scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction spectroscopy were used to analyze the microstructure of the cemented sand columns, as well as the mineral phases and distribution of the biomineralization products mechanistically. The results demonstrated that the single-phase injection treatment could only achieve local solidification of the Pisha sandstone sand column, whereas the self-absorption two-phase injection method could result in a more uniform spatial distribution of bacteria and a monolithic specimen, in which the calcium carbonate yield increased with increasing low concentration CaCl2 injection. The compressive strength appeared to increase significantly, and the effect of the applied liquid CO(NH2)2 was not obvious. Montmorillonite underwent dissolution during the mineralization process, eliminating the characteristics of Pisha sandstone swelling in water. Under the effect of biomineralization, calcium carbonate crystals are formed to wrap around the Pisha sandstone particles, changing their particle size and increasing the interparticle roughness. Meanwhile, the interstices between particles are filled via calcium carbonate precipitation, effectively forming cementation points that can significantly improve the strength of the Pisha sandstone. The results of this study provide a theoretical basis for the application of biomineralization technology in the ecological restoration of Pisha sandstone areas.

Stress corrosion cracking behavior of super 13Cr tubing in phosphate packer fluid of high pressure high temperature gas well

This paper describes a stress corrosion cracking (SCC) failure case of super 13Cr tubing serviced for six years in a high-pressure, high-temperature (HPHT) gas well. The failure causes and processes were studied by various characterizations and SCC simulation testing. The failure mode of super 13Cr tubing is brittle transgranular fracture, and the dominating role in SCC failure is the synergistic action of KH2PO4 phosphate packer fluid and CO2, leading to corrosion damaged zone, which is a selective corrosion along the martensitic multiscale boundaries. Under tensile stress, cracks developed at the corroded martensitic multiscale boundaries and propagated due to the low structural strength of the corrosion-damaged zone. Due to its compatibility with 13Cr tubing, formate is recommended as an alternate packer fluid for high-pressure, high-temperature (HPHT) gas wells.

Maize stalk stiffness and strength are primarily determined by morphological factors

The maize (Zea mays) stem is a biological structure that must balance both biotic and structural load bearing duties. These competing requirements are particularly relevant in the design of new bioenergy crops. Although increased stem digestibility is typically associated with a lower structural strength and higher propensity for lodging, with the right balance between structural and biological activities it may be possible to design crops that are high-yielding and have digestible biomass. This study investigates the hypothesis that geometric factors are much more influential in determining structural strength than tissue properties. To study these influences, both physical and in silico experiments were used. First, maize stems were tested in three-point bending. Specimen-specific finite element models were created based on x-ray computed tomography scans. Models were validated by comparison with experimental data. Sensitivity analyses were used to assess the influence of structural parameters such as geometric and material properties. As hypothesized, geometry was found to have a much stronger influence on structural stability than material properties. This information reinforces the notion that deficiencies in tissue strength could be offset by manipulation of stalk morphology, thus allowing the creation of stalks which are both resilient and digestible.

Effect of material characteristics on thermal, tribological and mechanical properties of high temperature cast iron coatings

Coating is a desideratum part of material science and effort must be entailed for obtaining desired high temperature properties. Properties of high temperature cast iron coatings depend on various factors and each of them must be precisely tailored for meeting the huge industrial demand. The unlimited architecture of novel materials has led to growth of various coating with tailored properties. Single/multi layered structure offers remarkable possibilities to improve coating properties. Cast iron has been increasingly used in gas turbine, internal combustion engine manufacturing industries due to its low cost and structural strength. Numerous researches have been done on cast iron coatings to study the thermal, tribological plus mechanical properties. Novel material for improving the thermal, mechanical and tribological properties of Thermal barrier coatings (TBCs) required an extensive property-characteristics connection realization. Yttria Stabilized Zirconia (YSZ) is the most established coating material for thermal barrier application. YSZ alone cannot withstand the huge industrial demand. YSZ in engine application suffers failure due to non-uniform thermal field, thermal stress and sintering during the cycle of operation. YSZ cannot withstand engine thermal properties including thermal cycle life and thermal durability. Wear rate gets affected as a result of brittle fracture, speed of operation and abrasion. High temperature exposure leads to microcracks and this affects the mechanical properties. TBCs are usually made of ceramic top coat and it suffers failure. Few material characteristics and factors include thermal aging, fracture toughness of the material. This paper summarizes confirmed experimental results by researchers concerning different factors including thermal conductivity, thermal shock resistance, thermal cycle life, wear resistance, coefficient of friction, hardness and material characteristics in regard with cast iron coatings for high temperature applications. This paper also recommends feasible materials and material combinations to overcome certain limitations and failures.

Ptosis and Bottoming out Following Mastopexy and Reduction Mammoplasty. Is Synthetic Mesh Internal Breast Support the Solution? A Systematic Review of the Literature.

Relying on soft tissues with low structural strength to provide long-term support of the breasts following aesthetic mammoplasty may be inadequate in many instances. To address the inherent weakness of tissues still present after surgery, use of synthetic non-autologous additional tissue reinforcement has been described. The current review is aimed at analyzing available evidence about safety and effectiveness of mesh support in various reduction mammoplasty, mastopexy, or augmentation-mastopexy. An advanced PubMed and Medline search was conducted to identify clinical studies about the use of synthetic mesh in aesthetic breast surgery. An additional search of breast symmetrization and synthetic mesh was also conducted. Ten studies were identified and judged to be relevant to this review. Retrieved studies were relatively few, characterized by marked heterogeneity, lack of well-defined outcome end points, poor outcome measures, and inherent bias in outcome documentation providing low level of evidence. Despite reported relative safety and some promising results, available data indicate that meshes do not effectively prevent recurrent ptosis and bottoming out; they may not be superior to described techniques with superior pedicle and hammocks or "balcony" flaps. Well-conducted studies are yet to be conducted comparing internal bra technique to procedures with innovative autologous tissue support. This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine Ratings, please refer to Table of Contents or online Instructions to Authors www.springer.com/00266 .