Ultimate Mechanical Properties Research Articles

Pure Chitosan Biomedical Textile Fibers from Mixtures of Low- and High-Molecular Weight Bidisperse Polymer Solutions: Processing and Understanding of Microstructure-Mechanical Properties' Relationship.

Natural polymers, as extracted from biomass, may exhibit large macromolecular polydispersity. We investigated the impact of low molar mass chitosan (LMW, DPw~115) on the properties of chitosan fibers obtained by wet spinning of chitosan solutions with bimodal distributions of molar masses. The fiber crystallinity index (CrI) was assessed by synchrotron X-ray diffraction and the mechanical properties were obtained by uniaxial tensile tests. The LMW chitosan showed to slightly increase the crystallinity index in films which were initially processed from the bimodal molar mass chitosan solutions, as a result of increased molecular mobility and possible crystal nucleating effects. Nevertheless, the CrI remained almost constant or slightly decreased in stretched fibers at increasing content of LMW chitosan in the bidisperse chitosan collodion. The ultimate mechanical properties of fibers were altered by the addition of LMW chitosan as a result of a decrease of entanglement density and chain orientation in the solid state. An increase of crystallinity might not be expected from LMW chitosan with a still relatively high degree of polymerization (DPw ≥ 115). Instead, different nucleation agents—either smaller molecules or nanoparticles—should be used to improve the mechanical properties of chitosan fibers for textile applications.

Tri-branched gels: Rubbery materials with the lowest branching factor approach the ideal elastic limit.

Unlike hard materials such as metals and ceramics, rubbery materials can endure large deformations due to the large conformational degree of freedom of the cross-linked polymer network. However, the effect of the network’s branching factor on the ultimate mechanical properties has not yet been clarified. This study shows that tri-branching, which entails the lowest branching factor, results in a large elastic deformation near the theoretical upper bound. This ideal elastic limit is realized by reversible strain-induced crystallization, providing on-demand reinforcement. The enhanced reversible strain-induced crystallization is observed in the tri-branched and not in the tetra-branched network. A mathematical theory of structural rigidity is used to explain the difference in the chain orientation. Although tetra-branched polymers have been preferred since the development of vulcanization, these findings highlighting the merits of tri-branching will prompt a paradigm shift in the development of rubbery materials.

Investigations on the interfacial heat transfer coefficient during hot stamping of ultra-high strength steel with Al-Si coating

The ultimate mechanical properties and microstructures of hot-stamped components are highly related with the interfacial heat transfer coefficient (IHTC) between blank and tool. In this paper, an advanced temperature acquisition system was established to measure the temperature of boron steel Usibor 1500P with Al-Si coating during the hot stamping process, and an inverse algorithm was developed to calculate the IHTC. Moreover, the influences of contact pressure and tool roughness on IHTC were discussed. The results show that the IHTC only increases with pressure when it is less than 1MPa, while remains almost unchanged under higher pressure. Meanwhile, little effects on the IHTC are generated by tool surface roughness. Those phenomena are quite different from the commonly studied uncoated blanks. Further exploration demonstrates that the effect of high pressure and tool surface roughness on IHTC is not significant due to the oxidation resistance and the spalling of the Al-Si coating respectively.

Rubber aging life prediction based on interpolation and improved time-temperature superposition principle

With focus on quickly and accurately predicting and evaluating the aging performance degradation of rubber at room temperature, the pseudo-failure life at each different acceleration temperature is proposed to be calculated by interpolation method based on indoor high temperature accelerated aging data, and on the basis of the obtained pseudo-failure life. By introducing the time-temperature equivalence principle, a shift factor obeying to an Arrhenius law is derived, and master curves are built as well for the compression set as for the ultimate mechanical properties. The concept of the sum of squares of dispersion coefficient errors is proposed to evaluate the prediction accuracy. Meanwhile a quantitative calculation method that considers the effect of temperature on the performance degradation curve and the shift factor is innovatively proposes. The results show that the proposed optimization method based on the traditional time-temperature superposition principle can quickly process the aging life at room temperature, and the prediction results are distributed within the 3-fold dispersion line, which can well meet the engineering requirements. The reduction of the DSC value from 1.4164 to 1.0828 further demonstrates the effectiveness of the proposed method above. This method can provide some reference for other related polymer materials accelerated aging data processing and life prediction.

3D printing of tough hydrogels based on metal coordination with a two-step crosslinking strategy.

We demonstrate the self-supporting 3D printing of complex hydrogel structures based on simultaneous crosslinking reactions while printing. The printing strategy is based on the Schiff base reaction and metal coordination with a two-step crosslinking process. The printing ink was first prepared by dispersing oxidized sodium alginate (OSA) and adipic dihydrazide (ADH) in poly(acrylamide-co-acrylic acid) (P(AAm-co-AAc)) polymer solutions, and was mixed and printed into 3D structures with an extrusion-based coaxial printing platform. Because of the rapid chemical crosslinking reaction between the aldehyde group in OSA and the hydrazide group in ADH, the printed structures can be solidified quickly, and are further crosslinked by forming carboxyl-Fe3+ coordination complexes to enhance their mechanical properties. The dynamic time-sweep rheological properties of the gel composed of different proportions of OSA and ADH were systematically investigated for the characteristic gelation time, and compression tests were carried out to measure the mechanical properties of the gel composed of OSA and ADH. Combining the gelation time and mechanical properties of the gel, the weight ratio of OSA and ADH was selected as 1 : 0.44 for an optimized setting in the subsequent printing. To evaluate the printability of inks, the material formula and printing parameters were systematically varied. The ink exhibited a wide printing range, self-supporting properties, and good printability. Tensile tests of the printed single fiber crosslinked by Fe3+ show that its strength and toughness are tunable. Complex 3D structures such as pyramids, cylinders, and noses were constructed to demonstrate the printability of the ink. This printing method provides a facile approach for tough hydrogel fabrication without changing the rheological properties of the ink or sacrificing the ultimate mechanical properties of the printed materials.

Analysis of Welding Behaviour of Friction Stir Welded UHMW-PE Plate

Polypropylene is one of the thermoplastic materials used in the lot of engineering applications such as marine, aerospace, automotive, toys and etc. Friction Stir Welding (FSW) is a solid-state method of used for joining metals. FSW process was successfully extended to join thermoplastic materials. Friction Stir Welding (FSW) is a solid-state process in joining thermoplastic materials. Polymers are engineering materials used for future technological development as the polymer processing and fabrication techniques have developed novel plastic products and components in major industries. Particularly UHMWPE is one of the polymer materials with a lot of engineering applications and a study on the behavior of the joining properties of polyethylene by FSW is necessary at this stage. In this investigation, FSW process has to be applied to join a polyethylene plate of 8 mm thickness with specially designed threaded tool pin profile. The research will be applied Taguchi Method on UHMWPE specimen of dimensions 100 × 100 × 8 mm, which have following parameters: various RPM, Feed and Axial Load. The experiments will be done in following aspects: Ultimate tensile strength and other mechanical properties. The main objective of the experimental of factors affecting to mechanical property of UHMWPE with FSW at different welding parameters and it has to be followed by L4 arrays.

Processing and Tensile Properties of Twisted Core-Shell Yarns Fabricated by Double Nozzle Electrospinning Device

In this study, a double nozzle electrospinning device consisting of a take-up/twister unit applied to develop core-shell structured nanofibrous yarns with potential applications in the production of functional textile materials. The process was performed by introducing a pre-electrospun core yarn into the electrical field between two oppositely charged nozzles, where nanofibers cover it by a certain arrangement owning to the twisting procedure. Herein, the main goal was to investigate how the core and shell structures influenced by process parameters, can contribute to the ultimate mechanical properties of the electrospun core-shell yarn. Accordingly, the same solution of nylon 66/formic acid was used for electrospinning of both core and shell nanofibers. The response surface methodology (RSM) was applied to study the effect of twisting rate, take-up speed, and twist amount of core yarn (called pre-twist) on the morphological and tensile properties of electrospun core-shell yarns. SEM images confirmed that the nanofibers were assembled surround the core with a certain angle to the axis to form a twisted core-shell yarn. The take-up speed caused considerable effects on the yarn diameter by changing the number of fibers formed in the triangle zone of the electrospinning process. The diameter of the core-shell yarns decreased significantly by increasing the twist rate and take-up speed. Improvements in tensile stress were generally realized at low twist levels and higher take-up speeds. The contribution of the core on the mechanical behavior of core-shell yarn was also considered in detail. The electrospun core-shell yarns showed superior (∼37 %) max stress when using a core yarn with low pre-twist amounts.

Properties of microfibrillar cellulose filled thermoplastic natural rubber: Morphology, mechanical properties, and melt flow index

Microfibrillar cellulose (MFC) was derived from local pineapple leaf fiber (PALF) and employed as filler in thermoplastic natural rubber (TPNR), that was a blend of natural rubber (NR) and polypropylene (PP). The blends were prepared by dynamic vulcanization. The performance of MFC-filled TPNR on morphology, mechanical properties, aging behavior, and melt flow index (MFI) were evaluated in this study. MFC was synthesized through several processing steps, including extraction, bleaching, acid hydrolysis, and ball-milling mechanical treatment. The blends were made in a fixed ratio of 60/40 (NR/PP) with various fiber concentration. The blending of NR/PP was performed by Rheomix internal mixer at 160 °C for 15 minutes with turning rotor of 90 rpm. The results showed that the particle size distribution of MFC was rather broad with some web-like agglomerates. An improvement in tensile strength was observed at low fiber loading. Fiber improved the tear strength, but decreased the elongation at break as expected. The hardness was independent to fiber loading. The morphology of fractured sample of TPNR showed fibrous structure indicating plastic deformation. MFC decreased the melt flow index of TPNR. The MFI value ranges between 3 – 5 g/10 min. One phr of MFC resulted in ultimate mechanical properties with considerable melt flow.

Hierarchical thermoplastic biocomposites reinforced with flax fibres modified by xyloglucan and cellulose nanocrystals

This work is focused on the modification of the interphase zone in short flax fibres / polypropylene (PP) composites by a bio-inspired modification of fibres called “nanostructuration” that uses the adsorption of biomass by-products, i.e. cellulose nanocrystals (CNC) and xyloglucan (XG), to create hierarchical flax fibres. The wettability and interfacial adhesion study reveals a strong decrease in the polar character of CNC modified flax fibres, hence increasing the work of adhesion with PP. Moreover, combining XG/CNC modified interphases with MAPP coupling agent enhances the ultimate mechanical properties of biocomposites with higher tensile strength and work of rupture, and modifies failure mechanisms as revealed by in situ micro-mechanical tensile SEM experiments. Bio-based hierarchical composites inspired by naturally occurring nanostructures open a new path for the development of sustainable composites with enhanced structural properties.

Experimental Investigation of ABS Matrix and Cellulose Fiber Reinforced Polymer Composite Materials

ABSTRACT The present study explores the effect of cellulose fiber-reinforced Acrylonitrile-Butadiene-Styrene (ABS) matrix composites on their mechanical, thermal, and impact properties, produced by using different weight percentage. The ABS matrix materials are high molten viscosity, causing less resin flow. Cellulose fiber reinforcement has a good reinforcing filler with an excellent aspect ratio and it renders ultimate mechanical properties generated by hydrogen bonding with high crystallinity. The results exposed the higher tensile strength, thermal and heat deflection temperature by ABS matrix and cellulose reinforced composites than those by using conventionally fabricated Cellulose fiber and ABS matrix composite materials. It is noted that the weight percentage of 20% of cellulose fiber and 80% of ABS polymer composite materials could produce the maximum mechanical and thermal properties of Cellulose fiber and ABS composites, compared to 100% ABS material. Compared to the base matrix ABS, the composite materials increased the tensile strength by 37%. The morphological examination was performed by Field Emission Scanning Electron Microscopy (FESEM) and the results showed the appearance of Cellulose fiber and ABS composites in the fractured tensile specimen.

Thermo-oxidative ageing of a SBR rubber: effects on mechanical and chemical properties

In this work, the effects of thermo-oxidative ageing at different temperatures and for different exposure durations on the mechanical and the chemical properties of a styrene butadiene rubber (SBR) are presented. Uniaxial tensile tests, hardness measurements, Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) spectra analysis, and swelling tests are carried out on as-received and aged samples. Accelerated ageing process was conducted at different temperatures ( $$50^\circ C$$ , $$70^\circ C$$ , $$90^\circ C$$ , and $$100^\circ C$$ ) and for different exposure durations $$(7, 14, 21, 28, 35, 45$$ and $$60$$ days).This work confirm that accelerated ageing lead to a decrease of the ultimate mechanical properties and of the molar mass between cross-links in one hand, and an increase of the cross-linking density and of the material hardness, in another hand. ATR-FTIR analysis shows significant changes in the chemical structure of aged SBR samples dominated by the thermo-oxidative process, which is, mainly pronounced at high temperature and long exposure time. The ultimate mechanical properties are related to the average molar mass between cross-links. A threshold value of this property corresponding to a complete degradation of the rubber can be determined. Finally, the time–temperature equivalence principle is applied to build master curves describing the evolution of certain quantities such as molar mass between crosslinks, tensile strength, and strain at break versus the reduced time. A predictive modeling of the stress and strain at break as function of the effective ageing time is proposed which give satisfactory results.

Influence of forging velocity on temperature and phase transformation characteristics of forged Ti-6Al-4V aeroengine drum

This study is prompted by the reality that the forging velocity greatly impacts the phase transformation of titanium forged parts, further affecting the ultimate microstructures and mechanical properties of their final parts. In this paper, to predict the transformation characteristics of temperature and phases in Ti-6Al-4V drum hot forging procedure, a simplified FE model embedded with phase transformation equations is developed and verified by experiments. Then, based on two evaluation indexes, the effect of forging velocity on the transformation characteristics of temperature and phases within the forged drum is analyzed quantitatively. The results suggest that unlike little effect on lamellar α+β phase, appropriately enhancing the forging velocity can increase the general levels of temperature and β phase and improve the uniformities of temperature, α phase, and β phase distributions within the forged drum.

Influence of forging velocity on temperature and phases of forged Ti-6Al-4V turbine blade

This study is prompted by the reality that the forging velocity significantly impacts the temperature and phases of titanium forged parts, further directly affecting the ultimate mechanical properties. In present work, a 3D FE model combined with phase transition equations is established to predict the characteristics of temperature and phases in Ti-6Al-4V blade forging procedure. Then, based on four evaluation indexes, the effect of forging velocity on temperature and phases in terms of extreme values, general levels and distribution uniformities are analyzed quantitatively. Finally, the validation of simulation results is confirmed by comparing with experimental ones. The results reveal that unlike little influence on lamellar α + β phase, enhancing the forging velocity can obviously increase the general levels of temperature and β phase, while decrease the distribution uniformities of α and β phases within the forged blade.

A route to improving elongation of high-temperature laser sintered PEKK

Abstract Laser sintering (LS) is one of the most popular additive manufacturing (AM) techniques as it produces parts of complex geometry with high dimensional accuracy and good mechanical strength. However, the nature of the LS process often leads to brittle behavior characterized by a low elongation at break if compared to conventional polymer processing techniques, e.g., injection molding (IM). For poly(ether ketone ketone) (PEKK), such elongation is currently below 3%. This study determines and then optimizes the relationship between cooling time and crystallization of PEKK during LS and the resulting elongation at break. The elongation at break of PEKK was successfully improved by using shorter cooling times. The combination of the slow crystallization kinetics of PEKK and a short cooling time of 1 h increased elongation at break to 14%; this is a striking result never achieved for PAEKs in LS before. A calibration curve was developed that can be used to correlate PEKK structure and mechanical properties to cooling conditions according to the application. This methodology can also be applied to select and optimize the mechanical properties of other LS polymers sharing similar kinetics of crystallization and processing temperatures. This work suggests a great potential for a wide range of “post-processing” heat treatments to be used in AM to tailor the ultimate mechanical properties.

Unlocking Sustainability Potentials in Heat Treatment Processes

Energy consumption, greenhouse gas emissions, environmental impact levels, and the availability of materials as well as their sustainable usage are all topics of high current interest. The energy intensive processes of casting production such as heat treatment are particularly affected by the pursuit of sustainability. It has been estimated that up to 20% of the total energy demand in a non-ferrous foundry is required to provide the heat energy necessary during heat treatment processes. This paper addresses the application-oriented development of a sustainable configuration of the heat treatment process at the example of the aluminium-casting alloy A356 (AlSi7Mg0.3). Based on calculations of the physically necessary operating modes and under investigation of previous parameter recommendations, experimental studies were carried out to investigate the effects of various heat treatment parameters on the ultimate mechanical properties of the alloy. Since the achievable mechanical properties of the finished casting are decisive, the static and dynamic casting properties resulting from the heat treatment with optimized process parameters were compared with those of conventional process control. Significant optimization potential is shown for reducing the treatment time and thus lowering the energy consumption.

Transformation characteristics of temperature and phases within Ti-6Al-4V aeroengine drum in hot forging and air cooling procedures

The phase distribution and evolution characteristics of titanium forged parts influenced by hot forging and subsequent air cooling procedures have an essential effect on the ones of final parts, resulting in a direct impact on their ultimate mechanical properties. This study presents a simple and effective modelling approach to predict the transformation characteristics of temperature and phases during Ti-6Al-4V drum hot forging and air cooling processes. In particular, 2D FE half-symmetry models coupling with phase transformation equations are developed, and then verified by experiments. Subsequently, the distributions of temperature and phases within the forged and cooled drums are numerical investigated in details. Finally, their temperature and phase evolutions in terms of general level and distribution uniformity are analyzed quantitatively. The results indicate that the distributions of different phases within the forged and cooled drums are extremely inhomogeneous, which are related to temperature variations. Besides, the α phase distribution within the forged drum remains constant in the subsequent air cooling operation, which is related to cooling rates.

High modulus thermoplastic segmented polyurethane/poly(L‐lactide) blends as potential candidates for structural implantable drug delivery systems: I. Structure‐properties relationship study

AbstractStructural polymers with a high modulus of elasticity have the necessary properties for drug eluting implants which are continuously subjected to long‐term loading. The range of polymers that can be used for this purpose is severely limited by the demanding requirements, such as biocompatibility, specific mechanical properties and the capability to control the release rate of incorporated drugs. A new series of blend compositions was developed for these applications, based on a combination of a high modulus thermoplastic segmented polyurethane (TSPU) (shore D hardness) and poly(L‐lactide) (PLLA). The microstructures and properties of these blends were characterized by DSC, tensile tester, FTIR, SEM, WAXD, transmitted light microscopy and elastic recovery testing. The two polymers were found to be immiscible, irrespective of the weight ratio between them, but still compatible, although the compatibility was reduced in blends with equal weight ratios or thereabouts. The dispersed phase in any case spontaneously organized into microspheres embedded within the continuous phase. PLLA could recrystallize as both the dispersed and the continuous phases. The ultimate mechanical properties and the elastic load‐recovery of the blends were largely dependent on the ratio between the polymers. The ability of these blends to create self‐assembled microspheres and the fact that they did not require a compatibilizer or plasticizer point to the potential of these systems to be used for structural implantable drug delivery systems.

The Role of POSS Functionality on Induced Deviation of Nano and Micromechanical Properties of UV Curable Urethane Acrylate Nanocomposite Coatings

The inorganic-organic hybrid coatings based on UV curable urethane acrylate contained two types of polyhedral oligomeric silsesquioxane (POSS) were prepared. The hydroxyl functionalized POSS nanoparticles, pristine POSS, and the one modified by acrylic acid utilized to study the effect of nanoparticle content and its functionality on ultimate mechanical properties of the hybrid coatings. The nanomechanical properties and scratch resistance of nanocomposite coatings were evaluated with nanoindentation and nanoscratch techniques, respectively. Consequently, the results compared with previously obtained DMTA data. The results revealed that by embedding acrylic acid modified POSS, the surface hardness and elastic modulus improved. Further increasing acrylic acid modified POSS content to 5%, the surface elastic modulus and hardness improved to 37% and 35%, respectively, relative to neat coating and also the enhanced scratch resistance was observed. Furthermore, the discrepancy between the surface elastic modulus and bulk elastic modulus was rationalized and the contributed parameters that led to this deviation such as pile-up area, holding time and hydrostatic stress was investigated. The results showed that apart from the inefficiency of Oliver and Pharr method to precisely evaluate the nanomechanical properties of hybrid coatings, the targeted placement of POSS nanoparticle is responsible for this disparity between surface mechanical properties and its bulk values.

Combining foam injection molding with batch foaming to improve cell density and control cellular orientation via multiple gas dissolution and desorption processes

In contrast to solid parts fabricated through conventional injection molding (CIM), foamed parts manufactured via foam injection molding (FIM) exhibit substantial variations in mechanical properties, which are attributed to differences in the cellular structure. In this study, parts with different cellular structures are fabricated via FIM, during which the gas dissolution and desorption processes are controlled by subjecting the gas‐laden melt to reciprocating compression and expansion operations. The results suggest that the cell density can be drastically improved by rapidly decreasing the pressure caused by the mold opening and that the cell orientation obviously occurs in the direction perpendicular to the mold‐opening direction. Moreover, the cell density and cellular orientation can be adjusted by utilizing appropriate mold opening and closing operations, leading to improvements in the resultant ultimate mechanical properties. In particular, the foamed samples fabricated with controlled mold opening‐closing operations exhibit excellent tensile strength and strain‐at‐break, indicating that samples containing a high density of cells oriented along the tensile test direction facilitate the formation of superductility and an increase in tensile strength. Hence, a method that combines FIM with batch foaming has been proposed for improving the cellular structure and controlling the cellular orientation.

Thermo-Mechanical Behavior and Hydrolytic Degradation of Linear Low Density Polyethylene/Poly(3-hydroxybutyrate) Blends

In this work, a commercial linear low density polyethylene (LLDPE) utilized for packaging applications was melt compounded with different amounts (from 10 wt% up to 50 wt%) of poly(3-hydroxybutyrate) (P(3HB)), with the aim to evaluate the possibility to partially replace LLDPE with a biodegradable matrix obtained from renewable resources. The processability, microstructural and thermo-mechanical behaviour of the resulting blends was investigated. Melt flow index (MFI) values of the LLDPE matrix were not much affected until a P(3HB) content of 20 wt%, while for higher P(3HB) concentrations an evident decrease of the viscosity was detected. Scanning electron microscope (SEM) observations on the blends highlighted that at limited P(3HB) concentrations the secondary phase was homogeneously dispersed in form of isolated domains, while at a P(3HB) content of 50 wt% a continuous layered morphology could be detected. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR) did not evidence any chemical or physical interaction between the two polymer phases. Quasi-static tensile tests and dynamical mechanical analysis showed that the introduction of P(3HB) led to a pronounced stiffening effect, while the progressive drop of the yield and ultimate mechanical properties could be attributed to the weak interfacial adhesion and poor compatibility between the two matrices. The resistance to hydrolytic degradation of the LLDPE/P(3HB) blends was evaluated over a period of 100 days of immersion in water at 50 °C. It was observed that the weight variation and the decrease of the tensile properties due to the hydrolytic process on the biodegradable phase were evident only for a P3HB content of 50 wt%. In conclusion, this work showed that the partial replacement of LLDPE with a biobased P(3HB) could lead to the development of an innovative blend with good processability and mechanical properties, until a P(3HB) amount of 20 wt%.