Ductile Polymer Research Articles

Determination of the Fracture Resistance of Ductile Polymers: The ESIS TC4 Recent Experience

Abstract Round-robin (RR) tests carried out under the direction of the Technical Committee 4, “Polymers, Polymer Composites and Adhesives,” of the European Structural Integrity Society (ESIS TC4) showed that the multispecimen methodology employed for the construction of the crack growth resistance curve (J vs crack extension, Δa) of polymers often does not provide reliable data because of the uncertainties associated with the measurement of Δa. With this in mind, the ESIS TC4 attention has been more recently focused on the analysis of a testing scheme based on the load separation criterion, which does not require the measurement of Δa. With the aim to employ this new approach into a standardized procedure, the degree of reproducibility of the results obtainable with the application of this testing scheme to ductile polymers has been assessed by means of multilaboratory RR testing exercises that started in 2011. An ESIS TC4 reference draft protocol was prepared and 10 laboratories participated in the RR activities. The present work describes the load separation criterion-based testing procedure recently examined by ESIS TC4 and gives a summary of the results obtained in the RR activities, which appear encouraging.

All-atom molecular dynamics study of impact fracture of glassy polymers. II: Microscopic origins of stresses in elasticity, yielding, and strain hardening

Polycarbonate (PC) is a typical ductile polymer which first undergoes shear yielding followed by plastic deformation when subjected to stress. Although numerous theoretical models have been proposed to explain the yielding, its microscopic origin remains unclear. Therefore, we analyzed herein our previous all-atomistic molecular dynamic (MD) trajectories to investigate the microscopic origins of the yielding stress as well as the entire fracture process via rigorous stress decomposition, which considered the contributions of the system alignment and the bond, angle, and van der Waals (vdW) interactions. The affine deformation in the elastic region occurs due to bond stretching, angle bending, and separation of the vdW interacting atom pairs. Plastic deformation, which initiates upon yielding, reduces the growth rate of the bond, angle stresses and vdW stresses via the interchain collisions; meanwhile, the system alignment also contributes to an increased stress. Finally, the main chain bonds and angles are further stretched after strain softening, thereby causing strain hardening.

Scratch behavior of polycarbonate by Rockwell C diamond indenter under progressive loading

Rockwell C diamond indenter with a spherical tip of radius of 108 μm was used to slide on bulk polycarbonate under linearly increasing normal load from 5 mN to 20 N in order to investigate deformation and damage of ductile polymers during scratching. The peak value of the ratio of residual depth over penetration depth corresponds to a critical normal load, which can be used as the piecewise point for the subsection functions describing variation of tangential load, penetration depth and residual depth with normal load. The true adhesion interfacial friction coefficient was found to be 0.3 for contact between PC and diamond by three-dimensional model. Residual groove widths were found to be proportional to the square root of normal load, and provided useful information to characterize the radius of spherical tip. Values of fracture toughness of PC obtained by scratch-based methodologies under sufficiently large normal loads associated with brittle fracture and crack plane, are in excellent agreement with literature values. The material is under triaxial compressive stress state, making the flow and shear stresses much larger than the ones for uniaxial tensile test.

Mechanical and Shrinkage Behaviors of Ductile Fiber-Reinforced Polymer Repair Mortar

Repair mortar (RM) with dense texture and high anti-crack is cost-effective for application onto the surface of concrete structure to effectively delay the detachment of concrete protective layer and the corrosion of steel bars. Here, the effects of ductile fiber and polymer latex on the flexural strength (ft), compressive strength (fc), bond strength (fb), and shrinkage rate (δr) of fiber-reinforced polymer repair mortar (FP-RM) were comprehensively studied. Results show, the individual doping of polymer latex can improve the ft, fb, and the toughness (the ft/fc ratio) of P-RM, and the fb is increased by 75.78% with respect to the plain mortar, which imply polymer ingredient is essential to P-RM. Some ductile fibers individual dosage also can enhance the ft, fc, and δr of F-RM, respectively; When the polymer latex and ductile fiber are simultaneously doped into the FP-RM mortar together, the ft and fb of FP-RM can be increased up to 9.4MPa, and 2.52MPa, respectively, in tandem with 40.54% reduction of δr, showing superior synergy effect.

Physical insights into stress–strain process of polymers under tensile deformation via machine learning

ABSTRACT Strain localization is a ubiquitous phenomenon of soft matters subjected to strain. Polymeric materials are a very important class of soft materials and widely used nowadays. Polymers have unique strain localization behavior such as crazing during tensile deformation. How to understand the mechanism at the molecular level of strain localization in polymeric materials has become an important topic in material science. In this work, tensile deformation process of polymers both under a melt state and a glassy state are investigated in MD simulations using a generic coarse-grained model. We use a machine learning technique, i.e., support vector machine (SVM) algorithm, to understand the local molecular structure and the dynamical properties during tensile deformation. By defining “softness” from the SVM model, we investigate the stress–strain behavior of both ductile polymer above glass transition temperature and brittle polymer glass during tensile deformation. We demonstrated that the softness can be used to predict physical properties efficiently; the softness provides deep physical insights into the non-equilibrium stress–strain process. We also find that the Hookean behavior of polymer glasses is mostly contributed by the hard regions of the system, and the elastic limit is quantitatively discussed as well.

Prediction of the Effective Stress-Strain Curves of Ductile Polymer 1D-Reinforced Composites Filled with Hollow Fibers Using Parameterized Model Based on Bezier Curves

The article presents the results of numerical and experimental studies of stress-strain curves of 1D-reinforced polymer composite materials based on hollow porous fibers and epoxy matrix. The two-scale nature of the composite under research was taken into account. A surrogate easily parameterized model based on Bezier curves was developed and used to approximate the stress-strain curve of ductile material. The calculations were performed using reversible homogenization and finite element methods, which were implemented in computational subsystem of DCS GCD. Representative volume elements of the investigated materials were created using the geometry generating subsystem of DCS GCD. Test samples were made using three-axis milling machine and compression tests were carried out. Computational results of effective stress-strain curves determination were obtained and compared with experiments.

Engineering thermally and electrically conductive biodegradable polymer nanocomposites

There is an urgent demand for producing biodegradable polymer based composites with good thermal and/or electrical conductivity to mitigate the plastic pollution introduced by electronic waste. Here, we have designed and engineered a mechanically strong, melt processable, biodegradable polymer based nanocomposite with excellent thermal and electrical conductivity using filler dispersion principle and the work of adhesion (Wa) as guides. In the design, graphene nano-platelets (GNPs) were dispersed into a highly ductile biodegradable polymer - poly (butylene adipate-co-butylene terephthalate) (PBAT). Blending with another biodegradable polymer, poly (lactic acid) (PLA) that has low affinity to GNPs, confined the dispersion of GNPs within PBAT matrix, thereby facilitating the formation of a percolated network. As a result, high thermal conductivity (3.15W/m⋅K) and electrical conductivity (338S/m) were achieved for the nanocomposite at 40 wt% of GNPs loading, and the mechanical performance remained strong even at such filler loading due to the strong interaction between GNPs and PBAT. This study provides a new strategy for effectively producing high thermally and/or electrically conductive polymer nanocomposites.

3-D printing takes on the body

One in every 2,000 children is born with tracheobronchomalacia, a life-threatening weakness of the walls of the trachea that can collapse the airway, making it difficult or impossible to breathe. In 2012, Glenn Green, a pediatric otolaryngologist at C.S. Mott Children’s Hospital in Ann Arbor, Michigan, was treating five people—four babies and one adolescent—with the condition. He teamed up with Scott Hollister, a professor of biomedical engineering at the University of Michigan, to create splints that would support the trachea. Guided by computed tomography scans of the patients, the pair 3-D printed splints out of the ductile and bioresorbable polymer polycaprolactone for the babies and out of high-performance polyether ketone ketone (PEKK) for the teenager. The splints allowed the children to breathe normally until their own tissues grew strong enough for them to do so unassisted. To date, some 60 splints have been placed in 30 people. Green, who sees

Molecular-engineered hybrid carbon nanofillers for thermoplastic polyurethane nanocomposites with high mechanical strength and toughness

The development of strong-yet-ductile polymer nanocomposites is critical, but still faces a great challenge. Herein, a hybrid carbon nanofiller consisting of one-dimensional carbon nanotube (CNT) and two-dimensional graphene (G) was molecularly engineered, and the as-prepared G-CNT hybrid nanofiller was then solution-casted with thermoplastic polyurethane (TPU) for the fabrication of TPU nanocomposite films. Due to the formation of unique network structure with multiple interactions of G-CNT hybrid within TPU matrices, the as-fabricated TPU nanocomposites showed simultaneously improved mechanical strength and toughness. With addition of only 1 wt% G-CNT hybrid, the tensile strength and toughness of TPU/(G-CNT) nanocomposite reached 69.5 MPa and 246.2 MJ m−3, 1.9 and 2.9 times over that of neat TPU, respectively. The significant and simultaneous enhancement in mechanical strength and toughness of TPU nanocomposites is attributed to homogeneously distributed G-CNT hybrid and its unique energy dissipations due to factures at different deformation stages upon tensile deformation. The work therefore provides a simple and efficient strategy for fabricating mechanically strong and ductile polymer nanocomposites with molecular-engineered hybrid carbon nanofillers.

Investigation of the Effects of Mold Temperature, Test Temperature and Strain Rate on Mechanical Behaviour of Polypropylene

This paper investigates the time and temperature dependence of a ductile polymer, polypropylene, from the mechanical point of view. As the polymeric specimens have been manufactured by the injection molding process, the influence of mold temperature has been also subjected to the study. Compression tests have been performed both in quasi static and dynamic range (10–3/103 s−1), at different test temperatures (− 20/40 °C), and the results have been analysed in terms of yield stress and failure behaviour. The strength of the polymer is strongly influenced from all the input parameters. Mold temperature has a relevant effect only at room and high temperature. Polypropylene reveals a ductile to brittle transition when it is loaded at high rates, except when test temperature is high. The relation between test temperature, strain rate and yield stress is found to be well described by the Ree–Eyring model, adapted to the two relaxation mechanisms involved.

Strengthening, toughing and thermally stable ultra-thin MXene nanosheets/polypropylene nanocomposites via nanoconfinement

Developing high-performance polymer nanocomposites with outstanding heat resistance and high strength, and superior ductility is vital for their practical applications. However, it is a great challenge to achieve simultaneous improvements in the strength and ductility due to the presence of antagonistic mechanisms in the both parameters. Nanoconfinement structure affording heat resistance and high strength, and good extensibility is inspired by the natural world, such as spider web, silkworm net, and honeycomb consisting of three-dimensional networks generated by vast hydrogen bonds. Here, we reported the methods of oxygen-free fast drying assisted solution casting and melt blending for fabricating advanced ultrathin two-dimensional (2D) titanium carbide (Ti3C2Tx)/polypropylene nanocomposites with significantly enhanced initial degradation temperature (79.1 °C increase), tensile strength (35.3% increase), ductility (674.6% increase) and storage modulus (102.2% increase). The thermal stability and mechanical properties improvements induced by Ti3C2Tx nanosheets are superior to those of similar 2D nanomaterials, such as graphene, molybdenum disulfide, montmorillonite, layered double hydroxide, and even 1D carbon nanotubes because of the combination of H-bonds induced nanoconfinement structure with the physical barrier effect of ultrathin Ti3C2Tx nanosheets. This work provides a facile nanoconfinement-inspired strategy for the design of thermally stable, mechanical strong and ductile polymer materials and a paradigm for broadening the application of 2D MXenes in polymeric materials.

Effect of Type of Adhesive Material on the Strength of Bullet-Proof Glass: A Parametric Study

Laminated glass armors play key roles in defense and architecture due to its high aesthetics and inviolability. Armor designing is based upon Wave propagation phenomena produced during a high velocity impact. Laminated glass or bulletproof glass usually comprises of two or more glass sheets bonded by an interfacial layer of adhesives and backed with a high strength ductile polymer (polycarbonate). Typical bonding agents include either high performance adhesives or synthetic resins such as Polyurethane or Polyvinyl butyral (PVB). Recent literature review infers that the ballistic performance of armors is markedly influenced by both the type and thickness of bonding agents, which may be explained by the acoustic impedance mismatch between laminate materials. This paper documents a novel simulation parametric study, considering the material of adhesive layer and thus concluding that, type of adhesive layer plays an important role in maintaining the strength of the bullet-resistant glass. This study took into account the effect of three types of bonding agents (epoxy, polyurethane and PVB) having constant thickness of 1.50 mm, on the ballistic performance of the laminated glass, against AK-47 ammunition projectiles. Simulations were performed in Ansys Explicit Dynamics using a finite element method approach.

Tensile properties of graphene nanoplatelets/epoxy composites fabricated by various dispersion techniques

This study investigates the tensile behavior of ductile and brittle epoxy polymers reinforced by graphene nanoplatelets (GNPs) dispersed by various methodologies. Dispersion of GNPs into the epoxy matrix using sonication or sonication in combination with high shear mixing was investigated. Also, the effectiveness of the dispersion of GNPs into epoxy resin or hardener was assessed. Then, the effect of GNP concentration on the tensile properties of GNP/epoxy composites fabricated by three selected dispersion techniques was explored. A large number of specimens were tested under uniaxial tensile loading and the results were analyzed in terms of tensile strength, fracture elongation, and tensile modulus. Finally, scanning electron microscopy imaging was used to assess the fractured surface of the selected specimens. Results indicate that the addition of GNPs into brittle epoxy does not provide significant improvements in tensile properties. On the other hand, up to 41% increase in tensile strength and 19% increase in tensile modulus were observed for ductile epoxy with 1 wt% GNPs. However, the addition of GNPs into either brittle or ductile epoxy reduces the fracture elongation of the developed composites.

Calibration of a Cohesive Model for Fracture in Low Cross-Linked Epoxy Resins.

Polymer-based composites are becoming widely used for structural applications, in particular in the aeronautic industry. The present investigation focuses on the mechanical integrity of an epoxy resin of which possible damage results in limitation or early stages of dramatic failure. Therefore, a coupled experimental and numerical investigation of failure in an epoxy resin thermoset is carried out that opens the route to an overall micromechanical analysis of thermoset-based composites. In the present case, failure is preceded by noticeable plasticity in the form of shear bands similar to observations in ductile glassy polymers. Thus, an elastic-visco-plastic constitutive law initially devoted to glassy polymer is adopted that captures the rate- dependent yield stress followed by softening and progressive hardening at continued deformation. A general rate-dependent cohesive model is used to describe the failure process. The parameters involved in the description are carefully identified and used in a finite element calculation to predict the material’s toughness for different configurations. Furthermore, the present work allows investigation of nucleation and crack growth in such resins. In particular, a minimum toughness can be derived from the model which is difficult to evaluate experimentally and allows accounting for the notch effect on the onset of failure. This is thought to help in designing polymer-based composites.

(Invited) Distinctive Characteristics of Internal Fracture in Tough Double Network Hydrogels Revealed By Various Modes of Stretching

(Invited) Distinctive Characteristics of Internal Fracture in Tough Double Network Hydrogels Revealed By Various Modes of Stretching

Physical properties of green composites based on poly-lactic acid or Mater-Bi® filled with Posidonia Oceanica leaves

This work focuses on the evaluation of Posidonia Oceanica leaves as effective reinforcing agent for ecofriendly, fully biodegradable polymer composites. Posidonia leaves were washed, ground and sieved in order to achieve two different size distributions and aspect ratios. They were then added to either a stiff or a ductile biodegradable polymer matrix, respectively poly-lactic acid (PLA) and MaterBi® (MB), at two different filler contents (10 wt% and 20 wt%). The materials were fully characterized from a spectroscopic, morphological, rheological, and mechanical point of view. In particular, the outcomes of tensile tests were statistically analyzed by using a Full Factorial Design in order to examine the main effect of type of polymer, filler size and filler content (as well as their mutual interactions) on two properties of great concern, such as elastic modulus and toughness.

Thermoreversible Folding as a Route to the Unique Shape-Memory Character in Ductile Polymer Networks.

Ductile, cross-linked films were folded as a means to program temporary shapes without the need for complex heating cycles or specialized equipment. Certain cross-linked polymer networks, formed here with the thiol-isocyanate reaction, possessed the ability to be pseudoplastically deformed below the glass transition, and the original shape was recovered during heating through the glass transition. To circumvent the large forces required to plastically deform a glassy polymer network, we have utilized folding, which localizes the deformation in small creases, and achieved large dimensional changes with simple programming procedures. In addition to dimension changes, three-dimensional objects such as swans and airplanes were developed to demonstrate applying origami principles to shape memory. We explored the fundamental mechanical properties that are required to fold polymer sheets and observed that a yield point that does not correspond to catastrophic failure is required. Unfolding occurred during heating through the glass transition, indicating the vitrification of the network that maintained the temporary, folded shape. Folding was demonstrated as a powerful tool to simply and effectively program ductile shape-memory polymers without the need for thermal cycling.

Influence of heating on spectroscopic, mechanical, and thermal properties of reduced graphene oxide-poly(vinyl alcohol) composite films

The spectroscopic, uniaxial tensile, and thermal properties of reduced graphene oxide (rGO)-poly(vinyl) alcohol (PVA) composite films produced by using rGO synthesized using modified Hummer's method with varying weight ratio of rGO up to 10% were investigated by using an uniaxial tensile tester, Fourier Transform Infrared (Ft-IR)/Attenuated Total Reflectance (ATR) spectroscopy, and Differential Scanning Calorimeter (DSC) methods after heating process at different temperatures from room temperature to 200 °C. For unheated composite films, with increasing rGO weight ratio up to 10%, the Young's modulus of rGO-PVA films increased by approximately 2–3 times, conversely the strain at break values decreased greatly by about 40–50%. The heating process led to the significant improvement in the tensile strength and the Young's modulus, and great decrease in extensibility with respect to the unheated samples, e.g., the tensile strength and Young's modulus increased by 3–4 times and 8–10 times, respectively, for the heating temperatures up to 150 °C. With increasing heating temperature, the stress-strain curve of rGO-PVA composite films turned from that of ductile and amorphous polymer to that of more brittle and rigid polymers. With increasing rGO content, the glass transition, the crystallization, and the melting temperatures increased and heating process generally shifted these temperatures to higher values. The great contribution of rGO content and the heating to the improvement of especially mechanical properties of rGO-PVA composite films thanks to the strong intermolecular interactions of rGO nanosheets with the PVA matrix molecules through hydrogen bonding and to the changes in the crystallinity ratio of PVA was shown in detail in the analysis of the spectral results. The relationship between the changes in the mechanical, structural, and thermal properties of the studied composite films was determined by using tensile testing, Ft-IR/ATR, and DSC methods.

Effect of Babassu Natural Filler on PBAT/PHB Biodegradable Blends: An Investigation of Thermal, Mechanical, and Morphological Behavior.

Blending of biodegradable polymers in combination with low-price organic fillers has proven to be a suitable approach to produce cost-effective composites in order to address pollution issues and develop products with superior mechanical properties. In the present research work PBAT/PHB/Babassu composites with 25, 50, and 75% of each polymer and 20% of Babassu were produced by melting extrusion. Their thermal, mechanical, and morphological behavior was investigated by differential scanning calorimetry (DSC), tensile testing, and scanning electron microscopy (SEM). Blending PBAT with PHB inhibited the crystallization of both polymers whereas adding Babassu did not significantly change their melting behaviour. Incorporation of Babassu reduced the tensile strength of its respective blends between 4.8 and 32.3%, and elongation at break between 26.0 and 66.3%. PBAT as highly ductile and low crystalline polymer may be seen as a crystallization tool control for PHB as well as a plasticizer to PBAT/PHB blends and PBAT/PHB/Babassu composites. As PBAT content increases: (i) elongation at break increases and (ii) surface fracture becomes more refined indicating the presence of more energy dissipation mechanisms. As PBAT/PHB/Babassu composites are biodegradable, environmental friendly, and cost effective, products based on these compounds have a great potential since their mechanical properties such as ductility, stiffness, and tensile strength are still suitable for several applications even at lower temperatures (−40 °C).

Dynamic tension of ductile polymers: Experimentation and modelling

Two main problems are recurrently mentioned when performing dynamic tension tests of polymers: The weak transmitted signal, often close to the level of the signal noise, and the (in)ability to achieve dynamic force equilibrium. In this work, we investigated the dynamic tensile response of commercial polycarbonate (PC) using the Kolsky-Bar apparatus and digital image correlation (DIC). We conducted 2D numerical simulations to explore the sensitivities of the experimental apparatus and to simulate the dynamic tests. By specifically focusing on the material behavior within the dynamically evolving neck, calibration of a stress strain curve was achieved in combination of numerical modeling. We introduce a “4-point model” that can successfully predict the location of the neck and the strain distribution along the specimens. The static tensile, dynamic tensile and dynamic compressive behavior of commercial PC are compared, and our analysis suggests a strong correlation between the three. This observation could facilitate the measurement of the dynamic tensile strength of polymers – to be possibly extracted from dynamic compression or static tension measurements only.