Craze Initiation Research Articles

Analysis of mode I crack propagation in glassy polymers under cyclic loading using a molecular dynamics informed continuum model for crazing

Craze and crack propagation in glassy polymers under cyclic mode I loading are investigated by employing a recently developed continuum-micromechanical model for crazing. This model accounts for the local morphology change from microvoids to fibrils during craze initiation, viscoplastic drawing of bulk material into fibrils, and viscoelastic creep recovery of the fibrillated craze matter during unloading. To ensure consistency between the bulk and craze model parameters, the material parameters of the craze model are normalised and calibrated based on a hybrid approach integrating experimental findings from the literature and molecular dynamics results. This yields a generic, yet representative glassy polymer response.In the framework of 2D plane strain finite element simulations, we study brittle as well as ductile glassy polymers and assess the results by drawing comparisons to the experimental and numerical literature. For brittle materials, characterized by a purely elastic bulk behaviour, the model reproduces craze characteristics such as the craze opening contour, the craze length-to-width ratio, a double stress peak at the craze and crack tip, and a non-proportional stress redistribution during loading-unloading cycles. In ductile glassy polymers, the interaction of shear yielding in the bulk and crazing along the ligament is analysed. In particular, shear bands emanate from the crack tip in each loading cycle and arch forward towards the craze. This plastic zone shares resemblance to the so-called epsilon-shaped deformation zone. The current simulations capture normal fatigue crack propagation, where craze and crack growth occur near the peak load in every cycle and the craze length remains relatively constant across the loading cycles. Moreover, findings from this study suggest that plasticity-induced unloading of the craze adjacent to the crack tip impedes crack growth.

Achieving high impact toughness in injection-molded SMMA foams via the synergistic effects of cell size and SBS

Optimizing the cellular structure of polymeric foams has long been considered one of the most economical and effective methods for enhancing their impact properties. However, the relationship between cell size and impact strength has rarely been systematically studied. In this study, styrene-butadiene-styrene (SBS) was used as a toughening agent to improve the toughness of Poly(styrene-co-methyl methacrylate) (SMMA). The expansion ratio of pure SMMA foams and their blended counterparts were controlled by fixing the mold-opening distance, while the cell size was adjusted by changing the blowing agent content and packing time. Notably, the impact strength of the optimized SMMA/SBS blend foam reached 15.5 kJ/m2, representing a significant increase of 1400 % compared to that of the pure SMMA foam. In addition, the impact strength of pure SMMA foams did not change significantly as the cell size increased from 29.7 μm to 278.9 μm. However, for SMMA/SBS blend foams, an optimal cell range (100−150 μm) was identified, where the impact strength was approximately twice that of foams with smaller cell sizes (1−25 μm). Then, examination of the impact-fractured surfaces at different cell sizes revealed that the synergistic effects of an appropriate cell size (100−150 μm) and the presence of SBS particles promoted craze initiation and expanded the plastic deformation area during crack initiation, leading toenhanced impact toughness in the blended foam. This work not only systematically elucidates the relationship between cell size and impact strength but also provides new insights into the synergistic effects of rubber particle and cell size on the mechanical properties of polymeric foams.

Effect of interface on elastic and toughening behavior in poly lactic acid/thermoplastic starch blends: Micromechanical finite element analysis

AbstractIt is frequently emphasized that the action of interfacial adhesion is a critical parameter to improve the stiffness and toughness of polylactic acid/thermoplastic starch (PLA/TS) blends. In this work, the micromechanical behavior of PLA/TS blends with droplet morphology selected from literature is predicted and analyzed systematically by finite element analysis. A quantitative assessment of the effect of interface (perfect or imperfect) on the elastic behavior and craze initiation for toughening of PLA/TS blends is presented. For the elastic behavior, the PLA phase is the blend's load‐bearing component as the TS is more compliant than PLA, so an interface perfectly bonded reduces the blend's elastic modulus when compared to the modulus obtained if the interface is weakly bonded. Regarding the toughening behavior, as a compliant phase, the TS has the potential to nucleate stable crazes in the host PLA matrix independently of the degree of interfacial adhesion because the highly stressed region lies near the equator of the particle; nonetheless, the critical stress for craze initiation is very sensitive to the TS particle size. On the other hand, as the TS is less capable than PLA to develop large hydrostatic stresses, the TS has a low potential to dissipate energy by cavitation.

Crazing Initiation and Growth in Polymethyl Methacrylate under Effects of Alcohol and Stress

Polymer crazing is typically a precursor to damage and considerably reduces the mechanical performance of polymer materials. The concentrated stress caused by machines and the solvent atmosphere created during machining exacerbates the formation of crazing. In this study, the tensile test method was employed to examine the initiation and progression of crazing. The research focused on polymethyl methacrylate (PMMA), both regular and oriented, and the impact of machining and alcohol solvents on the formation of crazing. The results showed that the alcohol solvent influenced PMMA through physical diffusion, whereas machining primarily affected crazing growth via residual stress. Treatment reduced the crazing stress threshold of PMMA from 20% to 35% and produced a threefold increase in its sensitivity to stress. The findings revealed that oriented PMMA exhibited 20 MPa higher resistance to crazing stress compared with regular PMMA. The results also indicated that the extension of the crazing tip and thickening were in conflict, with the crazing tip of regular PMMA severely bending under tensile stress. This study provides valuable insight into the initiation of crazing and the methods of its prevention.

Fabrication of super-toughened polypropylene-based nanocomposite with low elastomer content through tailoring the microscale damage mechanisms

Structural composites with high fracture toughness have wide applications in various fields. However, achieving high toughening efficiency at low fabricating cost is still challenging. Here, one-dimensional carbon nanofibers (CNFs) were incorporated into the ethylene-octene copolymer (POE)-toughened polypropylene (PP) blends. The fracture behaviors of the samples were systematically researched under different load conditions, such as impact load condition, three-point bending test and single-edge notched tensile (SENT) measurement, and then the toughening mechanisms were proposed. The results indicated that CNFs dramatically enhanced the fracture toughness of the PP/POE blends due to that CNFs prevented the initiation and propagation of crazes. Furthermore, through incorporating an additional annealing treatment, the fracture mechanism of composites changed from crazing fracture to shear yielding-crazing fracture, which endowed CNFs with more apparent toughening effect. For example, at POE and CNF contents of 25 wt% and 1 wt%, the annealed composite sample showed the impact strength of 69.4 kJ/m2 and elongation at break of 395.3%, which were increased by 208.4% and 92.0% compared with the unannealed binary blend, respectively. This work gives a novel insight to understand the toughening effect of one-dimensional nanofillers in the plastics/elastomer blends, and the methods may be used to toughen the other blend composites.

A combined experimental and computational study of environmental stress cracking of amorphous polymers

Environmental stress cracking (ESC) refers to a phenomenon causing the failure of polymeric materials such as thermoplastics. Although the precise mechanism is not fully understood, a few key incidents are known to be involved in the full failure process. Adsorbed fluid molecules on the polymer surface and their migration into the bulk material cause craze initiation and growth as well as eventually parts breakdown. The diffusion of the media inside the polymer, causing plasticization, is accepted to be one of the key incidents involved. Such materials degradation is a self‐sustained aging mechanism: local defects creating more free volume for fluid molecules to migrate into, causing more degradation until critical failure. Molecular modelling (MM) simulation techniques have been used in the present study for the first time in the ESC research to scrutinize the applicability of these techniques to the description of molecular events responsible for the various degradation steps. Molecular cross section, polymer‐fluid compatibility (as measured by the fluid solvation free energy), adsorption energies, etc, are possible descriptors to build up a quantitative structure‐activity relationship (QSAR) model to allow the prediction of the ESC risk presented by a fluid for a given polymer. Three amorphous polymers, namely, polystyrene (PS), polymethylmethacrylate (PMMA), and polycarbonate (PC), and various fluids (such as water and ethanol) have been modelled, using the above‐mentioned MM techniques.

Effect of Poly(styrene-ran-methyl acrylate) Inclusion on the Compatibility of Polylactide/Polystyrene-b-Polybutadiene-b-Polystyrene Blends Characterized by Morphological, Thermal, Rheological, and Mechanical Measurements.

A poly(styrene-ran-methyl acrylate) (S-MA) (75/25 mol/mol), synthesized by surfactant-free emulsion copolymerization, was used as a compatibilizer for polystyrene-b-polybutadiene-b-polystyrene (SBS)-toughened polylactide (PLA) blends. Upon compatibilization, the blends exhibited a refined dispersed-phase morphology, a decreased crystallinity with an increase in their amorphous interphase, improved thermal stability possibly from the thicker, stronger interfaces insusceptible to thermal energy, a convergence of the maximum decomposition-rate temperatures, enhanced magnitude of complex viscosity, dynamic storage and loss moduli, a reduced ramification degree in the high-frequency terminal region of the Han plot, and an increased semicircle radius in the Cole–Cole plot due to the prolonged chain segmental relaxation times from increases in the thickness and chain entanglement degree of the interphase. When increasing the S-MA content from 0 to 3.0 wt %, the tensile properties of the blends improved considerably until 1.0 wt %, above which they then increased insignificantly, whereas the impact strength was maximized at an optimum S-MA content of ~1.0 wt %, hypothetically due to balanced effects of the medium-size SBS particles on the stabilization of preexisting crazes and the initiation of new crazes in the PLA matrix. These observations confirm that S-MA, a random copolymer first synthesized in our laboratory, acted as an effective compatibilizer for the PLA/SBS blends.

Creep life assessment craze damage evolution of polyethylene methacrylate

AbstractTests have been conducted to investigate the influence of temperature, stress, and crazing for polyethylene methacrylate (PMMA) creep life. Creep life decreases with stress in a logarithmic relation, and rupture strains increase with decreasing stress. To assess the creep life of PMMA, the Larson–Miller parameter is first introduced to correlate stress and temperature with rupture time. The important role of crazing in polymer material damage, especially, the craze density and craze growth rate in the stable creep stage, is demonstrated. The craze initiation time is related to the tensile strength and to the stress and temperature at the elastic limit, and the craze initiation time can be predicted by an empirical model subsequently presented in this paper. Based on the connection between craze growth and creep rupture time, thereafter, a modified Monkman–Grant model is presented to predict creep life. The curve of craze density over creep time has the same two distinct stages as the curve of strain over time. In the final sections, a critical assessment of creep damage is also presented to provide a convenient method to predict residual life using only craze density.

Network approach towards understanding the crazing in glassy amorphous polymers

We have used molecular dynamics to simulate an amorphous glassy polymer with long chains to study the deformation mechanism of crazing and associated void statistics. The Van der Waals interactions and the entanglements between chains constituting the polymer play a crucial role in crazing. Thus, we have reconstructed two underlying weighted networks, namely, the Van der Waals network and the entanglement network from polymer configurations extracted from the molecular dynamics simulation. Subsequently, we have performed graph-theoretic analysis of the two reconstructed networks to reveal the role played by them in the crazing of polymers. Our analysis captured various stages of crazing through specific trends in the network measures for Van der Waals networks and entanglement networks. To further corroborate the effectiveness of network analysis in unraveling the underlying physics of crazing in polymers, we have contrasted the trends in network measures for Van der Waals networks and entanglement networks in the light of stress-strain behaviour and voids statistics during deformation. We find that the Van der Waals network plays a crucial role in craze initiation and growth. Although, the entanglement network was found to maintain its structure during craze initiation stage, it was found to progressively weaken and undergo dynamic changes during the hardening and failure stages of crazing phenomena. Our work demonstrates the utility of network theory in quantifying the underlying physics of polymer crazing and widens the scope of applications of network science to characterization of deformation mechanisms in diverse polymers.

Influence of liquid media on the craze initiation in amorphous polylactide

The influence of various liquid media (aliphatic alcohols, water-alcohol solutions, saturated hydrocarbons, and organosilicon liquids) on the development of uniaxial deformation and nucleation of crazes in amorphous polylactide (PLA) films has been studied. It has been found that in the presence of the liquids the tensile stress during the polymer deformation decreases by 3–4 times. According to the Griffith's theory, linear dependences (with an accuracy of R2 = 0.96–0.97) have been revealed between the yield point or the flow stress of PLA and the square root of product of the elasticity modulus of the polymer in a liquid and the interfacial surface energy at the polymer-liquid medium interface calculated using the Owens-Wendt equation. The data obtained may be used to predict the critical mechanical stress at which crazes will be initiated in PLA.

The Roles of Blending and of Molecular Weight Distribution on Craze Initiation

Craze initiation stress was measured in three-point bending isochronal creep tests on a series of entangled bimodal blends of polystyrenes of narrow dispersity, on three polystyrenes of broad dispersity, and on four blends of polystyrenes of broad dispersity. Crazing stress was found to increase rapidly with small additions of the higher molar mass component, quickly reaching a plateau. A simple model based on the weighted addition of the crazing stress contributions of the individual weight fractions obtained from an established piecewise linear crazing law was able to predict the crazing stress accurately in the bimodal blends using a power law exponent of 2.59 (90% CI [1.75–17.34]). In broad dispersity systems, in particular where short unentangled chains dilute the polymer, it was found necessary to modify the model using dynamic tube dilution theory. Dilution leads to a change in the entanglement length and hence in the molar mass at which transitions to disentanglement and chain scission crazing occur. With the improved model, crazing stress could be predicted even for the broad dispersity polymers with wide and bimodal distributions. This represents an opportunity for the molecular design of polymers by blending to achieve improved resistance to craze initiation.

Study of microstructure and fracture properties of blunt notched and sharp cracked high density polyethylene specimens

This paper examines the effect of a broad range of crosshead speed (0.05 to 100 mm/min) and a small range of temperature (25 °C and 45 °C) on the failure behaviour of high density polyethylene (HDPE) specimens containing a) standard size blunt notch and b) standard size blunt notch plus small sharp crack – all tested in air. It was observed that the yield stress properties showed linear increase with the natural logarithm of strain rate. The stress intensity factors under blunt notch and sharp crack conditions also increased linearly with natural logarithm of the crosshead speed. The results indicate that in the practical temperature range of 25 °C and 45 °C under normal atmosphere and increasing strain rates, HDPE specimens with both blunt notches and sharp cracks possess superior fracture properties. SEM microstructure studies of fracture surfaces showed craze initiation mechanisms at lower strain rate, whilst at higher strain rates there is evidence of dimple patterns absorbing the strain energy and creating plastic deformation. The stress intensity factor and the yield strength were higher at 25 °C compared to those at 45 °C

In situ investigation of the effect of stretching speed and annealing conditions on the crazing of as-spun IPP fibres using Pluta polarizing interference microscope

The effect of annealing conditions and stretching speed on the crazing and fracture behaviour of thermally treated as-spun IPP fibres during stretching with constant strain rate was investigated. The study includes observations and quantifying the crazing that occurs on stretched fibres via the estimation of the areal craze density as a function of annealing duration and stretching speed. An automated two-beam polarizing light interference Pluta microscope with the aid of modified mechanical stretching device are used for this investigation. Increasing the strain rate for thermally treated samples tends to delay the craze initiation and lowers the amount of areal craze density. A technique based on microscopic image analysis system(MIAS) with suitable designed software were used to observe the general feature of a single craze and calculate its wide and length. The value of strain rate is more effective than thermal history (annealing temperature and duration) on the crazing process of IPP fibres.

3D study of the competitions between shear yielding and crazing for a variable thickness on ductile polymers

The influence of the thickness in mode I fracture tests is investigated with extensive 3D finite elements calculations. A realistic constitutive law for the bulk is used. Failure by crazing is described with a cohesive model. A transition from ductile tearing to fracture by crazing is observed with increasing thickness. Ductile tearing takes place for thin samples even if possible crazing is allowed. Crazing takes place for thick samples with a noticeable difference in the load level for craze initiation to that for crack propagation. The consequence on the appropriate geometry for the estimation of the minimum toughness is discussed.

Mechanics of dynamic fracture in notched polycarbonate

Fracture toughness of brittle amorphous polymers (e.g. polymethyl methacrylate (PMMA)) has been reported to decrease with loading rate at moderate rates and increase abruptly thereafter to close to 5 times the static value at very high loading rates. Dynamic fracture toughness that is much higher than the static values has attractive technological possibilities. However, the reasons for the sharp increase remain unclear. Motivated by these observations, the present work focuses on the dynamic fracture behavior of polycarbonate (PC), which is also an amorphous polymer but unlike PMMA, is ductile at room temperature. Towards this end, a combined experimental and numerical approach is adopted. Dynamic fracture experiments at various loading rates are conducted on single edge notched (SEN) specimens with a notch of radius 150μm, using a Hopkinson bar setup equipped with ultra high-speed imaging (>105fps) for real-time observation of dynamic processes during fracture. Concurrently, 3D dynamic finite element simulations are performed using a well calibrated material model for PC. Experimentally, we were able to clearly capture the intricate details of the process, for both slowly and dynamically loaded samples, of damage nucleation and growth ahead of the notch tip followed by unstable crack propagation. These observations coupled with fractography and computer simulations led us to conclude that in PC, the fracture toughness remains invariant with loading rate at Jfrac=12±3kN/m for the entire range of loading rates (J̇) from static to 1×106kN/m−s. However, the damage initiation toughness is significantly higher in dynamic loading compared to static situations. In dynamic situations, damage nucleation is quickly followed by initiation of radial crazes from around the void periphery that initiate and quickly bridge the ligament between the initial damaged region and the notch. Thus for PC, two criteria for two major stages in the failure process emerge. Firstly, a mean stress based defect initiation is suggested. The value of the critical mean stress for defect initiation under dynamic loading is found to be 115±5MPa, which is significantly higher than its static value of 80MPa. The critical normal plastic stretch needed for crazes to nucleate from the nucleated defect is estimated to be about 1.78±0.2.

Specific features of the environmental crazing of poly(ethylene terephthalate) fibers

Tensile drawing of glassy PET fibers via environmental crazing is studied. Direct microscopic observations show that this process includes the development of macroscopic porosity due to the initiation and growth of multiple crazes with their specific fibrillar-porous structure. Environmental crazing of fibers is characterized by the early collapse of the thermodynamically unstable structure of crazes. This process commences at the stage of craze widening and proceeds until the crazes lose their porosity and become fully monolithic. The collapse is provided by the coagulation of the flexible nanoscale craze fibrils via their interaction by side surfaces. The collapse is markedly intensified when the active liquid is removed from the volume of crazes and, as a result, the crazed fibers acquire a specific surface relief with alternating thick and thin regions corresponding to the bulk unoriented polymer and the collapsed crazes. Practical advantages of the stage of the collapse of the crazed nanoporous structure of crazes for the creation of the fiber-based nanocomposite materials are discussed.

Novel blends of polylactide with ethylene glycol derivatives of POSS.

Polylactide (PLA), a main biodegradable and biobased candidate for the replacement of petrochemical polymers, is stiff and brittle at room conditions. It is therefore of high interest to formulate new PLA-based materials suitable for applications demanding flexibility and toughness. In this work, novel blends of PLA with polyhedral oligomeric silsesquioxanes (POSS) grafted with longer (P1) and shorter (P2) arms of ethylene glycol derivatives were prepared and studied. It was hypothesized that, owing to their architecture with the central POSS cage grafted with arms, miscibility and stability of the blends could be improved. Indeed, PLA/P1 blends were homogeneous despite P1 relatively high M w of 9,500 g mol−1. The blend with 20 wt% of P1, having T g at 16 °C, was transparent and flexible, elastomer-like material with excellent drawability. The blend remained homogeneous and retained its good drawability as well as flexibility during 6 months of aging at room temperature: a 2 % secant modulus of elasticity well below 100 MPa, a low yield stress below 2 MPa, and and a large strain at break of 8 (800 %). Contary to that, PLA/P2 blends were only partially miscible. Nevertheless, owing to the liquid state of the dispersed phase, the blend with 15 wt% of P2 was transparent and ductile, with T g at 49 °C, a relatively high yield strength of 29 MPa, and a large strain at break of 2.3 (230 %). The toughening mechanism involved the initiation of crazes and facilitation of their propagation by the liquid inclusions via the local plasticization effect.

On-line interferometric study on the mechanical fracture behaviour by crazing observed in stretched polypropylene fibres

Two-beam polarizing light interference microscope devised by Pluta is used for in situ investigation of fracture mechanism of as-spun isotactic polypropylene (iPP) fibres by crazing during cold drawing process. The study includes characterization of crazing of polypropylene fibres (involving craze initiation, craze propagation and craze breakdown) as a function of crazing strain, crazing temperature and stretching speed. The investigation of craze damage showed that it is increased rapidly with stretching speed and then increased slowly to level off. Also it was found that, stretching iPP fibre at relatively low speed (lower than 0.015 cm/sec) at constant temperature 19 °C would reduce the effect of fracture on the stretched filaments. The time to crazing, the time to failure and the areal craze density of iPP fibres during cold drawing process are estimated. Finally, observing the craze formation at different temperatures showed that there was a critical value of stretching temperature for the formation of crazes in iPP fibre, and it was found to be 40 °C. The obtained results are correlated to the corresponding variations in some optical and structural properties of iPP fibres due to stretching.

Nanoporous structure of the cell walls of polycarbonate foams

Using CO2 to prepare microcellular polycarbonate foams resulted in a pore diameter of 45 nm and a pore density of 108 cm−2 on the walls of microscale cells, which created nano/micro foams with an open cell structure. In this study, the craze nucleation theory and the bubble nucleation theory of foaming were combined to explain the mechanism of the foaming-induced nanopores (microvoids) on the cell walls. In the foaming process, the strain energy was developed in the cell walls by bubble nucleation and growth. With large strain energy, a nanoporous structure of the cell walls was formed by initiation of crazing. Because the foaming temperature affected the strain energy in the cell wall, the temperature became a key factor of forming microcellular structure as well as the nanopores on the cell walls. Our experimental results showed that the diameter and density of the nanopores were determined by the competitive movements between chain stretching and relaxation. Furthermore, certain solvents, such as acetone, were found to increase the nanopore density of the walls by exploiting the plasticization effect of the solvent on the reduction of surface tension and viscosity.

Regularities of the Rehbinder effect in polymers. Review

Data are analyzed that are related to particular details of manifestations of the Rehbinder effect in the case of polymer deformation in adsorption-active media (AAMs). It is shown that, as a result of such a deformation, a vitreous polymer is dispersed, forming a system of fibrillar aggregates of oriented macromolecules with a diameter of 1–20 nm separated in space by microcavities of similar size (the crazing phenomenon). It is shown that polymer crazing in AAMs is a complex multistage process. Its first stage is initiation of crazes related to the microscopic surface defect structure of the actual polymer and is controlled by the Griffith criterion. Craze growth and broadening are typical thermally activated processes of plastic deformation of the polymer. The main areas for practical application of the Rehbinder effect in polymers are pointed out.