Effect of a tie layer on the delamination toughness of polypropylene and polyamide‐66 microlayers

The effect of a thin tie layer on the adhesion of polypropylene (PP) and polyamide‐66 (PA) was studied by delamination of microlayers. The microlayers consisted of many alternating layers of PP and PA separated by a thin layer of a maleated PP. The peel toughness and delamination failure mode were determined using the T‐peel test. Without a tie layer, there was no adhesion between PP and PA. A tie layer with 0.2% MA provided some adhesion; however, delamination occurred by interfacial failure. Increasing the maleic anhydride (MA) content of the tie layer increased the interfacial toughness. With 0.5% MA, the interfacial toughness exceeded the craze condition of PP, and a transition from interfacial delamination to craze delamination occurred. Crazing ahead of the crack tip effectively reduced the stress concentration at the interface and dramatically increased the delamination toughness. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1461–1467, 1999     

Crazing phenomena in PC/SAN microlayer composites

The crazing behavior of coextruded microlayer sheets consisting of alternating layers of polycarbonate (PC) and styrene–acrylonitrile copolymer (SAN) was investigated as a function of PC and SAN layer thicknesses. In this study, the total sheet thickness remained essentially constant and the PC and SAN layer thicknesses were changed by varying both the total number of layers from 49 to 1857 and the PC/SAN volume ratio. Photographs of the deformation processes were obtained when microspecimens were deformed under an optical microscope. Three different types of crazing behavior were identified: single crazes randomly distributed in the SAN layers, doublets consisting of two aligned crazes in neighboring SAN layers, and craze arrays with many aligned crazes in neighboring SAN layers. The transition from single crazes to doublets was observed when the PC layer thickness was decreased to 6 microns. Craze array development was prevalent in composites with PC layer thickness less than 1.3 microns. It was concluded that SAN layer thickness was not a factor in formation of arrays and doublets; formation of craze doublets and craze arrays was dependent only upon PC layer thickness. © 1994 John Wiley & Sons, Inc.     

Mechanisms of interactive crazing in PC/SAN microlayer composites

Mechanisms are proposed for the two types of interactive crazing that have been observed in PC/SAN microlayer composites when the PC layer thickness is on the micron-size scale. It is demonstrated that when the PC layer is thin enough the deformation zone that forms at a craze tip can interact with the next-neighboring SAN layer. By measuring the dimensions of the craze tip in scanning electron micrographs, it was found that the craze-tip opening does not depend on the SAN layer thickness, i.e., the length of the SAN craze. Consequently, the size and shape of the resulting plastic zone in the PC layer are also independent of layer thickness. The zone that forms in the PC layer consists of a colinear plastic zone together with a pair of micro-shearbands that grow at an angle of about 45°. When the PC layer is less than 6 μm, the elastic stress concentration from the colinear plastic zone increases the probability of crazing in the neighboring SAN layer with the formation of craze doublets that consist of two aligned crazes in neighboring SAN layers. By taking into consideration the Weibull distribution of crazing, a craze doublet fraction comparable to the 30% observed experimentally was predicted with a stress intensification factor in the range of 1.03–1.05. When the PC layer thickness is less than 1.3 μm, the length of the colinear plastic zone is comparable to the PC layer thickness. Formation of a craze at the point of impingement of the plastic zone on the neighboring SAN layer leads to craze arrays with many aligned crazes in neighboring SAN layers. At higher strains, the micro-shearbands grow through the PC layers and extend into several adjacent SAN and PC layers. This produces a change in deformation mechanism in the SAN layers at the yield instability, from craze opening to shear yielding. © 1994 John Wiley & Sons, Inc.     

Comparison of irreversible deformation and yielding in microlayers of polycarbonate with poly(methylmethacrylate) and poly(styrene‐co‐acrylonitrile)

Microlayers of polycarbonate (PC) with poly(methylmethacrylate) (PMMA) or poly(styrene‐co‐acrylonitrile) (SAN) were processed with varying layer thicknesses. Adhesion between PC and PMMA was found to be an order of magnitude higher than between PC and SAN, as determined with the T‐peel method. To probe the effect of the adhesion difference on yielding and deformation of PC/PMMA and PC/SAN microlayers, the macroscopic stress–strain behavior was examined as a function of layer thickness and strain rate, and the results were interpreted in terms of the microdeformation behavior. During yielding, crazes in thick SAN layers opened up into cracks; however, PC layers drew easily because local delamination relieved constraint at the PC/SAN interface. Adhesion of PC/PMMA was too strong for delamination at the interface when PMMA crazes opened up into cracks at low strain rates. Instead, PMMA cracks tore into neighboring PC layers and initiated fracture. At higher strain rates, good adhesion produced yielding of thick PMMA layers, a phenomenon not observed with thick SAN layers. The change in microdeformation mechanism of PMMA with increasing strain rate produced a transition in the yield stress of PC/PMMA microlayers. Microlayers of both PC/SAN and PC/PMMA with thinner layers (individual layers 0.3–0.6 μm thick) exhibited improved ballistic performance compared to microlayers with thicker layers (individual layers 10–20 μm thick), which was due to cooperative yielding of both components. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1545–1557, 2000     

Adhesion and delamination failure mechanisms in alternating layered polyamide and polyethylene with compatibilizer

The effects of compatibilizer and the number of layers on the interfacial adhesion and delamination model of coextruded microlayer samples consisting of alternating layers of high‐density polyethylene (HDPE) and polyamide 6 (PA6) were studied with T‐peel test. When more maleic anhydride‐grafted HDPE was incorporated into HDPE layer, the interfacial delamination model changed from adhesive to cohesive failure in the case of bilayer samples. For high‐layer samples, the results of X‐ray photoelectron spectroscopy showed that the areal density of copolymers at the interfaces increased with increasing number of layers due to strong and durable shearing forces during microlayer coextrusion. Scanning electron microscopy observation revealed that the interfacial delamination model changed from single‐ to multiple‐interface delamination when the number of layers increased from 16 to 32. The crack propagation included a large number of layer–layer jumps. The peel strength of microlayer samples was found to be greatly influenced by the interfacial delamination mechanisms. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

The craze-tip deformation zone in PC/SAN microlayer composites

Crazing in styrene-acrylonitrile copolymer (SAN) layers of polycarbonate (PC)/SAN microlayer composites and the deformation zone that formed in the PC layer in response to the stress concentration created at the PC/SAN interface by the craze tip were examined by optical and scanning electron microscopy. A 49-layer composite with relatively thick layers, on the size scale of tens of microns, was chosen in order to confine interactions to the region of a single PC/SAN interface. The SAN craze density, which increased with applied stress, was shown to fit the Weibull distribution function. The distance between crazes was described by a correlation length, and the distribution of correlation lengths indicated that crazing occurred randomly in the SAN layers. In the PC layer, the crazetip deformation zone consisted of a colinear plastic zone together with a pair of microshearbands that grew away from the craze tip at an angle of about 45°. Assuming a blunted craze tip, the plastic zone was analyzed using the slip-line field theory. The dependence of the micro-shearband length on remote stress was similar to that predicted by both the BCS and Vitek models. © 1994 John Wiley & Sons, Inc.     

The damage zone in microlayer composites of polycarbonate and styrene—acrylonitrile

The irreversible deformation behavior of coextruded microlayer composites, consisting of 49 alternating layers of polycarbonate (PC) and styrene—acrylonitrile polymer (SAN), was examined in the triaxial stress state achieved at a semicircular notch during slow tensile loading. Variations in the proportions of PC and SAN were manifest as changes in the relative thicknesses of PC and SAN layers. When the SAN layers were thicker than the PC layers (PC/SAN 25/75 v/v) or the layer thicknesses were about the same (53/47 v/v) the composites were only slightly more ductile than SAN and the deformation behavior of the layers mimicked that of the components. Examination of optical micrographs showed that the damage zone closely resembled that of SAN and consisted of internal notch crazes in the SAN layers that grew out from the notch surface in conformity with a mean stress condition. Shear processes became more evident when the PC layers were thicker than the SAN layers (PC/SAN 65/35 and 74/26 v/v). An unusual transition was observed in the SAN layers from internal notch crazing to interactive crazing and shear banding. The internal notch crazes ceased to grow when they terminated in a pair of microshear bands in the SAN layer. Subsequently a macroscopic shear-yielding mode was observed as two sets of intersecting slip lines that grew out from the notch surface in both PC and SAN layers. Stress intensification caused by the plastic zone was responsible for the appearance of a second family of internal crazes in the SAN layers that originated in front of the notch tip. © 1993 John Wiley & Sons, Inc.     

An experimental investigation of the effect of interface adhesion on the fracture characteristics of a brittle-ductile layered material

The effect of interface adhesion on the failure characteristics of brittle-ductile layered material was experimentally investigated. Single-edge-notched fracture specimens were prepared by bonding two Homalite-100 layers to a thin aluminum layer using three different types of adhesives. The specimens were loaded under three-point bending and photoelasticity was used for full-field observation of the failure process. Fracture tests revealed two competing modes of failure: delamination along the Homalite-aluminum interface, and crack re-initiation in the Homalite layer across the reinforcing aluminum layer. The failure modes were directly influenced by the characteristics of the adhesive bond. Maximum load retention and energy dissipation capability during the fracture process was observed for a urethane based adhesive that formed an interfacial bond that was resistant to delamination, and additionally exhibited low modulus and large strain-to-failure, thereby suppressing crack re-initiation.     

Oligomer and random copolymer layer in relation to enhancement of interfacial adhesion at polystyrene/poly(styrene‐co‐acrylonitrile) interface

This study examines the interfacial adhesion between poly(styrene) (PS) and poly(styrene‐co‐acrylonitrile) (SAN) interfaces reinforced with poly(styrene‐co‐vinyl phenol) (PS‐ran‐PSPh) random copolymers using an asymmetric double‐cantilever beam (ADCB) test. The effects of oligomer and copolymer composition on interfacial adhesion were investigated. The results showed that the interfacial adhesion of the PS/SAN interface was increased significantly after removing the residual oligomer from the SAN. The interfacial adhesion was also measured for five‐purified SAN materials in the range 17–31 wt%. The highest level of PS/SAN adhesion was observed for 17% AN (acrylonitrile) materials. In addition, the interfacial adhesion of a mixture of diblock and random copolymer was measured in order to investigate which is the most effective method. The results showed that mixture systems are more cost‐effective with higher adhesion, which is independent of temperature. Atomic force microscopy showed that a single craze ahead of the crack is a possible failure mode during PS/SAN interface fracture. Copyright © 2004 Society of Chemical Industry     

On crack evolution with texturization of bonding layer in thermal barrier coating

The interface morphology of the bonding layer has a considerable effect on the damage and failure of sandwich-structured thermal barrier coatings. This work investigated the comprehensive effects of a grooved texture produced using laser ablation on the local surface strain, interfacial stress and strain, and crack behavior of the bonding layer in a thermal barrier coating system. The distribution and evolution of the local surface strain was obtained using the digital image correlation method. The interfacial stress, and the strain between the ceramic and bonding layers, were determined through a simulation of the plane-strain model, and the morphology and propagation of cracks were observed in thermal barrier coatings under an external tensile load. The results indicated that the local surface strain of the thermal barrier coating increased with the texturization of the bonding layer, whereas the fluctuation decreased. There were two inflection points in the local surface strain–time curves, corresponding to the initiation of surface cracks and that of interfacial transverse cracks. The surface cracks were initiated earlier than those without the texturization of the bonding layer. However, the behavior of the interfacial cracks was more complicated. If the roughness of the texture, defined as Rc, was small, the surface cracks propagated vertically to the interface between the ceramic and bonding layers, and turned into transverse cracks, leading to a separation of the ceramic layer. If Rc was greater than 22 μm, the surface cracks went further down to the interface between the bonding layer and substrate, and propagated horizontally, resulting in the separation of both the ceramic and bonding layers. Meanwhile, interfacial cracking and separation were deferred. A large roughness resulted in good cohesion between the ceramic and bonding layers, and a high stiffness for the coating, which improved the damage resistance and extended the life of the coating.     

Structural design and energy absorption mechanism of laminated SiC/BN ceramics

The laminated silicon carbide/boron nitride (SiC/BN) ceramics with different structural designs were fabricated by pressureless sintering at 1900?°C for 1?h in argon flow. The alumina (Al₂O₃)-and yttrium(III) oxide (Y₂O₃)-doped SiC ceramic exhibited a significant intergranular fracture behavior, which could be attributed to the yttrium aluminum garnet (YAG) phase located at the grains boundaries. The bending strength and fracture toughness were used to characterize the crack propagation including the delamination cracking, crack kinking, and crack deflection. The energy absorption in the process of crack propagation was characterized by the work of fracture (WOF) and damping capacity. The mode of crack propagation changed with the change in the structure and variation of BN content in the BN layer. The delamination cracks occurred inside the BN layer or at the interface between SiC and BN layers. The sample with a gradient structure exhibited the combination of delamination cracks occurring at the interface and inside the BN layer, which showed the maximum WOF of 2.43?KJ?m^?2, bending strength of 300?MPa, and fracture toughness of 8.5?MPa?m^1/2. The damping capacity varied with the change of the structure and the amplitude. The sample with a gradient structure exhibited the damping capacity of 0.088 and the maximum loss modulus of 9.758?GPa.     

Solvent stress crazing in PMMA: 1. Geometrical effects

Single crazes were propagated in poly(methyl methacrylate) (PMMA) by loading sheet specimens containing ‘starter’ cracks and immersed in various alcohols. Craze lengths and propagation velocities were measured (as functions of the applied load) using an ultrasonic scanning technique, for three types of behaviour. These were: (I) where craze arrest occurs after some growth; (II) where the craze grows at a constant velocity for an unlimited distance; and (III) where constant velocity growth continues after removal of the original ‘starter’ crack and re-application of the load. The results are analysed using the concept of an ‘equivalent crack’, whose length I turns out to be related to the length c₀ of the starter crack. At temperatures above a previously identified ‘characteristic temperature’, I is commensurate with c₀, but at lower temperatures, I?c₀. These results are discussed in terms of the strain hardening properties of the craze matter which fills the craze.     

Diffraction studies of craze structure

Recent work using small angle scattering techniques to study craze structure is reviewed. Three different radiations, electrons, X-rays, and neutrons were used to study four problems in the area. These were (1) the structure of crazes in thin films, (2) the structure of a single craze/crack in a bulk material, (3) the effect of organic environments, and (4), the effect of mechanical fatigue on craze structures. It is shown that the high intensity of a synchrotron source permits the examination of the process of growth and breakdown of a single craze in polystyrene and also the study in real time of the short- and long-term changes during fatigue of crazes. The H/D neutron contrast effects for neutrons were used to study environmental crazes with the environment still present.     

Crack Patterns in Thin Polymer Layers

Summary: During the solidification of thin polymer layers different crack patterns can occur. There are several mechanisms of the development of regular crack defects and layer fractures. In case of self‐organization caused by Marangoni instability at the fluid layer surface the substrate can be periodically uncovered by spreading motions when dewetting hinders a back flow from the higher spots of the layer. Another type of crack patterns is generated from shrinkage processes and stress differences in the drying layer. Mostly these patterns are characterized by intersecting straight cracks. In this paper some examples of unusual shrinkage‐crack patterns in polymer layers are presented. Their propagation is independent on surface flow and surface deformations caused by the Marangoni effect, although the strength of polymer layers is impaired by the interfacial instability. Especially at layer edges or spots with thickness differences one can observe periodic wavy or circularly bend shrinkage‐crack structures. As a third type ramified surface defects are studied in thin layers. Often they only propagate at the layer surface.

Wavy shrinkage‐cracks in a PMMA layer with longish surface elevations.     

Cold rolled ABS. Part 1: The effect of rubber particle size on the tensile properties of ABS before and after cold rolling

The effect of rubber particle size on the tensile properties of rolled and unrolled acrylonitrile-butadiene-styrene has been studied by considering model systems consisting of mixtures of a small particle (0.1 micron diam) rubber, S, and a large particle (0.56 micron diam) rubber, L, in an SAN matrix. Before rolling, tensile toughness is characterized by crazing. While both rubber induce matrix crazing, ABS systems containing only the S rubber exhibits early failure due to crack formation, before crazing is propagated very far along the tensile axis. The inefficiency of the small particle rubber is interpreted in terms of high composite yield stress and insufficient distance between particles to allow craze branching. The efficiency of the small particle rubber is improved via the addition of a small amount of large particle, L, rubber to the composite or by a slight degree of cold rolling, both of which enhance craze propagation in the tensile direction. With further rolling, the tensile deformation mode changes from one of localized crazing, which is propagated, to one of uniform deformation, which occurs without crazing.     

The effects of stress cracking on the fracture toughness of polycarbonate

The growth of crazes from a sharp crack in extruded polycarbonate sheets immersed in ethanol was measured. Below a critical level of the stress intensity factor craze growth was controlled by solvent diffusion through the end of the notch and fracture was prevented by craze arrest. Above a critical level, growth was controlled by either end diffusion or a combination of end diffusion and diffusion through the faces of the extruded sheet, and in both cases the final result was brittle fracture. The effects of annealing and quenching was studied at various sheet thicknesses. In thin specimens annealing and/or quenching had a significant effect on crack growth rate, which was predictable in terms of the state of stress. As the specimen thickness increased, causing a transition from plane stress to plane strain conditions, the previous thermal history had a diminishing effect on craze growth rate. The effects of thermal history and thickness on the fracture toughness of polycarbonate was also investigated. It was found that thickness was the more important variable and that at a ½ in. thickness the effects of thermal history were statistically insignificant. The effect of ethanol exposure on fracture toughness was studied. It was found that exposure to solvent initially caused an increase in k_IC with time to a maximum value, followed by a substantial decrease with time which eventually led to brittle fracture. This behavior was explained as a competition between plasticization of the crack tip and coalescence of crazes to form microcracks.     

Rate effects for mixed-mode fracture of plastically-deforming,adhesively-bonded structures

Mixed-mode fracture of an adhesively-bonded structure made from a commercial adhesive and a dual-phase steel has been studied under different rates. Since mixed-mode fracture occurs along the interface between the steel and the adhesive, the cohesive parameters for the interface were required. The mode-II interfacial properties were deduced in earlier work. In this paper the mode-I interfacial toughness and the mode-I interfacial strength were determined at different rates. The mode-I interfacial strength was not affected by rate up to crack velocities at levels associated with impact conditions, and was essentially identical to the cohesive strength appropriate for crack growth within the adhesive layer. The mode-I toughness was reduced by about 40% when the crack propagated along the interface rather than within the adhesive. Furthermore, transitions to a brittle mode of failure occurred in a stochastic fashion, and were associated with a drop in interfacial toughness by a factor of about five. The mode-I interfacial parameters were combined with the previously-determined mode-II interfacial parameters within a cohesive-zone model to analyze the mixed-mode fracture of the joints which exhibited both quasi-static and unstable fracture. The mixed-mode model and the associated cohesive parameters for both quasi-static and unstable crack propagation provide bounds for predicting the behavior of the bonded joints under various rates of loading, up to the impact conditions that could be appropriate for automotive design.     

Stress-strain behavior of the craze

The constitution and properties of crazes in glassy polymers and their relation to crack propagation are reviewed. New evidence is discussed which shows the craze to be much softer than the parent polymer but capable of sustaining large stresses and strains up to the point of failure. Craze failure is much more dependent on polymer molecular weight than is craze formation, and this difference is reflected in changes in both fracture surface morphology and crack toughness with molecular weight. Finally craze mechanical properties are thought to be integrally related to the mechanical behavior of high impact plastics.     

Adhesion of propylene–ethylene copolymers to high‐density polyethylene

The adhesion of some propylene–ethylene (P/E) copolymers to polypropylene (PP) and high density polyethylene (HDPE) was studied in order to compare them with other olefin copolymers as compatibilizers for PP/HDPE blends. A one‐dimensional model of the compatibilized blends was fabricated by layer‐multiplying coextrusion. The microlayered tapes consisted of many alternating layers of PP and HDPE with a thin tie‐layer inserted at each interface. The thickness of the tie‐layer varied from 0.1 to 15 μm, which included thicknesses comparable to those of the interfacial layer in a compatibilized blend. In the T‐peel test, the P/E copolymers delaminated at the HDPE interface. An elastomeric P/E with higher ethylene content exhibited substantially higher delamination toughness than a more thermoplastic P/E with lower ethylene content. Inspection of the crack‐tip damage zone revealed that a change from deformation of the entire tie‐layer to formation of a localized yielded zone was responsible. By treating the damage zone as an Irwin plastic zone, it was demonstrated that a critical stress controlled the delamination toughness. The temperature dependence of the delamination toughness was also measured. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Craze initiation and growth in high-impact polystyrene

Quantitative transmission electron microscopy and optical microscopy is used to study craze initiation and growth in thin films of high-impact polystyrene (HIPS). Dilution of the HIPS with unmodified polystyrene reduces the craze–craze interactions, permitting equilibrium growth and craze micromechanics to be studied. It is found that the equilibrium craze length depends on the size of the nucleating rubber particle, but not the internal structure; no short crazes less than a particle diameter are observed. The long crazes can be adequately modelled by the Dugdale model for crazes grown from crack tips. The effects of particle size and particle internal occlusion structure on craze nucleation have been separated. Craze nucleation is only slightly enhanced at highly occluded particles relative to craze nucleation at solid rubber particles of the same size. There is a strong size effect, however, which is independent of particle internal structure. Crazes are rarely nucleated from particles smaller than ～1 μm in diameter, even though these make up about half the total number. These craze nucleation and growth effects may be understood in terms of two hypotheses for craze nucleation: (1) the initial elastic stress enhancement at the rubber particle must exceed the stress concentration at a static craze tip and (2) the region of this enhanced stress must extend at least three fibril spacings from the particle into the glassy matrix. Since the spatial extent of the stress enhancement scales with the particle diameter, the second hypothesis accounts in a natural way for the inability of small rubber particles to nucleate crazes.