Surface microstructural modification and fracture behavior of tensile deformed polypropylene with different percentage crystallinity

Microstructural evolution during tensile deformation of injection molded polypropylene (PP) at the micro- and nano-scale level was studied using atomic force and scanning electron microscopy techniques. Atomic force microscopy (AFM) enabled microstructural changes of tensile deformed PPs with different percentage crystallinity to be captured. AFM of undeformed slow-cooled (SC-PP: high crystallinity) and water-quenched (WQ-PP: low crystallinity) PPs suggested that the fibrils are relatively more closely packed in the SC-PP with higher average surface height of 7.5 nm as compared to 5.4 nm in the case of WQ-PP. Tensile deformed SC-PP and WQ-PP at displacement rates of 125–500 mm/min (strain rates of 0.04 s⁻¹ to 0.16 s⁻¹) indicated that the fibrils/microfibrils are aligned along the tensile axis, with WQ-PP exhibiting enhanced stretching of fibrils/microfibrils/chain-folded lamellae in comparison to SC-PP. Three fracture morphologies were identified at different strain rates, and include crazing/tearing (C), brittle fracture in association with crazing/tearing (B1), and brittle fracture together with ductile pulling of ligaments (B2). The fracture morphology exhibited by both SC-PP and WQ-PP was similar, but the percent area fraction of the three identified morphologies varied. WQ-PP with lower crystallinity was characterized by a decrease in percent of crazing/tearing (C) and brittle+crazing/tearing (B1), and increase in brittle+ductile pulling of ligaments (B2). The fracture characteristics of PPs with differences in crystallinity was consistent with AFM observations.     

Atomic force microscopy of plastically deformed polyethylene subjected to tensile deformation at varying strain rates

Abstract

The surface damage induced during tensile deformation of polyethylene at different strain rates was studied by atomic force microscopy (AFM) operated in tapping mode, before and subsequent to uniaxial tensile plastic deformation. Atomic force microscopy revealed striking differences in the deformed microstructures up to the nanoscale range. The surface of undeformed polyethylene was characterised by ribbonlike fibrils of width 0·25 νm and surface features of height about 20–60 nm. Fibrils were considered to consist of microfibrils of width 0·03–0.04 νm. Small scan (30 × 30 nm) AFM images provided details of microfibrils containing chains of molecules of ~ 0·5 nm wide. Tensile deformation in the plastic region involved stretching of fibrils and microfibrils resulting in the formation of surface openings. The ability of the ribbonlike surface fibrils and microfibrils to stretch, merge, and acquire an oriented and flat structure increased with increase in strain rate in the uniaxial tensile test. Also, with increase in strain rate the chains of molecules unfold and align to produce an oriented and elongated structure. The impact of deformation on amorphous regions could only be observed at high strain rates.     

Phenomenological behaviour of long and short chain polypropylenes under tension

Abstract

Long and short chain polypropylenes, designated PP-L (long chains with lower melt flow index) and PP-R (short chains with higher melt flow index), were subjected to tensile deformation at strain rates in the range 10^-5-10^-1 s-¹ to examine microstructural evolution and tensile flow behaviour as a function of strain and strain rate. Scanning electron microscopy was used to characterise surface deformation processes and fracture surfaces of the polypropylenes. The surface of tensile deformed PP-L was characterised by closely spaced crazes. The array of crazes multiplied with an increase in strain and strain rate and grew inwards. The final separation occurred by tearing along the individual bundles of crazes growing from opposite faces, resulting in crazing–tearing at the edge and a brittle mode of fracture in the centre. In PP-R, the predominant mode of deformation at all strain rates was wedge cracking. Additionally, ductile ploughing was observed at low strain rates and less pronounced crazing at all strain rates investigated. Final fracture at various strain rates occurred through a combination of wedge cracking and brittle fracture, connected by plastic flow, around the wedge cracks. The modes of deformation are summarised and depicted in terms of rate of mechanical deformation–strain diagrams, providing a broad perspective of deformation processes operating in different deformation rate–strain regimes. The modes and domains of deformation processes varied depending on the physical and mechanical characteristics of the material (long chain and short chain). Regarding the tensile flow behaviour, the yield stress increased linearly with an increase in strain rate, and followed the thermal activation concept, with an activation volume consistent with the thickness of the lamellar crystallite. True stress–true strain plots indicated an increase in flow stress with increase in strain rate. Depending on the melt flow conditions, the flow behaviour of the two polypropylenes exhibited differences in strain rate sensitivity.     

Microstructural aspects of tensile deformation of high density polyethylene

Abstract

The paper describes tensile flow behaviour and related microstructural aspects of injection moulded polyethylene. The flow stress increased with increase in strain rate indicating that the tensile flow behaviour is sensitive to strain rate. Also, the yield stress exhibited a linear relationship with strain rate and followed the thermal activation concept with the activation volume consistent with the thickness of the lamellar crystallite. The behaviour of mechanically induced surface damage on polyethylene was studied by SEM. At low strain rates, the deformation process was characterised by features that looked like deformation bands. With increase in strain rate and strain, the deformation bands developed into a distinct array of crazes and grew inwards, followed by tearing. Also, with increasing strain rate the crazes multiplied and secondary crazes were generated that were at an angle to the tensile axis. The examination of the morphology of the fracture surface of polyethylene at various strain rates provided an insight into the mode of fracture process. At low strain rates polyethylene exhibited a ductile type of fracture with extreme fibrillation and, at intermediate strain rates, crazing - tearing was the predominant mode of fracture at the edges, while fibrillar failure occurred in the mid-thickness of the fractured surface. But, at higher strain rates the percentage of fibrillation was relatively small in comparison to lower strain rates. The different modes of deformation processes can be represented in the form of mechanical deformation - strain diagrams, which provide a broad perspective of the deformation processes operating in the different regimes.     

Micro- and nano-scale deformation processes during scratch damage in high density polyethylene

Abstract

Atomic force microscopy and scanning electron microscopy techniques have been used to examine the surface deformation experienced by high density polyethylene during the scratch test. The scratch deformation process involves stretching of fibrils and microfibrils resulting in the formation of surface openings. At the molecular level the chains of molecules unfold and align in the direction of the moving indenter. In the scratch test, the scratch velocity may suggest that low strain rates are valid, but the local strain rates can be many orders of magnitiude higher as exemplified by atomic force microscopy. A number of modes of deformation are encountered during scratching. They include deformation bands, crazing, tearing, microcracking, regular cracking, and grooving. Crazing-tearing is the predominant mode of scratch deformation. It is envisaged that the sequence of tearing along the craze involves formation of deformation bands, development of craze, followed by tearing. Atomic force and scanning electron microscopy of scratch surface damage indicated that the nature and modes of scratch deformation are qualitatively similar to the case of uniaxial tensile deformation, implying similarities in the deformation behaviour between scratch and tensile deformation.     

Grain size effect on high-speed deformation of Hadfield steel

The influence of the microstructure on the tensile properties and fracture behavior of Hadfield steel at high strain rate were studied. Hadfield steel samples with different mean grain sizes and carbon phases were prepared by rolling at medium temperatures and subsequent annealing. A sample with an average grain size larger than 10 μm, and a small number of carbides shows ductility with local elongation (post uniform elongation) at a high-speed tensile deformation rate of 10³ s⁻¹. In addition, the fracture surface changes from brittle to ductile with increasing strain rate. In contrast, a fine-grained sample with carbides undergoes brittle fracture at any strain rate. The grain size dependence is discussed by considering the dynamic strain aging as well as the emission of dislocation from cracks. The accelerated diffusion of carbon due to grain refinement is considered as one of the important reason for brittle fracture in the fine-grained Hadfield steel.     

Response of titanium alloys to high strain rate deformation

Abstract

The stress-strain response of samples of Ti64 and Ti550 at strain rates from 10^?1 s^?1 to 10³ s^?1 and samples of Ti811 and Ti153 at a strain rate of 10³ s^?1 have been assessed. It has been found that the influence of the imposed strain rate on the stress-strain response of Ti64 and Ti550 alloys is very similar – in both alloys the yield stress increases with increase of strain rate and the energy absorbed to fracture increases. At high strain rates localised deformation occurs in the form of shear bands in Ti64 and Ti550 but no shear banding was seen in Ti811 and Ti153. The fracture surfaces of Ti64 and of Ti550 show an increased tendency to brittle failure and an increase in necking with increase of strain rate. The influence of alloy microstructure and composition on the response to changes in imposed strain rate are discussed in terms of adiabatic heating and the factors controlling the flow stress in these alloys.     

Atomic force microscopy of scratch damage in polypropylene

Abstract

Atomic force microscopy (AFM) in tapping mode has been used to characterise surface damage on deformed polypropylenes induced during a scratch test. Atomic force micrographs revealed differences in microstructures that could be used to predict the deformation resistance of two different types of polypropylene. The undeformed surface of the two types of polypropylene (identified as polypropylene-L and polypropylene-R) was characterised by differences in arrangement (regular or irregular) of fibrils depending on their melt flow conditions. Polypropylene-L is a polymer with longer chains and with restricted flow, whereas polypropylene-R has shorter chains obtained by controlled rheology. The microfibrils in undeformed polypropylene-L bend, forming raised surface features of height in the region of 10- 50 nm. In comparison to polypropylene-L, the microfibrils in undeformed polypropylene-R exhibited surface features of relatively lower height (10 - 20 nm). 30 × 30 nm scan AFM images provided details of microfibrils containing chains of molecules of ~0.5 nm wide. Surface deformation induced by the scratch resulted in the formation of scratch tracks characterised by regions of quasi-periodic (consecutive) cracking. This type of deformation is attributed to higher applied loads or to higher contact strains. This is particularly important in semicrystalline polymers, where there is partial reorganisation of microstructure on the application of surface stresses because of their viscoelastic properties. Atomic force micrographs of mechanically deformed polypropylene-L and polypropylene-R at a scan size of 1 × 1 μm indicated a lesser amount of reorganisation of microstructure in polypropylene-L as compared with polypropylene-R. Surface profiles and section analysis of the AFM micrographs suggested that polypropylene-R is more scratch resistant in comparison to polypropylene-L under identical scratch test conditions, consistent with Raman spectroscopy observations of tensile deformed polypropylene.     

Atomic force microscopy characterisation of mechanically induced surface damage in ethylene-propylene diblock copolymeric materials

Abstract

Atomic force microscopy (AFM) in tapping mode has been used to characterise surface damage induced during the scratch test in ethylene - propylene diblock copolymers. The AFM enabled prediction of the deformation resistance of two different types of copolymer. The undeformed surface microstructure of the two copolymers (designated EP-M and EP-TC) was distinguished by differences in arrangement (regular or irregular) of fibrils, depending on their melt flow conditions. Type EP-M is a copolymer with long chains obtained by low melt flow rate, whereas EP-TC is a short chain copolymer with high melt flow rate. The microfibrils in both copolymers exhibited small kinks/nodules. Atomic force nanoscale images provided details of microfibrils containing molecular chains. The long chain EP-M copolymer exhibited non-uniformity in the alignment of molecular chains in comparison with short chain EP-TC. Surface deformation induced by the scratch test led to the formation of parabolic scratch tracks. The average surface height and peak - valley height of a track (considered a measure of the depth of the induced scratch) suggested that short chain EP-TC copolymer is more resistant to mechanically induced surface damage in comparison with long chain EP-M copolymer. Also, the average thickness of track was more for EP-TC, implying that the density of tracks/area is more in EP-M than in EP-TC, consistent with scanning electron microscope observations. Scanning electron microscopy investigations suggested localised plastic flow of material in the region surrounding the track, involving the formation of voids. Also, a comparative assessment of scratch damage was done, in terms of average surface height of the plastically deformed region, in relation to homopolymer and isotactic polypropylenes under identical test conditions.     

Creep failure of polypropylene: experiments and constitutive modeling

Observations are reported on isotactic polypropylene in uniaxial tensile tests with various strain rates, relaxation tests with various strains, and creep tests with various stresses at ambient temperature. Constitutive equations are derived for the viscoelastic–viscoplastic responses and damage of a semicrystalline polymer at three-dimensional deformations. Adjustable parameters in the stress–strain relations are found by fitting the experimental data. The model is applied to predict creep-failure diagrams in the entire interval of stresses. A phenomenological approach is proposed to determine a knee stress, at which transition occurs from ductile to brittle rupture. Accuracy of this method is evaluated by numerical simulation.     

Polypropylene/SiO<Subscript>2</Subscript> nanocomposites filled with different nanosilicas: thermal and mechanical properties,morphology and interphase characterization

Untreated and surface-treated SiO₂ nanoparticles with different alkyl chain length (described as C0, 3C1, C8 and C16 according to the number of carbon atoms) on particle surface were used as fillers for isotactic polypropylene (iPP). The iPP/SiO₂ composites containing 2.3 vol% of nanoparticles were prepared by melt blending and injection moulding. The dispersion quality of nanoparticles in matrix was examined using scanning electron microscopy (SEM). The crystallization behaviour of iPP was examined using differential scanning calorimetry (DSC). The mechanical properties of all samples were characterized by tensile test, compact tension (CT) test and dynamic mechanical thermal analysis (DMTA). The particle–matrix interphase behaviour was also examined and discussed. SEM images show that different silicas show different dispersion quality in matrix due to different hydrophobicity. The crystallinity and spherulite size of matrix are overall decreased in composites. The tensile properties of iPP/SiO₂ composites show clear relationship with alkyl chain length on particle surface, i.e. increasing alkyl chain length leads to decreased tensile modulus but increased tensile yield strength and strain, indicating increased interfacial interactions with increased alkyl chain length. The 3C1-composite shows the highest fracture toughness with an improvement by 9% compared to neat iPP, whereas the other composites show decreased values of fracture toughness.     

Deformation behaviour of coextruded multilayer composites with polycarbonate and poly(styrene-acrylonitrile)

The tensile properties of coextruded multilayer composites comprised of predominantly 49 alternating layers of polycarbonate (PC) and polystyrene- acrylonitrile (SAN) were investigated in the bulk and microscopically. The bulk was characterized by three types of behaviour: brittle fracture at low strains, ductile yielding with fracture during neck formation, and formation of a stable neck followed by drawing to high extension. Optical microscopy was utilized to correlate deformation mechanisms within each phase to the observed modes of deformation in the bulk. Optical microscopy showed that in all cases the initial irreversible deformation event was the formation of cracks or crazes in the SAN layers. Good adhesion between the layers resulted in the subsequent initiation of shear bands in the polycarbonate layers at the craze tips. Interaction of crazes and shear bands produced an expanded damage zone ahead of the propagating crack which delocalized the stress and delayed fracture. The ultimate mode of fracture depended on the relative thickness of the SAN and PC layers, as determined by the composition, and the strain rate.     

Phenomenological aspects of the double yield of polyethylene and related copolymers under tensile loading

The double yield point is shown to be a common feature to polyethylene and ethylene copolymers, regardless of the crystallinity level. Particular attention has been paid to the influence of draw temperature and strain rate which unambiguously indicate a combination of two thermally activated rate processes. Various thermal treatments have been investigated in order to check the influence of the crystal thickness distribution and the chain topology on the yield behaviour. Isothermal crystallization at high temperature is shown to have little effect compared with variations of crystallinity, temperature and strain rate in the case of compression-moulded samples. On the other hand, a strong effect has been observed in the case of solution crystallization which is well known to affect the chain-folding topology. The results are fairly consistent with the previous proposal by Takayanagi that (1) two processes govern the plastic deformation of the crystalline lamellae in semi-crystalline polymers, and (2) these processes are closely related to the viscoelastic relaxations in the crystal. The crystalline lamellae may deform plastically through sliding of crystalline blocks (brittle process) and/or homogeneous shear (ductile process). In order to account for the dependency of the brittle-to-ductile transition on the copolymer structure and crystallization method, a molecular model is put forward on the basis of the chain topology concepts borrowed from our former investigations on the tensile drawing and the melting behaviour of ethylene copolymers.     

Creep rupture and viscoelastoplasticity of polypropylene

Observations are reported on isotactic polypropylene in tensile tests with various strain rates, relaxation tests at various strains, and creep tests with various stresses at room temperature. Constitutive equations are derived for the viscoelastic and viscoplastic responses of semicrystalline polymers at three-dimensional deformations with small strains. The stress-strain relations involve eight material constants that are found by fitting the experimental data. The model is applied to the numerical analysis of creep failure of polypropylene under various deformation modes (uniaxial tension, equi-biaxial tension, shear, multiple-step creep tests).     

Tensile mechanical behaviour of quenched and annealed isotactic polypropylene films over a wide range of strain rates

The tensile mechanical behaviour of quenched and annealed isotactic polypropylene (iPP) films has been analysed over a wide range of strain rates, i.e. from 10^–3 to 3×10⁺²s^–1. Evolution of mechanical properties of such films versus strain rate has been analysed through the microstructure. Thus, both the Young's modulus and the yield stress could be mainly controlled not only by the crystallinity ratio but also by the physical cross-linking degree of the amorphous phase induced by crystalline entities. For a given crystallinity ratio, the drawability of quenched and annealed iPP films is mainly controlled by the sum of the effects induced by both the physical cross-linking degree of the amorphous phase and the perfection degree of the crystalline phase. The increase in annealing temperature leads to the opposite evolution of these two microstructural parameters and then to opposite effects on the drawability of films. Changes in original microstructure of quenched films induced by drawing at various draw ratios and at various strain rates are also discussed.     

Experimental study on tensile property of AZ31B magnesium alloy at different high strain rates and temperatures

As the lightest metal material, magnesium alloy is widely used in the automobile and aviation industries. Due to the crashing of the automobile is a process of complicated and highly nonlinear deformation. The material deformation behavior has changed significantly compared with quasi-static, so the deformation characteristic of magnesium alloy material under the high strain rate has great significance in the automobile industry. In this paper, the tensile deformation behavior of AZ31B magnesium alloy is studied over a large range of the strain rates, from 700 s⁻¹ to 3 × 10³ s⁻¹ and at different temperatures from 20 to 250 °C through a Split-Hopkinson Tensile Bar (SHTB) with heating equipment. Compared with the quasi-static tension, the tensile strength and fracture elongation under high strain rates is larger at room temperature, but when at the high strain rates, fracture elongation reduces with the increasing of the strain rate at room temperature, the adiabatic temperature rising can enhance the material plasticity. The morphology of fracture surfaces over wide range of strain rates and temperatures are observed by Scanning Electron Microscopy (SEM). The fracture appearance analysis indicates that the fracture pattern of AZ31B in the quasi-static tensile tests at room temperature is mainly quasi-cleavage pattern. However, the fracture morphology of AZ31B under high strain rates and high temperatures is mainly composed of the dimple pattern, which indicates ductile fracture pattern. The fracture mode is a transition from quasi-cleavage fracture to ductile fracture with the increasing of temperature, the reason for this phenomenon might be the softening effect under the high strain rates.     

Fibrillar level fracture in bone beyond the yield point

The nanoscale deformation and fracture mechanisms of parallel fibered bone are investigated using a novel combination of in-situ tensile testing to failure combined with high brilliance synchrotron X-ray scattering. The technique enables the simultaneous measurement of strain at two length scales – in the mineralized collagen fibrils (~100 nm diameter) along with the macroscopic strain (~1 mm diameter). Under constant rate tensile loading, we find that fibril strain saturates beyond the macroscopic yield point of bone at ~0.5 %, providing a correlation between the failure mechanisms at the nanoscale and the bulk structural properties. When bone stretched beyond the yield point is unloaded back to zero stress, the fibrils are contracted relative to their original state. We examine the findings in the context of a fiber – matrix shearing model at the nanometer level.     

中低应变率下闭孔泡沫铝动态力学性能研究

为探索闭孔泡沫铝的动态力学性能与吸能特性,基于万能材料试验机和高速液压伺服材料试验机在常温下分别对闭孔泡沫铝在准静态和中应变率下(0.001~100s^-1)的动态力学性能进行了测试,分析了不同应变率、不同相对密度和不同泡沫铝基体特性下闭孔泡沫铝的应力应变曲线特征和吸能特性变化。研究结果表明:中低应变率下的纯铝基体泡沫铝并不具备应变率效应,高脆性、相对密度较小的泡沫铝具备更好的吸能特性,塑性和脆性基体泡沫铝变形带分别呈现“V”形和“X”形,脆性基体泡沫铝同样不具备应变率效应。     

Influence of stress state and strain rate on the behaviour of a rubber-particle reinforced polypropylene

This article presents an experimental investigation of a ductile rubber-modified polypropylene. The behaviour of the material is investigated by performing tension, shear and compression tests at quasi-static and dynamic strain rates. Subsequently, scanning electron microscopy is used to analyse the fracture surfaces of the tension test samples, and to relate the observed mechanical response to the evolution of the microstructure. The experimental study shows that the material is highly pressure and strain-rate sensitive. It also exhibits significant volume change, which is mainly ascribed to a cavitation process which appears during tensile deformation. Assuming matrix-particle debonding immediately after yielding, the rubber particles might play the role of initial cavities. It is further found that the flow stress level is highly dependent on the strain rate, and that the rate sensitivity seems to be slightly more pronounced in shear than in tension and compression. From the study of the fracture surfaces it appears that the fracture process is less ductile at high strain rates than under quasi-static conditions.     

Ductility and brittleness of bone

Human, bovine and equine bones show brittle to ductile transitions as a function of strain rate. The transition is not sharp, but occurs around a strain rate of 10⁻¹ s⁻¹. At lower rates, the strength increases proportional to the logarithm of the strain rate, at higher rates it decreases. Additionally, the work of fracture peaks around 10⁻¹ s⁻¹. Thermal activation analysis gives an activation volume of (1 nm)³, an activation enthalpy of 1 eV and an activation energy of about 0.5 eV. Plastic deformation occurs both within and between collagen fibrils. In the fibrils, the existence of screw dislocations parallel to the collagen molecules with a Burger’s vector of 1 nm length is postulated. Deformation occurs by thermally activated kink pair formation in these defects.