Relationships between mechanical behaviour and craze morphology in thin films of polystyrene

A new technique has been developed to study the relationship between mechanical properties and craze microstructure in thin films (1 to 5m) of polystyrene. Thin film samples of thicknesses in this range could be strained in a conventional testing machine and subsequently examined in the deformed state in an electron microscope. The strain rate was systematically varied and its effect was investigated on both quenched and annealed samples. Correlations were found between craze morphology and ductility. A new craze parameter has been defined as the width of a strip of material which is involved in the development of a craze. From this value, the average strain and the average volume fraction of fibrils within a craze can be calculated.     

Shear and craze development in the fatigue of ductile amorphous polymers

The competitive interplay between shear and craze deformation in the fatigue of ductile amorphous polymers leads to the formation of intricate fatigue crack shapes and stable crack growth. The craze/shear dual deformation mode is expressed in a unique crack tip plastic zone, which has been observed in polycarbonate, polysulfone, a polyarylate block copolymer and a polyestercarbonate copolymer. The fatigue crack growth resistance in these polymers is high, and factors which affect the relative ease of shear flow and crazing are expected to affect their fatigue endurance.     

Numerical implementation of an elastic-viscoplastic constitutive model to simulate the mechanical behaviour of amorphous polymers

Due to their high deformation capabilities, polymeric materials are widely used in several industries. However, polymers exhibit a complex behaviour with strain rate, temperature and pressure dependencies. Numerous constitutive models were developed in order to take into account their specific behaviour. Among these models, the ones proposed by Richeton et al Polymer 46:6035–6043 (2005a), Polymer 46:8194–8201 (2005b) seem to be particularly suitable. They proposed expressions for the Young modulus and the yield stress with strain rate and temperature dependence. Moreover, these models were also implemented in a finite elastic-viscoplastic deformation approach using a flow rule based on thermally activated process. The increase of computational capabilities allowed simulating polymer forming processes using finite element (FE) codes. The aim of the study is to implement the proposed constitutive model in a commercial FE code via a user material subroutine. The implementation of the model was verified using compressive tests over a wide range of strain rates. Next, FE simulations of an impact test and of a plane strain forging process were carried out. The FE predictions are in good agreement with the experimental results taken from the literature.     

Crazing mechanisms and craze healing in glassy polymers

When held at temperatures above the glass transition temperature, crazes in certain polymers may be made to heal; that is, the crazed material may be made to recover the mechanical properties it had prior to crazing. Using thin films of a variety of polymers, we have investigated whether the mechanism of entanglement loss during crazing influences the heat treatment time necessary for healing. We find that under experimental conditions for which there is evidence of disentanglement during crazing, and if the crazes do not break down, healing occurs after heat treatment times of the order of those necessary to make the craze disappear optically. Similar heat treatments applied to scission mediated crazes however, do not result in healing. We argue that scission crazing results in a high proportion of chain fragments which are unable to contribute to the entanglement network. Since the heat treatments are not sufficiently long to disperse these fragments, and hence to restore the original local entanglement density, healing does not take place. Disentanglement should not result in such damage, so, consistent with our observations, healing times should be relatively short. Hence, these results provide independent evidence for disentanglement during crazing.     

The dynamic mechanical properties and fracture behaviour of phthalocyanine polymers

The dynamic mechanical properties of a series of phthalocyanine polymers with varying aliphatic chain length linking stable phthalocyanine nuclei have been studied. Dynamic shear modulus and polymer loss factor were determined using a torsion pendulum. Polymer fracture energies were also determined using both the standard compact tension specimens and double-tapered cantilever beam specimens. The results showed that both the dynamic shear modulus and the glass transition temperature increased with decreasing molecular flexibility and increasing extent of cure. The fracture energies increased with increasing molecular flexibility. It was also shown that extended thermal treatment could greatly embrittle the thermosetting polymers and hence reduce the fracture toughness, even though such treatments usually promoted crosslinking leading to higher moduli and glass transition temperatures for these polymers.     

Multiple formation of microcracks during mechanical loading of polymers

General kinetic and thermodynamic reasoning is used to propose a new mechanism for multiple crack formation caused by a mechanical action on a polymer. It is shown that the role of information “carrier” to “ weak” (in terms of thermodynamic stability) spots localized in the polymer matrix is assigned to phonons as a unique class of quasiparticles which exist in nonconducting and nonmagnetic dielectrics. Pis’ma Zh. Tekh. Fiz. 23, 80–85 (December 26, 1997)     

Correlation between structure and adsorption characteristics of oriented polymers

In a more or less swollen state the methods by which the fiber structure is determined provide more realistic information about their properties and structure in wet conditions, because of the swelling and alteration of fiber structure. These results, however, of sorption analyses are very rarely in accordance with those obtained from structural analyses. One such ”wet” method is the iodine sorption test. The iodine sorption test was originally proposed by Schwertassek [1] and since then has been constantly modified by other researchers [2–7]. In this presented work the correlation between different structural parameters, iodine sorption (ISV) and conventional sorption analyses (water vapor sorption and water retention power) of polyamide 6 (PA 6) and cellulose fibers was studied. The results demonstrate that the iodine sorption value best correlates with the degree of crystallinity and the perfection of the crystalline phase of both polyamide and cellulose fibers. The correlation is linear. Received: 25 August 2000 / Reviewed and accepted: 28 August 2000     

Correlation between domain wall properties and material parameters in amorphous SmCo-films

Amorphous Sm1-xcox-films with uniaxial in-plane anisotropy with various Co-concentrationsx (.67 leq x leq .91)and various film thicknesses d (10nm leq d leq 350nm) have been prepared by flash-evaporation of SmCo-alloy powders. The saturation magnetization decreases linearly with x in the whole concentration range, whereas the coercive field increases linearly with x down toxapprox74and then decreases. Drastic changes in all magnetic properties occur when at a given chemical composition the film thickness is reduced belowdapprox50nm. Head-on domains are separated by zig-zag walls whose amplitudes are analyzed as a function of the material parameters.     

Viscous behaviour and glass formation in Fe78B13Si9 amorphous alloy

Using a rotational viscometer, the viscosity of molten Fe₇₈B₁₃Si₉ alloy was measured over the temperature range 1165–1350 °C. The temperature dependence of the viscosity of liquid alloy is adequately described by the Arrhenius equation. However, this expression cannot be directly extended to the undercooled liquid temperature region. In this paper a proper equation derived from the free volume theory is proposed to describe the viscosity-temperature behaviour of Fe₇₈B₁₃Si₉ alloy in the temperature range of the undercooled liquid and above its melting point. Furthermore, the isothermal time-temperature-transformation curve for crystallization is constructed based on the assumption of homogeneous nucleation and crystal growth, together with the temperature dependence of the viscosity. The critical cooling rate required to form a glass of Fe₇₈B₁₃Si₉ alloy was calculated to be approximately 10⁵° Cs^–1.     

Correlation dependences between the mechanical characteristics of corrosion-resistant steels

A stability factor of austenite A _γ is proposed for corrosion-resistant steels. By using this factor, the equilibrium phase-structural state of steels can be quantitatively evaluated according to their chemical composition. The factor A _γ is determined by the values of the chromium and nickel equivalents. This factor, together with the generalized Scheffler diagram, enables one to deduce the correlation equations for the mechanical characteristics of steels σ_u, σ_0.2, δ, and ψ. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 1, pp. 54–60, January–February, 2007.     

Correlation between texture,earing behaviour and dissolved carbon and nitrogen

The present study compared the differences of texture and earing behaviour in DR tinplate steel sheets. Dissolved carbon and nitrogen contents were discussed as the principal factor. The {001}?110? texture component whose earing tendency is higher than that of heterogeneous {111} fibre texture, is harmful for earing property and needed to be reduced to minimum in most cases. The increase of the dissolved carbon and nitrogen contents is responsible for weaker {111} fibre texture and more intense {001}?110? component. Additionally, a strong {114}?110? texture component was observed on condition of higher dissolved carbon and nitrogen contents and its formation mechanism was discussed.     

Similarity between craze morphology and shear-band morphology in polystyrene

The formation of shear bands and crazes in thin films as well as in bulk samples of polystyrene were examined in the electron microscope using a variety of replication techniques. The morphologies of shear bands and crazes are quite similar both depending initially upon the relative shear displacement of 400 to 1000 Å domains. As deformation continues and orientation increases, fibrils varying from 50 to 700 Å are formed within the deformation zone, lateral constraint of the normal Poisson contraction causing voids to form in the crazes but not in the shear bands. Shear-band width was found not to be a unique function of either temperature or strain-rate and both craze and shear-band morphologies were found not to be strong functions of molecular weight. Regardless of molecular weight, fibrils formed within the deformation zone were always on the order of a few hundred Angstroms in diameter. However, for thin films of molecular weight less than 20 000 insufficient numbers of tie molecules between fundamental structural units or domains made it difficult for these fibres to span the craze width.     

Thermoelectric behaviour of amorphous magnetic alloys

The Thermoelectric emfs of thermocouples formed by amorphous METGLAS 2826 (Fe₄₀Ni₄₀P₁₄B₆) and METGLAS 2826B (Fe₂₉Ni₄₉P₁₄B₆Si₂) with standard thermocouple wires like copper, chromel, alumel, etc., were measured as a function of temperature between −196° C and 30° C to assess their suitability as thermoelectric temperature sensors. Thermoelectric emfs generated by METGLAS 2826/Cu and METGLAS 2826B/Cu thermocouples at −196° C are about an order of magnitude smaller when compared to thermal emfs of a standard copper/constantan thermocouple at the same temperature.     

Optical and mechanical properties of hydrogenated amorphous carbon

We have measured the refractive index and microhardness of hydrogenated amorphous carbon films grown on quartz and silicon substrates from a methane-argon mixture under various conditions: E/p = 40–180 V/mPa (E is the electric field between electrodes and p is the gas pressure in the deposition chamber) and substrate temperatures from 50 to 300°C. Such conditions enable the growth of diamond-, polymer-, and graphite-like a-C:H films with wide ranges of microhardness values and refractive indices.     

Correlation between microstructure and mechanical behaviour at high temperatures of a SiC fibre with a low oxygen content (Hi–Nicalon)

An oxygen free Si–C fibre has been studied in terms of the chemical, structural and mechanical properties produced as a function of annealing treatments. In spite of its high thermal stability with regard to a Si–C–O fibre the Si–C fibre was subject to moderate SiC grain growth, organization of the free carbon phase and densification within the temperature range 1200–1400°C. The strength reduction at ambient for temperatures ≤1600°C could possibly be due to SiC coarsening or superficial degradation. Bend stress relaxation (BSR) and tensile creep tests show that the as-received fibre undergoes a viscous flow from 1000°C. The thermal dependance of the creep strain rate strongly increases at temperatures ≥1300°C. This feature might be partly explained by the structural evolution of the fibre occurring above this temperature. Heat treated fibres (1400–1600°C) exhibit a much better creep strength, probably due to their improved structural organization. This revised version was published online in November 2006 with corrections to the Cover Date.     

Fatigue behaviour of some temperature-resistant polymers

Static and flexural (5 Hz, 20°C) fatigue properties of three linear polymers, polyetherimide (PEI), polysulphone (PSU), polyetheretherketone (PEEK) and one three-dimensional polymer, amine cross-linked epoxy (EPO), were compared. In all cases, the fatigue lifetime was governed by crack initiation and a brittle rupture occurred. The order of stress endurance limit is: PEEK (65 MPa) > EPO (28 MPa) > PEI (24 MPa) > PSU (18 MPa). For PEI and essentially PSU, this value increased with orientation. It thus appears that there is no relation between the fatigue durability and the ductility or toughness as established from static tests. The results suggest that crack initiation involves large-scale configurational changes such as crazing. These changes would be inhibited or reduced by chemical (EPO) or physical (semi-crystalline polymers such as PEEK), cross-linking.     

The fracture behaviour of ABS polymers

The fracture behaviour of three different grades of ABS has been investigated over a range of temperature and strain-rates. A two-stage fracture process has been observed consisting of a slow crack propagation region followed by catastrophic failure. The surface features in these two regions have been examined in detail using scanning electron microscopy. In all cases, the fracture initiation site was located at either a surface defect or at an internal inclusion much larger than the average rubber particle size.