Deformation behavior of polytetrafluoroethylene films under tensile stress at various temperatures

Deformation behavior of polytetrafluoroethylene (PTFE) films was investigated by thermomechanical analysis (TMA) under various tensile stresses (σ) up to 1.15 MPa in the temperature range from room temperature to 360°C. In the heating process above σ ≈ 0.25 MPa, a contraction of the PTFE film occurs in the melting temperature region. In the cooling process above σ ≈ 0.05 MPa, an elongation occurs in the crystallization temperature region, and above σ ≈ 0.5 MPa, it reaches 20–30% of the original length of the film. The PTFE films in the melt state above σ ≈ 0.5 MPa contract with increasing temperature up to 360°C and elongate with decreasing temperature. For the films that underwent deformation in the TMA, the crystalline orientation and the surface morphology were investigated by wide-angle X-ray diffraction and scanning electron microscopy, respectively. The degree of crystalline orientation in the deformed films increases with increasing σ and approaches a plateau at σ ≈ 0.4 MPa. On the surface of the deformed films, alignment of the bands and deformation of granules, which are formed by heat treatment above the melting point, are observed. © 1994 John Wiley & Sons, Inc.     

Molecular behavior of amorphous polystyrene under tensile stress

Summary Molecular level changes occurring due to external stress application have been studied by the dynamic infrared spectroscopic technique using Fourier Transform Infrared Spectrometer. The subtraction spectrum (stressed — 88% unstressed) of polystyrene has shown even minute changes in 540, 903, 1154, 1310, 1376, and 1602 cm¹ infrared bands assigned to various vibrational modes of phenyl side groups. Both infrared and x-ray results have indicated certain conformational rearrangements (under stress) of these bulky phenyl side groups present in the PS chains.     

Deformation and fracture of Mg(OH)2‐filled polyolefin composites under tensile stress

The tensile behavior of high‐density polyethylene (HDPE), polypropylene (PP), and linear low‐density polyethylene composites containing a titanate coupling agent and silicone‐oil‐treated magnesium hydroxide [Mg(OH)₂] was studied. The increase in the extent of the ultimate elongation of the composites was affected by the yield stress and the strain‐hardening tendency of the polymer matrix in the composites. Ethylene–propylene–diene rubber and octane–ethylene copolymer were introduced to adjust the yield stress of PP and HDPE. Although the ultimate elongation of PP/elastomer and HDPE/elastomer blends was higher than that of virgin PP or HDPE, the ultimate elongation of the filled composites dropped at a high content of Mg(OH)₂. Scanning electron microscopy showed that the difference in the uniformity of the interface exfoliation decreased with the yield stress of the matrix. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3248–3255, 2003     

Deformation and pore formation mechanism under tensile loading in isotactic polypropylene

Four different polypropylene (PP) samples were prepared through isothermally crystallizing at 0 °C (PP‐Q), 80 °C (PP‐80), 100 °C (PP‐100) and 120 °C (PP‐120). The results of differential scanning calorimetry, wide‐angle X‐ray diffraction, polarized light microscopy and tensile testing indicate that the spherulite structure gradually improves with increasing isothermal crystallization temperature. Meanwhile, the interface between spherulites becomes more obvious due to the larger dimension and the higher strength of spherulites. Therefore, the trend of interfacial debonding during stretching is enhanced distinctly. In addition, based on the structural characterization of samples at different draw ratios, two completely distinct morphological changes are demonstrated. There are no defects generated after longitudinal stretching within PP‐Q, because intra‐spherulitic deformation predominates, which is caused by the imperfect spherulites of PP‐Q. As a result, no microporous structure is produced after sequential biaxial stretching. And the improvement of the crystalline structure makes interfacial debonding more likely to occur. Therefore, fully developed crazes and cracks disperse between microfibril structures after longitudinal stretching. Furthermore, numerous microporous structures are produced through debonding of fully developed crazes and cracks after sequential biaxial stretching. Meanwhile, the quantity, dimension and uniformity of the microporous structures and the porosity are gradually improved. © 2017 Society of Chemical Industry     

Thermo-oxidative and photo-oxidative aging of polypropylene under simultaneous tensile stress

The effect of uniaxial tensile stress at constant load in the range of 0 to 6.86 MN/m² upon the behavior of stabilized and unstabilized types of isotactic polypropylene was investigated in the course of thermo-oxidative aging at 80° to 130°C and of photo-oxidative aging in the Xenotest 450 apparatus at 45° and relative humidity of 65%. From kinetic evaluation of the temperature dependence of weight changes of unstabilized polypropylene during thermo-oxidative aging, it was found that the weight losses of unstressed and tension-stressed specimens obey the kinetic equations for a reaction of the first order. The degradation of stressed specimens, however, proceeds at a considerably higher rate as compared with the unstressed state and is marked by a decrease in apparent activation energy as well as by an increased rate of crack development. The cracks develop in the direction perpendicular to the tensile stress and have the shape of hollow funnels widening outward. The tensile stressing of stabilized types of polypropylene in thermo-oxidative and photo-oxidative environments causes an accelerated embrittlement of the polymer, manifested particularly by a marked decrease of elongation at break. This accelerated embrittlement goes on under the action of tensile stresses exceeding the so-called safe stress, i.e., the stress below which no acceleration of aging (as measured by the relative decrease in the elongation at break of the stressed and unstressed specimens), can be detected. The values of this safe tensile stress decrease with the temperature and lie well below the tensile strength of the polymer. In the range of tensile stresses exceeding the safe stress, the dependence of the logarithm of time of a 50% decrease of the elongation at break on the applied tensile stress was found to be linear. The findings are in agreement with Zhurkov's kinetic concept of mechanical deterioration of polymers, where the effect of the applied mechanical stress is superimposed on the thermal fluctuation and leads to the scission of chemical bonds and to the formation of cracks in the polymer.     

Study on the long-term behaviour of glass fibre in the tensile stress field

Glass fibre is a random network structure composed of [SiO₄] tetrahedra. The structure contains a large number of defects, which act as crack initiation points. Under tensile stress, cracks undergo crack initiation, stable propagation, failure propagation, and fracture, and the stress that begins after unstable propagation is called the critical fracture stress. When the stress is less than the critical value, the crack is subject to the force of chemical bonds during the crack propagation process, and crack arrest occurs. When the stress is greater than the critical value, the glass fibre will undergo destructive fracture. In this paper, long-term tensile tests were carried out on glass fibre and a glass fibre composite under different constant tensile stress conditions. The fracture times of the glass fibre and glass fibre composite under different tensile stresses were obtained, the critical fracture stress of glass fibre was inferred, and the fracture mechanism was explained.     

Rubber-like tensile behaviour of yielded high-impact polystyrene

The tensile stress-strain properties of yielded high-impact polystyrene (HIPS) have been investigated at room temperature. Two HIPS samples were considered, having different rubber-phase particle sizes and morphologies. Specimens have been subjected to two subsequent tensile test runs, the first one to produce the yielded matter and the second one to test it. Different amounts of yielded matter content in the specimens and different strain rates for yielded matter production were examined by varying the test conditions in the first run. Yielded HIPS exhibits a rubber elastic stress-strain behaviour, which has been interpreted by assuming that a rubber-like material, proportional in amount to the plastic deformation, is present in the specimens as a consequence of the yielding process.     

Deformation behaviour of iron-doped alumina

Compressive creep tests in air were carried out on 1 cat.% Fe-doped alumina at a temperature T=1400 °C. Iron doping affected the plastic deformation by different ways in relation with Fe²⁺ cations population. Fe²⁺ cations sped up the deformation rates. FeAl₂O₄ spinel precipitates were identified and they were found (i) to interact with alumina grain boundaries (ii) to limit the grain growth within a range of strain. The Fe²⁺ cations underwent oxidation and this resulted in the dissolution of the some precipitates and in the decrease of deformation rates. It was suggested that deformation sped up this evolution through mass transport and that time was not a dominating parameter.     

Crystallization of poly(ethylene terephthalate) under tensile strain: crystalline development versus mechanical behaviour

The crystallization of poly(ethylene terephthalate) (PET) under tensile strain was investigated using wide angle X-ray diffraction. Real-time investigation of the crystallization state, including the crystalline ratio and the crystallite orientation, of the material could be undertaken due to the high brilliance of the synchrotron X-ray source used in our study. Initially amorphous PET specimens were stretched at different strain rates and draw ratios at the same temperature (above and close to T_g). Our experimental set-up was designed to undertake simultaneous recording of the X-ray diffraction patterns and the mechanical parameters. Up to the draw ratio of 500% and draw rate of 0.75 s⁻¹, the crystalline development dynamics corresponded to three different regimes. (i) For small enough extension rates, there was no measurable crystallinity during the drawing process. The crystallization developed after cessation of deformation. (ii) For intermediate extension rates, the whole crystallization process took place during the deformation. (iii) For the highest extension rate involved in our experiments, the crystallization started during the drawing process and continued after cessation of deformation. The mechanical behaviour of the polymer was simultaneously recorded and correlated with the induced crystalline microstructure. In particular, we were able to discriminate the influence of crystallite orientation and crystallization growth on the mechanical behaviour of the material.     

Post-peak cyclic behaviour of concrete in uniaxial tensile and alternating tensile and compressive loading

Deformation controlled uniaxial tests have been carried out on cylindrical concrete specimens in order to establish the complete stress-deformation curve in tensile loading. Furthermore, cyclic tests have been performed under three types of loading: cycling from a tensile low stress, cycling from a compressive lower stress, and cycling between a lower and an upper stress to the envelope curve, respectively. The results seem to show that the tensile envelope curve is unique and the residual strength is the more affected the lower the stress at unloading is.     

Influence of the wetting properties of polymeric adhesives on the mechanical behaviour of cork agglomerates

A series of adhesives, polyurethane prepolymers with alkane chains of different functionalities, were used in the production of cork agglomerates. The polymeric structure varied from long, linear chains in adhesive M1 (14% isocyanate groups) to short, branched chains in adhesive M4 (30% isocyanate groups). The wetting properties of the adhesives were studied through surface tension and contact angle measurements of the polymeric liquids deposited on cork substrates. The mechanical behaviour of the cork agglomerates was studied through compression and three-point bending tests, and the agglomerate structure was analysed by scanning electron microscopy (SEM). The relationship between the structural and wetting characteristics of the adhesives and the mechanical properties of the agglomerates was investigated. The experimental results indicate that it is the work of adhesion for the adhesive/cork system, and not only the contact angle, which determines the mechanical resistance of the cork agglomerate.     

Controlled supercontraction tailors the tensile behaviour of spider silk

The interest in the production of fibres that mimic the behaviour of natural silks has been boosted by the first successful attempts of spinning fibres based on spider drag line silk proteins. However, both the processing of biomimetic silk fibres and the basic studies on silk are hampered by the large variability of the fibre properties. Here we show that the tensile behaviour of spider silk can be predictably and reproducibly tailored by controlling the supercontraction effect, a large shrinkage of the longitudinal dimension of the fibre if unrestrained by its ends and immersed in water. This procedure allows to reproduce the tensile behaviour of natural drag line fibres and offers the possibility of obtaining silk fibres with predictable tailored properties in large quantities for experimental use. These results can be interpreted in the frame of the molecular model of drag line silk, as the result of re-orientation of the protein chains, leading to an explanation for the observed variability of natural drag line fibres.     

Deformation recovery behavior of a solid polymer after tensile yielding

The deformation recovery at room temperature of a polycarbonate after large tensile deformation and subsequent stress relaxation is measured as a function of time. The effect of the main parameters which affect the amount of recovered deformation is analyzed. This suggests a plot of the recovery versus time curves by means of proper coordinates which allow shifting of all curves towards a single master curve, thus indicating that most of the phenomena involved in the recovery process are taken into account.     

Analysis of the low temperature-dependent behaviour of a ductile adhesive under monotonic tensile/compression–shear loads

Various models exist to describe the non-linear behaviour of an adhesive in an assembly, taking into account the two stress invariants, hydrostatic stress and von Mises equivalent stress, which can be explained by the nature of the adhesive, i.e., a polymer. The identification of the material parameters of such pressure-dependent constitutive models requires a large experimental database taking into account various tensile–shear loadings. Under quasi-static loadings at low temperature, for a given strain rate range, viscous effects can be neglected, but only a few experimental results are available to model the behaviour of an adhesive in a bonded assembly accurately under realistic loadings. Moreover, edge effects often have a large influence on the mechanical response. This paper presents the possibility of combining the use of a modified Arcan device, which strongly limits the influence of the stress concentrations, with a usual thermal chamber. Experimental results, underlining the temperature-dependent non-linear responses of an adhesive, are presented in the case of various tensile/compression–shear monotonic loadings for a temperature range between 20 °C and −60 °C. The analysis of experimental results, obtained in the load-displacement diagram, focuses herein on the modelling of the initial temperature-dependent yield surface; but such results are also useful for the development of the flow rules in the case of pressure-dependent models.     

Investigation on non-uniform strains of a 2.5D woven ceramic matrix composite under in-plane tensile stress

In this study, the digital image correlation technique and strain gauges were applied to the in-plane tensile tests of a 2.5D woven ceramic matrix composite for the purpose of characterizing deformation and strain distribution features. The test results indicate a strong influence of the weave architecture on the strain distribution of the material. Mesoscale finite element models were established based on the weave architecture of the material. The experimental and simulation results show that: the strain distributions are periodic and fluctuating; the maximum strain fluctuation on the surface is about 30%, but the deviation of average strain decreases rapidly with the calculated area increase. The strain measurement methods were proposed to obtain the accurate average strain: the size of measurement/calculated area should be twice larger than the unit cell size; much more accurate result could be obtained when the area size is an integer multiple of the unit cell size.     

Shear and tensile relaxation behaviour in oriented linear polyethylene

The dynamic shear behaviour of oriented linear polyethylene has been studied with particular reference to previous studies of the dynamic tensile modulus. First, it has been shown that the increase in the ?50°C plateau shear modulus with draw ratio can be understood on a Takayanagi-type model in terms of an increase in crystal continuity. The crystal continuity is estimated from the longitudinal crystal thickness and the long period on the basis of the random crystalline bridge model. At a similar level of sophistication it is also possible to explain the cross-over in the ranking of samples of increasing draw ratio with change of temperature. The dynamic mechanical behaviour is then considered in terms of a simple extension of this Takayanagi model in which crystalline sequences which span two or more adjacent lamellae are regarded as the fibre phase in a short fibre composite. It can be shown that this model gives a satisfactory prediction of the changes in dynamic tensile modulus and loss with temperature, for a range of samples with different degrees of crystal continuity.     

Breakage patterns of agglomerates

The experimental information available in the literature regarding the patterns of breakage of agglomerate materials is scarce, particularly in dynamic loading. The primary objective of this paper is to present our findings on the breakage patterns of the agglomerates and the interparticle bond. A high-speed digital video imaging technique is used here to gain an insight into the impact behaviour of individual agglomerates against a target plate. Several breakage patterns are observed. Agglomerates may suffer localised damage only, with the disintegration of the damaged zone into very fine debris, or localised damage combined with fracture. The frequency of occurrence of these patterns depends on the impact velocity and agglomerate structure. The pattern of breakage affects significantly the size distribution of the impact product. An investigation of the breakage of individual interparticle bonds is also presented. Two forms of failure are observed, internal (cohesive) and interfacial (adhesive) failure. The morphology of the fractured surface depends greatly on the type of breakage. Internal breakage shows irregular surfaces due to crack jumping, whereas interfacial failure produces clean, smooth fracture surfaces. These observations should provide the necessary foundation for the development of a fundamental model of agglomerate breakage.     

Prediction of mechanical behaviour of adhesively bonded CFRP scarf jointed specimen under tensile loading using localised DIC and CZM

The present study focuses on the mechanical behaviour of both single and double tapered scarf adhesively bonded joint of Carbon fibre reinforced polymer (CFRP) laminate as adherend subjected to tensile loading. The layup sequence of the CFRP adherend having unidirectional (UD) [0⁰]₁₆ and quasi [+45/−45/0/90]_2S are studied. The adhesive used here is Araldite 2015 supplied by Huntsman which is a two part epoxy system of intermediate toughness grade. Here, 2D digital image correlation (DIC) technique is used for capturing the whole field longitudinal, peel and shear strain distribution over the adhesive bond line of the CFRP specimen. Further, a localised DIC measurement is also carried out using microscopic tube lens for precisely capturing strain field over concentrated zones where damage initiation occurs. The evolution of whole field strain distribution with increasing load is captured to predict the mechanical behaviour and failure mechanism of a tapered scarf joint specimen. In addition, 2-D finite element analysis (FEA) of scarf joint model is carried out for validating the DIC results. In the finite element model cohesive zone elements are used for the modelling of both adhesive layer and inter/intra laminar interface of the composite laminate. Initially, to verify the proposed numerical model, joint's initial stiffness, failure load and corresponding displacement obtained from FEA are compared against the experimental load – displacement results. Later, qualitative and quantitative comparison of longitudinal, peel and shear strain values obtained over the adhesive layer by DIC and FEA is carried out to confirm the accuracy of the DIC results. A decent correlation is found to exist between the DIC predictions and numerical results thereby confirming the accuracy of the DIC technique. Analytical solutions are also derived for the same problem based on mechanics of material and further it is compared with both FEA and DIC predictions for completeness.     

Tensile strength of agglomerates

Firstly, this paper gives a review of tensile strength of agglomerates; i.e. a discussion of the most important theoretical considerations, the various methods of measurement, a comparison of measuring techniques by experimental results and a brief look at deformation behaviour under unidirectional loading. In the second part new developments are dealt with.For the tensile strength of agglomerates in which forces are transmitted at points of contact, a new relationship is shown in which the displacements due to individual forces are taken into consideration. Experiments in which not only the tensile strength but also the force distribution are measured demonstrate the necessity of the new approach when short-range forces are present. However, the force—displacement characteristics of the attractive forces need not to be taken into account when long-range forces — e.g forces due to suitability large liquid bridges — act. The attractive forces may then be linearly superposed.The measurement of the tensile strength of agglomerates is discussed in detail. In particular the problematic nature of measurements using the split plate apparatus is pointed out, and a new method of interpreting results is suggested. The applicability of this method is tested by comparable experiments.