Thermal expansion behaviour of ultra-high modulus carbon fibre reinforced magnesium composite during thermal cycling

The thermally induced strain response of unidirectional P100S/AZ91D carbon fibre-reinforced magnesium composite was studied over five cycles in the ±100 °C temperature range. A temperature-dependent one-dimensional model was employed to predict the anticipated response to the cycling thermal environment. Strain hysteresis was observed during cycling and attributed to matrix yielding. First cycle residual plastic strains were modelled with reasonable agreement. Experimental results deviated from predictions during subsequent cycles with continued thermal ratcheting shifting the hysteresis loops to higher strains with increasing cycles. This was thought to be associated with interfacial debonding and frictional sliding at fibre/matrix interfaces. The effect of thermal treatment on composite expansion behaviour was investigated and the results discussed in terms of minimising thermally induced deformations during anticipated service conditions. Treatments were found to affect the first cycle behaviour, reducing in particular residual plastic strain generation. Matrix yield strength was exceeded over the thermal cycle due to a lack of sufficient hardening, and since interfacial conditions were unaltered, interfacial sliding and thermal ratcheting could not be eliminated. The potential for improvement of C/Mg composite thermal strain response was explored in the light of the current findings.     

The creep behaviour of isotropic polyethylene

Dead loading creep and constant strain rate yield experiments have been used to study the tensile creep behaviour of three grades of isotropic polyethylene. This has provided further evidence for the existence of two yield points in isotropic polyethylene. Two different models have been used to attempt to describe this behaviour. Although the results can be described by to both the two process model of Wilding and Ward and the co-operative jump model of Fotheringham and Cherry, it appears that the two process model provides a more convincing quantitative fit to the data. This revised version was published online in September 2006 with corrections to the Cover Date.     

The creep behaviour of Synroc and alumina

The creep of Synroc C and alumina in four-point bending in argon was investigated in terms of the relaxed symmetric stress and the reference asymmetric stress; the alumina being used as a reference material. The creep tests were undertaken in the temperature range from 850°C to 1300°C. The rupture behaviour of Synroc at 950°C indicated a high stress exponent, and that the creep ductility was unusual in that the strain increased with increasing test time. A scanning electron miscroscopy examination of Synroc after creep revealed the development of defect-free oxidised surface layers. For Synroc, neither prior exposure to pre-heating in air, nor prior indentation affected the creep rate behaviour. This was attributed to the formation of the oxidised surface layers and the associated healing effects of the damage produced by the indentations.     

The manufacture of ultra-high modulus polyethylenes by drawing through a conical die

Various grades of linear polyethylene have been drawn through a heated conical die at 100° C. It was found that after a suitable start-up procedure, continuous drawing was possible in all cases with a stable neck region extending beyond the die exit. The degree of deformation attainable was found to depend strongly on the draw velocity. Very high deformation ratios could be obtained and the Young's moduli of the die-drawn products were comparable to those of similar products obtained by solid state extrusion and tensile drawing, reaching values as high as 60 GPa. The effects of molecular weight and co-polymerization are substantial, but not exactly analogous to those previously observed in hydrostatic extrusion and tensile drawing. This is probably due to the non-isothermal nature of the final stage of deformation in the case of die drawing.     

Creep and stress-relaxation in ultra-high modulus linear polyethylene

It has been shown that the creep and stress-relaxation behaviour of oriented linear polyethylene can be satisfactorily described by a model where two thermally activated processes are acting in parallel. The creep behaviour of an oriented sample which does not reach a constant strain-rate can be described by the addition of a further plastic flow term where the viscosity increases linearly with increasing plastic strain.     

The mechanism of creep behaviour in glassy polymers

A model is presented to describe the creep behaviour of glassy polymers below the glass transition temperature. It consists of a Hookean spring in series with a non-Newtonian dashpot having an entropy spring in parallel. The shape of the response of this spring is deduced from a master curve, giving the extension as a function of logarithm of time, built from creep data, reported here and obtained on polycarbonate over a wide range of times and temperatures. The model takes into account a number of aspects of creep behaviour and predicts a threshold stress beneath which delayed yielding no longer occurs. Torsional creep data, obtained on Polyvinylchloride by Mallon and Benham are found to be in excellent agreement with the proposed model.     

The compression creep behaviour of silicon nitride ceramics

A comparison has been made of the compression creep characteristics of samples of reaction-bonded and hot-pressed silicon nitride, a sialon and silicon carbide. In addition, the effects of factors such as oxide additions and fabrication variables on the creep resistance of reaction-bonded material and the influence of dispersions of SiC particles on the creep properties of hot-pressed silicon nitride have been considered. For the entire range of materials examined, the creep behaviour appears to be determined primarily by the rate at which the development of grain boundary microcracks allows relative movement of the crystals to take place. Now with the BNF Metals Technology Centre, Wantage.     

Anisotropy of ultra-high modulus polymers drawn through a die

The influence of processing conditions on the microhardness of ultra-oriented linear polyethylene (LPE) and polyoxymethylene (POM) produced by drawing the material through a heated conical die has been examined. After indentation with a squarebased diamond the die-drawn rods exhibit an anisotropic impression which can be related primarily to a local elastic recovery of the material surface parallel to the fibre axis. It is found that the microindentation anisotropy MH is a unique function of the actual draw ratio achieved, which depends on the drawing speed. A correlation between MH and elastic modulus has also been found. These results confirm that the changes in structure with increasing draw ratio increase the elastic stiffness and improve the recovery behaviour. These changes in structure involve an increase in the number of taut tie molecules or intercrystalline bridges which leads to the increased modulus, and also produce a more effective oriented molecular network which gives rise to improved recovery behaviour. TheMH /E ratio shows for these die drawn materials values 10^–2 which are in the vicinity of those obtained for steel. The fibril orientation is partially destroyed at the die walls through friction effects giving rise to an anisotropy value at the surface which is smaller than that within the core of the die-drawn fibres. Finally the present data also emphasize the influence of the die dimensions on the anisotropy value.     

Transitions in creep behaviour

Attempts to identify the mechanisms operating during creep are often made by examining plots which yield apparent activation energies, or the stress or grain size-dependences of creep-rate. The forms of such plots are here examined and the ambiguities which arise near transitions from one regime to another are noted. The ranges of temperature, stress and grain size commonly used are inadequate and serious errors in interpreting the results of creep tests will continue to be made until a better understanding of the interaction of the basic processes is developed, so as to enable the positions of transitions to be predicted.At Dept. of Metallurgy, University of British Columbia, Vancouver 8, BC, Canada until 31 March 1970.     

Short-term tensile creep and shrinkage of ultra-high performance concrete

The tensile creep and free shrinkage deformations of ultra-high performance concrete (UHPC) were examined through short-term testing to assess the influences of stress/strength ratio, steel fiber reinforcement, and thermal treatment. The use of fibers and the application of thermal treatment decreased 14-day drying shrinkage by more than 57% and by 82%, respectively. Increasing the stress-to-strength ratio from 40% to 60% increased the tensile creep coefficient by 44% and the specific creep by 11%, at 14 days of loading. Incorporating short steel fibers at 2% by volume decreased the tensile creep coefficient by 10% and the specific creep by 40%, at 14 days. Also, subjecting UHPC to a 48-h thermal treatment at 90 °C, after initial curing, decreased its tensile creep coefficient by 73% and the specific creep by 77% at 7 days, as compared to ordinarily cured companion mixes. Comparison of tensile creep behavior to published reports on compressive creep in UHPC reveal that these phenomena differ fundamentally and that further evaluation is necessary to better understand the underlying mechanisms of tensile creep in UHPC. Results from this study also showed that the effects of both thermal treatment and fiber reinforcement were more pronounced in tensile creep behavior than tensile strength results of different UHPC mixes. This emphasizes the importance of conducting tensile creep testing to predict long-term tensile performance.     

Non-linear creep behaviour of PET

The non-linear creep behaviour has been studied on PET films at room temperature. A particular value of the stress, _c, was used to characterize the change between the linear to the non-linear domain. The variations of the elastic modulus, the relaxed modulus and _c revealed great sensitivity to the morphology of the crystallization. A molecular model of non elastic deformation, assuming (i) hierarchical correlated molecular motion, and (ii) nucleation and expansion of sheared-microdomains, was used to analyse the role of stress on anelasticity. To take into account the two-phase structure of semicrystalline films, a phenomenological series/parallel model was applied to express the mechanical coupling between amorphous and crystalline phases. Quantitative agreement was found between theoretical predictions and experimental data for low and high stresses. However, there was a discrepancy in the rate of recovery because the model predicts a strain recovery slower than the experimental behaviour. Consequently, it is proposed to develop further the molecular model mentioned above by specifying the energy profile of a sheared-microdomain and its stress dependence. Then, the difference between creep and recovery strain rate could be explained.Nomenclature A Anelastic equilibrium compliance - A Parameter proportional to the relaxation strength - b Shear vector - Correlation parameter - _i Particular value in the distribution - _e Average value in the material ( = 0.27) - Correlation parameter characterizing the ability of chain orientation - d _a Amorphous density - d _c Crystalline density - Parameter of the mechanical mixing law - E Tensile modulus - E _c Crystalline Young's modulus - g _i Statistical weight in the distribution - G ₀ Shear modulus at 0 K - J _max Creep compliance at the end of the creep time - J _max(0) Value ofJ _max for low stresses - J _u Unrelaxed compliance - J _i( _i;A _i) Calculated compliance for a couple ( _i;A _i) - J _exp Experimental creep compliance - J _a Compliance of the amorphous part - J _c Compliance of the crystalline part - J _sc Compliance of the semi-crystalline material - k Boltzmann's constant - Parameter of the mechanical coupling law - R Radius of a shear micro domain - ₀ Stress necessary to cross the energy barrier only by mechanical activation - T _c Crystallization temperature - t _c Creep time - _mol Time for a translational motion of a structural unit over a distance comparable to its size - Particular value of _mol in the time distribution - Characteristic time for the secondary relaxation - ₀ Time proportional to the Debye time - t ₀ Scaling time parameter determined by the experimental value of _mol - U Activation energy for an elementary molecular motion - X _c Crystallinity ratio - V _a Volume fraction of the amorphous part - V _c Volume fraction of the crystalline part     

The creep and fracture behaviour of tempered martensitic steels

Creep strength enhanced ferritic steels contain 9 to 12% Cr and were developed to exhibit excellent high temperature properties. These should be achieved when the microstructure exhibits a tempered martensitic matrix containing a substructure with a high dislocation density and a uniform dispersion of fine, second phase precipitates. It is interesting to note that when properly processed the typical alloy compositions for these steels provide reasonable strength but can exhibit brittle creep behaviour. The levels of ductility required in engineering applications necessitate proper control of composition (including trace elements), steel making and processing and all heat treatments. The properties needed for modern design methods can only be obtained using validated procedures for both uniaxial and multiaxial testing and documentation to establish the mechanisms controlling deformation and fracture for relevant stress states.     

The cyclic creep behaviour of Type 316 stainless steel

The cyclic creep behaviour of a Type 316 stainless steel at 625° C has been examined as a function of the maximum applied stress and frequency using trapezoidal loading cycling between zero and a maximum stress. The so-called static-to-dynamic creep transition observed is interpreted in terms of recoverable anelastic strain behaviour without using an internal stress argument. Over the range of experimental conditions examined, failure occurs by static creep modes, namely wedge crack nucleation and growth. The loading strain increments appear to be damaging to about the same extent as the much slower strain occurring at constant load, such that it is the overall strain rate that determines the rate of damage. A cursory examination of square wave load cycling shows that the behaviour is very similar to that observed during trapezoidal loading and suggests that the rate of loading and unloading does not play an important part in determining the creep and rupture behaviour.     

Short-term creep and strength of fibrous polypropylene structures

The short-term creep and strength of fibrous polypropylene structures are investigated. On the basis of these characteristics, we develop the models of linear and nonlinear viscoelastic deformation of materials, specify the fields of their applicability, and study criteria used for the evaluation of the static strength and durability of these composites. __________ Translated from Problemy Prochnosti, No. 6, pp. 77–90, November–December, 2007.     

Fracture behaviour of a polypropylene film

Ageing of polymers results from structural modifications at the molecular scale and kinetic modelling must be elaborated from analysis of the phenomenon at this scale. However, the change of mechanical properties results from modifications of structure at larger scale, especially the macromolecular scale (chain scission, crosslinking) and or at the macroscopic scale (skin-core structure linked to a superficial attack of the material).     

Creep behaviour of polyethylene and polypropylene

Tensile creep tests and stress reduction studies during creep have been carried out for polyethylene and polypropylene. The results obtained suggest that a consistent approach for the presentation of creep data for these polymeric materials can be obtained since the creep curves at 293K for polyethylene and polypropylene over a wide stress range can be superimposed by describing the variation of creep strain,, with time,t, as= ₀ + _p [1 – exp (–K t)] + t, where ₀ is the initial strain on loading, _p is the primary creep strain, is the secondary creep rate, andK is a constant.