Creep behaviour in unloading process of cellulose nitrate under superimposed tensile and hydrostatic loading

The creep behaviour in unloading process under various combinations of superimposed tensile and hydrostatic loading are quantitatively investigated using non-linear-viscoelastic cellulose nitrate heated at 65° C. The creep strain and the creep strain-rate in the unloading process are quite influenced by the effect of hydrostatic pressure, but are not so influenced as those in the loading process mentioned in a previous paper. The deformation properties in the unloading process are also discussed with experiments for proportional loading (namely, uniform rate of stress increases or decreases with time). The stress-strain relation in the unloading process of the creep behaviour under superimposed loading is deduced by using the invariant theory. The deduced relation gives good agreement with the actual observations under the superimposed loading.     

Cyclic tensile creep behaviour of cellulose nitrate and evaluation of constitutive equations

The cyclic creep behaviour under axial tension was quantitatively investigated using non-linear viscoelastic cellulose nitrate heated at 65°C. The instantaneous strains at the instants of loading and unloading for three creep cycles and the creep strain rates during the three creep cycles were found to be influenced by the cycle numbers. However, the effect of the cycle number on the loading process was quite different from that on the unloading process in the cyclic creep deformation. The evaluation of the creep constitutive equations for the loading and unloading processes at one loading cycle deduced in the previous papers is discussed for the cyclic creep deformation. The deduced creep equations give good agreement with the actual observations for the three creep cycles independent of the cycle numbers and of the creep stress levels.     

Deformation behaviour of self-compacting concrete under tensile loading

The deformation behaviour of self-compacting concretes of the compressive strength classes C30/37, C45/55 and C60/75 according to the European Standard EN 206-1 under sustained tensile loading was investigated up to 2.5 years. The long-term tensile strength was estimated to be 69% of the short-term tensile strength, determined at an age of 28 days. Load-free shrinkage has been measured on companion specimens. The usual way to determine creep is to subtract the measured shrinkage strain and the elastic or initial strain from the measured total strain. In doing so, a phenomenon was discovered which is called stress-induced shrinkage. It turned out that the drying shrinkage was larger for loaded specimens than for load-free specimens. Similar results have been found earlier. It seemed that the stress-induced shrinkage tends to increase with decreasing compressive strength. An important practical consequence is arising from stress-induced shrinkage: structural elements subjected to tensile␣load, tend to dry and to shrink faster than one␣would expect based on the assumption of load-free shrinkage. In the case of sustained deformation, this would raise the risk of cracking and would have a negative effect on the durability of a concrete structure. An erratum to this article can be found at     

Fatigue behaviour of alumina-fibre-reinforced epoxy resin composite pipes under tensile and compressive loading conditions

Fatigue tests have been conducted on composites consisting of epoxy resin reinforced with alumina fibres (AFRP) under cyclic tensile and compressive loading conditions with the variation of fibre orientation. The behaviour of the stress/strain curve for a ±45° sample is different from those for the ±15 and ±25° composite specimens, whereas, the monotonic strength decreases with increase in fibre angle for all specimens, which satisfies the maximum stress failure criterion. Fatigue results show that the applied stress decreases with an increase in the number of cycles to failure under both loading conditions for all composite pipes, but for the ±45° sample the decrease was slow. The results of fatigue tests on a macroscopic level indicate that the matrix crack density slowly increased with increase in the normalized number of cycles to failure in all the specimens. The normalized apparent stiffness therefore falls with an increase of the normalized number of cycles to failure. However, the maximum stress decreased with the increase in the number of cycles to failure in the case of the ±45° pipe. Finally, it is observed that matrix cracking and delaminations are occurring in the ±45° sample whereas delamination and fibre buckling are appearing in the ±15 and ±25° samples.     

Creep crack growth under history-dependent loading

We discuss the influence of loading history on creep crack growth. Our attention is mainly focused on the following three aspects of this problem: (i) principal laws of history-dependent creep strain of materials; (ii) creep behavior of cracks, including the choice of suitable fracture parameters that may help to predict cracking; (iii) the importance of taking the history-dependent response of the material into account. We performed numerical calculations based on the use of an appropriate constitutive model and fracture theory for (1) and (2), respectively, to analyze results of tests for (3).Battelle, Columbus, Ohio. UES Incorporated, Dayton, Ohio. Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 4, pp. 37–45, July – Augus, 1994.     

Mechanical behaviour and formulation of stress-strain relation for non-linear viscoelastic cellulose nitrate under cyclic loadings

The cyclic deformations under various repeated stresses are quantitatively investigated using non-linear viscoelastic cellulose nitrate heated to 60° C. The non-elastic strain or creep-plastic strain is remarkably influenced by the repeated stress and the stress rate. The cyclic deformations corresponding to the repeated stress less than a certain stress level attain the saturated state called the shake down after some cycles. The stress-strain relations of the non-linear viscoelastic media in the loading and unloading processes are deduced from the invariant theory using an hypothesis of creep potential. The non-linear viscoelastic observations obtained on the cellulose nitrate at 60° C under cyclic loadings are found to fit the deduced relations for the loading and unloading processes independent of the repeated stress and the stress rate.     

Equipment for creep testing under tensile stress and hydrostatic pressure

The design and operation of equipment is described that permits creep tests to be made under independently controlled tensile stresses and superimposed hydrostatic pressures at elevated temperatures. The range of usefulness and applications of the equipment are briefly indicated.     

Testing of anchorages in concrete under dynamic tensile loading

The tensile dynamic behaviour of anchors in concrete has been experimentally investigated by employing Hopkinson bar techniques. Dynamic pull-out tests with two anchor size diameters have been performed, concrete cone breakout failure has been predominantly induced, and full force-displacement curves have been obtained. The test results show that properly post-installed anchors can achieve the same load bearing capacity as the cast-in-place ones. It has also been verified that the three different types of anchors tested (with chemical adhesive, undercut, and headed studs) can develop comparable failure loads under similar embedment depths. Test results have demonstrated that force-displacement diagrams for dynamic loading tend in general to lie well above the corresponding static ones, i.e. additional ductility is available. Comparisons with predictions from design formulae have been conducted.     

Experiments on concrete under uniaxial impact tensile loading

A problem of practical importance for designing of structural elements is discussed in this paper—the behaviour of concrete subjected to uniaxial impact tensile loading. The “Split Hopkinson Bar” technique was adopted for testing concrete in uniaxial tension at stress rates between 2 and 60 N/mm²/ms. A remarkable increase in tensile strength was observed due to high stress rate. The ratio of impact and static tensile strength varied between 1.33 and 2.34 for various concrete mixes. The influence of maximum aggregate size, water-cement ratio, cement content, cement type and quality, specimen humidity, static compressive strength and loading/casting direction upon the uniaxial impact tensile strength was studied. The high stress rate resulted in an increase of the modulus of elasticity of concrete in uniaxial tension. An explanation for the observed phenomena is suggested.     

Braided composites for energy absorption under tensile loading

A new class of braided composites has been designed to maximise thetotal energy absorbed during tensile failure. Braided loops oflight, continuous fiber tows are configured in such a way that theymust be drawn through relatively large displacements before they comeinto direct contact with one another. Upon loop contact, thematerial hardens locally, forcing further damage to develop by thesame process elsewhere. In this way the entire gauge section absorbsenergy before ultimate failure. Levels of energy absorption per unitvolume reach 30 MJ/m³ and, per unit mass, 18 J/g. The mechanismsinvolved in damage delocalisation and failure are detailed andmodeled at a very simple level. While the current values of energyabsorption are already attractive, the simple models indicate muchhigher values for composites that have been optimised.     

Cycles of sub-critical tensile and shear alternating fracturing in diminishing dimensions,under tensile loading

A microscopic study reveals that when the curvature of striae that mark the fracture surface of PMMA glass with a chevron pattern increase beyond the critical angle, μc = 3° ± 2°, a breakdown into alternating tensile dark zones, and bright, ragged shear zones occurs. This breakdown was repeated in primary, secondary and tertiary cycles in diminishing scales. The secondary and tertiary breakdowns occurred exclusively in the shear zones. Similar breakdowns were found in chevron patterns on the fractured surface of a silicate glass ceramics. Due however to their different properties, certain differences were identified between the two materials in their breakdown characteristics, e. g. in the glass ceramic μc = 20° ± 2°. A similar primary breakdown was also identified on tensile fractures cutting rocks in geological outcrops. In the glass ceramic the interface angle , which the striae form with the fracture boundary, decreased from 32° ± 2°in the early stage of the striae growth at relatively low velocity, to 13° ± 2° during their advanced growth, at greater velocity, demonstrating that is a good tool for monitoring the change in fracture velocity in a given material. It was found that four interconnected factors determine the geometries and breakdown styles of the chevron pattern: (1) the curvatures of the fracture front and that of the striae which intersect each other orthogonally, (2) the influence of the fracture boundaries, (3) the material properties, such as stiffness, and (4) the fracture velocity in the material.     

Creep behaviour of AISI 316H stainless steel under stress-varying creep loading conditions: primary creep regeneration

Primary creep regeneration (PCR) is an important reported observation from creep under stress-varying conditions for several alloys. For a specimen deforming in the secondary creep regime, a stress reversal leads to an enhanced creep rate upon reloading due to reactivation of the primary creep regime (i.e. PCR). This paper focuses on an investigation of the PCR phenomenon during stress-varying creep loading for AISI 316H stainless steel at 650°C. The experimental observations clarify the influence of different parameters (e.g. forward creep stress level, reverse stress magnitude and forward and reverse accumulated inelastic strain) on the extent of PCR activation. In addition, a correlation between the extent of PCR activation and inelastic strain accumulation during the reverse loading period was found, which was employed to develop an empirical–phenomenological model for prediction of the creep behaviour of the alloy after stress transients (e.g. stress reversals).     

The tensile properties of pultruded GRP tested under superposed hydrostatic pressure

The failure mechanisms in waisted tensile specimens of pultruded 60% volume fraction glass fibre-epoxide were investigated at atmospheric and superposed hydrostatic pressures extending to 350 MN m^–2. The maximum principal stress at fracture decreased from 1.7 GN m^–2 at atmospheric pressure to 1.3 GN m^–2 at 250 MN m^–2 superposed pressure and remained approximately constant at higher pressures, as had been observed with carbon fibre reinforced plastic (CFRP) and a nickel-matrix carbon fibre composite. In the high-pressure region the failure surfaces were fairly flat, consistent with the fracture process being solely controlled by fibre strength. Pre-failure damage, in particular debonding, was initiated at 0.95 GN m^–2 at atmospheric pressure and this stress rose to 1.2 GN m^–2 at 300 MN m^–2 superposed pressure, i.e. by about 9% per 100 MN m^–2. Unlike the pressure dependence in CFRP, this contrasts with the pressure dependence of the resin tensile strength, about 25% per 100 MN m^–2, but can be associated with that of the fibre bundle/resin debonding stress, about 12% per 100 MN m^–2 superposed pressure. Consistent with this interpretation, glass fibres of the failure surfaces were resin-free, again in contrast to CFRP.     

Creep characteristics of glass reinforced cement under flexural loading

Tests are reported on the flexural creep behaviour and flexural strength characteristics of mortar specimens reinforced with 5.0% by weight glass fibres. The specimens were cut off from flat sheet materials produced by premixing and spray suction casting processes. It is shown that fibre reinforcement reduced the deflections under sustained flexural load. The results show unmistakably a very significant influence of the fibre reinforcement in reducing creep strains. Fibres were generally more effective in controlling compression creep than tensile creep. Strength reductions were observed with time which were insignificant in specimens produced with the spray suction method.