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     

Constitutive equation for transient creep of cellulose nitrate under hydrostatic pressure

The behaviour of polymers is influenced by the hydrostatic pressure during the unloading creep period as well as during loading creep. Moreover, the behaviour during the unloading creep is also influenced by the value of the unloading strain at the final point of the loading process. In this paper, a constitutive equation for transient creep in the unloading process is proposed by using the same postulates as in the loading process of the previous paper, which includes the effect of the hydrostatic pressure and the effect of the unloading strain. The proposed unloading creep equation is in good agreement with the actual creep data on cellulose nitrate.     

Electrochemical impedance behaviour of type 304L stainless steel under tensile loading

An interesting effect has been observed in the Electrochemical Impedance Spectroscopy (EIS) of type 304L stainless steel subjected to tensile loads. The impedance changes were found to correlate with the increase of activity as shown by Free Corrosion Potential. The effects are ascribed to fracture of the passive film and the associated increase in electrochemical activity as the passive film is broken.     

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.     

Tensile failure of pultruded glass-polyester composites under superimposed hydrostatic pressure

The tensile properties of five types of pultruded 0·52 V_f glass-fibre-polyester rods were investigated by extending waisted round specimens at atmospheric and superimposed hydrostatic pressures, −H, to 300 MPa. The maximum principal stress at fracture, −700 MPa, decreased, with the superimposition of −H, approximately by its magnitude. As −H increased the failure surfaces became flatter, the amount of fibre pull-out decreased and transverse cracks became shorter or were eliminated. Glass fibres in the failure surfaces were resin free, and failure of the glass fibre bundles appeared to control the fracture process in the entire pressure range for all materials. The decrease in maximum principal tensile stress with increasing −H indicates that the glass fibre failure process is not controlled by a critical tensile stress. Failure criteria are discussed, and in the tension-compression-compression octant of stress space the relevant criteria appear to be strain energy and deviatoric tensile stress, strain and strain energy for these GRPs and glass itself.     

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.     

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.     

Knitted composites for energy absorption under tensile loading

A new class of knitted composites has been designed to maximize the total energy absorbed during tensile failure. Knitted loops of light, continuous fiber tows are configured in such a way that they must be drawn through relatively large displacements before they come into direct contact with one another. Upon loop contact, the material hardens locally, forcing further damage to develop by the same process elsewhere. In this way the entire gauge section absorbs energy before ultimate failure. Levels of energy absorption per unit volume reach 40 MJ/m³ and, per unit mass, 25 J/g. The mechanisms involved in damage delocalization and failure are detailed and modeled in a simple manner. While the current values of energy absorption are already attractive, the models indicate that much higher values could be attained with optimized composites.     

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.     

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.