Biaxial impact fatigue of polycarbonate

An impact fatigue test has been designed for studying the repeated biaxial impact load response of polymers. In the test geometry, a thin plate-like sample is rigidly held by an annular clamp and is repeatedly impacted by a hemispherically-tipped plunger. Results have been obtained for polycarbonate. Under impact fatigue, cracks initiate on the bottom surface where high biaxial tensile stresses exist. These cracks propagate along radial lines on the sample surface and also into the sample normal to the surface. The load-bearing capability and the residual impact strength decrease as the cracks propagate. The development of impact fatigue cracks, the lifetime behaviour and the change in mechanical properties after partial impact fatigue are discussed.     

Residual fatigue strength assessment of concrete considering tension softening behavior

In this study, the residual strength of plain concrete beams under fatigue loading is assessed. The quasi-brittle nature of the material is considered by including the effect of tension-softening taking place in the fracture process zone. A two step approach is followed. In the first step, the effective critical crack length for unstable fracture to occur is determined by using two different methods, namely a modified LEFM based fatigue crack propagation law and the crack resistance method. In the second step, the moment carrying capacity as a function of increasing effective crack length is obtained in order to assess the residual strength of the member. A parametric study is performed by considering three different softening laws: linear, bilinear and power laws. It is seen that the bilinear softening law matches close to the experimental predictions of other investigators.     

Biaxial tension and ultimate deformation of knitted fabric reinforcements

In this study, the behavior under biaxial tension of two types of glass knitted fabrics, a rib and a milano weft knit, was investigated. The tests were performed using a homemade biaxial tester, which is capable of applying displacement velocity in two orthogonal directions independently. The ultimate deformation in both wale and course directions was determined at different displacement ratios. Furthermore, geometrical models were developed for the rib knit to predict the ultimate deformation. Good agreement with experimental results was achieved.     

Biaxial shear/tension failure behavior of Spectra single fibers

An experimental study is conducted to develop the biaxial failure surface of single Spectra 100d and Spectra 130d filaments in a torsion-tension environment. The cross-sectional profiles of single fibers are evaluated using scanning electron microscopy and X-ray computed tomography. In efforts to hold the polyethylene single fiber, a pin-gripping method is developed. Effects of pin diameter on failure stress for both Spectra types are characterized. Additionally, the effect of sample gauge length on fiber tensile strength is investigated. Quasi-static experiments are conducted using an MTS servo-hydraulic system to apply tensile loads on pre-twisted single fibers. A tension Kolsky bar is employed to study the biaxial shear/tensile behavior of single fibers at high strain-rates. A decreasing trend of tensile strength with increasing torsional strain is observed for both Spectra 100d and 130d. Furthermore, a torsional pendulum apparatus is used to determine the apparent torsional shear stresses in pre-twisted fibers at various levels of axial loading and a relationship between apparent shear stress and axial stress is developed. Finally, a biaxial shear/tension failure envelope of each fiber type is established.     

Sensitivity analysis of dynamic response and fatigue behaviour of various asphalt concrete mixtures

This paper presents the dynamic response (|E*|) and fatigue behaviour of various asphalt concrete mixtures subjected to sinusoidal compressive loading. Eight different wearing and base mixtures including Superpave, Asphalt Institute, British Standard dense bituminous macadam and Pakistan's National Highway Authority gradations were selected, and gyratory compacted specimens were fabricated. Laboratory investigations of |E*| at various temperatures (4.4 to 54.4 °C) and loading frequencies (0.1 to 25 Hz) were used to construct stress‐dependent master curves separately, for wearing and base course mixtures. The indicators of dynamic response and viscous (or elastic) properties of the mixtures were used to derive fatigue parameter to estimate the resistance to fatigue, and results revealed that Superpave wearing and NHA‐B base course had better resistance to fatigue for evaluated mixtures. Also, the sensitivity of the dynamic modulus to the variation in hot mix asphalt mix properties using different aggregate gradation, diverse loading frequencies and temperature were evaluated.     

Biaxial fatigue behavior of an epoxy polymer with mean stress effect

Biaxial fatigue behavior of an epoxy polymer was investigated under cyclic shear and proportional axial-shear combined loadings with mean strains. Axial and shear strains were simultaneously measured by a non-contact real-time strain measurement system during fatigue process. Then the mechanical parameters (stress/strain components, mean stress/strain, strain energy densities, etc.) of the specimens during entire fatigue life were able to be quantitatively retrieved from the recorded stress-strain data. Multiaxial fatigue life prediction models were established upon stress-, strain- and energy-based approaches with consideration of mean stress/strain effect, where better agreement was achieved in stress-based and energy-based approaches.     

Biaxial monotonic and fatigue fracture of some commercial ABS and PVC sheets

Biaxial loading of pre-cracked cruciform testpieces has been preformed in a novel rig attached to a uniaxial testing machine. Fracture toughness R or c of the ductile acrylonitride butadiene styrene (ABS) and polyvinyl chloride (PVC) determined by the Cotterell–Mai method is dependent on remote biaxiality. Least toughness is shown for equibiaxial tension; greatest for uniaxial tension. These monotonic fracture results may be modelled using void growth mechanics. Fatigue crack growth rates also depend on remote biaxiality. Paris/Walker representation of the data shows that the slopes n of log (da/dN) versus log K do not change much, but the constant of proportionality C decreases as the tensile mean stress increases. There may be a connection between the biaxial-dependent C and R or c. © 1998 Kluwer Academic Publishers     

Biaxial fatigue for proportional and non‐proportional loading paths

In order to study the use of a local approach to predict crack‐initiation life on notches in mechanical components under multiaxial fatigue conditions, the study of the local cyclic elasto‐plastic behaviour and the selection of an appropriate multiaxial fatigue model are essential steps in fatigue‐life prediction. The evolution of stress–strain fields from the initial state to the stabilized state depends on the material type, loading amplitude and loading paths. A series of biaxial tension–compression tests with static or cyclic torsion were carried out on a biaxial servo‐hydraulic testing machine. Specimens were made of an alloy steel 42CrMo4 quenched and tempered. The shear stress relaxations of the cyclic tension–compression with a steady torsion angle were observed for various loading levels. Finite element analyses were used to simulate the cyclic behaviour and good agreement was found. Based on the local stabilized cyclic elastic–plastic stress–strain responses, the strain‐based multiaxial fatigue damage parameters were applied and correlated with the experimentally obtained lives. As a comparison, a stress‐invariant‐based approach with the minimum circumscribed ellipse (MCE) approach for evaluating the effective shear stress amplitude was also applied for fatigue life prediction. The comparison showed that both the equivalent strain range and the stress‐invariant parameter with non‐proportional factors correlated well with the experimental results obtained in this study.     

Scale effect on concrete in tension

Scale effects in the mechanical behaviour of concrete are important phenomena and are the object of many studies internationally. The Laboratoire Central des Ponts et Chaussées has been working for several years on the scale effect on the distribution function of the tensile strength of concrete. Very many direct tensile tests have been performed on specimens of different volumes, made of concretes having different formulations. From these experimental investigations has been derived a general law that can be used to determine the distribution function (mean and standard deviation) of the tensile strength of the concrete versus the volume of the tensile specimen, for concretes having compressive strengths betwen 30 MPa and 130 MPa, from knowledge of just the compressive strength obtained on a standardized cylinder (in France) and the ratio of the volume of the tensile specimen to the volume of the coarsest aggregate in the concrete. It should be emphasized that this law is valid only if, on the one hand, the mode of preservation of the direct tensile specimens is identical to that of the compressive specimens and, on the other, the aggregates used in the concretes are common aggregates, which eliminates light aggregates and very hard aggregates with a high Young's modulus.

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Resume Les effets d'échelle dans le comportement mécanique du béton sont des phénomènes importants qui font l'object de nombreuses études au niveau international. Le Laboratoire Central des Ponts et Chaussées travaille depuis plusieurs années sur l'effet d'échelle existant sur la fonction de distribution de la résistance à la traction du béton. De très nombreux essais de traction directe ont ainsi été réalisés sur des éprouvettes de volume différent constituées de bétons de formulation également différente. De ces études expérimentales, il a été tiré une loi assez générale qui permet de déterminer, à partir de la seule connaissance de la résistance à la compression obtenue sur cylindre normalisé (en France), et du rapport du volume de l'éprouvette de traction au volume du plus gros grain de béton, la fonction de distribution (moyenne et écart-type) de la résistance à la traction du béton en fonction du volume de l'éprouvette de traction, cela pour des bétons dont la résistance à la compression est comprise entre 30 et 130 MPa. Il faut souligner que cette loi est valable uniquement si, d'une part le mode de conservation des éprouvettes de traction directe est identique à celui des éprouvettes de compression, et d'autre part les granulats utilisés dans les bétons sont des granulats courants, ce qui élimine les granulats légers et les granulats très durs et à fort module de Young.

     

On tension stiffening in reinforced concrete

A two phase mixture model for reinforced concrete is presented. The theory is intended to simulate phenomena such as tension stiffening in reinforced concrete members. Simulation capability includes steel-concrete bond degradation and slip, progressive concrete cracking, and nonlinear responses of steel and concrete. The resulting model is applied to the problem of tension stiffening 0of certain reinforced concrete elements and a closed form solution of the mixture relations is obtained for displacement-controlled tension tests. Theory versus experiment comparisons indicate that the mixture concept furnishes a capability to accurately mirror complex phenomena resulting from both concrete cracking and interface slip.     

Crack growth behavior of 7075-T6 under biaxial tension–tension fatigue

Crack growth behavior of aluminum alloy 7075-T6 was investigated under in-plane biaxial tension–tension fatigue with stress ratio of 0.5. Two biaxiality ratios, λ (=1 and 1.5) were used. Cruciform specimens with a center hole, having a notch at 45° to the specimen’s arms, were tested in a biaxial fatigue test machine. Crack initiated and propagated coplanar with the notch for λ = 1 in L–T orientation, while it was non-coplanar for λ = 1.5 between L–T and T–L orientations. Uniaxial fatigue crack growth tests in L–T and T–L orientations were also conducted. Crack growth rate in region II was practically the same for biaxial fatigue with λ = 1 in L–T orientation and for the uniaxial fatigue in L–T or T–L orientations, while it was faster for biaxial fatigue with λ = 1.5 at a given crack driving force. However, fatigue damage mechanisms were quite different in each case. In region I, crack driving force at a given crack growth rate was smallest for biaxial fatigue with λ = 1.5 and for uniaxial fatigue in T–L orientation, followed by biaxial fatigue with λ = 1 and uniaxial fatigue in L–T orientation in ascending order at a given crack growth rate.     

Cracking and permeability of concrete under tension

Cracking of concrete, whatever its origin (mechanical, physico-chemical, thermal,…) is a key factor for the materials durability. Knowledge of the transfer properties of sound and of cracked concrete is essential for predicting its durability since the deteriorating mechanisms (freezing, corrosion, leaching) depend on the flow of aggressive (liquid or gaseous) agents through the porous or cracked body. A series of experiments is intended to estimate the increase in permeability resulting from mechanically-induced cracking. A concrete specimen is subjected to uniaxial tension, and the water permeability is measured in a direction perpendicular to the axis of loading. The tests are designed and monitored in order to collect useful data for modelling concrete structures: the material is in tension (and not in compression as it is in more classical studies of damage-permeability coupling) and the permeability is measured through open cracks covering a large width range (0.1 μm–0.1mm). Thus, the transfer properties are being evaluated in the most unfavorable context. Herein we detail the principle of the tests and the design of the specimen, using numerical simulations, as well as present and comment the first experimental results.     

The static and fatigue response of metal laminate and hybrid fibre-metal laminate doublers joints under tension loading

Experimental and numerical studies have been undertaken on metal laminate (ML) doublers and hybrid fibre-metal (aluminium–Glare) laminate (FML) doublers to investigate their static and fatigue response under tension loading. Inevitably sheets in these laminates butt together and these butts can affect the joint strength. Progressive damage modelling, including the damage in the adhesive bondline, the butt, the metal and the fibre has been undertaken in both static and fatigue loading. This modelling was found to be in good agreement with the experiment data in terms both of the strength and the failure mechanisms. In ML, the butt influenced the static and fatigue response. In hybrid FML, the specimens either have the fibres parallel to the loading direction (spanwise) or perpendicular to the loading direction (chordwise). The spanwise specimen was found to have the highest strength followed by chordwise specimens without butts and finally chordwise specimens with butts. The most critical position for a butt was found to be adjacent to the doubler end. Without butts the static strength for spanwise and chordwise specimens was controlled by the failure in the Glare layer whilst the fatigue failure was precipitated by failure in the aluminium sheet.     

Method of testing materials for thermal fatigue with a specified Biaxial stress state

A method is devised for conducting thermal fatigue tests of specimens with an annular notch and specially shaped end surfaces. Packets of specimens are cyclically heated and cooled by the injection of hot gas and cold air through their internal cavity. A new design of specimen is proposed and is used as the base specimen in tests performed by the authors. It is shown that, if the heat-transfer conditions are the same for different parts of the specimen, the removal of a small amount of metal from certain sections of the specimen on a lathe changes the components of the maximum nonequilibrium thermal stresses and their proportions within the range from −∞ to +∞. Methods for calculating the thermal and thermal-stress states of specimens are described. This study was financed by the State Fund for Basic Research through the Ukrainian State Commission for Science (grant entitled “Extremal≓). Translated from Problemy Prochnosti, No. 5, pp. 45–52, May, 1996.     

Stochastic approach to study fatigue of concrete

While concrete is one of the most important materials in structural engineering, the fatigue life as a function of the stress amplitude displayed a large scatter and the mechanism of the fatigue behaviour has not been clarified yet. In the present paper, the mechanism was discussed and a stochastic approach to study the fatigue behaviour was described. The fatigue process was subdivided into three stages such as mesocracks initiation, microcracks accumulation and macrocrack propagation. Since the tertiary stage is very unstable, the fatigue life may be related to the transition from the secondary stage to the tertiary stage. From such a view point, a stochastic process theory was applied to estimate the fatigue life of concrete and the agreement with published experimental results was critically discussed.     

Reliability of reinforced concrete structures under fatigue

This paper focuses on time-variant reliability assessment of deteriorating reinforced concrete structures under fatigue conditions. A strategy combining two time scales, namely the micro-scale of instantaneous structural dynamics (or statics) and the macro-scale of structural lifetime, is proposed. Non-linear response of reinforced concrete structures is simulated by means of the finite element method with adequate material model. A phenomenological fatigue damage model of reinforced concrete is developed and calibrated against experimental results available in the literature. Reliability estimates are obtained within the response surface method using the importance/adaptive sampling techniques and the time-integrated approach. The proposed assessment strategy is illustrated by an example of a concrete arch under fatigue loading. The obtained results show a general inapplicability of local and linear fatigue models to system level of structures.     

Experimental behaviour of high-performance concrete in confined tension

Prestressed concrete is commonly used in a wide range of applications, yet on rare occasions spalling problems have been reported for concrete structures prestressed in two directions. The tensile behaviour of concrete may be affected by prestressing; consequently a number of experiments have been performed to reproduce this load combination on laboratory samples. Tests were carried out in a compression-tension triaxial cell suitable for the independent application of a lateral confining pressure, as well as a direct axial tension load. Results reveal a measurable but limited reduction in concrete tensile strength with a confining pressure of a similar magnitude as the prestress (order of magnitude of 10 MPa). Further experiments were conducted at higher confining pressures, in both confined tension and in triaxial extension, in order to generate an overview of limit state behaviour for the studied high performance concrete.     

Effects of static–fatigue (tension) on the tension–tension fatigue life of glass fibre reinforced plastic composites

The effects of static–fatigue interaction on tension–tension fatigue life of glass fibre reinforced plastic (GFRP) composites were investigated. This paper proposed a new static–fatigue model, which is capable of predicting residual strength after a period of static loading. Also an algorithm is proposed to calculate fatigue lives with the inclusion of static–fatigue interaction. Predictions from the proposed static–fatigue model show a good agreement with the experimental results. Static–fatigue interaction has shown a considerable effect on fatigue lives of GFRP composites at intermediate and lower applied stress levels possibly due to a longer exposure to applied loads. At higher load levels approximately greater than 65% of ultimate stress, and higher stress ratios range like 0.5 < R < 0.9, fatigue lives shown to be closer to material’s static–fatigue limits which is shorter than the expected lifetime by cyclic fatigue.