Application Of Cyclic Loading Research Articles

Effects of misoprostol and prostaglandin E2 on proteoglycan biosynthesis and loss in unloaded and loaded articular cartilage explants

The effects of misoprostol, a prostaglandin E1 analog, and prostaglandin E2 on proteoglycan biosynthesis and loss were studied in unloaded and mechanically loaded mature bovine articular cartilage explants. The prostaglandins were administered daily at dosages of 0, 10, 100 and 1000 ηg/ml for up to seven days, and proteoglycan biosynthesis determined by measurement of radiolabelled sulfate incorporation. The presence of misoprostol lead to a significant ( p<0.001) dose-dependent inhibition (30%–50%) in proteoglycan biosynthesis which was also dependent on exposure time ( p<0.05). A significant decrease in biosynthesis (34%) was also found for prostaglandin E2, but only at the highest dose (1000 ηg/ml). Proteoglycan catabolism rates were not affected by either substance as assessed by loss of newly synthesized proteoglycan. The application of a continuous cyclic mechanical compressive load (stress of 1.0 MPa at 1 hertz for 24 hours) resulted in a significant inhibition of proteoglycan biosynthesis (up to 50%) as compared to unloaded explants. However, there was no additive effect when mechanical load and misoprostol or prostaglandin E2 were combined. These results suggest that prostaglandins may have a role in the degenerative and repair process in various forms of arthritis where elevated intra-articular levels of prostaglandin E2 are present.

Tensile reinforcement of asphalt concrete using polymer coating

This study investigates the possibility of utilizing a polyester resin for reinforcing flexible pavements. The application of a thin-layer coating with a polymer, unsaturated polyester resin (UPR) on the surface of a laboratory-prepared unmodified asphalt concrete mixture was studied as a tensile reinforcement method for such a material. Selected laboratory performance tests were conducted and the results are compared with those of a normal (uncoated) asphalt concrete mixture and a modified asphalt mixture, both mixtures being widely used in Korea. The polymer coating was found to be effective in improving Marshall stability, tensile strength and flexural strength of asphalt concrete. These improvements can be explained as the effect of reinforcement by a thin polymer layer which is fully bonded to the specimen faces. The reinforcement was also effective in reducing the stiffness of the mixture whilst improving load-carrying capacity. This improvement in strength and reduction in stiffness resulted in a retardation of crack initiation resulting from cyclic load application and a significantly improved resistance to crack propagation. The study has shown that there is a possibility of using the polymer coating as a method of tensile reinforcement with flexible pavements.

Relationship between cyclic loading and degradation of piezoelectric properties in Pb(Zr,Ti)O3 ceramics

The degradation of piezoelectric properties with the application of compressive cyclic loading and the mechanical fatigue behaviour were investigated in Pb(Zr,Ti)O3 ceramics of tetragonal, morphotropic phase boundary and rhombohedral compositions. X-ray diffractometer analyses showed that the domains parallel to the electric field direction undergo rearrangement during the cyclic loading into random directions, which is responsible for the degradation of peizoelectric properties. For all three compositions, the fatigue resistance was lowered by poling treatment. The highest fatigue resistance was observed in the rhombohedral composition.

A test rig for the application of biaxial cyclic loads to structural models

The response of materials is often nonlinear may be highly dependent upon the load conditions and sometimes is also characterized by a large scatter of experimental results even when nominally equal test specimens are used. Consequently, experimental tests on a large number of specimens are essential in order to develop numerical models and to check their validity. By obtaining a reasonable compromise between the needs of extensive laboratory tests and the high cost of long experimental activities, a test rig for the application of biaxial cyclic loads to structural models was designed and established at the Department of Structural Engineering of the Politecnico di Milano, and a technical description is given here of the test rig, of the control equipment and of the data logging system. A test program developed for bricks, mortar and masonry specimens is then described, with particular emphasis given to the interaction between the experimental activity and the development of a numerical model for masonry. This model is based on homogenization and damage mechanics concepts, and numerical results are compared with experimental data.

Cold extrusion deformation of UHMWPE in total knee replacement prostheses

The use of metal-backed tibial plates in total knee replacement prostheses can result in the flow of ultra-high molecular weight polyethylene (UHMWPE) from the tibial insert into a cavity on the metal tray surface. A study of the relationship between the thickness of UHMWPE inserts and the amount of cold extrusion is reported here. An attempt was made to correlate the occurrence of cold extrusion with computer-aided analysis. UHMWPE samples of varying thickness, from 3 mm to 10 mm, were placed over cobalt-chrome (Co-Cr) discs. The Co-Cr discs had a 5 mm diameter hole placed centrally to simulate a tibial tray cavity. A cyclic load was applied at 20 Hz through a Co-Cr spherical indentor for a million cycles. The application of cyclic loading on UHMWPE samples resulted in cold-extrusion values comparable to those reported for retrieval analysis studies. Results after fatigue loading show that the samples do not suffer any gross surface damage. A shiny depression was visible at the load application site and the surface roughness value was decreased. The amount of cold extrusion increased with decreasing UHMWPE sample thickness. From the results, a minimum UHMWPE thickness of 12 mm is required if cold extrusion of UHMWPE is to be eliminated.

Assessment and Retrofit of a Reinforced Concrete Bridge Pier for Seismic Resistance

Typical structural deficiencies in some reinforced concrete bridges designed for seismic resistance before current seismic codes were introduced are briefly outlined. Results of simulated seismic load tests conducted on the critical region of a reinforced concrete bridge pier designed in New Zealand in 1936 are presented. The tests involved the application of compressive axial loading and quasi-static cyclic lateral loading to a full-scale subassemblage representing the column-cap beam region of the as-built pier. Subsequent loading tests were conducted on the pier retrofitted mainly by the addition of steel plates welded to the ends of the plain round longitudinal bars to improve anchorage. The improved bar anchorage enabled the pier to develop its full theoretical flexural strength, but the considerable loss of stiffness due to bond degradation which occurred in the initial test on the as-built pier was not restored.

Early strain-related changes in cultured embryonic chick tibiotarsi parallel those associated with adaptive modeling in vivo.

A model was developed for the application of cyclic mechanical loads to 17 day embryonic chick tibiotarsi in culture. A single 20 minute period of intermittent loading at 0.4 Hz, producing physiologic peak strains and strain rates, resulted in two peak strain magnitude-related responses that were previously reported in vivo: (1) a rapid increase in glucose 6-phosphate dehydrogenase activity in osteoblasts and osteocytes and (2) increased RNA synthesis, as shown by increased incorporation of [3H]uridine into extracted RNA. The RNA response was detectable 8 h following loading but was more pronounced by 24 h. Both responses were blocked by indomethacin (10(-6) M). These results demonstrate that embryonic chick bones in organ culture exhibit cellular responses to loading similar to those previously identified in adult canine cancellous bone cultures in vitro and adult avian cortical bone in vivo. These findings are consistent with a sequence of events between loading and new bone formation that includes an immediate strain magnitude-related, prostanoid-dependent increase in activity of the pentose monophosphate shunt in osteoblasts and osteocytes, followed by a similarly strain magnitude-related increase in RNA synthesis over the subsequent 24 h.

Modified approach to design of shear connectors in composite beams for fatigue loading: Naithani, K.C., Gupta, V.K. and Bhal, N.S. Conf. Fatigue and Fracture in Steel and Concrete Structures, ISFF '91, Madras, India, Dec 1991 Vol 2 (Oxford & IBH Publishing Co. Pvt. Ltd, New Delhi, India, 1991) 1361–1368

This paper reports on the behavior of headed shear stud connectors for use in steel frames with partially restrained connections and reinforced concrete infill walls, attached compositely to the steel frame around the perimeter of each wall panel (S-RCW system), subjected to seismic loading. In infill walls, the shear connectors lie in the plane of the concrete panel, which results in different behavior from studs in composite beams, for which the majority of past research has been conducted. In particular, shear connectors in infill walls are subjected to axial tension and compression forces due to sidesway and overturning of the S-RCW system, are influenced by the parallel edges of the wall, and are subjected to cyclic forces.An experimental program was developed to quantify the strength and deformation capacities of shear studs for use in S-RCW infill systems addressing the above issues, and to verify existing AISC, PCI, ACI, and Japanese design equations. A modification of the classic push-out test setup was made to accommodate the application of cyclic shear loading and axial tensile loading. Two different steel reinforcement configurations were used, one providing little confining reinforcement around the shear studs, and the other utilizing a steel reinforcement cage to provide ample confinement to the shear stud.

Experimental testing of ductile cladding connections for building facades

AbstractA ductile cladding connection is a special connecting device between a facade cladding panel and a building structure that is designed to dissipate energy when subjected to an earthquake. The effect of such a device is to reduce the overall seismic response of the structure. A special testing machine was designed to test the behavior of these advanced cladding connections. The machine, which is specially suited for the recording and evaluation of ductility and damping properties, is described. Its main characteristics are both the ability to load a specimen in shear simultaneously in two orthogonal directions without inducing any axial stress, and the flexibility to test a wide variety of different damping mechanisms with different geometric configurations. The machine was used to test a family of advanced cladding connections that employs ductile flexural action to achieve high and stable levels of ductility for repeated cyclic load applications. A lumped‐parameter mechanical model for these connections was identified from selected test data, and the predicted cyclic behavior was shown to agree well with test data. The model is simple and suitable for incorporation into nonlinear structural analysis programs.

Fatigue crack growth in polymers subjected to fully compressive cyclic loads

It is experimentally demonstrated in this work that the application of cyclic compression loads to polymeric materials, specifically high-density polyethylene and polystyrene, results in the nucleation and propagation of stable fatigue cracks. The cracks grow at a progressively slower rate along the plane of the notch in a direction perpendicular to the far-field cyclic compression axis. The overall characteristics of this compression fatigue fracture are macroscopically similar to those seen in metals, ceramics, as well as discontinuously reinforced inorganic composites. It is reasoned that the origin of this Mode I compression fatigue effect is the generation of a zone of residual tensile stress locally in the vicinity of the notch-tip upon unloading from the maximum far-field compressive stress. The residual tensile field is generated by permanent damage arising from crazing and/or shear deformation ahead of the notch-tip. Evidence for the inducement of residual tensile stresses on the crack plane is provided with the aid of micrographs of near-tip region where crazes are observed along the plane of the crack, i.e. normal to the compression loading axis. Compression fatigue crack growth in polystyrene is also highly discontinuous in the sense that the crack remains dormant during thousands of fatigue cycles following which there is a burst of crack extension, possibly in association with fracture within the craze. This intermittent growth process in cyclic compression is analogous to the formation of discontinuous growth bands during the tension fatigue of many crazeable polymers. The exhaustion of the near-tip residual tensile field and the increase in the level of crack closure with increasing crack length cause the fatigue crack to arrest. The universal features of this phenomenon are discussed in the context of ductile and brittle, non-crystalline and crystalline, as well as monolithic and composite materials.

Fatigue crack growth in unidirectional graphite-epoxy composites under cyclic compression

It is shown that in addition to the well-known failure process involving kink band formation, the application of cyclic compressive loads to notched plates of graphitr-epoxy composites can lead to the nucleation and growth of a single fatigue crack normal to the compression axis and to the fibre direction. This stable mode I compression fatigue fracture is a consequence of the generation of a zone of residual tensile stresses within the notch-tip region where deformation occurs in the form of matrix cracking, fibre breakage, fibre buckling nad/or interfacial debonding

Liquefaction of low plasticity silts

The influence of plasticity index on the liquefaction potential of silty soil of low plasticity (PI of 1·7 to 3·4) has been studied in detail. The change in plasticity index was achieved by mixing the low plasticity silt with 5 and 10% by weight of kaolinite clay. The test results indicate that the application of cyclic loads to saturated samples of silts result in a build up of pore water pressure equal to initial mean effective confining pressure. The samples also underwent failure in terms of 5, 10, or 20% double amplitude strains. It was further observed that the effect of increased plasticity (in the low plasticity range) was to reduce the liquefaction resistance of these soils. This has been related with their initial UU strength.

Nonlinear high-temperature fracture mechanics basis for strain-range partitioning: Kitamura, T. and Halford, G. R. NASA Lewis Research Center Report No N90-14642/4/XAB Oct. 1989 17 pp

A new creep-fatigue life prediction method has been developed in this paper to correlate high temperature low cycle fatigue (HTLCF) data on Cr-Mo steels. The new method has been developed within the premise that with the application of cyclic loading, which is plastic strain dominated under HTLCF conditions, damage accrues by means of viscous flow. Concepts of dynamic viscosity were used to describe such a flow behavior and assumed as damage parameter. When the ability of a material to accommodate viscous flow ceases, failure occurs. When these two terms namely; dynamic viscosity and material toughness, equated, produce a life prediction expression, that has been derived in this paper. The assessments of HTLCF data were performed with the new model developed in this paper for lCr-Mo-V, 2.25Cr-Mo, and 9Cr-1Mo steels. Numerous test conditions were employed by the National Research Institute for Metals, Tokyo, Japan, in generating HTLCF data where the assessments show that the new method correlates the data.

Thermoplasticity and fracture mechanics: Bui, H. D. and Inglebert, G. Mec. Mater. Electr. Sept. 1989 431, 8–14 (in French)

A new creep-fatigue life prediction method has been developed in this paper to correlate high temperature low cycle fatigue (HTLCF) data on Cr-Mo steels. The new method has been developed within the premise that with the application of cyclic loading, which is plastic strain dominated under HTLCF conditions, damage accrues by means of viscous flow. Concepts of dynamic viscosity were used to describe such a flow behavior and assumed as damage parameter. When the ability of a material to accommodate viscous flow ceases, failure occurs. When these two terms namely; dynamic viscosity and material toughness, equated, produce a life prediction expression, that has been derived in this paper. The assessments of HTLCF data were performed with the new model developed in this paper for lCr-Mo-V, 2.25Cr-Mo, and 9Cr-1Mo steels. Numerous test conditions were employed by the National Research Institute for Metals, Tokyo, Japan, in generating HTLCF data where the assessments show that the new method correlates the data.

Micromechanical model to predict sand Densification by Cyclic Straining

It has been observed that the application of cyclic shear loading to a dry sand and other cohesionless soils results in a progressive volume reduction and a relative density increase. Although this densification is larger and takes place faster in loose sands, it occurs in both loose and dense soils, and is caused by the tendency of any sand to contract under a small shear strain. A micromechanical statistical model was developed by the writers to interpret and compute the compaction of such a dry granular soil subjected to a strain‐controlled cyclic simple‐shear test. The method postulates that an important role is played in the compaction phenomenon by the spatial statistical distribution of porosity within the soil specimen. Numerical simulations of densification are presented using the proposed model, which are in excellent agreement with observed sand compaction in actual cyclic simple‐shear tests. These simulations provide new insight into the densification process, and especially on the significanc...

Fatigue crack growth in cementitious materials under cyclic compressive loads

This paper presents experimental results and a discussion of the mechanisms of fatique crack growth under uniaxial cyclic compressive loads in cementitious materials containing stress concentrations. It is demonstrated that the application of cyclic compressive loads to single edge-notched specimens of mortar induces Mode I fatigue cracks from the notch-tip in a direction normal to the far-field compression axis. The experiments of this study, in conjunction with the recent work of Suresh and coworkers on the compression fatigue behavior of a wide range of ceramics and ceramic composites, suggest that the driving force for Mode I crack advance is the generation of a zone of residual tensile stresses in the near-tip region of the stress concentration upon unloading from the far-field compressive stress. Numerical simulations of notch-tip residual stresses in brittle solids, including mortar and plain concrete, indicate that the residual tensile stresses are of sufficient magnitude to induce fracture along the plane of the notch. This Mode I fatigue crack growth mechanism in cyclic compression, observed at nominal compressive stress values far below the uniaxial compressive strength, is vastly different from the splitting mode of failure generally found in cementitious materials under monotonic compression. Practical implications of the cyclic compression fracture mechanism are highlighted.

Near-threshold fatigue crack growth properties at elevated temperature for 1Cr-1Mo-0.25V steel and 12Cr stainless steel: Matsuoka, S., Takeuchi, E., Nishijima, S. and McEvily, A.J. Metall. Trans. Apr. 1989 A 20A, (4), 741–749

The dependence of crack growth rate on various crack tip parameters was studied. Experiments were performed on thin sheets of 6063-T6 Al-alloy having a central notch, to find crack tip opening displacement, total strain range, plastic strain range, crack opening stress and crack growth rate. Crack tip opening displacement and crack opening stress were measured, using a surface measurement technique, with small crack opening displacement gauges. The theoretical predictions of crack tip opening displacement compare fairly well with the experimental values. It is found that crack propagation rate vs total strain range-plastic strain range gives a straight-line fit on a log-log graph and, for positive stress ratios, the fatigue crack growth rates are found to be independent of R.Experimental results show that the crack opening stress is not affected by the position of the gauge when it is mounted behind and near the crack tip.The effect of mechanical properties and loading on crack growth were also studied. The specimens were fatigue cracked to a predetermined length and some specimens were annealed and again loaded cyclically. The application of cyclic loads to annealed specimens caused significant increase in crack propagation rates in comparison with the specimens having no heat-treatment. The load-displacement record was found to stabilize in about 10 cycles; the crack then extended slowly as a fatigue crack. Crack propagation rates for different values of R for annealed and work-hardened material were plotted against a crack tip parameter, ΔK∗, based on notional crack lengths. Since the results of dadN vs ΔK∗ for both states of material (as-received and annealed) seem to lie on the same straight line on a log-log graph, the study provides a hope that the results for a material tested in any state (annealed or work-hardened) for positive values of R (0·0≤R≤0·3) will lie on this line, thus eliminating fatigue tests on the same material under different work-hardening conditions for different values of R. Models for dadN have been developed using various crack tip parameters.

Silicon Carbide Monofilament‐Reinforced Silicon Nitride or Silicon Carbide Matrix Composites

SiC‐monofilament‐reinforced SiC or Si3N4 matrix composites were fabricated by hot‐pressing, and their mechanical properties and effects of filaments and filament coating layers were studied. Relationships between frictional stress of filament/matrix interface and fracture toughness of SiC monofilament/Si3N4 matrix composites were also investigated. As a result, it was confirmed experimentally that in the case of composites fractured with filament pullout, the fracture toughness increased as the frictional stress increased. On the other hand, when frictional stress was too large (&gt;about 80 MPa) for the filament to be pulled out, fracture toughnesses of the composites were almost the same and not so much improved over that of Si3N4 monolithic ceramics. The filament coating layers were found to have a significant effect on the frictional stress of the SiC monofilament/Si3N4 matrix interface and consequently the fracture toughness of the composites. Also the crack propagation behavior in the SiC monofilament/Si3N4 matrix composites was observed during flexural loading and cyclic loading tests by an in situ observation apparatus consisting of an SEM and a bending machine. The filament effect which obstructed crack propagation was clearly observed. Fatigue crack growth was not detected after 300 cyclic load applications.

Characteristics of Birnbaum and Saunders model

Fatigue failure occurs when some dominant crack or cracks in a component extend to a critical level under the application of cyclic loading. By using the theory of stochastic processes Birnbaum and Saunders proposed a probability model to characterize the time (i.e. the number of cycles) required to propagate a fatigue crack past a critical value. The model is phenomenologically quite sound and provides a probabilistic interpretation of Miner's rule. In statistical literature a thorough treatment of the model is missing. For example, no work has been reported about the renewal and related functions of this model. This paper presents: (i) a summary of some known characteristics of the model; (ii) parameter estimation methods, and K– S test statistics for the model validation; (iii) the nature of hazard function in terms of the coefficient of life variation; (iv) the renewal function, renewal rate function and variance of number of renewals in graphical form; and (v) a comparison of a typical set of various functions.

Crack propagation rate as a function of crack tip characteristics

The dependence of crack growth rate on various crack tip parameters was studied. Experiments were performed on thin sheets of 6063-T6 Al-alloy having a central notch, to find crack tip opening displacement, total strain range, plastic strain range, crack opening stress and crack growth rate. Crack tip opening displacement and crack opening stress were measured, using a surface measurement technique, with small crack opening displacement gauges. The theoretical predictions of crack tip opening displacement compare fairly well with the experimental values. It is found that crack propagation rate vs total strain range-plastic strain range gives a straight-line fit on a log-log graph and, for positive stress ratios, the fatigue crack growth rates are found to be independent of R. Experimental results show that the crack opening stress is not affected by the position of the gauge when it is mounted behind and near the crack tip. The effect of mechanical properties and loading on crack growth were also studied. The specimens were fatigue cracked to a predetermined length and some specimens were annealed and again loaded cyclically. The application of cyclic loads to annealed specimens caused significant increase in crack propagation rates in comparison with the specimens having no heat-treatment. The load-displacement record was found to stabilize in about 10 cycles; the crack then extended slowly as a fatigue crack. Crack propagation rates for different values of R for annealed and work-hardened material were plotted against a crack tip parameter, ΔK∗, based on notional crack lengths. Since the results of d a d N vs ΔK∗ for both states of material (as-received and annealed) seem to lie on the same straight line on a log-log graph, the study provides a hope that the results for a material tested in any state (annealed or work-hardened) for positive values of R (0·0≤R≤0·3) will lie on this line, thus eliminating fatigue tests on the same material under different work-hardening conditions for different values of R. Models for d a d N have been developed using various crack tip parameters.