Preliminary Cyclic Loading Research Articles

Influence of proportional multiaxial fatigue loading on the residual mechanical properties of glass-reinforced plastic pipes

This work is dedicated to the experimental investigation of multiaxial cyclic loading influence on the residual mechanical properties of glass-reinforced plastic pipes (GRP pipes). Static, cyclic and combined (static after the preliminary cyclic loading) tests have been carried out on tubular specimens under tension with torsion. Residual stiffness curves have been made, their approximation has been carried out, using previously developed model, damage accumulation stages’ boundaries have been calculated. A specimen’s individual fatigue life prediction method, based on the initial parts of stiffness degradation curves, has been proposed. Influence of the preliminary multiaxial cyclic loading on residual Young’s modulus and tensile strength has been studied. Patterns of deformation and fracture processes have been studied, using digital image correlation method and acoustic emission method. A significant influence of fatigue damage on the residual properties of GRP pipes has been concluded.

Кинетика поврежденности предварительно циклированной малоуглеродистой стали с крупнозернистой и субмикрокристаллической структурой

The kinetics of accumulation of microcracks and fracture at various stages of tension of grade 20 steel specimens with coarse-grained and ultrafine-grained (UFG) structure obtained by the equal-channel angular pressing (ECAP) has been studied. To study the effect of mechanical degradation, a part of the samples was subjected to preliminary cycling to a relative lifetime of 50 %, followed by tensile tests, during which the parameters of acoustic emission, deformation fields obtained by digital image correlation were evaluated, and the intensity of the residual magnetic field was measured. The length and density of surface microcracks were measured by optical microscopy using computer image analysis. It is established that pre-cycling causes hardening of the material and a decrease in its plasticity. The fracture stages were revealed, and fatigue crack origins were found on the internal delaminations of specimens with a UFG structure after preliminary cyclic loading. It is shown that the formation of the UFG structure after the ECAP of specimens made of grade 20 steel leads to hardening, reduction of the fracture work, the area of the plastic zone, maximum major deformations and damage, as well as to an increase in the number of AE signals and the intesity of the residual magnetic field.

Influence of variable loading modes on the effectiveness of re-hardening with shot-peening

The regularities of the effectiveness of repeated hardening with shot-peening from the modes of preliminary cyclic loading of specimens from "30ХГСН2А" steel are established. Recommendations are given on the timing of repeated hardening, taking into account the level and duration of preloading for the complete restoration of the fatigue life of previously hardened parts. Keywords: fatigue strength, durability, resource recovery, shot-peening, re-hardening, high-strength steel, fatigue damages, cyclic operating time. gnkrav@mail.ru

Damage accumulation near the cold-expanded hole due to high-cycle fatigue by crack compliance method

The novel destructive method is implemented for quantitative assessment of fatigue damage accumulation in the stress concentration zone accompanied by residual stress due to cold expansion of the through-thickness hole. Damage accumulation is reached by preliminary cyclic loading of plane specimens with cold-expanded holes. Narrow notches, emanating from the hole edge at different stages of high-cycle fatigue, serve to manifest a damage level. These notches are inserted without applying external load. Deformation response to local material removing, caused by pure residual stress influence, is measured by electronic speckle pattern interferometry (ESPI) in terms of in-plane displacement components. Normalized values of the notch mouth open displacement (NMOD), in-plane displacement component at the initial point of the notch acting in the notch direction (U0), in-plane displacement component at the final point of the notch acting in the notch direction (U1) and the stress intensity factor (SIF) are used as current damage indicators. Numerical integration of curves, describing an evolution of each fracture mechanics parameter over lifetime, produces the damage accumulation function in an explicit form. It is established that all four fracture mechanics parameters give very close results.

Deformation and Fracture Mode of 10kp5 Steel Pretreated by Cyclic Loading and Pulse Current at Tension

The effect of preliminary cyclic loading and the pulse current on the deformation and fracture mode of the 10kp5 low-carbon steel under uniaxial tension at room temperature is investigated. Data on the steel mechanical properties in the initial state and after preliminary combined cyclic loading and exposure to pulse current in different sequences are presented. Fractographic studies of the samples’ fractures showed the viscous nature of the samples’ fractures in all cases considered.

Residual Strength of Cyclically Deformed Corrosion-Resistant Steel

The residual strength of 12Cr18Ni9Ti corrosion-resistant steel is determined in the initial state and after operation as the material of a vacuum chamber body. Specimens are subjected to preliminary cyclic loading and then subjected to tensile tests to estimate the characteristics of acoustic emission and residual strength. The stages in the change of the acoustic characteristics during the fracture of preliminary cycled specimens are studied. A correlation between the residual strength and the damage estimated from the of acoustic emission characteristics is found.

The effect of preliminary cyclic loading on acoustic emission parameters and damage of structural steels

Preliminary cyclic loading of structural low-carbon and 15Cr2MnMoV steels specimens to a relative lifetime of 0.3, 0.5 and 0.7 were conducted. After preloading, tensile tests with the acoustic emission (AE) registration were performed and the effects of preliminary cyclic loading on the intensity (dNAE/dt), the cumulated number (ΣNAE) and energy (ΣEAE)of acoustic emission signals, and the bAE-value (the slope coefficient of cumulative distributions of the AE signals amplitudes) were investigated.According to the test results, the dependencies of the AE parameters on the relative deformation were plotted. An analysis of these dependencies made it possible to distinguish the stages of changes in AE characteristics that correlate with the stages of changes in mechanical properties and material damage. New relations connecting acoustic emission parameters to the relative number of cycles before fracture of steels studied were obtained.

New acoustic parameter characterizing loading history effects

Abstract A new parameter of acoustic emission (TAE) is proposed, which determines the period of acoustic gap before the final fracture under tension of specimens of low- and medium-carbon steels after preliminary cyclic loading. The beginning of the acoustic gap period TAE corresponds to the achievement of the maximum load. TAE increases with the number of cycles of pre-cycling loading, the fracture magnitude and material strength; as the amplitude of the preliminary cyclic loading increases, it decreases. Values of the bAE and the total number of acoustic signals ΣNAE are estimated and the changes of these characteristics at various tension stages are considered. It is shown that the beginning of the plateau in the time dependence of the total number of acoustic signals is accompanied by a sharp decrease in the bAE-value.

Investigation into the Cyclic Strength of the Bodies of Steam Shutoff Valves from 10Kh9MFB-Sh Steel

Steam shutoff valves are operated under complex loading conditions at thermal and nuclear power stations. In addition to exposure to high temperature and stresses resulting in fatigue, these valves are subjected to cyclic loads in heating-up–cooling down, opening–closing, etc. cycles. The number of these cycles to be specified in designing the valves should not exceed the maximum allowable value. Hence, the problem of cyclic failure rate of steam shutoff valve bodies is critical. This paper continues the previous publications about properties of the construction material for steam shutoff valve bodies (grade 10Kh9MFB-Sh steel) produced by electroslag melting and gives the results of investigation into the cyclic strength of this material. Fatigue curves for the steal used for manufacturing steam shutoff valve bodies are presented. The experimental data are compared with the calculated fatigue curves plotted using the procedures outlined in PNAE G-002-986 and RD 10-249-98. It is confirmed that these procedures may be used in designing valve bodies from 10Kh9MFB-Sh steel. The effect of the cyclic damage after preliminary cyclic loading of the specimens according to the prescribed load conditions on the high-temperature strength of the steel is examined. The influence of cyclic failure rate on the long-term strength was investigated using cylindrical specimens with a smooth working section in the as-made conditions and after two regimes of preliminary cyclic loading (training) at a working temperature of 570°C and the number of load cycles exceeding the design value, which was 2 × 103 cycles. The experiments corroborated that the material (10Kh9MFB-Sh steel) of the body manufactured by the method of electroslag melting had high resistance to cyclic failure rate. No effect of cyclic damages in the metal of the investigated specimens on the high-temperature strength has been found.

Коэрцитивность металла как мера его микроповреждённости в задачах оценки усталости и восстановления механических свойств

Introduction. The problem of determining the current state of metal parts mechanical properties is always relevant. For its solution a simple, cheap and sufficiently informative NDT method is needed. Today the method of magnetic coercive force seems to be the most suitable. The purpose of this work was to determine patterns of change in a coercive force after cyclic loading of steel samples in the initial state and after surface plastic deformation (work hardening).
 Method. Two batches of 40X steel cylindrical samples in the delivery condition were put to cyclic tests. The both batches were tested in the same range of tension amplitudes but for the second batch the tests were carried out in two steps with intermediate surface plastic deformation. The change in the level of accumulated fatigue damage was determined by the coercive force method.
 Results. The most significant coercive force increase (up to 1.5%) was observed in the initial stages of the specimens’ elongation. For samples subjected to preliminary cyclic loading, surface plastic deformation leads to lower coercive force values (by 9%–17%) and to 2.8–3.4 times longer operating life. This effect can be explained by “healing” the accumulated defects by the surface treatment.
 Discussion. The results indicate high sensitivity of the coercive force to changes in structural state of the metal as a result of both fatigue damage accumulation and partial recovery of the metal structure due to defect “healing”.

The radiation swelling effect on fracture properties and fracture mechanisms of irradiated austenitic steels. Part I. Ductility and fracture toughness

The radiation swelling effect on the fracture properties of irradiated austenitic steels under static loading has been studied and analyzed from the mechanical and physical viewpoints. Experimental data on the stress-strain curves, fracture strain, fracture toughness and fracture mechanisms have been represented for austenitic steel of 18Cr-10Ni-Ti grade (Russian analog of AISI 321 steel) irradiated up to neutron dose of 150 dpa with various swelling. Some phenomena in mechanical behaviour of irradiated austenitic steels have been revealed and explained as follows: a sharp decrease of fracture toughness with swelling growth; untypical large increase of fracture toughness with decrease of the test temperature; some increase of fracture toughness after preliminary cyclic loading. Role of channel deformation and channel fracture has been clarified in the properties of irradiated austenitic steel and different tendencies to channel deformation have been shown and explained for the same austenitic steel irradiated at different temperatures and neutron doses.

ЭКСПЕРИМЕНТАЛЬНОЕ ИССЛЕДОВАНИЕ ВЛИЯНИЯ ДЕФЕКТОВ НА ПРОЧНОСТЬ КОМПОЗИТНЫХ ПАНЕЛЕЙ МЕТОДАМИ КОРРЕЛЯЦИИ ЦИФРОВЫХ ИЗОБРАЖЕНИЙ И ИНФРАКРАСНОЙ ТЕРМОГРАФИИ

The article is devoted to the experimental investigation of the defects influence on the residual strength of composites structure, as well as the possibility of using local repair operations. The objects of research are structurally similar elements of acoustical sandwich panels after a local repair of defects, such as through breakdown. The specimens were produced by serial technology from a fiberglass prepreg. Research was carried out using a universal electromechanical system Instron 5982 and servo-hydraulic system Instron 8801. For the analysis of the stress-strain state of the deformable elements the authors used the non-contact three-dimensional digital optical system Vic-3D, the mathematical apparatus which is based on the method of digital image correlation (DIC). To control the internal geometry of the specimen and assess the possible defect inspection infrared thermal imaging system FLIR SC7000 was used. The techniques of joint use of testing and measuring systems under static and cyclic tests were offered. For comparison, the repaired sandwich panel specimens were tested in tension and in tension under a preliminary cyclic loading with the registration of the deformation fields and thermal images. Their deformation and fracture mechanisms are analyzed, and their loading diagrams are obtained. The experimental data was obtained from the Vic3d system study of the evolution inhomogeneous fields of axial and transverse deformation on the surface of repaired sandwich panels under static loading and cyclic tests. By using infrared thermal imaging system internal structure, processes of the defects development and the temperature distribution on the surface of the test specimen were detected.

Influence of Cyclic Preloading on the Hydrogen Degradation of Materials

We present the results of numerical analysis of the influence of the mode of preliminary cyclic loading on hydrogen-induced crack growth in high-strength steel with regard for the stress-strain state and diffusion of hydrogen. The elastoplastic stress-strain state at the crack tip under cyclic loading followed by monotonic loading is simulated for the case of high strains. The diffusion of hydrogen at the crack tip is modeled by taking into account the evolution of the stress-strain state from postcyclic compression to tension in the course of hydrogenation. It is shown that the accumulation of hydrogen in the process zone depends on the distribution of residual stresses induced by cyclic preloading.

Variation of the creep limit and structure of a welded joint in PT-3V titanium alloy under the effect of cyclic loading. Report 2

Comparative analysis of the characteristics of amplitude-dependent internal friction (IF) was carried out to explain reasons leading to an increase of the creep limit of the butt welded joint in PT-3V titanium alloy (PT-2V filler material) after cyclic loading. IF was measured by the method of the direct torsional pendulum on cylindrical specimens representing the gage part of the specimens subjected to preliminary cyclic loading. The data were processed using the Granato-Lucke theory. The results show that the reduction of IF after cyclic loading took place as a result of an increase of the number and redistribution of pinning points of the dislocations. Consequently, the density of mobile dislocations decreases by 8% and the length of dislocation segments by 18%. These processes of stabilization of the structure were completed within 5·104cycles of preliminary loading. Stabilization of the dislocation structure of the material of the welded joint increases its creep limit.

Variation of the creep limit and structure of a welded joint in PT-3V titanium alloy under cyclic loading. Report 1

The creep of PT-3V titanium alloy and of a butt welded joint in this alloy (PT-2V filler material) was investigated on cylindrical specimens 7 mm in diameter with a gage length of 50 mm. The plane of the weld 10-mm wide, produced by manual submerged-arc welding, was situated in the center of the specimen in the direction normal to its longitudinal axis. The results show that in the examined cyclic stress amplitude range the preliminary cyclic loading lasting 5·104cycles with a frequency of 35 Hz increases the cyclic creep limit of PT-3V alloy by 11% and the static creep limit of the welded joint by 19%. This variation of the mechanical properties can be regarded as one of the reasons for the positive effect of the vibrotreatment technology, used in industry, leading to stabilization of the dimensions and form of components.

Deterministic and probabilistic evaluation of the fatigue-strength characteristics of carbon-fiber-reinforced plastics subjected to interlayer shear

We investigate the probabilistic characteristics of the fatigue sthrenght of carbon-fiber-reinforced plastics in interlayer shear, and also the residual sthrenght after preliminary cyclic loading

Effect of preliminary cyclic loading on wear resistance of steel 45

Results of tests on wear resistance of steel 45 in the initial state and with preliminary cyclic loading are presented and analyzed. It is shown that cyclic loading damages the surface layer of the material. This, in turn, increases wear and restricts for the tribosystem a range of loads at which normal mecahnochemical form of wear is realized. These changes are also investigated with surface layers removed.

Effect of filler type on the mechanical properties of latex films of natural rubber strained in liquid media

The structural mechanical properties of natural rubber with various types of fillers and different degrees of dispersion (chalk, TiO 2, polystyrene) subjected to strain in air and in the presence of an adsorption-active medium (alcohol) are studied. It is shown that in alcohol the strain mechanism depends on the nature of the filler. The structure of crazes formed in natural rubber filled with polystyrene subjected to strain in alcohol is studied. Preliminary cyclic loading can make the action of the adsorption-active medium on filled rubber less efficient.

Influence of preliminary cyclic loading on the resistance to failure of 16G2AF low-alloy steel

1. Preliminary loading of 16G2AF steel has a marked influence on its resistance to brittle fracture. The character of this influence is determined by the stage of preceding cyclic loading. The maximum degree of embrittlement (a 10–20°C increase in T50) is observed after loading to a number of cycles of\(\bar N\)=10% and more of the expected life, which is significantly (by two or four times) less than for low-carbon steel under similar cyclic loading conditions. 2. The intensity of the decrease in the specific work for the originaoand propagationap of a ductile crack after cyclic loading at normal temperature depends upon the amplitude of the stresses andap decreases steadily over the whole investigated range of number of load cycles. 3. As the result of preliminary cyclic loading the resistance of the steel to brittle fracture decreases despite the presence in the material of individual zones of local plastic deformation causing the appearance in the brittle zones of the fractures of areas of a ductile constituent. This is related primarily to the occurrence in the basic volume of the material of strain aging.

Influence of preliminary cyclic loading on the damping capacity and residual life of 40Kh steel

Influence of preliminary cyclic loading on the damping capacity and residual life of 40Kh steel