Action Of Cyclic Loads Research Articles

Study of pipe steel 06GB tendency towards corrosion-fatigue failure in chloride hydrogen sulfide-containing media

Results are provided for corrosion-fatigue testing of large steel 06GB specimens with and without a welded joint with a stress concentrator in air and in a hydrogen sulfide-containing medium according to NACE standard TM 0177-2006. It is shown that steel 06GB in air and in a corrosive medium is quite resistant to the action of cyclic loads and surpasses steels 08KhMFChA and Kh60. Stress intensity factor threshold coefficients K th are determined for basic metal and a welded joint. It is established that the basic metal failure mechanism in air and a corrosive medium is ductile.

Experimental results in fretting fatigue with shot and laser peened Al 7075-T651 specimens

This work focuses on determining the effect of shot and laser peening on fretting fatigue in the Al 7075-T651 alloy. These surface treatments generate a residual compressive stress field near the treated surface where contact under fretting fatigue produces high stress levels. The fretting fatigue resistance of shot and laser peened specimens was assessed in a series of tests involving measurements of the residual stress field, residual stress relaxation under the action of cyclic loads, the friction coefficient, surface roughness and material hardness. The obtained results are compared with those for untreated specimens. The tests show the beneficial effect of the compressive residual stresses and the improvement that surface roughness causes in fretting fatigue life, especially in shot peened specimens. Another important effect observed, is the partial residual stress relaxation produced during the fretting fatigue tests.

X-ray diffraction study of microstructural changes during fatigue damage initiation in steel pipes

Abstract Steel pipes used in the oil and gas industry undergo the action of cyclic loads that can cause their failure by fatigue. A consistent evaluation of the fatigue damage during the initiation phase should fundamentally be based on a nanoscale approach, i.e., at the scale of the dislocation network, in order to take into account the micromechanisms of fatigue damage that precede macrocrack initiation and propagation until the final fracture. In this work, microstructural changes related to fatigue damage initiation are investigated in the API 5L X60 grade steel, used in pipe manufacturing. Microdeformations and macro residual stress are evaluated using X-ray diffraction in real time during alternating bending fatigue tests performed on samples cut off from an X60 steel pipe. The aim of this ongoing work is to provide ground for further development of an indicator of fatigue damage initiation in X60 steel. This damage indicator could allow a good residual life prediction of steel pipes previously submitted to fatigue loading, before macroscopic cracking, and help to increase the reliability of these structures.

Ultrasonic Measurement of Residual Stresses in Welded Elements of Ship Structure

The objective of the study described in this paper is to identify the residual stress distribution and relaxation in standard welded specimens as well as in a large-scale welded panel imitating the critical, from the fatigue point of view, zones of ship structure. The residual stresses were measured after welding and in the process of fatigue loading of welded elements by the UltraMARS system that is based on using ultrasound. The measurements had shown that the maximum residual stresses near the welds (4-5 mm away from the weld) reach levels 290-320 MPa that are close to the yield strength of considered material both in welded specimens and in the large scale panel. Analysis of residual stress relaxation in the welded panel under the action of cyclic loading confirmed the fact that within the interval of applied stress ranges corresponding to the multi-cycle region of loading of the welded joints, the relaxation of residual stresses occurs mainly during the first cycle.

Structural features of the behavior of a high-carbon pearlitic steel upon cyclic loading

The structural evolution upon high-cycle fatigue (tension with the magnitude of stress in a cycle below the macroscopic yield stress) of the hypereutectoid steel U10 (1.03 wt % C), in which pearlite of different morphology (fine-lamellar, coarse-lamellar, and partially spheroidized pearlite) was formed, has been investigated by scanning and transmission electron microscopy. Based on the fractographic analysis, features of fracture of these structural states have been considered. At a significant distance (10 mm) from the fatigue fracture, features of structural transformations caused by cyclic loading have been revealed. It has been shown that upon high-cycle fatigue in the steel U10 with structures of lamellar and partially spheroidized pearlite, substantial structural changes occur, namely, fragmentation and partial dissolution of cementite plates, and in fine pearlite, additionally, spheroidizing of cementite and polygonization of the ferritic component are observed. A dependence of the character of fracture on the type of structure formed upon fatigue loading has been established. In the steel with a nonequilibrium structure of unannealed fine pearlite, an enhanced elasticity modulus, as compared to other more stable structures (coarse-lamellar, annealed fine, and spheroidized pearlite), and a reduction in the magnitude of the elasticity modulus under the action of cyclic loading have been found. It has been established that the structural changes in fine pearlite of laboratory specimens of the steel U10 upon cyclic tension are qualitatively similar to those in a railroad wheel of the steel 65G under the service conditions.

Failure Analysis and Reliability Evaluation of a Planetary Gear Box

With the qualities of smooth operation, high efficiency and small size, the planetary gear box was widely used in transmission field. In this paper, a 2-stage planetary gear box was studied. CAXA and Pro/E were used for modeling. The FEA analysis results were obtained by ANSYS, both the contact stress and the bending stress of the gear tooth root. The results showed that with the continuous action of cyclic loads, the excessive contact stress is the direct reason of the failure. Then, the system reliability evaluation was made, which can provide guidance for engineering practice.

A model of fatigue crack propagation in metals

A model of the formation and evolution of a local plastic deformation zone at the crack tip is proposed based on the analysis of the main physical processes taking place in a metallic material under the action of cyclic loads. An equation of fatigue crack growth rate curves, which explicitly accounts for the loading frequency, was derived. The equation applies to the whole range of crack lengths from short cracks to macroscopic ones.

Fatigue behaviour of post weld heat treated electron beam welded AA2219 aluminium alloy joints

This paper reports the effect of post weld heat treatment on fatigue behaviour of electron beam welded AA2219 aluminium alloy. An attempt has been made to enhance the fatigue strength of the electron beam welded joints through post weld heat treatment methods such as solution treatment, artificial aging, solution treatment and artificial aging. Electron beam welding machine with 100 kV capacity has been used to fabricate the square butt joints. Servo hydraulic controlled fatigue testing machine with a capacity of 100 kN has been used to evaluate the fatigue life of the welded joints. Of the three post weld heat treated joints, the solution treated and aged joints are enduring higher number of cycles under the action of cyclic loads.

Fatigue properties of a SAE 4340 steel coated with TiCN by PAPVD

An investigation has been carried out in order to study the fatigue performance, both in air and in a 3 wt.% NaCl solution, of a quenched and tempered (Q&T) SAE 4340 steel substrate coated with a TiCN film of ∼4 μm in thickness deposited by plasma assisted physical vapor deposition (PAPVD). The results obtained show that the presence of the coating provides a significant increase, in the range of ∼140–180%, in fatigue life, in comparison with the uncoated substrate, when the material is tested in air under rotating bending conditions at maximum alternating stresses in the range of ∼550–700 MPa. It is also shown that when the material is tested in the NaCl solution only a marginal increase (25% at the most) in the number of cycles to failure is observed, particularly at maximum alternating stresses above ∼570 MPa, which suggests that the TiCN film is susceptible to the corrosive effect of the solution under the combined action of cyclic loading. The increase in the number of cycles to failure in air is explained in terms of the high mechanical strength of the film, its compressive residual stress state and excellent adhesion to the steel substrate. It is shown that fatigue cracks nucleate at the surface of the coating and propagate normal to the substrate–coating interface without bifurcation along the latter, after going through the entire coating thickness. Under the assumption of the validity of a simple linear law of mixtures, the tensile strength of the film is estimated in ∼34 GPa, very close to its absolute hardness of ∼33 GPa, whereas its fatigue limit is found to be in the range of ∼12.6 GPa.

The stress field in a [0°/90°] laminated composite plate containing a finite line crack

This paper deals with the construction of an asymptotic solution for the stress field in a laminated composite plate, with (0/90) stacking sequence. The plate contains through the thickness a line crack of length 2c and its perimeter boundaries are sufficiently far away from the crack so that no edge effects are present. The stress field is derived explicitly, and includes a correction factor to account for the laminate effects in the third dimension. The stress σzz is a maximum at an angle of θ ≈ 83 ◦ (see Figure 3). The stress field may now be used to bridge the gap between macro and micro mechanics and to derive a series of fracture criteria, at the micro and macro level, which ultimately will provide us with a better understanding of the formation of the damaged zone ahead of the crack tip. For example, the stress field is used to derive one such approximate fracture criterion for mode I loading and for a self similar type of fracture, similar to that of Griffith. This criterion shows how the periodic length of the material lay-up microstructure effects the fracturing characteristics of the material system. Comparison with some experimental observations for two different material systems shows a fairly good agreement which substantiates the predicted influence. Despite careful design, practically every structure contains stress risers due to the presence of inclusions, holes or cracks. Bolt holes and rivet holes are necessary components for structural joints. It is not surprising, therefore, that the majority of service cracks nucleate in the vicinity of a stress riser. While the subject of stress risers is certainly familiar to engineers, the situation is significantly more complex in the case of high-performance laminated composite materials. The presence of a hole or a crack in the laminate introduces significant stress contributions in the third dimension which create a very complicated three-dimensional (3-D) stress state in the vicinity of such discontinuities. Moreover, this complex state of stress may depend on the stacking sequence of the laminate, the fiber orientation of each lamina as well as the material properties of the fiber and of the matrix. Ultimately, these stress risers form a primary source of damage initiation and property degradation, particularly in the presence of cyclic loading. Experimental investigations carried out by Bakis and Stinchcomb (1986) on graphite- epoxy laminates which have been weakened by a circular hole give us a better insight of this damage growth development under the action of cyclic loading. In general, the progression of this damaged process may be characterized as (i) debonding along fiber-matrix interfaces, (ii) matrix cracking parallel to the fibers, (iii) matrix cracking between fibers, (iv) delamination along the interface of two adjacent laminae with different fiber orientations, and (v) fiber breakage.

Behavior of Fe-Mn-Al-C Steels during Cyclic Tests

Alloys of the FeMnAlC system have been used for cryogenic purposes and for applications up to 673 K. At low temperatures, they have in general a better performance than austenitic Cr-Ni steels as far as fatigue is concerned, but are inferior to martensitic Cr steels. However, since the fatigue strength of FeMnAlC alloys in the temperature range of 523 to 823 K is higher than at room temperature, the present work has been conducted to describe the behavior of such alloys under the action of cyclic loading, including elasto-plastic deformation and cyclic temperatures. It has been concluded that components can be successfully subjected to cyclic loads in the elasto-plastic regime and to periodic changes in temperature under normal service conditions.

Effect of Hydrogen on Fatigue Resistance of VT6Ch Titanium Alloy

We studied the effect of hydrogen on the cyclic endurance of cylindrical specimens made of VT6Ch titanium alloy for various parameters of cyclic loading, coefficients of asymmetry of a cycle, coefficients of stress concentration, and load frequencies. The specimens were cut from plates with a lamellar structure at different angles with the rolling direction. In all cases where hydrogen was present in the solid solution, the limit of its bounded corrosion endurance in the initial state increased. This is probably caused by hardening of the solid solution and by separation of dispersed hydrides. At concentrations of hydrogen ≥0.03% (by mass), under the action of cyclic loading, hydrides separate from the supersaturated solution. They play the role of stress concentrators and lead to a decrease in cyclic endurance.

Editorial

Residual Stresses in material, tensile or compressive, are pent up furies that are released, unpredictably, over a period of time depending much upon temperature, ambient or applied. Tensile residual stresses add-on to applied stresses thereby increasing actual stress-situation in a component could promote premature failure - even more sounder the conjoint action of cyclic loading and ‘the environment’. On the other hand compressive residual stresses would hinder (surface) crack nucleation, somewhat obviating the situation In the material as under tensile residual stresses. In this issue of the IWJ authors of two papers, dedicated to the generic terms related to residual stresses in material ( P-7 & 24 ), describe in worthy details with wealth of references, difficulties both in theoretical modeling and in data accumulation processes. Readers, particularly In R & D works, would find useful references.

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

The metallographic methods and microanalysis were used for investigation of the welded butt-joint specimens from PT-3V titanium alloys. The specimens were in initial state and after cyclic loading. Duration of loading was 5·104 and 1·107 cycles on a frequency of 35 Hz. The stress amplitude was 150 MPa; this value is below the endurance limit of weld material. The polished sections for investigations were cut out from a working part of specimens, which contains all zones of weld including the base metal. The plane of the polished section was disposed parallel to the surface of the weld and the axis of the polished section was located perpendicularly to the axis of the weld. The results show that migration of interstitial impurities takes place to boundaries of weld zones and regions of primary segregation of substitution elements. This process occurs under the action of cyclic loading that is followed by reduction of relative density of movable dislocations and length of dislocation segments, which are described in Communication 2. The mentioned causes lead to stabilization of weld material structure and enable an increase in the cyclic and static creep limits as is described in Communication 1. The physical processes, responsible for stabilization of dislocation structure and growth of creep stresses, are mainly completed after 5·104 cycles of loading.

Crack propagation in panels of pressurized fuselages upon simultaneous action of cyclic loading by pressurization and high-frequency vibrations

Crack propagation in panels of pressurized fuselages upon simultaneous action of cyclic loading by pressurization and high-frequency vibrations

Paper 26: Cumulative Strain Behaviour of a Nickel-Chromium Alloy and an 11 Per Cent Chromium Martensitic Type of Steel under the Action of Cyclic Loading

Paper 26: Cumulative Strain Behaviour of a Nickel-Chromium Alloy and an 11 Per Cent Chromium Martensitic Type of Steel under the Action of Cyclic Loading