Crack Growth Rates In Specimens Research Articles

Pecularities of fatigue cracks growth in steel and composite sucker rods

To predict the fatigue life of sucker rods, it is important to take into account the effect of technological environments under various loading schemes on the conditional fatigue limit of steels used for their manufacture (in particular, such as steel 20N2M), and compare these properties with modern materials for the manufacture of fiberglass and hybrid sucker rods (with metal heads and polymer rod) as prototypes of steel elements of oil-producing equipment.Specimens of sucker rods with technological or operational defects were tested for fatigue life-time in aggressive technological environments. It has been established that the fatigue life-time and fatigue endurance of sucker rods decrease with an increase in their diameter. So, with a test base of 107 loading cycles, the endurance limits of rods with a diameter of 19, 22, and 25 mm were 95, 85, and 80 MPa, respectively. It was shown that technological defects practically did not affect the life-time of the rods. Minor differences were associated with the scatter of the fatigue characteristics of the metal. Defects formed in rods during their operation in wells have a more noticeable effect on their life-time under variable loads. It was found that the resource of sucker rods with a diameter of 19 mm decreased faster compared to rods with a diameter of 22 mm in the presence of operational defects of the same depth in both rods. This is due to the greater sensitivity of smaller diameter rods to defects.According to the curves of the crack growth rate in specimens made of sucker rods with a diameter of 22 mm, it was found that the critical state (according to the jump in the crack growth rate) is reached if the crack length along the perimeter of the rods is within 15...16 mm, and its depth is 4.5 ... 5.6 mm. Such cracks were fixed at a stress range of 100 MPa, which is commensurate with the operating loads of the rods. For rods of other standard sizes, the critical crack depth, which preceded the almost spontaneous destruction of the specimens, was 3...4, 4...5 and 5...6 mm for rods with a diameter of 16, 19 and 25 mm, respectively. In article we also evaluated fatigue life on samples of fiberglass sucker rods under cyclic loading by circular cantilever bending and obtained the corresponding fatigue curves. According to the results of these tests, probabilistic curves of fatigue endurance of samples with steel and fiberglass sucker rods in the technological environment of reservoir water were constructed with different probability of their non-destruction P(N). Fatigue endurance of samples with steel, fiberglass and hybrid sucker rods was compared.

Thermocyclic strength of grey cast iron SCh20 at high temperature shifts

To predict the durability of casting molds made of SCh20 cast iron under thermal fatigue during its filling by liquid metal the thermocyclic strength of the casting molds metal (grade SCh20 gray cast iron, analog of A48-30B AISI cast iron) was investigated. The specimens were tested by cyclic heating in the furnace to maximum temperatures from 700°С to 1000°C with following cooling in air and water. The Coffin equation and the dependence of crack growth rate in specimens are obtained on the basis of metallographic measurements of crack length in specimens and strain ranges FEM calculations. The investigations results allow one to predict the durability of casting molds made of SCh20 cast iron under thermal fatigue during its filling by liquid metal.

Fatigue Crack Propagation of Notched Steel Rebar in RC Beams Repaired with Externally Bonded CFRP

AbstractThis paper presents the results of an experimental study on the fatigue performance of RC beams strengthened with different externally bonded carbon fiber-reinforced polymer (CFRP) systems. Seven specimens were fabricated; three had no CFRP, while the remaining four had one of two CFRP systems. The objective of the research reported in this paper was to determine the effect the CFRP repair had on the growth rate of a fatigue crack initiating from a notch in the tensile rebars. The steel reinforcement was selected for focus because similar studies have found this reinforcement to be the limiting fatigue component in RC beams. The experimental results reported in this paper showed an extended fatigue life and a slowed crack growth rate in specimens repaired with both CFRP systems. The crack growth rates were used to determine material constants for the Paris law, which describes growth of a stable fatigue crack. These results are then used to propose recommendations for design of CFRP repair systems...

Fatigue Crack Initiation and Early Growth Behavior of Ferritic Stainless Steel in Laboratory Air and 3%NaCl Solution.

Crack initiation and crack growth behavior were studied on a ferritic stainless steel, SUS444, as a function of specimen orientation. Fatigue tests were conducted in laboratory air and in 3%NaCl solution using electro-hydraulic fatigue testing machine. Specimens with two different orientations, i.e. L and T direction, were prepared. In both orientations, cracks initiated from slip bands within ferritic grains. The effect of specimen orientation on fatigue lives was not observed in both environments. However environmental effect was found on fatigue lives, i.e. fatigue lives were shorter in 3%NaCl solution. In early crack growth region, while no discernible differences were found between both directions, crack growth rates in 3%NaCl solution were faster than in laboratory air. In long crack growth behavior, the anisotropy was only observed at low ΔK region, where the crack growth rates in specimens with L-T orientation were faster than those with T-L orientation. Taking account of crack closure, the crack growth in both orientations exhibited a similar behavior. Environmental effect was also found in the region of ΔK≥15 MPa √m for both orientations, where crack growth was enhanced in 3%NaCl solution.

The effect of cold expansion on fatigue crack growth from open holes at room and high temperature

In this paper a series of residual stress measurements and fatigue crack growth tests have been carried out using aluminium alloy 2650 specimens containing cold expanded and non cold expanded holes. Residual stress measurements have been done after cold expansion and after various loading and temperature conditions. In order to measure an angular variation of residual stresses, X-ray and a new technique called the Garcia–Sachs method have been employed. Results revealed that residual stress relaxation occurred as a result of exposure at 150°C. The magnitude of relaxation was shown to be dependent on the level and the sign of externally applied load. Fatigue crack growth tests have been carried out at 20°C and 150°C for both cold expanded and non-cold expanded conditions. Fatigue crack growth rates in specimens containing cold expanded fastener holes were affected significantly by elevated temperature exposure. Depending on the exposure time and loading conditions the fatigue life improvement was found to be between one and greater than 10 for tests at 20°C.

FATIGUE CRACK GROWTH BEHAVIOUR OF FIBRE‐METAL LAMINATE GLARE‐1 AND METAL LAMINATE 7475 WITH DIFFERENT BLUNT NOTCHES

Abstract— The fatigue crack growth behaviour of the fibre metal laminate “GLARE‐1” has been investigated for different blunt notches in Constant Amplitude (CA) tests. In order to investigate the influence of the fibres, the same laminate material but containing no fibres (Laminate 7475) was also tested. The fatigue crack growth properties of GLARE‐1 are superior to those of Laminate 7475. GLARE‐1 shows lower crack growth velocities at the same Knom values and in addition the crack growth rates decrease with increasing crack length. The Laminate 7475 shows typical metal behaviour for single crack propagation and accelerating crack growth with increasing crack length. In GLARE‐1, multiple crack propagation takes place. The cracks propagate independent of each other and have similar crack growth rates, in part due to closure effects caused by the unbroken fibre layers.The crack growth rates of specimens having a small root radius are higher in both materials than in specimens with a large notch radius. In GLARE‐1, the superiority of a larger notch radius is more pronounced than in the Laminate 7475 and is attributed to a stronger crack closure effect owing to fibre bridging. The reason for the higher bridging capability in specimens containing larger notches is that less fibres are broken or damaged in the notch vicinity.

Influence of specimen configuration and loading history on fatigue crack growth behaviour of 2024-T3 clad aluminium alloy

Experimental investigations have been carried out to study the effect of specimen configuration and loading history on the crack growth behaviour of 2024-T3 aluminium alloy at room temperature. It has been observed that fatigue crack growth rate in specimens loaded symmetrically is higher than that in specimens loaded asymmetrically. Further it has been observed that the crack growth rate under increasing stress intensity factor (SIF) range is higher than that under decreasing SIF range. These observations have been attributed to stress distribution ahead of the crack and the residual stresses developed during loading.

Effects of Residual Stresses on Stress Corrosion Crack Growth Rates in Aluminum Alloy

Abstract Stress corrosion crack growth rate data obtained as a function of the plane strain stress intensity using double cantilever beam specimens of 7079, 7075, and 7175 are presented. The effects of quenched-in residual stresses on crack growth rates in specimens of this design are discussed, and methods of eliminating the residual stress problem are presented.