Complete Crack Arrest Research Articles

Fatigue crack arrest in steel beams using FRP composites

Previous studies have demonstrated the effectiveness of strengthening with pre-stressed carbon-fiber reinforced polymer (CFRP) composites to increase the lifetime of cracked steel members. In some cases, complete crack arrest has been observed. This study aims to present a method that can estimate the minimum required prestressing that would result in a complete crack arrest in steel I-beams. Analytical and numerical models based on linear elastic fracture mechanics (LEFM) were developed and verified using a set of experimental results. Three steel I-beams with different crack lengths were strengthened with pre-stressed CFRP composites and later tested under a high-cycle fatigue loading regime. It was shown that the pre-stressed CFRP composites could result in a crack closure mechanism, in which the crack surfaces remained closed even under large external loads. Furthermore, it was shown that by considering the stiffness of the CFRP in the analytical formulation, the amount of prestressing required to arrest the fatigue crack growth can be reduced.

Mitigation of fatigue damage in self-healing vascular materials

The fatigue response of an epoxy matrix containing vasculature for the delivery of liquid healing agents is investigated. The release of a rapidly curing, two-part epoxy healing chemistry into the wake of a propagating crack reduces the rate of crack extension by shielding the crack tip from the full range of applied stress intensity factor. Crack propagation is studied for a variety of loading conditions, with the maximum applied stress intensity factor ranging from 62 to 84% of the quasi-static fracture toughness of the material. At the highest level of applied load, the rate of mechanical damage is so fast that the healing agents do not fully mix and polymerize, and the effect of healing is minimal. The self-healing response is most effective at impeding the slower propagating cracks, with complete crack arrest occurring at the lowest level of applied load, and reductions of 79–84% in the rate of crack extension at intermediate loads.

Fracture and fatigue response of a self-healing epoxy adhesive

A self-healing epoxy adhesive for bonding steel substrates is demonstrated using encapsulated dicyclopentadiene (DCPD) monomer and bis(tricyclohexylphosphine)benzylidine ruthenium (IV) dichloride (Grubbs’ first generation) catalyst particles dispersed in a thin epoxy matrix. Both quasi-static fracture and fatigue performance are evaluated using the width-tapered-double-cantilever-beam specimen geometry. Recovery of 56% of the original fracture toughness under quasi-static fracture conditions occurs after 24 h healing at room temperature conditions. Complete crack arrest is demonstrated for fatigue test conditions that render neat resin and control samples failed. Inspection of fracture surfaces by electron microscopy reveals evidence of polymerized DCPD after healing. These results are the first mechanical assessment of self-healing for thin (ca. 360 μm) films typical of adhesives applications.

Effects of Overload and Frequency on Fatigue Crack Propagation in Nanocrystalline Zr-Based Bulk Metallic Glass

A nanocrystalline (NC) bulk glass Zr 55 Al 10 Cu 30 Ni 5 (at%) containing nano-scale crystals embedded uniformly in a glassy matrix has both high tensile strength of 1.7 GPa and high ductility. The new alloy is therefore expected to have practical applications in machines and structures. The influences of frequency and overload on fatigue crack propagation behavior of the NC bulk glass were examined. The fatigue crack propagation rate da/dn less than 3 x 10 -5 mm/cycle was independent of frequency in the frequency range of 0.1 to 50 Hz at the stress ratio of 0.1 under sine and triangular waves. When the overload ratio (overload/baseline load) was large, a complete crack arrest occurred and the AK value just before a crack regrowth was three times larger than the threshold stress intensity factor range ΔK th . The reason for the crack arrest was not explained by the crack closure effect. The overloading induced the kinking and branching of the crack. The stress reduction near the crack tip due to the kinking and branching of the crack and the crack closure effect gave an appropriate explanation for the complete crack arrest and the larger threshold stress intensity factor range. When the overload ratio was small, a temporary crack arrest occurred and the kink and branching of cracks occurred intermittently at the crack front.

High frequency fatigue crack propagation behavior of a nickel-base turbine disk alloy

Fatigue crack propagation tests were conducted on the powder metallurgy nickel-base superalloy KM4 at room temperature. Two different heat treatments were investigated, one which produced a relatively coarse grain size around 55 μm, and another which produced a very fine grain size around 6 μm. Tests were conducted at 50 Hz and 1000 Hz in an advanced servohydraulic testing machine at R-ratios between 0.4 and 0.7. There was no effect of frequency on the fatigue behavior at room temperature, which is expected in this type of alloy, and this result yields confidence in the reliability of the servohydraulic fatigue testing system. The threshold stress intensity for fatigue crack propagation decreased with decreasing grain size and with increasing R-ratio, again as expected. With increasing grain size, the crack path tortuosity and the crystallographic facet size on the fracture surface both increased substantially, leading to increases in roughness-induced closure and a higher apparent threshold. The threshold ΔK values measured at 10 −10 m/cycle corresponded to essentially infinite lifetimes, as very small decreases in Δ K from the threshold values resulted in complete crack arrest, and led to difficulty in restarting the crack growth at higher Δ K levels. Finally, comparisons of the observed thresholds with existing models revealed significant discrepancies between the predicted and measured values.

The stress corrosion cracking and hydrogen embrittlement of titanium in methanol-hydrochloric acid solutions

Pre-exposure tests on unstressed specimens of IMI 125 (low-oxygen, commercially pure titanium) in MeOH/HCl solutions revealed that intergranular anodic dissolution was occurring. In addition it was also observed that for pre-exposure times of 25 h or more, absorbed hydrogen was responsible for an intergranular strain-rate dependent embrittlement when specimens were tested in air, in the strain rate range 5.56 × 10 −4-5.56 × 10 −6 s −1 immediately upon removal from the test environment. If an ageing period of 20 h at 150°C was inserted in between removing the specimens from the environment and testing, then the embrittlement effect could be eliminated. Constant crosshead speed stress corrosion tests were also performed in MeOH/HCl in the range of crosshead speeds 166.67 μm s −1 to 1.67 nm s −1. The mode of cracking was observed to be predominantly intergranular, with small regions of transgranular cleavage evident. Crack velocities of in excess of 1.2 mm h −1 were observed for the intergranular SCC. Additions of selenium, a cathodic poison, to the test environment produced a significant increase in the observed crack velocity, whilst additions of platinum were observed significantly to reduce crack propagation rates and, in some instances, cause complete crack arrest. Significantly less intergranular fracture was evident in specimens tested in platinum-containing solutions compared with those tested in platinum-free solutions. The above observations supported the argument that hydrogen embrittlement is the mechanism responsible for the intergranular SCC of titanium in MeOH/HCl solutions.

Fatigue crack growth retardation in plane stress and plane strain

The fatigue crack growth retardation phenomenon following a single peak overload applied tothick andthin SEN bend specimens has been studied for a low carbon structural steel. Fatigue tests were performed under a constant load ratio of 0.6, constant stress intensity range of 10 MPam1/2 and a constant overload ratio of 2.5. An immediate increase followed by a transient retardation in the fatigue crack growth rate, due to the applied overload, was observed for thethick specimen but complete crack arrest was obtained for thethin specimen. The immediate increase in the fatigue crack growth rate following the single peak overload in thethick specimen was attributed to the coincidence of monotonic fracture modes.

Creep brack growth behavior of Two Al-Li alloys

Creep crack growth behavior of two Al-Li-Cu-Mg-Zr alloys with and without the addition of Ge was determined under static load at 150 °C and the results were analyzed using the linear elastic and elastic-plastic fracture mechanics methods. The alloys were produced through conventional ingot metallurgy processing and extruded into a plate form. Texture is more pronounced in the alloy that does not contain Ge. Because of the texture, crack growth rates in the two alloys differ significantly for the L-T and T-L orientations. The growth occurs by the nucleation of microcracks ahead of the main crack which grow and coalesce with the main crack. While this leads to rapid crack growth in the T-L orientation, it causes crack tip bifurcation and even complete crack arrest in the L-T orientation.

Growth of cracks under far-field cyclic compressive loads: Numerical and experimental results

Crack growth from a notch tip, under the influence of fully compressive far-field cyclic loads, is investigated both numerically and experimentally in notched specimens of a lower strength steel and an aluminum alloy. Such cracks, emanating from the root of the notch, progressively decelerate until complete crack arrest occurs. Analyses of the stress state along the crack plane indicate that while the extent of the residual tensile stress field diminishes with an increase in crack length, there is a progressive increase in crack closure during crack advance. Approximate numerical calculations of the crack closure loads are in reasonable agreement with experimental results.

Crack initiation under far-field cyclic compression and the study of short fatigue cracks

A procedure involving crack initiation under far-field cyclic compression is used to study the fatigue behavior of small flaws which are ~0.3–0.5 mm in length and are amenable to linear elastic fracture mechanics (LEFM) characterization. This technique enables the determination of the threshold stress intensity range at which crack growth begins for small flaws and provides insight into some closure characteristics. Cracks were propagated in notched specimens of a bainitic steel subjected to fully compressive remote cyclic loads, until complete crack arrest occurred after growth over a distance of only a fraction of a mm at a progressively decreasing velocity. Following this, physically small flaws were obtained by machining away the notch. For the loads examined, the results indicate that the extent of damage left at the tip of the crack grown (and arrested) under remote compression is not large enough to affect subsequent tensile fatigue crack growth, when closure effects are not significant (e.g. at high load ratios). At high load ratios, the growth of small linear elastic cracks is identical to that of corresponding long flaws subjected to the same stress intensity range, which corroborates the similitude concept implicit in the nominal use of LEFM. At low load ratios, however, short tensile cracks propagate substantially faster than the longer flaws and exhibit lower threshold stress intensity range levels. Such apparent differences in their growth rates seem to arise, to a large extent, from the differences in their closure behavior, as indicated clearly from various aspects of the compression method. Global measurements of closure, with their inherent uncertainties, however, cannot account completely for the anomalous behavior of short flaws and for the effect of load ratio on short crack growth. Closure of short flaws begins to develop after growth over a minimum distance of about 0.5 mm in this steel. The significance and limitations of the compression technique are discussed and possible mechanisms responsible for the differences between long and short fatigue cracks are outlined.

Hold-time effects on high temperature fatigue crack growth in Udimet 700

Crack growth behaviour under creep-fatigue conditions in Udimet 700 has been studied, and the crack growth data were analysed in terms of the stress intensity factor as well as theJ-integral parameter. Crack growth behaviour is shown to depend on the initial stress intensity level and the duration of hold-time at the peak load. For stress intensities that are lower than the threshold stress intensity for creep crack growth, the crack growth rate decreases with increase in hold time even on a cycle basis, da/dN, to the extent that complete crack arrest could occur at prolonged hold times. This beneficial creep-fatigue interaction is attributed to the stress relaxation due to creep. For stress intensities greater than the threshold stress intensity for creep crack growth, the growth rate on a cycle basis increases with increase in hold time. For the conditions where there is no crack arrest, the crack growth appears to be essentially cycle-dependent in the low stress intensity range and time-dependent in the high stress intensity range. Both the stress intensity factor and theJ-integral are shown to be valid only in a limited range of loads and hold-times where crack growth rate increases continuously.