Initial Crack Growth Rate Research Articles

Fatigue Crack Development in Epoxy Coatings on Steel Substrate: The Role of Coating and Substrate Properties in Determination of the Onset of Fatigue Cracks

Development of service cracks in epoxy based corrosion protective coatings limits the life of the substrate structure. If cracks develop, corrosion protection is lost and costs of repair and re-protection of large marine structures can be crippling. Factors controlling development of cracks in the coating are poorly understood, and predictions of coating lifetime approximate. For bulk tanker applications service strains imposed on coatings arise from both low frequency thermal mismatch strains and mechanical strains from wave action. In this work the number of cycles to crack initiation has been measured during strain controlled fatigue of two selected 300 μm thick coatings applied to a 5.5 mm thick steel substrate. Cycling was performed at frequencies of 1 Hz, and strain amplitudes between ±0.16% and ±0.5%. Coating crack development was monitored optically. It was found that cycles to crack initiation depended on both strain amplitude and on the static ductility of the coating. After initiation crack growth rates in the coating reduced with increasing surface crack length. The significance of the results is considered in the light of requirements for quantitative models for service life prediction of coatings on metallic substrates.

Enhancement of the Stress Corrosion Sensitivity of AA5083 by Heat Treatment

In this study, the stress corrosion cracking (SCC) resistance of AA5083 is intentionally degraded by a series of progressively longer annealing treatments at 448 K (175 °C) that create a two-phase microstructure. Precipitation of strongly anodic Mg2Al3, known as β-phase, occurs heterogeneously with substantial precipitation along the grain boundaries, as observed by differential interference microscopy. Ultimate tensile strength, yield strength, and strain to failure of AA5083 alloy were found to be independent of the amount of β-phase precipitates, making AA5083 an ideal system to study the relative contributions of anodic dissolution and hydrogen embrittlement. Open circuit dropwise exposure SCC tests with precracked double cantilever beam (DCB) specimens made from the AA5083 alloy with different heat treatment conditions were conducted using 3.5 pct NaCl solution at an initial stress intensity factor (K I ) of $$ 1 5\,{\text{ksi}}\sqrt {\text{in}} .\;\left( { 1 6. 5\,{\text{MPa}}\sqrt {\text{m}} } \right). $$ Two SCC characteristics, initial crack growth rate and incubation time, were found to be strongly dependent on the amount of β-phase precipitates. Initial crack growth rate increased sigmoidally as a function of heat treatment time with an inflection point between 120 and 240 hours of sensitization time, while the incubation time decreases monotonically with sensitization time. Additionally, fracture surfaces investigated by scanning electron microscopy demonstrated characteristics of intergranular cracking with multiple crack tips. Discussion centers on the evidence supporting anodic dissolution of β-phase grain boundary precipitates as a primary mechanism of SCC in severely sensitized AA5083 alloy and the potential contribution of hydrogen embrittlement in the failure of grain boundary ligaments between β-phase grain boundary precipitates in less severely sensitized conditions.

Creep Crack Growth in P92 and P122 Alloy Steel Weldment

The structures connected by welding are fractured often in Heat Affected Zone (HAZ) when cracks grow under high-temperature and high-pressure conditions. In this study, the creep crack growth tests of P92 and P122 steels were performed and the creep fracture characteristics were evaluated. Degradation rate by cavities was investigated and the influence of degradation rate on crack growth rate was evaluated. Crack growth rate of new material was compared with that of the degraded material to confirm the influence of the degradation rate by cavities on the crack growth rate. From these results, the crack growth rates in welded joints were determined. The crack growth rate of the step test specimen was much faster than that of new material under the same test conditions of step temperature, while it was slower than that of new material under the same step loading conditions. It implies that crack growth rate of the welded specimen is faster when degradation rate by cavities is high even under the same temperature and stress conditions. HAZ and weld metal have faster cavity nucleation and growth rates than base metal. Therefore, the HAZ and weld metal specimens showed shorter crack initiation time and the initial crack growth rate (da/dt)c of the HAZ and weld metal was faster than the base metal. If (da/dt)c is fast, fracture life will be reduced.

圧縮応力による初期微小疲労き裂進展(停留き裂)(S14-2 切欠き効果と圧縮疲労強度特性,S14 金属材料の疲労特性と破壊機構)

It is well known that the initial crack growth rate is very high, when notched specimen is loaded by cyclic compressive stress. This reason is estiamted to be a occurrence of crack opening by the existence of a remained tensile stress. Therefore the influence of compressive stress for the crack opening phenomena and for the non-propagating crack was investigated. Used material was Alminum alloy and specimen figure was CT specimen. On the condition of three kinds of stress ratio R (R=0, R=1 and only compressive stress) crack propagation tests were performed. Even when absolute maximum values are same and crack dose not propagate by only tensile stress, there exists non-propagating crack on the condition of R=-1 and only compressive stress. Therefore it is found that non-propagating crack has a close relation to compressive stress.

Fatigue-fracture mechanism of slowly notched poly(ethylene terephthalate) polymers

Fatigue fracture behavior of slowly notched polyethylene terephathalate (PET) polymers were investigated at temperatures close to their β transition temperatures up to well above their glass transition temperatures. Detailed characterization on the morphology of the notched roots showed that the crack tip during crack propagation became more dull with increasing testing temperature. The failure cycle (Nf) of these samples increased with increasing temperatures until it reached the α transition temperatures of PET polymers, and most of the increase in Nf is due to the increased time consumed in the initiation period. On the other hand, the initial crack growth rate increased significantly and Nf of these samples decreased dramatically as the temperature increased well above the glass transition temperature. This interesting temperature dependence of fatigue behavior is explained due to the change of molecular motion of PET polymers at this temperature range.

An Empirical Model for Environmental Damage at the Crack Tip

ABSTRACTAn experimental method that is able to verify and to characterize the damage zone developed in front of crack tip under time-dependent fatigue crack propagation has been established. The crack growth rate was found to decrease exponentially in the damaged zone. Such a crack growth behavior can be described by a mathematical model which only requires three parameters: initial crack growth rate, (da/dN)q; normal crack growth rate (da/dN)o; and damaged zone size, D. An analyticat relationship has been developed to correlate the temperature and the time of sustained loading to the size of the induced damaged zone.

Crack initiation in cyclic compression and its applications

The characteristics and applications of crack initiation under far-field cyclic compressive loads are examined in notched specimens of a lower strength steel. The fatigue cracks, initiated at the notch root due to residual tensile stresses, grow at a progressively decreasing velocity before arresting completely. Simple correlations are explored between the total fatigue crack growth distance and the size of an effective damage zone estimated from the initial crack growth rate. Some important applications of crack initiation in cyclic compression in both long and short fatigue crack problems are demonstrated. Experimental results are presented to show that crack initiation in cyclic compression offers some interesting possibilities for obtaining accelerated estimates of slow fatigue crack growth rates, while minimizing some of the inherent uncertainties arising from the artifacts of conventional test techniques. Possible applications of this method are also discussed with respect to the study of physically-short flaws and of the evaluation of the progressive development closure during fatigue.

Study of stress waves in crack propagation by the finite-element method

1. An algorithm was devised for modeling rapid crack growth and was incorporated into a FEBIK program [9] to solve dynamic FEM problems. 2. The use of this program to study waves of unloading stresses in crack extension showed that the crack tip is a rather unusual source of stress waves. The first stress wave removes the load at the crack tip in all directions and produces a finite discontinuity, the value of which increases roughly parabolically with an increase in initial crack growth rate. In such a case, the first stress wave does not affect the formation of microcracks in front of the main crack. 3. The good agreement between the data obtained and the analytical results shows that the method used to numerically solve dynamic problems for solids with cracks is correct. A shortcoming of the method is the large amount of machine time required. The solution of a problem for one value of\(\mathop a\limits^ \bullet /\mathop a\limits^ \bullet _R\) requires 9 min of machine time on a CDC6600 computer, which corresponds to about 270 min on an ES-1040 computer.