Environmental Cracking Research Articles

AE: Acoustic emission testing of an HF acid sphere

The paper describes how an Acoustic Emission (AE) testing programme was used to detect environmental cracking in an HF storage sphere at an early stage, without the need for frequent internal inspections. The first AE test (in 1985) revealed a number of welding flaws in the top circumferential weld. These were verified by ultrasonic testing but repairs were not necessary. AE testing continued at approximate three-year intervals and recorded minimal levels of AE until the fourth (1995) test. This produced increased AE activity, giving the first indication of environmental damage. Follow-up examinations found fissuring and small blisters in the affected areas. This was not serious enough for immediate remedial action, so a follow-up AE test was scheduled to see if the condition changed after a further year of operation. This test, carried out in 1996, produced a further increase in AE activity and more areas were affected. The sphere was shut down for inspection and repair. Extensive cracking was found in the areas ofAE activity, most of these were removed by grinding but some weld repairs were needed. AE testing six-months after recommissioning showed AE activity had returned to the previous low levels.

Environmental Cracking Behavior of Submerged Arc-Welded Supermartensitic Stainless Steel Weldments

Supermartensitic stainless steel welds produced by submerged are welding were assessed for their microstructure and properties. Slow strain rate tests conducted on these specimens revealed that both the parent material and the weld metals are susceptible to cracking under conditions of hydrogen (H) charging.

Environmental Crack Growth under Variable Amplitude Loading of Pipeline Steel

Abstract An investigation was carried out on API X60 steel specimens with surface cracks to study environmentally assisted crack growth rates in simulated groundwater (NS-4 solution). The environmental conditions were similar to those for buried natural gas pipelines in service, and loading was aggressive. The maximum stress intensity factor at the tip of the crack on the surface ranged from 20.6 MPa√m to 31.4 MPa√m. A periodic underload was applied to pre-cracked cantilever bending specimens. Tests were carried out for a period of 40 days at a frequency of either 1 cycle or 40 cycles per day. Crack growth rates were obtained for two stress ratios (R = minimum/maximum load) of 0 or 0.5 under both constant and variable amplitude loading conditions. Depending on loading conditions, the crack growth varied from 0 μm to 2,640 μm. However, the majority of crack growth measured was <100 μm. The experimental results showed that there was an acceleration in growth following an underload. The growth was higher whe...

Effect of strain and strain rate on fatigue-accelerated biodegradation of polyurethane.

A diaphragm-type film specimen was used to study in vitro degradation of poly(etherurethane urea) (PEUU) under conditions of dynamic loading. This geometry allowed both uniaxial and biaxial loading in a single experiment. During testing, the film was exposed to a H(2)O(2)/CoCl(2) solution that simulated in vivo oxidation of PEUU. The combination of dynamic loading and biaxial tensile strain accelerated oxidative degradation. The effects of biaxial strain magnitude and strain rate were examined separately by increasing the frequency of fatigue loading from 0 to 1 Hz with constant maximum biaxial strain and by changing the maximum biaxial strain while maintaining constant strain rate. In the ranges of biaxial strain energy (0.17 to 0.55 MPa) and strain rate (0 to 46% s(-1)) tested, the rate of degradation increased with increasing strain rate whereas strain magnitude had essentially no effect on degradation rate. Although loading conditions affected the rate of oxidative degradation, ATR-FTIR analysis suggested that in all cases the mechanism of degradation did not change. Chemical degradation produced a brittle crosslinked surface layer marked by dimpling and pitting, as observed with scanning electron microscopy. Pits served as stress concentrators and initiated environmental stress cracks under dynamic loading but not under static (creep) loading. Small pits were sufficient to initiate cracks at higher strain rates whereas only large pits initiated cracks at lower strain rates. Consequently, a higher strain rate produced more profuse cracking.

Fracture analysis of a flow control device used in the petrochemical industry

This paper presents a service failure analysis of a steel control device assembled in a petrochemical industry division which resulted in a great fire. The purpose of this analysis was the determination of the reasons that caused fractures in the fixation screws of the body/cover of the valve during operation. The failure caused the leakage of hydrogen (H 2) and sulfide gas (H 2S), being responsible for an accident in Fortaleza, Brazil. Techniques such as visual inspection, electronic microscopy, optical microscopy, determination of mechanical properties, design analysis by computational simulation and spectrometry were used in this investigation. Several factors influenced the failure mechanism, such as: design failure, environmental cracking, device components out of fatigue and fracture specifications.

Environmentally assisted, sustained-load crack growth in powder metallurgy nickel-based superalloys

To examine the influence of niobium (Nb) on sustained-load crack growth (SLCG) in oxygen, three powder metallurgy (P/M) nickel-based superalloys, with nominal compositions similar to IN100, but with 0, 2.5, and 5 wt pct of Nb, are used. These alloys are gamma-prime (γ’) strengthened and have comparable volume fractions (53 vol pct) of γ’ precipitates. The SLCG experiments are conducted in high-purity oxygen and argon at 873, 923, and 973 K. The environmental cracking sensitivity (ECS) for the alloys with 2.5 and 5 wt pct of Nb is consistent with that of INCONEL 718 and supports the previously identified role of Nb-rich carbides in enhancing crack growth. The susceptibility of the Nb-free alloy to oxygen, however, is much greater than expected. The apparent activation energy for crack growth in oxygen was found to depend on stress-intensity-factor (K) levels for the Nb-containing alloys and ranged from about 320 to 260 kJ/mol for K levels of 35 to 60 MPa√m. It was nearly independent of K at about 250 kJ/mol for the Nb-free alloy. The results are discussed in terms of the rate-controlling process and of the mechanism for crack-growth enhancement.

Environmental fatigue crack growth in titanium aluminides and hydrogen evolution behaviour

The influence of environment on fatigue crack growth behaviour was investigated both in nearly lamellar and in duplex titanium aluminides, and the hydrogen evolution kinetics was analysed by thermal desorption spectroscopy. The tensile strength of the duplex material decreases in the order of the extent of the water molecule content in the environment: the strength in vacuum is the highest, and decreases in the order of laboratory air and finally in water. In the case of the lamellar material, the fatigue crack growth rate in dry air is higher in the R–C crack plane orientation than that in the L–C crack plane orientation. The crack growth rate becomes higher when the crack grows as the lamellae tear. However, in the case of the duplex material, the crack growth rate in the R–C crack plane orientation is smaller in the low ΔK (ΔKeff ) region. When cathodic charging is applied, the fatigue crack growth rate becomes higher than in dry air, particularly in the higher stress intensity factor range. The hydrogen evolution rate is increased by cathodic charging, with lower temperature peaks and higher ones. The peaks at lower temperatures are correlated with the decomposition of hydrides and de‐training of hydrogen from microstructural imperfections such as microvoids. As‐received materials also show an evolution peak at a higher temperature, and the evolution rate is almost independent of cathodic charging. In addition, the evolution rate at a high temperature (above 800 °C) is increased by cathodic charging. The hydrogen is considered to have an important role on fatigue crack growth acceleration.

Environment induced fatigue crack of type 40Cr steel in drilling muds

Drill pipes are subjected in service to aggressive environment (a wide range of drilling muds) and to a combination of high static and dynamic loads. Under such conditions, the high-strength steel is susceptible to various forms of environmental cracking, particularly to corrosion fatigue. The fatigue life of type 40Cr steel in salt drilling muds was studied with a self-made rotating bending corrosion fatigue testing apparatus. Environmental variables investigated included stress, frequency, temperature, and pressure of carbon dioxide. Experimental results show that carbon dioxide promotes the fatigue crack of 40Cr steel. These tests were supported by electrochemical corrosion measurements and hydrogen permeation.

Effect of artificial aging on the fatigue crack propagation resistance of 2000 series aluminum alloys

Artificial aging performed to impart higher strength generally degrades the fatigue crack propagation (FCP) resistance of naturally aged 2xxx-series aluminum alloys. This behavior is examined for commercial AA2024-T351 and a naturally aged Al–Cu–Mg–Li alloy stressed in high-humidity air. Environmental fatigue crack growth rate decreases with initial-artificial aging, then increases monotonically with increasing aging time. Overaging does not improve cracking resistance. This effect of microstructure is pronounced at low stress intensity factor range and persists for high stress ratio conditions that minimize crack closure. For both alloys, the highest resistance to fatigue crack growth correlates with the presence of artificial aging intensified solute clusters, and absence of distinct precipitates, as evidenced by electron microscopy and small-angle neutron scattering (SANS). Increased crack growth rates correlate with the dissolution of clusters and/or the formation of an increasing amount of S′ precipitates for AA2024 and T 1 precipitates for the Li-bearing alloy. The fundamental effects of very fine-scale clusters and precipitates on cyclic-slip mode and environment-sensitive crack tip damage are unresolved.

Effect of oxygen on the crack growth behavior of V–4Cr–4Ti at 600°C

Exploratory experiments were performed to evaluate the effect of oxygen on the crack growth response of V–4Cr–4Ti at 600°C under constant load. Tests were run in gettered argon, argon containing 2000 ppm oxygen, and laboratory air using fatigue pre-cracked compact-tension (CT) specimens. Crack growth was measured primarily by post-test fracture surface examination, but also by in-test compliance measurements. Crack growth rates measured in air and gettered argon were about 2–3 × 10 −3 mm/h at a stress intensity factor of about 40 MPa m . The crack growth rate in argon with 2000 ppm oxygen was about 7 × 10 −2 mm/h at the same stress intensity level. The crack growth rates were very sensitive to the stress intensity factor. Over a limited range of stress intensity values the crack growth rate in argon plus 2000 ppm oxygen appears to be power-law dependent on stress intensity with an exponent of about 8.9. The fracture mode in air and gettered argon was transgranular cleavage with 20–30% intergranular fracture. In the oxygenated argon environment crack growth occurred predominantly by transgranular cleavage.

Effect of Temper on Environmental Fatigue Crack Propagation in 7000-Series Aluminum Alloys

In contrast to stress corrosion, the effect of overaging temper on environmental fatigue crack propagation rate in the 7000-series aluminum alloy/aqueous-chloride solution system is nil, moderately beneficial, or detrimental depending on loading frequency. For both peak and overaged AA7075 plate, Paris regime fatigue crack growth rates are accelerated up to 10-fold by cyclic loading in 3.5% NaCl solution compared to fatigue in moist air for the SL-crack orientation. Crack growth is intersubgranular for the T6 microstructure, but transgranular-brittle for the T7 case. Cracking at low f is dominated by corrosion product induced crack closure that is enhanced by overaging. At intermediate to high f, environmental fatigue is due to hydrogen embrittlement and rate limited by H diffusion over a crack tip process zone distance established by local stress. The relationship between plateau crack growth rate and critical frequency for f-dependent EFCP is predicted by a H diffusion model. Plateau regime transgranular-brittle EFCP in overaged AA7075 is slower than intersubgranular cracking in the T6 microstructure, but persists to higher f CRIT , requiring either rapid H diffusion or reduced crack tip H for process zone embrittlement of the overaged case.

The Electrode Potential Dependence of Environment-Assisted Cracking of AA 7050

Thick plate, peak aged AA 7050 is susceptible to intergranular environment-assisted cracking in near-neutral 0.5 M Na 2 CrO 4 + 0.05 M NaCl solution. The effect of applied electrode potential (E APP ) on the Stage II crack growth rate (da/dt) is complex. While da/dt increases with increasing applied potential, slow growth at E APP less than -0.5 V SCE transitions after incubation to strongly E APP -dependent fast-rate EAC with hysteresis. A large potential gradient exists near the crack tip region, with the tip potential (∼0.8 V SCE ) independent of E APP , suggesting electrochemically decoupled crack tip and wake regions. Crack tip solution acidification (< pH 3.5) occurs from hydrolysis of a concentrated Al-salt solution formed by dissolution during crack advance. These crack chemistry changes are time dependent and cause the hysteresis in da/dt vs. E APP . Hydrogen uptake measured in the wake increases with increasing E APP due to increased acidification and overpotential for proton reduction. The hydrogen environment embrittlement mechanism for environmental cracking in AA 7050-T651 is supported by the correlation between E APP -dependent crack wake H concentration, local crack chemistry, and da/dt.

Variability of radiation-induced segregation in iron–chromium–nickel alloys

This work examines the variability in grain boundary segregation measurements in proton irradiated iron–chromium–nickel alloys made using Auger electron spectroscopy (AES) and scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDS). Variability occurs in segregation measurements made on different boundaries in the same sample and made on different samples irradiated under the same conditions. Variability occurs in each measurement technique, but is greater for AES. A portion of the greater variability in the AES measurements occurs because only the concentrations calculated from AES measurements are sensitive to changes in the shape of the energy intensity peak. A statistical analysis technique for testing the consistency of experimental conditions demonstrated that the variability is not attributable to uncertainty in irradiation temperature, dose, or material condition. Finally, the analysis of grain boundary composition distributions in Fe–20Cr–9Ni indicates that attempts to minimize environmental cracking by controlling grain boundary composition need to focus on both the average grain boundary compositions and the shape of the concentration distributions.

Environmental degradation of wood–HDPE composite

Long term degradations in weathering condition and in controlled environment conditions of different factors (ultraviolet light, oxidation and heat) were conducted for wood flake reinforced HDPE composites. An interaction of the mechanical recovery of the compressed wood cell structure and environmental stress cracks of HDPE plays an important role in the weathering degradation. Daily temperatures and sunshine times are believed be less crucial than rainfall and UV intensity. In controlled degradations, the oxidative degradation of the composite exposed to air was obvious but limited. After excluding the effect of dehydration the kinetic theory of the first order reactions could describe the thermal degradation of the composite, whose activation energy of 23.2 kJ/mol indicates an ease thermal degradation. UV exposure greatly promoted the environmental stress cracks of HDPE, but its impact on the composite was limited in the controlled dry condition.

The identification of hydrogen trapping states in an Al-Li-Cu-Zr alloy using thermal desorption spectroscopy

Thermal desorption spectroscopy (TDS) was utilized to identify several metallurgical states in an Al -2Li - 2Cu-0.1Zr (wt pct) alloy, which trap absorbed hydrogen. Six distinct metallurgical desorption states for hydrogen were observed for tempers varying from the T3 to peakaged condition. Lower energy thermal desorption states were correlated with interstitial sites, lithium in solid solution, and δ′ (Al3Li) precipitates. These states have trap-binding energies ≤25.2 kJ/mol. Under the charging conditions utilized, approximately 4 pct of the total (e.g., trapped and lattice) hydrogen content was associated with interstitial sites, consistent with the view that the intrinsic lattice solubility of hydrogen in aluminum is very low. In contrast, dislocations, grain boundaries, and T1 (Al2CuLi) particles were found to be higher energy-trap states with trap-binding energies ≥31.7 kJ/mol. Approximately 78 pct of all absorbed hydrogen occupied these states. Moreover, greater than 13 pct of the available trap sites at grain boundaries were occupied. Such a high hydrogen coverage at grain-boundary sites supports the notion that hydrogen contributes to grain-boundary environmental cracking in Al-Li-Cu-Zr alloys. Also, it points out the error in assuming that hydrogen cannot play a major role in cracking of Al-based alloys due to the low lattice solubility.

Cross-Sectional Sample Preparation and Analysis of Intergranular Stress Corrosion Cracks in FE-NI-CR Alloys.

Transmission electron microscopy in its modern high-resolution analytical form has great potential for elucidating mechanisms of environmental cracking in structural alloys and other materials. Apart from a few pioneering efforts, the difficulties of preparing suitable samples with the original corrosion structures preserved in narrow crack tips have limited contributions of TEM in this area. Recent work involving cross-sectional crack preparation has focused on intergranular stress-corrosion cracking of Fe- and Ni-base stainless alloys in high-temperature water. This presentation will illustrate preparation methods that have proven successful, and describe limitations of the analysis.The method for preparing cross-section samples suitable for high-resolution TEM characterization is based on earlier work by Lewis et al. It involves preparing a thin section containing cracks filled with epoxy, dimple grinding selected locations near the crack tip, and ion milling. Figure 1 schematically illustrates this method. The first step is to preserve the crack opening characteristics present under loading and protect the crack surfaces during thinning.

CARPENTER 20Mo-6 HS STAINLESS

Abstract Carpenter 20Mo-6 HS is an alloy in the generic family of 6% molybdenum alloys, which are very corrosion resistant. This steel also possesses an excellent resistance to environmental cracking. (See also Carpenter 20Mo-6, Alloy Digest SS-444, May 1984.) This datasheet provides information on composition, physical properties, hardness, elasticity, tensile properties, and bend strength. It also includes information on corrosion resistance as well as forming, heat treating, machining, and joining. Filing Code: Ni-556. Producer or source: Carpenter.

Stability of arrays of multiple edge cracks

The creation and subsequent shedding of arrays of edge cracks is a natural phenomenon which occurs in heat-checked gun tubes, rapidly cooled pressure vessels and rock, dried-out mud flats, paint and concrete and in ceramic coatings and permafrost. The phenomenon covers five orders of magnitude in crack spacing and the driving mechanisms may include fast fracture, environmental cracking and fatigue crack growth. A simple model is developed which indicates that the shedding behaviour is governed by the behavior of individual cracks rather than global energy changes. The model predicts that all cracks will deepen until a crack-spacing/crack depth ratio (2 h/ a) of 3.0 is achieved, at which stage crack-shedding will commence. Two out of every three cracks will be shed, leading to a new (higher) crack spacing/crack depth ratio at which stage growth of all currently active cracks will be dominant. An approach based upon rapid, approximate methods for determining stress intensity provides good indications of behaviour provided near-surface stress gradients are not excessive. In cases where stress gradients are high it is shown that it is necessary to employ numerical techniques in calculating stress intensity. Two specific examples are presented, the first at very small scale (heat-check cracking in a gun tube, typical crack spacing 1 mm) and the second at very large scale (permafrost cracking, typical crack spacing 20 m). The predicted ratios for the proportion of cracks shed and for crack spacing/crack depth are in agreement with experimental evidence for gun tubes, concrete and permafrost. The ratios also appear to match experimental observations of “island delamination” in ceramic coatings and paint films.

Compressive Thermal Yielding Leading to Hydrogen Cracking in a Fired Cannon

Investigation of environmental cracking of a 1100-MPa yield strength A723 steel cannon tube subjected to prototype firings is described. Metallographic results show cracking of the steel beneath a 0.12-mm protective layer of chromium. Cracks undermine and remove sections of chromium and lead to localized erosion that ruins the cannon. Key features of the firing thermal damage and cracking are: (i) recrystalization of the chromium to a depth of up to 0.08 mm; (ii) steel transformation to 0.19 mm below the chrome surface; (iii) two different periodic arrays of cracks normal to the hoop and axial directions, with mean depths of 0.23 and 0.46 mm, respectively. Time-temperature-depth profiles for the firing cycle were derived via bi-material finite difference analysis of a semi-infinite solid which incorporated cannon combustion gas temperatures and material properties that vary as a function of temperature. The temperature and depth associated with the steel transformation were used to solve iteratively for the convective heat transfer coefficient. This value was further confirmed by the depths of chromium recrystalization and of the crack arrays in the two orientations. A profile of maximum temperature versus depth is used to determine the near-bore applied and residual stress distributions within the tube. The measured volume change of steel transformation is used to determine an upper limit on applied and residual stresses. These stresses are used to determine crack-tip stress intensity factors for the observed crack arrays, and hence provide some explanation for the differential depths of cracking. The near-bore temperature and residual stress distributions are used to help determine the cause of hydrogen cracking and measures to prevent cracking. Compressive yielding due to thermal loading produces near-bore tensile residual stresses, and thereby causes hydrogen cracking. Prevention of cracking is discussed in relationship to hydrogen crack growth rate tests of alternative alloys and coatings.

Relation of Field Cracking to Layer Stiffness of Cold In Situ Recycled Materials

In 1996 the Alliance for Transportation Research Institute completed a comprehensive evaluation of 45 cold in situ recycling (CIR) pavement sections on New Mexico State Highway and Transportation Department routes built between 1984 and 1990. The results of further investigation into observed correlations between visual condition surveys, as an indicator of field performance, and the CIR stiffness measured on cores are reported. It was observed that CIR projects in New Mexico that are older than 9 years, with measured resilient modulus exceeding 6000 MPa and indirect tensile strength (ITS) exceeding 800 kPa, also exhibited reduced pavement condition index ratings based solely on environmental cracking distress. Reported is a direct comparison with some additional stiffness data and field performance for pavements recycled with different additives. Accelerated aging following Strategic Highway Research Program procedures was used to assess some samples tested. New sections evaluated were constructed by using hot lime slurry, cement, and a cement-fly ash blend. The criteria for cracking threshold observed in CIR materials appeared to be unaffected by addition of hot lime slurry and were quite different from those for layers recycled with cementious materials. It is proposed that criteria noted for resilient modulus and ITS may be used to guide the development of cold recycle laboratory mix design in combination with accelerated aging to simulate hardening under field conditions. By properly selecting the type of additive and the amount, more crack-resistant pavement sections may be constructed. Recommendations are developed for target values of stiffness in recycled materials.