Higher ΔK Values Research Articles

Structure, processing and performance of ultra-high molecular weight polyethylene (IUPAC Technical Report). Part 4: sporadic fatigue crack propagation

Abstract Fatigue tests were carried out on compression mouldings supplied by a leading polymer manufacturer. They were made from three batches of ultra-high molecular weight polyethylene (UHMWPE) with weight-average relative molar masses, M ¯ W ${\overline{M}}_{\mathrm{W}}$ , of about 0.6 × 106, 5 × 106 and 9 × 106. In 10 mm thick compact tension specimens, crack propagation was so erratic that it was impossible to follow standard procedure, where crack-tip stress intensity amplitude, ΔK, is raised incrementally, and the resulting crack propagation rate, da/dN, increases, following the Paris equation, where a is crack length and N is number of cycles. Instead, most of the tests were conducted at fixed high values of ΔK. Typically, da/dN then started at a high level, but decreased irregularly during the test. Micrographs of fracture surfaces showed that crack propagation was sporadic in these specimens. In one test, at ΔK = 2.3 MPa m0.5, there were crack-arrest marks at intervals Δa of about 2 μm, while the number of cycles between individual growth steps increased from 1 to more than 1000 and the fracture surface showed increasing evidence of plastic deformation. It is concluded that sporadic crack propagation was caused by energy-dissipating crazing, which was initiated close to the crack tip under plane strain conditions in mouldings that were not fully consolidated. By contrast, fatigue crack propagation in 4 mm thick specimens followed the Paris equation approximately. The results from all four reports on this project are reviewed, and the possibility of using fatigue testing as a quality assurance procedure for melt-processed UHMWPE is discussed.

On the fatigue crack growth behavior of Ti–10V–2Fe–3Al in gaseous hydrogen

The fatigue crack growth behavior of Ti–10V–2Fe–3Al in gaseous hydrogen (H2) was assessed through comparative experiments conducted in laboratory air and 8.3 MPa H2. The measured fatigue crack growth rate (da/dN) versus applied stress intensity factor range (ΔK) relationships and observed fracture morphologies for laboratory air and H2 were comparable up to ΔK ≈ 6.9 MPa√m, when tested at a load ratio of 0.1 and frequency of 10 Hz. At higher ΔK values, significant crack deflection and subsequent catastrophic failure occurred in the specimen tested in H2. This degradation was not observed in a specimen pre-exposed to 8.3 MPa H2 for 96 h and then immediately tested in laboratory air. X-ray diffraction of the failed H2-tested specimen revealed that the material remnants were predominantly composed of TiH2, suggesting that hydride formation was the catalyst for catastrophic failure in H2. The mechanistic implications of these results and their impact on current material compatibility assessments for Ti alloys in hydrogen service are then discussed.

Hydrogen-assisted fatigue crack propagation in a pure BCC iron. Part II: Accelerated regime manifested by quasi-cleavage fracture at relatively high stress intensity range values

Hydrogen effect on fatigue performance at relatively high values of stress intensity factor range, ΔK, of pure BCC iron has been studied with a combination of various electron microscopy techniques. Hydrogen-assisted fatigue crack growth rate is manifested by a change of fracture features at the fracture surface from ductile transgranular in air to quasi-cleavage in hydrogen gas. Grain reference orientation deviation (GROD) analysis has shown a dramatic suppression of plastic deformation around the crack wake in samples fatigued in hydrogen. These results were verified by preparing site-specific specimens from different fracture features by using Focused Ion Beam (FIB) technique and observing them with Transmission Electron Microscope (TEM). The FIB lamella taken from the sample fatigued in air was decorated with dislocation cell structure indicating high amount of plasticity, while the lamella taken from the quasi-cleavage surface of the sample fatigued in hydrogen revealed a distribution of dislocation tangles which corresponds to smaller plastic strain amplitude involved at the point of fracture. These results show that a combination of critical hydrogen concentration and critical stress during fatigue crack growth at high ΔK values triggers cleavage-like fracture due to reduction of cohesive force between matrix atoms.

Assessing Gaseous Hydrogen Assisted Fatigue Crack Growth Susceptibility of Pipeline Steel Weld Fusion Zones and Heat Affected Zones

Abstract The objective of this work was twofold: (1) measure reliable fatigue crack growth relationships for X65 steel and its girth weld in high-pressure hydrogen gas to enable structural integrity assessments of hydrogen pipelines, and (2) evaluate the hydrogen accelerated fatigue crack growth susceptibility of the weld fusion zone and heat-affected zone relative to the base metal. Fatigue crack growth relationships (da/dN versus ΔK) were measured for girth welded X65 pipeline steel in high pressure hydrogen gas (21 MPa) and in air. Hydrogen assisted fatigue crack growth was observed for the base metal (BM), fusion zone (FZ), and heat-affected zone (HAZ), and was manifested through crack growth rates reaching nearly an order of magnitude acceleration over rates in air. At higher ΔK values, crack growth rates of BM, FZ, and HAZ were coincident; however, at lower ΔK, the fatigue crack growth relationships exhibited some divergence with the fusion zone having the highest crack growth rates. These relative fatigue crack growth rates in the BM, FZ, and HAZ were provisional, however, since both crack closure and residual stress contributed to the crack-tip driving force in specimens extracted from the HAZ. Despite the relatively high applied R-ratio (R = 0.5), crack closure was detected in the heat affected zone tests, in contrast to the absence of crack closure in the base metal tests. Crack closure corrections were performed using the adjusted compliance ratio method and the effect of residual stress on Kmax was determined by the crack-compliance method. Crack-tip driving forces that account for closure and residual stress effects were quantified as a weighted function of ΔK and Kmax (i.e., Knorm), and the resulting da/dN versus Knorm relationships showed that the HAZ exhibited higher hydrogen accelerated fatigue crack growth rates than the BM at lower Knorm values.

Influence of welding parameters on fracture toughness and fatigue crack growth rate in friction stir welded nugget of 2024-T351 aluminum alloy joints

Friction stir welding (FSW) was performed on 2024-T351 aluminum alloy plates. Metallographic analysis, Vickers microhardness and XRD tests were conducted to determine the properties of the welded zone. FE simulation of the FSW process was implemented for the different welding conditions to extract the residual stress and stress intensity factor (SIF). Fracture and fatigue behaviors of the welds which have the initial crack in the nugget zone and the crack orientation along the welding direction, were studied based on standard test methods. Fracture behavior of the welds was also evaluated by shearography method. The results showed that the tool rotational and traverse speeds affect the fracture toughness and fatigue crack growth rate. FSW provides 18%–49% reductions in maximum fracture load and fracture toughness. A slight diminution in fracture toughness of the joints was observed for lower traverse speed of the tool, and at higher traverse or rotational speeds, increasing the probability of defects may contribute to low fracture toughness. Fatigue crack propagation rate of all welds was slower than that of the base metal for low values of stress intensity factor range ΔK (ΔK<13 MPa·m1/2), but is much faster for high values of ΔK.

Engineering Critical Assessment of nickel-based clad pipeline girth welds in a wet H2S-containing environment

Specimens from nickel-based clad pipeline girth welds were tested in H2S solution. The results of the fatigue crack growth rate curve at higher ΔK values were about 4 times higher than the in-air values. The rate at 180°C was generally higher than the in-air data for flaw assessments found in the British Standard BS7910 by a factor of up to 3 times. From the analyses of the diffusible hydrogen, the backing steel of such pipelines are predominantly protected from the sour environment by the corrosion-resistant alloy clad layer. The results from this study are relevant for desktop Engineering Critical Assessment of girth welds of such pipelines.

Measurement of Fatigue Crack Growth Relationships in Hydrogen Gas for Pressure Swing Adsorber Vessel Steels

Hydrogen-assisted fatigue crack growth rates (da/dN) were measured for SA516 Grade 70 steel as a function of stress-intensity factor range (ΔK) and load-cycle frequency to provide life-prediction data relevant to pressure swing adsorber (PSA) vessels. For ΔK values up to 18.5 MPa m1/2, the baseline da/dN versus ΔK relationship measured at 1 Hz in 2.8 MPa hydrogen gas represents an upper bound with respect to crack growth rates measured at lower frequency. However, at higher ΔK values, baseline da/dN data must be corrected to account for modestly higher crack growth rates at the lower frequencies relevant to PSA vessel operation.

Fatigue crack growth behaviour of forged Ti–6Al–4V in gaseous hydrogen

Fatigue crack growth (FCG) tests were performed to evaluate the fatigue behaviour of forged Ti–6Al–4V in air and high-pressure gaseous hydrogen (15MPa) at room temperature. The results indicate that the effect of gaseous hydrogen is dependent on the stress intensity factor (ΔK). The FCG rate was unaffected by hydrogen below a critical stress intensity, ΔK*≈20MPa√m. Above ΔK*, the FCG rate fluctuated and subsequently accelerated at higher ΔK values. The observed behaviour is attributed to the change in the fracture processes. A hypothesis is proposed that describes the FCG behaviour in gaseous hydrogen.

Fatigue crack propagation in API 5L X-70 pipeline steel longitudinal welded joints under constant and variable amplitudes

Fatigue crack propagation (FCP) under constant and variable amplitude loading in base metal (BM), weld metal (WM) and heat affected zone (HAZ) of longitudinal welded joints of an API X-70 pipeline steel was investigated. Constant amplitude loading tests were performed at R = 0.1 and 0.5, whereas for variable amplitude testing single peak tensile overloads (OLs) alternating between 75 and 100% of maximum load were applied at 2.5 mm intervals in crack growth. Results of SE(B) specimens tested under constant and variable amplitude loading revealed that BM, WM and HAZ regions subjected to R = 0.5 and low ΔK-values presented the highest crack growth rates. At higher ΔK values FCP rates in all the studied regions were similar and the R effect on FCP rate was no more observed. Crack growth retardation due to OLs was observed at the three studied regions, showing a decrease on the FCP delay with a decreasing on ΔK.

Fatigue-crack growth in Ti-6Al-4V-0.1Ru in air and seawater: Part I. Design of experiments, assessment, and crack-grown-rate curves

Fatigue-crack growth rates for different simulated ocean environments and loading conditions have been investigated for beta-annealed Ti-6Al-4V-0.1Ru (extra-low interstitials, (ELI)), a candidate material for oil production risers; the focus was on uncovering whether certain combinations of conditions could produce unexpectedly high crack growth rates. A two-level, one-quarter-fraction factorial design-of-experiments (DOE) approach was used to ascertain which testing variables and environmental conditions warranted further study. This study used eight different combinations of variables: parent/deformed material, 27 °C/85 °C temperature, 2 Hz/20 Hz loading frequency, 0.1/0.6 load ratio (R=σ min/σ max, where σ min is the minimum and σ max is the maximum stress during a fatigue cycle), and aerated/deaerated seawater. Comparisons were based on crack growth rates at ΔK=17 MPa $$\sqrt m $$ , roughly the middle of the Paris portion of the da/dN vs ΔK curves. The da/dN vs ΔK curves were also examined, and conclusions based upon these data were compared with those from the DOE. Consideration of the microstructure’s influence on the crack path is postponed until Part II of this article. Samples tested at the higher load ratio showed a statistically significant increase in the crack propagation rate compared to those tested at R=0.1; the same was true of specimens tested at 20 Hz vs those tested at 2 Hz, but the level of significance was lower. The parent material had somewhat higher crack growth rates than the deformed samples. Changes in environmental conditions other than frequency produced little effect on the crack growth rate. Comparison of crack growth rates over the ΔK range measured revealed details that would have not been uncovered in comparisons at a single ΔK value. The Paris exponent ranged between 3.7 and 6.7, and the only systematic variation observed was an increase in the exponent with increasing test frequency. In seawater, cold work (a 5 pct reduction in thickness by rolling) reduced fatigue-crack growth rates by a factor of 2 (compared to the parent material) at intermediate and high ΔK values. There was a crossover of crack growth rates for low ΔK values: below 10 MPa $$\sqrt m $$ , growth rates were lower for the parent material than for the cold-rolled material, suggesting a higher ΔK th for the parent material, while above this value, fatigue cracks grew more rapidly in the parent material than in the cold-rolled material. Crack growth rates were slightly higher in seawater than in air, but only slightly more than the sample-to-sample variation of crack growth rates, and cold work reduced fatigue-crack growth rates in air by about the same amount as in seawater. Somewhat more scatter was observed for the R=0.1 tests than for the R=0.6 tests. Differences in temperature (27 °C, 53 °C, and 85 °C) do not appear to affect fatigue-crack growth rates. For ΔK<20 MPa $$\sqrt m $$ , crack growth rates were similar for 0.2 and 2 Hz but were higher for 20 Hz; above 20 MPa $$\sqrt m $$ , the crack growth rates were similar for all three frequencies. One explanation for the unusual frequency dependence relies on the possibility that the environment produces different amounts of closure for different test frequencies. According to this view, closure is effective in air and in seawater at 0.2 and 2 Hz but not at 20 Hz: perhaps the higher loading rate breeches the passive layer at a rate more rapid than it can reform. Because the crack growth rate appeared independent of temperature, it is unlikely that there is a significant influence of thermally activated corrosion-fatigue mechanisms for the conditions tested. The results demonstrate that beta-annealed Ti-6V-4Al-0.1Ru (ELI) possesses a robust response to the combinations of environment and loading expected in oil production riser service. The value of the DOE approach was clear, and supplementary tests verified the main effects predicted by the DOE results. Comparison of the single-value DOE results with the da/dN curves reveals a limitation in the former: different slopes of the Paris curves and crossover effects are or would be missed for DOE comparing crack growth rates derived from constant ΔK tests. The use of constant Δσ data and a second level of interrogation, following DOE analysis and based on the da/dN curves, addressed this limitation effectively. A DOE comparison based, for example, on three ΔK values (the lower, middle, and upper portions of the Paris regime) might be another way of proceeding.

EFFECT OF LASER BEAM RADIATION ON FATIGUE CRACK PROPAGATION IN AISI 4150 STEEL

The effect of laser beam radiation on fatigue crack growth in AISI 4150 steel was performed on compact‐tension (CT) specimens, in which a composite region (CR) comprised of the hardened zones (HZs) on the top and bottom surfaces and the base metal (BM) in the interior, was aligned either along or normal to the crack growth direction. The microstructure of the HZs consisted of martensite, while lower bainite was present in the 300 °C preheated laser‐hardened specimens. When the crack propagated along the laser tracks (LTs), the fatigue crack growth rates (FCGRs) of the laser‐hardened specimen were lower than those of the base plate, particularly at low ΔK ranges. On the other hand, for a crack propagating normally to the LTs, decelerated FCGRs in the regions preceding the CR and accelerated FCGRs within the CR itself were found. However, enhanced FCGRs in the CR were not found in preheated specimens with a bainite structure in the HZs as the crack grew normal to the LTs. The enhancement of FCGRs in the CR, which became more accentuated at high ΔK values, was closely related to an embrittled microstructure (martensite) in the HZs.

Fatigue crack closure and crack growth behaviour in a titanium alloy with different microstructures

Fatigue crack closure and crack growth behaviour in Ti–2.5 wt % Cu alloy with two equiaxed and two lamellar microstructures have been investigated by constant-load amplitudetests. Plasticity-induced crack closure and roughness-induced crack closure have been characterized separately by experimental methods. A change in closure mechanism from plasticity-induced crack closure at high ΔK values (region of high stress intensity ranges)to roughness-induced crack closure at low ΔK values occurs in a solution-annealed equiaxed microstructure, while plasticity-induced crack closure is the operative closure mechanism in an over-aged equiaxed microstructure over the whole range of ΔK and roughness-induced crack closure occurs in two lamellar microstructures. The crack closing stress intensity factor for plasticity-induced crack closure increases continuously with increasing maximum stress intensity. The crack closing stress intensity factor for roughness-induced crack closure increases with increasing maximum stress intensity at low ΔK, and remains constant at high ΔK. Crack closure and crack path deflection have a significant influence on the crack growth rates. © 1998 Kluwer Academic Publishers

FATIGUE CRACK PROPAGATION IN AUSTENITIC STAINLESS STEEL UNDER LOW FREQUENCY LOADING AND SALT WATER CONDITIONS

Fatigue crack growth rates (FCGR) for a steel similar to AISI 316LN have been evaluated in air and salt water. At free corrosion potential, a short crack effect appeared for crack lengths shorter than 4 mm. For longer cracks, a segment (called region I) of the corrosion FCGR curve can be described by an equation with the same coefficients as for steels of ordinary resistance to corrosion. Thus, the process controlling crack propagation is presumably the same for both steels. Corrosion FCGR in region II (range of longer crack lengths and higher ΔK values, where the curve cannot be described by the previously mentioned equation) are slower than for ordinary steels. The plateau in FCGR observed at a cathodic potential corresponds to the higher plateau at the free corrosion potential. An interpretation is presented for the shape of the corrosion FCGR curves at both potentials.

The effect of microstructures on the fatigue crack growth in Ti6Al4V laser welds

The effect of microstructures on fatigue crack growth rates (FCGRs) in Ti6Al4V plates and laser welds was investigated. Experimental results revealed that the location of tensile fracture of laser-welded specimens was always in the base metal, owing to the presence of martensite in the narrow fusion zone (FZ). For welds aged at various temperatures, only minor differences in tensile properties were observed. The various heat treatments of such plate did not affect FCGRs significantly. The fracture surface was characterized by fatigue striation at high ΔK values. When the R ratio was increased from 0.1 to 0.4, higher FCGRs would be resulted. The decomposition of martensite in the aged FZ causes retardation of FCGRs in a weld, which is characterized by a mixed mode fracture. The more tortuous crack path within the FZ and HAZ was also indicative of high crack growth resistance. Roughness-induced crack closure caused by crystallographic fracture was responsible for decreased FCGRs.

Fatigue of in situ Reinforced Ti–8.5Al–1B–1Si

The effect of temperature on the fatigue and fracture properties of an in situ reinforced super α alloy Ti–8.5Al–1B–1Si (wt. %) was investigated. At room temperature the as-extruded composite has a strength of 631 MPa with limited ductility. On increasing the temperature to 700 °C only a marginal drop in strength to 610 Mpa was observed along with a significant improvement in ductility to 5.9%. Low-cycle fatigue results indicate a marginal decrease in fatigue life as temperature is increased from room temperature to 700 °C. Fatigue crack growth studies in the as-extruded microstructure indicate a strong influence of R-ratio on both the threshold for fatigue crack growth and crack growth rates in the Paris regimes. At elevated temperatures, the resistance to fatigue crack growth increases with temperature below approximately 500 °C. At 600 °C, however, there is an increase in the near threshold crack growth rate due to embrittlement effects. At higher δK values , the resistance to fatigue crack growth at elevated temperatures is always better than that at room temperature. This improvement is attributed to the increase in the inherent resistance

Fatigue crack propagation behavior of cellulose esters

AbstractThe effect of plasticizer concentration on fatigue crack propagation (FCP) rate in cellulose acetate‐propionate (CAP) was determined. Compact tension specimens were machined from 6.2 mm‐thick injection molded plaques and tested on an MTS servohydraulic testing machine using a sinusoidal waveform with a frequency of 1 Hz. Two FCP mechanisms were identified: a crazing mechanism, which dominated at low values of stress intensity factor range, ΔK, and a shear yielding mechanism, which dominated at high values of ΔK. The value of ΔK at the onset of the transition from the crazing mechanism to the shear yielding mechanism was a function of plasticizer concentration, and therefore yield strength of the CAP. The transition in crack propagation mechanism created a V‐shaped feature on the fracture surface, which could be used to weight the contributions from the two crack propagation mechanisms to the overall FCP rate.

The Influence of Testing Methodology on the Fatigue Crack Propagation Behavior of Cellulose Esters

Abstract In the fatigue crack propagation behavior of cellulose esters, a crazing mechanism controls growth at low stress intensity (ΔK) levels while a plane-strain-shear yielding mechanism becomes dominant at higher ΔK values. The growth rate associated with the transition between these two mechanisms has been found to be a function of the material's yield strength. Most recent data have shown that the onset of this transition is also related to the test methodology. Constant-load-amplitude, K-increasing tests produce the most conservative (highest fatigue rate) values for the transition, whereas with decreasing stress intensity tests, the transition occurs at much lower growth rate values. It is important to note, however, that test methodology does not affect the relation between crack growth rate and ΔK when growth is dominated by only a single mechanism.

DISLOCATION SUBSTRUCTURES IN FERRITE OF PLAIN CARBON DUAL‐PHASE STEELS AFTER FATIGUE FRACTURE

Abstract— The dislocation substructures of ferrite in plain carbon, ferrite plus martensite, dual‐phase steels in different stages of fatigue crack propagation were examined by transmission electron microscopy. The experimental results show that the dislocations are in random arrays in the ferrite before cyclic loading. At stages of low ΔK (near threshold) values, parallel clusters of dislocations and prolonged dislocation lines can be observed. At stages of intermediate ΔK values, the dislocations are rearranged into networks and loops while at stages of high ΔK (prior to failure) values, they are changed into dislocation cells or patches. It was also found that the volume fraction and carbon content of martensite have significant effects on the dislocation substructures of ferrite after cyclic deformation.

Environmental cracking of AISI 4145 steel in drilling muds

The environmental cracking of a quenched and tempered AISI 4145 steel in a water-based drilling mud has been investigated under slow strain-rate tensile and cyclic-loading conditions. The variables investigated were pH, potential, temperature and the addition of copper- and zinc-containing lubricants. These tests were supported by detailed electron fractography. Under tensile loading conditions, progressive loss of ductility occurs with decreasing potential and pH, and with increasing temperature. The addition of 25 wt% copper- and zinc-containing lubricants generally reduces this embrittlement. The corrosion fatigue crack propagation rate increases with decreasing potential and pH. At low ΔK values, temperature does not affect the corrosion fatigue crack propagation rate, but it is accelerated by increasing temperature at higher ΔK levels. In the presence of copper-containing lubricant, the crack growth rate is always lower than that in copper-free mud. The addition of zinc-containing lubricant to the mud accelerates the crack growth rate at low ΔK values, but does not affect it at higher ΔK values. It is concluded that the decrease in ductility and the increase in the fatigue crack growth rates in drilling muds, relative to air, are due to the appearance of brittle forms of fracture (intergranular, transgranular and quasi-cleavage fracture modes) which are the result of hydrogen-induced cracking.

AN EVALUATION OF FATIGUE AND FRACTURE MECHANICS PROPERTIES OF ULTRA-HIGH STRENGTH 7XXX SERIES Al-ALLOYS

— The fatigue and fracture mechanics properties of rapidly solidified ultra-high strength 7XXX series Al-alloys have been studied. With respect to conventional high-strength Al-alloys, these materials exhibited a better fatigue-endurance on both plain and notched specimens at low stress amplitude in constant amplitude fatigue tests, whereas the opposite occurred at high stress amplitudes. Fatigue crack growth tests indicated lower crack growth rates at low ΔK-levels, but at intermediate and high ΔK-values these materials were particularly prone to additional components of “static” crack propagation, which led to steeply inclined da/dN vs ΔK curves. Moreover, the increase in tensile strength was linked with some loss of ductility and fracture toughness. Overload regions were characterized by a large amount of intergranular decohesion, possibly facilitated by the presence of incoherent particles at grain boundary regions and by the large strength differential between the matrix and precipitate free zone. The best results in terms of elongation to rupture and toughness were obtained by reducing the amount of Cr/Mn incoherent dispersoid-forming elements, in order to lessen the tendency towards matrix-dispersoid interface decohesion at grain boundaries.