Superior Fatigue Crack Growth Resistance Research Articles

Effect of particle morphology on fatigue crack propagation mechanism of TiB2-reinforced steel matrix composites

Fatigue crack growth mechanisms of as-cast and hot-rolled TiB2-reinforced steel matrix composites (SMCs) fabricated by eutectic solidification are systematically investigated through in-situ scanning electron microscopy (SEM) fatigue experiment system. The experiment results show that the as-cast SMC exhibits a relatively superior fatigue crack growth resistance with respect to the hot-rolled SMC. A lower FCGR at the low and intermediate stress intensity factor (ΔK) range in as-cast SMC is found, which is related to the TiB2 particle morphology. Hexagonal and rectangular primary particles have a high Young’s modulus and strong interfacial bonding, which inhibits the advancing of plastic deformation in front of the crack tip. Three-dimensional interconnected eutectic particle phases composed of dendrites can produce a hardening effect at crack tip and restrain the generation of plastic deformation in this morphology region. The stress-strain field in front of the crack tip is changed by the hard TiB2 particles, which leads to the deflection of the crack path. Hence, the tortuous crack path created by facing these particles causes reduction in the effective driving force of the crack tip and slows the FCGR. The net mismatch between the surfaces lead to premature contact and roughness-induced crack closure further reducing the FCGR. In comparison, large number of hot-rolled micro-cracks in the hot-rolled SMC are detrimental to fatigue crack growth resistance through the coalesce mechanism of micro-cracks, thereby accelerating the FCGR.

Extrusion of Lightweight Aluminum and Magnesium Alloys Structures for Aviation Applications

The development of light-weight materials and fabricating parts/sub-assemblies of substantially large dimensions has become a major issue for the aerospace industry, which has boosted the development of more advanced materials with high specification properties. Recent aluminum and magnesium alloys developments are based on achieving superior fatigue crack growth resistance, better corrosion resistance, lower density, etc. Standard manufacturing techniques, such as extrusion, ought to be developed in order to find a beneficial solution allowing for structural weight reduction, which is a very efficient means of improving aircraft performance. It is associated with the problem of extrusion profiles with a complex cross-section shape. In this work the formability of aluminum and magnesium alloys in the extrusion process was determined by the upsetting test through specification of the flow stress in relationship to the deformation size and rate. The results of the upsetting test of Al (7075, 2024, 8090, 2099) and Mg ( AZ31, AZ61, AZ80, WE 43) alloys were used in determining the conditions of the extrusion of profiles of various cross section shape and extrusion ratio. The analysis of macro and microstructure of extruded products and their mechanical properties demonstrates strong influence of the shape of extrudate cross section on metal exit speed and extrusion force value. Macro-and microstructure of all the investigated alloys after extrusion are highly homogeneous in terms of the grain size and morphology of the phase components, compared to the macro- and microstructure in the initial state, which justifies the use of them in production of aviation profiles.

Towards superior fatigue crack growth resistance of TC4-DT alloy by in-situ rolled wire-arc additive manufacturing

Titanium alloys have many advanced applications where they are subject to fatigue. Here, we compare the fatigue crack growth resistance and microstructures of TC4-DT alloys fabricated by wire-arc additive manufacturing (WAAM; sample S1) and in-situ rolled WAAM (sample S2) followed by quasi-β heat treatment. Sample S2 had superior fatigue crack growth resistance, which is mainly attributed to its finer defects and finer α phase. The critical size of the defects related to fatigue performance was calculated. The majority of defects in sample S2 were smaller than the critical size, while several coarse defects (∼1.91 mm) were observed in sample S1. Multiple secondary cracks could be detected near the fracture surface in sample S2, leading to long and tortuous fatigue crack propagation paths. Therefore, in-situ rolling can improve the fatigue properties of titanium alloys fabricated by additive manufacturing.

Long fatigue crack growth behavior of Ti–6Al–4V produced via high-power laser directed energy deposition

The fatigue crack growth (FCG) behavior of long cracks was studied in Ti–6Al–4V alloy fabricated by high-power laser directed energy deposition. The results show the deposited parts under heat treated conditions have superior FCG resistance than that of the other reported additive manufactured Ti–6Al–4V, owing to the specimens with coarse prior-β grains and interior thick α-laths. Using linear elastic fracture mechanics and two (von Mises and Tresca) yielding criterions, we calculated the monotonic and cyclic plastic zone sizes, and found the FCG rates as a function of cyclic plastic zone sizes ahead of the crack tip during FCG region. A method was proposed to determine the typical microstructural characteristic unit of the additive manufactured Ti–6Al–4V parts with basket-weave structure, which shows the same length-scale as plastic zone sizes at the transition point in FCG curves.

In situ tailoring microstructure in laser solid formed titanium alloy for superior fatigue crack growth resistance

For damage tolerance (DT) titanium alloy, the fatigue crack growth resistance (FCGR) is a critical properties requirement for engineering applications. However, the Ti-6Al-4V-DT parts fabricated by laser solid forming (LSF) suffer from low FCGR, because of predominant basket-wave microstructure. Here, we have explored a novel LSF fabrication design to produce full colony microstructure, via in-situ controlled growth. The creation of such microstructures leads to superior FCGR, which markedly exceed conventional additive manufactured and mill-annealed samples. The present works provide a significant guidance for LSF-fabricated titanium alloy with high DT properties.

Metallurgical, impact and fatigue performance of electron beam welded duplex stainless steel joints

Fusion zones of electron beam welded (EBW) 2205 duplex stainless steel (DSS) joints pertaining to two different levels of heat input, exhibited variable microstructural morphology as well as ferrite-to-austenite (α/γ) ratio. Both the weld zones showed phase imbalance with ferrite being the dominating phase over austenite. However, as governed by the associated thermal cycles experienced by these welds, low heat input weld metal exhibited a relatively higher proportion of austenite as compared to the high heat input weld. Thermal aging at 850 °C/0.5 h promoted the formation of Cr and Mo-rich intermetallic σ-phase and carbides in the welds, whereas welding resulted into Cr2N precipitation, which consequentially influenced their impact toughness and fatigue crack growth behavior. The aged joints showed a severe degradation in their impact toughness, the extent of which was more in the base metal than in the weld metal. The role played by the prevalence of secondary phases, unlike impact toughness degradation, was not detrimental but rather beneficial, as aged weld metals exhibited significantly superior fatigue crack growth resistance as compared to their unaged counterparts, which probably could be attributed to dispersion strengthening that occurred due to intermetallic precipitation.

Effect of welding processes and consumables on fatigue crack growth behaviour of armour grade quenched and tempered steel joints

Quenched and Tempered (Q&T) steels are widely used in the construction of military vehicles due to its high strength to weight ratio and high hardness. These steels are prone to hydrogen induced cracking (HIC) in the heat affected zone (HAZ) after welding. The use of austenitic stainless steel (ASS) consumables to weld the above steel was the only available remedy because of higher solubility for hydrogen in austenitic phase. The use of stainless steel consumables for a non-stainless steel base metal is not economical. Hence, alternate consumables for welding Q&T steels and their vulnerability to HIC need to be explored. Recent studies proved that low hydrogen ferritic steel (LHF) consumables can be used to weld Q&T steels, which can give very low hydrogen levels in the weld deposits. The use of ASS and LHF consumables will lead to distinct microstructures in their respective welds. This microstructural heterogeneity will have a drastic influence in the fatigue crack growth resistance of armour grade Q&T steel welds. Hence, in this investigation an attempt has been made to study the influence of welding consumables and welding processes on fatigue crack growth behaviour of armour grade Q&T Steel joints. Shielded metal arc welding (SMAW) and Flux cored arc welding (FCAW) were used for fabrication of joints using ASS and LHF consumables. The joints fabricated by SMAW process using LHF consumable exhibited superior fatigue crack growth resistance than all other joints.

TIG 용접된 Al6013-T4 알루미늄 합금에서 피로균열전파저항의 변동성에서의 PWHT의 영향

The experimental investigation focuses on an influence of artificial aging time in longitudinal butt welded Al 6013-T4 aluminum alloy on the fatigue crack growth resistance. The preferred welding processes for this alloy are frequently tungsten inert gas welding (TIG) process due to its comparatively easier applicability and better weldability than other gas metal arc welding. Fatigue crack growth tests were carried out on compact tension specimens (CT) in longitudinal butt TIG welded after T82 heat treatment was varied in three artificial aging times of 6 hours, 18 hours and 24 hours. Of the three artificial aging times, 24 hours of artificial aging time are offering better resistance against the growing fatigue cracks. The superior fatigue crack growth resistance preferred spatial variation of materials within each specimen in the Paris equation based on reliability theory and fatigue crack growth rate by crack length are found to be the reasons for superior fatigue resistance of 24 hours of artificial aging time was compared to other joints. The highest of crack propagation resistance occurs in artificial aging times of 24 hours due to the increase in grain size (fine grained microstructures).

Effect of lamellar spacing on fatigue crack growth behaviour of a TiAl-based aluminide with lamellar microstructure

Abstract The fatigue crack growth resistance of a Ti–48Al–2Mn–2Nb (at.%) alloy with different lamellar spacings was investigated at room temperature in laboratory air. The crack growth resistance in the air-cooled specimen with a lamellar spacing of ∼0.9 μm was essentially higher than that in the furnace-cooled specimen with a lamellar spacing of ∼3.8 μm. The increased resistance is attributed to the zigzag crack growth across the fine lamellar plates in the air-cooled specimen. However, some furnace-cooled specimens actually exhibited superior fatigue crack growth resistance, comparable to that obtained by air cooling. In these specimens, evidence of crack branching and crack deflection, caused by interlamellar cracking away from the mode I crack growth direction, was frequently observed. Hence, the fatigue crack growth resistance can vary widely, especially for the furnace-cooled specimens of nominally similar fully lamellar microstructures.

Superior fatigue crack growth resistance, irreversibility, and fatigue crack growth–microstructure relationship of nanocrystalline alloys

While previous studies have reported that nanocrystalline materials exhibit poor resistance to fatigue crack growth (FCG), the electro-deposited nanocrystalline Ni–Co alloys tested in this paper show superior resistance to FCG. The high damage tolerance of our alloy is attributed to the following: alloying with Co, low internal stresses resulting in stability of the microstructure, and a combination of high strength and ductility. The high density of grain boundaries interact with the dislocations emitted from the crack tip, which impedes FCG, as predicted by the present model and measured experimentally by digital image correlation. Further, the addition of Co increases the strength of the material by refining the grain size, reducing the fraction of low angle grain boundaries, and reducing the stacking fault energy of the material, thereby increasing the prevalence of twinning. The microstructure is stabilized by minimizing the internal stress during a stress relief heat treatment following the electro-deposition process. As a result grain growth does not occur during deformation, leaving dislocation-mediated plasticity as the primary deformation mechanism. The low internal stresses and nanoscale twins preserve the ductility of the material, thereby reaching a balance between strength and ductility, which results in a superior resistance to FCG.

Fatigue crack growth resistance of gas tungsten arc, electron beam and friction stir welded joints of AA2219 aluminium alloy

AA2219 aluminium alloy square butt joints without filler metal addition were fabricated using gas tungsten arc welding (GTAW), electron beam welding (EBW) and friction stir welding (FSW) processes. The effect of three welding processes on fatigue crack growth behaviour is reported in this paper. Transverse tensile properties of the welded joints were evaluated. Microstructure analysis was also carried out using optical and electron microscopes. It was found that the FSW joints are exhibiting superior fatigue crack growth resistance compared to EBW and GTAW joints. This was mainly due to the formation of very fine, dynamically recrystallised grains and uniform distribution of fine precipitates in the weld region.

Fatigue Crack Growth Behavior of Gas Metal Arc Welded AISI 409 Grade Ferritic Stainless Steel Joints

The effect of filler metals such as austenitic stainless steel, ferritic stainless steel, and duplex stainless steel on fatigue crack growth behavior of the gas metal arc welded ferritic stainless steel joints was investigated. Rolled plates of 4 mm thickness were used as the base material for preparing single ‘V’ butt welded joints. Center cracked tensile specimens were prepared to evaluate fatigue crack growth behavior. Servo hydraulic controlled fatigue testing machine with a capacity of 100 kN was used to evaluate the fatigue crack growth behavior of the welded joints. From this investigation, it was found that the joints fabricated by duplex stainless steel filler metal showed superior fatigue crack growth resistance compared to the joints fabricated by austenitic and ferritic stainless steel filler metals. Higher yield strength and relatively higher toughness may be the reasons for superior fatigue performance of the joints fabricated by duplex stainless steel filler metal.

Effect of welding processes on fatigue crack growth behaviour of AISI 409M ferritic stainless steel joints fabricated using duplex stainless steel fillers

ABSTRACTThe present investigation aims to study the effect of welding processes such as shielded metal arc welding (SMAW), gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) on fatigue crack growth behaviour of the ferritic stainless steel (FSS) conforming to AISI 409M grade. Rolled plates of 4 mm thickness were used as the base material and AISI 2209 grade duplex stainless steel (DSS) was used as filler metal, for preparing single pass butt welded joints. Centre cracked tensile (CCT) specimens were used to evaluate the fatigue crack growth behaviour. From this investigation, it is found that the GTAW joints showed superior fatigue crack growth resistance compared with SMAW and GMAW joints. The reasons for the superior performance were discussed in detail.

Effect of weld metal properties on fatigue crack growth behaviour of gas tungsten arc welded AISI 409M grade ferritic stainless steel joints

The effect of filler metals such as austenitic stainless steel, ferritic stainless steel and duplex stainless steel on fatigue crack growth behaviour of the gas tungsten arc welded ferritic stainless steel joints was investigated. Rolled plates of 4 mm thickness were used as the base material for preparing single ‘V’ butt welded joints. Centre cracked tensile (CCT) specimens were prepared to evaluate fatigue crack growth behaviour. Servo hydraulic controlled fatigue testing machine was used to evaluate the fatigue crack growth behaviour of the welded joints. From this investigation, it was found that the joints fabricated by duplex stainless steel filler metal showed superior fatigue crack growth resistance compared to the joints fabricated by austenitic and ferritic stainless steel filler metals. Higher yield strength, hardness and relatively higher toughness may be the reasons for superior fatigue performance of the joints fabricated by duplex stainless steel filler metal.

Fatigue Crack Growth Behavior of AISI 409M Grade Ferritic Stainless Steel Welded Joints Using Austenitic, Ferritic and Duplex Stainless Steel Electrodes

This paper reveals the fatigue crack growth behaviour of the shielded metal arc welded AISI 409M grade ferritic stainless steel joints fabricated using austenitic stainless steel (ASS), ferritic stainless steel (FSS) and duplex stainless steel (DSS) electrodes. Centre cracked tensile (CCT) specimens were prepared to evaluate fatigue crack growth behaviour. Servo hydraulic controlled fatigue testing machine with a capacity of 100 kN was used to evaluate the fatigue crack growth behaviour of the welded joints. It is found that the joints fabricated by DSS electrode showed superior fatigue crack growth resistance compared to the joints fabricated by ASS and FSS electrodes. Higher yield strength and relatively higher toughness of the weld metal may be the reasons for superior fatigue performance of the joints fabricated by DSS electrode.

Fatigue Life Analysis of Al 8090 Helicopter Fuselage Panels

Considering the aerospace structures, the advantages of Al-Li alloys in comparison with conventional aluminium alloys comprise relatively low densities, high elastic modulus, excellent fatigue and toughness properties, and superior fatigue crack growth resistance. Unfortunately, these alloys have some disadvantages due to highly anisotropic mechanical properties and due to a very high crack growth rate for microstructurally short cracks. This could mean relatively early cracking in high stress regions such as rivet holes in helicopter fuselage panels. Consequently a more accurate approach in fatigue life analysis is requested. Considering that the 8090 T81 aluminium alloy has been widely used in an helicopter structure, in particular in the bolted connection between the stringers and the modular joint frame in the rear of the fuselage, it is extremely important to found a reliable procedure for the fatigue life assessment of the component. Thus, using the results of experimental tests made on panel specimens, a FE general model and two submodels of the critical zone (involved in fatigue damage during the tests) have been modelled in order to investigate the complex state of stress near the rivets holes. These stress values obtained have been elaborated for a fatigue assessment.

Microstructural Changes Due to Friction Stir Processing of Investment-Cast Ti-6Al-4V

Friction stir processing (FSP) was used to modify the coarse fully lamellar microstructure on the surface of investment-cast and hot isostatically pressed (HIP) Ti-6Al-4V plate. The α colony and platelet structure in the base material (BM) was refined such that the effective slip length was reduced from the α colony size of the BM, several hundred microns, to that of fine equiaxed primary α grains that are on the order of 1 μm. This change in the microstructure resulting from FSP is expected to increase fatigue crack initiation resistance making it beneficial for titanium components for aerospace applications. The as-cast coarse lamellar microstructure has superior fatigue crack growth resistance compared to other microstructures that can be obtained by thermomechanical processing. Thus, it is likely that an increase in fatigue life is obtainable by FSP. Given the growing interest in friction stir welding (FSW) and FSP of titanium alloys, we believe some consistent microstructural descriptors will help avoid confusion. Accordingly, we propose terminology to standardize the descriptions of the microstructures created during FSP and FSW of titanium alloys. We also describe the microstructure changes that occur in the stir zone (SZ), transition zone (TZ), and heat-affected zone (HAZ) during FSP of Ti-6Al-4V.

On the fatigue behavior of γ-based titanium aluminides: role of small cracks

Gamma-TiAl based alloys have recently received attention for potential elevated temperature applications in gas-turbine engines. However, although expected critical crack sizes for some targeted applications (e.g. gas-turbine engine blades) may be less than ∼500 μm, most fatigue-crack growth studies to date have focused on the behavior of large (on the order of a few millimeters) through-thickness cracks. Since successful implementation of damage-tolerant life-prediction methodologies requires that the fatigue properties be understood for crack sizes representative of those seen in service conditions, the present work is focused on characterizing the initiation and growth behavior of small ( a∼25–300 μm) fatigue cracks in a γ-TiAl based alloy, of composition Ti–47Al–2Nb–2Cr–0.2B (at.%), with both duplex (average grain size of ∼17 μm) and refined lamellar (average colony size of ∼145 μm) microstructures. Results are compared to the behavior of large ( a>5 mm), through-thickness cracks from a previous study. Superior crack initiation resistance is observed in the duplex microstructure, with no cracks nucleating after up to 500 000 cycles at maximum stress levels ( R=0.1) in excess of the monotonic yield stress, σ y. Comparatively, in the lamellar microstructure cracks nucleated readily at applied maximum stresses below the yield stress (85% σ y) after as few as 500 cycles. In terms of crack growth, measurements for small fatigue cracks in the duplex and lamellar microstructures showed that both microstructures have comparable intrinsic fatigue-crack growth resistance in the presence of small flaws. This observation contrasts previous comparisons of large-crack data, where the lamellar structure showed far superior fatigue-crack growth resistance than the duplex structure. Such “small-crack effects” are examined both in terms of similitude (i.e. crack tip shielding) and continuum (i.e. biased microstructural sampling) limitations of traditional linear elastic fracture mechanics.

The effect of bridging on fatigue crack growth behavior in Aramid Patched Aluminum Alloy(APAL)

A new hybrid composite (APAL: Aramid Patched Aluminum Alloy), consisting of a 2024-T3 aluminum alloy plate sandwiched between two aramid/epoxy laminate (HK 285/RS 1222), was developed. Fatigue crack growth behavior was examined at stress ratios of R=0.2, 0.5 using the aluminum alloy and two kinds of the APAL with different fiber orientation (0°/90° and 45° for crack direction). The APAL showed superior fatigue crack growth resistance, which may be attributed to the crack bridging effect imposed by the intact fibers in the crack wake. The magnitude of crack bridging was estimated quantitatively and determined by a new technique on basis of compliances of the 2024-T3 aluminum alloy and the APAL specimens. The crack growth rates of the APAL specimens were reduced significantly as comparison to the monolithic aluminum alloy and were not adequately correlated with the conventional stress intensity factor range(ΔK). It was found that the crack growth rate was successfully correlated with the effective stress intensity factor range (ΔK eff =K br -K ct ) allowing for the crack closure and the crack bridging. The relation between da/dN and theΔK eff was plotted within a narrow scatter band regardless of kind of stress ratio (R=0.2, 0.5) and material (2024-T3 aluminum alloy, APAL 0°/90° and APAL±45°). The result equation was as follow:da/dN=6.45×10−7(ΔK eff )2.4.

Fatigue crack growth behaviour of an 8090 AlLi alloy under mixed mode I – mode III loading conditions

Aluminum-lithium alloys have evoked considerable interest in recent years because of their high specific strengths and stiffness. An additional advantage of these alloys is their superior fatigue crack growth resistance as compared to the conventional aluminium alloys. However, most of the fatigue crack growth investigations have dealt with crack propagation behavior under mode 1 and loading. A recent study on monotonic fracture behavior of 8090 Al-Li alloys has shown that these alloys possess lower fracture toughness under mode 3 loading as compared to mode 1 loading. The fracture resistance under mixed mode 1 - mode 3 loading was also found to be lower than that under pure mode 1 loading. However, no experimental data is available on the mixed mode 1--mode 3 fatigue crack growth behavior in Al-Li alloys. The objective of the present paper is to study the effect of combined mode 1--mode 3 loading on the fatigue crack growth behavior of an 8090 Al-Li alloy.