Crack Retardation Research Articles

Effect of notch orientation and single overload on crack interaction in Ti–6Al–4V alloy

In this study, the effect of notch orientation and a single overload on the fatigue crack propagation behavior of Ti–6Al–4V alloy was investigated. The notches were formed in two different forms: symmetrical and asymmetrical. Two types of fatigue crack propagation tests were applied to the specimens, with constant amplitude and overload. It was determined that the increased notch angle delays the formation of crack at notch tip and causes the crack propagation rate to decrease. In asymmetric notched specimens, crack formation and crack propagation rate vary depending on the notch angle. At the edge where the notch angle is small, the crack is formed in a shorter time and the crack propagation rate is also high. The KI values decreased with increasing the crack angle, while the KII values showed only change in the threshold region due to crack angle. The crack growth rate in all specimens decreased with the application of a single overload, and the rate of crack retardation after overload increases with increasing notch angle.

On the role of Cu-Zr amorphous intergranular films on crack growth retardation in nanocrystalline Cu during monotonic and cyclic loading conditions

In the present study, molecular dynamics simulation was used to investigate the influence of amorphous intergranular films (AIF) and its thickness on possible crack retardation in nanocrystalline Cu under tensile and cyclic loading conditions. Structural analyses were carried out based on different methods such as centro-symmetry parameter, common neighbor analysis, and atomic strain. Results showed that specimens with AIF thickness as small as 0.5 nm can restrict the propagation of both perpendicular and parallel cracks during uniaxial tensile deformation. By increasing the AIF thickness, the crack propagation was found to be further retarded due to crack tip blunting. Atomic strain analysis also revealed that thicker AIF localized the strain distribution more strongly thus inhibiting the nucleation of multiple intergranular cracks and subsequent rapid fracture. In case of cyclic loading, the strain was accumulated through the formation of twins/stacking faults when the AIF thickness was 0.5 and 1 nm, which can interact with the AIF and eventually cause the nucleation of voids/cracks. However, increasing the AIF thickness to 2 nm can restrict such mechanism as the strain was mostly accommodated by the amorphous phase.

Cavitation shotless peening effects on fatigue crack growth behaviour under bending loads

The effects of partial surface shotless peening on the fatigue crack growth behaviour of a ferritic steel have been experimentally investigated in this paper. Seven fatigue tests were performed on four-point bend specimens fabricated from Optim700QL high-strength steel. Three distinct extents of partial cavitation shotless peening, with respect to the crack tip and specimen symmetry line, were applied on the specimen geometry. Alternating current potential difference (ACPD) was used to measure and monitor the crack depth during fatigue tests. The tests on partially surface peened specimens were conducted under two different load levels. The fatigue crack growth results from these experiments on partially peened specimens have been presented and compared with the results from an additional test on an unpeened specimen. The results show that the residual stress distribution fields formed because of the peening process play a significant role in the fatigue crack shape evolution, crack retardation, and crack propagation behaviour of the material.

Fatigue crack propagation along interfaces of selective laser melting steel hybrid parts

AbstractSelective laser melting (SLM) is an emerging additive manufacturing technology, capable of producing complex geometry components. The current work studied both the effect of substrate material and mean stress on the fatigue crack growth behaviour along interfaces of bi‐material specimens, substrate, and part by SLM. Fatigue tests were carried out in agreement with ASTM E647 standard, using 6‐mm‐thick compact specimens. The substrate steel has only a negligible effect both on the fatigue crack propagation rate and on the crack path. The failure occurs in the material additively manufactured by SLM, near the interface. The mean stress produced only a reduced influence on the fatigue crack propagation rate in the Paris regime. For larger values of ΔK, where Kmax approaches KIc, a significant influence of the mean stress was observed. In spite of nondetection of crack closure, the application of overloads promoted significant fatigue crack retardation, quite similar for both substrate materials, probably due to the crack bifurcation during the overload.

Effect of oxidation on crack propagation of Si nanofilm: A ReaxFF molecular dynamics simulation study

The effect of oxidation and oxide film on mechanical properties and fracture mechanisms of Si nanostructure is an important factor for the design and application of micro- and nano-scale devices but still remain several debates. Reactive force field molecular dynamics (ReaxFF MD) simulation provides a practical way to investigate such an issue, by which we focused on the role of initial stage oxidation on crack propagation of Si nanofilm. The fracture strain and fracture stress were increased by simultaneous oxidation during uniaxial tension, displaying that the crack propagation retarded and initiated from Si/SiOx interface. The modification of stress field due to oxidation may play as a main part on related strengthening behavior, i.e. the blocking of compression in oxide film and the tension relaxation at crack tip. Moreover, we found that the crack retardation may be associate to oxide film morphology, which indicating further retardation by a high quality oxide film with smoother interface. The mechanism detailed here explained previous simulation and experiment on oxide driven strength evolution of Si surface and may shed a light on strength design of nanofilm with oxide layers.

Analysis of Mode-I crack growth arresting mechanism in curved laminated joint

The critical damage for carbon fiber structures, which are being employed in aircraft structures, is known as delamination/crack. Fasteners are commonly installed to arrest the delamination by clamping the laminate together. Fasteners are installed in each corner of the delaminated zone to provide significant arrest capability, shifting the failure mode away from delamination under most conditions. The Finite Element Analysis (FEA) model is constructed to study the effectiveness of the fastener as crack arrest mechanism. The FEA results show that the fastener provides significant crack retardation capability in Mode-I condition. An analytical model is developed for the delamination embedded between the skin and stiffener interface of the joint. The fasteners are modeled with spring elements. The analysis is solved with Virtual Crack Closure Technique (VCCT) approach. The primary objective of the current research work is to enhance the safety of bonded joint by providing arrest mechanisms.

Cruciform specimens used for determination of the influence of T-stress on fatigue crack growth with overloads on aluminum alloy Al 6061 T651

The publication presents a cruciform specimen for the determination of cyclic crack growth data under biaxial loading. The design of the specimen with slotted loading arms allows good decoupling between the two loading directions. For different initial crack geometries, the solutions for the stress intensity factors KI and KII as well as the crack-parallel T-stress are calculated by linear elastic finite element analysis (FEA) with the program ABAQUS. For two specimens with the same geometry made of aluminum alloy 6061 T651, the crack growth behaviour is measured at different T-stresses at a stress ratio of R=0.7 and overloads. It is shown that the crack retardation after an overload with crack-parallel tensile stress is less than without it. The reason for this behaviour is considered to be the reduced plasticity at the crack tip due to the higher triaxiality of the stress state.

Microstructurally-sensitive fatigue crack growth in HCP, BCC and FCC polycrystals

Microstructurally sensitive fatigue crack growth in four material systems with BCC, FCC and HCP crystallography was investigated through integrated crystal plasticity eXtended Finite Element (XFEM) modelling and experiment. The mechanistic drivers for crack path tortuosity and propagation rate have been investigated and crack propagation found to be controlled by crack tip stored energy and the crack direction by anisotropic crystallographic slip at the crack tip. Experimentally observed microstructurally-sensitive fatigue crack path tortuosities and growth rates in titanium alloy (Ti–6Al–4V), ferritic steel, nickel superalloy and zirconium alloy (zircaloy 4) have been shown to be captured, supporting the underpinning mechanistic arguments. Very short crack growth is dominated by local slip, but with increasing length, crack tip stresses begins to predominate, increasing the availability of slip systems and giving smaller amplitude oscillations between slip systems. This leads to overall crack paths which are in fact crystallographic but which appear not to be. Key features of crack retardation at grain boundaries, changes in rate resulting from crystallography, and intragranular crack path deflections have been experimentally observed and captured.

An online‐offline prognosis model for fatigue life prediction under biaxial cyclic loading with overloads

AbstractThis paper presents a robust online‐offline model for the prediction of crack propagation under complex in‐phase biaxial fatigue loading in the presence of overloads of different magnitudes. The online prognosis model comprises a combination of finite element analysis and data‐driven regression to predict the crack propagation under constant loading, while the offline model is trained using experimental data to inform the post‐overload crack growth retardation behavior to the online model. The developed methodology is validated by conducting biaxial fatigue experiments using aluminum AA7075‐T651 alloy cruciform specimens. A close correlation is observed between the experimental results and model predictions. The results show that the model successfully predicts the crack retardation behavior under the influence of overloads with different magnitudes occurring at different stages of fatigue crack growth. Error analysis is conducted to investigate the sensitivities of the number of training points and crack increments to the prediction accuracy. In addition, the error propagation with respect to the crack length is studied, which provides constructive suggestions for further model improvement.

Improvement mechanisms of fatigue strength in weld joints by hammer peening

The hammer peening process is well known as one of the methods to improve fatigue lives in welded joints. Peening induces a compressive residual stress field near the weld toe in which fatigue crack usually initiates. When this method is applied to large scale welded strictures, the effects of the induced compressive residual stress on fatigue crack initiation stage and propagation stage should be separately comprehended because the balance of both stages in total fatigue life will be changed for different structure scales. In this study, the improvement mechanisms in fatigue crack initiation and propagation lives due to a compressive residual stress field was clarified by experimental observations of crack propagation behavior. The morphologies of propagating surface crack front were quantified in the specimens of plate and gusset welded joint with and without the hammer peening process. The experimental results suggested that the deeper aspect ratio of an elliptical surface crack just after crack initiation and propagation in the peened specimens induced fatigue crack retardation. Fatigue crack retardation in the peened specimens was discussed to clarify the fatigue crack life improvement mechanisms from the viewpoint of the stress intensity factor range suppression due to the crack aspect ratio.

Mechanical Treatment of Crack-Arrest Holes Subjected to Distortion-Induced Fatigue

Fatigue cracking is a major concern for many aging steel bridges in the United States. A common form of crack remediation is the drilling of crack-arrest holes, increasing the crack tip radius and thus reducing the applied stress intensity. However, the majority of cracking that occurs in bridge structures is caused by distortion-induced fatigue. Cross-frame forces generated by differential girder displacement produce multiaxial, mixed-mode fatigue loading at the intersection of the girder web, flange, and connection plate, producing cracking along web-to-stiffener welds and flange-to-web welds. Crack-arrest holes are often used as a first line of defense at these cracks. However, crack-arrest holes deform out-of-plane under distortion-induced fatigue loading, and have been found to be of limited utility in this application.Previous research has shown that mechanical treatment of crack-arrest holes has the potential to improve the efficacy of crack-arrest holes for halting crack propagation for in-plane fatigue, but the performance of such treatments is unknown for distortion-induced fatigue cracks. A research program sponsored by the Kansas Department of Transportation is investigating the effectiveness of one such technology for controlling crack propagation in steel bridge girders susceptible to distortion-induced fatigue. This technology produces a cold-worked region around the crack-arrest hole, introducing a field of compressive residual stresses aimed at retarding crack propagation. Evaluation of the mechanically treated crack-arrest holes is evaluated analytically for both in-plane and out-of-plane fatigue loading. This paper presents background on the problem and preliminary findings of the study.

A Study for Brittle Crack Prevention and Fatigue Life Improvement using a Welded Stiffener

Recently, thickness of steels used in ships is increasing along the increased size of vessels, e.g. container carriers in particular. In container vessels, thick plates are applied to hatch side coaming areas, and safety is also becoming important as steel plate thickness increases. Many studies are conducted on the brittle crack arrest characteristics to prevent catastrophic failure in those areas. A number of experiments have been conducted for investigating propagation characteristics of brittle crack from classification societies. Especially, ESSO test method is commonly employed to analyze the BCA (Brittle Crack Arrest) toughness. In order to prevent brittle crack, arrest holes, weld line shift and insert plates are suggested.Various researches have been carried out to arrest brittle crack in BCA steel. However, the actual structure thickness has to be applied to the experiment, and it is difficult to carry out experiments due to the large scale specimens. Thus, in this study, crack retardation method is researched considering the thickness, length, and location of a welded stiffener. Finite element analysis was conducted on the hatch side coaming to compare the allowable crack length according to the effect of stiffener and to compare fatigue life based on the characteristics of BCA steel. Key words: Brittle crack arrest steel, Brittle crack arrest toughness, ESSO test, Fatigue crack growth rate, Stiffener

Multiscale designs of the chitinous nanocomposite of beetle horn towards an enhanced biomechanical functionality

Operating mainly as a type of weapon, the beetle horn develops an impressive mechanical efficiency based on chitinous materials to maximize the injury to opponent and simultaneously minimize the damage to itself and underlying brain under stringent loading conditions. Here the cephalic horn of the beetle Allomyrina dichotoma is probed using multiscale characterization combined with finite element simulations to explore the origins of its biomechanical functionality from the perspective of materials science. The horn is revealed to be highly regulated from the macroscopic shape, geometry, and connection with the body to the meso- and microscopic architecture, moisture content, and chemical and structural characteristics. Varying kinds of gradients are integrated at all length-scales. Such designs are demonstrated to benefit the mechanical performance by mitigating stress concentrations, retarding crack propagation, and modulating local properties to better adapt to stress. Enhanced rigidity, robustness and stability are additionally generated from the constrained flexibility endowed by the nanocomposite plywood structure through the reorientation of chitin nanofibrils within the proteinaceous matrix. These findings shed light on the intriguing materials-design strategies of nature in creating synergy of offence and persistence. They may even offer inspiration for the synthesis of high-performance materials and structures, in particular beams to resist bending and torsion.

Deformation compatibility between nanotwinned and recrystallized grains enhances resistance to interface cracking in cyclic loaded stainless steel

Cracks often initiate at phase boundaries in conventional second phase reinforced alloys during cyclic loading, which limits their fatigue properties. Here, we prepared a nanotwin strengthened 316L stainless steel consisting of nanotwinned and recrystallized grains by using plastic deformation and subsequent partial recrystallization annealing. Fatigue tests revealed that interfaces separating hard nanotwinned grains from soft recrystallized ones exhibited excellent resistance to crack initiation. More than half of the cracks (57% in number fraction) are found in recrystallized grains while a small fraction (11%) is observed at the interfaces between nanotwinned and recrystallized grains. This is ascribed to the elastic homogeneity and cyclic deformation compatibility between nanotwinned and recrystallized grains. At small cumulative cyclic strains (below 4000 cycles at σa = 450 MPa), nanotwinned grains deform compatibly with the recrystallized grains without noticeable strain localization at their interfaces. Nanotwins can accommodate cyclic plastic strains by interaction of dislocations with twin boundaries, especially through the motion of the well-ordered threading dislocations inside the twin lamellae. At large cumulative strains, a moderate strain gradient is developed in recrystallized grains surrounding nanotwinned grains as a function of distance from the interfaces due to the occurrence of localized deformation in nanotwinned grains. The nanotwinned grains show high microstructural stability without notable de-twinnning, thus retarding crack initiation and propagation. Therefore, improved fatigue property with high fatigue limit of ∼350 MPa and high fatigue ratio of ∼0.45 is achieved in the nanotwin strengthened stainless steel, which is better than that of conventional second phase reinforced steels with comparable strength.

Quantifying fatigue overload retardation mechanisms by energy dispersive X-ray diffraction

The fatigue crack retardation mechanisms operating after an overload event are investigated for a bainitic steel using high spatial resolution energy dispersive synchrotron X-ray diffraction. The elastic crack-tip strain fields are mapped at mid-thickness of compact tension samples at R-ratios of 0.1 and 0.4. The same overload stress intensity factor (KOL = 60 MPa m1/2) is applied in each case with the cracks then propagating under the same applied stress intensity range, ΔKapp = 27 MPa m1/2. The competing retardation mechanisms are directly quantified and separated, with the associated fatigue crack growth (FCG) rates then being predicted according to a 2-parameter Walker-type assessment and validated against those measured. The stress intensity factor associated with the overload residual stress field is calculated using a weight function approach. For R=0.1, shielding from residual stress controls retardation when crack growth through the overload plastic zone, rpOL, is small (specifically <0.6rpOL). For more extensive crack growth, discontinuous crack closure controls the retardation behaviour, with significant load transfer across opposing crack faces being observed at minimum load (for R=0.1). These crack face tractions are associated with the plastic asperity created during overload. The traction forces holding the crack faces open at minimum load are, for the first time, used to directly quantify the associated stress intensity factor, Kmintract as a function of crack growth. While no crack shielding is expected, nor observed, for R=0.4, the variation in FCG rate after overload is explained by changes in effective R-ratio.

Laser-induced heating for enhanced fatigue life of aerospace aluminum alloys

According to airworthiness requirements, the primary structure of a commercial aircraft must be designed to be damage tolerant in order to guarantee the aircraft structural integrity. For this reason, the crack propagation analysis of metallic materials is performed during the structural design development. It is known that compressive residual stresses enhance the performance of structural components by retarding the crack propagation rate, extending the fatigue life of such components. For this reason, aircraft manufacturers may apply some process like mechanical peening, ball-burnishing and laser shock peening to improve the fatigue life of structural parts. This contribution aims to propose the laser surface treatment of aerospace aluminum alloys as a possible way to reduce the crack propagation rate in compact tension C(T) coupons. Since the laser technology has been already used in welding and cutting operations, the same setup could be used to produce discrete heating lines with low cost. Three types of aluminum alloys were analyzed Al–Zn–Mg–Cu (AA7475-T761), pure aluminum-coated Al–Cu (AA2024-T3) and Al–Si–Mn–Cu–Mg (AA6013-T4), for better understanding of phenomena related to the crack propagation. A carbon coating was proved to allow good absorptivity of the laser beam, since the aluminum surface is highly reflective to the fiber laser 1 µm radiation. It was verified that two laser tracks, with power 200 W, speed below 5 mm/s and beam diameter of about 2 mm, consistently results in a crack-retardant piece. According to the actual results, the laser processing barely doubled the fatigue life under some conditions. The main mechanism of crack retardation had been associated with the modification of stresses ahead of the crack tip, since the microstructure and hardness were almost unchanged after the laser treatment. The maximum measured compressive stresses ahead of the laser track for AA7475-T761, AA2024-T3 and AA6013-T4 were 30, 30 and 67 MPa, respectively.

Influence of lithium silicate coating on retarding crack formation in LiNi0.5Co0.2Mn0.3O2 cathode particles

LiNi0.5Co0.2Mn0.3O2 particles coated with lithium silicate were prepared via a precipitation method and their charge and discharge properties were investigated. The capacity retention and coulombic efficiency of the 0.5 wt% lithium silicate-coated LiNi0.5Co0.2Mn0.3O2 was improved at a high cut-off voltage of 4.6 V. Cross-sectional scanning electron microscope images revealed that extensive cracks were formed within the uncoated LiNi0.5Co0.2Mn0.3O2 particle along the grain boundaries after the charge and discharge cycling, while the crack generation was significantly inhibited for the coated particles. From cross-sectional transmission electron microscope analysis, it was shown that Si-rich regions were concentrated not only on the surface, but also at the grain boundaries even at a depth of 400 nm from the surface of the lithium silicate-coated sample. These results suggested that the Si-rich regions inhibited the crack formation and prevented the intrusion and the decomposition of the electrolyte solution within the particles. Consequently, lithium silicate coating is an effective way for improving the charge and discharge characteristics of LiNi0.5Co0.2Mn0.3O2.

Laser shock peening induced fatigue crack retardation in Ti-17 titanium alloy

Laser shock peening is an advanced surface treatment technique of great interest introducing beneficial compressive residual stress and further enhancing fatigue crack propagation resistance of metallic components. In this study, fatigue crack propagation and subsequent retardation of Ti-17 titanium alloy under laser shock peening are presented. Varying degrees of fatigue crack retardation were observed after peening with pulse energy of 20 J and 30 J. The fatigue life was increased up to 2.4 times that of the unpeened counterpart. The fatigue arrests were observed in the deceleration zone after peening, showing different angles with the fatigue crack path as the peening energy varied. The fatigue crack retardation mechanism based on the plastic zone size and crack propagation energy density drop at the crack tip was further discussed, and a crack tip energy density criterion was proposed to quantitatively understand the fatigue crack retardation.

The Possibility for Improving Damage Tolerance of Integral Airframe Structure by High Strength Bonded Straps

Integral stringer panels can attain weight reduction in primary aircraft structures, but do not contain the physical barriers for a fatigue crack growth. One of the promising techniques for prolonging a fatigue crack growth is bonded crack retarders made of materials with high stiffness. An experimental study was done on two specimens with different geometries. High strength bonded straps made of corrosion resistant steel AISI 301 were adhesively bonded to Center-Cracked Tension (CCT) specimens made of aluminium alloy 2024-T351 and fabricated by a high-speed machining process to promote fatigue crack growth retardation. Specimens were tested at a constant amplitude load. The study concludes that the fatigue crack growth life can be significantly improved. Experimental results were compared with a prediction based on the VCCT technique and the NASGRO equation.

In-situ observations of deformation twins and crack propagation in a CoCrFeNiMn high-entropy alloy

The interaction between crack propagation and twinning structures in an equi-atomic CoCrFeNiMn high-entropy alloy was investigated using in-situ transmission electron microscopy. Crack retardation mechanisms were strongly related with crack length. For a short and sharp crack, crack propagation was mostly retarded by crack deflection at the twin boundary and plastic relaxation resulting from twinning and detwinning. When a crack developed into a longer crack, its propagation was retarded by crack blunting and bridging effects at the deformation twin boundary.