Fatigue Crack Growth Rate Curves Research Articles

Automated crack extension measurement method for fracture and fatigue analysis using digital image correlation

A novel Automated Crack Extension Measurement (ACEM) method based on combining decorrelation in strain field and a threshold near maximum of full-field strain obtained from digital image correlation, is developed in this work. A manual method for measuring crack extension from images of the specimen surface and ImageJ, was utilized to validate the accuracy of the proposed method. Initially, ACEM method was validated for crack extension under mode-I fracture of single edge notch specimens with different initial notch sizes. A satisfactory agreement was obtained from this analysis along with limits on the different parameter values used in ACEM. The method was then applied to low-cycle fatigue specimens with different initial notch lengths. Comparison of crack extension between ACEM and manual method reveals satisfactory agreement reinforcing the fidelity of the developed method for fracture and fatigue analysis. Fracture resistance and fatigue crack growth rate curves were also obtained for the different specimens.

High-throughput experimental method for measuring fatigue crack growth rate curve of soft materials

The fatigue properties of soft materials, including fatigue crack initiation and propagation, fatigue life and threshold, have been a significant concern in the past few decades. Measuring the fatigue crack growth rate (FCGR) curve is a standard method to characterize fatigue fracture and has been extensively studied in the literature. Conventional methods employing a pure shear configuration measure only one data point on the FCGR curve per test run. Thus, obtaining the whole FCGR curve with numerous data points becomes excessively time-consuming. This work proposes a high-throughput experimental method for measuring the FCGR curve of soft materials. We prepare ten thermoplastic polyurethane (TPU) specimens in single edge notch tensile configuration with different precut crack lengths, use a mechanical mechanism to apply the same cyclic loads to the ten specimens simultaneously, and identify the crack growth of each specimen by image processing. The proposed high-throughput experimental method enables the measurement of over 90 data points on the FCGR curve in a single test run, facilitating the acquisition of the entire curve with just two runs. The obtained FCGR curve exhibits a linear relationship within its middle region on double logarithmic coordinates. The proposed method is further applied to study the effect of temperature on the FCGR curve of TPU specimens. The current experimental setup can be further optimized to accommodate a wider range of soft materials with diverse mechanical properties and specific loading conditions.

Microstructure, fatigue crack growth rate and low-cycle fatigue behavior of an Al[sbnd]Li alloy sheet with different aging

The microstructures, fatigue crack growth (FCG) rates and low-cycle fatigue (LCF) behaviors of 1445 Al-Li alloy sheet with T6 aging at 170 °C, single-step T8 (S-T8) aging at 150 °C and double-step T8 (D-T8) aging at 170 °C following aging at 150 °C for 4 h were investigated. The main precipitates in T6 and S-T8 samples are δ′ phases, but strain strengthening causes 40 MPa enhancement in the S-T8 sample yield strength (YS). Due to coherent shearable δ′ precipitates, the FCG rate curves of S-T8 samples are highly coincident. With the 2nd-step aging time extension, δ′ precipitates in D-T8 sample gradually coarsen, semi-coherent S′ phases form and their amount increases. The interaction between δ′ precipitates with dislocations transites from shearing to by-passing, leading to the YS enhancement and the FCG resistance decrease. Meanwhile, cyclic-hardening decreases and transfers to cyclic-softening during LCF at starin amplitude of 0.6% ∼ 0.9%. At 0.9% strain amplitude, due to greater cyclic-softening and cleavage fracture of S′ precipitates, the D-T8 sample with 2nd-step aging time of 120 h has a shorter LCF life. The D-T8 sample with 2nd-step aging time of 24 h possesses a comprehensive property of higher strength, lower FCG rate and better LCF life.

On Cyclic-Fatigue Crack Growth in Carbon-Fibre-Reinforced Epoxy-Polymer Composites.

The growth of cracks between plies, i.e., delamination, in continuous fibre polymer matrix composites under cyclic-fatigue loading in operational aircraft structures has always been a very important factor, which has the potential to significantly decrease the service life of such structures. Whilst current designs are based on a 'no growth' design philosophy, delamination growth can nevertheless arise in operational aircraft and compromise structural integrity. To this end, the present paper outlines experimental and data reduction procedures for continuous fibre polymer matrix composites, based on a linear elastic fracture mechanics approach, which are capable of (a) determining and computing the fatigue crack growth (FCG) rate, da/dN, curve; (b) providing two different methods for determining the mandated worst-case FCG rate curve; and (c) calculating the fatigue threshold limit, below which no significant FCG occurs. Two data reduction procedures are proposed, which are based upon the Hartman-Schijve approach and a novel simple-scaling approach. These two different methodologies provide similar worst-case curves, and both provide an upper bound for all the experimental data. The calculated FCG threshold values as determined from both methodologies are also in very good agreement.

An alternative and robust formulation of the fatigue crack growth rate curve for long cracks

Crack growth simulations are an important method for estimating the remaining service life of components and structures with cracks under cyclic loading. In particular, the evaluation of cracks emanating from clinched joints is a major challenge because the fracture mechanical properties of the sheet materials can change locally due to the joining process. Special specimens are used to determine the relevant crack growth rate curves. For the application of these curves within simulation programs, they are described using crack propagation equations. This requires an accurate formulaic description of the entire crack curve with clearly identifiable parameters that unambiguously describe certain characteristics of the crack growth rate curve. Known approaches either describe only parts of the crack growth rate curve or the single parameters affect different characteristics of the curve and thus influence each other. Therefore, a new alternative and robust mathematical fit of the crack growth rate curve is developed and validated with experimental results.

Fatigue Crack Growth Evaluation of Pipeline Steels and Girth Welds

Abstract Fatigue test specimens were prepared and tested with an API 5 L X70 spiral welded pipe steel and girth weld. For a few selected specimens, two unloading compliance techniques (elastic compliance and back-face strain compliance) were applied simultaneously to a single specimen for direct comparisons of in situ crack size estimation. This paper also includes fatigue crack growth rate (FCGR) data of other pipe steels and welds available in the literature. It was observed that most FCGR curves of pipeline steels (X65–X100) remained within the BS 7910 mean and upper bound design curves in the Paris region. It was noted that the FCGR of austenitic stainless pipe steel and girth weld obtained from Arora et al. (2014), “Fatigue Crack Growth Studies on Narrow Gap Pipe Welds of Austenitic Stainless Steel Material,” Procedia Eng., 86, pp. 203–208) showed a very superior fatigue property.

Fracture parameters analysis of compact tension specimens with deflected fatigue cracks: ZK60 magnesium alloy

Precise monitoring of crack growth and a comprehensive understanding of crack propagation rate are crucial for the evaluation of the structural integrity of magnesium alloys with defects. Different crack deflections caused by the coupling of specimen orientations and crack sizes significantly increase the challenge to magnesium alloy evaluation. In this paper, the systematic finite element analysis was conducted for compact tension (CT) specimens with deflection cracks. A wide range of geometric variations including initial crack length, crack deflection angle and crack propagation length were analyzed. Complete solutions of the stress intensity factor (KI and KII), T-stress and the compliance at the load line were determined. The results showed that the geometric sizes have a significant coupling effect on the fracture mechanics parameters, and the non-deflected cracked formula recommended by the ASTM standard is not applicable to the deflection crack specimens. Then, a prediction method of kinked fatigue crack length of CT specimen based on the compliance multiplier method was proposed. To further verify the importance of the fracture parameters obtained, CT fatigue crack growth (FCG) tests of ZK60 magnesium (Mg) alloy under different orientations and initial crack sizes were carried out. The FCG rate curves of ZK60 Mg alloy under different crack growth driving forces were compared and illustrated. The developed compliance multiplication method can accurately predict the real-time crack propagation length.

Effect of laser parameters on fatigue crack growth behavior of AZ31B magnesium alloy TIG welded bionic joint

The purpose of this study was to evaluate the effect of laser parameters on the fatigue crack growth (FCG) behavior of AZ31B Magnesium alloy TIG welded bionic joints with ‘soft-hard’ alternating structures prepared by laser bionic treatment (LBT). The changes of laser parameters promoted the growth of fine equiaxed grain with random texture, and average grain size was 5.12 ± 0.51–8.19 ± 1.02 μm, which significantly increased isotropy and microhardness. The average microhardness was 66.5 ± 1.68–70.85 ± 1.41 HV0.1, which increased by more than 23.11% compared with untreated welded joints. For bionic welded joints, the heat-affected zone (HAZ) had higher FCG resistance and cycle times than weld metal (WM), and the specimens with notch in the HAZ showed obvious crack deflection. It was found that FCG behavior showed significant microstructure sensitivity by comparing the FCG rate curve and crack opening displacement. The Paris parameters had a significant linear relationship and effectively reflect the FCG resistance of the different specimens. The fatigue fracture showed that the crack growth zone was mainly dominated by typical cleavage steps and secondary cracks. The fatigue fracture region contains quasi-cleavage and ductile fracture. A series of strengthening mechanisms were induced by LBT and could significantly affect FCG behavior, which promoted crack deflection and branching to increase the toughness of the joint, while the laser remelting zone increased the strength of the joint. Therefore, the synergistic effect of strength-toughness of the joint was obtained by LBT, which provided a new idea for optimizing the bionic design and improving the fatigue property of welded joints.

Characterization and analysis of the mode I interlaminar fatigue behaviour of thermoplastic composites considering [formula omitted]-curve effects

Through the application of innovative production processes, thermoplastic composites might help the aviation industry become more sustainable. However, there is currently not much experimental understanding on the fatigue behaviour, and validated analysis methodologies on thermoplastic composites are rather limited. In this work, the fatigue onset and propagation interlaminar properties of AS4D/PEKK-FC thermoplastic composite were characterized under mode I loading. Testing costs were reduced by using a multi-fatigue testing rig that allows loading 6 specimens at the same time. The offset in the fatigue crack growth rate curves tested at different severities was explained by the fatigue R-curve effects. The offset was modelled by means of superposed fatigue cohesive laws. Different approaches on how to consider fatigue damage were compared with experimental data, giving an insight on how fatigue damage evolves, and developing a novel modelling strategy based on a robust method for fatigue model parameter identification for fatigue delamination when several failure mechanisms interact.

Microstructure and Fatigue Performance of Ti6Al4V Produced by Laser Powder Bed Fusion after Post-Heat Treatment

With the development of additive manufacturing (AM), the Ti-6Al-4V alloy manufactured by laser powder bed fusion (LPBF) is becoming more widely studied. Fatigue fracture is the main failure mode of such components. During LPBF processing, porosity defects are unavoidable, which hinders the exploration of the relationship between fatigue performance and microstructure. In this study, a laser remelting method was used to reduce porosity defects inside the Ti-6Al-4V alloy. Three annealing treatments (AT) and three solution-aging treatments (SAT) were used to study the effect of the two-phase zone (α + β) microstructure on fatigue life and fatigue crack growth behavior. Fatigue life and crack growth rate (CGR) curves were obtained, and fatigue fracture surface and crack growth fracture surface were analyzed. The results show that microstructure influences fatigue life but has little effect on CGR. Compared with the as-built specimen, the fatigue life of the AT and SAT specimens increased significantly at 850℃ by 101 and 63.7 times, respectively. The thickness of the α lath and the location of crack nucleation together affect the fatigue life. In the stable growth stage, the layered microstructure of α colonies is the most resistant to crack growth.

An Improved Unique Fatigue Crack Growth Rate Curve Model and Determination of the Model Shape Exponents

An Improved Unique Fatigue Crack Growth Rate Curve Model and Determination of the Model Shape Exponents

A new effective stress intensity factor approach to determine thickness-independent fatigue crack growth rate curves

In this paper, it is experimentally demonstrated that the use of standardised methods to characterise the crack growth rate curves of metallic materials conforms to thickness-dependent results. To correct that dependency, the contribution of the stress intensity factor and crack closure on the effective stress intensity factor range is assessed by comparing analytical and numerical alternatives of calculation, which consider crack shapes experimentally determined. The findings reveal that (i) the crack closure is a thickness-independent edge effect, which allow developing analytical equations to determine the crack closure distribution along the thickness, and that (ii) the distribution of the effective stress intensity factor range is more influenced by crack closure than the maximum stress intensity factor. With the aim of obtaining a superposition of fatigue crack growth curves from different specimen thicknesses, an approach to calculate thickness-independent effective stress intensity factor range (ΔKeff,TI) is proposed and demonstrated.

Influence of microstructural deformation mechanisms and shear strain localisations on small fatigue crack growth in ferritic stainless steel

Microstructurally small fatigue crack growth (FCG) rate in body-centred cubic (BCC) ferritic stainless steel is investigated by using a novel domain misorientation approach for EBSD microstructural deformation analyses, in conjunction with in situ digital imaging correlation (DIC). The DIC analyses revealed that shear strain localisations occur ahead of the crack tip during propagation and correlate well with the FCG rate retardations. Grain boundaries can be found at both peaks and valleys of the FCG rate curve and alter the interaction between crack growth and shear strain localisations. At the microstructural level, the deformation is associated with the dislocation-mediated plastic deformation process, showing increased formation of grain sub-structures in the regions of the strain localisation. Consequently, material experiences local hardening causing the FCG retardation events. If the crack avoids the hardened material region through a macroscopic cross-slip mechanism, retardation is minor. On the contrary, if the crack penetrates the hardened region, retardation is significant.

Revealing the fatigue crack propagation mechanism of a Ni-based superalloy electron beam welded joint through in-situ SEM observation

In this study, in-situ scanning electron microscope (SEM) fatigue crack propagation experiments for electron beam welded joints of GH4169 Ni-based superalloy are carried out. Based on material characterization and analysis, the corresponding fatigue crack propagation mechanism of the welded joint (WJ) is investigated. Compared to the base material (BM), the WJ has comparable static strength, but worse plasticity. Significantly, the WJ has longer fatigue crack growth life than that of the BM. The fatigue crack growth rate (FCGR) curves of the WJ and the BM intersected with each other. In particular, under low stress intensity factors (ΔK) level, the FCGR of the WJ is lower than that of the BM. Under high ΔK level, the FCGR of the WJ is higher than that of the BM. The steep FCGR curve of the WJ can be attributed to the unique weld texture and a shielding effect caused by the weld strength mismatch.

Distribution of the through-thickness effective stress intensity factor range and its influence on fatigue crack growth rate curves

This paper focuses on an analysis of the through-thickness distribution of the effective stress intensity factor range of compact tension specimens, aiming to improve the characterisation of fatigue crack growth rate propagation curves. The maximum stress intensity factor and opening load are evaluated by standard analytical methods and three-dimensional numerical models for various crack lengths and crack front shapes to assess the relevance of considering real crack shapes. The findings reveal that standard procedures attributes only one value of velocity and one value of the fracture parameter for the entire front, whereas numerical methods report different values for both parameters along the thickness. Three-dimensional response surfaces are modelled and validated to estimate the fracture parameters, showing an incongruity in the use of fatigue crack growth rate curves obtained by standard two-dimensional procedures to assess the 3D crack growth.

The effect of crack length and maximum stress on the fatigue crack growth rates of engineering alloys

The fatigue crack growth rate (FCGR) curve of metallic alloys is usually divided into three regions. Region II is often referred to as the Paris regime and is usually modelled with a power law relationship with a single exponent. Regions I and III are located at the beginning and end of the FCGR curve, respectively, and are frequently modelled with asymptotic relationships. In this paper we hypothesize that fatigue crack growth is governed by power law behaviour at all crack lengths and all stress intensity factor ranges (ΔK). To accommodate for the changes in the FCGR slope at regions I - III mathematical pivot points are introduced in the Paris equation. Power law behaviour with the presence of pivot points enables direct fitting of the crack length vs. cycles (a-N) curve to obtain the FCGR as a function of ΔK. This novel approach is applicable to small and long crack growth curves and results in accurate multilinear FCGR curves that are suitable for reconstruction of the measured a-N curves. The method is subsequently applied to i) different alloys to show local changes in the FCGR curve for changes in alloy composition and heat treatments, ii) naturally increasing ΔK testing of microstructurally small cracks to obtain accurate small crack FCGR data. The comparison with accurate long crack data shows that small cracks are faster, but the transition from region I to region II occurs at a specific fatigue crack growth rate which results in an apparent shift in ΔK at the transition. iii) Long cracks, which show that the FCGR increases with maximum stress for a given ΔK and stress ratio when the maximum stress approaches the yield stress. The maximum stress phenomenon becomes important in the case of fatigue testing, where the initial crack lengths are usually small and maximum stresses are high. It is concluded that for long cracks the phenomenon explains why the Paris equation is applicable rather at low maximum stress, while the Frost-Dugdale equation is more applicable at high maximum stress.

Three-Dimensional Surface Crack Growth of Maraging Steel Spherical Pressure Shell

This study focused on the three-dimensional surface crack growth of a spherical pressure shell. Eight maraging steel 18Ni (250) samples were fabricated and tested, and the fatigue crack growth rate curves were obtained. Considering the influence of plastic closure effect and sample thickness on crack growth, the fitting formula of fatigue crack growth only related to materials was obtained. Based on the three-dimensional crack closure theory and the strip yield model, a three-dimensional surface crack growth model of spherical pressure shell was established. By using a self-written program and FRANC3D, the three-dimensional surface crack growth simulations of the spherical shell were completed. The influence of the initial shape ratio and initial depth of the crack on the crack growth and the fatigue life of the spherical shell was analyzed.

Warm deformation enhances strength and inhibits hydrogen induced fatigue crack growth in metastable 304 and 316 austenitic stainless steels

The effect of deforming temperature on fatigue crack growth rate (FCGR) was investigated for cold/warm deformed 304 and 316 stainless steels under 5.0 MPa hydrogen and argon gas atmospheres. The slope of the FCGR vs ΔK curve in both steels declined with increasing deforming temperature, especially in type 304 steel. Strain-induced α′ martensite was prone to flat-facetted, step-like and quasi-cleavage fracture. Warm deformation (400 °C) can inhibit the formation of α′ martensite around crack tip and cause cellular dislocation structures, which led to multi-directional crack pathway and lower FCGR. The microstructure was adjusted by warm deformation, and then hydrogen embrittlement resistance was improved on the basis of high-strength in metastable austenitic stainless steels.

Fatigue crack growth behavior of Ti-6Al-4V alloy fabricated via laser metal deposition: Effects of building orientation and heat treatment

This work studied the effect of building orientations and post-process heat treatments on the fatigue crack growth behavior of Ti-6Al-4V manufactured by laser metal deposition. The fatigue crack growth rate experiments were carried out in three sampling directions (YZ, ZY, XY) in order to evaluate the anisotropy. In addition, the heat-treated samples (YZ, ZY, XY) were used for comparison, which were heat-treated under the vacuum at 900 ℃ for 2 h followed by air cooling and 550 ℃ for 4 h followed by air cooling. The results showed that the ∆KT values before and after heat treatment were 13.6 and 15 MPa∙m1/2, respectively. In the hypo-transitional region, the effect of columnar grains orientation and uniaxial residual stress led to anisotropy in fatigue crack growth rate for as-built specimens. This effect is significantly attenuated after the post-process heat treatment. The coarsened α laths effectively changed the crack propagation behavior and improve the fatigue crack growth resistance of the material. In the hyper-transitional region, the fatigue crack growth was controlled by the inherent fatigue crack growth resistance rather than the microstructure, resulting in negligible differences in all fatigue crack growth rate curves.

Mechanical properties and fatigue crack growth in tungsten deposited by RF-plasma

Fracture toughness and fatigue crack growth rates of free-standing thick W deposits produced in a protective atmosphere using radio frequency inductively-coupled plasma process were investigated. Stress-strain behavior of the deposits was studied using four-point bending and their fracture toughness was estimated from three-point bending following the ASTM E1820 standard. The fatigue crack growth rates were measured in symmetrical bending using a resonance approach. The results were compared with commercially available bulk tungsten prepared by powder metallurgy that served as a reference. The results showed that the deposits had significantly lower stress-strain properties: the elastic modulus and tensile strength ranged from 30% to 50% of the reference bulk values. The crack growth rate tests together with fractographic analysis suggested that a static failure by inter-granular decohesion predetermines the fatigue properties of the deposits and causes high slopes of the corresponding fatigue crack growth rate curves. Contrary to this, the reference sheet failed trans-granularly. The inter-granular failure mode of the deposits was caused by grain boundary embrittlement, a phenomenon probably related to oxides. The lowest obtained oxygen concentration in deposit was nearly by two orders of magnitude higher, than in reference sheet. The distribution of oxygen was studied by TEM and the amorphous and crystalline oxide phases were identified at splat interfaces. The presence of oxygen in the deposits produced by the inherently non-oxidizing RF technology calls for control of oxygen content in the feedstock powder. The performed characterization provided baseline data of mechanical and fatigue properties of RF plasma sprayed tungsten.