Fatigue Crack Growth Rate of Ti-6Al-4V Considering the Effects of Fracture Toughness and Crack Closure

Fatigue fracture is one of the main failure modes of Ti-6A1-4V alloy,fracture toughness and crack closure have strong effects on the fatigue crack growth（FCG）rate of Ti-6A1-4V alloy.The FCG rate of Ti-6A1-4V is investigated by using experimental and analytical methods.The effects of stress ratio,crack closure and fracture toughness on the FCG rate are studied and discussed.A modified prediction model of the FCG rate is proposed,and the relationship between the fracture toughness and the stress intensity factor（SIF）range is redefined by introducing a correcting coefficient.Notched plate fatigue tests（including the fracture toughness test and the FCG rate test）are conducted to investigate the influence of affecting factors on the FCG rate.Comparisons between the predicted results of the proposed model,the Paris model,the Walker model,the Sadananda model,and the experimental data show that the proposed model gives the best agreement with the test data particularly in the near-threshold region and the Paris region,and the corresponding calculated fatigue life is also accurate in the same regions.By considering the effects of fracture toughness and crack closure,the novel FCG rate prediction model not only improves the estimating accuracy,but also extends the adaptability of the FCG rate prediction model in engineering.

Fatigue Life Evaluation Method for Foundry Crane Metal Structure Considering Load Dynamic Response and Crack Closure Effect

To compensate for the shortcomings of quasi-static law in anti-fatigue analysis of foundry crane metal structures,the fatigue life evaluation method of foundry crane metal structure considering load dynamic response and crack closure effect is proposed.In line with the theory of mechanical vibration,a dynamic model of crane structure during the working cycle is constructed,and dynamic coefficients under diverse actions are analysed.Calculation models of the internal force dynamic change process of dangerous cross-sections and a simulation model of first principal stress-time history are established by using the steel structure design criteria,which is utilised to extract the change of first principal stress of danger points over time.Then,the double-parameter stress spectrum is obtained by the rain flow counting method.The fatigue life calculation formula is corrected by introducing a crack closure parameter that can be calculated by the stress ratio and the effective stress ratio.Under the finite element model imported into Msc.Patran,crack propagation analysis is performed by the growth method in the fatigue integration module Msc.Fatigue.Taking the metal structure of a 100/40t-28.5m foundry crane with track offset as an example,the accuracy of calculation results and the feasibility and applicability of the proposed method are verified by theoretical calculation and finite element simulation,which provide a theoretical basis for improvement of the fatigue resistance design of foundry cranes.

A STUDY OF CRACK CLOSURE IN ELECTRIC-FIELD-INDUCED FATIGUE IN FERROELECTRIC CERAMICS

Considering the influence of the domain switching near the tip of a crack andapplying the idea of multiscale singularity fields in piezoelectric fracture, we have obtained anempirical criterion for the crack closure. Based on the domain switching in the electric yieldregion, referring to Yang's results on the small scale yield model for the electrical fatigue crack,a model of the crack closure during electric-field-induced fatigue is developed to analyze thecrack growth. In terms of the model we have obtained the formula of the rate of the crack growthunder cyclic electric loading. Finally we compare the theoretical predictions with the results givenby Cao and Evans experimentally. It should be pointed out that the model proposed is empirical andneeds to be verified by more experimental results.

Study on Crack Closure Considering Welding Residual Stress Under Constant Amplitude Loading and Overloading

Welding residual stress in the engineering structure has a non-negligible influence on crack propagation,and crack closure is a significant factor affecting the crack propagation.Based on the elastoplastic finite element method and crack closure theory,we studied crack closure and residual compressive stress field of butt-welded plates under constant amplitude loading and overloading regarding the stress ratio,maximum load,overload ratio,and number of overloads.The results show that the welding residual tensile stress can decrease the crack closure because of a decrease in the residual compressive stress in the wake zone,but the effect is gradually reduced with increased stress ratio or maximum load.And the combined effect of welding residual tensile stress and overload can produce a stronger retardation effect on crack propagation.

EFFECT OF CRACK CLOSURE ON SLOW PROPAGATION IN DUAL-PHASE STEEL

The effect of ferrite content in ferrite-martensite dual-phase steel on the initiation and prop- agation of fatigue crack and the plastic deformation at crack tip has been studied.In the range of ferrite content from 24.2 to 41.5％,the optimum seems to be 33.8％,of which the crack ini- tiation will be prolonged,the threshold value increased,the propagation rate decreased and the closure stress intensity factor increased.As the propagation force is described by effective stress intensity factor,three steels with various ferrite contents will show the same propagation behaviour on da/dN vs △ K_(eff)curve.It is shown that the closure effect increases with the decrease in △K at the fatigue crack tip.When △K equals to △K_(th),the closure effect reaches a maximum value of0.7 in a dual-phase steel with 33.8％ferrite.

FATIGUE CRACK CLOSURE MEASUREMENT OF 2024-T3 SHEET SPECIMEN

computer controled procedure is developed for automatic measurementsof the crack opening stress Sop dunng fatigue tests. A crack opening displacement gauge(COD meter) is used to obtain digital data on the load-COD curves. Three methods forderiving S op from the data sets are compared:(1) slope method, (2) tangent lines inter-secting method, and (3) tangent point method. The effect of the position of the CODmeter with respect to the crack tip on Sop is studied in tests of 2024-T3 specimens. Re-sults of crack growth and Sop are presented for CA loading with an overload and withan overload followed by an underload.

FATIGUE CRACK GSOWTH AND CRACK CLOSURE OF SIMULATED OVER-MATCHED WELDED JOINTS

Mismatching is one of the basic characteristics of welded joints.Overmatched welded joint with a crack in hard weld is simplified as a cracked body of CCP type SHS (a hard bar sandwiched between two soft material with a crier perallel to the interface ) in this paper.Effects of mechanical heterogeneity on fatigue crack growth and crack closure of this model are studied by experimental and elastic - plastic finite element methods. It is found that, residual plastic deformation remaining in the wake of a growing crack tip, which cases crack closure,is affected the plastic deformation in adjacent soft mate- rial near by the crack tip. Both the shape of crack tip plastic zone and stress distribution ahead of crack tip in heterogeneous cracked body are different from that of homogeneous one. Crack closure (open- ing) loads are estimated by investigating the changes of crack tip opening displacement during fatigue loading, and a new model of crack oening process is presented. Because of the difference in the effect of mechanical heterogeneity on displacement of non - fatigue crack and fatigue crack, conventional CTOD can not reflect the real deformation at crack tip of SHS specimens.

Closure Effect ofⅠ+ⅡMixed-mode Crack for EA4T Axle Steel

The crack-closure effect is a crucial factor that affects the crack growth rate and should be considered in simulation analysis and testing.A mixed-mode I+II loading fatigue crack growth test was performed using EA4T axle steel specimens.The variation of the plastic-induced crack closure(PICC)effect and the roughness-induced crack closure(RICC)effect during crack deflection in the mixed-mode is examined in this study.The results show that the load perpendicular to the crack propagation direction hinders the slip effect caused by the load parallel to the crack propagation direction under mixed-mode loading,and the crack deflection is an intuitive manifestation of the interaction between the PICC and RICC.The proportion of the RA value change on the crack side caused by contact friction was reduced by the interaction between PICC and RICC.The roughness of the crack surface before and after the crack deflection is different,and the spatial torsion crack surface is formed during the crack propagation process.With the increase of the crack length,the roughness of the fracture surface increases.During the crack deflection process,the PICC value fluctuates around 0.2,and the RICC value is increased to 0.15.

Simultaneously improved the strength and ductility of laser powder bed fused Al-Cr-Fe-Ni-V high-entropy alloy by hot isostatic pressing:Microcrack closure and precipitation strengthening

Hot isostatic pressing(HIP)is usually applied to reduce the defects including cracks and pores in the materials prepared by laser powder bed fusion(LPBF).In the present research,in order to improve the relative density and mechanical property,HIP was employed on the LPBF-processed Al-Cr-Fe-Ni-V high-entropy alloy(HEA)with microcracks and pores.The microstructure evolution and property improvement induced by HIP were investigated.In the LPBF-processed HEA,the microcracks were caused by residual stress and element segregation,and these microcracks as well as the pores reduced significantly after HIP treatments.Remarkably,HIP temperature has a more critical effect on the microcrack closure than the holding time,thus,microcracks and pores still existed after HIP-1 treatment(1273 K,8 h),while HIP-2 treatment(1473 K,4 h)could close the microcracks significantly.The crack closure was attributed to the interfacial diffusion of the alloying element under high temperature accompanied by high pressure,and the degree of element diffusion at both interfaces of the cracks determined the bonding strength after crack closure.Higher temperatures at high pressure induced more adequate element diffusion and higher bonding strength.The above high temperature and high pressure also induced the growth of the L1_(2) phase and the precipitation of the B2 phase in HEA.Consequently,the tensile strength and elonga-tion of the LPBF-processed HEA after HIP-2 treatment were simultaneously enhanced(80.7%and 222.5%higher than that of LPBF-processed HEA,respectively).This could be attributed to the combined effect of microcrack/pore closure and precipitation strengthening.The strengthening effect of the B2 phase and L1_(2) phase accounted for 53%(dislocation by-pass mechanism)and 47%(dislocation shearing mechanism)of the total precipitation strengthening,respectively.

Comparative study between crack closure model and Willenborg model for fatigue prediction under overload effects

A comparative study is performed between a crack closure model and the Willenborg model, which can calculate the fatigue crack growth rate under the overload effects. The modified virtual crack annealing(VCA) model is briefly reviewed, which is based on the equivalent plastic zone concept. In this method, the retardation phenomenon is explained by the crack closure level variation, which is derived from the interactions between forward and reverse plastic zones ahead of the crack tip. As a comparison, the Forman equation in conjunction with the Willenborg model is also reviewed. The retardation phenomenon is described by directly modifying the stress intensity factor. It is known that the large plastic zone created by the overload can decelerate the fatigue crack growth rate until the crack grows beyond this region. A relationship between the plastic zone and the modified stress intensity factor is developed, which is a mathematical fitting equation instead of physical-based formulation. The experimental data in aluminum alloys are used to validate these two models. Overall, good agreement is observed between the model predictions and the testing data. It is noted that the approach based on modified VCA model can give more accurate prediction curves than the Willenborg model.

DETERMINATION OF CLOSURE EFFECT IN FATIGUE CRACKING BY MEANS OF COMPLIANCE TECHNIQUE AND NUMERICAL METHOD

The crack tip strain gauge method in the compliance technique was used to determine the opening load of notched crack of axle steel,and the nonlinear finite element ADINA program, to which the cyclic stress-strain curve of axle steel was applied,was used to analyze the stress-strain field ahead of the crack tip and the opening load of notched crack.The results of both the compliance technique and the numerical method were in good agreement.In this pa- per,the concept of the sensitive point is proposed and the key to the determination of the crack opening load in the experiment is to place a strain gauge at sensitive point.It is certified by both experimental and numerical methods that the sensitive point has the best linear relation- ship character and the value of strain is much greater.

Anisotropic FCGR behaviour in hot-rolled Mg-3Al-0.5Ce alloy

In this work,anisotropic fatigue crack growth rate(FCGR)behaviour in a hot-rolled Mg-3wt%Al-0.5wt%Ce alloy was investigated using compact tension(CT)specimens with notch(an)parallel to the rolling direction(RD)and transverse direction(TD).The FCGR tests were conducted at a constant load ratio(R=0.1)and maximum stress intensity factor(KMax=15.6 MPa√m)to investigate the crack closure effect.For both constant R and KMax conditions:(i)the load-displacement curves for every loading cycle were linear for a_(n)∥ to RD and TD,indicating no crack closure;(ii)the FCGR was found to be lower for an∥RD than an∥TD over the entire stress intensity factor range(△K).The hot-rolled sample contained long-aligned Al11Ce3 intermetallic phase within grain boundaries that are elongated along RD.During the FCGR test,{10.12}<10.11>extension twins(ET)with lamellae∼⊥and c-axis∼∥to these elongated intermetallics along RD developed irrespective of the notch orientation.During the loading cycle,these intermetallics along RD generate back-stresses,reducing the in-plane tensile stress∼||and∼⊥to crack-tip to∼0 for a_(n)∥to RD and TD,respectively.Hence,lenticular ET∼||⊥and∼||,with c-axis∼||and∼⊥to crack path activates,leading to trans and inter lamellar crack for anto RD and TD,respectively,and anisotropic FCGR.Translamellar crack in a_(n)∥RD reduces the FCGR due to plastic energy dissipation as perceived by comparatively more geometrically necessary boundaries(GNBs).On the other hand,faster FCGR was obtained for a_(n)∥TD due to interlamellar cracking.Thus,the crack growth through the matrix-ET interfaces was favoured due to strain incompatibility.The Fractography for a_(n)∥RD shows smaller elongated grooves along crack propagation,which indicates crack arrest.However,larger elongated grooves for a_(n)||TD indicated easy crack propagation due to favourable interlamellar crack.

Influence of chemical liquids on the fatigue crack growth of the AZ31 magnesium alloy

The fatigue crack growth behavior of an AZ31 magnesium alloy was investigated by comparing the effect of zirconate and phos-phate chemical liquids. The morphology, components, and phase compositions of the chemical depositions at the fatigue crack tip were analyzed by employing scanning electron microscopy （SEM）, energy dispersive spectroscopy （EDS）, and X-ray diffraction （XRD）, respectively. For samples with and without the chemical liquids, their stress-intensity factor values at the fatigue crack tip were compared by using a stress-strain gauge. The results demonstrated that a zirconate film （ZrxOy-ZnxOy） and a phosphate film （Zn3（PO4）2·4H2O and MgZnP2O7） could be formed on the fatigue crack-surface at the fatigue crack tip. The stress distribution was changed because of the chemical depositions and the causticity of the chemical liquids. This could decrease the stress-intensity factor value and thus effectively cause fatigue crack closure, which reduces the fatigue crack growth rate. Moreover, it was found that the fatigue crack closure effect of zirconates was more positive than that of phosphates.

THRESHOLD OF SHORT FATIGUE CRACK FOR Al-Li ALLOY 8090

Threshold of fatigue for short through cracks for Al-Li alloy 8090 was evaluated to be lower than that of long cracks,and to decrease with the shortening of the crack length.The correla- tion among the threshold,crack wake,and crack tip shielding of the short fatigue cracks was discussed.The super demarcation length and lowest theoretical threshold of short through fa- tigue cracks were presented.

CORROSION FATIGUE CRACK GROWTH OF MEDIUM STRENGTH STEEL HT60 IN SYNTHETIC SEA WATER

The corrosion fatigue crack growth and near-threshold characteristics of a medium strength steel HT60 were investigated using compact tension specimens exposed to synthetic sea water. The da/dN-△AK_(eff) relation in air can give a conservative estimation of da/dN-△K relations in sea water. In the case of high R, however, crack growth acceleration at high △K regions appears to be cantrolled by the stress-assisted dissolution. The crack opening stress inlensity factor K_(op) detected by the back-face-strain method is the result of crack surface in contact with the corrosion products and therefore an overestimated value of K_(op) at the crack tip is given.

ROUGHNESS-INDUCED SHEAR RESISTANCE OF MODE Ⅱ CRACK GROWTH

It is obtained in this paper that the fatigue threshold value of mode H was 1.9 times of that of mode Ⅰ in dual-phase steel(DPS),and the normal stress intensity factor range oJ mode Ⅱ branch crack tip was 2.2 times of that of mode Ⅰ.Above results illustrate that the resistance of mode Ⅱ crack growth was higher than that of mode Ⅰ,the former resulting from roughness-induced shear resistance,the latter,crack closure. The mode Ⅱ component can play two important roles in near-threshold fatigue crack growth:(1)increasing crack tip plasticity which accelerates the crack growth and(2)intro- ducing crack surface contact and rubbing to reduce the crack propagation rate.By means of crack closure,the quantity of shear resistance was easily solved in this paper.The friction shear stress strength factor range of mode Ⅱ,K_,is still much higher than the closure stress strength factor range of mode Ⅰ,K_(Ⅰ,cl).This illustrated that the roughness enlarged the second role and played a role of shielding crack tip from mode Ⅱ crack.

TRANSFORMATION OF RETAINED AUSTENITE IN CARBURIZED CASE DURING FATIGUE CRACK GROWTH

The morphology of the retained austenite in the carburized case of 20CrNiMo steel and its transformation during fatigue crack propagation through the case were investigated by using X-ray and TEM analysis.In the carburized case both film and block shaped retained austenite were found.Due to the crystallographic orientation relationship at the interface,the fatigue crack is inclined to pass through the block shaped retained austenite and thereby stim- ulates its strain-induced martensitie transformation.During the process of the fatigue frac- ture,most of the retained austenite structures on the crack path are transformed into the martensite,and the untranaformed parts on the fracture surface remain less than 6%.The transformation of the retained austenite,which is restrieted mainly within the plastic zone,oc- curs only during the proeess of fracture,and is independent of the magnitudes of the external stress,stress ratio and cyclic number.The volume expansion accompanying the transforma- tion creates an additional residual displacement of about 0.44μm on fracture surfaces,which is equivalent to the magnitude of the plasticity-induced residual displacement.The phase transformation induced fatigue crack closure is believed to be an important factor affecting the fatigue crack behaviors in the high carbon laver of the carburized case.

Fatigue Crack Arrest in Mild Steel via Iron Electroplating

In this work, electrochemical plating treatments were applied to ASTM A36 steel specimens to study the efficiency and limitations of this method for arresting fatigue crack propagation. Electroplated iron was deposited onto the crack surfaces using a circuit in which Swedish Iron served as the anode in a solution of Ammonium Iron(II) Sulfate Hexahydrate. The iron ions were driven into fatigue cracks that were formed within ASTM E399 compact tension specimens. This work showed that an iron-plating treatment operated at 20°C can arrest fatigue crack propagation for a significant period of cycles. The propagation re-initiation lives that resulted ranged from 11,000 to 230,000 cycles. As observed in prior work, the propagation re-initiation life correlated strongly to the magnitude of the stress intensity factor range that was applied during cycling. As this stress intensity increased, the propagation re-initiation life decreased. Repeated treatments on the same crack provided extended service lives by as much as 370,000 cycles or 60% of the entire fatigue life of the component. Future work may show that re-application of the treatment, when conducted prior to crack re-initiation, could further extend the service life indefinitely. The Correia crack closure model was modified to provide an empirical expression for predicting the crack re-initiation life of the treated component. Interestingly, highly effective arrest behavior was still observed for cracks that were loaded to stress intensity factors of only 3 - 6 <img src="Edit_f69af9cd-e908-4aef-8ccb-3b1f36e16d08.png" alt="" />during the treatment but then subjected to 20 <img src="Edit_5ddb9cae-23ad-442d-8e6c-63b5e1d99a0f.png" alt="" /> during cyclic loading. Galvanic corrosion of the plated material exposed to simulated seawater was estimated to be 3 mpy. Future work will examine the use of less active plating alloys and the possibility of applying effective treatments into cracks that are in an unloaded state.

The Prediction of Self-Healing Capacity of Bacteria-Based Concrete Using Machine Learning Approaches

Advances in machine learning(ML)methods are important in industrial engineering and attract great attention in recent years.However,a comprehensive comparative study of the most advanced ML algorithms is lacking.Six integrated ML approaches for the crack repairing capacity of the bacteria-based self-healing concrete are proposed and compared.Six ML algorithms,including the Support Vector Regression(SVR),Decision Tree Regression(DTR),Gradient Boosting Regression(GBR),Artificial Neural Network(ANN),Bayesian Ridge Regression(BRR)and Kernel Ridge Regression(KRR),are adopted for the relationship modeling to predict crack closure percentage(CCP).Particle Swarm Optimization(PSO)is used for the hyper-parameters tuning.The importance of parameters is analyzed.It is demonstrated that integrated ML approaches have great potential to predict the CCP,and PSO is efficient in the hyperparameter tuning.This research provides useful information for the design of the bacteria-based self-healing concrete and can contribute to the design in the rest of industrial engineering.

Improved Fatigue and Damage Tolerant Material Design for Aerospace Industry

Various micro-mechanical and micro-structural influences on fatigue crack growth resistance of the material have been investigated over the years. It is widely recognized that resistance to fatigue crack growth can be differentiated into ‘intrinsic＇ and ‘extrinsic＇. The separation of intrinsic and extrinsic crack growth resistance has constituted a major theme of fatigue research in the last 30 years, with the concept of crack closure or crack tip shielding being used to rationalize a wide range of micro-structural and mechanical influences on fatigue crack growth behavior. An accurately quantitative understanding of intrinsic and extrinsic effects on crack growth is essential to directed alloy design for improved fatigue resistance, and/or improved structural service life. This paper presents a compliance-based crack closure measurement method and a multi-mechanism based analytical model for the separation of intrinsic and extrinsic material fatigue resistance, with application in characterizing the fatigue performance of two high strength damage tolerant airframe AI alloys.