Unstable Fatigue Crack Growth Research Articles

A study of the parameters influencing mechanical properties and the fatigue performance of refill friction stir spot welded AlMgSc alloy

Friction spot welds of 1.6-mm-thick AlMgSc alloy were investigated in this work. A design of experiment method was used to evaluate the effect of process parameters on the shear static strength. The optimized condition of parameters was employed in the assessment of the fatigue behavior. The typical hook feature was minimized by restricting the tool penetration into only the upper sheet. As a consequence, shear strength was sensitive to the extension of the welded region rather than the hook morphology. The fatigue performance was affected by the multiple crack initiation sites that resulted from a complex stress state during the axial loading. Striations were observed in practically the entire crack propagation region, suggesting that unstable fatigue crack growth did not take place in this specific weld configuration.

Total Life Estimation of a Compressor Blade with Corrosion Pitting, SCC and Fatigue Cracking

Since compressors have components rotating at high speed, the most common cause of failure in compressors is their high cycle fatigue. In the present study, one of the rotating blades on the first row of an axial flow compressor was prematurely fractured. The crack position and direction were determined, using nondestructive tests. The results indicated that fatigue cracking initiated at two corrosion pits on the surface of the pressure side of the blade in response to corrosive compounds in the site atmosphere. The total life of the blade, from corrosion pit initiation to unstable fatigue crack growth, was calculated. The blade life had three stages: pit initiation and pit growth, pit transition to initial crack and stable crack growth reaching a critical value where the final fracture occurred. Pit growth and its transition to initial cracking were described. The time an equivalent pit requires to reach initial fatigue cracking was subsequently calculated. The number of cycles was calculated via two methods until initial cracking reached unstable growth. The first calculation was performed, using ZENCRACK fracture mechanics code. These results demonstrated that the rate of fatigue crack growth was at first slow and stable. After a period of time, the stress intensity factor reached a critical value where the crack began to propagate unstably. A MATLAB code was written based on Paris model as the second method for estimating fatigue life. The results obtained from drawing a comparison between these methods revealed that ZENCRACK crack shape simulation was in agreement with the fatigue beach marks.

Influence of Grain Boundary on the Fatigue Crack Growth of 7050-T7451 Aluminum Alloy Based on Small Time Scale Method

Based on the small time scale method, the influence of grain boundary on the fatigue crack growth of 7050-T7451 has been investigated. The interaction between fatigue crack and grain boundary was investigated by in situ SEM testing. Results showed that the fatigue crack growth will be retarded by grain boundary when the angle between fatigue crack and grain boundary is greater than 90 degrees. Mechanism analysis showed that the fatigue crack tip would not be able to open until the loading reached the 55% of maximum load, and the fatigue crack had been closed completely before the loading was not reduced to the minimum value, which led to the crack growth retardation. When the 7050-T7451 aluminum alloy suffered from fatigue loading with constant amplitude, a behavior of unstable fatigue crack growth could be observed often, and results indicated that the bridge linked mechanism led to the behavior. The grain boundary was prone to fracture during fatigue loading, and it became the best path for the fatigue crack growth. The fatigue crack tip would be connected with fractured grain boundary eventually, which led to the fast crack growth in different loading stage.

Fatigue Life Assessment of Welded Joints by the Equivalent Crack Length Method

The fatigue life of welded structures is often dominated by crack initiation and growth from the weld toes, where the notch or an initial crack like flaw determines the fatigue endurance of the structure. In the present paper, the equivalent crack length method is proposed for predicting the crack propagation life of a welded joint from this initial flaw of length a0 to a final crack at fracture lc. The geometrical configuration of a welded structure with stress concentration is assumed approximately equivalent to an initial crack in an unwelded plate. The equivalent crack length a0 depends on the joint geometry and is determined from a single experimental data point under the low cycle fatigue region, where most of the fatigue life is spent on crack propagation and the crack initiation cycle can be ignored. Once a Paris law type equation is determined by the single experimental data point, the fatigue life of the same type welded joint can be calculated for other applied stress ranges. The critical crack propagation length lc for final fracture depends on the applied stress level in terms of ΔKfc; with higher stress levels the crack propagation length may be very short, whereas with very low stress levels the maximum propagation length is longer but it is limited for unstable fatigue crack growth. In this way, the final crack length is related to the critical stress intensity factor ΔKfc. The method gives conservative results in the high cycle fatigue region as only the fatigue crack propagation phase is taken into account in the fatigue life calculation. The proposed approach has been successfully applied to experimental data. The extension of the proposed method for fatigue under variable amplitude loading will be also discussed.

Interface fatigue crack propagation in sandwich X-joints – Part II: Finite element modeling

The aim of the second and final part of this study is to simulate fatigue crack growth in the tested Sandwich Tear Test specimens, described in Part I, using the finite element method. To accelerate the simulation, a cycle jump method is utilized and implemented in the finite element routine. The proposed method is based on conducting finite element analysis for a set of cycles to establish a trend line, extrapolating the trend line spanning many cycles, and use the extrapolated state as initial state for additional finite element simulations. The measured da/ dN relations of the face/core interface in Part I are used as input to the fatigue crack growth simulation routine. A fair accuracy with 99% saving in computation time is achieved in the simulation of the Sandwich Tear Test specimens with H100 core. However, for the Sandwich Tear Test specimens with H45 core a large deviation between the numerical and experimental results is seen due to a partial unstable fatigue crack growth which was not taken into account in the simulations.

UNSTABLE FATIGUE CRACK PROPAGATION AND FATIGUE FRACTURE TOUGHNESS OF STEELS

AbstractThis paper presents the results of fatigue crack growth and fatigue fracture toughness studies of a high‐pressure vessel steel with particular emphasis on the influence of heat treatment, low temperatures, plastic prestraining, the stress ratio and specimen dimensions.It has been shown that steels in an embrittled state, caused primarily by thermal treatment and low‐temperatures, exhibit unstable fatigue crack growth which is characterized by alternate crack jumps (cleavage zones) and zones of fatigue crack growth. The fatigue fracture toughness, which corresponds to the first crack jump, and final fracture can be appreciably lower (i.e. up to 50%) than the static fracture toughness under plane strain conditions at the corresponding temperature.An analysis has been performed of unstable and stable fatigue crack growth and a model of unstable crack propagation is proposed which accounts for the observed experimental behaviour.

Effect of cyclic deformation on the resistance of a material to brittle failure

A physicomechanical model is suggested for determining the critical brittle failure stress Sc in statically and cyclically deformed material. The model is based on ideas about retardation of microcracks in a deformed substructure which changes during material loading. The effect is determined by experiment of prior cyclic material deformation on resistance to cleavage. The calculated dependence of Sc on the Odquist parameter is compared with the experimental results obtained. Model ideas about the effect of material substructure on brittle failure are confirmed by fractographic studies. Unstable fatigue crack growth is studied on the basis of models for brittle and fatigue failure of BCC-metals.

Nature of scatter of fracture toughness in static loading

The previously proposed model of unstable fatigue crack growth is used to explain a large (in comparison with other mechanical characteristics) scatter of static fracture toughness for 15Kh2MFA and 15Kh2NMFA steels at temperatures below the tactile-brittle transition temperature. The results show that for the materials for which Kfc 1 < KIc the critical stress intensity factor KIc depends on the specific energy of inelastic strain W at the tip of the initial fatigue crack in its formation stage. The value of W is a function of the number of load cycles (in the conditions with a constant range of the stress intensity factor ΔK) as a result of irregular fatigue crack growth. Here Kfc 1 is the minimum cyclic fracture toughness. A method is proposed of evaluating the minimum fracture toughness of the material in static loading based on inspection of the process of irregular fatigue crack growth in the stage of crack initiation.

Growth of a fatigue crack. 3. Model of unstable crack growth

A quantitative model of unstable fatigue-crack growth is proposed on the basis of a determination of the specific plastic strain energy stored the distance X* from the crack tip. The model makes allowance for the energy irreversibly dissibated within the damage zone and the resistance of the material to brittle fracture during static loading. The model makes it possible to predict the transition from stable to unstable crack growth, the length of the brittle crack jump, and the number of load cycles and length of stable crack growth between brittle jumps. The proposed model is checked using steel 15Kh2MFA at 293 K (σ0.2 = 954 MPa). The theoretical and experimental data on the laws of unstable fatigue-crack growth is shown to agree satisfactorily.

Influence of loading frequency on the rules and mechanisms of fatigue crack growth in titanium alloys. Report 2

1. The use of the\(\Delta K - \Delta K\sqrt v\) auxiliary coordinates makes it possible to present the results of investigations of fatigue crack growth rate in the form of a kinetic curve consisting of five regions, the near-threshold, the intermediate between the near-threshold and the next, the region of a quadratic relationship of fatigue crack growth rate to stress intensity factor, the intermediate between this region and the next, and the region of unstable fatigue crack growth. In each of these regions there is observed a certain percent relationship of the failure micromechanisms realized, departure beyond the limits of which is accompanied by a break on the kinetic curve and a change in the form of fracture surface on the macrolevel. 2. The cyclic loading frequency does not influence the failure mechanisms, which are not a function of deformation rate, but does cause a decrease in the parameters dependent upon the time of the material under the action of the maximum load, which are the sizes of the cells and the bands of the dislocation structure and, as a consequence, the spacing of the fatigue striations. With an increase in loading frequency the values of the stress intensity factor corresponding to the start of the region of agreement of the micro- and macrorates of fatigue crack growth do not change.

Influence of loading cycle on the crack resistance characteristics of steels. Report 2

1. On the basis of experimentally obtained rules of unstable crack growth in constructional alloys, a model of transition from fatigue growth of a crack to brittle failure in cyclic loading of constructional alloys made it possible to propose plans of unstable fatigue crack growth and brittle failure of constructional alloys under cyclic loads, which makes it possible to predict the rules of unstable crack development in the transition from fatigue to brittle failure and to determine the criteria controlling the resistance to brittle failure of materials under cyclic loads (Kfc1, KIc, and KDc). 2. On the basis of the proposed model and plans of unstable fatigue crack development and also of knowledge of the relationship of the current value of KI to crack size, it is possible to predict the rules of unstable fatigue crack development in design elements and structures and to determine the criteria controlling the resistance to brittle failure in them, which makes it possible to more soundly formulate the principles of check calculations of design elements and structures for strength and life under cyclic loads in the presence of crack type defects in them.

Effect of load cyclicity on the fracture toughness characteristics of steels. Report 1

1. We studied the effect of teat temperature on fracture toughness characteristics and features of stable and unstable fatigue crack growth in the static and cyclic loading of steels 15Kh2MFA and 15Kh2NMFA (I, II). 2. It was shown that a reduction in test temperature leads to a substantial reduction in the fracture toughness characteristics K fc 1 , K fc k , and kIc of the investigated steels under static and cyclic loadings. Under certain conditions, for several of the materials the cyclicity of the loading substantially lowers the fracture toughness characteristics. For the steels investigated, at 293 °K loading cyclicity negligibly changes K fc 1 compared to kc. At low temperatures, k fc 1 changes substantially compared to KIc. For a given test temperature, the values of K fc 1 at which loading cyclicity has a significant effect depends on the initial value of KI at the beginning of the test and on the number of load cycles. 3. The laws of unstable fatigue crack growth in steels 15Kh2MFA and 15Kh2NMFA (I, II) under cyclic loading (number and sizes of crack jumps) are determined by the test temperature, the level of initial values of KI in the test, and the ratios of the fracture toughness characteristics K fc 1 , K fc k , and kIc. 4. A reduction in the test temperature leads to an increase in resistance to fatigue crack growth at low values of kI and a decrease in resistance to fatigue crack growth at higher KI. 5. A reduction in loading frequency ambiguously affects resistance to fatigue crack growth for different materials.