Short Crack Stage Research Articles

Efficient and accurate determination of crack propagation rate without unloading-induced crack closure

The accurate delineation of the fatigue crack growth rate (da/dN) model, encompassing the short crack stage, assumes paramount importance in the operation and maintenance of transportation vehicles such as aircraft. Nonetheless, the determination of the da/dN encounters challenges, notably crack closure engendered by unloading observations on the specimen and the attendant issue of exorbitant test expenditures due to low load amplitude (ΔP), thereby impeding the precise establishment of the da/dN model, particularly in the short crack stage. Hence, this study initially posits a novel crack observation methodology employing a self-engineered stress transfer apparatus to sustain the stress (σ) on the specimen at its closure stress (σcl) level post-unloading, thus mitigating the impact of crack closure on crack observation and augmenting the accuracy of da/dN curve determination. Subsequently, building upon this innovation, a fatigue crack propagation (FCP) test protocol tailored for the short crack stage is advanced, principally by substantially augmenting ΔP to surmount the cost constraints of the test, thereby broadening the scope of da/dN curve determination to 10−8 mm/cycle. Lastly, premised on the acquisition of high precision and comprehensive da/dN curves, a da/dN model is posited by integrating with the extant da/dN model, which can comprehensively and precisely delineate the FCP behavior across the entire crack growth spectrum, particularly in the short crack stage. Illustratively demonstrated through a case study on 42CrMo steel crankshaft, a fatigue life (Nf) prediction investigation conducted based on this da/dN model substantiates its efficacy in accurately assessing the FCP behavior of crankshaft materials vis-à-vis existing models, thereby reducing the Nf prediction error by 14.64 %.

Numerical prediction of fretting fatigue crack growth in scaled railway axles considering fretting wear evolution

To predict the fretting fatigue crack growth (FFCG) life of railway axles, a series of interrupted fatigue experiments were conducted on scaled railway axles. Subsequently, the evolutions of fretting wear and fatigue cracks in the press-fitted region were analyzed. Based on the test results, finite element models incorporating fretting wear evolution were established, and the FFCG was investigated using the maximum tangential stress criterion, cyclic resistance curve, and the modified NASGRO equation. The analysis revealed that the fretting wear evolution leads to stress redistribution at the press-fitted region, thereby promoting FFCG. When considering fretting wear evolution, the equivalent stress intensity factor (SIF) range of the crack remains above the threshold value throughout the short crack stage. However, neglecting fretting wear evolution results in the SIF range being below the threshold value for cracks shallower than 0.30 mm. This implies that considering fretting wear evolution enables life prediction throughout the short crack stage. As the crack length increases, the influence of fretting wear evolution on crack growth gradually diminishes. By accounting for fretting wear, a more accurate stress distribution in the press-fitted region can be obtained, leading to a more precise and conservative prediction of crack growth life.

Statistical modeling of microstructurally short crack growth in high cycle fatigue

Capturing the behavior of short cracks under fatigue conditions has become a challenge for researchers. The early stage of crack propagation is influenced by microstructural features, such as grain boundaries and orientations, which leads to significant statistical variations. Models such as the ones developed by Navarro and De Los Rios have been able to describe the process by considering the average properties of the material and estimating the number of cycles related to the short crack stage. However, fatigue should be seen as an extreme value process. In the present work, a more realistic model was developed to consider the statistical nature related to grain orientations to estimate fatigue life dispersion. Eleven crack propagation criteria have been used to compute the number of cycles spent in the short crack stage. Parameters such as the Schmid factor, the potential crack length, and the various angles describing the misorientation between adjacent grains were considered. One thousand random scenarios for short crack growth were studied for each propagation criterion and some statistical analyzes were run to estimate the worst-case scenario out of one million. Results can be well described using the generalized extreme value distribution, capturing the stochastic nature of the fatigue life.

Residual fatigue lives assessment of riveted lap joints based on a crack growth model

Many riveted joints bear the action of out-of-plane bending moments in steel truss bridges. The fatigue resistance of these joints isn’t stipulated in the current design criteria. Ensuring the safety of structures necessitates analysing the fatigue process of such riveted joints. This paper attempts to evaluate the fatigue life of riveted lap joints from the perspective of engineering applications. The study first found through fatigue tests with two types of fatigue details in riveted lap joints: plate surface cracking and rivet hole edge cracking. Then, the fatigue progression is divided into three stages: the microstructure short crack (MSC) stage, the physical short crack (PSC) stage and the long crack stage. The crack propagation model in the short crack stage is established by combining a scanning electron microscope test of the material and an empirical formula. The crack propagation model of the long crack stage is established by the crack propagation test and considering the crack closure effect. Results show that the fatigue life corresponding to the hole edge crack fatigue detail category is shorter than that to the plate surface crack. In each crack propagation stage, the crack propagation life decreases with increasing load and decreasing load ratio RL. For the two fatigue detail categories, the proportion of the long crack stage to the whole fatigue life is small, and the effect of the crack closure on the overall fatigue life is limited; therefore the effect of the load ratio RL on the fatigue life can be ignored.

The whole fatigue crack propagation life prediction of Ni-based single crystal super-alloy specimen with single hole based on EIFS and ‘Fish-eye’ theory

In this work, based on the ‘Fish-eye’ theory and EIFS method, a prediction model of the whole fatigue crack propagation life of the predictable structure under multi-axial stress fatigue is proposed. Then, the fatigue crack propagation life of single-hole nickel-based single crystal superalloy samples under nine working conditions was predicted and verified by finite element simulation. Finally, the effects of three loads and three initial crack lengths on the fatigue crack propagation life of single-hole nickel-based single crystal superalloy were discussed by finite element simulation. The results show that the analysis results of the theoretical model and the finite element model proposed in this work are basically consistent under certain conditions. It is found that the fatigue crack propagation life of the initial crack stage, short crack stage and long crack stage accounts for 2%, 6%–10% and 90% of the whole crack propagation fatigue crack propagation life, respectively. Under the same loading stress condition, the higher the proportion of short crack stage in crack propagation is, the faster the fracture speed of the sample is. At the same time, the progressive crack propagation process of single hole specimen under fatigue load can be seen through the calculation results of finite element model. The above conclusions provide a good theoretical reference for the design and fatigue crack propagation life prediction of nickel-based single crystal superalloy turbine blades with cooling film holes.

Study on Short Fatigue Crack Behaviour of LZ50 Steel Under Non-Proportional Loading.

The low cycle fatigue tests using the replica technique for LZ50 steel under non-proportional cyclic loading were carried out, and eight groups of effective test data were obtained. The evolution behaviour of short cracks was studied based on the effective short cracks criterion. The results show that short cracks generally originate in the grain or along the grain boundary. At the microstructural short crack stage, the crack propagation is influenced strongly by the microstructure of the material, and the growth rate of the short crack slows down several times according to the number of obstacles encountered. At the physical short crack stage, the crack propagation breaks through the banded structure of pearlite. Thus, the dominant effective short fatigue crack is formed, and the crack growth rate increases rapidly. Based on the modified parameters of the uniaxial short crack model, an approach is presented to calculate the growth rate of short cracks under multi-axial non-proportional loadings, and the new model can consider the non-proportional factor F. The fitting results of the multi-axial microstructural obstacles model are compared with test data. The comparison results show that this model can reflect the trend of short fatigue crack propagation rate under non-proportional loadings.

Applying fracture mechanics to fatigue strength determination – Some basic considerations

A discussion is provided on demands that must be met in order to apply fracture mechanics to the determination of overall fatigue lifetime and strength, i.e., S-N curves and fatigue limits. These comprise the determination of the cyclic crack driving force for all stages of fatigue crack propagation, in particular for the short crack stage where the crack driving force has to be determined for elastic-plastic deformation and the gradual build-up of the crack closure phenomenon. Special emphasis is put on a fatigue damage relevant specification of the initial crack size. Different approaches in the literature are discussed. Another important aspect is the adequate treatment of multiple crack propagation. Finally, the discussion is illustrated by an example of a butt weld made of a medium strength steel.

Fatigue strength and fracture mechanics – A general perspective

Common fracture mechanics based fatigue considerations are usually limited to the residual lifetime determination of so-called long cracks. The extension of this concept to the total lifetime, as in the S-N curve approach, requires an adequate description of short crack propagation which cannot be based on the ΔK concept, and it must consider the crack closure phenomenon as well as its gradual build-up at the short crack stage. Further, it has to provide a meaningful definition of initial crack dimensions and a solution for the multiple crack problem at stress levels higher than the fatigue limit as it is specific for some configurations such as weldments. This paper aims at a discussion of all these points and offers possible solutions which are illustrated by examples taken from the German IBESS project on fracture mechanics based determination of the fatigue strength of weldments, the results of which will be discussed in more detail in this Special issue.

Fractal Study on Collective Evolution of Short Crack for Low Cycle Fatigue at High Temperature

Short cracks play the dominant role in material fatigue damage process, from the physical point of view, the solid containing cracks is actually a non-linear dissipation system, so it can be described by fractal theory. The evolution of short fatigue crack is a fractal curve, the irregular process can be described by fractal dimension. The material 20 steel in high temperature low fatigue is experimentally studied, the evolution of short cracks are observed. By analyzing experiment results, the fractal dimensions evolving with the evolution process are as follows, with the development of the damage degree, the fractal dimension is increased, and the dimension of the crack implicates many physical parameters, it can provide a reference for understanding the damage degree of material, it also provide reference for the life of short crack stage of material.

Effect of split sleeve cold expansion on cracking behaviors of titanium alloy TC4 holes

The effect of split sleeve cold expansion on fatigue behavior of titanium alloy TC4 open hole was investigated by experiment. After cold expansion, fatigue life of titanium alloy TC4 open hole was increased to 1.5–3.0 times. The residual compressive stress drastically retards the crack propagation rate, especially in the short crack stage. Corner crack initiated at the entrance face for cold expanded holes, while face crack initiated at the mid-plane of non-expanded holes. Finite element results showed that the lowest compression stress occurred at the entrance surface, while the largest one occurred near the middle of the hole wall.

Experimental research on dominant effective short fatigue crack behavior for railway LZ50 axle steel

Behavior of dominant effective short fatigue crack, the crack results in the final failure of specimen, was experimentally investigated by a replica technique with seven smooth hourglass shaped specimens of railway LZ50 axle steel. Character of two-stages, i.e. the micro-structural short crack stage and the physical short crack stage, was revealed for the crack initiation and growth. Most importantly, the crack growth rate exhibited decelerations twice in micro-structural short crack stage. This behavior was related to the ferrite grain boundary firstly and then to the pearlite banded structure. The boundary appeared a barrier because there were pearlites around with significant higher micro-hardness values. Also because of the higher hardness values, the layered pearlite banded structure acted as another barrier for the crack to overcome. In physical short crack stage, the crack propagated with a decreasing resistance of micro-structural barriers as the crack length increased. Hence, the process of crack initiation and growth are subjected to the competition between the intrinsic resistances from the barriers and the increasing driving force from the growing crack size. Finally, a new short crack growth model is presented to describe the periodic influence of micro-structural barriers. Description to the test results of LZ50 steel has indicated the availability and reasonability of present model.

The effect of hole shape on the extent of fatigue life improvement by cold expansions

The cold expansion of circular holes is known to improve resistance to fatigue. In this study the effect of the cold expansion of a circular hole on fatigue life by means of a quasi-elliptical pin was investigated. Additional evaluations were conducted, including determinations of the effects of crack propagation from the hole. The major life extension was obtained through slower crack growth in the short-crack stage. The decrease in fatigue crack growth in cold-expanded specimens was related to higher crack-opening stresses which are a consequence of the presence of compressive residual stresses arising from cold expansion. In this study, an experimental investigation was carried out to quantify the effect of the cold expansion on the initiation and the propagation of the fatigue crack and was discussed. Fatigue life improvement of the cold-worked hole specimen was explained by determining the hardness results around the cold-worked hole. The results indicate that significant life improvements can be obtained through cold expansion applied with a quasi-elliptical pin in this work with the optimum results being obtained when the pin diameter is 4% larger than the diameter of the specimen hole. Also, a brief examination of the effect of the rivet shape on the fatigue life of a riveted specimen was carried out. To lengthen the fatigue life of a riveted plate which uses countersunk head rivets, the shape of the countersink and the rivet head were improved. The experimental results showed that the fatigue life of the riveted plate was improved where the improved rivet was used.

Evolution of fatigue damage using the fatigue damage map method

Using only readily available material properties and the concept of the fatigue damage map the work attends to develop a methodology able to predict crack tip propagation rates characterising each of the fatigue stages, namely crack arrest, microstructurally and physically short crack (Stage I), long crack growth (Stage II) and Stage III growth. Crack tip propagation rates are predicted through the combination of an Elasto-Plastic crack tip opening displacement model and the crack blunting theory. The methodology provides a theoretical framework for a 3D version of the fatigue damage map for damage tolerance design. In addition, the work attends to explain a number of fundamental problems starting from crack tip propagation discrepancies, found in the short cracking stage, as well as to provide an insight towards the effect of grain size on the fatigue limit/threshold stress intensity factor.

Fatigue life improvement in fatigue-aged fastener holes using the cold expansion technique

The use of cold expansion process as a life extension technique on aircraft structural joints was investigated. The primary focus was an experimental test programme consisting of open-hole and low-load transfer joint specimens made of 2024-T351 aluminium alloy and pre-fatigued to 25, 50 and 75% of the baseline fatigue life for plain holes. The FALSTAFF loading spectrum was applied. The results indicate that significant life improvements can be obtained through cold expansion applied at all percentages of fatigue life tested in this work with the optimum stage being around 25% of the baseline life. The major life extension was obtained through slower crack growth in the short crack stage. The life improvement factors for the open-hole and joint specimens were comparable provided that the degree of cold expansion is the same. Crack growth life of the open-hole specimen was predicted by employing an analytical residual stress model and the AFGROW computer code. The prediction results showed good agreement with the experimental results for cold expansion at build cases.