Total Fatigue Life Prediction Research Articles

Total fatigue life prediction of TC4 titanium alloy based on surface notch

A surface notch will be produced in crack grinding process of cracked blade, and fatigue life of surface notch is a critical measure to assess its remanufacturability. In this paper, a total fatigue life separation model based on surface notch is studied, which is defined as the sum of crack initiation life and crack propagation life. Tensile fatigue experiments were performed on surface notched plate specimens. At a given notch depth, there is a negative correlation between the secondary crack density and the notch width. Combined with Neuber's rule and elastic–plastic finite element results, a residual thickness correction coefficient Sε is proposed to reflect the effect of finite thickness on local strain. Depending on simulation results, it is found that Neuber's rule underestimates the notch-root strains of surface notch, which is contrary to the current conclusions. Crack initiation life is estimated by modifying Morrow method with the proposed Sε. The initial crack size a0 of surface notch is qualified by equivalent initial flaw size (EIFS) modified by fatigue strength calculated by Neuber approach. Crack initiation life is estimated by Labg equation with the improved a0. The proposed model is verified by fatigue experiment results with a prediction accuracy falling into the scatter band of factor 1.5. This study enriches the current notch fatigue analysis, which can provide reference suggestions by the model for crack grinding design in remanufacturing pre-treatment process.

Evaluation of different drilling quality and fatigue life prediction of nickel-based superalloy using equivalent initial flaw size

The initial quality assessment and total fatigue life prediction of film cooling holes under different drilling processes have great impact on key structural components. In this paper, the initial fatigue quality of nickel-based superalloys with different drilling processes were evaluated and the fatigue life was predicted based on the equivalent initial flaw size (EIFS) theory. We raised the modified K-T diagram method based on intrinsic stress intensity factor threshold considering the small crack data. The fatigue crack growth test at room temperature is carried out on the single edge notched specimens made of nickel-based superalloy GH4169 under two different drilling processes (HEDM and FEDM) to quantitatively evaluate EIFS values. Then, a new expression of fatigue crack growth rate is proposed based on EIFS. Finally, the total fatigue life within 2 times error band is predicted by combining EIFS and fatigue crack growth rate, which confirms the validity of fatigue models and tests.

Experimental investigation of corrosion effect on bending fatigue of the wire ropes

Wire rope has examined for the total fatigue life prediction methodologies based on the initial condition of the wire rope. The great majority of applications, e.g. cranes, lifts, and winches have been subjected the rope to bending fatigue. There are several parameters which effects on bending fatigue lifetime such as tensile load, sheave diameter, rope composition. Apart from that, according to application area, wire ropes have often exposed to atmospheric influences, and corrosion become inevitable. In this study, in order to determine the exact lifetime of wire rope, corrosion, as a major degradation influence has been investigated experimentally. Bending fatigue tests have been performed for pre-corroded wire ropes with different level of corrosion, and the data have been compared with the reference test results. For comparison of the combine effect of corrosion with tensile loads, and corrosion with sheave diameters, two different tensile loads and sheave diameters have been tested. In this paper, it has been reported the outcome of a series of test undertaken on 6 x 36 Warrington-Seale ropes in order to determine their exact fatigue endurance under different conditions.

Experimental and numerical study of fretting fatigue in dovetail assembly using a total life prediction model

The fretting fatigue in dovetail assembly was investigated by experiments and finite element method. A sub-scaled specimen and its fixture were designed and tested under cyclic loading of three load levels. The stress distribution in the contact zone was calculated and several fatigue parameters including SWT and FS parameter were employed to predict the direction and the location of the crack initiation and predict the initiation life. The crack growth path was determined by extended finite element method and crack growth life was predicted after introducing the effect of fretting-contact-induced crack closure. Then based on a total fatigue life prediction model, the total life was obtained by summing up the crack initiation life and the crack growth life using a fixed initial crack length, and the predicted results agreed well with experiments.

Total fatigue life prediction for welded joints based on initial and equivalent crack size determination

When the linear elastic fracture mechanics-based approaches are used to predict the fatigue life of welded joints, initial crack size is a key point, which eventually affects the accuracy of total fatigue life prediction. Meanwhile, the life prediction process under random loading is complicated. In this paper, a novel method is proposed to determine the initial crack size, which is based on the results of back-extrapolation approach. The proposed method expresses the stress intensity factor, and the boundary between crack initiation and propagation period is taken into consideration. Based on the proposed method, deterministic total fatigue life can be obtained with fewer tests and less cost. In addition, the concept of equivalent crack size and its calculation model are proposed to reduce the complexity of the calculation process of fatigue life prediction under random loading, and model uncertainty is included into the prediction model of probabilistic fatigue life based on equivalent crack size. It is feasible, which has been verified, to take the influence of stress level into account when determining the initial crack size. Meanwhile, the proposal of equivalent crack size simplifies the calculation process of probabilistic fatigue life, and the consideration of model uncertainty is more conducive to assess the safety and reliability of the materials or structures.

An evaluation of total fatigue life prediction of a notched shaft subjected to cyclic bending load

This article deals with the fatigue life prediction of a notched shaft in a bend loading state. The life prediction models applied include both initiation and propagation phases, without a prior definition of an initial crack length. Two criteria based on the critical plane approach were carried out to predict initiation fatigue life. The model proposed by Socie is applied to predict initial crack size in the propagation phase, and total fatigue life includes a combination of both initiation and propagation phases. In order to validate the estimated lives, results from experimental tests carried out on notched shafts were compared with predicted lives.

Multiaxial fatigue and life prediction of elastomeric components

Elastomeric components have wide usage in many industries. The typical service loading for most of these components is variable amplitude and multiaxial. In this study a general methodology for life prediction of elastomeric components under these typical loading conditions was developed and illustrated for a passenger vehicle cradle mount. Crack initiation life prediction was performed using different damage criteria. The methodology was validated with component testing under different loading conditions including constant and variable amplitude in-phase and out-of-phase axial–torsion experiments. The optimum method for crack initiation life prediction for complex multiaxial variable amplitude loading was found to be a critical plane approach based on maximum normal strain plane and damage quantification by cracking energy density on that plane. Rainflow cycle counting method and Miner’s linear damage rule were used for predicting fatigue life under variable amplitude loadings. The fracture mechanics approach was used for total fatigue life prediction of the component based on specimen crack growth data and FE simulation results. Total fatigue life prediction results showed good agreement with experiments for all of the loading conditions considered.

Total fatigue life prediction for Ti-alloys airframe structure based on durability and damage-tolerant design concept

Based on the concept of the damage-tolerance and durability design, the total fatigue life of titanium alloys is divided into three phases: crack initiation (0–0.3 mm), short crack growth (0.3–2 mm) and long crack growth (2 mm– a C). Among these three phases, different prediction models are accepted due to different failure mechanisms. A computer program was developed to predict the total fatigue life of the titanium alloy structure. Fatigue testing is also conducted for two types of ELI grade titanium alloy to verify the prediction models. The predicted fatigue life agrees well with experimental results.

The history, logic and uses of the Equivalent Initial Flaw Size approach to total fatigue life prediction

Total fatigue life is traditionally composed of the time to crack initiation plus the time for the initiated crack to grow to a critical crack size. Fracture mechanics does reasonably well in predicting the growth portion but there is still a lot of uncertainty about the definition of an initiated crack and scatter associated with the number of cycles to “initiation”. This paper will review some of the history, logic and uses of the Equivalent Initial Flaw Size (EIFS) approach to total life prediction. In short, this is a method where found cracks are analytically grown backwards to time equal zero (time or cycles) to determine an initial flaw, referred to as an EIFS. By growing a number of found cracks back to time equal zero a distribution of EIFS can be established. Example of establishing this distribution are given for the C-130 aircraft with a 7075 aluminum structure and for gas powered turbine blades made of directional solidified super-alloys.

The Ability of Current Models for Fatigue Life Prediction of Shot Peened Components

It is well known that shot peening has a marked benefit on fatigue life for the majority of applications. This effect is attributed mainly due to the compressive residual stress state at the component’s surface due to shot peening. The present paper evaluates the ability of several fatigue life prediction models, commonly used for general analyses, to predict the behaviour of components with compressive residual stress due to shot peening. Advanced elastic-plastic finite element analyses were carried out in order to obtain stress, strain, strain energy and fracture mechanics parameters for cracks within a compressive residual stress field. With these results several total fatigue life prediction models (including critical distance methods) and fracture mechanics based models were applied in order to predict fatigue life. Fatigue life predictions were compared with several experimental fatigue tests carried out on specimens, representative of a critical region of a compressor disc in a gas turbine aero engine. The results obtained showed that total fatigue life methods, even if combined with critical distance methods, give conservative results when shot peening is considered. Fatigue life was successfully predicted using the method proposed by Cameron and Smith, by adding initiation life to crack propagation life. This last method was also successfully applied for the prediction of non-propagating cracks that were observed during the experimental tests.

Predicting fatigue life of pre-corroded 2024-T3 aluminum

Much research has examined the viability of total fatigue life prediction methodologies based on the initial condition of the material. Further, corrosion attack is also of interest in terms of its damaging effects on structural durability. As a natural next step in understanding the effects of corrosion damage, this investigation was aimed at assessing the viability of a total fatigue life prediction methodology for material with pre-existing corrosion damage. The work covered here was mainly performed to further the breadth and scope of research and development, provide insights into several factors influencing fatigue resistance of corroded material with a basis in analytical and experimental data, and push closer to a fully conclusive analysis tool for truly predicting fatigue life in corroded materials. While the qualitative effects of corrosion and its effects on fatigue resistance are acknowledged by many researchers, a rigorous analytical technique for fully capturing the quantitative consequences of corrosion does not exist in closed form. Most current strategies call for overly conservative and costly repairs or make use of arbitrary factors of safety. For this study, fatigue specimens of nominal gage 0.063 in. aluminum alloy 2024-T3 were exposed to corrosion and tested in a laboratory setting. The test samples were taken from a single lot of material and corroded for 6, 24, and 72 h in three specimen orientations. The corroded specimens were cycled to failure at three stress levels. All failed specimens were examined to characterize the damage state(s) and failure mechanism(s). An analysis technique for predicting fatigue life results in the presence of corrosion damage was developed using a fracture mechanics foundation and the observed mechanisms of failure. In general, fatigue tests showed a decrease in life due to increased corrosion exposure. However, the fatigue tests from different specimen orientations exhibited similar lives to failure. The spread from shortest to longest fatigue lives among the different corrosion conditions decreased at the higher stress levels. Life predictions based on measured nucleation sites were generally conservative in nature and within 20–30% of the experimental lives. The main conclusion from this work was that a total fatigue life prediction methodology was shown to be a mostly successful tool in predicting the life of pre-corroded test specimens, while capturing the effects of three stress levels, three corrosion levels, and three specimen orientations.

SMALL CRACK GROWTH AND FATIGUE LIFE PREDICTIONS FOR HIGH‐STRENGTH ALUMINIUM ALLOYS: PART I—EXPERIMENTAL AND FRACTURE MECHANICS ANALYSIS

The small crack effect was investigated in two high‐strength aluminium alloys: 7075‐T6 bare and LC9cs clad alloy. Both experimental and analytical investigations were conducted to study crack initiation and growth of small cracks. In the experimental program, fatigue tests, small crack and large crack tests were conducted under constant amplitude and Mini‐TWIST spectrum loading conditions. A pronounced small crack effect was observed in both materials, especially for the negative stress ratios. For all loading conditions, most of the fatigue life of the SENT specimens was shown to be crack propagation from initial material defects or from the cladding layer. In the analysis program, three‐dimensional finite element and weight function methods were used to determine stress intensity factors and to develop SIF equations for surface and corner cracks at the notch in the SENT specimens. A plasticity‐induced crack‐closure model was used to correlate small and large crack data, and to make fatigue life predictions. Predicted crack‐growth rates and fatigue lives agreed well with experiments. A total fatigue life prediction method for the aluminium alloys was developed and demonstrated using the crack‐closure model.

Fatigue life prediction from inclusion initiated cracks

Fatigue cracks have often been observed to nucleate at inclusions or constituent particles in thin sheet 2024-T3 aluminum. Large amounts of scatter are commonly observed in the experimental fatigue lives and crack nucleation site data. This research attempts to address some of the stochastic issues associated with the fatigue process. An experimental program was conducted to investigate the influence of inclusions on fatigue crack formation. Inclusions were found to be the dominant crack nucleation site and the inclusion sizes that nucleated fatigue cracks were from the large particle tail of the overall inclusion size distribution. A probabilistic model has been developed to predict the observed variability in the experimental fatigue lives from the measured distribution of inclusion sizes. In order to model crack growth from a microstructurally sized defect (such as an inclusion), a total fatigue life prediction model accounting for the small crack effect must be utilized. The probabilistic model uses a Monte Carlo simulation and Newman's FASTRAN II model to predict fatigue life. The fatigue life distributions, that were generated with the probabilistic model, predicted the critical, lower experimental fatigue lives and the overall variability in fatigue lives under several loading conditions.

Some recent fatigue and fracture mechanics research in BIAM

The paper presents a brief review of some of the major research activities on fatigue and fracture mechanics in recent years at the Beijing Institute of Aeronautical Materials. Attention is mainly given to the studies on weight function methods for analyses of two- and three-dimensional crack problems, fatigue crack growth under variable amplitude loading, small crack effects and a fracture-mechanics-based total fatigue life prediction method.