Life Of Structural Components Research Articles

Residual Stress From Cold Expansion of Fastener Holes: Measurement, Eigenstrain, and Process Finite Element Modeling

Cold expansion (CX) is a material processing technique that has been widely used in the aircraft industry to enhance fatigue life of structural components containing holes. CX introduces compressive hoop residual stresses that slow crack growth near the hole edge. The objective of this paper is to predict residual stresses arising from cold expansion using two different finite element (FE) approaches, and compare the results to measurement data obtained by the contour method. The paper considers single-hole, double-hole, and triple-hole configurations with three different edge margins. The first FE approach considers process modeling, and includes elastic–plastic behavior, while the second approach is based on the eigenstrain method, and includes only elastic behavior. The results obtained from the FE models are in good agreement with one another, and with measurement data, especially close to the holes, and with respect to the effect of edge margin on the residual stress distributions. The distribution of the residual stress and equivalent plastic strain around the holes is also explored, and the results are discussed in detail. The eigenstrain method was found to be very useful, providing generally accurate predictions of residual stress.

Probabilistic shape design optimization of structural components under fatigue

Many failures of structural components are attributed to fatigue due to repeated loading and unloading conditions. The crack growth due to fatigue, represents a critical issue for the integrity and capacity of structural components. Apart from the loading conditions, the shape of the structural components plays an important role in their service life. In this study, extended finite element and level set methods are integrated into a probabilistic shape design optimization framework aiming to improve the service life of structural components under fatigue. In this context, the relation between the geometry of the structural components and their service life is investigated. The effect of uncertain material properties as well as the crack tip initialization, described by random variables is also examined. Comparisons between optimized shapes obtained for various targeted fatigue life values are addressed, while the location of the initial imperfection along with its orientation are found to have a significant effect on the optimal shapes for the components studied.

Fatigue testing of composite patches for ship plating fracture repair

ABSTRACTThe objective of this study is to examine, through physical testing, the use of composite patches for preventing crack growth and extending the service life of ship plating. Fatigue tests were conducted on steel plates to experimentally validate the effectiveness of using composite patches as a means to prevent crack growth and extend the fatigue life of structural components. Specimens were tested with and without the use of composite reinforcements. Previous results of numerical analysis studies had indicated that a one- to two-order of magnitude increase in service life can be expected for the conditions tested. The experimental results presented here show increases closer to a single order of magnitude. The difference is attributed to two factors: failure mechanisms associated with debonding of the patch and cracking of the patch itself. Of critical importance, therefore, are the implementation of a quality-controlled bonding procedure and optimisation of the geometry and properties of the patch systems based upon the parent plate's properties and fracture conditions.

Fatigue damage and life evaluation of SS304 and Al 7050-T7541 alloys under various multiaxial strain paths by means of energy-based Fatigue damage models

Three energy-based models of Macha, Ellyin and Varvani, respectively, are employed to evaluate fatigue life of SS304 and Al 7050-T7451 alloys undergoing variable multiaxial loading spectra. The Macha damage approach assesses the lives based on the equivalent strain energy densities over counted reversals on the critical plane. The Ellyin model evaluates fatigue life data based on the total strain energy density per cycle, consisting of two parts (elastic and plastic strain energies). These energies are quantified from areas within the axial and shear stress-strain hysteresis loops. The Varvani damage approach assesses fatigue lives through summation of the normal and shear energy ranges calculated on the critical plane where the largest stress and strain Mohr's circles over the counted reversals are determined. The reversals over variable multiaxial loading spectra are counted based on the Wang-Brown multiaxial cycle counting method. Damage values are calculated over reversals and then accumulated over a block loading spectrum. The total damage over block histories is then related to fatigue life Nf in the right-hand side of the damage model equations proposed by Macha, Ellyin, and Varvani, respectively. The choice of damage assessment models and how to determine the fatigue life of structural components under variable loading spectra are discussed.

Fatigue life of complex structures under random loads

A new method is proposed for calculating the life of structural components in random loading processes, when the number of extrema greatly exceeds the number of zero values.

Fatigue Life Prediction Based on Crack Growth Analysis Using an Equivalent Initial Flaw Size Model: Application to a Notched Geometry

Several methods for fatigue life prediction of structural components have been proposed in literature. The fatigue life prediction based on crack growth analysis has been proposed to assess the residual fatigue life of components, which requires the definition of an initial flaw. Alternatively, Fracture Mechanics crack growth-based fatigue predictions may be used to simulate the whole fatigue life of structural components assuming that there are always initial defects on materials, acting as equivalent initial cracks [1,2]. This latter approach is applied to a notched plate made of P355NL1 steel [3]. Fatigue crack growth data of the material is evaluated using CT specimens, covering several stress R-ratios. Also, S-N fatigue data is available for the double notched plate, for a stress R-ratio equal to 0 [3]. An estimate of the equivalent initial flaw size is proposed, using a back-extrapolation calculation [1,2]. The crack propagation model takes into account the elastic–plastic deformations in the crack-tip area within the calculation, based on the cyclic J-integral method. The performances of predictions are analyzed and deviations discussed.

Surface cracks in fatigued structural components: a review

ABSTRACTSurface cracks are a common occurrence in structural components due to the frequently highest values of the stress at the outer boundary of the body and to the presence of defects. Such flaws can heavily reduce the service life of structural components leading to their premature failure, especially under repeated loading. During last decades, many research works based on theoretical, numerical and experimental techniques have been performed, mainly devoted to the determination of the stress‐intensity factors for recurrent surface crack shapes, different loading types and boundary conditions. Further, fatigue behaviour of surface cracks has been examined through numerical analyses or by simplifying the expected crack growth process. In the present paper, a literature survey is carried out by discussing the main topics related to the safety assessment of structural components with surface cracks under static or fatigue loading.

A multi-temporal scale approach to high cycle fatigue simulation

High cycle fatigue (HCF) is a failure mechanism that dominates the life of many engineering components and structures. Time scale associated with HCF loading is a main challenge for developing a simulation based life prediction framework using conventional FEM approach. Motivated by these challenges, the extended space---time method (XTFEM) based on the time discontinuous Galerkin formulation is proposed. For HCF life prediction, XTFEM is coupled with a two-scale continuum damage mechanics model for evaluating the fatigue damage accumulation. Direct numerical simulations of HCF are performed using the proposed methodology on a notched specimen of AISI 304L steel. It is shown the total fatigue life can be accurately predicted using the proposed simulation approach based on XTFEM. The presented computational framework can be extended for predicting the service and the residual life of structural components.

A novel method and tool which enhance the fatigue life of structural components with fastener holes

The fatigue failure around fastener holes in metal structures is particularly dangerous, since it leads to inevitable accidents. The presence of residual compressive hoop stresses around these holes, imparted by means of mandrel coldworking methods (including split sleeve cold expansion and split mandrel coldworking) closes the existing cracks and impedes the formation of new ones and thereby extends the fatigue life of corresponding component. Although this type of pre-stressing of fastener holes has been of great advantage over recent years it has some disadvantages: significant and nonsymmetrical with respect to the plate middle plane axial gradient of the residual hoop stresses due to axial force flow passing through the plate, considerable surface upset, etc. This article presents a novel patented method and a tool ensuring “pure” radial cold hole expansion – introducing nearly uniform residual hoop stresses around the hole along its axis having a minimized and symmetric gradient with respect to the plate middle plane. Because of this advantage, the method has been called by the authors “symmetric cold expansion”. Another advantage of the method is that it ensures one-sided process like the split sleeve and split mandrel. The carried out finite element (FE) simulations proved that the residual stresses in the novel method are much more uniformly distributed in comparison with the mandrel coldworking methods. The fatigue life improvement of the novel method in comparison with mandrel coldworking methods has been proved experimentally through fatigue cyclic test.

Fracture Behaviour of 20MnMoNi55 Steel in DBT Region under Corrosive Environment

Different mechanical properties like strength, fracture toughness and fatigue life of structural components reduce due to corrosion of the material. In this paper the effect of corrosion on fracture toughness in ductile to brittle transition (DBT) region of 20MnMoNi55 steel is investigated. The corrosion properties of the material are measured by electrochemical impedance spectroscopy (EIS) test. The loss of ductility and fracture toughness due corrosion is measured by measuring master curve reference temperature in DBT region. Both single and multi–temperature method is used to measure the master curve reference temperature for both base material and corroded specimen. A comparative study is made to measure embrittlement that occurs due to corrosion in DBT region. From the EIS study it has been observed that the corrosion resistance properties decrease with time when the material corroded. Also the embrittlement occurring due to corrosion is reflected in the shift in master curve.

Dynamic substructuring and reanalysis methods in a surrogate-based design optimization environment

In light weight structure design, vibration control is necessary to meet strict stability requirements and to improve the fatigue life of structural components. Due to ever-increasing demands on products, it is generally more convenient to include vibration prerequisites in a design process instead of using vibration control devices on fixed designs. One of the main difficulties associated to design optimization of complex and/or large structures is the numerous computationally demanding Finite Element (FE) calculations. The objective of this research is to present a novel strategy for efficient and accurate optimization of vibration characteristics of structures. In the proposed strategy, a sub-structuring method is utilized. The FE model of the complete structure is partitioned, reduced and then reassembled. This increases the computational efficiency of dynamic analyses. Moreover, this method is coupled with a novel reanalysis technique to speed up the repeated structural analyses. These methods are finally embedded in a surrogate-based design optimization procedure. An academic test problem is used for the validation of this novel approach.

Towards Prediction of Failure in Ti‐6AI‐4V Aerospace Materials by Surface Observations

Abstract: This paper deals with the development of a methodology for the prediction of material failure in metallic aerospace alloys by evaluating changes in surface characteristics directly prior to unstable fatigue crack propagation. The study is based on in situ nondestructive characterisation of the depression zone ahead of the crack tip of fatigue‐pre‐cracked titanium alloy specimens subjected to static loading. A relationship between the surface characteristics of the deformation zone ahead of the crack and the stress intensity factor of the material was obtained. This relationship was common to a variety of microstructural conditions such as mill‐annealed and β‐annealed microstructures. Based on the analysis, prediction of the impending fracture in cracked samples of the material was enabled. The outcome of this study can be used for optimising the service life of structural components.

High temperature design curves for high nitrogen grades of 316LN stainless steel

Nitrogen alloyed low carbon grade 316L(N) stainless steel (SS) is a major structural material for high temperature structural components of sodium cooled fast reactors. With a view to significantly enhance the high temperature mechanical properties of 316L(N) SS and thereby increase the design life of structural components from 40 years to 60 years, the influence of nitrogen content on the tensile and creep properties of this steel has been investigated. Four heats of 316LN SS with 0.07, 0.11, 0.14, and 0.22 wt.% nitrogen were used in this investigation. Tensile tests were carried out at various temperatures between room temperature and 850 °C. Creep tests were carried out at 650 °C at various stress levels in the range of 140–225 MPa. The maximum rupture life in these tests was 16,000 h. The tensile and creep data were analysed according to RCC-MR nuclear code procedures and the design curves have been generated. The tensile and creep strength of 316L(N) SS have been found to improve significantly by increasing the nitrogen content.

Importance of Residual Stresses and Surface Roughness regarding Fatigue of Titanium Forgings

Abstract This paper presents the results of a long-term research program aimed at developing qualitative and quantitative design guidelines for the use of mechanical surface treatments designed to improve the fatigue life of structural components. High cycle fatigue tests were performed on planar four-point bending specimens derived from Ti-6Al-4V pancake forgings with a mill-annealed microstructure. The high cycle fatigue behavior of specimens with different surface conditions (as-forged and machined) in both an unpeened and a shot peened state was compared. In order to assess the fatigue failure mechanisms, detailed investigations of the surface layers were carried out. The as-forged surface state exhibits a stress distribution with significant compressive stresses near the surface, resulting in equilibrium tensile stresses in the depth. When the tensile stresses were exposed by machining a bordering surface, a distinct decrease in the fatigue strength was observed. In such cases, a shot peening treatment was shown to improve the fatigue strength. Square edges lead to a decrease in the fatigue strength, which could be aggravated by shot peening.

Evaluation of equivalent (by residual life) periods of thermal soaking during complex fatigue tests of samples of structural components

For the purpose of the substantiation of the parameters of forced fatigue tests, equivalent (by residual life) periods of thermal soaking during complex fatigue tests of smooth samples were determined by the calculation method. The variation of the lifetimes of samples with a hole and samples of riveting joints of D16AT and AK4-1AT1 aluminum alloys was experimentally investigated as a function of the previous stress history. The effect of aging processes on the fatigue strength of the samples that occur during soakings with different durations under natural conditions during the action of heightened temperature was evaluated. During the selection of the parameters of forced fatigue tests, it was shown to be necessary to take into account the different nature of the variation of the mechanical characteristics of structural components, which depended on the type of the structural component, the alloy, the soaking temperature, and modes of the fatigue tests. The results can be used during the design of experiments for the evaluation of effects of various factors on the fatigue life of structural components and during the forecasting of their operational life.

Four Point Bending Fatigue Tests of Forged Ti 6Al 4V

Abstract The paper derives from a long-term research program that is aimed at developing qualitative and quantitative design guidelines to influence mechanical surface treatments in order to improve the fatigue life of structural components. A four point bending test rig was developed using finite element analysis. High cycle fatigue tests were performed on plane specimens taken from Ti 6Al 4V forgings with mill-annealed or bimodal microstructure. The high cycle fatigue behaviour of specimens with two different surface conditions (as-forged and machined) was compared. In order to assess the fatigue failure mechanisms, detailed investigations of the surface layer were carried out. Residual stresses were shown to play an important role in fatigue.

A unique crack growth rate curve method for fatigue life prediction of steel structures

In this paper, a unique crack growth rate curve method, which is based on the equivalent stress intensity factor range (ESIFR) as the driving force, has been proposed and examined with crack growth rate data of base metals and as welded joints of some structural steels under constant amplitude external loading. By expressing the crack growth rate data with ESIFR instead of stress intensity factor range (SIFR) make it possible to establish a concise model for crack growth data under different R-ratios to the curve corresponding to R=0 both for base metals and welded joints. The most commonly tested crack growth rate constants under R=0 ∼0.1 are sufficient in fatigue crack growth life prediction of components subjected to tensile-tensile, tensile-compressive loading. Only two equations, one for Mean curve, and the other for Mean + 2SD curve replace the recommended crack growth rate curves in BS7910 for most structural steels. The phenomena that crack growth rates of as-welded joints under different applied loading ratios behaves independent of the applied loading ratio can be explained and the crack growth rate in residual stress field can be predicted well by the present model. The unique crack growth rate curve method does not only allow us to estimate the fatigue life of specimens of base metal and weld joints, but also the fatigue life of structural components under complex loading conditions.

Severe embrittlement of neutron irradiated austenitic steels arising from high void swelling

Data are presented from BOR-60 irradiations showing that significant radiation-induced swelling causes severe embrittlement in austenitic stainless steels, reducing the service life of structural components and introducing limitations on low temperature handling especially. It is shown that the degradation is actually a form of quasi-embrittlement arising from intense flow localization with high levels of localized ductility involving micropore coalescence and void-to-void cracking. Voids initially serve as hardening components whose effect is overwhelmed by the void-induced reduction in shear and Young’s moduli at high swelling levels. Thus the alloy appears to soften even as the ductility plunges toward zero on a macroscopic level although a large amount of deformation occurs microscopically at the failure site. Thus the failure is better characterized as “quasi-embrittlement” which is a suppression of uniform deformation. This case should be differentiated from that of real embrittlement which involves the complete suppression of the material’s capability for plastic deformation.

Modelling of residual stress relaxation around cold expanded holes in carbon steel

In cases of cold expanded holes, taking residual compressive stresses into account in strength estimation, without considering their relaxation under cyclic load, leads to inaccurate mathematical models for the prediction of fatigue life of structural components. This article presents the outcomes from experimental and numerical investigations of residual stress relaxation around cold expanded holes in medium-carbon steel under cyclic tension. An approach to experimental modelling of the relaxation is proposed. This approach reduces to minimum the volume of the experiment and at the same time provides a relatively simplified mathematical model applicable to engineering practice. A generalized mathematical model of residual stress relaxation has been obtained and it predicts the relaxation as a function of the number of cycles, parameters of the cycle and hole expansion rate. A metallographic analysis has been made for clarifying the physical nature of the residual stress relaxation. On this basis two FE models have been developed for quantitative estimation of the errors from the basic formulations in the study as well as for estimation of the residual stress relaxation. The experimental and the numerical outcomes are in a good agreement.

A Method to Estimate Operational Fatigue Life of Aeronautical Structural Components and Experimental Verification Thereof - An Outline

A Method to Estimate Operational Fatigue Life of Aeronautical Structural Components and Experimental Verification Thereof - An OutlineThe study presents an attempt to estimate fatigue life of structural components with further verification of the applied method by means of testing the specimens taken. Therefore, it has been assumed that a specimen subjected to tests is a structural component of a real system, and operational loads have been simulated in the form of a predefined loading scheme. All parameters necessary for the method have been derived from the analyses of both the specimens subjected to tests and properties of the preset load spectrum.The applied method of fatigue life estimation represents a probabilistic approach based on the Paris formula for the crack growth rate, and on difference equations that after some transformation result in an equation of the Fokker-Planck type. A probability density function of a crack length is a solution to this equation and depends on either the total time of operating the component in question or the number of load cycles applied.The probability density function of a crack length has been used to find out a formula for the probability of not exceeding the permissible crack length against the number of load cycles. The derived relationship has been applied after normalization to estimate fatigue life as based on results of experimental examination of specimens made from titanium alloy.The nomenclature of the aeronautical engineering is used throughout the paper due to the assumption that the component exposed to tests represents a part of an aircraft. This, in turn, has been intended to show some specific application(s) of the formulated model.