Life Assessment Of Components Research Articles

Failure analysis and structural optimization of abnormal cracking of through-channel support seats of digital rail vehicles

In this paper, the failure analysis and structural optimization of a new type of digital rail vehicle throughway support bracket with abnormal cracking are investigated. This paper addresses the failure case of a new digital rail vehicle throughway bracket. From the simulation point of view and the vibration test point of view of the service line, it verifies and reproduces the reasons for the fatigue fracture failure of the original through-channel bracket structure. The fatigue fracture failure of the throughway bracket is caused by external excitation. The shortcomings of the optimized structure of the through-channel support filled with steel plates are explained from the simulation point of view and the fracture mechanics damage tolerance theory. The feasibility of the vibration decoupling optimized structure is completely evaluated from the service line perspective and simulation perspective, and the results show that the service environment load of the vehicle meets the life requirements, and the research method can provide guidance for the service safety and fatigue life assessment of other components of the new type of digital rail vehicles.

Design and performance verification of thermal protection structure of high temperature probe in aeroengine

In order to concurrently measure the strain and temperature fields of the hot-end components in aeroengines, the research team has designed a sensor probe equipped to measure both temperature and deformation fields synchronously. The probe’s foremost section penetrates into the combustion chamber where subjected to a high-temperature, high-pressure polluted environment at 1600 K and 1.5 MPa, thus presenting significant operational challenges. In this paper, a thermal protection structure for the sensor probe is designed, which combines the high-temperature thermal barrier coating, the gas film cooling hole and the internal cooling structure. In the high temperature environment above 1700 K, the performance of the designed thermal protection structure is verified. The inner temperature of the probe far meets the temperature requirements of the internal lens, and the sapphire optical windows can be effectively cooled and cleaned. This study also evaluates the impacts of the internal turbulence structures, pressure and the water-cooling temperature on the overall heat transfer and flow performance. The result shows that the internal channel with turbulence enhancing structures has better cooling effect under same pressure drop. The research conducted significantly contributes to the temperature and strain measurement under extreme conditions. It holds considerable relevance for the life assessment and fault detection of hot-end components in aeroengines.

A new approach to evaluate Ramberg-Osgood parameters for SS316LN using different types of specimens

A novel methodology is proposed for evaluating the stress-strain curve and determining the Ramberg-Osgood parameters (n, α) of SS316LN material. This involves utilizing load-displacement data from various specimen types, including both smooth and notched tensile specimens with different notch radii, in conjunction with finite element analysis. An error minimization method was used for evaluating the material parameters. The proposed approach for the evaluation of unique Ramberg-Osgood parameters provides a better result than that of the material stress-strain data of smooth specimen test. Ramberg-Osgood parameters were evaluated at two temperatures, i.e., 25°C and 650°C and compared with RCC-MRx code. Stress-strain equation as provided in the RCC-MR, can’t be used in FE analysis if the plastic strain exceeds 1.5% at certain regions of structure containing discontinuity, e.g., regions like shell-nozzle junction, vessel head, T-junction etc. The approach developed in this work shall help to obtain the material stress-strain curve over a wide range of plastic strain values. Ramberg-Osgood parameters were evaluated for the whole temperature range of 25°C and 650°C, which can be used for structural integrity analysis of reactor components made of stainless steel grade SS316LN. The range of applicability of data for RCC-MR code was extended beyond 1.5% of plastic strain in this work. The equations derived in this work shall be useful for structural integrity analysis and life assessment of structural components of nuclear reactors.

Crack growth-based life prediction for additively manufactured metallic materials considering surface roughness

Additively Manufactured (AM) components are prone to fatigue damage due to defects accompanied by the manufacturing process, such as surface roughness, internal porosity, and anisotropy due to differences in build directions. This is especially true for as-built components with significant surface roughness, where the surface polishing and treatments are not possible due to resource or space limitations. This paper proposes a crack growth-based methodology for the fatigue life assessment of AM components subjected to uniaxial and multiaxial, constant and variable loading conditions. The work is based on a previously developed subcycle fatigue crack growth model. The developed FCG model is extended with the stress concentration factor due to surface roughness and an asymptotic stress intensity factor (SIF) interpolation method for notched specimens. The proposed model approximates the surface roughness as an equivalent notch having the same stress concentration as that posed by the irregularities on the surface. Fatigue life assessment is performed based on the concept of equivalent initial flaw size (EIFS) and FCG analysis. The proposed methodology is validated against in-house as well as experimental data from the available literature.

Creep Life Assessment of In-Service Components by Testing Sub-Standard Sized Samples in Bending: An Example Case of X22CrMoV12-1 Ferritic Steel Used in Steam Turbines

Creep Life Assessment of In-Service Components by Testing Sub-Standard Sized Samples in Bending: An Example Case of X22CrMoV12-1 Ferritic Steel Used in Steam Turbines

Effect of pre-strain on high cycle fatigue property of TWIP steel

A commonly used material in vehicle structure is twinning-induced plasticity (TWIP) steel. In addition, during auto safety structural parts process, the material is pre-strained to different levels. Therefore, the influence of pre-straining on fatigue life needs to be examined for design and life assessment. Based on the current test data, due to the different types of steel and loading methods, the fatigue life of the material will show an increase or decrease with the increase of pre-strain level. The results of this study are compared with the recommended S-N fatigue, and the applicability and conservative levels of fatigue life curves are discussed to consider the influence of material pre-strain on the fatigue life assessment of automotive structural components.

A probabilistic framework of creep life assessment of structural components at elevated temperatures

Current creep life assessment methods of components at elevated temperatures are mainly based on deterministic analysis strategies, which could not achieve the goal of probabilistic evaluation on creep failure. Based on this, a probabilistic framework of creep life assessment for components at elevated temperatures was provided. A method of determining distribution characteristics of material parameters was provided by randomly selecting a group of results at each stress level. Monte Carlo simulation was combined with finite element analysis technology to capture the distribution characteristic of creep rupture life of one typical structural component. Effect of standard derivation of material parameters on creep reliability assessment was discussed. Comparisons between probabilistic and deterministic creep design methods were made. Results indicated that the probabilistic analysis strategy can calculate the specific value of failure probability at various loading conditions, not two values of failure probability (i.e. 100% and 0%) by deterministic analysis strategy. The effect of standard derivation on mean values of effective stress and creep rupture life of the component is dependent on distribution characteristics of material parameters and related variables. A small standard derivation reduces the data scatter of effective stress and creep rupture life of the component.

Fretting fatigue under variable amplitude shear loading blocks considering partial slip regime: Experimental/numerical analysis

Generally, the fretting fatigue problem is treated with constant amplitude loading. However, this condition is not the reality for industrial applications. The objective of this work is to propose a numerical methodology for the life assessment of components subjected to fretting fatigue under variable amplitude loading and to study the effects of these loads. For this, experimental tests were carried out by applying H-L (High-Low) and L-H (Low-High) loading blocks to the tangential load, that is, the contact normal load and the amplitude of the bulk fatigue load were kept constant while the tangential load has its alternating amplitude. The numerical methodology proposed for fretting fatigue life assessment includes the wear effect and crack propagation. The multiaxial fatigue parameter SWT, the theory of critical distance TCD, and Miners’ linear rule were used to compute the damage in the crack nucleation phase. The node-displacement algorithm based on Archard’s law was applied to account wear. In the crack propagation phase, the CDM (critical direction method) was used to estimate the crack initiation angle considering the worn contact surface, and then the crack is modeled and propagated. The still limited range of experimental data done in this work for fretting fatigue under varying loading blocks seems to indicate that the loading sequence has an effect on the accumulated fatigue damage, mainly to the H-L loading sequences. Based on this observation, a new non-linear damage model was proposed. Using this new damage model, the life estimation results were within a scatter band of 1.2.

Residual Creep Life Assessment of High-Temperature Components in Power Industry.

A large percentage of power, petroleum, and chemical plants over the world were in operation for a long duration with the corresponding critical components being used beyond the design life of 30 to 40 years. It is generally more cost-effective to refurbish or modernize the degraded equipment or components, rather than to construct a new plant. Therefore, a reliable plant life extension assessment that can evaluate the critical components is needed. The key element in plant life extension is the residual life assessment technology. However, at present, there is still no general consensus among the industry players on the approach to adopt when performing residual life assessment for such a critical damage mechanism as creep. In this article, a three-level residual life assessment methodology is proposed as a general approach for high-temperature components prone to creep. A detailed validation of the selected guidelines and calculation models is also described. Eventually, an application of the three-level methodology to a real industrial case study is presented.

Fatigue life assessment of imperfective cast steel components in the medium-cycle regime by stress- and energy-based concepts

Manufacturing process-based imperfections, such as shrinkage porosities, severely limit the fatigue strength of cast steel components. Due to the complexly shaped spatial geometry of such defect structures, the application of local fatigue assessment concepts is recommendable. This work applies the Theory of Critical Distances (TCD) to assess the fatigue life of imperfective cast steel components made of G12MnMo7-4+QT in the medium-cycle regime. Based on a calibration procedure utilizing generalized S-N curves of plain and notched specimens at various load ratios, the relationship between the critical distance LM and the number of cycles to failure Nf is parametrized first. At second, the implemented planar framework is validated with specimens featuring a different notch geometry. The presented linear-elastic methodology is furthermore extended to arbitrarily shaped spatial imperfections, resulting in reasonable predictions of experimentally derived fatigue life results. Moreover, a strain energy density-based fatigue assessment methodology is formulated, introducing a relationship between the control radius Rc and Nf. The yielded fatigue strength assessment values correspond to the results of the initially implemented TCD-framework. Concluding, the presented stress- and energy-based concepts depict engineering-feasible methodologies to assess the fatigue life of cast steel components affected by complexly shaped shrinkage imperfections.

The RED criterion for fatigue life assessment of metals under non-proportional loading

In the present paper, a novel criterion for fatigue life assessment of engineering components under multiaxial low-cycle fatigue regime is proposed. The criterion, named Refined Equivalent Deformation (RED) criterion, allows to take into account, when the load is non-proportional, the fatigue strength decrease experimentally observed for those metals sensitive to non-proportionality. The criterion is applied to two different metals (304 stainless steel and 6061 aluminum alloy) characterised by different non-proportional sensitivities. The accuracy of such a criterion is compared with those of other criteria available in the literature.

A benchmark activity on the fatigue life assessment of AlSi10Mg components manufactured by L-PBF

One of the challenges associated with additive manufacturing (AM) is the definition of an assessment route which considers the main process signatures of the AM process. To this end, this work presents a complete benchmark activity for the assessment of an AlSi10Mg component produced by a laser powder bed fusion process, aimed at advancing the understanding of the fatigue resistance of AM materials with particular focus on the comparison between the fatigue performances of small coupons and demonstrators. Four builds of AlSi10Mg specimen geometries were manufactured to: (i) determine the fatigue curves for both as-built and machined conditions; (ii) measure the fatigue crack growth rate; (iii) produce and test under fatigue a benchmark component used as a reference for the validation of the fatigue assessment procedure. Tools and concepts of flaw tolerance were then used to perform the fatigue assessment of the benchmark component and were shown to be successful in the life prediction. Results obtained from this wide database (related to internal defects and surface features) show that only a fracture-based fatigue assessment is able to provide precise life estimates consistent with material crack growth properties. Eventually, all the experimental results including specimens design, analysis of fracture surfaces and raw tests’ data will be made available in a database which can be accessed and used by the industrial and scientific communities to calibrate and validate alternative fatigue assessment procedures of AM parts.

Assessment of three-dimensional multi-crack propagation for fatigue life prediction

The existence of multiple cracks affects the crack propagation life assessment of mechanical components. In the present work, numerical algorithms for multiple fatigue crack growth are studied with verification of finite element computations. The investigation confirms that the ratio of the crack center distance to the major crack length is a suitable parameter to evaluate the influence of multiple cracks in the fatigue life prediction. An efficient semi-analytical method is developed and verified to predict the fatigue crack propagation life, which can be applied for three-dimensional structural damage tolerance assessment.

Assessment of fatigue crack growth based on 3D finite element modeling approach

Fatigue crack growth life assessment enables manufacturers to quantify damage tolerance capability of high-risk components. Reduced order models that are based on simple geometries (i.e. surface crack in a plate, corner crack at a bolt hole) and on the assumption that cracks maintain an elliptical shape during propagation, are commonly employed in the damage assessment. A more comprehensive modeling process should consider the component geometry, service loading conditions and, eliminate assumptions related to crack front shape or planarity of the crack growth path. A 3D finite element-based approach is evaluated in this study as a more accurate alternative to reduced order models. For verification purposes, an analytical solution-based model was developed and implemented in MATLAB to predict fatigue crack growth life and crack front evolution for three different crack types: surface, corner and internal. The analytical model solutions are compared against 3D finite element (FE) based approach implemented in SimModeler Crack. The 3D FE modelling approach has been further tested and validated against experimental fatigue crack growth measurements from two Al 2024-T3 specimens containing multiple cracks. The verification and validation data presented herein show that the 3D FE-based modelling approach provides an accurate and effective modelling tool for crack propagation life assessment of structural components.

Prediction of creep crack initiation time based on constraint parameters in specimens with different geometries

Based on two creep constraint parameters (R* and Ac), the constraint-dependent creep crack initiation (CCI) time equations of a Cr–Mo–V steel at 566 °C have been established over a wide range of C* region. The equations are independent on the choice of the reference specimen. Based on the two-parameter (C*-R* and C*-Ac) and the constraint-dependent CCI time equations, the CCI time for the specimens with different geometries and dimensions (with a wide range of constraint levels) has been predicted over a wide range of C* region. The predicted CCI time agrees well with experimental data available and CCI time of finite element simulations based on the creep ductility exhaustion model. The specimen geometry with high constraint strengthens the effect of out-of-plane constraint on the CCI time. The two constraint parameters R* and Ac can give similar CCI time prediction results. The two-parameter approaches with considering constraint effect may be applied in the CCI life assessment of high-temperature components (such as pressure pipes and vessels) under static loads in operating plants.

316L(N) Creep Modeling with Phenomenological Approach and Artificial Intelligence Based Methods

A model that describes creep behavior is essential in the design or life assessment of components and systems that operate at high temperatures. Using the RCC-MRx data and the LCSP (logistic creep strain prediction) model, processed design data were generated over the whole creep regime of 316L(N) steel—i.e., primary, secondary, and tertiary creep. The processed design data were used to develop three models with different approaches for the creep rate: a phenomenological approach; an artificial neural network; and an artificial intelligence method based on symbolic regression and genetic programming. It was shown that all three models are capable of describing the true creep rate as a function of true creep strain and true stress over a wide range of engineering stresses and temperatures without the need of additional micro-structural information. Furthermore, the results of finite element simulations reproduce the trends of experimental data from the literature.

Fatigue life assessment in notched injection-molded specimens of a short-glass fiber reinforced Polyamide 6 with different injection gate locations

Based on a previously published Through Process Modeling (TPM) for multiaxial fatigue life assessment of injection-molded components, this work aimed to capture the difference of fiber orientation distribution ahead of notch tips in flat notched samples of PA6GF30 tested in tension at ambient temperature. The fiber orientation originated from different locations of the injection gate for an unchanged macroscopic geometry. Good lifetime estimations were obtained in Side- (resp. Top-) injected samples, regardless the choice of the sample type from which the fatigue criterion was identified. The experimentally observed differences of crack initiation zones were correlated to numerical simulations.

Creep fracture parameter C* solutions for circumferential surface cracks in pressurized cylinders

The creep fracture mechanics parameter C* is an important input parameter in creep life assessment of cracked components, and it should be accurately calculated. In the author's previous work, the C* solutions for axial surface cracks in pressurized cylinders have been investigated and given. In present work, the C* solutions for circumferential surface cracks have further been investigated and developed. Based on extensive finite element analyses, the creep influence function Hc values for calculating C* have been calculated, tabulated and fitted into equations for circumferential internal and external surface cracks in pressurized cylinders. The equations for estimating C* for materials with different creep exponent n are also investigated and given. The accuracy of all equations has been verified by finite element analyses. The effects of the crack sizes, orientation (axial and circumferential), location (internal and external) and cylinder geometries on the C* distributions along the crack fronts have been analyzed.

Creep fracture parameter C* solutions for axial internal and external surface cracks in pressurized cylinders

Accurate calculations of creep fracture mechanics parameter C* is very important for life prediction and assessment of high-temperature cracked components. The current two engineering estimation approaches (EPRI and reference stress approaches) for C* can seriously overestimate or sometimes underestimate C* of cracked components, thus it is desirable to develop accurate C* solutions. In this work, based on extensive three-dimensional (3D) finite element analyses, the C* solutions for axial internal and external surface cracks with a wide range of sizes in pressurized cylinders with different geometry sizes are investigated and developed. The creep influence function Hc for calculating C* along the crack fronts has been obtained and fitted into equations, and the accuracy of the equations has been verified. The correlation equations between C* and creep hardening exponent n are firstly developed which can be conveniently used to estimate C* for materials with different n values. The effects of crack sizes and cylinder geometries on the C* distributions along the crack fronts also are analyzed.

Effects of Axial and Multiaxial Variable Amplitude Loading Conditions on the Fatigue Life Assessment of Automotive Steering Knuckle

In this paper, the author has attempted to investigate the effects of different loading conditions including axial and multiaxial variable amplitude loading (VAL) on the fatigue life assessment of automotive components under various maneuvers. To this end, a case study was conducted on the cast iron steering knuckle of a passenger car. In fact, the various VAL histories are entered on the three joints of knuckle, namely steering linkage, lower control arm, and MacPherson strut. However, previous studies have shown that this high super-critical component fails through the steering linkage. Moreover, the rotation of the steering linkage is the most destructive load. Hence, in this research, different loading cases such as axial (destructive load as means 1 channel), multiaxial (only relates to loading on the joint of knuckle and steering linkage means 3 channels), and full multiaxial (including all loading time histories means 9 channels) were considered. Afterward, finite element analysis was performed for each case, and fatigue life of the component was predicted under different conditions. Next, fatigue life of the component was evaluated using the time histories of stress tensor in the root of steering linkage which is extracted by transient dynamic analysis and applying probabilistic approach based on the Liu–Zenner equivalent stress criterion. Eventually, the responses from both techniques were compared in different cases. The results reveal that life predicted using two methods are slightly different. But, the results of probabilistic approach are more accurate than the results of FEM in comparison with experimental data for the axial state. Also, one of the major achievements of this study is that for the components with complex geometry and under multi-input loading like the steering knuckle, it is essential to perform fatigue analysis by considering all real conditions and cannot be only focused to the destructive loading.