Local Strain Approach Research Articles

Fatigue Fracture Mechanism and Life Prediction of TA1 Titanium Alloy Clinched Joints

ABSTRACTThis study investigated the fatigue fracture mechanisms and life prediction of clinched joints made from titanium alloy TA1. The fatigue tests revealed that TA1 titanium alloy clinched joints exhibited failure characterized by fracture of the lower plate at three distinct fatigue load levels. Additionally, finite element analysis indicated that cold work hardening enhanced the fatigue performance of these joints. Observations of fracture surfaces using scanning electron microscopy identified the crack source and its propagation path, which correlated with the location of maximum principal stress from the finite element simulations. Fretting wear was also observed in this critical region. Furthermore, fatigue life predictions for TA1 titanium alloy clinched joints were made using Paris' law and the local strain approach. Both methods closely matched experimental results across different fatigue life intervals. Overall, the local strain approach exhibited superior predictive capability compared to Paris' law, taking into account various influencing factors.

Evaluation of the fatigue life of AISI 347 specimens with small notches based on local strain considerations

The use of materials data for cyclic loading determined on unnotched specimens to predict the fatigue life of notched components has a long tradition, specifically when it comes to nuclear engineering. The basic principle is that the material behaviour of the unnotched specimens can be transferred to what the material is exposed to be in a notch root. This principle has been proven for large notches. However, what happens when the notch is relatively small? Can the fatigue life still be predicted on the grounds of what has been considered as the local strain or Neuber approach? A key reference in that regard is the Neuber equation or generally the load versus notch stress, notch strain – or any combination of those – relationship. In a larger study, unnotched and notched specimens from the metastable austenitic steel AISI 347 have been tested and characterized providing a database that may give some answers to the question raised above.The unnotched specimens considered here had a cross-section diameter of 10 mm while the notch radii of the notched components were only 0.5 and 0.35 mm, respectively. Notch radii of such dimensions will easily reach a fully plastic condition once the material has exceeded the yield limit, because the difference in load between yield start in the notch and full plastification is relatively small. This fully plastic state exceeds the validity of the Neuber equation and requires respective corrections. In this paper it is shown how such corrections can be obtained through a finite element analysis and how this applies to the cases mentioned above.

A two-point fatigue strength assessment for surface-hardened notched components under consideration of residual stresses based on the local strain approach

The “Guideline non-linear” of the German Research Association for Mechanical Engineering provides a fatigue strength assessment for machine components based on the local strain approach. Currently, this assessment is limited to homogeneous components and without the possibility to consider residual stresses. The methods are thus inadequate for surface-hardened components, where the presence of material inhomogeneity and the introduced residual stresses significantly impacts fatigue life and the potential failure origin. In the following paper, an adaptation of the proof of structural durability of the guideline is shown, that firstly includes a two-point assessment and the estimation of load cycles to crack initiation at two failure-relevant points simultaneously. This enables the consideration of inhomogeneous material states, since the failure of surface-hardened components may initiate from the notch root as well as from the transition area between the low-strength core material and the high-strength surface layer. Secondly, the consideration of the residual stress state for both failure-relevant points is also introduced as part of the adapted fatigue strength assessment.

Optimizing fatigue life prediction of high strength bolts in bolted spherical joints

Bolted spherical joints (BSJs) are integral to large-span space grid structures commonly used in expansive public and industrial buildings. BSJs are susceptible to fatigue damage in high strength bolts due to variable random loads. This study summarized findings from prior fatigue tests, detailing the fatigue failure patterns and S–N curves for grade 10.9 M20, M30 high strength bolts, and grade 9.8 M39 high strength bolts in BSJs. By employing finite element analysis, the study determined the stress concentration factors for these bolts, which were then utilized in fatigue life estimations using the Heywood model and the local strain approach (LSA). Both methods could yield satisfactory predictions, but when it came to the Heywood model, particularly when modifying it in areas of high stress, it demonstrated superior accuracy in predicting the performance of M20, M30, and M39 high strength bolts. Additionally, incorporating initial defect considerations, fatigue life was predicted using the damage tolerance design method (DTDM). Finally, when comparing the fatigue life prediction results obtained from various methods, it became evident that the modified Heywood model outperformed the other methods. This not only provided a superior solution but also demonstrated user-friendliness, thereby establishing itself as the preferred option for predicting the lifespan of high strength bolts in BSJs.

Fatigue Analysis of Steering Knuckles Using the Local Strain Approach

The study investigates the mechanical properties and fatigue behavior of steering knuckles used on commercial vehicles. The steering knuckle is made of hot forged bainitic steel (18MnCrMoV6-4-8), which is known to demonstrate high levels of fatigue strength, toughness, and hardness. The local strain concept was adopted to assess the durability of the steering knuckle based on the stabilized cyclic material behavior. For this purpose, experimental investigations have been conducted on both the steering knuckle as well as fatigue specimens under constant and variable amplitude loadings. The fatigue specimens were removed from the area next to the crack initiation location, to represent the microstructure in the critical area of the component. Fatigue life estimations were performed under different load ratios using the FKM guideline nonlinear, employing damage parameters PRAM and PRAJ. The assessment enables a fatigue strength assessment for the steering knuckle by considering the local non-linear material behavior. The estimations of the material's fatigue lifetime using the FKM guideline nonlinear approach were unsatisfactory.

Numerical and experimental fatigue strength evaluation of thin-walled aluminum components for aerospace application manufactured by roll forming

The design of structural components for use in commercial aircraft places high demands on the safety under static and cyclic loading conditions during the total operating time. With immerging innovative developments in the production of thin-walled sheet metal components, savings in energy consumption and material are achievable by switching from metal-cutting production to forming operations. As the forming process has an impact the local material properties due to resulting residual stress state, cold working and surface finish, the fatigue life estimation requires the consideration of the forming history.The structural component under consideration from aluminum alloy EN AW-7475 T761 is manufactured by automated, robot-assisted roll forming. Potentially existing residual stresses are determined via process simulation of the angled profile utilizing commercial finite element solver LS-Dyna®. The fatigue life assessment is carried out using the Local Strain Approach for different loading conditions.

Cyclic elastoplasticity and XFEM based fatigue life assessment of out-of-plane gusset welded joints

Conventional design methods for high-cycle fatigue failure of engineering components often assume a linear-elastic material behaviour. The different driving forces of crack initiation and propagation fail to be considered, resulting in less accurate predictions of fatigue life. To address this issue, the present study proposes a novel method to predict the fatigue life of out-of-plane gusset welded joints based on cyclic elastoplasticity and extended finite element method (XFEM). Incorporating the welding residual stress simulated by thermal–mechanical analysis, the cyclic stress–strain response of out-of-plane gusset welded joints was assessed by Fatigue SS (FSS) constitutive model. The crack initiation life was therefore predicted by the local strain approach considering the mean stress correction. Based on the converged cyclic stress–strain response, the crack propagation behaviour and life were predicted by XFEM in combination with Virtual Crack Closure Technique (VCCT). The influence of mean stress, mixed-mode crack and residual stress on crack initiation and propagation life was also investigated. The results show that the proposed method can predict the fatigue life of out-of-plane gusset welded joints under the pure tension loading with high accuracy against experimental results.

Fatigue crack growth in anisotropic aluminium sheets — phase-field modelling and experimental validation

Fatigue crack growth is decisive for the design of thin-walled structures such as fuselage shells of air planes. The cold rolling process, used to produce the aluminium sheets this structure is made of, leads to anisotropic mechanical properties. In this contribution, we simulate the fatigue crack growth with a phase-field model due to its superior ability to model arbitrary crack paths. A fatigue variable based on the Local Strain Approach describes the progressive weakening of the crack resistance. Anisotropy regarding the fracture toughness is included through a structural tensor in the crack surface density. The model is parameterised for an aluminium AA2024-T351 sheet material. Validation with a set of experiments shows that the fitted model can reproduce key characteristics of a growing fatigue crack, including crack path direction and growth rate, considering the rolling direction.

Cyclic elastoplasticity-based life assessment of fatigue crack initiation and subsequent propagation in rib-to-deck welded joints

Conventional S-N curve methods for fatigue life assessment fail to consider the different cracking mechanisms of crack initiation and propagation, resulting in less accurate predictions of fatigue life. The present study aims to assess the fatigue crack initiation and subsequent propagation life of rib-to-deck welded joints separately by accurate prediction of local cyclic elastoplasticity. The welding residual stress was simulated with high precision by thermal-mechanical analysis based on the real bead geometry and welding parameters, and defined as initial stress condition. The performance of an unconventional Fatigue Subloading Surface constitutive model (FSS model, hereafter) to predict the cyclic elastoplasticity in the sub-yield stress state was proved by comparing to the experiment results and conventional plasticity theory. The FSS model was then embedded to distinct zones of welded joints for cyclic structural analyses. Considering the relaxation of residual stress, the fatigue crack initiation by the local strain approach based on the local cumulative plasticity damage, and propagation life was assessed by the analytical approach based on the LEFM parameters of materials. As a connection between the crack initiation and propagation, the initial crack featured by 0.5 mm depth and semi-circular shape was proposed. The current work is verified to assess the fatigue life of rib-to-deck welded joints with high accuracy under the pure bending load against experiment results.

Extension of methods for estimating the fatigue strength of components made of ultra-high strength steels

Various high and ultra-high strength steels are investigated experimentally by tensile and fatigue testing. The results are used to extend existing methods for the estimation of cyclic material behaviour for cases in which a limited amount of experimental data is available. Furthermore, fatigue tests of component-like high strength steel specimens are performed, including the determination of residual stresses and fractographic investigations of failure-relevant inclusions. The results serve to adapt and apply a proof of structural durability based on the local strain approach for components made of ultra-high strength steels.

Accuracy analyses of fatigue life predictions for multiaxially non-proportionally stressed notched components - a database evaluation

The local strain approach has been successfully applied in evaluating the fatigue life of notched components under multiaxial loading. In the present work, this approach is combined with the critical plane method. First, it is shown that by considering non-proportional strain hardening in the estimation of local stresses and strains, the accuracy of the results under non-proportional loading can be improved regardless of the applied multiaxiality hypothesis. Three different critical plane hypotheses, SWT (Smith Watson Topper, based on normal strain), FS (Fatemi Socie, based on shear strain) and a short-crack model are used to calculate the fatigue life of notched specimens and compare it to results from an extensive database of test results. The accuracy of the SWT approach is highly dependent on the loading situation. For tests under non-proportional and pure torsional loading, excessively long fatigue lives are predicted. A simple proposal is made for the FS approach to be applied in critical planes with multiple shear strain components. The FS and short-crack approaches predict fatigue lives with high accuracy regardless of the applied load.

Low cycle fatigue properties of a 9–12% Cr martensitic steel welded joint with Ni-based weld metal based on a local strain approach

The effect of mechanical heterogeneity on the low cycle fatigue (LCF) properties of a 9–12% Cr martensitic steel welded joint with Ni-based weld metal are studied using smooth and hourglass specimens based on a local strain approach. The results indicate that the smooth specimens of the welded joint fracture at the softened weld metal (WM) due to the strain concentration. Moreover, the strain–life curves of the smooth specimens of the welded joint are close to the strain–life curves of the WM smooth specimens after the local strain approach considering the mechanical heterogeneity of welded joints is adopted. The LCF properties of the inter-critically heat-affected zone (ICHAZ) are obtained through hourglass specimens based on the local strain approach. The LCF properties of the base metal (BM), WM, and ICHAZ in the welded joint are compared to investigate the relationships of the LCF properties of the welded joint and different microzones.

Low-Cycle Fatigue Performance of the Sealing Plate Connection in Aluminum-Alloy Bolt-Sphere Joints

The low-cycle properties of aluminum-alloy bolt-sphere joints were investigated in this study through experimental research, finite element simulations, and theoretical analyses. To obtain the fatigue performance of the material, low-cycle fatigue tests of 6061-T6 aluminum alloy were first performed as the basis for component fatigue estimation. The low-cycle fatigue tests were then used to investigate the failure mode and fatigue life of the sealing plate connection specimens. The specimens failed at the junction of the sealing plate and the aluminum-alloy tube. The fatigue life is primarily determined by the precise size of the connection part. A finite-element model for a sealing plate connection was created to investigate stress and strain distribution. The results show a high-stress concentration at the bottom thread's root. The maximum stress variation law is consistent with the low-cycle fatigue life variation law. Finally, a prediction for low-cycle fatigue life was attempted using the finite element result and the material's fatigue performance based on the local strain approach. The prediction results were consistent with the test results.

Design check of cyclically loaded weld seams

Abstract This paper discusses the design check of cyclically loaded weld seams with improved fatigue performance due to post weld treatment operations such as TIG dressing and grinding. The algorithm is mainly based on the local strain approach to design against fatigue. It accounts for the fundamentally different damage process and conceptual consequences of weld seams with post weld treatment compared to the usual approach based on nominal, structural or notch stresses.

Experimental field test on a multipiece steel wheel and influence of the material properties on its fatigue life evaluation

Wheels are important components for the general vehicle reliability. Their design validation is usually based on fatigue tests conducted in laboratory under constant amplitude load conditions, according to industrial standards. In the off-highway industry, the variety of applications and machines often makes it difficult to complete standard tests, due to the high load capacity requirements or to lack of information about specific applications. Therefore, strain data acquisition during field test on real machines can be a useful tool to understand actual load conditions. In this paper, a fatigue life assessment based on field test is applied to a multipiece wheel that experienced an unexpected crack development in a critical section. The local strain approach is used, together with rainflow counting method and Palmgren-Miner rule for the total damage evaluation. Mean stress influence is considered through the Smith-Watson-Topper method. The fatigue life estimation is completed for structural steel S355J2. The influence of extremely hard working conditions is also investigated and assessed to be the root cause of the failure. Finally, a possible design improvement regarding the use of a higher base material grade S690 is evaluated and proposed.

Fatigue assessment of weld seams considering elastic\u2013plastic material behavior using the local strain approach

In this contribution, the notch strain approach is applied to seam welds including butt welds and filet welds. This allows the fatigue assessment for all regimes of fatigue life, including the low cycle fatigue regime (N > 10). Linear-elastic finite element analyses are used to determine the local stresses. The modeling of the geometry to be assessed is similar to that of the effective notch stress concept. Necessary input values besides linear-elastic stresses are estimated from the hardness of the heat-affected zone. Subsequently, the elastic–plastic stresses and strains are estimated, and a service life calculation is carried out. The used algorithm for the notch strain approach is based on the German FKM-guideline “nonlinear” and includes the influence of stress gradients, highly stressed surfaces, and the influence of surface roughness. Up to now, this FKM-guideline has only been approved for non-welded components. However, this article shows that, with a few modifications, it can also be applied to welds.

Estimation of fatigue life for clinched joints with the Local Strain Approach

Widely used in automotive and aerospace industry, clinched joints are subject to failure due to neck fracture and disjoining if exposed to high static loads. For cyclic loading cases, the formation of eyebrow cracks can be observed. The main objective of ongoing research is the application of the Local Strain Approach to clinched joints joining extruded aluminum wrought alloy sheets, thus being able to predict crack initiation location and fatigue life. Complex geometrical features of a clinched joint and lacking of nondestructive methods to track local stresses and strains require a combined approach utilizing numerical and experimental techniques. Material characterization for an aluminum wrought alloy EN AW 6060T66 is accomplished by common uniaxial tension tests to determine flow curves and cyclic properties by strain controlled constant amplitude tests, respectively.Commercial finite element software LS-Dyna® is used to perform the process simulation in 2D, followed by mapping to 3D with constant amplitude loading to investigate local stresses and strains within the contact region. The Local Strain Approach with damage parameter PSWT is applied to estimate the fatigue lives of the clinched joint variants. Fatigue life estimations obtained from simulation results are compared to those from experiments.

Estimation of the fatigue strength of ultra-high strength steels

For the dimensioning of components made of low and medium-strength steels, there are established estimation methods for the material properties as well as transfer functions to transmit these parameters to components. However, for ultra-high strength steels, this level of knowledge is still insufficient and has not yet been specified in guidelines. In the present work, tensile and fatigue tests are performed with different high-strength and ultra-high-strength steels, which provide the data basis for an extension of the estimation methods. This is intended to enable the estimation of the cyclic material behaviour of these kind of steels on the basis of a limited amount of experimental input data and then to use these material parameters as input for the proof of structural durability based on the local strain approach for components made of ultra-high strength steels.

Determination of Local Stresses and Strains within the Notch Strain Approach: Efficient Implementation of Notch Root Approximations

An estimation of the elastic-plastic stress state using elasticity-theoretical input data is an essential part of the service life estimation with the local strain approach in general and a German guideline based on it, in particular. This guideline uses two different notch root approximations (an extended version of Neuber’s rule and an approach according to Seeger and Beste) for this estimation. Both require the implementation of Newton’s method to be iteratively solved. However, many options are left open to the user concerning implementation in program code. This paper discusses ways in which notch root approximation methods can be implemented efficiently for use in software systems and elaborates an application recommendation. The following aspects and their influence on the computational accuracy and performance of Newton’s method are considered in detail: influence of the formulation of the root finding problem, determination of the derivative required for Newton’s method and influence of the termination criterion. The investigation shows that the advice given in the abovementioned guideline indeed leads to a conservative implementation. By carefully considering the investigated aspects, however, the computational performance can be increased by approximately a factor of 2–3 without influencing the accuracy of the service life estimation.

Correlations between crack initiation and crack propagation lives of notched specimens under constant and variable amplitude loading

AbstractThis paper starts with an overview of the application of the three guidelines (GL) of the German Research Association of Mechanical Engineers (FKM). Each of these provides algorithms for calculating fatigue lives of components under constant or variable amplitude loading, however, with underlying different failure criteria, that is, technical crack initiation life (GL‐nonlinear), fatigue crack growth life (GL‐fracture mechanics), and total fracture life (GL‐linear). Since the GL‐linear only considers linear‐elastic material behavior in the fracture fatigue life calculation, the guideline is not capable for the LCF and is not able to take sequence effects into account. For a better approximation of fracture fatigue lives, the elastic–plastic material behavior has to be taken into account. This paper introduces the U‐Concept which has been evaluated from a large structural durability database from the literature with more than 1,500 experiments for constant and more than 700 experiments for variable amplitude loading. The U‐Concept is a small add‐on to the local strain approach (LSA) which is the backbone of the GL‐nonlinear. It enables (1) to directly calculate the fatigue life to total fracture based on elastic–plastic material behavior according to the LSA or (2) to estimate the remaining fatigue life from crack initiation to fracture without a crack growth simulation. Furthermore, the U‐Concept needs no experimental determined crack growth parameters, the only necessary material parameter is the ultimate tensile strength Rm. The U‐Concept is therefore easy applicable for industrial users and increases the accuracy of the approximated fatigue lives by considering the elastic–plastic material behavior.