Determination Of Local Stresses Research Articles

Fatigue strength assessment of preloaded cross-toothed flange connections based on the FKM guidelines

Cross-toothed flange connections are used in drivetrains for form-fit coupling of shafts. They belong to the group of non-shiftable couplings. Compared to frictional connections, tooth flange connections can transmit significantly higher torques in the same space. In contrast to other tooth couplings, the crossed tooth flange coupling investigated here is characterized by a particularly complex load distribution. The preload force, which is necessary to transmit operational loads, is generated by tensioning bolts, which can be located centrally or decentralized. Thus, the stress condition in the notch of the coupling is always determined by a superposition of stresses from preload force and operating loads.Extensive experimental and simulative investigations were carried out as a part of a research project. In addition to the development of a pragmatic, specially adapted calculation concept for cross-toothed couplings for practice, the experimental tests were accompanied by calculations with the FKM guideline “Analytical Strength Assessment” and the FKM guideline “non-linear”, The determination of local stresses was carried out with the help of elasticity theoretical and elastic-plastic FE full-contact calculations based on experimentally determined material data.This article deals with the possibilities of calculations based on the FE simulation results using the above mentioned FKM guidelines. The experimentally determined load-carrying capacity is critically compared to the calculation results. The focus is on the implementation of the fatigue strength analysis in the context of the corresponding boundary conditions. Topics such as the assumed knee point of the Wöhler curve to the fatigue strength range are discussed as well as the evaluation options for elastic-plastic FE results for the guideline „non-linear”.

Determination of Local Stresses and Strains within the Notch Strain Approach: The Efficient and Accurate Calculation of Notch Root Strains Using Finite Element Analysis

For the service life estimation of metallic components under cyclic loading according to strain-based approaches, a simulation of the elastic-plastic stress–strain path at the point of interest is necessary. An efficient method for determining this stress–strain path is the use of the load–notch-strain curve, as this is also implemented within the FKM guideline nonlinear. The load–notch-strain curve describes the relationship between the load on the component and the local elastic-plastic strain. On the one hand, this can be estimated from loads or theoretical elastic stresses by using notch root approximations. On the other hand, this can be determined in a finite element analysis based on the elastic-plastic material behaviour. This contribution describes how this latter option is carried out in general and how it can be optimised in such a way that the FEA requires significantly less calculation time. To show the benefit of this optimisation, a comparative calculation on an exemplary geometry is carried out.

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.

Using Quantitative Passive Thermography and Modified Paris-Law for Probabilistic Calculation of the Fatigue Damage Development in a CFRP-Aluminum Hybrid Joint.

Although metal to Carbon-fiber-reinforced-polymer (CFRP) hybrid-joints possess a high lightweight construction potential, their extensive application has to deal with interfacial stress concentrations promoting fatigue damage. Furthermore, the underlying damage processes and their influencing factors are still not completely understood. Besides interfacial property-gradients, generic shapes counteract a precise determination of local stresses or strains, respectively. Hence, new methods are required that combine non-destructive testing and fracture mechanics to account for the fatigue damage. In this work, data of mechanical fatigue testing of an aluminum-CFRP hybrid-structure is presented by means of the dynamic stiffness and the mechanical hysteresis. Additionally, in situ passive thermography allows for capturing the heat development due to delamination growth. Correlating the obtained data implies that faster delamination growth coincides with higher amplitude values of lock-in thermography and higher mechanical hysteresis. Supported by this observation, a model is formulated to calculate the local dissipation per loading cycle. Further integration into a Paris-law like formulation results in a calculation model to account for the mode-I fatigue delamination growth. Additional validation of the model parameters shows good agreement with the experimental data.

Stress Whitening as an Observation Method of Residual Stress in MABS Polymer Material through the Example of Holding Pressure in an Injection Molding Process

The effects such as warpage, dimensional instability and environmental stress corrosion, due to the presence of residual stresses in polymeric products, are strongly dependent on injection molding conditions. The holding time and holding pressure belongs to most important processing parameters, determining the dimensional stability and properties of injected goods. A new procedure based on a visualization technique was applied, where the levels of residual stresses of the samples were estimated. The experiments were performed for samples produced of translucent methacrylate acrylonitrile butadiene styrene (MABS), a commodity polymer with a high transparency, necessary for the optical visualization of the stress whitening. The samples produced by injecting molding were deformed to a constant elongation, to observe the dependent stress whitening effect subsequently used to evaluate the stress distribution. It was found that depending on the value of the injection holding pressure, various levels of residual stress and its distribution may be observed in MABS samples. These measurements conformed that the applied optical method is an easy-to-perform technique. The possibility to detect the residual stresses over the whole cross-section of the transparent product, without the necessity for local stress determination, is another significant advantage of this investigation procedure.

Determination of Local Stress Within Twins in a Twinning Induced Plasticity Steel

Discovered by Sir Robert Hadfield in 1881 [1], high manganese TWIP steels are currently one of the most attractive materials for structural applications in the automotive industry due to their...

Методы определения локальных напряжений в сварных соединениях труб (Обзор)

Методы определения локальных напряжений в сварных соединениях труб (Обзор)

Methods for determination of local stresses in welded pipe joints (Review)

Methods for determination of local stresses in welded pipe joints (Review)

Path independent limiting criteria in sheet metal forming

The Forming Limit Diagram (FLD) is a conventional failure diagram to estimate necking limits of sheet metal for proportional loading conditions. Previous studies reveal that the FLD is not suitable for predicting the influence of nonlinear strain paths. This paper presents methodical comparison among all common available strain path independent strain/stress based limiting criteria. All the strain path independent strain based limiting criteria (Traditional Failure Diagram (TFD), Extended Forming Limit Diagram (XFLD), Extended Stress Ratio Based Forming Limit Diagram (ESRFLD), and Polar Effective Plastic StrainDiagram (PEPSD)) and stress based limiting criteria (Traditional Stress based Failure Diagram (TFSD), Stress Based Forming Limit Diagram (FLSD), Stress Ratio and Stress Based Forming Limit Diagram (SRFLSD), Extended Stress Based Forming Limit Diagram (XFLSD), and Polar Effective Stress Diagram (PESSD)) are approximately path-independent for smaller amount of pre-straining and path dependent for large pre-straining conditions. From advance image correlation technique precisely determination of local strains near necked area is possible today. However direct determination of local stresses near necked area is not possible. Therefore, local stresses and equivalent stress are determined by employing both yield criterion and strain-hardening law. Similarly equivalent strain is calculated by the use of yield criterion. Therefore, the choice of yield criterion has an impact on the results for TFD, XFLD, ESRFLD and PEPSD. However, selections of both yield criterion and strain-hardening law have substantial influence on the results for TFSD, FLSD, SRFLSD, XFLSD and PESSD. The inherent calculation error can be minimized by generation of experimental data for each material and then selection of representable yield criterion and strain-hardening law. Improvement of experimental techniques and generation of rigorous material data bank in various strain paths may help researchers to diagnose and resolve the issue. TFD, TFSD and XFLSD have inherent variables to take care the effect of through thickness stress, however rigorous experimental verification is needed before the field application.

Laue-DIC: a new method for improved stress field measurements at the micrometer scale.

A better understanding of the effective mechanical behavior of polycrystalline materials requires an accurate knowledge of the behavior at a scale smaller than the grain size. The X-ray Laue microdiffraction technique available at beamline BM32 at the European Synchrotron Radiation Facility is ideally suited for probing elastic strains (and associated stresses) in deformed polycrystalline materials with a spatial resolution smaller than a micrometer. However, the standard technique used to evaluate local stresses from the distortion of Laue patterns lacks accuracy for many micromechanical applications, mostly due to (i) the fitting of Laue spots by analytical functions, and (ii) the necessary comparison of the measured pattern with the theoretical one from an unstrained reference specimen. In the present paper, a new method for the analysis of Laue images is presented. A Digital Image Correlation (DIC) technique, which is essentially insensitive to the shape of Laue spots, is applied to measure the relative distortion of Laue patterns acquired at two different positions on the specimen. The new method is tested on an in situ deformed Si single-crystal, for which the prescribed stress distribution has been calculated by finite-element analysis. It is shown that the new Laue-DIC method allows determination of local stresses with a strain resolution of the order of 10(-5).

Simplified Thermoelastoplastic Models for Determination of Global and Local Stress in Coke Drums

Coke drums as major pressure vessels in delayed coking process experience cyclic thermal and mechanical loading during operation. The elastic-plastic behavior of coke drums including global and local (hot and cold spot) stress distribution has been analyzed using finite element analysis (FEA) method in our previous study. Here, instead of using FEA simulations, simplified theoretical models (STM) for global and local stress determination are developed. The global stress model is developed based on a single cladded shell element with temperature and pressure loading cycle. The local stress model is built on an axisymmetric circular cladded plate experiencing a non-uniform temperature distribution history. With the same input data, stress and strain values estimated by the STM are consistent with those from the FEA. An ease-of-use software package with built-in STMs is developed. It is the objective of this paper to show that STM can be a very time- and cost-saving analysis tool for general industrial users compared to professional commercial FEA software.

Local Stress Analysis in an SMA during Stress-Induced Martensitic Transformation by Kossel Microdiffraction

The Kossel microdiffraction in a scanning electron microscope allows for local stress determination. This technique has been applied to monitor stress evolution within grains of austenite in the course of martensitic transformation in a shape memory alloy. Kossel diffraction patterns were recorded during in situ tensile straining of Cu-Al-Be alloy. These innovative measurements show large stress heterogeneities between grains, with the stress ratio exceeding two. As martensite variants are stress-induced, shear stress components appear in individual grains of austenite.

Local stress determination in chromia-former thanks to micro-Raman spectroscopy: A way to investigate spontaneous delamination processes

Spontaneous delamination process for α-Cr2O3 thermal oxide films growing on NiCr-30 alloys is studied thanks to micro Raman spectroscopy. In particular, stress maps are performed through and around buckles developed on chromia films. Depending on the cooling rate at the end of the oxidation process, different buckle types appear which are investigated. Associated residual stress distribution clearly evidences the stress release field. In addition, geometrical features are determined for the different buckle types, and from comparison with modelling describing buckle formation and propagation, it is possible to get the interface toughness distribution.

Determination of local stress–strain properties of resistance spot-welded joints of advanced high-strength steels using the instrumented indentation test

For spot-welded joints, the resistance to mechanical stress depends on the local strength properties and gradients in the weld area. The commonly used methods for investigating the stress–strain behaviour across the weld area are connected with a high level of sample preparation and with considerable limitations in local resolution. A promising method for determining locally resolved stress–strain curves is the instrumented indentation test in connection with the method of representative stress and strain (RS) and the method of artificial neural networks (NNs). The stress–strain properties of the weld nugget and the base metal determined by these two methods are compared and discussed, additionally in relation to the stress–strain curves obtained from the tensile test. The measured Vickers hardness across the weld area is compared with the evaluated local stress–strain properties. Three steels used in automobile manufacturing are investigated: mild steel DC04 and two advanced high-strength steels (TRIP steel HCT690T and martensitic steel HDT 1200M). The results of the two methods (RS and NN) show good correspondence for the base metal area but some significant differences for the weld nugget. Comparing the data across the weld area, no evidence of the presence of residual stress (which would influence the results) was found.

On characterisation of local stress–strain properties in friction stir welded aluminium AA 5083 sheets using micro-tensile specimen testing and instrumented indentation technique

Stress–strain property variations in welded joints are frequently characterized by determining local stress–strain curves across the joint. A commonly and widely accepted method is testing of micro-tensile specimens where the specimens have to be mechanically extracted form the joint and subsequently tested and analysed similar to that of a conventional tensile tests. A recently developed procedure for the determination of local stress–strain properties is based on instrumented indentation testing where the force–depth curves are analysed via artificial neural networks in order to derive local stress–strain curves. In order to verify this new method on welded joints, this method has been applied to a friction stir weld in 3mm thick aluminium AA 5083 sheet material. It turned out that the local stress–strain curves derived by instrumented indentation tests and neural networks are compatible with that obtained from micro-tensile testing. Further on, the article gives advises on a proper tests specimen preparation as well as on the treatment of possible residual stresses.

Fringe analysis by phase shifting technique for birefringent fluid studies

We have developed a new optical method to evaluate the tensio-optic behaviour of a fluid in a Couette cell. This method is based on the analysis of the phenomenon of light scattered from a laser beam passing through a sheared fluid. A phase shifting technique was added to increase the measurement accuracy and the range of possible applications. For example, with this modification the problem of low shear stresses can be analysed. This improvement enables measurements to be made even if there is less than one visible fringe. This method also allows the determination of local stresses within the fluid. For a classical Newtonian or shear thinning fluid, a linear correlation between fringe order and shear stresses is observed and the tensio-optic coefficient may then be calculated. A yield stress thixotropic fluid is also analysed to demonstrate the relevance of the developed method. This test shows two distinctive behaviours with two different tensio-optic coefficients relative to a solid state and a liquid state.

Local stress determination in shallow trench insulator structures with one-side and two-sides pad-SiN layer by polarized micro-Raman spectroscopy extraction and mechanical modelization

Different Si oxide dimension (OD) geometrical features, consisting of shallow trench insulator (STI) structure, and processes of fabrication such as one-side pad-SiN layer and two-sides pad-SiN layer are implemented to investigate the residual mechanical stress in Si OD through full process flow of modern semiconductor device. The Raman spectroscopy with polarized incoming laser light and the technology computer aided design (TCAD) simulation tool are used to estimate and extract the stress distribution, which will influence the device performance seriously, along the different axes. A technology computer aided design tool, ANSYS, is upgraded to yield stress fields in the deep submicron complementary metal-oxide-semiconductor devices. From a practical viewpoint in the modem manufacture of semiconductor, it is demonstrated and proposed in this work that the use of one-side pad-SiN (chemical vapor deposition (CVD)-SiN) layer and optimum STI/OD structure can allow significant reduction of the compressive stress in the active region, which will enhance the electron mobility due to the smaller STI trench volume for the filling of compressivelike STI oxide material and suitable stress distribution, respectively.

Multiscale strategy for the determination of magneto-elastic behaviour: Discussion and application to Ni-Zn ferrites

Micromagnetic models are usually based on energetic minimisation procedures at the magnetic domains scale. Two main difficulties associated to these approaches concern the determination of local stress and magnetic field. This key point, defined through specific localisation and homogenisation procedures, is discussed. A comparison between a recently proposed multiscale modelling and experimental data collected from isotropic Ni-Zn ferrites is finally presented as an illustration.

Measurement of residual stress in air plasma-sprayed Y 2O 3–ZrO 2 thermal barrier coating system using micro-Raman spectroscopy

Residual stress in air plasma-sprayed Y 2O 3 stabilized ZrO 2 thermal barrier coating (TBC) has been measured using micro-Raman spectroscopy. The frequency shift of Raman peak from tetragonal phase (≈640 cm −1) is used for determination of local stress in TBC. The relation between Raman peak shift and applied uniaxial stress is measured using a freestanding specimen. The relation is used to determine actual residual stress of the TBC layer coated on a stainless steel substrate. The uniaxial stress, σ ¯ t b c U , and Raman peak shift, Δ ν, on the freestanding TBC layer follows: Δ ν = Π ¯ σ ¯ t b c U ( Π ¯ = 25 c m − 1 GP a − 1 ) . The average in-plane residual stress in this layer is smaller than that obtained from thermal stress, although the trend of measured values versus thickness follows the estimated thermal stress-substrate thickness relation. This experimental evidence suggests that measurement by micro-Raman spectroscopy is an effective tool to determine local and average residual stress in the Y 2O 3 stabilized ZrO 2 TBC layer.

Comparative fatigue strength assessment of a structural detail in a containership using various approaches of classification societies

A comparative study on fatigue strength assessment procedures used by the classification societies has been performed by Committee III.2, ‘Fatigue and Fracture’, of the International Ship and Offshore Structures Congress (ISSC’2000). A pad detail on the longitudinal coaming of a Panamax container vessel was selected as an example. This detail was chosen because of the well-defined loading due to hull girder bending. Large differences in predicted fatigue lives were found, ranging from 1.8 to 20.7 years. The spreading of results is attributable to assumptions regarding loads, local stress determination and S–N curve. For comparison, a direct calculation of loads using the spectral method was performed. Also this calculation showed a relatively short fatigue life of 5.3 years, although the structural detail is considered not to be prone to fatigue failures.