Cyclic Material Properties Research Articles

The analysis of cyclic properties of Ti-6Al-4V titanium alloy at room and liquid nitrogen temperature

This paper presents the analysis of Ti-6Al-4V titanium alloy test results. Fatigue tests were carried out at room temperature and at the temperature of liquid nitrogen using single sample method. The values of plastic strain, cyclic modulus, cyclic yield stress and fatigue fracture surface were analyzed. The results of the tests were compared to the results from the monotonic tensile test. The presented analysis allowed to determine the value of the total strain at which there is a significant change in the material properties of the tested alloy. Two intervals are distinguished on the basis of this value, which are characterized by the range of low strain and the range of high strain. Different material properties were used in these intervals for analytical description of the cyclic material properties. The analytical description was based on the modification of the Ramberg-Osgood model.

List of abstracts

. Results obtained in the analysis of single layer and multilayered carbon stacks produced by PLD at low temperature will be presented. The mechanical properties of carbon single layers deposited at various laser pulse fluences, like hardness and elasticity, were determined by means of nano indentation measurements. It will be shown that a wide variety of mechanical properties can easily be adjusted by adapting the process parameters.

Determining Effective Length for 40 HM-T Steel by Use of Non-Local Line Method Concept

Abstract In the paper, the concept of non-local line method is presented and used for determining the effective length for notched elements. Experimental tests and calculations were performed for 40 HM-T (42CrMo4+QT) steel made specimens of two types, i.e. smooth specimens, and notched specimens with notch radius equal to 0.2 mm, 0.5 mm, 0.8 mm, and 1 mm. The performed FEM calculations took into account the multi-linear hardening model and cyclic material properties. The concept of the presented non-local line method bases on finding the position of critical plane and determining the effective length, meant as the fracture process zone. During numerical stress gradient simulations, also the weight function was implemented. It was observed that the effective length increases as the load increases.

Safety margin against the fatigue fracture under random loading

The fatigue lifetime of the structure in operation is express in the number of working cycles, kilometers or time depending units. By harmonic process of loading is possible to express the safety margin as reserve against the required lifetime in the form of number cycles or amplitude of loading. By nonharmonic character of loading process in operation is difficult to express the level of safety margin as the reserve of loading (safety margin in loading). On the case of towing trailer will be discussed more ways of expression the reserve (safety margin) of structure against the fatigue fracture for required fatigue lifetime. For experimental acquired loading process in the most loaded cross-section will be discussed concept of determination of the level safety as reserve in the form of loading parameters. In the case, when the peaks of loading process exceeding the material yield stress (in the most loaded point of the structure) will be discussed concept of expression of loading in the form of time-varying process of strain. By using the cyclic material properties obtained in mode with controlled total strain is possible the safety margin to express also as the multiple of process loading parameters.

Method for Describing Fatigue Processes in Structural Materials

The article studies one of the methods for phenomenological description of fatigue processes in structurally-inhomogeneous structural materials. The initial statement about the interrelationship between the static and cyclic material properties is investigated using the complete stress-strain curves or diagrams method (CSSD). Based on the analysis of the mathematic modeling results, the interrelationship is predicted to exist between the highly-localized fatigue process in a structurally-inhomogeneous material and the degradation of a static CSSD for a macroscopic specimen. It is noted that the conditions under which the specimen fails in a testing machine are similar to the ones for the material in a construction. Then the results are given for the direct experimental testing of the model predictions, illustrated with the examples of cyclic degradation of several structural materials. The tests on multiple-cycle fatigued specimens reveal degradation of several mechanical properties. The justification for the selection of the available plasticity of a material as a parameter representative of the fatigue process is given. The authors describe a few examples of building one-parameter cyclic degradation models for steels and some prospects of using the complete strain-stress diagrams for various purposes.

Influence of Material Structural Inhomogeneity on Fracture Strength of Constructional Elements

The issue of material and construction fracture is one of unresolved problems in mechanics. The phenomenological approach used by mechanics of softening materials is taken by the authors for the purposes of solving that problem. Deformation of the “machine – model specimen” system is analyzed in the article using catastrophe theory for various boundary conditions. Computer simulations using a material’s calculation model allowed to establish nonlocal character of fracture criteria and correlation between static and cyclic properties of the model material. It is shown that the fracture scenario for a mechanical system depends on the degree of structural inhomogeneity of the material and the loading conditions. Regarding the fracture as a loss of deformation stability leads to a new understanding of the mechanical system’s ultimate state. The obtained results could be used to improve methodologies for calculating life and durability of constructions.

Low-cycle multiaxial fatigue behaviour and fatigue life prediction for CuZn37 brass using the stress-strain models

The low-cycle, strain-controlled fatigue tests have been conducted on CuZn37 brass (ASTM: C27200, EN: CW508L). Machined specimens have been subjected to uniaxial, proportional and non-proportional axial-torsional loadings. The monotonic and cyclic material properties have been determined for the loadings used. The obtained fatigue lives have been analysed in reference to the total Huber-von Mises equivalent strain ranges, the hysteresis loops energy in the loading cycle and the total plastic strain energy in the fatigue test. The relations between the plastic strain energy and the total as well as the plastic strain have been described. On the basis of the obtained experimental results, the selected stress-strain multiaxial fatigue criteria have been evaluated. In the conclusion it was found that fatigue lives predicted using the Fatemi-Socie and Garud criteria are closest to the experimental ones.

Possibility of fatigue damage detection by non-destructive measurement of the surface hardness

The cyclic material properties of metals are reflection of their internal structure and chemical composition. The fatigue process inside material is the consequence of the cyclic plastic deformation localized in the points of stress concentration. The sleep processes inside material induced by cyclic loading gradually lead to creating slip curves, slip bands and then generating and spreading of microcracks. Those processes change the state and also properties at the surface of material. This fact led to the idea of fatigue damage level detection by measuring the hardness at the surface layers. The presented contribution will present results of surface hardness measurements of material specimens during cyclic loading. There will be also introduced the methodology of surface hardness measurement by Brinell´s method and hardness measurements of constitutive phases by Vickers method on steel specimens cyclically loaded. The results will be presented graphically in the form of dependency between that surface hardness respectively constitutive phases and the number of cycles to fracture. Measurements were made for several levels of loading stress amplitude and for two kinds of steel material. Obtained experimental results are analyzed and discussed in relation of other experimental measurement in detail.

Optimized Durability Prediction of Cast Iron Based on Local Microstructure

The microstructure of cast iron is decisively influenced by the composition, the applied metallurgy and the process conditions. Differences in microstructure mainly influence the local mechanical properties. In conventional durability predictions, local microstructure and local defects in the casting are not taken into account. The assumption of homogeneous material properties applying one single material dataset for the prediction of fatigue strength usually leads to a misinterpretation in the assessment of local risks. Furthermore, the material potential of the casting will not be used in full. Safety margins result in an over design and unnecessarily high weight of the component as well as increased casting part costs. The designer can only benefit from the potential of the material cast iron, if the real spectrum of local material properties is considered in durability calculations. Current casting process simulation tools allow the prediction of local microstructure and resulting static mechanical properties as a function of alloy composition, applied metallurgy and local solidification and cooling conditions. Within a German research project, the link between casting process simulation and cyclic material properties was established for different cast iron materials for the first time. S–N curves (Woehler curves), which depend on local microstructure close the gap between casting process simulation and lifetime analysis. This innovative approach leads to improvements in lifetime predictions. It was shown that the consideration of local fatigue strength based on this innovative approach leads as well to an increasing number of cycles to failure as to a correct prediction of the location of crack initiation. Compared to the conventional design of castings, a substantial qualitative and quantitative improvement in assessing the real performance of cast iron materials can be realized.

Identification of local cyclic stress‐strain curves at material inhomogeneities on the basis of digital image correlation

Low‐cycle fatigue (LCF) tests on butt‐welded joints revealed that material inhomogeneities become decisive for the fatigue behavior at elastic‐plastic strain amplitudes and the geometrical notch is no longer the failure location – as observed in high‐cycle fatigue (HCF) regime. Digital image correlation techniques enable the measurement of local strains at the surface of the welded region. A new method is proposed that is able to calculate related local stresses and identifies local cyclic material properties in structures of inhomogeneous material and geometry. This method combines strain measurements and Finite Element simulations. It is a fast iteration procedure that is able to perform the identification within less than ten iterative steps at several hundred local points. Transient material behavior is easy to capture and an application at high temperatures is anticipated due to the method is based on materials mechanics.

Ductile Crack Growth Modeling under Low Cycle Fatigue Loading Condition

This paper presents a numerical method using FE damage analysis to simulate ductile tearing in CT specimen under low cycle fatigue loading conditions. To define a cyclic material properties, the nonlinear kinematic hardening model is adopted. The damage model is defined based on the ductility exhaustion concept using the multi-axial fracture strain energy concept. The proposed model is then applied to simulate two cyclic fracture toughness tests with different R-ratios. Simulated results show overall good agreement with experimental results.

Calculation of Metalwork Life with Allowance for Cyclic Degradation of Material in Operation

The article describes an alternative method of calculating durability and life of metal construction elements. Parameters of complete deformation diagram are used as a representative of the fatigue process. Such diagram is plotted when a small sample of material is stretched in a rigid load-applying unit. Determination of process kinetics from a mechanical experiment enhances possibilities for phenomenological description of constructional elements’ fatigue fracture. Proper usage of material cyclic degradation model in the stage of fatigue crack development is possible when one explicitly allows for anomalies of a constructional element’s surface layer properties. Within the scope of general concept of correlation between static and cyclic material properties a scheme for differentiating between a component’s surface layer and base metal properties is presented. Explicitly allowing for anomalies of a component’s surface layer properties makes it possible to calculate its durability and life within the unified model of material.

Small-scale specimen testing for fatigue life assessment of service-exposed industrial gas turbine blades

Service-exposed industrial gas turbine blades made from conventionally cast nickel-base superalloy IN738 were investigated in order to study the effect of service-induced microstructural changes on the tensile and cyclic material properties. Optical microscopy and scanning electron microscopy were used for the microstructural characterization of the gas turbine blades. The microstructural examinations showed a degraded microstructure in the airfoil section, such as γ′ coarsening, MC carbide decomposition as well as M23C6 carbide formation. Uniaxial tensile and isothermal low cycle fatigue tests were carried out at 850°C using small-scale specimens manufactured from the blade root and airfoil section. The service-induced microstructural changes in the airfoil section yielded to a reduction of the tensile properties. Furthermore, a lower fatigue life of airfoil specimens compared to the blade root specimens was observed. Based on the determined fatigue data the residual fatigue life of the gas turbine blades was estimated. Finally, the fatigue crack paths were studied by optical microscopy.

Non‐local line method for notched elements with use of effective length calculated in an elasto‐plastic condition

AbstractThe paper presents a non‐local line method used to the fatigue life calculation of notched elements. The presented method is based on the concept of an effective length which determines the size of the equivalent fatigue zone. Effective values of normal stress calculated in the critical plane with a weight function were applied when determining the equivalent fatigue zone. Simulation studies were performed for two types of steel and two types of loading. Five different series of tests and simulations were used. Experimental studies were carried out for 40 HM‐T and EA4T steels. These materials are used in railway industry, including the manufacturing of coupling bars. The notched test specimens contained notches with a tip radius of 0.2, 0.5, 0.8 and 1 mm. Stress calculations were performed using the finite element method by adopting cyclic material properties described by the model of a multi‐linear hardening. Non‐local calculations were performed in a defined critical plane for normal stress distribution and a weight function. As a result, the function of variation of the effective length depending on the loading level and geometry of the notch has been determined.

Equivalent Fatigue Zone in a Notched Elements Determined by Use of Non-Local Line Method with Weight Function

The work presents non-local line method by which the equivalent fatigue zones were designated. These zones are one-dimensional efficient lengths in which operating stress variables cause initiation of fatigue cracks. The algorithm of the presented method considers the issues of multi-axial stress state, critical plane and weighting function. Calculations of stresses in the test element were performed by using the FEM assuming cyclic material properties, which are described with the model for multi-linear hardening. The empirical lifetime of elements with a notch was assumed to define length of the crack amounting to 0.1 mm. As the result of calculations, the dependence of effective length on nominal stress and radius of the notch was determined.

Fatigue assessment of full-scale welded crane runway girders*

Abstract This publication is focused on a critical plane approach to assess the influence of the multiaxial stress state on the lifetime of welded crane runway girders cyclically stressed by travelling wheel loads. Cyclic stress-strain curves of specimens both from the weld region and the base material of the girder are evaluated in strain-controlled reversible fatigue tests as a basis for the assessment of the lifetime. Finite element analyses are performed to estimate the multiaxial and non-proportional local stress condition acting on sharp notches of the girder, i. e. the weld toes and the weld root. The MPA AIM-life concept is used to evaluate the lifetime of crane runways on the basis of advanced fatigue parameters and the cyclic material properties. Furthermore, the notch stress approach according to IIW guideline [1] is applied to investigate the fatigue behavior. The estimated lifetimes are compared to experimental results of component fatigue tests conducted with full-scale welded crane runway girders.

Strain analysis at notch root in laser welded samples using material properties of individual weld zones

This paper presents results of the strain analysis at a notch root in a laser-welded joint subjected to cyclically variable stresses. The examined welds were made of S355JR (PN-EN 10025-2) ferritic steel. A theoretical analysis of strain was performed using cyclic material properties determined for individual weld zones. For the purpose of comparison, values of strains at a notch root were also determined using base metal properties. Experimental verification of determined strain values was performed using the laser grating interferometry method. Cyclic variations of material properties for base metal and welded joint zones were determined as per the standard and the methodology presented in previous study and described with the Ramberg–Osgood relationship. The above material properties were used to calculate strain at the root of two types of geometric notches machined in a laser-welded joint. Analytical calculations were performed using the Neuber’s rule, while numerical calculations with the use of the finite element method by means of the ABAQUS software.Test results confirmed good efficiency of the numerical strain analysis using cyclic material properties of the individual weld zones in low cycle fatigue.

Influence of changes of the bending plane position on the fatigue life

The paper concerns influence of changes of the bending plane position on the fatigue life. The obtained results were analyzed and compared with the fatigue results for oscillatory bending.The applied specimens were smooth, they had round sections, and they were made of the leaded brass CuZn40Pb2 (MO58). The results obtained under cyclic bending with the plane position change were compared with the results obtained for the specimens with the same parameters under pure oscillatory bending. A change of the bending plane position occurred every 10% fatigue life determined under pure oscillatory bending at the given amplitude of the bending moment, according to the defined fatigue characteristics. Calculated values of nominal stress in a cross section were recalculated according with cyclic material properties and values of elasto‐plastic stress were obtained.

High cycle fatigue properties of selective laser sintered parts in polyamide 12

Selective laser sintered parts in polyamide are increasingly being used in functional applications. The monotonic properties of these parts are well-known and documented. The cyclic material properties, on the other hand, are largely unknown. Therefore, in this paper a systematic analysis is made of fatigue properties and failure mechanisms of selective laser sintered parts subjected to fluctuating loading in tension/compression. Both plain and notched specimens are produced along perpendicular building directions. The fatigue behaviour of these parts is analysed and compared to injection moulded samples. In addition, the influence of the test frequency and the notch manufacturing method is studied.

Advanced Notch Strain Approach for Fatigue Calculation of Welded Components

The calculation methods for assessing the fatigue life of welded structures are gaining increasing importance in many technical applications, since they contribute in the decrease of the development costs and time. In various engineering applications, especially in the commercial vehicle sector with thick and thin components and welds of complex geometry and loading configurations, there is still a lack of experimentally verified guidelines for the calculation of fatigue life. The present paper shows an advanced procedure for FE modeling and fatigue life calculation of welds based on the Notch Strain Approach. It uses elastic-plastic cyclic material properties and can be applied in early stages of development when prototypes are not available. It can be applied to continuous and non-continuous welds, e.g. ones with pronounced terminations. Due to its local character, it covers both weld toe and root failures. To verify its accuracy, the proposed calculation procedure is applied to serial motor truck's rear axles providing complex 3D weld geometries subjected to cyclic loading. Calculated lifetime results are compared to experimental ones determined on a test rig. The agreement is satisfactory.