Load Increase Tests Research Articles

Acoustic emission of lattice structures under cycling loading relates process parameters with fatigue properties

Metamaterials, especially lattice structures, are of great interest for many application areas such as aerospace, automotive and medicine due to their adjustable mechanical properties and their low weight. Due to their complex geometry, lattice structures are usually manufactured additively, which causes a large variance in the manufacturing-related mechanical properties. In order to establish metamaterials in industrial applications under cyclic loading, the fatigue behavior needs to be investigated to evaluate the load capacity of these structures. Here we analyze the fatigue behavior of AlSi10Mg truss structures fabricated with L-PBF using a load increase test in combination with acoustic emission measurements. The acoustic signals are evaluated in terms of time-dependent amplitude signal and frequency spectrum. Increasing load and increasing specimen damage resulted in changes of the acoustic spectrum and the amplitude of the time signal. Based on the results, a correlation of specimen properties with build platform position in the manufacturing process could be established. Acoustic emission measurement as an in situ characterization method during cyclic loading is promising for surveillance of lattice structures in safety related applications.

Influence of Transformation Temperature on the High‐Cycle Fatigue Performance of Carbide‐Bearing and Carbide‐Free Bainite

This study investigates the high‐cycle‐fatigue (HCF) behavior of carbide‐bearing bainite (CBB) and carbide‐free bainite (CFB) fabricated at different transformation temperatures. The fatigue limit of each material is determined via staircase method using a 1 kHz resonant testing machine. A new load increase test is proposed as an efficient alternative to estimate the fatigue limit in HCF regimes. The assessment of the fatigue behavior is accompanied by data‐driven microstructural analyses via state‐of‐the‐art computer vision tools. The analyses reveal that the finer carbide distribution, which is obtained at lower transformation temperature, enhances the overall performance of CBB. Electron backscatter diffraction (EBSD) measurements of CFB before and after tensile testing evidence the transformation of retained austenite (RA) to martensite during deformation. The finer film‐like and stable RAs, which are promoted via reduction in transformation temperature, enhance the HCF properties by absorbing the energy required for fatigue crack propagation through improved transformation‐induced plasticity. However, blocky unstable RA and/or martensite‐austenite (MA) islands at prior austenite grain boundaries deteriorate the HCF properties of high‐temperature CFB. Furthermore, unindexed regions in EBSD maps are effectively used to differentiate the MA islands of CFB, as validated by scanning electron microscopy (SEM) images and deep learning‐based MA island segmentation.

A new short-time procedure for fatigue life evaluation based on the linear damage accumulation by Palmgren–Miner

The aim of the presented research is to develop a new short-time-procedure based on the linear damage accumulation according to Palmgren–Miner, which can provide an S-N curve in the HCF regime based on results of only one fatigue test. Load increase tests were carried out to determine the partial damage per load level from the material response by using NDT-methods. By converting the calculated partial damage of each load step into the corresponding number of cycles to failure, an S-N curve can be generated. This research focuses on a quenched-tempered 20MnMoNi5-5 steel. The validation was performed on three further steels.

Influence of the Inherited Structure Induced by Al and Si Alloying on Microstructure Evolution and Mechanical Properties of 100Cr6 Steels

The fatigue lifetime of high‐strength 100Cr6 steels can be improved by an increased content of retained austenite that can be induced by Al and Si alloying. The related deformation‐induced retention of the austenite–martensite transformation during cyclic loading increases their local strain hardening capacity. However, for those 100Cr6 steels containing retained austenite, sufficient dimensional stability must be ensured. In this study, two standard 100Cr6 steels alloyed either with 1.5 wt% Al or 1.5 wt% Si (to diminish carbide formation and accordingly promote austenite retention) are laboratory melted and processed to adjust a microstructure of bainite, retained austenite, and carbides. The segregation simulation in the as‐cast condition and the corresponding microstructures in the forging and heat‐treating conditions are investigated. The inheritance of chemical heterogeneity leads to structural heterogeneity on both the nano‐ (nm) and micro (μm) scales. This heterogeneity is much more pronounced in the Al‐alloyed steel, which can be attributed to inheritance from the as‐cast state. While the results of the quasistatic tensile tests are comparable for both alloys, the cyclic load increase tests indicate a higher fatigue strength of the Si‐alloyed steel, which can be explained with the more homogenous microstructure and the finer distribution of the retained austenite.

Short-Time Fatigue Life Estimation for Heat Treated Low Carbon Steels by Applying Electrical Resistance and Magnetic Barkhausen Noise

Tensile tests and fatigue tests on differently heat-treated low carbon (non- and low-alloy) steels were conducted and accompanied by non-destructive electrical resistometric (ER) and magnetic Barkhausen noise (MBN) measuring devices, in order to establish an improved short-time fatigue life estimation method according to StressLife. MaRePLife (Material Response Partitioning) is the hereby proposed method for calculating S-N curves in the HCF regime, based on the partitioning of material responses acquired during the above-mentioned mechanical tests. The rules were set to make use of the information gathered from pre-conducted tensile tests, which helps to determine the parameters of two load increase tests (LIT) and two constant amplitude tests (CAT). The results of the calculated S-N curves were satisfactory and could be verified by more separately conducted fatigue tests on specimens under different material conditions.

Characterization of the Fatigue Behaviour of Low Carbon Steels by Means of Temperature and Micromagnetic Measurements

Investigations on low carbon (non- and low-alloy) steels were conducted in form of load increase tests (LIT) and constant amplitude tests (CAT) to find the correlation among material behaviour, mechanical load, and the type of NDT method. With the help of preprogrammed load-free sequences, the thermal impact on magnetic Barkhausen noise (MBN) measurement can be avoided, so that the cyclic deformation properties of material responses can be interpreted more precisely. The results indicate differences between the change in temperature and the MBN-derived variable during LITs and CATs regarding the demonstration of the incubation stage and the cyclic hardening behaviour.

Comparison of process behavior, microstructure and mechanical properties of ultrasound enhanced friction stir welded titanium/titanium joints

AbstractTitanium as a high-performance material offers great potential for a wide range of applications with different aspects like lightweight-constructions, biocompatibility as well as design and is used in various industrial areas like the transportation sector or in medical and sports engineering. To exploit the full potential of titanium, there is still a need for innovative and efficient joining techniques compared to conventional methods. The process of friction stir welding as a solid-state joining method is able to produce high-quality similar joints of titanium. Nevertheless, there are still some challenging aspects when friction stir welding titanium like the formation of oxide layers that affect the mechanical properties especially in the field of fatigue negatively. The superposition of power ultrasound on the friction stir welding process can address this issue. Therefore, the hybrid process of ultrasound enhanced friction stir welding was applied on similar Ti6Al4V/Ti6Al4V joints in a butt joint configuration to reduce the amount of oxide layers and improve the fatigue lifetime of the joints. For the joint configuration, a positive impact of the power ultrasound could be identified by the breakup of oxide layers and an increase in the tensile strength of 16%. Also stepwise load increase tests revealed a higher estimated fatigue strength for ultrasound enhanced friction stir welded specimen in this butt joint configuration.

Fatigue and fracture of ultrasonically welded aluminum alloys to carbon fiber reinforced thermoplastics

AbstractThis work evaluates the fatigue behavior of ultrasonically welded aluminum AA5024) to fiber reinforced thermoplastic (PEEK) joints at a stress ratio of R > 0. Initial load increase tests were carried out and, based on a detected material reaction caused by fatigue damage, the load horizon for subsequent constant amplitude tests was derived. After identifying characteristic stress level, microscopic investigations of hybrid fracture surfaces were carried out. Damage behavior and failure mechanisms in comparison to monotonic tensile tests were investigated and discussed in detail. For a fatigue limit of 2·106 cycles, the applied maximum force corresponds to 35% of the maximum lap shear force for monotonic testing. A drop in stiffness to 88% close to failure was observed.

Cyclic deformation behavior of Mg–SiC nanocomposites on the macroscale and nanoscale

AbstractMetal‐ceramic nanocomposites are promising candidates for applications necessitating light weight and excellent fatigue resistance. We produced Mg–SiC nanocomposites from mechanically milled powders, yielding a homogeneous nanocrystalline structure and excellent quasistatic strength values. Little is known, however, about the fatigue behavior of such composites. Here, we used load increase tests on the macroscale to yield estimation values of the fatigue endurance limit. Fatigue strength increased significantly for the materials processed by the powder metallurgical route. We further investigated the cyclic deformation behavior under stress‐controlled conditions on the macroscale and nanoscale. Cyclic nanoindentation showed that indentation depth and cyclic plastic deformation decreased with increasing reinforcement content, hinting to a higher cyclic strength and corroborating the results from the macroscopic load increase tests. Our results therefore show that cyclic nanoindentation reliably determines the plastic deformation behavior of Mg nanocomposites offering the possibility of fast material analysis.

Effect of Quenching and Partitioning Heat Treatment on the Fatigue Behavior of 42SiCr Steel

The manufacturing of advanced high-strength steels with enhanced ductility is a persistent aim of research. The ability of a material to absorb high loads while showing a high deformation behavior is a major task for many industrial fields like the mobility sector. Therefore, the material properties of advanced high-strength steels are one of the most important impact factors on the resulting cyclic fatigue behavior. To adjust advanced material properties, resulting in high tensile strengths as well as an enhanced ductility, the heat treatment process of quenching and partitioning (QP) was developed. The quenching takes place in a field between martensite start and martensite finish temperature and the subsequent partitioning is executed at slightly elevated temperatures. Regarding the sparsely investigated field of fatigue research on quenched and partitioned steels, the present work investigates the influence of a QP heat treatment on the resulting microstructure by light and scanning electron microscopy as well as on the mechanical properties such as tensile strength and resistance against fatigue regarding two different heat treatment conditions (QP1, QP2) in comparison to the cold-rolled base material of 42SiCr steel. Therefore, the microscopic analysis proved the presence of a characteristic quenched and partitioned microstructure consisting of a martensitic matrix and partial areas of retained austenite, whereas carbides were also present. Differences in the amount of retained austenite could be observed by using X-ray diffraction (XRD) for the different QP routes, which influence the mechanical properties resulting in higher tensile strength of about 2000 MPa for QP1 compared to about 1600 MPa for QP2. Furthermore, the transition for the fatigue limit was approximated by using stepwise load increase tests (LIT) and afterwards verified by constant amplitude tests (CAT) in accordance with the staircase method, whereas the QP1 condition reached the highest fatigue strength of 900 MPa. Subsequent light and scanning electron microscopy of selected fractured surfaces and runouts showed a different behavior regarding the size of the fatigue fracture area and also differences in the microstructure of these runouts.

Performance and mechanisms of different tribological thermoplastic composites in sliding contact with steel

Polyetheretherketone-based tribological materials are nowadays firmly established in a wide variety of technical applications. In parallel, numerous publications on the improvement of tribologically usable PEEK materials by means of different fillers have been presented from academic research. This paper shows, using five tribological PEEK compounds provided by industry as examples, a comparative evaluation in a wide pv range in sliding contact against steel. Using a special test technique, so-called load increase tests, characteristic parameters such as the coefficient of friction in the stationary friction phase and the integral wear are determined and the wear manifestations are investigated. Element analyses lead to the conclusion that, beyond the addition of carbon fibers and solid lubricants, the best tribological properties can be achieved by adding a wide variety of inorganic fillers.

Testing Procedure for Fatigue Characterization of Steel-CFRP Hybrid Laminate Considering Material Dependent Self-Heating

Combining carbon fiber reinforced polymers (CFRP) with steel offers the potential of utilizing the desired characteristics of both materials, such as specific strength/stiffness and fatigue strength of fiber reinforced polymers (FRP) and impact resistance of metals. Since in such hybrid laminates multiple material layers are combined, a gradual failure is likely that can lead to changes in mechanical properties. A failure of the metal partner leads to an increase in stress on the FRP, which under fatigue load results in increased self-heating of the FRP. Therefore, a suitable testing procedure is required and developed in this study, to enable a reproducible characterization of the mechanical properties under fatigue load. The resulting testing procedure, containing multiple frequency tests as well as load increase and constant amplitude tests, enabled characterization of the fatigue performance while never exceeding a testing induced change in temperature of 4 K. In addition to the development of the testing procedure, an insight into the manufacturing induced residual stresses occurring in such hybrid laminates, which impacts the load-bearing capacity, was established using finite element simulation. The gathered data and knowledge represents a basis for future in-depth investigations in the area of residual stress influence on the performance of hybrid laminates and highlights its importance, since not only the used testing procedure determines the measured fatigue performance.

Analysis and evaluation of mechanical descriptors from micro compression tests on spherical samples

Within the development of a novel high-throughput method for the discovery of new structural materials, a compression test on spherical micro samples is established as a short-term characterization process. In this study, the experimental investigations regarding the elastic–plastic deformation behavior of microspheres are carried out on the bearing steel X46Cr13 in 10 different heat treatment conditions. Characteristic values, so-called descriptors, can be derived directly from continuously measured force–displacement curves or are normalized by taking the sample geometry into account. In addition to micro compression tests with quasi-static loading, load increase tests are performed, in which a spherical sample is repeatedly loaded and unloaded with progressively increasing loads. Loading and unloading phases and the development of the displacement amplitude of the hysteresis are analyzed. New descriptors of load increase tests are extracted and discussed regarding their statistical significance and validity as descriptor showing a high potential for a fast characterization of the material behavior during cyclic loading. The descriptor-based micro compression test proves to be a robust method and is a promising approach for a fast evaluation of different material conditions.

Adapted Process Strategies in Front Face Flow Drilling and Thread Forming of Lightweight Casting Materials

By using modern technologies to produce detachable joints in lightweight components, it is possible to reduce material consumption and manufacturing times. By front face flow drilling of lightweight cast materials, expanded bore walls are formed into thin-walled profiles, which are used as core holes for internal threads. The flow drilling process has to be adapted to the specific properties of the regarded materials AZ91 and AlSi10Mg. Thus, enhanced adjustments make it possible to increase the bore qualities significantly. Particularly in order to improve the roundness of the holes, the influence of decreased feed rates on the shape of the flow-drilled holes is analysed. Furthermore, this paper deals with the influence of the process temperature during flow drilling. By varying the predrilling diameter as well as the peripheral speed, the friction between the tool and the workpiece can be significantly influenced. This directly affects the thermally induced formability of the lightweight alloys. In order to produce high strength threads, cold forming is used instead of conventional tapping. In particular, in the context of the adapted flow drilling strategies, the surface qualities and the threads profiles are investigated in detail. Finally, continuous load increase tests were conducted to evaluate the fatigue properties of the formed internal threads using various strategies.

PhyBaLSL – Short-time procedure for the determination of the fatigue lifetime of metallic materials under service loading

Because components are usually not loaded with periodical waveforms and constant amplitudes, but with temporally variable, cyclic loadings, fatigue testing under near service loading conditions is essential to optimize the component’s design. However, this kind of fatigue tests requires high efforts in material, time and costs, which can be significantly decreased by using efficient short-time procedures. In the present work, a modification of the physically based lifetime calculation method (PhyBaLLIT) for service loading conditions, i.e., PhyBaLSL, was used to estimate the Gassner curve of ductile cast iron EN-GJV-400 resulting from cyclic loading based on the Car Loading Standard (CARLOS). For fatigue tests, the first 5 000 cycles of CARLOS were used for periodically repeated loading intervals (LIs) with defined maximum stress σmax, CARLOS. In the presented work single step tests (SSTs) with constant σmax, CARLOS as well as load increase tests (LITs), in which σmax, CARLOS was stepwise increased after each LI, were performed. Because for PhyBaLSL the determination of the material’s cyclic deformation behavior is a prerequisite, measuring intervals (MI) with constant stress amplitude σa, MI were performed after each LI. The influence of MIs on the fatigue lifetime and cyclic deformation behavior was analyzed by using different σa, MI, showing no significant influence but higher resolution of fatigue-induced changes with increasing σa, MI. Based on the measurements obtained in the LIT and two SSTs, the Gassner curve could be determined with PhyBaLSL showing a good correlation to the lifetime data obtained from additionally conducted SSTs. Furthermore, the results from LITs enable a rough estimation of the expected fatigue strength at 2 × 106 cycles. In addition to the results obtained from cyclic deformation behavior determined in MI, the change in electrical resistance ΔR was measured in the specimens’ gauge length during the fatigue tests. This measurement does not require a constant stress amplitude σa and hence, can be used to characterize the fatigue behavior without using MIs. The presented results demonstrate that ΔR enables an adequate estimation of the Gassner curve by using PhyBaLSL.

Investigation of the fatigue behavior of thermoplastic composites by load increase tests

Fatigue is one major load case in many structures for transport applications. New materials often lack the necessary data base for a fast application in cyclic loaded components due to time consuming testing series. The aim of this study is the evaluation of the load increase test as method to determine a possible fatigue limit of glass fiber reinforced polyamide 6. Under the working hypothesis that cracks are the main contributors for heat emission, the results show that the investigated material exhibits a different behavior in comparison to thermosets. Instead of crack formation experimental and numerical data indicate that the matrix relaxes under fatigue loading. This relaxation could potentially lead to crack prevention but might also result in the observed sudden failure behavior of the material. These findings suggest a totally different behavior of thermoplastic composites under fatigue loading.

Thermodynamic entropy as a marker of high‐cycle fatigue damage accumulation: Example for normalized SAE 1045 steel

AbstractA nondestructive thermographic methodology is utilized to determine the fracture fatigue entropy for evaluating the fatigue damage in metals within the high‐cycle fatigue regime. Thermodynamic entropy is shown to play an important role in the fatigue process to trace the fatigue damage as an irreversible degradation of a metallic material being subjected to cyclic elastic‐plastic loading. This paper presents a method to evaluate fatigue damage in the normalized SAE 1045 steel being based on the concept of thermodynamic entropy and its nonlinearities. The procedure looks to be applicable to constant and load increase tests proven by experiments.

Characterization of Damage Evolution on Hot Flat Rolled Mild Steel Sheets by Means of Micromagnetic Parameters and Fatigue Strength Determination.

In continuous casting processes, inevitable voids (damage) are generated inside the material. The subsequent forming process of hot flat rolling offers the potential of healing these defects by closing the voids and bonding the internal surfaces. In this paper, different forming conditions from hot flat rolling process were characterized with micromagnetic measurement techniques and the influence of the damage evolution on the fatigue behavior was investigated. To characterize the reduction of voids through hot flat rolling processes, nondestructive testing techniques are required. Therefore, micromagnetic measurements such as Barkhausen noise, incremental permeability, and harmonic analysis were carried out, correlated with the number of voids, and compared with each other. The influence of damage evolution of different forming conditions on the fatigue behavior was characterized based on instrumented constant amplitude and multiple amplitude (load increase) tests. A significant increase in fatigue strength due to the hot flat rolling process, which leads to a reduction in the number of voids, was observed. In addition, the fracture surfaces of the specimens were analyzed in the scanning electron microscope.

A nonlinear fatigue damage model: Comparison with experimental damage evolution of S355 (SAE 1020) structural steel and application to offshore jacket structures

Miner’s rule is commonly used in fatigue life estimations and is also recommended by design standards. Recently, a few models have been proposed, to capture loading sequence effects more precisely than Miner’s rule. However, practical applications of these models have not been found, due to the requirement for additional material parameters. The authors have recently proposed a nonlinear fatigue damage model, which does not require any additional material parameters, other than the S-N curve. It can be applied by practicing engineers, using partially known S-N curves given in design standards, including the corresponding detail categories. The model has been verified with several materials for both damage evolution curves and fatigue life estimations. Verification of this model for S355 (SAE 1020) structural steel used in offshore structures is desirable for the industry. Experimental techniques, based on characterizing fatigue damage using physical quantities, have also recently been developed by the authors and are used for this verification. Subsequently, damage evolution curves under load increase test and constant amplitude tests are developed by observing the changes in plastic strain amplitude, electrical resistance and temperature. The corresponding fatigue strength curve is developed. This curve is used, together with the proposed damage index, for further verification of the proposed model for S355 (SAE 1020) structural steel. Finally, application of the proposed model is shown for an existing offshore jacket structure, and the results are compared with those of the conventional approach. Hence, the importance and significance of the proposed model is further established.

An integrated experimental and numerical method to assess the fatigue performance of recycled rail

Recycling of rail is practised in the railway industry to promote sustainability and economic efficiency. The functional reliability of recycled rail has to be addressed to ensure safe application. Studies on the reliability of railway rails place great emphasis on fatigue failure. However, scarcity of public domain data on recycled rails and limitation of experimental hardware capability has constrained the study on the fatigue of recycled rails. The aim of the investigation is to propose a novel integrated approach for exploring the fatigue performance of recycled rail effectively and efficiently. A high cycle fatigue test was conducted on a recycled rail specimen to obtain data for the validation of the finite element (FE) numerical model. Following this, the FE numerical model was incorporated with the stepwise load increase test (LIT) method. The integrated method gave a more conservative prediction of the fatigue performance than the analytical method. The shot blasting process induced compressive residual stress which affected the specimen’s fatigue performance. This result was demonstrated by the integrated method. Furthermore, this integrated method also successfully reduced the overall required test time. The predicted endurance limit of the specimen was 130.37MPa, accomplishing the BS EN 13674-1:2011 standard.