Determination Of Fatigue Limit Research Articles

A new method for determination of fatigue limit of composite laminates based on thermographic data

In this paper, a new fatigue limit determination method based on the thermographic data is proposed to overcome the limit of Risitano and Luong’s method. The maximum stress corresponding to peak value of the angle change normalized by its amplitude is defined as the fatigue limit. The method was evaluated by the experimental data of Multidirectional (MD) CFRP laminates in this study, and by the data found in the literature for different materials. The analyze of the results from different methods demonstrated that this method can effectively apply to determine the fatigue limit of composite laminates whatever their stacking sequence is.

Fatigue response of additively manufactured as‐built 15‐5 PH stainless steel and effects of machining and thermal and surface treatments

AbstractAdditively produced 15‐5 PH stainless steel has wide industrial applications, but the combined effects of heat treatment, machining, and shot‐peening and their order have not been deeply investigated. This topic is addressed here by a 2‐by‐3 experimental plan that has involved S–N curve and fatigue limit determination, using vertically built cylindrical samples, tested under rotating bending. The obtained responses have been analyzed by an ANOVA‐based statistical approach for comparison of fatigue trends. Results indicate that heat treatment without machining may be even detrimental for fatigue due to embrittlement. Conversely, machining with subsequent shot‐peening, even without heat treatment, has a remarkable impact and leads to a doubled fatigue strength with respect to as‐built material. This strength is also quite close to that achievable for wrought material. The study has been completed by micrography and fractography, to reveal the dependence of microstructure, crack initiation sites, and failure mode on the performed treatments.

A hybrid method for determination of fatigue limit and non-destructive evaluation of composite structures after low-velocity impact loading

One of the most challenging and still unsolved problems within the design and operation of composite structures is the prediction of its behavior under fatigue loading. This problem is even more complex when a composite structure contains damage, especially low-velocity impact damage considered as one of the most dangerous types of damage in composite structures. In this paper, the authors presented the hybrid method, which combines the evaluation of a fatigue limit of structures with low-velocity impact damage (LVID) with a non-destructive evaluation of this damage using thermocouples and thermography in a single test. The tests were performed on carbon and glass fiber-reinforced composite structures with LVID of various energies. The relation between fatigue limit and impact energy was demonstrated, and the observed damage mechanisms analysis was performed. Moreover, the performance of LVID detectability is analyzed, defining minimal self-heating temperature increase necessary for detection of LVID. The proposed method demonstrated high sensitivity to impact damage and simultaneously an ability of evaluation of fatigue life of a tested structure. Finally, the recommendations on measurement setup and parameters of testing are provided. It was demonstrated that the proposed hybrid method could be successfully used for rapid, precise and non-destructive evaluation of residual life of composite structures after impact loading.

Determination of Fatigue Limit by Static Thermographic Method and Classic Thermographic Method on Notched Specimens

The aim of the present work is to investigate the possibility to evaluate the fatigue limit of such a component through the so-called Static Thermographic Method (STM). This new approach has been proposed in 2013 by Risitano and Risitano to determinate the fatigue limit of metallic materials through static tensile tests. Investigating the trend of the surface temperature of the material during static tensile test, the method proposes to correlate the first deviation from linearity of the surface temperature to a damage limit that has been proven to have relation with the fatigue limit. Static tensile tests at different stress rate have been carried out on V-notched specimens. Stepwise fatigue tests have been carried out in order to apply the classic Thermographic Method that has been hugely validated to determinate the fatigue limit of metallic components. Static tests at different stress rate showed that the method is dependent on this parameter; anyway, this dependence does not affect the results achieved in terms of fatigue limit. The experimental results showed a good agreement with the energetic methods considered in the present work.

Energy Dissipation Measurement in Improved Spatial Resolution Under Fatigue Loading

Recently, a technique for rapidly determining a material’s fatigue limit by measuring energy dissipation using infrared thermography has received increasing interest. Measuring the energy dissipation of a material under fatigue loading allows the rapid determination of a stress level that empirically coincides with its fatigue limit. To clarify the physical implications of the rapid fatigue limit determination, the relationship between energy dissipation and fatigue damage initiation process was investigated. To discuss the fatigue damage initiation process at grain size scale, we performed high-spatial-resolution dissipated energy measurements on type 316L austenitic stainless steel, and observed the slip bands on the same side of the specimen. The preprocessing of dissipated energy measurement such as motion compensation and a smoothing filter was applied. It was found that the distribution of dissipated energy obtained by improved spatial resolution measurement pinpointed the location of fatigue crack initiation. Owing to the positive correlation between the magnitude of dissipated energy and number of slip bands, it was suggested that the dissipated energy was associated with the behavior of slip bands, with regions of high dissipated energy predicting the location of fatigue crack initiation.

Comparison of Experimental Thermal Methods for the Fatigue Limit Evaluation of a Stainless Steel

This paper regards the rapid determination of fatigue limit by using thermal data analysis. Different approaches available in the literature to estimate the fatigue limit of cold-drawn AISI 304L bars are analyzed and compared, namely, temperature- and energy-based methods. Among the temperature-based approaches, the Risitano Method (RM) and the method based on material temperature evolution recorded during a static tensile test were analyzed. Regarding the energy-based approaches, the input mechanical energy density stored in the material per cycle (i.e., the area of the hysteresis loop), the heat energy dissipated by the material to the surroundings per cycle, and the “2nd-harmonic-based” methods were considered. It was found that for the material analyzed, all the considered energy-based approaches provided a very good engineering estimation of the material fatigue limit compared to a staircase test.

Thermographic applications for the rapid estimation of fatigue limit

The AIAS group studying Energetic Methods for Experimental Analysis, MEAS, is performing round robin experimental tests for the rapid determination of fatigue limit on steels by different thermographic techniques. This work is part of the project and describes the experimental activity performed at Politecnico di Milano, based on stepwise cyclic tests. Thermograms are processed in terms of: 1) amplitude of the first order harmonic, in-phase with respect to the loading signal; 2) amplitude of the second harmonic, out-of-phase; 3) slope of the thermal signal with respect to the number of cycles. These values typically show bilinear trends, allowing to define a breakup point and a corresponding stress which is the thermographic estimation of the fatigue limit. The paper presents and discusses the results of tests with different stress ratios, i.e. fully reversed cycling with R=-1 and tensile-tensile cycling with R=0.1.

НОВЫЕ ПАРАДИГМЫ В ОПИСАНИИ УСТАЛОСТИ МЕТАЛЛОВ

Present processes of damages accumulation in metals under regular cyclic loads were analyzed. Based on the concept of physical mechanics, a sequence of damage accumulation mechanisms was considered in accordance with the stress level increasing. It was shown that the evolution of metals behavior takes place in the direction from micro-, to meso-, and then macroscale levels in accordance with the bifurcation diagram under consideration. It was explained why metals mechanical characteristic called fatigue limit cannot be used for simulation of structures durability and in-service life-time. The problem of the bimodal durability distribution for different kinds of metals was discussed when the bifurcation transition takes place from one scale level to another one. It was shown that in the bifurcation region metals can experience a constant stress level but its reaction appeared in two ways with a different probability because the difference in damage accumulation belonged to the scale above and below the bifurcation region. Mechanisms of the subsurface fatigue cracking in a very-high-cycle-fatigue regime were reviewed. It was demonstrated that the dominant process in damages accumulation under the metal surface and appearance of the subsurface cracking origin is related to sliding deformation and material torsion during material uploading. Test data for fatigue limit determination of aviation structural materials in accordance with the standard were reviewed. The influence of mechanical characteristics of the fatigue limit value was analyzed. It was demonstrated that the major part of the material realized all three scale levels during the stress level increasing from one unit to another. The realization of the low-cycle-fatigue is not the computationally recommended case for the operated complex structures when the mesoscale of metal fatigue does not exist.

Determination of fatigue limit of polymeric composites in fully reversed bending loading mode using self-heating effect

In this paper, the fatigue limit in fibre-reinforced polymeric composites loaded in the fully reversed bending loading mode is investigated by using the self-heating effect occurring during such a type of testing. This approach provides the possibility of fast estimation of fatigue limit without the necessity of performing time-consuming high cycle fatigue tests. In order to validate the determined fatigue limit, the obtained results of temperature evolution were compared with the results of acoustic emission measurements. Based on the performed observations and comparisons, a structural degradation of polymeric composites subjected to cyclic loading with accompanied self-heating effect was described in terms of critical self-heating temperature concept. This allows for estimating the criticality of fatigue from the point of view of both mechanical and thermal failure.

Fatigue limit estimation using IR camera

Determination of fatigue limit by traditional method is associated with testing of large number of specimens. This is caused by more scatter in test results and therefore a higher amount of tests are needed to generate a statistically reliable trend. The paper presents rapid and reliable estimation of fatigue limit using infrared thermography. By analysing the temperature of the material surface during cyclic loading it is possible to evaluate the dynamic behaviour of a structure and determine the fatigue limit. The method provides reliable results using a very little number of specimens in a very short time compared with traditional method.

Rapid evaluation of fatigue limit on thermographic data analysis

Based on infrared thermography, the graphic methods such as Luong’s method and Risitano’s method are proved to be rapid and efficient for fatigue limit determination comparing to conventional methods. However, the determination procedure involves visual inspection so contains man-made uncertainties, which restricts their usage. In the present paper, we propose three new treatment methods in terms of relation curve between experimental temperature response (or dissipated energy) and the applied stress amplitude so as to determine the fatigue limit with uniqueness. Those three methods were all evaluated by applying to the experimental data from literature and the error of results were discussed and analyzed. In addition, numerical experiments were carried out to investigate the influence of loading stepped length and random error on each new treatment method.

Fatigue limit evaluation of structure materials based on thermographic analysis

The work presented here is dealing with implementation of new approach to fatigue limit determination. The approach is based on application of thermography on specimen that is step-by-step increasingly loaded. Detected temperature changes at different stress levels are evaluated and final fatigue limit level is determined.. Thermography analysis seems to have a great potential to reduce the material demand and to achieve the minimum testing time while the quality of test results remains comparable to standard approach. This is extremely useful in cases where the experimental material is strictly limited, such as new materials development, residual service life of in-service components determination or also, nowadays, for additive manufacturing components, where specimens preparation is expensive.Each specimen was tested at different stress amplitudes, where the test procedure consists of at least 8 loading steps. Results obtained with the thermography technique were compared with those obtained from standard high-cycle force-controlled fatigue tests under constant loading until failure in accordance with the ASTM E466-07 standard. All tests were done at room temperature with the cycle asymmetry coefficient R = -1.

Short-time creep, fatigue and mechanical properties of 42CrMo4 - Low alloy structural steel

The proper selection of materials for the intended use of the structural member is of particular interest. The paper deals with determining both the mechanical properties at different temperatures and the behavior in tensile creep as well as fatigue testing of tensile stressed specimens made of low alloy 42CrMo4 steel delivered as annealed and cold drawn. This steel is usually used in engineering practice in design of statically and dynamically stressed components. Displayed engineering stress - strain diagrams indicate the mechanical properties, creep curves indicate the material creep behavior while experimental investigations of fatigue may ensure the fatigue limit determination for considered stress ratio. Also, hardness testing provides an insight into material resistance to plastic deformation. Experimentally obtained results regarding material properties were: tensile strength (735 MPa / 20°C, 105 MPa / 680°C), yield strength (593 MPa / 20°C, 76 MPa /680°C). Fatigue limit in the amount of 532.26 MPa, as maximum stress at stress ratio <i>R</i> = 0.25 at ambient temperature was calculated on the basis of experimentally obtained results. Regarding the creep resistance it is visible that this steel can be treated as creep resistant at high temperatures (including 580°C) when applied stress is of low level (till 0.2 of yield stress).

Accelerated Determination of Fatigue Limit and S-N Curve by Means of Thermographic Method for X5CrNi18-10 Steel

Abstract A new thermographic method that enables simultaneous accelerated determination of the fatigue limit and the S-N curve was presented in this paper. The fatigue limit determination method was based on a constant rate of temperature rise occurring in second phase of the specimen fatigue life. The S-N curve determination method was based on energetic parameter with assumption of its dependency on the stress amplitude. The tests made on X5CrNi18-10 steel under reversed bending revealed that the fatigue limit value obtained from accelerated thermographic tests as compared to the value obtained from full test differs by 5.0 %. The S-N curve obtained by accelerated thermographic method fits inside 95 % confidence interval for the S-N curve obtained from full test.

Self-heating Method for Fatigue Limit Determination on Thermoplastic Composites

To achieve weight reduction of structures, composite carbone fabric materials are used extensively in different industrial applications as aerospace and automotive. Their high mechanical performances and good fatigue durability offer advantages compared to more traditional metallic materials. To be able to design composite parts, it is necessary to characterize their fatigue behavior and specifically to evaluate fatigue limites. All fatigue tests generally used for this determination are time-consuming and potentially highly costly.We propose herein after to apply on woven carbon fibres thermoplastic matrix composites, the self-heating method, which has been developed and validated initially on unidirectional carbon/epoxy composite laminates.The material under consideration is a balanced woven made of carbon fibers and PA66 matrix. Static and fatigue tests are conducted on two stacking sequences ([0°] and [+/-45°]) in order to identify their mechanical properties. Our findings demonstrate that during self-heating tests at room temperature, the mechanisms leading to the increase in temperature are different compared with those observed in thermoset matrix composite systems. Anew analysis of these results is mandatory to obtain the limit of fatigue. The fatigue limit obtained within hours, in our work appears comparable with the results obtained with the classical SN curves method.

Determination of Fatigue Limit by Locati Methodusing S-N Curve Determined by Means of Thermographic Method

Selected results of experimental research showing possibilities to combined accelerated methods, i.e. thermographic determination of the fatigue curve and estimating the fatigue limit, were presented in this paper. The tests were performed using the reversed bending fatigue-testing machine with a rotational frequency of 77 Hz. The main item of the test station was the thermographic camera. Test specimens were made of C45 (1.0503) steel drawn bars. The full tests results were presented for the fatigue S-N curve (23 specimens at 10 load levels) and fatigue limit by the Staircase method (25 specimens) based on 107 cycles to failure was determination. The accelerated test for determining the slope of the S-N curve and the fatigue limit were made by a single specimen based on gradually increasing loading. Methods previously presented in the literature are based on the phenomenon of temperature stabilization during most of the fatigue process. The results presented in this work did not show the occurrence of such a phenomenon. For that reason, the new approach to the determination of the slope of S-N curve was proposed. Combined application of both experimental methods, Locati and thermographic, with a new approach to the determination of this slope might allow one to develop automatic equipment for the accelerated determination of the fatigue limit which would contribute to significant reduction of the test costs and time, thus increasing availability of such tests.

Rapid evaluation of the fatigue limit in composites using infrared lock-in thermography and acoustic emission

Fatigue limit determination via the conventional Wöhler-curve method is associated with extended experimental times as it requires testing of a large number of specimens. The current paper introduces a methodology for fast, reliable and experimentally economic determination of the fatigue limit in monolithic and composite materials by means of combined usage of two nondestructive inspection methods, namely infrared (IR) lock-in thermography and acoustic emission (AE). IR thermography, as a real-time and non-contact technique, allowed the detection of heat waves generated due to thermo-mechanical coupling as well as of the energy dissipated intrinsically during dynamic loading of the material. AE, on the other hand, was employed to record the transient waves resulting from crack propagation events. Aluminum grade 1050 H16 and cross-ply SiC/BMAS ceramic matrix composites were subjected to fatigue loading at various stress levels and were monitored by an IR camera and AE sensors. The fatigue limit of the monolithic material, obtained by the lock-in infrared thermography technique and supported by acoustic emission was found to be in agreement with measurements obtained by the conventional S–N curve method. The fatigue limit of the ceramic matrix composite was validated with acoustic emission data.

Microplastic Limit of Steels as a Means of Fatigue Limit Determination

The experimental determination of fatigue limit in case of steels represents an expensive and time consuming process not speaking about the necessity of a sufficient volume of representative material necessary for the specimens manufacturing, manufacturing of numerous test specimens, the need of an expensive (electrohydraulic) test equipment etc.Microplastic strains occurring at stresses below the yield point play a significant role at cyclic loading of metals within high cycle fatigue region. The microplastic strains cannot be measured by common methods due to the discontinuity of their distribution and a varying character of their level in the metal structure, but however, owing to the magnetoelastic effect the threshold stress at which dislocation pile-up stresses begin to obstruct the magnetic domains in rotation to the direction of the tensile stress can be determined by means of changes in magnetic permeability during tensile loading.In the paper, the changes of the magnetic permeability were measured indirectly – by means of the changes of the electrical impedance (a.c. resistance). The measurement was performed on the normalized low-carbon structural steel CSN 41 1375. The microplastic limit was determined by evaluation the ΔR-σ records. A comparison with the fatigue limit in a reversed fatigue loading showed that the MPL was below the fatigue limit. The microplastic limit can thus be considered to separate non- damaging and damaging cyclic stresses.The paper presents the results of the abovementioned characteristics what may shorten and make cheeper the determination of the fatigue limit and non-damaging cyclic stresses of structural materials.

Rapid Determination of the High Cycle Fatigue Limit Curve of Carbon Fiber Epoxy Matrix Composite Laminates by Thermography Methodology: Tests and Finite Element Simulations

Self-heating measurements under cyclic loading allow a fast estimation of fatigue properties of composite materials. The tensile fatigue behavior of a high stress carbon fiber epoxy-matrix composite laminates is examined at room temperature. Tension- tension cyclic fatigue tests are also conducted under load control at a sinusoidal frequency of 5Hz to obtain high-cycle fatigue stress curves (S-N). The fatigue limits of the different composite lay-ups tested were successfully compared with data resulting from the self-heating test method on the same laminates. This comparison reveals a good agreement between the two methods dedicated to stress fatigue limit determination. In addition, a tomographic analysis is used to perform comparisons at the microscale between both fatigue methods. The nonlinear heat transfer laminate theory is used for self-heating tests simulation. Self-heating simulations involving conduction, convection, and boundary radiation are performed with the Finite Element code Cast3M.

A methodology to determine a conditional probability density distribution surface from S–N data

A conditional probability density distribution surface (CPDDS) is constructed by two functions in this paper. One is the function of population mean and the stress level; the other is the function of standard deviation and the stress level. The surface represents the probability density curve of fatigue life changing with the stress level. A new definition of fatigue limit as well as the determination method is given in this paper according to the small probability event of CPDDS. The proposed method can save considerable cost and time in the determination of fatigue limit.