Effects of external operating environments on fatigue of heat‐exchanger copper tubes

The purpose of this study is to analyze the effects of surface defects (eg, notches) and external environment conditions (eg, operating temperature, the number of re‐weldings) on the static strength and fatigue of C1220T‐O copper tubes used in the heat exchangers of air conditioners. Instead of using standardized specimens, as is done in general rotary bending fatigue tests, special specimens were fabricated in this study by inserting metal plugs on both ends of the copper tubes to perform fatigue tests on the actual tube product, and then the fatigue characteristics were evaluated using stress‐life (S‐N) curves. Regarding the welding conditions (maximum 1000°C and 10 seconds), the grain size grew (grain size number decreased), and the hardness decreased as the number of re‐weldings increased. The effects of the operating temperatures on the fatigue life were examined at a room temperature of 25°C and a heat exchanger operating temperature of 125°C, resulting in the same fatigue limit (70.21 MPa) at both room and operating temperatures. However, the fatigue limit of 37.46 MPa measured in the notched specimens (radius of 3 mm, depth of 0.2 mm) was lower than that obtained from those without notches. The material constant (1.07) used in the Peterson equation was then computed from the fatigue notch factor (1.87 = 70.21/37.46), and the stress concentration factor (2.18) of the notched tube specimens was obtained from the structural analysis. This material constant can be used to predict a decrease in the fatigue limit over varying notch sizes in copper tubes (C1220T‐O).     

On the strengthening effect of increasing cycling frequency on fatigue behavior of some polymers and their composites: Experiments and modeling

Effect of cycling frequency on fatigue behavior of neat, talc filled, and short glass fiber reinforced injection molded polymer composites was investigated by conducting load-controlled fatigue tests at several stress ratios (R = −1, 0.1, and 0.3) and at several temperatures (T = 23, 85 and 120 °C). A beneficial or strengthening effect of increasing frequency was observed for some of the studied materials, before self-heating became dominant at higher frequencies. A reduction in loss tangent (viscoelastic damping factor), width of hysteresis loop, and displacement amplitude, measured in load-controlled fatigue tests, was observed by increasing frequency for frequency sensitive materials. Reduction in loss tangent was also observed for frequency sensitive materials in DMA tests. It was concluded that the fatigue behavior is also time-dependent for frequency sensitive materials. A Larson–Miller type parameter was used to correlate experimental fatigue data and relate stress amplitude, frequency, cycles to failure, and temperature together. An analytical fatigue life estimation model was also used to consider the strengthening effect of frequency in addition to mean stress, fiber orientation, and temperature effects on fatigue life.     

Nonlinearity of bituminous mixtures

This paper presents an experimental characterization of the strain dependency of the complex modulus of bituminous mixtures for strain amplitude levels lower than about \(110~\upmu\mbox{m}/\mbox{m}\). A series of strain amplitude sweep tests are performed at different temperatures (8, 10, 12 and 14°C) and frequencies (0.3, 1, 3 and 10 Hz), during which complex modulus is monitored. For each combination of temperature and frequency, four maximum strain amplitudes are targeted (50, 75, 100 and \(110~\upmu\mbox{m}/\mbox{m}\)). For each of them, two series of 50 loading cycles are applied, respectively at decreasing and increasing strain amplitudes. Before each decreasing strain sweep and after each increasing strain sweep, 5 cycles are performed at constant maximum targeted strain amplitude.Experimental results show that the behavior of the studied material is strain dependent. The norm of the complex modulus decreases and phase angle increases with strain amplitude. Results are presented in Black and Cole–Cole plots, where characteristic directions of nonlinearity can be identified. Both the effects of nonlinearity in terms of the complex modulus variation and of the direction of nonlinearity in Black space seem to validate the time–temperature superposition principle with the same shift factors as for linear viscoelasticity.The comparison between results obtained during increasing and decreasing strain sweeps suggests the existence of another phenomenon occurring during cyclic loading, which appears to systematically induce a decrease of the norm of the complex modulus and an increase of the phase angle, regardless of the type of the strain sweep (increasing or decreasing).     

Effects of strain rate and mean strain on cyclic behavior of aluminum alloys under isothermal and thermo-mechanical fatigue loadings

In this paper, effects of strain rate and mean strain on the cyclic behavior and the lifetime of aluminum–silicon alloys are investigated under thermo-mechanical and isothermal fatigue loadings. To achieve these goals, low cycle fatigue tests are accomplished at evaluated temperatures under various strain rates (by changing the loading frequency) and different strain ratios (minimum to maximum strain). Thermo-mechanical fatigue experiments are performed in an out-of-phase condition where the temperature varies between 50 and 250 °C. Various heating/cooling rates are taken into account to assess the strain rate effect and different starting temperatures are considered to study the mean strain effect.     

Dynamic magneto-electric multiferroics PZT/CFO multilayered nanostructure

Highly oriented PbZr_0.53Ti_0.47O₃/CoFe₂O₄ (PZT/CFO) multilayered nanostructures (MLNs) were grown on MgO substrate by pulsed laser ablation using La_0.5Sr_0.5CoO₃ (LSCO) as conducting bottom electrode. The effect of various PZT/CFO (PC) sandwich configurations having three, five, and nine layers while maintaining total thickness of PZT and CFO be identical has been systematically investigated. X-ray diffraction (XRD) and micro-Raman spectra revealed the existence of pure PZT and CFO phases without any intermediate phase. Intact MLNs were observed by transmission electron microscopy (TEM) with little inter-diffusion near the interfaces at nano-metric scale without any impurity phase. Impedance spectroscopy, modulus spectroscopy, and conductivity spectroscopy were carry out over a wide range of temperatures (100–600 K) and frequencies (100 Hz–1 MHz) to investigate the grain and grain boundary effect on electrical properties of MLNs. Temperature dependent real dielectric permittivity and dielectric loss illustrated step-like behavior and relaxation peaks near the step-up characteristic, respectively. Cole–Cole plots indicate that most of the dielectric response came from the bulk (grain) MLNs below 300 K, whereas the grain boundary and the electrode–MLNs effects are prominent at elevated temperatures. The dielectric loss relaxation peak shifted to higher frequency side with increase in temperature, it was out of the experimental frequency window above 300 K. Our Cole–Cole fitting of dielectric loss spectra indicated marked deviation from the ideal Debye-type of relaxation, which is more at elevated temperature. Master modulus spectra supported the observation from the impedance spectra; it also indicated that the magnitude of the grain boundary compared to grain becomes more prominent with increase in number of layers. We have explained these electrical properties of MLNs by Maxwell–Wagner type contributions arising from the interfacial charge at the interface of the ML structures. Three different types of frequency dependent conduction processes were observed at elevated temperatures (>300 K), which fitted well with the double power law, indicating that the low frequency (<1 kHz) conductivity may be due to long-range ordering (frequency independent), mid frequency conductivity (<10 kHz) may be due to short-range hopping, and high frequency (<1 MHz) conduction due to the localized relaxation hopping mechanism. Ferroelectric polarization decreased slowly in reducing the temperature from 300 to 200 K, with complete collapse of polarization at ~100 K, but there was complete recovery of the polarization during heating, which was repeatable over many different experiments. At the same time, the temperature dependent remanent magnetization of the MLNs showed slow enhancement in the magnitude till 200 K with three-fold increase at 100 K compared to room temperature. This enhancement in remanent magnetization and decrease in remanent ferroelectric polarization on lowering the temperature indicate temperature dependent dynamic switching of ferroelectric polarization. The magnetic and ferroelectric properties of MLNs were quite different compared to individual layers suggesting its improper ferroelectric characteristics. The fatigue test showed almost 0–20% deterioration in polarization. Fatigue and strong temperature and frequency dependent magneto-electric coupling suggest MLNs utility for Dynamic Magneto-Electric Random Access Memory (DMERAM).     

Complex electrical conductivity measurements of a KTB amphibolite sample at elevated temperatures

In the present work, complex electrical conductivity measurements in the frequency range of 10 mHz–1 MHz and at elevated temperatures (423 K–1373 K) were carried out in amphibolite sample, originating from the KTB drilling. Impedance Cole–Cole plots were used to discriminate the contributions of grain interior, grain boundaries and electrode polarization effects to the measured conductivity spectra. At frequencies above 10 Hz where the electrode effects are negligible, ac-conductivity exhibits frequency dispersion according to a two-term power law behaviour, in most cases of the isothermal spectra. A conductivity relaxation step is observed during heating of the sample up to 773 K, which is attributed to strongly bound water, either in the form of hydroxyls in the crystal lattice of grains, or in the form of molecules trapped between the grain boundaries. The hysteresis plot of dc-conductivity shows an anomalous behaviour at around 1000 K during the heating procedure, due to the dehydroxylation of the sample. The electric modulus M* representation provides additional information over the low temperature range and the conductivity relaxation times were estimated for a limited temperature range. Thermal activation energies of Arrhenius type vary from 0.58 eV up to 1.50 eV, and they are ascribed to proton conduction at low temperature and hopping conduction of small polarons at higher temperatures.     

Effects of frequency on fatigue behavior of CVI C/SiC at elevated temperature

Effects of frequency on fatigue behavior of a chemical vapor infiltrated carbon fiber reinforced silicon carbide composite (C/SiC) were investigated at an elevated temperature of 550 °C. Tension–tension fatigue tests were conducted at three frequencies: 0.1, 10 and 375 Hz to establish stress versus cycles to failure (S–N) relationships. There was an increase in cycles to failure at a given stress level as frequency increased from 0.1 to 375 Hz at elevated temperature. This trend was different at room temperature where cycles to failure decreased when frequency changed from 40 to 375 Hz but remained almost same below 40 Hz. There was a reduction in cycles to failure at frequencies less than 40 Hz but cycles to failure remained same at a higher frequency of 375 Hz when test environment changed from room temperature to 550 °C. Analysis of damage mechanisms showed that the oxidation of carbon fibers was the major difference between the room and elevated temperatures, which caused a reduction in cycles to failure with lower frequencies at elevated temperature in comparison to that at room temperature. However, oxidation of carbon fibers was almost absent or negligible at higher frequency at elevated temperature, which caused practically no reduction in cycles to failure at elevated temperature in comparison to their counterparts at room temperature.     

Tensile and fatigue behavior of superelastic shape memory rods

The tensile and fatigue behavior of superelastic shape memory alloy (SMA) bars heat-treated at three different temperatures were examined. Low cycle fatigue tests at variable load rates were carried out to determine the effect of stress and frequency on residual strain and energy dissipation in a fatigue cycle. The mechanism of energy dissipation was studied by monitoring the temperature changes in the fatigued samples as a function of applied stress and frequency of testing. Results from the tensile tests revealed that the stress for the Austenite to Martensite transformation decreased from 408 MPa to 204 MPa with an increase in temperature of heat treatment from 300 to 450 °C. The ultimate strength of the SMA increased from 952 MPa to 1115 MPa when the heat treatment temperature was increased from 300 to 450 °C. Fatigue testing prior to conducting the tensile test decreased the ultimate strength of the SMA and also reduced the failure strain. The energy dissipation in fatigue tests was found to decrease as test frequency increased from 0.025 Hz to 0.25 Hz and the change in sample temperature during the test at the lower test frequency was found to be considerably higher than at the higher frequency.     

Thermomechanical fatigue evaluation and life prediction of 316L(N) stainless steel

An attempt has been made to understand the thermomechanical fatigue (TMF) behaviour of a nitrogen-alloyed type 316L austenitic stainless steel under different temperature domains. Smooth, hollow specimens were subjected to in-phase (IP) and out-of-phase (OP) thermal–mechanical cycling in air under a mechanical strain control mode, at a strain rate of 6.4 × 10^?5 s^?1 and a strain amplitude of ±0.4%. For the sake of comparison, total strain controlled low cycle fatigue (LCF) tests were also performed at the peak temperatures of TMF cycling on similar specimens employing the same strain rate and strain amplitude. Life was found to depend on the thermal/mechanical phasing and temperature. Creep was found to contribute to life reduction in IP tests when the peak temperature of cycling was above 600 °C. A few TMF tests were performed in vacuum in order to assess environmental influence on life. Thermomechanical fatigue cycling led to the development of significant amounts of mean stresses and the stress response was generally higher compared to that of LCF tests at the peak cyclic temperatures. Also, the isothermal tests at the peak temperature of TMF cycling resulted in lower lives compared to those obtained under TMF. An attempt was made to predict the TMF life using the isothermal database and satisfactory predictions were achieved using the Ostergren’s frequency modified damage function (FMDF) approach.     

In‐phase and out‐of‐phase thermomechanical fatigue behavior of 4Cr5MoSiV1 hot work die steel cycling from 400 °C to 700 °C

The hysteresis loops, stress and strain behavior, lifetime behavior and fracture characteristic of 4Cr5MoSiV1 hot work die steel at a wide range of mechanical strain amplitudes (from 0.5% to 1.3%) during the in‐phase (IP) and out‐of‐phase (OP) thermomechanical fatigue (TMF) tests cycling from 400 °C to 700 °C under full reverse strain‐controlled condition were investigated. Stress‐mechanical strain hysteresis loops of 4Cr5MoSiV1 steel are asymmetric, and stress reduction appears at high‐temperature half cycles owing to a decrease in strength with increasing temperature. 4Cr5MoSiV1 steel always exhibits continuous cyclic softening for both types of TMF tests, and the cyclic softening rate is larger in OP loading condition. OP TMF life of 4Cr5MoSiV1 steel is approximately 60% of IP TMF life at the same mechanical strain amplitude and maximum temperature. Lifetime determined and predicted in both types of TMF tests is adequately described by the Ostergren model. Fracture surfaces under IP TMF loading display the striation and tear ridge, showing quasi‐cleavage characteristics, and the cracks are less but longer. However, fracture surfaces under OP TMF loading mainly display the striation and dimple characteristics, and the cracks are more and shorter.     

Effect of ageing and temperature on the fatigue behaviour of bitumens

Bitumen ageing plays a significant role in determining the resistance of asphalt mixes to fatigue cracking. Regardless of the type of ageing (oxidation during manufacture or during the service life), hardening effects increase the risk of cracking. The objective of this work is to examine the combined effect of the loss of volatiles and oxidation produced during ageing on the fatigue behaviour of the bitumen. To this end, different types of bitumen were subjected to accelerated ageing in the laboratory, simulating long-term ageing (RTFOT + PAV). They were then subjected to traditional tests (penetration, softening point, Fraass fragility point, dynamic viscosity, etc.), Dynamic Shear Rheometer tests (frequency and temperature sweep), and the EBADE test (a fatigue strain sweep test at different temperatures). Different temperatures have been used to evaluate the effect of visco-elastic phenomena on aged binder fatigue. The results showed that, in terms of their response to ageing, modified binders show a higher rate of variation in their general properties than conventional binders. In addition, it was shown that temperature plays an important role in the impact of ageing on the fatigue response of bituminous binders, and in the same way, in the mechanical response of these materials.     

Influence of the annealing and defects on the VHCF behavior of an SLM AlSi10Mg alloy

In the paper, the influence of the annealing temperature on the Very High Cycle Fatigue (VHCF) of AlSi10Mg specimens produced through selective laser melting (SLM) is experimentally assessed. VHCF tests at 20 kHz are carried out on Gaussian specimens subjected to a heat treatment suggested by the system supplier (heating for 2 hours to 320°C and air cooling) and to a heat treatment proposed by the authors (heating for 2 hours to 244°C and air cooling). The defects originating failure and the conditional P‐S‐N curves are compared. Experimental results show that an annealing temperature of 320 ° C induces the spheroidization of the Si network, which enhances the ductility but has a negative effect on the VHCF response. On the contrary, by reducing the heating temperature to 244 ° C , the original as‐built microstructure is not altered and the minimization of the residual stresses permits to enhance the VHCF response.     

Strain-controlled fatigue life and modeling of conduit polymers

Strain-controlled fatigue lives of conduit polymers, viz., nylon 6, polypropylene (PP) and calcium carbonate filled black colored polypropylene (PP-blk) were studied. Thermal and mechanical analyses were conducted before fatigue tests. Thermal characteristics, such as the degree of molecular degradation, glass transition temperatures, and melting points were determined. Tensile strength, elastic modulus, and Poisson’s ratio were obtained from tests under quasi-static loading. Fatigue lives were measured under different displacement ranges and temperature conditions. Four different temperatures were selected to represent low (−40 °C), room (25 °C), and high (65 and 125 °C) temperature conditions. Hysteretic heating was found to be significantly operative in PP specimens. By optimizing the previously developed unified strain model [1], strain fatigue lives were predicted based on the studied materials.     

Off-axis tensile fatigue assessment based on residual strength for the unidirectional 45° carbon fiber-reinforced composite at room temperature

The tensile fatigue behavior of unidirectional carbon fiber-reinforced thermoplastic and thermosetting laminates was examined at room temperature. Tension-tension cyclic fatigue tests were conducted under load control at a sinusoidal frequency of 10 Hz to obtain stress-fracture cycles (S-N) relationship. The fatigue limits of carbon fiber-reinforced thermoplastic laminates (CF/PA6) and thermosetting laminates (CF/Epoxy) were found to be 28.0 MPa (48% of the tensile strength) and 56.2 MPa (63% of the tensile strength), respectively. Two types (in constant and incremental loading way) of loading-unloading low cycle fatigue tests were employed to investigate the modulus history of fatigue process for announcing the fatigue mechanism. The residual tensile strength of specimens that survived fatigue loading maintained with the increase of fatigue cycles and applied stress. Examination of the fatigue-loaded specimens revealed that the more flexible/ductile trend of resins and the formation of micro-cracks at the interface between fiber and matrix was facilitated during high fatigue loading (⩾fatigue limit stress), while no interfacial/matrix damage in resins was detected during low fatigue loading (<fatigue limit stress), which was consider to be the governing mechanism of strength maintain during fatigue loading.     

Fatigue and fatigue crack growth of aluminium alloys at very high numbers of cycles

The application of ultrasonic frequency (20 kHz) loading to test fatigue and fracture mechanical properties of materials is briefly reviewed and recent investigations on high strength aluminium alloys are reported. Very high cycle endurance tests and near threshold crack growth experiments were performed with the 2024-T351 aluminium alloy. Lifetimes are approximately 10–100 times lower, if cycled in distilled water instead of ambient air. Fatigue experiments under randomly varying loads showed that linear damage summation calculations overestimated lifetimes by approximately a factor 2. Fracture mechanics studies in ambient air, dry air and in vacuum served to investigate the role of air humidity on near threshold fatigue crack growth at ultrasonic frequency. The threshold value was 2.1 MPa√m in ambient air and 3.3 MPa√m in vacuum. The aluminium alloy AlZnMgCu1.5-T66 and the aluminiumoxyde particle reinforced alloy 6061-T6 were tested at 100 Hz and 20 kHz to investigate frequency influences on high cycle fatigue properties, and similar lifetimes were found at both frequencies.     

Mechanical properties and strain fatigue lives of insulation polymers

Insulation polymers are not well characterized for their mechanical properties particularly in terms of fatigue strains. This article aims to examine the durability and strain fatigue lives of three commonly used cable insulation polymers, viz., (1) polyvinyl chloride (PVC), (2) crosslinked polyethylene (XLPE), and (3) polyphenylene ether (PPE) under selected strain and temperature ranges. The tensile properties of these materials were measured using an Instron testing machine at constant and controlled loading rates. Fatigue tests were performed at three selected temperatures, −40, 25, and 65 °C, to characterize the temperature effects on fatigue life. From the tensile test results, it was observed that PVC and XLPE are ductile and exhibit significantly more elongation prior to breaking, while PPE exhibits brittle behavior. When the loading rate is increased, there is an improvement in the tensile strength of PPE and elastic modulus of PPE and PVC. The durability of XLPE under strain fatigue testing was the largest, followed by PVC and PPE. The strain fatigue lives of PVC and XLPE decreased drastically at −40 °C and demonstrated a noted increase at 65 °C compared to fatigue lives at room temperature. This trend was not observed in PPE where the strain fatigue life showed improvement at both lower and higher temperatures.     

The effect of frequency on the giga‐cycle fatigue properties of a Ti–6Al–4V alloy

The effect of frequency on giga‐cycle fatigue properties was investigated in smooth and 0.3 mm‐hole‐notched specimens at three heats (Heats A, B, and C) for a 900 MPa‐class Ti‐6Al‐4V alloy. Fatigue tests were performed at frequencies of 120 Hz, 600 Hz, and 20 kHz using electromagnetic resonance, high‐speed servohydraulic, and ultrasonic fatigue testing machines, respectively. Heats A and B developed internal fractures, and in these cases, frequency effects were negligible. On the other hand, Heat C developed only surface fractures. In this case, high‐frequency tests showed a higher fatigue strength, indicating frequency effects were not negligible. The tests using the notched specimens showed almost no frequency effects regardless of the heat. The frequency effects observed in the cases of surface fracture were believed to be related to a delay in local plastic deformation in response to high‐frequency loading, since temperature increases in these specimens were successfully suppressed. The delay in the plastic deformation was observed to be reduced in the notched specimens because of stress concentration and limitation in the plastic deformation zone. In turn, the significant conclusion of this research is that high‐frequency tests can be applied not only to internal fractures but also to notch problems, but are not applicable to surface fractures of smooth specimens.     

Experimental study of type 316 stainless steel failure under LCF/TMF loading conditions

A detailed investigation of low cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) of a 316FR type stainless steel is presented in this paper in order to identify the failure mechanism based on the experimental results and the subsequent metallography of the samples. The LCF–TMF servohydraulic testing with a temperature uniformity of less than ±5 °C within the gauge section of the specimens was employed to conduct the experimental tests. Fully-reversed, strain-controlled isothermal tests were conducted at 650 °C for the strain ranges of Δɛ = ±0.4%, ±0.8%, ±1.0% and ±1.2%. Strain-controlled in-phase (IP) thermo-mechanical fatigue tests were conducted on the same material and the temperature was cycled between 500 °C and 650 °C. Additionally, the creep–fatigue interactions were investigated with the introduction of symmetrical hold time under both LCF–TMF tests. The cyclic behaviour was further studied by performing microstructural investigations using the scanning electron microscope (SEM).     

Ultrasonic fatigue of a single crystal Ni-base superalloy at 1000 °C

An ultrasonic fatigue testing system capable of operating at temperatures up to 1000 °C has been developed and utilized to study the fatigue behavior of a single crystal superalloy (PWA 1484) at a temperature of 1000 °C and loading frequency of approximately 20 kHz. The stress-life data generated from the ultrasonic testing system were comparable to those from conventional servo-hydraulic fatigue tests for similar single crystal alloys. Interior Ta-rich carbides were the major microstructural feature responsible for crack initiation in the alloy. Crack growth under ultrasonic loading frequency at 1000 °C for PWA 1484 occurred in a crystallographic manner on {1 1 1} octahedral slip planes, in contrast to the normal Mode-I growth mode typically observed for single crystal superalloys at high temperature (>850 °C) with conventional servo-hydraulic loading frequencies (<100 Hz).     

Frequency effect and influence of testing technique on the fatigue behaviour of quenched and tempered steel and aluminium alloy

Influences of testing technique and frequency on the fatigue behaviour of 50CrMo4 and EN AW-5083 were investigated. To clarify the effect of test frequency on the fatigue behaviour, tests with 20 kHz and f < 400 Hz were performed. The frequency effect can be caused by temperature, environment and strain rate. For the aluminium alloy, the influence of environment is responsible for the dependence of fatigue lifetime on the frequency. The fatigue lifetime of the steel showed in both environments similar frequency dependency, i.e. the strain rate is assumed to be responsible for the differences in fatigue lifetime.