bluuebunny/crossref_metadata_2025 huggingface dataset:
Prediction of Crack Growth Under Creep-Fatigue Loading Conditions
Authors: Nikbin KM , Webster GA
Published: February 2009

Crack growth data are presented on a brittle and a ductile low alloy steel and a nickel base superalloy which have been subjected to combined static and cyclic loading at elevated temperatures. The cyclic tests have been conducted between fixed load and fixed displacement ranges. Distinct regions of fatigue and creep dominated cracking have been identified. It is shown that crack growth eventually ceases because of load relaxation in the constant displacement amplitude tests. Fracture of a component made of the ductile steel is unlikely to occur under this type of low-cycle fatigue loading, but significant cracking is possible with the brittle steel. Where cyclic crack growth is creep controlled, it is shown that crack propagation rates can be predicted from static creep data using the creep fracture mechanics parameter C*.
Fatigue Crack Propagation of Nickel-Base Superalloys at 650°C
Authors: Gayda J , Gabb TP , Miner RV
Published: February 2009

The 650°C fatigue crack propagation behavior of two nickel-base superalloys, René 95 and Waspaloy, were studied with particular emphasis placed on understanding the roles of creep, environment, and two key grain boundary alloying additions, boron and zirconium. Comparison of air and vacuum data showed the air environment to be detrimental over a wide range of frequencies for both alloys. In-depth analysis of René 95 showed that at lower frequencies, such as 0.02 Hz, failure in air occurred by intergranular, environmentally assisted creep crack growth, while at higher frequencies, up to 5.0 Hz, environmental interactions were still evident but creep effects were minimized. The effect of boron and zirconium in Waspaloy was found to be important where environmental and/or creep interactions were present. In those instances, removal of boron and zirconium dramatically increased crack growth. It is therefore plausible that effective dilution of these elements may explain, in part, a previously observed trend in which crack growth rates increased with decreasing grain size.
A Model for Predicting Low Cycle Fatigue Life under Creep-Fatigue Interaction
Authors: Rie K-T , Schmidt R-M , Ilschner B , Nam SW
Published: February 2009

An attempt is made to predict fatigue life under high-temperature low-cycle fatigue conditions with superimposed hold times. It is well recognized that a reduction in life in this region is caused by the nucleation and growth of cavities along grain boundaries. The cavities can be formed by vacancy clustering, dislocation pileup, and grain boundary sliding. The subsequent growth leads to grain boundary cracking and intergranular fracture.

The possibility of unstable crack advance is given if the crack tip opening displacement becomes equal to a critical cavity configuration. To describe this configuration we postulate a power function for cavity growth.

Experimental data and the predicted values show excellent agreement, indicating the important role of creep cavitation for creep-fatigue life prediction.
High-Temperature Low Cycle Fatigue Behavior and Lifetime Prediction of a Nickel-Base ODS Alloy
Authors: Nazmy MY
Published: February 2009

The high-temperature low-cycle fatigue (HTLCF) behavior of mechanically alloyed oxide dispersion strengthened MA 754 nickel-base alloy was studied under strain-controlled condition at 850°C. Specimens were cut from the longitudinal and the long transverse directions of the bar stock. Triangular strain cycles as well as strain cycles with hold times were employed in this study. In general, longitudinal specimens exhibited the longest lives, long transverse specimens the shortest lives. The cycles with tensile hold times were the most damaging to the HTLCF lives of specimens of both orientations. The difference between the HTLCF response of the alloy in the longitudinal and the long transverse orientations is interpreted in terms of factors such as modes of crack initiation and propagation, anisotropy in crack growth rates, and the directionality in creep properties. Three models for HTLCF lifetime were evaluated for their ability to correlate the fatigue data on MA 754 in both orientations. These models are the Strain Range Partitioning Model (SRP), the Frequency-Separation Model (FS), and the Frequency-Modified Damage Function (FMDF). The correlations of the data, having the smallest standard deviations of 0.22 and 0.14 for the longitudinal and the long transverse orientations, respectively, were by the FS model. The FMDF and the SRP models had standard deviations, for the longitudinal orientations, of 0.25 and 0.33, respectively.
Influence of Temperature and Environment on the Fatigue Mechanisms of Single-Crystal and Polycrystal 316L
Authors: Driver JH , Gorlier C , Belrami C , Violan P , Amzallag C
Published: February 2009

Single-crystal and polycrystal 316L specimens were fatigued at constant plastic strain amplitudes in the range 10−4 to 2 × 10−2 in air and in vacuum (about 3 × 10−4 to 10−3 Pa) at 20, 300, and 600°C. The temperature and strain dependencies of the single-crystal cyclic stress-strain response are qualitatively similar to those of the polycrystalline material.

In air, the fatigue lives of both polycrystals and single crystals decrease by about an order of magnitude at 600°C compared with the results at 20 and 300°C. The fatigue lives of polycrystal specimens were prolonged in vacuum (compared with air) by factors of ∼2 or 3 at room temperature, to ∼20 at elevated temperatures and low plastic strains. The crack initiation mechanism of polycrystals tends to become partially intergranular at 600°C, but this change is not thought to significantly affect fatigue life. The reduction in fatigue resistance in air of both single and polycrystals at 600°C is attributed to oxidation—enhanced Stage I crack initiation.
A Model for Life Predictions of Nickel-Base Superalloys in High-Temperature Low Cycle Fatigue
Authors: Romanoski GR , Antolovich SD , Pelloux RM
Published: February 2009

Extensive characterization of low-cycle fatigue damage mechanisms was performed on polycrystalline René 80 and IN100 tested in the temperature range from 871 to 1000°C. Low-cycle fatigue life was found to be dominated by propagation of microcracks to a critical size governed by the maximum tensile stress. A model was developed which incorporates a threshold stress for crack extension, a stress-based crack growth expression, and a failure criterion. The mathematical equivalence between this mechanistically based model and the strain-life low-cycle fatigue law was demonstrated using cyclic stress-strain relationships. The model was shown to correlate the high-temperature low-cycle fatigue data of the different nickel-base superalloys considered in this study.
An Integrated Approach to Creep-Fatigue Life Prediction
Authors: Agatonovic P , Heidenreich R
Published: February 2009

There has been special interest recently in developing new, reliable analytical design methods for components under higher temperature conditions. However, at present, the use of material properties is still limited to arrays of single characteristics which do not interact with each other. In this work, several creep fatigue experiments on smooth specimens of IN 800 H have been carried out at 830°C. In some tests, these have also been combined with inside hysteresis loops to investigate the different effects on deformation and damage behavior which originate in a creep and fatigue environment. As a result of these tests, it has been found that the material behavior under creep-fatigue conditions can be significantly changed compared to the material behavior under simple load conditions. Therefore there is a need for life analysis methods to be expanded to include possible variations in properties; for greater accuracy, the material properties must be treated as a complex interacting system of parameters. The examination has been extended to a typical component used under high-temperature conditions. The results of the numerical analysis show that the stress-strain history in the critical area of that component is not simply strain controlled, as it is in the typical laboratory creep-fatigue interaction life test containing a tensile or compressive dwell at constant peak strain level. At high temperatures, the conditions in the component are more severe, causing the life to be reduced compared with the typical laboratory test. In this paper, these conditions are successfully simulated with the help of a generalized Neuber law: σ · εp = constant. Based on this ratio, the engineering method for evaluating component geometry and loading conditions and their effects on material behavior can be established.

On the basis of the results of this study, it follows that a very large amount of information on material behavior and on its component dependence is needed when the material properties are time-dependent, as in the design of hot components. For that reason, the satisfactory solution to designing components for high-temperature conditions requires an integrated approach, with full consideration of different interaction effects from the various influences in the main areas of the material deformation and damage behavior, and of the component effects. The results of this study may apply to the standard methodology of life prediction in high-temperature areas, which shall be developed in the future.
Mapping of Low Cycle Fatigue Mechanisms at Elevated Temperatures for an Austenitic Stainless Steel
Authors: Kanazawa K , Yamaguchi K , Nishijima S
Published: February 2009

Mapping of low cycle fatigue mechanisms at elevated temperatures is attempted for a better understanding of the elevated temperature, low cycle fatigue behavior of an austenitic stainless steel. Results of strain-controlled, uniaxial low cycle fatigue tests on a solution-treated Type 310 stainless steel were used in the analysis. The experiments were performed at 15 temperatures that ranged from room temperature to 800°C and under four strain rate conditions that ranged from 6.7 × 10−3 s−1 to 6.7 × 10−6 s−1. Modes of variation, both in stress amplitude and in fatigue life, are classified and plotted against temperature-strain rate coordinates. By superposing the two maps, one can easily find the principal factors that govern the fatigue life for any given conditions of temperature and strain rate. The proposed idea of mapping is believed to have wide applicability (e.g., for selecting materials and predicting fatigue behavior under service conditions).
Oxidation-Fatigue Interactions in a Single-Crystal Superalloy
Authors: Wright PK
Published: February 2009

This research was conducted to identify, understand, and model the interactions of oxidation and low cycle fatigue (LCF) in a single crystal nickel-base superalloy, René N4. The LCF testing was conducted at 1093°C (2000°F) in three test environments: purified helium (<1 ppm oxygen), laboratory air, and enriched oxygen (80% oxygen at 2 atm pressure). Two cycle frequencies, 20 and ½ cpm, were investigated.

The fatigue life and behavior of René N4 at 1093°C was found to depend on both oxygen concentration and cyclic frequency. These parameters influenced the portions of fatigue life required for crack initiation and crack propagation to different extents, although both portions were decreased under conditions which were environmentally more severe. A relatively small oxygen pressure dependence of fatigue life was seen (Nfα PO2−0.23). The fatigue process in René N4 showed a mixed time and cycle dependency leading to reduced fatigue life for longer cycles. Crack initiation was distinctly more time dependent than crack propagation and varied from ½ to 1/10 or less of the total fatigue life over the conditions investigated.

Crack initiation in René N4 occurred by fatigue assisted cracking and spalling of oxide products to produce a roughened and pitted surface. This process also accelerated the observed oxidation rate. With further cycling, these pits developed into oxide spikes and then into sharp fatigue cracks. A modification of an oxidation-fatigue model by Antolovich was satisfactory in describing this process. The crack propagation process was also influenced by oxidation. However, the precise mechanism could not be clearly defined.
Fatigue of Solder Joints in Surface Mount Devices
Authors: Shine MC , Fox LR
Published: February 2009

Lifetime studies of a 16 I/O surface-mounted solder joint array undergoing isothermal cyclic fatigue in torsion shear under fixed plastic strain range show a strong correlation with creep fatigue and a creep-cracking mechanism. Experimental lifetime data follow an inverse dependence on matrix creep. Experimental measurement of the steady-state shear creep rate versus shear stress defines the creep characteristic that is sensitive to changes in metallurgical structure. The amounts of grain boundary and matrix creep taking place during a fatigue cycle are derived from experimental creep data combined with stress-strain hysteresis data obtained in steady-state cycling. Initially, thicker solder joints have a larger grain size than thinner solder joints, giving more matrix creep during fatigue and a faster failure rate. Fatigue increases the mean grain size of the solder joint as determined by the creep-rate-versus-stress characteristic and microstructure. Effects of grain size and joint thickness on lifetime are discussed. A maximum in the creep fatigue rate occurs at 333 K (60°C).
Thermal-Mechanical Cyclic Stress-Strain Responses of Cast B-1900+Hf
Authors: Marchand N , L'Espérance G , Pelloux RM
Published: February 2009

A study was undertaken to develop an understanding of the fatigue response of superalloy B-1900+Hf under combined thermal and mechanical strain cycling in air. Comparative evaluations were made with existing thermal-mechanical data of B-1900+Hf and with results of a comprehensive study of the fatigue behavior of the same alloy under isothermal conditions. The thermal-mechanical fatigue (TMF) response was investigated for constant amplitude, fully reversed, mechanically strained cycling of uniaxially loaded specimens in the temperature range from 400 to 925°C. Experiments were conducted both with maximum strain in-phase with maximum temperature and out-of-phase with maximum temperature.

The TMF cycling was observed to cause more cyclic hardening than in isothermal fatigue experiments at the maximum and minimum temperatures. In terms of mean stress or plastic strain range, out-of-phase cycling was shown to be more deleterious than in-phase or isothermal cycling. However, few differences were observed in terms of the stabilized stress ranges. The asymmetric cyclic hardening/softening behavior is explained in terms of coarsening of the γ′ and associated strain field. For TMF cycling, the high temperature flow stress depends on the density of the misfit dislocations, whereas the low temperature flow stress is controlled by the magnitude and sign of the applied stress. The TMF cracking modes are discussed. The results show that the fracture criterion under TMF cycling is stress based.
Thermomechanical Fatigue Behavior of a Superalloy
Authors: Malpertu JL , Rémy L
Published: February 2009

A thermal-mechanical fatigue test facility is described to study the stress-strain behavior and fatigue damage under anisothermal conditions of materials for gas turbine blading applications. Hollow specimens are heated by a radiation furnace; a microcomputer is used to generate simultaneous strain and temperature signals and tests are conducted under closed-loop control of axial strain. A typical mechanical strain-temperature loop has been used from 600 to 1050°C (873 to 1323 K) with peak strains at intermediate temperatures. Application to IN 100, a nickel base superalloy, is reported. Crack initiation and early crack growth are shown which are deduced from a plastic replication technique. The role of oxidation has been emphasized and the life of thermal mechanical fatigue specimens was found to be in good agreement with low-cycle fatigue results at 1000°C (1273 K).
Thermal Fatigue Testing of Coated Monocrystalline Superalloys
Authors: Holmes JW , McClintock FA , O'Hara KS , Conners ME
Published: February 2009

Induction heating of stepped-disk specimens to study thermal fatigue of coated superalloys is well suited for studying anisotropic alloys and is easily adaptable for corrosive or inert atmosphere testing. Details of the experimental apparatus and results of a thermoelastic finite element analysis to find the stress-strain history are given.

To illustrate the technique, the effect of cyclic thermal strains on the durability of an aluminide coating applied to a typical monocrystalline nickel-base superalloy was studied in air. Results show that both the compressive strain encountered on specimen heatup and the tensile strain encountered on cooldown critically affect the aluminide coating degradation. After 6000 cycles of heating from 520 to 1080°C in 5 s, followed by 30 s cooling, 80% coating penetration by scalloping was observed. A similar treatment with 6 s cooling resulted in alumina-filled “cracks” extending into the substrate.
Thermal Mechanical Fatigue of Cast René 80
Authors: Cook TS , Kim KS , McKnight RL
Published: February 2009

Hot path components of aircraft engines contain complex thermal and mechanical fields which can interact to produce thermal mechanical fatigue (TMF) failure. Development of a model of thermal mechanical fatigue for cast René 80 is underway through TMF experiments, a series of isothermal tests, and finite element analysis. The modelling effort consists of two parts, the determination of a suitable damage parameter and a technique for calculating the quantities required by the damage parameter. Classical plasticity theory was employed in a finite element code to model the TMF deformation. The predicted hysteresis loops were compared to the experimental values; the correlation was excellent for both the in-phase and out-of-phase cycles at 871 to 982°C but not as good as 760 to 871°C. The thermal cycle variation in these tests is limited, but the model's ability to correlate the TMF lives and predict the constitutive behavior of a material whose behavior changes with temperature provides optimism that this analysis may be applied to other TMF situations.
Influence of Nitrogen Solutes and Precipitates on Low Cycle Fatigue of 316L Stainless Steels
Authors: S Degallaix , G Degallaix , J Foct
Published: February 2009

Improvement of monotonic and cyclic mechanical properties of 316 steels by nitrogen is studied for different nitrogen contents between 0.03 and 0.25 wt% at 20 and 600°C. At 20°C, yield and ultimate tensile stress increase almost linearly with nitrogen content to the detriment of ductility. Low-cycle fatigue (LCF) life also increases linearly with nitrogen solutes until 0.12 wt% nitrogen where a saturation of this effect occurs. This improvement is attributed to a more homogeneous distribution and to a better reversibility of plastic strain.

The influence of aging treatment at 600°C is studied on tensile and LCF properties at 20 and 600°C. An LCF life decrease with aging is more noticeable when nitrogen content is high. Furthermore, the higher the strain range, the more marked is the decrease in life at room temperature; this tendency is inverted at 600°C.

At 20°C, stress evolution during the hardening-softening stage is virtually independent of aging time but strongly dependent on nitrogen content. For high strain ranges at 600°C and high nitrogen content, the stress increases continuously until rupture, due to significant intergranular precipitation and embrittlement.
A New Approach to Low Cycle Fatigue Behavior of a Duplex Stainless Steel Based on the Deformation Mechanisms of the Individual Phases
Authors: T Magnin , JM Lardon , L Coudreuse
Published: February 2009

This paper concerns the low-cycle fatigue (LCF) behavior of a duplex (austenitic-ferritic) stainless steel with a 50% ferrite content. The study focuses on the damage mechanisms leading to crack initiation in the duplex alloy cycled at imposed plastic strain amplitude. Furthermore, the behavior of two other alloys of compositions close to those of the α and γ phases is simultaneously examined and correlated to the duplex alloy cyclic properties. The LCF properties of the duplex alloy approach those of the ferrite phase at high strain amplitudes and those of the austenite phase at low strain amplitudes. Many observations confirm this result: the Coffin-Manson curves, the hardening/softening curves, the cyclic stress-strain curves, and the crack initiation sites indicate a change in the duplex behavior near a plastic strain amplitude of 10−3. The proposed analysis of the LCF behavior of the duplex alloy is based on the cyclic plastic deformation mechanisms of the α phase (which exhibits twinning and pencil glide) and of the γ phase (with planar slip). Finally, the influence of a 3.5% NaCl solution on the LCF properties of the duplex alloy clearly underlines the occurrence in this alloy of two different types of behavior according to the applied plastic strain amplitude.
Quantification of Slip Lines in Fatigue
Authors: WG Fricke
Published: February 2009

Although fatigue, even low-cycle fatigue, can occur at stresses below the engineering yield strength of a material, plasticity is involved because slip lines may appear on the surface of a test material. These slip lines increase in number in the early stages of fatigue and are indeed the precursors of ultimate failure. In a cyclic stress test, the initial rapid increase in number of slip lines produces slip saturation, after which other phenomena such as crack initiation become predominant. New slip lines are added at a decreasing rate.

Quantitative measurements have been made of slip lines in fatigue specimens and compared with the number in a tensile test of the same material. In fatigue, the equation describing the rate of addition of new slip lines requires that the stress be described in terms of how much the test stress exceeded a quantity, characteristic of the material, which can be thought of as a “yield strength” in fatigue. This quantity corresponded closely to the measured endurance limit of the material.

A comparison is made with similar measurements in a tensile test.
Effect of Elevated Temperatures on the Low Cycle Fatigue of 2.25Cr-1Mo Steel—Part II: Variable Amplitude Straining
Authors: J Polák , A Vašek , M Klesnil
Published: February 2009

A 2.25Cr-1Mo steel was subjected to variable amplitude block straining at test temperatures of 22, 350, 450, and 550°C. A loading block having 500 strain peaks with an exponential probability density distribution and a constant diametral strain rate was generated. The loading block was analyzed by the rain flow procedure, closed hysteresis loops were identified, and peaks forming a cycle with stress amplitude smaller than half of the fatigue limit were deleted. The strain block characterized by the maximum strain amplitude was applied to a set of cyclindrical specimens in a computer-controlled electrohydraulic testing system.

Stress amplitudes of all closed hysteresis loops versus the number of blocks (i.e., the variable amplitude cyclic hardening-softening curves) were plotted. Similar hardening-softening behavior as in constant amplitude straining was observed. A plot of stress amplitudes versus plastic strain amplitudes in a block at the half-life (i.e., the service stress-strain curve) was obtained. The parameters of the service stress-strain curves were evaluated in dependence on the maximum strain amplitude and on the temperature. They were compared with the basic cyclic stress-strain curves.

The fatigue life in variable amplitude block loading (i.e., the number of blocks to failure) was plotted versus maximum strain amplitude. The experimental results were compared with the predictions using a linear cumulative rule combined with a rain flow counting procedure and the Manson-Coffin fatigue life curve. The actual to predicted life ratio was 0.4. The observed discrepancy was discussed in terms of the mechanism of fatigue damage under spectrum loading.
Influence of High Frequency Vibrations on the Low Cycle Fatigue Behavior of a Superalloy at Elevated Temperature
Authors: J Ouyang , Z Wang , D Song , M Yan
Published: February 2009

The influence of relatively high-frequency, low-amplitude vibrations on the low-cycle fatigue (LCF) behavior and its fractographic feature for a GH36 Fe-Ni alloy at 600°C is studied. It is found that the influence is rather complicated and significant. The nature and extent of the influence depended upon the ratio of the amplitude of high-cycle to low-cycle stress range, Q. As Q approaches Qc (the critical value of Q), there is no appreciable influence on the total LCF life; as Q < Qc, it will prolong the lifetime slightly; and as Q > Qc, the fatigue life will decrease progressively. The variation of fatigue life was ascribed to the occurrence of some considerable interactions between creep, high-cycle fatigue (HCF), LCF, and environmental effect under different values of Q at elevated temperature, and the change of damage mechanism of the material. The total LCF life, especially crack propagation life, will be markedly reduced by the high-cycle vibrations when Q is much greater than Qc. Meanwhile, the mixed failure of creep, HCF, LCF, and environmental attack will gradually change into combined cycle fatigue failure with increasing Q. The fractographic analysis is in agreement with these results.
Relations Between the Strain Energy Density Distribution and Elastic-Plastic Stress-Strain Fields Near Cracks and Notches and Fatigue Life Calculation
Authors: G Glinka
Published: February 2009

Relations between the strain energy density distribution and the elastic-plastic strain-stress fields near notches have been analyzed. It was found that the strain energy density distribution in the plastic zone ahead of the crack or notch tip can be satisfactorily estimated on the basis of the theoretical elastic stress field. The strain energy density can be translated into the elastic-plastic strain and stress in the plastic zone if the nonlinear material stress-strain curve is known. Good agreement with finite element calculations has been achieved for cracks under plane strain conditions. It has also been shown that the equivalent strain energy approach can be useful for the notch-tip elastic-plastic strain calculation under both monotonic and cyclic loading.

Very good agreement between the experimentally measured and calculated notch-tip strains has been obtained up to general yielding. The elastic-plastic notch-tip strain histories calculated on the basis of the equivalent strain energy density concept were used for fatigue life calculation of key-hole specimens tested under SAE transmission loading history. Significant improvement of fatigue life prediction has been achieved in comparison to the calculations based on Neuber's rule.

Possible applications of the equivalent strain energy density concept for fatigue crack growth prediction are also discussed.
Evaluation of Low Cycle Creep-Fatigue Degradation for Notched Components
Authors: SN Singhal , RA Tait
Published: February 2009

Four approaches have been examined to predict the endurance of circumferentially notched, uniaxially loaded test pieces subjected to fully reversed strain-controlled cyclic loading at elevated temperatures. Results are compared with uniaxial data derived from smooth test pieces. The strain-time waveforms include both trapezoidal and triangular shapes to distinguish between fatigue and fatigue-creep damage. The four materials investigated are HP-45 (1144 K), Incoloy 800H (1144 K), Incoloy 800 (922 K), and 1¼Cr-½Mo (811 K). The analysis has reflected the various degrees of cyclic hardening and creep deformation exhibited by these materials. The predictive approaches include Neuber notch strain analysis and finite element analyses to identify the maximum plastic strain range, the equivalent plastic strain range, and the total plastic work per cycle.

The study provides a simple, pragmatic approach to predicting whether cyclic plant operation is lifetime limiting. The primary conclusion is that the total plastic work-per-cycle approach is the preferred method for evaluation of fatigue-limited plant lifetimes.
Strain Energy Partitioning Approach and Its Application to Low Cycle Fatigue Life Prediction for Some Heat-Resistant Alloys
Authors: Z Duan , J He , Y Ning , Z Dong
Published: February 2009

An improved life prediction approach of time-dependent fatigue, strain energy partitioning (SEP), was verified with three alloys of different strengths. SEP synthesizes the energy concept of the Ostergren model and the thought of inelastic strain partition of the SRP model, and, thus, the partitioned strain energy component (Uij) is the single independent variable in SEP. The results show that the life predictability of SEP to all three alloys appears to be the most promising in comparison with those of strainrange partitioning (SRP) and frequency separation (FS) methods. It seems that the higher the strength level of the tested material, the better the predictability of SEP over that of SRP and FS.
High-Temperature Low Cycle Fatigue Crack Propagation and Life Laws of Smooth Specimens Derived from the Crack Propagation Laws
Authors: R Ohtani , T Kitamura , A Nitta , K Kuwabara
Published: February 2009

The objectives of this work are (1) to make clear the behavior of high-temperature low-cycle fatigue (LCF) crack propagation, (2) to verify the applicability of the J-integral to the fracture mechanics equations of crack propagation rates, (3) to derive two types of LCF life laws of smooth specimens based on the two types of crack propagation equations (i.e., cycle-dependent and time-dependent), (4) to show a resemblance to or a difference from the Manson-Coffin equation and the strain-range partitioning equations, and (5) to characterize the cycle-dependent and time-dependent fatigue laws. The effectiveness of the proposed failure-life equations was examined using experimental data on isothermal and thermal fatigue of smooth specimens.
Reconsideration of Macroscopic Low Cycle Fatigue Laws Through Observation of Microscopic Fatigue Process on a Medium Carbon Steel
Authors: S Harada , Y Murakami , Y Fukushima , T Endo
Published: February 2009

In order to correlate macroscopic low-cycle fatigue laws such as the Manson-Coffin relation and Miner's rule with the actual microscopic fatigue process, cyclic strain-controlled lowcycle fatigue tests were conducted on a medium carbon steel under constant-, two-step-, and random-strain amplitude conditions. Through continual observation of the microscopic fatigue process of plain specimens and holed specimens with a very small hole (40 μm diameter), it was found that the total fatigue life is mostly occupied by the microcrack propagation life. Both the Manson-Coffin relation and Miner's rule are derived using the microcrack propagation law. Then, background for the applicability of Miner's rule is discussed.

The results of the random loading fatigue tests showed that microcrack propagation life can be predicted on the basis of the microcrack growth law in conjunction with the rain flow method for counting fatigue damage. Finally, the microcrack growth law-aided approach was applied to interpret anisotropy in fatigue strength of a thick forged steel plate.
Grain Boundary Oxidation and an Analysis of the Effects of Oxidation on Fatigue Crack Nucleation Life
Authors: Y Oshida , HW Liu
Published: February 2009

A study on the effects of pre-oxidation on subsequent fatigue life was conducted. The surface oxidation and grain boundary oxidation of a nickel-base superalloy (TAZ-8A) were studied at 600 to 1000°C for 10 to 1000 h in air. The surface oxides were identified and the kinetics of surface oxidation was discussed.

The grain boundary oxide penetration and morphology were studied. The pancake-type grain boundary oxide penetrates deeper and its size is larger, therefore, it is more detrimental to fatigue life than the cone-type grain boundary oxide. The oxide penetration depth, am, is related to oxidation temperature, T, and exposure time, t, by an empirical relation of the Arrhenius type. The effects of T and t on the statistical variation of am were analyzed according to the Weibull distribution function. Once the oxide is cracked, it serves as a fatigue crack nucleus. The statistical variation of the remaining fatigue life, after the formation of an oxide crack of a critical length, could be related directly to the statistical variation of grain boundary oxide penetration depth.
Compressive Properties and Laser Absorptivity of Unidirectional Metal Matrix Composites
Authors: Chang DJ , Steckel GL , Hanna WD , Izaguirre F
Published: February 2009

Metal matrix composite (MMC) use has been impeded because there is no well qualified property data base. Elevated temperature compressive properties and laser absorptivity data are in great demand in the design, modelling, and analysis of MMC structures in elevated temperature applications and survivability assessment. Testing methods were developed for room-temperature and elevated-temperature compressive modulus and strength measurements of single-ply unidirectional graphite-aluminum (Gr/Al) and graphite-magnesium (Gr/Mg), and to generate laser absorptivity data for these two types of materials.

An effective gage length was established for valid compression tests to eliminate buckling failure. The measured failure strength values for Gr/Al and Gr/Mg are approximately 50 to 60% lower than the material tensile strength. Small gage strain gages are being used to measure the compressive modulus as a function of temperature.

The laser absorptivities of Gr/Al and Gr/Mg were measured by the use of an in-house developed differential calorimeter. The calorimeter has been used to measure total hemispherical emittance and normal laser absorptance of specimens as a function of temperature from 200 to 700°C.

Test results indicated that the laser absorptances of Gr/Al and Gr/Mg at 10.6 μm wavelength remain more or less constant over most of the temperature range tested.
Mechanical Behavior of Three-Dimensional Braided Metal Matrix Composites
Authors: Majidi AP , Yang J-M , Chou T-W
Published: February 2009

The results of an on-going experimental study to characterize the mechanical properties and failure mechanisms of a novel 3-D braided composite of α-alumina fiber (fiber FP) in a matrix of aluminum +2.5% lithium have been presented. Quasi-static tension, compression, and shear tests have been conducted. Acoustic emission data have been collected during tension tests to provide additional information about failure. Fracture toughness and drop-weight impact characteristics have been also evaluated and compared with those of unidirectional FP/aluminum-lithium composites.

The 3-D braiding of fibers not only provides reinforcement in the thickness direction and reduces or eliminates potential for interlaminar failure, but also appears to be an effective approach to increasing the resistance of the composite to crack propagation under both static and impact conditions of loading.
An Evaluation of the Failure Behavior of 3-D Braided FP/Aluminum-Lithium Composites Under Static and Dynamic Blanking
Authors: Ko F , Razavi A , Rogers HC
Published: February 2009

The adiabatic shear stability of unreinforced, unidirectionally reinforced and 3-D braid reinforced aluminum-lithium were studied by static and dynamic blanking at various deformation levels. Comparing to Hadfield steel, no apparent adiabatic shear was detected in any of these materials. The failure mechanism of these structures were identified by microstructural analysis.
Factors Affecting the Determination of Thermophysical Properties of Metal Matrix Composites
Authors: Tye RP , Smith SE
Published: February 2009

Methods, techniques, and models available for the measurement or estimation of thermal conductivity, electrical conductivity, thermal diffusivity, thermal expansion, and heat capacity, are reviewed with respect to their applicability to metal matrix composite materials and systems.

Various factors influencing the choice of method used to obtain reliable properties information are discussed. These include: (a) application and use of the material or system; (b) continuous and discontinuous reinforcement of the matrix; (c) layered and other combination of materials; (d) size and form; (e) anisotropy; and (f) temperature and accuracy requirements.

Considering the unique problems represented by these materials and systems, recommendations are made regarding the most appropriate means currently available to obtain reliable thermal properties information. Areas are identified where further information and work are required in order to improve existing models and measurement techniques.
Pressure Dependence of the Elastic Constants of Silicon Carbide/2014 Aluminum Composite
Authors: Dandekar DP , Frankel J , Korman WJ
Published: February 2009

Adiabatic elastic constants of a composite composed of 25 volume percent of silicon carbide (SiC) whiskers in 2014 aluminum matrix are measured to 1.5 GN/m2. Whereas the pressure derivative of bulk modulus of the composite exceeds those of its two constituents, the pressure derivative of the shear modulus of the composite lie between those of its constituents. Calculated values of the upper and lower bounds of these moduli of the composite appear to be insensitive to the pressure derivative of the shear modulus of SiC. A new modified and simple model of such a composite predicts the values of the elastic constants of this composite fairly well at both the ambient pressure and 1.5 GN/m2.
Micromechanical Modeling of Yielding and Crack Propagation in Unidirectional Metal Matrix Composites
Authors: Adams DF
Published: February 2009

A two-dimensional finite element micromechanics analysis was used to predict the response of a unidirectional graphite/aluminum composite subjected to transverse tensile, longitudinal shear, or axial-transverse tensile biaxial loadings. Processing-induced thermal residual stresses and the temperature dependence of the highly nonlinear aluminum matrix properties were included, along with the anisotropy of the graphite fibers. A parametric study was made of the influence of matrix heat treatment condition and fiber-matrix interface bond strength on composite response to the various loadings. The correlations with available experimental data suggested that the actual composite material may have been in an intermediate heat treat condition, and a moderate interface strength degradation state.
Minimechanics Analysis and Testing of Short Fiber Composites: Experimental Methods and Results
Authors: Awerbuch J , Goering J , Buesking K
Published: February 2009

This paper describes a study performed to experimentally investigate the constitutive behavior of silicon carbide whisker reinforced aluminum and to provide data for a companion material modelling study. The experimental program included measurement of material stress-strain curves, assessment of the acoustic emission response of the material, and examination of the material failure surfaces.

The results of the mechanical tests showed that as the whisker content increases, stiffness, strength, and rate of strain hardening increases, whereas the strain to failure decreases. Changes in the extrusion ratio have little effect upon the composite properties although the extruded bulk aluminum was significantly different from commercial aluminum alloy 6061-T6. The acoustic emission results showed that the failure modes of the composites and unreinforced materials were similar. The scanning electron microscopy results showed that failure of the materials was due to microyielding within the aluminum. Therefore, the composite failure is believed to be caused by matrix plasticity and whisker pullout.
Noncontact Ultrasonic Evaluation of Metal Matrix Composite Plates and Tubes
Authors: Reed RW
Published: February 2009

Noncontacting electromagnetic acoustic transducers (EMATs) are used to generate and receive wideband pulses of ultrasound in graphite/aluminum (Gr/Al) plates and tubes. High precision velocity and attenuation measurements are used to locate defects and to determine elastic moduli. Both static global and scanned local values of the attenuation and velocity are measured by using large or small receiver EMAT separations, respectively. Nondispersive, shear horizontal, and lowest order symmetric Lamb waves are employed along with a four-transducer method which eliminates measurement uncertainties associated with EMAT lift-off variations. Elastic moduli of Gr/Al plates are determined from ultrasonic data.
Nondestructive Evaluation of Fiber FP Reinforced Metal Matrix Composites
Authors: Widrig JE , McCabe DD , Conner RL
Published: February 2009

Fiber FP is a continuous polycrystalline alumina fiber suited for reinforcement of metals, plastics, and ceramics. Fiber FP/metal composites are very attractive for weight and stiffness critical applications. In order for these materials to be accepted in the marketplace, quality control methods needed to be developed. In this study, we developed methods based on computed tomography (CT) and ultrasonic scanning to identify porosity, delamination, and other discontinuities in Fiber FP reinforced composites.

Prior to this work, Fiber FP/metal plates and coupons were inspected using a combination of X-ray and ultrasonic methods—both with significant limitations. The ultrasonic evaluation utilized a defect threshold technique with inherent sensitivity uncertainties of defects close to the threshold level. This study demonstrated that defect intensity level scanning was superior to threshold scanning for Fiber FP reinforced composites. We developed color indicators for the various intensity levels to facilitate data interpretation.

The ultrasonic technology was also limited to simple flat shapes. Fiber FP development programs have recently shifted to fabrication of more complex prototype components such as connecting rods and helicopter transmission housings. CT offers several advantages over traditional radiographic techniques. Standard radiography creates an obscure 2-D image of a 3-D body by superimposing defect indications. A CT scanner has the ability to look at 2-D slices of a 3-D component. A series of 2-D images can be taken, and then interfaced with a computer to show the 3-D nature of the object. This technique has been demonstrated for prototype connecting rods and confirmed via destructive testing.
Deformation and Failure Characteristics of Center-Notched Unidirectional Boron/Aluminum at Room and Elevated Temperatures
Authors: Madhukar MS , Awerbuch J , Koczak MJ
Published: February 2009

The global and local deformation characteristics of center-notched unidirectional B/Al composites were examined both in the elastic and inelastic regions of the load-displacement curves. The global deformation was measured at room temperature by applying the conventional compliance gage, while the local deformation was measured by means of the interferometric displacement gage (IDG) technique at room and elevated temperatures. The effects of notch length and test temperature on the deformation characteristics and on damage initiation and progression were determined. The local compliance calibration curves, obtained with the IDG technique, were found to be highly sensitive to notch length. With increasing load, significant nonlinearity in the load-crack-opening displacement (COD) and jumps in COD have been observed, resulting from the crack-tip damage progression in the form of fiber breakage, matrix inelastic (plastic) shear deformation, and matrix cracking. The results obtained with the IDG technique were qualitatively correlated with high magnification (× 150) visual observations in rela-time via a closed circuit television. The IDG technique could be easily applied at elevated temperatures. Results from the IDG indicate that significant changes in the deformation characteristics occur at temperatures above 204°C (400°F). The experimental global and local compliance calibration curves were compared with predictions and an excellent agreement has been established. The predictions of COD were made by applying an existing analytical model which assumes the existence of a zone of longitudinal inelastic shear deformation emanating from the notch-tip. Good agreement between prediction and experiment could be established. The comparison provided a simple procedure to evaluate the in situ matrix shear yield stress. The experimental results indicate that the analytical modeling for the inelastic deformation of the subject material should incorporate both the matrix strain hardening and the mechanism of sequential failure.
Compressive Testing of Metal Matrix Composites
Authors: Bethoney WM , Nunes J , Kidd JA
Published: February 2009

A newly developed direct compressive test fixture for metal matrix composites is described and evaluated in this paper. This fixture was designed to be reusable, provide accurate and reproducible data, and to eliminate premature “brooming” failures. A comparative analysis between this test method and ASTM D 3410 for compressive properties of unidirectional or cross-ply fiber-resin composites using the Illinois Institute of Technology Research Institute (IITRI) indirect compression test method was also investigated.

In utilizing the direct compressive test method, the affects of specimen size and geometry were studied on a uniaxially oriented FP-alumina fiber reinforced magnesium composite. Cylindrical specimens 6.35, 9.53, and 12.7 mm in diameter, and a 9.53-mm-square specimen were tested at a constant 1/d ratio of 2.0. The tabbed specimen size and geometry that was employed in the IITRI test method was 2.54 mm thick by 6.35 mm wide by 152.4 mm long with a 12.7 mm untabbed gage length. All IITRI test specimens were tabbed by employing an improved method for bonding the aluminum tabs to the FP-alumina/magnesium surface. This tabbing procedure was developed to overcome earlier difficulties that were encountered in obtaining sufficient tabbed bond strength.

All of the test specimens investigated were from a single plate of as-cast composite material that contained a fiber volume fraction of 55%.

Tests were conducted with the applied load parallel to the fiber axis. Each specimen was strain gaged to provide a continuous load-strain curve to failure, to determine various materials properties, and to monitor any bending of the specimen. Data obtained included the ultimate compressive strength, the tangent modulus, the apparent proportional strain, strain to failure, and Poisson's ratio. Results from both the direct and the indirect compressive test methods were statistically compared. Finally, post test metallographic and fractographic examinations were performed to determine the principal failure modes.

Final results indicate that the cylindrical specimens, regardless of their respective diameters, display the least scatter in their ultimate compressive strengths (UCS). Also, higher stress values for the cylindrical specimens was obtained; as compared to the IITRI specimens.
Elevated Temperature Testing of Metal Matrix Composites Under Rapid Heating Conditions
Authors: Frankle RS , Baetz JG
Published: February 2009

The Atlantic Research Corporation has designed and built facilities for testing metal matrix composites at elevated temperatures. The facilities are capable of performing tension or compression tests of sheet-type specimens to 1927°C (3500°F) at heating rates in excess of 38°C/s (100°F/s). Tension tests were performed on unidirectional and cross-ply graphite/aluminum, unidirectional graphite/copper, and discontinuous silica carbide/aluminum materials. The continuously reinforced materials exhibited superior room temperature properties and elevated temperature property retention when compared with the matrix metals alone. The elevated temperature response of the discontinuously reinforced SiC/Al was comparable to that of aluminum. These test results indicate the potential advantages of using metal matrix composites in rapid heating, elevated temperature applications, where the temperature may approach the melting point of the matrix material.