bluuebunny/crossref_metadata_2025 huggingface dataset:
Computerization of Fracture Features and Failure Analysis of Automotive Composite Materials
Authors: Saliba SS , Saliba TE
Published: March 2009

An experimental program was conducted to characterize the failure modes of fiber reinforced composite materials which are important to the automotive industry for both structural and non-structural applications. The composite materials utilized for this study included a randomly oriented, chopped glass reinforced polyester (SMC), Azdel's P-100 glass mat reinforced polypropylene, reaction injection molded (RIM) glass mat reinforced polycarbonate, Wellman's magnesium silicate filled nylon (Wellamid), and Dow's glass filled polyurea (Spectrum HT). The study included (1) conditioning and mechanical testing of samples, (2) fractographic examination, (3) development of a computerized atlas of all fractographs, and (4) design of a preliminary failure analysis expert system. Fractographs of all five materials unconditioned, and subjected to thermal aging or humidity aging identifying typical topographical features and fracture surfaces are presented for specimens subjected to tension, compression, flexure, impact, tension-compression fatigue, and flexure fatigue testing. The fractographic information obtained was summarized in two computerized atlases. In addition, the results were used to develop an expert system that guides the user in establishing potential failure modes based on the type of material, environmental conditions, and features observed on fracture surfaces. It should be noted that the expert system has been utilized for test specimens only and has not been tested on actual parts fabricated of the materials utilized in this study.
Statistical Analysis of Profilometric Sampling for Roughness Parameters
Authors: Drury WJ , Gokhale AM
Published: March 2009

In quantitative fractography, roughness parameters RL and RS are often used as important descriptors of fracture surface geometry. These parameters are usually estimated by analyzing fracture profiles acquired by sampling the surface of interest with metallographic sections perpendicular to the global plane of fracture. Because of the limited sampling, there is an inherent statistical error to the measured RL and RS values. This work addresses the determination of this sampling error using basic statistical concepts. In addition, a simple parameter is introduced that facilitates estimation of the amount of profile data required to achieve a specified precision to the roughness parameters. Experimental results of sampling error and effort are provided from ‘typical’ steel (AISI 4340) fracture surfaces.
Quantitative Fracture Surface Analysis of Fatigue Crack Propagation Under Variable Amplitude Loading
Authors: Ranganathan N , Benguediab M , Henaff G , Adiwijayanto F
Published: March 2009

A method for quantitative analysis of fatigue fracture surfaces has been developed in our laboratory. Firstly, the salient features of the proposed method are presented based on the results obtained under constant amplitude (CA) tests at five R ratios for the 2024 T351 aluminum alloy.

Secondly, the proposed method is applied to fatigue crack growth under variable amplitude loading. Two kinds of tests were studied, simple block loading (BL) consisting of four mean load levels with different number of cycles at the highest R ratio. These tests were also carried out on the 2024 T351 alloy. Finally, variable amplitude loading (VAL) tests using a transport aircraft wing spectrum were carried out on three different aluminum alloys, including an aluminum lithium alloy.

In the case of BL tests the spatial distribution of striations and dimples are quantified with respect to the maximum stress intensity factor. After comparison with CA results, it is shown that the peak to peak load ratio and the highest load ratio in a block are important parameters in determining the fracture surface appearance.

For VAL conditions, the results obtained for the 2024 T351 alloy show that the spatial distribution of the significant fractographic features is quite similar to that obtained at R ≃ O under CA conditions at Kmax levels corresponding to the maximum load in the spectrum. Comparison of fracture aspects under VA conditions for the different aluminum alloys shows that a larger amount of striation like features rather than dimples produces a better crack growth resistance. It is also shown that the presence of secondary cracks may also have a beneficial effect on the crack growth resistance.

In the discussion, a technique for the determination of Kmax and equivalent R ratio for VAL conditions is proposed.
Fractographic Study of Notch Fatigue Crack Closure and Growth Rates
Authors: Sunder R , Prakash RV , Mitchenko EI
Published: March 2009

A fractographic study was carried out on fatigue crack closure and crack growth rates in an Al-Cu alloy under conditions of monotonic and cyclic yield at the notch root. Comparative tests were also conducted on smooth specimens subject to elastic loading conditions. A notch root fatigue crack can see substantial difference between crack closure and opening stress. This difference is considered responsible for local mean stress effects observed in notch fatigue response to complex load sequences. The difference between closure and opening stresses diminishes to negligible proportions as stress excursions reduce to elastic interval and is therefore hardly observed in long cracks.

To support validation effort on predictions, notch root crack growth rates were determined from optical fractography under a programmed load adaptation of the FALSTAFF load sequence. The results of these measurements suggest that scatter in short crack growth data is similar to that in long cracks at comparable growth rates. Life to initiation of (naturally initiating) multiple notch root cracks is dissimilar. The crack having been in existence longest appears to assume the role of the dominant crack and accelerates others (smaller) cracks.
Tensile and Tension-Tension Fatigue Fracture Behavior of γ-Al2O3/Al Metal Matrix Composite at Room and Elevated Temperature
Authors: Komai K , Minoshima K , Funato T
Published: March 2009

Tensile and tension-tension fatigue (stress ratio: 0.05) fracture behavior of a continuous γ-Al2O3 fiber reinforced aluminum (Al-0.5Ba) matrix metal matrix composite (fiber volume fraction: 50%) manufactured by a squeeze casting process was investigated in laboratory air at room and elevated temperature (573 K). Special attention was paid to fracture processes: in situ observation of static fracture processes was also made in a scanning electron microscope (SEM). Fracture surfaces were closely examined with an SEM, and we have discussed the tensile and fatigue fracture mechanisms. The fiber reinforcement improved the material properties, including tensile strength, elastic modulus and fatigue strength of both [0°], [0°/90°], and [ ±45°] composites, compared with those of unreinforced equivalent matrix materials, 1XXX series aluminums. The in situ observation in the SEM showed that the static fracture of [0°] and [0°/90°] composites was controlled by failures of 0° plies, and the fracture initiated at broken fibers at the specimen surface. In the case of [±45°] composites, intralaminar failures at the middle position in thickness occurred, leading to interlaminar deformation and failures, and finally unstable fracture. The tensile strength of [0°] composites at 573 K was higher than that at room temperature. In the case of [0°/90°] composites, there was little difference in strength between room temperature and 573 K. However, strength at 573 K of [ ± 45°] composites decreased from that at room temperature. Both at room temperature and at 573 K, few fiber pullouts were observed and pullout fibers were covered with matrix. Failures of [90°] composites occurred in matrix near fiber/matrix interfaces and fiber splitting also existed. These indicated that the composite had excellent fiber/matrix interfacial strength. Failure mechanisms of fatigue in [0°] composites differed depending on stress level; at high stress levels, final failures were brought about by a macroscopic transverse crack, whereas at low stress levels, longitudinal crack first initiated and propagated in the longitudinal direction, that is, loading axis direction. This longitudinal crack reached the gripped position, and then a macroscopic transverse crack was initiated and propagated, resulting in final failure. In the case of [±45°] composites, macroscopic failures occurred in the transverse direction, irrespective of stress level, and the S-N curve was very flat.
Failure Analysis of Aramid Fiber Reinforced Aluminum Laminates with Surface and Through Thickness Fatigue Cracks
Authors: Macheret Y , Bucci RJ , Nordmark GE , Kipp TR
Published: March 2009

Fiber reinforced aluminum laminates are able to retard fatigue crack growth by transfer of load from cracked aluminum layers to intact fibers. The resulting crack bridging effect leads to superior laminate fatigue crack growth response compared to that of monolithic aluminum. In real laminate structures, fatigue cracks emanating from an open hole, as well as surface cracks initiated in an outer laminate aluminum layer, can be present. In this investigation, fatigue crack growth data on surface notch and open hole (through thickness notch) initiated fatigue cracks are presented. The failure analysis reveals that the surface initiated cracks propagate only in the outer aluminum layer, with fibers remaining intact. The open hole initiated cracks, on the other hand, propagate through the laminate thickness in all aluminum layers. In this case, depending on the laminate residual stress state (controlled by a post stretch procedure), a complete fiber failure may occur. When the fiber integrity is preserved, however, the laminates fatigue response in all cases is superior to that of monolithic aluminum.
Fractographic Analysis of Quasi-Isotropic Laminates Loaded in Compression
Authors: Pinnell WB , Crasto AS , Kim RY
Published: March 2009

Graphite/epoxy specimens with two different quasi-isotropic layups ([0/45/90/ - 45]s and [0/±60]s) were tested in axial compression in an IITRI test fixture using both miniature sandwich specimens and all-laminate coupons. Fractographic examinations of the tested specimens' cross-sections and fracture surfaces were conducted. The initial failure events, sequence of failure events and failure modes were determined for each specimen. Stress analysis was used to calculate the ply stresses within the laminate and determine the interlaminar stresses at each interface, and thereby predict the failure mode and location of failure within the laminate in conjunction with a failure theory. Analytical results supported the conclusions of the fractographic investigation.
Quantitative Fractography of the Cracked Lap Shear Composite Specimen
Authors: Fortson BH , Armanios EA
Published: March 2009

Understanding composite failure requires knowledge of the material behavior near the tip of a propagating crack. Microscopy permits examination of small regions of the fracture surface, allowing close study of the crack tip region. This work represents an effort to link visual observations of the crack surface to the mechanical history of the specimen. Scanning electron microscopy examines the fracture surfaces of failed cracked lap shear (CLS) specimens. Quantitative fractography tracks the area fraction of four fracture morphologies as a function of position on the fracture surface. Statistical hypothesis testing determines locations of significant shifts in fracture morphology. These locations are compared to regions of crack arrest during testing, and the correlation is found to be poor. It is suggested that these changes in fracture surface morphology are linked to variations in the thickness of the resin layer along the plane of the crack.
Thermal Stability Testing of Aviation Turbine Fuel — A History
Authors: Strauss KH
Published: March 2009

The paper traces the history of high temperature problems in aircraft jet fuel systems and outlines the parallel progress of fuel specifications to control the problems. The development of laboratory equipment to implement these controls is described together with the testing used to validate the control limits. Shortcomings of the various types of control equipment are reviewed.
Physicochemical Aspects of Aviation Fuel Thermal Stability
Authors: Hazlett RN
Published: March 2009

Physical and chemical parameters effect significant control over thermal stability. Temperature is the most important of the physical factors as deposition of solids increases as the temperature rises. Other physical factors which play an important role in thermal stability are system pressure, flow regime, test duration, and characteristics of the heated surface. Chemical aspects which are important in thermal stability are oxidation, fuel composition, and metal catalysis. Free-radical autoxidation is the trigger which initiates the reactions that ultimately form insoluble material. Hydroperoxides play a crucial role in deposition. Sulfur, nitrogen, and oxygen are found in large concentrations in fuel system deposits compared to their presence in unstressed fuel. Compounds containing these elements are more readily oxidized than hydrocarbons and appear to be instrumental in increasing polarity and thus reducing solubility of oxidation products in the low-polarity jet fuel. Our understanding of the chemistry of thermal stability is complicated by the fact that a very small fraction (<0.1 ppm) of fuel is converted into deposits and filterable solids.
Air Frame Considerations in Fuel Thermal Stability for Commercial Supersonic Flight
Authors: Schmidt JE
Published: March 2009

Airframe fuel system thermal environments for anticipated supersonic flight in the Mach 2.0 to Mach 4.5 range are calculated and discussed. Aerodynamic heating can result in tank fuel temperatures as high as 100°C, and airframe cooling requirements may raise the temperature at the engine pump inlet by another 50°C. Results of earlier tests on formation of fuel deposits in tanks show tank deposits to be inconsequential for speeds below Mach 3.0, although verification of this for nonmetallic tank materials and current fuels is recommended. Adaptions to fuel thermal stability specifications and testing may be required for supersonic flight. These could include higher specification test temperature limits, extended residence time or recirculation in the thermal stability test section, heat presoaking of fuel test samples, and elimination of prefiltering before the test section. Development of a test for the formation of fuel tank deposits may be required if speeds above Mach 3.0 are anticipated.
High Temperature Fuel Requirements and Payoffs
Authors: Croswell BM , Biddle TB
Published: March 2009

This paper describes a study performed under contract to Air Force Wright Laboratory, Wright-Patterson Air Force Base, Dayton, OH in association with the Naval Air Propulsion Laboratory, Trenton, NJ. The study projected fuel temperature capability requirements for future tactical fighter applications and the payoffs that would be realized by achieving these capabilities. The study was approached on the basis of the maximum benefit that might be realized through the use of high temperature fuels, i.e. elimination of the recirculation system. Heat loads were projected for different missions spanning IHPTET technology phases I, II, and III at different flight conditions. Fuel temperatures across fuel system components were calculated at these heat loads. Shortfalls of the current 163°C fuel capability were shown and minimum fuel temperature requirements defined. The study concluded that elimination of the recirculation system is not feasible but shows how high temperature fuel capability can minimize the weight penalty associated with fuel recirculation. In this way, potential payoffs for high temperature fuel development were shown in the form of reduced weight penalties that would normally be encountered as larger and larger recirculation systems are required to accommodate the increasing heat loads projected for advanced aircraft.
Fuel Thermal Stability Outlook for GE Aircraft Engines in 1991
Authors: Lyon TF
Published: March 2009

Although the thermal stability of fuels currently used in commercial engines is adequate, and usually exceeds the specification requirements by a considerable margin, pressure may be developing to cause a trend toward poorer fuel thermal stability. Factors influencing this trend are economic pressure, poorer quality crude, and environmental legislation. This adverse trend may be initiated as engines are being developed which tend to thermally stress fuel to a greater extent than engines currently in production. This situation exists while the adequacy of the thermal stability test procedure is being investigated by a CRC Task Force. In addition, engine sales to the Soviet Union (Aeroflot) and other east European airlines will expose the engines to fuels from these areas which have not previously been used in the engines on a continuous basis. These factors create a situation where potential effects of poor fuel thermal stability are a cause for some concern.
The Effect of Manufacturing Processes on Aviation Turbine uel Thermal Stability
Authors: Taylor WF
Published: March 2009

The effect of jet fuel manufacture processes on thermal stability quality is discussed in general terms. Processing sequences used to prepare jet fuel blend stocks vary widely depending on factors such as overall refinery design and crude type, and can have a significant effect on thermal stability quality. Processes which remove compounds which have a deleterious effect on thermal stability such as reactive olefins and heteroatom compounds have the greatest potential for thermal stability quality enhancement. Thus, hydrocracking and hydrotreating processes, in general, have greater potential for producing blend stocks with higher thermal stability quality than sweetening processes which convert mercaptans to disulfides which are left in the jet fuel. Treating with adsorbates such as Attapulgus clay also has the potential to remove deleterious compounds.
The Effect of Manufacturing Processes on Aviation Turbine Fuel Thermal Stability — Handling
Authors: Fletcher R
Published: March 2009

Thermal stability problems encountered over the past few years during the distribution of jet fuel will be reviewed. Surprisingly almost all of the problems have occurred during transportation by ship with very few being encountered with rail and multi-product pipeline movements. The reason for the failures will be discussed in relation to the distribution system and to the manufacturing processes.
Critical Review of the JFTOT
Authors: Datschefski G
Published: March 2009

The generation of JFTOT deposits and their subsequent assessment are reviewed, using results of a round-robin exercise involving four UK laboratories to study the precision of determining specification breakpoint temperature. A number of areas are identified in which the usefulness of the JFTOT as an aircraft fuel system simulator could be enhanced. Considerable variability was found in deposit assessment methods, and recommendations are made on how to overcome these for fuel research and specification purposes.
Simulation, Precision and Decision in JFTOT Testing
Authors: Wilson GR
Published: March 2009

JFTOT testing is a form of performance testing simulating the effects of a fuel/oil heat exchanger and other hot engine parts on the thermal oxidative stability of a jet fuel. Over the years, many questions have been raised about the dynamic aspects of the simulation, the reliability of the data, and the methods by which the data are used to satisfy specification needs. This paper examines these issues and shows how the test is as technically reliable as it has been practically reliable in use.
Measures of Fuel Thermal Stability — Which Answer is Correct?
Authors: Clark RH , Bishop GJ
Published: March 2009

It is highly desirable to be able to predict the degree of fuel thermal degradation within a jet engine fuel system from simple chemical parameters or rapid small-scale testers. However, given the complexity of the problem, which includes such variables as system components, fuel heat history, fuel flow regimes and residence times, it is likely that no one single measurement can hope to provide the answer.

Shell Research's approach to investigating jet fuel thermal stability is proceeding along three parallel fronts:- • Large-scale rigs realistically simulating engine components, so enabling predictions of fuel performance in the real environment. • Critical examination of the JFTOT specification test, assessing its ability to predict service fuel performance unequivocally. • Fundamental mechanistic studies to provide the necessary chemical background for the studies. These include analyses of fuel composition and kinetic measurements of fuel oxidation.

This paper describes each method, the advantages, disadvantages and nature of the measurement. The differences between the various types of measurement are explained. Finally, the results from each test are drawn together to yield a simple mechanism for fuel thermal degradation.
An Automated Device to Quantitatively Measure Thermal Deposits from JFTOT Heater Tubes by Interferometry
Authors: Morris RE
Published: March 2009

The practical value of the ASTM D3241 JFTOT procedure. for evaluating jet fuel thermal oxidation stability is limited by the lack of a quantitative measurement of heater tube deposition. Both the visual and the TDR methods have been shown to be disproportionately responsive to highly colored deposits and neither method appropriately deals with so-called “peacock” films. Destructive methods which determine deposits as total carbon require the availability of expensive, specialized analyzers and it is often difficult to obtain reliable measurements from thin deposits. This paper will describe an automated device which has been developed to determine deposit volumes by measurements of light interference. Data acquisition, data manipulation and volume calculations are performed under control of menu-driven software on a personal computer. Within the applicable thickness range, the device provides a means to obtain a reliable measurement of deposit volume which is not affected by the tube composition.
Corrosion-Resistant Outdoor Electronics
Authors: Schubert R , Veca A , Fischer E
Published: March 2009

The operating telephone companies are committed to assuring reliable and continuous quality telephone service. Environmental durability must be designed into the components of the electronic coin telephone that is often located in uncontrolled environments and in areas of continuous exposure to corrosive pollutants. To observe and quantify the effect of the environmental pollutants on coin telephone equipment, functional but unhoused electronic printed circuit board assemblies, a fully assembled, unhoused electronic chassis and coin acceptor, and a fully housed electronic chassis and coin acceptor were placed in a chamber and exposed to a pollutant-containing environment along with copper, nickel, and electroplated gold control coupons. The test pollutant atmosphere was a Battelle Laboratories Class III atmosphere consisting of air at 30°C and 70% relative humidity with H2S, Cl2, and NO2 at 100, 20, and 200 ppb, respectively. We report the results of Auger electron spectroscopy with Ar+ ion depth profiling that was done on various electronic components from housed and unhoused circuit packs and the control coupons. In general, corrosion film thicknesses on circuit components were less than coupon film thicknesses. This is attributed to the circuit pack geometry and component shrouding. A theoretical model supports the experimental results. Repeated functional testing at 95% relative humidity of both the housed coin telephone and unhoused assemblies was performed after exposure in the polluted atmosphere. After exposure, all circuits performed according to specification with respect to laboratory central office equipment and a fully active coin operation telephone line.
Formation of Copper Sulfide in Moist Air-Sulfur Dioxide
Authors: Chawla SK , Rickett BI , Payer JH
Published: March 2009

Films formed on copper exposed to various sulfur-bearing environments were analyzed by XPS and electrochemical reduction to study the formation of copper sulfide. The S-2p photoelectron band showed the presence of copper sulfide in the film in addition to oxysulfur species. Coulometric reduction analysis of the film suggested that the sulfide was nonstoichiometric. The formation of copper sulfide was also noted on copper immersed in “sulfurous” acid and in sulfuric acid. The properties of the sulfide that formed reductively on copper from these oxysulfur environments were compared with those of copper sulfide that formed from a moist hydrogen sulfide environment. Formation of copper sulfide from the oxysulfur environments showed that sulfur could participate as a reducible species in the film-forming corrosion reactions.
The Effect of Temperature, Humidity, and Silicon Content on the Oxidation of Fine Iron Particles
Authors: Hadad AS , Pizzo PP
Published: March 2009

In this study, the oxidation and electrochemical corrosion of Fe-Si particles used in magnetic recording are investigated under controlled thermal and relative humidity conditions. The relative humidity, above a threshold value, dominates the oxidation behavior. Oxidation kinetics increase greatly above this threshold, and a portion of the exposed particle population becomes completely oxidized. X-ray diffraction data confirm the appearance of α-Fe2O3 in addition to the usual γ-Fe2O3 following exposure to high humidities. Greater silicon content increases the threshold humidity, above which rapid degradation in magnetic properties occurs.
Corrosion of Soft Magnetic, Controlled Expansion, and Glass Sealing Alloys
Authors: DeBold TA , Masteller MS , Werley TN
Published: March 2009

There is relatively little data available on the corrosion resistance of soft magnetic, controlled expansion, and glass sealing alloys. This has been due in part to the relatively innocuous traditional service environments. The primary purpose of this paper is to provide corrosion data on a wide variety of iron-base, nickel-base, and cobalt-base alloys typically used for magnetic, controlled expansion, and glass sealing applications. The data presented systematically characterize the corrosion resistance of these alloys using humidity and salt spray testing and provide a database for the selection of materials as well as the development of new test procedures.
Electrochemical and Structural Characterization of Permalloy
Authors: Lee CH , Stevenson DA , Lee LC , Bunch RD , Walmsley RG , Juanitas MD , Murdock E , Opfer JE
Published: March 2009

The objective of this study is to characterize the passive behavior of electroplated permalloy films (EP) and bulk permalloy samples. The motivation for this work is the optimization of the magnetic properties of these films consistent with the required etching steps used in the fabrication of read/write thin film magnetic heads and the corrosion behavior of these heads in service conditions. The different samples of permalloy were evaluated with electrochemical potentiodynamic anodic polarization (PAP) at different scan rates in electrolytes with various pH and chloride content. The most significant difference in the passive behavior of the two types of samples was the predominantly active behavior of the EP samples, in contrast to a classic passive behavior of the bulk samples. The structure of EP permalloy was evaluated with EPM, XRD, and TEM in order to explain the difference in passive behavior. The most significant difference was the columnar grain structure with 20 to 30 nm in diameter by 100 nm in length and a (200) texture for the EP films.
Monitoring Small-Crack Growth by the Replication Method
Authors: Swain MH
Published: March 2009

The suitability of the acetate replication method for monitoring the growth of small cracks is discussed. Applications of this technique are shown for cracks growing at the notch root in semicircular-edge-notch specimens of a variety of aluminum alloys and one steel. Cracks were allowed to initiate naturally along the surface or at the corner of the notch root, a stress condition representative of a fastener hole or fillet in aircraft components. The calculated crack growth rate versus ΔK relationship for small cracks was compared to that for large cracks obtained from middle-crack-tension specimens. The advantages and limitations of the acetate replication method in comparison to other commonly used methods for small crack research are delineated. The primary advantage of this technique is that it provides an opportunity, at the completion of the test, to go backward in time towards the crack initiation event and “zoom in” on areas of interest on the specimen surface with a resolution of about 0.1 (μm (0.0001 mm). The primary disadvantage is the inability to automate the process. Also, for some materials, the replication process may alter the crack-tip chemistry or plastic zone, thereby affecting crack growth rates.
The Experimental Mechanics of Microcracks
Authors: Davidson DL
Published: March 2009

The experimental mechanics of small cracks require high spatial resolution measurements that are only obtained by using the scanning electron microscope. This paper describes how cyclic loading stages developed for the scanning electron microscope have been used successfully to study growing microcracks, both at ambient and elevated temperatures. In addition, the stereoimaging technique has been used to measure crack-tip parameters that characterize the mechanics and driving force for small fatigue cracks.
An Ultrasonic Method for Measurement of Size and Opening Behavior of Small Fatigue Cracks
Authors: Resch MT , Nelson DV
Published: March 2009

A surface acoustic wave (SAW) technique has been observed to be effective in the detection of initiation and measurement of growth behavior of surface microcracks during fatigue cycling. The experimental procedure involving excitation of Rayleigh waves on the surface of a specimen under investigation is described in detail in conjunction with automated data acquisition of the reflected echo from a small surface crack. The effectiveness of a split spectrum processing algorithm to separate specular reflections of isolated cracks from non-specular reflections of microstructural features is also described. A simplified model for predicting the relationship between crack size and the amplitude of the reflected echo from a crack is included along with a description of the technique for measuring opening behavior of individual cracks in-situ during fatigue cycling. Examples of effective utilization of the SAW technique in contemporary investigations of small fatigue crack growth behavior are presented. Finally, the advantages and limitations of the technique are discussed.
Use of Mechanistic Life Prediction Methods for the Design of Damage-Tolerant Composite Material Systems
Authors: Reifsnider KL
Published: March 2009

An increasing number of engineering applications depend on the use of material systems such as fiber-reinforced composites. For the most part, the manner in which these systems are “designed” is presently heuristic. Although much analysis and understanding of “how such materials are made” is available, there is comparatively less systematic rigor that addresses “how such materials should be made”. This is a serious inhibition to the exploitation of these materials and material systems.

During the last few years, a variety of approaches has been developed for the analysis of composite materials, especially fiber-reinforced systems. The body of literature is especially replete in the technical area of “effective stiffness” models, many of which are sophisticated and well founded—and reasonably well validated. A comparable body of work which addresses “effective strength” is not available. However, the author and his colleagues have developed a mechanistic approach of this type, that is generally referred to as the “critical element concept,” whereby careful laboratory work is used to define representative volumes of material that enclose a “typical” failure mode. This representative volume is divided into a “critical element” that controls the final failure event, and “subcritical elements” that alter the local stress state around the critical element.

The present paper extends this concept to the fiber/matrix level by introducing micromechanical strength models to be used in the critical elements. The result of this advance is that mechanistic models that include explicit representations of the parameters that describe the manner in which the material systems are made can be used to estimate remaining strength when those parameters change during the lifetime of the material. Moreover, the model can then be used to “design” or tailor a material system for specific long-term performance. This last topic is the focus of the present paper. The approach will be demonstrated, and the influence of several parameters will be discussed. This discussion will then be used to advance several concepts for the rigorous design of material systems for damage tolerance.
Calculation of Spectrum Load Notch Root Crack Growth Rate Under Elastic and Inelastic Conditions
Authors: Sunder R , Prakash RV , Mitchenko EI
Published: March 2009

A hysteretic model of crack opening stress variation forms the basis for spectrum load notch fatigue crack growth analysis. The model is based on the observation of a significant difference between crack opening and crack closure stress, particularly under high stress-strain cycling. The two stress levels are controlled by the largest cycle in the load sequence. The analysis uses rainflow cycle count, constant amplitude long crack growth rate constants and cyclic stress-strain curve as inputs apart from closure data. Growth rate calculations for 6 mm thick L168 (2014-T651) Al-Cu alloy coupons under FALSTAFF load spectrum compare well with experimental data obtained on a long crack and at notch root crack depth ranging from under 50 microns to 2 mm.
Fatigue Damage Due to Sub-Threshold Load Cycles Between Periodic Overloads
Authors: Zhou Z , Zwerneman FJ
Published: March 2009

Overloads separated by below-threshold minor cycles are applied to ASTM A588 steel specimens. Fatigue crack propagation data were recorded and acoustic emission (AE) signals were monitored throughout each test. Estimates of crack growth based on a linear summation are less than experimentally observed rates, even when below-threshold cycles are conservatively assumed damaging. Acoustic emission data provide additional evidence that below-threshold cycles within a variable amplitude load history contribute to fatigue damage. The intensity of the AE signals increases with the number of minor cycles between overloads, even though all minor cycles are below the threshold stress intensity.
Exact Determination of ΔKeff and Crack Propagation Prediction for Selected Loading Sequences
Authors: Zhang SJ , Döker H , Nowack H , Schulte K , Trautmann K-H
Published: March 2009

In the present study the correlation between the crack opening and crack propagation behavior under selected loading histories was systematically investigated on center cracked Al 7475-T7351 specimens. The development of the crack opening stress intensity Kop as the lower limit of the effective stress intensity factor range ΔKeff, has been determined using the near threshold crack growth (NTCG) method as proposed by Döker and Bachmann. Significant crack closure (i.e., ΔKeff < ΔK) was found for all sequences, and also for those at very high R-ratios (R > 0.83). The crack growth behavior was determined by the DC-potential drop method and by striation measurements and showed significant retardation. The Kop values correlated with the da/dN measurements.

The experimentally determined Kop and da/dN behavior was compared to predictions as performed by J. C. Newman, Jr. applying his advanced FASTRAN II model. The predictions of the development of the Kop and of the da/dN values compare quite well except in one case, where the model predicted complete crack arrest. The affected cycle numbers for the retardation periods were also satisfactorily predicted (except in that one case). From the investigations performed in the present study it is concluded that exact measurements of the trends in Kop as caused by load sequence effects form an important basis for further improvements of crack propagation calculations.
FEM Analysis of Cyclic Deformation Around the Fatigue Crack Tip After a Single Overload
Authors: Oliva V , Kuneš I
Published: March 2009

A two-dimensional elastoplastic finite element method (FEM) program was developed for analysis of cyclic deformation around the fatigue crack tip under variable amplitude loading. Edge fatigue crack growth in a thin rectangular AlCu4MglMnl plate under repeated tensile loading with a single 50% overload cycle was modeled. The computations show in detail the specific effects of an overload leading to the delayed retardation of the crack.

Results of the analysis were used for a direct estimation of crack growth rate according to Sih's energetic theory. A concept of “dead cycles” without crack advance was introduced to eliminate the influence of an unrealistically large crack jump in a model cycle. The crack growth rate was computed from the strain energy density change in the dead cycle and from the critical strain energy density resulting from low-cycle fatigue tests. For constant amplitude loading, the theoretical crack rate approximately agrees both with the experiments and with the crack rate determined from the crack opening stress by the ΔKeff method.
Thermal-Mechanical Fatigue Lifetime Prediction of an Austenitic Stainless Steel
Authors: Shi HJ , Robin C , Pluvinage G
Published: March 2009

In-phase and out-of-phase low-cycle thermal-mechanical fatigue tests were carried out on 316L austenitic stainless steel specimens controlled by computer. A nonlinear kinematic hardening model with internal variables was developed to describe the cyclic stress strain behavior of thermal-mechanical fatigue and a very good approximation of the hysteresis loops was obtained by comparing with the experiments. In order to predict the lifetime, a typical stabilized hysteresis loop of thermal-mechanical cycling was considered to be the combination of a great deal of parts at different temperatures. Each part has a different contribution rate of the energy density to the damage by introducing a damage factor depending on the temperature. With isothermal fatigue data an equivalent total strain energy density was deduced as a damage parameter and with which the calculation for thermal-mechanical fatigue lifetime prediction gave satisfactory results corresponding to test data.
Fatigue Lifetime Prediction of Angle-Plied Fiber-Reinforced Elastomer Composites as Pneumatic Tire Materials
Authors: Lee BL , Medzorian JP , Hippo PK , Liu DS , Ulrich PC
Published: March 2009

An initiative is underway to realign the test procedure where the emphasis is placed on more cost-effective and less empirical laboratory predictions of fatigue life of aircraft tires. The initial goals are to define the deformation and fracture mechanisms of various critical regions of aircraft tires and to identify the parameters that control the process of fatigue damage accumulation for cord-reinforced elastomer composite elements. The first phase of our study is focused on the delamination of bias aircraft tire carcass in the shoulder area. The study examined the effect of a broad range of the combination between different loading parameters on the fatigue resistance of angle-plied elastomer composites. Among them, stress amplitude was found to play a dominant role in determining fatigue lifetime of composites. The damage accumulation process of composites was accompanied by a continuous increase of cyclic strain (i.e., dynamic creep), temperature and acoustic emission (AE). The dynamic creep rate and the rate of temperature increase were inversely proportional to the fatigue life according to a power law. When the minimum cyclic stress was high enough, distinctly different rates of AE signal accumulation could be assigned to the cord-matrix debonding and delamination failure modes. The results demonstrated a great potential of the measurement of local strain change, heat generation, and AE as a viable experimental technique for real-time monitoring of the damage accumulation process. Finite element analysis was used to approximate the stress and strain components in the composite coupon specimens. Two finite element models were created using 75 and 1500 elements, respectively. The 1500 element model was shown to produce much better results for the σz stress distribution, while both models produced similar results for the σx and σxz stress distributions. The predictions were in good agreement with the total axial load measured experimentally.
Problems with Current Methodology in Using the Arrhenius Equation to Predict the Long-Term Behavior of Polymeric Materials in Geotechnical Environments
Authors: Bright DG
Published: March 2009

Polymeric materials used in earth structures and waste containment sites have minimum performance requirements to be maintained over a service life ranging from 1 to 100 years. The ability to predict the material performance in these applications with time has great appeal. However, it is necessary to know the mode, mechanism, and profile of behavior of these materials over an anticipated lifetime. Today, there is a proliferation in the use of rate expressions supposedly representing a material's response with time to a specific environmental condition. The objective is to review the impact of current protocol and simplifying assumptions on the prediction of long-term behavior of polymeric materials.

The problems with the current methodology are multiple. For a specific environmental condition associated and polymeric material, literature shows that the rate expression and its order vary with the experimenter and his or her parameters of choice in measuring the material's behavior. Thus, the integrated expressions predict different behaviors with time. Elevated temperatures are commonly used to accelerate a deteriorative mode sufficiently to collect data within a reasonable time frame. Typical range of elevated temperatures commonly traverse at least one polymeric phase transition. Deteriorative mechanisms change with physical state thus affecting the development of rate parameters representative of another physical state. Within the Arrhenius equation, the rate constant (k) is to be a function solely of temperature, and the activation energy (E) is to remain constant over the temperature range of evaluation and extrapolation and with time in long-term predictions. Literature has established that a ±1.0 kcal/mol (4.186 kJ/mol) error in E will cause an error of ±100% at a 95% confidence level in the extrapolation of kinetic rate predictions from 225 to 25°C.

The engineer and designer will see the dangers and the consequences associated with using rate expressions supposedly representing a deteriorative mode, and the impact of methodology and simplifying assumptions commonly employed to predict long-term behavior.
Neutron Fluence Determination for WER-440 Reactor Pressure Vessel Ageing Surveillance
Authors: Ilieva KD , Apostolov TG , Belousov SI , Antonov SY
Published: March 2009

The neutron flux and fluence determination on Bulgarian WER-440/230 (Units No 1–4 of Kozloduy NPP) Reactor Pressure Vessels is discussed. In these reactors weld 4 of RPV undergoes the most severe irradiation embrittlement. The choice of the neutron transport calculational parameters is substantiated. The inconsistency of 3D neutron flux values obtained by the 3D code TORT and by the synthesis method of DOT 2D results in the point of most severe irradiation embrittlement is within 3% limits range. Radiochemical analysis of the templets sampled out from the Unit No 2 vessel after 16 cycles of exploitation, before and after the annealing, was carried out. The calculated and experimental values of the neutron fluence on the Unit 2 RPV are compared.
Analysis of the TIHANGE-2 Surveillance Neutron Dosimetry Using the Lepricon Code System
Authors: Abderrahim HA , Minsart GL , D'hondt PJ
Published: March 2009

The TIHANGE-2 is a three-loop 900 MW(e) Framatome PWR. Two surveillance capsules were unloaded, after 3 reactor operating cycles for the first one (at 19.7° azimuth) and 6 cycles for the second one (at 17° azimuth). A complete set of radiometric (Fe, Ni, Ti and Cu) and fissile (Np-237 and U-238) dosimeters was present in these capsules. The core burn-up and the resulting changes in the fission distribution have been taken into account for the analysis of these capsules. These experimental results provided an opportunity to apply for the first time at SCK/CEN the LEPRICON code system on a real PWR reactor. In this paper we are showing the improvements obtained in comparison with the former methodology we were using for the dosimetry analysis but also some problems we encountered when using the LEPRICON methodology.
Lepricon, Combination of Correlated Data and Peelle's Puzzle
Authors: Perel RL , Wagschal JJ , Yeivin Y
Published: March 2009

The purpose of this discourse is elaboration of the least-squares formalism for averaging any number of correlated data. A procedure is considered, in a rather general formulation, which serves to determine m parameters by n correlated experimental data on possibly different functions of the parameters. If m > n, the very same procedure applies to the adjustment of a given library of m correlated parameters by a set of n relevant correlated data which might even be correlated to the given parameters. However, casual or indifferent application of the procedure might yield erroneous or even meaningless results. A very simple, and relevant, example is the combination of averages over partially overlapping sets of data. The combination yields a result which is different from the true average of the data, i.e. the average over the union of partial data sets. Further, when the measured quantities are nonlinear functions of the parameters, special care should be taken in order not to exceed the range of validity of the conventional, i.e. linearized, algorithm. One should also consider the mutual consistency of the given data, for even few outliers, which in a limited data set are highly improbable, might severely bias the resulting estimates of the unknown parameters. We discuss several observations and offer some comments on the applicability of the least-squares combination of correlated data.
Assessment of Uncertainty in Reactor Vessel Fluence Determination
Authors: Lippincott EP
Published: March 2009

Evaluation of neutron fluence for reactor vessels is important for evaluation of irradiation damage that may limit vessel life. The uncertainty in calculated fluence is considered to be typically about 20%. Benchmark measurements have improved confidence in the uncertainty assessments and enable testing of improved cross section data and calculational procedures. Use of appropriate plant-specific measurements can lower uncertainty substantially below 20% for some cases and also can limit the effects of unknown errors that may cause larger deviations.