bluuebunny/crossref_metadata_2025_split huggingface dataset:
Failure Analysis of a Bonded Honeycomb Structure: A Case Study Employing SEM
Authors: Bailey CD 
Published: March 2009

The scanning electron microscope (SEM) was used in determining the causes of fracture which occurred in an adhesively bonded aluminum honeycomb sandwich panel. The panel fractured during structural tests conducted while it was submerged in water. Similar panels structurally tested in air did not fracture indicating that applied loads were not the major cause of fracture. A thorough examination of the fractured panel was conducted, and the following factors were identified as the causes of fracture : ( 1 ) Water entered the panel through the adhesive bond on the edge close-out members and then migrated from cell to cell around the edges of the honeycomb core. Porosity in the supported adhesive film and capillaries through the film, which were caused by incomplete bonding to the adhesive carrier, provided paths for water migration. (2) Water caused corrosion both on the honeycomb cell walls and at the edges of the core which were against the adhesive carrier. (3) Adhesive bonding to the core was either nonexistent or destroyed in the presence of water.
The unique features of the SEM provided a new approach to fracture analysis of specimens with complex topographical surfaces. The advantages of directly viewing the specimen in three dimensions at high magnifications are illustrated in SEM photomicrographs.
Evaluation of Unidirectional Glass-Graphite Fiber/Epoxy Resin Composites
Authors: Kalnin IL 
Published: March 2009

As an all-fiberglass reinforcement is progressively replaced by graphite fiber, the longitudinal composite fracture stress and strain decrease rapidly for all loading modes except shear, accompanied by a substantial increase in the tensile and compressive stiffness. Conversely, upon introducing some fiberglass into a graphite composite, its strength declines in proportion to the decrease in graphite fiber content. The maximum torsional shear stress is approximately 8 ksi for all combinations of glass and graphite. Most of the transverse mechanical properties decrease linearly upon progressive replacement of the fiberglass by graphite. The transverse torsional shear properties are about 20 percent lower than the longitudinal ones.
The magnitude of the flexural modulus, which depends not only on the graphite content but even more so on the distance of the graphitic plies from the center plane of the laminate, may be calculated by means of the existing theory of bidirectional laminates. It is concluded that the large increase in composite stiffness upon addition of a small amount of oriented graphite fiber is the most salient characteristic of glass-graphite hybridization.
Special Fractographic Techniques for Failure Analysis
Authors: Whiteson BV , Phillips A , Kerlins V , Rawe RA 
Published: March 2009

In order to assist the investigator of service failures, work was performed using electron fractographic methods to resolve three separate problems that have not been solvable using the more conventional macro- or light microscopic techniques. Three independent problems were examined, and solutions were achieved. These were: (1) determination of fracture direction in thin sheet metal components, (2) differentiating between hydrogen embrittlement and stress corrosion in high-strength steels, and (3) determination of applied cyclic stress as a function of fatigue striation spacing.
The results of the investigation indicated that: 1. Fracture direction in thin sheet-metal components can be determined by replicating the acute angle shear lip. The direction of open dimples with respect to the fracture edge indicated the fracture direction in the plane of fracture. 2. There is reasonable evidence that stress corrosion and hydrogen embrittlement fractures in high-strength steel can be separated by the following: (a) Hydrogen embrittlement fractures initiated subsurface, while stress corrosion fractures initiated at the free surface. (b) The corrosion attack in hydrogen embrittlement fractures, if present, was random while in stress corrosion the oxidation products were more concentrated at the nucleus. (c) Stress corrosion fractures had stronger indications of secondary cracking than hydrogen embrittlement failures. (d) The fracture features on the hydrogen embrittled specimens were more clearly defined than those on the stress corrosion fractures. 3. A correlation was established between cyclic stress and fatigue striation spacing for several aluminum alloys over a wide range of alternating and mean stresses in thicknesses of 0.050 and 0.500 in. The correlation was empirically derived.
Use of Aggregates Producing High Shrinkage with Shrinkage-Compensating Cements
Authors: Polivka M , Mehta PK 
Published: March 2009

Coarse aggregate has a large influence on drying shrinkage which is one of the major causes of cracking in concrete structures. The stiffer an aggregate, the lower is the drying shinkage of concrete. Since it is not possible to alter the stiffness characteristic of an aggregate, a practical method of minimizing cracking of concrete when high-absorption or low-modulus aggregates must be used can be the application of a shrinkage-compensating cement in combination with these aggregates of marginal quality.
Data are presented showing that the use of high-absorption aggregates in combination with a Type K shrinkage-compensating cement produced concretes whose net contraction due to drying was equal to or lower than the drying shrinkage of a Portland cement concrete made with a low-absorption aggregate. The shrinkage-compensating cement concretes developed higher strength than the corresponding Portland cement concretes.
High-absorption or low-modulus aggregates when used in appropriately proportioned shrinkage-compensating concretes can meet even the most rigid shrinkage limitations specified to minimize cracking and deflections in concrete structures.
Interactions Between Creep and Low-Cycle Fatigue in Udimet 700 at 1400 F
Authors: Wells CH , Sullivan CP 
Published: March 2009

The cumulative damage concept in low-cycle fatigue with intermittent hold times was assessed for Udimet 700 at 1400 F. A 20 or 60 min period of tensile stressing in each strain cycle reduced the cyclic life in conservative agreement with the linear cumulative damage rule. However, the effect of holding compressive strain was more damaging than holding tensile strain. Similarly, the effect of creep prestrain on the subsequent fatigue behavior was more deleterious when applied in compression than in tension. Furthermore, the creep rate in compression was less than half that in tension and continuously decreased with strain.
Observations of the Bauschinger effect indicated that high internal stresses accompanied both creep and plastic deformation; these stresses are attributed to the pileup of dislocations at grain boundaries. It is postulated that the pileup is relaxed by cavitation and that the rate of deformation is governed by the growth of these cavities as they annihilate dislocations. These cavities were observed to be a source of intergranular cracking in creep and low-cycle fatigue. The rate of growth of the cavities, in turn, is affected by the hydrostatic component of stress, accounting for the decelerating creep in compression. In addition the shape of the cavities influences the rate of crack propagation and fracture toughness: round cavities promote toughness, while cavities flattened by compressive creep provide easy propagation paths.
A Statistical Evaluation of Atmospheric, In-Service, and Accelerated Corrosion of Stainless Steel Automotive Trim Material
Authors: Black HL , Lherbier LW 
Published: March 2009

Stainless steel panels with various surface finishes were exposed to winter salt and ash conditions on trucks and automobiles, salt atmosphere, industrial atmosphere, and rural atmosphere for periods of one to four years. They were also evaluated by six accelerated tests either presently used by, or proposed by, the automobile industry.
Comparisons of the average results of the multiple specimens were made in an attempt to determine the relative superiority among the grades of stainless and among the surface finishes in each exposure environment. Statistical correlation coefficients were calculated to determine the degree of corrosion resistance correlation among the sites and to determine the correlation between “in-service” corrosion and the accelerated tests.
No clear superiority of any grade in all environments could be determined. With the exception of chromium flash plating, no surface finish tested was significantly superior to any other. The automobile and truck exposures showed that the varying weather conditions from one winter to another could cause unpredictable, conflicting results. The correlation between corrosion resistance and temperature, snow-fall, and de-icing agents was not clear. The statistical correlation analysis conducted showed that very limited or no correlation exists between in-service corrosion and and accelerated tests. Even among the tests, the correlation was poor or not existent. This study indicates that none of the accelerated tests evaluated can be expected to predict stainless steel corrosion performance in service.
Fatigue Crack Growth Data for Various Materials Deduced from the Fatigue Lives of Precracked Plates
Authors: Pook LP 
Published: March 2009

Data previously obtained for a wide range of metallic materials, using thin plates containing edge cracks between 0.3 and 7.5 mm (0.01 and 0.3 in.) long, have shown for both zero mean load and the general tensile stress cycle σm±σa where σm &gt; σa that whether a crack grows or remains dormant can be predicted from the value of ΔK, the range of stress intensity factor during the fatigue cycle: a critical value ΔKc is necessary for crack growth, its value in general depends on the ratio σm/σa.
Fatigue crack growth data for many materials can conveniently be represented by the equation da/dΝ = Β(ΔK)m, where Β and m are material constants. If this equation is integrated the initial rate of crack growth can be calculated from the total fatigue life of a cracked specimen. Data for low crack growth rates, obtained in this way from the broken plate specimens, are in good agreement with data obtained from conventional crack growth tests. The method has the advantage that no crack growth monitoring instrumentation is necessary, so that data on crack growth in difficult environments can readily be obtained.
Fatigue Crack Propagation Growth Rates under a Wide Variation of ΔK for an ASTM A517 Grade F (T-1) Steel
Authors: Bucci RJ , Clark WG , Paris PC 
Published: March 2009

Fatigue crack propagation experiments on an ASTM A517 Grade F (T-1) steel were carried out within the framework of linear-elastic fracture mechanics.
At low ΔK levels an examination of very slow fatigue crack propagation rates of the order of 10-9 in./cycle revealed the existence of a stress intensity threshold level, ΔKT, below which fatigue crack propagation rates become diminishingly small (order of 10-9 in./cycle). Stress ratio, R (ratio of σmin/σmax ≃ Kmin/Kmax), was found to alter the threshold ΔKT with higher R ratios resulting in somewhat depressed threshold values. The influence of stress ratio was also felt at ΔK levels above the threshold with increased growth rate being associated with increased values of R.
Low ΔK level fatigue crack growth rate tests were conducted at room temperature in air, distilled water, and dry hydrogen gas. These environments did not effect the fatigue threshold. Identical values of ΔKT were recorded in each environment studied. Furthermore, for the range of slow growth rates, order of 10-9 to 10-7 in./cycle, and frequencies, 120 to 180 Hz, considered, these environments seemed to produce little effect on fatigue crack growth rate results.
For higher cyclic growth rates (&gt; 10-7 in./cycle) and ΔK levels, the effect of material condition and heat treatment was evaluated. Plate material rated as ultrasonically good and poor was evaluated in two heat treated conditions. Examination of fatigue crack growth rates indicated that ultrasonic ratings do not necessarily give good indications of crack growth resistance.
Predictive Testing in Elevated Temperature Fatigue and Creep: Status and Problems
Authors: Coffin LF , Goldhoff RM 
Published: March 2009

The paper selects two well recognized problem areas of high temperature material behavior, namely, creep and fatigue, and considers those factors which contribute to the prediction of the useful service life of the component. The information available is obtained almost entirely in a laboratory environment, in which simplified load conditions and specimen geometries and controlled environments are employed for material evaluation. We do not consider transferring this information to actual design, other than to identify the importance of such problems as the analytical determination of stresses and strains in the structure, the effect of highly complex loading, or the behavior in a complex environment, but we will draw on several practical problems occurring in high temperature power generating machinery as examples of the topics we wish to discuss namely, creep, fatigue, and creep-fatigue interaction.
The topics of creep and fatigue are considered separately, the discussion being built around three central topics: (1) real problems in prediction, (2) complicating instabilities including the effects of environment, deformation, and microstructure; and (3) some current predictive methods. Consideration of the less well defined problem of creep-fatigue interactions is also included.
Applications of Exoelectron Emission to Nondestructive Evaluation of Alloying, Crack Growth, Fatigue, Annealing, and Grinding Processes
Authors: Hoenig SA , Savitz CA , Ott WA , Russel TA , Ali MT 
Published: March 2009

It has been demonstrated that exoelectron emission phenomena can be used to observe and measure alloying, crack growth, fatigue, annealing, and grinding processes. For observation of alloying, crack growth, annealing, and grinding a fixed plate or wire collector biased at +90 V is used to collect the electrons. The current is measured by a picoammeter and recorded directly via a strip chart recorder. To find the fatigued area in a specimen a heat source is scanned along the coupon and the exoelectron emission is picked up by a moving collector wire connected to a picoammeter.
These experiments are usually done in ambient air without any special control of temperature and humidity. The initiation of crack growth is observed as a sudden change in exoelectron emission level. In a fatigued but uncracked specimen a significant increase in exoelectron emission is observed when the heat source and collector move over the area where the fatigue damage is most intense.
Most of the data to date has been obtained with aluminum but we feel that this technique can be extended to other aircraft structural materials.
Testing Methods for Stress Corrosion Cracking
Authors: Ketcham SJ 
Published: March 2009

Stress corrosion testing methods are discussed from a general point of view with an emphasis on the desirability of standardization. The usefulness of smooth, notched, or precracked specimens is compared and some results using the different types are presented. Some precautions that should be taken are described: these include avoidance of crevice corrosion, galvanic effects, or hydrogen embrittlement results. Finally, since it is possible to find some type of environment that will crack almost every engineering alloy, it is important to use realistic loading methods and test media that relate to material serviceability for a particular application.
The work of ASTM Committee G-1 on Corrosion of Metals in preparing recommended practices for various test specimens such as C-ring, bent beam, tension, U-bend, and precracked specimens is summarized; as is, similar work with environments such as the boiling MgCl2 test.
The Resistance of Wrought High Strength Aluminum Alloys to Stress Corrosion Cracking
Authors: Brown RH , Sprowls DO , Shumaker MB 
Published: March 2009

An up to date review of the stress corrosion cracking performance of wrought high strength aluminum alloys is presented. Special emphasis is placed on factors of microstructure and its directionality and the effect of ancillary alloying elements, and environmental factors.
Examples of typical service problems encountered with stress corrosion cracking of high strength aluminum alloys are described along with means of combating them, such as peening, coatings, or the use of special stress corrosion resistant alloys.
NASA Mechanical Impact Testing in High-Pressure Oxygen
Authors: Bryan CJ 
Published: March 2009

Many materials will ignite or initiate high energy reactions when in contact with gaseous oxygen (GOX) or liquid oxygen (LOX) if subjected to stimuli such as a mechanical impact, adiabatic compression (pneumatic impact), or an electrical discharge in the form of a spark. Such materials must therefore be characterized as to compatibility with LOX or GOX to define the degree of hazard with their use.
Generally, materials are more sensitive to mechanical impact in gaseous oxygen than in liquid oxygen, and impact sensitivity is known to increase with increasing pressure. Thus, the evaluation of the sensitivity of materials in GOX is required to supplement LOX impact test data.
Three independently developed high-pressure GOX mechanical impact testers have been used to evaluate materials for the space shuttle program. This paper presents a description of a test program evaluating the performance of these three high-pressure oxygen mechanical impact test systems. Two of the systems were fabricated from the same design drawing, whereas the third system, even though similar, was of different design. Four unpressurized mechanical LOX impact testers were also evaluated.
The most suitable materials identified were Teflon® TFE for gaskets and seals, Krytox® 240AC grease as a lubricant, and the fluorocarbon elastomers where a rubber is absolutely necessary.
Oil Film Hazards in Oxygen Systems
Authors: Werley BL 
Published: March 2009

A number of known or assumed hazard mechanisms for oil films in oxygen systems are itemized. Each mechanism is discussed for understanding in terms of its hazard and pertinent data available from the literature and laboratory tests at Air Products and Chemicals, Inc. The methods of collection, values, and shortcomings of these available data are reviewed to place the various hazards in perspective. Areas not fully understood and in need of further work or that require specific evaluation procedures are itemized.
The principal oil film hazard in oxygen systems is flammability. However, an indirect hazard is migration of nonflammable films and accumulation to flammable levels. Generally, flammability of oil films has not been reported below roughly 0.108 to 1.08 g/m2 (10 to 100 mg/ft2) film concentrations. In comparison, migration of films of thickness above roughly 0.215 to 0.431 g/m2 (20 to 40 mg/ft2) can be substantial, while films of thickness below this range migrate more slowly. Migration along a surface can be greatly inhibited by choice of surface finish. Vapor phase migration is complex. Traditional cleaning methods provide systems of cleanliness well below flammable thresholds. However, migration of oil films can eventually produce hazardous regions, and, in combination with the rate of introduction of new contaminants, dictates required cleaning intervals.
A Reliability Approach to the Fatigue of Structures
Authors: Payne AO 
Published: March 2009

In recent years, due to the progressive development of higher performance aircraft, the fatigue strength of aircraft structures has become an increasingly important problem.
In the present paper, a method of assessing structural safety in fatigue is proposed in which a statistical model for the fatigue process is used to carry out a reliability analysis, enabling the probability of failure to be estimated at any stage of the life. The statistical variability in crack-propagation rate and residual strength of the cracked structure is included together with the effect of any prescribed inspection procedure.
The method is applied to a structure of high strength steel typifying a “safe-life” structure and to a redundant aluminum alloy structure representative of the “fail-safe” construction.
It is concluded that the reliability analysis can be applied to both fail-safe and safe-life structures and provides a quantitative basis for ensuring their safe operation, including the planning of an inspection procedure if feasible. In this regard the method represents an advance on the existing procedures but inherent in the quantitative approach it employs is the adoption of an acceptable safety level. The most appropriate way of defining safety level is discussed and a suitable measure is proposed. An extensive amount of data is required in applying the procedure but it is suggested that in the case of aircraft structures this difficulty can be overcome by using results from the comprehensive structural testing program normally carried out, together with relevant data from similar structures.
Intergranular Fatigue Fracture of Chemical Lead at Room Temperature
Authors: Kitagawa M 
Published: March 2009

Intergranular fatigue fracture was investigated by performing a series of torsional reversed creep and fatigue tests on tubular specimens of chemical lead at room temperature.
It is shown that the intergranular fatigue life, 2Nfgb, is related to the amount of grain-boundary sliding per reversal, ΔUgb, by a simple expression 2NfgbΔUgb=constant This simple relationship is valid for all cyclic test results in this investigation regardless of stress amplitude and strain rate.
A“strain rate modified strain range” parameter is found to be useful for describing the strain rate dependence of the intergranular fatigue fracture.
Cyclic Deformation and Failure of Polymers
Authors: Johnson TA 
Published: March 2009

The phenomenological fatigue behavior of many metals can be described by equations in which the material constants are determined from cyclic deformation and monotonic fracture properties. This approach makes it possible to describe and predict fatigue behavior with a minimum of testing and provides a rational method for selecting metals to resist fatigue. There have been few attempts to determine what cyclic and monotonic properties are necessary to describe and predict the fatigue failure of polymers, however, and this knowledge may be useful when and if these materials are modified to be used for structural purposes. To this end the cyclic deformation and failure behavior of some common plastics is examined.At least three failure phenomena occur in polycarbonate and polymethylmethacrylate tested in completely reversed strain control at room temperature and at low frequencies: catastrophic fracture, cyclic fracture, and delayed cyclic softening. At high cyclic strains the definition of failure is often arbitrary, and the onset of failure can depend upon the test frequency. In this region it does not appear feasible to relate the fatigue behavior to any monotonic material properties. At low cyclic strains the number of cycles to fracture appears to be independent of the frequency of testing, and the behavior is often described by an equation similar to the Coffin-Manson equation. Moreover, when log strain is plotted against log life, the slope appears to be relatively constant for a variety of polymers. In addition, the plot of stress amplitude versus log life is linear for nylon and polycarbonate in the low strain region and the extrapolation to one reversal is very well approximated by the fracture load divided by the cross sectional area after fracture.