bluuebunny/crossref_metadata_2025_split huggingface dataset:
Evolution of Notch-Tip Damage in Metal-Matrix Composites During Static Loading
Authors: Bakuckas JG , Awerbuch J , Tan T-M , Lau ACW 
Published: March 2009

The evolution of notch-tip damage during static loading in center-notched unidirectional boron/aluminum and silicon carbide/titanium composites was characterized through a two-prong investigation involving experiments and analysis. The effects of heat treatment on the failure process in boron/aluminum were addressed. In the experimental phase, the notch-tip damage initiation and progression was monitored and recorded in real time through a high-magnification (× 150) closed-circuit television system, acoustic emission, and load-deformation responses. Fracture surface morphologies were examined via a scanning electron microscope in order to identify the microfailure mechanisms.
In the analytical phase, a numerical technique was employed to predict the failure process in the materials studied without specifying the crack path a priori. The predictions elucidated the failure mechanisms and their interaction in the evolution of damage ahead of an existing crack. Correlations were made with experimental results in terms of the observed failure process and load-deformation responses. Good agreement was obtained between the observed failure process and predictions. The computational predictions captured the salient features in the observed failure processes.
Notch-tip damage progression was quite different in the various materials studied. A complex state of damage developed in the vicinity of the notch-tip consisting of several dominant failure mechanisms. The various failure mechanisms were identified, and the development of the notch-tip damage zones was determined.
Compression-Compression Fatigue of a Stitched Uniwoven Graphite/Epoxy Composite
Authors: Vandermey NE , Masters JE , Poe CC , Morris DH 
Published: March 2009

Stitched uniwoven graphite/epoxy laminates show great promise as a damage-tolerant composite material which is easily fabricated into large net-shaped parts and structures. They also offer the potential of being less costly than other damage-tolerant systems such as toughened matrix composites. Work to date with stitched uniweaves has included evaluations of static properties and damage tolerance as measured by impact resistance, compression after impact, and delamination propagation [1-7]. This paper focuses on experimental characterization of the fatigue response of stitched uniweave composite materials.
The performance of a stitched uniweave material system with an underlying AS4/3501-6 quasi-isotropic lay-up subjected to compression-compression fatigue loading was investigated. In addition, the material's strength and modulus were measured in a limited static compression study. Performance of unnotched specimens having stitch rows at either 0 or 90° to the loading direction was compared. Specimen performances were generally the same regardless of stitch orientation.
Special attention was given to the effects of stitching-related manufacturing defects. The stitches locally compacted the surface layers; microcracks were also evident along the length of the stitches. Damage evaluation techniques included monitoring stiffness and residual compressive strength, X-ray radiography, and edge replication. It was found that the manufacturing defect of inclined stitches had an adverse effect on material performance but only under low-cycle, high-load fatigue conditions. The microcracks did not generally initiate damage or propagate during the compression-compression cycling. The compression failure mode was transverse shear for all specimens, both in static compression and fatigue cycling tests.
Finally, manufacturing advances have improved material performance compared to previously evaluated stitched systems.
Acoustic Resonance Methods for Measuring Dynamic Elastic Modulus of Adhesive Bonds
Authors: Sinclair AN , Dickstein PA , Spelt JK , Segal E , Segal Y 
Published: March 2009

An investigation is made into acoustic resonance methods for measuring the dynamic elastic modulus of the adhesive layer in a bonded metal-to-metal joint. Empirical studies have indicated that this parameter may be used to infer the cohesive strength of the bond; in theory, an accurate measurement of two or more resonant frequencies should give both the elastic modulus and thickness of the adhesive layer. Verifying the accuracy of the technique was greatly hampered by the strong dependence of elastic properties in the adhesive on the test frequency.
The resonant frequencies of aluminum-to-aluminum adhesive joints were determined by both contact and immersion acoustic resonance techniques; receptance analysis was then used to determine the elastic modulus. Results were consistent with those achieved by time-of-flight measurements on bulk adhesive specimens. Based on these tests, inherent limitations of the Fokker Mark II Bondtester for assessing the cohesive strength of a bond were explored.
Microstructural Change in Ferritic Steels Under Heavy Ion Irradiation
Authors: Hide K , Sekimura N , Fukuya K , Kusanagi H , Taguchi M , Satake T , Arai Y , Iimuna M , Takaku H , Ishino S 
Published: March 2009

Ferritic steel is one of the most promising materials for fuel cladding of fast breeder reactors and fusion first wall. However, not enough data on microstructural evolution under irradiation have been obtained, especially at high doses. In the present study, microstructural changes in several ferritic steels under ion irradiation up to 200 dpa were investigated.
The specimens were irradiated with 200 keV C+ or 3 MeV Ni2+ ions between 698 and 898 K. Post irradiation microstructures were observed with transmission electron microscopes. Few voids were detected in MA957. Dislocation structure and dispersed oxide particles (Y2O3) were found to be stable under irradiation up to 200 dpa. Uniform distributions of fine voids were observed in HT-9, Fe-12Cr-2Mo, and pure Fe-12Cr alloy irradiated with 200 keV C+ ions to 150 dpa. However, 3 MeV Ni2+ irradiation of HT-9 between 748 and 848 K caused heterogeneous nucleation of voids near lath boundaries. As-received metallic compound-strengthened steel consisted of martensitic and ferritic phases. Large voids were formed in limited region in martensitic phase during ion irradiation, whereas few voids were found in the ferritic phase. Radiation-enhanced formation of Laves phase (Fe2Mo) was observed in ferritic phase.
On the Role of Helium in High Temperature Embrittlement of Irradiated Austenitic Steels
Authors: Kesternich W , Nandedkar RV 
Published: March 2009

Helium that is produced by nuclear transmutations can cause severe degradation of the mechanical properties in structural components of fusion and fast fission reactors. For studying the mechanisms of irradiation embrittlement, the helium effects have been simulated by helium implantation from cyclotrons or by making use of (n,α) reactions from thermal neutron fluxes. As is discussed in the present paper, the results from neutron irradiations and helium implantations are contradictory with respect to the helium levels at which helium embrittlement appears. To study the combined effect of helium and neutron irradiation on embrittlement, two titanium stabilized austenitic steels of identical composition, with the exception of their boron content of 40 and 2 ppm, respectively, have been neutron-irradiated at 845 K to thermal neutron fluences of 2.6 × 1025 n/m2. The precipitate and helium bubble microstructure has been quantitatively characterized by transmission electron microscopy (TEM) and has been correlated with the embrittlement behavior, studied in post-irradiation creep experiments at 973 K. Contrary to the expectation the material with the lower helium content was found to exhibit the higher reduction in creep strength after irradiation.
This result and the differences between low-dose neutron and alpha irradiations can only be explained if a new assessment of the role of helium in high temperature irradiation embrittlement is made. Helium embrittlement and a proposal for an additional embrittling mechanism operative under neutron irradiation are discussed.
Simulation of Solid Inert Gas Bubbles in Metals
Authors: Srinivasan S , Tyagi AK , Krishan K 
Published: March 2009

Recent work on the precipitation of inert gases in metals has shown that heavier inert gases such as argon, krypton, and xenon implanted into metals at ambient temperatures can form three-dimensional solid precipitates (referred to as solid bubbles). The formation of solid inert gas bubbles in nickel, aluminum, copper, gold, silver, molybdenum, iron, titanium, and zinc has been reported. We have examined some of these experimental results and found a strong correlation between the solid inert gas lattice parameter and that of the host matrix. In particular, we have carried out an atomistic computer simulation study of solid argon bubbles in nickel. The general features obtained from these results are, however, applicable to other gas-metal systems also.
Three different types of defect configurations in nickel are reported: (1) three-dimensional argon bubbles; (2) argon platelets in (111) planes; and (3) small clusters of argon atoms and vacancies. The relaxation fields of argon and nickel atoms, surface energy at the bubble-matrix interface, and bubble pressure have been computed. The results show that argon atoms can show both inward and outward relaxation with a very strong relaxation field at the surface. The hydrostatic stress shows strong variations at different shells in solid argon although the average bubble pressure is consistent with the experimentally determined pressures. We have also examined the possibility of large relaxation fields in the nickel matrix that could lead to loop punching but do not find any evidence for this.
An HVEM Study of Displacement Cascade Damage in Nb3Sn at 13 K
Authors: Kirk MA , Baker MC , Kestel BJ , Weber HW , Kampwirth RT 
Published: March 2009

We have begun a series of experiments to study the defect structure produced by ion irradiation in A15 superconductors. Low temperature irradiations and microscopy are performed in the high voltage electron microscope (HVEM) accelerator facility at Argonne National Laboratory, Argonne, IL. The first transmission electron microscope (TEM) images of the martensitic structure in niobium three tin (Nb3Sn) were recorded at various temperatures between 12 and 30 K. The transition to the superconducting state was observed by a slight deflection of the electron beam due to a perturbation of the magnetic field of the objective lens at the sample. The primary interest is in irradiation effects due to fast neutrons from a fusion reactor, and therefore, ion irradiation conditions that are somewhat comparable to this were selected. These consisted of 50 keV Xe+ irradiations to simulate neutron produced cascades in a near surface region (&lt;20nm) and 1.5 MeV Kr+ to produce cascade damage through a greater sample thickness (0.5μm). Defect images were obtained as a function of ion dose at 13 K using both fundamental and superlattice dark field reflections in two-beam conditions.
For 50 keV Xe+ irradiations at 13 K, the defect yields, which are black-white contrast features in fundamental reflections or black dot contrast features in superlattice reflections, were quite low at low ion doses (2 × 1011 cm-2), where individual defect cascades are well separated. At higher ion doses (3 × 1012 cm-2), when significant cascade overlap is present, defect density and contrast became greater than expected. Annealing to room temperature produced a large increase in visible defect density. Irradiation by 1.5 MeV Kr+ at 15 K removed the martensitic structure by an ion dose of 5 × 1012 cm-2, Further irradiation to 3.5 × 1013 cm-2 produced evidence for the initial stage of amorphization.
Modeling of Electrode Processes and Surface Chemistry in Carbon Dioxide Containing Solutions
Authors: Turgoose S , Cottis RA , Lawson K 
Published: March 2009

Corrosion processes and the rates of the individual electrode reactions in carbon dioxide containing solutions are well known to be affected by solution composition and hydrodynamics. There are many different mechanistic explanations for the observed, from rotating-disk studies, cathodic current-potential-rotation speed relationships, most of which obtain the observed currents by the simple addition of two or more currents due to processes (for example, chemical reactions and diffusion) that are considered to act independently. The validity of this is not clear, since many of the species contributing interact in the boundary layer.
A model is described that derives cathodic current densities and surface solution chemistries for the rotating-disk electrode over the range of conditions of practical relevance. The model allows for the slow homogeneous chemical step of carbon dioxide hydration, diffusion, convection, charge transfer processes, and the rapid acid-base equilibria. It is shown that the experimental data are adequately modeled without the need to consider a heterogeneous surface hydration of carbon dioxide, and that the electrochemical response under conditions not previously studied is predicted. Further, the increase in cathodic limiting current at pH &gt; 6 is shown to be a consequence of the solution chemical equilibria and the direct reduction of bicarbonate is not necessary to explain the experimental data. None of the mechanisms previously proposed is valid over the entire pH range of interest.
The concentration profiles in the near surface region throw light on the rate determining steps in the cathodic process and can be used to predict the possibility of surface film formation.
Use of the Chevron-Notched Short Bar Test to Guarantee Fracture Toughness for Lot Release in Aluminum Alloys
Authors: Bray JW 
Published: March 2009

Chevron-notched short bar fracture toughness (KIvM per ASTM E 1304) has been correlated with plane strain fracture toughness (KIc or KQ per ASTM E 399 or ASTM B 645) for commercial 2124-T851, 7050-T7451, 7050-T7651, and 7475-T7351 plate.
It was found that the chevron-notched and KIc(KQ) test methods correlated regardless of alloy, orientation, or temper. However, larger scatter in the higher toughness materials resulted in expected retest rate advantages (% of time the correlative test would fail and the referee test would pass lot release) when separating the results into “low” and “high” toughness databases. An A value minimum approach, via computation of derived properties (as defined in paragraphs 9.2.10 and 9.2.11 of Military Handbook 5E), was used to calculate the lot release correlations between KIvM and KIc(KQ). In addition, statistical correlations were performed on the entire database in order to utilize the chevron-notched short bar test more effectively for research and development programs.
Chevron-Notched, Flexure Tests for Measuring the Elevated-Temperature Fracture Resistance of Structural Ceramics
Authors: Jenkins MG , Ferber MK , Ghosh A , Peussa JT , Salem JA 
Published: March 2009

Chevron-notched, three-point flexure specimens were used to study the quasistatic fracture behaviour of a variety of structural ceramics at temperatures to 1400°C. Types of materials tested included monolithic ceramics (SiC., Si3N4, MgAl2O4), self-reinforced monoliths (acicular-grained Si3N4, acicular-grained mullite), and ceramic matrix composites (SiC whisker/Al2O3 matrix, TiB2 particulate/SiC matrix, SiC fibre/CVI SiC matrix, Al2O3 fibre/CVI SiC matrix).
Fracture resistance behavior of the materials was quantified as three distinct regimes of the fracture histories. At the initial part of the crack propagation, the apparent fracture toughness was evaluated as the critical stress intensity factor for the chevron notch, KIvM. During stable crack propagation, the crack growth resistance was characterized by the instantaneous strain energy release rate, GR, using a compliance method assuming linear-elastic unloading to calculate the effective crack lengths. At final fracture, the complete fracture process was quantified using the work-of-fracture, γWOF, which can be equated to the fracture surface energy for linear-elastic materials.
Results indicate that the chevron-notched, three-point flexure specimen facilitates the study of fracture behaviour in a wide range of brittle and quasi-brittle materials at elevated temperatures. The unique features of the chevron geometry, which are automatic crack initiation and inherent stable crack growth, are crucial to the successful evaluation of the fracture tests.
Evaluation of Impact Damage in Composite Materials Using Acoustic Emission
Authors: Bolduc M , Roy C 
Published: March 2009

The Defence Research Establishment, Valcatier (DREV) has been studying the application of composite materials for use in a lightweight anti-tank recoilless gun. Since thermoset resins are very sensitive to impact, it is important to determine the severity of damage using a nondestructive technique in order to establish the serviceability of the tube.
Acoustic emission (AE) was identified as a very promising method to measure the level of damage accumulation due to low-velocity impact, and a study was undertaken. The experimental program consisted of impacting 28 of 36 tubes with energies varying from 3 to 18 J. These tubes, fabricated by a wet winding process using carbon fiber/epoxy resin, were 25.4 cm long, and had a 9-cm internal diameter and a 3.6-mm wall thickness. The winding angle was ±71°. The AE was measured while pressurizing these tubes to rupture. Results indicate the potential of acoustic emission as a means of estimating the serviceability of a composite tube.
Effects of Water and Jet Fuel Absorption on Mode I and Mode II Delamination of Graphite/Epoxy
Authors: Hooper SJ , Subramanian R 
Published: March 2009

The environmental effects of water and jet fuel absorption on AS4/3501-6 graphite/epoxy were measured in terms of the changes in the interlaminar fracture toughness. Saturated double cantilever beam (DCB) and end notched flexure (ENF) specimens were tested to determine the environmental effects of moisture sorptions on critical strain energy release rates, GIc and GIIc, respectively. These data are compared with baseline data obtained for specimens tested in the room temperature dry (RTD) condition. This phase of the study revealed that moisture absorption of either water or jet fuel tends to toughen this material and that this change is more significant for Mode I than for Mode II.
This paper also presents the results of a fractographic analysis of the fracture surfaces developed in these unidirectional specimens. These results document the changes in the surface morphology due to the environmental conditions studied. Significant differences were observed for Mode I fracture surfaces. SEM fractographs of delamination surfaces for DCB specimens reveal that the moisture absorption condition results in fracture surfaces where the fibers are covered with resin. This corresponds to the Mode I toughening, which is quantified in terms of an increase in GIc. The differences in the fractographs of saturated and room temperature dry ENF specimens are less profound, with the hackle formation in the former case being more deformed than the latter.
Micromechanics of Tensile Strength in Composite Systems
Authors: Gao Z , Reifsnider KL 
Published: March 2009

This paper deals with the tensile strength of composites. The fibers of the composites are considered to have a statistical distribution of flaws of the Weibull type. The tensile strength formulation developed explicitly contains the Weibull shape factor, m, stress concentration factors due to fiber fractures, Ci, and the ineffective length, δ. Here, m describes the uniformity of the fiber strength. The stress concentration factors, Ci, characterize the disturbance of stress in neighboring unbroken fibers when i numbers of fiber have broken together. They are related directly to the subsequent event of fracture propagation perpendicular to the fiber direction, which is an important failure mechanism in ceramic composites. The ineffective length, δ, hinges on the concept of “the weakest link” along the fiber direction and is the dominant factor in the bundle strength representation of the strength of polymer-based composites. There is a lack of systematic analysis addressing the competition of these two important failure mechanisms and their influence on the tensile strength of the composites. In this paper, we combine the theory of bundle strength with the mechanics of local stress concentration to develop a micromechanical model for tensile strength, and study the influence of micromechanical properties on the tensile strength of composites and optimal design for tensile-controlled loading applications.
Using the strength formulas developed in this paper, we study how tensile strength is influenced by the changes of the micromechanical properties, m, Ci, and δ, and also by the properties of the fiber and matrix, Ef and Gm. It is shown that the existence of stress concentrations due to fiber fractures increases the chance of local crack propagation and, therefore, decreases the strength values predicted by the bundle strength theory. Stress concentrations play a dominate role in determining the magnitudes of tensile strength, especially for large values of m. However, since the stress concentrations remain virtually constant, for composites with different Ef and Gm and regularly spaced fibers, the variation of the strength curve as a function Ef and Gm is still controlled by the changes of the ineffective length, δ. The predictions of strength from the present model are compared to experimental measurements to validate the accuracy of the formulation.
The strength consideration is more complicated when the existence of irregular fiber spacing and the lack of quantitative descriptions of the spacing are taken into account. We show that, even for polymeric composites, optimal strength (when material properties Ef and Gm are taken as design variables) can be achieved due to certain fiber spacing variations. Therefore, the fiber spacing of composites, although difficult to control in the manufacturing process and hard to determine for a specimen, must be considered carefully to eliminate, isolate, or correctly represent its effects on strength and other macromechanical properties of engineering importance.
Fatigue Damage in Thick, Cross-Ply Laminates with a Center Hole
Authors: Wolterman RL , Kennedy JM , Farley GL 
Published: March 2009

An experimental investigation was conducted to characterize the response of three composite material systems to long-term cyclic loading. Thick cross-ply laminates of uniwoven AS/4 carbon fabric were produced using a brittle matrix with and without stitching and a tough-matrix material. Quasi-static tension and compression tests were conducted on specimens with and without circular holes to determine strength, modulus, and failing strain. These tests showed that the measured static mechanical properties were insensitive to the type of matrix material because the laminate response was dominated by the 0° fibers. The stitched specimens had significantly lower static compressive strengths.
A series of fatigue tests (tension-tension, compression-compression, and tension-compression) were conducted to study the influence of an open hole on damage and residual strength. These tests showed that the matrix material and stitching influenced the fatigue behavior of the composite. The tough-matrix material developed less damage than the corresponding brittle-matrix specimens and survived one million cycles of fatigue loading at all stress levels. The stitched composite developed less longitudinal damage than the unstitched materials, but it developed more transverse damage. Both the brittle-matrix and the stitched materials failed prior to one million cycles of tension-compression fatigue loading. The residual strength of the materials was affected by the matrix material and the damage state in the specimen.