R.H. Zee

Impact resistance and energy absorption mechanisms in hybrid composites

Abstract The response of hybrid composites to low-velocity impact loading has been investigated. The energy absorbing mechanisms of laminates containing various fibers were studied primarily by means of the instrumented falling dart impact testing technique. Static indentation tests and scanning electron microscopy (SEM) were also employed to assist in the identification of failure mechanisms. The composites containing polyethylene (PE) fibers, which were of high strength and high ductility, were found to be effective in both dissipating impact energy and resisting through penetration. Polyester (PET) fiber reinforced epoxy also exhibited superior impact characteristics even though the PET fabric layers without epoxy did not have good modulus or ductility. Good energy absorbing capability was also observed in epoxy reinforced with woven fabrics made of high-performance Nylon fibers. Nylon, PE and PET fibers were found to enhance the impact resistance of graphite fiber composites. Upon impact loading, the composites containing either PE or PET fibers in general exhibited a great degree of flexural plastic deformation and some level of delamination, thereby dissipating a significant amount of strain energy. Hybrids containing Nylon fabric showed analogous behavior, but to a lesser degree. The stacking sequence in hybrid laminates was found to play a critical role in controlling plastic deformation and delamination. This implies that the stacking sequence is a major factor governing the overall energy sorbing capability of the hybrid structure. The penetration resistance of hybrid composites appeared to be dictated by the toughness (strength plus ductility) of their constituent fibers. The fiber toughness must be measured under high strain rate conditions.

Infrared sensing techniques for penetration depth control of the submerged arc welding process

Abstract This paper presents an investigation into the development of a rugged, low cost, point infrared sensor to monitor and control the welding process in harsh fabrication environments. Perturbations occurring during the welding process create changes in the temperature distributions of the plates being welded. By monitoring the changes in these temperature distributions, action can be implemented to eliminate or mitigate defects that may form due to the process perturbations. Heat transfer analyses were performed to study the effects of disturbances to the welding process on the surface temperature of the plates being welded. A point sensor was used to monitor changes in the plate surface temperatures occurring during the welding process. The objective was to demonstrate that weld bead penetration depth could be monitored and controlled during both gas tungsten arc welding (GTAW) and submerged arc welding (SAW) processes to eliminate or reduce weld defects. The infrared energy exchange between a defined area on the topside plate surface and the sensor was monitored during the welding process and compared to predictions of the heat transfer analyses. Changes in the plate geometry (gap size, plate thickness, and cooling sinks representing stiffeners) were introduced during the experiments to perturb the welding process. Using the infrared sensor, constant depth of penetration was maintained in the presence of these perturbations by feedback control of the welding process parameters.

Effects of deviations from stoichiometry on the strength anomaly and fracture behavior of B-doped FeAl

Abstract Tensile tests were conducted at temperatures ranging from 145–1123 K on four different FeAl alloys, containing 40, 43, 45, and 48 at.% Al, each doped with 0.12 at.% B. The alloys were initially heat treated to obtain a relatively large grain size (~200 μm), after which they were given a long, low-temperature anneal (673 K for 5 d), to minimize, respectively, the effects of grain boundary strengthening and thermal vacancies on the measured yield strengths. Each alloy displayed bcc-type behavior at low temperatures (yield strength decreasing with increasing temperature), followed by a strength anomaly at intermediate temperatures (yield strength increasing with increasing temperature), and a sharp drop in yield strength at elevated temperatures (beyond the anomalous strength peak). Thermal vacancies that are generated during the hold time at the test temperature may contribute to the production of the strength anomaly. In specimens not given the vacancy-minimizing anneal, quenched-in vacancies were found to substantially increase low-temperature strength, thereby masking the yield strength anomaly. As the Al concentration of FeAl increased, the prominence of the yield strength anomaly decreased. Ductility also exhibited a peak at elevated temperatures, first increasing with temperature until it reached a maximum value and then decreasing with further increases in temperature. The peak in ductility occured at lower temperatures as the Al content increased. The fracture mode in all four alloys was mixed (intergranular + transgranular) at cryogenic temperatures, predominantly intergranular at around room temperature, dimpled rupture at peak ductility, and intergranular cavitation at elevated temperatures where the ductility dropped.

IN SITU FORMATION OF THREE-DIMENSIONAL TIC REINFORCEMENTS IN TI-TIC COMPOSITES

Three-dimensional, single-crystal reinforcements of TiC were producedin situ during manufacture of Ti-TiC composites. The composites, containing 40 to 50 vol pct TiC, were produced using standard casting procedures. The presence of aluminum in Ti-TiC composites showed enhanced strength without loss of ductility at room and elevated temperatures. Aluminum additions were found to solid solution strengthen the Ti matrix and increase the strength of the TiC phase. The morphology of the TiC, which was controlled by processing parameters, influenced the properties of the Ti-TiC composites investigated. Refinement of the secondary dendrite arm spacing of the three-dimensional (3-D) TiC particles was found to dramatically improve the ultimate tensile strength (UTS) and ductility of the Ti-TiC composites.

Room-temperature mechanical behavior of FeAl: Effects of stoichiometry, environment, and boron addition

Abstract The intrinsic ductility of FeAl (in ultrahigh vacuum) decreases with increasing Al content, from around 16% in Fe–37Al to zero in Fe–48Al. The sharpest decline occurs around the composition where the fracture mode changes from transgranular to intergranular. Boron shifts this ductile–brittle transition to higher Al levels by segregating to the grain boundaries and suppressing grain-boundary fracture. However, its effectiveness decreases with increasing Al concentration, even though the amount of B segregating to the grain boundaries remains the same, independent of alloy stoichiometry. Consequently, even the B-doped alloys become brittle and fracture intergranularly as the stoichiometric composition is approached. Low-pressure hydrogen embrittles FeAl, although not as severely as atmospheric moisture. Environmental embrittlement is most noticeable in Fe-rich FeAl; with increasing Al concentration, the grain boundaries become intrinsically weak, and brittle fracture persists even after environmental effects are eliminated.

Yielding and flow behavior of Mo5Si3 single crystals

Abstract Deformation behavior of Mo 5 Si 3 was studied using single crystals under compression. It was found that yielding and flow behavior were strongly dependent on temperature, strain rate and crystal orientation. A stress exponent and an activation enthalpy of lower yield stresses were estimated to be ≈6 and 4.5 eV, respectively.

Energy absorption processes in fibrous composites

Abstract The objective of this investigation is to determine the contribution of fibers in polymer matrix composites during impact. Experimental measurements were made using specialized specimen designs and test methodologies to isolate the roles of fiber deformation and fracture as well as matrix–fiber interaction. Three types of fibers were examined: Spectra-900 PE, Kevlar-49 and graphite. Experimental results show that PE possesses the highest deformation energy density (normalized to volume), followed by Kevlar and graphite. A similar trend was observed for fracture energy. This trend correlates well with the toughness of these fibers estimated from the strength and ductility of the materials. The energy absorption capacity of the fibers examined was found to increase with increasing strain rates as a result of the generation of stress wave during high velocity impact. Experimental measurements were made to determine the energy absorption characteristics of fabric, fibers and composite structures. Results show that the fiber contribution is significant in the composite structures examined.

Irradiation growth of zirconium single crystals: A review

The purpose of irradiation growth studies using single crystals of zirconium is to attempt to derive the underlying mechanisms of growth through the use of specimens with well characterized microstructures, where grain boundaries are absent. Early work at low fluences showed that growth of annealed crystals takes the form of an expansion parallel to the a-axes and a contraction along the c-axis. More recent experiments have shown that, at temperatures of 353 and 553 K, a growth plateau is reached at fluences of 1025 n/m2 and strains of ~10−4. At higher fluences (~ 3 × 1025 n/m2 at 553 K) an increase in growth rate occurs, similar to the “breakaway” phenomenon observed with Zircaloy-2, which has been associated with the nucleation and growth of 〈c〉-component vacancy loops on the basal planes. The growth mechanism at high fluences is therefore somewhat analogous to that proposed by Buckley. Crystals that were severely deformed show exceptionally high growth rates that continue in a steady-state manner to high fluences. The growth is consistent with the annihilation of an excess of interstitials at the 〈a〉-type dislocations, together with the loss of the corresponding vacancies to a high density of oriented twin boundaries and to 〈c〉-component dislocations. Further experiments on single crystals with controlled microstructures could provide additional information on the mechanism of growth in zirconium. The examination of post-irradiation microstructures would also be of value, to confirm the relationship between growth and microstructure.

Hydrogen-boron interaction and its effect on the ductility and fracture of Ni3Al

Abstract Ni 3 Al alloys, of nominal composition Ni-24 at.% Al and three different B concentrations (50, 100 and 500 wppm), were tensile tested at room temperature in high-purity H 2 gas at pressures ranging from ∼6 × 10 −8 to 7 × 10 3 Pa. The highest elongations to fracture were obtained in ultrahigh vacuum: ∼36, 45 and 60% for the 50, 100 and 500 wppm B alloys, respectively. With increasing H 2 pressure, the ductility of all three alloys dropped precipitously. Accompanying this drop in ductility was a change in the fracture mode from predominantly transgranular to predominantly intergranular. An intriguing result of our present study is that, at the higher H 2 pressures employed, B-doped Ni 3 Al (50 or 100 wppm) is brittle and fractures predominantly intergranularly, whereas B-free Ni 3 Al is ductile and fractures predominantly transgranularly. This result indicates that B—by possibly promoting the dissociation of molecular H 2 into atomic H— embrittles Ni 3 Al in a dry H 2 environment, unlike in H 2 O-containing environments, where B suppresses grain boundary fracture and improves the ductility of Ni 3 Al.

Effect of low-pressure hydrogen on the room-temperature tensile ductility and fracture behavior of Ni3Al

Abstract The effect of low-pressure (≤1.3 × 10 3 Pa) hydrogen gas on the ductility and fracture behavior of polycrystalline Ni 3 Al (23.4 at% Al) was investigated. Room-temperature tensile ductilities remained high over the entire pressure range tested: from 41% elongation to fracture at 5.7 × 10 −8 Pa pressure to 31% at 1.3 × 10 −3 Pa. Over this pressure range, the amount of transgranular fracture also remained quite high and scaled with the tensile ductility, increasing from ~60% in the samples with 31% ductility to ~70% in the specimen with 41% ductility. The ionization gage—used to measure hydrogen pressure—had a dramatic (deleterious) effect on the ductility of Ni 3 Al: at any given hydrogen pressure, the ductility measured with the ion gage on was about half to a quarter of that measured with the ion gage turned off. Accompanying this decrease in ductility was a change in fracture mode from predominantly transgranular to predominantly intergranular. The role of the ion gage is believed to be hot-filament-assisted dissociation of molecular H 2 into atomic H, which is quickly absorbed and embrittles the crack-tip regions. In the absence of any H-induced embrittlement (either by filament-assisted dissociation of H 2 or by Al-induced reduction of H 2 O), polycrystalline Ni 3 Al is found to be quite ductile, with tensile elongations exceeding 40% and predominantly (>70%) transgranular fracture. Since these ductilities are similar to those of the most ductile B-doped alloys, the main role of boron is to suppress environmental embrittlement. Our results indicate that, at room temperature, low-pressure H 2 does not dissociate very efficiently into atomic H on the surfaces of Ni 3 Al and that, at comparable pressures, hydrogen is not as harmful to ductility as moisture (H 2 O).