Ductile Region Research Articles

R‐Curve Behavior and Stable Crack Growth at Elevated Temperature (1500°–1650°C) in a Si3N4/SiC Nanocomposite

The crack growth resistance behavior and the stable crack growth regime of a Si3N4/SiC composite have been examined at high temperature (1500°–1650°C). SENB specimens were used and the load/unloading technique, with high deflection rates to ensure an elastic behavior, has been employed to estimate the crack lengths. Rising R‐curves have been obtained with a maximum crack growth resistance almost twice as high as the initial value. Above the Tg of the intergranular glassy phase, the behavior changes from brittle to visco‐plastic and, consequently, the fracture characteristics become strongly rate dependent. It is observed experimentally that in the enhanced ductile region the crack extension velocity during the stable crack propagation from a preexisting flaw decreases rapidly with time. This phenomenon has been tentatively attributed to dynamic crack‐tip stress relaxation resulting from the rapid flow of the glassy intergranular phase in the process zone. Thus, the rheological properties of the composite appear to be of major importance to gain insight into the mechanical behavior at such elevated temperatures.

Lower-crustal deformation during terrane dispersion along strike-slip faults

After accretion, terranes caught between obliquely converging plates are often displaced several hundreds of kilometers. How this motion is accommodated at depth is not well known. Spurred by our Trans-Alaskan Crustal Transect (TACT) seismic studies of the Yukon-Tanana terrane, Alaska, I propose that the lower crust responds ductilely to accommodate differential movement between upper crust and upper mantle during margin-parallel translation. The locations of these ductile regions are dependent on temperature, composition and strain rate. For geotherms based on moderate surface heat flow (60–90 mWm −2) and reasonable geological strain rates (100% in 3 Ma), typical lower-crustal rocks can deform ductilely. The resultant subhorizontal layered fabric should be detectable with seismic techniques. Two terranes that have experienced hundreds of kilometers of margin-parallel translation, the Yukon-Tanana terrane sampled by our seismic refraction/wide-angle reflection profiles and the Salinian block (California) sampled by both seismic refraction and reflection, have reflective lower crust. Both of these terranes have moderate to high surface heat flows, have relatively thin crust, and are interpreted seismically to be underlain by a quartzo-feldspathic lower crust; conditions favorable for ductile deformation in the lower crust. Interpretations of lower-crustal layering for these two terranes is based on an increase in lower-crustal reflectivity. This layering is attributed to tectonic banding consistent with ductile deformation.

Magnetotelluric evidence for a brittle‐ductile transition, peninsular ranges batholith, southern California?

A magnetotelluric traverse of the Peninsular Ranges in southern California has revealed a pervasive zone of lower resistivity beginning at a uniform depth of 10 km and extending to depths of 60–90 km. Resistivities above 10 km depth are similar to those found in batholiths; very high values correspond to outcrops of crystalline basement. Because seismicity below 11–12 km is sparse, others have concluded that the brittle‐ductile transition is shallow beneath the range. The zone of low resistivity in the lower crust corresponds well to the ductile region, and we conclude that the lower values are caused by fluids trapped below the transition. Because the range has experienced vertical uplifts during the Pliocene era and the top of the low resistivity zone is flat, the present brittle‐ductile transition must have been formed in the last 5 M.y. A possible source for the fluids is the rift to the east in the Salton Trough.

Substantiation of two-criterial, failureestimation diagram for pressure vessels and pipelines with axial through cracks

A form of diagram for failure estimation is tested on the basis of results of full-scale experiments on the failure of pressure vessels and pipelines with axial through cracks. Use of the two-criterial approach to predict the failure of structural components containing a crack in the brittle, mixed, and ductile regions of failure is substantiated. For example, the brittle failure of cylindrical bodies with through cracks is monitored by criteria of linear fracture mechanics, and ductile failure by the criterion established when the load attains its limiting value (P=Pld).

Microstructurally toughened particulate-reinforced aluminum matrix composites

Silicon carbide particulate-reinforced aluminum matrix composites with dramatically higher energy absorption capability and damage tolerance have been demonstrated. The approach, referred to as microstructurally toughened composites, consists of segregating the composite into particulate-reinforced regions and continuous ductile toughening regions. Composites consisting of silicon carbide particulate-reinforced 6061 Al (SiCp/6061) with monolithic 6061 and commercially pure (CP) titanium toughening regions were fabricated. As much as an order of magnitude increase in notched Charpy impact energy absorption capability was demonstrated relative to conventional SiCp/6061 composites, with the higher values being associated with those samples that deflected the crack front more extensively during failure. The longitudinal tensile strength of the composites was shown to be independent of the scale of the microstructure or the magnitude of the toughening/reinforced region’s interfacial strength. The high impact energy 6061 toughened composites displayed low transverse tensile strength values, while the CP titanium toughened composites simultaneously displayed high energy absorption and high transverse tensile strength. A model was also developed to predict the minimum and maximum energy absorption capability of the composites, as well as provide a quantitative estimate of the composite energy absorption based on the measured crack front deflection length in the failed impact samples.

A nonlinear rheology model for mid‐ocean ridge axis topography

This paper describes a mechanical model of the mid‐ocean ridge axis based on a nonlinear rheology. The oceanic lithosphere is modeled as a strongly temperature‐dependent “power law” material with the Byerlee's friction law describing the brittle behavior of the upper oceanic lithosphere. We examined two simple models. A mantle‐only model predicted an axial rift valley 1–2 km deep and 10–15 km wide at slow half‐rates of 5–25 mm/yr which is in fair agreement with observations. However, the rift valley did not completely vanish at fast spreading rates even though its width and relief became much smaller. Thus, the mantle‐only model failed to explain the observed transition in axial topography with spreading rate. By including a uniform thickness oceanic crust with theological properties different from the mantle, our second model produced the first order variation of axial topography with spreading rate: a pronounced rift valley with 1–2 km relief at slow rates, a smaller relief (<500 m) rift valley at intermediate rates, and no rift valley at fast rates. There are two additional results of the nonlinear rheology model which are distinctively different from the previous uniform viscosity mantle models. First, horizontal velocity at the seafloor changes sharply in a very narrow zone at the ridge axis, i.e., the stresses are concentrated in a very narrow zone for the nonlinear rheology. This conclusion is remarkably independent of the boundary conditions at the seafloor. Second, the computed width of the axial plastic failure zone decreases with spreading rate, a result very different from that of a uniform viscosity model. This is caused by the strong temperature dependence of the power law rheology and the fact that the temperature of the oceanic lithosphere depends greatly on the spreading rate. Beyond some distance from the ridge axis, the brittle plate (roughly the region above the 750°C isotherm) deforms very little; both the horizontal strain rate ė11 and the maximum shear strain rate ėmax are less than 10−17 s−1 which is in good agreement with the 10−18 s−1 average for oceanic brittle plate deformation inferred from seismic moment data. Within the ductile region (area beneath the 750°C isotherm) the strain rates are in the range of 10−14–10−16 s−1, except within a narrow band near the ridge axis where higher strain rates are located. We found that the thermal structure of the oceanic lithosphere is not very sensitive to the details of the mantle flow pattern (or the rheology) but is very sensitive to the cooling by hydrothermal circulation near the ridge axis. Axial structure is also influenced by factors other than the spreading rate. Within the range of slow and intermediate rates, our calculations show that the size of an axial rift valley decreases with an increase in either the thickness of the oceanic crust or the mantle temperature; in other words, a thinner crust or a cooler mantle temperature will produce a more pronounced rift valley. In the vicinity of hotspots, the absence of an axial rift valley even at slow spreading rates, such as the Reykjanes Ridge, can be well explained by the anomalously thicker crust and higher mantle temperatures. In contrast, at the Australian‐Antarctic Discordance a deeper than normal section of an intermediate spreading ridge has very rough topography not present on the adjacent sections of the ridge; this can be explained by our model as due to an anomalously low mantle temperature as has been suggested or perhaps also due to a thinner oceanic crust

AES analysis of grain boundary segregation and precipitation in electron beam welded molybdenum and Mo-0.56% Nb alloy

Electron beam melted Mo and a Mo-0.56% Nb alloy have been welded using an electron beam welding technique. The microstructure, tensile properties and fracture behavior of these welds have been examined and compared with those of sintered welds between 77 and 1273 K. Tensile tests have revealed that the strength of the weld metal is equivalent to that expected for the recrystallized structure in the ductile region. Brittle behavior is observed with fracture initiating at grain boundaries and propagating by intergranular or transgranular mode below room temperature. Welds exhibited a general loss of ductility compared with the corresponding values for the base metals. Postweld heat treatment was found to enhance the ductility mainly because of the recrystallization of the base metal. Auger electron spectroscopy has eliminated oxygen, nitrogen or other impurities as sources of grain boundary embrittlement in welds. It was found that carbon precipitation was effective in enhancing the grain boundary cohesion in welds of melted materials.

Late brittle deformation in a caledonian ductile thrust wedge: new evidence for gravitational collapse in the Moine Thrust sheet, Sutherland, Scotland

In the ductile regions of orogenic wedges, late “brittle” structures formed at shallow crustal depths have received little attention. However, they are important as they of ten record the final stages in deformational history during uplift and erosion of the orogen. In the Moine rocks of Sutherland, late Caledonian brittle folds and low-angle faults post-date ductile structures formed during WNW-NW-directed thrust movements earlier in the same orogenic event. In the western Moine Nappe, the late structures display geometries which indicate that they formed during regionally ESE-directed extensional movements, down the dip of the pre-existing regional foliation. These late structures are correlated with similar, WNW-directed extensional low-angle faults recognised in the Moine Thrust Zone south of the Assynt culmination. Both are thought to have formed in response to late Caledonian gravity-induced thinning of the Moine Thrust wedge, and both pre-date Devonian extension associated with the formation of the West Orkney Basin. Along-strike changes in the location and sense of extensional movements, north and south of the Assynt culmination, are explained by changes in the pre-existing structural geometry of the Caledonian ductile thrust nappes within the Moine. Hence, earlier deformation patterns are thought to exert important controls over later events in an evolving orogenic wedge.

Gravity‐driven deformation of the crust on Venus

Intense horizontal deformation features are observed on the surface of Venus despite the apparent absence of terrestrial‐style plate tectonics. High surface temperature, low erosion rate, and regions of elevated terrain suggest that gravity spreading may be responsible for some of these features. Gravity spreading on the Earth is known to cause extensive deformation in certain thin‐skinned tectonic regions, where the uppermost layers become detached along a decollement. While the high pore pressures and soft sedimentary rocks frequently observed along decollements on the Earth are not plausible for Venus, temperature induced regions of ductile deformation may exist. Simple viscous models of gravity spreading above a ductile region of the crust are developed to examine the effects of rheology, thermal gradient, surface slope and crustal thickness on deformation. For a reasonable range of parameters, the models show that surface‐slope‐driven ductile deformation at relatively shallow depths in the crust is possible and may be capable of detaching the uppermost rigid portion of the crust, resulting in horizontal translation.

Response of two‐phase fluids to fracture configurations within submarine hydrothermal systems

Submarine hydrothermal systems at mid‐ocean ridges evolve within a region of brittle deformation overlying a region of ductile deformation. The interplay between brittle and ductile modes of deformation and the geometry of fracture systems within the brittle zone can exert a significant influence on fluid flow, especially under conditions leading to aqueous two‐phase separation. Within the brittle region, fractures vary between large conduits directly open to the seafloor and small veins and microcracks with constricted or indirect connections to the seafloor. In the ductile region, fluids may be trapped under near‐lithostatic pressure, while fluids within throughgoing veins in the brittle region will experience hydrostatic pressure. In a narrow transition zone between these two extremes, a steep pore fluid pressure gradient must exist. In the brittle region itself, fluid pressures will vary between lithostatic and hydrostatic as fluid occupies interstices varying from isolated pores through tortuous microcracks to channels directly connected to the main upflow conduits. A hydrostatic gradient modified by the decreased density of hot fluid and the effects of the flow itself should be applicable in major conduits open to the seafloor. Fluctuating pressure conditions in the transition zone may induce phase changes in a fluid remaining at constant depth. As a result of the large volume increase that accompanies two‐phase separation, a two‐phase fluid must either accelerate upward or exert an increased pressure on its surroundings, possibly leading to enhanced fracturing. The fracture configuration will also affect the behavior of the mixed fluid after phase separation. A two phase‐fluid experiencing rapid turbulent flow in a large open conduit is more likely to remain a well‐mixed suspension of vapor and liquid, whereas a two‐phase mixture in a constricted, branching series of channels, moving more slowly in a laminar flow regime, is more likely to undergo physical isolation of the phases, leading to venting of a vapor phase and retention of a saline brine phase at depth. We propose a possible configuration for the fracture pattern within a submarine hydrothermal system consisting of a feeder zone comprised of anastomozing microcracks and veinlets, progressively increasing in diameter and decreasing in abundance, feeding into a main upflow/stockwork zone in which flow directly to the surface takes place in relatively large, unconstricted conduits. Fluid movement within a feeder zone may be modified by capillary flow, tortuous channelways, disconnected fractures, blocking of cracks by mineral deposition, and opening of new fractures.

The effect of temperature on cold-rolled nylon-6

The effect of pre-orientation on nylon-6 at various temperatures reveals three types of fracture behaviour—(1) brittle (−196 and −150°C), (2) brittle/ductile (−100°C), and (3) ductile (above 100°C). Multiple fracture occurred in the brittle region, particularly with increasing degrees of pre-orientation. In the brittle/ductile transition region, there is evidence of ductility in the fractured specimen. Some evidence of flow is seen in the specimens pre-oriented up to 25%, but beyond that multiple fracture occurs although less marked than at the lower temperatures. In the third region, i.e. the ductile region, the material exhibits considerable ductility.

Elastodynamic fracture analysis of large crack-arrest experiments

Recent elastodynamic fracture analysis results are summarized from Heavy-Section Steel Technology (HSST) studies in two major areas that related to assessing nuclear reactor pressure vessel integrity under pressurized-thermal-shock (PTS) conditions. These areas are crack run-arrest behavior in wide plates under nonisothermal conditions and fracture behavior of a thick-wall vessel under combined thermal and pressure loadings. The WP-1 series of HSST wide-plate crack-arrest tests are being performed at the National Bureau of Standards (NBS), Gaithersburg, MD, using specimens from HSST Plate 13A of A533 grade B class 1 steel. The six tests in the WP-1 series are aimed at providing crack-arrest data at temperatures up to and above that corresponding to the onset of the Charpy upper-shelf, as well as providing information on dynamic fracture (run and arrest) processes for use in evaluating improved fracture analysis methods. Elastodynamic analyses have been completed for the actual test conditions of the four tests, WP-1.1 through WP-1.4, conducted thus far in the WP-1 series. In this paper, the computed results are compared with data for crackline strain-time response, crack-propagation speed, arrest location and post-arrest tearing. The paper includes a summary of the arrest toughness calculations compiled in the four tests at temperatures that range from transition to upper-shelf values for the wide-plate material. These same elastodynamic fracture analysis techniques have been applied to the analysis of the first pressurized-thermal-shock experiment (PTSE-1) performed at ORNL. The experiment addressed warm-prestressing phenomena, crack propagation from brittle to ductile regions, and crack stabilization in ductile regions. Test and analysis results are summarized in the paper.

Some advances in fracture studies under the heavy-section steel technology program

Recent results are summarized from HSST studies in three major areas that relate to assessing nuclear reactor pressure vessel integrity under pressurized-thermal-shock (PTS) conditions. These areas are irradiation effects on the fracture properties of stainless steel cladding, crack run-arrest behavior under non-isothermal conditions, and fracture behavior of a thick-wall vessel under combined thermal and pressure loadings. Since a layer of tough stainless steel weld overlay cladding on the interior of a pressure vessel could assist in limiting surface crack extension under PTS conditions, its resistance to radiation embrittlement was examined. A stainless steel overlay cladding, applied by a submerged arc, single-wire, oscillating-electrode method, was irradiated to 2 × 10 23 neutrons/m 2 (> 1 MeV) at 288°C. Yield strength increases up to 27% and a slight increase in ductility were observed. Charpy V-Notch data showed a ductile-to-brittle transition behavior caused by temperature-dependent failure of the 8-ferrite phase. The type 308 cladding, microstructurally typical of that in reactor pressure vessels, showed very little degradation in either upper-shelf energy or transition temperature due to irradiation. Crack-arrest behavior of A533 grade B class 1 steel was examined for temperatures extending above the onset of Charpy upper-shelf. Crack-arrest experiments that use wide-plate specimens have shown crack arrest occurring prior to transition to tearing or tensile instability. High values of crack-arrest toughness have been recorded (static values above 400 MPa m that are well above the maximum value that safety assessment criteria assume such materials can exhibit. A validation experiment was performed by exposing an intentionally flawed HSST intermediate test vessel to combined pressure and thermal transients. The experiment addressed warm-prestressing phenomena, crack propagation from brittle to ductile regions, and crack stabilization in ductile regions. Test and analysis results are summarized.

The heavy-section steel technology pressurized-thermal-shock experiment, PTSE-1

A pressurized-thermal-shock (PTS) facility was developed in the Heavy-Section Steel Technology Program at Oak Ridge National Laboratory for performing experiments that challenge predictions of analytical methods applicable to full-scale reactor pressure vessels under combined loading. The first experiment (PTSE-1) was designed to address three principal issues: (1) warm-prestressing phenomena; (2) crack propagation from brittle to ductile regions; and (3) transient crack stabilization in ductile regions. The paper presents a description of the PTS facility at ORNL and a review of the objectives and results of the first test. Also included are elastodynamic finite-element analyses of the two crack run-arrest events that occurred in the second and third phases of the test. Finally, some conclusions and recommendations are presented based on the outcome of the first experiment.

EFFECT OF SOAKING IN ALCOHOL ON MECHANICAL PROPERTIES OF POLY (METHYL METHACRYLATE)

Effects of temperature and ultrasonic irradiation on the deterioration of poly(methyl methacrylate) (PMMA) with the soaking in both methyl and ethyl alcohols were investigated by measuring the changes of tensile strength, elongation and elastic modulus.PMMA was assumed to have the two-phase structure of rubbery shell and glassy core with Case II diffusion by the soaking in alcohol. The changes of mechanical properties depended upon the penetration of alcohol. The applications of ultrasonic irradiation and heat were effective for accelerating degradation test. The change of elastic modulus of PMMA was described by the additivity of mechanical contribution from both rubbery shell and glassy core, but not for the tensile strength. At the initial stage of soaking, the value of tensile strength was smaller than the expected value from the additivity. At the late stage of soaking, it was larger than the expected value from the additivity. Namely, the elongation of original region in the two-phase structure increase as the ductile region increases because of the sorption of solvent in front of sharp boundary.

I Interaction of Plastic Local and Lateral Buckling

Attention is given to the interaction between local and lateral buckling of I‐sections and their influence on plastic flexural ductility in regions of moment gradient. It is shown by means of simple theoretical formulations and the results of a series of tests to failure that rotation capacity in the plastic range is influenced significantly not only by conventional codified parameters, but also by the ratio of span to flange thickness and coincident axial force. In addition, the interaction between local and lateral buckling in the plastic region is demonstrated, and a model is formulated for quantifying this strain‐weakening effect. The results are relevant to the plastic design of structures as well as more general research into methods of improving flexural ductility of I‐sections.

The effect of irradiation on the toughness of pressurized water reactor vessel steels under different service conditions

The different toughness tests performed on two pressure vessel steels with very different upper shelves served to make a number of observations concerning the shifts in the transition temperature due to the effect of irradiation, as well as changes in toughness with temperature in the ductile region. With respect to shifts in the transition temperature, the following was observed: the shift obtained with precracked charpy test specimens was narrower than with the others; the shift obtained with charpy V impact tests was substantially equal to that obtained with CT test specimens — wider in the case of steel A, but slightly narrower in that of steel H. With respect to toughness values in the ductile region: the toughness values obtained using precracked charpy test specimens are significantly higher than those obtained with CT test specimens for static tests; 25and 12.5 mm thick CT test specimens display comparable variations in J 1C and d J/d a, but with wide scattering; the effect of irradiation, if any, is of the same order of magnitude as the scattering of the results — however, a test temperature effect is observed; the variation in toughness with temperature is determined more easily by considering a J value corresponding to a stable crack propagation of 1 mm, so that J Δal = J 1C + d J d a × 1 ; this variation of J Δal with temperature is substantially the same for both steels, or about −30% at 70 or 80°C, and −50% at 290°C.

高電流密度・短時間スポット溶接法によるアルミニウム合金板と軟鋼板の接合 Ｉ 表面処理法の溶接性におよぼす効果

Effect of faying surface treatment on the tensile shear strength (TSS) of joints has been studied in the weld of aluminum alloy (0.8t 5052) to mild steel (0.8t SPCC) using uni-directional weld current from SPCC to 5052.Welded joints had 380 kg/spot TSS under the condition of wire-brushing on both SPCC's and 5052's faying surfaces. Slightly reduced TSS value was obtained under the condition of wire-brushing on SPCC's faying surface alone. But on the contrary, wire-brushing on 5052's faying surface alone, gave TSS value merely about 100 kg/spot. Therefore, to give wire-brush or not on SPCC's faying surface before the weld affects strongly on TSS of joints. Further, peeled fracture surface of the joint was observed by scanning electron microscope and it was found that joints under SPCC wire-brushing showed ductile deep dimple region while the other presented relatively flat bisttle face.The above mentioned results indicate that the surface treatment of metals with higher melting point is important in the welding of dissimilar metals.

四国地方における<i>V</i><sub><i>p</i></sub>/<i>V</i><sub><i>s</i></sub>と地震活動との関連

The velocity ratio Vp/Vs is examined in relation to the P wave velocity structure or the seismic activities in central Shikoku, Japan. The following four layers distinguished from one another according to seismic velocities have been observed in Shikoku: the superficial, granitic, basaltic and mantle layers. The occurrence of earthquakes has been active mainly in the granitic layer and the uppermost mantle. A ratio Vp/Vs=1.73 is observed for both of these seismically active layers. The Vp/Vs of the ductile region of the mantle where earthquakes have scarcely occurred is inferred to be 1.80 or over, as was expected. A Vp/Vs=1.75 is applied for the basaltic layer, but the value is not necessarily reliable. It is remarkable that a Vp/Vs=1.58-1.65 observed for the superficial layer is unexpectedly low. This low value may reflect the high fracture of rocks and the resultant low seismic activity in that layer.

Tensile failure of crystalline polymers

Abstract In the creep experiment the brittle fracture of the unoriented semicrystalline polymers at very small and very high tensile load with the intermediate ductile region may be explained by the competition between crazing and shear band formation during the microcrack growth phase. The former type of microcrack growth leads to brittle fracture while the latter type yields necking which transforms the original lamellar structure into the final fibrous structure. The actual fate of the strained sample depends on the growth time of the craze, tg, and of the shear band formation time, ts. If tg ts, the material will deform plastically. The failure of the fibrous material seems to occur when the ratio between the average distance and diameter of the microcracks reaches a value about 3. The microcracks seem to form primarily at defects of the microfibrillar structure, i.e., at the ends of microfibrils where the axial connection of subsequent crys...