Showing papers on "Tensile testing published in 1991"

Hot ductility of steels and its relationship to the problem of transverse cracking in continuous casting

Abstract: The influence of composition and cooling rate on the hot ductility of steels has been reviewed. Models to predict hot ductility behaviour have been discussed and the parts of the trough which can be used to predict the likelihood of cracking occurring are highlighted. On tensile testing both deformation induced ferrite in sufficient quantity to improve ductility and dynamic recrystallisation occur but not when straightening during continuous casting; the strain being too low. This limits the use of the hot ductility curve in predicting cracking behaviour. The temperature range in which straightening of the continuously cast strand should be carried out is either 30°C below the Ar 3 when there is a large amount of ferrite (∼40%) present before deformation or above the T d, the temperature at which dynamic recrystallisation starts to take place in a tensile test; this being when the ferrite film no longer forms and precipitates are sufficiently coarse and few in number to influence the ducti...

A direct-tension split Hopkinson bar for high strain-rate testing

Abstract: A direct-tension split-Hopkinson-bar apparatus is introduced. In this apparatus the specimen is loaded by a tensile wave that is generated by the release of a stored load in a section of the input bar. The system can be used for experiments with test durations of up to 500 μs. The effect of specimen geometry (length to diameter ratio) is investigated. Consistent results are obtained when the ratio is larger than about 1.60. Results from tests with 6061-T651 aluminum are in agreement with published data.

Plastic deformation and fracture of binary TiAl-base alloys

Abstract: The mechanical behavior of binary TiAl alloys containing 46 to 60 at. pct Al has been studied in bulk materials preparedvia rapid solidification processing. Bending and tensile tests were carried out at room temperature as a function of Al concentration. A few alloys were also tested from liquid nitrogen temperature to ∼ 1000°C. Deformation substructures were studied by analytical transmission electron microscopy and fracture modes by scanning electron microscopy (SEM). It was found that both microstructure and composition strongly affect the mechanical behavior of TiAl-base alloys. A duplex structure, which contains both primary y grains and transformedγ/α 2 lamellar grains, is more deformable than a single-phase or a fully transformed structure. The highest plasticities are observed in duplex alloys containing 48–50 at. pct Al after heat treatment in the center of theγ + α phase field. The deformation of these duplex alloys is facilitated by 1/2[110] slip and {111} twinning, but very limited superdislocation slip occurs. The twin deformation is suggested to result from a lowered stacking fault energy due to oxygen depletion or an intrinsic change in chemical bonding. Other factors, such as grain size and grain boundary chemistry and structure, are important from a fracture point of view. The results on the deformation and fracture modes as a function of test temperature are also discussed.

Fiber-Matrix Adhesion and Its Effect on Composite Mechanical Properties: II. Longitudinal (0°) and Transverse (90°) Tensile and Flexure Behavior of Graphite/Epoxy Composites:

Abstract: An optimum level of interfacial bond strength between reinforcing fiber and a polymeric matrix in which it is placed is essential for acceptable composite mechanical properties and performance. The...

Ductility enhancement in NiAl (B2)-base alloys by microstructural control

Abstract: An attempt to improve ductility of NiAl (B2)-base alloys has been made by the addition of alloying elements and the control of microstructure. It has been found that a small amount of fccγ phase formed by the addition of Fe, Co, and Cr has a drastic effect not only on the hot workability but also on the tensile ductility at room temperature. The enhancement in ductility is mainly due to the modification of Β-phase grains by the coexistence ofγ phase. The effect of alloying elements on the hot forming ability is strongly related to the phase equilibria and partition behavior amongγ,γ′ (L12 structure), and Β phases in the Ni-Al-X alloy systems. The ductility-enhancement method shows promise for expanding the practical application of nickel aluminide.

Prediction of concrete tensile strength from compressive strength: evaluation of existing relations for normal weight concrete

Abstract: The 0.5 power relation adopted by ACI Committee 318 predicting the splitting tensile strength of concrete from its compressive strength has been investigated. Research has consistently indicated that the relation in the Building Code Requirement does not agree particularly well with test results. Consequently, researchers have proposed alternative relations. The research for this paper investigated the validity and accuracy of these alternate relations. Tensile strength preditions from these relations were compared with test results assembled from various sources. It was found that the splitting tensile strength is not proportional to the 0.5 power of compressive strength. Although most of the alternate relations appear to be good, the assembled test data revealed that 0.69 is the most accurate power relation. Thus 0.69 power relation is proposed as an alternative to the ACI 318 relation.

Toughening of MoSi2 with a ductile (niobium) reinforcement

Abstract: The effect of interfacial reactions and Y2O3 coatings on toughening of MoSi2 by ductile phase Nb reinforcements has been investigated. In the absence of coating the interfacial reaction layer exhibits parabolic growth with Mo5Si3, (Mo, Nb)5Si3, (Nb, Mo)5Si3 and Nb5Si3 phases forming. In precracked laminates subjected to tensile loads the ductile phase deformation is partially constrained, with debonding occurring within the interfacial reaction zone. Dense Y2O3 coating inhibits interdiffusion and results in more extensive debonding. In either case, significant toughening is expected with measured work of rupture values χ ≈ 5.7 to 6.3. Bulk composite MoSi2 reinforced with 20 vol.% Nb particles subjected to a chevron-notched three point flexure test had a work of rupture almost five times larger than the unreinforced MoSi2 matrix.

The anisotropic mechanical properties of a Ti matrix composite reinforced with SiC fibers

Abstract: The anisotropic mechanical properties of a Ti alloy composite reinforced with SiC fibers have been investigated and rationalized using analytical models. The appropriate material model for this composite involves the following features: an interface that debonds and slides, a flaw insensitive ductile matrix, and high-strength elastic fibers subject to residual compressive stress caused by thermal expansion mismatch. This, model is broadly consistent with the longitudinal, transverse, and shear properties of the composite.

Environmental effect on mechanical properties of recrystallized L12-type Ni3(Si,Ti) intermetallics

Abstract: The environmental effect on the mechanical properties of boron-doped and undoped Ni3(Si, Ti) polycrystals was investigated by tensile testing in air from room temperature to 1073 K, and the results were compared with those obtained previously by tensile testing in vacuum. The environmental effect for the Ni3(Si, Ti) alloys was significant at ambient temperatures whereas that for the boron-doped Ni3(Si, Ti) alloys was considerable at elevated temperatures. When these samples at associated temperatures were tensile tested in air and also at low strain rate, intergranular fracture was dominant. It was suggested that the environmental embrittlements at low and high temperatures were due to hydrogen and oxygen absorbed from the air, respectively, and were caused by the weakening of the grain-boundary cohesion. It was proposed that boron competing with hydrogen, for site occupation or for its effectiveness at grain boundaries, has the effect of suppressing hydrogen embrittlement, whereas it was suggested that the low-melting phases, consisting of boron and oxygen (and/or constituent atoms), may be responsible for the ductility loss in the boron-doped Ni3(Si, Ti) alloys.

The deformation and fracture of constrained metal sheets

Abstract: A brittle solid can be toughened by dispersing ductile inclusions in it. The degree of toughening depends on the properties, volume fraction and size of the ductile inclusions and on the strength of the interface between inclusion and matrix. Experiments and models are described which quantify the influence of interface strength. The experiments use a sandwich of lead bonded between two glass plates, in such a way that debonding can be controlled. Cracks are introduced into the glass, and the behaviour of the lead at and near the crack tip is examined. The way in which debond and crack-orientation influence the work of fracture is explored.

Tensile ductility of superplastic ceramics and metallic alloys

Abstract: Superplastic ceramics and metallic alloys exhibit different trends in tensile ductility in the range where the strain-rate-sensitivity exponent, m, is high (m⩾0.5). The tensile ductility of superplastic metallic alloys (e.g. fine-grained zinc, aluminium, nickel and titanium alloys) is primarily a function of the strain-rate-sensitivity exponent. In contrast, the tensile ductility of superplastic ceramic materials (e.g. zirconia, alumina, zirconia-alumina composites and iron carbide) is not only a function of the strain-rate-sensitivity exponent, but also a function of the parameter ⋗e exp (Qc/RT) where ⋗e is the steady-state strain rate and Qc is the activation energy for superplastic flow. Superplastic ceramic materials exhibit a large decrease in tensile elongation with an increase in ⋗e exp (Qc/RT). This trend in tensile elongation is explained based on a “fracture-mechanics” model. The model predicts that tensile ductility increases with a decrease in flow stress, a decrease in grain size and an increase in the parameter (2γs−γgb), where γs is the surface energy and γgb is the grain boundary energy. The difference in the tensile ductility behavior of superplastic ceramics and metallic alloys can be related to their different failure mechanisms. Superplastic ceramics deform without necking and fail by intergranular cracks that propagate perpendicular to the applied tensile axis. In contrast, superplastic metallic alloys commonly fail by intergranular and transgranular (shearing) mechanisms with associated void formation in the neck region.

Fracture of particles in a particle/metal matrix composite under plastic straining and its effect on the Young's modulus of the composite

Abstract: Fracture of Al 2 O 3 particle in an Al 2 O 3 particle/aluminium alloy metal matrix composite (MMC) under plastic straining was observed by in-situ tensile testing in a scanning electron microscope. The fracture of larger sized particles was found to be preferred to that of smaller sized ones for a given plastic strain. The Young's modulus (E c ) of plastically strained MMC was reduced with increase in plastic strain (e p )

Microstructure-property relationships of in situ reacted TiC/AlCu metal matrix composites

Abstract: A novel technique was utilized for the fabrication of in situ titanium carbide reinforced aluminum alloy metal matrix composites. The reacted, cast, extruded and heat-treated samples exhibited a homogeneous distribution of fine (0.1–3 μm) TiC platelets in a fine-grained recrystallized Al4.5wt.%Cu matrix. Elevated temperature tensile testing indicated that the composite retains its room temperature strengths up to 250 °C and compared favorably with composites fabricated by more complex and costly processes. When compared with Al4.5wt.%Cu alloy processed similarly but without the TiC reinforcement, the additions of TiC resulted in a yield strength and tensile strength increase of 130% and 65% respectively. Fractographic analysis indicated ductile failure, although the ductility and strength were limited by the presence of coarse titanium aluminides (Al3Ti).

Modes of deformation in rubber-modified thermoplastics during tensile impact

Abstract: Real-time small-angle X-ray scattering (RTSAXS) studies were performed on a series of rubber-modified thermoplastics. Scattering patterns were measured at successive time intervals as short as 1.8 ms and were analysed to determine the plastic strain due to crazing. Simultaneous measurements of the absorption of the primary beam by the sample allowed the total plastic strain to be computed. The plastic strain due to other deformation mechanisms, e.g. particle cavitation and macroscopic shear deformation was determined by the difference. Samples of commercial thicknesses can be studied at high rates of deformation without the inherent limitations of microscopy and its requirement of thin samples (i.e., plane strain constraint is maintained on sample morphology).

Tensile deformation and fracture toughness of 2014 + 15 vol pct SiC particulate composite

Abstract: Tensile tests and fracture toughness experiments were conducted on 2014 aluminum alloy with 15 vol pct SiC particulate in a stage which fit within a scanning electron microscope. Strains associated with tensile deformation and the transition from slow to rapid crack growth were determined using the stereoimaging technique. Overall tensile elongations were measured at 1.6 to 2.4 pct, while localized strains were up to ≈50 pct but depended on the dispersion of SiC particles. Measured fracture toughness values ranged from 18.7 to 29:5 √m. Fractography revealed the virtual absence of dimpled rupture on both types of specimens. The fracture toughness values measured could be accounted for by computing the work done in forming new crack surfaces. To do this, the strain gradients determined during the tests were used in a previously developed model. Matrix ductility and particle dispersion are identified as the factors controlling toughness.

Creep and creep fracture in hot-pressed alumina

Abstract: The creep and creep fracture behavior of two hot-pressed aluminas are presented, for both flexural and tensile testing. Steady-state power-law creep is observed with a stress exponent of about 2 for each material. Three distinct fracture regimes are found. At high stress in flexure, fracture occurs by slow crack growth with a high stress dependence of the failure time. At intermediate stresses, in both flexure and tension, creep fracture occurs by multiple microcracking after modest strains. Failure times exhibit a modest stress dependence (stress exponent of 2.5 in tension and 3 in flexure), with a constant failure strain equal to 0.09. The failure times are considerably longer in flexure than in tension, because of the constraint imposed on crack growth by the bending geometry. We conclude that flexure cannot be used for creep lifetime assessment, even in simple, single-phase materials such as Al2O3. At low stresses, in tension, failure also exhibits a modest stress dependence but with a much higher failure strain. The material shows the onset of super-plastic behavior.

The effect of alloying on slip systems in 〈001〉 oriented NiAl single crystals

Abstract: An investigation of the effect of alloying on slip behavior in NiAl as a function of temperature has been conducted. Single Cyrstal specimens have been deformed in tension and compression in 〈110〉 and 〈001〉 orientations. It was found that the addition of Cr and other alloying additions promote the activation of 〈111〉 slip over deformation by kinking in NiAl based alloys. This is believed to result from differential proportional hardening of the 〈100〉 vs 〈111〉 slip systems. No increase in RT tensile elongation is observed in these alloys. Increased tensile ductility observed at higher temperatures is due to the movement of b = 〈110〉 dislocations.

An analysis of the effect of residual stresses on deformation and damage mechanisms in AlSiC composites

Abstract: The effect of thermally induced residual stresses on the mechanical properties and ductility of AlSiC composites was investigated numerically. The predicted behavior in uniaxial loading was calculated with and without the inclusion of the residual stresses which result from the mismatch in thermal expansion between aluminum and SiC. In this analysis, void nucleation by interfacial debonding at the whiskers' ends was assumed to be the limiting failure mechanism. Two cases, both with a fiber volume fraction of 20% and fiber aspect ratio of 4, but with different fiber spacings, were considered. The residual stresses had a small effect on the predicted ductility of the composite, even when a relatively weak interface strength was assumed. The residual stresses are shown to redistribute as interfacial failure is approached. A close end-to-end fiber spacing gives a greater flow strength in compression than in tension and the residual stresses which arise during thermomechanical processing tend to enhance this effect.

Analysis of crack propagation in asphalt concrete using a cohesive crack model

Abstract: A cohesive crack model, which is similar to the Dugdale-Barenblatt model, was proposed to simulate the progressive crack development in asphalt concrete. Tensile strength, fracture energy, and the stress-separation relationship are the basic material properties associated with this model. To evaluate the material properties, indirect tensile tests and three-point bend tests were performed. From these experimental results, the effects of temperature on Young's modulus, the fracture energy, and the indirect tensile strength were evaluated. To determine the stress-separation relationship, a numerical simulation (or curve-fitting method) was used. Using the material properties obtained from the experimental study, temperature effects on different fracture parameters (i.e., critical stress intensity factor and critical J-integral) were studied. The theoretical predictions were found to be in good agreement with the available experimental results. This finding also indicates the potential applications of the proposed model in evaluating the performance of asphalt concrete pavements.

In situ formation of three-dimensional TiC reinforcements in Ti-TiC composites

Abstract: 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.

Splitting tensile strength and compressive strength relationships at early ages

Abstract: The paper reports on investigation of the relationship between concrete compressive strength and its splitting tensile strength, especially at early ages, as well as the examination of the applicability of some of the existing relations between these properties to concrete at early ages. Analyses of test results show that the compressive strength and the spliltting tensile strength are related, and an increase in one, in general is similarly reflected in an increase in the other. The commonly accepted 0.5 power relationship between the compressive strength and the spliltting tensile strength was found to be inaccurate at all ages. In fact the tensile strength was found to be proportional to the 0.79 power of the cylinder compressive strength. An alternate relationship between the tensile strength and the compressive strength is proposed. Tables of results and figures supporting these observations and conclusions are included.

Mechanical behavior of cast particulate SiC/AI (A356) metal matrix composites

Abstract: Mechanical tests were carried out to study the deformation behavior of particulate SiC-reinforced Al (A356) matrix composites produced through direct casting using the molten metal mixing method. The matrix alloy-Al (A356) was also tested as a control material for comparison. The elastic constant and yield strength of the composite material were found higher than those of the control alloy, but the ultimate tensile strength (UTS) and the ductility were lower. The Tsai-Halpin equation was found applicable for calculating the elastic constant if an average particle aspect ratio could be determined. The strain-hardening behavior of the tested composite material appeared very different from that of the control alloy. The high strain-hardening rate in the early stage of plastic deformation of the composite was rationalized by the interaction between the hard particles and the ductile metal matrix; on the other hand, the low hardening rate recorded from intermediate strain amplitude to fracture was attributed to the early coalescence of voids and other microdamages. Particle-matrix interface debonding, particle cracking, and void for-mation in the metal matrix were considered to be responsible for the low ductility. Deformation asymmetry of the composites was noticed, not only through the Bauschinger effect, but also through the difference in virgin specimens’ yield stresses in tension and compression.

Room temperature deformation behavior of multiphase Ni20at.%Al30at.%Fe and its constituent phases

Abstract: The microstructure of the multiphase alloy Ni20Al30Fe (where the composition is in approximate atomic per cent), processed through a casting and hot-extrusion route, consisted of a pro-eutectic β′ (ordered b.c.c. or B2-structured) phase in a fine (0.5 μm lamellae width) eutectic. The eutectic consisted of β′ + γ (f.c.c.) phases; the γ phase itself contained γ′ (ordered f.c.c. or L12-structured) precipitates. When tested in tension at room temperature, this alloy exhibited up to 20% elongation and a yield strength of up to 850 MPa. Crack stopping by the γ–γ′ phase was concluded to be an important factor contributing to the ductility of the alloy. In contrast, β′ Ni30Al20Fe, which had a composition similar to that of the pro-eutectic phase of the multiphase alloy, exhibited a similar yield stress of 800 MPa but only 2% elongation. This alloy deformed by «001a slip. Finally, the alloy Ni12Al40Fe, which was similar to the γ–γ′ phases of the multiphase alloy, exhibited 28% tensile elongation and a yield strength of 507 MPa and deformed by «110a slip.

Fracture initiation at hydrides in zirconium

Abstract: The effect of hydride size and stress state on fracture initiation at hydrides in a reactor grade Zr material has been studied Uniaxial and triaxial states of stress were imposed by using smooth and notched tensile specimens, respectively Crack initiation at hydrides was monitored using acoustic emission (AE) The specimens contained, nominally, either 018 or 090 at pct hydrogen Plate-or needle-shaped hydrides having different lengths were produced by varying the cooling rate to room temperature from the hydrogenation temperature Initial orientation of the plate normals of the hydrides with respect to the tensile axis was mainly random After deformation, the hydrides near the fracture surface were all oriented with their plate normals perpendicular to the tensile axis direction Regardless of the hydride size, fracture at hydrides commenced at stress levels just above the proportional limit under uniaxial deformation Average plastic strain values at initiation were ~02 pct Slightly lower values of plastic strain were needed to initiate fracture at hydrides under triaxial loading Fracture of the hydrides was always through-thickness and specimen fracture ductile This is in contrast to previous results on hydride fracture obtained using the pressure tube alloys In these materials, the fracture of hydrides with their plate normal oriented parallel to the tensile axis became less ductile as the hydride length increased

On the prediction of tensile properties from hardness tests

Abstract: The possibility of correlating the hardness to the tensile properties of a material has been investigated using Assab 760 steel, mild steel and API Std 5LX grade X60 pipeline steel that have been heat-treated for different times at various tempering temperatures and 6063-T1 aluminium that has been solution heat-treated. It is found that the strain hardening coefficient and the strength coefficient of all materials tested were linearly related to the hardness, irrespective of the type of hardness measurement used. Using these relationships, equations were defined to estimate the yield and ultimate tensile stresses of the materials. Good agreement between experimental results and estimated values was obtained for all materials studied. The feasibility of using the present findings in non-destructive field testing is discussed.

On the constraint factor associated with the indentation of work-hardening materials with a spherical ball

Abstract: It is generally accepted that the constraint factor (CF) associated with the indentation of metallic materials by a much harder indentor is in the range of 2.8 to 3.0. Invariably, the CF is assumed to have a constant value in the above range irrespective of the material indented while correlating the hardness of the material indented with its uniaxial strength properties. The objective of the present investigation is to assess the above assumption by evaluating the CF associated with the indentation process as a function of strain in the case of metallic materials exhibiting a wide range with respect to elastic modulus, strength, and strain-hardening rate. The results indicate that the CF is not really a constant but is dependent on the various material properties. The experimental CF-strain relationship observed in all of the test materials has been rationalized on the basis of elastic-plastic and fully plastic indentation models. While the theoretical models can explain the trend of the data, they are not capable of making adequately accurate quantitative predictions.

Mechanical properties of recrystallized L12-type Ni3(Si, Ti) intermetallics

Abstract: The mechanical properties of the Ni3(Si, Ti) alloys undoped and doped with 50 p.p.m. boron, both of which were polycrystalline specimens prepared by recrystallization, were investigated by tensile testing. The yield stress was found to increase with increasing test temperature to a maximum at 800 K, followed by a decrease. The tensile elongation was highest at room temperature and tended to decrease with increasing temperature for both alloys, but was consistently higher in the boron-doped Ni3(Si, Ti) alloys than in the undoped ones over all the test temperatures. The change in the ultimate tensile stress (UTS) with temperature was similar to that of tensile elongation. The transgranular fracture became dominant as the elongation increased, regardless of the alloys and the testing temperature. Thus, this work again verified that the alloying method proposed by the present authors is useful for improving the grain-boundary cohesion of L12-type ordered alloys.