Decreasing Test Temperature Research Articles

Temperature, strain rate, stress state and the failure of HY-100 steel

The influence of temperature and strain rate on the deformation and failure behavior of HY-100 steel has been examined as a function of stress state using notched and un-notched axisymmetric tensile specimens. Behavior over the range of temperatures/strain rates from −85°C and 1 s −1 to 27°C and 10 −3 s −1 shows an equivalence of decreasing test temperature or increasing strain rate on deformation behavior in a manner that can be predicted by the thermally activated flow theory. Over the entire range of temperatures/strain rates, the influence of stress state on failure is such that two void coalescence mechanisms control failure; at low stress triaxialities, relatively equiaxed voids grow to impingement, while at high triaxialities, a void-sheet process intervenes linking elongated MnS-initiated voids by a shear instability. The failure strains decrease rapidly with increasing stress triaxiality ratio in a similar manner for all temperatures and strain rates except for an intermediate stress triaxiality condition where the void-sheet mode of failure extends to lower stress triaxialities under cryogenic test conditions.

The Influence of Strain Rate and Temperature on the Deformation Behaviour of 63Sn/37Pb and 60Sn/40Pb Solder Alloys

The influence of strain rate and temperature on plastic deformation behaviour of 63Sn/37Pb and 60Sn/40Pb solder alloys is investigated by means of a computer controlled servo-hydraulic machine over a temperature range of -40 to 80°C at constant strain rates of 10 -3 , 10 -2 and 10 -1 s -1 The fracture characteristics of deformed specimens are analyzed using scanning electron microscopy and correlated with macroscopic behaviour. The results indicate that the flow stress of both alloys is largely dependent on strain rate and temperature. The 63Sn/37Pb alloy, however, is stronger than 60Sn/40Pb over the range of 25 to 300°C. The change in flow behaviour is related to differential strain rates and temperature sensitivities. Over the strain rate and temperature range studied, 60Sn/40Pb exhibits higher strain rate and temperature sensitivities than 63Sn/37Pb. 60Sn/40Pb rupture resistance is found superior to 63Sn/37Pb in light of fracture observations revealing an absence of damage as well as an absence of flow instability. 63Sn/37Pb fracture is catastrophic at a strain rate of 10 -3 s -1 and is characterized by shear. With decreasing test temperature, more edge cracks appear due to enhanced brittleness. The use of a deformation constitutive equation in combination with the parameter values obtained through these tests allows for an accurate description of the deformation behaviour of both tin-lead alloys over the range of conditions used in this study.

Effect of the Rolling Ratio, Stress Amplitude and Test Temperature on the Fatigue Property of Cold Rolled PP/LCP Blends.

Effects of the rolling ratio, the stress amplitude and the test temperature on the fatigue property of the rolled PP/LCP blends have been examined from the variation of dynamic viscoelasticity factor during the fatigue process. The results show that : (1) Under a low rolling ratio (within 40%), the specimen was softened and easily to generate heat. Therefore, it ruptures for the ductile fracture earlier than the unrolled specimen. (2) For the 60% rolled specimen, with the increase of stress amplitude level the E' decrease while the tan δ, Ts, ΔL, Ld increase. The ductile fracture tendency became more evident. But, the specimens had not ruptured under all stress level and test temperature. It shows an excellent fatigue resistance property. (3) The resistance to fatigue becomes better as the test temperature decreases. Especially, it shows a high ductile, high strength and high fatigue resistance even under -10°C when the blends material, which lost the ductile property even under the room temperature, is pressed at 60%.

Techniques for Determining Errors in Asphalt Binder Rheological Data

Numerous new methods for specifying asphalt binders, based on more complex theoretical approaches, have emerged in the past years, and researchers are increasingly using these methods for studying asphalt binders. These methods require an increased degree of accuracy and precision in the laboratory data. A number of simple techniques are detailed that can be successfully used to identify errors in rheological data obtained for asphalt binders. Asphalt binders do not exhibit sudden changes in their behavior with respect to time or temperature. Therefore, any discontinuities revealed by the visual inspection of the test data in graphical format can be attributed to testing errors. A powerful yet simple tool in identifying potential problems with the dynamic shear rheometer test data is the Black diagram. The Black diagram is a plot of phase angle versus log | G*|, which, unlike master curves, does not require shifting of the data generated at different temperatures. However, pseudo Black diagrams should not be used with the bending beam rheometer (BBR) data because of the errors in calculating m-values associated with the use of the polynomial approximation. The c-coefficient can be used for a quick check of BBR data on the basis of the fact that the slope of the m-values decreases as the test temperature decreases. For the direct tension test, the secant modulus calculated from the stress-strain data provides a better tool to identify errors in the test data.

Heat Resistance and Thermal Acclimation Rate in Tropical Tetra Astyanax bimaculatus of Venezuela

Venezuelan river tetra, Astyanax bimaculatus juveniles of 34.1–36.7 mm standard length and 0.83–1.0 g wet weight were acclimated for four weeks to 24–33°C, which are approximate average minimum and maximum river temperatures throughout the year. The fish acclimated to 24, 27, 30, and 33°C were exposed for 10 000 minutes at 35, 36, 37, 38, and 39°C to determine individual heat resistance times. To determine acclimation rates, the juveniles acclimated to 24 and 30°C were tested for individual heat resistance times at 39°C by changing acclimation temperatures. The individual heat resistance times were increased in accordance with an increase in acclimation temperature and a decrease in test temperature, indicating that acclimation level has a great influence on thermal resistance of the fish tested. As the fish were transferred from 24 to 30°C (upward acclimation), they completed their acclimation level in a few days, while those transferred from 30 to 24°C (downward acclimation) required about 14 days. It has reaffirmed the following general behavior: the rate of gain in thermal resistance is fast and the loss in heat tolerance is very slow. This physiological phenomenon is very important for tropical fish, which acclimates rapidly in rising temperature during the hot day and does not lose this level in decreasing temperature during the cool night. Consequently, a tropical fish can maintain its maximum resistance level, adapt well in thermally fluctuating tropical waters, and survive in lethally high temperatures caused by a sudden increase in temperature during hot day.

Impact behaviour of polypropylene/polyethylene blends

Due to the large volume consumption of polyolefins, the treatment of the resulting solid waste is becoming a major concern. One possible utilization of the polyolefin wastes is to form blends from recycled polypropylene (PP) and polyethylene (PE). This study is a preliminary investigation on the impact fracture behaviour for the PP/PE blends. The impact testing method employed in this study includes the conventional and instrumented Izod impact tests, the instrumented Charpy impact test, and the instrumented drop weight plate impact test (IDWPIT). In both conventional and instrumented Izod impact testing, PP homopolymer and PP/LDPE blend have similar impact strengths, while PP/HDPE blend exhibits slightly lower impact strength. However, the instrumented Charpy impact test indicates that both PP/LDPE and PP/HDPE have similar impact strengths, and both are slightly lower than PP homopolymer. This suggests that the type of impact test employed is playing an important role in the impact fracture behaviour of the PP/PE blends. The effect of temperature on the impact fracture behaviour for the PP/PE blends was evaluated by means of the Charpy and drop weight plate impact tests. During the tests, impact strengths reduced with decreasing testing temperature. In the Charpy impact test, the impact strength for PP homopolymer was higher than those of the two PP/PE blends at both 20 and 0°C. The Etotal measured from the IDWPIT indicated PP/LDPE has the highest impact strength at both 20 and 0°C.

Fatigue Crack Growth Properties and Scanning Electron Microscopy of Fatigue Fracture Surface of a Ti-Ni-Co Shape Memory Alloy

Mechanical fatigue crack growth characteristics of a Ti-49.7at.%Ni-1.38at.%Co alloy were investigated as a function of heat-treatment and test temperature. The fatigue crack growth characteristics of the alloy were examined as to center-cracked-tension type specimens by using a computer controlled servo-hydraulic type machine For the 623 K aged alloy ( M s = 297 K ), the crack growth resistance decreases with decreasing test temperature, and that at 320±1 K shows the best performance than those at lower test temperatures The best performance for the 623 K aged alloy was comparable to the performance of 723 K aged alloy ( M s = 272 K ) tested at 295±1 K. Fatigue fracture surfaces of the aged alloys were examined by using a scanning electron microscope with a field emission electron gun, and striation patterns were observed on them. The micro crack growth rate estimated from the striation patterns was in good agreement with the crack growth rate obtained from the examination of center-cracked-tension type specimens.

Mechanical Behavior of Carbon Steels in the Temperature Range of Mushy Zone.

Tensile strength and ductility of carbon steels have been measured in the temperature range of mushy zone by the in-situ melting tensile test technique with Gleeble system. The specimen was melted and cooled to the test temperature before the tensile deformation in order to get the mechanical properties subject to the continuous casting process. During hot tensile test, a ceramic fiber tube was used to reduce the radial temperature gradient in the heated specimen. Tensile strength of carbon steels in the temperature range of mushy zone increased with decreasing test temperature, and was well described by the modified yield criterion for porous metals. The measured zero strength temperature (ZST) and zero ductility temperature (ZDT) were related to the solid fractions evaluated by the numerical simulation of microsegregation developed by Ueshima et al. The characteristic solid fractions of ZST and ZDT which corresponded to 0.75 and 0.99, respectively, were well described by the prediction equation on ZST and ZDT at given steel compositions and cooling rates.

Influence of temperature, strain rate and specimen geometry on the microscopic cleavage fracture stress

The aim of this work is to determine the influence of temperature, strain rate and specimen geometry on the microscopic cleavage fracture stress σ f*. Besides, the dependence of the initiation temperature for shear fracture T i and the temperature for general yield T gy on strain rate is investigated. Finally, the local values of stress triaxiality and equivalent plastic strain at the occurrence of cleavage fracture for several steels and specimen types are compared with the failure curve for ductile fracture to check the validity of the theory of stress controlled cleavage fracture and the strain/triaxiality controlled shear fracture in the transition region. Based on experimental tests, the results are obtained by finite element analyses. The investigation shows, that σ f* is dependent on temperature and strain rate and increases with decreasing test temperature and increasing strain rate. The transition temperatures T i and T gy increase with increasing strain rate. The theories of stress controlled cleavage fracture and of shear fracture controlled by equivalent plastic strain and stress triaxiality seem to be valid. That fracture mechanism occurs for which the critical condition is reached first.

Structural and magnetic properties of Fe clusters embedded in Ag matrix

Evaporation–gas–aggregation (EGA) co-deposition for the preparation of thin films of Fe clusters embedded in a silver matrix is developed in this paper. Transmission electron microscope and electron diffraction investigations and X-ray energy spectrum show that Fe clusters are well encapsulated in silver coatings and are in the form of body centred cubic Fe and face centred cubic Ag in as-prepared samples. Vibrating sample magnetometer measurements reveal that samples show superparamagnetic behaviour and saturation magnetization increases with decreasing test temperature. At lower temperature, the coercivity is much higher than that of bulk α-Fe, and it decreases with increasing test temperature. © 1998 Kluwer Academic Publishers

Cryogenic fracture behaviors and polarization characteristics according to sensitizing heat treatment on structural material of the nuclear fusion reactor

The cryogenic fracture behaviors of austenitic stainless steel HN2 developed for nuclear fusion reactor were evaluated quantitatively by using the small punch(SP) test. The electrochemical polarization test was applied to study thermal aging degradation of HN2 steel. The X-ray diffraction(XRD) analysis was conducted to detect carbides and nitrides precipitated on the grain boundary of the heat treated HN2 steel. The mechanical properties of the HN2 steel significantly decreased with increasing time and temperature of heat treatment or with decreasing testing temperature. The integrated charge(Q) obtained from electrochemical polarization test showed a good correlation with the SP energy(ESP) obtained by means of SP tests. From the results observed in the x-ray diffraction and anodic polarization curve, it was known that the material the grain boundary. Combining SP test and electrochemical polarization test, it could be useful tools to non-destructively evaluate the cryogenic fracture behaviors and the aging degradation for cryogenic structural material.

Strength and toughness properties of two nitrogen alloyed stainless steels and their submerged arc weldments at cryogenic temperatures

Submerged arc weldments of the two nitrogen-alloyed stainless steels X2 CrNiN 18 10 (similar to AISI 304 LN) and X4 CrNiMoN 18 14 (similar to AISI 316 LN) welded with the fully austenitic filler metal X2 CrNiMnMoN 20 16 were investigated. Tensile, impact toughness and single-specimen J-integral tests at room temperature, 77 K and 4 K were performed. The strength of the materials increased whereas the impact toughness and the fracture toughness decreased with decreasing temperature. The toughness of the steel X4 CrNiMoN 18 14 was superior to that of X2 CrNiN 18 10, and for the austenitic weld a good combination of strength and toughness was also found. On the fracture surface of the compact tension specimens, a stretch zone was found, the width of which was reduced with decreasing test temperature. For all three materials at all three temperatures the critical values, JIc, of the J integral determined according to ASTM E 813 are approximately twice the respective values for the J integral at physical crack initiation determined according to the German specification DVM 002 using the width of the stretch zone. © 1998 Chapman & Hall

Edge cracking in hot-rolled coils of semi-killed steels

Various processing parameters leading to edge cracking in hot-rolled coils are described, along with a study of the effect of these parameters on the occurrence of edge cracking during commercial production of semi-killed hot-rolled coils. High-temperature tensile testing revealed that ductility decreased with a decrease in test temperature from 850 to 750 °C and improved with a further drop in temperature. This phenomenon is attributed to the closed-packed structure of single-phase austenite at higher temperatures, which transforms to ferrite + austenite at the intermediate temperature range. Edge cracking was frequently observed in hot-rolled coils that were finish rolled within this intermediate temperature range. Optical microscopy revealed large ferrite grains near edges of the coil as well as at inner surfaces of cracks caused by decarburization at higher temperatures. A minimum manganese/sulfur ratio of 12 to 8 for a sulfur level of 0.03 to 0.04%, proper teeming practice, adequate soaking, and a finish rolling temperature above 875 °C resulted in overall improvement in the edge quality of hot-rolled coils.

Evaluation of cryogenic fracture toughness in SMA-welded 9% Ni steels through modified CTOD test

As the first step of the study for the safety performance of LNG storage tank based on the concept of fitness-for-purpose, the change of cryogenic toughness within the X-grooved weld HAZ (heat-affected zone) of SMA (shielded metal arc)-welded QLT (quenching, lamellarizing, and tempering)-processed 9% Ni steels, was investigated qualitatively and quantitatively. In general, CTOD (crack tip opening displacement) test is widely used to determine the fracture toughness of steel weldments. But there is no standard or draft for evaluating the toughness of thick weldment with X-groove such as in this case. Therefore, in this study, modified CTOD testing method for fatigue precracking. calculation of CTOD, examination of fractured specimen was proposed and used. And the results of modified test were compared with those of conventional CTOD test and Charpy V-notch impact test. In addition, the relationship between the fracture toughness and microstructure was analyzed by OM, SEM and XRD. The cryogenic toughness in HAZ decreased as the evaluated region approached the fusion line from base metal. The decrease in toughness was apparently caused by the reduction of the retained austenite content and the absence of grain refinement effect in the coarse-grained zone in HAZ. The austenite reduction resulted from the decrease in nucleation sites for α’γ reverse transformation due to the increase in fraction of coarse-grained zone within HAZ. More complex thermal cycles in the mixed zone of weld metal and base metal caused the poor stability of retained austenite in the zone by the redistribution of alloying element in retained austenite. Due to this reason, the toughness drop with decreasing test temperature in F.L. (fusion line)-F.L.+3 mm was larger than that in F.L.+5 mm and F.L.+7 mm.

Microstructure of a sintered 16.5 mol % CeO2-ZrO2 alloy at cryogenic temperature

The microstructure of a fine grained 16.5 mol% CeO2-stabilized tetragonal zirconia polycrystal (Ce-TZP) has been investigated by transmission electron microscopy observations. The results show that the samples fractured at 298, 77 and 4.2 K change significantly. At 298 K, there is no stress-induced martensite but a few anti-phase boundaries (APB) in the tetragonal (t) parent phase. With a decrease in testing temperature, monoclinic (m) product, or martensite within retained t phase, appears. Its morphologies are characterized as lenticular and block-like at 77 K and as lenticular and butterfly-like at 4.2 K. The relationship between microstructure and mechanical properties is also discussed.

Loading rate and test temperature effects on fracture ofIn Situ niobium silicide-niobium composites

Arc cast, extruded, and heat-treatedin situ composites of niobium suicide (Nb5Si3) intermetallic with niobium phases (primary—Nbp and secondary—Nbs) exhibited high fracture resistance in comparison to monolithic Nb5Si3. In toughness tests conducted at 298 K and slow applied loading rates, the fracture process proceeded by the microcracking of the Nb5Si3 and plastic deformation of the Nbp and Nbs phases, producing resistance-curve behavior and toughnesses of 28 MPa√m with damage zone lengths less than 500μm. The effects of changes in the Nbp yield strength and fracture behavior on the measured toughnesses were investigated by varying the loading rates during fracture tests at both 77 and 298 K. Quantitative fractography was utilized to completely characterize each fracture surface created at 298 K in order to determine the type of fracture mode (i.e., dimpled, cleavage) exhibited by the Nbp. Specimens tested at either higher loading rates or lower test temperatures consistently exhibited a greater amount of cleavage fracture in the Nbp, while the Nbs, always remained ductile. However, the fracture toughness values determined from experiments spanning six orders of magnitude in loading rate at 298 and 77 K exhibited little variation, even under conditions when the majority of Nbp phases failed by cleavage at 77 K. The changes in fracture mode with increasing loading rate and/or decreasing test temperature and their effects on fracture toughness are rationalized by comparison to existing theoretical models.

Fatigue crack propagation in Al-Li 8090 alloy at room (300K) and cryogenic (77K) temperatures

It has been reported that Al-Li alloys display enhanced ductility and fracture toughness at lower temperatures than room temperature. Therefore, Al-Li alloys have been considered as an attractive material for cryogenic fuel tanks as well as for aircraft structure parts. While many investigations have been carried out on the tensile and fracture behavior of Al-Li alloys at cryogenic temperatures, cryogenic fatigue properties have been investigated only recently. Previous study indicates that the fatigue strength increases as the test temperature decreases. The improved fatigue strength at cryogenic temperatures has been attributed to the increased tendency for more homogeneous plastic deformation and delamination toughening. The objectives of the present work are to examine the fatigue crack propagation behavior of an Al-Li 8090 alloy at room (298 K) and cryogenic (77 K) temperatures and also to study the microstructural effect on the cryogenic fatigue properties.

Fatigue and fracture behavior of aluminum-lithium alloys at ambient and cryogenic temperatures

The fatigue strength of Al-Li-Cu-Mg-Zr alloys (T6) increases as the test temperature decreases, especially in the long life regime. The fatigue strength is much improved at both room and low temperatures when more zirconium is added. However, the alloy with higher Zr exhibits less-temperatures dependence of the fatigue strength, particularly in the short life regime. Change in fatigue behavior between room and low temperatures are associated with changes is the fracture mode which can be affected significantly by the addition of zirconium.

The influence of temperature on microstructural damage during uniaxial compression of aluminium matrix composites

The Al{sub 2}O{sub 3} and SiC reinforced aluminum composites exhibited different mechanisms of damage in response to compressive deformation. The F3S matrix SiC reinforced composite exhibited particle fracture, the amount of fracture increasing with decreasing test temperature due to the increase in local stress within the particles. However, at low temperatures both the damage and deformation become localized in the form of intense shear bands. In 2618 matrix Al{sub 2}O{sub 3} reinforced materials the as-received material contained small voids at the interfaces of the majority of the particles which through their growth prevented particle fracture due to the relaxation of the stress build up within the particles. At the high temperatures which would be employed in forging, neither material showed appreciable damage. However, the higher strain rates used in industrial forging practice could potentially cause damage in both composite materials.

The small punch toughness test: some detailed fractographic information

The present paper describes a detailed fractographic survey on a series of small punch tests conducted on a CrMoV bolting steel over the temperature range −200°C-ambient. Two specific material conditions were investigated, viz. fully and partially embrittled bolts which experience toughness degradation by reverse temper embrittlement (RTE) during service at elevated temperatures approaching 490°C. The following pertinent fractographic observations were established: (1) in the upper shelf regime, ductile failure by microvoid coalescence (MVC) occurred over the full through-thickness test section; (2) in the transition region an initial, limited amount of slow ductile MVC growth occurred before final failure ensued; and (3) at the lower shelf brittle region, wholly brittle fracture, which was predominantly transgranular cleavage containing some isolated facets of intergranular failure, occurred. The initial slow ductile MVC growth was semi-elliptical in nature and the extent decreased with decreasing test temperature. Finally it was shown that the level of biaxial fracture strain, q f , exhibited a smooth continuous transition curve over the temperature range ambient-−200°C; also the level of fracture strain explained the change in the amount of initial ductile MVC growth through the transition region.