Hold Time Conditions Research Articles

Effect of Steam Explosion on the Structure, Chemical Composition and Pyrolytic Volatile Composition of Tobacco Waste

Tobacco stems are an important part of the tobacco leaf, containing a lot of cellulose, result in the low utilization of tobacco leaves in cigarettes. The pre-treatment technology of tobacco stems is a key to increase the utilization of tobacco stems. In this work, steam explosion treatment was carried out on tobacco stems with different moisture contents containing 2%, 5% and 10%, respectively. The steam explosion is carried out under different pressure and pressure holding time conditions, the changes in physical structure, chemical composition and pyrolysis volatiles of tobacco stem was investigated. The results indicate that steam explosion has a negligible impact on the crystalline structure of cellulose and low-order polysaccharide crystals, while it has an obvious effect on KCl. Steam explosion increases the specific surface area by destroying the amorphous structure on tobacco stems. The contents of chemical constituents and pyrolytic volatiles indicate that there is a significant leaching effect of lignin, but no change in cellulose content. Soluble polysaccharides, the main component of hemicellulose, were converted into pyrolytic volatiles e.g., furfural and furfuryl alcohol. In conclusion, the technology of steam explosion promotes the leaching, decomposition and chemical transformation of amorphous structures on tobacco stems, thus destroying the structure of the cell walls.

Experimental Study on Nanomechanical Properties of Yunnan Red-Bed Mudstone

As a natural building material and building environment, red-bed mudstone has special engineering properties due to its special formation conditions. The engineering disaster caused by these special properties is becoming increasingly prominent. Therefore, it is of great theoretical significance and practical engineering value to study the macro- and micromechanical properties of red-bed mudstone. Nanoindentation mechanical tests are carried out on the red-bed mudstone from Chuxiong, Jingdong, and Nanhua in Yunnan. The microstructure, residual indentation morphology, and nanomechanical parameters of red-bed mudstone in different areas are compared and analyzed. The results show that the load-displacement curves can be divided into four stages, which are the compaction stage, linear elastic stage, holding stage, and unloading stage. Under the same loading and holding time conditions, sample 3 (Nanhua) has the deepest residual indentation, and SEM tests show that the damage to indentation is obvious. Sample 2 (Jingdong) has the shallowest residual indentation. The indentation morphology is complete and the size is small by SEM tests. The residual indentation depth of sample 1 (Chuxiong) is between sample 2 and sample 3. The average elastic modulus of sample 1 (Chuxiong) is 38.0 ± 2.29 GPa. The average hardness is 1.36 ± 0.42 GPa, and the fracture toughness is 0.966 ± 0.048 MPa·m0.5. The average elastic modulus of sample 2 (Jingdong) is 40.1 ± 2.03 GPa. The average hardness is 1.71 ± 0.35 GPa, and the fracture toughness is 1.142 ± 0.053 MPa·m0.5. However, the average elastic modulus of sample 3 (Nanhua) is 25.1 ± 2.53 GPa. The average hardness is 0.61 ± 0.21 GPa, and the fracture toughness is 0.507 ± 0.043 MPa·m0.5. Overall, the elastic modulus and the hardness of sample 2 (Jingdong) are the greatest because of its high quartz crystal content. It shows that the nanomechanical properties of red-bed mudstone in Jingdong are better than those in Chuxiong and Nanhua. The results can provide a basis for revealing the regional differences in macroscopic mechanical properties of Yunnan red-bed mudstone.

Development of a boron-free filler metal for brazing nickel-based superalloys utilizing the Cu–Mn–Ni system

A new material for boron-free brazing of nickel-based superalloys is described that utilizes a transient liquid-phase bonding (TLPB) mechanism. Commercially available AMS 4764 powder (52.8Cu–37.5Mn–9.7Ni) was mixed with pure Ni powder to increase Ni content in the filler metal. This alloy is a good candidate for TLPB in Ni superalloy applications because of its high strength, low liquidus temperature, low cost and the absence of brittle intermetallic phases. Analysis of in situ braze joint formation with IN625 base metal using differential scanning calorimetry measurements allowed for optimization of Ni content, braze temperature and hold time conditions of the brazing process. Calculated ternary phase diagrams coupled with EDS analysis of braze joints identified a low-temperature copper-rich segregated region in braze joints, which, through interdiffusion with the base metal, increases melting temperature with increasing hold time. This work concludes that a 50-µm half braze joint can be formed through full isothermal solidification with an 80-min hold at 1050 °C. Solidification occurs primarily through interdiffusion of base metal and filler metal.

Investigating crack propagation behavior and damage evolution in G115 steel under combined steady and cyclic loads

The creep-fatigue crack growth (CFCG) behavior and damage evolution of G115 steel were investigated based on modified multiaxial damage constitutive model. Multiaxial stress was taken into consideration in the fatigue damage model, and the simulated and experimental results were in close agreement, which verify the validity of the creep-fatigue model. Simulations of tests with different hold times ranging from 60 s to 7200 s were conducted and the parameters ΔK and Ctavg were utilized to characterize the crack propagation data. Results revealed that both frequency ranges of creep-fatigue interaction, and the frequency for the most significant interaction, shifted to higher frequencies as ΔK was increased. Meanwhile, for longer hold times or in higher Ctavg regions, variations of crack growth rate, (da/dt)avg became limited and CFCG data tended to pure creep crack propagation data, indicating domination of time-dependent creep. Additionally, the creep and fatigue damage at crack tip were extracted to investigate the damage evolution during crack growth process. It was verified further that the CFCG of G115 steel studied was a result of the effect of dominant creep damage, low-order fatigue, and creep-fatigue interactions, especially for longer hold time conditions.

Creep-fatigue crack growth behavior of G115 steel under different hold time conditions

The creep-fatigue crack growth behavior and fracture mechanisms of G115 steel at 650 °C were studied using compact tension specimens in a constant load control mode. The crack growth length and load-line displacement analyses reveal that G115 steel is a creep-ductile material and the crack growth life and initiation time increase with increasing dwell time. Meanwhile, the parameter ΔK was used to characterize the crack growth behavior and the effect of dwell time on da/dN depends on ΔK. These data were also compared with those getting from P91 steel under 625 °C and P92 steel at 630 °C. When the crack growth rate, da/dtavg, is represented by Ctavg, all the data seem to collapse within a single narrow scatter band. Further, the approximate Nikbin-Smith-Webster (NSW) crack growth model was utilized to predict the creep-fatigue crack propagation behavior of G115 steel. And the microstructures of the cross-sections along the crack growth direction were studied through scanning electron microscopy to reveal the crack growth mechanisms of G115 steel.

Separation of iron-nickel alloy nugget from limonitic laterite ore using self-reduction

To efficiently extract nickel from low grade limonitic laterite ore, a separation method of iron-nickel alloy nugget by selfreduction of coal composite limonitic laterite ore briquette was investigated. In this investigation, in order to decrease the separation temperature of iron-nickel alloy nugget, the selective reduction by control of C/O ratio was introduced and reductant added in the briquette was inadequate for the reduction of nickel and iron. Nickel was preferentially reduced in the reduction process, while iron was partially reduced due to the lack of reductant. After reduction, a certain amount of FeO existed in the reduced product. This residual FeO had a great role in the formation of low melting point slag, which could promote the formation and the separation of iron-nickel alloy nugget at relatively low temperature. In this investigation, the reduction experiments were all conducted at 1300?C. To evaluate the formation and the separation of iron-nickel alloy nugget in the reduction process, we observed the patterns of reduced products under different C/O ratio, CaO addition ratio and holding time conditions. And then the effect of C/O ratio and CaO addition ratio on nickel content of nugget and nickel recovery ratio were investigated. The results showed that 0.7 of C/O ratio, 8% of CaO addition ratio and 40min of holding time were suitable for the separation of iron-nickel alloy nugget from limonitic laterite ore. Nickel and iron content of the nugget and the nickel recovery ratio were 4.75%, 89.51% and 85%, respectively. Nuggets were easily separated from slag by crushing and screening. This separation method could be applied to any limonitic laterite ore by adjusting C/O ratio and CaO addition ratio.

Relation between crack growth behaviour and crack front morphology under hold-time conditions in DA Inconel 718

The crack growth behaviour of Direct Aged Inconel 718 was studied at 550°C. Experiments were carried out under pure fatigue cycles, hold-time cycles of different durations and a mix of both. Hold-time cycles were systematically associated with complex crack front morphologies. A new numerical approach was developed to assess the effect of crack front morphology on the direct current potential drop technique, mechanical fields at the crack tip and ultimately, measured crack growth rates. Using this approach, a clear relation was established between crack front morphology and its evolution, and the crack growth behaviour under hold-time conditions. Complex crack front morphologies are demonstrated to be responsible for increased crack growth rates. From this, a crack growth mechanism under hold-time conditions is proposed. Finally, the numerical framework here presented is to be considered as a new, easily reproducible, way to properly analyse experimental data when dealing with complex loading cycles and complex crack front morphologies.

Crack growth threshold under hold time conditions in DA Inconel 718 – A transition in the crack growth mechanism

Aeroengine manufacturers have to demonstrate that critical components such as turbine disks, made of DA Inconel 718, meet the certification requirements in term of fatigue crack growth. In order to be more representative of the in service loading conditions, crack growth under hold time conditions is studied. Modelling crack growth under these conditions is challenging due to the combined effect of fatigue, creep and environment. Under these conditions, established models are often conservative but the degree of conservatism can be reduced by introducing the crack growth threshold in models. Here, the emphasis is laid on the characterization of crack growth rates in the low ?K regime under hold time conditions and in particular, on the involved crack growth mechanism. Crack growth tests were carried out at high temperature (550 °C to 650 °C) under hold time conditions (up to 1200 s) in the low ?K regime using a K-decreasing procedure. Scanning electron microscopy was used to identify the fracture mode involved in the low ?K regime. EBSD analyses and BSE imaging were also carried out along the crack path for a more accurate identification of the fracture mode. A transition from intergranular to transgranular fracture was evidenced in the low ?K regime and slip bands have also been observed at the tip of an arrested crack at low ?K. Transgranular fracture and slip bands are usually observed under pure fatigue loading conditions. At low ?K, hold time cycles are believed to act as equivalent pure fatigue cycles. This change in the crack growth mechanism under hold time conditions at low ?K is discussed regarding results related to intergranular crack tip oxidation and its effect on the crack growth behaviour of Inconel 718 alloy. A concept based on an “effective oxygen partial pressure” at the crack tip is proposed to explain the transition from transgranular to intergranular fracture in the low ?K regime.

Modelling of Fatigue Crack Growth in Inconel 718 under Hold Time Conditions - Application to a Flight Spectrum

Gas turbine operating cycles at high temperatures often consist of load reversals mixed with hold times; the latter occurring either as cruise for aero engines or at continuous power output for land based turbines, but also at low frequency loading conditions, e.g. slow “ramp up” of engine thrust. The hold time conditions cause the crack to grow by intergranular fracture due to material damage near the crack tip, thus rapidly increasing the crack growth rate. Since the damaged zone will affect the crack propagation rate due to cyclic loadings as well, the complete load history of a component therefore has to be considered. The crack propagation model presented in this paper is based on the damaged zone concept, and considers the history effect in the form of damaged zone build up during hold times, and subsequent destruction as the crack propagates onwards by rapidly applied load reversals. By incorporating crack closure for handling different R-values, an aero engine component spectrum is evaluated for a surface crack at 550 °C. The result shows a good correlation to model simulation, despite the complexity of the load spectrum.

A load history dependent model for fatigue crack propagation in Inconel 718 under hold time conditions

Modelling of high temperature fatigue crack growth in Inconel 718 under the interaction of fast cyclic loading and hold times at maximum load has been conducted. A model, based on the concept of a damaged zone in front of the crack tip has been applied for three different temperatures, 550, 600 and 650°C, with good agreement for both calibration and validation tests. A statistical evaluation of 22 tests in total was also conducted, which shows that the developed model gives a reasonable scatter factor at a probability of failure of 0.1%.

Modelling of high temperature fatigue crack growth in Inconel 718 under hold time conditions

Inconel 718 is a frequently used material for gas turbine applications at temperatures up to 650°C. The main load cycle for such components is typically defined by the start-up and shut-down of the engine. It generally includes hold times at high temperatures, which have been found to have a potential for greatly increasing the fatigue crack growth rate with respect to the number of load cycles. However, these effects may be totally or partly cancelled by other load features, such as overloads or blocks of continuous cyclic loading, and the actual crack propagation rate will therefore depend on the totality of features encompassed by the load cycle. It has previously been shown that the increased crack growth rate found in hold time experiments can be associated with a damage evolution, where the latter is not only responsible for the rapid intergranular crack propagation during the actual hold times, but also for the increased crack growth during the load reversals. In this paper, modelling of the hold time fatigue crack growth behaviour of Inconel 718 has been carried out, using the concept of a damaged zone as the basis for the treatment. With this conceptually simple and partly novel approach, it is shown that good agreement with experimental results can be found.

High temperature fatigue crack growth behaviour of Inconel 718 under hold time and overload conditions

Inconel 718 is a frequently used material for gas turbine applications at temperatures up to 650°C. For such components, the main load cycle is typically defined by the start-up and shut-down of the engine. In this main loading cycle, hold times at high temperature are commonly present in critical components. These high temperature hold times may greatly increase the fatigue crack growth rate with respect to the number of cycles unless other beneficial factors such as for example initial overloads are present. The latter can be caused by abnormal service conditions but can also occur on a more regular basis and are then typically observed in components with strong thermal transients during engine start-up. In this paper, focus has been placed on the effect of overloads on the hold time fatigue crack growth behaviour and its subsequent description. More specifically, crack propagation in Inconel 718 has been studied at the temperatures 550°C and 650°C with and without an overload at the start of the cycle. The effect of initial overloads was found to be substantial. A simple model for describing the effect of these loading conditions has also been developed based on the concept of the damaged zone, present around the crack tip. Irregular crack fronts and unbroken ligaments left on the fracture surfaces seen in complementary microscopy studies seem to support this approach. Furthermore, the stress state in front of a crack tip in a 2D model was investigated both with and without an initial overload. The results were related to the observed crack growth retardation behaviour found in the material testing.

PS15 環状切欠試験片を用いたひずみ制御高温クリープ・疲労重量条件下におけるP92鋼のき裂成長寿命の繰返し速度特性(フェロー賞対象ポスターセッション)

W strengthened 9Cr ferritic heat resistant steel has been applied to boiler materials for the most efficient electric power plant technology and it will be used under the conditions of high temperature creep-fatigue multiplication. In this study, mechanical tests under the conditions of high temperature creep-fatigue multiplication were conducted for W strengthened 9%Cr ferritic heat resistant steels (ASME-Code P92) with various stress holding time conditions. Based on these results, the characteristics of load frequency of crack growth life were investigated by analyzing cycle-dependent and time-dependent mechanisms on the characteristic of load frequency of crack growth life.

Joining of Aluminum Foam/Aluminum Metal by Spark Plasma Sintering Process

Aluminum foam is lightweight structures, energy absorption and thermal management. In this reason, there is a lot of attention on the aluminum foam as a structural material. The present papers showed various conventional joining techniques can be applied for foam-sheet structures, i.e., riveting, screwing, welding, gluing and soldering. This research presents new joining technique of Aluminum foam/Aluminum metal using spark plasma sintering (SPS) process. The Aluminum foam/Aluminum metals were fabricated by changing of various SPS holding temperature and holding time conditions. With increasing holding temperature and holding time, the tensile stress increased. The specimen sintered at 550°C for 20 min shows σts = 1.47 MPa. Also, it was found that the SPS holding time is dominant factor than the holding temperature for sound joining of two joint materials.

Predicting safety and quality parameters for UHT‐processed milks

A spreadsheet was developed to evaluate safety and quality parameters for milks subjected to different heating and cooling profiles and holding time conditions in a continuous heat exchanger. Safety parameters evaluated were lethality ( Fo) and microbial inactivation ( B*) values; quality parameters were chemical change ( C*), thiamin loss, lactulose formation, Maillard browning and hydroxymethylfurfural (HMF) formation. The spreadsheet was used to predict these parameters for a pilot plant, heating milks at 120–150°C for 4, 27 and 58 s, using its heating and cooling profiles. Milks were processed in the same plant at these conditions and a selection of the milks were analysed to establish the validity of the spreadsheet. All samples with a C* value < 3 gave a positive Aschaffenburg turbidity. The activation energy that gave the best agreement between the predicted and experimental results for lactulose was found to be 122 kJ/mol. One set of optimized conditions for this product was 135°C for 10 s, giving B* = 1.24 and C* = 0.55. This milk was compared with two others and its sensory characteristics were similar to that with a C* of 1.5 but different to that with a C* value of 6. Browning was only found to be noticeable in the most severely heat‐treated sample immediately after processing, but would become apparent on storage at 30°C and above. The spreadsheet is useful for comparing the characteristics of products from plants with different heating profiles or as a means for optimizing UHT plant performance where the heating and cooling rates are fixed.

Effect of hold-time on low cycle fatigue behaviour of nitrogen bearing 316L stainless steel

Axial strain controlled low cycle fatigue (LCF) behaviour and creep–fatigue interaction behaviour of 316L(N) stainless steel (SS) in solution annealed condition has been investigated at various strain amplitudes. Creep–fatigue interaction behaviour of the material has been evaluated by employing tension and compression holds at peak strain. Tension holds up to 90 min were studied. Under all the testing conditions, the material showed initial hardening followed by stress saturation. Hold-time conditions generally showed lower cyclic stress response compared to continuous cycling. The decrease in cyclic stress response with hold-time is attributed to enhanced recovery of the substructure and increase in the grain boundary damage accumulated during the stress relaxation period. The fatigue life is observed to be lower in tensile-hold conditions and the endurance decreased with increase in the duration of the hold. The factors that contribute to the decrease in fatigue life with hold-time have been identified from metallographic studies as the development of creep cracks and cavities and crack initiation and propagation assisted by oxidation. Creep–fatigue damage values based on the damage summation rule have been computed and compared with the RCC-MR bi-linear creep–fatigue interaction diagram suggested for nitrogen alloyed 316L SS at 600°C.

Thermal fatigue on MANET II

Out-of-phase thermal fatigue experiments in air are performed in a test facility, which has been designed to allow temperature cycling of the sample by ohmic heating. The test specimens are hollow, hourglass-shaped and cylindrical samples of a ferritic-martensitic stainless steel, MANET II, in the tempered condition. The effects of different thermal cycle ranges and the imposition of several hold time conditions on the mechanical behaviour are investigated. A continuous softening is the characteristic feature observed in all thermal fatigue tests. Larger temperature changes and the application of temperature hold periods produce an accelerated softening process, leading to smaller numbers of cycles to failure. Compared to a prior heat MANET I, the thermal fatigue endurance of MANET II is found to be in the same scatterband.

一方向凝固鋳造超合金Ｒｅｎｅ′８０Ｈの全ひずみエネルギに基づく疲労・クリープ相互作用下の寿命評価

This paper examines the strain controlled fatigue-creep interaction (FCI) life of directionally solidified (DS) superalloy Rene 80H.The author previously proposed a frequency-modified total strain energy parameter, obtained from the total tensile strain energy and tensile loading time, for use in evaluating the failure life of equiaxial Ni and Co based superalloys under FCI conditions. The present paper examines the applicability of this parameter for predicting the FCI life of DS Rene 80H.The FCI life prediction by the total strain range was first evaluated. It was found that the FCI life of the specimens cut in parallel to the DS direction of a directionally solidified plate was longer than that of the specimens cut in transverse to the DS direction. In addition, the specimen life under tensile hold time conditions was longer than under compressive hold time conditions.The FCI life prediction by the frequency-modified total strain energy parameter was evaluated next. For the specimens in both directions, there is a linear correlation between this parameter and the cycles to failure. Therefore, it is concluded that the FCI life prediction for DS Rene 80H can be made by using this parameter.

Fatigue crack propagation in type 316 stainless steel in vacuum and air environments

The fatigue crack propagation performance of unirradiated type 316 stainless steel was investigated in vacuum for comparison with results in an air environment under continuous cycling and hold time conditions. The materials included both fast breeder reactor (FBR) and magnetic fusion energy (MFE) reference type 316 steels. The continuous cycling results showed that the crack propagation rates of annealed FBR and MFE materials were essentially identical in a vacuum of 6.7 × 10 −6 Pa at 550° C. However, a significant reduction in crack propagation rates was observed for the FBR material in vacuum at 593°C when compared with the results in the air environment at 593°C. For 20 percent cold worked steel, the results showed that the crack propagation rates for the MFE material at 550°C in vacuum were lower than for the annealed MFE material at this temperature. These results were consistent with data for 20 percent cold worked FBR material at 593° C which showed that cold work reduced the rates of fatigue crack propagation when compared with annealed material. Hold time effects for both steels were significantly larger for evaluations conducted in vacuum or in air after irradiation as compared with previous unirradiated air results. Scanning electron microscope examination of the specimens tested in vacuum showed that the mode of crack propagation was entirely ductile and transgranular in character for continuous cycling tests and typically intergranular for hold time evaluations. The vacuum results obtained in this study were compared with the data for type 316 stainless steel tested in air for both the irradiated and the unirradiated conditions. The relationships developed from these comparisons suggest that the existing data will be of value to fusion design when consideration is given to the effects of environment and irradiation.

Elevated-temperature low-cycle fatigue behavior of different heats of type 304 stainless steel

The low-cycle fatigue results of three heats of Type 304 stainless steel have been ob-tained at 593°C under selected cyclic-loading conditions. The results are compared with those generated for a reference heat of steel for which extensive low-cycle fatigue data are available. Observation of the microstructures of specimens in the pretest condition after a given heat treatment and examination of fatigue fracture surfaces were con-ducted by means of optical and scanning electron microscopy and X-ray analysis. The three heats of stainless steel, which exhibit different microstructural features, show approximately the same continuous-cycling low-cycle fatigue behavior as that of the re-ference heat. However, the three materials show improved fatigue strength during tensile hold-time conditions where significant creep occurs. The fatigue properties determined in the present study for the different heats of steel are consistent with the observed mi-crostructural features. Finally, the creep-fatigue properties of the heats as well as the microstructural observations are discussed in terms of a damage-rate approach re-cently developed by the authors.