Strength and Fracture of SPKh18N9T Steel and SP36KNM Alloy under Dynamic Loading

We present results of an experimental investigation of the strength properties of SPKh18N9T steel and SP36KNM alloy and the characteristics of fracture processes in these materials under dynamic loading. It is shown that, under conditions of uniaxial static tension, the standard mechanical characteristics of these materials are very close to each other. Moreover, as the strain rate increases to 10³ sec⁻¹, they undergo slight changes (thus, the yield limit and the ultimate strength increase by at most 13% and 6%, respectively, and the relative elongation at rupture decreases by 5%). At the same time, under impact loading, the impact strength of SPKh18N9T steel is twice that exhibited by SP36KNM alloy. This fact can be partially explained by the difference in the initial porosities of these materials. Translated from Problemy Prochnosti, No. 6, pp. 122–125, November–December, 1997.     

A Method for the Evaluation of Fatigue Life under Nonproportional Low-Cycle Loading

We consider a method for the evaluation of fatigue life under multiaxial nonproportional low-cycle loading based on the concept of equivalent strains. The expression for the equivalent strain range is a function only of the strain path and contains a constant depending on the additional hardening of the material under nonproportional loading. We propose a new parameter of the material based on the work of plastic strains in a cycle. This parameter is universal when applied to materials with both low and high degrees of additional hardening. It is in good agreement with the results of testing of 08Kh18N10T stainless steel and VT9 titanium alloy under nonproportional low-cycle loading.     

钛合金BT9和08X18H10了不锈钢多轴低周疲劳损伤的微观机理

研究了钛合金BT9和08X18H107不锈钢在比例和非比例载荷下的低周疲劳特性，非比例载荷下钛合金BT9附加强化程度很小，08X18H10T不锈钢则产生了明显的附加强化，而二者的疲劳寿命均降低明显。采用透射电镜（TEM）对钛合金BT9及08X18H107不锈钢的疲劳位错亚结构进行了对比分析，结果表明：非比例载荷下，钛合金BT9中局部高密度位错，以及08X18H107不锈钢面塑性变形方式从平面状滑移向波纹状滑移转化，是其低周疲劳损伤程度加剧及寿命降低的主要原因。     

钛合金BT9和08X18H10T不锈钢多轴低周疲劳损伤的微观机理

研究了钛合金BT9和08X18H10T不锈钢在比例和非比例载荷下的低周疲劳特性,非比例载荷下钛合金BT9附加强化程度很小,08X18H10T不锈钢则产生了明显的附加强化,而二者的疲劳寿命均降低明显。采用透射电镜(TEM)对钛合金BT9及08X18H10T不锈钢的疲劳位错亚结构进行了对比分析,结果表明:非比例载荷下,钛合金BT9中局部高密度位错,以及08X18H10T不锈钢面塑性变形方式从平面状滑移向波纹状滑移转化,是其低周疲劳损伤程度加剧及寿命降低的主要原因。     

Failure of AA 6061 and 2099 aluminum alloys under dynamic shock loading

Microstructural aspects of the deformation and failure of AA 6061 and AA 2099 aluminum alloys under dynamic impact loading are investigated and compared with their responses to quasi-static mechanical loading in compression. Cylindrical specimens of the alloys, heat-treated to T4, T6 and T8 tempers, were subjected to dynamic compressive loading at strain rates of between 2800 and 9200 s⁻¹ and quasi-static compressive loading at a strain rate of 0.0032 s⁻¹. Plastic deformation under the dynamic impact loading is dominated by thermal softening leading to formation of adiabatic shear bands. Both deformed and transformed shear bands were observed in the two alloys. The shear bands offer preferential crack initiation site and crack propagation path in the alloys during impact loading leading to ductile shear fracture. While cracks propagate along the central region of transformed bands in AA 6061 alloy, the AA 2099 alloy failed by cracks that propagate preferentially along the boundary region between the transformed shear bands and the bulk material. Whereas the AA 2099 alloy shows the greatest propensity for adiabatic shear banding and failure in the T8 temper condition, AA 6061 alloy is most susceptible to formation of adiabatic shear bands and failure in the T4 temper. Deformation under quasi-static loading is dominated by strain hardening in the two alloys. Rate of strain hardening is higher for naturally aged AA 6061 than the artificially aged alloy, while the strain hardening rate for the AA 2099 alloy is independent of the temper condition. The AA 2099 alloy shows a superior mechanical behaviour under quasi-static compressive loading whereas the AA 6061 shows a higher resistance to impact damage.     

Cryogenic-alloy strength and cracking resistance under dynamic loading

Test results are given on 12Kh18N10T and 03Kh20N16AG6 steels and on AMg6 alloy designed to give the dynamic strength and cracking resistance at strain rates of 0.36 m/sec in the temperature range 77–293 K. Cooling raises the strength, plasticity, cracking resistance, and resistance to crack growth in AMg6 alloy. Cooling the two steels increases the strength and cracking resistance, but reduces the plasticity and crack propagation resistance. Compact specimens with small natural thicknesses can be used to determine the correct critical values for the cracking resistance J_Ic for 03Kh20N16AG6 steel and AMg6 alloy. It is possible to determine correctly the critical values for the cracking resistance for 12Kh18N10T steel at 77 K.Translated from Problemy Prochnosti, No. 9, pp. 48–53, September, 1992.     

Investigation of the regularities of deformation of heat-resistant steels under conditions of creep in the complex stressed state

We study the regularities of deformation of 1Kh18N9T steel under long-term static loading along different paths of radial loading with regard for the influence of the type of stressed state. The viscoplastic properties of 08Kh18N10T-VD steel subjected to stepwise static loading under conditions of complex stressed state are analyzed.     

Effect of fatigue damage on the dynamic tensile behavior of 6061-T6 aluminum alloy and AISI 4140T steel

Usually material properties are determined from damage free materials, but it is not well known how these properties vary with respect to previous fatigue damage. In the present work the dynamic response of fatigue damaged 6061-T6 aluminum alloy and AISI 4140T steel specimens subjected to impact loading was investigated. Samples subjected to previous damage under high cycle fatigue and low cycle fatigue were tested. Different fatigue damage levels were considered. In addition, the effect of previous fatigue damage on the quasi-static behavior, ductility and fracture mechanism was also evaluated for both materials. A tensile Hopkinson bar apparatus was used in this work to investigate the dynamic response of the pre-fatigued specimens. Projectile speeds ranged from 18 m/s to 30 m/s giving strains rates from 550 to 2850 s⁻¹. The quasi-static mechanical properties of aluminum are not affected by the way the fatigue damage is induced. The dynamic properties, however, are sensitive to the previous fatigue damage, but are not affected by the strain rate. In the steel case, and when damage is induced by strain control, the mechanical properties are influenced by the previous fatigue damage. The dynamic properties are sensitive to the previous fatigue damage and depend on the strain rates. The analysis results show an increase in the ductility of the aluminum alloy when increasing the fatigue damage level; the steel exhibits an opposite behavior, a decrease in the ductility when increasing the damage level. The results show how the previous fatigue damage can modify the quasi-static and dynamic mechanical properties of the tested materials.     

Shot peening of plasma nitrided steel for fretting fatigue strength enhancement

Abstract

The potential of fretting fatigue strength enhancement by a duplex surface engineering process involving shot peening of plasma nitrided steel, termed duplex SP/PN, is demonstrated. Specimens of 709M40 steel were individually plasma nitrided, shot peened, or duplex SP/PN treated. Fretting fatigue properties of the surface engineered specimens were evaluated. Surface roughness, residual stress, and hardening effect following the various surface treatments were examined and compared. It has been found that the duplex treatment can significantly improve the fretting fatigue strength of the investigated low alloy steel. Under the present testing conditions, the duplex SP/PN treatment increased the fretting fatigue strength (at 10⁷ cycles) of 709M40 steel by more than 70% relative to the nitrided, 120% to the shot peened, and 500% to the untreated steel. The improvement has been explained in terms of the significantly increased surface hardness and compressive residual stress in the near surface region following the duplex SP/PN treatment. By analysing the stress distributions in a shot peened surface, the influence of surface roughness on fretting fatigue strength is also discussed.     

Effect of lithium-lead eutectic and hydrogen on the fatigue behavior of some austenitic alloys

We investigate low-cycle and high-cycle fatigue of 04Kh16N11M3T and 03Kh20N45M4BCh austenitic alloys under rigid loading by pure bending in air, hydrogen, and lithium-lead eutectic at 80, 250, and 350°C. The amplitude of deformation did not exceed ±3%. We discovered a decrease in durability at 250 and 350°C in the presence of lithium-lead eutectic under low-cycle loading and under high-cycle loading in the stage of limited endurance. The greater the amplitude of loading, the sharper the decrease in durability, and this effect is more pronounced at 250°C than at 350°C. Under the action of gaseous hydrogen, the durability of 04Kh16N11M3T alloy increases under low-cycle loading as compared to that in air. In the same situation, the durability of 03Kh20N45M4BCh alloy decreases. Under high-cycle loading, no influence of hydrogen on the limited endurance of these alloys was detected. Both in hydrogen and in lithium-lead eutectic, we observed a decrease in the amplitude of deformation at the fatigue limit on a base of 10⁷ cycles.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 101–106, January – February, 1995.     

Energy Consumption for Deformation to Fracture of a Circular-Clamped Thin Plate under Impact Loading

An experimental procedure for evaluating energy spent for fracture of a circular-clamped sheet structural element upon transverse flexure, induced by impact loading of a spherical head-face body, is briefly outlined. Flexure test results for two sheet metals (a 20 mild steel and a D16T aluminum alloy 1.0 and 0.75 mm thick, respectively) and a 2.0-mm PA6 shock-resistant composite are cited. Experimental data analyses and stress-strain state calculations for a plate material within the circular boundary upon flexure made it possible to establish the relation between the work of deformation and the dynamic strength and plasticity. Sheet structural element materials are comparatively evaluated by their specific energy spent for deformation under transverse static and impact loading.     

Dynamic Compression of High-Strength Steel and Titanium Alloy

We describe a procedure of impact compression of 20Kh2N4A high-strength steel and VT-8 titanium alloy and present some experimental results accumulated for these materials. The analysis of these data demonstrates that compression strength significantly increases as the investigated range of strain rates becomes larger by an order of magnitude. Moreover, the compression strength of the studied materials strongly increases with the strain rate.     

Low-cyclic fatigue behaviour of an Al–Cu–Mg–Ag alloy under T6 and T840 conditions

The low-cyclic fatigue (LCF) behaviour of an AA2139 alloy belonging to the Al–Cu–Mg–Ag system was investigated under T6 and T840 conditions. The T840 treatment involves cold rolling with a 40% reduction prior to ageing, and this was effective in increasing the tensile strength of the alloy. Under cyclic loading at total strain amplitudes (ε_ac) of ±0.4 to ±1.0%, the mechanical behaviour is defined as the prevalence of elastic over plastic deformation processes under both the T6/T840 conditions. The initial weak hardening during one to two cycles of loading at ε_ac?>?0.55% and an insignificant softening upon following the cyclic loading to fracture was observed for the T6/T840 conditions. The LCF behaviour of the alloy under the T6/T840 conditions is described by the Basquin–Manson–Coffin relationship.

This paper is part of a Themed Issue on Aluminium-based materials: processing, microstructure, properties, and recycling.     

The change in mechanical properties of iron, 12Kh18N10T steel,and tantalum in impact loading of near-spalling intensity

The results of investigation of the influence of impact loading on the mechanical properties of iron 12Kh18N10T steel, and tantalum are presented. The remaining mechanical properties were determined on small specimens cut from samples subjected to impact loading and control samples. It is shown that the method of determination of the residual mechanical properties together with metallography may be used as a diagnostic one for determination of the conditions of origin of spalling microdamages in materials.Translated from Problemy Prochnosti, No. 1, pp. 104–108, January, 1996.     

The effects of aging treatment and strain rates on damage evolution in AA 6061 aluminum alloy in compression

AA 6061 aluminum alloy in T4, T6 and T8 temper were subjected to quasi-static compressive loading at a strain-rate of 3.2 × 10⁻³ s⁻¹ and dynamic compressive loading at strain-rates between 7.0 × 10³ and 8.5 × 10³ s⁻¹. The effects of strain rates and temper condition on the deformation behavior of the alloy are discussed. Under the quasi-static loading, deformation was relatively homogeneous and controlled by strain hardening, which is more pronounced in the naturally aged than the artificially aged alloys. Thermal softening played a dominant role under impact loading leading to strain localization along narrow bands called adiabatic shear bands (ASBs). Both deformed bands consisting of aligned second phase particles and transformed bands consisting of fine recrystallized grains were observed. The average size of the recrystallized grains in the transformed bands is about 600 nm and varies slightly depending on the temper condition. The fine grains are suggested to form by dynamic recrystallization. The T4 alloy showed the highest propensity for thermal softening, strain localization and cracking under impact loading while the T8 alloy showed the least tendency. The degree of recrystallization in the transformed band is influenced by temper condition with T8 alloy having the highest fraction of unrecrystallized grains inside the transformed bands. This is related to the temperature rise in the transformed bands that was estimated to be highest in the T4 alloy and lowest in the T8 alloy. The combined effects of high temperature and severe strain inside the transformed bands caused dissolution of second phase particles and induced microstructural changes that resulted in less silicon inside the transformed bands than in the adjacent region.     

Application of quenching–partitioning–tempering process and modification to a newly designed ultrahigh carbon steel

The applicability of quenching–partitioning–tempering (Q–P–T) process to an ultrahigh carbon steel (UHCS) has been investigated by means of optical microscopy (OM), scanning electronic microscopy (SEM) combined with energy-dispersive spectrometry (EDS), X-ray diffraction (XRD) and mechanical property tests. The molten steel was modified with a multi-component modifier-rare earth and a low melting point alloy (Al–Bi–Sb) before casting into iron molds. Observations showed that the carbide exists as partly isolated and fine blocky structure in as-cast microstructure, indicating good effect of modification. After the Q–P–T treatment, carbon was partitioned into austenite from martensite, creating a mixture of carbon-depleted martensite, carbon-enriched retained austenite and fine carbides. This kind of microstructure leads to a much higher impact toughness, 32 J/cm², in comparison with the value, i.e., no more than 20 J/cm², of the conventional quenching and tempering (Q–T) treatment at the same hardness level. Furthermore, wear-resisting property of the steel has also been investigated. It showed that the Q–P–T treated steel has better abrasive wear resistance, about 18% increased, compared with the Q–T treated alloy under high load conditions.     

Tests of Sheet Materials for Structural Elements in Transverse Shear

An experimental procedure to evaluate the energy spent for fracture of a circular-clamped thin plate in transverse shear, induced by impact loading with a flat-face body, is briefly outlined. Test results for the specimens of the three materials: mild steel, thickness = 1.0 mm, a D16T aluminum alloy, = 0.75 mm, and a PA6 shock-resistant composite, = 2.0 mm, are given. The analysis of experimental and calculated stress-strain data (within the circular area between the clamping contour and the loading surface) demonstrates the qualitative relation between the energy spent for deformation to fracture and dynamic strength and plasticity. Test results allow one to compare different materials by the energy spent for their fracture under impact loading. The specific work of deformation in shear characterizes this value more comprehensively than that determined in flexure, induced by impact of a spherical body, and is applicable over an expanded range of loading rates.     

Structure and mechanical properties of materials after hydroexplosive forming

The relationship between structure and mechanical properties of nickel, nichrome and steel Kh18N10T deformed at strain rates of 10⁴–10⁵ sec^–1 was studied. X-ray diffraction analysis was carried out, and static and high-speed deformation of these materials were compared.     

Influence of preliminary plastic deformation of 12Kh18N12T steel on its mechanical properties

We have established that the preliminary plastic deformation of 12Kh18N12T austenitic steel causes cold-work hardening, which depends on the strain rate. With increase in the strain rate of specimens from 8∙10⁻⁴ to 417∙10⁻⁴ sec⁻¹, both strength (ultimate strength) and plasticity (percentage elongation) characteristics of 12Kh18N12T steel decrease. After holding of the preliminarily work-hardened steel at a temperature of 650°C, its strength increases, and its plasticity decreases. At the same time, the isothermal influence for 1 and 10 h does not facilitate intercrystalline corrosion of the steel during its holding in a corrosive medium for 24 h.