Identification of Materials Properties with the Help of Miniature Shear Punch Test Using Finite Element Method and Neural Networks

This paper describes an approach to identify the mechanical properties i.e. fracture and yield strength of steels. The study involves the FE simulation of shear punch test for various miniature specimens thickness ranging from 0.20mm to 0.80mm for four different steels using ABAQUS code. The experimental method of the miniature shear punch test is used to determine the material response under quasi-static loading. The load vs. displacement curves obtained from the FE simulation miniature disk specimens are compared with the experimental data obtained and found in good agreement. The resulting data from the load vs. displacement diagrams of different steels specimens are used to train the neural networks to predict the properties of materials i.e. fracture and yield strength. Two different feed forward neural networks have been created and trained in order to predict the Fracture toughness and yield strength values of different steels. L-M algorithm has been used in the networks to form an output function corresponding to the input vectors used in the network. The trained network provides the output values i.e., fracture toughness and yield strength of unknown input values, which are within in the range of data that is used for the training of network.     

Near-threshold fatigue crack propagation of several high strength steels

In support of Douglas Aircraft Co. structural damage tolerance analysis, crack propagation rates were measured in humid air from 10^?8 to 10^?3 mm/cycle of several high strength martensitic steels at stress ratios of $\text{R = 0.1}$ and $\text{R = 0.5}$. Increasing the stress ratio was found to decrease the threshold stress intensity. Increasing the tensile yield material strength in these quench and tempered steels resulted in a nearly linear inverse relationship with the threshold stress intensity. The threshold stress intensity appeared to be independent of the fracture toughness of the alloy steel.     

Rißfortschritt in ultrafesten Stählen und deren Schweißverbindungen bei dynamischer Beanspruchung

Crack propagation in ultra-high-strength steels and their welded joints under dynamic loading . Reported are results of investigation into the propagation of cracks in the base metal and weld metal of an ultra-high-strength steel. The material used in the investigations was a Ni? Co? Mo? alloy maraging steel with a yield point of 170 kp/mm². The steel was arc welded and TIG welded. The joints exhibited a drop of static strength in the range of 5 to 8 percent related to the base metal. Under zero-to-tension stress cycles the fatigue strength corresponded that of other high-strength steels, under tension-compression stress cycles the steel exhibited a higher fatigue strength. It was possible to show striations with the aid of scanning microscopy. Comparing the track propagation calculated in the microscopic range with the results obtained from the crack growth curves produced approximate agreement.     

LOW- AND HIGH-CYCLE FATIGUE PROPERTIES OF VARIOUS STEELS SPECIFIED IN JIS FOR MACHINE STRUCTURAL USE

Abstract —Low-cycle fatigue properties were investigated on four carbon steels and five low alloy steels specified in JIS (Japanese Industrial Standard) for machine structural use, which are the most commonly used in Japan. Several different heats from each of several representative manufacturers were sampled so as to represent the average fatigue characteristics of current materials. The cyclic deformation behaviour of material was denned by comparing the monotonie yield stress on the extrapolated tensile work hardening curve with the cyclic yield stress in the cyclic stress-strain curve determined by incremental step test. The normalized ferrite-pearlitic steels cyclically hardened, while the quench-tempered martensitic were cyclically stable or softened. The S–N relations derived from the strain-controlled low-cycle tests were compared with the results obtained by load-controlled high-cycle tests. The extrapolated S–N curves based on the cyclic stress-strain curve predicted the fatigue strength in the high-cycle range to be stronger for cyclic-hardening steels, but weaker for cyclic-softening steels. The predicted S–N curves for stable steels coincided with the high cycle test data. The fatigue limit had a proportional relationship with cyclic yield stress, slightly depending on the cyclic deformation behaviour. On the other hand, the cyclic yield stress was found to exhibit a very good linear correlation with the monotonie tensile strength, independent of cyclic deformation behaviour. This explains the empirical law that the fatigue limit is approximately proportional to the tensile strength.     

冷弯非薄壁方管材料特性试验及分析模型

为了能够更好掌握冷弯非薄壁方管(壁厚大于6mm)的受力性能,考虑不同的壁厚、宽度、加工工艺和钢号,对冷弯非薄壁方管的平板部位、弯角部位以及平板与弯角交界部位进行了材性试验。结合国内外以往的研究成果,对比发现:所测试的方管角部的屈服强度及抗拉强度都要大于平板部位,角部的屈服强度的提高幅度要大于其抗拉强度;平板与弯角交界部位的材料强度与平板部位相比,并没有表现出明显的提高;虽然方管所有部位的屈服强度及抗拉强度均有不同程度的提高,但它们的强屈比及拉断后的伸长率却有不同程度的降低;材料的应力-应变全过程曲线呈现“无明显屈服平台”的特性,弯角部位的材料强化模量要明显大于平板部位,先圆后方工艺的方管材料的强化模量要大于直接成方的方管。最后,在试验结果的基础上,提出了冷弯非薄壁方管材料力学性能的分析模型。     

Tensile and work hardening properties of low carbon dual phase strip steels at high strain rates

Abstract

As a result of their unique combination of strength and ductility dual phase steels play an important role in reducing weight in automobile components and improving crashworthiness. The purpose of this paper is to quantify the crash performance of dual phase steels, as defined by the influence of low and high strain deformation rates (0·001 s^-1 and 100 s^-1 respectively), on the tensile and work hardening properties of a range of commercial dual phase products. The objective is to establish whether dual phase steels maintain their desirable mechanical property characteristics of low yield strength, high tensile strength and high work hardening rates during plastic deformation under the application of a high strain rate loading. The results confirmed that the yield/proof strength and tensile strength increased with increasing volume fraction of second phase constituents and increasing strain rate. In particular, a dual phase steel with a microstructure consisting of a significant volume fraction (>10–15%) of additional second phase material (bainite) is shown to display superior energy absorption properties. However, this is accompanied by poor ductility and work hardening characteristics.     

Perforation resistance of five different high-strength steel plates subjected to small-arms projectiles

Thin plates of high-strength steel are frequently being used both in civil and military ballistic protection systems. The choice of alloy is then a function of application, ballistic performance, weight and price. In this study the perforation resistance of five different high-strength steels has been determined and compared against each other. The considered alloys are Weldox 500E, Weldox 700E, Hardox 400, Domex Protect 500 and Armox 560T. The yield stress for Armox 560T is about three times the yield stress for Weldox 500E, while the opposite yields for the ductility. To certify the perforation resistance of the various targets, two different ballistic protection classes according to the European norm EN1063 have been considered. These are BR6 (7.62 mm Ball ammunition) and BR7 (7.62 mm AP ammunition), where the impact velocity of the bullet is about 830 m/s in both. Perforation tests have been carried out using adjusted ammunition to determine the ballistic limit of the various steels. In the tests, a target thickness of 6 mm and 6 + 6 = 12 mm was used for protection class BR6 and BR7, respectively. A material test programme was conducted for all steels to calibrate a modified Johnson–Cook constitutive relation and the Cockcroft–Latham fracture criterion, while material data for the bullets mainly were taken from the literature. Finally, results from 2D non-linear FE simulations with detailed models of the bullets are presented and the different findings are compared against each other. As will be shown, good agreement between the FE simulations and experimental data for the AP bullets is in general obtained, while it was difficult to get reliable FE results using the Lagrangian formulation of LS-DYNA for the soft core Ball bullet.     

Advanced cold rolled steels for automotive applications

Advanced multiphase steels offer a great potential for bodies‐in‐white through their combination of formability and achievable component strength levels. They are first choice for strength and crash‐relevant parts of challenging geometry. The intensive development of high‐strength multiphase steels by ThyssenKrupp has led to hot dip galvanizing concepts with an outstanding forming potential. Hot rolled, hot dip galvanized complex phase steels are currently produced in addition to cold rolled DP and RA steels. New continuously annealed grades with tensile strength levels of up to 1000 MPa in combination with sufficient ductility for applications mainly in the field of structural automobile elements make use of the classic advantages of microalloying as well as the principles of DP and TRIP steels. Further improvement of properties will be reached by the new class of high manganese alloyed steels.     

Stainless steels suited for solution nitriding

Solution nitriding is a new heat treatment to yield a high nitrogen case on stainless steels at 1100 ± 50°C. Combining experimental results and thermodynamic calculation steels are selected to give a hard martensitic or high strength austenitic case. Especially developed steels are discussed as well as the suitability of standard grades. A martensitic case is combined with a martensitic core in steel Cr13C0.2 and with a softer ferritic‐martensitic core in steel Cr13C0.1. The nitrogen content of an austenitic case increases with the Cr/Ni ratio, e.g. in the order of Cr17Ni12Mo2, Cr18Ni10, Cr22Ni5Mo3N0.2. The duplex microstructure of the latter provides the highest yield strength in the core. It is essential to stay clear of the austenite/austenite + M₂N boundary and avoid precipitates which deteriorate the fatigue and corrosion resistance. Seventeen steels are assessed in this report.     

The design of advanced performance high strength low-carbon martensitic armour steels: Microstructural considerations

Neither a higher hardness nor higher mechanical properties (yield strength, ultimate tensile strength, impact energy, and %elongation) appear to be exclusive or even reliable criteria for predicting the ballistic performance of martensitic armour steels, as shown in our previous work [K. Maweja, W.E. Stumpf, Mater. Sci. Eng. A (February), submitted for publication]. An alternative design methodology for tempered martensitic armour steels is, therefore, proposed which is based on the effect of retained austenite on the ratio of the yield to ultimate tensile strength (YS/UTS), the microstructure of the tempered martensite and its martensite start temperature M_s. This approach was developed using 6 mm thick armour plates and later was successfully applied to the design of eight experimental armour steels with plate thicknesses ranging from 4.7 to 5.2 mm and tested by the standard R4 (5.56 mm rounds) ballistic test.     

1.0 GPa low carbon medium Mn heavy steel plate with excellent ductility

ABSTRACT

A vanadium-bearing medium Mn heavy steel plate was designed, and the yield strength reaches around 1.0?GPa and the total elongation reaches around 18% after the annealing treatments. Meanwhile, the steel also possesses relatively good cryogenic impact toughness. Although the dislocation density of ～2.65?×?10¹⁴?m^?2 in the hot-rolled steel decreases to ～1.13?×?10¹⁴?m^?2 in both annealed steels and the volume fractions of retained austenite reach ～7.9% and ～13.5% for the steels annealed at 620°C for 1 and 2?h, there is no or small decrease in yield strength owing to numerous nanometer-sized carbides precipitated in martensite matrix.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

Einige Gesichtspunkte für die Auswahl von Stählen in der chemischen Höchstdrucktechnik

Some Aspects of Selecting Materials in the Chemical High Pressure Technology . Steels for chemical industry in the pressure range between 2000 and 3000 bar must have high yield strengths in order to be able to bear the static internal pressure. Even with high yield strengths the wall thickness is so high that steels must be selected which can be thoroughly and evenly quenched and tempered. They should be insensitive to tempering embrittlement. For components stressed by static internal pressure the safety against bursting and the bursting behaviour play a major rǒle. In addition to that, under pulsating pressures, the fatigue characteristics of a component are of importance. Steels with the high fatigue strengths necessary in high pressure application are notch sensitive. It is shown which measures are to be taken from the metallurgical side and in fabrication to increase their fatigue strength. Results of fatigue tests on high pressure tubes with different surface treatment are given. The fatigue strength can be improved by improving tube manufacturing, mechanically or electrochemically smoothening the inner tube surface, nitriding or autofrettage. For special purposes it is sometimes necessary to use steels whose strength is too low for high pressure applications. One must then apply a strength calculation conceding partial plastic deformation. This is illustrated for an age hardening austentitic steel.     

Wear behavior of light-weight and high strength Fe-Mn-Ni-Al matrix self-lubricating steels

The good combination of mechanical and tribological properties for self-lubricating materials is crucial. In this work, novel self-lubricating Fe-16.4 Mn-4.8 Ni-9.9 Al-xC(wt%) steels containing graphite phase were fabricated using mechanical alloying and spark plasma sintering. The compositions of the steels were designed by using thermodynamic calculation, and the effect of carbon addition on the microstructure was further investigated. The steel possesses high hardness of 621 HV, high yield strength of 1437 MPa and good fracture toughness at room temperature. The yield strengths are still above 600 MPa at 600?C.The tribological behavior and mechanical properties from room temperature to 800?C were studied, and the wear mechanisms at elevated temperatures were discussed. The steel has a stable friction coefficient of 0.4 and wear rate in a magnitude of 10~(-6) mm~3/N·m below 600?C. The good tribological properties of the steels were mainly attributed to the high hardness, lubrication of graphite and stable surface oxide layer.     

Tensile flow behavior of ultra low carbon,low carbon and micro alloyed steel sheets for auto application under low to intermediate strain rate

This paper is concerned with tensile characteristics of auto grade low carbon, ultra low carbon and micro alloyed steel sheets under low to intermediate strain rates ranging from 0.0007 to 250 s⁻¹. Experimental investigation reveals two important aspects of these steels under intermediate strain rate deformation. Firstly, the yield stress increases with strain rate in all these steels. Of course yield stress increment is higher for low carbon and ultra low carbon steel sheets. Secondly, the strain hardening rate drastically decreases with strain rate for low carbon and ultra low carbon steel sheets, whereas it remains steady for micro alloyed steel sheets. Based on these observations, a constitutive model has been proposed which predicts the strain rate sensitive flow behavior of all these grades within the strain rate range of automotive crash event.     

Constitutive modeling of the mechanical behavior of high strength ferritic steels for static and dynamic applications

A constitutive relation is presented in this paper to describe the plastic behavior of ferritic steel over a broad range of temperatures and strain rates. The thermo-mechanical behavior of high strength low alloy (HSLA-65) and DH-63 naval structural steels is considered in this study at strains over 40%. The temperatures and strain rates are considered in the range where dynamic strain aging is not effective. The concept of thermal activation analysis as well as the dislocation interaction mechanism is used in developing the flow model for both the isothermal and adiabatic viscoplastic deformation. The flow stresses of the two steels are very sensitive to temperature and strain rate, the yield stresses increase with decreasing temperatures and increasing strain rates. That is, the thermal flow stress is mainly captured by the yield stresses while the hardening stresses are totally pertained to the athermal component of the flow stress. The proposed constitutive model predicts results that compare very well with the measured ones at initial temperature range of 77 K to 1000 K and strain rates between 0.001 s⁻¹ and 8500 s⁻¹ for both steels.     

CORRELATION OF FATIGUE CRACK GROWTH RATE AT DIFFERENT STRESS RATIOS FOR QUENCHED AND TEMPERED STEELS AND OTHER ALLOYS

Abstract— Measurements of the effect of stress ratio on the constant amplitude fatigue crack growth rates in four quenched and tempered steels in the Paris regime are reported. This data and published data for other alloys (including lower strength steels and non-ferrous alloys) are evaluated, and a correlation function suitable for practical fatigue life calculations is derived. In addition to stress intensity factor range and stress ratio, other significant parameters are the yield stress of the material and its thickness. For the four steels on which new measurements were made, the degree of dependence of the crack growth rate on stress ratio may be related to sensitivity to environmental conditions.     

Estimation of oxide scale growth and temperature increase of high (9–12%) chromium martensitic steels of superheater tubes

The improved ferritic steels that have very good creep strength such as grades 91 and 92 have been widely used for power plant applications. For example, a chromium–molybdenum–vanadium steel tube (SA213-T91) has been available in the market for two decades. The SA213-T91 material has better high temperature strength and oxidation resistance than those of the more widely used materials such as T11 and T22. In this paper, estimations of the oxide scale growth and temperature increase of 9–12% Cr martensitic steels of superheater tubes are carried out using an incremental procedure, utilizing the finite element simulations and the relation between the Larson Miller Parameter (LMP) and the scale thickness over a period of time. In order to show the superiority of T91 steels over T22 steels, two different steam temperatures are considered in the models for comparisons. The estimations showed as expected that the oxide scale growth and temperature increase of the 9–12% Cr steels were significantly lower compared to those of 2.25% Cr steels.     

Fatigue strength of high strength dual-phase steel sheet

Fatigue tests have been carried out on lean-alloyed dual-phase steels with tensile strengths ranging from 300–800 MPa. Smooth specimens and specimens with punched holes were tested. The fatigue strength of dual-phase steel was found to be similar to that of other types of steel (eg solution hardened or microalloyed steels) of equal tensile strength. The fatigue strength increases with increasing yield strength. For notched specimens it is also related to the yield ratio. Work and bake hardening increase the fatigue strength of smooth specimens in proportion to the increase in yield strength. For notched specimens this effect is less and is dependent on the yield ratio. Bake hardening of material which was not work hardened also increased the fatigue strength. The notch sensitivity of low yield ratio dual-phase steel is found to be low. The notch sensitivity seems to increase with increasing yield ratio.     

Fracture and strength properties of selected austenitic stainless steels at cryogenic temperatures

Austenitic stainless steels have an excellent combination of mechanical and physical properties for load-bearing structures of large superconducting magnets for plasma containment in magnetic fusion experiments. To assess their relative suitability fracture toughness, fatigue crack growth, and tensile properties data for five austenitic steels at 295, 76, and 4 K have been obtained. The steels were AISI 304, 316, 304LN, and 316LN, and an Fe-21cr-12Ni-5Mn alloy with a higher nitrogen content than the other four grades. The two principal findings were the systematic variation of yield strength with nitrogen content and a systematic inverse correlation between fracture toughness and yield strength. Data from previous studies are reviewed which confirm the trends of the present data.     

飞机起落架用超高强度不锈钢的研究及应用进展

本文结合飞机起落架的设计理念,梳理了飞机起落架用超高强度钢及高强不锈钢的应用及发展历程,重点阐述了典型超高强度不锈钢的成分、组织和力学性能以及强韧化机理。建议通过材料热力学动力学计算创新设计新的超高强度不锈钢钢种;提出新型超高强度不锈钢的组织设计,将更关注多类型或高密度的共格析出强化以及高力学稳定性残余奥氏体的强韧化作用机制;最后指出采用最新的一些加工工艺技术,如等温多向锻造工艺技术,可显著提高超高强度不锈钢的综合力学性能。