A model for the dynamic fracture toughness of ductile structural steel

We assume in this paper that the dynamic fracture toughness K_Id of ductile structural steels is dependent on void nucleation and void growth. The void nucleation-induced dynamic fracture toughness K_Id·n and the void growth-induced dynamic fracture toughness K_Id·g were obtained by modifying the void nucleation-induced and void growth-induced static fracture toughness models, respectively, considering the effect of strain rate and local temperature. By the relationship between the void nucleation-induced dynamic fracture toughness K_Id·n and the void growth-induced dynamic fracture toughness K_Id·g((K_Id)²=(K_Id·n)²+(K_Id·g)²) dynamic fracture toughness K_Id could be quantitatively evaluated. With this model the dynamic fracture toughness of two structural steels (X65 and SA440) was assessed, and the causes for the differences between the static and dynamic fracture toughness were also discussed.     

Failure analysis of a crankshaft made from ductile cast iron

This paper describes the failure analysis of a diesel engine crankshaft used in a truck, which is made from ductile cast iron. The crankshaft was found to break into two pieces at the crankpin portion before completion of warranty period. The crankshaft was induction hardened. An evaluation of the failed crankshaft was undertaken to assess its integrity that included a visual examination, photo documentation, chemical analysis, micro-hardness measurement, tensile testing, and metallographic examination. The failure zones were examined with the help of a scanning electron microscope equipped with EDX facility. Results indicate that fatigue is the dominant mechanism of failure of the crankshaft. It was observed that the fatigue cracks initiated from the fillet region of the crankpin-web. The absence of the hardened case in the fillet region and the presence of free graphite and nonspheroidal graphite in the microstructure of the crankshaft made fatigue strength decrease to lead to fatigue initiation and propagation in the weaker region and premature fracture.     

Modelling of fracture toughness data in the ductile to brittle transition temperature region by statistical analysis

A fracture toughness database for a ferritic 22NiMoCr37 steel forging for 12.5, 25, 50 and 100 mm thick specimens tested at nine different temperatures has been analysed statistically. The method employed uses a statistical procedure based on competing risks to evaluate the fracture toughness and quantify the probability of cleavage fracture as a function of temperature, specimen thickness and ductile crack growth. This paper describes the application of the competing risks statistical methods to the fracture toughness database obtained from the joint European Project.     

Effect of pre-strain on fracture toughness of ductile structural steels under static and dynamic loading

The ductile fracture process consists of void nucleation, growth and coalescence. The whole ductile process can be divided into two successive steps: (I) the initial state to void nucleation, followed by (II) void growth up to void coalescence. Based on this suggestion, resistance to ductile fracture could be divided into the resistance to stage I and stage II, and accordingly the whole fracture toughness could be regarded to be due to contributions from stages I and II. The fracture toughness contributed from the two steps is, respectively, denoted as void nucleation-contributed fracture toughness and void growth-contributed fracture toughness. The effect of plastic pre-strain on the fracture toughness of ductile structural steels under static and dynamic loading (4.9 m/s) within the ductile fracture range was evaluated by summing contributions due to void nucleation-contributed and void growth-contributed fracture toughness. The effect of strain rate on fracture toughness was also investigated by the same means. The results show that both plastic pre-strain and high-speed loading decrease the void nucleation-contributed fracture toughness while their effects on the void growth-contributed fracture toughness depend on the variations in strength and ductility. Moreover, fracture toughness of structural steels generally decreases with increasing strain rate.     

Determination of dynamic crack resistance of ductile cast iron using the compliance ratio key curve method

The compliance ratio method is an analytical approach for instantaneous crack length determination in dynamic single-specimen J-R curve testing of ferritic ductile cast iron (DCI). Comparison testing at room temperature and −40 °C was applied to PCVN and SE(B)15 specimens to examine their performance and suitability for the dynamic key curve method for DCI. An experimental reference database of dynamic crack resistance curves was set up by low-blow multiple-specimen tests and used to validate the results of the CR method. The influence of test temperature, microstructure, loading rate and specimen geometry on fracture behavior of the tested DCI was investigated in great detail and these parameters were linked to fracture mechanical properties. The results obtained show that the CR method is suited to establish valid dynamic crack resistance curves for both types of specimen. Nevertheless, SE(B)15 specimens are preferred for dynamic J-R curve determination of DCI based on their advantages such as higher accuracy.     

Experimental and numerical investigation of thickness effect on ductile fracture toughness of steel alloy sheets

Effect of thickness on ductile fracture toughness of plates made of steel alloy GOST 08Ch22N6T is investigated experimentally. Multiple specimen tests for determining fracture toughness have been conducted using compact tension (CT) specimens with thicknesses of 1.25, 1.64 and 4.06 mm according to standard test method ASTM E813. The results show the significant effect of thickness on fracture toughness. It is observed that in low thickness, J_c increases with the thickness increase until it reaches a maximum; however, further increase in the thickness causes the J_c-value to decrease. Two-dimensional finite element analysis is also performed to reproduce the experimental results. The comparison shows a very good agreement.     

Experimental and numerical assessment of fracture toughness of dual-phase austempered ductile iron

The effects of the microstructure topology on the fracture toughness of dual-phase austempered ductile iron are studied in this paper by means of finite element modelling and experimental testing. To this end, specimens with matrix microstructures ranging from fully ferrite to fully ausferrite were studied and the preferential zones and phases for crack propagation were identified in every case. The effectiveness of the ausferrite phase as a reinforcement of the ferritic matrix via the encapsulation of the brittle and weak last-to-freeze (LTF) zones was confirmed. The toughening mechanism is consequence of the increment in the crack path longitude as it avoids the encapsulated LTF zones. Besides, the presence of small pools of allotriomorphic ferrite increase the crack propagation resistance of the ausferrite-ferrite matrices.     

Improvement of toughness of high chromium white cast iron: duplex ferritic-austenitic matrix

It is attempted to enhance the impact toughness of industrially used high chromium white cast iron (WCI) without sacrificing wear resistance. The microstructure is engineered by cyclic annealing to obtain features such as duplex grain matrix, where austenite envelops ferrite grain, refined M₇C₃ carbide. The newly cast and heat-treated alloy shows remarkable impact toughness i.e. 13J with improved wear resistance. The fracture micro-mechanism is studied through extensive scanning electron microscopy and it is ascertained that enhanced impact toughness results from crack arrest at duplex grain boundaries. A few other toughness enhancing features are also discussed. The results are compared with standard ASTM grade Class-III high chromium WCI and are found to be encouraging.     

A model for the static fracture toughness of ductile structural steel

Fracture of ductile structural steels generally occurs after void initiation, void growth and void coalescence. In order for ductile fracture of structural steels to occur, energy must be spent to induce void initiation and void growth. Therefore, fracture toughness for ductile fracture should be contributed from void initiation and void growth. On the basis of this suggestion static fracture toughness (K_IC) of ductile structural steels is decomposed into two parts: void nucleation-induced fracture toughness (denoted as K_IC.n) and void growth-induced fracture toughness (K_IC.g). K_IC.n, defined as the stress intensity factor at which voids ahead of a crack begins to form, is calculated from crack tip strain distribution and void nucleation strain distribution. In contrast, K_IC.g is determined by the void growth from the beginning of void nucleation to void coalescence. Therefore, K_IC.g relates to the void sizes and void distribution. In this paper, the expression for K_IC.g is given from the void sizes directly from fracture surfaces. The relationship between K_IC.n, K_IC.g and K_IC is expressed in the form (K_IC)²=(K_IC.n)²+(K_IC.g)². The newly developed model was applied to the fracture toughness evaluation of three structural steels (SN490, X65 and SA440), and the theoretical calculation agrees with the experimental results.     

An experimental and numerical analysis to correlate variation in ductility to defects and microstructure in ductile cast iron components

This paper describes a statistical test plan to determine tensile properties and fracture properties for ductile cast iron considered for the Swedish nuclear waste canisters and associated analysis. Large variations were found in the ductility between tested canister inserts and between specimens taken from different locations in each insert. A large number of tested tensile specimens were subsequently analysed by fractography and metallography to relate low ductility values to size and type of casting defects. Loss of ductility could be related to slag defects and to a lesser extent to high pearlite content, low nodularity and chunky graphite. Slag defects were modelled by an elasto-plastic fracture mechanics model for penny-shaped slag defects and semi-empirical models for the other defect types. The fracture model was incorporated into a probabilistic scheme to compute distribution of elongation for the inserts and the associated defect size. The computed ductility distribution agrees very well with measured data whereas the computed defect size distribution is underestimated. By including crack growth resistance and various aspect ratios of defects a much better agreement with observed defects can be achieved.     

Fracture toughness and growth of short and long fatigue cracks in ductile cast iron EN‐GJS‐400‐18‐LT

Heavy components of ductile cast iron frequently exhibit metallurgical defects that behave like cracks under cyclic loading. Thus, in order to decide whether a given defect is permissible, it is important to establish the fatigue crack growth properties of the material. In this paper, results from a comprehensive study of ductile cast iron EN‐GJS‐400‐18‐LT have been reported. Growth rates of fatigue cracks ranging from a few tenths of a millimetre (‘short’ cracks) to several millimetres (‘long’ cracks) have been measured for load ratios R=?1, R= 0 and R= 0.5 using a highly sensitive potential‐drop technique. Short cracks were observed to grow faster than long cracks. The threshold stress intensity range, ΔK_th, as a function of the load ratio was fitted to a simple crack closure model. Fatigue crack growth data were compared with data from other laboratories. Single plain fatigue tests at R=?1 and R= 0 were also carried out. Fracture toughness was measured at temperatures ranging from ?40 °C to room temperature.     

Using torsional bar testing to determine fracture toughness

A new method to determine fracture toughness K _IC of materials is introduced. A round-rod specimen having a V-grooved spiral line with a 45° pitch is tested under pure torsion. An equibiaxial tensile/compressive stress state is effectively created to simulate conventional test methods using a compact-type specimen with a thickness equivalent to the full length of the spiral line. K _IC values are estimated from the fracture load and crack length with the aid of a three-dimensional finite element analysis. K _IC of 7475-T7351 aluminium is estimated to be 51.3 MPa √m, which is higher than the vendor's value in the TL orientation by ∼0.8% and higher than 0.5T compact tension (CT) value by 6%; A302B steel yields 54.9 MPa √m being higher than CT test value by ∼2%. Good agreement between the K _IC values obtained by different methods indicates the proposed method is sound and reliable.     

Influence of nitriding on the fatigue behavior and fracture micromechanisms of nodular cast iron

Surface modification processes are increasingly used to fully exploit material potential in fatigue critical applications because fatigue strength is sensitive to surface conditions. Nitriding is extensively adopted with ferrous materials because it forms a hard and strong surface layer and a system of superficial compressive residual stresses. Fatigue, however, is strongly dependent also on defects and inhomogeneity. When nitriding is applied to nodular cast iron, the relatively thin hardened layer (about 300 μm) contains graphite nodules (diameter of the order of 30 μm), casting defects and a heterogeneous matrix structure. The paper presents and discusses the influence of nitriding on the fatigue response and fracture mechanisms of nodular cast iron. A ferritic nodular cast iron and a synthetic melt with different content of effective ferrite were initially gas-nitrided. Then, (i) structural analysis of nitrided layers, (ii) fatigue testing with rotating bending specimens, and (iii) fatigue fracture surface inspection were performed. Performance and scatter in fatigue performance is discussed by selective inspection of fracture surfaces and identification fracture micromechanisms. A semiempirical model explains observed trends in test results and is used for the process optimization. __________ Translated from Problemy Prochnosti, No. 1, pp. 85–88, January–February, 2008.     

Effect of heat treatment on ultrasonic velocity in ductile cast iron

Abstract

The measurement of the ultrasonic velocity is a common method in the foundry industry for the evaluation of the nodularity in ductile iron castings. Practical experience has shown that heat treatment can reduce the ultrasonic velocity compared to the as cast condition. Using ductile iron samples with different heat treatments in order to vary the ferrite and pearlite content respectively confirmed this decrease in the ultrasonic velocity compared to the as cast state. Further investigations showed that with all the heat treatments applied, irrespective of their effect on the microstructure, the density was decreased. The decrease in density correlated with the decrease in ultrasonic velocity for all heat treatments. The mechanisms involved in the reduction in the density are discussed.     

陶瓷材料动态断裂韧性测试

为了测试陶瓷材料动态断裂韧性,利用Hopkinson压杆实验原理和改装的Hopkinson压杆装置,并将试件加工成单边切口梁进行了三点弯曲动态试验.利用改装的Hopkinson压杆装置可直接测得透射应力波,从而直接得到试件变形过程中作用在试件上的支反力.本文定义了无量纲挠度和挠度变化率,给出了几种陶瓷材料在不同挠度变化...     

Master curve analysis of the “Euro” fracture toughness dataset

Brittle fracture in the ductile to brittle transition regime is connected with specimen size effects and - more importantly - tremendous scatter of fracture toughness, which the technical community is currently becoming increasingly aware of. The size effects have the consequence that fracture toughness data obtained from small laboratory specimens do not directly describe the fracture behavior of real flawed structures. Intensive research has been conducted in the last decade in order to overcome these problems. Different approaches have been developed and proposed, one of the most promising being the master curve method, developed at VTT Manufacturing Technology.For validation purposes, a large nuclear grade pressure vessel forging 22NiMoCr37 (A508 Cl.2) has been extensively characterized with fracture toughness testing. The tests have been performed on standard geometry CT-specimens having thickness 12.5, 25, 50 and 100 mm. The a/W ratio is close to 0.6 for all specimens. One set of specimens had 20% side-grooves. The obtained data consists of a total of 757 results fulfilling the ESIS-P2 test method validity requirements with respect to pre-fatigue crack shape and the ASTM E-1921 pre-fatigue load. The master curve statistical analysis method is meticulously applied on the data, in order to verify the validity of the method. Based on the analysis it can be concluded that the validity of all the assumptions in the master curve method is confirmed for this material.     

Evaluation of dynamic fracture toughness KId by Hopkinson pressure bar loaded instrumented Charpy impact test

A novel method for measuring the dynamic fracture toughness, K_Id, using a Hopkinson pressure bar loaded instrumented Charpy impact test is presented in this paper. The stress intensity factor dynamic response curve (K_I(t)−t) for a fatigue-precracked Charpy specimen is evaluated by means of an approximate formula. The onset time of crack initiation is experimentally detected using the strain gauge method. The value of K_Id is determined from the critical dynamic stress intensity factor at crack initiation. A K_Id value for a high-strength steel is obtained using this method at a stress-intensity-factor rate () greater than 10⁶ MPa .     

Study of the fracture of ferritic ductile cast iron under different loading conditions

This work is a continuation of the studies presented in a recent paper by the authors, where the fracture surfaces of pearlitic ductile cast iron under different loading conditions were exhaustively analysed. In this study, fracture surfaces of ferritic ductile cast iron (or ferritic spheroidal graphite cast iron) generated under impact, bending and fatigue loading conditions were characterised and compared. The fracture surfaces were characterised qualitatively and quantitatively from the observation under a scanning electron microscope. The fracture mechanisms in each case were identified. For impact tests, as test temperature increases, the dominant fracture mechanism changes from brittle to ductile. For bending tests, a fully ductile fracture micromechanism dominates the surface. In fatigue tests, the surface shows a mix of flat facets that appear to be cleavage facets and ductile striations, but the typical fatigue striations are not easily found on the fracture surface. Methodologies for the determination of the macroscopic direction of main crack propagation in both ductile and brittle failure modes are proposed. These allow identifying main crack propagation direction with good approximation. The results are potentially useful to identify the nature of loading conditions in a fractured specimen of ferritic spheroidal graphite cast iron. The authors believe that it is necessary to extend the methodologies proposed in samples with different geometry and size, before they can be used to provide additional information to the classical fractographic analysis.     

Determination of dynamic fracture parameters using a semi-circular bend technique in split Hopkinson pressure bar testing

Fracture initiation toughness, fracture energy, fracture propagation toughness, and fracture velocity are key dynamic fracture parameters. We propose a method to simultaneously measure these parameters for mode-I fractures in split Hopkinson pressure bar (SHPB) testing with a notched semi-circular bend (SCB) specimen. The initiation toughness is obtained from the peak load given dynamic force equilibrium. A laser gap gauge (LGG) is developed to monitor the crack surface opening displacement (CSOD) of the specimen, from which the fracture velocity and the fracture energy can be calculated. The feasibility of this methodology for coarse-grained solids is demonstrated with the SHPB-SCB experiments on Laurentian granite.