Microfractography in failure analysis of cold forging dies

In this paper microfractographic features in fracture surfaces for tensile, fatigue, impact and three point bending of cold forging die steels with Rockwell C scale hardness number of 52–68 is presented. The emphasis is placed upon the stretched zone formation ahead of fatigue crack and the relation between the stretched zone width and fracture toughness of these cold forging die steels. Finally it is briefly described that the quantitative analysis for cold forging die failure can be possible by measuring the stretched zone width.     

Einfluß der Probengröße auf die bruchmechanischen Eigenschaften von Sinterstahl

Influence of Specimen Size on the Fracture Mechanical Behaviour of Sintered Steel Fracture mechanics testing was carried out with small and big specimens using high-temperature sintered Fe-2%Cu-2.5%Ni-alloys in the densities of ρ = 7.1 and 7.4 g/cm³. These steels are often used in the manufacturing of PM-parts. Due to the different dismensions the crack propagation is for the bigger sizes faster than for the smaller sizes. Also the conditional fracture toughness of the big specimens is superiour to the toughness of the small specimens. But under consideration of a plain strain state for the big specimens and of a plain stress state for the small specimens valid fracture toughness values being independent from the specimen size can be calculated applying linear-elastic fracture mechanics. These results were obtained for both densities investigated. The increase of the density delivers principally better fracture mechanical data. Hereby the relation of strength data with the microstructure is also discussed.     

Anwendung des instrumentierten Kerbschlagbiegeversuchs zur Abschätzung der Rissauffangzähigkeit

Application of Charpy V‐notch testing to estimate the crack‐arrest toughness Modern structural integrity assessment relies upon fracture mechanics, thus utilizing fracture mechanical parameters describing the material fracture resistance against crack initiation and crack propagation as well as the material crack‐arrest behaviour. However, crack‐arrest fracture toughness values are usually difficult and expensive to determine. In this paper correlations are proposed for estimating the nil‐ductility temperature (T_NDT) and the crack‐arrest fracture toughness (K_Ia) from a transition temperature, based on instrumented Charpy‐V crack‐arrest load information. The transition criteria used are the 4 kN crack‐arrest force and the mean crack‐arrest fracture toughness of 100 MPa√m according to the master curve approach. Correlations between transition temperatures, T(F_a = 4 kN), T(K_Ia), and T_NDT, which were proposed for various structural steels, work very well for the 18Ch2MFA material.     

Simple method to measure the crack resistance of ceramic materials

A new and simple method to study the change in crack resistance during the process of crack growth in ceramic materials has been developed. The method is based on using the chevron-notched short-bar test which is generally accepted as a convenient method for measuring fracture toughness. The simple modification described here allows one to measure fracture toughness using the assumption that fracture toughness changes in the process of crack growth (presence of crack resistance curve, or R-curve). This method presents many advantages, especially the long stable crack growth under mode I fracture specimen and plane strain, small specimen size and no need of pre-cracking for measuring R-curve behaviour in ceramic materials.     

Characterization of fracture toughness (<Emphasis Type="Italic">G</Emphasis><Subscript>c</Subscript>) of PVC and PES foams

The fracture behavior of polyvinyl chloride (PVC) and polyethersulfone (PES) foams has been examined using the single-edge notch bend and the double cantilever beam (DCB) tests. PVC foam densities ranging from 45 to 100 kg/m³ and PES foam densities ranging from 60 to 130 kg/m³ were examined. The PVC foams failed in a linear elastic brittle manner, whereas the PES foams displayed much more ductility and substantially larger toughness at a comparable foam density. The cell wall thickness of the PES foams was almost twice the thickness of the PVC foams which may have contributed to the high fracture toughness here defined as critical energy release rate (G _c). The PES foam, further displayed low initiation toughness, due to the sharp artificial crack tip and large toughness corresponding to propagation from a natural crack. The results show that the ductile PES foams have toughness close to its solid counterpart whereas the toughness of the PVC foams falls substantially below its solid counterpart.     

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.     

低温环境下测试复合材料Ⅰ型层间断裂韧性的简易方法

目前ASTM复合材料Ⅰ型层间断裂韧性测试标准需不断观测裂纹扩展长度。然而在低温环境下,裂纹扩展长度不易测量且过程繁琐。为克服这一缺陷,本文采用双柔度法测试复合材料低温环境下Ⅰ型层间断裂韧性,该方法的步骤与ASTM标准基本相同,但不需观测裂纹扩展长度便能获得低温下Ⅰ型层间断裂韧性。为了验证该方法的可靠性和精度,采用5件碳纤维增强环氧树脂基复合材料双悬臂梁（DCB）试样在-10℃环境下进行Ⅰ型层间裂纹扩展实验,应用ASTM标准所推荐的三种方法及本文的双柔度法进行数据处理获得复合材料Ⅰ型层间断裂韧性。结果表明:ASTM标准的三种方法与双柔度法得到的Ⅰ型层间断裂韧性结果一致,相对差别小于5%,而本文的双柔度法不需测量裂纹扩展长度,因此更简单,为测试低温环境下Ⅰ型层间断裂韧性提供了一种准确、简单的新方法。      

Fracture toughness measurement by cylindrical specimen with ring-shaped crack

A method for measuring the plane strain fracture toughness of metals by means of cylindrical specimen in tension with axi-symmetrical ring-shaped crack is discussed. Owing to the fact that the crack tip of such a specimen is closer to ideal plane strain state, the K_1c value measured is effective and reliable. This investigation has fairly satisfactorily solved the problems of crack prefabrication, experimental technique, data processing and requirements for specimen dimensions.In both safety evaluation and life estimation of engineering components by linear elastic fracture mechanics, it is necessary to measure the fracture resisting parameter—fracture toughness under plane strain. According to the ASTM-E399-74 standard[1], when measuring the fracture toughness K_1c values of medium and low strength steels with a standard compact tension specimen or three-point bending specimen, it is necessary to use specimens of large dimensions, great tonnage fatigue testing machine and universal testing mechine. Naturally, this presents great difficulties to the investigation and application of fracture mechanics and it is precisely for the purpose of overcoming these difficulties that we have studied the method of measuring the plane strain fracture toughness by a cylindrical specimen in tension with axi-symmetrical ring-shaped crack. This method has fairly satisfactorily solved the problems of crack prefabrication, experimental technique, data processing and requirements for specimen size. Owing to the fact that the field around the crack tip of such a specimen is closer to ideal plane strain state, the results obtained are values smaller than those by using compact tension and three-point bending specimens and are more reliable fracture resisting constants for materials in linear elastic fracture mechanics analysis. Moreover, this method is more practical and economical because no expensive large fatigue testing machine is needed and the specimen size is small.     

Bruchzähigkeit und Ermüdungs-Rißwachstum von Gußeisen

Fracture Toughness and Fatigue Crack Growth of Cast Irons Fracture toughness, elastic moduli and fatigue crack growth rates in air and in vacuum were measured for 17 different cast irons. The graphite shape in the cast irons varied from flakes to nodules, the matrix varied from ferrite to pearlite to martensite. In the fatigue crack growth rate tests, using fracture mechanics methods, it was observed that the fatigue crack growth rate increases significantly as the cyclic stress intensity range approaches the critical value for stable crack growth. This phenomenon was used to determine the fracture toughness of the cast irons. Such toughness data agree well with literature data on the fracture toughness of cast irons. An extensive review of the effects of strength on the fracture toughness of commercial cast irons is presented. In cast irons with flake graphite, cyclic loading results in a reduced modulus of elasticity. This is attributed to the rupture of the graphite flakes under cyclic loading.     

Meßmethoden und technische Anwendungen der Bruchmechanik

Measuring methods and technical application of fracture mechanics. A method of measuring the plane-strain fracture toughness K_lc using compact-tension-specimens according to American (ASTM) and British (BISRA) standards is described. There exist conditions for the minimum thickness and minimum crack length necessary for the determination of valid K_lc-values. The K_lc-values decrease with decreasing temperature. Own results and pictures of the fracture surfaces of the tempering steel 30 Cr Ni Mo 8 are shown as examples. With low strength steels linear elastic fracture mechanics characterized by K_lc is normally not valid at room temperature. Therefore it is supplemented by the crack-opening-displacement concept (COD-concept) applicable up to macroscopic yielding. — After a survey of three failure categories for cylindrical pressure vessels with axial through the wall cracks and the controlling material properties two examples are given for the application of fracture mechanics to test pressure vessels containing longitudinal slits.     

New Al–Mg–Li–C dispersion strengthened alloy Part 2 – Optimisation of material for forging applications

Abstract

This paper describes work aimed at optimising the forging process and mechanical properties of a new powder route Al–Mg–Li–C dispersion strengthened alloy, specifically for forging applications. The alloy, of the AA 5091 type, has a nominal composition of Al–1.2Li–5Mg–0.35C (wt-%), and is non-heat treatable. Materials were supplied by AMC Ltd, and evaluated in laboratory based forging trials to determine the influence of forging parameters on mechanical properties, including an investigation of using extruded bar as forging stock. The process was scaled up to produce an industrial forging, using an EH101 helicopter part, which was cut up for detailed mechanical property assessment. It was found that adequate hot working of the material is needed for good toughness, and that the balance of strength to toughness can be controlled by the forging temperature. The new alloy, in the as forged condition, meets the minimum tensile and fracture toughness levels set in the draft AECMA Specification for AA 5091 forgings.     

Fatigue and fracture of a Ni–P amorphous alloy thin film on the micrometer scale

Fracture and fatigue tests have been performed on micro‐sized specimens for microelectromechanical systems (MEMS) or micro system technology (MST) applications. Cantilever beam type specimens with dimensions of 10 × 12 × 50 μm³, approximately 1/1000th the size of ordinary‐sized specimens, were prepared from a Ni–P amorphous thin film by focused ion beam machining. Fatigue crack growth and fracture toughness tests were carried out in air at room temperature, using a mechanical testing machine developed for micro‐sized specimens. In fracture toughness tests, fatigue pre‐cracks were introduced ahead of the notches. Fatigue crack growth resistance curves were obtained from the measurement of striation spacing on the fatigue surface, with closure effects on the fatigue crack growth also being observed for micro‐sized specimens. Once fatigue crack growth occurs, the specimens fail within one thousand cycles. This indicates that the fatigue life of micro‐sized specimens is mainly dominated by a crack initiation process, also suggesting that even a micro‐sized surface flaw may be an initiation site for fatigue cracks which will shorten the fatigue life of micro‐sized specimens. As a result of fracture toughness tests, the values of plane strain fracture toughness, K_IC, were not obtained because the criteria of plane strain were not satisfied by this specimen size. As the plane strain requirements are determined by the stress intensity, K, and by the yield stress of the material, it is difficult for micro‐sized specimens to satisfy these requirements. Plane‐stress‐ and plane‐strain‐dominated regions were clearly observed on the fracture surfaces and their sizes were consistent with those estimated by fracture mechanics calculations. This indicates that fracture mechanics is still valid for such micro‐sized specimens. The results obtained in this investigation should be considered when designing actual MEMS/MST devices.     

Fracture toughness KIC analysis of Co-doped 7YSZ

Abstract

Co-doped samples of 7YSZ with Yb³⁺, Ce⁴⁺ and Nb⁵⁺ having high porosity are subject to Vickers hardness testing. Fracture toughness K_IC values are obtained by measuring linear and non-linear crack geometries. Three separate means are used to calculate the fracture toughness and to investigate the associated trends. It is confirmed that high amounts of retained tetragonal zirconia improve fracture toughness, while elevated amounts of monoclinic zirconia lower overall fracture toughness. The experimental trend for increasing K_IC is Nb:7YSZ<Yb:7YSZ<7YSZ<Ce:7YSZ; however, this trend is qualitative as the Young’s modulus values for different samples are corrected for porosity using an equation that does not generally apply to indentation techniques.     

Forging behaviour and properties of metal matrix composites based on mechanically alloyed AI-Mg-Li alloy

Abstract

A range of Al-Mg-Li-C MMCs (25 vol.-% of 3 μm SiC particles) were produced by mechanical alloying and powder processing at Aerospace Metal composites, Farnborough. Upset forging trials were conducted at 350 and 550°C and 0.01 and 0.1 S^-l. No SiC particle cracking or wedge cracking at grain boundary triple points was observed. Forging at 550°C caused some edge cracking and a coarser grain structure. Large billets of varying composition were forged to ～15 mm thick plate at 350°C and 0.0l S-^l or 550°C and 0.l S^-l. Monotonic testing showed the MMCs to exhibit high stiffnesses at moderate strength levels but rather low ductility and fracture toughness. Significant strengthening was found to accrue from dispersion hardening (C content) and solid solution strengthening (Mg and Li solutes). A link was found between lower proof stress and coarser grain structure after forging at 550°C. Fracture toughness K_1C was found to decrease with increasing yield strength which was attributed to higher strain concentrations in the smaller crack tip plastic zone.     

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.     

Notch fracture toughness of high-strength Al alloys

In this study, the notch fracture toughness (NFT) of high-strength Al alloys was examined by a non-standardized procedure. The NFT is defined as the critical notch stress-intensity factor (NSIF) K_ρ,c, which is determined by using several methods of analysis and computing. A set of specimens with different notch root radii made from overaged 7xxx alloy forging was selected. The influence of the notch radius on the fracture toughness of the material was considered. It was found that the notch radius strongly affects the fracture behavior of forged 7xxx alloy in overaged condition. The notch fracture toughness was higher than the fracture toughness of a cracked specimen and increased linearly with notch radius. The critical notch radius was related to the spacing of intermetallic (IM) particles which promote an intergranular or transgranular fracture mechanism according to their size. It appeared that ductile transgranular fracture generated by the formation of dimples around dispersoids and matrix precipitates was predominant which indicates that intense strains are limited to a much smaller zone than the coarse IM particles spacing. This double mechanism is also operate for crack propagation of ductile fatigue. The nature and morphology of IM particles exert significant effects on the rate of fatigue crack growth and fracture toughness properties.     

Mode II fracture testing method for highly orthotropic materials like wood

In this paper a mode II fracture testing method has been developed for wood from analytical, experimental and numerical investigations. Analytical results obtained by other researchers showed that the specimen geometry and loading type used for the proposed mode II testing method results in only mode II stress intensity and no mode I stress intensity at the crack tip. Experiments have been carried out to determine mode II fracture toughness K _IIC and fracture energy G _IIF from the test data collected from both spruce (pice abies) and poplar (populus nigra) specimens. It was found that there existed a very good relation between fracture toughness K_IIC and fracture energy G _IIF when the influence of orthotropic stiffness E _II ^* in mode II was taken into account. It verified that for this mode II testing method the formula of LEFM can be employed for calculating mode II fracture toughness even for highly orthotropic materials like wood. In the numerical studies for the tested spruce specimen, the crack propagation process, stress and strain fields in front of crack tips and the stress distributions along the ligament have been investigated in detail. It can be seen that the simulated crack propagating process along the ligament is a typical shear cracking pattern and the development of cracks along the ligament is due to shear stress concentrations at the crack tips of the specimen. It has been shown that this mode II fracture testing method is suitable for measuring mode II fracture toughness K _IIC for highly orthotropic materials like wood.     

Partially stiffened elastic half-plane with an edge crack

A technique, using the Brazilian disk specimen, for measuring the fracture toughness of unidirectional fiber-reinforced composites, over the entire range of crack-tip mode mixities, was developed. The fracture toughness of a graphite/epoxy fiber-reinforced composite was measured, under both mode-I and mode-II loading conditions. We found that for certain material orientations the mode-II fracture toughness is substantially higher than the mode-I toughness. The complete dependence of the fracture toughness on the crack-tip mixity was determined for particular material orientations and the phenomenological fracture toughness curves were constructed. Using the Brazilian disk specimen, together with a hydraulic testing machine, the fracture toughness of the composite under moderate loading rates was measured. We observed that the mode-I fracture toughness was not sensitive to the loading rate at the crack tip, K, while the mode-II ‘dynamic’ fracture toughness increased approximately 50 percent over the quasi-static fracture toughness. A qualitative explanation of the dependency of fracture toughness on crack-tip loading rate is discussed. Finally, a mechanical fracture criterion, at the microscopic level, which governs the crack initiation under mixed-mode loading conditions is presented; these theoretical predictions closely follow the trend of experimental measurements. This revised version was published online in August 2006 with corrections to the Cover Date.     

Failure analysis of cold forged 37Cr4 alloy M10x28 bolts

This paper covers experimental and numerical studies on the crack evolution on cold forged ³⁷Cr₄ high strength steel M10x28 bolts. Numerical simulations of forging operations were prepared on SIMUFACT FORMING finite element software. Failure evolution in simulations was predicted by using Cockroft-Latham damage model. Possible surface defects on work-pieces were also considered in the study. Artificial surface defects were generated on work-pieces by using electric discharge machine to determine the effects of defects on crack formation and propagation. As followings, numerical predictions on crack evolution were verified by conducting forging experiments. It was determined that deviation of the punch during forging led formation of shear cracks on the bolt head. Artificial surface defect was found to trigger crack evolution and affect the shape and propagation of the crack. Shear cracks propagated though the head of the bolt with 52° inclination angle while cracks triggered by surface defect propagated perpendicularly though longitudinal axis of the bolt.     

The effect of ceria co-doping on chemical stability and fracture toughness of Y-TZP

The fracture toughness and ageing resistance of yttria, ceria-stabilized tetragonal zirconia polycrystals (Y, Ce-TZP) were evaluated as a function of grain size and ceria content. Very fine grained, fully dense materials could be produced by sinter forging at relatively low temperatures (1150–1200 °C). The ageing resistance in hot water (185 °C) of 2 mol% Y₂O₃-stabilized TZP is strongly enhanced by alloying with ceria. The ceria content necessary to avoid degradation completely, decreases with grain size. The toughness of fully dense Y, Ce-TZP is 7–9 MPa m^1/2 for grain sizes down to 0.2 m. No or very little transformation took place during fracturing and no clear variation with grain size was observed for the toughness at grain sizes up to 0.8 m. Reversible transformation and crack deflection may explain the observed toughness values.