The cracking of zinc spangles on hot-dipped galvanized steel

The cracking of zinc spangles on hot dipped galvanized sheet steel was studied under controlled conditions. It was confirmed that there are two major modes of failure—boundary cracking and intragranular basal cleavage. There are four types of surface topographiesviz “mirror-like”, “feathery”, “dimpled” and “ridged”. Of these only the last type, “ridged”, was found to have its basal planes lying at an angle to the plane of the sheet. These spangles also tended to fracture most readily when strained. Hence a relationship between surface topography, crystallographic orientation and fracture behavior has been established. The implications of these findings for paint adherence is discussed. Formerly Senior Lecturer, University of Newcastle.     

Crystallography of Fluted Fracture in Near-<Emphasis Type="Italic">α</Emphasis> Titanium Alloys

The fracture topography of two-phase titanium alloys is generally complex and reflects features of the underlying microstructure, including crystallographic orientation. This article describes the correlation between crystallographic orientation and the elongated dimples, more commonly known as flutes, that are often observed on fracture surfaces of α and near-α titanium alloys and other hcp metals. The correlations are made by employing quantitative tilt fractography and electron backscatter diffraction (EBSD).     

The fracture energy of bimaterial interfaces

This article describes a framework applicable to the measurement and interpretation of the fracture energy of bimaterial interfaces. A major conclusion of this study is that the fracture energy, Γ_i, is not unique and usually exhibits values substantially larger than the thermodynamic “work of adhesion. ” The lack of uniqueness is related to mode mixity (shear/opening) effects experienced by interface cracks, as characterized by the phase angle of loading, ψ: typically, Γ_i, is found to increase as ψ increases. These trends are attributed to crack shielding caused by roughness of the interface fracture surface, to material nonlinearity,etc. The phase angle is, in turn, influenced by the choice of test specimen, resulting in values of Γ_i that differ between specimens in a manner attributed to the locus of Γ_i with ψ. Preliminary models that relate Γ_i to roughness, plasticity, segregation,etc., are described, leading to insights concerning microstructural aspects of “weaklrd and “strong” interfaces. This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

Microstructure-property relationships in materials

Microstructures of materials differ fundmentally from the structures of isolated objects, in that objects normally possess specific form and size, whereas microstructure goes on and on as a continnum, never appearing quite the same in any two places and having no beginning or ending. Such “unbounded irregular structures” are amenable to precise characterization by the use of parameters that can be expressed in terms of totals in unit volume of material, sometimes called the “global parameters”. The methods of quantitative microscopy have, thus far, provided eight parameters of this kind, namely: length of line, area of surface, volume fraction, curvature of line, torsion of line, curvature of surface, number, and connectivity. Whenever a physical property can be related simply and directly to one of these parameters, the relation is insensitive to the value of any other geometric property of the structure, that is, it is shape-insensitive. Average geometric properties of microstructure can be had by taking ratios of the fundamental parameters. These can be used to formulate structure-property relationships when the average is made up of uniquely defined components. This condition has been found to prevail when the structure is in a configuration of minimum energy.     

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

The effect of the welding cycle on the fracture toughness properties of high-strength low alloy (HSLA) steels is examined by means of thermal simulation of heat-affected zone (HAZ) microstructures. Tensile tests on notched bars and fracture toughness tests at various temperatures are performed together with fracture surface observations and cross-sectional analyses. The influence of martensite-austenite (M-A) constituents and of “crystallographic” bainite packets on cleavage fracture micromechanisms is, thus, evidenced as a function of temperature. Three weakest-link probabilistic models (the “Master-curve” (MC) approach, the Beremin model, and a “double-barrier” (DB) model) are applied to account for the ductile-to-brittle transition (DBT) fracture toughness curve. Some analogy, but also differences, are found between the MC approach and the Beremin model. The DB model, having nonfitted, physically based scatter parameters, is applied to the martensite-containing HAZ microstructures and gives promising results.     

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

The effect of the welding cycle on the fracture toughness properties of high-strength low alloy (HSLA) steels is examined by means of thermal simulation of heat-affected zone (HAZ) microstructures. Tensile tests on notched bars and fracture toughness tests at various temperatures are performed together with fracture surface observations and cross-sectional analyses. The influence of martensite-austenite (M-A) constituents and of “crystallographic” bainite packets on cleavage fracture micromechanisms is, thus, evidenced as a function of temperature. Three weakest-link probabilistic models (the “Master-curve” (MC) approach, the Beremin model, and a “double-barrier” (DB) model) are applied to account for the ductile-to-brittle transition (DBT) fracture toughness curve. Some analogy, but also differences, are found between the MC approach and the Beremin model. The DB model, having nonfitted, physically based scatter parameters, is applied to the martensite-containing HAZ microstructures and gives promising results.     

A general method for estimation of fracture surface roughness: Part I. Theoretical aspects

An assumption-free general method is developed for quantitative estimation of fracture surface roughness from the measurements performed on the fracture profiles generated by sectioning planes which are normal to the average topographic plane of the fracture surface. The input data are the profile roughness parameter,R _L , and angular orientation distribution of line elements on the fracture profile,f(α). It is shown that (fx1193-01) whereR _S is the fracture surface roughness parameter andψ is the profile structure factor, which is completely determined by the profile orientation distribution function,f(α).R _L · ψ the expected value of the productR _L · ψ on a set of sectioning planes normal to the average topographic plane of the fracture surface; measurement ofR _L andψ on few such sectioning planes can give a reliable estimate of the fracture surface roughness,R _S . The result is geometrically general, and it is applicable to fracture surfaces of any arbitrary complexity and anisotropy.     

Temper embrittlement of Ni-Cr Steels by phosphorus

Temper embrittlement in 3.5 pct Ni, 1.7 pct Cr steels doped with P and isothermally aged at several temperatures was studied by measurements of ductile-to-brittle transition temperature and hardness, which were correlated with observations of the intergranular fracture surfaces by Auger electron spectroscopy and scanning electron fractography. It is shown that if all other factors remain constant, the effect of a small change in the matrix hardness can be very large; “overaging” (a maximum in embrittlement with respect to aging time) was found to result from softening rather than from a reversal of segregation of P. Nickel was found to be segregated at the grain boundaries, and both Ni and Cr appear to enhance the amount of segregated P. The major role of Cr was found to be its effect of increasing matrix hardness (by enhancing hardenability and resistance to softening during tempering), resulting in an increased susceptibility to temper embrittlement. The effect of variations in the roughness of grain boundary topography appears to be small. It is shown that the segregation of P to grain boundaries can be accounted for by diffusion from the matrix and is consistent with the hypothesis of equilibrium (Gibbsian) segregation. The results are in qualitative agreement with the thermo-dynamic theory of Guttmann. Formerly a Research Fellow at the Department of Materials Science, University of Pennsylvania, Philadelphia, PA 19174.     

Modeling of local strains in ductile fracture

The concept of dividing microvoid coalescence (MVC) ductile fracture into three constituent processes, nucleation, growth, and coalescence, is discussed, with emphasis on needs for additional analytical and experimental work. Statistical and stochastic aspects of the problem are presented. Recent work on modeling of local strains during ductile fracture, and particularly as components of fracture toughness, is summarized and discussed in light of current knowledge of ductile fracture. Such local strain modeling is especially attractive because it permits micromechanisms of fracture to be explicitly included in the fracture model. This paper is based on a presentation made at the symposium “Stochastic Aspects of Fracture” held at the 1986 annual AIME meeting in New Orleans, LA, on March 2-6, 1986, under the auspices of the ASM/MSD Flow and Fracture Committee.     

Weibull statistical fracture theory for the fracture of ceramics

The Weibull statistical fracture theory is widely applied to the fracture of ceramic materials. The foundations of the Weibull theory for brittle fracture are reviewed. This theory predicts that brittle fracture strength is a function of size, stress distribution, and stress state. Experimental multiaxial loading results for A1₂O₃ tubes are compared to the stress state predictions of the Weibull theory. For the most part, the Weibull theory yields reasonable predictions, although there may be some difficulties in dealing with shear stress effects on fracture. This paper is based on a presentation made at the symposium “Stochastic Aspects of Fracture” held at the 1986 annual AIME meeting in New Orleans, LA, on March 2-6, 1986, under the auspices of the ASM/MSD Flow and Fracture Committee.     

Influence of cyclic to mean load ratio on creep/fatigue crack growth

Crack growth data under combined creep and fatigue loading conditions are presented on a nickel base superalloy and a brittle and ductile low alloy steel. The main variables that have been examined are minimum to maximum load ratioR and frequency. It is shown that at high frequencies transgranular fatigue failure dominates and at low frequencies time dependent mechanisms govern. Where fatigue processes control, it is demonstrated that crack growth/cycle can be described by the Paris law and that the influence ofR ratio can be accounted for by crack closure caused by fracture surface roughness, oxidation, and creep and plastic strain developed at the crack tip. At the low frequencies where time dependent processes dominate, it is shown that crack growth can be characterized satisfactorily in terms of the creep fracture mechanics parameterC * using a model of crack extension based on ductility exhaustion in a creep damage zone at the crack tip. This model leads to enhanced resistance to creep/fatigue crack growth with increase in material creep ductility. This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue” presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee.     

Role of third order statistics in discriminating among models of fatigue crack growth in metals

Fatigue crack growth is looked upon as an irreversible nondecreasing stochastic process. The suitability of several well-known distributions for describing the time for a crack to progress a specified amount is discussed relative to the coefficient of skewness. Based upon available data, it is shown that “weakest link” distributions that have appeared in the literature are not suitable. Further, it is demonstrated that to the contrary, “strongest link” distributions are consistent with the data. Finally, implications for stochastic models of fatigue crack growth are discussed. This paper is based on a presentation made at the symposium “Stochastic Aspects of Fracture” held at the 1986 annual AIME meeting in New Orleans, LA, on March 2-6, 1986, under the auspices of the ASM/MSD Flow and Fracture Committee.     

A general method for estimation of fracture surface roughness: Part I. Theoretical aspects

An assumption-free general method is developed for quantitative estimation of fracture surface roughness from the measurements performed on the fracture profiles generated by sectioning planes which are normal to the average topographic plane of the fracture surface. The input data are the profile roughness parameter,R _L, and angular orientation distribution of line elements on the fracture profile,f(α). It is shown that

Mechanical properties of “Thick” amorphous metallic wires produced by the Ulitovskii-Taylor method

The strength and plastic characteristics of thick (d = 40–120 μm) amorphous wires made of a model Co-based alloy and fabricated by the Ulitovskii-Taylor method are studied. They are found to have stable strength and plastic characteristics along the length. The plasticity of the thick wires is high, and they can form a full knot and undergo a load of 0.5 ultimate tensile strength in this state. The surface state and fracture surfaces of the amorphous wires are analyzed by scanning electron and optical microscopy. The wires are found to have a smooth lustrous lateral surface almost free of defects and to retain stable geometrical parameters along their length. Zones with different positions and frequencies of shear bands can form on the wire surface depending on the type of deformation action. The fracture surfaces of the thick amorphous wires are specific: a venous zone consists of several large pronounced principal “veins” and a rare network of adjoining secondary veins.     

Micromechanisms of brittle fracture

Mechanical processes operating in materials on the scale of the microstructure have come to be called “micromechanisms.” The fundamental science and the micromechanisms of brittle fracture are reviewed here, with particular emphasis on cleavage and intergranular fracture. Extant micromechanisms for these fracture types are evaluated. The role of solutes, particularly in intergranular fracture, is also discussed in terms of the fundamentals of brittle fracture. This article is based on a presentation made in the symposium “Quasi-Brittle Fracture” presented during the TMS fall meeting, Cincinnati, OH, October 21–24, 1991, under the auspices of the TMS Mechanical Metallurgy Committee and the ASM/MSD Flow and Fracture Committee.     

Damage mechanisms and fiber orientation effects on the load-bearing capabilities of a NEXTEL/BLACKGLAS low-cost ceramic composite

In this article, the fiber orientation effect on the load-bearing capabilities of a NEXTEL/BLACKGLAS low-cost composite was experimentally investigated, and damage mechanisms were analyzed. Ultrasonic nondestructive evaluation (NDE) was performed to interrogate the composite samples before testing. A four-point bending test was conducted to study the mechanical behavior. Scanning electron microscopy (SEM) fractography and SEM energy-dispersive X-ray spectroscopy (EDXS) along with SEM/EDXS elemental mapping were employed to characterize damage mechanisms, microstructures, and the microchemical distribution of elements following mechanical tests. A mechanistic understanding of the fracture behavior of the NEXTEL/BLACKGLAS ceramic composite was provided. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Damage mechanisms and fiber orientation effects on the load-bearing capabilities of a NEXTEL/BLACKGLAS low-cost ceramic composite

In this article, the fiber orientation effect on the load-bearing capabilities of a NEXTEL/BLACKGLAS low-cost composite was experimentally investigated, and damage mechanisms were analyzed. Ultrasonic nondestructive evaluation (NDE) was performed to interrogate the composite samples before testing. A four-point bending test was conducted to study the mechanical behavior. Scanning electron microscopy (SEM) fractography and SEM energy-dispersive X-ray spectroscopy (EDXS) along with SEM/EDXS elemental mapping were employed to characterize damage mechanisms, microstructures, and the microchemical distribution of elements following mechanical tests. A mechanistic understanding of the fracture behavior of the NEXTEL/BLACKGLAS ceramic composite was provided. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

mechanisms of fatigue crack retardation following single

In ingot metallurgy (IM) alloys, the number of delay cycles following a single tensile overload typically increases from a minimum at an intermediate baseline stress intensity range, ΔK_B, with decreasing ΔK_B approaching threshold and increasing ΔK_B approaching unstable fracture to produce a characteristic “U”-shaped curve. Two models have been proposed to explain this behavior. One model is based on the interaction between roughness and plasticity-induced closure, while the other model only utilizes plasticity-induced closure. This article examines these models using experimental results from constant amplitude and single overload fatigue tests performed on two powder metallurgy (PM) aluminum alloys, AL-905XL and A A 8009. The results indicate that the “U”-shaped curve is primarily due to plasticity-induced closure, and that the plasticity-induced retardation effect is through-thickness in nature, occurring in both the surface and interior regions. However, the retardation effect is greater at the surface, because the increase in plastic strain at the crack tip and overload plastic zone size are larger in the plane-stress surface regions than in the plane-strain interior regions. These results are not entirely consistent with either of the proposed models.     

The relation of microstructure and fracture properties of electron beam melted,modified SAE 4620 steels

A method is described for the transmission and scanning electron microscope study of the relationship between the microstructure and the fracture properties of two quenched and tempered, electron beam melted, modified SAE 4620 steels consisting of tempered low carbon martensite. Among all the microstructure constituents considered, the constituentR (randomly oriented, “tempered low carbon martensite, TLCM”) achieved the highest probability for dimple fracture. The thick TLCM laths (designated as the microstructure constituent II) exhibited higher probability of dimple plus quasi-dimple mode of fracture than the thin laths (I). It is concluded that the steel EB1035 derived the high toughness from a) the high concentration of the “high toughness” microstructure constituentsR and II, b) “non-embrittled” prior austenite grain boundaries with 50 pct probability for smooth plus quasi-smooth mode and 50 pct dimple plus quasi-dimple mode of intergranular fracture. In contrast, besides having low content ofR and II, the steel EB1014 displayed “completely embrittled” prior austenite grain boundaries with 100 pct probability for smooth plus quasi-smooth intergranular fracture. The conclusions derived from the microconstituentsR, II and I seemed to reflect the “embrittling” effect of decreased spacings between the pseudo twin related laths and between the lath boundary cementite films, and the “toughening” effect of the randomly oriented laths. Auger spectra obtained from the fracture surface before and after sputtering is analyzed to determine the presence of grain boundary sulfur segregation.     

Mode I stress intensity factors induced by fracture surface roughness under pure mode III loading: Application to the effect of loading modes on stress corrosion crack growth

A model to estimate the reduction of effective crack tip Mode III stress intensity factors by frictional and asperity interaction of an idealized fracture surface is described. An extension of the model is used to calculate the Mode I stress intensity factors due to the crack tip opening displacement induced by the mismatch of the fracture surface asperities. The results of calculations based on a “reasonable” fracture surface profile are used to analyze experimental studies designed to determine the relative significance of hydrogen embrittlement and crack tip dissolution in stress corrosion crack growth in Al alloys by comparison of Mode I and Mode III stress corrosion cracking (SCC) resistance. It is concluded that a pure Mode III stress state is not possible for cracks with microscopically rough surfaces and that the magnitude of the induced Mode I stress intensity factor is sufficient to cause stress corrosion crack growth.