Inter-crack Spacing Research Articles

Combined Infrared Thermography, X-ray Radiography, and Computed Tomography for Ink Drying Studies

This work describes the comparison of drying behavior of ink deposited on two substrates used commonly in polymer electrolyte fuel cells (PEFCs): microporous layer (MPL) and Nafion. For the same drying conditions and ink composition, ink deposited onto the MPL dried faster and also formed cracks faster. During drying, ink deposited on the MPL had more cracks and smaller intercrack spacing compared to ink deposited on Nafion. Smaller intercrack spacing for ink on the MPL substrate is explained by the higher critical stress predicted by the model. A novel combination of infrared (IR) thermography, X-ray radiography, and computed tomography (CT) is used to characterize ink drying behavior. X-ray radiography with fast temporal resolution showed the existence of skin on the top of drying film that formed due to slow convective ink transport within the film and fast evaporative drying. From X-ray CT, detailed three-dimensional data was obtained on crack morphology within the drying film and was correlated to te...

Stress evolution mechanism of NiCr thin films on titanium alloy

The stress distribution associated with the film under tensile loading of the substrate was investigated by the analytical model proposed by F. Ahmed et al. The interfacial shear stress was evaluated by the modified analytical model by B. F. Chen et al. The validity of both the models has been tested on samples with NiCr thin film and titanium alloy substrate. These samples were progressively tensile tested and subjected to intermittent scanning electron microscopy for microstructural investigations. Insitu microscopy was adopted to identify the onset of cracks during tensile loading and the crack spacing is measured. Finite Element Analysis (FEA) is performed on the film-substrate system to study the stress pattern in the film and at the interface. The interfacial shear stress follows the modified shear lag model and is in good agreement between the stress profiles proposed in the model and the FEA results. The evolution of the tensile stresses and the cracking behavior are mapped as a function of the applied strain in the analytical model. The concept of critical intercrack spacing is introduced and the stress relaxation as a function of the inter crack spacing is discussed. The interfacial shear stress evaluated is found to be in agreement with the yield shear strength of the substrate.

Stress intensity factors for multiple cracks in thick-walled cylinder

The stress intensity factor (SIF) is the linear elastic fracture mechanics parameter that relates remote load, crack size and structural geometry. It predicts very accurately the stress state. In this work, cylinders with multiple cracks are considered. The following parameters are varied during the analysis of the cylinders: the number of cracks, (the variation in number of cracks ultimately led to a variation in the inter-crack spacing), the crack length to cylinder thickness ratio (a/t), the diameter ratio of the cylinders. Very good agreement between the finite element stresses and the theoretical stresses is seen.

Effect of local stress on the heat-checking morphology in high temperature tool steels under thermal fatigue: Transition from multi-axiality to uniaxiality

Thermal fatigue experiments are performed on a high temperature tool steel X38CrMoV5 (AISI H11), under various maximum temperatures and heating rates. A microscopic interconnected crack pattern (named “heat-checking”) forms on the oxidised surface of the laboratory tubular specimen. A gradual transition is observed, from a “cell-type” cracking at the centre of the specimen to a “parallel cracking” at its extremities. This variation of the morphology is well demonstrated by geometrical and topological characteristics of the crack network (micro-crack orientations, cell shape and node density), which change along the longitudinal axis of the specimen. The thermo-elasto-plastic loading of the specimen is estimated by Finite Element Calculations using ABAQUS™. Whatever the thermal fatigue conditions, a linear correlation can be established between the longitudinal and hoop stress amplitude ratio Δσzz/Δσθθ and the hoop and longitudinal inter-crack spacing ratio dθθ/dzz. It is shown that a stress amplitude ratio close to 1 results in a multi-axial heat-checking, while a uni-axial cracking is generated when Δσzz/Δσθθ is close to or below 0.6. This means that the morphology of the heat-checking pattern (cell or parallel type) can be used as an indicator for the local stress ratio of the thermal fatigue specimen or industrial real tools.

Cracking Behavior of Cr2O3 Scales on Fe–25Cr Alloys Under Creep Deformation at 973 K in Ar Atmosphere

An Fe–25Cr alloy was oxidized in Ar at 973 K with or without external stresses of 30, 35, and 40MPa. A 0.1-μm thick Cr2O3 scale was formed during pretreatment, after which it grew obeying a parabolic rate law without formation of flaws, i.e., cracks and exfoliation. After the pretreatment, tensile stresses of 30, 35, and 40 MPa were applied to monitor cracking behavior of the oxide scale. Cracking commenced at the alloy grain boundaries by the end of a second-stage creep. In a third stage, cracks formed in the alloy grains, arrayed almost perpendicular to the direction of the tensile stress, with regular intercrack spacings. The average intercrack spacings (L) at grain boundaries are smaller than the intercrack spacing of the grains, and the spacings are given by the power law relation: L∝(e⋅)n, with strain rate exponents n of -0.22 for grains and −0.44 for grain boundaries. It appears that the local strain rate at grain boundaries is much larger than that of the grain as well as that the toughness of the oxide formed on the grain boundaries may be less than that of the grains. Indentation with a Vickers micro hardness tester indicated that the adhesion and fracture toughness of the oxide scale decreased both with growth of the oxide scale and creep deformation of the alloy substrate.

Influence of shape and locations of initial 3-D cracks on their growth in uniaxial compression

Mechanisms of 3-D crack development in uniaxial compression have been studied. Several tests undertaken on different transparent casting resin, cement and mortar samples with single internal cracks of different shapes and sizes, produced by different techniques, demonstrated that 3-D crack growth in compression was qualitatively different from the two-dimensional case. Unlike 2-D cracking, there were intrinsic limits on 3-D growth of wing cracks produced by a single pre-existing crack. This limitation was related to the wrapping (curving) of emerging wings around the initial crack. On the other hand, in samples with few initial cracks, a special crack arrangement existed which could produce a large tensile fracture parallel to the direction of loading. Particularly, for two coplanar initial cracks, the tensile fracture was produced only when the inter-crack spacing was below a critical value. In samples where the initial cracks were present in high concentrations, the influence of interaction always generated a number of tensile fractures parallel to the direction of loading.

Numerical and experimental study of the circular cracks observed at the contact edges of the indentations of coated systems with soft substrates

In this work, the finite element method (FEM) was used, in conjunction with experiments, to study some of the crack patterns observed during the indentation of coated systems. In particular, the study was centered on the array of circular cracks observed close to the contact edge of the spherical indentations of systems with soft substrates. A sequence of steps was considered during the FEM formulation. Initially, the deposition (intrinsic) and thermal (extrinsic) stresses were introduced to account for all the residual stresses present in the deposited films. Later, a loading–unloading sequence was applied on the pre-stressed system, with a maximum normal load of 50 N. The FEM also simulated the formation of the circular cracks by considering the presence of fifteen cracks distributed over the film surface and their propagation in the direction perpendicular to the film/substrate interface. Two different types of substrates and two different coatings were analyzed during the experiments. Substrates made of AA 6061 aluminum alloy and AISI 304 stainless steel were coated with (i) titanium nitride (TiN) by unbalanced magnetron sputtering; and (ii) silicon carbide (SiC) by plasma-enhanced chemical vapor deposition. Three thicknesses were studied in each of the four systems, which were later indented with normal loads of 50 N, applied by a sphere. The results indicated that, in each case, the array of circular cracks observed close to the contact edge usually develops with a similar inter-crack spacing, which has a strong relation with the film thickness and properties.

Structural changes of the friction surface and wear resistance of a ZrO2-Y2O3 ceramic

Quantitative parameters of the surface structures and the wear resistance of a ZrO2-Y2O3 ceramic tested by friction on a steal disk have been determined. It is shown that the friction contact gives rise to quasiperiodic cracks on the friction surface of the ceramic and leads to the formation of a transfer layer. It is established that the wear intensity correlates with the intercrack spacing along the sliding direction and the amount of the steel counterbody elements on the ceramic surface.

Surface cracking during creep of a Ti-25Al-11Nb alloy

The surface cracking behavior of a Ti-25Al-11Nb alloy has been quantitatively evaluated at 923 K. The influence of prior exposure at 923 K for 30–488 h on surface crack nucleation and growth has been determined at 160, 200 and 250 MPa by interrupted creep tests. A critical strain exists for surface cracking. The value of critical strain depends on the applied stress and the pre-exposure duration. The stress dependence of crack opening and intercrack spacing agrees with Riedel model for surface cracking. Microstructure insensitive transgranular crack growth suggests local embrittlement and cracking at the crack tip as crack growth mechanism.

Brittle Fracture from a Sliding Hertzian Contact

A simplified method of analysing the characteristic horseshoe cracks appearing behind a spherical indenter sliding along the surface of a brittle material, is described. The technique provides a reliable means of estimating the fracture toughness of the material and an explanation of the inter-crack spacing, which is found to be independent of the sliding speed.

Periodic oxide cracking on fe2.25Cr1 Mo produced by high-temperature fatigue tests with a compression hold

Long, straight cracks perpendicular to the stress axis are seen on the oxidized surface of specimens of Fe2.25Cr1Mo cycled with a compressive hold at high temperatures. The cracks in the oxide are periodically spaced. They resemble cracks observed in a brittle film on a ductile substrate after a tension test of the substrate. They also resemble the parallel multiple fractures that occur in a brittle matrix of a composite with ductile fibers undergoing tension. We apply both the model of a brittle film on a ductile substrate and of the brittle matrix composite to explain the observed intercrack spacing. Cracks in the oxide film lead to localized oxidation of the metal in the region around their intersection with the oxide-metal interface. These cracks are seen to penetrate the metal. Stress concentrations from deep grooves that form during compression hold fatigue, together with crack initiation from the oxide, lead to a shortened cycle life.

Stress intensity factor approximate formulae for uniform crack arrays in pressurized or autofrettaged cylinders

The presently available large number of stress intensity factor (SIF) values for uniform arrays of radial cracks, emanating from the bore of partially or fully autofrettaged cylindrical pressure vessels, results from extensive research into the fatigue and fracture processes of these vessels. The data consist of more than 400 values for K I P , the SIF due to internal pressure, for different crack arrays and for a wide range of crack lengths, as well as more than 200 values for K I A , the negative SIF due to the compressive residual stress field induced by the autofrettage process, for different configurations of crack arrays and various levels of autofrettage. A detailed analysis of all the data herein reveals that any of the above results pertains to only one of two possible categories designated “sparse crack arrays”, or “dense crack arrays”. While for sparse arrays both K I P and K I A are directly proportional to the crack length, l, in the dense case, they depend primarily on the inter-crack spacing, d, and are practically crack length independent. Furthermore, the inter-crack aspect ratio, l d , is found to be the sole parameter determining to which of the two categories each case belongs. Based on these results, and using a least-squares approach, SIF approximate formulae for K I P and K I A for sparse and dense crack arrays are developed, yielding simple expressions of very good engineering accuracy which are applicable to the whole gamut of existing results.

The effect of fiber-matrix debond energy on the matrix cracking strength and the debond shear strength

The matrix cracking model given by Budiansky, Hutchinson and Evans [ J. Mech. Phys. Solids 34, 167 (1986)] is extended to include the effect of fiber-matrix bonding in a unidirectional fiber-reinforced composite. By use of an energy balance analysis, the work of debonding Γ d is related to a characteristic critical interface shear stress τ d and to the frictional interface sliding resistance τ f by the expression τ d − τ f = √4 G m Γ d/ av, where G m is the matrix shear modulus, a, the fiber radius, and v the function of fiber volume exceeds the background fiber tensile stress far from the crack by a fixed calculable amount. Finally, the combined effect of τ f and adhesion on the intercrack spacing is predicted for the case where multiple cracks develop in the matrix.