Crack Growth Plane Research Articles

Geometric tunability of interlaminar resistance

The inherent weakness and variability in the interlaminar properties of composites and associated delamination pose significant challenges, compromising the performance of composite structures. This study investigates a strategy to tailor the apparent interlaminar resistance by altering the geometry of the interface to control crack propagation. We have developed analytical models, which propose a formulation that links the geometry, i.e. the interface width change rate and shape factor of the crack growth plane, with the reaction force–displacement curve. The results obtained, a family of power law relations followed during interlaminar crack growth, indicate that the analytical formulation is valid for all geometries tested, including those with significant width change rates; however, it may not always be easy to use or interpret. An approximated solution, constrained to slowly varying geometries, is also proposed. The limits of using this model are presented and discussed. The final validation of the models is performed using numerical and experimental approaches, considering the delamination of carbon fiber laminates. This work offers new insights into the design of composites and the fracture process in general, featuring unique shapes and improved crack resistance. It also addresses practical issues, e.g., related to composite repairs, which can enhance the performance and longevity of composite structures. Specifically, the proposed formulations can be easily adopted to obtain optimized patch geometries.

Structural integrity of additively manufactured stainless steel with cold sprayed barrier coating under combined cyclic loading

Integration of metal additive manufacturing (AM) and cold spray (CS) technologies provide an unprecedented opportunity to manufacture coated material systems with complex geometrical features. The application of these material systems in functionally critical components requires adequate structural integrity, particularly in the presence of cyclic loading. This work researches the multiaxial fatigue (axial-torsional cyclic loading) behavior of a coated material system consisting of 15Cr-5Ni precipitation-hardening stainless steel (15-5 PH SS) substrate additively manufactured by direct metal laser sintering with a layer of chromium carbide nickel (CrC-Ni) barrier coating deposited by CS. The influence of AM and CS-induced residual stresses on fatigue performance of test specimens was thoroughly studied. Additionally, the effect of surface roughness and processes induced defects were considered to explain the crack growth mechanism. Stresses assessed by synchrotron X-ray diffraction indicated a substantial accumulation of residual stresses, particularly in the outer surface of the as-fabricated 15-5 PH SS specimens. The state of residual stress was changed notably following the deposition of CrC-Ni coating in the axial, hoop, and radial directions of the fatigue test specimen. Also, CS deposition of CrC-Ni coating caused significant improvement in the surface quality of the additively manufactured components. Fatigue test results indicated, CS deposition of CrC-Ni substantially enhances the fatigue life of the AM-produced 15-5 PH SS substrate in all loading conditions, particularly in the high cycle fatigue regime. The improvement in the fatigue life of the specimens with coating was associated with a reduction in equivalent residual stress at the substrate surface and improvement in the specimens' surface condition (i.e., reduced surface roughness). The fractographic analysis of the specimen indicated the cracks tend to initiate in the surface of both as-fabricated and cold-sprayed specimens. However, the mechanism of crack growth changed notably following the deposition of CrC-Ni coating. The cracks tended to propagate in the planes parallel or with a small deviation from the build layers of the AM-produced specimens. On the other hand, deposition of CrC-Ni coating increased the deviation of crack growth plane from the build layers of the substrate.

Crack growth rates in lithium disilicates with bulk (mis)alignment of the Li2Si2O5 phase in the [001] direction

Processing-induced alignment of Li2Si2O5 crystals in biomedical lithium disilicate glass-ceramics have been shown to increase the monotonic resistance to fracture and better simulate the mechanical behavior of natural dental tissues. The (mis)orientation of the [001] direction in the Li2Si2O5 crystal phase relative to the crack growth plane was induced by injection-molding of a high aspect ratio composition in order to determine the effect of crystal alignment. The role of particulate geometry was assessed by comparing two compositions having randomly oriented Li2Si2O5 crystals of similar phase fraction but dissimilar crystal aspect ratios. Static and cyclic fatigue experiments in varying frequencies and R-ratios were conducted to clarify the degradation of crack bridging mechanisms induced by the crystal phase during repetitive opening and closure of the crack faces. Both crystal alignment and aspect ratio showed to play important roles in affecting the subcritical crack growth resistance the lifetime of lithium disilicate biomedical components.

The effect of crack path on tearing resistance of a narrow-gap Alloy 52 dissimilar metal weld

Fracture mechanical testing of welds with strength mismatch can lead to deviation of the crack path from the initial crack growth plane. Current fracture mechanics standards do not give guidance for analyses of the effect of crack path on the tearing resistance. In this study, the effect of crack path on tearing resistance was investigated by fracture mechanical testing and crack path analyses of a narrow-gap Alloy 52 dissimilar metal weld. The results show that tearing resistance of HAZ cracks deviating towards the same zone close to the fusion boundary is larger for cracks initially further from the fusion boundary. The results were obtained for a specific dissimilar metal weld, but the results can also be used to interpret the effect of crack path on tearing resistance in other type of welds.

Fracture anisotropy in texturized lithium disilicate glass-ceramics

Lithium disilicate glass-ceramics show great potential in the fabrication of anisotropic structures due to their high-strength needle-form crystalline phase subjective to processing-induced alignment. Here we explore this strategy by focusing on the mechanistic aspects of fracture. Through different variations of the fracture toughness test we demonstrate the effect of bulk crystal orientation on fracture energy, toughening mechanisms, crystal size and aspect ratio. Using the anisotropic Poisson's ratio of the Li2Si2O5 crystal phase obtained by Resonant Ultrasound Spectroscopy, we apply the new geometry factor developed by Strobl et al. to provide a more accurate insight into the anisotropic crack propagation behavior in LS2 glass-ceramics. Raman spectra and X-Ray Diffraction patterns are resolved for the Li2Si2O5 phase aligned at plane parallel, anti-plane and random orientations.We show that Li2Si2O5 crystallites oriented perpendicular to the crack growth plane tended to induce large scale crack deflection unless the crack was forced into a straight path, thereby promoting crystallite fracture, increased KIc-values and accentuated R-curve behavior. Crystal aspect ratio and residual stresses in the glass have been identified as important influencing factors on crack growth behavior and toughening mechanisms.

Effect of inhomogeneous distribution of non-metallic inclusions on crack path deflection in G42CrMo4 steel at different loading rates

An inhomogeneous distribution of non-metallic inclusions can result from the steel casting process. The aim of the present study was to investigate the damaging effect of an inhomogeneous distribution of nonmetallic inclusions on the crack extension behavior. To this end, the fracture toughness behavior in terms of quasi-static J-?a curves was determined at room temperature. Additionally, dynamic fracture mechanics tests in an instrumented Charpy impact-testing machine were performed. The fracture surface of fracture mechanics specimens was analyzed by means of scanning electron microscopy. It was shown that an inhomogeneous distribution significantly affected the path and, therefore, the plane of crack growth. Especially clusters of non-metallic inclusions with a size of up to 200 ?m exhibited a very low crack growth resistance. Due to the damaging effect of the clusters, the growing crack was strongly deflected towards the cluster. Furthermore, crack tip blunting was completely inhibited when inclusions were located at the fatigue precrack tip. Due to the large size of the non-metallic inclusion clusters, the height difference introduced by crack path deflection was significantly larger than the stretch zone height due to the crack tip blunting. However, the crack path deflection introduced by a cluster was not associated with a toughness increasing mechanism. The dynamic loading ( 1 0.5 5 s MPam 10 ? ? K? ) did not result in a transition from ductile fracture to brittle fracture. However, the crack growth resistance decreased with increased loading rate. This was attributed to the higher portion of relatively flat regions where the dimples were less distinct.

Load sequence effects on fatigue crack growth in notched tubular specimens subjected to axial and torsion loadings

Fatigue crack growth behavior of tubular specimens with a through thickness circular hole made of a carbon steel subjected to axial and torsional loads was investigate. Loading sequence effect on crack growth rate was also studied by alternating between axial and torsion cycles in a loading block. Mode I crack growth was observed. Torsion fatigue crack growth lives were shorter and crack growth rates were higher than for axial loading. This is explained by a larger plastic zone size produced by a compressive tangential stress acting parallel to the crack growth path. In block loading with dominated torsion cycles crack grown rate was slower in comparison with pure torsion, while in block loading with dominated axial cycles a faster crack growth rate occurred in comparison with pure axial loading. Effects of the stress state on the plane of crack growth and of one pair of cracks on a second pair are considered to explain these observations. Crack growth rates were correlated with stress intensity factor range with or without considering the T-stress effect. Short crack growth behavior near the threshold region is also discussed.

Ratchetting strain as a driving force for fatigue crack growth

Ratchetting deformation has significant implications on material damage and fatigue life of components under service loading conditions. In this work, we explore the concept of ratchetting strain as a driving force in controlling crack growth, both time-independent and time-dependent, utilising elasto-plastic, visco-plastic and crystal-plasticity constitutive models. Characteristics of crack tip deformation were examined for both stationary and growing cracks using the finite element method. Whilst the strain range and the stress range remained largely unchanged throughout the loading cycles, distinctive strain ratchetting behaviour near the crack tip occurred in all cases, leading to progressive accumulation of tensile strain normal to the crack growth plane. It seems plausible that this tensile strain, or ratchetting strain, may be responsible for material separation leading to crack growth. The effects of material constitutive model, grain orientation and loading frequency on the development of ratchetting strain were also examined. Ratchetting strain was found to be related to accumulated plastic strain; and both were used in a crack growth criterion with which the crack growth rates of a nickel alloy were predicted. The preliminary results seem to be encouraging.

In situ synchrotron X-ray imaging of high-cycle fatigue crack propagation in single-crystal nickel-base alloys

Fatigue crack propagation in single-crystal nickel-base superalloys was studied in situ with synchrotron X-ray imaging in a very high-cycle regime. Experiments were performed at ambient temperature with a newly developed portable ultrasonic fatigue instrument. The fatigue apparatus produced stable fatigue crack propagation with positive mean stresses for specimens as thin as 150 μm at a loading frequency of 20 kHz. Fatigue cracks propagated in a mixed-mode crystallographic manner along {1 1 1} slip planes. Phase-contrast imaging permitted analysis of the interaction of the crack front with microstructural features. Three-dimensional linear elastic finite-element analyses were performed to investigate the driving forces for crack propagation in the sheet specimen geometry. For the experimental conditions examined, fatigue crack growth is driven by the resolved shear and normal stresses on the active crack tip slip systems. An octahedral stress intensity is proposed as the relevant driving force for crack growth which determines the fatigue crack growth plane in nickel-base single crystals at room temperature.

Modelling of the carbon nanotube bridging effect on the toughening of polymers and experimental verification

The effect of aligned and randomly oriented carbon nanotube (CNT), with respect to the crack growth plane, on the fracture toughness of polymers is modelled in this paper using the Elastic Plastic Fracture Mechanics. According to a critical length, two dominant toughening mechanisms for CNT-modified polymers are presented, i.e. CNT pull-out and CNT rupture. The model is then used to identify the effect of CNTs geometrical and mechanical properties on the enhancement of fracture toughness in CNT-modified polymers. The key CNT properties are the CNT radius, average length, ultimate strength, elongation before failure, interfacial shear strength between CNTs and the polymer and nanotube volume fraction. Finally, experimental results are compared with the model predictions. The correlation shows that processing of long, aligned CNTs remains the main barrier in achieving major fracture toughness enhancement.

A viscoplastic study of crack-tip deformation and crack growth in a nickel-based superalloy at elevated temperature

Viscoplastic crack-tip deformation behaviour in a nickel-based superalloy at elevated temperature has been studied for both stationary and growing cracks in a compact tension (CT) specimen using the finite element method. The material behaviour was described by a unified viscoplastic constitutive model with non-linear kinematic and isotropic hardening rules, and implemented in the finite element software ABAQUS via a user-defined material subroutine (UMAT). Finite element analyses for stationary cracks showed distinctive strain ratchetting behaviour near the crack tip at selected load ratios, leading to progressive accumulation of tensile strain normal to the crack-growth plane. Results also showed that low frequencies and superimposed hold periods at peak loads significantly enhanced strain accumulation at crack tip. Finite element simulation of crack growth was carried out under a constant Δ K-controlled loading condition, again ratchetting was observed ahead of the crack tip, similar to that for stationary cracks. A crack-growth criterion based on strain accumulation is proposed where a crack is assumed to grow when the accumulated strain ahead of the crack tip reaches a critical value over a characteristic distance. The criterion has been utilized in the prediction of crack-growth rates in a CT specimen at selected loading ranges, frequencies and dwell periods, and the predictions were compared with the experimental results.

Assessment of bending fatigue limit for crankshaft sections with inclusion of residual stresses

The fatigue limit of rolled, ductile cast iron crankshaft sections under bending was investigated experimentally and analytically. The relationship of different failure criteria – specifically, surface crack initiation, resonant shifts, and two-piece failures – on the fatigue limit of the crankshafts was explored from testing. A comparison showed that using the surface crack failure criterion decreased the apparent fatigue limit of the crankshaft significantly. This result supported the theory of crack arrest in the fillet area of crankshafts and raises a significant challenge to the use of the surface crack failure criterion due to its inconsistent correlation to two-piece failure. Also an engineering practice for residual stress simulation and fatigue limit estimation due to a fillet rolling process on a crankshaft was explored. The complete analytical process was validated by the resonant bending fatigue test results. The normal stress distribution from the fillet surface-to-depth of a crankshaft section under bending was first obtained by a three-dimensional elastic finite element analysis. The residual stress distribution near the crankshaft fillet induced by a fillet rolling process was then determined by a three-dimensional elastic–plastic finite element analysis. A resultant normal stress distribution along any possible crack growth plane originating from the fillet surface and extending into the component was defined by superimposing the normal stress components due to the bending moment and the rolling load. Based on these calculated normal stresses, two fatigue models (such as the SWT equivalent stress and the effective stress intensity factor range) used to estimate the fatigue limits on surface cracks and in-depth cracks were examined by the experimental data. The results showed that the SWT equivalent stress was not a good fatigue damage parameter for surface cracks and the effective stress intensity factor range derived on the technique of crack modeling did correlate reasonably to the experimental data.

Fatigue crack path prediction in UDIMET 720 nickel-based alloy single crystals

The effect of orientation, applied stress state, environment, and temperature on the crack growth and crack-path behavior of single-crystal specimens of UDIMET 720 has been examined. Stage I cracking has been promoted by mixed-mode loading, plane stress, and vacuum conditions; increasing the test temperature to 600 °C does not suppress stage I crack growth in vacuum. Consideration of the local resolved shear-stress intensity and local resolved normal-stress intensity for each slip system as it intersects the nominal crack-growth plane allows the prediction of stage I crack paths, clarifying the importance of secondary single-crystal testing orientation (i.e., nominal crack-growth direction as well as effective tensile axis). A combination of both opening and shearing are found to promote stage I crack growth, and boundary conditions have been established within which stage I cracking is promoted. Highly deflected stage I cracking gives rise to significant shielding effects, but under suitable mixed-mode loading, highly oriented, coplanar stage I crack growth can be produced. Intrinsic stage I cracking under mixed-mode loading appears to be greatly accelerated compared with mode I-dominated stage II crack growth for comparable stress-intensity levels.

Mode I Fracture Toughness of a Small‐Grained Silicon Nitride: Orientation, Temperature, and Crack Length Effects

The Mode I fracture toughness (KIC) of a small‐grained Si3N4 was determined as a function of hot‐pressing orientation, temperature, testing atmosphere, and crack length using the single‐edge precracked beam method. The diameter of the Si3N4 grains was <0.4 µm, with aspect ratios of 2–8. KIC at 25°C was 6.6 ± 0.2 and 5.9 ± 0.1 MPa·m1/2 for the T–S and T–L orientations, respectively. This difference was attributed to the amount of elongated grains in the plane of crack growth. For both orientations, a continual decrease in KIC was observed through 1200°C, to ∼4.1 MPa·m1/2, before increasing rapidly to 7.5–8 MPa·m1/2 at 1300°C. The decrease in KIC through 1200°C was a result of grain‐boundary glassy phase softening. At 1300°C, reorientation of elongated grains in the direction of the applied load was suggested to explain the large increase in KIC. Crack healing was observed in specimens annealed in air. No R‐curve behavior was observed for crack lengths as short as 300 µm at either 25° or 1000°C.

Anisotropic Fracture Toughness Effects on Failure Modes of Piping

This paper summarizes various cases where anisotropic fracture toughness properties caused the failure mode to change during ductile fracture experiments on piping. It is noted that in particular for carbon steel piping, the anisotropy can cause an initial circumferential crack to propagate in a helical or even axial direction, even though there are only applied bending loads. This has implications that under combined loading, such pipes may have lower longitudinal stresses at failure than may be calculated by a leak-before-break analysis that only considers the longitudinal stresses and the toughness in the circumferential crack growth plane.

Rayleigh waves excited to acoustic emission

Analytical results are presented that describe the Rayleigh waves excited by the acoustic emission from a crack that suddenly starts to grow. At first a two-dimensional crack is considered, then the results are extended to three dimensions. The plane of crack growth is inclined to the surface. The fracture mode can be brittle or some plastic yielding at the crack edge can take place. The analysis consists of two parts. First, wavefront approximations to the acoustic emission are calculated. Second, representation integrals for the Rayleigh waves are constructed using the compressional and shear emissions in combination with an appropriate Green's tensor. The integrations are taken over the wavefronts of the emitted signals and the integrals are evaluated asymptotically. Graphs of the resultant surface motion are given. [Work supported by NSF(MEA-8104738).]

Metallographic observations on the developing hydride morphology at the crack tip during hydrogen induced delayed cracking in a Zr-2.5 Nb alloy

A comprehensive metallographic investigation has been made to study the morphology of the hydrides precipitated during Hydrogen Induced Delayed Cracking (HIDC) in Zr-2.5 Nb compact tension specimens after removing successive layers from various cross-sections of the specimen. The major findings are: (1) Hydrides are precipitated in the crack tip region in narrow bands of adjacent platelets parallel to the crack growth plane. (2) The crack front region is not uniformly hydrided but is discontinuous. This peculiar morphology of hydrides is responsible for the fracture surface which is characteristic of HIDC in this material. (3) The growth of the hydrides is not limited to the region within the plastic zone.

モードＩＩ形繰返し負荷を受ける軟鋼およびアルミニウム合金の疲労き裂進展挙動

The behavior of fatigue crack growth under mode II loading has been investigated on steels (JIS SPCC and SM 50B) and aluminum alloys (7075-T6 and 2017-T4). Tests were made by a specially designed apparatus under various combinations of ΔKII and KIS, where ΔKII is the range of the repeated mode II loading and KIS is the statically applied KI. Under mode II loading, two modes of fatigue crack growth, mode II growth and tensile mode growth, were usually observed, though in some cases only one was observed. Here, mode II growth and tensile mode growth mean the fatigue crack growth in the plane of precrack and that in the plane which makes about 60°to the precrack, respectively. In the latter, the plane of crack growth may be taken as the plane of the maximum tensile stress.Aluminum alloys showed continuous mode II growth in the region of ΔKII larger than ΔKII-threshold for mode II growth. In the case of mild steel (SPCC), however, tensile mode growth occurred in the region of ΔKII larger than ΔKII-threshold for tensile mode growth, and mode II growth was observed only in the narrow region of ΔKII smaller than the above ΔKII-threshold for tensile mode growth. In weldable structural steel (SM 50B) only tensile mode growth was observed under mode II loading. The mode II growth rate in aluminum alloys was much larger than that of mode I growth, while the mode II growth rate was very small in mild steel specimens.