Crack-like Notches Research Articles

Fracture mechanics of additive manufactured crack-like notches by digital image correlation

The rapid development of a variety of Additive Manufacturing (AM) techniques is witnessed by the large interest of the scientific community. Among the numerous new features offered by these techniques, very lately the possibility of including cracks of any shape, direction, and position inside AM samples to validate FEM models was explored. Therefore, the necessity arose of full-field measurement techniques that would allow the evaluation of the fracture mechanics parameters in the cases of both linear elastic and partially plastic materials, as a function of the fracture modes. Invented in the ’80 of the XX century, the Digital Image Correlation (DIC) in recent years has taken place in nearly every laboratory. It is used to measure displacements, and by numerical differentiation or coupling with Finite Element Method, to calculate strains at different spatial scales. In particular, two procedures that were successfully employed on a specific kind of specimens, based on DIC at a small scale near the crack-tip, in this paper are shown in detail.

Investigation by Digital Image Correlation of Mixed-Mode I and II Fracture Behavior of Polymeric IASCB Specimens with Additive Manufactured Crack-Like Notch.

In this work, the fracture mechanics properties of polyamide (PA) specimens manufactured by the selective laser sintering (SLS) technology are investigated, in which an embedded crack-like notch was inserted in the design and produced during the additive manufacturing (AM) phase. To cover a wide variety of mode I/II mixity levels, the inclined asymmetrical semicircular specimen subjected to three points loading (IASCB) was employed. The investigation was carried out by analyzing the full displacement field in the proximity of the crack tip by means of the digital image correlation (DIC) technique. To characterize the material, which exhibits a marked elastic-plastic behavior, the quantity J-integral was evaluated by two different methods: the first one exploits the full fields measured by the DIC, whereas the second one exploits the experimental load–displacement curves along with FEM analysis. The DIC methodology was experimentally validated and proposed as an alternative method to evaluate the J-integral. It is especially suited for conditions in which it is not possible to use the conventional LDC method due to complex and possibly unknown loading conditions. Furthermore, results showed that the AM technique could be used effectively to induce cracks in this type of material. These two aspects together can lead to both a simplification of the fracture characterization process and to the possibility of dealing with a wider number of practical, real-world scenarios. Indeed, because of the nature of the additive manufacturing process, AM crack-like notches can be sintered even having complex geometry, being three-dimensional and/or inside the tested structure.

Parameter identification of crack-like notches in aluminum plates based on strain gauge data

The identification of crack parameters and stress intensity factors in aluminum plates under tensile loading is in the focus of the presented research. In this regard, data of strain gauges, distributed along the edges of the samples, are interpreted. In the experiments, slit-shaped notches take the role of cracks located in the interior of the specimens. Their positions, inclinations and lengths as well as the magnitudes of external loadings are identified solving the inverse problems of cracked plates and associated strain fields. Exploiting the powerful approach of distributed dislocations, based on Green’s functions provided by the framework of linear elasticity, in conjunction with a genetic algorithm, allows for a very efficient identification of the sought parameters, thus being suitable for in situ monitoring of engineering structures. Tested samples exhibit one or two straight crack-like notches as well as a kinked one.

Equivalence between shallow notch and shallow crack in structural failure caused by plastic instability

Introducing crack-like notches is essential for investigating the mechanical properties of pre-cracked structures because fatigue pre-cracking occasionally fails to ensure some rigorous investigation conditions. However, the results obtained by previous studies related to crack-like notches under small-scale yielding are invalid for shallow notches with extensive plasticity, particularly when plastic instability instead of unstable crack propagation governs the structural strength. This study focused on the plastic instability occurring under the plane strain condition without the assistance of pre-crack propagation. The boundary condition, geometric configuration, and elastoplastic fields dominating the overall work hardening in notched and cracked cross-sections were considered in finite element analyses. From the perspective of asymptotic and phenomenological analysis, this study proposes that a shallow crack-like notch should satisfy the following requirements: (1) the structural strength should be independent of the notch geometry, except for the notch depth; (2) the elastoplastic fields in the notched cross-section should be broadly convergent to those in the pre-cracked cross-section; (3) the failure mode characterized by damage pattern should be similar. The underlying reason for crack-like notches existing in extensive plasticity is that different elastoplastic field gradients close to the notch root or crack tip may still result in the same overall work hardening in notched and pre-cracked cross-sections. This concept was experimentally verified using interstitial-free steel, which is a typical strain-hardening ferrite steel with excellent ductility and simple metallurgical microstructure. The generality of the identification method and the significance of influential factors for shallow crack-like notches are discussed in this study.

Investigation by Digital Image Correlation of Mixed Mode I and II Fracture Behavior of Metallic IASCB Specimens with Additive Manufactured Crack-Like Notch

This work investigates the fracture behavior of maraging steel specimens manufactured by the selective laser sintering (SLS) technology, in which a crack-like notch (sharp notch) was directly produced during the additive manufacturing (AM) process. For the evaluation of the fracture toughness, the inclined asymmetrical semi-circular specimen subjected to three points loading (IASCB) was used, allowing to cover a wide variety of Mode I and II combinations. The effectiveness of manufacturing crack-like notches via the SLS technique in metals was evaluated by comparing the obtained experimental results with the ones obtained with pre-cracks induced by fatigue loading. The investigation was carried out by using the digital image correlation (DIC) technique, that allowed the evaluation of the full displacement fields around the crack tip. The displacement field was then used to compute the stress intensity factors (SIFs) for various combinations of Mode I and II, via a fitting technique which relies on the Williams’ model for the displacement. The SIFs obtained in this way were compared to the results obtained with the conventional critical load method. The results showed that the discrepancy between the two methods reduces by ranging from Mode I to Mode II loading condition. Finally, the experimental SIFs obtained by the two methods were described by the mixed mode local stress criterium.

Generalized Formulation for Fatigue Assessment of Laser Stake-welded T-joints Varying Thicknesses and Loading Conditions

The present work considers the evolution of the crack tip plasticity ahead of the crack-like notches with varying the loading condition and the thickness of laser stake-welded T-joints. A general procedure is described for the fatigue assessment of the joints under different loading conditions and made of different thicknesses. The new method is based on the analyses of the first order plastic radius (according to Irwin) for pure mode I, and it is generalized to mixed mode (I+II) by using the Minimum Plastic Zone Radius (MPZR) criterion. The method is validated, in the end, with a case study and explains why the fatigue slope m varies for joints loaded under bending/tension and when the thickness is reduced. The results prove that the procedure gives an excellent estimation of the fatigue life.

Toward Robotic Inspection of Dry Storage Casks for Spent Nuclear Fuel

Extended dry storage of spent nuclear fuel makes it desirable to assess the structural integrity of the storage canisters. Stress corrosion cracking of the stainless steel canister is a potential degradation mode especially in marine environments. Sensing technologies are being developed with the aim of detecting the presence of chloride-bearing salts on the surface of the canister as well as whether cracks exist. Laser-induced breakdown spectroscopy (LIBS) methods for the detection of Chlorine are presented. In addition, ultrasonic-guided wave detection of crack-like notches oriented either parallel or perpendicular to the shear horizontal wave vector is demonstrated using the pulse-echo mode, which greatly simplifies the robotic delivery of the noncontact electromagnetic acoustic transducers (EMATs). Robotic delivery of both EMATs and the LIBS system is necessary due to the high temperature and radiation environment inside the cask where the measurements need to be made. Furthermore, the space to make the measurements is very constrained and maneuverability is confined by the geometry of the storage cask. In fact, a large portion of the canister surface is inaccessible due to the presence of guide channels on the inside of the cask's overpack, which is strong motivation for using guided waves for crack detection. Among the design requirements for the robotic system are to localize and track where sensor measurements are made to enable return to those locations, to avoid wedging or jamming of the robot, and to tolerate high temperatures and radiation levels.

Additively Manufactured PLA under static loading: strength/cracking behaviour vs. deposition angle

Abstract: This paper aims to assess the existing interactions between strength/fracture behaviour and infill angle in additively manufactured PLA subjected to static loading. Plain specimens and samples containing crack-like notches of 3D-printed PLA were manufactured horizontally by making the deposition angle vary from 0° to 90°. A direct inspection of the fracture surfaces revealed that, irrespective of the infill orientation, static failures were caused by two mechanisms, i.e.: (i) initial shear-stress-governed de-bonding between adjacent filaments and subsequent normal-stress-governed breakage of the filaments themselves. The results being generated demonstrate that, from an engineering point of view, the influence of the deposition angle on the overall strength/fracture resistance of additively manufactured PLA can be neglected with little loss of accuracy. The profile of the stress vs. strain curves being obtained experimentally suggests also that the mechanical behaviour of the 3D-printed PLA being investigated can be modelled accurately without requiring the use of complex non-linear material models, with this resulting in a great simplification of the design problem.

Fatigue strength of laser-welded foam-filled steel sandwich beams

Laser stake-welded steel sandwich panels are widely used in engineering due to their high stiffness-to-weight ratios. The welds are thinner than the plates they join so that there are two crack-like notches on each side of a weld. As a consequence, the welded joints are susceptible to fatigue. In this study, as a remedy to the fatigue problem, low-density H80-grade Divinycell polyvinylchloride foam is bonded adhesively to the voids of stake-welded web-core sandwich beams. The foam reduces shear-induced stresses in the stake-welds. The choice of Divinycell H80 is founded on earlier J-integral-based finite element fatigue assessments of sandwich panels. Empty and the H80-filled sandwich beams are tested in three-point-bending for stiffness, ultimate strength and fatigue (load ratio R=0.05). The failure modes in the weld joint region are studied using scanning electron microscopy. The experimental results show that the filling increases the stiffness of the sandwich beams by a factor of three while the weight is increased only by 6%. The ultimate strength is increased by 2.7 times. As for the fatigue behavior, the slope increases from m=4.508 of empty panels to m=7.321 of filled panels while the load level at 2millioncycles increases by a factor of 8.5.

Electromagnetic acoustic transducers for nondestructive inspection of dry storage canisters for used nuclear fuel

Inspection for stress corrosion cracking in stainless steel canisters that store used nuclear fuel must be done within a harsh environment that requires robotic delivery of the sensing system. The system must tolerate high temperatures and radiation and it must operate in constrained spaces. In order to be delivered robotically noncontact transducers are preferred. Electromagnetic acoustic transducers (EMATs) are noncontact transducers that can be made from materials capable of resisting high temperature and radiation and therefore are an excellent choice for this application. This presentation will describe the requirements of the dry storage environment, the ability of compact EMATs to detect cracks oriented both transverse and parallel to the wave vector using shear horizontal guided waves, and the nondestructive inspection methodology developed for full penetration weld regions. Laboratory experiments indicate that the pulse-echo mode is effective at detecting crack-like notches. B-scans constructed from the heat affected zone of welds show both the defects and the welds, which are distinguishable by their time-of-flight.

Fatigue strength assessment of laser stake-welded T-joints using local approaches

This study investigates the applicability of local stress- and energy-based approaches to the fatigue strength assessment of laser stake-welded T-joints. The T-joint has two crack-like notches with infinitesimal tip rounding on each side of the weld. The local approaches used are fictitious tip rounding of 1 and 0.05mm and the approaches that assume zero rounding within this study, namely the stress intensity factor, the average strain energy density and the J-integral.It is shown that the slopes of the fatigue resistance curves vary between 4 and 8 under different loading conditions imposed on the joint. The slope value exhibits a linear relation with the dimensionless gradient of the maximum principal stress evaluated at the critical notch tip. The same linear relation between the slope and the gradient is valid for all approaches. Because of the slopes, which differ from the usual 3, the agreement of the T-joints with other steel joints is obtained at five million load cycles. The exception is the fictitious rounding concept of 0.05mm, in which case the recommended design S–N curve with a slope of 3 appears overly conservative in a high-cycle regime.

Application of the Notch Stress Intensity and Crack Propagation Approaches to weld toe and root fatigue

Several fatigue failures are initiating from the root of non-penetrating welds, which are widely applied in several industrial sectors. The structural optimization and reduction of weld material increase the danger of root cracking. The fatigue assessment during design is mainly based on the nominal, structural hot-spot or notch stress approach. These approaches are only partly applicable to weld roots, give rough estimates such as the nominal stress approach or over-conservative life estimation such as occasionally the notch stress approach when keyhole notches are modelled. An alternative assessment is possible with the crack propagation approach assuming the non-welded root gap and a short crack at the weld toe as initial cracks. A new approach is the notch stress intensity approach (N-SIF approach) which is able to assess the fatigue life of V-shaped notches at weld toes as well as crack-like notches at weld roots using the strain energy density around the notch. Both approaches are applied to different joints where toe and root failures are probable. The results are compared with a previous analysis using the notch stress approach. Fatigue test results are also available for a comparison which allows to draw conclusions with respect to the approaches for an appropriate fatigue strength assessment.

Influence of the root radius of crack-like notches on the fracture load of brittle components

Narrow notches often cause damage that can lead to the destruction of components. The stress field in the vicinity of such crack-like notches in two-dimensional (2D) structures is similar to the stress field around equivalent cracks. Therefore similar investigations are necessary to predict the fracture load for components with cracks or narrow notches. Thus, the asymptotical stress field for a narrow notch with a rounded notch root is deduced from an Airy’s stress function. Based on this stress field a fracture criterion is developed. Comparing the theoretical fracture limit curves derived from the fracture criterion with experimental results it can be shown that for brittle material the local stress state at the fracture initiation point is the same for mode I, mixed-mode and mode II loading.

The validation of some methods of notch fatigue analysis

This paper is concerned with the testing and validation of certain methods of notch analysis which the authors have developed theoretically in earlier publications. These methods were developed for use with finite element (FE) analysis in order to predict the fatigue limits of components containing stress concentrations. In the present work we tested and compared these methods using data from standard notches taken from the literature, covering a range of notch geometries, loading types, R‐ratios and materials: a total of 47 different data sets were analysed. The greatest predictive success was achieved with critical‐distance methods known as the point, line and area methods: 94% of these predictions fell within 20% of the experimental fatigue limits. This was a significant improvement on previous methods of this kind, e.g. that of Klesnil and Lucas [(1980) Fatigue of Metallic Materials, Elsevier Science]. Methods based on the Smith and Miller [(1978) Int. J. Mech. Sci. 20, 201–206] concept of crack‐like notches were successful in 42% of cases; they experienced difficulties dealing with very small notches, and could be improved by using an ElHaddad‐type correction factor, giving 87% success. An approach known as ‘crack modelling’ allowed the Smith and Miller method to be used with non‐standard stress concentrations, where notch geometry is ill defined; this modification, with the same short‐crack correction, had 68% success. It was concluded that the critical‐distance approach is more accurate and can be more easily used to analyse components of complex shape, however, the crack modelling approach is sometimes preferable because it can be used with less mesh refinement.

Stresses near narrow rectangular notches, with rounded corners, in beams in bending

The results of finite element calculations have been used to show that an analytical solution for the stresses in the vicinity of crack-like notches with elliptical and hyperbolic shaped ends gives accurate results for narrow, semi-circular ended notches. It is also shown that the peak stresses can be obtained from the stress intensity factor for an equivalent crack and the notch tip radius. Finite element solutions for narrow, rectangular notches with rounded corners, have also been used to show that the peak stresses in such notches can be obtained by using a notch shape factor to modify the peak stress values for semi-circular notches. The shape factor depends only on the notch width to corner radius ratio.

Engineering formulae for fatigue strength reduction due to crack-like notches

Engineering formulae for fatigue strength reduction due to crack-like notches