Transitions in Charpy Energy and Splitting for X80 Pipe Steel

Using data obtained for a recent production 48-inch diameter X80 PSL2 pipe, the transition in fracture behavior between fully brittle and fully ductile modes was examined to assess any effects of transverse splits or delaminations. The additional surface area formed at the splits can increase and decrease the Charpy energy by expending energy to form more fracture surface and reduce constraint on the propagating crack as it extends through the unnotched Charpy ligament. Fitting of full Charpy energy transition curves for the tested material with standard hyperbolic tangent functions does not represent the behavior well. An important transition is noted in the behavior between splitting that initiates at the initial notch tip and splitting that occurs further down in the ligament. Spitting across the notch tip increases Charpy energy by releasing constraint on crack initiation behavior at the machined notch and occurs at temperatures just above the lower shelf temperature where brittle fracture dominates. The splitting behavior can also be correlated to effects on fracture toughness in the same orientation for CTOD specimens in the base material and for correlated effects on CTOD behavior in the weld heat affected zone when the crack advance is in the through thickness direction.

On the topological enrichment for crack modeling via the generalized/extended FEM: a novel discussion considering smooth partitions of unity

Purpose It has been usual to prefer an enrichment pattern independent of the mesh when applying singular functions in the generalized/eXtended finite element method (G/XFEM). This choice, when modeling crack tip singularities through extrinsic enrichment, has been understood as the only way to surpass the typical poor convergence rate obtained with the finite element method (FEM), on uniform or quasi-uniform meshes conforming to the crack. Herein, the topological enrichment pattern is revisited in the light of a higher-order continuity obtained with a smooth partition of unity (PoU). Aiming to verify the smoothness' impacts on the blending phenomenon, a series of numerical experiments is conceived to compare the two GFEM versions: the conventional one, based on piecewise continuous PoU's, and another which considers PoU's with high-regularity. Design/methodology/approach The stress approximations right at the crack tip vicinity are qualified by focusing on crack severity parameters. For this purpose, the material forces method originated from the configurational mechanics is used. Some attempts to improve solution using different polynomial enrichment schemes, besides the singular one, are discussed aiming to verify the transition/blending effects. A classical two-dimensional problem of the linear elastic fracture mechanics (LEFM) is solved, considering the pure Mode I and the mixed-mode loading. Findings The results reveal that, in the presence of smooth PoU's, the topological enrichment can still be considered as a suitable strategy for extrinsic enrichment. First, because such an enrichment pattern still can treat the crack independently of the mesh and deliver some advantage in terms of convergence rates, under certain conditions, when compared to the conventional FEM. Second, because the topological pattern demands fewer degrees of freedom and impacts conditioning less than the geometrical strategy. Originality/value Several outputs are presented, considering estimations for the J–integral and the angle of probable crack advance, this last computed from two different strategies to monitoring blending/transition effects, besides some comments about conditioning. Both h- and p-behaviors are displayed to allow a discussion from different points of view concerning the topological enrichment in smooth GFEM.

Crack arrest and propagation in impact loaded shock resistant PMMA: mesh-free numerical simulation

The use of shock resistant RT-PMMA in engineering structures potentially subject to accidental overloading requires an evaluation of its crack arrest capability under impact loading. Based on experimental results obtained from a series of Kalthoff and Winkler (KW)-type impact tests, the present study aims at numerically reproducing the conditions for brittle-like crack initiation and propagation in impact-loaded RT-PMMA. For that purpose, three-dimensional SPH numerical simulations were conducted and the performance of various failure criteria was evaluated. The numerical model together with a combination of stress- and strain- dependent failure criteria were shown to fairly reproduce the experimental results in terms of finite crack advance and orientation.

Evaluation of Fatigue Crack Growth Characteristics on Stainless Steel SS 316 LN Using Acoustic Emission Technique

Results of online acoustic emission (AE) monitoring during fatigue crack growth rate (FCGR) experiments on a stainless steel SS 316 LN are presented in this paper. Two specimen geometries — viz., standard compact tension (C(T)) specimens as well as side-grooved C(T) specimens were considered for experiments at ambient temperature and at 600°C (873K). There is a good correspondence between crack length increment and the increase in AE cumulative count and cumulative energy during the experiments. The side grove introduced on the thickness direction of the test specimen constrains the plastic zone ahead of the crack tip, thereby enforcing plane strain conditions at the crack. Reduced AE activity at initial stages of crack growth was observed for side grooved samples. The transition to Stage-II crack growth was observed using acoustic emission (AE) technique which otherwise was not visible from the fatigue crack growth plot. The work further attempts to correlate the AE parameters obtained during elevated temperature (873K) fatigue crack growth in stainless steel. For the purpose of acquiring AE signals outside the heated zone, a waveguide was used to transmit the acoustic waves from the specimen at high temperature. A correlation between crack advance and AE parameters was obtained from the elevated temperature tests.

СТРУКТУРА И СВОЙСТВА ДЕТАЛЕЙ, ПОЛУЧЕННЫХ СЕЛЕКТИВНЫМ ЛАЗЕРНЫМ СПЛАВЛЕНИЕМ ПОСЛЕ ИСПЫТАНИЯ НА МАЛОЦИКЛОВУЮ УСТАЛОСТЬ

Results of low cycle fatigue (LCF) testing, investigation of microstructure, and fractures of specimens, which was, obtain by selective laser melting process (SLM) of the powders from Inconel 718 alloy have been carried out in the present article. Chemical composition of the considered specimens, microstructure before and after testing, results at room and elevated temperatures, which were built in XY direction (horizontal), were carrying-out. The specimens in the as-build state were exposed by hot isostatic pressing (HIP) with subsequent inherent Inconel 718 heat treatment. It was established that in specimens in as-built state observe accurately expressed zones of layer-by-layer smelting with 100 μm height. After HIP with subsequent heat treatment strengthening of the considered alloy provides by intermetallic γ'' – Ni3Nb phase, γ'-phase and carbides, also was identified lamellar δ-phase in the microstructure. LCF-testing was conducted in a so-called «soft» cycle of loading with a predetermined interval of the strain for providing of fatigue life in 103, 5х103, 104 cycles. The obtained data was built logarithmic curve in coordinates «Strain σ – Number of cycles N», that allows, with sufficient reliability, determine a rational strain level at LCF-testing for providing a predetermined number of cycles. The following ratios of σ – N were established (Rσ=0, υ=1 Hz): for providing N=103, σ≤1020 МPа, N=5х103, σ=931…961 МPа, N= 104, σ≤941 МPа. Results of the fractures investigation after LCF-testing at 20°С and 550°С shows that during the cyclic elastoplastic deformation at room temperature multicentricity of crack initiations zones observes. Mainly fixed a viscous pattern of fracture with a fatigue grooves presence, which size increases in the process of crack advance to the zone of fracture area. At elevated temperature in the zones of fatigue crack initiation and spreading quasi-brittle character of destruction observes, and turns in viscous in the zone of fracture area. Authors should be pointed out that the application of additive technologies in the manufacture of aerospace parts requires extensive R&D works, and testing efforts to confirm the repeatability of alloy characteristics.

Mesoscale bicontinuous networks in self-healing hydrogels delay fatigue fracture

Load-bearing biological tissues, such as muscles, are highly fatigue-resistant, but how the exquisite hierarchical structures of biological tissues contribute to their excellent fatigue resistance is not well understood. In this work, we study antifatigue properties of soft materials with hierarchical structures using polyampholyte hydrogels (PA gels) as a simple model system. PA gels are tough and self-healing, consisting of reversible ionic bonds at the 1-nm scale, a cross-linked polymer network at the 10-nm scale, and bicontinuous hard/soft phase networks at the 100-nm scale. We find that the polymer network at the 10-nm scale determines the threshold of energy release rateG0above which the crack grows, while the bicontinuous phase networks at the 100-nm scale significantly decelerate the crack advance until a transitionGtranfar aboveG0. In situ small-angle X-ray scattering analysis reveals that the hard phase network suppresses the crack advance to show decelerated fatigue fracture, andGtrancorresponds to the rupture of the hard phase network.

On the micromechanism of inclusion driven ductile fracture and its implications on fracture toughness

The objective is to identify the micromechanism(s) of ductile crack advance, and isolatethe key microstructural and material parameters that a?ect these micromechanisms andfracture toughness of ductile structural materials. Three dimensional, ?nite element, ?nitedeformation, small scale yielding calculations of mode I crack growth are carried out forductile material matrix containing two populations of void nucleating particles using anelasto-viscoplastic constitutive framework for progressively cavitating solid. The larger par-ticles or inclusions that result in void nucleation at an early stage are modeled discretelywhile smaller particles that require large strains to nucleate voids are homogeneously dis-tributed. The size, spacing and volume fraction of inclusions introduce microstructure-basedlength-scales. In the calculations, ductile crack growth is computed and fracture toughness ischaracterized. Several features of crack growth behavior and dependence of fracture tough-ness on microstructural and material parameters observed in experiments, naturally emergein our calculations. The extent to which the microstructural and material parameters a?ectthe micromechanisms of ductile crack advance and, hence, the macroscopic fracture tough-ness of the material is discussed. The results presented provide guidelines for microstructuralengineering to increase ductile fracture toughness, for example, the results show that for amaterial with small inclusions, increasing the mean inclusion spacing has a greater e?ect onfracture toughness than for a material with large inclusions.

Fracture energy characterisation of a structured interface by means of a novel J-Integral procedure

In the present investigation, a J-Integral formulation for non-flat crack paths, in the framework of the cohesive zone model, is developed. The formulation allows fracture energy properties in a direction that is not necessarily coplanar with the global crack advance to be analysed. Specifically, the effective fracture energy, [Formula: see text], has been examined based on the horizontal projection of the crack advance, [Formula: see text] (also called effective crack length). The use of [Formula: see text] is convenient in several situations as the case of patterned interfaces in adhesive joints. Finite-element analysis of double cantilever beam specimens including a trapezoidal patterned interface were employed to check the accuracy of this new definition of the contour integral. Post-process of the finite-element model, including those variables involved in the fracture energy calculation, is discussed together with some considerations that distinguish the energy evaluation procedure for flat profiles from structured designs. Finally, [Formula: see text] values obtained using the modified J-Integral formulation are compared with [Formula: see text] values obtained from the load–displacement curve method for comparison purposes.