Critical Stress Concentration Factor Research Articles

Improving fatigue performance of metal parts with up-facing inclined surfaces produced by laser powder bed fusion and in-situ laser remelting

• Three-point bending fatigue life significantly improved by in-situ laser remelting. • In-situ laser remelting proved to be applicable to two different materials. • No microstructural defects were observed in remolten zone or interface with bulk. It is well known that the relatively poor surface quality of metal parts produced by Laser Powder Bed Fusion (LPBF) compared to conventionally manufactured parts has a negative effect on their fatigue life. To enhance fatigue life, time consuming post-processing (e.g. machining, polishing) is typically applied to improve the surface quality. Laser remelting is a promising technique for surface quality improvement, applicable to variable shapes. However, laser remelting is traditionally only used as post-process since surfaces inclined with respect to the powder bed are covered with loose powder during building. Recently, the authors have developed a dual laser technique, which enables improving the quality of up-facing inclined surfaces during building. The technique consists of two steps: (1) selectively removing powder from inclined surfaces by laser-induced shock waves, (2) laser remelting newly exposed surfaces. This paper shows a clear improvement in three-point bending fatigue of such treated samples from maraging steel 18Ni-300 and titanium alloy Ti-6Al-4V, compared to as-built state. This performance was quantified by the number of load cycles until failure at a fixed maximum tensile stress level. The improved performance could be linked to reduced surface roughness, total notch depth, critical stress concentration factor and a lower density of surface asperities. The remolten layer exhibited no visible defects and a decreased microhardness, which could beneficially impact the crack nucleation. This technique shows large potential for reducing the time and cost of metal parts produced by LPBF with enhanced fatigue performance by limiting or avoiding the need for surface post-processing.

Influence of weld geometry on stress concentration factor distributions in tubular joints

Fatigue design of welded tubular joints relies on an extensive number of parametric equations that allow calculation of the stress concentration factors at characteristic locations around the weld circumference. These equations were developed with finite element models that either do not include a weld geometry or include a simplified representation thereof. This paper presents a new approach for modeling American Welding Society D1.1 standard compliant welds with different geometries. The generated finite element models are used to study the effect of different weld geometries on the stress concentration factor distributions in tubular joints. The results show that the geometry of the weld has a strong influence on the magnitude of the critical stress concentration factor, the shape of the stress concentration factor distribution and the expected failure location. Finally the authors highlight the need for more comprehensive guidelines regarding the determination of complete stress concentration factor distributions from finite element simulations.

PPG-Terminated Tetra-Carbamates as the Toughening Additive for Bis-A Epoxy Resin.

We synthesized PPG-terminated tetra-carbamates as a new toughening additive for epoxy thermosets through facile addition reaction of hexamethylene diisocyanate (HDI) with poly(tetra-methylene glycols) (PTMG) and poly(propylene glycols) (PPG). The effects of prepared tetra-carbamates on the rheological behavior of neat epoxy resin were studied along with the various cured properties of their modified epoxy systems. Four carbamate groups (–NHCOO–) endow the prepared additives not only with good intramolecular interactions, but also with optimal intermolecular interactions with epoxy polymers. This results in the suitable miscibility of the additives with the epoxy matrix for the formation of the typical biphasic structure of microparticles dispersed in the epoxy matrix via polymerization-induced microphase separation. The impact strength and critical stress concentration factor (KIC) of cured modified epoxy systems with the additives are significantly higher than those of unmodified epoxy systems, without sacrificing the processability (Tg) and flexural strength. The toughening mechanism is understood as a synergism combination among the phase separation mechanism, the in situ homogeneous toughening mechanism, and the particle cavitation mechanism.

Synthesis and characterization of zirconia toughened alumina ceramics prepared by co-precipitation method

Abstract Undoped and 3 mol% yttrium doped ZrO 2 –Al 2 O 3 composite powders with partially stabilized ZrO 2 (PSZ) content varying from 0 to 30 wt% were prepared by a co-precipitation route using inorganic precursors Al(NO 3 ) 3 , ZrOCl 2 and Y(NO 3 ) 3 . The precipitates were characterized by DTA and subsequently calcined at 1200 °C for 4 h to achieve fine grained composite powders. The calcined powders were characterized by FTIR and XRD. In order to enhance the sinterability, the calcined powders were wet milled in a high energy ball mill. Powders were uniaxially pressed to form pellets and sintered at 1600 °C for 5 h to achieve greater than 96% relative density. Microstructural analysis of the sintered compacts revealed the uniform distribution of the zirconia particles among the alumina matrix. It was also observed that the faceted intergranular zirconia grains were present at the grain boundaries and junctions in the alumina matrix. Vickers indentation was carried out at 1 kgf load for hardness and 2 kgf load for estimating the critical stress concentration factor (K c ). Microscopic studies of the indented samples showed that cracks were propagating around the grain boundaries. Highest K c ∼8.40 ± 0.4 MPa√m and hardness ∼16.31 ± 0.58 GPa was obtained for the 30 wt% PSZ-Al 2 O 3 composite. The sintered density and critical stress intensity factor (K c ) achieved were compararble to that achieved earlier by hot press and SPS.

Overloading effect on the fatigue strength in resistance spot welding joints of a DP980 steel

Dual phase steel DP980 sheets were joined by resistance spot welding (RSW) process. Mechanical resistance of the welds was characterized by microhardness, tensile shear and fatigue tests. A significant hardness decreases was observed in the RSW lap joints with respect to the base material, which was attributed to phase transformations during the heating and cooling of the steel. Fatigue Wöhler curves using a fixed load ratio of 0.1 were obtained. It was found that the spot weld at the nugget interface close to the fusion zone induced a critical stress concentration factor, which decreased the fatigue life of the joints in the as-welded condition. Failure of the welds was initiated at the interface between welded sheets. Two predominant fatigue fracture modes were observed associated with mode I/III cyclic loading, which were correlated with the fatigue crack initiation and propagation stages. Compressive residual stresses were induced by a loading-unloading cycle on the spot welds, which tends to increase the fatigue life of the joints when compared to the as-welded condition.

Numerical and Experimental Investigations of Key Assumptions in Analytical Failure Models for Sheet Metal Forming

In this paper, the key assumptions in the M-K and effective stress ratio models are investigated for AISI 1018 steel specimens with a thickness of 0.78 mm using experimental and numerical data from Marciniak tests. The experimental procedure included Digital Imaging Correlation (DIC) to measure the major and minor in-plane strains. Strain components were obtained at points inside (i.e., the defect region) and adjacent (i.e., the safe regions) to the high strain concentrations for four different strain paths. In the numerical analysis, FEA simulations with Marc Mentat were performed with shell elements to investigate the four specimen geometries. The key assumptions of interest are the incremental major strain ratio from M-K model and the critical stress concentration factor from effective stress ratio model. Thus, the mechanics- and material-based failure phenomena in these two analytical models are examined in this paper to provide insight into the material behavior at failure. Also, data are presented that shows clearly the localization (both size and strain value) for the various strain paths.

A fracture criterion for blunted V-notched samples

This paper shows how the cohesive zone model can help in predicting fracture loads of brittle components with blunted V-notches. Linear Elastic Fracture Mechanics cannot be applied in such cases because there are no singularities; there is no crack, and neither is the notch sharp. Numerical predictions based on the cohesive zone model were checked succesfully against experimental measurements for PMMA at −60 °C (made by the authors) and steel, zirconia, silicon nitride and alumina by other researchers. The concept of a critical stress concentration factor, well established for sharp V-notches, is generalized, under certain circumstances, to blunted V-notches, and a non dimensional formulation of the fracture criterion is proposed.

Prediction on fatigue limit of notches with non-propagating crack

Theoretical estimation of fatigue limit of notches under axial push-pull loading was made with an emphasis on the formation of non-propagating crack. A critical notch radius and a critical stress concentration factor were determined so as to distinguish notch fatigue limits with crack initiation and crack non-propagation. Errors in calculating fatigue limit of notches and comparisons between theoretical and experimental results were discussed.

Micromechanical characteristics of hot-pressed titanium nitride from ultradispersed powders

The method of microindentation was used to examine the relationships governing the elastoplastic deformation and brittle failure of hot-pressed specimens of titanium nitride from ultradispersed powders with different density. Their micromechanical characteristics were determined. The results show that with an increase of density the microhardness and Young's modulus become stable, microbrittleness decreases, whereas the microstrength at the critical stress concentration factor increased.

Tapping of melt by veins and dikes

Abundant cross‐cutting dikes have been observed in mantle peridotites. These dikes are inferred to have formed while the material was a mostly crystalline mush beneath a ridge axis. The presence of these dikes indicates that the mush was able to fracture. The physics of the growth of vein and dikes and the extent to which they tap melt are investigated in this paper. Once small veins are initiated, they tend to open if the fluid pressure within them exceeds the least principal stress. Porous flow into the veins is necessary for them to open. This implies a melt pressure gradient and that the melt pressure in growing veins is less than the regional pressure. The opening rate and pore pressure decrease are obtained considering both porous flow and viscous deformation around the vein. The stress concentration factor driving vein lengthening is largest for veins near the compaction length. If the critical stress concentration factor is sufficiently low, vein growth and tapping of the veins by dikes reduces the melt pressure to near the least principal stress. Then the matrix of a material undergoing deviatoric strain is compacted because the melt pressure is less the “pressure” of the solid. Retention of significant melt directly beneath the axes of fast ridges is expected because the deviatoric strains are low there. However, computed contours of melt fraction are weakly dependent on the efficacy of tapping of melt by dikes. Numerical models indicate that about half of the melt is tapped by dikes if the shape of the conduit is thermally controlled. Tapping of melt by density‐driven porous flow is enhanced if the melt viscosity is low owing to high temperatures. This process, rather than dikes, was probably important in the Archean and is now important beneath hotspot ridges, such as Iceland. Shear strain rates in the asthenosphere are large enough that significant quantities of melt should be extracted if partial melt exists in intraconnected pores and the critical stress concentration factor for vein growth is exceeded.