Slip Strength Research Articles

Crystal plasticity modeling for mechanical property prediction of AA2195-T6 friction stir welded joints

The gradient material exposure to frictional heat and plastic flow during friction stir welding leads to local variations in microstructure and mechanical properties among different joint zones. This study aims to predict strengthening contributions of local microstructural evolutions including precipitate distribution, crystal morphologies, grain size, and dislocation density for friction stir welded joints of AA2195-T6. These effects are included by a unified microstructure-sensitive physics-based constitutive modeling approach into joint local variations in yield strength, plastic hardening, and ultimate tensile strength. Local precipitate phase distributions are observed by the transmission electron microscopy method. X-ray diffraction process is conducted to measure joint local variations in dislocation density. Electron backscattered diffraction mapping is utilized to observe joint local crystal morphologies. Evolutions in the crystal slip strength due to each of mentioned microstructure attributes are inserted into a crystal plasticity finite element frame work. Modeling parameters are calibrated for AA2195-O and T6 conditions, and simulated outputs are validated by comparing with experimentally observed joint local tensile properties using the digital image correlation method. Subsequently, obtained local tensile properties in the grain scale are inserted into a macroscale global model containing all friction stir welded joint areas. An acceptable match is observed between simulated and experimentally obtained data on local and global friction stir welded joint tensile stress-strain curves and tensile failure location.

Cold dwell fatigue analyses integrating crystal-level strain rate sensitivity and microstructural heterogeneity

Cold dwell fatigue remains an important life-limiting factor in aircraft engine titanium alloys. Microstructure-level creep and stress accumulation during each loading cycle are controlled by strain rate sensitivity. Here, an integrated experimental and computational framework is used to link crystal-level slip properties to microstructure-sensitive cold dwell debit of a forged rotor graded Ti-6Al-4V alloy. Slip strengths and anisotropic strain rate sensitivities are extracted from micro-pillar compression tests for different slip systems, incorporated within α+β microstructurally-faithful polycrystal representations. Dwell and non-dwell cyclic loading in alloy Ti-6Al-4V are investigated for two differing microstructures, and the cycles to failure predicted based solely on the crystal c-axis tensile strength, and the dwell debit quantified. The dwell effect is predicted to diminish to zero below a peak applied stress of about 790 MPa in the alloy studied.

Role of interface-affected dislocation motion on the strength of Mg/Nb nanolayered composites inferred by dual-mode confined layer slip crystal plasticity

In this work, we present a nanostructure-sensitive crystal plasticity model for the deformation response of nanolaminate composites. The model is applied to investigate the strength of Mg/Nb nanocomposites, wherein the Mg phase has either a hexagonal close-packed (HCP) or a body-centered cubic (BCC) crystal structure. To account explicitly for the effects of layer thickness and biphase interface on crystallographic slip, the model features a hardening law, called dual-mode confined layer slip (CLS). The model is applied to a suite of stress-strain measurements made on Mg/Nb nanocomposites, varying layer thickness, texture, and interface structure. Experiments show that the BCC/BCC Mg/Nb nanocomposites achieve substantially higher strength than the HCP/BCC nanocomposites. Apart from the finer layer thicknesses, the model indicates that the pseudomorphic BCC Mg phase contributes to strength by increasing the slip strengths of the 〈111〉 slip modes compared to the 〈a〉 slip modes in HCP Mg. It also suggests that the coherent interface poses less resistance to dislocation motion than the incoherent interface. It is, therefore, found that the BCC/BCC composite strength benefited from both the confinement on dislocation motion imposed by the reduced layer thickness and higher inherent strength of its BCC phase, but that it would be even higher if the interface was not a sharp coherent interface.

Effect of static magnetic field treatment on fresh concrete and water reduction potential

The current research proposes to conduct experimental studies to examine the optimal method of magnetic treatment for the mixes and provides guidelines to use in the concrete mix design. Six magnetic flux densities (25, 50, 100, 200, 400 and 500 millitesla) were considered. More than 180 specimens were considered in the experimental program. In each experiment the fresh mix was divided into two portions; the first portion was untreated, while the other portion passed through a magnetic field. The mechanical properties named workability, compressive strength, spitting tensile strength, flexural and bond slip behavior were examined. Experimental results clearly demonstrated noticeable increases in the compressive strength of the concrete mixes up to 16 %. The workability was increased by about 7 %–26 % and the maximum increases in workability were recorded for 400 m T flux density treatment. Splitting tensile strength, flexural and bond slip strength were also enhanced when the magnetic treatment is carried out for the fresh mixes, also all treated specimens showed significant ductile behavior. The current study showed that the water reduction potential can be utilized and recommended with maintaining similar workability when the magnetic treatment is conducted for mixes. The characteristic of resulted concrete improved in terms of compressive and tensile strength in about 29 % and 27 % respectively. However, with these advantages, the current study can be considered as first step towards production of fresh treated mixes and this might give engineers more confidence to use magnetic field treatments to the concrete mixes during production and placing process.

Mechanical Chuck Design and Slip Strength Analysis

Chuck is one of the key parts of coal mine tunnel drill, its performance directly affects the performance of the machine. In view of the existing problems of the hydraulic chuck, the structure of the new hydraulic chuck is designed and its working principle is briefly described. The strength of slips is analyzed by finite element method. The calculation results show that the initial design of slips meets the requirements of tunnel drilling rigs.

Effect of surface treatment of countersunk hole on slip strength of friction grip joints with high strength countersunk head bolts

Effect of surface treatment of countersunk hole on slip strength of friction grip joints with high strength countersunk head bolts

Microstructural fracture mechanics: Stored energy density at fatigue cracks

This paper addresses the mechanistic drivers of short fatigue crack growth using theory, computational crystal plasticity and experimental test and characterisation. The asymptotic theory shows that the crack tip stored energy density is non-singular and finite and can be related to stress intensity, but unlike the latter, it depends on the crystal Burgers vector and intrinsic slip strength. The computational methods allow the stored energy to be calculated accurately at crack tips and show that good agreement is obtained for static cracks with the theory. The experiments allow the crack tip stored energy to be measured, demonstrating intimate microstructural sensitivity, direct correlation with experimental crack growth variations and good quantitative agreement with both asymptotic theory and computational modelling. Hence a new microstructurally-sensitive fracture mechanics has been presented in the context of short cracks within crystalline materials.

Micro-scale computational modeling of graphene-based nanocomposites – Influence of filler-matrix interface failure

The effect of failure at the filler-matrix interface on the tensile stress-strain response of graphene-based polymer nanocomposites is studied using micro-scale finite element (FE) modeling. Interfacial failure can occur in the form of debonding or slip. Based on previous studies (Jiang et al., 2014; Anagnostopoulos et al., 2015; Li et al., 2017), interfacial slip, which causes friction-like interaction between the filler and matrix, is identified as the major failure mechanism that affects the nanocomposite response. By establishing single-filler models in ABAQUS, it is demonstrated that interfacial slip affects the stress-strain response of the nanocomposite in two ways. Firstly, it limits the load that can be transferred to the filler. Secondly, it causes a shear displacement discontinuity across the interface, and this generates additional strain over and above those of the matrix and filler; this reduces the level of stress enhancement in the nanocomposite that the incorporation of fillers contribute, and under certain conditions, even weaken the nanocomposite. Models with fully aligned and randomly oriented fillers are also established to examine the influence of parameters such as filler orientation, interfacial slip strength, and filler aspect ratio, on the tensile response of the nanocomposite. The model developed is employed to describe the tensile response of an actual polyvinyl alcohol (PVA)- graphene oxide (GO) nanocomposite, and good correlation with experiments is observed.

The effect of fly ash and granulated blast furnace slag on slip and tensile adhesion strength of tile adhesives mortars

In the formulations of adhesive mortars for tiles almost exclusively CEM I Portland cements are used. Practically no CEM II - CEM V cements with additives are used, although they are often produced with strength classes 32.5 R and some even 42.5R. Relationship between strengths of tile adhesives in which cement was partially replaced with fly ash and granulated blast furnace slag was studied. A fly ash was used in three different replacement levels from 5% to 25% by weight of either cement. The tensile adhesion were determined at 28 and 90 days after various conditioning conditions of the samples. The influence on the flexibility of mortars was also assessed. In small substitution levels, fly ash replacement increased the tensile adhesion strength. The results indicate that the optimal amount of fly ash and granulated blast furnace slag additive replacing a given amount of cement allows to obtain adhesive mortars for tiles with high functional parameters.

Micromechanical approaches to understand dwell fatigue: from titanium a-b microstructures to disc thermal alleviation

Micro-pillar tests on α and α-β colony Ti alloys in combination with crystal plasticity finite element analysis has enabled the extraction of a and b phase slip strength and rate sensitivity properties. Faithfully representative α-β microstructure polycrystal plasticity models have then been established in order to investigate dwell fatigue in isothermal rig test behaviour and anisothermal thermomechanical flight loading conditions. The role of thermal alleviation in diminishing dwell sensitivity has been demonstrated.

Polycrystal plasticity modeling for load reversals in commercially pure titanium

In this work, we use polycrystal modeling to study the interactions between slip and twinning during load reversals of commercially pure titanium. The constitutive response incorporates anisotropic elasticity, crystal plasticity, a dislocation density based hardening law for prismatic slip, basal slip, and pyramidal type I 〈c + a〉 slip, and micromechanical model for twin reorientation on two types: {1012¯} extension twinning and {112¯2} contraction twinning. The key feature of the model is the inclusion of slip-system level backstress development due to dislocation density accumulation. To demonstrate, the model is used to simulate the stress-strain response and texture evolution in a series of compression-tension and tension-compression tests carried out to different strain levels and applied in two different load directions to a strongly textured CP-Ti plate. Material parameters associated with the slip strengths for the three slip modes are reported. The model identifies the few systems within the pyramidal 〈c + a〉 slip mode as developing the most backstress among the three slip modes. It also indicates that the backstresses that develop in the forward loading path promote pyramidal slip in the reversal loading path. We also find that reverse loading changes negligibly the relative slip mode contributions from monotonic loading but it strongly affects the twinning-detwinning behavior. This work highlights the ability of polycrystal modeling to account for the co-dependent nature of multiple crystallographic slip and twinning modes, the hysteresis in plastic response during the forward-reversal cycle, and the two sources of hardening engendered by history-dependent dislocation density accumulation.

Crystal plasticity finite element modelling of coarse-grained [formula omitted]-uranium

The deformation behaviour of coarse-grained α-uranium is studied using crystal plasticity finite element simulations. The constitutive model includes 8 slip and 2 twin systems, based on dislocation densities as state variables. Polycrystal simulations are carried out to reproduce the manufacturing procedure: a quenching stage is followed by cutting out a sub volume to allow relaxation of the internal stresses. Room temperature tension and compression experiments are simulated for the textured samples that are generated by this process. Calibration of the model parameters is obtained by comparing simulated experiments against measured stress-strain curves and lattice strains from published in situ neutron diffraction studies. The comparison of the internal elastic strains allows the activity of slip and twin modes to be quantified, and to estimate their critical resolved shear stresses. The comparison between the present simulation results and previous models for fine-grained α-uranium provides information about the grain size dependence of the strength of slip and twin systems. The critical resolved shear stress of the most prominent twinning mode {130} is found to be similar to single crystal samples and much smaller than for fine-grained samples, showing the strong grain size dependence of this deformation mode.

Strength evaluation of bolted connections with slotted holes in non‐structural components

ABSTRACTIn the bolted connections of the non‐structural exterior wall to structural frames, slotted holes are often used to improve the workability. In Japan, in case the slotted holes are used in the bolted connections of structural frame, a reduction factors according to European Recommendations for bolted connections in structural steel work(1) have to be employed to reduce the slip strength of the connections. However, the strength reduction ratio equals to 1.0 when the major axis direction and the stress direction of the slotted holes are orthogonal; thus, the strength reduction factor is not considered to be substantial in many cases. Furthermore, these provisions are not employed for connections of non‐structural components to structural frames.In this study, a tension test of bolted connection with slotted hole was carried out to evaluate the slip coefficient and confirm the strength reduction. In order to consider influences of the surface treatment of the specimen on slip proof stress, five different types of surface treatment are considered in the test, which are hot dip galvanizing with and without antirust processing, electroplated zinc colored chromate coating with and without antirust coating, and stainless steel. From the test results, it is found that the slip strength ratio of the bolted connection with the slotted holes to that of with the conventional holes was ranging from 0.70 to 0.80. As next step, a series of slip coefficient evaluation experiments of actual non‐structural components are planned to investigate that the reduction factor obtained from the element tests could be adopted to the components. In that case, eccentricity and stress distribution in the connection should be considered.

New Study of Mgo Nps in Drilling Fluid to Reduce Stick-Slip Vibration in Drilling System

Stick-slip is kind of vibration which associated with drilling operation in around the bottom hole assembly (BHA) due to the small clearance between drill string & the open hole and due to the eccentric rotating of string. This research presents results of specific experimental study that was run by using two types of drilling mud (Fresh water Bentonite & Polymer), with/without Nanoparticle size materials of MgO in various ratios and computes the rheological properties of mud for each concentration [Yield point, plastic viscosity, Av, PH, filter loss (30 min), filter cake, Mud Cake Friction, Friction Factor]. These results then were used to find a clear effects of Nanoparticle drilling mud rheology on stick - slip strength by several perspectives through a special “Torque and Drag” software which simulate the torque amount expected on BHA during drilling a vertical well in different conditions using real drilling string design that usually used in Iraqi oil fields. Thus to mitigate or to prevent stick–slip and cure the sequence events that could happen to both of drilling string and the well, i.e. Bit/BHA wear, pipe sticking, borehole instability and low Rate of penetration. Our study concluded that there are good reduction in the torque from (2031lb-ft) to (1823lb-ft) using polymer mud and torque reduction from (4000lb-ft) to (3450lb-ft) using Fresh Water Bentonite, these results do not include any breaking in the satisfactory range of other mud rheology.

Strength of the shaft/hub joint using a finite element model

This article describes the effect of the roughness size on the axial slip strength between the parts of shaft/hub joints with interference fit. The surface roughness was obtained from a turning process with different finishes (fine, medium and rough). A finite element modeling was developed, which uses a normal contact stiffness equivalent to the size of the surface roughness between the joint pieces to represent the real contact. In order to validate the numerical model, theoretical results of contactpressure and extraction force of the shaft/hub joint with smooth elements were compared with the corresponding numerical results obtained. The numerical results from studies that considered the size of the surface roughness showed that the axial load capacity of the joint decreased with larger roughness.

Unexpected influence of prismatic plate-shaped precipitates on strengths and yield anisotropy in an extruded Mg-0.3Ca-1.0In-0.1Al-0.2Mn (at.%) alloy

The stress-strain response of an extruded Mg-0.3Ca-1.0In-0.1Al-0.2Mn (at. %) alloy containing prismatic plate-shaped precipitates was measured. An Elasto-Viscoplastic Self-Consistent polycrystal plasticity code was used to evaluate the effect of precipitation hardening on individual deformation modes. The results reveal the precipitates (whose shape is ideal according to the Orowan bowing model) are sheared by basal dislocations, limiting their strengthening effect, and the ratio of the strength of prismatic slip to that of extension twinning is increased, resulting in an increase in the tension-compression yield anisotropy.

Slip Coefficient and Ultimate Strength of High-Strength Bolted Friction Joints with Compact Bolt Spacing and Edge Distance

In current design codes, the minimum bolt spacing and edge distance for high-strength bolted friction joints are specified based on the bolt diameter, regardless of the plate thickness or steel strength. However, the mechanical performance of joints with a more compact bolt spacing than that given in the current design specifications has been little examined. In this study, the slip and ultimate strength of high-strength bolted joints with a compact bolt spacing and edge distance were investigated. Tensile tests were conducted on friction-type joints with various bolt arrangements. The use of a compact bolt arrangement was not observed to have a negative influence on the slip strength in these tests. The estimated strengths corresponding to the experimentally observed failure modes were found to agree fairly well with the experimental results within a 10% error. A parametric finite element analysis was also conducted to investigate the influence of the bolt arrangement on the slip coefficient. It was concluded that the bolt spacing and edge distance have little influence on the slip coefficient.

ULTIMATE SLIP BEHAVIOR OF PERFOBOND STRIP UNDER THE LATERAL CONFINEMENT PARALLEL TO THE PBL PLATES

Perfobond strip (called PBL, hereafter) proposed by Leonhardt in Germany is widely used as the validated shear connector in steel-concrete composite structures. Fujii et al. proposed a fracture mechanism of PBL as follows: Slip force (shear force) acting on a PBL causes the expanding force which splits the concrete around the PBL. If restraint force, which is caused by cover concrete, rebar etc., resisting the expanding force exist, the slip strength is enhanced. Consequently, the fracture of PBL will happen when the expanding force exceeds the restraint force. Also, based on the above concept, they have given the evaluation formula of PBL slip strength and verified it by comparing previous experimental results. However, bonding force and frictional force are not taken into account in the formula. Through the loading tests of the composite girder, we found that the bonding and the frictional force, which appear along the boundary surface between concrete deck and steel flange, work as resistance force to not only the slip strength but also the expanding force. In this paper, the effects of the bonding force and the frictional force on the ultimate slip behavior of PBL are investigated experimentally, and an evaluation formula of PBL slip strength is improved by taking their effects into account.

ULTIMATE SLIP STRENGTH OF PERFOBOND STRIP CONFINED BY CONCRETE COVER

The ultimate slip strength of a perfobond strip (called "PBL" hereafter) depends significantly on the concrete confinement conditions around PBL hole as well as concrete shear fractures at the hole. If the hole is small, the effect of concrete confinement will still more strongly appear. In this paper, focusing on the concrete confinement due to concrete cover, the ultimate slip strength of PBL with a small hole is investigated experimentally through push-out tests, in which the diameter of PBL hole is set at 30 mm and the thickness of the concrete cover is varied from 0 to 100 mm. Test results show that the concrete separates into two blocks with concrete crack parallel to the PBL plate and that the ultimate slip strength of a 50-mm-thick concrete cover is 34.4 kN, which is about 10 times of the slip strength without concrete cover (3.5 kN). Thus, it can be concluded that thicker concrete cover makes the slip strength remarkably larger, in the case of small-hole PBLs.

Investigation of Size Effects in Slip Strength of Titanium Alloys: α Nodule Size Dependence of the Critical Resolved Shear Stress

Reported critical resolved shear stress (CRSS) values for basal and prismatic slip of titanium alloys are presently reviewed. Despite different average compositions, CRSS variations appear as mostly related to the α nodule size. Investigations of tensile tests performed in a scanning electron microscope reveal that the underlying size effect results from back stress generated by dislocations piling up at nodule boundaries. A relationship is proposed to predict CRSS for alloys with a given nodule size.