High Tensile Steel Research Articles

Bond-slip models for the interface between steel fabric reinforced cementitious matrix and concrete substrate

Composite materials for strengthening existing structural members represent a valid alternative to the traditional strengthening techniques. In particular, Fabric Reinforced Cementitious Matrix (FRCM) composites have been successfully employed to strengthen existing reinforced concrete and masonry structures.Among the different FRCM composites available on the market, the attention of the authors was recently focalized on systems composed of high tensile strength steel cords embedded in a cementitious matrix (systems also well known as Steel Reinforced Grout, SRG).The main failure mechanism of FRCM is represented by the debonding at the matrix-fiber interface. Therefore, the study of the bond behavior of FRCM composites is a key topic to develop reliable design procedures. In this paper, the applicability of different existing bond-slip interface models for FRP-concrete interface to SRG system is studied. The different parameters characterizing the models are calibrated by using an experimental database available by the authors and including results of direct single-lap shear tests on concrete prisms bonded to SRG strips. All the models are calibrated using a classical technique which minimizes the difference between the measured and computed interfacial shear stress values at different slip levels. The results indicate that all the models predict relatively well the slope of the ascending branch of the shear stress-slip curve, but they give substantially different descending branch profiles.The calibrated models are, then, compared with some other laws proposed in the literature for PBO and basalt FRCM systems; some of them are also implemented within a numerical procedure developed by the authors for the estimation of both the maximum axial stress at debonding and the corresponding effective bonded length of the SRG strip. The comparison with the experimental data has allowed for verifying the accuracy of the calibrated bond-slip models.

Influence of joint interface on mechanical properties in dissimilar friction welds

ABSTRACT The solid-state joining process of friction welding became an interesting option as it provides sound welds without cracks compared to the conventional arc welding techniques. The joining of dissimilar materials is not feasible using arc welding methods due to the difference in their physical properties. Friction welding allows the successful joining between dissimilar materials with excellent mechanical and metallurgical properties. In this study, the dissimilar joint of austenitic stainless steel to high tensile alloy steel was welded using friction welding. The interface microstructure, axial shortening, joint temperature, microhardness and tensile strength of the welds were evaluated. The thickness of the intermixing zone was identified at an interface and its variation with the welding parameters. The effect of intermixing zone in dissimilar welds and its effect on the mechanical properties was investigated. The thickness of the intermixing zone is proportional to the tensile strength of the welds. The microhardness profiles were recorded that the highest hardness obtained at the intermixing zone and dynamically recrystallised zone. Temperature of the joint interface was influenced by the thickness of the intermixing zone and the axial shortening of the substrates at the mating surfaces.

Moment redistribution and flexural performance of RC continuous T-beams strengthened with NSM FRP or steel bars

This paper discusses the flexural strengthening of continuously reinforced concrete (RC) beam constructions in hogging or sagging regions using near-surface mounted (NSM) technique. The main difficulties in strengthening RC continuous beams in hogging are the vertical middle column which prevents continuity of the NSM reinforcement bars and end anchorage of the NSM reinforcement. A test program consisted of seven two-span T-beams was used; one control beam, which was used for comparison, four beams were strengthened at the hogging region, and two beams were strengthened at the sagging regions. Two types of strengthening material were adopted in this study, carbon fiber reinforced polymers (CFRP) rebars, and high tensile steel bars. An RC column loaded with a constant load (120 kN) was used as the middle support. The main test parameters were the type of the reinforcement bars, continuous NSM bars beside the middle column, or NSM bars anchored inside the middle column, as well as the strengthened region. Besides, the impact of using embedded through-section (ETS) end anchorage of NSM CFRP bars was investigated as well. The experimental results were evaluated for the effects of strengthening technique and materials on the load-carrying capacity of beams, deformation, ductility index, and moment redistribution. The experimental results showed that strengthening with NSM steel bars could improve the service load carrying capacity by 23–40%, whereas the NSM CFRP rebars improved the service loads by 33–36%. The ETS end anchorage at the hogging region and the NSM steel bars at the sagging region helped to achieve the flexural rigidity by a very significant amount relative to the other strengthened beams and control beam by 270%. The moment redistribution achieved after the beam cracking load until the failure load varies in accordance with the strengthening region and the strengthening material type.

To study the laser cladding of ultra high strength AerMet-100 alloy powder on AISI-4340 steel for repair and refurbishment

High Tensile Steels such as AISI-4340 is extensively used steel in various engineering sectors such as aerospace, defense, automotive, heavy engineering and power sectors for components such as pinion housing, gears, shafts, connecting rods, propeller shafts, & landing gears of aerospace vehicles etc. Such component faces extensive wear at the contact areas during operation resulting in loss of performance leading to replacement. Directed energy deposition or laser cladding for refurbishment of such components can reduce the cost of replacement and can also be advantageous in countering environmental concerns. In this study, feasibility of using AerMet-100 alloy powder for refurbishment of AISI4340 steel was explored using laser cladding process. A 6 kW diode laser with 6 + 2 axes robot was used for the experimentation where laser clads were generated by melting and depositing the powder that was fed through a powder-feeding nozzle. AerMet-100 clads were found to have good compatibility with the substrate and defect free clads were generated. In order to homogenize the microstructure at Heat Affected Zone, a post-clad heat treatment (PCHT) at 470 °C for 1hr was carried out. The as-cladded microstructure shows lath martensite with retain austenite, whereas post clad heat-treated sample shows slightly reduced retained austenite. Micro-hardness evaluation revealed increased in clad hardness after PCHT due to formation of fine carbides. Micro-tensile test of Post heat-treated clad showed max Tensile stress of 1752 MPa compared to substrate 1240 MPa. It can be concluded that AerMet-100 material can be a potential candidate for repair and refurbishment of AISI4340 steel.

Microstructure-dependent hydrogen diffusion and trapping in high-tensile steel

In this work, the hydrogen embrittlement (HE) characteristics of high-tensile steel sheets with different microstructural characteristics were investigated. The sheets were fabricated via cold rolling (CR), water quenching (WQ), baking hardening (BH), and low-temperature annealing (LT), and their HE characteristics were clarified by examining the relationships between the microstructural characteristics and the severity of HE. Severe HE occurred in the WQ sample with hydrogen trapping at the boundaries of the retained austenite phases, resulting in intergranular and cleavage-like brittle failure. A reduction in HE was realized after the BH and LT processes. In these cases, hydrogen trapping was divided between the ε-carbide in the lattice spacings and at the boundaries of retained austenite, resulting in a mixed ductile/brittle failure mode. The extent of HE in the CR sample was similar to those in the BH and LT samples. However, the trapping sites were different; hydrogen trapping in the CR sample occurred in the slip band and around dislocations, resulting in delamination-like brittle failure on the slip planes. The extent of HE was also affected by the strain rate. More severe HE occurred in both the WQ and BH samples loaded slowly at 0.01 mm min−1 compared to the samples loaded 1.0 mm min−1 (i.e., intergranular failure). In this case, HE was affected by the large amount of hydrogen atoms trapped at the boundaries of the retained austenite phases. The hydrogen atoms in the lattice structure and ε-carbide migrated to the boundaries via dislocation movement. The extent of deterioration in tensile strength was two times higher in the samples loaded at the higher speed of 1.0 mm min−1 compared to those loaded at 0.01 mm min−1. Finally, the hydrogen trapping and failure mechanisms on the nano and atomic scales were discussed based on the results of the microstructural analyses.

On the prediction of ductile fracture in ship structures with shell elements at low temperatures

Abstract This paper proposes a practical method for addressing the effects of low temperatures on the ductile fracture behavior of high tensile strength steel plates and modeling using large shell finite elements. The test data of marine high-tensile strength steel grades, DH32, DH36, and EH36, at sub-zero temperatures were utilized. The tensile tests at sub-zero temperatures were simulated using fine solid elements to identify hardening law parameters. Simple localized necking loci were derived using the hardening curves of the steel grades considered. By defining the onset of localized necking as a failure condition, a simple fracture model for large shell finite elements that consider the low temperature effect was proposed. Several case studies were included to demonstrate the application and limitations of the proposed model.

Comparative study on ductile fracture prediction of high-tensile strength marine structural steels

ABSTRACT Accidental events such as collision, grounding, and explosion in marine structures can cause plastic deformation and structural damage. Various fracture models based on different mechanical concepts have been proposed to predict damage extents involving ductile fracture. In this paper, some fracture models were evaluated: Maximum shear stress, Cockcroft-Latham, Johnson-Cook, Modified Mohr-Coulomb, Lou-Huh, and Hosford-Coulomb models. The model parameters were identified for two shipbuilding steel grades: AH36 and DH36. A hybrid experimental-numerical approach was adopted. The parameters of six fracture models were identified using an optimisation algorithm based on the extracted loading paths from the numerical simulations and adopting a linear damage accumulation law. The calibrated fracture models are presented in stress triaxiality and Lode angle parameter space as a 3D fracture locus, and under plain stress condition as a function of the stress triaxiality as a 2D fracture locus. The predictive capabilities and flexibility of the evaluated fracture models were discussed.

Experimental failure assessment of high tensile stiffened plates with openings

A series of compressive tests of high tensile steel stiffened plates with multiple openings have been conducted. Different degrees of openings represented by different sets of openings are considered, and their effect on the ultimate carrying capacity is explored. The results are discussed, and several conclusions are stated. Based on the experimental results, several relationships predicting the ultimate axial capacity of high tensile steel stiffened plates with multiple openings as a function of remaining volume and degree of openings are developed. Also, a failure assessment diagram of stiffened plates with openings is defined. Finally, a comparison with other published experimental results for deteriorated mild steel plates with multiple opening is performed.

Laser Transformation Hardening of EN24 Alloy Steel

Abstract The intention of improving the surface hardness, wear resistance, sliding and rolling contact fatigue properties without affecting the material core or in other words minimizing the distortion caused to the inner core of the material due to heat treatment processes has led to the development of Selective Hardening Methods (SHM), like flame hardening, induction hardening, laser hardening, electron beam hardening, ion implantation, selective carburizing and nitridingetc,. An experimental investigation of laser surface hardening of EN24 alloy steel using Nd-YAG laser in atmospheric medium was conducted to understand the influence of laser parameters like laser power, travel speed on the microstructure and material hardness. The laser heat input is allowed to harden the surface of the material by varying laser beam power (500W-3000W) and travel speed (1500-5500mm/min) of the worktable. The laser heating provides localized surface treatment without altering the mechanical properties of the material, leaving the core properties unaffected. The conventional heat treatment followed by rapid quenching distorts the material leaving differential material properties, downgrading the material unfit for highly stressed mechanical applications. EN24 is high tensile alloy steel used in applications like gears, axles, kicker rods, bolts pump shafts and turbine rotors. Microstructure study revealed the transformation of martensitic structure with evident carbides formation with substantial increase in surface hardness of EN24 alloy steel. The case depth relationship with laser parameters were analyzed and documented. The microstructure of laser hardened EN24 alloy steel revealed the formation of martensite along the laser hardened zone (widthwise) boundary and martensite along with transformed carbides beneath the laser hardened surface (depth wise). The surface hardness ranging between 50 HRC and 57 HRC at 2.1 kW laser power with precise directional travel of 3500 mm/min to 4000 mm/min with diffusion less martensitic transformation was observed. Undistorted phase transformation, consistent surface hardness with precise process controls can be achieved using laser hardening of mechanical components

Corrosion Fatigue Properties of High Tensile Steel Plates (YP40 steel) for a Ship Hull

Corrosion Fatigue Properties of High Tensile Steel Plates (YP40 steel) for a Ship Hull

Magnetic pulse welding conditions for high-tensile steel of 1 GPa class and 6061-T6 aluminum alloy sheets

Magnetic pulse welding conditions for high-tensile steel of 1 GPa class and 6061-T6 aluminum alloy sheets

Experimental Analysis and Prediction of Springback in V-bending Process of High-Tensile Strength Steels

Experimental setup has been designed, to study the effects of thickness, width, bend angle and machine tool parameters on the springback, in two high-tensile strength steel grades, namely JSC440 and JSC590, during the V-bending process. Relationship between the springback and the parameters are analyzed using plots. Optimal combination of parameters for the minimum springback is evaluated. Analysis of variance has been carried out to analyze the magnitude of influence of these parameters on the springback. Using the experimental results, analytical models for the prediction of springback for the combinations of blank thickness, width, bend angle and machine tool parameters have been developed. Results reveal that in V-bending of JSC440, thickness and width are the dominant factors influencing the springback, whereas in JSC590 steel, insignificant change in springback is observed with the change in width of blank and using the hydraulic press with holding. However, thickness of steel sheet and bend angle influence significantly the springback in JSC590 steels.

The Structural Behaviour of a Ship’s Sub-Block During Lifting Operation

The process of lifting a ship’s block is one of the important process in shipbuilding. The purpose of lifting or transfering of the blocks is to relocate the block to a new position before joining with other blocks. Prior to the lifting operation of ship’s sub-block, some stress and deflection analysis shall be done, in order to assess the magnitude of the deformation and the stresses acting on the structural members of the block. Based on the assessment, some changes in lifting operation can be intiated for example changing the lifting eye pad location to minimize plate deformation and bucking of structural members. In this study, the structural behaviour of a block was investigated using basic finite element method in order to simulate the deflections and stresses during the lifting operation. The initial dimension of the sub-block that was being investigated are with its dimensions: 21 meters in length, 16 meters in width, 4 meters in height. The type of material of all the structural components used in the sub-block is High Tensile Steel DH36. The investigation used Multiframe software. There are four case studies conducted with varying padeye location and number of lifting eye pads used in a single lifting. The sub block is approximately at 160 tonnes. These results were assessed with the DNV-GL acceptance criteria which can be found in DNV-GL Rules for finite element analysis. The highest yield ulitilization factor was found in case study 3 at 0.864, where it did not comply with the permissible coarse mesh yield utilization factor which is limited to 0.80. The lowest yield utilization factor was found in case study 2 at 0.278. Using the DNV-GL acceptance criteria it is concluded that case study 2 is the best configuration in lifting the sub-block.

Experimental bond behavior of Steel Reinforced Grout systems for strengthening concrete elements

Composite materials consisting of high tensile strength steel cords embedded in a cementitious matrix (Steel Reinforced Grout, SRG) are becoming an attractive solution for strengthening existing structures thanks to the lower cost than “traditional” FRP systems, the isotropy of the steel and the ability of the mortar to resist fire. With respect to Steel Reinforced polymer (SRP) systems – whose mechanical behavior is predominantly dependent on the strength of the substrate to which the steel fibers are epoxy bonded – the mechanical properties of SRG systems are mainly dependent on the cement-based matrix which is sensitive to curing the installation conditions and methods.The experimental results presented and discussed in this paper contribute to expanding the existing knowledge on the bond behavior between the SRG and concrete substrate. To this purpose, a number of SRG strips were bonded to concrete blocks by an inorganic matrix and the lap joint was subsequently subjected to direct shear tests performed in displacement control.Since mortars are very sensitive to environmental conditions and humidity as previously mentioned, two different curing conditions were used. In the first case – termed “curing a” – specimens were cured at room temperature for 28 days and wet cloths were placed on top of the composite surface each day for two weeks. In the second case – termed “curing b” – specimens were just cured at room temperature for 28 days.In addition to the curing condition, the following study parameters were considered: (a) the concrete surface roughness in the bonded region, (b) the density of the dry steel fabric, (c) the bonded interface length, and (d) the concrete strength. Relatively to the latter parameter, concrete prisms were broadly divided into two strength groups: Normal Strength Concrete (NSC) having an average concrete cylindrical strength between 13 and 25 MPa and High Strength Concrete (HSC) having an average strength equal to 40 MPa.Experimental evidence underlines that SRG systems mainly exhibited a debonding failure at steel fibers-matrix interface (due to the sliding phenomena) irrespective of the concrete strength and surface finish. Furthermore, like the SRP systems, the low tape density showed better performances with respect to the higher densities considered in this paper, both in terms of the efficiency factor (i.e., the ratio between the tensile stress experienced by SRG in each test and the average tensile ultimate strength of dry steel fabric) and of the maximum force transferred by the system.

The Structural Behaviour of a Ship‟s Sub-Block During Lifting Operation

The process of lifting a ship’s block is one of the important process in shipbuilding. The purpose of lifting or transfering of the blocks is to relocate the block to a new position before joining with other blocks. Prior to the lifting operation of ship’s sub-block, some stress and deflection analysis shall be done, in order to assess the magnitude of the deformation and the stresses acting on the structural members of the block. Based on the assessment, some changes in lifting operation can be intiated for example changing the lifting eye pad location to minimize plate deformation and bucking of structural members. In this study, the structural behaviour of a block was investigated using basic finite element method in order to simulate the deflections and stresses during the lifting operation. The initial dimension of the sub-block that was being investigated are with its dimensions: 21 meters in length, 16 meters in width, 4 meters in height. The type of material of all the structural components used in the sub-block is High Tensile Steel DH36. The investigation used Multiframe software. There are four case studies conducted with varying padeye location and number of lifting eye pads used in a single lifting. The sub block is approximately at 160 tonnes. These results were assessed with the DNV-GL acceptance criteria which can be found in DNV-GL Rules for finite element analysis. The highest yield ulitilization factor was found in case study 3 at 0.864, where it did not comply with the permissible coarse mesh yield utilization factor which is limited to 0.80. The lowest yield utilization factor was found in case study 2 at 0.278. Using the DNV-GL acceptance criteria it is concluded that case study 2 is the best configuration in lifting the sub-block.

Fresh and sea water immersion corrosion testing on marine structural steel at low temperature

ABSTRACT The aim of the present study is to experimentally examine the corrosion progress characteristics of steel associated with fresh and sea water immersion at low temperature. Three types of steel, namely mild steel (Grade A) and high tensile steel (Grades A and D) are tested under various corrosive conditions in the fresh water, in the sea water and in the air at a temperature of 18°C, 0°C and −10°C. Mass loss of test specimen due to corrosion is measured at a monthly interval and it is converted to a loss of steel plate thickness. Based on the test database, the effects of parameters affecting the corrosion progress are discussed. Test database obtained in the present study is documented.

Bending Property of Multi-Material Hat Shaped Member of CFRTP and Aluminum Alloy

CFRTP (Carbon Fiber Reinforced Thermoplastics) are expected to be used in the automobile parts due to their properties such as high specific strength and high specific stiffness in order to reduce the weight of car body. However, due to the expensive carbon fiber, the cost of CFRTP is high and its application to automobiles is limited. For mass-produced automobiles, a multi-material concept using a combination of dissimilar materials has been attracting attention. While the multi-material concept is often used to joint dissimilar materials, such as composite materials, high tensile strength steel, Al alloy and so on, the method of combining dissimilar material into one part is also expected. For the development of automotive parts, CAE technology is required and in order to apply multi-material concept to mass-produced automobiles, optimization method of materials and shape is necessary to be developed in the future. In this study, multi-material hat shaped members of CFRTP and aluminum alloy were molded by using hybrid molding method and their bending properties were evaluated. Their FEM analysis was also carried out and the relationship between material cost and mechanical properties were evaluated. When the surface of Al alloy is treated by electrolytic polishing, delaminating between Al alloy and CFRTP was suppressed and high stiffness multi-material hat shaped member can be obtained. The multi-material hat shaped member of CFRTP and Al alloy has superior specific stiffness as compared with molded products using only CFRTP for surface material, although the specific maximum bending load was lower.

Out-of-plane seismic retrofitting of masonry walls with Textile Reinforced Mortar composites

Masonry walls are particularly vulnerable against out-of-plane seismic actions. Steel tie-bars and crowing beams in reinforced masonry can prevent their overturning, but collapse may take place also by bending, leaf separation or disaggregation. Textile Reinforced Mortar (TRM) composites, comprising high strength fabrics and inorganic matrices, can be applied to effectively improve the seismic capacity of masonry load-bearing walls and infill panels. Nevertheless, a deeper knowledge on the dynamic response and ultimate capacity of retrofitted walls still needs to be gained before TRM systems can be confidently used in engineering practice. This work describes a shake table test carried out on two full-scale wall specimens, one made of regular tuff blocks and one of two leaves of rubble stones, subjected to seismic out-of-plane vertical bending. The walls were tested unreinforced, repaired and strengthened with TRMs and tested again. A unidirectional textile of ultra high tensile strength steel was used on the tuff wall, whereas a bidirectional basalt mesh was applied over the entire surface of the stone wall, with the addition of transversal steel connectors. The responses of the specimens before and after retrofitting are compared to show the improvement of acceleration and displacement capacity entailed by TRM retrofitting and the modification of deflection profiles, failure modes, damage development and dynamic properties. Test outcomes prove the effectiveness of TRM composites for the protection of existing masonry structures, including architectural heritage, in earthquake prone areas and provide information on the reliability of analytical predictions for seismic assessment.

Prevention of cold cracking by the welding process for reducing diffusible hydrogen in high-tensile thick plate welding

ABSTRACT Cold cracking sensibility is a severe problem in the welding fabrication of large-scale steel structures. It is well known that diffusible hydrogen content in the weld metal strongly influences this cracking. In recent years, a novel welding process using a special welding torch was researched and developed. By using this process, the amount of hydrogen content in the weld metal can be greatly reduced than the conventional process. In this study, the effect of cold cracking prevention and required preheating temperature reduction by new welding process were confirmed. The window type restraint weld cracking test was applied. Both base metal and welding material were 780 MPa class high-tensile steel. During the test, the preheat and interpass temperatures were controlled strictly. Moreover, the temperature level was changed from 50°C to 0°C. As a result, cold cracking (transverse crack) occurred with the conventional method in which the temperature was kept under 50°C. In contrast, there is no crack with the developed one under 50°C and 25°C. When the controlled temperature was lowered to 0°C, many cracks were observed with the developed process. From the above, the effect of cold cracking prevention by the novel welding process for reducing diffusible hydrogen was confirmed and about 50°C preheating temperature reduction effect was demonstrated. These test results confirmed the consistency with past research results. A very good agreement was obtained when comparing the test results and the past research results of cold cracking control indexes in terms of root cracking and multi-pass weld transverse cracking. Therefore, the validity of this study was demonstrated.

Microstructure Characterization in Dissimilar TIG Welds of Inconel Alloy 718 and High Strength Tensile Steel

Nickel based super alloy Inconel 718 is widely used in aerospace and power generation industries. The dissimilar joining of high strength tensile steel to Inconel 718 was joined using TIG welding. The welding current and travel speed parameters were varied to identify the suitable welding conditions for achieving highest joint strength. Microstructural characterization of the weldments were evaluated by using optical, scanning electron microscope and energy dispersive spectroscopy analysis. The fracture surfaces of the tensile tested joints were investigated under scanning electron microscope to reveal the mode of fracture and its influence on the performance of the joints. Microhardness and tensile tests were conducted to evaluate mechanical properties of the joints. The microstructure evolution revealed that the formation of precipitates at the fusion boundary between the weld zone and Inconel 718 alloy. The hardness of the fusion boundary area id higher than the base metal and fusion zone on Inconel 718 side, whereas the highest hardness obtained in heat affected zone on high tensile strength steel. The strength of the joints were failed in the weld zone along the deposition of the filler metal, which strength is similar to the joint strength. The elemental line scan analysis confirmed the formation of Nb and Ti rich precipitates in weld zone and along the fusion boundary of Inconel alloy.