Ultimate Strength Levels Research Articles

Effect of thermo-mechanical and low-cycle preloading on the strength of carbon fiber-reinforced ceramic matrix composites

Fiber-reinforced ceramic matrix composites (CMCs) exhibit excellent thermo-mechanical properties including outstanding resistance against damage and fatigue. Some CMCs show occasionally even a strength enhancement after fatigue, often interpreted with relieve of internal stresses and interfacial degradation. This study reports the influence of low-cycle thermo-mechanical preloading on the bending and tensile strength of carbon fiber-reinforced silicon carbon (C/C-SiC). For this purpose two C/C-SiC materials with the same fiber architecture but different assumed internal stress states were subjected to single and cyclic mechanical preloads up to 90% of their ultimate strength level at room temperature and at 350 °C. Statistical evaluations of the experiments show that the ultimate strength values were surprisingly unchanged after preloading. The results are discussed regarding the thermal residual stresses (TRS).

Advanced High-Strength Steels for Automotive Applications: Arc and Laser Welding Process, Properties, and Challenges

In recent years, the demand for advanced high-strength steel (AHSS) has increased to improve the durability and service life of steel structures. The development of these steels involves innovative processing technologies and steel alloy design concepts. Joining these steels is predominantly conducted by following fusion welding techniques, such as gas metal arc welding, tungsten inert gas welding, and laser welding. These fusion welding techniques often lead to a loss of mechanical properties due to the weld thermal cycles in the heat-affected zone (HAZ) and the deposited filler wire chemistry. This review paper elucidates the current studies on the state-of-the-art of weldability on AHSS, with ultimate strength levels above 800 MPa. The effects of alloy designs on the HAZ softening, microstructure evolution, and the mechanical properties of the weld joints corresponding to different welding techniques and filler wire chemistry are discussed. More specifically, the fusion welding techniques used for the welding of AHSS were summarized. This review article gives an insight into the issues while selecting a particular fusion welding technique for the welding of AHSS.

A Comparative Evaluation of Third-Generation Advanced High-Strength Steels for Automotive Forming and Crash Applications.

While the third generation of advanced high-strength steels (3rd Gen AHSS) have increasingly gained attention for automotive lightweighting, it remains unclear to what extent the developed methodologies for the conventional dual-phase (DP) steels are applicable to this new class of steels. The present paper provides a comprehensive study on the constitutive, formability, tribology, and fracture behavior of three commercial 3rd Gen AHSS with an ultimate strength level ranging from 980 to 1180 MPa which are contrasted with two DP steels of the same strength levels and the 590R AHSS. The hardening response to large strain levels was determined experimentally using tensile and shear tests and then evaluated in 3D simulations of tensile tests. In general, the strain rate sensitivity of the two 3rd Gen 1180 AHSS was significantly different as one grade exhibited larger transformation-induced behavior. The in-plane formability of the three 1180 MPa steels was similar but with a stark contrast in the local formability whereas the opposite trend was observed for the 3rd Gen 980 and the DP980 steel. The forming limit curves could be accurately predicted using the experimentally measured hardening behavior and the deterministic modified Bressan–Williams through-thickness shear model or the linearized Modified Maximum Force Criterion. The resistance to sliding of the three 3rd Gen AHSS in the Twist Compression Test revealed a comparable coefficient of friction to the 590R except for the electro-galvanized 3rd Gen 1180 V1. An efficient experimental approach to fracture characterization for AHSS was developed that exploits tool contact and bending to obtain fracture strains on the surface of the specimen by suppressing necking. Miniature conical hole expansion, biaxial punch tests, and the VDA 238-100 bend test were performed to construct stress-state dependent fracture loci for use in forming and crash simulations. It is demonstrated that, the 3rd Gen 1180 V2 can potentially replace the DP980 steel in terms of both the global and local formability.

Case study on the mineralogical and petrophysical analysis of reinforced concrete slabs of a highway viaduct of the S.G.C. Orte-Ravenna

For the maintenance of the safety and security of people traveling on the road, renovation of the reinforced concrete is necessary, especially for bridge or viaduct slabs. Being able to quantify the degradation of slabs and propose methodologies for its retrofit or maintenance is crucial not only in the mineralogical-engineering field but also for socio-economic implications. In this study, samples of deck slabs of a viaduct from the highway E45 near the locality of Bagno di Romagna (Emilia Romagna, north of Italy) were subjected to a testing program to evaluate mechanical and mineralogical parameters through thermographic analyses, compression, and Ultrasonic Pulse Velocity tests and morphological observation. The analyses allow to better recognize the damage of the reinforced concrete samples and they have shown that the cause of the detachment of the asphalt has mainly a natural origin, due to temperature variations and precipitation, with a secondary cause in the anthropogenic impact. The work aims to understand the relationships between the structure of the aggregates and the characteristics of concrete to understand the development of degradation. This knowledge could be used to prevent future damages to the highways.Article HighlightsConcrete slabs analysed provided information about the relation between damage level, mechanical behaviour, size and distribution of aggregates.Thermographic analyses were useful to understand how deep the damage was in the slabs and to detect any detachment of the asphalt.Compressive tests, through the failure load value, shown that the ultimate strength level decreased due to formation of microcracks.

Reliability analysis of hull girder ultimate strength for large container ships under whipping loads

The two incident reports for MSC (Mediterranean Shipping Company) Napoli in 2008 and MOL (Mitsui O.S.K. Lines) Comfort in 2015 both pointed out that whipping response was one main reason for inducing the catastrophes. Subsequently, International Association of Classification Societies (IACS) proposed a Unified Requirement (UR) S11A for ultimate strength evaluation of container ships, and main classification societies also updated their guidelines for whipping response. However, there are big differences in these requirements, and the safety level of ultimate strength under whipping loads is still unclear. This article is to find the effect of the whipping loads on hull girder reliability. Two large container ships 14500TEU and 19000TEU designed by China Classification Society (CCS) Rules are selected as case studies. Hull girder ultimate capacity in mid-ship section under hogging conditions is adopted as the dangerous failure mode, which is predicted by the increment-iterative method in IACS Common Structural Rules (CSR). The whipping effect on hull girder reliability is accounted for by a factor attaching on wave-induced moment. Then, a new evaluation criterion is proposed to evaluate hull girder ultimate strength for container ships under whipping loads, and sensitivity analysis of whipping effect is carried out. The conclusions in this article can provide a guidance for revising rule requirements in classification societies and optimising container ship structural design.

Ultimate strength formulation of stiffened panels under in-plane compression or tension with cracking damage

The aim of the present study is to develop closed-form formulations for predicting the ultimate compressive and tensional strength of stiffened steel panels with crack damages. First, a numerical database is generated. This database includes the ultimate strength levels of stiffened steel panels with cracks subjected to axial compressive or tensile loads. It was carried out with a series of nonlinear FEM analyses by varying the size of crack damage. In the following sections, regression analysis is used for deriving the empirical formulations. The results of the present paper can be used for the reliability and risk assessment of structures, including stiffened steel panels with cracks.

Modeling of the Irreversible Deformation of Soils and Rock Masses by the Methods of the Theory of Elasticity

We analyze the existing algorithms, models, and procedures of evaluation aimed at the investigation of the irreversible processes of land-sliding and deformation of soils and rock masses. We establish the advantages both of the Cundall procedure [based on the cyclic (in time) scheme of evaluation of the incremental strain rates according to Newton's second law of motion and the levels of stresses and strains according to generalized Hooke’s law] and of the Coulomb–Mohr strength model modified by introducing the ultimate tensile strength. The dissipation of the kinetic energy caused by the temporal disbalance of forces is described by the introduction of damping forces proportional to the active forces and strain rates. The novelty of results of our investigations is explained by the combination of the indicated algorithms and models on the basis of the finite-difference scheme of numerical analyses guaranteeing the possibility of reliable calculations of large irreversible deformations of rocks, which makes it possible to reproduce the actual history of their landslides and deformations beyond the level of ultimate strength. The practical significance of performed investigations is confirmed by the comparison of accumulated data with the data of geodesic observations of the landslides of ground slopes.

Numerical investigation on the ultimate strength of aluminium integrally stiffened panels subjected to uniaxial compressive load

The objective of this study is to analyse numerically the ultimate strength of stiffened aluminium panels built with extruded aluminium profiles. A finite element code is used to reproduce the mechanical response of the stiffened panels subjected to axial compression. The fabrication related imperfections, such as initial deformations, material softening in Heat-Affected Zone (HAZ) and residual stresses are simulated. The numerical simulations are compared with the experimental response curves, and sensitivity to geometric parameters, initial imperfections and material properties are analysed. The results show that for the considered panel: 1) The ultimate strength is more sensitive to the cross-section dimensions than to the length; 2) The initial deformation has a strong effect not only on the level of ultimate strength but also on the failure mode as well; 3) Both the width of the HAZ and the yield strength in the HAZ has little effect on the ultimate strength of the considered aluminium integrally stiffened panels; 4) The residual stresses will improve the ultimate strength for the considered panels.

Influence of Al-Cu-Mn-Fe-Ti Alloy Composition and Production Parameters of Extruded Semi-Finished Products on their Structure and Mechanical Properties

Studies have been conducted as to the effect of Cu, Mn, Fe concentration changes in Al-Cu-Mn-Fe-Ti alloy, the conditions of thermal and deformational treatment of ingots and extruded rods 40 mm in diameter on the microstructure, phase composition and mechanical properties. It has been determined that changing Al-6.3Cu-0.3Mn-0.17Fe-0.15Ti alloy to Al-6.5Cu-0.7Mn-0.11Fe-0.15Ti causes an increase in the strength characteristics of extruded rods at the room temperature both after molding and in tempered and aged conditions, irrespective of the conditions of thermal treatment of the initial ingot (low-temperature annealing 420 °С for 2 h, or high-temperature annealing at 530 °С for 12 h). Increasing the extruding temperature from 330 to 480 °С, along with increasing Cu, Mn and decreasing Fe in the alloy Al-Cu-Mn-Ti, is accompanied by the increased level of ultimate strength in a quenched condition by 25% to 410 MPa, irrespective of the annealing conditions of the original ingot. An opportunity to apply the Al-6.3Cu-0.3Mn-0.17Fe-0.15Ti alloy with low-temperature annealing at 420 °С for 2 h and the molding temperature of 330 °С has been found to produce rods where, in the condition of full thermal treatment (tempering at 535 °С + aging at 200 °С for 8 hours), a structure is formed that ensures satisfactory characteristics of high temperature strength by resisting to fracture for more than 100 hours at 300 °С and 70 MPa.

Stability of Retained Austenite in High-Strength Martensitic Steels with Low Ms Temperature

An important factor in enhancing the mechanical properties of high-strength steels is the stabilization of an appropriate amount of metastable retained austenite in martensitic matrix. Various novel heat treatment and thermomechanical processing routes have been developed recently which benefit from the effects of retained austenite in terms of achieving better elongation at still high ultimate strength levels. One of these procedures is the Q&P process (Quenching and Partitioning). It produces martensite and retained austenite to obtain strengths of more than 2000 MPa and elongation levels of about 10%. For this investigation, four steels with low Ms temperatures have been selected. Their chemistries contained manganese, silicon, molybdenum and chromium.The development of heat treatment sequences involved trials with various austenitizing temperatures, cooling rates, quenching temperatures, and carbon partitioning temperatures for the stage in which austenite becomes stabilized. The experimental heat treatment led to microstructures consisting of martensite with retained austenite in all the steels. Their strengths were in the range of 1750-2400 MPa and their A5mm elongation was 10-15 %. The largest fraction of retained austenite, according to X-ray diffraction, was 10 %. Specimens with the largest fraction of retained austenite obtained from one schedule were used for studying the stability of retained austenite under cold and hot.

Effect of solution temperature on microstructures and tensile properties of high strength Ti–6Cr–5Mo–5V–4Al alloy

The effect of solution treatment on microstructural characteristics and tensile properties of a new high strength Ti–6Cr–5Mo–5V–4Al alloy was studied. The microstructure–mechanical property relationship is also discussed. The results show that, the Ti–6Cr–5Mo–5V–4Al alloy in solution treated condition has moderate strength levels (800–1000MPa) with an excellent ductility (−22% of the elongation and 50–60% of the reduction of area). The alloy in the α/β-solution and aged (α/β-STA) condition shows a more attractive combination of strength and ductility than the alloy in the β-solution and aged condition (β-STA). The α/β-STA results in the occurrence of mixed α precipitates at nano-scale (−100nm) and at micron-scale (1μm), which contributes to the high ultimate strength levels (1400–1600MPa). However, the fine β grains below 10μm and the primary α phase contribute to good ductility (6–13.5% of the elongation and 22–52% of the reduction of area). The β-ST leads to equiaxed and coarsen β grains (40–100μm), and subsequent aging contributes to the α phase with the size of 1–5μm. The β-STA microstructure contributes to 1200–1400MPa of the ultimate strength with 7–10% of the elongation and 22–28% of the reduction of area, which shows less attractive behavior on the properties than that of the α/β-STA.

횡방향 가력실험 및 충격실험을 통한 강판콘크리트(SC) 전단벽의 감쇠비 평가

Steel plate concrete (SC) composite structure is now being recognized as a promising technology applicable to nuclear power plants as it is faster and suitable for modular construction. It is required to identify its dynamic characteristics prior to perform the seismic design of the SC structure. Particularly, the damping ratio of the structure is one of the critical design factors to control the dynamic response of structure. This paper compares the criteria for the damping ratios of each type of structures which are prescribed in the regulatory guide for the nuclear power plant. In order to identify the damping ratio of SC shear wall, this study made SC wall specimens and conducted experiments by cyclic lateral load tests and vibration tests with impact hammer. During the lateral loading test, SC wall specimens exhibited large ductile capacities with increasing amplitude of loading due to the confinement effects by the steel plate and the damping ratios increased until failure. The experimental results show that the damping ratios increased from about 6% to about 20% by increasing the load from the safe shutdown earthquake level to the ultimate strength level.

Correlation Between Shear Punch and Tensile Strength for Low-Carbon Steel and Stainless Steel Sheets

The deformation behavior of AISI 1015 low-carbon steel, and AISI 304 stainless steel sheets was investigated by uniaxial tension and the shear punch test (SPT). Both materials were cold rolled to an 80% thickness reduction and subsequently annealed in the temperature range 25-850 °C to produce a wide range of yield and ultimate strength levels. The correlations between shear punch and tensile yield and ultimate stresses were established empirically. Different linear relationships having different slopes and intercepts were found for the low-carbon and stainless steel sheets, and the possible parameters affecting the correlation were discussed. It was shown that, within limits, yield and tensile strength of thin steel sheets can be predicted from the shear data obtained by the easy-to-perform SPT.

Structure and mechanical properties of the 03Kh14GNF steel after deformation and annealing

It has been shown that the annealing of cold-worked 03Kh14GNF steel is accompanied by several strengthening and softening processes that take place in different temperature intervals. Therefore, the temperature dependence of its hardness has a complicated form; i.e., it contains several maxima and minima. The following processes should be noted: recovery and recrystallization of δ ferrite, precipitation of M 2(CN) carbonitrides and M 23C6 carbides from the α martensite, their dissolution in the γ phase, the formation of austenite with different stability, polygonization of the α martensite, etc. Tempering for 30 h can produce the level of mechanical properties required according to technical specifications for two processing regimes. These regimes are the annealing at 680°C and at 600°C, but in the second case the high plasticity is probably caused by the formation of a highly stable austenite that is retained down to room temperature. However, the degree of plasticity of this γ phase at room and lower temperatures is unknown. After annealing at 660°C, the steel does not achieve the level of ultimate strength of 480 N/mm2, which is required by the technical specifications. However, since the relative elongation of 35% exceeds the required magnitude, the required level of mechanical properties can apparently be produced by a decrease in the duration of annealing to 20–25 h.

A combined optimization of alloy composition and aging temperature in designing new UHS precipitation hardenable stainless steels

Alloy composition and proper heat treatment conditions are of paramount importance in maximising mechanical properties of precipitation hardenable stainless steels. Three base stainless steel alloys for ultimate strength levels employing carbides, Cu particles and/or Ni-rich precipitates are designed via a computational approach coupling a genetic algorithm with optimization criteria based on thermodynamic, kinetic and mechanical principles. The combined effects of 11 alloying elements (Al, C, Co, Cr, Cu, Mo, Nb, Ni, Si, Ti and V) are investigated on the basis of: a suitable martensite start (Ms) temperature, the suppression of undesirable phases, a minimal Cr concentration in the matrix and the potency of the precipitation strengthening contribution. The optimal aging temperature is derived from precipitation strengthening optimization and predicted values match experimental optima for existing alloy grades rather well. This thermodynamic justification of optimal precipitation temperatures in UHS steels has not been given before. For the optimized alloys considered the results of a sequential optimization of composition and precipitation temperature do not differ significantly from those of an integrated optimization.

Influence of sterilization treatments on continuous carbon-fiber reinforced polyolefin composite

To evaluate the influence of sterilization treatment on continuous carbon-fiber reinforced polyolefin composite (CFRP) so as to provide experimental reference for selection of sterilization method for CFRP. Seventy bars of CFRP were divided into 7 equal groups to undergo sterilization by autoclave, 2% glutaraldehyde soaking, 75% alcohol soaking, ethylene oxide sterilization, and Co-60 gamma ray irradiation of the dosages 11 kGy, 25 kGy, and 18 kGy respectively, and another 10 bars were used as blank controls. Then the bars underwent three-point bending test and longitudinal compression test so as to measure the biomechanical changes after sterilization treatment, including the maximum load, ultimate strength, and elastic modulus. Three-point bending test showed that the levels of maximum load of the all experimental groups were lower than that of the control group, however, only those of the 3 Co-60 irradiation groups were significantly lower than that of the control group and that Co-60 radiation lowered the level of maximum load dose-dependently; and that the levels of ultimate strength of all the all experimental groups were lower than that of the control group, however, only those of the 3 Co-60 groups were significantly lower than that of the control group and that the higher the dosage of Co-60 radiation the lower the level of ultimate strength, however, not dose-dependently. The elastic modulus of the Co-60 25 KGy group was significantly higher than that of the control group, and there was no significant difference in the level of ultimate strength among the other groups. Longitudinal compression test showed that the levels of maximum load and ultimate strength of the 3 Co-60 irradiation groups, autoclave group, and circular ethylene groups were significantly lower than that of the control group, and there was no significant difference in elastic modulus among different groups. During sterilized package of CFRP products produced in quantity autoclave sterilization and Co-60 gamma ray irradiation sterilization should be avoided. Ethylene oxide is proposed as the best sterilization method. If gamma ray irradiation is to be used further technology improvement is necessary.

SOFTENING EFFECTS ON THE SEISMIC RESPONSE OF NON-DUCTILE CONCRETE FRAMES

An approach for predicting the response of non-ductile reinforced concrete (RC) frames subjected to seismic loading is proposed. A beam-column element is developed based on component tests. The main characteristic of the developed element is its ability to represent the drop in the strength of non-ductile RC members after reaching the ultimate strength level. The term softening is used to define the reduction in the component strength capacity under cyclic loading after reaching the ultimate strength limit. Moment and deformation capacities of the frame members are estimated. The analytical predictions are compared with experimental measurements of the response of members of non-ductile frames. The approach is applied to study the seismic response of a three-storey non-ductile frame. The response calculated using the softening model is compared with the prediction using traditional models that do not represent the strength softening of the frame members. It is concluded that ignoring the strength softening underestimates the deformation as members of the frames pass their ultimate strength.

Shear capacity of large fabricated steel cylinders: A truss model

Failure of large diameter steel cylinders fabricated from hot rolled sheet steel plates and subjected to transverse beam shear occurs when large diagonal buckles develop in the panels formed by the circumferential stiffeners. The load-carrying capacity then drops to a slightly lower level than the ultimate strength level. Whereas the ultimate strength has been shown to be significantly reduced in the presence of fabrication residual stresses, the postbuckling load level is not affected. Further, this load level is stable. It is advocated, therefore, that the postbuckling load be used as a conservative design base for calculation of shear capacity. A general truss model is developed herein as a description of the load-carrying mechanism in the postbuckling range. Predictions obtained using the model are compared with available test results and a design chart is developed that will enable designers to handle most practical cases.

Notch sensitivity of linearly softening materials exhibiting tensile fracture

Notch sensitivity diagrams are numerically derived for tensile specimens of linearly softening materials, exhibiting tensile fracture. The notch types are center crack, single and double edge notch, and center hole. Computations are based on Finite Element modeling of the specimen geometry and the Damage Zone Model (DZM) for the linear softening fracture process and static strength prediction. The diagrams are presented in a non-dimensional format, representing failures ranging from linear elastic fracture to net section failure at the ultimate strength level. Furthermore, the diagrams reveal the effect of the notch and specimen size, and the stiffness, fracture energy and strength of the material.

Stiffness behaviour of trabecular bone specimens

Trabecular bone specimens were tested by non-destructive technique with the purpose of investigating stiffness behaviour and optimizing stiffness determination. Cylindrical specimens ( n = 25) were loaded repetitively (0.1 Hz, 30 cycles) by axial compression to 50% of predicted ultimate strength and finally compressed to failure. Analyses of single compression curves showed increasing stiffness ( E′) until a stress level about 50% of ultimate stress followed by decreasing stiffness. Curve fit analysis of the elastic part of the compression curve showed the best fit, when a second order polynomial was used ( r = 0.94, p < 0.001). The stiffness determined non-destructively at the 25% level of ultimate strength increased significantly to the tenth loading cycle followed by a steady state. The precision of stiffness determination as an average of five consecutive measurements at steady state was E′ ± < 5% (95% confidence limits). A reproducibility test by repetition of the test sequence after 3 h rest showed qualitatively the same stiffness behaviour. The variation of stiffness determination between the two test sequences was ± 27% at the first loading cycle falling to ± 12% at steady state.