Cyclic Response Behavior Research Articles

Micro-scale mechanism for cyclic deformation behavior in Zirconium under low-cycle-fatigue loading

Cyclic deformation behaviors and damage mechanisms of pure Zr were systemically investigated at fully reversed strain-controlled tension-compression fatigue experiments (R = −1), followed by detailed microstructural characterization using scanning electron microscopy with electron backscatter diffraction and transmission electron microscopy. The results demonstrated that three stages (Ⅰ, Ⅱ, Ⅲ) of cyclic response behavior were observed for all applied strain amplitudes, and the trend of the cyclic stress response mainly originated from the substructure evolutions. The initial cyclic softening was attributed to the formation of alternating high- and low-dislocation-density regions, enhancing the free path of dislocation motion. The fatigue life prediction model was established based on the hysteresis loops strain energy. SEM/EBSD slip traces analysis and TEM double beam diffraction analysis showed that prismatic <a> dislocation slip was the dominating deformation mechanism, and pyramidal <c + a> dislocations and 101¯2 twins played an essential role in accommodating plastic deformations. Further, fatigue damage behavior was not only influenced by initial texture, but also other factors played an essential role.

The cyclic response behavior, failure mechanism, and life prediction model of 9% Cr‐based steel under different strain ratios in the low cyclic fatigue regime at 430°C

AbstractDuring steam turbine rotor operation, the startup and showdown processes can cause asymmetric loading stress, leading to fatigue damage. To research the cyclic deformation behavior and fatigue failure mechanisms of 9% Cr‐based steel under asymmetric loading conditions, the strain ratios including −1, 0, and 0.1 are introduced into the low cyclic fatigue tests at 430°C. The results show the cyclic softening behavior is observed under three strain ratios at 430°C. As the strain ratio increases, the crack initiation sources, plastic strain, and consumed energy present an increasing trend. Comparing the microstructure morphology at different strain ratios, the coarsening lath and carbides affect fatigue crack initiation. Besides, the grain rotation and dynamic recrystallization can be observed, resulting in the cyclic softening behavior during the fatigue test. Finally, a new fatigue life prediction model is proposed at 430°C.

Nonlinear Dynamic Analyses Utilising Macro-Models of Reinforced Concrete Building Structures and Site-Specific Accelerograms

This paper aims to guide structural engineers on how to apply the rapid nonlinear time history analysis (RNLTHA) procedure effectively to predict seismic demand, taking into account ductility and overstrength, and effects of dynamic phenomena including cyclic degradation of strength and stiffness in structures, in a direct and expedient manner. The shortcoming of the conventional force-based approach of design involving the use of a force reduction factor to account for nonlinear effects is well recognised. Nonlinear static (pushover) analysis and dynamic nonlinear time history analysis (NLTHA) are offered as alternative methods of analysis by major codes of practices to achieve better optimisation in the use of materials. NLTHA has advantages over pushover analysis in being more direct and capable of capturing cyclic response behaviour. Despite the merits of NLTHA, its adoption in the industry has been limited, mainly because of the complexity and the higher analysis cost involved. RNLTHA proposed in this article uses a macroscopic model of the building to fulfil the purpose of NLTHA, whilst saving computational time and achieving a good degree of accuracy, as verified by comparison with results generated from SeismoStruct.

Fretting fatigue behavior and damage mechanism of cast Al-Si-Cu-T6 alloy under two surface roughness conditions

This paper experimentally evaluates the fretting fatigue properties of cast Al-Si-Cu-T6 alloys against nodular cast irons with two surface roughnesses. A softening phenomenon contrary to the cyclic response behavior of plain cyclic loading is pointed out and discovered to be affected by persistent damage and cumulative plastic deformation. Two considerably different roughness conditions produce two entirely distinct damage mechanisms, and a schematic diagram is drawn to illustrate the discrepancy. The initiation and propagation of cracks are not only affected by fretting wear damage but also significantly depend on the microstructure, especially the α-Fe and (β + ω) phases play key roles. The realization of this research comprehensively reveals the fretting damage mechanism of Al-Si cast alloys under two characteristic contact surface states, which serves the fretting failure analysis of aluminum castings in the automotive industry.

Thermomechanical Fatigue Behavior of Spray-Deposited SiCp/Al-Si Composite Applied in the High-Speed Railway Brake Disc

The thermomechanical fatigue (TMF) behaviors of spray-deposited SiCp-reinforced Al-Si alloy were investigated in terms of the size of Si particles and the Si content. Thermomechanical fatigue experiments were conducted in the temperature range of 150-400°C. The cyclic response behavior indicated that the continuous cyclic softening was exhibited for all materials, and the increase in SiC particles size and Si content aggravated the softening degree, which was attributed to dislocation generation due to differential thermal contraction at the Al matrix/Si phase interface or Al matrix/SiC particle interface. Meanwhile, the TMF life and stress amplitude of SiCp/Al-7Si composites were greater than those of Al-7Si alloy, and increased with the increasing SiC particle size, which was associated with “load sharing” of the direct strengthening mechanism. The stress amplitude of 4.5μmSiCp/Al-Si composite increased as the Si content increased; however, the influence of Si content on the TMF life was not so significant. The TMF failure mechanism revealed that the crack mainly initiated at the agglomeration of small-particulate SiC and the breakage of large-particulate SiC, and the broken primary Si and the exfoliated eutectic Si accelerated the crack propagation.

Behaviour of steel–concrete–steel sandwich composite beams with lacing subjected to reversed cyclic loads

Steel–concrete–steel (SCS) sandwich composite system consists of steel plates covering both sides of the concrete core and connected by mechanical means such as shear connectors. In conventional steel–concrete–steel system, shear connectors are welded to the steel cover plates. Laced steel–concrete composite (LSCC) system is a new form of steel–concrete–steel, proposed earlier by the authors. In LSCC system, steel cover plates are connected in a novel way using lacings and cross rods and hence is devoid of welding. Proposed sandwich composite system is being evaluated systematically for its structural behaviour under various modes of loading for use in special structures under severe loading such as blast loading. Damage under cyclic loading and energy absorption are extremely important, which are highlighted in this paper. An experimental investigation on the cyclic response behaviour of two LSCC beams is carried out. Angle of lacing is the parameter that is varied between the two beams. Both the beams are found to exhibit similar behaviour on most of the aspects. The envelope of hysteretic response indicates mild softening behaviour after reaching peak value. Maximum load resisted under both sagging and hogging moment conditions is found to be nearly equal, thus making the LSCC system suitable for situations where reversal of loads are encountered. Dissipated energy is observed to be nearly the same for the load applied in the upward as well as in the downward direction. Analytical prediction on energy absorption capacity is carried out by adopting a hysteretic model with strength deterioration. Cyclic ductility factor is evaluated to be about 20 for LSCC beams, while support rotation is calculated to be about 8° and 10° for beams with 45° and 60° angles of lacing, respectively. Spalling of concrete is prevented in LSCC beams by the steel cover plates.

Retrofit of RC hollow piers with CFRP sheets

Hollow bridge piers, particularly those built before the seventies, often have insufficient shear capacity due to inadequate transverse reinforcement details. Therefore, special attention must be given to this very important aspect when reinforced concrete (RC) piers with hollow sections are analysed and retrofitted. This paper covers the experimental analysis of retrofit solutions using CFRP sheets along the piers’ entire height to prevent shear failure. Experimental cyclic tests were carried out to evaluate the shear retrofit strategy efficiency on a set of RC piers with square hollow sections. This work also covers the study of design procedures for CFRP shear retrofitting and the evaluation of the associated ductility capacity improvement. The various transverse reinforcement detailing scenarios were assessed to determine their shear-failure prevention efficiency. The corresponding cyclic response behavior was also evaluated. The most relevant experimental information is presented in the paper, such as the evolution of the outer damage pattern. Finally, shear retrofit solutions, with a 40% increase over the maximum flexural force, show that this strategy is adequate to allow satisfactory ductility behavior.

Modeling of pre-cast concrete hybrid connections by considering the residual deformations

This paper presents an alternative section analysis and hysteretic modeling for the response of pre-cast concrete hybrid connections which have different level of mild steel contributions to the connection flexural capacity. Well-known classical reinforced concrete section analysis approaches cannot be directly applied to the pre-cast concrete hybrid connections due to the strain incompatibility between the concrete section, and the partially bonded mild steel, and the un-bonded pre-stressing tendons. In the proposed section analysis, initially the moment-rotation behavior of the hybrid connection is modeled by providing a new de-bonding length formulation for the mild steel. Later, a hysteretic response model is proposed by considering the residual displacements measured during the hybrid connection subassembly tests. The general trends and the energy dissipation predictions of the proposed model are compared with the previously published reversed cyclic test results. It is observed that the proposed moment-rotation envelope model and the cyclic response behavior model both exhibit satisfactory agreement with the previously published test results. Key words: Precast concrete, post-tensioning, hybrid connections, moment-rotation envelope, de-bonding length, residual deformations, hysteretic modeling.

Cyclic Response and Fatigue of Steels Subjected to Strain Control with Non-Zero Means

Abstract Baseline fatigue data are presented through a series of strain-controlled tension-compression-type fatigue tests on smooth specimens of a Type S1214 mild steel under various combinations of strain amplitudes and nonzero mean strains. The strain magnitudes were chosen in the upper limit of the elastic range, in the plastic range, or in the transition area between the elastic and the plastic range. Experimental results show that for strain amplitudes in the upper limit of the elastic range, positive mean strains prolong fatigue life and negative mean strains shorten fatigue life. The observations are explained by the analysis of the basic material properties associated with the material concerned. The cyclic response behavior of the materials, namely the Bauschinger effect, the cyclic softening, and the mean stress relaxation processes have been studied also to reveal the underlying fatigue mechanism. Several modified models have been proposed for life prediction under nonzero mean strain and/or mean stress based on experimental observations. The comparison of the experimental results with the predictions made from the proposed models and from an existing model are also presented.

Effect of reinforcement size on low cycle fatigue behavior of SiC particle reinforced aluminum matrix composites

From the examination of the particle size effect on the cyclic deformation in SiC/Al composites, the authors offer the following conclusions: SiCp/Al composite displayed continuous cyclic softening after initial hardening for any two or three cycles. In contrast, the unreinforced matrix showed initial cyclic hardening, cyclic stability and second hardening. Higher stress in response to a given strain amplitude were observed with the composite with fine (10 {micro}m) SiC particles in comparison with the composite with coarse (32 {micro}m) SiC particles. In addition, the evolution of cyclic softening became faster when the particle size increased. The low cycle fatigue endurance of the composites was generally lower than that of the unreinforced matrix. The decrease in particle size led to a higher fatigue resistance at lower strain amplitudes, but reduced the fatigue life at higher strain amplitudes. The particle size effect on cyclic response behavior can be explained when the difference in interparticle spacing and thus the difference in plastic deformation ability between the coarse and fine particle composites are taken into account. The particle size effect on fatigue-life behavior was explained in terms of the fracture modes.