Subsequent Mechanical Behavior Research Articles

Tensile Properties of Open Hole and Unhole Sugar Palm ‘Ijuk’ (SPI) Fibre Composite Treated with Sodium Hydroxide (NaOH)

This study evaluates the effects of sodium hydroxide (NaOH) treatment on tensile properties of sugar palm 'ijuk' (SPI) fibre-reinforced polymer (FRP) composites, with and without an open hole that acts as a stress concentrator. The NaOH treatment is aimed to improve the interfacial adhesion between SPI fibres and polymer matrix. Composite specimens were prepared using the hand lay-up method, incorporating SPI fibres in various orientations, and featuring a 6 mm diameter hole. Tensile tests were conducted to evaluate the mechanical performance of SPI FRP composite, including ultimate tensile strength, Young's modulus, and elongation at break. The research also compared the properties of SPI to those of synthetic glass fibre in fibre-reinforced polymer composites. The results showed that NaOH treatment significantly improves the fibre-matrix adhesion with 26% increase in tensile strength, leading to enhanced tensile properties in both samples, regardless of hole presence. The 0° orientation provides the highest strength and stiffness when the load is applied in the direction of the fibres. While for the 90° orientation, strength reduces by 14%. The impact of the hole on stress concentration and the subsequent mechanical behaviour of the open-hole specimens is substantial. The findings of this study offer insightful perspectives on the potential use ofNaOH-treated SPI fibre in structural and other applications, demonstrating its ability to withstand tensile stresses, even with geometric discontinuities like holes.

Effects of seepage flow on liquefaction resistance of uniform sand and gap-graded soil under undrained cyclic torsional shear

Internal erosion is the transportation of soil particles from within or beneath geotechnical structures, caused by seepage flow, that impacts the subsequent mechanical and hydraulic behaviour of the soil. However, it is difficult to predict the liquefaction resistance of eroded soil due to several factors related to the soil fabric. The present study investigates the impact of seepage flow on the undrained cyclic behaviour of two types of soil: uniform sand and gap-graded soil with a fines content of 20%, using a novel erosion hollow cylindrical torsion shear apparatus. From the results for the uniform sand, the soil fabric formed by moist tamping (MT) leads to higher liquefaction resistance than that formed by air-pluviation (AP). However, after applying seepage flow, the liquefaction resistance of the eroded MT specimens becomes even lower than that of the non-eroded AP specimen. Therefore, the liquefaction resistance of soil is expected to decrease due to the rearrangement of the initially stable coarse particles during seepage flow. On the other hand, the liquefaction resistance of the gap-graded soil tends to increase after the removal of fines as the number of stable contacts between the coarse particles is increased. Under these test conditions, the latter effect is found to be greater for the given gradation, leading to a slight increase in the liquefaction resistance of the tested gap-graded soil after internal erosion. Furthermore, the intergranular void ratio and small-strain shear modulus are seen to be well correlated with the liquefaction resistance of the tested soil.

On the influence of cyclic preloadings on the liquefaction resistance of sands: A numerical study

Soil deposits may be subjected to preloading episodes due to different factors such as previous earthquakes, excavations, refilling, compaction, construction of overlying structures, storms under onshore/offshore conditions, among many others. It is well-known that preloading episodes remarkably influence the subsequent soil mechanical behavior and liquefaction resistance. In order to accurately describe the influence of cyclic preloadings, advanced constitutive models which are able to realistically reproduce the soil mechanical behavior are necessary. In this work, a numerical study was carried out to investigate the influence of cyclic preloadings on the liquefaction resistance of sands. The numerical analyses were performed considering three advanced and well established constitutive models, namely: the hypoplastic model for sands by Von Wolffersdorf (1996) with the Intergranular Strain extension by Niemunis and Herle (1997), the same hypoplastic model for sands extended with Intergranular Strain Anisotropy by Fuentes et al. (2020), and the bounding surface plasticity model Sanisand by Dafalias and Manzari (2004). The simulations were performed based on the experimental databases on Zbraslav sand by Duque et al. (2023a,b). Remarks about the model capabilities and limitations on tests with different types of cyclic preloadings and their repercussion on boundary value simulations are given at the end.

Effect of the anisotropy of martensitic transformation on ferrite deformation in Dual-Phase steels

The volume increase and shape change during austenite to martensite transformation in dual-phase (DP) steels are largely accommodated in the microstructure by the deformation of the surrounding ferrite matrix. Accurate estimation of transformation-induced deformation of ferrite via experiments and modeling is essential for predicting the subsequent mechanical behavior of DP steels. This study aims to illustrate the disadvantages of simplifying the anisotropic transformation deformation of martensite to isotropic dilatation for modeling the transformation-induced deformation of ferrite. A novel methodology is developed comprising sequential experimental and numerical research on DP steels to quantify transformation-induced strains in ferrite. This methodology combines the results of prior austenite grain reconstruction, phenomenological theory of martensite crystallography and electron backscatter diffraction (EBSD) orientation data to estimate variant-specific transformation deformation. Subsequently, by comparison of full-field micromechanical calculation results on a virtual DP steel microstructure with experimental EBSD kernel average misorientation and geometrically necessary dislocation measurement results it is shown that neglecting the shear deformation associated with the martensitic transformation leads to significant underestimation in the prediction of transformation-induced strains in ferrite.

ELASTO-PLASTIC BEHAVIOR OF NATURALLY DEPOSITED CLAY DURING/AFTER SAMPLING

Changes in soil structure and overconsolidation in highly structured natural deposited clay as a result of sampling and setting in laboratory tests, together with their effects on the subsequent mechanical behavior, are numerically simulated in terms of unconventional elasto-plastic mechanics. To describe the mechanical behavior, a Super/subloading Yield Surface Cam-clay model is employed. This model allows a description of the decay of soil structure, loss/ increase of overconsolidation and evolution of anisotropy with ongoing plastic deformation from the process of the soil sampling through the unloading with swelling up to the subsequent tests. The sampled clay subjected to isotropic unloading and/or one-dimensional unloading with swelling down to the first stage of vertical loading level, displays a response to some degree, different from that of the soil at the original sampling site; for example, an overconsolidated sample subjected to isotropic unloading with swelling will display rewinding behavior in a subsequent undrained triaxial compression test, and secondary consolidation/delayed compression in a one-dimensional consolidation test. However, one-dimensional unloading with swelling, which is followed by one-dimensional compression test, will give better prediction of the in-situ consolidation yield stress than isotropic unloading. Artificially disturbed clay subjected to repeated undrained shearing after sampling suffers a significant loss of structure. This results in a drop in peak strength under subsequent monotonic undrained shearing tests, and in the compression index in one-dimensional compression tests, as described by Schmertmann.

Mechanical behavior of shape memory alloy fiber reinforced aluminum matrix composites

This study aims to investigate mechanical behavior of NiTi shape memory alloy (SMA) long-fiber reinforced aluminum (Al) matrix composites, experimentally and numerically. The composites with three different pre-memorized shapes of SMA fibers, which were straight-, wave- and rectangle-shaped fibers, were fabricated by powder metallurgical techniques. Three-point bending and tensile test results demonstrated that wave-shape SMA/Al composites showed the highest elastic modulus and strength, and wave-shape and rectangle-shape SMA/Al composites showed high Poisson’s ratio (lateral to longitudinal strain ratio in elastic-plastic regime). The experimental results were examined based on finite element analysis (FEA). It was concluded that internal stresses in the composites, which were introduced during the fabrication processes, highly affected their subsequent mechanical behavior of the composites.

Multiscale Modeling of the Strength and Ductility Paradox for High-Pressure Torsion Samples With Gradient Microstructure

Abstract Severely plastically deformed microstructures of pure copper were produced by subjecting cylindrical copper samples to high-pressure torsion. The effects of this procedure on introducing gradient microstructure and subsequent mechanical behavior were investigated by utilizing electron backscatter diffraction and performing Vickers hardness/tensile testing. A crystal plasticity-continuum dislocation dynamics modeling effort was performed to predict the mechanical performance of these samples. The model includes mechanisms based on the gradient of dislocation density and grain size, back stress fields of grain boundaries, dislocation density transmission across grain boundaries, and stress/strain gradient effects.

Fabric steering technology for variable stiffness panels: Manufacture and mechanical testing

A novel low-cost manufacturing process is introduced, referred to as ‘Fabric Steering’. By manipulating biaxial fabrics, curvilinear fibre paths can be created to manufacture variable stiffness panels, similar to those produced using Automated Fibre Placement [1]. With low equipment costs and the capability to steer multiple-layers of fabrics simultaneously, this technique potentially offers faster production rates and lower manufacturing costs compared to Automated Fibre Placement. A computer aided engineering tool, SteerFab [2,3], is used to guide the design and manufacture process by predicting: (a) the optimum 2D fibre paths, (b) the subsequent mechanical behaviour of the resulting variable stiffness panel (including improvements in buckling resistance and strength) and (c) step-by-step manufacturing instructions. Compared to conventional quasi-isotropic straight-fibre laminates, experimental buckling tests show slightly heavier (by 4.6%) steered-fibre laminates achieve improvements of ~9% and ~17% in buckling resistance load and failure load, findings that fit well with numerical predictions.

The effects of energy density and heat treatment on the microstructure and mechanical properties of laser additive manufactured Haynes 282

The nickel-based superalloy Haynes 282 is a promising candidate material among the existing batch of aerospace alloys for manufacture via laser powder bed fusion (LPBF). LPBF Haynes 282 has a strong preference for epitaxial grain growth in the (001) orientation, promoting inhomogeneous grain morphologies and anisotropic mechanical behaviour. In this paper, LPBF Haynes 282 specimens have been extracted from perpendicular and parallel orientations in respect to the primary vertical build direction and studied in their original as-built form and when exposed to a solution and age heat treatment. The effect of alternative energy densities is also considered in the different conditions. Results show that the numerous processing variables discussed in this research have a direct influence on the morphology of the final grain structure. Although a strongly anisotropic microstructure was present in the as-built material in both respective orientations, this behaviour was eradicated following the solution and aging heat treatment through recrystallisation, and the alleviation of local texture and misorientation to help produce a more uniform equiaxed grain morphology. The subsequent mechanical behaviour has been assessed through hardness, tensile and creep stress rupture testing, and results have corroborated the microstructural findings to confirm a more isotropic material was successfully achieved.

Influence of reversible and non-reversible fatigue on the microstructure and mechanical property evolution of 7075-T6 aluminum alloy

Evolution of microstructural features and defects caused by high cycle fatigue loading have the potential to affect post-fatigue global behavior in different ways depending on the material, fatigue loading parameters, and subsequent loading strain rate. This work focuses on the influence of reversible and non-reversible, macroscopic elastic fatigue on the microstructure and subsequent mechanical properties of 7075-T6 aluminum alloy. Subsurface material obtained from interrupted fatigue experiments is used for both microstructural characterization and post-fatigue mechanical property evolution. Crystal plasticity modeling, in conjunction with machine learning, is used to help determine mechanisms contributing to the mechanical property evolution.It was found that the subsequent mechanical behavior was dependent on both the fatigue stress ratio and subsequent loading strain rate. There was a 7% decrease in quasi-static strength after being fatigued with a stress ratio of zero. However, quasi-static strength remained unchanged after being fatigued at other stress ratios. In addition, dynamic tension experiments did not reveal any decreases in strength due to prior fatigue loading. Fatigue-induced voids potentially developed in grains where slip transmission across the grain boundary was less likely to occur. Iron-rich inclusion cracking took place throughout the bulk fatigued material and was mainly limited to larger sized inclusion particles, but there was no evidence it contributed to significant mechanical property changes. Results from this work show how different stress ratios and degrees of dislocation reversibility during high cycle fatigue loading influence the microstructure and post-fatigue global behavior of 7075-T6 aluminum alloy.

PROPERTIES OF THE HAND MIXED PMMA BASED CEMENT FOR BIOMEDICAL APPLICATIONS

This paper presents insights into the recent trends in development of PMMA bone cements considering their improvements for applications in clinical practice. Experimental investigation of hand mixed PMMA bone cement was realized, aiming to determine mechanical behavior of the material during nanoindentation. Standard multi-cycle indentation tests were applied, with maximum load of 15 N and immediate load relaxation down to 5 N, with sharp Vickers indenter. Indentation curves were obtained and analyzed as the function of the normal load vs penetration depth, for three different numbers of cycles (100, 200 and 300 cycles) and different indentation positions on the sample surface. Resulting indents were analyses from the aspect of the final material structure and its subsequent mechanical behavior. Agglomeration of PMMA beads was observed in the final hardened cement in some surface zones, thus indicating non-homogenous material structure. Changes in the number of cycles did not show significant influence on the mechanical response of the sample. However, sites with agglomerated PMMA beads showed significantly different indentation curves, thus indicating that hand-mixing of PMMA bone cement can produce non-homogenous final material structure.

Manual 2-dimensional Fabric Steering, for the Manufacture of Variable Stiffness Panels

In recent years, Automated Fibre Placement has been applied in manufacturing variable stiffness panels containing continuously changing fibre paths. The aim is to achieve superior mechanical properties, compared to straight-fibre laminates [1,2]. This paper proposes a novel low-cost manufacturing technique, referred to here as ‘manual 2D fabric steering’. The process is able to manufacture similar variable stiffness panels to those produced using Automated Fibre Placement. By manipulating biaxial fabrics, curvilinear fibre paths can be created. The technique involves very low equipment costs and can be used to steer multiple-layers of fabrics simultaneously (in contrast to the tow-by-tow and layer-by-layer fibre deposition method in Automated Fibre Placement), leading to faster production rates and lower costs, at least for smaller production runs. A computer aided engineering tool, SteerFab [3], is used to guide the design and manufacture process by predicting: (a) the optimum 2D fibre paths, (b) the subsequent mechanical behaviour of the resulting variable stiffness panel (including improvements in buckling resistance) and (c), the step-by-step instructions for the actual manufacturing process. In this paper, manual 2D fabric steering is described and the effect of the manipulation sequence on the final steered fabric preform is investigated. The repeatability of the process is also examined.

Microstructural evolution of Alloy 709 during aging

The creep-resistant austenitic stainless steel Alloy 709 (Fe-20Cr-25Ni (wt%) based steel) is being investigated as a candidate structural material for the next generation fast neutron reactors at service temperature of 500–550 °C. However, the study of microstructural evolution of Alloy 709 during aging is lacking. In this study, thus, the microstructure of Alloy 709 has been investigated using electron microscopy in the as-received state and after static aging at 550, 650 and 750 °C. The results show that the prominent precipitate in the as-received Alloy 709 is Nb(CN), with the rod-like Z phase (CrNbN) observed very occasionally. After aging at 550 °C even up to 2000 h, no significant microstructure change was observed, which means that Alloy 709 is fairly stable at 550 °C. Aging at 650 °C produced globular M23C6 phase on grain boundaries, plate-like M23C6 carbides at twin boundaries and in the grain interior, and blocky M23C6 carbide nucleated on Nb(CN). Fine dispersoid Z phases were found on dislocations after aging at 650 °C for 500 h; their amount increases with aging time and temperature. (Cr,Mo)3(Ni,Fe)2SiN θ phase forms at grain boundaries after aging at 650 °C for 1000 h. After aging at 750 °C, θ phase nucleated on M23C6 carbide and a transformation of M23C6 to θ phase was found, which suggests that θ phase is the more stable. Aging at 550 °C promotes the segregation of Cr and Mo to grain boundaries whereas clear Cr depletion was observed at 650 °C due to the precipitation of Cr-rich M23C6 carbides at grain boundaries. Such depletion nearly disappears at 750 °C. The implications of aging for the subsequent mechanical behaviour of Alloy 709 are discussed briefly.

A study of moisture-induced cracking during a short-term rain event in a pre-cracked asphalt concrete pavement with an expansive base layer

ABSTRACT A model is developed herein for predicting moisture-induced crack growth during a short-term rain event, considered herein to be of duration not greater than 2 hr, in a pre-cracked asphalt concrete pavement built on an expansive base layer. The model is in two parts, the first being the prediction of moisture transport into the pavement, and this part is accompanied by an experimental component focused on measuring the diffusivity of moisture in asphalt concrete. The second part of the model deals with the prediction of the effect of moisture transport on the subsequent mechanical behaviour of the pavement, with emphasis placed on the initiation and propagation of an additional surface crack due to expansion of the base layer. Both components of the model are developed computationally via the use of an in-house one-way coupled finite element code, and a description of this algorithm is briefly reviewed herein. The model developed suggests that the pre-existing crack must extend completely through the asphalt concrete layer into the highly permeable and expansive base layer for there to be significant moisture transport during a short-term rain event. In the case wherein the initial crack extends into the base layer, the stress concentration induced due to dilatation of the base layer upon moisture absorption is predicted to cause additional cracking rapidly.

Limit Deformation Analysis of Unsaturated Expansive Soils During Wetting and Drying Cycles

Experimental observation on unsaturated expansive soils subjected to wetting and drying cycles presents a stabilized limit equilibrium state at the end of the suction cycles. Models developed previously for this behavior introduce a large number of parameters which lead to a time-consuming procedure to calibrate. This paper develops a limit analysis method with the Zarka concept for the mechanical behavior of expansive soils subjected to wetting and drying cycles. The required parameters of the proposed limit analysis method are calibrated by the experimental results obtained for bentonite/sand mixtures subjected to cyclic suction loadings in an odometric test. The comparison between the test results and the model predictions demonstrates the capacity of the proposed method to calculate the limit deformation during suction cycles and simulate the subsequent mechanical behavior of the expansive soils after suction cycles.

Investigation of the Unloading Yield Effect in Aluminum and Magnesium Sheet Metal Alloys at Room Temperature

The reduction of CO2-emissions and the lowering of fuel consumption are two main objectives in the automotive industry. To reach these targets, conventional materials like deep drawing steels are substituted by new modern lightweight materials such as aluminum and magnesium alloys. During forming of sheet metal parts, the material experiences a plastic deformation, which can affect the part quality regarding the amount of springback or the occurrence of stretcher strain marks. In this context, a time dependent change of the material behavior can emerge after removing the part from the forming tool. Within this contribution, the influence of pre-strain and the unloading yield effect on the subsequent mechanical behavior of the aluminum alloy AA7020 and the magnesium alloy AZ31B are investigated. Additionally, the time dependency of mechanical properties is analyzed for different aging times from 5 seconds to one week after pre-straining. The results show a significant increase of unloading yield effect with increasing pre-strain during uniaxial tension.

Forming limits of an age hardenable aluminum sheet after pre-straining and annealing

Abstract A two-stage stamping with an intermediate annealing method was developed to improve formability of age hardenable aluminum alloys, which are more challenging than the non-age hardenable aluminum alloys because of the effects that intermediate annealing will have on dissolution of solutes, precipitation of strengthening particles, and the subsequent mechanical behavior of the materials. The process could readily be applied locally to highly strained areas of a complex panel for 6000 series aluminum alloys in order to form a good part without fracture; however, there will be a decrease of precipitation strengthening of those locally treated, highly strained areas. In this study, the strain-based forming limit curves (FLC) of as-received AAx610-T4PD alloy sheets were shown to increase after pre-straining to ~15% and annealing at 425 °C for 10 s. However, since strain-based FLCs are very sensitive to the pre-strain history, both stress-based FLCs and polar-effective-plastic-strain (PEPS) FLCs, which are path-independent, were used to evaluate the forming limits after pre-straining and annealing. Due to the change of mechanical behavior during annealing, this paper describes the procedure to calculate the stress-based FLC in which a residual-effective-plastic-strain (REPS) is determined by overlapping the hardening curve of the pre-strained and annealed material with that of material heat treated without pre-strain. After converting the strain-based FLCs using the constant REPS method, it was found that the stress-based FLCs and the PEPS FLCs of the post-annealed materials are quite similar and both tools are applicable for evaluating the forming limits of Al–Mg–Si alloys for the two-step stamping process with intermediate annealing heat treatment.

Effect of lateral pre-compression on the compressive behavior of a CuZr-based bulk metallic glass composite containing B2-CuZr phase

The effect of pre-loading on the compressive behavior of a ductile Cu48Zr48Al4 bulk metallic glass composite (BMGC) containing a B2-CuZr phase was investigated by a lateral pre-compression method. It was found that noticeable plasticity and fracture strength degradations occurred for the BMGC after the lateral pre-compression. The degraded mechanical properties of the BMGC were correlated with the martensitic transformation from B2-CuZr phase to B19′-CuZr phase induced by the pre-compression. Based on the compressive behavior of the B2-CuZr and B19′-CuZr phases, influences of the two crystalline phases on the shear band multiplication in the BMGC were compared. It is expected that the pre-martensitic transformation of the B2-CuZr phase during the pre-compression would reduce the contribution of the martensitic transformation on the mechanical properties of the BMGC during subsequent compression, causing degradations of its plasticity and work-hardening-like characteristics. The results suggest that mechanical prehistory of the CuZr-based BMGC may be an important factor on its subsequent mechanical behavior.

Phase Stability in the Mo-Ti-Zr-C System via Thermodynamic Modeling and Diffusion Multiple Validation

Alloys in the Mo-rich corner of the Mo-Ti-Zr-C system have found broad applications in non-oxidizing environments requiring structural integrity well beyond 1273 K (1000 °C). Alloys such as TZM (Mo-0.5Ti-0.08Zr-0.03C by weight %) and TZC (Mo-1.2Ti-0.3Zr-0.1C by weight) owe much of their high temperature strength and microstructural stability to MC and M2C carbide phases. In turn, the stability of the respective carbides and the subsequent mechanical behavior of the alloys are strongly dependent on the alloying additions and thermal history. A CALPHAD-based thermodynamic modeling approach is employed to develop a quaternary thermodynamic database for the Mo-Ti-Zr-C system. The thermodynamic database thus developed is validated with diffusion multiple experiments and the validated database is exercised to elucidate the effects of alloying and thermal history on the phase equilibrium in Mo-rich alloys.

Effect of deformation temperature on microstructure and mechanical behaviour of warm working vanadium microalloyed steels

Plane strain compression tests of two V microalloyed steels and one plain C–Mn steel have been done to analyse the influence of the deformation temperature, in the warm working range, on the final microstructure and subsequent mechanical behaviour. In the case of V microalloyed steels, the reheating temperature has an effect on the amount of vanadium in solution prior to deformation. This factor influences the austenite evolution during warm deformation and the transformation during cooling. As a consequence, in the microalloyed steels complex multiphase microstructures are obtained that lead to a wide range of strength–toughness combinations. In contrast, in the case of the plain C–Mn steel minor effects are observed in the deformation range from 800 to 870 °C.