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Mechanical properties of materials
Published in Ash Ahmed, John Sturges, Materials Science in Construction: An Introduction, 2014
Figures 4.5–4.7 are stress–strain plots for a ductile material; up to this point all deformation is uniform. However, at this maximum stress a small neck (constriction) begins to form and all subsequent deformation is confined to this neck. This phenomenon is known as ‘necking’, and is defined as the reduction of the cross-sectional area of a material in a localised area caused by uniaxial tension. During necking, the material can no longer withstand the maximum stress and the strain in the specimen rapidly increases. Fracture ultimately occurs at the neck, as illustrated in Figure 4.8. The fracture strength corresponds to the stress at fracture (vf in Figure 4.7).
Mechanical Characterization Techniques for Composite Materials
Published in Amit Sachdeva, Pramod Kumar Singh, Hee Woo Rhee, Composite Materials, 2021
Partha Pratim Das, Vijay Chaudhary
Stress is usually expressed in N/m2 or Pascal (1 N/m2 = 1 Pa). The stress value from the experiment is determined by dividing the amount of force (F) exerted by the device by its cross-sectional area (A) in the axial direction, which is measured before the experiment is carried out. Mathematically, it is expressed in Equation 8.1. Strain values that do not have units can be calculated using Equation 8.2. In the equation, L is the instantaneous length of the specimen and L0 its original length. σ=FAε=L−L0L0 The stress–strain curve is characteristic of ductile metallic constituents. Another interesting aspect is that we usually speak about the “engineering stress–strain” curve [9]. When a material approaches the stress-strain curve’s maximum, it will significantly reduce its cross-sectional area, a phenomenon known as necking. The computer program assumes when plotting the stress–strain curve that the cross-sectional area will remain constant during the experiment, even throughout the necking process, allowing the curve to slope downwards. The “real” stress–strain curve could be plotted by directly installing a gauge to calculate the change in the specimen’s cross-sectional area during the experiment.
A multi-objective framework for identification of material parameters based on multiple mechanical tests
Published in Mechanics of Advanced Materials and Structures, 2023
Tensile tests of ductile materials involve the following stages: (i) uniform elastic deformation; (ii) plastic deformation under uniform stress states; (iii) instability onset; (iv) necking and growth of triaxial stress states; and (v) catastrophic/macroscopic failure. Experimental investigations have shown that the specimen size plays a small role when obtaining the inelastic parameters up to stage (iii) owing to uniformity of the stress states [41]. In this case, the classical strategy can be adopted, which uses true stress true strain curve up to the maximum load or instability point [42]. However, at such stage, plastic deformation is generally small, thereby rendering the inelastic parameters unsuitable to predict accurately stress states and loading in most metal forming operations.
Superior tensile properties in additively manufactured Ti alloys
Published in Australian Journal of Mechanical Engineering, 2021
A. Zafari, P. Chandran, K. Xia
As discussed above, AM of Ti-64 has been extensively studied and much effort has been made to create a good combination of high yield strength and total elongation. However, the work hardening rate in Ti-64, whether with fully martensitic α′ or fully lamellar α+β microstructures, is very low, resulting in mostly small uniform elongation of < ~6%, as shown in Figure 3, for various AM-processed Ti alloys. In fully martensitic α′ Ti-64, a large number of substructural dislocations and solid solution strengthening effect of V and Al increase yield strength to the same level as that in fully lamellar α + β Ti-64 which is strengthened by fine lamellar spacing (Xu et al. 2015a; Cao et al. 2018). The high yield strength in Ti-64 reduces the difference between yield strength and ultimate tensile strength, leading to low work hardening rate and small uniform elongation with dimples or cracks forming at α′/α′ or α/β interfaces, resulting in necking and fracture. Further, although total elongation can be very scattered for SLM-fabricated Ti-64 (Figure 1), uniform elongation for the same set of materials has consistently been reported to be ~2-6% (Figure 3). Hence, as suggested by Voisin et al. (Voisin et al. 2018), it would be more appropriate to evaluate ductility using uniform elongation instead of total elongation.
Effects of annealing and extrusion on the microstructure and tensile properties of ultrafine-grained Al fabricated by spark plasma sintering
Published in Powder Metallurgy, 2021
Lei Cao, Yuehuang Xie, Yifei Luo, Jiamiao Liang, Jun Wang, Deliang Zhang, Limin Wang
As shown by the tensile engineering stress–strain curves in Figure 11, the as-SPSed sample fractured prematurely before macroscopic yielding at an average stress of 210 MPa (Table 3). The fracture surfaces of the specimens (Figure 12(a)) showed features reflecting the shapes of prior Al powder particles, suggesting that the specimens fractured by crack initiation and propagation along the IPBs. The annealed sample showed the tensile mechanical behaviour of a typical ductile metallic material, demonstrating work hardening, uniform plastic deformation and necking. As shown in Table 3, the average YS, UTS and EL of the annealed sample were 176, 223 MPa and 5.6%, respectively, with an average uniform elongation of 4.0%. The extruded sample also showed a typical ductile tensile mechanical behaviour with the average YS, UTS and EL being 251, 288 MPa and 12.7%, respectively. The fracture surfaces of the tensile test specimens (Figure 12(b,c)) exhibited a large number of dimples, confirming that they fractured in a ductile mode. They also showed a few large and deep pits with sizes in the ranges of 8–22 and 1.5–5.0 μm for the annealed and extruded samples, respectively. It is suspected that these large and deep pits formed from the separation of prior powder particles at locations where the bonding strength of IPBs was low.