Acoustic emission in hafnium under tensile deformation

The paper addresses the dependence of acoustic emission produced in rolled hafnium GFé-1 under tensile deformation on the material’s structural state. A correlation has been established between the material structure, the level of mechanical properties and the values of acoustic parameters. Acoustic emission in non-recrystallized hafnium under tensile deformation is recorded only at the stages that precede fracture. Upon recrystallization annealing at temperatures of 1123 and 1373 K acoustic emission occurs at all the stages of tensile deformation. The highest level of acoustic emission activity in hafnium is observed during the transition from elastic to elastic-plastic strain.     

Micro-deformation mechanisms in thermoformed alumina trihydrate reinforced poly(methyl methacrylate)

Micro-deformation mechanisms involved in thermoforming of alumina trihydrate (ATH) reinforced poly(methyl methacrylate) (PMMA) was investigated in a new experimental method replicating industrial heavy-gage thermoforming procedure. Uniaxial tension tests under non-steady thermal conditions were carried out at different forming rates and forming temperatures. Stress–strain curves and load–displacement histories of thermoformed samples were studied in terms of specimen temperature at different forming conditions. Neat PMMA samples were stretched to 50% strain under identical thermoforming conditions as PMMA/ATH for comparison purposes. Stress whitening in thermoformed PMMA/ATH samples was monitored with optical microscope and degree of stress whitening was characterized by an index obtained from optical image histograms. Micro-deformation features on the surface of PMMA and PMMA/ATH samples were examined by scanning electron microscopy (SEM). Micro-deformation in neat PMMA was in the form of homogenous drawing and did not include any type of void formation. SEM images of PMMA/ATH samples showed that particle cracking is the dominant deformation mechanism at low-forming temperatures, while at high-forming temperatures, combined particle disintegration and interfacial failure are dominant mechanisms. Stress whitening was not observed in neat PMMA which was attributed to absence of micro-voids or craze-like structures. On the other hand, PMMA/ATH samples displayed different levels of stress whitening depending on density, size and type of micro-deformation features.     

Effect of strain rate on quasistatic tensile flow behaviour of solution annealed 304 austenitic stainless steel at room temperature

Room temperature tensile test results of solution annealed 304 stainless steel at strain rates ranging between 5 × 10⁻⁴ and 1 × 10⁻¹ s⁻¹ reveal that with increase in strain rate yield strength increases and tensile strength decreases, both maintaining power–law relationships with strain rate. The decrease in tensile strength with increasing strain rate is attributed to the lesser amount of deformation-induced martensite formation and greater role of thermal softening over work hardening at higher strain rates. Tensile deformation of the steel is found to occur in three stages. The deformation transition strains are found to depend on strain rate in such a manner that Stage-I deformation (planar slip) is favoured at lower strain rate. A continuously decreasing linear function of strain rate sensitivity with true strain has been observed. Reasonably good estimation for the stress exponent relating dislocation velocity and stress has been made. The linear plot of reciprocal of strain rate sensitivity with true strain suggests that after some critical amount of deformation the increased dislocation density in austenite due to the formation of some critical amount of deformation-induced martensite plays important role in carrying out the imposed strain rate.     

Stress relaxation of high impact polystyrene

The stress relaxation behaviour of high impact polystyrene has been correlated with the microstructural changes observed in tensile tests. The inhomogeneity of plastic deformation, manifested as stress whitening, has been measured using microhardness tests. This method has been found to be sensitive to the amount of crazing in the material. The stress relaxation behaviour changed at the onset of crazing, but did not change appreciably as the volume fraction of crazes increased. An analysis of the relaxation in terms of a site population model based on White's approach suggests the macroscopic stress relaxation is related to the crazes in the boundary regions between the stress whitened and unwhitened material.     

Tensile dilatometric studies of deformation in polymeric materials and their composites

The theory for volume changes in deformation for polymeric materials is presented, together with a brief literature review of the general area of tensile dilatometry. The theory has been used to enable the prediction of the volumetric response of a material to a deformation, which allows for the detection of the onset of cavitation (volume increasing)-type mechanisms in materials displaying such responses. A series of experiments has been performed using an instrumented tensile dilatometry technique on PMMA and on talc-filled reinforced polypropylene at 23 and 60 °C. The engineering constants, tensile modulus and lateral contraction ratio were measured and found to be viscoelastic. The determination of strain in three mutually perpendicular directions during the instrumented tensile test resulted in the measurement and prediction of the volumetric strain response with applied load. A significant cavitation-type mechanism was recorded in the case of the talc-filled reinforced polypropylene, whereas PMMA showed a deviatoric type mechanism. The volume strain has been found to be directly related to the bulk modulus for these materials. Finally, a new method of presenting volumetric strain versus applied stress data is shown and its relevance explained.     

Deformation and fracture of styrene-acrylonitril copolymer - rubber blends: Microscopy studies of deformation zones

A styrene-acrylonitril copolymer (SAN) was toughened by SAN-grafted polybutadiene core-shell rubber particles. Notched tensile specimens were fractured with a tensile speed ranging from 10-4 to 10 m s-1. The deformation processes close to the fracture surface were studied by means of transmission electron microscopy. A marked difference in the structure of the deformation zone was observed between low speed (10-3 m s-1) and high speed (≥1 m s-1) deformed samples. At low tensile speed the structure of the deformation zone correlated closely with fracture mechanics theory. When the tensile speed was increased the deformation zone had a layered structure. In the zone 400–1.5 μm below the fracture surface the deformation structure was similar to that at low speed. In the layer 1.5–0.5 μm from the fracture surface the rubber particles were strongly deformed, but no cavities or crazes could be observed. Directly next to the fracture surface the high speed deformation zone showed a small layer (0.5 μm) where all the deformation had vanished. It is suggested that due to high strain-rate plasticity at the crack tip a temperature rise occurs which is high enough to cause complete relaxation of the deformation in this layer. Therefore, locally the glass transition temperature of the matrix material was reached. The interaction between thermal effects and deformation processes at the crack tip is discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Tensile behaviour of as deposited and heat-treated electroless Ni-P deposits

Electroless Ni-P coatings were deposited on mild steel substrates and the effect of heat-treatment on their structure and tensile behaviour was studied, with the following conclusions. The as-deposited electroless Ni-P coating is amorphous and it remains amorphous up to 300 °C. At 400 °C the coating becomes crystalline and consists of a Ni₃P matrix containing areas of metallic nickel. For the selected coating/substrate thickness ratio, the contribution of the coating in the tensile properties of the coating-substrate system is negligible as expressed by the values of yield strength, ultimate tensile strength and fracture strain in mild steel substrates and coated as-deposited and heat-treated specimens. Extensive cracking of the coating accompanied by spalling was occurred during the tensile tests. The density of cracks was found to increase close to the fracture surface of the tensile specimen and with increasing heat-treatment temperature. The cracks observed on the surface of the coatings are believed to form due to the inability of the brittle coating to accommodate the strain generated in the ductile substrate. Their orientation to the tensile axis is in close relation to the structure of the coating and the failure mechanism that is dictated by this structure. The first cracks on the surface of the coatings were found to form after the yield strength of the tensile specimen has been reached and plastic deformation of the substrate takes place. Their density increases with the accumulation of strain up to fracture.     

Analysis of the microstructure evolution during tensile testing at room temperature of high-manganese austenitic steel

The microstructure and texture evolution of low-stacking fault energy high-manganese austenitic steel during tensile testing at room temperature was studied by means of interrupted tests. Untested material shows fully recrystallized austenitic grains and an almost random texture. During deformation, two deformation mechanisms, mechanical twinning and dislocation gliding, compete with each other. The governing mechanism of a single grain will depend on its crystallographic orientation relative to tensile direction. As a result of the strong interaction between grain orientation and twinning activity at low tensile strain, both grains with and without deformation twins can be observed. However, at high strain all the grains exhibited twin bands. Tensile tested samples were characterized by the presence of a fiber texture with the <111> and <100> directions parallel to the tensile direction.     

Determination of mechanical strength properties of hemp fibers using near-infrared fourier transform Raman microspectroscopy

Fourier transform near-infrared (FT-NIR) Raman microspectroscopy was adopted for analyzing the micro mechanical tensile deformation behavior of cellulosic plant fibers. Mechanical strength parameters such as tensile strength, failure strain, and Young's modulus of diversified hemp fibers were determined within the range of single fiber cells and fiber filaments. The analysis of fiber deformation at the molecular level was followed by the response of a characteristic Raman signal of fiber cellulose that is sensitive to the tensile load applied. The frequency shift of the Raman signal at 1095 cm(-1) to lower wavenumbers was observed when the fibers were subjected to tensile strain. Microstructural investigations using electron microscopy under environmental conditions supported the discussion of mechanical properties of hemp fibers in relation to several fiber variabilities. Generally, mechanical strength properties of diversified hemp fibers were discussed at the molecular, microstructural, and macroscale level. It was observed that mechanical strength properties of the fibers can be controlled in a broad range by appropriate mercerization parameters such as alkali concentration, fiber shrinkage, and tensile stress applied to the fibers during the alkaline treatments.     

医用级热塑性聚氨酯的拉伸性能测试

文中以医用级高分子聚氨酯(TPU)为原材料,通过热压成型制备厚1.2 mm TPU片材,并制取75×4哑铃型拉伸试样。在电子万能试验机上以较高的恒定拉伸速率进行拉伸,预先设定应力到某一设定值后停止拉伸,然后应力以一定速度进行减载。实验探索了整个拉伸-回复循环中应力与应变的关系,研究了拉伸速度对医用级TPU拉伸强度及断裂伸长率的影响,并对拉伸过程中出现应力发白现象进行探究。结果表明,TPU是一种强而韧的硬弹性高分子材料,在高应力大形变下由弹性提供的可逆形变约80%,粘性提供的不可逆形变约20%。在高应力下材料内部孔洞尺寸变大,由此引起材料由透明转而发白,此光学现象可反复发生。     

Tensile impact behavior and deformation mechanism of duplex TiAl intermetallics at elevated temperatures

Investigations are made on the effects of strain rates on the tensile behavior and deformation modes of Duplex Ti–46.5Al–2Nb–2Cr (DP TiAl) at temperatures ranging from room temperature to 840 °C and under strain rates of 0.001, 320, 800, and 1350 s⁻¹. The dynamic strength is higher than quasi-static strength but does not change much over the high strain rate range. Yield stress anomaly is not found. Brittle-to-ductile transition temperature (BDTT) increases with the increased strain rates. A Zerilli–Armstrong constitutive model with appropriate coefficients is chosen to describe the high strain rate flowing behavior. TEM analysis indicates that both ordinary dislocations and superdislocations are found and dislocation pile-up only appears in samples deformed under quasi-static loadings at elevated temperatures. The deformation twins are common in equiaxed grains and the proportion of twinned grains increases with the increased strain rate from 46–72% under quasi-static loadings to 69–95% under high strain rate loadings. No deformation twins are found in lamellar colonies.     

A study on micro-mechanical deformation behaviour of heat-treated 20MnMoNi55 steel

Several heat treatment procedures are designed considering critical temperatures of phase transformation evaluated through dilatometric testing of 20MnMoNi55 steel to transform low carbon bainitic as-received material into ferrite-martensite dual-phase steels consisting of varied martensite fractions. A thorough metallographic study correlated with the micro-hardness of constituent phases ensures morphological characteristics along with its fractional variations in as-received and dual-phase steels. The impact of fractional variation in constituent phases on the uniaxial monotonic deformation characteristics of dual-phase steels has been observed with a correlation study between experimental tensile and finite element simulated results. Therefore, a physical-based model with a 2-dimensional representative volume element has been established, addressing actual morphological characteristics obtained from metallographic studies. Moreover, the constitutive flow behaviours of ferrite and martensite are also derived from the dislocation-based hardening model to address the actual deformation phenomenon. Finally, an inhomogeneous deformation behaviour among constituent phases and localization of plastic strain in ferrite matrix has been observed with von-Mises stress, and equivalent plastic strain distribution through finite element simulated results. This phenomenon is again confirmed with kernel average misorientation mapping and geometrically necessary dislocation density evaluation through electron backscattered diffraction of tensile samples subjected to different degrees of plastic strain.     

Analysis of the tensile behavior of a TWIP steel based on the texture and microstructure evolutions

The texture and microstructure evolutions of a fine-grained TWIP steel subjected to tensile tests at room temperature were investigated in relation to the mechanical behavior. This steel combines both high ductility and strength owing to the TWIP effect. Also the steel exhibits a high strain hardening rate that evolves according to five stages, which are related to the microstructure and texture evolutions and characteristics. The formation of nano-twins in the initial stage of deformation leads to an increase in strain hardening rate. The development of the pronounced <1 1 1> fiber in the tensile direction sustains mechanical twinning and maintains the strain hardening rate on a high level. The resulting microstructure exhibits several types of twin configurations and sub-boundaries with high misorientations due to intense activities of dislocation glide. The twin volume fraction was estimated to be 9% at the final stage of tensile deformation. The new orientations generated by mechanical twinning do not change considerably the final texture.     

A method of assessing the strength of metal surface using film samples on titanium alloy Ti‐1023

The fatigue life of aerospace components depends greatly on the mechanical properties of the finished surface layer. However, no independent strength test of this layer has been reported because of the lack of suitable samples. Therefore, a direct method of assessing the surface tensile strength using film samples with thicknesses of approximately 40 μm is proposed in this paper. The immediate objective of this research is to demonstrate the fundamentals of surface strength testing and prove the feasibility of preparing films by tracking the evolution of the surface integrity. The test results show that layer‐by‐layer grinding and polishing is a feasible method for preparing film samples with sufficient area, controllable thickness, and well‐maintained surface integrity. During the preparation of the film samples, the roughness and micro‐hardness of the test side (the side kept unprepared for testing) are protected, and those of the processed side (the side that is ground and polished) are controlled. The residual stress on both sides is released to zero. The film specimens exhibit regular fracture behaviour in the tensile tests, and their stress–strain curves can be explained as weighted averages of the stress–strain functions of multiple layers.     

Plastic deformation and solid-phase transformation in polybutene-1

The stress-strain behaviour under uniaxial tension was investigated for polybutene-1 specimens crystallized in Modification II with various types of spherulitic microstructure and degrees of crystallinity. As for Modification I, the tensile properties were found to depend greatly on the initial microstructure, but Modification II specimens exhibit a much more pronounced plastic behaviour than specimens fully transformed into Modification I prior to deformation. The deformation of spherulites within the samples was followed either by direct microscopic observation or by means of small-angle light scattering patterns. In situ microscopic observations of individual spherulites showed that the characteristic dark band, which appeared in the early stages of the deformation of Modification I spherulites and which corresponds to the whitening of the strained samples when observed with reflected light, occurred for much larger strains in the case of specimens initially crystallized in Modification II. Differential scanning calorimetry and wide-angle X-ray scattering experiments performed before and after the tensile tests ensured that the unstable tetragonal Modification II was transformed into the stable hexagonal Modification I during the plastic deformation. Eventually, the evolution of this phase transformation was determined as a function of the applied tensile strain by use of X-ray diffractometry.     

Micromechanics of rubber–toughened polymers

A new micromechanical model is provided to account for the full interaction between rubber particles in toughened polymers. Three-dimensional large deformation elastic–plastic finite element analysis is carried out to obtain the local stress and strain fields and then a homogenization method is adopted to obtain the effective stress–strain relation. The dependence of the local stress and strain distributions and effective stress–strain relation on phase morphology and mechanical properties of rubber particles is examined under various transverse constraints. The profile for the effective yield surface is obtained at four different particle volume fractions. It is shown that stress triaxiality affects significantly the effective yield stress and the local stress concentrations. Rubber cavitation and matrix shear yielding are two coupled toughening mechanisms; which one occurs first depends on the properties of rubber particles and matrix and the imposed triaxiality. Rubber cavitation plays an important role in the toughening process under high tensile triaxial stresses. Axisymmetric modelling may underestimate, and two-dimensional plane-strain modelling may overestimate, the inter-particle interaction compared with three-dimensional modelling.     

Axi-symmetric compression of solid cylinders

Cylinders of EN 24 steel and commercial aluminium were compressed at 0.2 and 10.0 s⁻¹ (rapid loading conditions). The temperature rise due to plastic deformation increased with strain rate and was significantly more in steel than in aluminium. The shape of the observed true stress-true strain curves was similar to the temperature rise-true strain plots. In steel, beyond a certain strain, the flow stress decreased with increasing strain, but in aluminium, a direct relation between the observed true stress and the true strain existed over the entire deformation range. Under rapid loading conditions the ring compression test was more reliable than the Cook and Larke method. In both materials, in specimens of constant diameter up to a true strain of 30%–40%, the compressive yield stress, σ_o, was proportional to H^1/8, where H is the instantaneous height of the specimen. Beyond this strain level, σ_o increased with the diameter-to-height ratio (as seen during slow loading). The various factors that can influence the shape of the observed true stress-true strain curves have been considered. Semi-empirical equations have been developed which ensured that the friction-corrected data covering four to five decades of strain rate superimposed fairly well, following suitable temperature or temperature and strain-rate corrections.     

Evolution of deformation structures under varying loading conditions followed in situ by high angular resolution 3DXRD

With high angular resolution three-dimensional X-ray diffraction, individual subgrains are traced in the bulk of a polycrystalline specimen and their dynamics is followed in situ during varying loading conditions. The intensity distribution of single Bragg reflections from an individual grain is analyzed in reciprocal space. It consists of sharp high-intensity peaks arising from subgrains superimposed on a cloud of lower intensity arising from dislocation walls. Individual subgrains can be distinguished by their unique combination of orientation and elastic strain. The responses of polycrystalline copper to different loading conditions are presented: during uninterrupted tensile deformation, formation of subgrains can be observed concurrently with broadening of the Bragg reflection shortly after onset of plastic deformation. With continued tensile deformation, the subgrain structure develops intermittently. When the traction is terminated, stress relaxation occurs and number, size and orientation of subgrains are found to be constant. The subgrain structure freezes and only a minor clean-up of the dislocation structure is observed. When changing the tensile direction after pre-deformation in tension, a systematic correlation between the degree of strain path change and the changes in the dislocation structure quantified by the volume fraction of the subgrains is established. For obtaining the subgrain volume fraction, a new fitting method has been developed for partitioning the contributions of subgrains and dislocation walls.     

The compressive behavior of isocyanate-crosslinked silica aerogel at high strain rates

Aerogels are low-density, highly nano-porous materials. Their engineering applications are limited due to their brittleness and hydrophilicity. Recently, a strong lightweight crosslinked silica aerogel has been developed by encapsulating the skeletal framework of amine-modified silica aerogels with polyureas derived by isocyanate. The mesoporous structure of the underlying silica framework is preserved through conformal polymer coating, and the thermal conductivity remains low. Characterization has been conducted on the thermal, physical properties and the mechanical properties under quasi-static loading conditions. In this paper, we present results on the dynamic compressive behavior of the crosslinked silica aerogel (CSA) using a split Hopkinson pressure bar (SHPB). A new tubing pulse shaper was employed to help reach the dynamic stress equilibrium and constant strain rate. The stress-strain relationship was determined at high strain rates within 114–4386 s⁻¹. The effects of strain rate, density, specimen thickness and water absorption on the dynamic behavior of the CSA were investigated through a series of dynamic experiments. The Young’s moduli (or 0.2% offset compressive yield strengths) at a strain rate ∼350 s⁻¹ were determined as 10.96/2.08, 159.5/6.75, 192.2/7.68, 304.6/11.46, 407.0/20.91 and 640.5/30.47 MPa for CSA with densities 0.205, 0.454, 0.492, 0.551, 0.628 and 0.731 g cm⁻³, respectively. The deformation and failure behaviors of a native silica aerogel with density (0.472 g cm⁻³), approximately the same as a typical CSA sample were observed with a high speed digital camera. Digital image correlation technique was used to determine the surface strains through a series of images acquired using high speed photography. The relative uniform axial deformation indicated that localized compaction did not occur at a compressive strain level of ∼17%, suggesting most likely failure mechanism at high strain rate to be different from that under quasi-static loading condition. The Poisson’s ratio was determined to be 0.162 in nonlinear regime under high strain rates. CSA samples failed generally by splitting, but were much more ductile than native silica aerogels.     

Developing aluminium–zinc-based a new alloy for tribological applications

In order to develop aluminium–zinc-based a new alloy for tribological applications, six binary Al–Zn and seven ternary Al–25Zn–(1–5)Cu were prepared by permanent mould casting. Their microstructure and mechanical properties were investigated. Dry sliding friction and wear properties of the ternary alloys were investigated using a pin-on-disc machine. Surface and subsurface regions of the wear samples were studied with scanning electron microscopy (SEM). The highest hardness and tensile strength were obtained with the Al–25Zn alloy among the binary ones. The microstructure of this alloy consisted of aluminium-rich α and eutectoid α + η phases. Addition of copper to this alloy resulted in the formation of θ (CuAl₂) phase. The hardness of the ternary alloys increased with increasing copper content. The highest tensile and compressive strengths and wear resistance and the lowest friction coefficient were obtained from the ternary Al–25Zn–3Cu alloy. The dimensional change measured on ageing (stabilization) of this alloy was found to be much lower than that obtained from the copper containing zinc-based alloys. Microstructural changes were observed below the surface of the wear samples of the Al–25Zn–3Cu alloy. These changes were related to the heavy deformation of the surface material due to normal and frictional forces, and smearing and oxidation of wear material. Adhesion was found to be the main wear mechanism for the alloys tested.