Tensile Stress-strain Behavior Research Articles

Direct assessment of tensile stress-crack opening behavior of Strain Hardening Cementitious Composites (SHCC)

The process of designing Strain Hardening Cementitious Composites (SHCC) is driven by the need to achieve certain performance parameters in tension. These are typically the pseudo-strain hardening behavior and the ability to develop multiple cracks. The assessment of the tensile load-deformation behavior of these materials is therefore of great importance and is frequently carried out by characterizing the material tensile stress–strain behavior. In this paper an alternative approach to evaluate the tensile performance of SHCC is investigated. The behavior of the material in tension is studied at the level of a single crack. The derived tensile stress-crack opening behavior is utilized to analyze and compare the influence of various composite parameters on the resulting tensile behavior. The deformations occurring during tensile loading are furthermore examined using a digital image-based deformation analysis technique to gain detailed insight into the crack formation, propagation and opening phases.

Pressure Dependent Yield Criteria Applied for Improving Design Practices and Integrity Assessments against Yielding of Engineering Polymers

Conventional yield criteria for ductile materials, such as Tresca and von Mises, predict that yielding is independent on the hydrostatic stress state (pressure), which means that tensile and compressive stress-strain behaviors are considered equal and are equally treated. This approach is reasonable for ductile metallic materials but sometimes inaccurate for polymers, which commonly present larger compressive yield strength, therefore being characterized as uneven. Some pressure dependent theories are available, but there is no consensus concerning the choice of the most appropriate criterion, its use and benefits. As a step in the direction of improving structural integrity practices taking advantage of unevenness, this work performs three key-activities: i) first, a critical review about existing theories and its accuracy; ii) second, a series of experiments under tension and compression including four selected polymers (PA-66, PA-6, PP, and HDPE) to assess real unevenness levels; iii) third, a numerical evaluation of the potential benefits of using modified criteria. Stress states, safety, and stiffness were evaluated for a typical application to illustrate the proposals. Mass reductions up to 39% could be achieved even with simple geometric changes, while keeping original safety and stiffness levels.

Ti–50.8at.% Ni wire with variable mechanical properties created by spatial electrical resistance over-ageing

Abstract This work attempts to develop superelastic TiNi wires with spatially varied shape memory characteristics along their length. A short section of the wire was over-aged by electrical resistance heating, which resulted in the deterioration of its superelasticity, whilst the reminder of the wire length remained superelastic. The tensile stress–strain behaviour of such treated wires is characteristic of two discrete stress plateaus during the stress-induced martensite transformation, corresponding to the original and over-aged sections.

Strain- and strain-rate-dependent mechanical properties and behaviors of Cu3Sn compound using molecular dynamics simulation

This work aims at investigating the mechanical properties and behaviors of orthorhombic Cu3Sn crystals at room temperature through molecular dynamics (MD) simulation. The focuses are placed on the tensile stress–strain behaviors and properties of the Cu3Sn single crystal and also their dependence on applied strain and strain rate. An attempt to characterize the deformation evolution of the Cu3Sn nanostructure during the stress–strain test is also made. In addition, the elastic properties of bulk polycrystalline Cu3Sn are estimated, as a function of strain rate and applied strain, by using the monocrystal results. The effectiveness of the MD model is demonstrated through comparison with the nanoindentation results and also published theoretical and experimental data. The calculated orthotropic elastic and shear moduli and Poisson’s ratio of Cu3Sn single crystal reveal not only high anisotropy, but also the great effects of applied strain and strain rate only as the strain rate exceeds a threshold value of about 0.072% ps−1. Specifically, raising the strain rate increases the orthotropic elastic properties and also the ultimate tensile and shear strengths of the nanocrystal, whereas increasing the applied strain reduces them.

Fatigue behavior of a Hi-Nicalon™/SiC–B4C composite at 1200°C in air and in steam

Effects of steam environment on fatigue behavior of a non-oxide ceramic composite with a multilayered matrix were investigated at 1200 °C. The composite was produced via chemical vapor infiltration (CVI). The composite had an oxidation inhibited matrix, which consisted of alternating layers of silicon carbide and boron carbide and was reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had pyrolytic carbon fiber coating with boron carbon overlay applied. Tensile stress–strain behavior and tensile properties were evaluated at 1200 °C. Tension-tension fatigue tests were conducted at 0.1 Hz and at 1.0 Hz for fatigue stresses ranging from 100 to 140 MPa in air and in steam. Fatigue run-out was defined as 105 cycles at 0.1 Hz and as 2 × 105 cycles at 1.0 Hz. Presence of steam had little influence on fatigue performance at 1.0 Hz, but noticeably degraded the fatigue lifetimes at 0.1 Hz. Specimens that achieved run-out were subjected to tensile tests to failure to characterize the retained tensile properties. Prior fatigue in air and in steam caused significant reduction in tensile strength and modulus. Composite microstructure, as well as damage and failure mechanisms were investigated.

Effect of frequency and environment on fatigue behavior of a CVI SiC/SiC ceramic matrix composite at 1200 °C

The fatigue behavior of a SiC/SiC CMC (ceramic matrix composite) was investigated at 1200 °C in laboratory air and in steam environment. The composite consists of a SiC matrix reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had boron nitride fiber coating applied and were then densified with CVI SiC. Tensile stress–strain behavior and tensile properties were evaluated at 1200 °C. Tension–tension fatigue tests were conducted at frequencies of 0.1, 1.0, and 10 Hz for fatigue stresses ranging from 80 to 120 MPa in air and from 60 to 110 MPa in steam. Fatigue run-out was defined as 10 5 cycles at the frequency of 0.1 Hz and as 2 × 10 5 cycles at the frequencies of 1.0 and 10 Hz. Presence of steam significantly degraded the fatigue performance. In both test environments the fatigue limit and fatigue lifetime decreased with increasing frequency. Specimens that achieved run-out were subjected to tensile tests to failure to characterize the retained tensile properties. The material retained 100% of its tensile strength, yet modulus loss up to 22% was observed. Composite microstructure, as well as damage and failure mechanisms were investigated.

Effects of steam environment on creep behavior of Nextel™720/alumina–mullite ceramic composite at elevated temperature

The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1000 and 1100 °C in laboratory air and in steam. The composite consists of a porous alumina–mullite matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The tensile stress–strain behavior was investigated and the tensile properties measured. Tensile creep behavior was examined for creep stresses in the 70–140 MPa range. The presence of steam accelerated creep rates and dramatically reduced creep lifetimes. The degrading effects of steam become more pronounced with increasing temperature. At 1000 °C, creep run-out (set to 100 h) was achieved in all tests. At 1100 °C, creep run-out was achieved in all tests in air and only in the 87.5 MPa test in steam. Composite microstructure, as well as damage and failure mechanisms were investigated.

Effects of Steam Environment on Fatigue Behavior of Two SiC/[SiC+Si3N4] Ceramic Composites at 1300°C

The fatigue behaviors of two SiC/[SiC+Si3N4] ceramic matrix composites (CMC) were investigated at 1,300°C in laboratory air and in steam. Composites consisted of a crystalline [SiC+Si3N4] matrix reinforced with either Sylramic™ or Sylramic-iBN fibers (treated Sylramic™ fibers that possess an in situ BN coating) woven in a five-harness satin weave fabric and coated with a proprietary boron-containing dual-layer interphase. The tensile stress–strain behaviors were investigated and the tensile properties measured at 1,300°C. Tension–tension fatigue behaviors of both CMCs were studied for fatigue stresses ranging from 100 to 180 MPa. The fatigue limit (based on a run-out condition of 2 × 105 cycles) in both air and steam was 100 MPa for the CMC containing Sylramic™ fibers and 140 MPa for the CMC reinforced with Sylramic-iBN fibers. At higher fatigue stresses, the presence of steam caused noticeable degradation in fatigue performance of both composites. The retained strength and modulus of all run-out specimens were characterized. The materials tested in air retained 100% of their tensile strength, while the materials tested in steam retained only about 90% of their tensile strength.

Mechanics of SFRC under tension, part I: analytical study

This is the first of a two-part paper involving a fundamental study of mechanics of short-fibre-reinforced concrete under tension. The theory derived in this paper is validated in a second paper using experimental investigation. The composite material approach, which is based on micromechanics, is employed to describe the phenomenon in terms of constitutive properties of fibre, matrix, fibre–matrix interface and their proportioning. The tensile stress–strain behaviour of short-fibre-reinforced concrete is described with four distinct stages, namely the elastic stage, the stage of closed isolated crack development, and the stages of through-crack development both in the absence of fibre rupture and in the presence of fibre rupture. The analytical development does not require pre-assumed composite behaviour such as peak load, initial tangent modulus etc. The formulation is validated using experimental investigation, supplemented by the results of the available models in the existing literature. Given the extent of the study, it was decided to present the entire work in two parts. The general theoretical development is presented here in part I and the comparisons with test results are presented in the companion paper, as part II.

Soybean oil-based, aqueous cationic polyurethane dispersions: Synthesis and properties

A variety of soybean oil-based, aqueous cationic polyurethane dispersions (PUDs) have been successfully synthesized from methoxylated soybean oil polyols (MSOLs) with hydroxyl functionalities ranging from 2.4 to 4.0. The effects of the hydroxyl functionality of the MSOLs on the particle size of the PUDs and the thermal and mechanical properties of the resulting polyurethane films have been carefully investigated by Fourier transform infrared, transmission electron microscopy, dynamic mechanical analysis, differential scanning calorimetry, thermal gravimetric analysis and measurement of the mechanical properties. The particle size diameter of the PUDs ranges from 45 to 115 nm. The resulting polyurethane films are thermally stable up to 200 °C and exhibit tensile stress–strain behavior ranging from elastomeric polymers to ductile plastics, depending on the hydroxyl functionality of the MSOLs. This work provides a new way of utilizing biorenewables for the preparation of value-added polymers with high performance, contributing to a sustainable chemical industry.

Creep behavior of Nextel™720/alumina–mullite ceramic composite with ±45° fiber orientation at 1200 °C

The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite with ±45° fiber orientation was investigated at 1200 °C in laboratory air, in steam and in argon. The composite consists of a porous alumina–mullite matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The tensile stress–strain behavior was investigated and the tensile properties measured at 1200 °C. The elastic modulus was 38.6 GPa and the ultimate tensile strength (UTS) was 37 MPa. Tensile creep behavior was examined for creep stresses in the 13–32 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run-out (set to 100 h) was achieved in all test environments for creep stress levels ≤20 MPa. At creep stresses >20 MPa, creep performance was best in laboratory air and worst in steam. The presence of either steam or argon accelerated creep rates and significantly reduced creep life. Composite microstructure, as well as damage and failure mechanisms were investigated. Matrix degradation appears to be the cause of early failures in argon and in steam.

Effects of Steam Environment on Creep Behavior of Nextel™610/Monazite/Alumina Composite at 1,100°C

The tensile creep behavior of a N610™/LaPO4/Al2O3 composite was investigated at 1,100°C in laboratory air and in steam. The composite consists of a porous alumina matrix reinforced with Nextel 610 fibers woven in an eight-harness satin weave fabric and coated with monazite. The tensile stress-strain behavior was investigated and the tensile properties measured at 1,100°C. The addition of monazite coating resulted in ~33% improvement in ultimate tensile strength (UTS) at 1,100°C. Tensile creep behavior was examined for creep stresses in the 32–72 MPa range. Primary and secondary creep regimes were observed in all tests. Minimum creep rate was reached in all tests. In air, creep strains remained below 0.8% and creep strain rates approached 2 × 10−8 s−1. Creep run-out defined as 100 h at creep stress was achieved in all tests conducted in air. The presence of steam accelerated creep rates and significantly reduced creep lifetimes. In steam, creep strain reached 2.25%, and creep strain rate approached 2.6 × 10−6 s−1. In steam, creep run-out was not achieved. The retained strength and modulus of all specimens that achieved run-out were characterized. Comparison with results obtained for N610™/Al2O3 (control) specimens revealed that the use of the monazite coating resulted in considerable improvement in creep resistance at 1,100°C both in air and in steam. Composite microstructure, as well as damage and failure mechanisms were investigated.

Stochastic approach to multiple cracking in composite systems based on the extreme-values theory

A stochastic model of fragmentation of brittle constituents, which has been pioneered for ceramic matrix composites, is revisited. It consists in a stochastic description of the brittle fracture of successive fragments having a decreasing size. Fracture of a fragment is caused by its most severe flaw. The approach is based on the low extremes of fragment flaw strength distributions. The paper is aimed at assessing the approach and its potential application to various composite systems containing brittle constituents, such as polymer matrix composites or multi-layers. Expressions for fragments and fibers failures involve various random variables. The influence of the random variables on tensile stress–strain behaviour predictions was investigated. Fragment strength–size relationships were established using tensile tests performed on C/SiC minicomposites in the chamber of a scanning electron microscopy (SEM). Experimental data and predictions validated the approach. Important implications for the prediction of multiple cracking and resulting stress–strain behaviour are discussed. Then simple analytical expressions were derived.

Development of a Bamboo-Based Composite as a Sustainable Green Material for Wind Turbine Blades

Bamboo has many engineering and environmental attributes that make it an attractive material for utilization in wind turbine blades. This paper examines the mechanical properties of a novel bamboo-poplar epoxy laminate which is being developed for wind turbine blades. Information provided in this paper includes an overview of the laminate construction and initial data for the monotonic tensile and compressive stress-strain behavior and tension-tension fatigue life of panels formed by hot-pressing. In addition, a discussion of fracture resistance of the bamboo-poplar laminate, under Mode I and Mode II loading conditions is provided.

Creep of Nextel™720/alumina–mullite ceramic composite at 1200 °C in air, argon, and steam

The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1200 °C in laboratory air, in steam and in argon. The composite consists of a porous alumina–mullite matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The tensile stress–strain behavior was investigated and the tensile properties measured at 1200 °C. The elastic modulus was 74.5 GPa and the ultimate tensile strength was 153 MPa. Tensile creep behavior was examined for creep stresses in the 70–140 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run-out (set to 100 h) was achieved in laboratory air for creep stress levels ⩽91 MPa. The presence of either steam or argon accelerated creep rates and reduced creep lifetimes. Composite microstructure, as well as damage and failure mechanisms were investigated.

Strain-Induced Crystallization of Fractionated Natural Rubber from Fresh Latex

Effects of non-rubber contents on strain-induced crystallization and tensile mechanical properties were investigated using fractionated samples from fresh latex of natural rubber (NR). The upper fraction (UF) was mostly composed of cis-1,4-polyisoprene (namely, NR molecules), while the bottom fraction (BF) contained non-rubber contents of more than 13%. Vulcanized samples were prepared from the UF and the BF. Wide-angle X-ray diffraction patterns and tensile stress data were collected during 6 cycles of uniaxial deformation for the vulcanized samples. Both UF and BF samples showed the similar degree of the strain softening (the Mullins effect) in the tensile stress-strain behavior. On the other hand, the larger depression of crystallization in the 2nd cycle compared with the 1st cycle was observed for the sample prepared from the BF. The strain ratio at which crystalline reflections disappeared was almost unchanged regardless to the number of deformation cycles and the difference of the samples. On the basis of these results, the non-rubber contents in the BF were suspected to suppress strain-induced crystallization not by reducing thermodynamic driving force but by affecting the kinetic factors of crystallization.

Ductile-to-brittle transition in tensile failure of particle-reinforced metals

We present an analytical micromechanical model designed to simulate the tensile stress–strain behaviour and failure of damaging composites containing a high volume fraction of reinforcing particles. One internal damage micromechanism is considered, namely particle fracture, which is assumed to obey a Weibull distribution. Final composite tensile failure occurs when one of two possible failure criteria is reached, given by (i) the onset of tensile instability, or (ii) an “avalanche-like” propagation of particle breaks to neighbouring particles. We show that an experimentally observed transition from failure by tensile instability to abrupt failure resulting from an increase of matrix strength can be mimicked by the model because local load-sharing (i.e. load transfer from a broken particle to its immediate neighbours) is accounted for.

Soybean-Oil-Based Waterborne Polyurethane Dispersions: Effects of Polyol Functionality and Hard Segment Content on Properties

The environmentally friendly vegetable-oil-based waterborne polyurethane dispersions with very promising properties have been successfully synthesized without difficulty from a series of methoxylated soybean oil polyols (MSOLs) with different hydroxyl functionalities ranging from 2.4 to as high as 4.0. The resulting soybean-oil-based waterborne polyurethane (SPU) dispersions exhibit a uniform particle size, which increases from about 12 to 130 nm diameter with an increase in the OH functionality of the MSOL from 2.4 to 4.0 and decreases with increasing content of the hard segments. The structure and thermophysical and mechanical properties of the resulting SPU films, which contain 50-60 wt % MSOL as renewable resources, have been studied by Fourier transform infrared spectroscopy, differential scanning calorimetry, dynamic mechanical analysis, thermogravimetric analysis, transmission electron microscopy, and mechanical testing. The experimental results reveal that the functionality of the MSOLs and the hard segment content play a key role in controlling the structure and the thermophysical and mechanical properties of the SPU films. These novel films exhibit tensile stress-strain behavior ranging from elastomeric polymers to rigid plastics and possess Young's moduli ranging from 8 to 720 MPa, ultimate tensile strengths ranging from 4.2 to 21.5 MPa, and percent elongation at break values ranging from 16 to 280%. This work has addressed concerns regarding gelation and higher cross-linking caused by the high functionality of vegetable-oil-based polyols. This article reports novel environmentally friendly biobased SPU materials with promising applications as decorative and protective coatings.

Effects of environment on creep behavior of two oxide/oxide ceramic–matrix composites at 1200 °C

The tensile creep behavior of two oxide/oxide ceramic–matrix composites (CMCs) was investigated at 1200 °C in laboratory air, in steam, and in argon. The composites consist of a porous oxide matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, have no interface between the fiber and matrix, and rely on the porous matrix for flaw tolerance. The matrix materials were alumina and aluminosilicate. The tensile stress–strain behavior was investigated and the tensile properties were measured at 1200 °C. Tensile creep behavior of both CMCs was examined for creep stresses in the 80–150 MPa range. Creep run-out defined as 100 h at creep stress was achieved in air and in argon for stress levels ≤100 MPa for both composites. The retained strength and modulus of all specimens that achieved run-out were evaluated. The presence of steam accelerated creep rates and reduced creep life of both CMCs. In the case of the composite with the aluminosilicate matrix, no-load exposure in steam at 1200 °C caused severe degradation of tensile strength. Composite microstructure, as well as damage and failure mechanisms were investigated. Poor creep performance of both composites in steam is attributed to the degradation of the fibers and densification of the matrix. Results indicate that the aluminosilicate matrix is considerably more susceptible to densification and coarsening of the porosity than the alumina matrix.

Characterization of fibre/matrix interfaces in carbon/carbon composites

The present paper proposes an approach to characterizing fibre/matrix (F/M) interface in carbon/carbon (C/C) composites with respect to both modes of loading that may be expected: opening or shearing. Push-out and tensile tests were used. The former tests involve the shearing mode whereas the latter ones involve the opening one. Push-out tests use a diamond indenter to load the fibres. The interface sliding shear stress was obtained from the load-fibre displacement curve. The tensile tests were conducted on specimens having fibres oriented at 90° with respect to loading direction in order to preferentially open the interfaces. Interface opening strength was extracted from the composite tensile stress–strain behaviour. The specimens were examined under load and after ultimate failure by optical microscopy (OM). The mechanical properties of the F/M interfaces were then discussed.