Tensile Fractured Specimens Research Articles

Fabrication and Characterization of Sisal/Glass and Polypropylene Hybrid Composites

The present work summarizes an experimental study on the dynamic mechanical behaviour of sisal – glass fiber reinforced PP composites (SGRP). Variation in mechanical strength, storage modulus (E’), loss modulus (E’’) and damping parameters (tan δ) with the addition of fibers and MAPP were investigated. Mechanical tests revealed an increase in the tensile, flexural and impact strength in the SFRP composites with an increase in the fiber loading from 10% to 30%. Further the SGRP composites prepared using 15 wt% of glass and 15 wt % of sisal fiber, exhibited improved mechanical performance as compared with the virgin matrix as well as 30% SFRP composites. The fiber matrix morphology of the interface region in the composites was examined employing SEM analysis of the tensile fracture specimens. Thermal stability in the composites was studied using TGA/DTG thermo grams. Dynamic mechanical analysis data showed an increase in the storage modulus of the treated composite. The tanδ spectra presented a strong influence of fiber content and coupling agent on the α and γ relaxation process of PP. FTIR spectra of the treated and untreated sisal fibers was also studied to ascertain the existence of the type of interfacial bonds.

Thermo gravimetric analysis and morphological behavior of castor oil based polyurethane-polyester nonwoven fabric composites

Castor oil (CO) based polyurethane (PU)– polyester nonwoven fabric composites were fabricated by impregnating the polyester nonwoven fabric in a reactive composition containing CO and diisocyanate. Composites were fabricated with two different isocyanates such as toluene-2, 4-diisocyanate (TDI) and hexamethylene diisocyanate (HMDI). Thermogravimetric analysis (TGA) studies of the composites were performed to establish the thermal stability and their mode of thermal degradation. It was found that degradation of neat PU takes place in two steps and that of polyester nonwoven fabric reinforced PU composites takes place in three steps. From the TGA thermograms, a little improvement in thermal stability incase of polyester nonwoven fabric reinforced PU composites were noticed compared to unreinforced PUs. Degradation kinetic parameters were obtained for the composites using Broido, Coats and Redfern, and Horowitz and Metzger methods. Tensile fractured composite specimens were used to analyze the morphology of the composites by scanning electron microscopic technique. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Effect of cold work and aging on mechanical properties of a copper bearing microalloyed HSLA-100 (GPT) steel

Investigations have been carried out on the effect of cold work and subsequent aging on mechanical properties of a Cu-bearing HSLA-100 steel microalloyed with Nb and Ti. Aging at 400°C after various degrees of cold work (25–70 pct) exhibits multiple hardness peaks. The treatments cause significant improvement in hardness and tensile strength, but at the cost of impact strength. Cold work also causes deterioration in ductility, which again improves on subsequent aging. The C70A treatment involving 70 pct deformation exhibits maximum response to age hardening giving a hardness of 465 VHN and a UTS of 1344 MPa, but with low values of ductility (5 pct) and impact energy (24 J). C50A treatment involving 50 pct cold work and aging results in an optimum combination of mechanical properties. This treatment in the second hardness peak stage yields a hardness of 373 VHN, UTS of 1186 MPa together with a ductility value of 11 pct and impact energy of 109 J. Scanning electron microscopic studies of fracture surfaces reveal that the impact fracture occurs by formation of dimples and nucleation and growth of voids and cracks. Fracture in tensile specimens is caused by formation of voids and cracks at high density striations. Formation of voids and cracks is also assisted by the presence of precipitated carbide particles.

Processing and tensile properties of hydroxyapatite-whisker-reinforced polyetheretherketone

Polyetheretherketone (PEEK) was reinforced with 0–50 vol% hydroxyapatite (HA) whiskers using a novel powder processing and compression molding technique which enabled uniform mixing at high whisker content. Texture analysis showed that viscous flow during compression molding produced a preferred orientation of whiskers along the specimen tensile axis. Consequently, the elastic modulus or ultimate tensile strength of HA-whisker-reinforced PEEK was able to be tailored to mimic human cortical bone. PEEK reinforced with 40 and 50 vol% HA whiskers exhibited elastic moduli of 17 and 23 GPa, respectively. Elastic constants were measured using ultrasonic wave propagation and revealed an orthotropic anisotropy also similar to that measured in human cortical bone. PEEK reinforced with 10 and 20 vol% HA whiskers exhibited an ultimate tensile strength of 90 and 75 MPa, respectively. Tensile specimen fracture surfaces showed evidence of brittle failure in both reinforced and un-reinforced PEEK. Whisker pullout was observed with PEEK adhered to HA whiskers, suggesting a relatively strong interface between the PEEK matrix and HA whisker reinforcements.

Interfacial, dynamic mechanical, and thermal fiber reinforced behavior of MAPE treated sisal fiber reinforced HDPE composites

AbstractThe present article summarizes an experimental study on the mechanical and dynamic mechanical behavior of sisal fiber reinforced HDPE composites. Variations in mechanical strength, storage modulus (E′), loss modulus (E″), and damping parameter (tan δ) with the addition of fibers and coupling agents were investigated. It was observed that the tensile, flexural, and impact strengths increased with the increase in fiber loading up to 30%, above which there was a significant deterioration in the mechanical strength. Further, the composites treated with MAPE showed improved properties in comparison with the untreated composites. Dynamic mechanical analysis data also showed an increase in the storage modulus of the treated composites The tan δ spectra presented a strong influence of fiber content and coupling agent on the α and γ relaxation process of HDPE. The thermal behavior of the composites was evaluated from TGA/DTG thermograms. The fiber–matrix morphology in the treated composites was confirmed by SEM analysis of the tensile fractured specimens. FTIR spectra of the treated and untreated composites were also studied, to ascertain the existence of type of interfacial bonds. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3306–3315, 2006

In-situ crack propagation observation of a particle reinforced polymer composite using the double cleavage drilled compression specimens

In this study, we investigate the feasibility of in-situ crack propagation by using a double cleavage drilled compression (DCDC) specimen showing a slow crack velocity down to 0.03 mm/s under 0.01 mm/s of displacement control. Finite element analysis predicted that the DCDC specimens would show at least 4.3 fold delayed crack initiation time than conventional tensile fracture specimens under a constant loading speed. Using DCDC specimens, we were able to observe the in-situ crack propagation process in a particle reinforced transparent polymer composite. Our results confirmed that the DCDC specimen would be a good candidate for the in-situ observation of the behavior of particle reinforced composites with slow crack velocity, such as the self-healing process of micro-particle reinforced composites.

Dynamic mechanical and thermal properties of MAPE treated jute/HDPE composites

The present paper summarizes an experimental study on the mechanical and viscoelastic behavior of jute fibre reinforced high density polyethylene (HDPE) composites. Variations in mechanical strength, storage modulus ( E′), loss modulus ( E″) and damping parameter (tan δ) with the addition of fibres and coupling agents were investigated. It was observed that the tensile, flexural and impact strengths increased with the increase in fibre loading upto 30%, above which there was a significant deterioration in the mechanical strength. Further, the composites treated with MAPE showed improved properties in comparison to the untreated composites. Dynamic mechanical analysis data showed an increase in the storage modulus of the treated composites The tan δ spectra presented a strong influence of fibre content and coupling agent on the α and γ relaxation process of HDPE. The thermal behavior of the composites was evaluated from TGA/DTG thermograms. The fibre–matrix morphology in the treated composites was confirmed by SEM analysis of the tensile fractured specimens. FTIR spectra of the treated and untreated jute fibres was also studied to ascertain the existence of type of interfacial bonds.

Effect of the interfacial adhesion on the tensile and impact properties of carbon fiber reinforced polypropylene matrices

Thermoplastic composites have been applied in a wide variety of industrial products, showing recently a great potential to be used in aeronautical field. The objectives of this work were to evaluate the fiber/matrix interface of carbon fiber reinforced polypropylene-based matrices after tensile and impact tests and also to compare the mechanical test results of the manufactured laminates. The laminates were prepared by stacking carbon fiber fabric style Plain Weave (CF) and films of four different polypropylene matrices, described as (a) polypropylene-PP, (b) polypropylene-polyethylene copolymer-PP-PE, (c) PP-PE with an interfacial compatibilizer-AM1 and (d) PP-PE containing an elastomeric modifier-AM2. The composites were processed using hot compression molding. The mechanical testing results showed that the CF-AM1 laminate family presented the lowest impact strength and the highest tensile strength values when compared to the other laminates. SEM analysis observations of both tensile and impact fractured specimens of the CF-PP/PE-AM1 specimens revealed a stronger fiber/matrix interface. The CF-PP/PE-AM2 laminate showed a lower tensile strength and higher impact strength values when compared to the CF-PP/PE-AM1 one. PP-PE and PP laminates presented the lowest impact strength values.

Numerical investigation of constraint effects on ductile fracture in tensile specimens

This study explores the capabilities of a computational cell framework into a 3-D setting to model ductile fracture behavior in tensile specimens. The cell methodology provides a convenient approach for ductile crack extension suitable for large scale numerical analyses which includes a damage criterion and a microstructural length scale over which damage occurs. Laboratory testing of a high strength structural steel provides the experimental stress-strain data for round bar and circumferentially notched tensile specimens to calibrate the cell model parameters for the material. The present work applies the cell methodology using two damage criterion to describe ductile fracture in tensile specimens: (1) the Gurson-Tvergaard (GT) constitutive model for the softening of material and (2) the stress-modified, critical strain (SMCS) criterion for void coalescence. The present work first applies the cell methodology to investigate effects of constraint (stress triaxiality) on ductile crack initiation of notched tensile specimens. An application also follows to determine the dependence of ductility on stress triaxiality for the tested steel. These exploratory 3-D studies using computational cells clearly demonstrate its capability to predict the strong effects of constraint on measured stress-strain response for tensile specimens.

真空圧延接合法によるクラッド材の製造とその特性（第４報） Ａｌ／Ｔｉクラッド材接合部の微細構造の観察

Clad materials consisting of aluminum and titanium plates were produced using the vacuum roll bonding method. This method produces an excellent bond without the intermetallic phase formation observable with conventional optical microscope. Bond interfacial structures in the Al/Ti clad material were observed by TEM, and the bonding mechanism were discussed. Tensile fracture occurred in the aluminum base metal of the clad material on the tensile test and the maximum tensile strength was 95 MPa. In addition, no cracks near the bonded zone were observed in the tensile fractured specimen. Nano-crystals and ultra fine-crystals were observed within the bonded zone using TEM. Within the ultra fine-crystal zone, the compositions of Al and Ti were approximately 80 and 20 at%, respectively. Precipitates those identified as a super lattice structure of the L12 type and an intermetallic phase TiAl3 (D022 type), which are nano-meter size particles, were observed within the ultra fine-crystal zone having Al solid solution (FCC crystal) supersatulated with Ti. The bonding mechanism of aluminum to titanium on the vacuum roll bonding method can be considered that very small thin regions of aluminum surface to be bonded are melted by frictional heat due to the great relative sliding between aluminum and titanium plates during the roll bonding. After that the bonded zone is cooled rapidly due to the diffusion of the heat from the bonded zone toward each base metal and rolls, therefore, the nano-crystals and the ultra fine-crystals are formed in the bonded zone.

Rate dependent yield and post-yield behavior of engineering polymers

To characterize the rate dependent fracture behavior of various engineering polymers, instrumented impact tests were performed with bending and tensile type fracture specimens in the testing rate range of 10 -5 m/s up to 8 m/s. Load-time signals were recorded using an instrumented striker and a fixture equipped with a piezoelectric load cell and strain gages, respectively. Furthermore, the time-to-fracture was detected with different strain gage types applied to the specimen side surfaces in the vicinity of the crack tip. The data reduction to determine rate dependent fracture toughness values was carried out according to different procedures (conventional force based analysis and dynamic key curve method) taking specific local crack tip loading rates into account. In the quasi-brittle failure regime, good agreement was found between the fracture toughness values determined by force based and dynamic data reduction schemes.

Biodegradable composites based on l-polylactide and jute fibres

Biodegradable polymers can potentially be combined with plant fibres to produce biodegradable composite materials. In our research, a commercial l-polylactide was converted to film and then used in combination with jute fibre mats to generate composites by a film stacking technique. Composite tensile properties were determined and tensile specimen fracture surfaces were examined using environmental scanning electron microscopy. Degradation of the polylactide during the process was investigated using size exclusion chromatography. The tensile properties of composites produced at temperatures in the 180–220 °C range were significantly higher than those of polylactide alone. Composite samples failed in a brittle fashion under tensile load and showed little sign of fibre pull-out. Examination of composite fracture surfaces using electron microscopy showed voids occurring between the jute fibre bundles and the polylactide matrix in some cases. Size exclusion chromatography revealed that only minor changes in the molecular weight distribution of the polylactide occurred during the process.

Ultrafine-grained Al–5 wt.% Fe alloy processed by ECAP with backpressure

A process for increasing both strength and ductility in a brittle Al–5 wt.% Fe alloy by Equal Channel Angular Pressing (ECAP) was investigated. The increase in solid solubility of iron in the Al matrix produced by intense deformation under ECAP permitted age hardening in this alloy although it is not hardenable by conventional processing. An ultrafine-grained microstructure of 325–450 nm was obtained in a brittle, cast Al–5 wt.% Fe alloy by ECAP with various numbers of passes and backpressure levels from 40 to 275 MPa. A supersaturated solid solution with a maximum solubility of 0.6 wt.% of iron in an aluminium matrix was obtained during ECAP, which allowed ageing of the conventionally non-hardenable alloy. Strength, ductility and microhardness of the cast alloy processed by the ECAP technique were significantly enhanced (e.g. strength and ductility from 102 MPa and 3.4% to 261 MPa and 5.8%, respectively). The subsequent artificial ageing resulted in a further increase in strength to 272 MPa. It was shown by scanning electron microscopy (SEM) that the type of fracture of tensile specimens taken from material subjected to ECAP was predominantly ductile. An increase in backpressure retards cracking of intermetallic particles and enhances the workability and ductility of such alloys processed by ECAP.

Using a micromechanical finite element parametric study to motivate a phenomenological macroscale model for void/crack nucleation in aluminum with a hard second phase

A multi-scale methodology that includes microscale finite element simulations, physical experiments, and a macroscale phenomenological model was used to determine the appropriate first-order influence parameters relating to void/crack nucleation. The finite element analyses were used to examine the role of seven independent features (number of silicon particle sites, uniformity of particle sizes which were micron size, shape of particles, additional microporosity, temperature, prestrain history, and loading conditions) in debonding and fracture of hard silicon particles in a cast A356 aluminum alloy. Owing to the wide range of features that can affect void/crack nucleation, an optimal matrix of finite element calculations is generated using a statistical method of design of experiments (DOE). The DOE method was used to independently screen the parametric influences concerning void/crack nucleation by second phase fracture or interface debonding. The results clearly show that the initial temperature was the most dominant influence parameter with respect to the others for both fracture and debonding. Experiments were then performed at three temperatures to quantify the void/crack nucleation from notch tensile specimen fracture surfaces. The data verified the importance of the temperature dependence on void/crack nucleation and showed that as the temperature decreased, the void nucleation rate increased. The Horstemeyer–Gokhale void/crack nucleation model was modified to include the temperature dependence and material constants were determined based on the experimental data. This study exemplifies a methodology of bridging various size scale analyses by sorting out the pertinent cause–effect relations from the structure–property relations.

Study of inclusions in a failed aero-engine component

The cause of the fatigue failure in the retaining ring of the compressor region of an aero-engine turbine was found to be the presence of a high concentration of nonmetallic inclusions. The results of chemical analysis were used to estimate the phases present. The most frequently observed inclusions were spinel solid solutions of the type MO · N2O3, where M=Fe, Mn, or Mg and N=Cr or Al. The detrimental inclusions were corundum, calcium aluminates, cristobalite, and silicates. The most detrimental phases were traced on the surfaces of the specimens fractured using impact loading; the comparison is being made with the polished surfaces and the tensile specimen fracture surfaces. The inclusions in the failed retaining ring were compared with the ones in a similar component obtained from a used engine. In the case of the latter, a large number of fine and elongated (Mn, Cr, Fe)S inclusions were present along with spinels. The nondeformable, rigid oxide particles are considered more undesirable than the sulfides as far as fatigue life of the component is concerned. It has been reported that the presence of sulfides may eliminate the stresses due to oxides.

Effect of temperatures on the mechanical and morphological properties of polyphenylene sulfide composites

Mechanical properties and corresponding microstructures of polyphenylene sulfide(PPS)/glass fiber composites were investigated according to the glass fiber content and testing temperature change. Tensile test results showed that maximum stress and elastic modulus increased markedly with increasing glass fiber content at below the glass transition temperature(Tg) of PPS matrix. Notched Izod impact strength showed a maximum value at the glass fiber content of 40 wt% regardless of testing temperatures. Scanning electron microscope (SEM) observation of tensile and impact fractured specimens showed that, at the same testing temperature condition, the fracture surface became more rugged as the glass fiber content increased. Polarized optical microscope(POM) observation showed the crack tip bifurcation at the glass fiber surface in tensile fractured specimens, whereas glass fiber breakdown in impact fractured specimens at higher glass fiber content.

Temperature dependence of additional cyclic hardening of a nickel-base superalloy during out-of-phase multiaxial deformation

Work-hardening of alloy IN738LC at room temperature is almost nil (UTS/{sigma}{sub pl, 0.2%} < 1.1). The reason for this behavior is an extremely inhomogeneous slip distribution which causes fracture of tensile specimens at an elongation of only 2%. In other words, cyclic hardening as well as additional OP-hardening cannot develop at room temperature, because the specimen life is restricted to only a few cycles. The difference in hardening behavior at RT and 450 C could be attributed to the much more homogeneous slip distribution and hence higher ductility at the latter temperature. The disappearance of additional hardening above 700 C is due to the breakdown of planar slip which is caused by enhanced cross slip and climb processes. At 450 C a maximum of additional cyclic hardening in out-of-phase (OP) tests was obtained. The value of 110MPa or 12% of the maximum stress of the in-phase (IP) tests is not very pronounced. Only temperatures lower than 750 C produce any additional hardening effect. TEM investigations showed that up to 600 C planar slip dominates. At higher temperatures dislocations can evade {gamma}{prime} particles and other obstacles by cross slip and climb. At 450 C plastic deformation is generated by a/2 {l_angle}110{r_angle}more » dislocations cutting {gamma}{prime} particles as APB-coupled pairs on {l_brace}111{r_brace} slip planes. During OP tests additional slip systems on different {l_brace}111{r_brace} planes are activated resulting in a higher density of immobile dislocations stored in the lattice.« less

Why Direct Tension Test Specimens Break Flexing to the Side

Contrary to the traditional view, unnotched direct tension test specimens of quasi‐brittle materials that exhibit post‐peak strain softening do not deform symmetrically. After passing the peak load, the equilibrium path bifurcates and the secondary postbifurcation branch represents flexing to the side. The bifurcation is shown to be analogous to Shanley's bifurcation in elastoplastic columns. According to the thermodynamic criterion of stable path, the flexing to one side must occur even if the geometry is perfect and if the straightening effect of the moment of the axial force about the centroid of the deflected cross section is taken into account. The lateral flexing favors failure of the specimen at midlength. The phenomenon (which is similar to the recently discovered behavior of notched tensile fracture specimens) is first illustrated using a simple model in which the specimen consists of two rigid bars of unequal lengths, joined by a strain‐softening link. It is shown that flexing to the side is ret...

ガラスおよび窒化けい素の引張破壊および引張・圧縮繰返し疲労破壊破面のフラクトグラフィ的研究

A fractographic analysis was made on two kinds of silicon nitride, glass ceramics and borosilicate glass fractured under monotonic tensile loading and tension-compression cyclic loading. Emphasis in the fractographic investigation was placed on the fracture initiation area. The relation between mirror size and fracture stress was investigated under various stress conditions. The mirror sizes observed on the fracture surfaces of tensile fractured specimens were approximately 60% of those for bending tests when the comparison was made at the same fracture stress. The fractographic observation on the area of fracture initiation revealed that the fracture in silicon nitride-B always occurred from the defects formed during sintering, while in silicon nitride-A, in which fracture always occurred from the surface, no defect was observed in the fracture initiation area. This result means that the fracture in silicon nitride-B occurs by the crack growth from these defects (formed during sintering), while in silicon nitride-A, fracture initiation occurs by the cracking formed along the grain boundaries by SCC in the surface layer, and then these cracks grow by SCG to final failure. The results of EDX analysis on the fracture initiation area of silicon nitride-A showed that the relative content of Y, which is the main element of sintering aid additions, was higher in the region inside of the critical crack size than in the region outside this region. This will probably show that the crack growth occurred by SCG in this area and by rapid growth in the area outside of the critical crack size. This observation seems to support the fracture intiation process discussed above on the fracture of silicon nitride-A.

The determination of fracture toughness for a porous elastic-plastic solid

The near tip stress and porosity fields in a plane strain mode I crack under small scale yielding (s.s.y.) conditions have been studied using a constitutive model for porous elastic-plastic solids. These fields have been used in conjunction with criteria for fracture of tensile specimens to predict values of KIc for a mild steel over a range of temperatures. The regimes of dominance of fracture by slip-induced cleavage and void coalescence mechanisms have been determined. The predictions are shown to compare well with experimental data.