Tensile Property

we use a tensile load-deformation curve (see Fig. 3.50) to examine the mechanical behavior of a packaging material. This curve relates the applied load to the resultant deformation of a sample. Commonly, it is measured using an apparatus that provides for a constant slow rate of deformation, and uses a load cell to measure and record the force required to produce that deformation. 

A stress-strain curve (Fig. 3.51) standardizes the load-deformation curve by normalizing the load to a unit cross-sectional area and normalizing the deformation to a unit length. Stress is then the applied tensile load divided by the original cross-sectional area of the specimen, and strain is the increase in length divided by the original length of the sample. 

The basic equation for stress, the force per unit area, is F 

as mentioned, the behavior of the sample is affected by the rate of stretching, stress-strain curves are typically obtained at low strain rates. 

The equation for determining the modulus of elasticity, the ratio between the stress applied and the strain produced in the elastic portion of material behavior, E, is  

In a typical tensile test, the plastic sample in the shape of a rectangular (or dog-bone) piece is held at its extremities in a pair of grips and slowly pulled along its long axis. The deformation (or the increase in length of the sample) and the force F applied are recorded continuously. The strain increases at a prescribed rate during the tensile test. Basic quantities involved in the test are defined as follows (Fig. 3.9)  

FIGURE 3.9 Upper Standard dog-bone-shaped test piece used in tensile tests. Lower Tensile deformation of a rectangular test piece. Notice shrinking of the width. Direction of strain shown by the double-headed arrow at right. 

FIGURE 3.10 Left Change in shape of the dog-bone test piece. Source Reprinted with permission from Coppieters et al. (2011). Right Tensile stress-strain curves for glass bead-filled LDPE at different volume fractions of beads. Source Reprinted with permission from Liang et al. (1998). 

The standard tensile test, is conducted on a uniaxial specimen with a reduced cross section. Standard tensile specimens have an overall length of 8 in. with a 2 in. gage length and cross section of 0.500 in. width and thickness t. Smaller specimens of one-half standard size are sometimes used for polymers. An extensometer is mounted on the central portion of the specimen to measure the elongation over the 2-in. gage length, and the conventional engineering strain is calculated from 

The conventional engineering tensile stress in the specimen is calculated from 

FIGURE 2.1 Stress (strain) curves and the effect of strain rate, (a) Engineering stress/strain curve, (b) ductile material with yield point, (c) stress/strain curves for different behavior, and (d) effect of strain rate. 

The dispersion of clay platelets in the polymer matrix can improve the mechanical properties significantly. The incorporation of clay platelets in the polymer system has improved the mechanical properties by any one or more of the reasons specified below. 

Presence of high strength, stiff or elastomeric compatibilizer 

In crystalline PP, the clay incorporation has improved the elongation at break along with significant improvement in strength and modulus, which is due to the reinforcement effect of clay (Table 9.10). 

The compatibilizer also plays a definite role in improving the properties of polymer. The presence of high strength and stiff PP-g-MA compatibilizer in clay has improved the stiffness and the strength of the PP significantly with reduction in ductility. Table 9.10 clearly depicts the effect of compatibilizer on mechanical properties. 

The elastomeric compatibilizer like polyethylene octene has improved the ductility (elongation at fracture) significantly. However the presence 

Similar to bulk modulus, the shear modulus of isotropic materials also has a simple relationship with tensile modulus and Passion s ratio  

The relationship between shear modulus, tensile modulus and Poisson s ratio of anisotropic fibers deviates from Equation 15.19. In general, the ratio of tensile modulus to shear modulus for most anisotropic fibers is greater than what is predicted by Equation 15.19. 

In many practical applications, tensile properties are the most important mechanical properties of fibers since they typically are under tension or complex stress states that include tensiom This section focuses on the typical stress-strain behavior and factors that affect the stress-strain behavior of polymer fibers. The elastic recovery of polymer fibers also is discussed. 

Polyesters exhibit excellent physical properties. They have high tensile strength, high modulus, they maintain excellent tensile properties at elevated temperatures, and have a high heat distortion temperature. They are thermally stable, have low gas permeability and lo v electrical conductivity. For these reasons, polyesters are considered engineering polymers. 

There have been numerous papers on the mechanical properties of PEEK (16) (58) (59). The tensile properties have been examined 

In general, the contributions of both the soft and hard segments in the polyurethanes can be correlated with the properties observed. The soft rubbery block primarily affects resiliency, wear, tear, compression set and low temperature properties, while the hard block affects hardness, modulus and tensile properties [11]. 

The Durometer hardness was measured after the cast elastomers were aged for at least 7 days at 23 °C and 50% relative humidity. The details are given in Table 8.3. From the results, it is evident that as the crystalline hard segment content of the elastomer is decreased, the elastomer hardness also decreases. By analysing the soft and hard segments and their relation to hardness, it appears that the amorphous hard segment behaves like the polyether soft segment. 

0- H2CH2CO-R -OH2CH2C-OOCHN-R-NHCOO-(Polyether)-OOCHN-R-NHCOO(CH2CH20)2.3-R -(OCH2CH2)2 3-0 

1— Crystalline Hard Segment —I Segment I I— Amorphous (Hard) Segment —I 

For the moment, it can be concluded that the reinforcing effect of nanoparticles on the polymeric matrices can be realized as long as the particles are grafted and a proper dispersion of the modified particles can be formed. In addition, the tensile properties of the nanocomposites can be purposely adjusted according to the interfacial viscoelastic properties provided by different grafting monomers. 

As a parameter closely related to the static stress transfer at the interface. Young s modulus shows another aspect of the role played by the grafting polymers (Table 1). The increase in stiffness of the nanocomposites is obviously a result of the high modulus of the particulate fillers. However, taking into account that the tensile modulus was determined within a small strain range, the formation of a relatively compliant layer at the interface (e.g., PBA, PVA, and PEA) tends to hinder the complete stress transfer under such a low stress level and thus masks 

Grafting polymer GraftingM,7r) li Homopolymer fraction (%) Tensile strength (MPa) j, l Young s modulus (GPa) Elongation to break (%) z,  

Area under tensile stress-strain curve (MPa) 

Compared with elongation to break, the area under the tensile stress-strain curve can characterize more reasonably the toughness potential under static tensile loading. In Table 1, it can be seen that the addition of modified nano-Si02 helps to improve the ductility of PP, except for the case of Si02-g-PEA. It is believed that 

Similar results were obtained with EPDM. 

K torsional constant of wire, Nm L fiee length of test piece, m a angle of twist measured,  

Tensile testing machines are usually equipped with chambers, which maintain low or elevated temperatures. This is specially important in testing properties of plasticized materials. The standard also describes a machine for determination of tensile set. Equipment required to cut ring samples and fixtures needed to use them in testing equipment are also discussed in detail. Standard also evaluates within and between laboratory testing preei-sion. 

the strain rate sensitivity for both coarse- and fine-grained microstructures of Ti3SiC2 is quite high [142, 148] and is more characteristic of super-plastic solids than of typical metals or ceramics. This does not imply that the deformation mechanisms of the MAX phases are in any way comparable to those of superplastic solids. The latter have comparable strain rate sensitivities, but only at grain sizes that are at least two orders of magnitude smaller than those of the CG Ti3SiC2 samples. 

Testing has been covered in Chapter 17 and it is most important to compare the test results of like with like. Cost and time taken will always be a consideration when testing for routine control purposes and for more meaningful results, it may be necessary to undertake expensive extended composite testing. 

Initially, filament testing was used, but this demanded considerable operator skill, with abihty to handle fine filaments and required many tests to get a reasonable statistical result, which tended to be operator dependent. However, it did produce relatively speedy results, ideal for research purposes. Tow testing subsequently became the preferred test procedure and Table 20.8 shows a comparison of the test procedures for very early Courtaulds fiber. 

It is not surprising that the single filament test gives the highest value for modulus, since it is a single filament correctly aligned, whereas with a tow test, it is not possible to align 

Class GPa Manufacturer GPa Type gcm Strength GPa Modulus GPa Density Specific strength Specfic modulus 

Silicon-carbide Glass quartz Nippon carbon Nical on 2.7 193 2.55 1.06 76 

Nothing ruins the truth like stretching it (unkown source) 

But tnith is not a material... suggest we see if it applies to polymer stretehing 

Tensile tests are used to control product quality and for determining the effect of chemical or thermal exposure on an elastomer. 

As shown [127], the tensile stress or tensile modulus is the force per unit of original cross-sectional area required to stretch the specimen to a stated elongation. 50%, 100% and 300% extensions are commonly used. Usually, the test is performed on a dumbbell-shaped specimen [258]. 

Ultimate elongation or elongation at break is the elongation at the point that the sample breaks [127,258]. 

Most structures are designed to ensme that only elastic deformation will result when a stress is applied. A structme or component that has plastically deformed—or experienced a permanent change in shape—may not be capable of functioning as intended. It is therefore desirable to know the stress level at which plastic deformation begins, or where the phenomenon of yielding occurs. For metals that experience this gradual 

Concept Check 6.1 Cite the primary differences between elastic, anelastic, and plastic 

FigMre 6.11 Typical engineering stress-strain behavior to fracture, point F. The tensile strength TS is indicated at point M. The circular insets represent the geometry of the deformed specimen at various points along the curve. 

Mechanical Property Determinations from Stress-Strain Plot 

Inasmuch as the line segment passes through the origin, it is convenient to take both o-j and ej as zero. If 0-2 is arbitrarily taken as 150 MPa, then 2 will have a value of 0.0016. Therefore, 

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In the late 1980s, new fully aromatic polyester fibers were iatroduced for use ia composites and stmctural materials (18,19). In general, these materials are thermotropic Hquid crystal polymers that are melt-processible to give fibers with tensile properties and temperature resistance considerably higher than conventional polyester textile fibers. Vectran (Hoechst-Celanese and Kuraray) is a thermotropic Hquid crystal aromatic copolyester fiber composed of -hydroxyben2oic acid [99-96-7] and 6-hydroxy-2-naphthoic acid. Other fully aromatic polyester fiber composites have been iatroduced under various tradenames (19). 

Tensile properties of importance include the modulus, yields, (strength at 5% elongation), and ultimate break strength. Since in many uses the essential function of the film may be destroyed if it stretches under use, the yield and values are more critical than the ultimate strength. This is tme, for example, where film is used as the base for magnetic tape or microfilm information storage. In some cases, the tensile properties at temperatures other than standard are critical. Thus if films are to be coated and dried in hot air ovens, the yield at 150°C or higher may be critical. 

Biaxial Orientation. Many polymer films require orientation to achieve commercially acceptable performance (10). Orientation may be uniaxial (generally in the machine direction [MD]) or biaxial where the web is stretched or oriented in the two perpendicular planar axes. The biaxial orientation may be balanced or unbalanced depending on use, but most preferably is balanced. Further, this balance of properties may relate particularly to tensile properties, tear properties, optical birefringence, thermal shrinkage, or a combination of properties. A balanced film should be anisotropic, although this is difficult to achieve across the web of a flat oriented film. 

In conventional tenter orientation, the sequence of steps is as described above (MD—TD). In some cases it is advantageous to reverse the draw order (TD—MD) or to use multiple draw steps, eg, MD—TD—MD. These other techniques are used to produce "tensilized" films, where the MD tensile properties are enhanced by further stretching. The films are generally unbalanced in properties and in extreme cases may be fibrillated to give fiber-like elements for special textile appHcations. Tensilized poly(ethylene terephthalate) is a common substrate for audio and video magnetic tape and thermal transfer tape. 

Finally, a modification has been carried out in which a polyacrylate emulsion is added to a normal tetrakis(hydroxymethyl)phosphonium sulfate [55566-30-8] (THPS), urea, and TMM fire-retardant treatment in an attempt to completely alleviate the strength loss during the finishing. Indeed, better retention of tensile properties is achieved with no loss in fire resistance (85). 

Table 2 Hsts properties of PVF films. Various multilayer cast PVF films have been reported (82). Physical and tensile properties of the film depend on the extent of its orientation (83).

Tensile strength and modulus of rigid foams have been shown to vary with density in much the same manner as the compressive strength and modulus. General reviews of the tensile properties of rigid foams are available (22,59,60,131,156). 

Those stmctural variables most important to the tensile properties are polymer composition, density, and cell shape. Variation with use temperature has also been characterized (157). Flexural strength and modulus of rigid foams both increase with increasing density in the same manner as the compressive and tensile properties. More specific data on particular foams are available from manufacturers Hterature and in References 22,59,60,131 and 156. Shear strength and modulus of rigid foams depend on the polymer composition and state, density, and cell shape. The shear properties increase with increasing density and with decreasing temperature (157). 

The copolymer fiber shows a high degree of drawabiUty. The spun fibers of the copolymer were highly drawn over a wide range of conditions to produce fibers with tensile properties comparable to PPT fibers spun from Hquid crystalline dopes. There is a strong correlation between draw ratio and tenacity. Typical tenacity and tensile modulus values of 2.2 N/tex (25 gf/den) and 50 N/tex (570 gf/den), respectively, have been reported for Technora fiber (8). 

Sample designation Melt flow rate, dg/min Stiffness, MPa Tensile properties ... 

Fig. 9. Variation of tensile properties and grain stmcture with cold working and annealing A, elongation B, yield stress and C, ultimate tensile stress.

Table 1. Effect of Heat Treatment on Tensile Properties of Al—4.5% Cu ...

Physical characteristics of metals have a significant impact on machinabihty. These include microstmctural features such as grain size, mechanical properties such as tensile properties, and physical properties such as thermal conductivity. 

Additions of selected alloying elements raise the recrystaUization temperature, extending to higher temperature regimes the tensile properties of the cold-worked molybdenum metal. The simultaneous additions of 0.5% titanium and 0.1% zirconium produce the TZM aUoy, which has a corresponding... 

Plastisols may also be semi-geUed for storage, ie, enough heat is imparted to convert the plastisol into a soHd but without the full development of tensile properties brought about by full fusion. 

ASTM D638, Test Methodfor Tensile Properties of Plastics, Vol. 8.01, ASTM, Philadelphia, Pa., 1991. 

ISO 527, Plastics Determination of Tensile Properties, ISO, Geneva, Swit2erland, 1991. 

Tensile Properties. Tensile properties of nylon-6 and nylon-6,6 yams shown in Table 1 are a function of polymer molecular weight, fiber spinning speed, quenching rate, and draw ratio. The degree of crystallinity and crystal and amorphous orientation obtained by modifying elements of the melt-spinning process have been related to the tenacity of nylon fiber (23,27). 

Table 1. Tensile Properties of Nylon-6 and Nylon-6,6 Continuous-Filament Yams...

Definitions of the commonly measured tensile properties are as follows Unear density (tex) is the weight in grams of 1000 m of yam. Tenacity is the tensile stress at break and is expressed in force-per-unit linear density of unstrained specimen, N /tex. Knot tenacity is the tensile stress required to mpture a single strand of yam with an overhand knot tied in the segment of sample between the testing clamps. It is expressed as force-per-unit linear density and is an approximate measure of the britdeness of the yam. Toop tenacity is the tensile stress required to mpture yam when one strand of yam is looped through... 

Bisphenol A diglycidyl ether [1675-54-3] reacts readily with methacrylic acid [71-49-4] in the presence of benzyl dimethyl amine catalyst to produce bisphenol epoxy dimethacrylate resins known commercially as vinyl esters. The resins display beneficial tensile properties that provide enhanced stmctural performance, especially in filament-wound glass-reinforced composites. The resins can be modified extensively to alter properties by extending the diepoxide with bisphenol A, phenol novolak, or carboxyl-terrninated mbbers. 

Because of the simplicity of the first test method, most of the comparisons are made usiag this technique. The effects of the aging process are usually measured on tensile properties such as tensile strength, elongation, and stress (modulus) at 300% elongation (42). 

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