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Tensile & Flexural

Tensile Strength, Modulus and Extension  
(ASTM D638)

 Tension is analogous to pulling from opposite directions. Tensile strength is a common and very important property.  It is probably true that the number of applications for plastics which load the material in pure tension are few. All forms of mechanical stress, however, have components of tensile loading. In flexure, the layer of material on the outside of the bend radius is in tension. In shear, the material at 90o to the direction of shear is in tension. In torsion, the material on the entire circumference is in tension. And even in compression, there is a component of tensile loading through the center of the sample due to the elastic properties of plastic. Because of this wide effect of tensile strength, it is the most common property referred to when considering the general strength of a material.

 In addition to tensile strength, tensile testing provides modulus and elongation data. Modulus is the ratio of load to the deformation of material resulting from the load, or it can be interpreted as a measure of the stiffness of a material. Elongation is how much the material stretches or deforms in the direction of loading. There can be one or two phases during the failure of a polymer under tension. The material may first yield, which results in a reduction of its load carrying capacity, but continue to elongate.

 The second phase is brittle and rapid failure, i.e., it breaks. Some polymers, especially reinforced grades, will not yield before they break. Yielding is caused when the load to overcome the intermolecular secondary forces is less than that required to break molecular bonds. The molecules begin to uncoil and slip past each other. The material will continue to elongate until so much molecular orientation has occurred that the load begins to be resisted by primary molecular bonding. At this point the load carrying capacity will increase until the primary bond strength is exceeded and the material undergoes rapid brittle failure.

 Reinforcements in resin generally prevent uncoiling and slipping of molecules due to their higher affinity for the particles of reinforcement. The weak link in this case is usually the primary molecular bonds. To differentiate, the two types of elongation are referred to as "@ yield" or "ultimate".

 Tensile testing is generally done on samples shaped like "dog bones", and are often referred to by this name. The overall size of the sample is not nearly as important as the relative shape. The central third to half of the sample should be narrower than the ends. This assures and nearly all of the deformation will take place in the narrower section. This is important in producing accurate and repeatable data. The load bearing capacity of the material must not be influenced by the hardware used to anchor r the samples in the testing machine. The width of the narrow section or "gage" section should be as dimensionally uniform as the manufacturing method allows. Accurate data depends on the load being distributed uniformly throughout the gage section. If non-uniform, the true length in which deformation occurred will not be known and using the gage length in the calculations will produce erroneous values.

 Surface defects and contamination are other common causes of erroneous tensile data. Nicks, scrPDChes, bubbles, splay or other defects on the surface of test samples serve as an initiation site for fractures. As the test starts, stress builds uniformly throughout the gage section of the sample, as the molecules distribute the stress evenly among themselves. The molecules bordering a surface defect cannot distribute the stress to the molecules on the other side of the defect.

 If the defect is solid contamination, the adhesive attraction of the molecules to the defect is low. The stress is channeled from the molecules bordering the defect to the molecules located where the defect stops. Concentration of the stress occurs where the two sides of the defect meet at 90o to the direction of loading. The stress will increase rapidly at the concentration point and primary molecular bonds will break. A crack will develop which will then serve as a defect itself and will propagate through the material by the same action that initiated at the original defect. The angle at which the two faces of the defect perpendicular to the load meet is important. If the angle is small, i.e., the defect sharp, the concentration effect will be greater. This is why bubbles or voids have less effect on reducing the strength of a material sample than a scrPDCh.

 Experiments have been done which show that reinforcement particles with sharp corners have less reinforcing ability than particles with rounded edges. Reinforcements act as stress distributors and are dependent on strong surface adhesion to the polymer molecules. The crystals in crystalline polymers also act as stress distributors. Therefore, amorphous unreinforced polymers are therefore the most susceptible to failure from defects and contamination.

 Rate of deformation is an important determinant of the strength values when testing. The stress in a sample is actually the polymers response to deformation. A tensile testing machine pulls on a sample causing deformation. The resistance to the deformation is what is measured and used to calculate tensile properties. A high rate of deformation mean less time for molecules to mobilize and uncoil to locally  relieve the stress.  This results in less deformation before primary bonds are broken and potentially a lower stress value at failure. Significant reductions can also be seen in yield and ultimate elongation. (This may cause modulus values to be increased.) When comparing material properties it is important to have equal testing rates, but also rates suitable for the type of polymer.  ASTM D638 recommends general testing speeds. It is often useful at the beginning of a test sequence to run several samples at different speeds to evaluate test speed on the material being tested.


To determine the tensile properties of unreinforced and reinforced plastics in the form of standard dumbbell-shaped test specimens when tested under defined conditions of pretreatment, temperature, humidity, and testing machine speed.

 This test method is designed to produce tensile property data for the control and specification of plastic materials.  These data are also useful for qualitative characterization and for research and development.


Test specimens are molded or cut to specified dimensions and conditioned prior to testing.  At least five specimens of each sample are tested.  Tensile bars are mounted in the grips, the extension indicator is attached, and the testing speed is selected by the specification of the material being tested.

Tensile strength, percent elongation, and the modulus of elasticity are calculated from the load-extension curve.

Flexural Strength and Modulus (ASTM D790)

 Flexural strength and modulus can more often be related to functional requirements of parts. Flexural modulus is the most widely used property for comparing the relative stiffness of different materials. Samples can be molded or milled from laminates or parts. They are generally 12.7 mm (0.500 in) wide and 127 mm (5.00 in) long. The thickness is usually 3.2 mm (0.125 in) but can vary. Samples are supported in a horizontal position at a span of 102 mm (4.0 in). Different span lengths can be used and adjusted for in the calculations. A load is applied to the sample at a continuous rate at the center of the span until yield or brittle failure. Strength and modulus are calculated from the linear portion of the load/deformation curve, as with tensile properties.

 There are two other versions of flexural testing. The first is similar to the method described above, except the load is applied at two points, separated by one third the distance of the support span, and centrally located. This method is used for materials which flex very easily and would "bottom out" without reaching yield or brittle failure using the single point load. The second method is called "Stiffness in Flexure" and is covered by ASTM D747). This method uses the same type of sample but in a cantilevered geometry. Elastic modulus cannot be calculated by this method.  It is acceptable for determining relative stiffness but is not that widely used in the industry.


To determine the flexural properties of unreinforced and reinforced plastics in the form of rectangular bars molded directly or cut from sheets.  These test methods are applicable to rigid and semi rigid materials.  However, flexural strength cannot be determined for materials that do not break or that do not fail in the outer fibers. 

 Flexural properties are especially useful for quality control and specification purposes.


A bar of rectangular cross section is tested in flexure as follows: the bar rests on two supports and is loaded by means of a loading nose midway between the supports.  The specimen is deflected until rupture occurs in the outer fibers or until the maximum fiber strain of 5% is reached, whichever occurs first.

 At least five specimens are tested for each sample.  Flexural strength and flexural modulus are determined from the load deflection curve.

Compressive Strength, Sheer Strength and Hardness

Compressive strength and modulus, shear strength, and hardness are other mechanical properties less often tested and reported. These tests are somewhat more specific to end use applications. and of less general significance.

 Compressive Strength and Modulus (ASTM D695)

 Compressive testing is analogous to tensile testing except the strain is in the opposite direction. Samples can be of any simple geometric cross-section including: cylindrical, tubular, square, or triangular. The length of the sample should be twice the width. Where elastic modulus and offset yield stress data are desired, double the length to cross-section ratio. (See ASTM D695) (Samples of this geometry are difficult to obtain.  Molding at these thicknesses will result in sink marks and internal voids caused by continued polymer shrinkage at the interior of the sample after the exterior has solidified. Machining from stock will produce better samples. Specimens are mounted upright in a compression fixture. Load is applied at a constant rate of strain perpendicular to the central axis of the sample. Compressive strength is calculated as the pressure or stress required to rupture the sample, or in the case of ductile polymers, deform it by a given percentage of the original height. Compressive modulus is the ratio of the critical stress to the percentage of deformation at the critical stress. (Critical stress is used to mean the stress at which the previously defined failure point is reached.) Compressive properties are not generally analogous to real life situations. Very few applications involve only compressive loading. These values are rarely used in design calculations.

 Shear Strength (ASTM D732)

 Shear testing is done on square or circular samples of at least a 50.8 mm (2.0 in) diameter equivalent, and 0.125 mm (0.005 in) to 12.5 mm (0.500 in) in thickness. Samples can be molded or machined from sheet or stock. Testing is done using a punch type fixture. Downward force is applied to a 25.40 mm (1.000 in) punch. The sample shears along a line at the outer diameter of the punch and the inner diameter of the punch bushing. Shear strength is calculated as the maximum load sustained over the area sheared (sample thickness multiplied by circumference of punch). Shear strength is more important in sheet products than molded injection molded plastic parts.