# Chpr3 experimental

Engineer en Central Institute of Plastics Engineering and Technology
8 de Mar de 2013
1 de 11

### Chpr3 experimental

• 1. EXPERIMENTAL Chapter-III 3. Experimental 3.1 Tensile strength 3.1.1 Definition The maximum load applied in breaking a tensile test piece divided by the original cross- sectional area of the test piece. Originally quoted as tons/sq.in. It is now measured as Newtons/sq.mm. Also termed Maximum Stress and Ultimate Tensile Stress. 3.1.2 Specimen Shape The specimen's shape is usually defined by the standard or specification being utilized, e.g., ASTM D638. Its shape is important because you want to avoid having a break or fracture within the area being gripped. So, standards have been developed to specify the shape of the specimen to ensure the break will occur in the "gage length" (2 inches are frequently used) by reducing the cross sectional area or diameter of the specimen throughout the gage length. This has the effect of increasing the stress in the gage length since stress is inversely proportional to the cross sectional area under load, Vertical alignment of the specimen is an important factor to avoid side loading or bending moments created in the specimen. Mounting the specimen in the upper grip assembly first then allowing it to hang freely will help to maintain alignment for the test. There are three definitions of tensile strength: 1. Yield strength The stress at which material strain changes from elastic deformation to plastic deformation, causing it to deform permanently. 2. Ultimate strength The maximum stress a material can withstand. Breaking strength 3. Rupture The stress coordinate on the stress-strain curve at the point of rupture. CIPET CHENNAI P a g e | 31
• 2. EXPERIMENTAL Figure 3.1 ASTM D638 Lloyds UTM Metals including steel have a linear stress-strain relationship up to the yield point, as shown in the figure. In some steels the stress falls after the yield point. This is due to the interaction of carbon atoms and dislocations in the stressed steel. Cold worked and alloy steels do not show this effect. For most metals yield point is not sharply defined. Below the yield strength all deformation is recoverable, and the material will return to its initial shape when the load is removed. For stresses above the yield point the deformation is not recoverable, and the material will not return to its initial shape. This unrecoverable deformation is known as plastic deformation. For many applications plastic deformation is unacceptable, and the yield strength is used as the design limitation.After the yield point, steel and many other ductile metals will undergo a period of strain harening, in which the stress increases again with increasing strain up to the ultimate strength. If the material is unloaded at this point, the stress-strain curve will be parallel to that portion of the curve between the origin and the yield point. If it is re-loaded it will follow the unloading curve up again to the ultimate strength, which has become the new yield strength.After a metal has been loaded to its yield strength it begins to "neck" as the cross-sectional area of the specimen decreases due to plastic flow. When necking becomes substantial, it may cause a reversal of the engineering stress-strain curve, where decreasing stress correlates to increasing strain because of geometric effects. This is CIPET CHENNAI P a g e | 32
• 3. EXPERIMENTAL because the engineering stress and engineering strain are calculated assuming the original cross-sectional area before necking. If the graph is plotted in terms of true stress and true strain the curve will always slope upwards and never reverse, as true stress is corrected for the decrease in cross-sectional area. Necking is not observed for materials loaded in compression. The peak stress on the engineering stress-strain curve is known as the ultimate strength. After a period of necking, the material will rupture and the stored elastic energy is released as noise and heat. The stress on the material at the time of rupture is known as the tensile strength. Ductile metals do not have a well defined yield point. The yield strength is typically defined by the "0.2% offset strain". The yield strength at 0.2% offset is determined by finding the intersection of the stress-strain curve with a line parallel to the initial slope of the curve and which intercepts the abscissa at 0.002. 3.2 Flexural strength 3.2.1 Definition The strength of a material in bending, expressed as the stress on the outermost fibers of a bent test specimen, at the instant of failure. In a conventional test, flexural strength expressed in Psi is equal to Where P= the load applied to a sample of test length L, width b, and thickness d. The flexure test method measures behavior of materials subjected to simple beam loading. It is also called a transverse beam test. Maximum fiber stress and maximum strain are calculated for increments of load. Results are plotted in a stress-strain diagram. Flexural strength is defined as the maximum stress in the outermost fiber. This is calculated at the surface of the specimen on the convex or tension side. Flexural modulus CIPET CHENNAI P a g e | 33
• 4. EXPERIMENTAL is calculated from the slope of the stress vs. deflection curve. If the curve has no linear region, a secant line is fitted to the curve to determine slope A flexure test produces tensile stress in the convex side of the specimen and compression stress in the concave side. This creates an area of shear stress along the midline. To ensure the primary failure comes from tensile or compression stress the shear stress must be minimized and controlling the span to depth ratio does this; the length of the outer span divided by the height (depth) of the specimen. For most materials S/d=16 is acceptable. Some materials require S/d=32 to 64 to keep the shear stress low enough. Figure 3.2 ASTM D790 Three Point Bending Flexure testing is often done on relatively flexible materials such as polymers, wood and composites. There are two test types; 3-point flex and 4-point flex. In a 3-point test the area of uniform stress is quite small and concentrated under the center loading point. In a 4-point test, the area of uniform stress exists between the inner span loading points (typically half the outer span length). The 3-point flexure test is the most common for polymers. Specimen deflection is usually measured by the crosshead position. Test results include flexural strength and flexural modulus. 3.3 Impact strength Energy per unit thickness required to break a test specimen under flexural impact. Test specimen is held as a vertical cantilevered beam and is impacted by a swinging pendulum. The energy lost by the pendulum is equated with the energy absorbed by the test specimen. Several methods are used to measure the impact resistance of plastics - CIPET CHENNAI P a g e | 34
• 5. EXPERIMENTAL Izod, Charpy, Gardner, tensile impact, and many others. These impact tests allow designers to compare the relative impact resistance under controlled laboratory conditions and, consequently, are often used for material selection or quality control a pendulum swings on its track and strikes a notched, cantilevered plastic sample. The energy lost (required to break the sample) as the pendulum continues on its path is measured from the distance of its follow through. Sample thickness is usually 1/8 in. (3.2mm). The test method of ASTM D256. The result of the Izod test is reported in energy lost per unit of specimen thickness (such as ft-lb/in or J/cm) at the notch ('t' in graphic at right). Additionally, the results may be reported as energy lost per unit cross-sectional area at the notch (J/m² or ft-lb/in²). In ISO 180 methods the results reported based only on the cross-sectional area at the notch (J/m²). Polymeric materials that are sensitive to the stress concentrations at the notch ('notch-sensitive') will perform poorly in the notched izod test. Engineers use this knowledge to avoid using such polymers in designs with create high stress concentrations such as sharp corners or cutouts. Figure 3.3 ASTM D256. The Izod impact strength test apparatus. CIPET CHENNAI P a g e | 35
• 6. EXPERIMENTAL Unnotched specimens are also frequently tested via the Izod impact method to give a more complete understanding of impact resistance. Izod impact tests are commonly run at low temperatures - down to -40°F (-40°C) or occasionally lower - to help gauge the impact resistance of plastics used in cold environments. The impact resistance of a given grade of polymer is a function of the base resin plus the presence of any impact modifiers (such as Elastomers) and reinforcing agents that may be added by the manufacturer/compounder. Environmental factors other than temperature also play a role in impact resistance For example, nylons (polyamides) generally experience higher impact strength in the conditioned state (in equilibrium with atmospheric moisture) than in a dry-as-molded state because of the plasticizing effect of absorbed moisture. Figure 3.3 Notched Impact Strength Specimen 3.4 Heat deflection temperature The deflection temperature is a measure of a polymer's resistance to distortion under a given load at elevated temperatures. The deflection temperature is also known as the 'deflection temperature under load' (DTUL), 'heat deflection temperature', or 'heat distortion temperature' (HDT). The two common loads used are 0.46 MPa (66 psi) and 1.8 MPa (264 psi). The Heat Distortion Temperature is determined for ASTM D648. The test specimen is loaded in three-point bending in the edgewise direction. CIPET CHENNAI P a g e | 36
• 7. EXPERIMENTAL Figure 3.4 ASTM D648 Heat Deflection Apparatus The outer fiber stress used for testing is either 0.455 MPa or 1.82 MPa, and the temperature is increased at 2 °C/min until the specimen deflects 0.25 mm.The deflection temperature is the temperature at which a test bar, loaded to the specified bending stress, deflects by 0.010 inch (0.25 mm). The value obtained for a specific polymer grade will depend on the base resin and on the presence of reinforcing agents. Deflection temperatures of glass fiber or carbon fiber reinforced engineering polymers will often approach the melting point of the base resin. The deflection temperature rest results are a useful measure of relative service temperature for a polymer when used in load-bearing parts. However, the deflection temperature test is a short-term test and should not be used alone for product design. Other factors such as the time of exposure to elevated temperature, the rate of temperature increase, and the part geometry all affect the performance. Limitations that are associated with the determination of the HDT is that the sample is not thermally isotropic and, thick samples in particular, will contain a temperature gradient. The HDT of a particular material can also be very sensitive to stress experienced by the component which is dependent on the component’s dimensions. The selected deflection of 0.25 mm (which is 0.2% additional strain) is selected arbitrarily and has no physical meaning. CIPET CHENNAI P a g e | 37
• 8. EXPERIMENTAL 3.5 Melt Flow Index The Melt Flow Index is a measure of the ease of flow of the melt of a thermoplastic polymer. It is defined as the mass of polymer in grams flowing in 10 minutes through a capillary of specific diameter and length by a pressure applied via prescribed alternative gravimetric weights for alternative prescribed temperatures. The method is given in ASTM D1238 .Melt flow rate is an indirect measure of molecular weight, high melt flow rate corresponding to low molecular weight. At the same time, melt flow rate is a measure of the ability of the material's melt to flow under pressure. Melt flow rate is inversely proportional to viscosity of the melt at the conditions of the test, the viscosity for any such material depends on the applied force. Ratios between two melt flow rate values for one material at different gravimetric weights are often used as a measure for the broadness of the molecular weight distribution.Melt flow rate is very commonly used for polyolefins, polyethylene being measured at 190°C and polypropylene at 230°C. The plastics converter should choose a material with a melt index so high that he can easily form the polymer in the molten state into the article intended, but on the other hand so low that the mechanical strength of the final article will be sufficient for its use. Figure 3.5 ASTM D1238 Melt Flow Index Apparatus CIPET CHENNAI P a g e | 38
• 9. EXPERIMENTAL 1. A small amount of the polymer sample (around 4 to 5 grams) is taken in the specially designed MFI apparatus which is nothing but a miniature extruder. The apparatus consists of a small die inserted into the extruder, with the diameter of the die generally being around 2 mm. 2. The material is packed properly inside the extruder barrel to avoid formation of air pockets. 3. A piston is introduced which acts as the medium that causes extrusion of the molten polymer. 4. The sample is preheated for a specified amount of time: 5 min at 190°C for polyethylene and 6 min at 230°C for polypropylene. 5. After the preheating a specified weight is introduced onto the piston. Examples of standard weights are 2.16 Kg, 5 Kg, etc. 6. On account of the weight shear is exerted on the molten polymer and it immediately starts flowing through the die. 7. A sample of the melt is taken after desired period of time and is weighed accurately. 8. MFI is expressed as grams of polymer/10 minutes of flow time. Melt Flow Index are Melt Flow Rate and Melt Index. More commonly used are their abbreviations: MFI, MFR and MI. 3.6 Differential scanning calorimeter Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature. Both the sample and reference are maintained at nearly the same temperature throughout the experiment. Generally, the temperature program for a DSC analysis is designed such that the sample holder temperature increases linearly as a function of time. The reference sample should have a well-defined heat capacity over the range of temperatures to be scanned.The basic CIPET CHENNAI P a g e | 39
• 10. EXPERIMENTAL principle underlying this technique is that, when the sample undergoes a physical transformation such as phase transitions, more or less heat will need to flow to it than the reference to maintain both at the same temperature. Whether more or less heat must flow to the sample depends on whether the process is exothermic or endothermic. For example, as a solid sample melts to a liquid it will require more heat flowing to the sample to increase its temperature at the same rate as the reference. This is due to the absorption of heat by the sample as it undergoes the endothermic phase transition from solid to liquid. Likewise, as the sample undergoes exothermic processes (such as crystallization) less heat is required to raise the sample temperature. By observing the difference in heat flow between the sample and reference, differential scanning calorimeters are able to measure the amount of heat absorbed or released during such transitions. DSC may also be used to observe more subtle phase changes, such as glass transitions. DSC is widely used in industrial settings as a quality control instrument due to its applicability in evaluating sample purity and for studying polymer curing.The result of a DSC experiment is a curve of heat flux versus temperature or versus time. There are two different conventions: exothermic reactions in the sample shown with a positive or negative peak; it depends by the different kind of technology used by the instrumentation to make the experiment. This curve can be used to calculate enthalpies of transitions. This is done by integrating the peak corresponding to a given transition. It can be shown that the enthalpy of transition can be expressed using the following equation: ΔH = KA where ΔH is the enthalpy of transition, K is the calorimetric constant, and A is the areaunder the curve. The calorimetric constant will vary from instrument, and can be determined by analyzing a well-characterized sample with known enthalpies of transition. CIPET CHENNAI P a g e | 40
• 11. EXPERIMENTAL 3.6.1 Applications Differential scanning calorimetry can be used to measure a number of characteristic properties of a sample. Using this technique it is possible to observe fusion and crystallization events as well as glass transition temperatures (Tg). Figure 3.6.1 a schematic DSC curve demonstrating the appearance of several common features DSC can also be used to study oxidation, as well as other chemical reactions. Glass transitions may occur as the temperature of an amorphous solid is increased. These transitions appear as a step in the baseline of the recorded DSC signal. This is due to the sample undergoing a change in heat capacity; no formal phase change occurs. As the temperature increases, an amorphous solid will become less viscous. At some point the molecules may obtain enough freedom of motion to spontaneously arrange themselves into a crystalline form. This is known as the crystallization temperature (Tc). This transition from amorphous solid to crystalline solid is an exothermic process, and results in a peak in the DSC signal. CIPET CHENNAI P a g e | 41