The Analysis of Plastic Hangers - Report Example

Failure Analysis The Failed Object Figure A broken hanger. Figure 2. Sketch of a hanger showing the dimensions and the fracture. Explanation and Analysis A hanger is usually made up of engineering plastic although some might be made from wood and metal. Plastics are technically synthetic polymers that are made up of many monomers linked together through the process of polymerization (“Characterization and Failure Analysis of Plastics”, 3). Plastics are made using natural gas and petroleum chemicals. Some of the main constituents of plastics are synthetic polymer resins and other additives. The resins are rubber type organic compounds that are made up of hydrocarbons. This allows the polymer to have greater flexibility, rigidity, heat resistance, weather resistance, processibility, and also gives it color. Basically, the properties and characteristics of a polymer depend upon its structure. The strength of the plastics can be determined from a measurement known as the Young’s modulus. Young’s modulus is the ratio of stress over strain, i.e. elasticity and is measured in units of Pascal (Pa). Unfilled plastics usually have a Young’s modulus of less than 3.5 GPa at room temperature (“Characterization and Failure Analysis of Plastics”, 53). Their strength is also temperature sensitive and can be affected by environmental factors as well. The strength can be improved by using fillers and fibers as reinforcements in order to enhance the mechanical properties of the plastic. The tensile strength of most plastics is less than 35 MPa. A lesser tensile stress means a lesser rigidity (MatWeb, n.pag.) but this can be increased by using resin of higher Young’s modulus so as to provide with better reinforcements. The designing of the plastic also has a significant role in improving the strength of the plastic (“Characterization and Failure Analysis of Plastics”, 53). Lower quality or recycled plastics have a lower tensile strength and can break at a very low magnitude of stress. This is because after recycling the plastics are “down cycled” causing them to become less rigid and more amorphous thereby causing them to break at a lower stress level. The load on the above hanger is caused by hanging clothes. In this case the force or load is caused by the weight of the clothes which depends upon the thickness of the fabric. On average clothes weigh about 2 to 5 pounds (lbs) with the weight being centered at the middle point of the hanger. Continuous weight on one point of the hanger results in breakage. The fracture shown in Figure 1 illustrates that breakage has occurred midway along the length of the hanger leaving 18.5 cm on both sides. Material Properties and Comparison to Stress Analysis The hanger being made of plastic is prone to deformation occurring as a result of high temperature as well as the time factor. Mechanical plastics are visoelastic materials (“Characterization and Failure Analysis of Plastics”, 185). The time factor involves the rate of stress application, duration of stress and the history of stress. Thermal properties of plastics impact its temperature dependence as they differ according to different types of plastics such as amorphous thermoplastics (the one in our case), crystalline thermoplastics, and thermosets (“Characterization and Failure Analysis of Plastics”, 185). Deformation in plastics results in about half of the energy being released as heat and the rest causes cavitation or changes in the orientation of the polymer chains. Hence the entire polymer molecules get disoriented around their centers of gravity (“Characterization and Failure Analysis of Plastics”, 185). A number of mechanical tests are therefore performed on plastics in order to determine their mechanical properties so as to understand whether they can stand the stress which might be involved in their usage. These tests include those recommended by the Organization for Standardization (ISO) and European Standards for testing the tensile properties, compressive properties, stiffness, resistance, static and kinetic coefficients of friction, and so on (“Characterization and Failure Analysis of Plastics”, 187). Since polymers consist of long chains of monomers their properties differ from metals and woods. As a result plastics have a lower stiffness, higher elasticity or recoverable strain, greater range of Poisson’s ratio and time dependent deformation due to visoelasticity. Most unreinforced plastics have tensile strengths ranging from 50 to 70 MPa and have a recoverable strain of more than 500% (“Characterization and Failure Analysis of Plastics”, 185). During deformation, the long chains of polymers are kept from flowing past each other by introducing cross links that hold the chains together. Glassy polymers have a recoverable strain of 5% due to the presence of weak van der Waals forces holding the chains together. Therefore the test have to standardized to take into account both small elastic strains and large elastic strains. Also, since polymers are also affected by the time span, the testing that is performed based on a short term can result in misrepresented data. So the tests should consider the time and temperature aspect of polymers so as to obtain data that is relevant. Furthermore, fatigue testing is performed as loading can lead to fatigue fractures in plastics just as the one shown in the hanger in Figure 1. Most failures in plastics occur as a result of a fatigue fault or irreversible accumulated loss. The fatigue performance of plastics is affected by many factors including, mechanical variables, molecular variables, reinforcements, and the design of the fatigue test. Generally the fatigue performance of a plastic is proportional to the molecular weight and increases as the molecular weight increases. Also, the fatigue performance is enhanced due to the presence of rubber reinforcements as it helps to reduce the brittle property of the plastic. During fatigue testing and failure, the loading conditions as well as other factors including frequency of loading, upper and lower loading limit, and the loading waveform are significant is testing the fatigue behavior. Two mechanisms have been identified in fatigue failure depending upon the amplitude of stress and the frequency of load application. The first one involves yielding (thermal softening) which occurs before the crack propagates thereby leading to failure. This mechanism happens at high stress amplitude. At low stress amplitudes, the mechanism involves fatigue crack propagation (FCP) (“Characterization and Failure Analysis of Plastics”, 249). In FCP a damage formation takes place before the crack is introduced and propagated. Environmental Factors Involved Environmental factors that may result in failure include heat, physical aging, moisture, photolytic degradation, and microbial degradation. Temperature extremes and deformation leads to a buildup of internal stresses. These stresses affect the permeability, dimensional stability and their resistance to hostile environments. Increase in stress can be detrimental as it can result in cracks and spaces that reduce the mechanical properties of plastics. Amorphous polymers face three types of stresses: thermal or cooling stresses, orientation ad orientation stresses, and quenching stresses. The thermal or cooling stress in plastics can be caused by the variation of mechanical properties with temperature. The Young’s modulus itself varies with temperature and inhomogenous cooling during solidification leads to thermal stress. In the case of plastics hangers repeated loading and unloading of clothes can lead to fracture. The weight of the clothes hung is of equal importance as a weight higher than the tensile strength results in deformation and eventually breakage. The frequency of repeated loading also plays a vital part in failure. Surrounding temperature and humidity may also gradually corrode the plastic resulting in deformation of the chains of polymers affecting the hanger’s ability to bear the weight of clothes. Details of Fracture Figure 3. Hanger with a coat weighing 2 lbs. Figure 4. The resulting bend in the hanger. Figure 5. Hanger with the fracture. Conclusion According to Vishu Shah, the material has an important part in the strength of an object (Shah, n.pag.). The manufacturers take into consideration various factors while selecting the plastic material for producing its product. The most important one may include the application of the product. If the product involves industrial application, then the manufacturer must select a plastic material that is strong, rigid and has a suitable modulus and tensile stress. If the product is a basic home appliance such as a hanger then the plastic need not to be very strong. Various other factors are also important that includes use of product after its lifetime, misuse and excessive stress resulting in deformation of the product. A lot of manufacturers use low quality recycled plastics to create hangers so as to reduce their costs. Such hangers are substandard and cannot tolerate high stress and load. The dimensions of the broken hanger shown in Figure 2 had inconsistent thickness along its length and cross section. Therefore the hanger was unable to support the weight of the clothes uniformly. Also, the breakage point is at the centre indicating the pressure exerted was concentrated midway along its length. Recommendations The design of a particular plastic must follow the guidelines provided by the supplier. The designing rules are different for different materials as well as applications (Shah, n.pag.). The radius of fillet must be appropriate because a lower thicknesses of fillet leads to a very high stress concentration factor. The material selection should also be done depending upon its application and a suitable form of polymer must be chosen that can bear the stress. Tests should be performed so as to assess the plastics resistance, flexibility, mechanical performance, fatigue endurance, etc. This allows an evaluation of the overall performance of the product so as to avoid failures. References Shah, Vishu. “Why Plastics Fail (Part 1) .” UL IDES. N.p., 11 Apr. 2007. Web. 25 Aug. 2013. . “Tensile Property Testing of Plastics.” MatWeb. N.p., n.d. Web. 25 Aug. 2013. . “Characterization and Failure Analysis of Plastics.” Materials Park, OH: ASM International, 2003. Print. Read More