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Task 7: Quality Control Systems and Testing to ASTM Standards
ASTM, which stood for American Society of Testing Materials is a not for profit organisation which provides a forum for the development and publication of international standards for materials, products and systems. They provide PDF books which provide set standards on many different applications, materials, processes, properties, measurements and testing methods. For glass fiber, the ASTM have published standard test methods for different properties of glass fibers including glass density, softening point of glass, annealing point and strain point of glass. Some properties such as tensile strength and modulus of elasticity are measured directly on the fibers. Young’s modulus or modulus of elasticity highlights tensile elasticity. It is defined as ‘the tendency of an object to deform along an axis when opposing forces are applied along that axis.’ Also, the modulus of rupture is an important test for fibreglass. Modulus of rupture or flexural test is when a specimen is placed on two supporting pins a fixed distance away from each other before a force is applied to the top of the specimen until it fails. Other properties such as glass density and glass refractive index are measured on the fibers and bulk samples in annealed or unannealed form.
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ASTM D790 is on standard test methods for flexural properties of plastics and electrical insulating methods. This document outlines the method of flexural tests on reinforced plastics, unreinforced plastics and electrical insulating materials. The specimen is placed on two supporting beams with support to span depth of 16:1. The load will be placed on the specimen until rupture occurs in the outer surface of the test material or until a maximum strain of 5% is reached. For quality control, the flexural test gives important results on the properties of different materials as if a material fails before the 5% strain point, it would not be useful for certain uses although these properties may change with different factors like specimen depth and temperature.
The flexural test is performed on a calibrated machine in which the error in the load measuring system does not exceed 1%. Also, the machine must be stiff enough so that the total elastic deformation of the machine does not exceed 1% of the total deflection of the specimen while it is being tested. The machine loading nose and support beams for the specimen are cylindrical in shape and must be within radius of 4.9 to 5.1mm so that there isn’t excessive indentation done on the specimen and so that there isn’t failure due to stress concentration. In relation to micrometers, there should be an incremental discretion of at least 0.025mm for the measurement of width and length of rigid and semi-rigid plastics. This can be hand measured by using a micrometer and ratchet. For non-rigid plastics, a different apparatus must be used which has a contact measuring pressure of 22.5kPa to 27.5kPa, a moveable circular contact foot between 6.325mm and 6.375mm in diameter and a lower fixed anvil which is capable to extend beyond the contact foot in every direction while being parallel to the foot within 0.005mm.
Before the test is undertaken, the test specimen must be prepared appropriately for the test. The specimen can be either cut from sheets and plates or it can be moulded to specific dimensions. There must be at least 5 test samples for isotropic and moulded specimens. For each anisotropic material in sheet form, flatwise and edgewise tests will be carried on specimens cut in crosswise and lengthwise directions. For the flexural test, lengthwise represents designates the axis of anisotropy which should be interpreted to mean the direction of the sheet which is strong in flexure. On the other hand, crosswise represents the sheet direction which is known to be weaker in flexure than lengthwise. Crosswise should be at 90 degrees to lengthwise. Also, the test specimens must be conditioned between 21-25 degrees Celsius and between 45-55 percent relative humidity for no less than 40 hours before commencing the test. The test will also be done in these conditions.
On top of this, if the test specimen is 1.6mm or more in thickness, the depth should be the same as the thickness for a flatwise test. For an edgewise test, the width of the specimen should be the thickness of the sheet and the depth should be less than the width. For both types of tests, the support span should be 15 – 17 times the depth of the beam. The width of the specimen should not be greater than a quarter of the support span for specimens which are greater than 3.2mm in depth while specimens that are less than 3.2mm in depth should be precisely 12.7mm in width. The length of the specimen should be great enough so that it hangs over at least 5% of the support span on either side. There should be no less than 6.4mm on each end. The specimen must be overhanging so that it doesn’t end up slipping through the supports when being tested. For materials which are less than 1.6mm in thickness, the specimen should be 50.8mm in length by 12.7mm in width while being tested flatwise on a 25.4mm support span.
For each measurement taken during the test, an untested specimen must be used.
The test steps are as followed:
Firstly, measure the width and depth of the specimen to the nearest 0.03mm at the centre of the support span. If the depth of the specimen is 2.53mm or less, then measure it to the nearest 0.003mm. Also, make sure that the support span meets all requirements which were outlined above for materials which are greater than 1.6mm in thickness and for materials which are less than 1.6mm in thickness. Next, the flexural fixtures must be set up and measured before the start of the testing phase. For flexural fixtures which have adjustable spans which can be changed continuously, measure the span accurately to the nearest 0.1mm for spans which are less than 63mm and to the nearest 0.3mm for spans which are at least 63mm in size. For all calculations, the actual measured value of the span must be used so that the calculations are accurate. For flexural fixtures which have fixed span positions, make sure that the span distance is the same as the adjustable spans for each machined position. This specified distance will become the span for that positioned and will be used in the calculations. To determine the rate of crosshead motion which must be set on the machine, the following calculation must be worked out to obtain the rate of crosshead motion in mm/min.
This answer should not differ from the actual crosshead motion by more than 10% positively or negatively. Next, the loading nose and the supports must be aligned so the axes of the cylindrical surfaces are parallel with each other. Then centre the testing specimen onto the support in a way so that the long axis of the specimen is perpendicular to the loading nose and supports. After this has been set up, apply the load onto the specimen at the crosshead rate which was found in the calculation above and then make a note of the simultaneous load – deflection results/data. The deflection can be measured by using a gauge beneath the specimen while in contact with the centre of the support span, measuring the motion of loading nose relative to the support beams or by mounting the stationary gauge relative to the specimen supports. Then, plot the load – deflection values onto a graph to then determine the flexural strength, the tangent modulus of elasticity and the total work which is measured by calculating the area under the load – deflection graph. The test is complete when the maximum strain of the outer layer of the specimen reaches 0.05 mm/mm or when the specimen’s outer layer breaks prior to reaching the 0.05mm/mm strain point. The deflection point at which this strain occurs can be worked out by substituting 0.05mm for r in the following equation:
If the fibreglass sample does not yield or break within the 5% strain limit, then repeat the test on an untested sample of fibreglass but change the rate of straining from 0.01mm/mm/mm to 0.1mm/mm/mm. If the fibreglass sample still does not break in the outer surface, then the test will be discontinued.
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The other set of ASTM standards, ASTM D2584 is on standard test method for ignition loss of cured reinforced resins. This document outlines the determination of the ignition loss of cured reinforced resins. This test method involves placing a fibreglass specimen in a crucible and igniting it and letting it burn until there is only ash and carbon remaining. This carbon is heated it in a furnace at 565 degrees Celsius to reduce it to ash. It is then cooled in a desiccator and then weighed. This testing method can be used for different uses like obtaining the ignition loss of a cured reinforced resin sample. Also, the ignition loss can be the resin content of the sample if the glass filament is used as reinforcement to the resin which is decomposed to materials which evaporate when ignited in the furnace. Although, this testing method does not provide a way to measure the content of resin which is contained in reinforcing materials that lose weight when ignited in the furnace or resins which do not decompose to materials that evaporate quickly.
For this test, there is only two pieces of equipment needed. They are a porcelain or platinum crucible of 30ml capacity and an electric muffle furnace which can maintain a temperature between 537 to 593 degrees Celsius. There should also be at least three specimens tested for each sample which weigh roughly 5 grams each and are 2.5cm by 2.5cm in thickness. These test specimens should be conditioned between 21 to 25 degrees Celsius and between 45 to 55% relative humidity for a minimum of 40 hours before starting the test. The test will also be done in these exact conditions mentioned above. The test method starts off by heating the crucible to a temperature between 500 to 600 degrees Celsius for 10 minutes and then letting cool back down to room temperature. A test specimen is then placed in the crucible and then weighed to the nearest milligram. The specimen and crucible will then be heated until they catch fire. This temperature will be maintained so the specimen burns at a steady rate until there is only carbon and ash left over. After this, heat the residue and crucible in the furnace between 537 to 593 degrees Celsius so that all carbon has disappeared. Then cool the crucible back down to room temperature and weigh it again. The ignition loss of the specimen can be found by the following formula:
Do this equation for all three specimens and then average them to get the mean.
Also, the standard deviation of this test can be found by the following formula:
The test and calculations are completed once the ignition loss range is calculated. This is done by subtracting the lowest specimen ignition loss from the highest.
Quality Control Systems
One way that manufactures make sure that their fibreglass products are up to standard is through inspections. This involves testing, examining and measuring the product to make sure that all specifications and dimensions are accurate. If this product is not up to a level of quality that it should be, it will not be sold. Failure testing is another quality control technique. This involves testing a product by applying stress until it fails. This is like the ASTM testing methods stated above but for actual finished products. Another quality control technique is process checklists. This involves a manager following the production of a product and ticking off an activity from a list of crucial steps that must be completed before selling the good.
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