types of accelerated weathering test and their interpretations

There are three major accelerated weathering tests:

1. Exposure to carbon arc lamps

2. Exposure to fluorescent UV lamps

3. Exposure to xenon arc lamps

4. Accelerated exposure to sunlight using the Atlas type 18 FR Fade-O-Meter

The xenon arc, when properly filtered, most closely approximates the wavelength distribution of natural sunlight.

Fluorescent UV Exposure of Plastics (ASTM D4329)

This method simulates the deterioration caused by sunlight and dew by means of artificial ultraviolet light and condensation apparatus. Solar radiation ranges from ultraviolet to infrared. Ultraviolet light of wavelengths between 290 and 350 nm is the most efficient portion of terrestrial sunlight that is damaging to plastics. In the natural sunlight spectrum, energy below 400 nm accounts for less than 6 percent of the total radiant energy. Since the special fluorescent UV lamps radiate between 280 and 350 nm, they accelerate the degradation process considerably. The test apparatus basically consists of a series of UV lamps, a heated water pan, and test specimen racks. The temperature and operating times are independently controlled for both UV and the condensation effect. The test specimens are mounted in specimen racks with the test surfaces facing the lamp. The test conditions are selected based on requirements and programmed into the unit. The specimens are removed for inspection at a predetermined time to examine color loss, crazing, chalking, and cracking.

Xenon Arc Exposure of Plastics Intended for Outdoor Applications

(ASTM D 2526)

Specimen:

This method is applicable when light and water exposure approximation are used for artificial weathering.

Procedure:

A water-cooled xenon-arc-type light source is one of the most popular indoor exposure tests because Xenon arc have been shown to have a spectral energy distribution when properly filtered. This closely simulates the spectral distribution of sunlight at the surface of the earth. The xenon arc lamp consists of a burner tube and a light filter system consisting of interchangeable glass filters used in combination to provide a spectral distribution that approximates natural sunlight exposure conditions. The apparatus has a built-in recirculating system that recirculates distilled or deionized water through the lamp. The water cools the xenon burner and filters out long wavelength infrared energy. For air-cooled lamps, this is accomplished by the use of optical filters.

It is highly recommended that a controlled irradiance exposure system be used. This is best accomplished through the use of a continuously controlled monitor that can automatically maintain uniform intensity at preselected wavelengths or wavelength range, when broadband control is being used.

Significance:

There is no precise correlation existing between the data obtained by Xenon arc method and outdoor weathering and other laboratory weathering devices because the emitted energy from Xenon lamps decay with time and the parameters of temperature and water do not represent specific known climatic conditions.

Accelerated exposure to sunlight using the Atlas type 18 FR Fade-O-Meter:

It is used to check and compare the color stability. Besides determining the ability of various pigments needed to provide both standard and custom colors, the Fade-O-Meter is helpful in studying various stabilizers, dyes and pigments compounded in plastics to prolong their useful life. It is for testing material to be used in articles subject to indoor exposure to sunlight.

It was extensively used in the development of UV absorbing acetate film for store windows to protect merchandise (good for sale) displayed in direct sunlight.

Exposure in the Fade-O-Meter cannot be directly related to the exposure in direct sunlight because other weather effects are always present in outdoors.   

Interpretations and Limitations of Accelerated Weathering Test Results:

There has been a severe lack of understanding on the part of users regarding the correlation between the controlled laboratory test and the actual outdoor test and application. The questions often asked are: “How many hours of exposure in a controlled laboratory enclosure is equal to one month of outdoor exposure?” “How do the results obtained from one type of weathering device compare to another type?” There is a general agreement among the researchers, manufacturers, and users that the data from accelerated weathering tests are not easily correlated with the results of natural weathering. However, accurate ranking of the weatherability of most material is possible using improved test methods and sophisticated equipment.

Accelerated weathering tests were devised to study the effect of actual outdoor weather in a relatively short time period. These tests often produce misleading results that are difficult to interpret or correlate with the results of actual outdoor exposure. The reason for such a contradiction is that in many laboratory exposures, the wavelengths of lights are distributed differently than in normal sunlight, possibly producing effects different from those produced by outdoor weathering. All plastics seem to be especially sensitive to wavelengths in the ultraviolet region. If the accelerated device has unusually strong emission at the wavelength of sensitivity of a particular polymer, the degree of acceleration is disproportionately high compared to outdoor exposure. The temperature of the exposure device also greatly influences the rate of degradation of a polymer. The higher temperature may cause oxidation and the migration of additives which, in turn, affects the rate of degradation. One of the limitations of accelerated weathering devices is their inability to simulate the adverse effect of most industrial environments and many other factors present in the atmosphere and their synergistic effect on polymers. Some of the newly developed gas-exposure cabinets have partially overcome these limitations. These units are capable of generating ozone, sulfur dioxide, and oxides of nitrogen under controlled conditions of temperature and humidity. Improved ultraviolet sources and more knowledge of how to simulate natural wetness now make it possible to achieve reliable accelerated weathering results if the following procedures are observed:

1. Include a material of known weather resistance in laboratory tests. If such a material is not available, use another similar product that has a history of field experience in a similar use.

2. Measure or estimate the UV exposure, the temperature of the product during UV exposure, and the time of wetness under service conditions of the product.

3. Do not use abnormal UV wavelengths to accelerate effects unless testing small differences in the same material. Evaluating two different materials by this technique can distort results.

ACCELERATED WEATHERING TESTS

Specimen:

Any shape, size up to 5” x 7” x 2”

Procedure:

Artificial weathering as defined by ASTM is:

The exposure of plastic to cyclic laboratory conditions involving changes in temperature, RH and UV radiant energy with or without direct water spray in an attempt to produce changes in the material similar to those observed after long term continuous outdoor exposure. A variety of light sources are used to simulate the natural sunlight. The artificial light sources include carbon arc lamps, xenon arc lamps, fluorescent sun lamps, and mercury lamps. These light sources, except the fluorescent, are capable of generating a much higher intensity light than natural sunlight. In the same wavelength band, xenon arc lamps can be operated over a wide range from below peak sunlight to twice the sunlight levels. Quite often, a condensation apparatus is used to simulate the deterioration caused by sunlight and water as rain or dew. Modern instruments have direct specimen spray on the front and/or back side of the specimen.

Significance:

Most data on the aging of plastics are acquired through accelerated tests and actual outdoor exposure. The latter is a time-consuming method; accelerated tests are often used to expedite screening the samples with various combinations of additive levels and ratios. Though there is no precise correlation between artificial laboratory weathering and natural outdoor weathering the standard laboratory test conditions produce results which are in general agreement with data obtained from outdoor exposures. Moreover the conditions can be easily reproduced.

list of weather resistance test methods and outdoor weathering of plastic (ASTM D 1435)

WEATHER RESISTANCE:

The effects of weather on plastics can be predicted by any of the following methods:

1.       Outdoor weathering (ASTM D 1435)

2.       Accelerated weathering (ASTM G 23)

3.       Water cooled Xenon arc type (ASTM D 2565)

4.       Accelerated exposure to sunlight using Atlas type 18 FR Fade-O-meter  

Out of which the first is test procedure using natural weathering and remaining three uses artificial environmental conditions. This methods conclude that there is no substitute for natural weather.

OUTDOOR WEATHERING OF PLASTICS (ASTM D 1435)

Specimen:

Any standard molded specimen or cut pieces of sheet or machined sample.

Procedure:

Exposure test specimens of suitable shape or size are mounted in a holder directly applied to the racks. Specimens are mounted outdoor on racks slanted at 45⁰ angle, facing south or facing the equator. Many other variations in the position of the racks are also employed, depending upon the requirements. The specimens are removed from the racks after a specified amount of time and subjected to appearance evaluation, electrical tests, and mechanical tests. The results are compared with the test results from control specimens. It is recommended that concurrent exposure should be carried out in many varied climates to obtain the broadest results. Since weathering is a comparative test, control samples are always utilized and retained at standard conditions of temperature and humidity. The control samples must also be covered with inert wrapping to exclude light exposure during the aging period. However, dark storage does not insure stability.

Significance:

Since one quarter of all polymers end up in outdoor applications, outdoor weathering tests have become very popular. It is the most accurate method of obtaining a true picture of weather resistance but the test time required is of several years exposure.

The test is devised to evaluate the stability of plastic materials exposed outdoors to varied influences that comprise weather exposure conditions that are complex and changeable.

Factors affecting:

Climate, time of year, and the presence of industrial atmosphere. It is recommended that repeated exposure testing at different seasons and over a period of more than one year be conducted to confirm exposure at any one location. Test sites are selected to represent various conditions under which the plastic product will be used. Arizona is often selected for intense sunlight, wide temperature cycle, and low humidity. Florida, on the other hand, provides high humidity, intense sunlight, and relatively high temperatures.

Outdoor Accelerated Weathering

To accelerate outdoor weathering, a reliable method for predicting long-term durability in a shorter time frame had to be developed. The method employs Fresnel-reflecting solar concentrators that use 10 fl at mirrors to uniformly focus natural sunlight onto specimens mounted in the target plane. High-quality, first surface mirrors provide an intensity of approximately eight suns with spectral balance of natural sunlight in terms of ultraviolet integrity. The test method provides an excellent spectral match to sunlight, correlating well to subtropical conditions such as southern Florida as well as an arid desert environment such as Arizona.

The test apparatus is a follow-the-sun rack with mirrors positioned as tangents to an imaginary parabolic trough. The axis is oriented in a north–south direction, with the north elevation having the capability for periodic altitude adjustment. The target board, located at the focal line of the mirrors, lies under a wind tunnel along which cooling air is deflected across the specimens. A nozzle assembly is employed to spray the specimens with deionized water in accordance with established schedules. Nighttime spray cycles can be used to keep specimens moist during the non tracking portion of the test. The entire three-year real-time Florida exposure test can be carried out in just six months depending on the program start date. The test is widely used in automotive, agriculture, building, textile, and packaging industries.

SPECIFIC GRAVITY (ASTM D 792)

Specific gravity is defined as the ratio of the weight of the given volume of a material to that of an equal volume of water at a stated temperature. The temperature selected for determining the specific gravity of plastic parts is 23°C. Specific gravity values represent the main advantage of plastics over other materials, namely, light weight. All plastics are sold today on a cost per pound basis and not on a cost per unit volume basis. Such a practice increases the significance of the specific gravity considerably in both purchasing and production control. Two basic methods have been developed to determine specific gravity of plastics depending upon the form of plastic material. Method A is used for a specimen in forms such as sheet, rods, tubes, or molded articles. Method B is developed mainly for material in the form of molding powder, flakes, or pellets.

Method A

This method requires the use of a precision analytical balance equipped with a stationary support for an immersion vessel above or below the balance pan. A corrosion-resistant wire for suspending the specimen and a sinker for lighter specimens with a specific gravity of less than 1.00 is employed. A beaker is used as an immersion vessel. The test specimen of any convenient size is weighted in air. Next, the specimen is suspended from a fine wire attached to the balance and immersed completely in distilled water. The weight of a specimen in water (and sinker, if used) is determined. The specific gravity of the specimen is calculated as follows:

Specific gravity = a (a + w) / b

Where,

a = weight of specimen in air;

b = weight of specimen (sinker, if used) and wire in water;

w = weight of totally immersed sinker (if used) and partially immersed wire.

Method B

This method, which suitable for pellets, flakes, or powder, requires the use of an analytical balance, a pycnometer, a vacuum pump, and a vacuum desiccator. The test is started by first weighing the empty pycnometer. The pycnometer is filled with water and placed in a water bath until temperature equilibrium with the bath is attained. The weight of the pycnometer filled with water is determined. After cleaning and drying the pycnometer, 1–5 g of material is added and the weight of the specimen plus the pycnometer is determined. The pycnometer is filled with water and placed in a vacuum desiccator. The vacuum is applied until all the air has been removed from between the particles of the specimen. Last, the weight of the pycnometer filled with water and the specimen is recorded. The specific gravity is calculated as follows:

Specific gravity = a (b + a / m)

Where,

a = weight of the specimen;

b = weight of the pycnometer filled with water;

m = weight of the pycnometer containing the specimen and filled with water.

If another suitable immersion liquid for the water is substituted, the specific gravity of the immersion liquid must be determined and taken into account in calculating the specific gravity.

OXYGEN INDEX TEST (ASTM D 2863)

Introduction

Oxygen index is defined as the minimum concentration of oxygen, expressed as volume percent, in a mixture of oxygen and nitrogen that will just support flaming combustion of a material initially at room temperature under specified conditions.

The oxygen index test is considered one of the most useful flammability tests because it allows one to precisely rate the materials on a numerical basis and simplifies the selection of plastics in terms of flammability. The oxygen index test overcomes the serious drawbacks of conventional flammability tests. These drawbacks are variation in sample ignition techniques, variation in the description of the endpoint from test to test, and operation of tests under non equilibrium conditions.

Test Procedures

The test determines the minimum concentration of oxygen in a mixture of oxygen and nitrogen flowing upward in a test column that will just support combustion. This process is carried out under equilibrium conditions of candle like burning. It is necessary to establish equilibrium between the heat removed by the gases fl owing past the specimen and the heat generated from the combustion. The equilibrium can only be established if the specimen is well ignited and given a chance to reach equilibrium when the percent oxygen in the mixture is near limiting or critical value. The equipment used for measuring the oxygen index consists of a heat-resistant glass tube with a brass base. The bottom of the column is filled with glass beads, which allows the entering gas mixture to mix and distribute more evenly. A specimen-holding device to support the specimen and hold it vertically in the center of the column is used. A tube with a small orifice having propane, hydrogen, or other gas flame, suitable for inserting into the open end of the column to ignite the specimen, is used as an ignition source. A timer, flow measurement, and control device are also used. The test specimen used in the experiment must be dry since the moisture content of some materials alters the oxygen index. Four different types of specimens are specified. They are physically self-supporting plastics, flexible plastics, cellular plastics, and plastic film or thin sheet. The dimension of the specimen varies according to the type. The specimen is clamped vertically in the center of the column. The flow valves are set to introduce the desired concentration of oxygen in the column. The entire top of the specimen is ignited with an ignition flame so that the specimen is well lighted. The specimen is required to burn in accordance with set criteria, which spell out the time of burning or the length of specimen burned. The concentration of oxygen is adjusted to meet the criteria. The test is repeated until the critical concentration of oxygen, which is the lowest oxygen concentration that will meet the specified criteria, is determined. 

The oxygen index is calculated as follows:

Oxygen index percent = (100 × O2)/ (O2 + N2) 

where O2 = volumetric flow of oxygen at the concentration determined; 

N2 = volumetric flow of nitrogen.

Factors Affecting the Test Results

1. Thickness of Specimen. As the specimen thickness increases, the oxygen index also increases steadily.

2. Fillers. Fillers such as glass fibers tend to increase the oxygen index up to a certain percentage loading. In case of polycarbonate, the oxygen index peaks at about 25 percent loading. Higher loading beyond this point subsequently decreases the oxygen index.

3. Flame Retardants. Flame retardants increase the oxygen index, making polymers more suitable for applications requiring improved flammability.

ESCR ASTM D 1693

Environmental Stress Cracking Resistance (ASTM D 1693)

Specimen: LDPE measuring 1/8 x ½ x 3/2 inches, annealed in water or steam at 100 ⁰ C for 1 hour and then conditioned at room temperature for 5 to 24 hours.

Procedure:

 The specimen is placed in an air circulating oven and then inserted in a test tube which is then filled with a fresh reagent [Igepal = RC₆H₄O(CH₂CH₂O)_nCH₂CH₂OH  where R is C₆H₁₇ or higher homolog]. The tube is stoppered with an Aluminum covered cork and placed in a constant temperature bath at 50 ⁰C. These are inspected periodically and any visible crack is considered as failure. The duration of the test is recorded along with the percentage of failure.

 Significance:

The cracking obtained in this test indicates what can be expected from a wide range of other stress cracking agents. Though the information cannot be translated directly into end use service prediction, but it serves to rank various types and grades of PE in categories of resistance to stress cracking. This test can also be used on high and medium density used on high and medium density material.

flow test and Spiral flow test, cup flow test

Flow Tests

The ability of the material to flow is measured by filling a mold with the plastics material under a specified condition of applied temperature and pressure with a controlled charge mass. The flow tests are used as a quality control test and as an acceptance criterion for incoming raw materials.

Factors Affecting Flow

Resin Types. All resins flow differently because of basic differences in the structure of the polymers. For example, melamine formaldehyde exhibits longer flow than urea formaldehyde. Phenolics, because of the variety of resin types, enable the molder to select the flow best suited for a particular design.

Type of Fillers. The small particle size of wood fl our, mica, and minerals creates less turbulence and less frictional drag during mold filling. The size of the glass fibers, short or long, can adversely affect the flow.

Degree of Resin Advancement. The degree of advancement is generally controlled by the resin manufacturers. Molders can advance resin polymerization with oven or radiant heat or electronic preheating.

Storage Time. All resins have a natural tendency to polymerize in storage, causing partial precure which reduces flow. An exception might be polyester in which catalyst decomposition slows or prevents curing, which increases flow duration.

Spiral Flow of Low-Pressure Thermosetting Compounds (ASTM D 3123)

The spiral flow of a thermosetting molding compound is a measure of the combined characteristics of fusion under pressure, melt viscosity, and gelation rate under specific conditions. The test requires a transfer molding press, a standard spiral flow mold, and a thermosetting molding compound. The molding temperature, transfer pressure, charge mass, press cure time, and transfer plunger speed are preselected as specified. The preconditioned compound is forced through a sprue into a spiral flow mold. Once the curing is complete, the part is removed and the spiral flow length is read directly from the molded specimen. Compounds are classified as low (1–10), medium (11–22), and high (23–40) plasticity.

Cup Flow Test (ASTM D 731)

Molding Index of Thermosetting Molding Powder. This test is primarily useful for determining the minimum pressure required to mold a standard cup and the time required to close the mold fully. The preconditioned and preweighed material is loaded into the mold. The mold is closed using sufficient pressure to form a required cup. The pressure is reduced step by step until the mold cannot close. The next higher pressure and time to close the mold is reported as the molding index of the material.

Melt Flow Index (ASTM D 1238)

To measure flow rate by extrusion Plastometer.

Specimen:

Any form which can be introduced into the cylinder bore may be used. e.g. powder, granules, strips of film. Conditioning required varies with material.

Procedure:

The apparatus is preheated to 190^oC for PE. Material is put into the cylinder and the loaded piston (@ 43.25 psi) is put into place. After 5 minute the extrudate issuing from the orifice is cut off, flush and again one minute later. These cuts are discarded. Cuts for the test are taken at 1, 2, 3 or 6 minutes depending on the material or its flow rate. The melt index is calculated and recorded as gm/10 minute.

Significance:

It is primarily useful to raw material manufacturer as a method of controlling material uniformity. The melt index value is strongly indicative of relative “flowability” of various kinds and grades of PE. The ‘property’ measured by this test is basically melt viscosity or ‘rate of shear’. In general, the higher molecular weight materials are more resistant to flow.

Melt index is an inverse measure of molecular weight. Since fl ow characteristics are inversely proportional to the molecular weight, a low-molecular-weight polymer will have a high melt index value and vice versa.

1. Preheat Time. If the cylinder is not preheated for a specified length of time, there is usually some non uniformity in temperature along the walls of the cylinder even though the temperature indicated on the thermometer is close to the set point. The causes the flow rate to vary considerably. There should be zero thermal gradient along the full length of the test chamber.

2. Moisture. Moisture in the material, especially a highly pigmented one, causes bubbles to appear in the extrudate which may not be seen with the naked eye. Frequent weighing of short cuts of the extrudate during the experiments reveals the presence of moisture. The weight of the extrudate is significantly influenced by the presence of the moisture bubbles.

3. Packing. The sample resin in the cylinder must be packed properly by pushing the rod with substantial force to allow the air entrapped between the resin pellets to escape. Once the piston is lowered, the cylinder is sealed off, and no air can escape. This causes variation in the test results.

4. Volume of Sample. To achieve the same response curve repeatedly, the volume of the sample in the cylinder must be kept constant. Any change in sample volume causes the heat input from the cylinder to the material to vary significantly.

Interpretation of Test Results

The melt index values obtained from the test can be interpreted in several different ways. First, a slight variation in the melt index value should not be interpreted as indicating a suspect material. The material supplier should be consulted to determine the expected reproducibility for a particular grade of plastic material. A significantly different melt index value than the control standard may indicate several different things. The material may be of a different grade with a different flow characteristic. It also means that the average molecular weight or the molecular weight distribution of the material is different than the control standard and may have different properties. Melt index is an inverse measure of molecular weight. Since flow characteristics are inversely proportional to the molecular weight, a low-molecular-weight polymer will have a high melt index value and vice versa.

Ultrasonic measurement techniques in NDT

The three basic ultrasonic measurement techniques most widely used today are:

1. Pulse echo

2. Transmission

3. Resonance

Pulse-Echo Technique

The pulse-echo technique is the most popular of the three basic ultrasonic, nondestructive testing techniques. The pulse-echo technique is very useful in detecting flaws and for thickness measurement. The initial pulse of ultrasonic energy from a transducer is introduced into the test specimen through the couplant. This sound wave travels through the thickness of the specimen until a reflecting surface is encountered, at which time the sound wave reflects back to the transducer. This is called the back-wall echo. If the wave encounters a fl aw in its path, the fl aw acts as a reflecting surface and the wave is reflected back to the transducer. The echo in this case is referred to as a flaw echo. In both cases, the reflected wave travels back to the transducer, causing the transducer element to vibrate and induce an electrical energy that is normally amplified and displayed onto a CRT or other such device. The echo wave coming from the back wall of the specimen is marked by its transit time from the transducer to the back wall and return. Similarly, the transit time for the fl aw echo can also be determined by this technique. Since transit time corresponds to the thickness of the specimen, it is quite possible to calculate the thickness of the specimen using simple computer logic. One other technique, known as the immersion test technique, has generated tremendous interest among the manufactures that are in favor of automated inspection techniques. In the immersion technique, the specimen is completely immersed in the liquid.

Transmission Technique

In this technique, the intensity of ultrasound is measured after it has passed through the specimen. The transmission technique requires two transducers, one to transmit the sound waves and one to receive them. The transmission testing can be done either by direct beams or reflected beams. In either case, the flaws are detected by comparing the intensity of ultrasound transmitted through the test specimen with the intensity transmitted through a reference standard made of the same material. The best results are achieved by using the immersion technique since this technique provides uniform and efficient coupling between transducers and test specimen. The main application of the transmission technique is in detecting flaws in laminated plastic sheets.

Resonance Technique

This method is primarily useful for measuring the thickness of the specimen. This is accomplished by determining the resonant frequencies of a test specimen.

APPLICATION OF ULTRASONIC NDT IN PLASTICS

Ultrasonic nondestructive testing (NDT) has gained popularity in the past decade along with the growth of the plastic industry and along with an increasing emphasis placed on automation and material saving. Two major areas in which ultrasonic testing concepts are applied extensively are fl aw detection and thickness measurement. The pulse-echo technique is used to detect a flaw such as voids and bubbles in an extruded rod of rather expensive materials such as Teflon and nylon. The flaw detection unit and other auxiliary equipment can be programmed so that the specific portion of the rod with a flaw is automatically cut off and discarded without disturbing the continuous extrusion process. The transmission technique is commonly used to detect flaws in laminates. Thickness measurement by ultrasonic equipment is simple, reliable, and fast. This NDT technique simplifies the wall thickness measurement of parts with hard-to-reach areas and complex part geometry. Automated wall thickness measurement and control of large diameter extruded pipe is accomplished by using the immersion technique. An ultrasonic sensing unit is placed in a cooling tank to continuously monitor wall thickness. In the event of an out of control condition, a closed-loop feedback control system is activated and corrections are made to bring the wall thickness closer to the set point. Many such systems are commercially available. The ultrasonic NDT technique is used extensively by gas companies to examine the integrity of plastic pipe socket joints after they have been solvent cemented together. Ultrasonic measurements can also be used for determining the moisture content of plastic. In materials like nylons, the attenuation and the acoustic velocity change with the change in moisture content. The use of ultrasonics in testing reinforced plastics and missiles and rockets has been discussed.

Ultra sonic testing as NDT

Non Destructive Test -Ultrasonic Testing:

Ultrasonic testing is one of the most widely used methods for nondestructive inspection. In plastics, the primary application is the detection of discontinuities and measurement of thickness. Ultrasonic techniques can also be used for determining the moisture content of plastics, studying the joint integrity of a solvent-welded plastic pipe and fittings, and testing welded seams in plastic plates.

The term ultrasonic, in a broad sense, is applied to describe sound with a frequency above 20,000 cycles/sec. Commercial ultrasonic testing equipment generally employs the testing frequency in the range from 0.75 to 20 MHz. To provide a basis for understanding the ultrasonic system and how it operates, it is necessary to introduce the following terms:

Frequency Generator. A device that imposes a short burst of high-frequency alternating voltage on a transducer.

Transducer. A transducer or a probe is a device that emits a beam of ultrasonic waves when bursts of alternating voltage are applied to it. An ultrasonic transducer is comprised of piezoelectric material. Piezoelectric material is material that vibrates mechanically under a varying electric potential and develops electrical potentials under mechanical strain, thus transforming electrical energy into mechanical energy and vice versa. As the name implies, an electrical charge is developed by a piezoelectric crystal when pressure is applied to it and reverse is also true. The most commonly encountered piezoelectric materials are quartz, lithium sulfate, and artificial ceramic materials such as barium titanate.

Many different types of ultrasonic transducers are available, differing in diameter of the probe, frequency, and frequency bandwidth. Each transducer has a characteristic resonant frequency at which ultrasonic waves are most effectively generated and received. Narrow bandwidth transducers are capable of penetrating deep, as well as detecting small flaws. However, these transducers do a poor job of separating echos. Broad bandwidth transducers exhibit excellent echo separation but poor flaw detection and penetration. Transducers of a frequency range of 2–5 MHz are most common. For plastic materials, transducers in the range of 1–2 MHz seem to yield the best results.

Couplants. Air, being one of the worst transmitters of sound waves at high frequencies owing to a lack of impedance matching between air and most solids, must be replaced by a suitable coupling agent between the transducer and the material being tested. Many different types of liquids have been used as coupling agents. Glycerine seems to have the highest acoustic impedance. However, oil is the most commonly used couplant. Grease, petroleum jelly, and pastes can also be used as couplants, although a wetting agent must be added to increase wettability as well as viscosity. Some couplants have a tendency to react with the test specimen material and therefore chemical compatibility of the couplant should be studied prior to application. Couplants that are difficult to remove from test specimens should also be avoided. Any type of contamination between the test specimen and the transducer can seriously affect the thickness measurements, especially in the case of thin films. Therefore, it is absolutely necessary to remove these contaminants before applying the couplant. The basic sequence of operation in any ultrasonic measurement system is:

1. Generation of ultrasonic frequency by means of a transducer.

2. Use of a coupling agent (couplant), such as oil or water, to help transmit the ultrasonic waves into the material.

3. Detection of the ultrasonic energy after it has been modified by the material.

4. Displaying of the energy by means of a recorder, cathode-ray tube, or other devices.

Sandwich structure, FRM process of FRP

v  SANDWICH STRUCTURE PROCESSING:

Definition:

Panels composed of a light weight core material like honey comb, foamed plastic etc. to which relatively thin, dense, high strength faces or skins are adhered to produce material with lower and lower density and higher and higher stiffness.

Honey comb:

Manufactured product consisting of sheet metal or a resin impregnated sheet material (paper, fibrous glass etc.) which has been formed into hexagonal shaped cells.

v  FOAM RESERVOIR MOULDING (FRM) or ELASTIC RESERVOIR MOULDING (ERM):

Process:

a.       It is a method of preparing a sandwich structure.

b.      An open cell urethane foam is impregnated with a controlled amount of epoxy or other structural resins.

c.       Dry glass fabric or mat facing sheets are placed on either side of the foam and the assembly is placed in a mould.

d.      As the mould closes the dry glass is pressed by the foam against the mould surface and the air from the foam is escaped through the dry glass.

e.       When the mould is closed, the resin escapes from the foam and saturates the facing sheets.

f.       After curing, the laminate with considerably low density (than FRP) results.

g.      Advantages: The physical properties are superior to SMC parts and lower pressures than SMC parts is used.

h.      Capital investment for large presses is less, hence saving is considerable.

i.        Process cannot be used for parts with sharp curves, ribs or bosses.

j.        Fabrication of automobile body parts is made by FRM

Sandwich structures are also made by using an integrally woven 3-dimensional structure, with the facing attached to the core during the weaving operation.

Additional compression and bending strength can be obtained by placing foam or PP inserts between the webs. The woven structure can ne impregnated before or after the inserts are in place. However, curing is done after the inserts are placed. The foam inserts are usually not removed to get additional strength while PP inserts are removed to minimize the weight with the hollow core.

FILAMENT WINDING PROCESS AND OTHER INGENIOUS TECHNIQUES IN FRP

v  FILAMENT WINDING:

Definition:

Roving or single strands of glass, metal or other reinforcement are wound in a predetermined pattern on a suitable mandrel. The pattern is so designed as to give maximum strength in the direction required. The strands can either be run from a creel through a resin bath before winding or pre-impregnated mat (Randomly distributed interlocked felt of glass fiber) can be used. When the right number of layers have been applied the wound mandrel is cured at room temperature or in an oven.

Material:

Epoxy resins are usually used with fiber glass, graphite, or Kevlar (aromatic polyamide fiber) roving reinforcements. When graphite is used, a pre-pegged fiber (reinforcing material containing or combined with full complement of resin) or tape is used because it can be made with closely controlled resin content. Certain angles of winding which cannot be done with ‘wet’ fiber winding due to slippage, can be done using pre-pegged tapes.

Equipment:

Any type of body of revolution which can be withdrawn from the cured part, can be used. Mandrels are made of steel, Aluminum and plaster. Some tools are made collapsible to facilitate their removal. Some tools are made of breakaway plasters, or of material which can be dissolved out. Inflatable mandrels are also used. The winding machine acts like a lathe. (Machine for turning wood, metal etc. or with revolving disc for throwing and turning pottery)

Process:

Continuous strands (roving) are fed through a catalyzed resin bath and then wound over a mandrel to form a part. OR Woven or unidirectional pre-peg tapes are used.

      Advantages:

a.       Higher production rates.

b.      Certain wind patterns.

c.       Possibility of more uniform resin content.

d.      Parts with highest strength in one direction or in several direction can be made because the ratio of reinforcement to resin is very high.

e.       Mostly the parts are made using automated equipment, hence very little operator skill is required.

f.       Parts can be very uniform.

g.      High quality parts can be made very economically because reinforcement material is cheap.

      Disadvantages:

a.       Higher costs.

b.      The use of a woven tape does not result in a part with the maximum content of unidirectional fibers, hence in some application this does not result in the highest strength.

Extremely high strengths can be obtained depending on the angle of wind, particularly if all the winding is done in the circumferential direction.

The winding machine lays the strands perpendicular to the axis of the part or the machine is programmed to wind the strands at some helix angle to result in more or less reinforcement in the axial and circumferential direction. Some of the wind patterns are:

a.       Hoop or circumferential.

b.      Helix with wide ribbon.

c.       Helix with narrow ribbon and medium or high angle.

d.      Helix with low winding angle.

e.       Zero or longitudinal.

f.       Polar wrap.

g.      Cone.

h.      Simple spherical.

i.         Simple Ovaloid.

j.        True spherical.

k.      Miscellaneous.

Different equipment are used with different considerations for each pattern.

Application:

Corrosion resistant pipe, high pressure resistant tank, rocket motor nozzles, tank closures, metal fittings, metal threaded sections, it is excellent adhesive because epoxy resin is used. It is used when metal inserts are required. Used in small volume project connected with the space program or for the military, this include very large filament wound tank, rocket motor nozzles, entry cones etc.

Ingenious techniques:

In order to accelerate the normal filament winding process and/or to eliminate the need for very large sized processing equipment used for application of pressure to the part, number of ingenious techniques are developed.

1.         Snap cure method for tape winding.

2.         Cable-clave process.

3.         Hydro-clave process.

4.         Deep submergence technique.

Snap cure method:

a.       The pre impregnated tape is heated with a quartz lamp, just before placing it on the mandrel.

b.      This quickly sticks the resin and starts curing.

c.       The tape is then pressed down on the mandrel with a hydraulically actuated roller at a pressure of 400-800 psi.

d.      To assure good density, several rollers are used at a time.

e.       This method, mainly using phenolic impregnated tape, result in a fast cure, eliminates the volatiles and produces very dense and uniform parts.

Cable-clave process:

a.       Large part is wound either with filament roving or tape.

b.      Two form fitting rubber bags are placed on the wrapped part and then a number of loose, thin metal strips are placed over the bags to cover the total bag area.

c.       A steel cable is then wrapped completely around the entire structure.

d.      Fluid at pressure up to 2000 psi is pumped into the space between the two rubber bags.

e.       The steel strips prevents the bag extrusion between the cables and the cables restrains the bag hence very dense structures can be made.

f.       Heating is done either by heating the structure in an oven or heating the pressurized fluid.

Hydro-clave process:

Filament wound parts are placed in a rubber bag and the entire assembly is then placed in a pressure vessel which is filled with a liquid as a pressurizing agent. By choosing a suitable liquid, pressures up to 1000 psi at reasonable temperature are expected. However, requirement of an enormously costly pressure vessels for a very large part is the major disadvantage of the process.

Deep submergence technique:

The parts are enclosed in a rubber bag and are surrounded by electric heating coil. Another bag surrounding the assembly will completely seal the structure which is then submerged in the ocean up to a sufficient depth to get satisfactory pressurization. However, transportation of the assembly to the sea, application of the large power requirement, and to hold the parts steady during the cure, make the process impractical for the fabrication. Still no parts are made by this process.