Flex Test Research Articles

Mechanics of the Dynamic Flex Test

Abstract The dynamic flex test is a comparative test used for screening synthetic cords. The test objective is to subject the cord through accelerated test conditions as observed in an underinflated and deflected tire. This test ultimately determines the percent break strength retained for flexed synthetic tire cords. It consists of a two-layer composite laminate subjected to a constant tensile load while dynamically flexing around a spindle. After flexing for two to six hours (the time depends on the cord material and construction), the break strength is determined for the flexed and for an unflexed cord and the percent retained break strength is reported. The objective of this paper is to quantify the stress and strain distributions in the cord under conventional laboratory conditions and identify the conditions required for prescribing the desired tension and compression cyclic strain for a cord construction. The tension-compression behavior is controlled by the location of the neutral plane which, in turn, is a function of the static tensile load, fiber reinforcement and rubber matrix moduli, epi, geometry of the laminate cross section, and the diameter of the spindle.

Characterization Methods for the Mechanical Behavior of Thermoplastic Composites

The tensile and shear moduli of four thermoplastic composites are deter mined using three-point beam flex tests at different spans. The experimental values are compared to the predictions of the rule of mixtures and the rule of mixtures modified by the stress partitioning function. Tensile and flexure strengths are also determined. The results are compared to data for a thermoset composite. The measured shear moduli of thermoplastic composites is lower than that predicted when a stress partitioning function of .4 is used. Laminates with large areas of voids had lower shear moduli, and lower ten sile and flexure strength than void-free laminates.

SEM and x-ray EDS studies on the adhesion of novel bisoxazoline-phenolic copolymers to carbon and glass fibers, and metallic filler surfaces

A very promising new entry into the family of polymers for advanced composites, for aerospace and heavy duty industrial applications requiring high service temperature and/or low flammability, are bisoxazoline-phenolic resins derived from the step-growth copolymerization of 1,3 (or 1,4)-phenylene bisoxazoline (PBOX) and phenol-free phenolic resins. This new resin system offers a very attractive group of properties, including high modulus, high compressive and shear strengths, excellent toughness, high heat distortion temperature, no volatiles evolution and low shrinkage (<1%) during cure, low flammability and low smoke production, good electrical properties, relatively low coefficient of thermal expansion, very good thermal shock resistance and machinability, and excellent adhesion to carbon fibers, fiberglass and powdered (5 to 20 μm) metal fillers (e.g., iron and aluminum).In this paper are presented example scanning electron microscopy (SEM) micrographs of PBOX-phenolic interfacial regions with both reinforcing fibers and metallic fillers. Carbon fiber reinforced composite specimens were examined by SEM (CamScan Series 4 instrument) after testing to failure using standard ASTM shear, flex and compression tests.

An Experimental Study of Particle Emissions From Cleanroom Papers

Six types of paper, four of which are "cleanroom" papers and two of which are conventional bond papers, were evaluated for aerosol particle emissions in the Texwipe modified flex tester. Under this type of stress, all cleanroom papers emitted significantly fewer particles than the bond papers. Significant differences also existed among the samples of the four cleanroom papers.

From polymerization to product: The effect of molecular structure on the properties of butadiene‐acrylonitrile copolymer elastomer

AbstractButadiene–acrylonitrile copolymers were prepared with such variations in molecular structure as molecular weight, degree of long branching, and gel. The acrylonitrile content was varied from 28% to 44%. Viscoelastic behavior of gum elastomers, uncured compounds, and vulcanizates were characterized. Thus, the effects of molecular architecture on the properties were followed through the processing steps of compounding and vulcanization. Also, the effects of molecular parameters on the large deformation of vulcanizates were examined by the tensile stress–strain behaviour. The fracture toughness was examined with the tensile failure characteristics and De Mattia Flex Test

化学銅めっき皮膜の疲労柔軟性

Copper films were prepared by chemical plating from the baths containing EDTA or Quadrol as a complexing agent and the effects of 2, 2′-bipyridyl and potassium hexacyanoferrate (II) on the mechanical properties of the copper deposits were studied in detail. Fatigue Ductility Flex Tester was employed for evaluating the fatigue ductility of the copper films. Factors affecting fatigue ductility of the copper films were discussed on the basis of crystallographic features of the deposits. The addition of 2, 2′-bipyridyl and potassium hexacyanoferrate (II) in the chemical plating baths resulted in an increase in the elongation and fatigue ductility of the copper films, because strain-free copper deposits having larger crystallite size were formed. The best fatigue ductility can be obtained from the EDTA bath containing 20mg/l 2, 2′-bipyridyl and 20mg/l potassium hexacyanoferrate (II). In general, the harder deposit having the larger ultimate tensile strength was found to be composed of smaller crystallite. It was concluded that the fatigue life of the copper films was improved by the increase in fatigue ductility and the ultimate tensile strength.

A statistical method for computer treatment of elastomer flex data

AbstractIn flex testing of elastomers, the length of a cut is measured periodically as a function of the number of flexing cycles. In most cases a large number of measurements is accumulated for each test piece which would lead one to believe that the results should have a high degree of precision. This, however, is not the case, mainly because not all of the measurements are used to express the final result. Instead of being a precise test, flex testing is in fact notorious for its irreproducibility. A new method is proposed for computer treatment of flex data in which all of the flex measurements are fitted to an empirical equation by means of nonlinear regression analysis. From the fitted equation, more precise estimates of rate of growth and growth initiation point can be obtained than is possible using the data treatment procedures proposed by ASTM. Also a new index of flex life is defined which is conveniently scaled from 0–100. This so‐called flex index is a convenient index of overall flex behavior.

A Rapid Quantitative Method for Measuring Ozone Cracking and Its Use in Outdoor Dynamic Testing

Abstract A fairly rapid quantitative technique for measuring the cracking damage done to rubber by ozone has been described. The method consists of microscopic measurement of the length and width of selected ozone cracks. The product of the “average” length and width is termed the “severity of ozone cracking” and is abbreviated, SOC. The SOC value for a specimen is approximately 2 times the void area in arbitrary units. The use of the SOC technique in conjunction with an outdoor dynamic ozone test showed how various antiozonants perform in NR, SBR, and OE-SBR both with and without wax. Some effects of the amount and type of extending oil and of wax melting point are illustrated. Weather plays an important role in regard to ozone attack especially for wax containing compounds. During the warmer months higher melting waxes perform better in natural and OE-SBR while higher melting waxes perform slightly poorer in non-oil extended SBR. Good correlation was found between the outdoor flexing tests and service type outdoor tire tests carried out on an outdoor wheel. Some of the ramifications of this type of testing are pointed out.

Preliminary considerations for flexural endurance testing of semicrystalline polymers for biomedical materials standards.

AbstractSemicrystalline polymers used as restorative prostheses in the cardiovascular system require more detailed consideration of flexural fatiguing with simple physical characterization and mechanical measurements as one basic means for developing test standards and specification levels for biomedical research materials. Such initial means are considered with polytetrafluoroethylene as a reference polymer nominally inert chemically used to illustrate one approach to assessing morphological and structural changes with a flex tester. Density measurements and void content have been carried out to provide one means of assessment whereby the corresponding crystallinity and permeation can be considered for more detailed correlation to cytological interactions as the polymers are used as replacement components in the cardiovascular system. Progressive flexural cycling up to 106 cycles at a 3 cps rate indicate significant decreases in crystallinity simultaneously with increasing void content. As an accelerated test, the implications for extrapolation to 25‐year fatiguing in normal physiological cycling rate are conjectured to provide a starting point for more realistic, cyclic ex vivo screening of biomaterials.

Methods of single fiber evaluation

AbstractResearch and development directed at improved fibers and fibrous materials normally utilizes relatively conventional apparatus for fiber, yarn, and fabric characterization. When desired, material properties are dictated by end‐use requirements which represent extremes in temperature resistance, strength‐to‐weight ratio, high impact velocities, etc.; novel equipment and techniques are likely to be necessary for such research. This is the case with the Air Force's effort in the area of materials evaluation. Techniques now in use are, for the most part, suitable for materials being researched, but improvements must be made for the next generation of fibrous materials. New methods of characterization will be needed to overcome shortcomings of present techniques and to adapt to an ever‐increasing requirement for the properties of all types of materials. For example, evaluation of thermally stable abrasion‐resistant finishes will require long‐term, high temperature flex testing. The laboratory equipment herein described represents a small fraction of the total in‐house capability of the Air Force Materials Laboratory for conducting exploratory research and development on advanced fibrous materials for aerospace systems.

Evaluation Techniques for Fibrous Materials

Research and development directed at improved fibers and fibrous materials normally utilizes relatively conventional apparatus for fiber, yarn, and fahric characterization. When desired material properties are dictated by end-use requirements, which represent extremes in temperature resistance, strength-to-weight ratio, high impact velocities, etc., novel equipment and techniques are likely to be necessary for such cesearch. This is the case with the Air Forces' effort in the area of materials evaluation. Techniques now in use are, for the most part, suitable for materials being researched, but improve ments must be made for the next generation of fibrous materials. New methods of characterization will be needed to overcome shortcomings of present techniques and to adapt to an ever increasing requirement for the properties of all types of materials. For example, evatuation of thermally stable abrasion-resistant finishes will require long- term, high-temperature flex testing. The laboartory equipment herein described repre sents a small fraction of the total in-house capability of the Air Force Materials Labora tory for conducting exploratory research and development on advanced fibrous materials for aerospace systems.

An Improved Flexing Machine. I. Description and Application to Studies of Cut Growth

Abstract A new flex testing machine has been developed which yields accurate information on cut growth of vulcanizates. Its unique features are accurate temperature control of the test chamber (25°–200° ± 1° C), adjustable work input upon the test specimen by varying both the frequency of flexing (100–600 ± 1 rpm) and flex angle, and accomodation of a large number of test specimens (90–120). Furthermore, provision is made to test with a specific testing atmosphere, either oxidative or inert, depending on the required severity of conditions. Advantages of the test machine are: a) simple operation, once the test conditions are established: b) rapidity of testing, since a large number of specimens can be tested at the same time; and c) excellent reproducibility in preparing test specimens, varying and maintaining test conditions, and measuring cut growth. The maximum standard error for the test is about 12%. A further advantage is that compounds of varying degrees of hardness, for example, tire treads, shoe soles, and belt compounds, can be tested on the same machine by adjusting the severity of the test conditions. Effects of polymer and compounding variables on cut growth are discussed using data obtained with this new flexing machine.

Effect of liquid media on fatigue strength of polyamides

The results of an experimental study of polyamide fatigue strength are presented. Multiple flexing tests were performed on specimens rotating at 3000 cycles/min with a symmetrical load cycle in various media, including air (with and without blowing), volatile liquids (petroleum ether, ethanol, water), glycerin, and transformer oil. Fatigue strength is determined from (σN — log N) curves for 106 cycles. Dissipation of heat is found to be the main factor determining the effect of slightly aggressive media on fatigue strength. Values of the ratio fatigue strength/static strength (coefficient K) are presented.

Cut Growth and Flex Testing of Natural Rubbers and Synthetic Elastomers

Abstract In the last decades, numerous studies have appeared concerning the behavior of tears in natural rubber and synthetic elastomers. In the United States' and England there are even standardized test methods. It can be seen from all the investigations and test instructions that the results are very poorly reproducible and only rarely agree with practical experience. In investigations on the behavior of tears, the two stages—the beginning of the tear and the growth of the tear (tear propagation)—must be differentiated. The following factors must be given primary consideration in starting the tear: external damage, the action of ozone and oxygen, frequently changing tensile or compression stresses. Here oxygen plays a role similar to that in the corrosion of metals. Fatiguing mechanical stress undoubtedly leads to the appearance of tears. But it can be assumed that the beginning of a tear observed in the area of maximum stress takes place at a preferred point on the surface. Such preferred points are always present. As is well known, the surface of a material— even on a high polymer—must not be regarded as completely “smooth”. In place of this supposed smoothness, there appears under microscopic or electron-microscopic observation a kind of “mountain system”. Observed under sufficient magnification, the “valleys” or cuts in the mountains are comparable to tears. Under mechanical stress, such cuts will then continue to grow particularly rapidly (nick effect). Now it is largely dependent on the kind of observation and the observer as to whether a small irregularity in the surface is called a tear. The beginning of the tear cannot, therefore, be defined clearly. The case is quite different with tear growth. The initial conditions—the tear present at the beginning of measurement—are given and only the growth of the tear has to be followed by measurement. The growth of the tear therefore seems to us to be well suited for detailed investigation.

Belt-Flexing Test of the Du Pont Type

Abstract In general, flexing tests are subject to large errors, some of which have been studied using a test of the du Pont belt type. An improvement in the standardization of the test conditions can be effected by using a suitably designed belt balance arm and regularly adjusting the length of the belt. The rate of cracking and also the reproducibility of the results is affected by the design of the test-piece and also by the type of fastener used. Although the error between the results on replicate test-pieces is extremely large, more important are the large errors due to lack of reproducibility of replicate molded slabs, and this variability probably arises during vulcanization. Extremely large differences can also occur between nominally identical repeat experiments. This may be due to differences in the judgment of the stages of cracking in different experiments. Graded photographs were found not to be of any value in improving the reproducibility of results between two different operators. Attention is drawn to the differences in types of cracking obtained with the du Pont and De Mattia test-pieces. It is pointed out that with the du Pont test-piece the cracking tends to develop in depth more than is the case with the De Mattia, and therefore it is more difficult to construct or use graded photographs with the du Pont test. Accepting the fact that variations in the disposition of the stages of cracking do occur between two different experiments, a method of treating flex-cracking results graphically is suggested. By this method random variations in results are ignored and the average flex-cracking results obtained. It is suggested that flexing properties may be defined by two characteristics, viz., (1) Crack initiation time, and (2) Crack propagation time, and an example of the method of determining these characteristics is given.

Mechanism of Rubber Cracking

Abstract Rubber and rubberlike polymers, in common with other materials which deteriorate on exposure to oxidizing or similar agents, eventually crack under normal atmospheric service conditions. The present paper confines itself to the factors affecting the type and appearance of the cracking which eventually occurs when a surface film of a rubber object (particularly when stressed) hardens and deteriorates in strength from any cause whatsoever. The importance of an understanding of the effects of exposure conditions on the character of the cracking produced cannot be too highly emphasized. This is so even if interest is confined solely to simple tests on new protective paints on an acceptance basis rather than research into surface deterioration. It is probable that many protective agencies have been accepted or rejected for extensive production tests because the initial laboratory tests have either not been representative of service or have been inadequate in scope. This still remains a major problem. The theory presented here has gradually taken shape over a period of some ten years and appears to describe adequately the general findings on the whole gamut of cracking, viz., fine check cracking and isolated deep cracks produced under service and laboratory conditions, the latter covering flexing, roof exposure, ozone and similar tests.

Determination of Brittle Point of Rubber Materials in Cold Air

Abstract The method described herein for determining brittle point is relatively simple, and the results are indicative of the ability of the material to withstand impact combined with bending. It is believed that this combination will more correctly evaluate brittle point than a bent loop test or other moderate flexing tests. If the material is exposed to low ambient air temperatures in service, then tests to determine brittle point should be made in cold air and not in an immersion bath. Appreciable differences in brittle point are obtained by judicious modifications in compounding ingredients of Buna-N and chloroprene synthetic rubbers.

The Standardization of Rubber Flexing Tests

Abstract The present paper offers a method for standardizing rubber breakdown tests which is an adaptation of discriminant function analysis recently developed by R. A. Fisher, and hitherto applied only to the treatment of biological data. The data which form the subject of the present study are rather inadequate, but it is worth while to use them to make the method available for more extensive investigations at a later date. It has, at least, revealed a gross lack of control in experimental technique, and shows that great improvements must be made on the practical side to give the new statistical method a chance to show its real sensitivity.

Laboratory Flexing Tests as an Aid in Investigating the Pneumatic Tire Carcass

Abstract TIRE tests, both service and test wheel methods, are generally used for carcass development, supplemented to a limited extent by laboratory tests, which include stress-strain tests on the compound and the cord, friction tests to measure the force necessary to pull the rubber compound from the cord, and pulley flexing tests. Owing to the cost and difficulty of reproduction of tire tests, it is desirable to use laboratory tests in so far as possible, although it will be impractical to eliminate tire tests as final checks before putting laboratory information into production and as a general method of attack for special problems in design. Among the important problems that lend themselves especially well to laboratory methods are: (1) Insulation of tire cord, including thickness of skim and spacing of the cord; and (2) rubber compounds (both before and after aging), including composition and state of cure. In choosing laboratory methods, it was decided to use flexing methods, with tension tests on the compound as an aid in judging state of cure. Friction tests were not made since they would be meaningless for indicating the bond between rubber and cotton when the cords are well separated in rubber.