THE INDIA RUBBER WORLD 



TESTING MACHINES." 



A SURVEY of the types of testing machines used in various 

 countries for testing textile materials divides them into two 

 distinct classes: (1) constant increment of load; (2) constant 

 increment of stretch. 



The constant-increnient-of-Ioad machines apply the load by 

 uniformly increasing the tension in very small increments. There 

 are a few isolated cases in which the load is increased by appre- 

 ciable amounts at stated intervals of time. 



The constant-increment-of-stretch machines apply the load by 

 stretching the material at a uniform rate, and because of their 

 simplicity and rapidity of operation have found much favor with 

 textile men both in this country and abroad. 



The progress of testing-machine development in England, par- 

 ticularly during the war, is rather interesting. For a long time 

 the two types of machines were in general use, but the constant- 

 increment-of-stretch machine was the one most used. .'Kt the be- 

 ginning of the war the problem of studying aircraft fabrics was 

 given to the National Physical Laboratory, which has much the 

 same relation to English industry as the Bureau of Standards 

 has to American industry. In the study of such fabrics, it is ex- 

 tremely desirable that a machine be used that will give tests 

 which do not involve machine characteristics. In this connection 

 the Avery testing machine has been developed and the English 

 Government insists that all aeronautical fabrics be tested on that 

 particular machine. 



The Avery machine is illustrated in Fig. 1. Its principle is 

 not new, but the mechanical design is quite an improvement 

 upon the Goodhand and Smith machines of this type. It con- 

 sists briefly of a compound lever, 

 having at one end of the system 

 a shot container so arranged to 

 allow of a uniform rate of in- 

 flow and at the other end a 

 fabric clamp. The shot con- 

 tainer is allowed to be uniformly 

 increased in weight and the bal- 

 ance beam kept at zero by taking 

 up the stretch of the sample. 

 Provision is made to cut ofif the 

 supply of shot at the time of 

 breaking of sample. A spring 

 balance interposed between the 

 shot container and balance arm 

 allows the increase of weight of 

 the container to be quickly ob- 

 served. 



It has been observed that 

 within certain limits a change in 

 the rate of loading produces prac- 

 Fic. l.-AvERY Fabric-Testing ''"^^''^ "° '^'^^"S^ '" '^^ ^^^''' 



Machine. 



A, Lower Grip; B, Upper Grip; 

 Hand Wheel; D, Reservoir; 

 Can; F, Steelyard; G, Steely 

 Pivot; H, Lever; I. Link; 

 Switching Mechanism; L, Spr 

 Balance; M, Checking Can; 

 Sliding Poise; O, Receptacle; 

 Screw; R, Vertical Scale. 



ical properties of the material. 

 C The Avery machine is operated 

 E. in accordance with specifications, 

 J, within these limits and as a re- 

 N^ suit the rate of loading varies 

 P' with the kind of material being 

 tested. 



The development and use of this machine show a very care- 

 ful study and consideration of the tensile properties of the ma- 

 terial to be tested. The machine is theoretically very good, but 

 the personal equation and time of operation is so large that its 

 practical utility is questioned. 



If the constant-increment-of-stretch machine had no machine 

 characteristics, the results of tests on this type of machine would 



correspond with those obtained from a conslant-incrcment-of- 

 load machine, provided both were operated between the limits that 

 deline the relations of stretch, load and time, within which a 

 change in the rale of load application produces only a slight 

 change in properties. 



The constant-increment-of-stretch machines may be classified 

 according to the method of recording the load transmitted by the 

 fabric: (1) inclination balance; (2) elastic system (such as a 

 spring). 



The rigidity of construction, ease of operation and so-called 

 "dead weight" feature of the inclination balance type of head — 

 together with the fact that springs were not, in the past, con- 

 structed to give constancy of operation — 'has caused the first 

 type to be used universally in this country and in Germany. This 

 type is illustrated in Fig. 2. 



The theory and caUbration of an inclination balance machine 

 assumes a null method of weighing, that is, the balance or poise 

 arm is at rest. This condition is entirely changed during the 

 test by the inertia of a moving 

 balance arm. The calibration is, 

 therefore, not applicable to the 

 machine as used. The error may 

 be expressed as a function of the 

 mass of the moving body multi- 

 plied by the acceleration. Ob- 

 viously the acceleration changes 

 with each diiTerent kind of mate- 

 rial, shape and size of specimen, 

 and speed of operation, and the 

 total inertia component changes 

 with the design and capacity of 

 the machine. This is particularly 

 emphasized in testing the same 

 fabric on machines of different ca- 

 pacities ; it is an established fact 

 that results are different. From 

 this it is readily seen that the re- 

 sults of tests of different fabrics 

 on machines of the same capacity 

 are influenced materially by the 

 machine characteristics. All tests 

 made on such machines include 

 machine characteristics which vary with the variables of the test 

 specimen, including nature of material, dimensions of test speci- 

 men, and rate of testing. 



The use of this machine to check up deliveries is quite rea- 

 sonable, for it is assumed that the machine characteristics are 

 included in the specifications automatically and are constant for 

 any one fabric and any one size and type of testing machine. 

 Specifications based on tests made on this type of machine should 

 specify the size, type and make of machine and rate of operation, 

 as well as the specimen dimensions. 



For investigational work, however, such as the comparison of 

 the properties of different fabrics and the determination of the 

 effect of varying the size of sample, it is clear that the results 

 are very misleading if they include machine characteristics. The 

 errors introduced become infinitesimally small as the speed of the 

 moving arm approaches zero as a limit. 



The use of an elastic system for recording the load transmitted' 

 eliminates the variables introduced by the inclination balance, 

 provided the mechanical design is good, and it has the advantage 

 of rapid and positive operation. Unfortunately, very little atten- 

 tion has been paid to the design of such machines and the vari- 

 ance introduced has caused them to be much in disfavor. 



As the situation presents itself, there are two courses open for 

 improvement and standardization of machines for testing textile 

 materials : 



Fig. 2.— Schopper Fabric- 

 Testing Machines, Inclin- 

 ation Balance Type. 



A, Lower Grip; B. Upper Grip; 

 C, Inclination Balance: D, 

 Cvlinoer; E, Ratchet Scale; 



