Journal of Research of the National Bureau of Standards Vol. 49, No.6, December 1952 Research Paper 2376 Stiffness of Paper F. T. Carson and Vernon Worthington An instrument to determine the stiffness of paper is described. The spccimen is bent through a given angle, and the bending moment is measured as the torque in two lengths of piano wue, between which the clamp that holds the specimen is suspended. Stiffness of paper is defi ned as the bendmg moment per unit of width of sp ecimen producing unit curvature. This quantity was evaluated for eight types of paper, for various lengths a nd widths of specimen, and for bendi ng angles from 5° to 30°. The stiffness val ue for a given specimen remamed substantially constant when the width and bending angle were varied, but increased somewhat as the length of the speClmen was increased. 1. Introduction unLil a certain minimum angle of fo ld (angle between the tangents at the two ends of the specimen) was Yarious me thocls and manv devices have been attained, the minimum angle being proportional to proposed for the measurement of the stiffness of the thiclmess of the material tested. The fiexural paper, textiles, and highly fiexible materials, and work was then evaluated from the torque and angle stiffness has been defined in various ways. Most readings. Nfeasurements were also made on paper. paper-stiffness tester now available measure stiffness The Institutc of Paper Chemistry, i.n a di scussion in an arbitrary and relative manner, evaluating such preliminary to the study of certain commercial paper quantities as the force required to bend a strip of stiffness tes ters, defuled some fl exural properties paper through a certain angle or to deflect the end related to stiffness [4]. Flexural rigidity was de­ a given amount. Some determine the length on Hned as the bending moment required to produce whieh a given forcc must act to produce a given unit curvature of bend, and rigidity was defined as effect. Stiffness of paper is therefore usually defined the fiex ural rigidity pel' unit width, or the bending implicitly as some force, moment, angle, or length moment per unit width required to produce unit that will produce a given effect on a specimen of curvature of bend. given dimensions strained in a specified manner . A Clark [1], in formulatinO' expressions for data review of various types of stiffness testers especially obtained with his apparatus, in which the specimen designed for paper testing is contained in an article is disposed as a cantilever bending under its own by Clark [1] . weight through large angle, defined stiffness as the Stiffness is sometimes defined according to the cube of the critical length multiplied by an arbitrary function the strained member is expected to perform. constant. Stiffness appears to be proportional to In engineering mechanics the stiffness of a beam is the modulus of elasticity and the moment of inertia, measured by the load that it can carry with a given and inversely proportional to the weigh t per unit defiection. In this sense the stiffness depends on area [4] . how the beam is supported and how it is loaded. Sharman [5] measured the stiffness of paper with On the other hand, various fiexural properties may a pendulum damped by a flexing paper ring, an.d be specifically defined so as to be more or less inde­ defined stiffness as the bending moment pCI' umt pendent of dimensions and conditions imposed on width that produces unit curvature. H e pointed the stressed mcmber. out, however, that the modulus of elasticity has In the fi eld of textiles Peirce [2] has discussed in different values in various directions in the sheet of some detail the problems and theoretical difficulties paper, and that it is necessary to evaluate a stiff­ of formulating the stiffness and related properties of ness for machine direction, and a stiffness for fabrics. H e measured the stiffness of textile fabrics by suspending specimens as cantilevers bending under cross direction. their own weight and expressed the results as (a) an Abbott [6] measured the stiffness of fabrics, which effective bending length, equal to the length of the had been rated subj ectively by experts, by means specimen multiplied by a somewhat complex function of several methods and found that measurements of the bending angle, (b) a quantity called flexural made with an apparatus similar to that of Peirce [2] rigidity, defined as the bending moment for unit and expressed as fiexural rigidity, agreed best with curvature pel' unit width of the material, and (c) the subjective ranl;:ing. bending modulus, or intrinsic stiffness of the material, H ebeler and coworkers [7] devised a "Hexometer" proportional to "fl exural rigidity" and inversely with which a torque was applied to a cantilever proportional to the cube of the thickness. specimen by means of a rotating clamp, and. ~he Schiefer [3], with his Flexometer, evaluated fiexural reaction force was measured by means of a senSItIve work, a property closely related to stiffness, and also electronic strain gage. Stiffness was expressed as other fiexural properties of textile fabrics. By means the elastic modulus (bending modulus) calculated by of calibrated springs, a measure was obtained of the means of the conventional equation for a cantilever torque necessary to fold the specimen back on itself beam loaded at the end. 385 ----- ----- e FIGU R E 2. Stiffness tester . FIGURE 1. Schematic drawing to illustrate the principle of the stiffness tester. s 2. Testing Instrument , The principle of the apparatus described herein , was outlined some years ago by the authors [8] in , conn.ection with an attempt to use stiffness as a , , m~asure of artificial wear of currency paper as a FIGURE 3. Diagram showing the bent specimen held at each result of repeated crumpling. A test was required end in a rotatable clamp, the torque and reaction axes, and that would reflect the continuous deterioration of the bending angle, O. the paper resulting from the crumpling treatment. However, tests madlil with a crude, but adequately (about ~~ mm in diameter), the Quter ends of the sensitive, model showed that the stiffness did not wires being fixed at points A and B to a pivoted decrease continuously, but actually increased during frame, F . The wires are fastened to the clamp in the early crumpling treatment, because of the cor­ line with the clamping edge. One end of the paper rugating effect that increased the effective thickness specimen, S, is fastened in the torque clamp, C, and the moment of inertia in bending. This idea whereas the other end is held by a similar clamp, K. for the evaluation of the stiffness of paper, not In operation, a bending torque is transmitted through being applicable to the problem, was laid aside for the clamp, C, while clamp, K, applies the reaction. a time, but more recently was reviewed and further The latter clamp is also mounted with its axis developed. through the front edge. This axis corresponds to Figure 1 is a schematic drawing intended to illus­ the free end of a cantilever and must be perfectly trate the principle of operation, figure 2 shows the free to turn, or to move in the plane of the two finished apparatus, and figure 3 shows the bent speci­ axes, so as to allow the specimen to bend freely and men in relation to the clamps. A clamp, C, is naturally, being restrained only from displacement suspended between two lengths of piano wire, W, about the axis WW. The pivots of clamp, K , can 386 1- move freely in slots in the supporting bracket; link counterbalanced with paper held in clamp C until it L supports the clamp. returns to zero. A small weight is fastened on the The torque is applied by rotating frame F , to pointer at a known distance from the piano-wire which the outer ends of the piano wires are attached. axis. This, of course, causes the pointer to move The torque, transmitted through clamp C, bends the downward. The instrument is then rotated around paper through an angle 8 (fig . 3), shown by the posi­ the piano-wire axis until the pointer is again hori­ tion of the pointer P , which is integral with clamp C, zontal. The moment increment (added weight in on the fixed scale, D (fig. 1). Simultaneously, the grams times lever arm in centimeters) divided by the torque is measured by the position of the pointer angle read opposite the pointer on the torque scale on the torque scale, T , attached to the pivoted gives the calibration constant in gram-centimeters frame, F, that is, by the angular displacemen t of per degr ee. This procedure is repeated with different clamp C with respect to the torque frame, F . added weights and lever arms, and on both sides of The apparatus owes its sensiti vity in considerable zero over an interval on the scale representing the degree to the design that eliminates the need of range of readings usually encountered. The average bearings in tho torque measuring device. Further­ value of the calibration constant found in this way more, because the bending axis is vertical, the meas­ was 0.092 g-cm/deg. urement is uncomplicated by a gravity component. The torque angle, read on scale T , multiplied by To increase the range and usefulness of the instru­ 0.092 gives the bending momen t in gram-centimeters ment, a m eans: is provided for varying the length of for a specinlen of a chosen width and length (distance the specimen, that is, of changin g the distance in centimeters between clamps) and for the angle 8 between the two clamps.