COMPARISON OF TWO SPECIMEN SHAPES FOR SHORT COLUMN TEST OF CORRUGATED FIBERBOARD

Transcription

1 U.S DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY MADISON, WIS. In Cooperation with the University of Wisconsin U. S. FOREST SERVICE RESEARCH NOTE FPL-0109 OCTOBER 1965 COMPARISON OF TWO SPECIMEN SHAPES FOR SHORT COLUMN TEST OF CORRUGATED FIBERBOARD

2 COMPARISON OF TWO SPECIMEN SHAPES FOR SHORT COLUMN TEST OF CORRUGATED FIBERBOARD By J. W. KONING, JR, Technologist 1 Forest Products Laboratory, Forest Service U. S. Department of Agriculture --- Introduction A recent article by McKee and other 2 indicated the significance of the edgewise compressive strength of corrugated fiberboard and its relationship to box compressive strength. McKee developed a formula for predicting the top-tobottom compressive strength of corrugated fiberboard boxes. It is based on (1) board thickness; (2) box perimeter: and (3) the edgewise compressive strength of the combined board, as determined from testing a rectangular short column specimen having wax-reinforced loading edges. Research conducted at the Forest Products Laboratory indicated that 3 the edgewise compressive strength of the combined board could be determined using a neckeddown short column specimen. The purpose of this study was to make a comparative evaluation of the two types of short column specimen shapes to determine if there was a significant difference between them. 1 Maintained at Madison, Wis., In cooperation with the University of Wisconsin. 2 McKee, R. C., Gander, J. W., and Wachuta, J. R. Compression Strength Formula for Corrugated Boxes. Paperboard Packaging, Vol. 48, No. 8, pp Koning, J.W., Jr. A Short Column Crush Test of Corrugated Fiberboard. TAPPl Vol. 47, No. 3, pp FPL-0109

3 Method of Investigation Corrugated fiberboards of A-, B-, and C-flute single-wall, A-B and C-B flute double-wall, and one C-A-A flute triple-wall were used in this evaluation. The specific grades and flute combinations of these boards are given in table 1. The specimens were prepared and tested according to the following outline: 1. Scope. 1.1 This method is for determining the resistance to compressive forces acting in the plane and parallel to the flutes of flat single-wall, double-wall, or triple-wall corrugated fiberboard. 2. Definitions. None. 3. Apparatus. 3.1 Testing machine. The apparatus shall. consist essentially of a compression testing machine having: (1) An upper and Power platen, one of which may be fixed and the other movable. The surfaces of the platens shall remain smooth, flat, and parallel to each other within one part in 2,000 throughout the test. The platens shall be so mounted to have not more than inch lateral movementm (2) Means for measuring and indicating the applied load within 1 pound. (3) A capacity of not less than 500 pounds, and the indicating mechanism should be such that it can be accurately checked with dead weight loads, load cell, or proving ring Guide blocks. Two metal blocks--1 inch high, 2 inches long, and 8/10 inch. wide--are used, to vertically aline the specimen in the testing machine. The blocks shall have a 1/l6-inch-deep and 3/8-inch-wide notch cut out of the 8/10-inch face. The 8/10- by 5/8-inch face shall be finished perpendicular to the 8/1.0- by 1-15/16-inch face which shall also be a finished surface (fig. 1). 4 Details for the guide blocks and the procedure for waxing the loading edges of the rectangular specimens are based on the research and suggestions of The instilute of Paper Chemistry. FPL

4 Figure 1--Narrow guide blocks. NOTE: Though the guide blocks described in section 3.2 were used for testing both the necked-down and rectangular specimens the following guide blocks are suggested for use with just the rectangular shape: Two metal blocks--7/8 inch high, 1-1/2 inches long, and 1-3/4 inches wide--are used to vertically aline the specimen in the testing machine. The blocks shall have a 1/16-inch-deep and 3/8-inch-wide notch cut out of the 1-3/4-inch face. The 1-3/4- by 1/2-inch face shall be finished perpendicular to the 1-3/4- by 1-7/16-inch face which shall also be a finished surface (fig. 2). Figure 2.--Wide guide blocks. FPL

5 4. Specimens. 4.1 Unless otherwise stated, the method of sampling; shall be random. 4.2 At least 10 specimens shall be tested from each sample. 4.3 Specimen shape A.--Necked-down(fig. 3) The finished specimens shall be 1.50 ±0.06 inches high, 2 inches wide, and necked-down to 1 ±0.03 inch. The flates shall be parallel to the 1.5-inch dimension. First cut the bearing surfaces of the specimen parallel to each other and perpendicular to the axis of the flutes. Then the circles shall be out to 0.5-inch radius to form the necked-dow portion of the specimen. The last cuts shall be made 2 inches apart to complete the specimen. The specimen shall be cut with a sharp, no-set, hollow ground, table saw blade. The necked portion shall be cut using a drill press equipped with an adjustable circle cutter. The cutting tool shall be made from a piece of tool steel 3/16 inch square and 2 inches long. The circle cutter and details of the cutting tool are shown in fig Specimen shape B.--Rectangular (fig. 5) The finished rectangular specimens shall be 1.25 ±0.06 inches high, 2 ±0.03 inches wide, with the loading edges reinforced with parrafin. The flutes shall be parallel to the 1.25-inch dimensions. The specimen shall be cut with a sharp, no-set, hollow ground, table saw blade. The hearing surfaces of the specimen, shall be parallel to each other and perpendicalar to the axis of the flutes. Each loading edge shall be dipped in a pan containing molten paraffin (melting point 125 F., approximately) to a depth of 1/4 inch until the wax visibly begins to migrate beyond the 1/4 inch dip zone--usually about 3 seconds. (Hotplate temperature--170 to 180 F.; liquid wax temperature--156 to 166 F.) The wax should impregnate the specimen but not migrate appreciably beyond the 1/4-inch dip zone. (If excessively fast migration is encountered, reduce the temperature of the wax.) After dipping, each edge shall be blotted momentarily on paper toweling which rests on a hotplate maintained at 170 to 180 F. to remove excess paraffin from the loading edges. 4 FPL

6 Figure 4.--Circle cutter. Figure 5.--Rectangular specimen shape. FPL

7 NOTE: An alternative to dipping is to touch the specimen edge against a paraffin-saturated pad, such as paper toweling, which rests on a hotplate maintained at 170 to 180 F. The paraffin migrates from the pad into the specimen. Although this method of impregnation generally requires a longer time than dipping, it offers better control of the depth of the zone and is helpful with specimens in which migration proceeds very rapidly. 5. Conditioning. 5.1 The specimens shall be conditioned and tested as follows: (a) Precondition in air at 80 ±3.5 F. and less than 35 percent relative humidity for at least 24 hours. (b) Condition not less than 48 hours and test in air at 73 ±3.5 F. and 50 ±2 percent relative humidity. 6. Procedure. 6.1 Measure the width of each specimen to the nearest 0.01 inch Center the specimen on the bottom platen of the testing machine with the guide blocks on each side to hold the specimen with its flutes perpendicular to the platen. The guide blocks are recessed to allow for the additional thickness of the waxed edge. 6.2 Apply compressive force to the specimen by driving the movable platen at inch per minute when testing the necked-down specimens and at inch per minute when testing the rectangular specimens The guide blocks shall be removed when 10 pounds of load are applied to the specimen. 6.3 Observe and record the maximum load to cause failure of the specimen. 7. Report. 7.1 The type of testing machine, machine speed, maximum load in pounds per inch of width required to crush the specimen, the specimen dimensions, and the identity of the specimen shall be reported for each specimen tested. Any departure from the method described above shall be noted. FPL

8 Statistical Analysis Using the following statistical method, as suggested by Natrella 5, each corrugated fiberboard was treated as paired observations for determining if the average maximum loads attained by each specimen shape differed significantly. First the significance level of the test was chosen as 5 percent. Then X d for each pair of observations, X d, and s d were computed using the following (1) Where X 1 is the compressive strength of the rectangular specimen shapes and X 2 is the compressive strength of the necked-down specimen shapes. (2) Where X d is determined from formula (1) and pis the number of paired observations. (3) Knowing s d a value of u was determined using the following formula: 5 Where t 1-a/2 = t.975 = for 29 degrees of freedom (Source: Table A-4 ). Thus if the absolute value of X d is greater than u, then the averages differ; if the absolute value of X d is less than u, then there is no reason to believe they differ. (4) 5 NatreIla, M. G. Experimental Statistics. National Bureau of Standards Handbook FPL

9 Results Obtained The average maximum loads, for both specimen shapes, are given in table 1 and plotted in figure 6. Based on the statistical analysis, the absolute value of X d was less than u, thus the average maximum loads as determined by each method, did not differ significantly at the 5 percent level. The average coefficient of variation for the rectangular-shaped specimen was 5.9 percent and for the necked-down shape 5.8 percent. The corrugated fiberboards failed in the area between the wax-reinforced loading edges for the rectangular shape and in the necked-down area of the other. However, some flaring of the facings was noted at the loading edges of the rectangular specimens. One drawback to the rectangular specimen is the problem of determining its moisture content. This is due to the presence of wax, which is affected by elevated temperatures when drying the specimens. This problem, however, may be eliminated by submitting samples of the same material without waxed edges to the same conditions and for the same length of time as the test specimens. Considering the time and equipment required to prepare each specimen shape, the 2.0-inch-wide inch-high type with the wax-reinforced loading edges appears to be the most desirable. FPL

10 Table 1.--Edgewise compressive strength of single-, double-, and triplewall corrugated fiberboard as determined from rectangular and necked-down specimens 1 Number refers to nominal bursting strength and letters refer to flute type. 2 Xd = difference between average maximum load attained by rectangular specimen and necked-down specimen. Used in the statistical analysis. 3 C-flute consisted of two nominal 26-pound corrugated mediums laminated together. 4 C-flute was nominal 33-pound basis weight. 5 1,100 refers to puncture test value. FPL

11 Figure 6.--Edgewise compressive strength of corrugated fiberboard as determined by necked-down and rectangular specimens. FPL

12 PUBLICATION LISTS ISSUED BY THE FOREST PRODUCTS LABORATORY The following lists of publications deal with investigative projects of the Forest Products Laboratory or relate to special. interest groups and are available upon request: Architects, Builders, Engineers, and Retail Lumbermen Box, Crate, and Packaging Data Chemistry of Wood Drying of Wood Fire Protection Fungus and Insect Defects in Forest Products Furniture Manufacturers, Woodworkers, and Teachers of Woodshop Practice Growth, Structure, and Identification of Wood Logging, Milling, and Utilization of Timber Products Mechanical Properties of Timber Structural Sandwich, Plastic Laminates, and Wood-Base Components Thermal Properties of Wood Wood Fiber Products Wood Finishing Subjects Glue and Plywood Wood Preservation Note: Since Forest Products Laboratory publications are so varied in subject matter, no single catalog of titles is issued. Instead, a listing is made for each area of Laboratory research. Twice a year, January 1 and July 1, a list is compiled showing new reports for the previous 6 months. This is the only item sent regularly to the Laboratory s mailing roster, and it serves to keep current the various subject matter listings. Names may be added to the mailing roster upon request.