SPEED OF TESTING MACHINE

In instructions for making static tests the rate of application of the stress, i.e., the speed of the machine, is given because the strength of wood varies with the speed at which the fibres are strained. The speed of the crosshead of the testing machine is practically never constant, due to mechanical defects of the apparatus and variations in the speed of the motor, but so long as it does not exceed 25 per cent the results will not be appreciably affected. In fact, a change in speed of 50 per cent will not cause the strength of the wood to vary more than 2 per cent.[⁵⁸]

Following are the formulæ used in determining the speed of the movable head of the machine in inches per minute (n):

(1)	For endwise compression	n	=	Z l
				Z l²
(2)	For beams (centre loading)	n	=	------
				6h
				Z l²
(3)	For beams (third-point loading)	n	=	------
				5.4 h

		Z	=	rate of fibre strain per inch of fibre length.
		l	=	span of beam or length of compression specimen.
		h	=	height of beam.

The values commonly used for Z are as follows:
Bending large beams	Z	=	0.0007
Bending small beams	Z	=	0.0015
Endwise compression-large specimens	Z	=	0.0015
Endwise compression-small specimens	Z	=	0.003
Right-angled compression-large specimens	Z	=	0.007
Right-angled compression-small specimens	Z	=	0.015
Shearing parallel to the grain	Z	=	0.015

Example: At what speed should the crosshead move to give the required rate of fibre strain in testing a small beam 2" × 2" × 30". (Span = 28".) Substituting these values in equation (2) above:

		(0.0015 × 28²)
n	=	-----------------	=	0.1 inch per minute.
		(6 × 2)

In order that tests may be intelligently compared, it is important that account be taken of the speed at which the stress was applied. In determining the basis for a ratio between time and strength the rate of strain, which is controllable, and not the ratio of stress, which is circumstantial, should be used. In other words, the rate at which the movable head of the testing machine descends and not the rate of increase in the load is to be regulated. This ratio, to which the name speed-strength modulus has been given, may be expressed as a coefficient which, if multiplied into any proportional change in speed, will give the proportional change in strength. This ratio is derived from empirical curves. ([See Table XVII].)

TABLE XVII
SPEED-STRENGTH MODULI AND RELATIVE INCREASE IN STRENGTH AT RATES OF FIBRE STRAIN INCREASING IN GEOMETRICAL RATIO. (Tiemann, loc. cit.)
(Values in parentheses are approximate)
Rate of fibre strain. Ten-thousandths inch per minute per inch		2/3			2			6			18			54			162			486
COMPRESSION	Speed of crosshead. Inches per minute	0.000383			0.00115			0.00345			0.0103			0.0310			0.0931			.279
	Specimens	Wet	Dry	All	Wet	Dry	All	Wet	Dry	All	Wet	Dry	All	Wet	Dry	All	Wet	Dry	All	Wet	Dry	All
	Relative crushing strength				100.0	100.0	100.0	103.4	100.8	101.5	107.5	102.7	103.8	113.9	105.5	107.9	121.3	108.3	116.4	128.8	110.0	118.9
	Speed-strength modulus, T				0.017	(0.006)	(0.009)	0.033	0.012	0.016	0.047	0.021	0.029	0.053	0.027	0.039	0.060	0.023	0.049	(0.052)	(0.015)	(0.040)
BENDING	Speed of crosshead. Inches per minute	0.0072			0.0216			0.0648			0.194			0.583			1.75			5.25
	Specimens	Wet	Dry	All	Wet	Dry	All	Wet	Dry	All	Wet	Dry	All	Wet	Dry	All	Wet	Dry	All	Wet	Dry	All
	Relative crushing strength	97.4	99.0	98.2	100.0	100.0	100.0	105.1	102.1	103.7	111.3	105.8	108.1	117.9	108.6	112.7	123.7	109.6	116.3	126.3	110.3	118.9
	Speed-strength modulus, T	(0.014)	(0.005)	0.012	0.033	0.014	0.026	0.049	0.026	0.037	0.053	0.033	0.038	0.049	0.014	0.035	0.038	0.006	0.025	(0.023)	(0.004)	(0.014)
NOTE.—The usual speeds of testing at the U.S. Forest Service laboratory are at rates of fibre strain of 15 and 10 ten-thousandths in. per min. per in. for compression and bending respectively.