Poisson's ratio

Poisson ratio is one of the important criteria to be considered in pipe selection where the system may be subjected to sub-atmospheric pressure (e.g. Siphonic drainage systems). In general, a lower value in Poisson ratio provides better strength against collapse of pipe due to sub-atmospheric pressure.

In evaluating the limiting or maximum pressures, forces and deflections on plastic pipe, fittings and components covered by this standard, the material properties listed in Table 8.1 are assumed at an operating temperature of 20°C (68°F).

Table 8.1:Material Properties at an Operating Temperature of 20°C (68°F)
Property/ Material	Tensile Modulus of Elasticity	Creep Modulus	Poisson’s Ratio	Rate of Thermal Expansion
Symbol	$E_t$	$E_c$	$\mu$	$C_{tx}$
Units	Mpa(psi)	Mpa(psi)	dimensionless	cm/cm/°C (ft/ft/°F)
ABS	2,206 (320,000)	Varies (refer to 8.2.2)	0.35	10.3 E-5 (5.7 E-5)
HPDE	862 (125,000)		0.45	18.0 E-5 (10.0 E-5)
PVC	2,827 (410,000)	0.35	5.2 E-5 (3.0 E-5)
Test Method	ASTM D638	ISO 899	ASTM E132	ASTM D696

Calculation of allowable pressure $(Pa)$: The minimum required pipe wall thickness $(t)$ shall be based on Equations 8.1 and 8.2 in Table 8.2 and the mechanical and dimensional properties of the pipe material evaluated. Equations 8.1 and 8.2 apply for a long cyclindrical tube of length L and wall thickness t under uniform external pressure with $(R/t) /> 10$.

Table 8.2 Pipe Wall Thickness Calculations
Equation 8.1 When, $L \ge 4.9R \sqrt{\frac{R}{t}}$	Equation 8.2 When $L \le 4.9R \sqrt{\frac{R}{t}}$
$P_{cr}= \frac{E_c}{3(1-\mu^2)} \Bigg[ \frac{t}{R} \Bigg]^3$	$P_{cr}= 0.807 \Bigg[ \frac{E_c t^2}{LR} \Bigg] \Bigg[ \Big( \frac{1}{1-\mu^2} \Big) \Big( \frac{t}{R} \Big)^2 \Bigg] ^{0.25}$
When, $L \ge 4.9R \sqrt{\frac{R}{t}}$	When, $L \le 4.9R \sqrt{\frac{R}{t}}$

The parameter $E_c$ is the elastic creep modulus at the specified time of loading. The allowable pressure $(Pa)$ shall be the calculated critical buckling pressure divided by the factor of safety (FS) as in equation below:

Equation 8.3:
$P_a = \frac{P_cr}{FS} \ge 14.7 psia (1.0 bar)$