Cylinder Theory Derivation Andrew Ning We will find the stress in a - - PDF document

Cylinder Theory Derivation

Andrew Ning

We will find the stress in a cylinder by using a free-body diagram approach (Fig. 1). σr is called the radial stress, σθ is the tangential stress or hoop stress, and σz is the longitudinal or axial stress. There are no shear stresses in this coordinate system. Figure 1: Stress on a radial slice within the cylinder, and then on a small wedge within that slice. A force balance yields (σr + dσr)(r + dr)dθdz − σr(rdθdz) = 2σθ(drdz) sin(dθ/2) (1) Because we are looking at a differential element sin θ ≃ θ, and after simplification yields σrdr + dσr(r + dr) = σθdr (2) The product of differentiables drdσr can be considered negligible compared to the other terms. This gives σrdr + rdσr = σθdr (3) 1

which can be written as (σθ − σr) = rdσr dr (4) This same result can be found purely mathematically by starting with the compatibility equations for a linearly elastic structure (Eq. (5)), which is just a statement of static equilibrium. ∇ · σ = 0 (5) We will expand this equation in cylindrical coordinates ∂σr ∂r + 1 r ∂τrθ ∂θ + ∂τrz ∂z + 1 r(σr − σθ) = 0 (6) 1 r2 ∂(r2τrθ) ∂r + 1 r ∂σθ ∂θ + ∂τθz ∂z = 0 (7) 1 r ∂(rτrz) ∂r + 1 r ∂τθz ∂θ + ∂σz ∂z = 0 (8) We assume symmetry such that deformations are independent of θ and assume we are far from an end so that deformations are independent of z (thus, all partial derivatives with respect to θ and z drop out). Then, the second and third equation result in τrθ = C1 r2 (9) τrz = C2 r (10) Since the shear stress is zero at the free surface, the constants C1 and C2 must be zero, and thus both of these shear stresses are zero throughout the cylinder. The first equation then simplifes to give the the same result as Eq. (4). Just as we is done in deriving beam bending equations, we will assume that plane sections remain plane. This means that the longitudinal strain ǫz must be constant across the cross-section. We also assume that the longitudinal stress is constant (at least away from the walls). Then from the stress-strain relationships ǫz = 1 E [σz − ν(σr + σθ)] (11) we know that σr + σθ must also be a constant. Let us set that constant as 2A σr + σθ = 2A (12) If we sub σθ from Eq. (12) into Eq. (4) we have (2A − 2σr) = rdσr dr (13) We now multiply through by r to get 2rσr + r2 dσr dr − 2Ar = 0 (14) That was done so that we can pull out a differential d(r2σr − Ar2) = 0 (15) This implies that r2σr − Ar2 is a constant which we set to B. This gives σr = A + B r2 (16) 2

and substituting back into Eq. (12) we have σθ = A − B r2 (17) These equations are called the Lam´ e equations and are the basis for our equations on cylindrical stress. As we will see the longitudinal stress is also related σz = A (18) The boundary conditions of the problem will determine the unknown constants. We can solve for the radial and tangential stress in a pressurized thick cylinder by applying the boundary

• conditions. At σr(ro) = −po and σr(ri) = −pi. In both cases, the sign is negative because the pressure

causes compression. If we plug in our boundary conditions into Eq. (16) we get A + B r2

i

= −pi A + B r2

• = −po

(19) This gives us two equations to solve for the unknown constants. The result is A = pir2

i − por2

• r2
• − r2

i

B = (po − pi)r2

i r2

• r2
• − r2

i

(20) We now have the radial and tangential stress in a pressurized cylinder σr(r) = pir2

i − por2

• + r2

i r2

• (po − pi)/r2

r2

• − r2

i

(21) σθ(r) = pir2

i − por2

• − r2

i r2

• (po − pi)/r2

r2

• − r2

i

(22) The longitudinal stress can be found from a simple force balance (see Fig. 2): pir2

i − por2

• = σz(r2
• − r2

i )

⇒ σz = pir2

i − por2

• r2
• − r2

i

σz = A (23) Note that σz is halfway between σr and σθ. 3

Figure 2: Longitudinal stress balances by pressure acting on cylinder. 4