# Calculate the change in the arc

1. Dec 3, 2009

### Dell

in the following question,

E=65 GPa
V=0.3

find the new length of the arc BD??

i have found the stresses

xx=-56Mpa
yy=0
xy=-28Mpa

using hookes law i can find the strains

xx=-8.615e-5
yy=2.58e-4
0.5*xy==-1.12e-3

but how do i calculate the change in the arc using this? i would know how to solve this if i had some kind of angular strain- i need to use a polar system not Cartesian. is there any way to do this?

also how do i know the new angle DAB? i know that the XY axis' new angle is 90.06417, and the n,t system (axes tilted 45 degrees to XY) is also 90.06417 but how do i find DAB,? generally is there any way of knowing how the axis is strained, for example, has the X axis dropped 0.06417 degrees, or the Y axis opened up 0.06417 degrees, or a bit each??
in this specific case can i say that since there is no yy strain the x axis stays at the same angle?

DA*=DA(1+tt)=4.999569cm
AB*=5.0012923cm

can i do this:

using the transformation equations, i know

εnn= (εxx + εyy)/2 + (εxx - εyy)/2*cos(2ϴ) + εxxsin(2ϴ)

since i have already found xx, yy, xy, instead of looking for a specific εnn can i take the whole eqaution and say

ΔL=$$\int$$εnndL {dL=r*dϴ}

=$$\int$$εnn*r*dϴ with my integral going from 0 to pi/4

is this a possibility?

the correct answer is meant to be 2.2167e-5m

how do i know whhat my lims are for integratin, i thought maybe pi/2 -> 3pi/4 but cant get it

Last edited: Dec 3, 2009
2. Dec 5, 2009

### Dell

i tried the following logic,
since in this special specific case, i have found that $$\epsilon$$xx = $$\epsilon$$AD, i know that the radiiii will stay rhe same lengths as each other after deformation therefore preserving the circular shape of the arc

knowing that the volume of the shape with an area of an eighth of a circle (DAB) before deformation is V and after deformation is V'
lets say the thickness of the board is "t"

the new angle DAB is " a' " after deformation

$$\Delta$$=($$\epsilon$$xx +$$\epsilon$$yy + $$\epsilon$$zz)

V=(pi*R2)t/8

V'= (a')(R')2(t')/2

but i also know that

V'=$$\Delta$$*(1+V)
R'=R(1+$$\epsilon$$xx)
t'=t(1+$$\epsilon$$zz)

(a')(R')2(t')=(pi*R2)t/8*(1+$$\epsilon$$xx +$$\epsilon$$yy + $$\epsilon$$zz)

(a')(R(1+$$\epsilon$$xx))2(t(1+$$\epsilon$$zz))=(pi*R2)t/8*(1+$$\epsilon$$xx +$$\epsilon$$yy + $$\epsilon$$zz)

therfore i get

(a')= (pi/4)*(1+$$\epsilon$$xx +$$\epsilon$$yy + $$\epsilon$$zz)/[(1+$$\epsilon$$xx))2((1+$$\epsilon$$zz))]

once i have the new angle, since it is still an arc of a circle

L'=R'*a'
L=R*pi/4

$$\delta$$L=L'-L

but this gives me an incorrect answer

is this a correct method and do i maybe have something wrong in y calculations

i get $$\delta$$L=1.016463761198405e-005