Question

Cite five factors that lead to scatter in measured material properties. Upon what three criteria are factors of safety based? A large tower is to be supported by a series of steel wires. It is estimated the load on each wire will be 15,000 N. Determine the minimum required wire diameter, assuming a factor of safety = 2.0 and a yield strength of 800 MPa (125,000 psi) for the steel. Reflect by commenting on the meaning of your answer.

Answer 1

Three questions there, let me try to organize them.

a) Cite five factors that lead to scatter in measured material properties.

• Material inhomogeneities and/or compositional differences.
• Variation in specimen fabrication procedure
• Test method
• Apparatus calibration
• Operator bias

b) Upon what three criteria are factors of safety based?

• Accuracy of measurement of mechanical forces and/or material properties
• consequences of failure
• Previous experience

A large tower is to be supported by a series of steel wires. It is estimated the load on each wire will be 15,000 N. Determine the minimum equired wire diameter, assuming a factor of safety = 2.0 and a yield strength of 800 MPa (125,000 psi) for the steel. Reflect by commenting on the meaning of your answer.

`\sigma_w = (\sigma_y)/(N)`

Where,

`\sigma_w=`Working stress

`\sigma_y=`Allowable working unit stress

N= Factor of safety

Given the information we have,

`\sigma_w=(800)/(2)=400Mpa`

Diameter and force are related by,

`A_0=(F)/(\sigma_w) = \pi ((d_0)/(2))^2`

`d_0 = \sqrt{(4F)/(\pi \sigma_w)} = \sqrt{(4*15kN)/(\pi(400*10^6))}`

`d_0=4.77mm = 0.185in`