question need help

Possible reasons

• Overpressure • Environmental effects • Rate of loading

• HEAT CHECKING  network of cracks with a depth of ~0.13 cm

S. Sopok, C. Rickard, S. Dunn, Thermal–chemical–mechanical gun bore erosion of an advanced artillery system part one: theories and mechanisms, Wear, Volume 258, Issues 1–4, January 2005, Pages 659-670

NOTE THAT PROPERTIES ARE AT 21C DIFFICULT TO FIND ELEVATED TEMPERATE VALUES

In general a minor reduction is expected in KIC with increasing temperature above BDTT but the exact temperature history here is a mess…

Comparison with other cannons

“Correlation” of fatigue life with KIC and Flaw size

Axial (if closed)

Hoop

Radial

Stresses

σθθ= 543 MPa (axial if closed 99 MPa, and radial -345 MPa)

(deviatoric 444, 0, -444 MPa) Equivalent stress 770 MPa

Assuming NO residual stresses.

Residual stresses (autofrettage)

plastic

elastic

Expand the internal diameter of the tube so that only part of the cross section is yielding plastically

Upon unloading the inner section is subjected to compressive residual stresses

Overstrain = % of section yielding

Impressive! In theory, the compressive residual stress in the inner diameter nullifies any tensile stress from the explosive. But is it true?

BUT WHAT ARE THE REAL RESIDUAL STRESSES

• The analysis does a good job for the intact barrel but NOT for a cannon in use for a long time – Thermal history (and associated chemical and

microstructural evolution) – Local damage and erosion

• Can we measure them?

Residual stresses

Residual stress measurement hole drilling ASTM Standard E837

Make a hole Monitor the change of diameters via strain gauges Back calculated the stresses from strains (not trivial)

Assuming that hole drilling does not affect the material locally

Similar in theory are the slit method and the curvature methods

Residual stress measurement X-ray

• Stress is an extrinsic property so it must be measured indirectly

Need modulus, Poisson

Similarly with neutron diffraction and to greater depth and accuracy (and cost)

Not from the same cannon but it indicates clearly that the true residual stress is much lower than the 60% of yield stress that the analysis shows. ASSUME ~200MPa compressive

K ~ 1.7 p sqrt(πα) ~ 99MPa m0.5

Does not take into account residual stresses

t=thickness c=semiaxis along cylinder length a=semiaxis along thickness

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St re

ss In

te ns

it y

Fa ct

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Pa sq

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Residual Stress (MPa)

Need to explore plain strain and small scale yielding