I.O. Smith

Linearly increasing stress test (LIST) for SCC research

This paper presents a new testing apparatus for stress corrosion research. In this linearly increasing stress test (LIST) method, plain un-notched specimens are simultaneously exposed to an environment and subjected to an applied stress increasing linearly at a controlled rate. Experiments are repeated at various applied stress rates and the parameters measured include SCC initiation stress (the stress at which stress corrosion cracks start to propagate), fracture stress and average crack velocity. The LIST test is similar to the constant extension rate test (CERT) test, with the essential difference that the LIST test is load-controlled whereas the CERT test is displacement-controlled. The LIST test is particularly aimed at service conditions of loaded parts and structures; that is, service conditions described as load-controlled. This paper describes the design, construction and operation of the test apparatus and some typical results are given to illustrate its performance. Experiments have been conducted using as-quenched 3.5Ni-Cr-Mo-V turbine rotor steel specimens in aerated distilled water at 90 degrees C at stress rates varying from 0.00034 MPa s-1 to 2.08 MPa s-1.

The role of crack tip strain rate in the stress corrosion cracking of high strength steels in water

Creep tests have been performed on fracture mechanics specimens of as-quenched 4340 and 3.5NiCrMoV rotor steel to confirm the importance of crack tip strain rate in causing stress corrosion cracking. By allowing creep in a noncracking environment, dry air for the high strength steels tested, cracking did not occur when water, the corrosive solution, was later added to the system. Thus, it is possible to inhibit stress corrosion in spite of conditions otherwise conducive to crack growth. Conditions necessary to restart cracking were also tested. The importance of this result in terms of the mechanism of stress corrosion and difficulties in measuring KISCC is discussed.

Metallographic aspects of surface damage, surface temperature and crack initiation in fretting fatigue

Fretting fatigue experiments were conducted on flat fretting junctions of 1040 steel subjected to axial cyclic zero-tension-zero-type loading. It was found that fracture often occurred in the central part of the fretted area and metallographic examination showed this was then associated with the high degree of mechanical damage of that zone. Caverns of martensite were found in the damaged region. The presence of the martensite indicates that temperature peaks of over 850 °C occur locally on the fretted surface. Fretting fatigue cracks were observed to originate from the martensitic structure.

Microstructural coarsening of the PbAg eutectic

Abstract The coarsening behaviour of PbAg eutectic alloys has been investigated under isothermal and temperature gradient environments. No evidence was found for fibre migration or fibre coalescence. Under both isothermal and temperature gradient conditions the rate of coarsening occurred with an activation energy equivalent to the sum of the activation energies for self-diffusion of lead and dissolution of silver in lead.

Stress drop experiments during creep of Zr-1 wt.% Nb Alloy

Abstract Strain transients following small stress decrements were investigated during creep of Zr-1 wt.% Nb alloy at temperatures in the range 295 – 773 K. These transients consisted of two regions of constant strain rate (stages I and II) separated by a sharp transition or breakaway. Delay times were observed under conditions of strong dynamic strain aging only. The breakaway transients are rationalized in terms of a recovery creep theory in which either of two separate recovery processes may be rate controlling. Cold work followed by recovery prior to testing resulted in the breakaway transients being replaced by a smooth curve. The creep rates in stages I and II of the transients correlated well with the effective stress and the stress dependence of these creep rates took on values of less than unity at low effective stresses.

A stress analysis of a shaft with a press-fitted hub subjected to cyclic axial loading

This paper presents a calculation of the stress distribution in a shaft with a press-fitted hub subjected to axial fretting fatigue. Both normal contact stresses and frictional shear stresses at the shaft-hub interface are included in the model. The solution of Airys function is obtained by means of Fourier integrals. The results are presented for different combinations of hub length, shaft radius and non-slip area and for various values of the coefficient of friction. Special consideration was given to the axial component of the stress, σz, as this is the most important stress component in the initiation and propagation of fatigue cracks. Finally, the paper deduces the implications arising from the stress analysis on the fretting fatigue of the junction studied.

Accelerated Atmospheric Corrosion Of Copper And Copper Alloys

The corrosion behaviour of a range of readily available commercial copper based alloys was examined in three accelerated atmospheric corrosion tests: acidified salt spray, neutral salt spray at 80°C, and the GM (salt spray with thermal cycling) test. The samples exposed to the GM test evinced very little corrosion. During exposure, the weight of all the samples fluctuated in a random fashion, indicating that the corrosion products were only partially adherent and became detached from the surface at random times. The sequence of corrosion product formation on the specimen surfaces during exposure to the neutral salt spray test was very similar to that observed on coppers exposed to the atmosphere, indicating that this test can be used to give accelerated indications concerning atmospheric corrosion in humid marine regions. Four 24 h cycles of salt spray exposure are approximately equivalent to 1year’s exposure to the atmosphere. Alloying copper with phosphorus, tin, zinc, or silicon does not improve atmospheric corrosion resistance. The alloy containing 30%Zn exhibited dezincijication. The nickel silvers exbibited corrosion rates in the neutral salt spray at 800C that were lower by a factor of at least 13 than those of the high purity coppers. For six of the alloys studied, the as received surface condition correlated with a slightly, but statistically significantly, higher corrosion rate, attributed to residual contaminants from rolling.

Instantaneous strain and creep transients in an Al-7.72at.%Mg alloy

Strain transients following stress decrements were investigated during the creep of an Al-7.72at.%Mg alloy at 300°C. After an instantaneous reverse strain the transients for small stress reductions consisted of two regions of constant strain rate (stages I and III) separated by a fairly short transition or breakaway region (stage II). The instantaneous reverse strain was always greater than that expected from purely elastic behaviour. The transients in stages I and III are compared with previous data for Zr-1wt.%Nb and copper and are rationalized in terms of a recovery creep theory in which either of two separate recovery processes may be rate controlling.

Thermally activated deformation in age-hardenable ferritic FeNiAlTi alloys

The early stages of plastic deformation in two age-hardenable Fe Ni Al Ti alloys have been investigated by means of the stress relaxation and incremental unloading techniques. The effective stress was found to be a similar function of ageing time to the 0.1% proof stress versus ageing time relationship. However, the magnitude of the change in effective stress was more than an order of magnitude lower. This change in effective stress with ageing and the plastic strain-rate versus effective stress relationship are explained in terms of double kink nucleation being the rate controlling process with the sideways expansion of the kink pairs being limited by the precipitate particles. The mobile dislocation density in the microstrain region appears to be a function of effective stress. When this is incorporated in the model, the activation area versus effective stress relationship is fully described and independent evaluations of activation area by single strain-rate change confirm this relationship.

Work-hardening rates during the high temperature creep of magnesium determined from the instantaneous strain on sudden stress changes

Work-hardening rates during the steady state creep of polycrystalline magnesium at 100, 200 and 300°C have been determined from the instantaneous strains on a sudden increase and a sudden decrease in the creep stress. The work-hardening rate h varied from 0.45 to 1.22 times the value for Youngs modulus and decreased with increasing creep stress or increasing temperature. Such work-hardening rates are of the same order of magnitude as those previously published for f.c.c., b.c.c. and h.c.p. metals and alloys. The stress exponent n of steady state creep of 3.70-4.65 indicates recovery (climb-controlled) creep. Climb-controlled creep in a dislocation substructure containing intracrystalline barriers, e.g. subgrain boundaries, is also indicated by the measured value of the stress exponent m of recovery of about 4.