The influence of residual stress

Suppose that a residual equi-biaxial membrane stress (Tm is present in the film prior to application of the pressure p. The buckling stress for this configuration is CTb = —1.22 5/a, and it is assumed from the outset that (Tin (Tb so that the undeflected configuration of the film is stable against buckling deformation. The purpose of this section is to examine the influence of (Tm on the transverse deflection of the film once the pressure p is applied. 

For p/pr 1, the linear relationship (5.81) between wo and p is unchanged because, in this regime of behavior, membrane and bending effects are not coupled to each other. For p/pr 1, the membrane analysis leading to (5.87) is unchanged by the presence of the residual stress except that the compatibility equation (5.85) must be replaced by 

it appears that the role of the residual stress (Tm for large p/pr is to stiffen or soften the response of the bulging film to applied pressure compared to its behavior in the absence of residual stress, depending on whether (Tm 0 or (Tm 0, respectively. 

The results are insensitive to the aspect ratio a/hi as long as a/hi 20. A study of the sensitivity of the response of a pressurized bulge to initial stress and other system parameters was reported by Small and Nix (1992). 

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Hsu Y-J, Chang Y-H, Chai Y-L, and Chen G-J, The influence of residual stress and membrane thickness on the performance of Fe—Ni—Cr = Invar bimetal freestanding membrane for microdevices. Sens Actuators A 2002 101 160. 

Most stress corrosion service failures occur from the influence of residual stresses. These arise during component manufacture, and plant assembly, including welding. In laboratory work the stress is usually applied externally since it is then much easier to control and measure. The oldest and simplest test is to measure how long a specimen takes to break — the time-to-failure, tf. Typical specimens might be loaded in tension and surrounded by the corrodent or bent into a U, clamped and then immersed. Typically, the value t varies as shown in Figure 1. Two points can be made. First, the tf changes less markedly at stresses above the 0.1X proof stress than below it. Secondly, it is not always clear whether a threshold stress exists below which stress corrosion failures will not occur. This is of obvious importcuice but it has proved a... 

A. Okamoto, H. Nakamura, The influence of residual stress on fetigue cracking, J. Press. Vessel Tech-nol. ASME 112 (1990) 199-203. 

Hardness values, indentation moduli, strain hardening exponents and viscoelastic properties can be measured with the instrumented indentation test, also the fracture toughness of very brittle polymers as well as the influence of residual stresses. If needed and a suitable device provided measurements can be done with high spatial resolution and with very small indentation depths. A special application of the testing devices is the characterization of the elastic behaviour of miniaturized components or the realization of micro compression tests, i.e. using the machines like a small universal testing machine. 

This programme was presented last year (ref. 2). The purpose of TERFIS 5 tests carried out in 1996 was to quantify the influence of residual stresses on crack initiation and growth in weldments. They were obtained on stainless steel 316 L tubes where a defect was machined in a welded joint. The tubes were submitted to a low uniform tension (29 Mpa) and to thermal stocks (ex in the case of a circunferential defect AT in the thickness 180°C and 10 cycles in the case of a semi-elliptic defect AT in the thickness 300°C and 1500 cycles). 

Interpretation of the data are in progress. It seems that for the low primary stresses the influence of residual stresses is small because of large cycling strain conditions. This work will be presented at the SMIRT 14 Conference. It is planned to complete the TERFIS 5 tests with lower strain conditions. 

Figure 5. R curve results for segmentation cracks in attached and stand alone TBCs. For comparison R curve of attached TBC corrected from the influence of residual stress.

Another point worthwhile considering is the influence of residual stresses in the surface layer on the strength of 2HA. Because of the difference in thermal expansion coefficients of alumina and zirconia, tensile stresses should be present in the alumina layer. However, it seems that these do not have an important effect on the strength. 

The influence of residual stresses and hardening introduced during the indentation [x-ocess can also be studied. The endurance limit HIM has been determined for dents made on 32CrMoV13 steel surface in pure rolling while neglecting residual stresses and hardening due to the indentation process. This endurance limit, noted as... 

Akay M and Ozden S (1995) The influence of residual stresses on the mechanical and thermal properties of injection molded ABS copolymer, J Mater Sci 30 3358-3368. Cesari M, Perego G and Mazzei A (1965) The crystal structure of poly(tetrahydro-furan), Makromol Ghem 83 196-206. 

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