Fatigue materials selection

We shall now examine material selection for a pressure vessel able to contain a gas at pressure p, first minimising the weight, and then the cost. We shall seek a design that will not fail by plastic collapse (i.e. general yield). But we must be cautious structures can also fail by fast fracture, by fatigue, and by corrosion superimposed on these other modes of failure. We shall discuss these in Chapters 13, 15 and 23. Here we shall assume that plastic collapse is our only problem. 

The material selected for the pin was 070M20 normalized mild steel. The pin was to be manufactured by machining from bar and was assumed to have non-critical dimensional variation in terms of the stress distribution, and therefore the overload stress could be represented by a unique value. The pin size would be determined based on the —3 standard deviation limit of the material s endurance strength in shear. This infers that the probability of failure of the con-rod system due to fatigue would be very low, around 1350 ppm assuming a Normal distribution for the endurance strength in shear. This relates to a reliability R a 0.999 which is adequate for the... 

Altshuler, T. L., Fatigue Life Predictions for Materials Selection Guide, ASM Software, 1988. 

When selecting a bellows valve, it is important to pay some special attention that the material selection is in accordance with the process conditions. Some SRV manufacturers use as standard bellow material INCONEL alloy 625LCF-UNS N06625 (ASME SB0443). This material is not perfect either but, compared to simple stainless steel, has an enhanced resistance to mechanical fatigue and sour gases it is commonly used in refinery FCC systems for expansion joints. 

Materials selection process can be depicted in terms of Figure 1.40. Materials selection involves many factors that have to be optimized for a particular application. The foremost consideration is the cost of the material and its applicability in the environmental conditions so that integrity can be maintained during the lifetime of the equipment. When the material of construction is metallic in nature, the chemical composition and the mechanical properties of the metal are significant. Some of the important mechanical properties are hardness, creep, fatigue, stiffness, compression, shear, impact, tensile strength and wear. 

The outputs from finite element analysis will tell the chemist many of the important predictive characteristics of the part. The effects of load (stress, strain, and deflection) are measured. Material selection based on strength and part dimensions and tolerances are determined and easily adjusted. The effects of heat (heat transfer and thermal stresses) are determined as well as material selection based on conduction and insulation. Special applications such as bending and vibration characteristics, crashworthiness, fatigue, and noise can be determined. The design requirements, such as expected loads, load cycle cost, mass targets, and budget targets, can be predicted and met with proper application of the FEA tool. 

Corrosion fatigue—Occurs when corrosive and fatigue effects occur simultaneously. Corrosion can reduce fatigue life by pitting the surface and propagating cracks. Material selection and fatigue properties are the major considerations. 

In several eases, non-metallie materials such as polymers, rubbers, ceramics, wood or concrete must also be taken into consideration (see Section 10.1.11). With respect to materials selection for screws, bolts, nuts, rivets or oflier small parts for use under possible galvanic corrosion conditions, it should be checked that they are a little more noble (have a little more positive corrosion potential in the actual enviroiunent) than the components they are binding together, or are in metallic contact with in any way. In addition, one should be particularly careful to avoid catastrophic deterioration forms such as hydrogen embrittlement, stress corrosion cracking and corrosion fatigue in such parts. 

Another form of design error can be poor material selection. A selection error may result from lack of knowledge or data about particular materials. Similar materials may have different properties that are critical. A selection error may result from lack of knowledge or from lack of field data or test data about a use environment. Selection of incompatible materials may induce or accelerate corrosion and fatigue, increase brittleness, or cause other effects that reduce the strength of the material. 

Table 7.1 Y-TZP materials selected for fatigue testing [26] (with kind permission of Elsevier)...

Reversed bending fatigue tests were made at 75 and -300 F for each of the three test materials selecting appropriate stress levels so that smooth S-N curves could be developed from 10 cycles to 10 cycles at 75 F and to 10" cycles at 300°F. With few exceptions two specimens were tested at each stress level. 

Corrosion, erosion, mechanical fatigue, operation at design limit and/or beyond it, calibration failure, design fault (such as wrong material selection), obsolescence, etc. are major contributing factors for degradation of equipment/system, and/or electrical control and instmmentation (EC I) items. 

Operation beyond designed temperature range Operation beyond design limits Bad material selection/fatigue test not carried Sustained operation beyond corrosion limit Change of services Extremely corrosive atmosphere... 

Manufacturers handbooks give data on tensile and flexural yield or fracture stress, tensile elongation at break, notched Charpy or Izod impact strength, and fatigue life. Whilst these data are useful for materials selection, they are not always sufficient for quantitative design. To date, fracture mechanics has been used to only a limited extent in design with plastics, and data are not provided in data books. The lack of approved standards for testing of polymers is currently a major obstacle to the wider use of fracture mechanics. 

A typical material selection involves many properties which are not easily quantifiable in numerical terms (such as weathering, warpage, surface finish, ease of machining, etc.) or which may have very obscure units (such as transparency, fatigue, wear. 

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