Strain-limited design

This is an example of strain-limited design. We apply the pseudo-elastic design method, specifying the duration of loading as S hours (which is 18000 seconds). In order to determine the modulus, we need the isochronous stress-strain curve for 18000 s. Substituting in the equation,... 

A sandwich structure is composed of two skins and a core material. The same or different materials are combined in the form of sandwich construction (Figure 7.46). They can be used in products with an irregular distribution of the different materials, and in the form of large structures or sub-structures. Overall load-carrying capabilities depend on average local sandwich properties, but materials failure criteria depend on local detailed stress and strain distributions. Design analysis procedures for sandwich materials composed of linear elastic constituents are well developed. In principle, sandwich materials can be analyzed as RP structures, but incorporation of viscoelastic properties will be subject to the limitations discussed throughout this book. 

In designs that are strain-limited, the maximum strain for the purposes of specifying creep modulus is simpfy the maximum allowable strain for the material in question. In designs that are deflection-limited, the maximum strain is not known in advance. In these cases, a short iteration is used an initial estimate is made, and the stress anafysis is repeated a few times until consistent results are obtained. 

Poisson s ratio v=0A. Die design strain limit for MDPE is 0.02 (apply this to the maximum print ial strain). 

Part of a meter valve mechanism will consist of a rectangular cantilever beam spring injection-moulded in acetal. The free end will be held at a constant deflection in order to maintain a moment of at least 0.5 N m over a period of 3 years at a temperature of 45X. The depth of the beam has alreacty been fixed for reasons of space at 5 mm. Taking the design strain limit as 1%, calculate the minimum width of the beam, using data given in Problem 8.21 for the acetal polymer. 

P(9) Similarly, at no point shall the strain exceed the limiting design value. 

A design strain limitation of 3000 ncmcm had been imposed and this, coupled with the cost of composite manufacture, made various composites non-competitive. This spurred resin manufacturers to introduce a class of tough resins that could meet design strain limits as high as 6000ncmcm [69],... 

Another popular approach to evaluate the design of snap fits is to calculate their strain rather tlian their stress. Then compare this value with the allowable dynamic strain limits for the particular plastics. In designing the beams it is important to avoid having sharp comers or structural discontinuities that can cause stress risers. Tapered finger provides a more uniform stress distribution, which makes it advisable to use where possible. 

In the mechanical acceleration method of creep testing, the stress levels used in the laboratory tests are significantly higher than the contemplated design stress levels, so the limiting design strains are reached in a much shorter time than in actual service. The data taken in the mechanical acceleration method are plotted as stress level versus time for a family of constant strain curves all run at a constant temperature. 

To interpret such data In terms of the permissible stresses for a pressure tube material, it Is necessary to decide how much strain can be tolerated In the particular design of pressure tube. Por a vertical tube reactor such as the SGHWR, this question becomes one of how much Increase of diameter (and associated wall thinning) is acceptable. Considerations such as the effect of pressure tube diameter on coolant flow, on fuel element support or on calandria tube gap do not provide a strain limit of less than 5. ... 

What constitutes failure (section 1.4) will be dictated by the design, but a 10% drop in stiffness is becoming accepted in fatigue. For creep, a strain limit is usually imposed by the design. 

The maximum strain criterion is becoming popular for plastics materials subjected to long-term loads upper strain limits for representative plastics are suggested in Table 2.2. Thus the appropriate design stress at a particular time and temperature can be found in the relevant creep data. 

Figure 11.21 Design stress domains recommended forNb-lZr, Ta-8W-2Hf, TZM [112], and V tCr-4Ti [113], The creep strain limit was made by two-thirds of creep rupture stress for 10 h for V-4Cr-4Ti and 1% creep strain in 7 years for the others.

Because many geotextile reinforcement design codes specify a limit to allowable creep strain, it is convenient to present the loads at which these strain limits are reached. In this context, the creep data may be presented in an isochronous plot consisting of an array of load—strain curves, similar to the one from a tensile test, but with each curve representing a different duration. Each isochronous (ie, constant time) curve is created by taking load and strain levels from each creep curve at a given constant time and plotting them to form an isochronous curve. Isochronous curves are not an extrapolation tool, but instead are an interpolation tool. 

---