Material Utilisation (U)
Material utilisation refers to the percentage of elastic strain that can be used for any particular purpose, e.g. working, design, test, accidental, etc.
The limits of utilisation generally fall into two distinct camps: 1) limit-state design, and 2) safety factor. Limit-state design is generally applied to structural applications and safety factor is generally used for all other applications, such as machine parts and pressure containment.
Both of the above practices are normally controlled by recognised design codes and standards.
There are numerous codes and standards that apply equally well to all forms of design but in order to simplify this explanation we shall concentrate on pressure vessels, which includes pipes and pipelines. The same approach may be equally well applied to any other system
Codes and Standards
The most recognised design codes and standards for pressure vessels are: ASME, ANSI, API, BS and DNV. BS5500 is based upon ASME VIII and shall therefore be ignored for the purposes of this comparison.
The following is a summary of the principle pressure vessel (which includes pipe and pipeline) utilisation factors that should be used for a material at its design temperature, albeit many of these standards offer alternative values based upon differing operational and test temperatures.
| Utilisation based upon a factor of SMYS | |||
|---|---|---|---|
| Code | Working | Test | ULS |
| Author's Experience | 0.6 | 0.9 | <1 |
| ASME VIII (Appendix P) | 0.67 | 1 | <1 |
| ANSI B31.4 | 0.72 | 0.9 | 0.8 |
| BS8010 | 0.72# | 1 | 0.96 |
| IP6 | 0.72 | <0.95 | 0.9 |
| DNV Pipelines | 0.67 | 0.96 | 1 |
| DNV Flexible Pipes | 0.67## | 1 | 0.9 |
# pipelines, ## end terminations
All of the above are based upon well-defined load conditions and material that has been manufactured and tested according to a recognised specification and fully certified.
Where one exists, you should always use the governing design code or standard.
If, on the other hand, a design code or standard is not available, you should either self-impose a relevant standard or you should devise your own values based upon the following criteria:
knowledge of the loads
and
knowledge of the material(s)
The following summary of the above table: Working: 0.6, Test: 0.9 and ULS: 1, represents the highest level of safety given a good knowledge of both loads and materials for projects with no governing design code or standard.
Philosophy
Improving your knowledge of the expected loads can usually be achieved by sufficient calculations and optimising/limiting operational procedures.
Whilst this is not usually the case for unknown or uncertified materials, there are measures you can take to minimise the likelihood of failure. For example:
a)
you could send the material away to a suitable testhouse for evaluation
or, if this not a practical option;
b)
you could test the material yourself
If you do not know what material you have but you need to use it, you can perform a test on the material prior to use in order to estimate its yield stress.
A simple non-destructive procedure for the determination of yield stress in your workshop is provided below.
Yield Test
Fig 1. Yield Stress Measurement
The following is an improvised test procedure that can be carried out in a workshop and will give you reasonable confidence in the final result. The most important piece of equipment you will need is an accurate load measurement device such as a load cell or the means of inducing a known load.
The smaller the diameter of the pin-punch you use, the lower the load you will require. The larger the pin-punch the more accurate the result.
For improved confidence, you can perform this test in a number of locations around the surface of the material.
Fig 2. Test Result
A simple setup is described in Fig 1 whereby a vice is used to force a pin-punch into the material you are testing and a load cell is used to measure the load.
If you make sure that the end face of the pin-punch is parallel to the test material then you need to find the highest load you can apply without actually generating a permanent mark in the metal. The yield stress of the material will be the force divided by the area of the end face of the pin-punch:
σ = F ÷ A (Fig 2: result A).
If you generate a partial depression, the yield stress of the material will be the force divided by the area of the dent generated by the end face of the pin-punch (Fig 2: result B). The area of the partial dent may be calculated as follows:
Area = ⅛ز.{θ - Sin(θ)}
The following table shows the force (in Newtons) you will need to apply to verify the yield stress of a material using a pin-punch of the specified diameter.
| SMYS | Pin-Punch Diameter (mm) | |||||
|---|---|---|---|---|---|---|
| (N/mm²) | 1 | 2 | 3 | 4 | 5 | 6 |
| 100 | 78.5 | 314.2 | 706.9 | 1256.6 | 1963.5 | 2827.4 |
| 150 | 117.8 | 471.2 | 1060.3 | 1885.0 | 2945.2 | 4241.2 |
| 200 | 157.1 | 628.3 | 1413.7 | 2513.3 | 3927.0 | 5654.9 |
| 250 | 196.3 | 785.4 | 1767.1 | 3141.6 | 4908.7 | 7068.6 |
| 300 | 235.6 | 942.5 | 2120.6 | 3769.9 | 5890.5 | 8482.3 |
| 350 | 274.9 | 1099.6 | 2474.0 | 4398.2 | 6872.2 | 9896.0 |
| 400 | 314.2 | 1256.6 | 2827.4 | 5026.5 | 7854.0 | 11309.7 |
| 450 | 353.4 | 1413.7 | 3180.9 | 5654.9 | 8835.7 | 12723.5 |
| 500 | 392.7 | 1570.8 | 3534.3 | 6283.2 | 9817.5 | 14137.2 |
| 550 | 432.0 | 1727.9 | 3887.7 | 6911.5 | 10799.2 | 15550.9 |
| 600 | 471.2 | 1885.0 | 4241.2 | 7539.8 | 11781.0 | 16964.6 |
| 650 | 510.5 | 2042.0 | 4594.6 | 8168.1 | 12762.7 | 18378.3 |
| 700 | 549.8 | 2199.1 | 4948.0 | 8796.5 | 13744.5 | 19792.0 |
| 750 | 589.0 | 2356.2 | 5301.4 | 9424.8 | 14726.2 | 21205.8 |
Tips
You need to consider the work-hardening properties of carbon steel and other metals when testing.
1) Your first attempt should assume a low yield stress, e.g. 200MPa in concert with the appropriate load for the pin-punch diameter(s) you have available and follow this up either by increasing the test load or decreasing the punch diameter.
2) If you generate a full diameter penetration it is likely that the measured yield stress will be based upon the material in its work-hardened state and your test result will be unreliable.
3) If you generate a partial diameter penetration it is likely that the measured yield stress will be based upon the material in its work-hardened state. You should therefore retest using a lower force and improving the parallel relationship between the material surface and the end face of the pin-punch.
Further Reading
You will find further reading on this subject in the relevant design codes and standards as described above