16.3.1 Overview
Development testing aims to confirm the theoretical analysis carried out on the joint. This also applies to long-term characteristics such as fatigue. All joints show a statistical variation of performance, and it is important to take this into account when obtaining approval based on test results. Development testing can include any or all of the following approaches:
- a. A large number of samples simply tested under normal loading conditions.
- b. A small number of samples simply tested but at loads enhanced in relation to the number of samples.
- c. A small number of samples tested in great detail such that the internal functioning of the joint can be understood.
It is important that above approaches include both static and dynamic tests (if appropriate) and also long-term effects.
16.3.2 The Test Factor
When a test is made on a single specimen or a number of randomly chosen specimens, it is important that the results show an extra margin of strength, or ‘test factor’, over the design value to allow for the possibility that the chosen specimens were stronger than average and also to ensure that the weakest specimen produced has acceptable strength. Past experience has shown that for conventional metallic structures the strength can be expressed as a normal distribution with a specified coefficient of variation (the ratio of standard deviation to the mean). This distribution can be used to deduce the magnitude of the test factor that is used for a given number of test specimens, in order to establish with an acceptable probability that the weakest specimen has the required strength.
A coefficient of variation of 0,03 has been established for joints made to aircraft standards from the following materials:
- • Aluminium Alloy
- • Titanium
- • Steel
If the coefficient of variation is not known, tests are necessary to establish a value.
The probability that the mean strength realised on test divided by the test factor is less than the required design strength given by,
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[16.3.1]
where P is, Smt is the mean strength found during testing, Ft is the test factor, and Sreq is the required strength.
Table 15.3.1 shows the test factors to be used to give a range of values of P under for a range of variation coefficients using Equation [16.3.1].
| Number of Tests | Probability of Failure (P) | Coefficient of Variation (V) | ||||
|---|---|---|---|---|---|---|
| 0,03 | 0,05 | 0,07 | 0,10 | 0,20 | ||
| 1 Test | 1 in 10 | 1,056 | 1,095 | 1,136 | 1,200 | 1,452 |
| 1 in 100 | 1,104 | 1,180 | 1,262 | 1,400 | 2,069 | |
| 1 in 1000 | 1,141 | 1,247 | 1,366 | 1,577 | 2,890 | |
| 1 in 10 000 | 1,172 | 1,276 | 1,461 | 1,749 | 4,241 | |
| 3 Tests | 1 in 10 | 1,046 | 1,078 | 1,112 | 1,167 | 1,385 |
| 1 in 100 | 1,086 | 1,150 | 1,220 | 1,339 | 1,943 | |
| 1 in 1000 | 1,117 | 1,207 | 1,310 | 1,495 | 2,716 | |
| 1 in 10 000 | 1,143 | 1,257 | 1,392 | 1,650 | 4,023 | |
Table 16-1 indicates that if only one test can be done on a joint with a coefficient of variation of 0,10 and the probability of joint failure in service is to be better than 1 in 1000 then the test factor to be used is 1,577. However, if three tests were done against the same requirement then the test factor to be used is 1,495.
The alternative method to the relatively straightforward tests factors described above is undertaking testing that provides detailed knowledge of how the joint works. In this approach tests are conducted which provide detailed knowledge of the loads, stresses and strains within the joint as a function of the external applied loads. These results are compared to an equally detailed theoretical analysis of the joint using a suitable method (such as the Finite Element Method) to ensure that measurements are being made in the most relevant areas.
16.3.3 Specific Development Tests
16.3.3.1 Overview
It is important to consider very carefully the environment to be withstood by a joint before specifying the development test programme. In general, higher loads are applied to the structure while it is in a benign environment, whereas lower loads are applied whilst there are extremes of temperature and temperature gradient. It is important that the effects of such temperatures and temperature gradients are investigated during the testing, especially if dissimilar materials are involved, and care is taken to ensure that the joint is loaded in a manner compatible with the real application.
The following sections describe the most important categories of benign environment tests.
16.3.3.2 Load - Deflection
A load - deflection curve can provide knowledge on the following:
- • Stiffness of the joint
- • Hysteresis characteristics
- • Backlash
- • Load sharing
- • Failure mechanism
16.3.3.3 Ultimate Strength
Ultimate strength tests are used to prove the suitability of the joint. After failure, it is important to examine the joint carefully to determine the mode of failure.
16.3.3.4 Fatigue Testing
Fatigue testing is normally only relevant on re-useable space equipment. It provides information in the following areas:
- • Fatigue Life
- • Vibration Loosening
- • Crack growth
- • Fretting
- • Creep
16.3.3.5 Corrosion Testing
Corrosion can considerately alter the characteristics of a joint both in terms of its strength and life. It is important to consider corrosion testing in accordance with the relevant environmental conditions, and that the effects of the corrosion are observed and measured.
16.3.3.6 High/Low Temperature Testing
One of the objectives of testing at extreme temperature conditions is to reproduce the appropriate material characteristics as well as thermal strains and stress. Hence, not only temperatures be simulated but also thermal gradients. Due to the difficulties and expense, it is important to minimise this testing. It is important under this condition, to derive the information on joint strength and stiffness and variation from similar tests carried out at normal ambient to be noted.
16.3.3.7 Creep Testing
Creep of joints can significantly reduce the preload over time. Loss of preload can be caused by creep of materials in the joint interface such as thermal spacers, CFRP, peelable shims, thermal fillers, etc. It is important to consider creep testing in accordance with the relevant environmental conditions, in particular for thermal exposure and thermal cycling.
16.3.3.8 Vibration Loosening
One objective of proto-flight or acceptance dynamic testing (sine, random, acoustic) is to verify, that the bolted junctions are not loosening under vibration loads.