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Strain Survey & Failure Analysis

The Issue

Strain surveys of static and linearly moving parts help engineers understand how their designs can be improved to increase life and performance while reducing material costs. Strain calculations are typically done using a Finite Element Analysis (FEA) model. While FEA models can be useful in estimating strain, more sophisticated methods are required to gather accurate information when high-speed rotating parts are involved. This is especially important when those rotating parts include odd shapes, welds, or fillets, or parts made from composite materials.

Why It Matters

The higher the operational speed of rotating parts, the more important it is to understand the impact of strain. When high speed parts will be spun up and spun down often, rather than running in a steady state condition, the need for empirical data of strain information (not just a theoretical model) becomes critical. Should strain problems go undetected and find their way into the field, the results can be costly in terms of system performance, personal safety and business reputation.

What’s more, identifying areas of concern in a component as early as possible in a design process can save substantial dollars. The US Military stated at a conference that the cost of an unidentified problem increases from the start to the end of a program by the following multipliers:

What You Can Do

Test Devices offers comprehensive Strain Surveys of rotating parts. Information gathered in these tests can be used to test complex parts, understand assembly stresses and determine the properties of materials (including composites) when exposed to operational strain under realistic conditions. The data collected can be used to calibrate an FEA model. Test Devices' service is highly cost-effective – typically costing substantially less than a complete FEA – and provides the advantage of actual test data instead of a model.

Reasons for Strain Testing

Calibrate an FEA (Finite Element Analysis) model

Test odd shapes, welds, fillers, etc.

Understand assembly stresses

Determine material properties of composites

Perform elastic/plastic analysis

Measure damping inherent in a system (Q)

Calibrate strain and clamping of bolts

Measure thermal strain superimposed on rotational strain

Monitor the propagation of a crack (usually in an LCF test)

Understand the stress distribution of multi-material component assemblies

Advantages of Strain Survey and Failure Analysis

1. Empirical Data vs. Modeling: FEA software programs are quite good at modeling one-piece part designs, but empirical testing can bring forth issues that do not show up in modeling. Test Devices Strain Surveys can gather information on areas where stresses concentrate (e.g., assemblies, odd shapes, welds, fillets, etc.) and potentially lead to failure.
2. Testing Composites: Typically, composite structures are made up of layers of "fabric" which can shift during rotation, creating stress and strain that cannot be predicted through modeling. A Test Devices Strain Survey can provide data on composite products and the quality of their manufacturing processes.
3. Elastic/Plastic Analysis: Parts spun at high speed grow as a result of the high centrifugal forces. Some of the growth is recoverable (elastic) while some is permanent (plastic). Determining the ratio of elastic to plastic growth provides data vital for determining the useful life of a part.
4. Dynamic Strain Surveys: Test Devices can measure the damping inherent in a system to minimize vibration and provide information on the effectiveness of damping added to an assembly. Thermal gradient strain also can be accurately measured.
5. Strain Gauging: The clamping force of a bolt is proportional to, but not necessarily directly related to, the torque applied. As a result, it is difficult to determine. Strain gauging bolts can calibrate the effects of friction, galling, lubrication, etc., and provide a more accurate conversion of clamping force from torque.
6. Predicting Failure Risk: In jet aircraft, failure of rotating parts can be serious – even fatal. Strain surveys can provide information on the propagation of cracks while rotating or cycling from start through operating speeds and back. This can provide valuable information on imminent failure risk.
7. Strain/Burst Combination: A Strain Survey can be done in conjunction with other tests, such as a burst test, where the strain can be measured accurately up to the point of destruction.
8. Cost-Effective: In most cases, Test Devices can run a complete Strain Survey for significantly less cost than a complete FEA analysis. This saves money and also provides data which is more dependable than that produced from a model because the analysis has been taken one step further – real information on a real part.

Test Devices’ robust equipment allows speeds all the way up to and including burst speeds. Unlike one-time brittle coatings sometimes used, Test Devices can incorporate cycling the device under test multiple times and vary the test temperatures resulting in a Strain Survey that more accurately represents design performance.

In the graphic above, sensors positioned on the device under test (DUT) were used to calibrate the FEA model shown just below the DUT. A profile of the DUT over the designer’s chosen range of speeds was created from the resulting data.

In another example, Test Devices was able to extrapolate the burst speed to within 100 rpm from test data generated using high elongation sensors, providing considerable savings over burst testing.