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Product category: FEA and structural analysis software
News Release from: Algor | Subject: Alibre Design
Edited by the Engineeringtalk Editorial Team on 14 March 2008

Software saves weeks of engine testing

Using Alibre Design, Hamler created a CAD model of a clamp that could attach to the compressor blade and support a weight on each side of the torsion mode nodal axis.

When Hamler Test and Analysis (HTA) needed to perform shaker fatigue testing of a compressor blade from a gas turbine engine, it used linear dynamic finite element analysis (FEA) software from Algor to simulate the test and determine the optimal setup "Algor FEA allowed me to do 'virtual' testing", said Jesse Hamler, President and Chief Technical Officer of HTA

"I was able to simulate different ideas and hardware configurations without having to spend time or money to physically make the hardware for each setup, which saved weeks of trial-and-error testing, if not months".

HTA provides testing, design and FEA consulting services.

"We specialise in vibration measurement and experimental vibration analysis tools such as tap testing, modal analysis, operating deflection shapes analysis and rotating machinery analysis, particularly for engine vibration" Hamler said.

"In conjunction with our experimental capabilities, we offer a broad range of product design and finite element analysis services".

"We believe that experimental validation and correlation of computer models are vital to reducing the cost and cycle time of product development and product support".

For one application, HTA was contracted to perform physical fatigue testing of a compressor blade from a stationary gas turbine engine used for power generation.

"Electro-dynamic shaker testing was required to verify the client's analytical vibratory high-cycle fatigue life prediction methodology", said Hamler.

"The first phase of this shaker testing involved vibrating several blades to failure at the first bending mode and the second phase involved testing several blades to failure at the first torsion mode".

Hamler described the first phase of testing: "The primary challenge involved attaining an acceptable blade tip response displacement at the first bending mode frequency of 2900Hz".

"The target response was dictated by blade stress levels required by the customer during testing".

"Because displacement is directly proportional to acceleration but inversely proportional to the square of frequency, the target displacement would be difficult to achieve at 2900 Hz, even with the gain in response at resonance".

"This challenge was overcome by mass loading the end of the blade with a special investment cast clamp, which lowered the first bending mode frequency to approximately 850Hz".

"This simple change made it possible to achieve the desired bending mode tip displacement and, consequently, the required blade stress levels".

"The optimisation of the correct amount of mass was determined by experimental trial and error".

"I knew that finding a solution to failing a blade at the first torsion mode for the phase two testing would be far more challenging".

Hence, a trial-and-error solution was not feasible due to time constraints and Algor FEA was used to direct the testing in the right direction.

Hamler said "It is difficult to excite an angular mode shape with linear movement".

"Initial sine sweeps of the mass-loaded test setup from phase one indicated that the first torsion mode had dropped from 8400 to 1450Hz with the addition of the tip mass and an adequate response from the new 'mass-loaded' torsion mode did not appear to be attainable on the shaker".

The gain in response obtained at a torsion mode is minimal when the excitation is linear motion.

There was not a commercially available linear or rotary shaker system that could provide enough excitation to achieve the desired torsion mode response with the test setup from phase one.

However, using Algor FEA, it was evident that a sufficient torsion mode response could be attained with reasonable levels of linear excitation by substantially increasing the mass moment of inertia about the nodal axis or nodal line of the first torsion mode (axis of rotation with zero displacement)".

Hamler continued, "An initial Algor analysis predicted that, by making such modifications, the first torsion mode would occur at a lower frequency than the first bending mode and the desired blade stress levels could be obtained with acceleration levels that were attainable from the shaker".

However, Algor results also predicted that if the total mass increase was too large, the torsion and bending modes would become coupled.

Hence, increasing the mass moment of inertia about the torsion mode nodal axis would require concentrated masses to be applied at a significant radial distance from the nodal axis, while also minimising the mass increase near the nodal axis.

Designing hardware that could meet these requirements and physically attach to the blade would prove to be challenging.

Furthermore, such hardware would have to be feasible to manufacture.

Using Alibre Design, Hamler created a CAD model of a clamp that could attach to the compressor blade and support a weight on each side of the torsion mode nodal axis.

The blade-weights-clamps assembly measured 89 x 29 x 22mm.

Alibre Design was used to create this CAD model of the weighted compressor blade assembly.

The compressor blade is highlighted in orange and the weights and clamps are highlighted in green.

Hamler opened the CAD model in Algor in order to set up for linear dynamic analysis.

"Algor's direct support for Alibre made my job very easy", he said.

In the FEA model, the surface at the base of the compressor blade was fully constrained.

Loads were applied as 17G acceleration in the Z direction with excitation at the first torsion mode and first bending mode natural frequencies of 420 and 622 Hz, respectively.

Damping was determined experimentally and included in the FEA model.

"Algor modal and frequency response analyses were performed to predict the proper size and location of each weight", Hamler said.

"In this way, shaker experiments and the fabrication of weight assemblies were minimised".

The initial test setup design incorporated a 35g trailing edge weight and a 22g edge weight.

FEA results indicated that this design would produce an acceptable response on the shaker, but each weight was initially made to have a weight of 28g, with the assumption that the resulting experimental response would still be in the vicinity of the FEA prediction Therefore, if needed, the shaker excitation level could be adjusted to compensate for any difference.

However, the first run with the 28g weights indicated that the response was significantly lower than the FEA prediction of the 35 and 22g weight setup, even at the maximum excitation level capable from the shaker.

The next logical step was to tune the test setup to replicate the FEA model.

Two pieces of steel flat stock were bolted to the top of the trailing edge weight, giving it a total weight of 22g Then the edge weight was milled down to 22g.

"This small weight adjustment provided a night-and-day difference in response on the shaker, as predicted by FEA" Hamler said.

Experimental results closely matched FEA predictions.

"Testing was a success and the blades failed as predicted", said Hamler.

"I was surprised that the FEA model was able to predict the real-world response of a structure so accurately".

"This was a valuable exercise in how FEA can drastically reduce experimentation and troubleshooting involved with nonstandard testing objectives".

"I would say that using FEA cut the testing time in half at the very least".

"Without FEA, virtual testing is not an option and an experimental solution becomes an expensive and time-consuming iterative process".

Hamler plans to continue using Algor software for upcoming HTA projects.

"I plan on using Algor FEA to complete modal and harmonic analyses in support of a proposal I am drafting for a potential project".

"Furthermore, at some point in the near future, I plan to expand my software capability to include nonlinear material models".

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