Low cycle fatigue life improvements realized by reduced thermal strain due to flange separation in bolted joints

摘要

Low cycle fatigue (LCF) life is an essential aspect in aircraft engine component design, particularly concerning structural members. Realistic finite element modeling is critical in obtaining life predictions that accurately represent fielded parts. One of the most challenging designs to model accurately is the bolted joint. Bolted joints are critical in aircraft engines as they connect parts and transfer loads. In complex joints, modeling can be difficult and is often simplified with the use of conservative assumptions. Recent commercial experience on structural hardware exposed to high engine temperatures and pressures has indicated that fielded part life for bolted joint members may be significantly higher than simplified finite element modeling, which includes a number of conservative assumptions, would predict.Several factors are critical to the quality of a finite element model of a bolted joint. These factors can significantly impact the results for predicted LCF life and can include: proper geometric matching with the actual hardware, appropriate material properties, realistic boundary conditions and suitable heat transfer. This paper will compare simplified 2D modeling of a three flange bolted joint with more accurate analysis taking into account flange separation and leakage. This paper will also attempt to demonstrate that leakage assumptions can impact LCF life predictions. For the joint of concern in this paper, flange leakage is shown to reduce thermal gradients and improve LCF life by approximately 50%. While this paper focuses on only one of the many important facets in bolted joint methodology, effort is made to show the benefit in the inclusion of flange leakage assumptions.The successfully fielded bolted joint considered in this paper will be examined thoroughly with industry standard methodologies. Finite element models and approaches will be compared between an original model, developed for certification of the hardware, and an updated model which uses the latest in modeling technology. The original model, which used beam elements to simulate the bolt in the joint, did not allow for the full range of flange motion and separation witnessed on fielded hardware. As a result, the applied thermal model did not account for any joint separation or leakage. Temperatures at the joint assumed the flanges were essentially fused together. This resulted in increased thermal gradients on the flange members and lower LCF life then would be expected if the joint were allowed to open. The significant thermal strain resulting from these gradients yields lower LCF life than field experience would suggest. In fact, examination of high life hardware shows no sign of fatigue and this hardware seems capable of service of well beyond the original designed life of the part. This is undoubtedly due to the reasonably conservative nature of the analysis. Again, this paper will seek to understand how the assumptions surrounding joint leakage can impact this analysis. More detailed modeling techniques were recently applied that allow for appropriate separation. Complex thermal models were also modified to rationally account for flange leakage. Although these models yield increased LCF life, flange leakage is only one of the conservative assumptions necessary in bolted joint design. This paper will also touch on how pairing leakage with material property and convection multiplier assumptions can impact predicted life. Although conservatism is imperative in aircraft component design, this paper will attempt to strip away some of the essential moderation and achieve life predictions that more accurately represent fielded hardware. Future work will focus on quantifying how these assumptions impact life analysis as engineers strive to create detailed models of bolted joints.