Joint design failure analyses

Design and failure analysis of composite bolted joints for aerospace composites... 

This chapter presents both experimaital and finite element approaches that are commonly used to assist in the design and failure analysis of composite bolted joints. The work presented mostly stems from the authors work in the field over the past decade. Results from both local (i.e. detailed) finite element and global (Le. efficient) closed form and finite element approaches are presented, and expmmental results from the open literature are used to both calibrate and validate the modelling procedures. The primary variable under consideration is bolt-hole clearance, which is chosen as it induces significant three-dimensional stresses into the joint, and significandy alters the bolt-load distribution in multi-bolt joints, and so provides a rigorous test case for analysis. 

A future trend in composite bolted joint design is the use of more advanced continuum damage mechanics approaches to model joint failure, so that non-linear shear behaviour and gradual stiffness degradation can be implemented. An important element of joint failure is delamination, and many efforts are being undertaken to implement delamination in finite element models. However, for bolted joints analysis this poses many problems due to the large number of delamination interfaces in thick laminates. 

In terms of failure modes, design and analysis this joint is practically identical to the angle joints discussed above. The only difference is that the joint is formed by bonding two pre-formed angles attaching either side of the web piece, i.e. the leg of the tee, to the flange piece (the base of the tee) to form the overall tee connection. 

The level to which a particular system must be certified is selected by a process of failure analysis and input from the device manufacturers and the certifying authority (FAA or Joint Aviation Authority [JAA]), with the final decision made by the certifying authority. Note that different software components do not need to be certified specifically at each designated level. Certification at any level automatically covers the lower-level requirement but, obviously, the converse is not true. Software certified at Level A can be used in any avionics application. It should be noted that following the advent of RTCA/DO-254 and SAE/ARP-4754, the term SL is now often referred to as DAL in order to be in alignment with the terms used in these documents. 

The immediate cause was failure of the ring joint seal on the solid rocket booster. Yet, a root-cause analysis revealed a much more complex scenario. According to information published after the investigation, post flight evidence from as far back as early 1984 showed that the joint seals were failing to meet design specifications. 

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