Issue 30

M. N. James, Frattura ed Integrità Strutturale, 30 (2014) 293-303; DOI: 10.3221/IGF-ESIS.30.36 298  Explicit consideration of inspection intervals at the design stage  Identification of critical areas in the structure via full-scale testing  Feed-back into design from failure analysis  Codified design procedures The success of this type of approach, when coupled with the use of fracture mechanics and defect-tolerance, can be illustrated by contrasting the consequences of fatigue cracking and subsequent structural damage in two famous aircraft failures:  De Havilland Comet 1 in 1954, where growth of a fatigue crack led to rupture of the fuselage and complete break-up of the aircraft in the air through explosive decompression at an altitude of about 35,000 feet [10, 11, 12].  Aloha Airlines Boeing 737 in 1988, in which a fatigue crack caused an explosive decompression and structural failure at an altitude of 24,000 feet [11]. According to the official US National Transportation Safety Board report [13], approximately 5.5 m of the pressure cabin skin and structure aft of the cabin entrance door and above the passenger floor line separated from the aircraft during flight. The flight crew made an emergency descent and landed the aeroplane safely. The fail-safe approach is attempting to design ‘defect-tolerant’ structures, although prior to the development of fracture mechanics it was not possible to predict crack growth rates or remnant life in the presence of a defect. Fracture mechanics and the quantification of defect-tolerance The development of fracture mechanics was spurred by some notable failures that occurred over the period 1940 to 1980. These ranged across ships, aircraft, bridges and pressure vessels. Several useful reviews of the development of defect tolerant methodologies in these various industries are available, including the US Air Force Handbook for Damage Tolerant Design [14] and a review of strategies to combat aircraft structural failures by Brot [15]. The basic concepts in fracture mechanics are that:  Crack tip stresses, strains or displacements reach a critical value (which under a specific set of conditions is represented by a material constant value of resistance to crack growth, the fracture toughness).  The rate of energy absorption in incremental crack advance is < the rate of energy supplied by release through crack growth of the stored strain energy in the structure. A critical factor in fast fracture is that the presence of a crack or sharp crack-like defect induces a triaxial stress field near the crack tip. This makes plastic deformation more difficult and increases the constraint on plastic flow, particularly in thicker sections of material. Extensive work in the period between 1950-1975 led to the definition of parameters to characterise crack tip fields, and to the development of standardised fracture toughness tests which give ‘material constant’ values (under specified conditions) for the resistance to crack growth of a material [16]. The three usual characterising parameters are the stress intensity factor K, the crack tip opening displacement COD, and the J-integral. K is an elastic parameter which characterises critical and subcritical crack growth under conditions of macroscopically elastic behaviour in a cracked body, i.e. constrained and limited plasticity. The J-integral is a nonlinear elastic parameter based on energy integration along a path around the crack tip, which has been shown to characterise macroscopically elastic-plastic behaviour. COD is a parameter which explicitly considers the existence of extensive plasticity at the crack tip. Current codes for the engineering assessment of criticality of defects in structures [8] allow for use of any of the three, depending on the level of plasticity experienced during fracture. The codes also consider a two- parameter failure mode where the possibilities of fracture and plastic collapse are assessed independently and the probability of failure is then plotted on a failure assessment diagram (FAD). Work in the 1960’s established that, in a similar manner that K characterises the onset of fracture in the presence of a ‘critical’ crack, the range of the applied stress intensity factor ΔK characterised the growth of stable cracks under cyclic loading, i.e. ‘subcritical’ cracks. For the first time, quantitative predictions of remnant life were possible once a crack was detected. Thus defect-tolerant design involves living with defects and an explicit acceptance that manufacturing and fabrication processes introduce cracks or crack-like defects so that a structure enters service in a ‘flawed’ state. The steps in the procedure generally include:  Size the initial defect population on service entry via NDT (perhaps coupled with proof testing to establish a possible maximum size of defect that could be present but has not been detected)  Perform a fracture mechanics based life assessment  Set inspection intervals and the level of inspection required