B. Nageswara Rao

A comparative study on failure pressure estimations of unflawed cylindrical vessels

There is a need to design a reliable lightweight solid rocket motor case, pressure vessel for a launch vehicle or a missile system. The rocket motor case used in the advanced solid propulsion system is essentially a lightweight shell acted upon by static internal pressure and dynamic and thermal loads during flight, but for practical structural integrity purposes, consideration of internal pressure is all that is necessary. This paper examines existing test data, theories and procedures, frequently used for evaluating the maximum pressure in closed ended cylindrical vessels.

Evaluation of fracture energy GIC using a double cantilever beam fibre composite specimen

Abstract Studies are made to determine the mode I interlaminar fracture toughness, or critical energy release rate, GIC of composites using a slender double cantilever beam (DCB) specimen. Fracture analysis has been carried out on DCB specimens made of carbon-fibre/PEEK and carbon-fibre/epoxy composites. The calculated critical load and the corresponding displacement for the measured crack size of DCB specimens using the determined fracture energy, are found to be in good agreement with the published test results.

Residual strength of aluminum-lithium alloy center surface crack tension specimens at cryogenic temperatures

Abstract A three parameter fracture criterion which correlates the stress and the stress intensity factor at failure, is followed for the residual or fracture strength estimations of cracked configurations made of aluminum–lithium (Al–Li) alloys. The three fracture parameters are determined from the fracture data of Al–Li alloy center surface crack tension (SCT) specimens at cryogenic temperatures. It is found that the estimated fracture strength values compare well with the test results.

Instability load for cracked configurations in plate materials

Abstract An empirical relationship between the failure stress and the elastic stress intensity factor at failure established by means of three fracture toughness parameters ( K F , m , and p ) of a material is examined to estimate the failure load of cracked configurations. The theoretical results obtained using this fracture criterion compare well with existing test results.

Fracture of solid rocket propellant grains

Abstract The importance of fracture criteria in the failure assessment of solid propellant grains is briefly described. Computation of the crack tip stress intensity factor and the development of the crack growth rate equation through fracture properties essential for fracture analysis are also discussed.

Fractured analysis of a surface cracked plate under tension

Abstract A relation between the elastic stress intensity factor and the elastic nominal stress (net section) at failure is established using the crack growth resistance curve ( R- curve ), generated from a compact tension specimen. The fracture strength of a surface cracked plate under tension is determined from this relation and is compared with the experimental failure stress. The results are found to be in good agreement with each other.

Fracture strength of flawed cylindrical pressure vessels under cryogenic temperatures

Damage tolerant and fail-safe approaches have been employed increasingly in the design of critical engineering components. In these approaches, one has to assess the residual strength of a component with an assumed pre-existing crack. In other cases, cracks may be detected during service. Then, there is a need to evaluate the residual strength of the cracked components in order to decide whether they can be continued safely or repair and replacement are imperative. A three-parameter fracture criterion is applied to correlate the fracture data on aluminium, titanium and steel materials from test results on cylindrical tanks/pressure vessels at cryogenic temperatures. Fracture parameters to generate the failure assessment diagram are determined for the materials considered in the present study. Failure pressure estimates were found to be in good agreement with test results.

Tensile fracture strength of boron/aluminum laminates with holes and slits

Abstract This paper improves the stress fracture criteria proposed by Whitney–Nuismer known as the point stress criterion (PSC) and the average stress criterion (ASC), to predict the strength of composite laminates containing holes and slits. A simple relation is used for the characteristic length to improve the accuracy while evaluating the notched strength of laminates. The applicability of the proposed approach is examined by considering the notched strength data on carbon/epoxy and boron/aluminum laminates. The analytical results obtained in the present study correlate well with the test results of different notched configurations. This study confirms that the present modification in the stress fracture criteria can be used for notched strength evaluation of composite laminates.

Tension and Compression Strength Evaluation of Composite Plates with Circular Holes

Modifications were made in the well-known inherent flaw model as well as in the stress fracture models namely, point stress criterion and average stress criterion for accurate prediction of notched tensile and compressive strength of composite laminates containing holes. The adequacy of these modified fracture models were examined by considering the fracture data of center-hole specimens made of carbon/epoxy composites, weft-knitted glass fiber composites, and pultruded composites. Any one of these three fracture models can be utilized to predict the notched strength.

Tensile fracture of HTPB based propellant specimens

Abstract The failure of a viscoelastic material, such as solid propellant, is a complex phenomenon with a requirement to address the following two fundamental and complementary aspects in the failure criteria. First, what is the failure criterion for a solid propellant in stress–strain space, and what are the variations in failure data when failure is influenced by time and temperature effects? Second, how are conditions for failure influenced by the microstructure of the propellant? In fact, the usual practice is to present failure data in the form of two failure loci: one in the stress–strain plane where failure points are generated from tests at different strain rates from very low to very high values, and in others these failure stresses and failure strains are plotted against the corresponding strain rates. Uniaxial tensile specimens are generally used to determine the standard failure properties such as tensile strength and the percentage of elongation under constant strain rate and temperature. In order to understand the failure behaviour of propellant materials, the fracture data of a HTPB (hydroxyl terminated polybutadiene) based propellant generated from the uniaxial tensile specimens, is considered. Following the work of Smith and Stredry, the fracture data are represented by the curve of strain at maximum stress versus temperature reduced strain rate, and the failure boundary curve of maximum stress with the corresponding strain. Uniaxial and strip biaxial tension specimens are analysed here for various loading rates. Failure analysis has been carried out by considering the master stress relaxation modulus data as well as the failure data of the propellant.