Reliability Fracture Mechanics Research Articles

A Reliable Fracture Mechanics

A branch of human knowledge, which treats the behavior of cracked structures, is called fracture mechanics. Since there is no intact structure in the world, then the paramount importance of fracture mechanics in human life is accentuated. The main parameter of fracture mechanics is called crack compliance, which is the amount of flexibility added to the flexibility of the intact structure due to the presence of a crack with specified size. The compliance, similar to flexibility, is the sole characteristics of the cracked structure. In this way for a given structure with a specified crack, there should be a single compliance. Unfortunately, in classical fracture mechanics that is not the case! The number of crack compliances for a clacked structure is equal to the number of researchers who treated the case! This diversity in the results stems from the presence of epistemic uncertainty in the mathematical basis of classical fracture mechanics. In view of the need for remedy, the Abdolrasoul Ranjbaran Team (ART), investigated the case and proposed a reliable fracture mechanics, which is based on sound logical reasoning. The proposed reliable fracture mechanics is described in the presented paper. The paper is managed via fourteen titles as, introduction, the mathematical basis of the classical fracture mechanics, birthplace of the state based philosophy, strong form of governing equation, analytical solution by Laplace transform, the weak form equation, the finite element equation, logical basis of the state based philosophy, state functions, Persian curves, reliable crack compliance, energy release rate, stress intensity factor, and weight function for the stress intensity factor in sections one to fourteen respectively. The paper concludes with a list of cited references.

Comparison of J-integral methods for the characterization of tough polypropylene grades close to the glass transition temperature

The fracture mechanical characterization of very tough Polypropylene at room temperature is of great interest for many technical applications. Precision and validity of obtained values can significantly be influenced by notching and crack length determination, which has thoroughly been shown in literature. Additionally, the proximity to the glass transition temperature can induce further uncertainties.The aim of this study is to compare different J-integral methods of elastic plastic fracture mechanics to determine J-R curves for tested materials. Methods consist of the classical multi-specimen, a single specimen method (load separation) and a statistical approach (Weibull). Specific data reduction procedures based on physical principles were applied to reduce scattering and achieve reliable values. In this way, a reliable fracture mechanics characterization of Polypropylene at room temperature was achieved with a critical verification of the used methods. Finally, the determined fracture toughness value showed a good agreement with the surface texture of inspected fracture surfaces.

Fracture mechanics testing for environmental stress cracking in thermoplastics

Under the combined influence of an aggressive environment and applied stress, engineering thermoplastics may undergo a phenomenon known as environmental stress cracking (ESC). This can result in adverse effects such as embrittlement and premature failure in service, due to the growth of environmentally-induced cracks to critical sizes, with little to no fluid absorption in the bulk material. Fracture mechanics is proposed as a suitable scheme to study and quantify ESC, with the aim being to obtain characterising data for different polymer-fluid combinations of interest, as well as to develop a reliable fracture mechanics test protocol. In the proposed method, slow crack growth is monitored to assess the effect of a range of applied crack driving forces (K, or alternatively G) on observed crack speeds, as opposed to simply measuring time-to-failure. This paper presents the results of experiments performed on the following materials: linear low density polyethylene (LLDPE) in Igepal solution and high impact polystyrene (HIPS) in sunflower oil. A discussion of the various issues surrounding the data analysis for these long-term tests is also included, as the attainment of consistent and repeatable results is critical for a method to be internationally standardised, which is a goal of the European Structural Integrity Society (ESIS) Technical Committee 4 from whose interest this work is drawn.

Instrumented impact testing of a glass swirl mat‐reinforced reaction injection‐molded polyamide block copolymer (NBC)

AbstractThe fracture behavior of a reaction injection‐molded (RIM) polyamide block copolymer (NBC) reinforced with a continuous glass strand mat (swirl mat) was studied in instrumented high‐speed impact‐bending tests. Investigations were carried out on Charpy and Izod specimens of different size and notching direction to elucidate specimens' size effects. From the tests performed at ambient temperature and −40°C, fracture mechanical parameters (fracture toughness, fracture energy) and E‐modulus were derived. Changes observed in the above parameters of the matrix were attributed to a molding‐induced skin‐core morphology. Both fracture toughness and energy of the composites depended strongly on the type, i.e., on the ligament width (W) and length‐to‐span ratio (L0/L) of the specimens. Reliable fracture mechanics data can be determined in three‐point bending when W > 12 mm and L0/L > 1.7 for the specimens. The fracture toughness and E‐modulus increased monotonously, whereas the fracture energy reached a plateau as a function of fiber mat content. Their values were only slightly influenced by the temperature. It was also shown how molding‐and machining‐induced flaw size can be defined. The failure mode of the materials was studied by fractography and the failure events were summarized in a model and discussed.

The efficient maintenance of offshore structural integrity using reliability analysis

AbstractEfficient maintenance of offshore structures is one of the priority development areas of the offshore industry world‐wide. The maintenance of the installations in a hostile environment such as the North Sea poses a particular problem. Owing to the large number of wave‐induced stress cycles experienced by the structures, the integrity is subject to progressive degradation under fatigue crack growth. If cracks are discovered in service, the operators of the structures will need to decide on the appropriate course of action. The priorities for actions will also need to be determined for cracks of various sizes found in different locations of the structures.In the past, the above decisions have depended very much on engineering judgement. However, the latest developments in reliability fracture mechanics (RFM) analysis have for the first time provided an objective criterion for this type of decision making. Moreover, this criterion is consistent with the established structural design criteria based on reliability analysis of strength—load interaction. Therefore, it has become a real possibility to incorporate structural integrity considerations by design.The reliability‐fracture‐mechanics‐based fatigue analysis includes studies on the uncertainties of load history, materials crack growth behaviour, fracture mechanics modelling, non‐destructive inspection and other environmental factors. This paper will review the development in this methodology.In order to integrate the above analysis into the overall design process, it is useful to carry out reliability analysis for the complete structural system. The current state of the art of structural system reliability analysis is therefore briefly reviewed. The necessary development in order to accommodate fatigue reliability degradation is then discussed.