Hamed Yazdani Nezhad

Prediction of Transient Creep Response Under Combined Primary and Secondary Loading

Residual stress effects on creep deformation and fracture play a significant role in structural integrity assessments of engineering components. The focus of the current work is to investigate creep behaviour of mechanically loaded cracked structures in the presence of residual stress fields. Finite-element analyses have been carried out on single edge notch bend, SEN(B) , and tension, SEN(T) , specimens at different residual stress and mechanical stress levels. The redistribution time and associated stress relaxation for combined primary and secondary (residual) stresses have been determined (from the finite-element analysis) and interpreted using the transient fracture mechanics parameter, C(t). The observed trends are consistent with earlier studies involving combined thermal and mechanical stress.Copyright

Study of Creep Relaxation Behaviour of 316H Austenitic Steels Under Mechanically Induced Residual Stress

Compact tension 316H austenitic steel specimens, extracted from an as-received ex-service pressure vessel header, have been pre-compressed to different load levels in order to introduce a residual stress field. Finite element (FE) analysis has been performed to predict the load level required to obtain a high magnitude tensile stress field over a significant distance ahead of the notch while preventing a large plastic zone in the specimen. The predicted residual stress profiles along the crack path are compared with those measured using neutron diffraction (ND). Comparisons have also been provided between the ND results of this work with recent work carried out on 316H and 347 stainless steels under different loading levels. The creep relaxation behaviour of the steel has been studied numerically. A proposed method to estimate the steady state creep crack tip parameter, C*, has been examined using the obtained displacement rates for the case of combined loading. Creep relaxation data for combined stresses are compared with the earlier studies.Copyright

Effect of morphological changes due to increasing carbon nanoparticles content on the quasi-static mechanical response of epoxy resin

Mechanical failure in epoxy polymer and composites leads them to commonly be referred to as inherently brittle due to the presence of polymerization-induced microcrack and microvoids, which are barriers to high-performance applications, e.g., in aerospace structures. Numerous studies have been carried out on epoxy’s strengthening and toughening via nanomaterial reinforcement, e.g., using rubber nanoparticles in the epoxy matrix of new composite aircraft. However, extremely cautious process and functionalization steps must be taken in order to achieve high-quality dispersion and bonding, the development of which is not keeping pace with large structures applications. In this article, we report our studies on the mechanical performance of an epoxy polymer reinforced with graphite carbon nanoparticles (CNPs), and the possible effects arising from a straightforward, rapid stir-mixing technique. The CNPs were embedded in a low viscosity epoxy resin, with the CNP weight percentage (wt %) being varied between 1% and 5%. Simplified stirring embedment was selected in the interests of industrial process facilitation, and functionalization was avoided to reduce the number of parameters involved in the study. Embedment conditions and timing were held constant for all wt %. The CNP filled epoxy resin was then injected into an aluminum mold and cured under vacuum conditions at 80 °C for 12 h. A series of test specimens were then extracted from the mold, and tested under uniaxial quasi-static tension, compression, and nanoindentation. Elementary mechanical properties including failure strain, hardness, strength, and modulus were measured. The mechanical performance was improved by the incorporation of 1 and 2 wt % of CNP but was degraded by 5 wt % CNP, mainly attributed to the morphological change, including re-agglomeration, with the increasing CNP wt %. This change strongly correlated with the mechanical response in the presence of CNP, and was the major governing mechanism leading to both mechanical improvement and degradation.

Electrospun Piezoelectric Polymer Nanofiber Layers for Enabling in Situ Measurement in High-Performance Composite Laminates

This article highlights the effects from composite manufacturing parameters on fiber-reinforced composite laminates modified with layers of piezoelectric thermoplastic nanofibers and a conductive electrode layer. Such modifications have been used for enabling in situ deformation measurement in high-performance aerospace and renewable energy composites. Procedures for manufacturing high-performance composites are well-known and standardized. However, this does not imply that modifications via addition of functional layers (e.g., piezoelectric nanofibers) while following the same manufacturing procedures can lead to a successful multifunctional composite structure (e.g., for enabling in situ measurement). This article challenges success of internal embedment of piezoelectric nanofibers in standard manufacturing of high-performance composites via relying on composite process specifications and parameters only. It highlights that the process parameters must be revised for manufacturing of multifunctional composites. Several methods have been used to lay up and manufacture composites such as electrospinning the thermoplastic nanofibers, processing an inter digital electrode (IDE) made by conductive epoxy–graphene resin, and prepreg autoclave manufacturing aerospace grade laminates. The purpose of fabrication of IDE was to use a resin type (HexFlow RTM6) for the conductive layer similar to that used for the composite. Thereby, material mismatch is avoided and the structural integrity is sustained via mitigation of downgrading effects on the interlaminar properties. X-ray diffraction, Fourier transform infrared spectroscopy, energy dispersive X-ray spectroscopy, and scanning electron microscopy analyses have been carried out in the material characterization phase. Pulsed thermography and ultrasonic C-scanning were used for the localization of conductive resin embedded within the composite laminates. This study also provides recommendations for enabling internally embedded piezoelectricity (and thus health-monitoring capabilities) in high-performance composite laminates.

Impact damage response of carbon fibre-reinforced aerospace composite panels

This research looks into the damage response and energy absorption behaviour of unidirectional carbon fibre-reinforced polymer (CFRP) composite panels subjected to low-velocity impact events. The response of CFRP composite materials with thermoset (TS) resin and thermoplastic (TP) PEEK polymer matrix are investigated. Evolution of impact force and absorbed energy with time during impact are presented for each TS and TP panel. Comparisons are provided between damage area obtained optically and using C-scanning technique. The investigations are based on the scanned images along with the characteristic force and absorbed energy curves for two material systems with TS and TP polymers, having similar stacking sequences, carbon volume fraction and thickness.

Influence of Prior Deformation on Creep Crack Growth Behaviour of 316H Austenitic Steels

The influence of pre-strain and pre-stress on creep crack growth behaviour of 316H austenitic steels is studied experimentally and numerically in this paper. Compact tension, C(T), specimens (25mm thickness) have been extracted from two steam headers, one as-received and one uniformly compressed to the strain value of 8%. The C(T) specimen extracted from the as-received header was compressed, introducing a non-uniform strain field. Creep crack growth (CCG) tests were performed at 550°C. Comparisons have been provided with the results from as-received C(T) specimens. Finite element (FE) analysis has been carried out to simulate the CCG behaviour of the C(T) specimens. By choosing the problem parameters appropriately, good agreement may be achieved between the FE predictions and the creep data.© 2012 ASME