Dynamic Contrast-Enhanced MR Imaging in Head and Neck Cancer: Techniques and Clinical Applications

ABSTRACT SUMMARY: In the past decade, dynamic contrast-enhanced MR imaging has had an increasing role in assessing the microvascular characteristics of various tumors, including head and neck cancer. Dynamic contrast-enhanced MR imaging allows noninvasive assessment of permeability and blood flow, both important features of tumor hypoxia, which is a marker for treatment resistance for head and neck cancer. Dynamic contrast-enhanced MR imaging has the potential to identify early locoregional recurrence, differentiate metastatic lymph nodes from normal nodes, and predict tumor response to treatment and treatment monitoring in patients with head and neck cancer. Quantitative analysis is in its early stage and standardization and refinement of technique are essential. In this article, we review the techniques of dynamic contrast-enhanced MR imaging data acquisition, analytic methods, current limitations, and clinical applications in head and neck cancer. ABBREVIATIONS: AIF ϭ arterial input function; DCE-MR imaging ϭ dynamic contrast-enhanced MR imaging; EES ϭ extracellular extravascular space; GCA

Strain Sensor’s Network for Low-Velocity Impact Location Estimation on Carbon Reinforced Fiber Plastic Structures: Part-I

116-124In this work, we have investigated the strain response (angular/spatial) from fiber Bragg grating (FBG) sensor & resistance strain gauge (RSG) sensors bonded to the composite structure due to the projectile low velocity impact (LVI). The number of sensor & its orientating has been optimized based on such experimental data and designed an optimum sensor network for faithful LVI detection. In order to study the efficacy of the sensor network, an impact localization algorithm based on peak strain amplitude from the sensor bonded to the structure was used in this study. Further the detection efficiency of the algorithm has been improved using weighted average value around the peak amplitude of strain experienced by the sensor. We found that for the high energy (~35 J) LVI the maximum distance error (Euclidian distance) was 50 mm for 80% of total trail case. Furthermore, we have developed and compared the relative performance of the algorithm cited in the literature, will be presented in PART-II of the same Journal

Novel Design of Cocured Composite ‘T’ Joints with Integrally Woven 3D Inserts

Composites can be exploited to their full potential when cocured, wherein different parts are made and bonded together in a single cure operation to realise an integral structure. The key element in a typical cocured construction is T-joint, which forms the primary load transfer mechanism between the skin and stiffener in a structural assembly. T-joints are particularly vulnerable for pull off loads and researchers are looking at various techniques to improve the pull strength viz. stitching, tufting, 3D weaving, multilayer weaving, 3D braiding and the like. The present work uses a novel technique to improve the strength of T-joints by employing a hybrid design wherein an integral 3D ‘T’ insert is interleaved with a conventional T-joint. Inserts were woven using 3K and 6K carbon tows and incorporated in T-joints using CSIR-NAL proprietary process called ‘Vacuum Enhanced Resin Infusion Technology (VERITy)’ process. Several configurations of T-joints were tested in an UTM in the pull mode till the failure to assess the efficacy of integrally woven 3D inserts. It was observed that the initial failure load was nearly the same across the various T-joint configurations tested whereas the maximum failure loads were quite different. The normalised strength of T-joints with integrally woven 3D inserts in pull off mode was enhanced by about 30% when compared T-joints without the insert and thus vindicating the usage of integrally woven 3D insert in a cocured T-joint. The insert is conceived in such a way that it can be easily incorporated in the design of cocured structures

Structural Health Monitoring of Composite Aircraft Structures Using Fiber Bragg Grating Sensors

Aircraft industry is continually striving towards reducing the acquisition, operation and maintenance costs. Usage of advanced composite materials in primary aircraft structures have resulted in significant weight savings owing to their higher specific strength and specific stiffness. Composite structures, in spite of their inherent advantages, are prone to various damages. To detect and repair various structural damages that can occur during the service life of the aircraft, a thorough inspection schedule is implemented through conventional visual and Non Destructive Evaluation methods. Such scheduled inspections lead to considerable increase in maintenance cost & down-time of the aircraft. An online structural health monitoring (SHM) system consisting of well-designed sensor networks incorporated in the structure along with necessary hardware and software can provide information about the structure, thereby leading to reporting of flaws or damages in real time. Such a system can provide inputs for condition based maintenance which can result in reduced maintenance cost. This paper presents the work carried out at CSIR-National Aerospace Laboratories towards developing a flight-worthy SHM system and its demonstration on an unmanned aerial vehicle (UAV). Sensor selection, characterization, instrumentation design, algorithm development towards damage detection & load estimation at lab level and implementation of the technology on a UAV are discussed in this paper

Challenges in Static Testing of Co-Cured Co-Bonded Composite Aircraft Structures

Due to several advantages, advanced composite structures have been successfully used for developing severalrimary aircraft structures. One of the certification requirements, as per FAR 25, is to ensure their compliance to the design loads by conducting static tests. Based on the requirements given by the designer, the experiments are conducted. To achieve realistic behavior of the structure in the static test loading and attachments should be properly simulated. This requires considerable time and is a very challenging assignment. The general philosophy is to carryout limited testing, and then uses analytical models to simulate other load cases. A good correlation between testing and analysis is essential to gain confidence in the design of the structure. The total aerodynamic loads are to be distributed as lumped loads without effecting much change in the bending moments and shear forces diagrams. Testing also involves collection of reliable information from strain gauges, dial gauges and acoustic emission sensors. Adequacy of the test rigs and proper simulation of all attachments need to be addressed. The design of the whiffletree to distribute the loads in the desired fashion is one of the key parameters in the testing process. The paper discusses these aspects and the challenges in carrying out static test through a few examples

Damage Tolerance of Stiffened Skin Composite Panels

This work discusses design, fabrication and damage tolerance evaluation of co-cured skin stringer carbon fiber composite panels. Such stiffened panels are typically found in aircraft wing skins. Composite panels representing a portion of an aircraft wing box are designed and fabricated using Carbon fiber and epoxy matrix using resin infusion process. Fixtures to support the panels during low velocity impact tests are also designed and fabricated. A drop tower is used to conduct impact tests. Panels are subjected to various impacts to study the effect of impact energy on damage visibility and damage size. Impacts are also categorized according to their location: (a) Impact exactly above stringer, (b) Impact above skin, and (c) Impact above stringer flange. The extent of damages is studied based on non-destructive inspection techniques such as ultrasonic inspection. Further, one of the panels containing impact damages is subjected to residual strength test. Displacements and strains are measured using digital image correlation technique and resistance strain gages. Finite element model of the panel is also developed. Deformations and strains obtained from FE simulations are compared with test data. Results show that impact damages did not alter the load path significantly in the composite panel

Composites airframe panel design for post-buckling – An experimental investigation

Three series of airframe composite panels with T-stiffener, I-stiffener and J-stiffener are designed and optimized to have the same local skin buckling load and weight. All the panels are designed to undergo local skin buckling between Design Limit Load (DLL) and Design Ultimate Load (DUL), approximately at 120% of DLL to utilize the reserve strength in structures. Additionally, identical panels are designed and fabricated from each series to study the effect of various extrinsic parameters such as disbond, delamination, impact damage and repeated loading to identify the best performing series for post-buckling design. All the panels are tested under compression to demonstrate no onset of damage before DUL. The influence of defects such as disbond, delamination and impact damage on the post-buckling behavior is also demonstrated. One pristine panel of each series is repeatedly loaded 1000 times beyond buckling to determine the onset of damage if any. The panels with I-stiffener and J-stiffener found to be the potential design choices for post-buckling design philosophy due to the high margin between skin buckling and collapse load even in the presence of damage. The results from this study would help in moving closer to the post-buckled composite design philosophy for airframe structure

Tufting thread and density controls the mode-I fracture toughness in carbon/epoxy composite

Herein, interlaminar crack initiation and its growth in tufted carbon/epoxy composite under Mode-I loading are investigated for different thread materials and tuft densities. Two configurations of Double Cantilever Beam (DCB) test specimens are fabricated – one with 2 rows of tuft (hereafter referred to as lower tuft density) and another with 3 rows of tuft (will be referred to as higher tuft density). The crack front is arrested and delamination growth is delayed by tufting, which increases interlaminar fracture toughness . Higher enhancement in fracture toughness is observed for carbon thread tufted specimens followed by Kevlar and then glass thread tufted specimens. Fracture toughness of tufted specimens is about 4.5 to 10 times of untufted specimen depending on the thread material and tuft density. An increase in tuft density increases fracture toughness from 175% for Kevlar to 272% for glass threads. Fiber bridging from the parent laminate layer is observed in the untufted specimen, whereas in the tufted specimen, this phenomenon is insignificant except for bridging due to tufting thread. Fracture analysis shows that the failure is mainly due to the rupture of thread at the interface. Thread pull-out or slippage is absent exhibiting good adhesion with epoxy matrix

Unfolding the effects of tuft density on compression after impact properties in unidirectional carbon/epoxy composite laminates

Post impact compressive residual strength of epoxy-based composite laminates is studied in the presence of through-thickness reinforcement (TTR). The tufting technique is used to introduce TTR in compact carbon unidirectional (UD) composites. Composite laminates are tufted using Kevlar thread with different tuft density by varying tuft pitch and spacing. The specimens are impacted with low-velocity impact to achieve barely visible impact damage (BVID). The damaged area is quantified using the ultrasonic C-scan method. Tufted specimens exhibited a significant reduction in the damaged area. The reduction in damage area is about 26% to 51% depending upon the tuft density. Residual strength post impact is determined by compression after impact (CAI) test. Upon increasing the tuft density up to 0.56%, the damaged area decreases by 51%, and CAI strength increases by 43%. Above this tuft density, damage area slightly decreases and CAI strength remains almost the same. The improvement in CAI strength is due to an increase in apparent interlaminar strength attributed to the enhancement in bridging effect due to TTR. Different failure modes such as delamination, fiber crushing, kink-band formation, etc. are observed in both the untufted and tufted specimens