NON-DESTRUCTIVE EVALUATION OF DEFECTS IN COMPOSITE BI-MATERIAL STRUCTURES AND ESTIMATION OF FRACTURE FRONT USING DATA DRIVEN TERAHERTZ TIME DOMAIN ANALYSIS

The high luminosity Large Hadron Collider (HL-LHC) will collide particles at unprecedented rates to search for new physics and make high precision measurements to challenge the standard model with emerging technologies that pose high demands for the materials of charged particle tracking detector support structures. Tracking detectors at current (and future) colliders are encounter high-radiation environment where polymeric and carbon fiber composite materials are used in the mechanical support structures of the detectors. The accumulated radiation dose for these materials and thermal loads lead to defects like voids and cracks due to de-gassing and thermal cycling. Terahertz time domain spectroscopy is used to map these strains in a bi-material strip and trace the locations of fractures in a thermal interface material (TIM) layer or an adhesive layer. Statistical data driven terahertz scan image processing analysis is used for predicting the fracture propogation behavior to validate the cohesive FEA model for the fracture observed.
EXTENDED ABSTRACT

SIZE EFFECTS OF COMPOSITE CEMENT AND FUNCTIONALIZED PLASTIC BEAMS: TOWARDS INCREASED DUCTILITY AND ENERGY ABSORPTION

Polyethylene Terephthalate (PET) plastic particles, having been functionalized using a simple, cost-effective, and scalable treatment technique, presented in patented application 17484834, were used as a cement replacement ingredient in plain cement beams. The functionalization increases the affinity of PET to water, and thus their hydrophilicity, enabling the particles to form bonds with Ordinary Portland Cement (OPC) hydration products. The particles were randomly distributed into cement powder during the mixing process. Size effect beams of 4 different geometrically similar sizes were cast in three different percentages (families) of cement replacement with functionalized PET in notched beams to be tested in three-point bending. Bažant’s Type 2 Size Effect Law was used to elucidate the size effects and initial fracture energies (Gf) of all families. The Hillerborg Work-of-fracture method was used to find the total fracture energy (GF). Preliminary results indicate that beams with adequately bonded PET demonstrated improved ductility, caused by crack bridging, as well as increased i) fracture process zone (FPZ) size, ii) Gf and iii) GF, compared to reference OPC beams, while closely preserving the bending strength for larger sizes.
EXTENDED ABSTRACT

DEBOND FRACTURE AND KINKING IN MULTILAYER SYSTEMS: THEORETICAL SOLUTIONS AND PRACTICAL APPLICATIONS

Debond fracture is a dominant failure mechanism in multilayer systems used for various current applications, from laminated and sandwich structural components to protective coatings and thermal barrier coatings; from microelectronic devices, in the electronics and flexible electronics fields, to biomedical devices. Debond cracks originate and propagate at the interfaces between the layers, which often have disparate mechanical and thermal properties; they may kink out of the interfaces and lead to unexpected collapses, such as those observed in marine sandwich composites where these mechanisms may yield to the detachment of entire portions of the core from the outer facesheets. The presentation reviews elasticity techniques and closed form solutions recently derived by the authors for the fracture parameters of interface cracks in edge cracked orthotropic layers, bimaterial layers and sandwich beams and for the crack tip compliance coefficients (root rotations and displacements) in bimaterial isotropic and orthotropic layers. Practical applications of the solutions will be discussed: operative formulae for the characterization of the interfacial toughness in classical and novel fracture mechanics specimens; calibration of the parameters of one-dimensional model; and analytical criteria for kinking in multilayer systems.
EXTENDED ABSTRACT

EFFECTS OF TEMPERATURE ON FIBER TENSION FRACTURE TOUGHNESS OF COMPOSITE LAMINATES AT HIGH LOADING RATE

The understanding of fracture toughness associated with fibre dominated tensile failure is of great important for safety design of composite structrures threatened by extreme loading conditions, such as high/cold temperature and high rate loading. The dynamic fracture toughness of composite laminates in fibre tension is characterized under different temperatures (e.g. -55 °C, 23 °C and 90 °C) with compact tension (CT) sample, at loading rate of 8 m/s using a tension Hopkinson bar intergrated with a experimental chamber. Digital image correlation (DIC) with high-speed imaging is employed for obtaining the full-field strain fields and crack tip location.
EXTENDED ABSTRACT

FATIGUE CHARACTERIZATION OF ADHESIVELY-BONDED GFRP JOINTS VIA SELF-HEATING

High cycle fatigue (HCF) of composite structures is known to exhibit self-heating coupled with stiffness degradation due to progressive damage accumulation. Recent advances have been made in correlating fatigue damage accumulation to full-field temperature fields during the HCF of composite structures. This work uses a thermal medium wave infrared camera to quantify self-heating in adhesively bonded unidirectional glass fiber reinforced polymer (UD-GFRPs) specimens under tension-tension fatigue loading.
EXTENDED ABSTRACT

FAILURE MECHANISMS OF STEEL FIBERS EMBEDDED IN HSFRSCC

The bond stress transfer between fibers and matrix is the basic resistant mechanism of fiber-reinforced composite materials. Interfacial bond properties and failure mechanisms of the composite are commonly evaluated through single pullout tests on unreinforced matrices. The small size of the molds used to cast the samples prevents the fibers from being randomly distributed in the matrix and makes it difficult to compact the mixture. Krahl et al. (2020) developed an innovative portable pullout machine that allows testing fibers embedded in fiber-reinforced matrices with larger sample sizes. This paper discusses the experimental results of single fiber pullout tests carried out with the portable machine on high-strength fiber-reinforced self-compacting concrete (HSFRSCC). The bond behavior of hooked-end steel fibers and their relationship with the failure mechanisms are analyzed for fiber contents of 0% and 0.75%. The results show that bond and failure mechanisms were influenced by the presence of fibers in the matrix.
EXTENDED ABSTRACT

AN ANALYTICAL APPROACH FOR THE FRACTURE CHARACTERIZATION IN CONCRETE UNDER CYCLIC LOADING CONDITION

Many civil engineering infrastructures frequently encounter repetitive loading during their service life. Due to the inherent complexity observed in concrete, like quasi-brittle materials, understanding the fatigue behavior in concrete still poses a challenge. Moreover, the fracture process zone characteristics ahead of the crack tip have been observed to be different in fatigue loading than in monotonic cases. Therefore, it is crucial to comprehend the energy dissipation associated with the fracture process zone (FPZ) due to repetitive loading. It is well known that stiffness degradation due to cyclic loading provides a better understanding of the fracture behavior of concrete. Under repetitive load cycles, concrete members exhibit a two-stage stiffness degradation process. Experimentally it has been observed that the stiffness decreases initially with an increase in crack length and subsequently increases. In this work, an attempt has been made to propose an analytical expression to predict energy dissipation and, later, the stiffness degradation as a function of crack length.
EXTENDED ABSTRACT

FRACTURE AND FATIGUE STUDIES ON META-SANDWICH AUXETIC CORE

Inspired by the Parker solar probe’s heat shield, a carbon-carbon semi-auxetic laminate sandwiching a lightweight carbon auxetic core has been designed in this work. The fracture and fatigue crack propagation in 2D and 3D auxetic core at ambient and extreme temperatures have been predicted and compared with conventional honeycomb cores and foams. Comparative studies have been performed between the results obtained by in-house codes of phase-field fracture (PFF) in FEniCS and the extended finite element method (XFEM) in ABAQUS™.
EXTENDED ABSTRACT

ON THE DESIGN OF CRACK-ARRESTING LAYERS IN POLYPROPYLENE BASED MULTILAYER COMPOSITES

In natural materials, outstanding properties can be attained through an advantageous combination of different materials in intricate microstructures. Usually, a matrix material contributes high strength and stiffness (“hard phase”), while interlayer (IL) materials are often proteins with low strength and modulus (“soft phase”) but high strains at break. The combination of both phases leads to crack arresting properties, thus dramatically increasing the fracture toughness and damage tolerance of the composite. Renowned examples of such effects are nacre as well as deep-sea sponges. The same concepts shall be mimicked to increase the fracture toughness of talcum reinforced polypropylene (PP). The challenge lies in preserving specimen stiffness while also utililzing the crack arresting properties of soft ILs. In this contribution, two types of PP with different mechanical properties are used as ILs. Normalized parameters for fracture toughness and specimen stiffness are used in order to assess the overall properties of multilayer composites. The trade-offs between stiffness and toughness are illustrated, while optimized structures also demonstrate that both properties can be attained simultaneously.
EXTENDED ABSTRACT

TAGUCHI BASED – FUZZY METHOD OPTIMIZATION OF PROPOSED ULTRA-HIGH STRENGTH STEEL /UHMWPE HELMET UNDER VARIABLE IMPACTOR CONDITIONS.

Ultra high strength steel (UHSS) is known for its high strength, high modulus and high energy absorption ability through plastic deformation. However, it is less appealing for lightweight applications due to it relatively high density despite their cheap price. Therefore, all advanced helmets are made from advanced lightweight polymeric materials which are very expensive. The advantages of these classes of material can be harnessed by combining the synergies between them. This study seeks to investigate the application of thin layers of high strength steel and ultra-high molecular weight polyethylene (UHMWPE) in designing cheap and low weight helmet under impact application. Optimization of the proposed helmet under variable impact conditions has been performed through a Taguchi-based fuzzy logic approach. The optimization study investigated the integrity of the proposed helmet considering variable helmet weights, impact velocities and impactor masses. Optimum combinations of these design parameters were obtained through the utilization of Taguchi orthogonal array matrix. Maximization of fracture energy and reaction forces between inner shell and cushion of helmet were considered as criteria for the optimization procedure. Input data for the optimization process were obtained through numerical simulation using the explicit finite element program – LS-PrePost.
EXTENDED ABSTRACT