Balamurugan M. Sundaram

Dynamic Crack Propagation in Layered Transparent Materials Studied Using Digital Gradient Sensing Method: Part-II

Dynamic fracture behavior of bi-layered PMMA samples are studied using Digital Gradient Sensing (DGS) in conjunction with high-speed photography. DGS exploits elasto-optic effect exhibited by transparent solids subjected to a non-uniform state of stress causing deflection of light rays propagating through the material. The current work builds on authors’ previous report concerning crack trapping, interfacial bifurcation, and mixed-mode penetration into the second layer when a dynamically growing mode-I crack in the first layer encounters a normally oriented interface of different strengths. The current work specifically focuses on the role of the interface location within the bi-layered specimen relative to the initial crack tip on fracture behavior with an intention of examining the effect of the incident crack speed and the associated stress intensity factors for select interfacial fracture toughness. The location of the interface is varied sequentially from ‘near’ to ‘far’ from the initial notch tip to accomplish this task. Preliminary results suggest that crack growth in both the interface and the second layer are greatly affected by the location of the interface. That is, the velocity and stress intensity factors of the incident crack affect the outcome of the overall fracture behavior of bi-layered samples including interfacial trapping and penetration into the second layer.

Dynamic Crack Branching in Soda-Lime Glass: An Optical Investigation Using Digital Gradient Sensing

Transparent brittle materials such as soda-lime glass (SLG) with relatively low fracture toughness and high stiffness pose unique challenges for performing full-field optical measurement of deformations and stresses to characterize their fracture behavior. The current work builds on authors’ previous report wherein the feasibility of Digital Gradient Sensing (DGS) was demonstrated for measuring stress wave induced crack-tip deformations in SLG. In this study, ultrahigh-speed photography (>1 million frames per sec) was used in conjunction with DGS and a Hopkinson pressure bar to load V-notched SLG plates to investigate the crack branching phenomenon. The experimental parameters were controlled such that a single mode-I crack that initiated at the V-notch tip propagated through the glass plate before branching into two prominent mixed-mode daughter cracks. The optical measurements of angular deflection fields that represent stress gradients in two orthogonal in-plane directions were obtained. Using higher order finite-difference based least-squares integration (HFLI) scheme, stress invariant fields (σxx + σyy) were evaluated near dynamically propagating crack-tip throughout the branching process.

Fracture and Failure Characterization of Transparent Acrylic Based Graft Interpenetrating Polymer Networks (Graft-IPNs)

IPNs are made of two or more polymer networks, each polymerized in the presence of the other/s. They can be suitable alternatives to traditional polymers made from single monomer as desirable characteristics of the constituent polymers can be engineered into IPNs. In this study, an acrylic-based transparent graft Interpenetrating Polymer Networks or simply graft-IPNs were processed and their mechanical properties in general and fracture/failure behaviors in particular were characterized. Good optical transparency, high fracture toughness, and high stiffness were among the attributes targeted in the graft-IPNs for potential transparent armor applications. The graft-IPNs were synthesized by sequential polymerization of compliant elastomeric polyurethane (PU) phase and a stiff acrylate-based copolymer (CoP) phase to generate crosslinks (or, ‘grafts’) between the two networks. A series of such graft-IPNs were synthesized by varying the ratios of CoP:PU. Uniaxial tension tests were performed on the resulting IPNs to measure the elastic modulus and strength whereas mode-I fracture toughness was measured under both quasi-static and dynamic loading conditions. A Hopkinson pressure bar was used in conjunction with an optical technique called Digital Gradient Sensing (DGS) and ultrahigh-speed photography to measure the fracture behavior during stress wave loading. The results show significant enhancements in the crack initiation toughness for some of the graft-IPN compositions relative to the constituents as well as commercially procured PMMA and polycarbonate (PC) sheet stocks. Besides the optical transparency, the increase in fracture toughness is attributed to the grafts or crosslinks generated between the PU and CoP networks.

Digital Gradient Sensing Method to Visualize and Quantify Crack-Tip Deformations in Soda-Lime Glass Under Static and Dynamic Loading

Transparent ceramic materials such as soda-lime glass pose unique challenges for visualizing and quantifying deformations to characterize fracture and failure. The relatively low fracture toughness coupled with high stiffness and elastic wave speeds create spatio-temporal challenges as deformations tend to be confined to a very small region around an almost mathematically sharp crack that tends to propagate at ∼1500 m/s. Soda-lime glass show weak birefringence to be able to use conventional photoelasticity. Interferometric methods often need elaborate coherent optics and are less attractive in terms of experimental simplicity. Digital image correlation (DIC) techniques, on the other hand, require creating speckles on the specimen surface which makes it difficult to locate a propagating crack-tip for evaluating fracture parameters precisely. Also, achieving a good measurement resolution at conventional magnifications using currently fielded high-speed cameras is difficult. Motivated by these overlapping factors, the feasibility of Digital Gradient Sensing (DGS) method to measure crack-tip deformations near stationary and growing cracks in soda-lime glass is investigated in this work. Both quasi-static and dynamic problems associated with crack initiation and growth are successfully demonstrated.

Dynamic Mixed-Mode Crack Initiation and Growth in PMMA and Polycarbonate

Mixed-mode dynamic crack initiation and growth in polymethymethacrylate (PMMA) and polycarbonate (PC) are studied experimentally. A simple specimen geometry and loading configuration is used to generate various mode-mixities during dynamic crack initiation and fracture. A Hopkinson pressure bar is used to rapidly load free-standing edge cracked samples in reverse impact configuration. Using eccentric loading relative to the crack line, different mode-mixities at crack initiation are achieved by increasing the initial crack length while keeping all other experimental parameters unchanged. A relatively new full-field optical technique, Digital Gradient Sensing (DGS), along with ultrahigh-speed photography is used to perform full-field measurements. DGS can measure instantaneous angular deflections of light rays due to elasto-optic effects and provides two orthogonal stress gradients under plane stress conditions. The mode-I and -II stress intensity factor histories of PMMA are evaluated via overdeterministic analysis of optically measured data. By quantifying critical stress intensity factors evaluated at crack initiation, dynamic fracture envelopes can be developed.

Dynamic Penetration and Bifurcation of a Crack at an Interface in a Transparent Bi-Layer: Effect of Impact Velocity

Dynamic fracture behavior of layered PMMA sheets is studied using transmission-mode Digital Gradient Sensing (DGS) technique. DGS is a relatively new optical method that exploits elasto-optic effects exhibited by transparent solids allowing a direct quantification of two orthogonal in-plane stress gradients simultaneously and hence crack tip parameters when used to study fracture mechanics problems. The current work builds on authors’ previous two reports on this topic. Interfacial trapping, bifurcation and mixed-mode penetration into the second layer of a dynamically growing mode-I crack in the first layer encountering a normally oriented interface in a bi-layered configuration was reported in the first report [1]. In the second, the role of the location of a weak interface relative to the initial crack tip within the given geometry of the specimen was studied and interfacial penetration vs. bifurcation mechanisms was demonstrated and analyzed [2]. The current work focuses on the effect of impact velocity and the resulting loading rate on crack branching/penetration phenomenon when the mode-I crack encounters a normally oriented interface. In this ongoing work, a select location of interface relative to the initial crack tip is re-examined by varying the impact velocity. Using DGS for visualization and quantification, fracture mechanisms associated with crack growth are explained for the bi-layered system.