Interfacial stresses and debonding failures in plated beams

Extensive research and recent developments in structural engineering has shown that adhesive bonding of fibre-reinforced polymer (FRP) composite, steel or any other metallic plate to the tension face of a reinforced concrete (RC), metallic or timber beam can effectively enhance its strength and other aspects of structural performance. This technique is now popularly adopted for retro-fitment and rehabilitation of existing structures. These plated beams often fail prematurely well before attaining the full flexural capacity by either plate end debonding (PED) or intermediate crack-induced interfacial debonding (ICD) failure. Concentration of higher interfacial shear and normal stresses at the plate end due to a geometric discontinuity is believed to be responsible for PED that initiates at the plate end and propagates inwards. PED includes concrete cover separation and interfacial debonding initiated at the plate end; and such failure initiated at a critical diagonal crack. ICD initiates at an intermediate major flexural or flexural-shear crack in the soffit of the original beam due to high bond stress and propagates towards one of the plate ends (type-1) or an adjacent crack (type-2). This thesis presents a study of interfacial stresses and debonding failures in plated beams. It first presents a simple and novel theoretical solution of interfacial stresses applicable to any loading considering major deformations like axial and flexural deformations in the beam and plate within linear elastic range. This solution is then enhanced with the inclusion of the effect of adherends’ shear deformation by approximating the displacement field for interfacial shear stress and using Timoshenko’s beam theory for interfacial normal stress, achieving a better understanding of the effect of shear deformation which is ill-understood. This resulted in a first ever solution to include the effect of adherends’ shear deformation under both interfacial shear and normal stresses. This solution is further advanced by developing a rigorous and a versatile closed-form solution fully based on Timoshenko’s beam theory that offered a significant insight. Interfacial stresses at the plate end cannot be measured directly using available measurement techniques, and may only be interpreted indirectly from measured plate strains. The conventional interpretation is based on the assumption that the plate is under pure tension. A significant drawback of this is that the interfacial normal stresses iii cannot be deduced. A new technique is developed to deduce both interfacial shear and normal stresses from strain measurements. The thesis presents three PED strength models for the special case of an RC beam with the plate terminated in the constant moment region: a theoretical model based on interfacial fracture mechanics with a reasonable accuracy; a semi-empirical model with greater accuracy; and an empirical model that is slightly less accurate but simpler to apply than the semi-empirical model. This is followed by the development of a shear debonding model to predict the debonding failure in an RC beam with the plate terminated in high shear and a very low or zero moment region. The two models for PED failure in pure bending and pure shear zones are then combined to result in an accurate shear-bending interaction debonding model. An assessment of these models against a carefully constructed large test database shows that they are more accurate than existing models and suitable for implementation in design codes or guidelines. Finally, a structural mechanics formulation for an FRP-to-concrete bonded joint between two adjacent cracks is developed. It considers axial forces, transverse shear forces and bending moments in the adherends and uses a linearly softening bond-slip model. A section analysis with partial interaction and a rotational spring method are used to relate the applied loading to the interfacial deformation. A closed-form solution is obtained that may form the basis of a rational ICD design method

Numerical Simulation of Ballistic Impact on Armour Plate with a Simple Plasticity Model

Ballistic impact of a steel projectile on armour steel plate is examined by numerical simulations using 3-D nonlinear dynamic explicit finite element code ANSYS LS-DYNA. Simulations are attempted using a simple strain rate dependent plasticity model that can capture large strain, strain rate hardening and fracture encountered at high velocity ballistic impacts. Initial simulations are carried out for a cylindrical bullet with a semi-spherical nose shape impacting a military vehicle door at two different velocities as a test problem. This is then extended to simulate a real problem of armour piercing shot impact on a thick armour steel plate at ordnance velocity regime. The former is compared with results reported in published literature while the latter is assessed with the experimental findings. The deformation pattern generated in the deformed armour plate, residual projectile velocity and displacement of the projectile are taken as the necessary parameters for evaluating the results of simulation. The study presented in this paper demonstrates the effectiveness of the adopted simple plasticity model to simulate a highly nonlinear phenomenon to reasonably predict the physically measurable impact parameters.Defence Science Journal, Vol. 64, No. 1, January 2014, DOI:10.14429/dsj.64.452

Experimental studies on snaking in 3D-printed cylindrical shells under axial compression using photogrammetry

<p>The buckling instability of cylindrical shells under axial compression has been one of the most renowned problems in structural engineering for several decades. Many pioneering works in the 20<sup>th</sup> century have provided insights into understanding the shells' infamous imperfection sensitivity and led to reliability-based designs. However, a recent surge in numerical studies of the snaking phenomenon explores the development of a localised stable post-buckling mode in axially compressed cylindrical shells. Hitherto, none of the experimental studies report on the evolution of azimuthal snaking. In this work, experimental studies are carried out with the objective of revealing the snaking phenomenon. The axial compression experiments are performed on 3D-printed shells made of thermoplastic polyurethane (TPU). The work's novelty lies in the usage of TPU shells for slowing down the propagation of circumferential dimples and making it feasible to capture them using Photogrammetry. Despite the match between the experimental and numerical mode shapes, the experiments reveal multiple routes for the snaking sequence. Furthermore, mode transitions such as reduction in circumferential wave number and transformation of symmetric mode into an asymmetric one are observed. These experimental results provide insights into the localised phenomenon of snaking and validate numerical solutions.</p><p>The dataset of Numerical results of buckling of 3D printed shells was generated using ABAQUS, a Finite element software.</p> <p>The dataset of Experimental results of buckling of 3D printed shells consists of load-displacement response under compression. In addition, the post-buckling mode shapes of the shell acquired using Photogrammetry were also uploaded.</p&gt