Response of CFRP retrofitted steel cylindrical shells to external blast loads

Shuaib, Mujtaba Mahmoud Nasr

Response of CFRP retrofitted steel cylindrical shells to external blast loads

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Mechanical Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

Cylindrical steel shells are commonly used in critical infrastructure such as pipelines, offshore platforms, and bridge piers, where structural integrity under extreme loading is essential. These components are susceptible to degradation over time due to environmental exposure, which can adversely affect their mechanical performance. Structures can also be vulnerable to extreme dynamic loading conditions, such as explosions and impacts, which can result in significant damage to these structures. Explosion is known as one of the security threats that can have a significant impact when directed at key infrastructures such as oil and gas pipelines. Thus, retrofitting solutions that improve the performance of the existing structures to mitigate damage from potential risks while preventing severe deterioration of the element over its lifespan are becoming increasingly significant. The use of carbon fibre-reinforced polymers (FRP) for retrofitting cylindrical shells has gained increasing attention as a means of extending structural service life. Previous studies [1-3] have demonstrated that FRP wrapping improves resistance to quasi-static and impact loads by enhancing longitudinal and circumferential stiffness. FRP offers practical advantages including ease of application, minimal disruption to service, and low added weight. While FRP retrofitting as method for blast protection has been previously introduced [4-8], limited attention has been given to conducting detailed evaluations of its effectiveness, compared to extensive research conducted on concrete elements under blast, and steel tubes under non-blast conditions. This study contributes to bridging existing gaps in evaluating the effectiveness of FRP in mitigating external air-blast effects, through a combined experimental and numerical investigation focused on deformation and failure characteristics. Unlike prior research, which typically employed simplified classifications of steel tube damage (e.g., ruptured vs. unruptured) and focused on contact blast scenarios or large-diameter exposed and buried pipelines, this work evaluates damage progression across a range of impulse loads (6.4–14.98 Ns). Using consistent tube dimensions (Ø101.6 mm) and steel-to-CFRP thickness ratios, four different CFRP lay-up configurations A series of controlled blast experiments were conducted on 33 steel tube specimens, comprising 7 bare and 26 CFRP-retrofitted tubes. Four CFRP lay-up configurations were applied using combinations of longitudinal (L), transverse (T), and ±45° angled plies (A/N): TLTL (90/0/90/0°), ANAN (±45/±45°), TLAN (90/0/±45°), and ANTL (±45/90/0°), to assess the effect of fibre orientation on blast performance. The specimens were subjected to localised blast loads generated by detonated PE4 plastic explosives, with charge masses ranging from 4g to 10g. Quantitative analysis showed that CFRP-retrofitted steel tubes exhibited up to 34% reduction in midpoint deflection and withstood impulse levels approximately 60% higher before tearing occurred, compared to bare specimens. The steel tubes exhibited distinct response modes, ranging from large inelastic deformation (Mode I) at lower impulses to tearing (Mode II) at higher impulses. The experiments revealed combined CFRP failure modes caused by localised blast loads, such as metal-CFRP debonding, interlaminar delamination, and fibre rupture, all of which occurred in the localised blast zone. Simplified dimensional analysis based on the Wierzbicki–Hoo Fatt model enabled scalable comparison, with results showing that CFRP retrofitting reduced the dimensional impulse required for a given deflection, indicating enhanced energy dissipation efficiency compared to bare steel tubes. Preliminary numerical simulations using LS-DYNA were developed based on earlier blast tests on CFRP-retrofitted steel plates to establish the modelling framework for the tube numerical analysis. Simulations were validated using prior experimental data on deflections and impulses, providing insights into the deformation profiles, energy absorption characteristics, and CFRP damage characteristics in the retrofitted plates. While plate specimens showed up to 52% reduction in midpoint deflection with CFRP retrofitting, the tube experiments demonstrated a comparable reduction of up to 34%, alongside a 55.4% increase in tearing failure threshold. These results confirmed the applicability of the plate-based modelling approach to cylindrical geometries and provided practical insights into the blast response of CFRP-retrofitted steel tubes. Numerical modelling was then developed to assess the blast response of CFRP-retrofitted steel cylindrical shells, incorporating strain-rate sensitive plasticity, composite damage models, and cohesive zone modelling. Simulations were validated against experimental data from bare and retrofitted tubes subjected to detonated PE4 explosive charges (4-9g). Although deflection prediction accuracy varied at the low and high charges (4g and 9g), simulations for retrofitted tubes under 5g and 6g charges showed good agreement with experimental deflections within ±10%. The simulated steel tubes exhibited large inelastic deformation and CFRP layers showing fibre fracture and debonding consistent with experiments. Parametric studies explored partial and internal CFRP applications, with internal retrofitting improving midpoint deflection by 15%. While the model captured global deformation trends and failure modes, its accuracy was constrained by literature-based material parameters, absence of strain-rate characterisation, and limited insight into some failure mechanisms such as the steel rupture and CFRP-steel debonding. In conclusion, the experimental and numerical results clearly demonstrate that CFRP retrofitting enhances the blast resistance of steel cylindrical tubes by effectively reducing midpoint deflection and delayed tearing onset. Validated LS-DYNA simulations captured key failure modes, including fibre rupture and metal–CFRP debonding. These findings clearly highlight the potential of FRP retrofitting as an effective method for mitigating structural damage and failure in cylindrical steel tubes subjected to external air-blast loading.

Keywords

blast load

cylindrical steel shells

Reference:

Collections

PhD / Doctoral

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