Post-impact Residual Strength Research Articles

Comparison of post-impact residual strength ratio between hybrid filament-wound cylinder and hybrid plate specimen

The filament-wound (FW) cylinders possess numerous benefits as a high-pressure storage device but are susceptible to impact loads. The presence of post-impact residual strength in an operating FW cylinder has the potential to accelerate the onset of failure. Ensuring the structural integrity and safety of the equipment requires thorough impact testing, which can be expensive and time-consuming, resulting in limited use. This article presents an innovative and cost-effective approach for conducting low-velocity impact testing on a hybrid Al/CFRP filament-wound cylinder by utilizing a simple hybrid Al/CFRP plate specimen. The main focus is to conduct low-velocity impact testing on both types of specimens, followed by post-damage testing that includes tensile tests on the hybrid plate specimen and burst tests on the FW cylinder. The results of laboratory experiments were compared to explicit and implicit numerical models conducted in a broad spectrum of impact energy range and found to be consistent. Using a parameter known as the post-impact residual strength ratio, it is possible to determine the expected impact loading conditions on the hybrid plate specimen that are equivalent to those experienced by the FW-cylinder. This low-cost impact testing method offers a practical and feasible solution to ensure the dependability and safety of FW cylinders throughout their operational lifespan.

High-fidelity post-impact residual strength assessment for composite aircraft sustainment

AbstractHigh-fidelity simulation tools have significant potential to support composite aircraft sustainment, though further study is required on incorporating the complex impact damage field. In this paper, compressive residual strength assessment is investigated using the high-fidelity computational tool BSAMTM. The experimental impact damage was mapped and modelled at a high-fidelity level, which included ply-by-ply definition of the geometry of the impact indentation, fibre fracture in the plies and delamination in ply interfaces. It was shown that applying a small lateral displacement or ‘pseudo-impact’ step was highly effective in generating matrix cracks in the impact region, which provided a suitably realistic representation of the interconnected damage map through-the-thickness. It was found that inclusion of all damage modes in the post-image damage map at a high-fidelity definition was essential due to the strong degree of interaction between damage modes. The results support improved sustainment of defence platforms, through enhanced predictive capability and understanding.

Reprocessed Materials Used in Rotationally Moulded Sandwich Structures for Enhancing Environmental Sustainability: Low-Velocity Impact and Flexure-after-Impact Responses.

In the rotational moulding industry, non-used, scrap, and waste purge materials have tremendous potential to be reprocessed and applied in skin-foam-skin sandwich structures to replace and reduce the use of virgin polymers. This approach not only encourages the re-use of these waste materials but also significantly contributes to reduce environmental impacts associated with the use of virgin polymers in this sector. The demand of rotationally moulded sandwich structures is rapidly increasing in automotive, marine, and storage tanks, where investigating their impact and after-impact responses are crucial. Hence, this study investigated the low-velocity impact (LVI) and flexure-after-impact (FAI) responses of rotationally moulded sandwich structures manufactured using reprocessed materials. Results obtained from LVI induced damage at two different incident energy levels (15 J, 30 J), and the residual flexural strength of impacted structures evaluated by three-points bending tests were compared with non-reprocessed sandwich structures (virgin materials). The impact damage progression mechanism was characterized using the X-ray micro-computer-tomography technique. Reprocessed sandwiches demonstrated 91% and 66% post-impact residual strength at 15 J and 30 J respectively, while for non-reprocessed sandwiches, these values were calculated as 93% and 88%. Although reprocessed sandwich structures showed a lower performance over non-reprocessed sandwiches, they have a strong potential to be used in sandwich structures for various applications.

Quantifying the sensitivity of plate aspect ratio and edge boundary conditions on CFRP plate impact and post impact residual strength

Much literature exists on composite plate impact damage and residual strength post impact, but there is limited work which systematically examines how plate behaviour alters with plate aspect ratio and edge rotational restraint. Through simulation of composite stiffened plates under low velocity impact and Compression After Impact loading, the presented work demonstrates that laminate impact response and subsequent residual strength is highly sensitive to both plate aspect ratio and boundary conditions. Increasing edge restraint for low aspect ratio plates results in a moderate increase (up to 13.3%) in residual strength. Conversely, for higher aspect ratio plates (representative of aircraft stiffened panel components), the same change in edge restraint results in an equivalent decrease in residual strength (up to 13.0%). In particular, the interaction between impact induced damage and the onset of plate buckling observed in higher aspect ratio plates, and the subsequent effect on predicted residual strength, is not captured when examining low aspect ratio plates utilised in traditional material coupon testing practices.

Low- and high-fidelity modeling of sandwich-structured composite response to bird strike, as tools for a digital-twin-assisted damage diagnosis

The constant requirement of aerospace industry to enhance the structural efficiency has driven to the usage of high-performance composite materials, either monolithic or sandwich. However, aerospace composite structures are prone to damage due to high-velocity impact events such as bird strike, hail impact, etc. These impact events can result in extensive damage including structure perforation, which will eventually degrade its post-impact residual strength. Therefore, the early detection of damage in composite structure is imperative to avoid catastrophic failure. This paper develops the computational models which predict the dynamic behavior of a helicopter composite sandwich structure undergoing a bird strike. The models are aimed to be used as virtual tools for a future digital-twin-assisted fault detection technique. Firstly, a high-fidelity (HF) FE/SPH model was developed in LS-DYNA, and it was validated against the soft body impact experiments. Afterwards, a computationally efficient low-fidelity (LF) model was developed and correlated with the high-fidelity model. It was concluded that the high-fidelity model can sufficiently accurately predict the strain history experimentally recorded by the FBG sensors, and that size of the predicted delamination area at the front face of the sandwich structure agrees very well with the experimentally observed delamination area. It was also shown that the LF model can rapidly predict the global dynamic response of sandwich panel under the impact loading, through the good agreement between the numerical strain histories with the FBG measurements. Consequently, the LF model can be used as a quick numerical guide for the identification of the loading condition, whereas the HF model can be used as virtual damage detector and estimator of damage extension before the scheduled inspection.

Low velocity impact (LVI) and flexure-after-impact (FAI) behaviours of rotationally moulded sandwich structures

There is limited academic knowledge and industrial understanding available on low velocity impact (LVI) properties, impact damage propagation mechanism and post impact residual strength of rotationally moulded skin-foam-skin sandwich structures. In this work, two roto-moulded sandwich structures (sandwich-1 and sandwich-2) were manufactured at the same thickness and skin layers configurations, with two different commercially available foam core material types and densities. They were tested using an instrumented drop weight impact tester at 25 J, 35 J and 45 J energy levels to analyse the force–time, force–deflections, and energy–time properties. The damage mechanism was investigated with a high-resolution X-ray micro Computed Tomography (μ-CT) technique which correlated the measured impact properties for both type of sandwich structures. Flexure-after-impact (FAI) test was carried out to characterise the effects of impact induced damage on the residual strength of impacted sandwich specimens for the first time in this study. In comparison of two sandwich structures, the lower density foam material manufactured sandwich structures (sandwich-2) showed a better impact property, damage resistance and FAI strength compared to higher density foam material sandwich structures (sandwich-1). This result was not industrially expected and could be related to the incomplete decomposition of blowing agents or forming of immature foam cells in the foaming process of higher density foam material in sandwich-1 leading to its less impact and FAI test load bearing capacities.

Post-impact residual strength and resilience of sandwich panels with natural fiber composite faces

In this paper, the post-impact residual flexural behavior of sandwich panels with flax fiber-reinforced polymer (FFRP) faces and polyisocyanurate (PIR) foam cores is investigated experimentally. The faces were manufactured using a wet lay-up procedure with a balanced bidirectional 2x2 twill flax fiber fabric and a bio-based epoxy with a bio-content of 30%. Each specimen was 1200 mm long x 75 mm wide x 80 mm thick. The main parameters in the study were the face thickness (one, two or three FFRP layers, representing core-to-skin thickness ratios of approximately 54, 28 and 20) and core density (32, 64 or 96 kg/m3); a total of nine combinations. In this study, 27 specimens (three specimens for each combination) were tested under impact loads and the surviving specimens were tested under monotonic three-point bending. Each of the three identical specimens was tested under different impact condition, namely 100%, 75% and 50% of the energy resistance of an intact specimen, with the last two impacted 50 times. The results of the post-impact residual flexural tests were compared to three-point bending tests of intact specimens. The beams demonstrated remarkable resilience in that the impact events did not have a negative effect on their flexural strength or stiffness. In fact, those tested at higher energy levels exhibited a slight increase in strength after impacts. This shows their suitability for use in infrastructure applications such as building cladding panels, flooring and roofing as they retain their strength and stiffness even after multiple impacts.

Experimental Damage Evaluation of Honeycomb Sandwich with Composite Face Sheets under Impact Load

Aerospace structures are highly vulnerable to impact loads whose damage tolerance, and its resistance vary over the range of impact velocity. Honeycomb sandwich structures are used in aerospace industries where mass efficient and impact resistant structures are needed. However, the structural integrity of these structures is reduced by impact load due to tool drop, runway debris, hailstones and improper handling of the structure. A thorough investigation of the damage behaviour of honeycomb sandwich under low-velocity impact and the post-impact residual strength determination is required to design a crashworthy lightweight structure. This paper presents the experimental evaluation of specific energy absorption using Charpy impact, residual compressive strength by compression after impact and damage evaluation of honeycomb sandwich structures having composite face sheets. Parametric studies on composites and honeycombs are carried out by varying the cell size, cell thickness, core height, impact velocity, thickness and orientation of lamina. Densely packed thick honeycombs provide higher fracture energy. Under transverse compressive loading, the honeycomb core undergoes cell wall buckling, crushing and densification. Load-displacement history under in-plane compression and compression after impact for different impact energies is observed. The present study contributes for the understanding how various parameters affect the characteristics of face sheet indentation and plastic buckling of honeycomb sandwich structures with composite face sheets, thereby providing useful guidelines for its potential applications in impact engineering.

Experimental investigation on the effect of glass fiber orientation on impact damage resistance under cyclic indentation loading using AE monitoring

ABSTRACT This paper investigates the effect of glass fibre orientation on the impact damage evolution and post-impact residual strength of glass/epoxy laminates, using mechanical and acoustic emission (AE) analysis. The glass fibre reinforced polymer (GFRP) composite laminates with stacking sequence such as 0°, 90°, [±45°] and [0°/90°] were tested. The indentation-induced damage behaviour was examined in GFRP composite laminates by conducting drop weight impact and quasi-static indentation (QSI) tests to induce barely visible impact damage (BVID). Both tests provided essentially the same results for damage modes, dent depth, back surface crack size and load-deflection history. Mechanical parameters such as peak force, incident energy, absorbed energy, rebound energy and linear stiffness were used for the quantification of the indentation response. The AE parameters, such as amplitude, rise time, counts and energy were considered for monitoring damage progression during QSI loading. Also, other parameters linked to AE monitoring, such as the rise angle (RA), Felicity ratio (FR) and sentry function (SF) were measured for evaluating damage progression in each cycle of indentation. These results showed the [±45°] laminates having higher indentation damage resistance compared to other fibre orientation, whereas residual compressive strength was high for 0° laminates.

Acoustic emission based investigation on the effect of temperature and hybridization on drop weight impact and post-impact residual strength of hemp and basalt fibres reinforced polymer composite laminates

This paper investigates the effect of temperature and hybridization on the impact damage evolution and post-impact residual strength of hemp/epoxy, basalt/epoxy and their hybrid laminates, using mechanical and acoustic emission (AE) based analysis. To start with, the specimens were impacted by a drop weight impact tower machine at two temperatures of 30 °C and 65 °C and then they were subjected to a three-point bending test for the assessment of their residual strength, while online AE signals were recorded during the test. The mechanical behavior of the laminates was evaluated through measurement of the impact force and absorbed energy. AE response of the slope of cumulative rise angle (RA) was used for identification of the severity of the impact-induced damage in the laminates. In addition, the sentry function was computed on the basis of the correlation between the mechanical strain energy stored in the materials and the acoustic energy propagates by fracture events, enabled evaluation of the amount of impact-induced damage. These results showed the hybridized laminates having a better resistance to impact damage at the elevated temperature (65 °C) compared with the non-hybridized laminates, whereas, in the case of the ambient temperature (30 °C), basalt/epoxy laminates had a higher impact damage resistance than other configurations. This study reveals the capability of the proposed AE-based methods to investigate the effect of temperature and hybridization of composite laminates.

Influence of barely visible impact damage on post-impact residual flexural properties of hybrid Fibri Rock – Aero Eco-composites

This study experimentally investigates the effects of barely visible impact damage (BVID) under low-velocity impact and its influence on post-impact residual flexural properties of hybrid flax-basalt Eco-composites Fibri Rock –Aero of 2 and 4 mm in thickness using drop weight impact and 3-point bending tests. The damage-induced was characterised by X-ray micro computed tomography (X-ray µCT) technique. The results show that the post-impact residual flexural strength (PI-RFS) and modulus of the composites are dependent on their thickness and incident impact energies employed. A significant damage was evident for both types of specimens caused by microscopic inter-laminar delamination and cracks induced especially at the highest incident energy with a noticeable reduction on PI-RFS and modulus.

Compression after impact of flax/PLA biodegradable composites

A study of the impact behaviour and the post-impact residual strength of fully biodegradable composites is presented in this work. To this end, low-velocity impact tests and compressive residual strength tests were carried out on flax/PLA laminates. The results were compared with carbon/epoxy laminates, showing some important advantages in terms of absorbed energy and normalized residual strength. The reason was attributed to different energy absorption mechanisms; the main failure mode in flax/PLA laminates is fibre failure while residual strength of carbon/epoxy laminates is dominated by delaminations.

Foreign Object Damage in an Oxide/Oxide Ceramic Matrix Composite Under Prescribed Tensile Loading

Foreign object damage (FOD) behavior of an N720/alumina oxide/oxide ceramic matrix composite (CMC) was characterized at ambient temperature by using spherical projectiles impacted at velocities ranging from 100 to 350 m/s. The CMC targets were subject to ballistic impact at a normal incidence angle while being loaded under different levels of tensile loading in order to simulate conditions of rotating aeroengine airfoils. The impact damage of frontal and back surfaces was assessed with respect to impact velocity and load factor. Subsequent postimpact residual strength was also estimated to determine quantitatively the severity of impact damage. Impact force was predicted based on the principles of energy conservation.

Experimental determination of the level of damage suffered by rigid PVC pipes under quasi-static impact load

This paper presents a approach to determining the level of damage on rigid PVC pipes (polyvinyl chloride), destined for the supply of drinking water, and previously subjected to quasi-static impact. The originality of the work has been to experimentally characterize the level of damage via readily measurable physical quantities obtained under operating conditions. For this purpose, a wide range of experiments were performed. Firstly, trials involving impact with a hemispherical-end hammer weighing 16kg, dropped from a height of between 0 - 2m on portions of rigid PVC pipe. Thereafter, the dimensions of the affected area on the portions of the pipe are measured using a Tridimensional Measuring Machine (TMM). Finally, tensile tests on flat test pieces from around the impacted area of the pipe section were used to determine the post-impact residual mechanical strength. This investigation on post-impact properties allows the definition of the level of damage reached via the unified theory model of Bui Quoc. By analogy to the latter, the damage is expressed as a function of the area of the impacted section.

Porosity Effect on Residual Flexural Strength following Low Energy Impact of Carbon Fibre Composites

Studies of the combined effects of the presence of porosity (as it may result from partially effective cure cycles) and of low-energy impact damage on the residual properties of CFRP laminates have led so far to controversial results. In particular, it is not clear from the literature whether the presence of voids would blunt crack propagation following impact or rather would promote damage development. These effects would respectively either increase or reduce post-impact residual strength, relative to that of the laminate with virtually no voids, as the result of an optimal manufacturing procedure. With this in mind, different cure cycles have been applied to produce carbon fibre-reinforced polymer (CFRP) composites with various levels of void content, which were subjected to low energy impact damage (3, 4.5 and 6 J) and then to post-impact flexural strength measurement. Damage assessment using micro-focus computed tomography (μCT) was used to complement traditional ultrasonic C-scans, which proved ineffective on the high-porosity samples. Three cure-cycles were investigated: one which led to high porosity (average void content 4 vol%) and two conventional low-porosity cure cycles, only one of which included a post-cure cycle. This study has found that, despite a lower initial flexural strength, higher residual flexural strength was retained after impact in the high-porosity material than in the low-porosity one. This is explained by the lower extent of impact damage observed in the high porosity material, where voids had the effect of suppressing delamination propagation.

Cure cycle effect on impact resistance under elevated temperatures in carbon prepreg laminates investigated using acoustic emission

Voids in composites have been a perennial problem, since the amount of porosities is deemed to bear a strong relation with the degradation of service performance of laminates. On the other side, the effect of porosity on impact resistance of laminates is often dependant on their distribution in the material, especially with respect to the location and severity of impact damage in it. In this study, the influence of void content percentage on the residual flexural strength of CFRP laminates impacted at very low energy, in the region of 1 J, at different temperatures was investigated. Laminates were fabricated using 16 layers of Cycom 977-2 prepreg material in a [0/90/90/0]2S layup with different void contents in the range from 1 to 7% by varying cure conditions. Low velocity impact tests were conducted on three categories of laminates, defined as high pressure cured, low pressure debulked and low pressure non debulked respectively, each of these at ambient temperature (30 °C) and elevated temperatures of 50, 70 and 90 °C. Post-impact residual flexural strength of the laminates was measured by three-point bending tests followed in real time by acoustic emission (AE) monitoring. From the separation of frequency bands and of their amplitude levels, identification of the different failure modes, such as matrix cracking, delamination and fibre failure, was performed. The results indicated that in the case of very low impact energy high porosity laminates, such as non debulked ones, may possess slightly higher residual flexural strength and an enhanced delamination resistance with respect to debulked ones.

Prediction of post-impact residual strength and fatigue characteristics after impact of CFRP composite structures

The residual strength and fatigue life of CFRP (Carbon Fiber-Reinforced Plastic) composite structures with impact damage were predicted by using the characteristic length of the composite with the hole corresponding to the impact damage area. Since the specimen has a C-shaped structure that is different from the shape of a standard specimen, the shape factor was obtained from the concentration of the plate specimen and that of the structure using finite element analysis. The fatigue life of composite structures with impact damage can be predicted accurately by applying the prediction model, which takes into account the residual strength after impact, the shape factor which considers geometric characteristics, and the fatigue characteristics of virgin composites that have not been impacted to the model for strength degradation following impact.

A Repair Criterion for Impacted Composite Structures Based on the Prediction of the Residual Compressive Strength

Abstract The residual strength in particular the compression strength after damages due to low velocity impact is one of the most critical issue for composite laminates. If the impact event induces relevant damages on composite components, a repair of the degraded parts is necessary. The aim of this work is to develop a repair criterion to be applied on the composite structures of the European launch vehicle VEGA. The criterion is based on a numerical procedure which was validated in a previous research work and which allows to predict the impact damage and the post impact residual strength as function of the impact energy, making possible to determine the energy threshold beyond which it is necessary to repair the component.

Size Scale Effects on Post-impact Residual Strength of Hybrid Glass/Carbon/Epoxy Nano-composites

Understanding the material response to macroscopic scaling along the thickness direction in fiber-reinforced composite materials is important to reduce the effects of scaling-up during design. This paper presents the effect of size-scale of composites obtained through two methods of scaling: a) ply-level (0gn/90cn)s and b) sub-laminate level (0g/90c)ns where, (n= 2,3,4), on the residual strength and post-impact response. The strength parameters were determined with and without nano-clay addition (Nanomer 1.30E, clay surface modified with 25-30 wt% octa-decylamine). The hybrid glass/carbon/epoxy composite and nano-composite (1.5% nano-clay of weight of epoxy) specimens were prepared using compression molding technique. The ultimate tensile strength of thickness scaled composite and nano-composite specimen was examined. Impact tests were done at two velocities, viz., 4.49 m/s and 3.78 m/s for both thickness scaled composite specimens with and without nano-composite addition. It was observed that ply-level scaled laminates absorbs more impact energy compared to sub-laminate level scaled specimens. The residual tensile strength was found out by conducting tensile test on the impacted specimens. The reduction in residual strength is more in nano-composite specimens compared to plain-composite material. The damage developed in the ply-level scaled specimen is more compared to sub-laminate level scaled specimens.

Effect of resin modification on the impact strength of glass-polyester composites

Summary — Fiber-reinforced plastics (FRP) are nowadays used commonly for constructions subjected to impacts of different energies and velocities, therefore the problem of their impact resistance is very important. One of the methods to improve impact properties of the composites is resin modification with elastomeric phase. In this study the effect of resins addition on properties of composites of unsaturated polyester (UP) resins with glass fibers was investigated. At the first stage, the properties of Polimal 109-32K UP resin modified by the addition of reactive liquid rubbers were tested. The wettability of glass fibers by resin compositions was also tested. Composites were then prepared with the addition of 2 and 6 phr of epoxy or vinyl terminated butadiene-acrylonitrile rubbers (ETBN or VTBN, respectively), as well as 5 and 15 wt. % of Polimal 150 elastic UP resin. These composites were subjected to mechanical tests. Impact properties were tested using Charpy method, as well as by a ballistic impact using a gas gun. After the ballistic impact, damage extent and residual strength as well as water leakage through the composites were evaluated. The damage was also investigated under a microscope. The damage extent was confirmed to be linearly dependent on the impact energy. The addition of rubber was found to decrease the damage extent and increase post-impact residual strength, as well as decrease water leakage rate.