Final Report Summary - IMPTEST (Impact test campaign)
Executive Summary:
IMPTEST has involved gas gun impact testing of shields made of composite materials for protection of aircraft against small metal fragments generated by failure of rotating engine components. Three different sizes of steel cylinders were used to represent engine fragments in different applications. The test campaign included three phases, where Phase 1 involved impact perpendicular to the panels to find the ballistic limit velocity for penetration of metallic shields and three different composite shield concepts, using three different impactors at velocities below 600 m/s. Phase 2 involved studies of the influence of impact angle for the two selected design concepts, while Phase 3 studied the influence of material aging for the selected shield concepts. Furthermore, fibre bundles were tested at -40°C, +23°C and +80°C or +120°C, and the results were used in impact simulations to study the influence of temperature. High speed photography was used to record the impact response history of all specimens. The damage in all specimens was characterised quantitatively and qualitatively using various fractographic methods and by 3D scanning of the deformed shape after impact.
The work in IMPTEST is closely linked to the project IMPSHIELD, where the impact shields were designed and manufactured. Both projects involved participation from Imperial College and Swerea SICOMP. Evaluation of the tests in Phase 1 revealed that the two design concepts with polymer fibres were clearly superior, and these were selected for oblique impact testing in Phase 2. The penetration velocity per unit weight of the selected composite shields was 2-3 times higher than for corresponding metal shields. The oblique impacts in Phase 2 demonstrated that the penetration velocity is fairly proportional to the impact velocity component perpendicular to the impact shield. Hence, small impact angles require extremely high velocities for penetration. Furthermore, non-penetrating oblique impacts on laminated composite shields result in a peculiar response, where the projectile is trapped inside the laminate and slides between the plies until it is fully arrested. Panels which had been aged in hot/wet conditions were tested in Phase 3, but the difference in impact performance was insignificant. Tensile tests demonstrated that temperature changes between -40°C and +23°C only had a moderate influence on fibre strength, but that the strength dropped significantly at temperatures around +100°C. The stiffness measurements can be used for future more detailed simulations of the influence of temperature.
The project has demonstrated that ballistic shields made from polymer fibre composites provide efficient lightweight protection of aircraft against engine debris and that their performance per unit weight is 2-3 times better than for corresponding metal shields. Such shields will facilitate the use of open rotor engines mounted on the rear aircraft fuselage, and the reduced weight will contribute to reductions in fuel consumption and harmful emissions. Aging in hot/wet conditions appears to be a minor problem, but use at higher temperatures must be carefully considered, as the mechanical properties of polymer fibres change at higher temperatures. The most efficient composite shields experience significant deflections during impact, which requires consideration during design. Furthermore, the tests highlight the need to consider the attachment of the shields to the substructure to avoid local failure, e.g. at bolt holes.
Project Context and Objectives:
Reduced weight and more efficient engines of future aircraft are vital to satisfy European and international goals for reducing the emissions and climate impact of aviation. The structural weight is primarily reduced by increased use of polymer composites, which combine superior mechanical performance with low weight. Open rotor jet engines with an uncontained fan section and are considered as a promising concept for reduced fuel consumption. For several reasons designers prefer to mount such engines directly on the rear part of the fuselage.
Failure of rotating aircraft engine components may result in fragments thrown outwards at high velocity, and such fragments may easily damage the fuselage or other primary structure of the aircraft. The risk is particularly serious for uncontained engines mounted close to the fuselage, as envisioned for future open rotor concepts. Thus, a reliable and lightweight shielding is crucial for the fuselage sections exposed to such high velocity impact.
The objective of the accompanying project IMPSHIELD is design and manufacture of three alternative design concepts for protective shields of composite material, capable of preventing penetration for a range of impactor sizes and velocities relevant for the aircraft manufacturers involved in the project. The objectives of the project IMPTEST is to compare the impact performance of the composite shields manufactured in IMPSHIELD with conventional metal shields exposed to projectiles with a velocity of several hundred m/s. More specifically the following issues were studied:
• Determination of the penetration threshold velocity (V50) for each shield and projectile size
• Ranking of the composite shields and metal shields considering performance versus weight
• Quantification of residual deflections after impact and maximum deflections during impact
• Inspection and description of the resulting damage after impact
• Influence of impact angle on penetration velocity and damage
• Influence on the impact performance after aging in hot/wet conditions
• Measurement of fibre properties at reduced and elevated temperatures to allow simulations of impacts beyond the room temperature test conditions.
Project Results:
The main results include quantification of the penetration velocity for the three composite shield concepts and some reference metal shields exposed to projectiles of different size and velocity.
The projectiles included three sizes (small, medium and large) representative for the aircraft of the associated manufacturers.
The main result was that the composite shields with polymer fibres had a penetration threshold velocity per unit weight which was 2-3 times higher than conventional metal shields, while composite shields with glass fibres had a performance similar to the metal shields. The best performance was found for shields with ultra high molecular weight polyethylene fibres, while the performance of shields with aramid fibres was somewhat lower.
Comparison of the results for various impact angles revealed that penetration is more or less directly related to the velocity component perpendicular to the shield. Thus, oblique impact requires significantly higher impact velocities for penetration. During oblique impact projectiles in composite shields were deflected and then slipped between two interior plies while projectiles in metal shields followed a path which was an extension of the direction before impact.
The polymer shields were manufactured by hot pressing layers of aramid textile weave with a binder powder or pre-impregnated layers of UHMW polyethylene fibres. Fibre bundles extracted from the weave or pre-impregnated layers were tested in tension at different temperatures. The strengths at −40°C and +23°C did not differ significantly. Increasing the temperature to 120°C caused a moderate strength reduction for the aramid fibres while a severe reduction was observed already at +80°C for the UHMW polyethylene fibres. The influence of temperature on stiffness and elongation at failure were also measured.
The data on temperature influence on fibre properties may be used in simulations of impact at temperatures deviating from the room temperature used in gas gun tests.
Potential Impact:
The project has quantified the ballistic performance of three composite shield concepts and compared them with conventional metal shields. The two shields with polymer fibres provided 2-3 times higher perforation threshold velocity per unit weight, i.e. 2-3 times lower weight for protection at a given velocity.
The service temperature should be considered in the choice of material as shields with ultra high molecular weight polyethylene fibres perform better at low and moderate temperatures, but are unsuitable at temperatures approaching +80°C, while aramid fibres show an acceptable performance up to +120°C.
The studies of the influence of impact angle, temperature and material aging for a range of different projectile sizes provides increased confidence in the use of composite shields as an alternative to metal shields for ballistic protection. The introduction of composite shields for ballistic protection of aircraft is likely to cause significant weight savings in future aircraft where such protection is required. These weight savings are directly transferred into reduced fuel consumption and less harmful emissions affecting the climate and human health.
All the results of the project have been disseminated to the industrial partners linked to the project, i.e. Dassault Aviations, Airbus Innovation and Airbus in France and Spain. Due to time constraints, and the fact that most experimental results were obtained during the final year of the project no results have yet been published in scientific journals or conferences.
The following articles in peer reviewed journals are planned after completion of the project:
1. One article presenting a comparison of the performance of the different shield concepts
2. One overview article on the project aims and different issues studied in the project
3. One article presenting the influence of temperature on the properties of polymer fibres
The exploitation of the concepts for design of composite shields for ballistic protection will be further considered in feasibility studies for future aircraft performed by the industrial partners of the project.
The knowledge and experience gained in modelling and testing of high velocity impact on composite materials will be used by Swerea SICOMP and Imperial College to strengthen their further research in this area. This will benefit the general European research community in high velocity impact via the joint network of both organisations.
List of Websites:
No public project website is available. Further information can be obtained from the project partners:
Dr Robin Olsson (project coordinator), Swerea SICOMP (www.swerea.se/en/sicomp)
Prof. Lorenzo Iannucci, Dept of Aeronautics, Imperial College (www.imperial.ac.uk/engineering/departments/aeronautics)
IMPTEST has involved gas gun impact testing of shields made of composite materials for protection of aircraft against small metal fragments generated by failure of rotating engine components. Three different sizes of steel cylinders were used to represent engine fragments in different applications. The test campaign included three phases, where Phase 1 involved impact perpendicular to the panels to find the ballistic limit velocity for penetration of metallic shields and three different composite shield concepts, using three different impactors at velocities below 600 m/s. Phase 2 involved studies of the influence of impact angle for the two selected design concepts, while Phase 3 studied the influence of material aging for the selected shield concepts. Furthermore, fibre bundles were tested at -40°C, +23°C and +80°C or +120°C, and the results were used in impact simulations to study the influence of temperature. High speed photography was used to record the impact response history of all specimens. The damage in all specimens was characterised quantitatively and qualitatively using various fractographic methods and by 3D scanning of the deformed shape after impact.
The work in IMPTEST is closely linked to the project IMPSHIELD, where the impact shields were designed and manufactured. Both projects involved participation from Imperial College and Swerea SICOMP. Evaluation of the tests in Phase 1 revealed that the two design concepts with polymer fibres were clearly superior, and these were selected for oblique impact testing in Phase 2. The penetration velocity per unit weight of the selected composite shields was 2-3 times higher than for corresponding metal shields. The oblique impacts in Phase 2 demonstrated that the penetration velocity is fairly proportional to the impact velocity component perpendicular to the impact shield. Hence, small impact angles require extremely high velocities for penetration. Furthermore, non-penetrating oblique impacts on laminated composite shields result in a peculiar response, where the projectile is trapped inside the laminate and slides between the plies until it is fully arrested. Panels which had been aged in hot/wet conditions were tested in Phase 3, but the difference in impact performance was insignificant. Tensile tests demonstrated that temperature changes between -40°C and +23°C only had a moderate influence on fibre strength, but that the strength dropped significantly at temperatures around +100°C. The stiffness measurements can be used for future more detailed simulations of the influence of temperature.
The project has demonstrated that ballistic shields made from polymer fibre composites provide efficient lightweight protection of aircraft against engine debris and that their performance per unit weight is 2-3 times better than for corresponding metal shields. Such shields will facilitate the use of open rotor engines mounted on the rear aircraft fuselage, and the reduced weight will contribute to reductions in fuel consumption and harmful emissions. Aging in hot/wet conditions appears to be a minor problem, but use at higher temperatures must be carefully considered, as the mechanical properties of polymer fibres change at higher temperatures. The most efficient composite shields experience significant deflections during impact, which requires consideration during design. Furthermore, the tests highlight the need to consider the attachment of the shields to the substructure to avoid local failure, e.g. at bolt holes.
Project Context and Objectives:
Reduced weight and more efficient engines of future aircraft are vital to satisfy European and international goals for reducing the emissions and climate impact of aviation. The structural weight is primarily reduced by increased use of polymer composites, which combine superior mechanical performance with low weight. Open rotor jet engines with an uncontained fan section and are considered as a promising concept for reduced fuel consumption. For several reasons designers prefer to mount such engines directly on the rear part of the fuselage.
Failure of rotating aircraft engine components may result in fragments thrown outwards at high velocity, and such fragments may easily damage the fuselage or other primary structure of the aircraft. The risk is particularly serious for uncontained engines mounted close to the fuselage, as envisioned for future open rotor concepts. Thus, a reliable and lightweight shielding is crucial for the fuselage sections exposed to such high velocity impact.
The objective of the accompanying project IMPSHIELD is design and manufacture of three alternative design concepts for protective shields of composite material, capable of preventing penetration for a range of impactor sizes and velocities relevant for the aircraft manufacturers involved in the project. The objectives of the project IMPTEST is to compare the impact performance of the composite shields manufactured in IMPSHIELD with conventional metal shields exposed to projectiles with a velocity of several hundred m/s. More specifically the following issues were studied:
• Determination of the penetration threshold velocity (V50) for each shield and projectile size
• Ranking of the composite shields and metal shields considering performance versus weight
• Quantification of residual deflections after impact and maximum deflections during impact
• Inspection and description of the resulting damage after impact
• Influence of impact angle on penetration velocity and damage
• Influence on the impact performance after aging in hot/wet conditions
• Measurement of fibre properties at reduced and elevated temperatures to allow simulations of impacts beyond the room temperature test conditions.
Project Results:
The main results include quantification of the penetration velocity for the three composite shield concepts and some reference metal shields exposed to projectiles of different size and velocity.
The projectiles included three sizes (small, medium and large) representative for the aircraft of the associated manufacturers.
The main result was that the composite shields with polymer fibres had a penetration threshold velocity per unit weight which was 2-3 times higher than conventional metal shields, while composite shields with glass fibres had a performance similar to the metal shields. The best performance was found for shields with ultra high molecular weight polyethylene fibres, while the performance of shields with aramid fibres was somewhat lower.
Comparison of the results for various impact angles revealed that penetration is more or less directly related to the velocity component perpendicular to the shield. Thus, oblique impact requires significantly higher impact velocities for penetration. During oblique impact projectiles in composite shields were deflected and then slipped between two interior plies while projectiles in metal shields followed a path which was an extension of the direction before impact.
The polymer shields were manufactured by hot pressing layers of aramid textile weave with a binder powder or pre-impregnated layers of UHMW polyethylene fibres. Fibre bundles extracted from the weave or pre-impregnated layers were tested in tension at different temperatures. The strengths at −40°C and +23°C did not differ significantly. Increasing the temperature to 120°C caused a moderate strength reduction for the aramid fibres while a severe reduction was observed already at +80°C for the UHMW polyethylene fibres. The influence of temperature on stiffness and elongation at failure were also measured.
The data on temperature influence on fibre properties may be used in simulations of impact at temperatures deviating from the room temperature used in gas gun tests.
Potential Impact:
The project has quantified the ballistic performance of three composite shield concepts and compared them with conventional metal shields. The two shields with polymer fibres provided 2-3 times higher perforation threshold velocity per unit weight, i.e. 2-3 times lower weight for protection at a given velocity.
The service temperature should be considered in the choice of material as shields with ultra high molecular weight polyethylene fibres perform better at low and moderate temperatures, but are unsuitable at temperatures approaching +80°C, while aramid fibres show an acceptable performance up to +120°C.
The studies of the influence of impact angle, temperature and material aging for a range of different projectile sizes provides increased confidence in the use of composite shields as an alternative to metal shields for ballistic protection. The introduction of composite shields for ballistic protection of aircraft is likely to cause significant weight savings in future aircraft where such protection is required. These weight savings are directly transferred into reduced fuel consumption and less harmful emissions affecting the climate and human health.
All the results of the project have been disseminated to the industrial partners linked to the project, i.e. Dassault Aviations, Airbus Innovation and Airbus in France and Spain. Due to time constraints, and the fact that most experimental results were obtained during the final year of the project no results have yet been published in scientific journals or conferences.
The following articles in peer reviewed journals are planned after completion of the project:
1. One article presenting a comparison of the performance of the different shield concepts
2. One overview article on the project aims and different issues studied in the project
3. One article presenting the influence of temperature on the properties of polymer fibres
The exploitation of the concepts for design of composite shields for ballistic protection will be further considered in feasibility studies for future aircraft performed by the industrial partners of the project.
The knowledge and experience gained in modelling and testing of high velocity impact on composite materials will be used by Swerea SICOMP and Imperial College to strengthen their further research in this area. This will benefit the general European research community in high velocity impact via the joint network of both organisations.
List of Websites:
No public project website is available. Further information can be obtained from the project partners:
Dr Robin Olsson (project coordinator), Swerea SICOMP (www.swerea.se/en/sicomp)
Prof. Lorenzo Iannucci, Dept of Aeronautics, Imperial College (www.imperial.ac.uk/engineering/departments/aeronautics)
