Damage tolerance criteria for composite laminates under tension and compression load

Any composite aircraft structure exposed to impact has to be designed accounting for damage tolerance (DT). Impact damage has to be endured throughout the structure's service life. The damage tolerant design has to guarantee a load-sustaining capability above the limit load at any time. An exceptional challenge to the damage tolerant design of composites is the multiplicity of failure modes in a composite laminate. Considerable effort is necessary to characterize the damage behavior and the respective residual properties through physical experiments or numerical high-fidelity methods. The common proceeding during the preliminary design phase is a fixed strain allowable that may not be exceeded. To relax the existing single allowable and to open the design space allowing composite structures with arbitrary laminates, specific allowables for each laminate have to be determined. The methods presented in this work shall enable this specific calculation for the preliminary phase of the aircraft design. In this phase, efficient DT criteria building on elementary material parameters are required. To derive suitable calculation methods, the driving failure mechanisms of the damage evolution are identified for cyclic tension and compression loading, separately. Under tensile loading, the mechanism driving the damage propagation results from an interaction of delamination and fiber fracture. The corresponding mode II energy release rate (ERR) GIIx at the delamination crack tip can be calculated through an elementary energy balance approach and the principle of maximum energy release. Under compression load, fiber kinking and delamination growth characterize the damage evolution of an impact damage. A sublaminate buckling criterion combined with a virtual crack extension method permits one to calculate the ERRs GIx and GIy in each propagation direction. The results are validated through the data of cyclic tension after impact and compression after impact tests

Virtuelle Zertifizierung - Mit uns rechnen? Aber sicher!

Die Anwendung einer Berechnungsmethode für die Zertifizierung von Luftfahrtstrukturen birgt neue Herausforderungen für die Methodenentwicklung und die Umsetzung in Software. Mitarbeitende des Instituts entwickeln seit vielen Jahren Bewertungsmethoden und Versuche zu deren Validierung. Das Virtuelle Produkthaus (VPH) des DLR nutzt seit 2019 einen Teil dieser Ansätze für die virtuelle Auslegung, die simulationsbasierte Fertigung und den virtuellen Test von Luftfahrtstrukturen. Erste Anwendungsfälle sind multifunktionale Flugzeugsteuerflächen und Wasserstofftanks. Ziel des VPH ist es, die verwendeten Ansätze zunehmend für eine virtuelle Zertifizierung einzusetzen. Die Forschenden stellen sich die Frage: Welche Anforderungen müssen numerische Bewertungen und die damit erzeugten Daten erfüllen, um gegenüber Zertifizierungsanforderungen als zugelassenes Nachweismittel anerkannt zu werden

An Experimental Study of the Cyclic Compression after Impact Behavior of CFRP Composites

The behavior of impact damaged composite laminates under cyclic load is crucial to achieve a damage tolerant design of composite structures. A sufficient residual strength has to be ensured throughout the entire structural service life. In this study, a set of 27 impacted coupon specimens is subjected to quasi-static and cyclic compression load. After long intervals without detectable damage growth, the specimens fail through the sudden lateral propagation of delamination and fiber kink bands within few load cycles. Ultrasonic inspections were used to reveal the damage size after certain cycle intervals. Through continuous dent depth measurements during the cyclic tests, the evolution of the dent visibility was monitored. These measurements revealed a relaxation of the indentation of up to 90% before ultimate failure occurs. Due to the distinct relaxation and the short growth interval before ultimate failure, this study confirms the no-growth design approach as the preferred method to account for the damage tolerance of stiffened, compression-loaded composite laminates

Ist der Tank noch ganz dicht?

Wasserstoffdrucktanks sind als Energiespeicher ein Grundbaustein für zukünftige Antriebstechnologien. Entsprechende Druckbehälter stellen zweierlei Grundanforderungen an das Material für die Tankhülle. Es muss die mechanische Belastung durch den Innendruck aushalten und die Dichtheit gegen Gasaustritt gewährleisten. Konventionell gebaute Wasserstoffdrucktanks bestehen aus einer zweischichtigen Struktur: einer innenliegenden isotropen Isolierschicht (Liner), um den Austritt von Wasserstoff zu verhindern, und einer lasttragenden Außenschicht aus Faserverbundmaterial, um die hohen Innendrücke auszuhalten. Eine gute Möglichkeit bisher ungenutzte Leichtbaupotenziale zu erschließen, stellt dabei die Fusion beider Aufgaben in einer Schicht dar. Durch neue FaserverbundTankkonzepte ohne Liner soll das lasttragende Laminat zusätzlich die Funktion der Abdichtung gewährleisten. Hieraus resultiert für die Auslegung die besondere Herausforderung, den Gasaustritt durch die Diffusion von Wasserstoff durch das Faserverbundmaterial berechnen zu können. Kohlenstofffasern weisen beispielsweise einen deutlich höheren Permeationswiderstand auf als Epoxidharz. Um die Permeation des Gases berechnen zu können, ist diese Heterogenität des Verbunds mit angepassten Methoden zu berücksichtigen. Die resultierende Permeabilität für das zweiphasige Verbundmaterial liegt dabei zwischen den theoretischen Ober- und Untergrenzen, die durch die jeweiligen Einzelmaterialien gegeben sind. Der Faservolumengehalt ist der bestimmende Parameter, welcher die Permeabilität des Faserverbunds definiert. Aufgrund der heterogenen Zusammensetzung einer Laminatschicht ist die Diffusion zusätzlich abhängig von der aus Fasern und Matrix gebildeten Mikrostruktur und ist daher mit zu berücksichtigen

Operational Loads Monitoring and CFRP Damage Accumulation for Predictive Maintenance

A flexible aircraft maintenance scheme based on predictive maintenance allows a higher structural uti-lization. This leads to reduced mass for new aircraft designs or longer maintenance intervals for existingaircraft. Current aircraft are designed to endure predefined operating load and to fulfil standard main-tenance requirements with fixed intervals for damage inspection. For a predictive maintenance, theutilization the actual load history of an aircraft as well as evaluating the development of a damage will beanalysed. The workflow in figure 1 uses the concept of a digital twin by evaluating data from on-boardsensors. The Operational Loads Monitoring System (OLMS) transforms this data into operating globalloads. These are converted to local strain spectra and fed into a damage tolerance calculation estimatingthe remaining useful life (RUL) of a structu

An experimental damage tolerance investigation of CFRP composites on a substructural level

The damage tolerance (DT) allowables for the design of a composite structure are typically determined through experiments on the coupon level. The present study examines the transferability of the DT behavior from the coupon level to a structural level. For that purpose, a DT critical panel with two stiffeners was designed and tested. In one quasi-static and two cyclic compression after impact tests, the damage evolution behavior was studied and compared with results achieved on the coupon level. Similar phenomena were found on both scales: a long interval of load cycles, without any detectable damage evolution is succeeded by the sudden propagation of the delamination and the fiber fracture. Afterward, the ultimate failure occurs within few load cycles. Even though the stiffened panel offers a significant possibility to transfer load from the damaged skin, a significant damage stabilization could not be achieved. The no-growth interval was found to be shorter on the structural scale, however, an analytical DT analysis suggests the different damage size as the most likely cause. However, it was found that the stiffeners slow down the damage propagation. Eventually, the study confirms the no-growth design approach as the preferred method to account for the DT of stiffened, compression-loaded composite structures

Calibration of a Digital Twin for Structural Testing

An efficient physical test of a composite structure shall provide a maximum of information while the efforts for preparation, testing, and evaluation shall be minimized. To deliver deeper insight into the structural behavior, a test-accompanying simulation through a virtual test rig and a model of the specimen is a state-of-the-art procedure. Typically, the specimen in this analysis is a nominal model representing the specimen as-designed. To improve the accuracy, the nominal model can be replaced by a digital twin (DT) of the individual specimen. This DT digitally represents the specimen as a physical entity. The present work presents a particular DT creation method for the purpose of structural testing. A DT shall be created from a nominal model by optimizing the deviations between the measurement data obtained from the physical entity and the respective values in the virtual space. This adoption of the DT concept is new to the field of structural composite testing and permits us to increase the accuracy of a test-accompanying simulation

About the Damage Tolerance of Composite Structures

In aerospace engineering, a structural design has to account for Damage Tolerance (DT). According to the DT design principle, a flawed structure still has to sustain the design load until that flaw becomes discovered and is repaired. The DT analysis is well-established for metallic materials. However, composite structures and conventional metal structures differ significantly in their damage behavior. The DT of composite structures is particularly complex and the available methods are still in their infancy. The particular case of designing damage-tolerant composite structures was addressed within the DLR project KonTeKst, through the work packages of the department of structural mechanics of the Institute for Composite Structures and Adaptive Systems. This topic was addressed in three steps: 1) A theoretical investigation, based on a screening of the literature 2) the development of an analytical DT analysis method 3) experiments on coupons and stiffened panels

A combined analytical and numerical analysis method for low-velocity impact on composite structures

Understanding impact damage is essential to building lightweight composite structures. The present doctoral thesis proposes a comprehensive impact analysis approach that combines analytical and numerical methods. This approach assists in understanding the details of laminate damage modes and in analyzing the impact scenarios of a typical aircraft structure. This thesis approaches numerical impact analysis through an examination of various existing methods. An explicit finite element model that captures the laminate on the meso-scale can be used to plausibly predict impact-induced delamination, inter-fiber failure, and fiber failure. This model involves two fracture-mechanical methods: first, cohesive surfaces catch the delamination damage while, second, continuum damage mechanics addresses the intra-ply failure modes. Building on this model, the present work describes the development of an appropriate degradation method for inter-fiber cracks in oblique fracture planes. Using a rank-eight damage tensor, this method enables the calculation of the resulting stiffness tensor including the coupling effects of shear and normal deformation. A simplified approach in fracture plane coordinates is derived on the basis of this tensorial degradation. Compression experiments with oblique fracture planes and coupon impacts serve as validation of this new progressive damage model. The computational cost of this high-fidelity approach impedes a direct application on the structural level. However, a typical property of damage resulting from impact with low velocity and large mass helps to reduce the scope of the structural model: as the impact damage is small in comparison to the full structure, the relevant zone for damage analysis is limited to a small cross-section around the impact location. This model reduction permits a very efficient analysis of structural impact. An analytical transfer approach allows the reduced model to comply with the original structural impact. A newly developed spring-mass model captures the damage that occurs. In this model, a damage element objectively describes the damage for a laminate configuration. Thus, the spring-mass model offers a method for transferring the damage behavior to any sufficiently similar impact configuration. Wherever qualitatively similar damage occurs, this model scales the impact energy for damage similarity. In this manner, a structural impact scenario can be analyzed on a numerical or experimental reference coupon of minimal size. Impact experiments validate the method and show its range of applicability. Finally, the transfer method enables impact analysis on sizeable structural areas through the areal evaluation of the damage description in the spring-mass model. This development allows for the establishment of a damage-tolerant design based on the actual impact threat to structures