Damage Tolerance Of Structures Research Articles

Effect of Post-Cured through Thickness Reinforcement on Disbonding Behavior in Skin-Stringer Configuration.

An experimental investigation of interlaminar toughness for post-cured through-thickness reinforcement (PTTR) skin-stringer sub-element is presented. The improvement in the crack resistance capability of skin-stringer samples was shown through experimental testing and finite element analysis (FEA) modeling. The performance of PTTR was evaluated on a pristine and initial-disbond of the skin-stringer specimen. A macro-scale pin-spring modeling approach was employed in FEA using a non-linear spring to capture the pin failure under the mixed-mode load. The experimental results showed a 15.5% and 20.9% increase in strength for the pristine-PTTR and initial-disbond PTTR specimens, respectively. The modeling approach accurately represents the overall structural response of PTTR laminate, including stiffness, adhesive strength, crack extension scenarios and progressive pin failure modes. This modeling approach can be beneficial for designing damage-tolerant structures by exploring various PTTR arrangements for achieving improved structural responses.

Overload-induced anisotropy of fatigue crack growth in laser deposited Ti–6Al–4V alloy

Considering the anisotropy of microstructure of additive manufactured metal materials, quantitative assessment of the anisotropy of fatigue crack growth (FCG) behavior is necessary for structural damage tolerance design and evaluation. This study conducted an experimental and mechanistic investigation of the FCG behavior of the laser metal deposited (LMD) Ti–6Al–4V alloy, focusing on the anisotropy of FCG and the effect of overload on FCG. Both the FCG experiments with constant amplitude loading (CAL) and with a single overload (OL) were performed in different directions. Under CAL, the FCG rate exhibits a bilinear correlation with the stress intensity factor. Before the transition point (ΔKT = 16.9 MPa√m), since the grain boundaries are sparser along the columnar crystal growth direction, the FCG rate in the XY specimen is higher than that in the other directions. When the FCG rate further increases, the hindrance effect of grain boundaries on FCG weakens, and the anisotropy of FCG rates disappears. An interesting new finding is that when an overload is applied after ΔKT, the FCG rates in the XZ and ZX directions become different, and the overload-induced anisotropy increases with the overload ratio (OLR). The difference in FCG rates between the two directions exceeds 23.2% when OLR is 2.5. Based on microscopic and theoretical analysis, it was demonstrated that the overload induces residual strain in front of the crack-tip and enhances FCG resistance. The FCG resistance is positively related to the size of the plastic zone and negatively related to the yield strength in the loading direction.

Enhancing structural resilience by using 3D printed complex polymer reinforcement for high damage tolerant structures

Conventional cementitious structures demonstrate high compressive strength, yet they are inherently prone to brittleness. The development of materials or building blocks with high damage tolerance is essential for constructing robust buildings and structures, capable of withstanding various forms of damage. In this study, we introduce a novel approach that involves incorporating 3D printed complex schwarzites and zeolite based biodegradable polymer porous structures (soft phase) as a reinforcement medium within cement matrices (brittle phase). In the case of composite using primitive schwarzites and primitive gyroids, the specific resilience increases by 128.1% and 121.30%, respectively, compared to the control 3D printed structures. The most remarkable enhancement was observed in zeolites based structures, where the specific resilience was improved by 125.63% w.r.t the cement cube and an 505.33% compared to the control pristine 3D printed structure, respectively. This transformation effectively would mitigate the inherent brittleness of cement blocks, imparting a more ductile and damage-tolerant charecteristic to the composite material. These findings highlight the superior resilient structure and energy absorption capabilities of the composite blocks with different 3D printed polymer structures compared to pure cement cubes.

A study of a bio-inspired impact resistant carbon fiber laminate with a sinusoidal helicoidal structure in the mandibles of trap-jaw ants.

The majority of living organisms demonstrate remarkable attributes and have evolved effective mechanisms for synthesizing impact-resistant and damage-tolerant structures. One exemplary instance is the rapid mandible strikes exhibited by trap-jaw ants, which are a highly aggressive species of terrestrial social organisms. An impact-resistant sinusoidal helicoidal architecture is discovered in the mandibles of trap-jaw ants. The bioinspired laminate with a bi-sinusoidal helicoidal structure was manufactured using unidirectional carbon fiber prepreg by mold press forming. This study examines the impact resistance and damage tolerance of a bionic laminate through low velocity impact, computed tomography, and compression after impact tests. The results demonstrate that bionic laminates effectively limit damage propagation within the plane while enhancing energy dissipation capacity. The sinusoidal helicoidal configuration enhances cushioning capability against impact forces, retards penetration under higher loads, hinders crack propagation, and improves residual strength. Bionic laminates provide a valuable solution for damage tolerance through the resistance to through-the-thickness loads. STATEMENT OF SIGNIFICANCE: Helicoidal and sinusoidal helicoidal microstructures have been identified in the cross-section of the jaws of trap-jaw ants. The multiple waviness ratio parameters are designed for fabricating a sinusoidal helicoidal structure laminate using unidirectional carbon fiber prepreg through the mold press forming technique. This results in a damage-tolerant mechanism characterized by reduced delamination damage, which leads to a stiffer mechanical response. Meanwhile, it enhances resistance to crack propagation, leading to the formation of discontinuous delamination areas and the accumulation of sub-critical failures. Additionally, the sinusoidal helicoidal structure laminate combines the cushioning performance of bi-sinusoidal arrangements with the enhanced impact resistance of helical arrangements. This design delays penetration at higher loads, resulting in increased residual strength.

Investigation of bionic composite laminates inspired by the natural impact-resistant helicoidal structure in the mandibles of trap-jaw ants.

Many living organisms exhibit exceptional capabilities and have evolved effective strategies to synthesize impact-resistant and damage-tolerant structures. One such example can be observed in the rapid mandible strikes ofOdontomachus monticola, a species of trap-jaw ants from the ponerine subfamily. During trap-jaw strikes, the mandibles can achieve peak speeds of 35.42 m s-1, and the maximum acceleration can reach 71 729 g within an average duration of 0.18 ms. The extreme acceleration results in instantaneous mandible strike forces that can exceed 330 times the ant's body weight, withstanding thousands of impacts. A natural impact-resistant fibrous helicoidal structure is found in the mandibles of trap-jaw ants. This microstructure is characterized by periodic modulus oscillations that increase energy absorption and improve stress redistribution, offering added protection against damage from impact loading. A carbon fiber reinforced helicoidal composite is fabricated based on the microstructure of the trap-jaw ant's mandibles. The results show that the helicoidal composite with a 12° helical-fiber exhibits higher residual strength, making it more capable of withstanding strong collisions. The catastrophic propagation of damage along the thickness direction is prevented by in-plane spreading and redirection of cracks. This research provides useful references for fabricating bionic impact-resistant composites.

Numerical analysis on the strength prediction of composite bonded repairs under different loading scenarios

In the last years, aeronautical industry has developed certified composite patch bonded repairs for damage tolerant structures, but only restoring metallic components. The mechanical behaviour of composite bonded repairs has been investigated since the development of composite damage tolerant structures is directly dependent on this technology. This study presents a strength prediction on carbon-epoxy adhesively repaired component, using ARALDITE® 2015 for the repairing and recurring to ABAQUS®, applying cohesive zone modelling (CZM), four different load conditions: tension, compression, upward bending and downward bending were tested. Four types of repairs are analysed: Single-Strap, Double-Strap, Scarf and Two-Stepped. The obtained results state that the Double-Strap repair withstand higher loads considering all the applied load cases when compared with the other geometries. Scarf and Two-Stepped repairs could achieve promising behaviours and they should be optimized, considering the advantages related to embedded patch repairing. Generally, the overlap length revealed to favour higher strengths, specially increasing from 5 to 10 mm. Failure paths were demonstrated, revealing proper relations with strength predictions.

3D fatigue crack path deflection and residual stresses in 17-4PH stainless steel rod

Damage tolerant structures require accurate fatigue crack growth rate models for life prediction. Central to these models are fracture mechanics similitude and empirically gathered growth rates. In this work, standard test methods failed to achieve similitude for 17-4PH stainless steel round-rod but succeeded for plate. Material product form differences are interrogated through constant ΔK tests, crack path analysis, residual stress (RS) characterization, and growth rate simulation. Analyses revealed that quench-induced RS in the round-rod promoted a non-planar, 3D crack path with closure effects. Strategies to mitigate the RS/path effects are evaluated by controlling constraint, closure, environment, and heat treatment.

Remarkable strength-impact toughness conflict in high-strength Al-Mg-Sc-Zr alloy fabricated via laser powder bed fusion additive manufacturing

Laser powder bed fusion (LPBF) of AlMgScZr alloys have a combination of high strength and good ductility, which may apply to damage-tolerant structures. However, there have been only limited studies on the toughness performance of this promising alloy system. In the present work, the impact toughness of an LPBF-processed AlMgScZr alloy, which is critical for the application that involves high-strain-rate loading, was systematically investigated using instrumented Charpy impact testing. The results demonstrated that the AB(as-built)-LPBF-processed AlMgScZr alloy had a good combination of yield strength and impact toughness (338 ± 3 MPa, 27.47 ± 1.04 J), which were about 2 and 4 times those of the AC(as-cast)-processed AlMgScZr alloy (160 ± 1 MPa, 6.21 ± 0.66 J). However, post-aging treatment greatly deteriorated the impact toughness. On one hand, the strain-hardening capability of the LPBF-processed AlMgScZr alloy was reduced after post-aging; on the other hand, the coplanar slip associated with the L12 superlattice structure of the secondary Al3(Sc,Zr) phase promoted the intergranular brittle fracture. This resulted in the remarkable strength-impact toughness conflict in the AA(as-aged)-LPBF-processed AlMgScZr alloy (512 ± 2 MPa, 4.27 ± 0.09 J). This study not only provides valuable insights into the dynamic fracture resistance of the LPBF-processed AlMgScZr alloys but also sheds light on the importance of choosing whether to post-age the LPBF-processed AlMgScZr alloy based on service loading conditions.

A mixed-mode energy-based elastoplastic fatigue induced damage model for the peridynamic theory

In recent years a considerable effort has been dedicated to the development of analysis tools that enable the design of damage tolerant structures, particularly in aerospace applications where weight reduction is a crucial requirement. One of these tools developed in recent years is the peridynamic theory, employed to solve numerically complex elastodynamics problems. One of its advantages reported in the literature is the natural ability to simulate the initiation and crack growth without the need for additional numerical procedures commonly employed in other numerical approaches, like in the conventional finite element formulation. Within this context, this paper presents a novel elastoplastic fatigue-induced damage model whose formulation is based on a strain energy framework combined with a smeared crack approach to simulate the damage process without the need of knowing the location of the crack tip and its length within the domain. The proposed model also can predict the mixed-mode damage propagation in ductile materials without knowing a priori the mixity mode ratio. This approach incorporates an analytical methodology based on the material properties that correlate the strain energy calculated away from the crack tip to the expected propagation rate predicted by the Paris Law. The accuracy of the proposed model is verified by comparing the results obtained using an in-house peridynamic FORTRAN code with the experimental results available in the open literature. Some improvements for the peridynamic material parameters are also presented in this paper, which is also verified by using the in-house code.

Novel ceramic fibre - Zirconium diboride composites for solar receivers in concentrating solar power systems

Ultra-high temperature ceramic matrix composites (UHTCMCs) are a new class of composites which combine boride phases with carbon fibres to achieve new structural and functional properties and were originally designed for aerospace applications. For the first time, this work investigates the optical response of UHTCMCs in view of their possible application as high temperature solar absorbers in Concentrating Solar Power systems. Carbon fibre-reinforced ZrB2 pellets with different fibre type and arrangement were manufactured and characterized from the point of view of optical properties and surface microstructure. The composites had high fractions of ZrB2 (∼40 vol%) and carbon fibres (up to 60 vol%). Spectral hemispherical reflectance in the range 0.3–16 μm wavelength was assessed and compared to pure boride and graphite phases. Irrespective of the fibre architecture, the solar absorptance significantly increased with respect to the pure boride, reaching or even overpassing the solar absorptance of silicon carbide. Thermal emittance, calculated from 800 K to 1500 K temperature, increased as well in comparison to the pure boride, but always remained lower than that of SiC. These optical properties, combined with structural damage tolerance and light weight, suggest that UHTCMCs are of great interest as high temperature solar absorbers, overpassing the weaknesses of current solar receivers, as well as those of the most promising alternative materials to date.

Assessment of three-dimensional multi-crack propagation for fatigue life prediction

The existence of multiple cracks affects the crack propagation life assessment of mechanical components. In the present work, numerical algorithms for multiple fatigue crack growth are studied with verification of finite element computations. The investigation confirms that the ratio of the crack center distance to the major crack length is a suitable parameter to evaluate the influence of multiple cracks in the fatigue life prediction. An efficient semi-analytical method is developed and verified to predict the fatigue crack propagation life, which can be applied for three-dimensional structural damage tolerance assessment.

Mechanical properties and damage analysis of S-glass: A reactive molecular dynamics study

Glass fibers are widely used as reinforcements in composites for applications ranging from lightweight and damage tolerant structures to protective materials providing high levels of energy absorption during ballistic impact. Understanding the atomistic origin of mechanical response and damage modes under various strain rate loading conditions is important to improve the properties of glass fibers. In this paper, molecular dynamics (MD) simulations with a reactive potential relate the composition of S-glass fiber to the mechanical properties and progressive damage mechanisms. Stress-strain response of Magnesium Aluminosilicate S-glass fiber using our newly developed Mg/Al/Si/O ReaxFF interatomic potential is predicted from 1e7/s to 1e15/s. Overall strain rate dependent modulus and strength data exhibit a characteristic S-shaped curve on a semi-log plot indicating strain rate dependent properties followed by a steep rise in properties at approximately 1e12/s to a strain rate-independent plateau. Detailed analysis of the atomic structure during high strain rate tensile loading provides insight into the dependency of properties on the rate of atomistic reconstruction, suggesting that the characteristic time-to-reconstruct is an important factor governing strain rate-dependent progressive damage. The origin of the higher modulus of S-glass over silica arises from electrostatic interactions associated with Al-O and Mg-O bonds in S-glass. Investigation on Al-O-Mg, Mg-BO/NBO interaction, and Al/Si-O ring structure indicates that the reconstruction of local structure leads to a more ductile-type response and progressive damage evolution in S-glass fiber than silica glass . In addition, we introduce a methodology to construct stress-strain response for low strain rates (1E-3/s) from a series of high strain rate loading to prescribed strain levels followed by stress-relaxation to an equilibrium stress level. The method is computationally efficient, and the results agree with the quasi-static modulus of both S-glass and silica.

Free vibrations analysis of cracked variable stiffness composite plates by the eXtended Ritz method

Variable stiffness composite laminates show advantageous structural features related to their enlarged design space. They are attractive candidates for advanced engineering applications where the assessment of static and dynamic behavior and strength in the presence of cracks is often required. In the present work, a single-domain extended Ritz formulation is proposed to study the free vibrations of cracked variable stiffness composite plates. The plate model is based on the first-order shear deformation theory whose primary variable, i.e. displacements and rotations, are approximated via a set of orthogonal polynomial trial functions enriched with a set of special crack functions. These functions are able to inherently account for crack opening and crack tip singular fields. The plate governing equations are deduced by the stationarity of the energy functional and the formulation has been implemented in a computer code. The method has been validated by comparing the present results with literature solutions for cracked isotropic plates and uncracked variable stiffness plates as, to the best of author’s knowledge, no data on cracked variable stiffness plates free vibrations are available. An explicative and representative study on the free vibrations of variable angle tow composite laminates is finally presented with the aim of illustrating the approach capabilities, providing benchmarck results and identifying distinctive features and opportunities of the variable stiffness concept for the design of advanced damage tolerant structures.

Investigation of Laser Treatment as a Method for Fatigue Crack Growth Retardation in Aluminum Alloy 2198-T851

Among the third-generation Al-Li alloys, AA2198 stands out for its lower density, formability and increased stiffness, being suitable for use in aircraft fuselage sheets and other inner structures in order to reduce weight and improve performance. An important topic related to damage tolerant structures is the development of techniques to retard fatigue crack propagation, such as the localized heating by a laser source. The aim of the present work was to find the most suitable parameters for the production of laser heating lines in 2198-T851 alloy sheets in order to reduce the fatigue crack growth rate in this material. Laboratory tests using C(T) specimens under two loading conditions (R = 0.1 and 0.5) provided a useful dataset on the laser heated material. The experimental results indicate a 200 W laser beam power at treatment speeds of 1 and 10 mm/s was sufficient to retard crack growth in the current setup. The more expressive results were obtained for 200 W laser power with a speed of 1 mm/s and cyclic loading with stress ratio R = 0.1.

An experimental and finite element investigation of compression-after-impact (CAI) behaviour of biaxial carbon fibre non-crimp-fabric (NCF) based composites

The damage mechanisms of biaxial non-crimp-fabric (NCF) reinforced composites with a quasi-isotropic [(45/-45)(90/0)]4s layup subject to low-velocity impact (LVI) and compression-after-impact (CAI) loadings are investigated. Experimental results indicate that fibre compressive failure in the 0° plies controls the peak load during CAI, which is influenced by the out-of-plane transverse deformation of the impacted specimen. The study proposes a novel finite element (FE) model to predict the CAI strength of the NCF specimen. While the number of delaminations in the LVI model is less than the actual number of the multiple delaminations in the test specimen, the same out-of-plane transverse deformation measured by the experiment is still captured. The strength predicted from the subsequent CAI model is in good agreement with the experiment. The study provides a potential numerical design tool for the use of NCF materials in damage-tolerant structures.

Progress in interlaminar toughening of aerospace polymer composites using particles and non-woven veils

AbstractFibre-reinforced polymer (FRP) composites generally have a layered architecture and are commonly manufactured with thermosetting resins—making them susceptible to interlaminar fracture (i.e. delamination), which is often a major concern in structurally critical applications. As a result, various approaches have been explored to enhance interlaminar fracture resistance. This review focuses on third-phase toughener inclusions, which offer opportunities to create damage resistant and damage tolerant structures without significantly adding weight or reducing in-plane mechanical properties. These toughener inclusions, typically introduced in the interlaminar regions, are divided into two categories herein: particle fillers and non-woven fibre veils. The advantages and limitations of both types are discussed, and the potential of the two approaches is evaluated using published data, aiming to provide an overview of the current understanding and challenges in designing and manufacturing safe and reliable composite structures.

Damage tolerance and notch sensitivity of bio-inspired thin-ply Bouligand structures

Different bio-inspired Bouligand thin-ply Carbon-Fibre-Reinforced Plastic (CFRP) laminates with a pitch angle as low as 2.07∘ are realised, which is the smallest pitch angle realised in literature. The angle is therefore close angles found in biological microstructures. Low-Velocity Impact (LVI) and residual compressive strength tests determined the damage tolerance of the structures. Investigated were two different interlaminar fracture toughnesses and two different metal-Bouligand-CFRP-layups. The low pitch angle results in significantly higher residual strengths than 45∘ quasi-isotropic (QI) layups, despite the significantly lower proportion of 0∘ fibres. Higher fracture toughness and hybridisation with steel layers lead to reduced matrix damage without increasing residual compressive strength. In-plane plane tension properties are determined with a pitch angle of 2.59∘. The results reveal, that the unnotched tensile strength is significantly lower. However, only helicoidal, sub-critical matrix cracking and no delaminations occur before final failure. The sub-critical matrix cracking leads to almost no notch sensitivity and a similar open-hole-tensile strength to 45∘-QI layups despite the low number of 0∘-fibres.

Multiaxis three-dimensional (3D) glass fiber preform/cementitious matrix concrete composites: Experimental characterizations by panel test

Multiaxis three-dimensional (3D) alkali-resistant continuous glass fiber/cementitious matrix concrete composites were developed using the “winding and molding” method. The panel properties of the concretes were experimentally investigated and compared with the neat concrete. The experiment revealed that the load-bearing capability, strength and energy absorption properties of the multiaxis three-dimensional concretes were significantly influenced by the placement and orientation of the reinforcement fiber. The newly developed concrete composite was found to have the strain hardening behavior which attributed to an improved energy absorption performance of the concrete. It was also identified that four directional glass structure (ARG-4D) demonstrated somewhat better energy absorption performance compared to that of the biaxial (ARG-2D) and uniaxial (ARG-1D) structures because of the fiber orientation in the multiaxial direction in the concrete. Further investigation of the fractured four directional glass fiber/cementitious matrix concrete (ARG-4D) showed that the fiber-matrix interfacial debonding in each yarn set, local matrix breakages, yarn bridging due to tensile strengthening, tensile pull-out of outer filament and stick-slip frictions between the filament TOWs and the cementitious matrix were the principal fracture mechanisms. The micro-cracks occurred in multiaxis concretes were restricted by the multiaxis 3D fiber architecture. The multiaxis three-dimensional alkali-resistant continuous glass fiber/cementitious concrete can be considered as the damage tolerant structures with regard to neat concrete.

Electromagnetic damage tolerance for radar absorbing composite structures with impact damage

Radar absorbing structures (RAS) based on fiber reinforced plastics, which is applied to the external structures of low-observable aircrafts, is vulnerable when exposed to a variety of external impacts during aircraft operations. The general composite structure is designed with the concept of damage tolerance to ensure durability and structural integrity. Composite RAS requires both structural and electromagnetic characteristics. Therefore, it is necessary to establish the concept of structural and electromagnetic damage tolerance considering the degradation of electromagnetic performance due to impact damage. In this study, a concept of electromagnetic damage tolerance of a RAS is proposed. Impact damage to RAS specimens and structural damage is detected using non-destructive inspection. Moreover, rate of variation in electromagnetic (EM) wave absorption performance with impact energy is quantified through the measurement of return loss of EM wave before and after impact using free-space measurement. This is obtained by investigating the relationship between electromagnetic properties and structural damage tolerance for the laminated type of RAS.

On the impact toughness of gradient-structured metals

Gradient-structured (GS) materials are capable of displaying high strength without compromising ductility, which can result in damage-tolerant structures. However, due to the difficulties in fabricating bulk GS materials, there has been only limited studies on the fracture behavior in GS metals. In the present work, the impact toughness of the macroscale GS pure Ni plates was investigated using instrumented Charpy impact testing. The gradient orientation was found to have a significant influence on the impact toughness of GS Ni. For gradient structures that transition from coarse grains (CG) to nano-grains (NG), termed CG→NG gradients (in the present study from ∼8 μm to ∼30 nm), the absorbed energy and the tensile strength were increased, respectively, by 1.6 and 2.3 times from those exhibited by uniform coarse-grained structures, demonstrating a simultaneous enhancement in strength and impact toughness. Analysis of load-displacement curves revealed that the resistance to both crack initiation and propagation were significantly enhanced as the crack penetrated through the CG→NG gradient structure, leading to markedly rising dynamic R-curve behavior estimated from nonlinear-elastic fracture mechanics J-based measurements. The superior fracture resistance in the CG→NG gradient structure was found to originate from sustained ductile fracture by microvoid coalescence, taking place not only in the initial CG zone, but also within the latter NG regions where adiabatic shear bands form during impact; in these latter regions, plasticity becomes enhanced due to grain coarsening induced by recrystallization under the dynamic loading. The present work not only reveals how the dynamic fracture resistance can be significantly enhanced in GS metals, but also provides structure-design strategies for developing superior metallic materials for impact engineering applications.