Quasi-static Indentation Loading Research Articles

Damage Assessment of 3-D Printed Ceramic Sandwich Structures with an Auxetic Honeycomb Core Subjected to Quasistatic Indentation Loading

Abstract Ceramic sandwich structures (CSS) have become an important material in the aerospace industry because of their high strength and excellent thermal insulation properties. However, the brittle nature of ceramic makes them vulnerable to damage from foreign objects, which can reduce their load-bearing capacity. In this paper, a series of tests were designed to investigate the response of CSS to impacts from foreign objects. To realize the damage characteristics and failure modes under the indentation force, a quasistatic indentation (QSI) test was conducted on CSS. Additionally, an acoustic emission device was used to capture damage signals during the QSI testing. Thereafter the extent of damage was evaluated by analyzing the damaged area and the compression after indentation properties. The results of these tests revealed the failure mechanism maps and indicated that the compressive strength of the damaged CSS had a stronger correlation with damage to the honeycomb core than to the face sheet.

Experimental study on composite spherical-roof contoured-core (SRCC) sandwich panels under the V-shaped quasi-static indentation loading

The paper investigated the experimental study of composite spherical-roof contoured-core (SRCC) sandwich panels under a V-shaped quasi-static indentation (QSI) test. The carbon fiber, glass fiber, aramid fiber, hybrid carbon/aramid, and glass fiber chopped strand mat sheets were used to manufacture the composite cores, which used the hand layup and hot compression techniques. The polylactide (PLA) core was fabricated using fused deposition modelling (FDM). The V-shaped indenter had a 65° angle and was set at 1 mm/min as indentation speed in the experimental test. It was found that the maximum indentation deflection was 35.49 mm on the SRCC sandwich panel with a hybrid carbon/aramid core compared to other composite cores. Furthermore, it was localized fiber fracture, matrix damage, delamination, inwards fronds, and debonding damage mode on composite SRCC sandwich panels, which was visually shown at the central SRCC core unit cell. It is highlighted that aramid fiber and hybrid carbon/aramid materials can provide outstanding impact characteristics, which have great potential to be used as impact-resistant sandwich structures.

Model-experiment comparison of quasi static loading on a composite overwrap pressure vessel (Part 2)

A methodology to develop a 3D Finite Element (FE) model of a full metal-lined Composite Overwrapped Pressure Vessel (COPV) was developed and is presented in this paper. The model is intended for prediction of the metal-composite delamination and residual dent depth developed in the metal liner as a result of quasi-static indentation loading. Cohesive elements are used to model the composite and the metal-composite interfaces. Experimental and numerical comparisons of force–displacement curves, delamination area and residual dent depth are presented. Numerical results are in good agreement to the experimental data.

Effect of solution-blown nanofibrous web on quasi-static punch shear test results and quasi-static indentation behavior of carbon fiber-reinforced epoxy matrix composites

This study investigated the effects of the nanofiber interlayer in carbon fiber-reinforced epoxy matrix composites on quasi-static loading behaviors such as quasi-static punch shear (QS-PST), quasi-static indentation (QSI), and short-beam shear (SBS) loading. The solution-blowing technique was used to produce polyamide-6 (PA6) nanofiber with 1 g/m2 areal density. A 2×2 twill weave carbon fiber fabric with an areal density of 200 g/m2 was covered with solution-blown nanofibers. The vacuum-assisted resin transfer molding (VARTM) method was used to manufacture nanofiber interlayered composites and non-interlayered composites. QS-PS experiments were conducted at three different span to punch ratios (SPR), namely 1.1, 2, and 4. The PA6 nanofiber had a synergetic effect on the composite structure. In the QS-PST experiments, it increased the amount of work achieved by the specimen at rates of 29.5%, 13.5%, and 5.66% for SPR 1.1, 2, and 4, respectively while it provided increases in the maximum force at rates of 15.3%, 15%, and 5.7%, respectively. Additionally, the PA6 nanofiber contributed to the composite material properties determined based on the QSI test including the maximum force, punch crush strength, crush stiffness, and through-thickness modulus at rates of 7.5%, 7.56%, 2.72%, and 5.76%, respectively. Finally, an increase at a rate of 11.43% was determined in the SBS strength of the nanofiber interlayered composite, while it showed an increase in maximum force observed during the SBS experiment at a rate of 12.28%. The enhancements of mechanical properties of structural composites make them more reliable for high technology applications like aerospace, and automotive.

Study of damage behavior and repair effectiveness of patch repaired carbon fiber laminate under quasi-static indentation loading

Damage caused due to low-velocity impacts in composites leads to substantial deterioration in their residual strength and eventually provokes structural failure. This work presents an experimental investigation on the effects of different patch and parent laminate stacking sequences on the enhancement of impact strength of Carbon Fiber Reinforced Polymers (CFRP) composites by utilising the adhesively bonded external patch repair technique. Damage evolution study is also performed with the aid of Acoustic Emission (AE). Two different quasi-isotropic configurations were selected for the parent laminate, viz., [45°/45°/0°/0°]s and [45°/0°/45°/0°]s. Quasi Static Indentation (QSI) test was performed on both the pristine laminates, and damage areas were detected by using the C-scan inspection technique. Damaged laminates were repaired by using a single-sided patch of two different configurations, viz., [45°/45°/45°/45°] and [45°/0°/0°/45°], and employing a circular plug to fill the damaged hole. Four different combinations of repaired laminates with two configurations of each parent and patch laminate were produced, which were further subjected to the QSI test. The results reveal the effectiveness of the repair method, as all the repaired laminates show higher impact resistance compared to the respective pristine laminates. Patches of [45°/0°/0°/45°] configuration when repaired by taking [45°/45°/0°/0°]s and [45°/0°/45°/0°]s as parents exhibited 68% and 73% higher peak loads, respectively, than the respective pristine laminates. Furthermore, parent and patch of configuration [45°/0°/45°/0°]s and [45°/0°/0°/45°], respectively, attain the highest peak load, whereas [45°/45°/0°/0°]s and [45°/45°/45°/45°] combinations possess the most gradual decrease in the load.

Investigation of quasi-static indentation on sandwich structure with GFRP face sheets and PLA bio-inspired core: Numerical and experimental study

Composite materials incorporated in naval, aerospace, and automobile structures are exposed to unforeseen damages, which seriously damage their mechanical performance. In particular, the existence of hardly visual impairment on the in-service composite structures is a critical issue that highlights the severity of structural safety in an aircraft. The ability of these materials to sustain multiple impacts requires an understanding of the impact energy absorption to improve the material’s damage tolerance. The present study investigates the energy absorption, load response, failure modes, and damage mechanics under the quasi-static indentation of a sandwich structure. The structure comprises bio-inspired Poly-lactic Acid (PLA) core and Glass Fibre Reinforced Polymer (GFRP) laminates, used in real naval and aeronautical structures, under repeated low-velocity impact with reduced energies and quasi-static indentation. In addition, a lightweight sandwich structure’s damage performance is greatly influenced by its core geometry. Therefore, choosing an appropriate core structure is critical. A Numerical model is developed for the composite structure under quasi-static indentation loads, considering suitable material properties and damage models for the core and face sheets. These results are then compared with the experimental investigation. The load–displacement response and the energy absorption characteristics of the sandwich structure showed good agreement for the experimental and numerical analysis results.

Delamination initiation in fully clamped rectangular CFRP laminates subjected to out-of-plane quasi-static indentation loading

To further investigate the effects of in-plane and out-of-plane stresses on the delamination initiation for composite laminates under out-of-plane loading, this paper reports a joint experimental and numerical study, in which the fully clamped rectangular CFRP composite laminates were subjected to out-of-plane quasi-static indentations. The results show that the combination of the out-of-plane shear and in-plane tensile stresses together determined the initiation of delamination, whereas the influences of the out-of-plane compressive stress on the delamination initiation can be neglected. For the purpose of understanding the effects of deformations on the out-of-plane shear and compressive stress distributions, a concise analytical model was developed, which was validated against the numerical and experimental results. As a key take-away, this study reveals that the common impact tests at the geometric centre of the panel may not resemble sufficient similitude with stiffened panels where panel flexure is suppressed by various geometrical stiffening concepts.

Impact resistance of cork-skinned marine PVC / GRP sandwich laminates

The introduction of a thin cork layer was investigated as a means to improve the impact resistance of composite foam sandwich laminates whilst maintaining structural performance. Six configurations of this ‘cork-skinned’ sandwich architecture were evaluated via extensive experimental comparisons with a ’baseline’ GRP/ PVC foam sandwich laminate as typically used in marine structures. The concept improved perforation resistance (by up to 60%) for both quasi-static indentation and impact loading rates, albeit with an increase in laminate weight. However, initial (slight) damage resistance and bending strength can be compromised if the detailed laminate architecture is not designed with care.

Delamination Initiation in Fully Clamped Rectangular CFRP Laminates Subjected to Out-of-Plane Quasi-static Indentation Loading

Delamination Initiation in Fully Clamped Rectangular CFRP Laminates Subjected to Out-of-Plane Quasi-static Indentation Loading

3D Microstructure-Based Finite Element Simulation of Cold-Sprayed Al-Al2O3 Composite Coatings Under Quasi-Static Compression and Indentation Loading.

This study developed microstructure-based finite element (FE) models to investigate the behavior of cold-sprayed aluminum–alumina (Al-Al2O3) metal matrix composite (MMC) coatings subject to indentation and quasi-static compression loading. Based on microstructural features (i.e., particle weight fraction, particle size, and porosity) of the MMC coatings, 3D representative volume elements (RVEs) were generated by using Digimat software and then imported into ABAQUS/Explicit. State-of-the-art physics-based modeling approaches were incorporated into the model to account for particle cracking, interface debonding, and ductile failure of the matrix. This allowed for analysis and informing on the deformation and failure responses. The model was validated with experimental results for cold-sprayed Al-34 wt.% Al2O3 and Al-46 wt.% Al2O3 metal matrix composite coatings under quasi-static compression by comparing the stress versus strain histories and observed failure mechanisms (e.g., matrix ductile failure). The results showed that the computational framework is able to capture the response of this cold-sprayed material system under compression and indentation, both qualitatively and quantitatively. The outcomes of this work have implications for extending the model to materials design and for applications involving different types of loading in real-world application (e.g., erosion and fatigue).

A Hyper-Viscoelastic Continuum-Level Finite Element Model of the Spinal Cord Assessed for Transverse Indentation and Impact Loading.

Finite Element (FE) modelling of spinal cord response to impact can provide unique insights into the neural tissue response and injury risk potential. Yet, contemporary human body models (HBMs) used to examine injury risk and prevention across a wide range of impact scenarios often lack detailed integration of the spinal cord and surrounding tissues. The integration of a spinal cord in contemporary HBMs has been limited by the need for a continuum-level model owing to the relatively large element size required to be compatible with HBM, and the requirement for model development based on published material properties and validation using relevant non-linear material data. The goals of this study were to develop and assess non-linear material model parameters for the spinal cord parenchyma and pia mater, and incorporate these models into a continuum-level model of the spinal cord with a mesh size conducive to integration in HBM. First, hyper-viscoelastic material properties based on tissue-level mechanical test data for the spinal cord and hyperelastic material properties for the pia mater were determined. Secondly, the constitutive models were integrated in a spinal cord segment FE model validated against independent experimental data representing transverse compression of the spinal cord-pia mater complex (SCP) under quasi-static indentation and dynamic impact loading. The constitutive model parameters were fit to a quasi-linear viscoelastic model with an Ogden hyperelastic function, and then verified using single element test cases corresponding to the experimental strain rates for the spinal cord (0.32–77.22 s−1) and pia mater (0.05 s−1). Validation of the spinal cord model was then performed by re-creating, in an explicit FE code, two independent ex-vivo experimental setups: 1) transverse indentation of a porcine spinal cord-pia mater complex and 2) dynamic transverse impact of a bovine SCP. The indentation model accurately matched the experimental results up to 60% compression of the SCP, while the impact model predicted the loading phase and the maximum deformation (within 7%) of the SCP experimental data. This study quantified the important biomechanical contribution of the pia mater tissue during spinal cord deformation. The validated material models established in this study can be implemented in computational HBM.

A methodological approach to model composite overwrapped pressure vessels under impact conditions

A combined experimental-numerical method is proposed to predict the mechanical behavior of composite overwrapped pressure vessels under impact conditions. Material characterization tests are conducted on filament-wound samples to determine the key material parameters, whilst taking into account manufacturing defects due to the production process. These material parameters serve as input for the numerical model. The model captures the cylindrical area of a pressure vessel with diminishing levels of detail away from the impactor. The performance of the model is optimized using a comparison with quasi-static indentation tests on cylinders with different thicknesses and lengths. It is shown that the mechanical behavior of a pressure vessel can be predicted accurately both under quasi-static indentation loads as well as under impact loads.

An experimental study on quasi-static indentation, low-velocity impact and damage behaviors of laminated composites at high temperatures

Nowadays, fiber reinforced laminated composites are widely used in many applications due to their high strength/weight ratio. However, these materials are very sensitive to transverse loading. The low-velocity impact test has been widely used by researchers to simulate the transverse loading. However, the low-velocity impact tests are highly toilsome, and this test requires expensive hardware and software systems. To reduce the experimental costs of the low-velocity impact test, it will be more attractive, much simpler, cheaper and more widely available to achieve impact behavior using quasi-static tests. Thus, to compare both tests, in this work the absorbed energy and force-deflection curves obtained by low-velocity impact and quasi-static indentation loading in two different fiber reinforced epoxy composites have been investigated. The Carbon-Kevlar hybrid fabrics and S2 glass fabrics were used as reinforcements. For low-velocity impact tests, a range of energies was used between 20 and 80 J. For quasi-static indentation test, the crosshead speeds were increased gradually from 1 mm/min to 60 mm/min. In addition, tests at 23°C, 40°C, 60°C and 80°C were made to examine the effect of temperature on these tests. As a result of the quasi-static tests performed, the amount of energy required to perforate the samples at a certain test speed is at the same level as the low-velocity impact test. Thus, the required energy amount for the perforation of the materials can be found by performing a quasi-static test at an appropriate speed, rather than the low-velocity impact test.

The Effect of a Coupling Agent on the Impact Behavior of Flax Fiber Composites

Abstract The effects of a coupling agent on the behavior of flax fiber-reinforced composites have been investigated by testing the specimens under both quasi-static (QS) indentation and high-velocity impact loading. The specimens are manufactured embedding a commercial flax fiber fabric in a polypropylene (PP) matrix, neat and premodified with a maleic anhydride-grafted PP, the latter acting as a coupling agent to enhance the interfacial adhesion. QS compressive tests were performed using a dynamometer testing machine equipped with a high-density polyethylene indenter having the same geometry of the projectile employed in the impact tests. The impact tests were conducted setting three different impact velocities. Digital image correlation maps of out-of-plane displacement were employed to compare the specimens with and without the coupling agent. The QS testing results indicate that the coupling agent has an enhancing influence on the bending stiffness of tested flax composites. The testing results show that the coupling agent improves the mechanical behavior by decreasing the out-of-plane displacement under impact loading. This approach gives rise to new materials potentially useful for applications where impact performance is desired while also providing an opportunity for the incorporation of natural fibers to produce a lightweight composite.

Investigation of novel multi-layer sandwich panels under quasi-static indentation loading using experimental and numerical analyses

In this study, the effect of multi-layering in sandwich panel composite structures with different configurations of corrugated cores under the effect of quasi-static indentation loading is investigated experimentally and numerically. Composite plates and corrugated cores with equal weight fraction were manually made using ML506 epoxy resin with 15% hardener and the overall volume fraction of 45% for woven glass fibers. Experiments were performed using a cylindrical indenter with a diameter of 20 mm and a semi-spherical nose shape, and the behavior of the composite structure was evaluated in the case of energy absorption, contact force, and fracture mechanisms for different corrugated cores (rectangular, trapezoidal and triangular). Progressive damage analyses of the composite samples were investigated using well-known 3D failure criteria developed in ABAQUS software, and an acceptable agreement between the experimental and numerical results was observed. Experimental results show that multi-layering of composite sandwich panels not only increases the structural strength in quasi-static indentation process but also increases the energy absorption of specimens by increasing the peak load, instantaneous force, and dislocation displacement up to the complete penetration. By comparing different geometries of the corrugated cores in terms of maximum force, energy absorption, and specific energy, it was shown that the functionality of specimens with rectangular cores is the best. Among the most important damage mechanisms of the examined sandwich panel specimens (visual analysis) in the quasi-static indentation process, matrix cracking, fiber breakage, delamination, cell walls buckling and crushing, face sheets and core debonding, and specimens complete penetration are noticeable.

Identifying the imaging correlates of cartilage functionality based on quantitative MRI mapping - The collagenase exposure model

Cartilage functionality is determined by tissue structure and composition. If altered, cartilage is predisposed to premature degeneration. This pathomimetical study of early osteoarthritis evaluated the dose-dependant effects of collagenase-induced collagen disintegration and proteoglycan depletion on cartilage functionality as assessed by serial T1, T1ρ, T2, and T2* mapping under loading. 30 human femoral osteochondral samples underwent imaging on a clinical 3.0 T MRI scanner (Achieva, Philips) in the unloaded reference configuration (δ0) and under pressure-controlled quasi-static indentation loading to 15.1 N (δ1) and to 28.6 N (δ2). Imaging was performed before and after exposure to low (LC, 0.5 mg/mL; n = 10) or high concentration (HC, 1.5 mg/mL; n = 10) of collagenase. Untreated samples served as controls (n = 10). Loading responses were determined for the entire sample and the directly loaded (i.e. sub-pistonal) and bilaterally adjacent (i.e. peri‑pistonal) regions, referenced histologically, quantified as relative changes, and analysed using adequate parametric and non-parametric statistical tests. Dose-dependant surface disintegration and tissue loss were reflected by distinctly different pre- and post-exposure response-to-loading patterns. While T1 generally decreased with loading, regardless of collagenase exposure, T1ρ increased significantly after HC exposure (p = 0.008). Loading-induced decreases in T2 were significant after LC exposure (p = 0.006), while changes in T2* were ambiguous. In conclusion, aberrant loading-induced changes in T2 and T1ρ reflect moderate and severe matrix changes, respectively, and indicate the close interrelatedness of matrix changes and functionality in cartilage.

Mechanical behavior of resin pin-reinforced composite sandwich panels under quasi-static indentation and three-point bending loading conditions

This paper reports the mechanical behavior of resin pin-reinforced composite sandwich panels made from polyvinyl chloride core and glass/epoxy face sheets under indentation of a hemispherical indenter and three-point bending loading conditions. The goal was to study the effects of reinforcing parameters such as the number, arrangement, and diameter of the through-the-thickness resin pins on the indentation maximum load, the bending strength, and energy absorption characteristics of the tested samples under these loading conditions. The results revealed that using the resin pins to reinforce the polyvinyl chloride foam core led to increase the indentation maximum load up to 47%, and the maximum bending load up to 34%, compared to nonreinforced foam–core sandwich structures. Also, the presence of resin pins led to the change in the failure modes of specimens (i.e. from local to global deformation and failure) and consequently increased the energy absorption capability of sandwich structures by 31% and 68% respectively under indentation and bending loads.

Effect of thickness on indentation response and tensile loading behaviour of glass/epoxy laminates under acoustic emission monitoring

ABSTRACT This research work investigates the effect of thickness on the quasi-static indentation and tensile loading behaviour of glass/epoxy laminates. Cross-ply [0°/90°]3S laminates fabricated at different process pressures (40 kg/cm2, 50 kg/cm2 and 60 kg/cm2) were subjected to quasi-static indentation loading. Furthermore, the residual load-bearing capacity was estimated by performing tensile after indentation (TAI) test. The damage development and corresponding failure mechanisms associated during loading were characterised by acoustic emission monitoring. The results show that the damage onset load, residual dent and damage area were dependent on the thickness of the laminates. Type II samples (processed at 50 kg/cm2) exhibited higher damage onset load and contact stiffness by an average of 20% and 25% than type I and type III laminates. Higher peak force, reduced damage area and permanent dent were attributed to greater elastic response. In contrast, Type I samples (processed at 40 kg/cm2) with thick resin-rich zone exhibited severe delamination through transverse shear cracking. This observation was also evident from the intense AE signals related to debonding/delamination modes, which correlates well with the damage area.

Fiber push-in failure in carbon fiber epoxy composites

Micromechanical quasi-static and cyclic fiber push-in tests performed with an indenter in-situ in a scanning electron microscope on polished slices (thickness around 300 μm) of carbon-fiber epoxy composites provide information on the effect of fiber density on the fiber-matrix debonding and the fiber failure. Specifically, "close-packed", i.e., fibers surrounded by six nearest neighbor fibers (called "hexagonal" hereafter) versus "isolated" fibers, defined as fibers at least one fiber diameter distant from the next nearest fiber, were compared. In spite of the more compliant behavior of isolated fibers under quasi-static and cyclic indentation loads, initiation of fiber-matrix debonding and push-in failure (fiber splitting into two or three parts) occurred at roughly comparable loads for both, "hexagonal" and "isolated" fibers. Earlier tests on thinner slices of such composites (thickness around 30 μm) had resulted in full debonding and fiber push-out without fiber failure. The fibers in the thicker slices showed first indenter imprints on the surface and an increasing hysteretic behavior in the load-displacement curves before debonding. The analysis of the cyclic load-displacement curves yields the elastic-plastic and the hysteretic (debonding) energy contributions.

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.