Quasi-static Indentation Loading Research Articles

Energy dissipation studies of SMA reinforced composites under multiple quasi-static indentation loading

The aim of this paper is to find out the increase in energy dissipation by adding pseudoelastic shape memory alloy (PE-SMA) in glass fibers reinforced (GFRP) composites. Nature of loading is quasi-static, performed multiple times until material fails completely and indenter goes inside the sample. This is done to find out the alteration in the ultimate failure load due to the addition of SMA. It also gives the pattern of degradation in energy dissipation and stiffness over the number of cycles in both GFRP and SMA/GFRP samples. Reduction in the effective properties exhibits the advantages of embedding SMA.

Damage Modelling in Thermoplastic Laminates Reinforced with Steel and Glass Fibres under Quasi-Static Indentation Loading at Low-Velocity

This paper deals with experimental and numerical investigations of the composites damages with ductile and fragile reinforcement under quasi-static indentation loading. The main goal of the work is to increase the post-damage residual strength and ductility of thermoplastic composite. Two types of composite laminates with polypropylene (PP) matrix are tested: glass fibre laminate (GFPP) and steel fine wire mesh laminate (SWPP). The specimens are [0° 90°]2s stacking sequence and prepared by using a compression moulding technique. Quasi-static indentation tests were performed with two distinct penetration scales under low velocity (1.2 mm/min). The diameter of the hemispherical indenter is 16 mm. The failure mechanisms of composite layers were examined by the field emission scanning electron microscope (SEM). The results show that the failure mode of SWPP laminates is principally dominated by the plastic deformation component. In contrast, the GFPP laminate exhibits a fragile behaviour which is related to the fragile failure of glass fibres. In addition, the SEM shows that matrix cracking, fibre breakage, debonding and fibre pull out are the major damages observed around the indentation area. A model based on the combined use of plasticity, damage and fracture, was developed and applied to simulate quasi-static indentation behaviour and predict the resulting damage.

Effect of Thickness and Denting Behavior of Glass/Epoxy Laminates Subjected to Quasi-Static Indentation (QSI) Loading Under Acoustic Emission Monitoring

The present work focuses on investigating the effect of laminate thickness on quasi static indentation behavior of glass/epoxy laminates under acoustic emission monitoring. The load bearing and energy absorbing capability of the glass/epoxy samples at different indentation depth were examined. The damage development and damage mechanisms associated with the tests were characterized in both thin and thick samples, and their results were correlated. The results showed that the thickness of the glass/epoxy laminates significantly influenced the damage onset load, residual deformation and damage area. It was observed that the thick samples exhibited higher peak force, greater absorbed energy, and residual flexural load by an average of 2.08, 2.53, and 2.13 times respectively compared to thin samples. In contrast, the permanent deformation and damage area was observed to be 32% deeper and larger in thick samples while the thin samples showed reduced damage area and residual deformation which attributes to greater elastic response caused by large global deformation.

The mechanical response characteristics of sapphire under dynamic and quasi-static indentation loading

Sapphire is widely used as optical materials and substrate materials due to its excellent physical and chemical properties. The mechanism of crack propagation and fracture damage evolution has important significance for improving the manufacturing quality and application performance of sapphire parts. In this study, dynamic and quasi-static indentation tests have been performed on the c-plane and a-plane of sapphires by Hopkinson pressure bar tester and continuous indentation tester, respectively. The crack propagation path in sapphire has been captured by High-speed camera and the crack velocity has been calculated. The crack propagation and fracture damage evolution has been analyzed based on the fracture morphology of specimen. It was found that the bearing capacity of sapphire is related to the loading velocity, while the crack propagation is affected by the crystal orientation. Under the indentation loading, the cracks in sapphire first propagate steadily, and then the cracks begin to propagate uncontrollably after reaching the critical conditions, where the crack propagation velocity obviously increases, typically from 204 m/s to 1006 m/s (dynamic indentation) or from 0.0032 m/s to 820 m/s (quasi-static indentation). And the crack propagation velocity depends on the loading speed at stable stage. The r-planes of sapphire are weaker than other crystal planes and are prone to crack propagation.

A comparison of quasi-static indentation and low-velocity impact on composite overwrapped pressure vessels

The equivalence of quasi-static indentation and low-velocity impact loading regimes has been assessed for composite overwrapped pressure vessels. Test specimens were assessed in detail in terms of the force–displacement response, and micro-focus computed tomography was used for qualitative and quantitative assessment of the associated damage to the constituent materials/interfaces. The results show that the force–displacement response follows an essentially similar pattern between the two loading regimes (within 10% for all cases). Quantitative assessment of the projected composite damage area and permanent deformation of the aluminium substrate as a function of peak indentor displacement also showed a high degree of equivalence between the loading regimes. It is concluded that quasi-static indentation represents a usable analogue for mechanistic assessment of low-velocity impact damage in the tested composite overwrapped pressure vessels.

Clustering of interlaminar and intralaminar damages in laminated composites under indentation loading using Acoustic Emission

This study focuses on the clustering of the indentation-induced interlaminar and intralaminar damages in carbon/epoxy laminated composites using Acoustic Emission (AE) technique. Two quasi-isotropic specimens with layups of [60/0/-60]4S (is named dispersed specimen) and [604/04/-604]S (is named blocked specimen) were fabricated and subjected to a quasi-static indentation loading. The mechanical data, digital camera and ultrasonic C-scan images of the damaged specimens showed different damage evolution behaviors for the blocked and dispersed specimens. Then, the AE signals of the specimens were clustered for tracking the evolution behavior of different damage mechanisms. In order to select a reliable clustering method, the performance of six different clustering methods consisting of k-Means, Genetic k-Means, Fuzzy C-Means, Self-Organizing Map (SOM), Gaussian Mixture Model (GMM), and hierarchical model were compared. The results illustrated that hierarchical model has the best performance in clustering of AE signals. Finally, the evolution behavior of each damage mechanism was investigated by the clustered AE signals with hierarchical model. The results of this study show that using AE technique with an appropriate clustering method such as hierarchical model could be an applicable tool for structural health monitoring of composite structures.

An analytical model for the response of carbon/epoxy-aluminum honeycomb core sandwich structures under quasi-static indentation loading

An analytical model is developed to predict the loading and unloading response, as well as the residual dent diameter and dent depth, of carbon/epoxy-aluminum honeycomb core composite sandwich structures undergoing quasi-static indentation loading. The model considers damage created using spherical indenters and is valid up to the barely visible external damage threshold. The initial low load regime (until the onset of core crushing) is modeled using a combination of local Hertzian indentation of an elastic half-space and small deflection plate theory of a circular plate on an elastic foundation. For loads above those required to cause core crushing, the model uses the Rayleigh-Ritz method of energy minimization with the total system energy determined using a combination of face sheet bending energy, face sheet membrane energy and work done to the core during both elastic deformation and crushing. Degraded face sheet properties are used in the model beyond the onset of face sheet delamination, which is predicted using Griffith’s energy criterion. The model is validated using experimental results for sandwich structures consisting of quasi-isotropic 8- (thin) and 16- (thick) ply carbon/epoxy face sheets and aluminum honeycomb cores. The results show that the overall mechanics of the model are fundamentally correct and reflective of physical behavior. Thus, in its present form the model shows promise as a preliminary design tool.

T2 MR imaging vs. computational modeling of human articular cartilage tissue functionality

The detection of early stages of cartilage degeneration remains diagnostically challenging. One promising non-invasive approach is to functionally assess the tissue response to loading by serial magnetic resonance (MR) imaging in terms of T2 mapping under simultaneous mechanical loading. As yet, however, it is not clear which cartilage component contributes to the tissue functionality as assessed by quantitative T2 mapping. To this end, quantitative T2 maps of histologically intact cartilage samples (n=8) were generated using a clinical 3.0-T MR imaging system. Using displacement-controlled quasi-static indentation loading, serial T2 mapping was performed at three defined strain levels and loading-induced relative changes were determined in distinct regions-of-interest. Samples underwent conventional biomechanical testing (by unconfined compression) as well as histological assessment (by Mankin scoring) for reference purposes. Moreover, an anisotropic hyperelastic constitutive model of cartilage was implemented into a finite element (FE) code for cross-referencing. In efforts to simulate the evolution of compositional and structural intra-tissue changes under quasi-static loading, the indentation-induced changes in quantitative T2 maps were referenced to underlying changes in cartilage composition and structure. These changes were parameterized as cartilage fluid, proteoglycan and collagen content as well as collagen orientation. On a pixel-wise basis, each individual component correlation with T2 relaxation times was determined by Spearman's ρs and significant correlations were found between T2 relaxation times and all four tissue parameters for all indentation strain levels. Thus, the biological changes in functional MR Imaging parameters such as T2 can further be characterized to strengthen the scientific basis of functional MRI techniques with regards to their perspective clinical applications.

Characterization of indentation damage resistance of hybrid composite laminates using acoustic emission monitoring

Abstract This paper focuses on the experimental investigation of impact damage resistance in hybrid composite laminates. In this case, the low velocity impact behaviour of quasi-isotropic glass/epoxy, glass/basalt/epoxy (G/B/G, B/G/B) and glass/carbon/epoxy (G/C/G, C/G/C) composite laminates was simulated by carrying out quasi-static indentation (QSI) tests with online acoustic emission (AE) monitoring. The dent depth, back surface crack size and load-deflection behaviors were examined and there is no distinct differences could be seen between low velocity impact tests and quasi-static indentation tests. The QSI tests were performed on specimens with rectangular section, of size 150 mm x 100 mm, which were loaded at the centre by a hemispherical steel indenter with 12.7 mm diameter. The indentation response was evaluated by measuring peak force, absorbed energy and linear stiffness. The residual strength of the laminates following indentation was measured by testing them under compression load in a 100 kN universal testing machine, once again with AE monitoring. AE parameters, such as amplitude, rise time, cumulative counts and cumulative energy were considered for monitoring damage progression during quasi-static indentation loading. Also other parameters linked to AE monitoring, such as the rise angle (RA) and Felicity ratio (FR) were measured for evaluating the damage resistance in each cycle of indentation. In addition, sentry function was also computed based on the combination of mechanical strain energy accumulated in the materials and of the acoustic energy propagates by fracture events made it possible to evaluate the amount of induced damage. These results showed that the combination of glass and carbon fibres in glass/carbon/epoxy (C/G/C) laminates improved their interlaminar shear strength at a level well above the other configurations tested.

Real Area of Contact in a Soft Transparent Interface by Particle Exclusion Microscopy

Soft matter mechanics are characterized by high strains and time-dependent elastic properties, which complicate contact mechanics for emerging applications in biomedical surfaces and flexible electronics. In addition, hydrated soft matter precludes using interferometry to observe real areas of contact. In this work, we present a method for measuring the real area of contact in a soft, hydrated, and transparent interface by excluding colloidal particles from the contact region. We confirm the technique by presenting a Hertz-like quasi-static indentation (loading time > 1.4 hrs) by a polyacrylamide probe into a stiff flat surface in a submerged environment. The real contact area and width were calculated from in situ images of the interface processed to reduce image noise and thresholded to define the perimeter of contact. This simple technique of in situ particle exclusion microscopy (PEM) may be widely applicable for determining real areas of contact of soft, transparent interfaces.

Effect of post-cure temperature and different reinforcements in adhesive bonded repair for damaged glass/epoxy composites under multiple quasi-static indentation loading

This paper investigates the individual and associated effects of post-cure temperature and reinforcement phases on the local bending response of adhesive reinforced repairs in damaged glass/epoxy composite laminates. Multiple quasi-static indentation tests were performed on repaired composite laminates: damage propagation, absorbed energy and residual deflection data assisted in the evaluation of phase changes, which cause the transition from brittle to ductile nature and control the visco-elastic behavior the material is undergoing under the influence of temperature. Three phases of fiber reinforcements, namely particulate glass fibers, chopped short glass fibers and continuous glass fibers, were tested to study the fiber reinforcement’s effect on indentation strength and damage mechanism. The effects of post-cure temperature on the local bending response of the repaired glass/epoxy specimens were investigated by performing indentation tests on laminates exposed to post-cure at ambient temperature (30°C) and elevated temperatures up to glass transition temperature of epoxy resin (50, 70 and 90°C). Multiple quasi-static indentation test results indicated that the post-cure temperature and fiber reinforcements have considerable effect on the indentation response of the repaired specimens. Damaged glass/epoxy specimens repaired using chopped short fibers and exposed to a post-cure temperature of 50°C showed the most favorable indentation behavior.

Investigation of the response to low velocity impact and quasi-static indentation loading of particle-toughened carbon-fibre composite materials

This work investigates damage caused by low velocity impact and quasi-static indentation loading in four different particle-toughened composite systems, and one untoughened system. For impact tests, a range of energies were used between 25 and 50J. For QSI, coupons were interrupted at increasing loading point displacement levels from 2 to 5mm to allow for monitoring of damage initiation and propagation. In both loading cases, non-destructive inspection techniques were used, consisting of ultrasonic C-scan and X-ray micro-focus computed tomography. These techniques are complemented with instrumentation to capture force–displacement data, whereby load-drops are associated with observed damage modes. Key results from this work highlight particular issues regarding strain-rate sensitivity of delamination development and an earlier onset of fibre fracture associated with particle-toughened systems. These issues, in addition to observations on the role of micro-scale events on damage morphology, are discussed with a focus on material development and material testing practices.

Finite size effect does not depend on the loading history in soft matter indentation

Due to the finite size effect (including the finite sample thickness and the indenter tip size), Hertz's solution cannot accurately describe the indentation behaviour of soft matter. In addition, soft matters are typically viscous and are thus loading history dependent. There might be a coupling between the indentation loading history and the finite size effect, and thus the indentation loading curve of soft matters might be too complex to determine the mechanical properties from it. Using finite element modelling, the indentation loading response of soft matters is investigated based on the commonly used viscous models, including the power-law rheology model and the standard linear solid model, as well as the general viscous model that we have proposed. The results show that the finite size effect does not depend upon the loading history, which also suggests that the finite size effect does not depend upon the material model but only relates to the geometric parameters, such as indenter tip size and indentation depth. With this finding, the mechanical properties of soft matter with finite size (e.g. biological cells) can be determined by indentation tests, not only from the frequency domain but also from the time domain (the quasistatic indentation loading curve), which makes the indentation technique a powerful tool to measure the mechanical properties of soft matters with finite size.

Quasi-Static Flexural and Indentation Behaviour of Polymer-Metal Laminate

Metal-polymer laminates were subjected to quasi-static flexural and indentation loading. The laminates were made of two aluminium skins heat-bonded (laminated) to a core made of polyethylene plastic material. The samples were trimmed into standard-sized beams and panels which were then tested in flexural and indentation using the Instron universal testing machine at loading rates of 1 mm/min, 10 mm/min and 100 mm/min. The load-displacement and energy absorption curves of the composite beams were recorded. It was found that the loading rate has a large effect on flexural and indentation behaviour of aluminium composite laminate.

Quasi-Static Indentation Behaviour of Glass Fibre Reinforced Polymer

Fibre reinforced polymer (FRP) plates subject to quasi-static indentation loading were studied. The plates were fabricated from three layers of chopped strand mat glass fibre and polyester resin using vacuum infusion process. Indentation tests were conducted on the plates with loading rates of 1 mm/min, 10 mm/min, 100 mm/min and 500 mm/min using a hemispherical tip indenter with diameter 12.5 mm. The plates were clamped in a square fixture with an unsupported space of 100 mm × 100 mm. The loads and deflections at the indented location were measured to give energy absorption-deflection curves. The results showed that the loading rate has a large effect on the indentation behaviour and energy absorbed.

Characterization of sandwich panels for indentation and impact

The integrity of sandwich structures which are susceptible to impact may deteriorate significantly due to collapse of the core material and delamination of the face sheets. The integration of a thin polyurethane interlayer between the composite face sheet and foam core is known to protect the core material and substantially improve the resistance to impact. The objective of the present work is to characterize the response of sandwich panels, as well as that of the constituents to impact. In particular, the response of polyurethane and foam samples under a range of quasi-static and dynamic loading rates is determined experimentally. Furthermore, the response of sandwich panels to quasi-static indentation and low velocity impact is examined to quantify the extent of damage and how it is affected by the integration of polyurethane interlayers in their construction. This information is useful in the modelling of high velocity impact of sandwich panels; an effort which is currently underway. The results illustrate the benefit of using polyurethane interlayers within the construction of sandwich panels in enhancing their performance under quasi-static indentation and impact loads.

Nanoscopic dynamic mechanical properties of intertubular and peritubular dentin

An experimental evaluation of intertubular and peritubular dentin was performed using nanoindentation and Dynamic Mechanical Analysis (DMA). The objective of the investigation was to evaluate the differences in dynamic mechanical behavior of these two constituents and to assess whether their response is frequency dependent. Specimens of hydrated coronal dentin were evaluated by DMA using single indents over a range of parametric conditions and using scanning probe microscopy. The complex (E∗), storage (E′) and loss moduli (E″) of the intertubular and peritubular dentin were evaluated as a function of the dynamic loading frequency and static load in the fully hydrated condition. The mean complex E∗ (19.6 GPa) and storage E′ (19.2 GPa) moduli of the intertubular dentin were significantly lower than those for peritubular dentin (E∗=31.1 GPa, p<0.05; E′=30.3 GPa, p<0.05). There was no significant influence of dynamic loading frequency on these measures. Although there was no significant difference in the loss modulus (E″) between the two materials (p>0.05), both constituents exhibited a significant increase in E″ with dynamic load frequency and reduction in the quasi-static component of indentation load. The largest difference in dynamic behavior of the two tissues was noted at small quasi-static indentation loads and the highest frequency.

The breakage behaviour of Aspirin under quasi-static indentation and single particle impact loading: Effect of crystallographic anisotropy

The influence of crystallographic structural anisotropy on the breakage behaviour of Aspirin under impact loading is highlighted. Under both quasi-static testing conditions, using nano-indentation, and dynamic impact tests, Aspirin demonstrates clear anisotropy in its slip and fracture behaviour. During nano-indentation on the (1 0 0) and (0 0 1) faces, cracks were propagated along the [0 1 0] direction. While the hardness was found to be comparatively similar for both these faces, it was observed that slip due to plastic deformation occurred more readily on the (1 0 0) than the (0 0 1) crystal planes suggesting the former as the preferred slip plane. Furthermore, the fracture toughness on the (0 0 1) planes was found to be distinctly lower than that of the (1 0 0) planes, indicating the former as the preferred cleavage plane. Observations of the crystal morphology of damaged particles after dynamic impact testing showed that both the chipping and fragmentation of Aspirin mostly occurred via cleavage in a manner consistent with the observed fracture behaviour following nano-indentation. This work highlights the importance of cleavage as a dominant factor underpinning the fracture mechanism of Aspirin under both quasi-static and impact loading conditions.

Damage assessment of thin SiC/SiC composite plates subjected to quasi-static indentation loading

Tri-dimensional woven SiC/SiC composite was subjected to quasi-static indentation tests at room temperature. For this purpose, hemispherical indentors and circular supports of various diameters were used. The extent of damage was evaluated with the help of optical and scanning electron microscopy. Formation of cone cracks initiating from the indented site is observed. The predominant internal damages (fibre bundle and matrix cracking) remain localised beneath the indentor. The indented specimens were tensile tested at room temperature to determine their residual strength. Results indicate that the strength reduction is proportional to the diameter of the damaged zone evaluated on the back side of the specimen.

Quasi-static indentation tests on aluminium foam sandwich panels

Abstract Mechanical response and energy absorption of aluminium foam sandwich panels subjected to quasi-static indentation loads were investigated experimentally. These sandwich panels consisted of two aluminium face-sheets and a closed cell aluminium foam core (ALPORAS®). Quasi-static indentation tests were conducted using an MTS universal testing machine, with sandwich panels either simply supported or fully fixed. Force–displacement curves were recorded and the total energy absorbed by sandwich panels was calculated accordingly. Videos and photographs were taken to capture the deformation of top face-sheets, foam cores and bottom face-sheets. Effects of face-sheet thickness, core thickness, boundary conditions, adhesive and surface condition of face-sheets on the mechanical response and energy absorption of sandwich panels were discussed.