Quasi-static Punch Tests Research Articles

Rate dependent fracture of monolithic and laminated glass: Experiments and simulations

Glass is a brittle material known to possess large scatter in its fracture strength, which is caused by the existence of microscopic surface flaws. Fracture in glass generally originates from stress concentrations around these flaws, which cause the fracture strength to be dependent on the flaw properties and the stress state on the glass surface. The fracture strength is also reported to increase with the loading rate. The current study aims to determine the probabilistic fracture strength of glass plates exposed to arbitrary loading and loading rates by a proposed rate-dependent strength prediction model (SPM). The SPM is based on the existence of microscopic surface flaws, and performs virtual experiments on glass plates through Monte Carlo simulations. To validate the SPM in some measure, we performed quasi-static punch tests and low-velocity impact tests on monolithic and laminated glass. The experimental work clearly demonstrated the stochastic fracture strength of glass, in addition to the load-rate dependency. The SPM managed to capture many of the trends observed in the experiments, such as the increase in fracture strength with the loading rate and the positions of fracture initiation in the glass.

Investigating the feasibility of a new testing method for GFRP/polymer foam sandwich composites used in railway passenger vehicles

This paper investigates the feasibility and validity of a recently proposed testing method when applied to foam core sandwich composites to determine the impact resistance of train car body shells. The published method proved itself useful when applied to monolithic fibre reinforced plastic laminates, however, further analysis is required if sandwich composites are used due to the subtle differences compared to monolithic laminates. It is especially important for the front nose section of high-speed trains as these car body parts can be subjected to ballast flight at high impact velocities. Due to the aerodynamic design aspects, front nose sections generally incorporate complex 3D geometries, and sandwich composite materials are advantageous in this respect due to the ease of manufacturing of complex 3D shapes. Quasi-static punch tests (QSPT) were performed on E-glass fibre/epoxy resin/PET foam core sandwich plates. Numerical finite element model (FEM) was developed and validated with experiments in terms of penetration resistance and structural damage. Numerical model was used to investigate three railway impact related standards and high-velocity impact parameters (energy transfer, contact force, projectile velocity) were analysed for each case. Additionally, the foreseeable contribution of this work to the railway industry was given from multiple aspects.

Critical thrust force predictions during drilling: Analytical modeling and X-ray tomography quantification

In the past, several experimental and analytical studies have been conducted aimed towards predicting the critical thrust force responsible for delamination at the hole exit during drilling. Among these analytical models, few take into account the coupling between bending and stretching often observed in multi-directional (MD) laminates with an un-symmetrical stacking sequence considering the presence of an elliptical crack. In addition, in these analytical models, the tool/composite contact region is modeled without taking into account the chisel edge effect. In the present study, a unique analytical model for critical thrust force prediction has been proposed that explicitly accounts for the effect of the chisel and cutting edges. The interaction zone (composite/chisel edge and composite/principal cutting edge) has been modeled using classical lamination plate theory (CLPT) and linear elastic fracture mechanics (LEFM) principles. In addition, a comparison between the proposed analytical model predictions and experimental results from quasi-static punch tests accompanied with different X-ray tomography observations led to choosing the accurate loading profile developed during drilling.

Flying Ballast Resistance for Composite Materials in Railway Vehicle Carbody Shells

This paper describes a simplified practical approach to the impact damage assessment of flying ballast or debris in composite materials, which could potentially be used for passenger rail vehicles. With the development of high-speed lines in order to guarantee the safety of primarily the driver and passengers, the vehicle body shell must be strong enough to resist the penetration of these types of objects into the vehicle. Currently the EN12663 standard has insufficient information on missile impact requirements for car bodies. Using the requirements of British Railway Group Standard GM/RT2100, GFRP composite material was analyzed in both static and dynamic events. Impact events at high velocities (V>50 m/s) lead to difficulties in testing due to the requirements for specific and expensive equipment and safety issues in the test application process. The simpler assessment and testing method described in this paper aims to provide a more cost effective, less time consuming, easily applicable method, with accurate results compared to existing methods. Previous research found that high-velocity impact behavior of composite materials can be mimicked by quasi-static punch tests (QSP). In this study, QSP tests were performed with E-glass/polyester composite laminates and FE simulations were carried out to match the experiments. The resulting numerical material and modelling data was used to simulate high velocity impact cases to assess the response of the material and to determine the connection between static and dynamic cases.

Shear-dominated plastic behavior of a cross-ply Dyneema® composite

Dyneema® composites derive some of their performance attributes from their elastic and plastic anisotropy. The principal objective of the present study is to develop a finite element framework for predicting the large-strain deformation response of these composites in a [0°/90°] configuration. We employ a binary model comprising a series of line elements to represent the axial fiber properties and a 3D effective medium to represent all off-axis properties. The model is assessed through comparisons with the results of two types of tests: (i) quasi-static punch tests and (ii) dynamic tests performed by firing (crushable) foam projectiles at high velocities. The measured load–displacement response in the punch tests and the evolution of the back-face deflection in the dynamic tests as well as the final shapes of the test specimens are represented well by the numerical simulations. Discrepancies occur only at the highest levels of displacements, wherein material damage becomes a significant factor.

The Effect of Hybridization on the GFRP Behavior under Quasi-Static Penetration

Quasi-static punch tests using three different penetrator shapes, namely blunt-ended, hemispherical, and conical, were conducted to investigate the mechanical behavior of glass/polyester laminates hybridized with a layer of Kevlar/polyester at four different positions. It was found that the addition of the Kevlar layer at any position improves the toughness (strength and ductility) of the composite laminates, which led to enhancement of the penetration resistance. Generally, it was also verified that the back face is the most appropriate location for the Kevlar layer for laminates penetrated quasi-statically. The results also indicate that the laminates response and damage mechanism are found to be highly sensitive to the penetrator geometry. The procedure and main findings can serve as a design guideline for dynamic applications, namely, high velocity impact loading.

Analytical models of composite material drilling

Drilling of composite material structures is widely used for aeronautical assemblies. When drilling, damage to the composite laminate is directly related to the cutter geometry and the cutting conditions. Delamination of the composite materials at the hole exit as directly related to the axial force (F Z) of the cutter is considered to be the major such defect. To address this issue, an orthotropic analytical model is developed in order to calculate the critical force of delamination during drilling and a number of hypotheses for loading are proposed. This critical axial load is related to the delamination conditions (propagation of cracks in the last layers) and the mechanical characteristics of the composite material machined. A numerical model is also drawn up to allow for numerical validation of the analytical approach. A comparison between these analytical and numerical modellings and experimental results from quasi-static punch tests led to the choice of the loading hypothesis closest to the experimental conditions. The selection of corresponding load permits to model the drilling critical thrust force on delamination and then to optimise the cutting conditions. The dimensions and geometrical shape of the cutter are of considerable importance when it comes to choosing this load. The present article focuses on the case of the twist drill, which is commonly used to drill thick plates. However, this work can be adapted to different cutter geometries.

Analytical modelling of high velocity impacts of cylindrical projectiles on carbon/epoxy laminates

In this work an analytical model has been developed in order to predict the residual velocity of a cylindrical steel projectile, after impacting into a woven carbon/epoxy thin laminate. The model is based in an energy balance, in which the kinetic projectile energy is absorbed by the laminate through three different mechanisms: linear momentum transfer, fiber failure and laminate crushing. This last mechanism needs the quantification of the through-thickness compressive strength, which has been evaluated by means of quasi-static punch tests. Finally, high velocity impact tests have been accomplished in a wide range of velocities, to validate the model.

Ballistic impact response for two-step braided three-dimensional textile composites

We are concerned with the ballistic impact response of three-dimensional two-step braided textile composites struck by a 36.1-g hemispherical tip-ended rigid cylindrical projectile. A series of quasistatic punch tests using a hemispherical indentor have been conducted to investigate the progressive damage modes of the target and to obtain the punch load-displacement relationship. In addition, a pneumatic launcher is used to propel the projectile with incident velocities ranging from 70 to 170 m/s. The ballistic limit is experimentally determined to be near 74.1 m/s. Commercially available finite element code (MARC) is incorporated with the constitutive relationship for three-dimensional two-step braided composites and the proposed static penetration model to simulate the dynamic impact response. An energy consideration is applied to predict the projectile's residual (or terminal) velocity. Numerical simulated result based on the static elastic properties of the target tends to overestimate the projectile terminal velocity. When the target's elastic moduli used in simulation are increased by 1.5 times the static values, good agreement is found between the simulated terminal velocities and test results for projectile incident velocities ranging from 70 to 180 m/s.

Predicting the ballistic limit for plain woven glass/epoxy composite laminate

This paper is concerned with predicting the ballistic limit of plain woven glass/epoxy composite laminates struck by a 14.9 gm bullet-like rigid projectile with a tip radius of 5 mm. The 4 mm thick square specimens were clamped along their 100 mm edges. A pneumatic gun was used to propel the bullet with incident velocities ranging from 140 to 200 m/sec. The ballistic limit was experimentally determined to be near 153 m/sec. A series of quasi-static punch tests was performed in order to investigate the progressive damage modes of the targets and to obtain the punch load-displacement relation. These quasi-static punch tests were conducted to characterize the penetration process. Similar to dynamic impact test results, the major damage modes for targets subjected to quasi-static punch loading were found to be governed by delamination and fiber breakage. After specimens were perforated, a steady friction force was observed from quasi-static punch tests. Test results also indicate that the rhombus-shaped delamination of impact damaged samples is greater than that of quasi-statically punched specimens. A partial hybrid stress finite element code was incorporated with the proposed static penetration model to simulate the dynamic impact process. An energy consideration was applied to predict the ballistic limit. The difference between the predicted ballistic limit and test findings was found to be approximately 24% if the target's static material properties were used in the code simulation. Due to the rate-sensitive nature of glass/epoxy composites, the effect of dynamic elastic properties on the predicted ballistic limit was further studied. Good agreement between the predicted ballistic limit and test results was found if the target's elastic moduli used in simulation were increased to two times the static values.