Perforation of high-strength fabric by projectiles of different geometry

This paper investigates the ballistic limits, energy absorption behaviour and the mechanisms that lead to perforation in Twaron^® CT 716 plain-woven, single-ply fabric by different shaped projectiles. The projectile shapes tested are flat head, hemispherical head, ogival head (CRH 2.5) and conical head (half angle of 30°).Results show that while the amount of energy absorbed by the fabric is quantitatively different for all four projectiles, they show similar trends—energy absorbed increases with impact velocity up to a critical impact velocity before it starts to decrease. The energy absorption capability of the Twaron^® fabric is explained by considering how impact energy is converted to strain energy and kinetic energy of the fabric. Each projectile shape was also found to perforate the fabric through different mechanisms—yarn rupture, fibrillation, failure by friction, and bowing.     

Experimental investigation of delamination buckling of stitched composite laminates

In the current investigation, effects of through-the-thickness stitching with two different types of aramid threads, Kevlar^® and Twaron^® threads, on the buckling loads of delaminated glass/epoxy composite laminates are studied. Buckling loads are predicted based on the Southwell, Vertical displacement and Membrane strain plot methods by using the experimental data. From the Southwell, Vertical displacement and Membrane strain plot methods it is observed that stitching either by Kevlar^® or Twaron^® threads is effective in improving the buckling strength of glass/epoxy composite laminates when the delamination length is greater than 0.5L, L being the length of the laminate. For long delaminations, Kevlar^® stitched glass/epoxy composite laminate is best in retaining its buckling strength when re-loading is done. Southwell plot method tends to overestimate the buckling loads as the data obtained from this method are influenced by the breakages in the glass/epoxy composite laminate buckling test specimens.     

Analytical modeling for the ballistic perforation of planar plain-woven fabric target by projectile

This paper presents an analytical model to calculate decrease of kinetic energy and residual velocity of projectile penetrating targets composed of multi-layered planar plain-woven fabrics. Based on the energy conservation law, the absorbed kinetic energy of projectile equals to kinetic energy and strain energy of planar fabric in impact-deformed region if deformation of projectile and heat generated by interaction between projectile and target are neglected. Then the decrease of kinetic energy and residual velocity of projectile after the projectile perforating multi-layered planar fabric targets could be calculated. Owing to fibers in fabric are under a high strain rate state when fabric targets being perforated by a high velocity projectile, the mechanical properties of the two kinds of fibers, Twaron^® and Kuralon^®, respectively, at strain rate from 1.0×10⁻² to 1.5×10³ s⁻¹, are used to calculate the residual velocity of projectile. It is shown that the mechanical properties of fibers at high strain rate should be adopted in modeling rate-sensitivity materials. Prediction of the residual velocities and energy absorbed by the multi-layered planar fabrics show good agreement with experimental data. Compared with other models on the same subject, the perforating time in this model can be estimated from the time during which certain strain at a given strain rate is generated. This method of time estimation is feasible in pure theoretical modeling when the perforation time cannot be obtained from experiments or related empirical equations.     

Characterization and micromechanical testing of the interphase of aramid-reinforced epoxy composites

The properties of the interphase between Twaron^® aramid fibres and polymer matrix systems can be optimized by a surface treatment process of the fibres. The relation between this surface treatment, the resulting chemical and physical surface structure, as measured with XPS, IGC and SFM, and the adhesion strength in fibre–epoxy systems, as measured with Raman spectroscopy and single-filament pull-out experiments, has been established and related to the macromechanical data of real composites. The concept of local bond strength was used and the fibre–epoxy failure mechanism investigated.     

The mechanics of stretch-graphitization of glassy carbon fibres

The mechanics of stretch-graphitization of glassy carbon fibres made from two pitch precursors was studied by determining the plastic deformation characteristics of monofilaments during their elongation to 50% strain in a continuous apparatus. By monitoring fibre tension under various processing conditions and analysing deformation profiles quenched into extending fibres, the temperature and strain-rate dependence of induced fibre stresses has been obtained. Over the temperature range 2200 to 3000° C and strainrate range 3.0×10^?4 to 2.5×10^?1 sec^?1, tensile stresses in the glassy carbon fibres ranged from 7000 to 50 000 psi. The deformation can be described by the empirical equation \(\dot \varepsilon = A\sigma ^n \) exp (? ΔH/RT), where A is a constant, n=8.6±1 and ΔH ～ 290±50 kcal mole^?1, independent of temperature and strain-rate, for both fibre types. The apparent activation energy is consistent with the controlling operation of a microfibrillar reorientation process, accompanied by atomic diffusion (ordering). The high strain-rate stress dependence, indicating an apparent activation volume ～ 4000 ų, also suggests a molecular-scale rate-limiting process. Results are compared with those of various high temperature processes in carbonaceous solids and deformation in organic polymers and it is suggested that stretch-graphitization can be considered analogous to the affine drawing of amorphous polymer fibres.     

Modellierung des Einflusses von Temperatur und Verformungsgeschwindigkeit auf die Fließspannung von Ck 45 bei Temperaturen T ≲ 0,3 Ts

Modelling of the temperature and strain-rate dependance of the flow stress of Ck 45 at temperatures T ≲ 0.3 T_s After presenting the theoretical background of thermally activated dislocation slip an evaluation method is discussed, which allows to describe the influence of temperature and strain-rate on the flow stress. For this purpose it is neccessary to measure the flow stresses in tensile tests as well as the thermal flow stress jumps in strain-rate jump tests at different temperatures and strain-rates. The quality of this method is shown using experimental results obtained at specimens of normalized Ck 45 (SAE 1045). The constitutive law allows a reliable extrapolation of flow stress values to strain-rates up to 10⁺⁴ s⁻¹. The influence of plastic strain on the constitutive constants of the modelling law is discussed.     

Study on mechanical properties and constitutive model of 5052 aluminium alloy

The stress–strain relationship of 5052 aluminium alloy was investigated via quasi-static tensile tests and split Hopkinson pressure bar tests. The specimens were exposed to various temperatures (25–500°C) and strain rates (10^?4–0.7?×?10^4?s^?1). At strain rates ranging from 0.001 to 3000?s^?1, the material underwent significant work hardening. When the strain rate exceeded 5000?s^?1, the work hardening effect decreased and the flow stress was relatively constant. The Johnson–Cook constitutive model was modified to describe the deformation behaviour of the material subjected to high temperatures and strain rates. The accuracy of the modified model was verified through ballistic impact testing.     

Effects of temperature and strain rate on the mechanical properties of lead-free solders

Recently studies of mechanical properties of lead-free solders show that large reductions in stiffness, yield stress, ultimate strength, and strain to failure (up to 35%) were found after 2 months of aging at room temperature. It also shows that the tensile properties of both lead-free solders and Sn–Pb solders tend to become relatively stable after 10 days of aging at room temperature. In this study, in order to minimize any room temperature aging contribution in the investigation of the dependence of temperature and strain rates, all specimens tested were preconditioned after 10 days of aging at room temperature. All tests were conducted under the same conditions. Testing has been performed at three strain rates, 10⁻³, 10⁻⁴, and 10⁻⁵ s⁻¹, in temperature range from −40 to 150 °C. A linear relationship was found between the temperature and the tensile properties (elastic modulus, yield stress, and ultimate stress), while a power law relationship was found between strain rate and tensile properties. Constitutive models have also been developed based on the experimental data with multiple variables of strain rate and temperature for both lead-free and lead content solders. With the obtained constitutive models, tensile properties of lead-free solders can be predicted at any testing strain rate and temperature.     

Dynamic strain-rate effect on uniaxial tension deformation of Ti5Al2.5Sn α-titanium alloy at various temperatures

The uniaxial tensile experiments for Ti5Al2.5Sn alloy were performed at strain rates ranging from 10^?3–10⁺³ s^?1 and test temperatures of 153–873 K. Experimentally measured stress-strain responses indicate the yield strength exhibits positive strain-rate dependency, while the yield strength increases as the test temperature is decreased. To understand the thermomechanical coupling of dynamic plastic deformation, a specially developed single-tensile-pulse loading technique was used, and the isothermal stress-strain curves for the rates of 180 and 450 s^?1 were obtained at temperatures of 203, 298 and 573 K. The plastic strain hardening measurements obtained here are essentially athermal and largely independent of strain rate, consistent with titanium and its alloys being bcc-structure-like in mechanical behaviour. Based on the experimentally obtained plastic deformation features of the alloy, the physically based Voyiadjis-Abed constitutive relationship was modified to model the dynamic tensile deformation of the Ti5Al2.5Sn alloy at low and high temperatures.     

The static and high strain rate behaviour of a commingled E-glass/polypropylene woven fabric composite

In this paper the effect of strain rate on the tensile, shear and compression behaviour of a commingled E-glass/polypropylene woven fabric composite over a strain rate range of 10⁻³–10² s⁻¹ is reported. The quasi-static tests were conducted on an electro-mechanical universal test machine and a modified instrumented falling weight drop tower was used for high strain rate characterisation. The tensile and compression modulus and strength increased with increasing strain rate. However, the shear modulus and strength were seen to decrease with increasing strain rate. Strain rate constants for use in finite element analyses are derived from the data. The observed failure mechanisms deduced from a microscopic study of the fractured specimens are presented.     

Effects of Steam Environment on Creep Behavior of Nextel™610/Monazite/Alumina Composite at 1,100°C

The tensile creep behavior of a N610™/LaPO₄/Al₂O₃ composite was investigated at 1,100°C in laboratory air and in steam. The composite consists of a porous alumina matrix reinforced with Nextel 610 fibers woven in an eight-harness satin weave fabric and coated with monazite. The tensile stress-strain behavior was investigated and the tensile properties measured at 1,100°C. The addition of monazite coating resulted in ~33% improvement in ultimate tensile strength (UTS) at 1,100°C. Tensile creep behavior was examined for creep stresses in the 32–72 MPa range. Primary and secondary creep regimes were observed in all tests. Minimum creep rate was reached in all tests. In air, creep strains remained below 0.8% and creep strain rates approached 2 × 10⁻⁸ s⁻¹. Creep run-out defined as 100 h at creep stress was achieved in all tests conducted in air. The presence of steam accelerated creep rates and significantly reduced creep lifetimes. In steam, creep strain reached 2.25%, and creep strain rate approached 2.6 × 10⁻⁶ s⁻¹. In steam, creep run-out was not achieved. The retained strength and modulus of all specimens that achieved run-out were characterized. Comparison with results obtained for N610™/Al₂O₃ (control) specimens revealed that the use of the monazite coating resulted in considerable improvement in creep resistance at 1,100°C both in air and in steam. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Effects of gage length, loading rates, and damage on the strength of PPTA fibers

Axial tension and transverse compression experiments on single fibers were performed to investigate the mechanical behavior of three high-performance fibers (Kevlar^®, Kevlar^® 129, and Twaron^®) with diameters in the order of 9-12 μm. The single fibers were manufactured from 1998 through 2008. A miniaturized tensile Kolsky bar was used to determine the tensile response of PPTA single fibers at a high strain rate. Gage length and strain rate were found to have minimum effects on the tensile strength of PPTA single fibers. Manufacturing time over a decade was found to have negligible effects on the tensile strength of the fibers. Initial transverse compression on the fibers reduces their ultimate tensile strengths. A high resolution scanning electron microscope (SEM) was also used to examine the fracture modes of transversely deformed fibers. Different types of fracture morphology were observed.     

Time and temperature dependent mechanical characterization of polymer‐based materials in electronic packaging applications

Abstract

The thermo‐mechanical testing of high performance polyimide films Type HPPST supplied by Dupont^® was conducted at different strain rates and in different temperature environments. The stress‐strain behavior of materials was investigated, and the dependence of Young's modulus on temperature and strain rate is reported. In view of the uncertainty of the Young's modulus determination, the specimens were tested with unloading‐reloading to verify the test results. Constant strain rate uniaxial tensile tests and long‐time creep tests at various temperatures were performed to characterize the time‐temperature‐dependent mechanical property precisely. Cyclic loading tests were also implemented on specimens to investigate cyclic stress‐strain behaviors. This research is expected to enhance finite‐element‐modeling accuracy and characterize material properties precisely.     

A numerical study of ductile void growth under dynamic loading conditions

A numerical study of void growth at differing global strain rates in the range 149 s^–1–2240 s^–1 and at start temperatures between 173 K and 573 K has been carried out for a material containing a three-dimensional periodic array of equally spaced, initially spherical voids. To take account of the effect of strain rate and temperature on the flow stress under dynamic adiabatic conditions, the well-established Zerilli-Armstrong constitutive relations for pure copper and iron have been employed. An instability criterion based on the maximum mean tensile stress has been used to identify the point at which unstable void growth occurs. For both materials, the strain at instability has been found to be dependent on stress triaxiality and start temperature but only weakly affected by strain-rate     

Derived Dynamic Ply Properties from Test Data on Angle Ply Laminates

Dynamic unidirectional tensile ply properties were extracted from the results of burst tests on angle ply filament wound GRP and KRP tubes under internal hoop loading. The extracted longitudinal and transverse tensile strengths as well as transverse tensile moduli exhibited strain-rate sensitivities. Shear properties were derived from test results on 55° and 65° tube angles. Derived shear stress-strain curves and shear strength values are presented at different strain rates; again clearly demonstrating rate effects on these properties. Complete sets of strain rate dependent lamina tensile properties are presented for GRP and KRP covering the strain rate regime of 1 to 10² sec^-1.     

Joint investigation of concrete at high rates of loading

Uniaxial tensile tests have been carried out on a micro-concrete with strain rates between 0.5 and 1.25 s⁻¹. The investigation is described and the results are discussed with respect to the influence of water content on the strain-rate sensitivity.     

Hot tensile deformation behaviors and constitutive model of 42CrMo steel

The hot tensile deformation behaviors of 42CrMo steel are studied by uniaxial tensile tests with the temperature range of 850–1100 °C and strain rate range of 0.1–0.0001 s⁻¹. The effects of hot forming process parameters (strain rate and deformation temperature) on the elongation to fracture, strain rate sensitivity and fracture characteristics are analyzed. The constitutive equation is established to predict the peak stress under elevated temperatures. It is found that the flow stress firstly increases to a peak value and then decreases, showing a dynamic flow softening. This is mainly attributed to the dynamic recrystallization and material damage during the hot tensile deformation. The deformation temperature corresponding to the maximum elongation to fracture increases with the increase of strain rate within the studied strain rate range. Under the strain rate range of 0.1–0.001 s⁻¹, the localized necking causes the final fracture of specimens. While when the strain rate is 0.0001 s⁻¹, the gage segment of specimens maintains the uniform macroscopic deformation. The damage degree induced by cavities becomes more and more serious with the increase of the deformation temperature. Additionally, the peak stresses predicted by the proposed model well agree with the measured results.     

Effects of strain rate and temperature on dynamic mechanical behaviour and microstructural evolution in aluminium–scandium (Al–Sc) alloy

Abstract

The present study applies a compressive split Hopkinson bar to investigate the mechanical response, microstructural evolution and fracture characteristics of an aluminium–scandium (Al–Sc) alloy at temperatures ranging from ? 100 to 300°C and strain rates of 1·2 × 10³, 3·2×10³ and 5·8 × 10³ s^?1. The relationship between the dynamic mechanical behaviour of the Al–Sc alloy and its microstructural characteristics is explored. The fracture features and microstructural evolution are observed using scanning and transmission electron microscopy techniques. The stress–strain relationships indicate that the flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, but decrease with increasing temperature. Conversely, the activation volume and activation energy increase as the temperature increases or the strain rate decreases. Additionally, the fracture strain reduces with increasing strain rate and decreasing temperature. The Zerilli–Armstrong fcc constitutive model is used to describe the plastic deformation behaviour of the Al–Sc alloy, and the error between the predicted flow stress and the measured stress is found to be less than 5%. The fracture analysis results reveal that cracks initiate and propagate in the shear bands of the Al–Sc alloy specimens and are responsible for their ultimate failure. However, at room temperature, under a low strain rate of 1·2 × 10³ s^?1 and at a high experimental temperature of 300°C under all three tested strain rates, the specimens do not fracture, even under large strain deformations. Scanning electron microscopy observations show that the surfaces of the fractured specimens are characterised by transgranular dimpled features, which are indicative of ductile fracture. The depth and density of these dimples are significantly influenced by the strain rate and temperature. The transmission electron microscopy structural observations show the precipitation of Al₃Sc particles in the matrix and at the grain boundaries. These particles suppress dislocation motion and result in a strengthening effect. The transmission electron microscopy analysis also reveals that the dislocation density increases, but the dislocation cell size decreases, with increasing strain rate for a constant level of strain. However, a higher temperature causes the dislocation density to decrease, thereby increasing the dislocation cell size.     

Determination of the high strain rate fracture properties of ductile materials using a combined experimental/numerical approach

The ‘flying wedge’ is a dynamic tensile test rig for determining the fracture properties of ductile materials over a range of strain rates from 10²–10⁴ s⁻¹. To accurately interpret the results from these tests, finite element simulations of the complete experiment as well as detailed models of the specimen subjected to different loading conditions are conducted using advanced materials constitutive relations. A new technique to carry out interrupted tensile testing at high strain rates on the flying wedge has been developed. Some typical results of the combined experimental and numerical technique are used to compare the predictions of different numerical ductile failure models which are also briefly reviewed in this paper.     

Unusual grain-size and strain-rate effects on the serrated flow in FeMnC twin-induced plasticity steels

The purpose of the paper is to clarify the serrated flow behavior and to explore the grain-size and strain-rate effects on serrations in coarse- and fine-grained FeMnC twin-induced plasticity (TWIP) steels at room temperature. Tensile tests were performed under extensometer-measured strain control, rather than under conventional cross-head displacement control, at strain rates ranging from 6 × 10^?6 to 6 × 10^?3 s^?1. The results indicate that both the coarse- and fine-grained steels show different types of serrations, which depend on strain rate, and demonstrate a nonmonotonic strain-rate sensitivity of the critical strain for the onset of serrations, i.e., a positive strain-rate sensitivity of the critical strain in the high strain-rate region typified by type A serrations, and a negative strain-rate sensitivity in the low strain-rate region typified by type C ones. As compared to the coarse-grained steel, the fine-grained steel increases the critical strains, showing an inverse grain-size effect. The findings suggest that the fine-grained FeMnC TWIP steel suppresses the serrated flow, possibly due to the enhanced dynamic recovery associated with the decreased planar slip length in the fine-grained low stacking-fault-energy steel.