A tensile impact test apparatus for composite materials

A tensile impact test apparatus capable of applying a pure axial tensile loading to even a highly orthotropic composite material, e.g., a unidirectionally reinforced composite, was designed and constructed. Existing impact test methods such as Charpy, Izod and plate impact induce very complex stress states, making the interpretation of results difficult. Details of the apparatus design, and instrumentation problems which had to be overcome, are discussed.was Graduate Student, Composite Materials Research Group, P.O. Box 3295, University of Wyoming, Laramie, WY 82071.     

Experimental study of a metal-matrix composite

A metal-matrix specimen was investigated using moiré interferometry with 2400 ℓ/mm (60,960 ℓ/in.). The specimen was a 6-ply [0/±45] _s boron-aluminum tensile coupon with a central slot. The unequal deformations in fibers and matrix were evident. Special attention was given to a plastic slip zone in the matrix. Normal and shear strains were calculated in the slip zone; the shear-strain concentration factor rose dramatically with the onset of plastic slip and continued to rise gradually with load to 95 percent of the failure load. The precipitous change from maximum tensile strain in one fiber to zero tensile strain in the neighboring fiber was accommodated by the very high shear strain in the matrix in the slip zone. Experimental considerations are discussed: shear strains are independent of rigid-body rotations; simplified strain calculations are justified; averaging across the narrow slip region and the influence of finite thickness of the specimen grating contribute to underestimation of peak strains. Paper was presented at the 1986 SEM Spring Conference on Experimental Mechanics held in New Orleans, LA on June 8–13.     

Multiple scattering of elastic waves in metal-matrix composite materials with high volume concentration of particles

A new approach is proposed to investigate the propagation of compressional (P) and shear (SV) waves in metal-matrix composite materials with high volume concentration of particles. The theory of quasicrystalline approximation and Waterman's T matrix formalism are employed to treat the multiple scattering resulting from the particles in composites. The addition theorem for spherical Bessel functions is used to accomplish the translation between different coordinate systems. The analytical expression of the Percus–Yevick correlation function is also given. Closed form solutions for the effective propagation constants and the dynamic effective elastic modulus of materials are obtained in the low frequency limit. At higher frequencies, only numerical results of them are presented. Numerical examples show that the phase velocities of P and SV waves in the composite materials with low volume concentration in the low frequency are in good agreement with the results in previous literatures. The effects of the incident wave number, the volume fraction of particles and the material properties of the particles and matrix on the dynamic effective elastic modulus are also examined.     

Phenomenological theory of failure criterion for composite materials

In this paper, some current anisotropic failure criteria in the forms of tensor polynomials are investigated. In order to determine the interaction coefficients of the failure criterion, a non-linear optimization method is proposed. The results obtained by different theories as well as the optimization method are compared with the test data of some composite materials. The comparison shows that the optimization method is effective.     

含切口损伤复合材料层合板拉伸失效行为研究

对含有不同切口损伤的复合材料层合板试件进行了拉伸试验,采用电阻应变计同步测量切口损伤前缘区域随载荷的变化,测定含切口损伤层合板的剩余强度,并讨论了损伤长度和损伤角度对剩余强度的影响规律.建立含切口损伤复合材料层合板有限元模型,分析了含切口损伤复合材料层合板的拉伸失效行为,计算了含切口损伤复合材料层合板的剩余强度,确定了剩余强度与切口损伤状态的关系.计算结果与试验结果具有较好的一致性.     

含圆孔复合材料层板在拉伸载荷下损伤破坏过程的模拟

本文应用有限元法模拟受拉含孔夏合材料层板损伤起始、累积直至破坏的过程。在模拟中引入了作者早些时建立的包含基体开裂层非对称约束影响的损伤本构关系，以及裂纹密度与损伤区应力关系函数等。本文给出损伤累积模拟方法并编制了程序，用该程序可以获得层板加载过程中各层的损伤状态、计算其刚度衰减和应力重分布以及最终破坏载荷。给出了数值模拟算例并与现有研究结果进行了比较。     

Measurement of nonlinear damping in a Gr/Al metal-matrix composite

This paper is concerned with the measurement of nonlinear (i.e., strain amplitude dependent) intrinsic material damping in continuous-fiber-reinforced metal-matrix composites (MMC). The particular MMC studied is a four-ply [±θ] _s P55Gr/6061 Al composite with θ=0, 15, 30, 45, 60, 75 deg. A popular method for measuring damping is the free-decay of flexural vibrations of a cantilevered beam. However, the strain field in a cantilevered beam is inhomogeneous. Therefore, for materials whose damping is nonlinear, the measured specimen damping is not equal to the intrinsic material damping. Using an elementary algorithm develeped by Lazan, the authors extract nonlinear intrinsic material damping from the nonlinear specimen damping.     

Nonlinear asymmetric deformation of composite shells formed from materials having different tensile and compressive strengths

Khar'kov Polytechnic Institute, Ukraine. Translated from Prikladnaya Mekhanika, Vol. 29, No. 11, pp. 80–86, November, 1993.     

Deformation rate effects on failure modes of open-cell Al foams and textile cellular materials

The compressive behavior of open-cell aluminum alloy foam and stainless steel woven textile core materials have been investigated at three different deformation rate regimes. Quasi-static compressive tests were performed using a miniature loading frame, intermediate rates were achieved using a stored energy Kolsky bar, and high strain rate tests were performed using a light gas gun.In agreement with previous studies on foam materials, the strain rate was not found to have a significant effect on the plateau stress of metallic foams. For all the tests, real time imaging of the specimen combined with digital image correlation analysis allowed the determination of local deformation fields and failure modes. For the Kolsky bar tests, the deformations in the foam specimen were found to be more distributed than for the quasi-static test, which is attributed to moderate inertia effects. The differences in failure mode are more dramatic for the gas gun experiments, where a full compaction shock wave is generated at the impact surface. The stresses in front and behind the shock wave front were determined by means of direct and reverse gas gun impact tests, i.e., stationary and launched specimen, respectively. A one-dimensional shock wave model based on an idealized foam behavior is employed to gain insight into the stress history measurements. We show that the predictions of the model are consistent with the experimental observations. Woven textile materials exhibited moderate dependence of strength on the deformation rate in comparison with open-cell foam materials.     

Uniaxial tensile testing of rubber-like materials

Experimental Techniques -     

铁电材料的疲劳失效行为

在航空航天、核能发电等重大装备技术领域, 作为高温传感/驱动/能量收集器件的敏感材料——铋层状结构铁电(BLSF)陶瓷在复杂载荷环境下的疲劳失效问题严重限制着器件寿命和可靠性的提高. 本文以BLSF陶瓷的应用需求为背景, 围绕铁电材料的疲劳裂纹扩展与电畴极化翻转及其相互作用机制等关键问题, 综述了铁电材料在热、力、电三种载荷及其耦合作用下疲劳失效行为的研究现状, 并根据当前铁电材料的一些新发展、新应用对其未来研究方向进行了展望, 旨在为高性能、长寿命铁电/压电器件设计提供参考.      

Compression failure modes in composites

The relationship between the tendency for growth of a delamination in a layered material and the presence of out-of-plane displacements due to Euler instability of the delaminated plies, is examined. By considering the work done by an external load, an improved expression for the energy release rate, G associated with a layered isotropic plate containing a through width delamination and subject to in-plane compressive loading, is derived and compared with finite element and experimental results.     

Strength of composite materials

Here we consider the class of composites in which the strength of the contact between the materials is less than the strength of the components. It is found that the strength of such a material is independent of the size of the initial defect within certain limits but is determined by the shape and size of the most hazardous [weakest] inclusion. A theoretical relationship is deduced for the strength in relation to the size of the largest inclusion, which agrees well with experiment [1], This mechanism probably plays a part in the failure of steel and may be one reason for the scale effect in steel.     

Finite element modelling of woven composite failure modes at the mesoscopic scale: deterministic versus stochastic approaches

Textile composites are composed of 3D complex architecture. To assess the durability of such engineering structures, the failure mechanisms must be highlighted. Examinations of the degradation have been carried out thanks to tomography. The present work addresses a numerical damage model dedicated to the simulation of the crack initiation and propagation at the scale of the warp yarns. For the 3D woven composites under study, loadings in tension and combined tension and bending were considered. Based on an erosion procedure of broken elements, the failure mechanisms have been modelled on 3D periodic cells by finite element calculations. The breakage of one element was determined using a failure criterion at the mesoscopic scale based on the yarn stress at failure. The results were found to be in good agreement with the experimental data for the two kinds of macroscopic loadings. The deterministic approach assumed a homogeneously distributed stress at failure all over the integration points in the meshes of woven composites. A stochastic approach was applied to a simple representative elementary periodic cell. The distribution of the Weibull stress at failure was assigned to the integration points using a Monte Carlo simulation. It was shown that this stochastic approach allowed more realistic failure simulations avoiding the idealised symmetry due to the deterministic modelling. In particular, the stochastic simulations performed have shown several variations of the stress as well as strain at failure and the failure modes of the yarn.     

Non-contact tensile viscoelastic characterization of microscale biological materials

Many structures and materials in nature and physiology have important “meso-scale” structures at the micron length-scale whose tensile responses have proven difficult to characterize mechanically. Although techniques such as atomic force microscopy and micro- and nano-identation are mature for compression and indentation testing at the nano-scale, and standard uniaxial and shear rheometry techniques exist for the macroscale, few techniques are applicable for tensile-testing at the micrometre-scale, leaving a gap in our understanding of hierarchical biomaterials. Here, we present a novel magnetic mechanical testing (MMT) system that enables viscoelastic tensile testing at this critical length scale. The MMT system applies non-contact loading, avoiding gripping and surface interaction effects. We demonstrate application of the MMT system to the first analyses of the pure tensile responses of several native and engineered tissue systems at the mesoscale, showing the broad potential of the system for exploring micro- and meso-scale analysis of structured and hierarchical biological systems.     

Ductile-to-brittle transition in tensile failure of particle-reinforced metals

We present an analytical micromechanical model designed to simulate the tensile stress-strain behaviour and failure of damaging composites containing a high volume fraction of reinforcing particles. One internal damage micromechanism is considered, namely particle fracture, which is assumed to obey a Weibull distribution. Final composite tensile failure occurs when one of two possible failure criteria is reached, given by (i) the onset of tensile instability, or (ii) an “avalanche-like” propagation of particle breaks to neighbouring particles. We show that an experimentally observed transition from failure by tensile instability to abrupt failure resulting from an increase of matrix strength can be mimicked by the model because local load-sharing (i.e. load transfer from a broken particle to its immediate neighbours) is accounted for.     

Dynamic tensile strength of composite laminate joints fastened mechanically

The tensile strength of composite joints is determined under dynamic loading conditions. The composites are laminates made from hybrid fiber reinforced plastic (HFRP) and carbon fiber reinforced plastic (CFRP). Three different mechanically fastened joint configurations are considered: they are the pin-connected, single-lap and double-lap type. The joint strength under dynamic load is found to be lower than that under quasi-static load. The pin-connected joint was the weakest. Investigated also are the influence of geometric parameters for pin-connected HFRP laminate joints. The results shed light on how to improve the bearing strength of mechanical joints when encountering dynamic loads.