Strength and failure behavior of stitched carbon/epoxy composites

This article presents a study of the effect of through-the-thickness stitching yarns upon the strength and failure behavior of multidirectionally reinforced composites. The in-plane yarns were placed in four directions (0, ±45, 90) to form a quasi-isotropic preform, which had open spaces between adjacent yarns. These interyarn spaces allowed easy insertion of the through-the-thickness stitching yarns without significant damage of the in-plane fibers. Fiber volume fractions of over 54 pct were obtained by this method. The through-the-thickness yarn sizes used in this study were 2, 4, and 6 kilo-filament (kf). Non-stitched preforms were also manufactured with the same fiber content and by the same procedure as the stitched preforms for the control experiments. All preforms were infiltrated with epoxy resin by the resin transfer molding (RTM) technique. In-plane tensile and compressive strength, interlaminar shear strength, and mode I fracture toughness of the carbon/epoxy composites were measured at three through-the-thickness yarn contents. Although the through-the-thickness yarns significantly enhanced the mode I fracture toughness, they tended to degrade the in-plane tensile and compressive strength. The failure process under interlaminar shear loading by double notch shear tests showed two distinct stages: the fiber-matrix interfacial failure followed by the breakage/debonding of the through-the-thickness yarns. The through-the-thickness yarns caused a reduction of the initial failure load in the first stage but could enhance the final failure load in the second stage. In composites with 6 kf through-the-thickness yarns, the final failure load could exceed the initial failure load. Scanning electron microscope (SEM) and optical microscopic examinations were also conducted for observing the failure mechanisms and fracture surfaces. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite-Materials Committee.     

Influence of Microcracking on Shear Localization

This work examines the influence of microcracking on a material’s tendency to shear localize under compressive loading. A two-dimensional (2D) finite-element framework with explicit crack representation using cohesive-element methodologies is employed. The influence of microcracking is examined by taking the fracture toughness of the cohesive elements as a free parameter. The simulations suggest that an optimum fracture toughness exists for promoting shear localization. This value corresponds to the limiting mode ?I fracture toughness, below which microscopic material defects lead to brittle compressive failure, as opposed to shear localization. While in the presence of confinement, this value is shown to be close to zero; in the absence of confinement, it is computed to be 28% of the shear band toughness for the specific case of ultrafine-grained tungsten. More generally, it is found that the ratio of mode?I fracture toughness to shear band toughness provides a crude indicator for predicting whether material defects are likely to lead to brittle failure or enhanced shear localization.     

Failure mechanisms of laminated carbon-carbon composites—II. Under shear loads

Failure mechanisms under both interlaminar and in-plane shear loading are determined for two-dimensional carbon-carbon composites by using a direct shear set-up. This set-up is applicable for both types of shear loading, “as manufactured” laminate thickness can be tested without the need to make long samples by gluing different pieces together. A detailed finite element analysis, which considers the microstructure of the composite shows that for woven laminates, the initial crimp angle morphology does not allow the composite to deform in a state of simple shear. In fact, normal tensile and compressive stresses of almost twice the magnitude of the peak shear stress are produced in the vicinity of the crimped bundles. Consistent with these predictions, we observed the shear fault following the crimp boundaries in 0°/90° and quasi-isotropic laminates. Therefore, experimental techniques which can secure a state of pure shear stress in aligned, unkinked, uniaxial fiber composites cannot do so in woven laminated composites.     

Fracture Behavior of Acrylonitrile Butadiene Styrene (ABS) Hybrid Composites Reinforced with Nano Zirconia and Poly Tetra Fluoro Ethylene (PTFE)

ABS composites were processed using melt blending technique by twin screw extrusion and further employing compression molding process. Microstructure and characterization analysis were carried out on the ABS composites as well as pure ABS through XRD, TEM and SEM. Mode I fracture toughness behavior were studied by conducting compact tension test. Short beam shear strength of the composites were determined by carrying out short beam strength test. Fractographic analysis was done using SEM in order to study the various toughening mechanisms involved. XRD studies revealed that nano zirconia and PTFE has formed an uniform structure with ABS polymer, which has also been confirmed with microstructural analysis. Addition of nano zirconia up to 1.5% increases the toughness, which can be attributed to crack bowing and crack deflection. With further addition of nano zirconia, fracture toughness get reduced as the composite become brittle in nature which improves strength but reduces the toughness. It has been observed that addition of PTFE enhances fracture toughness which is ascribed to the crack pinning, cavitation, crack bowing and crack deflection. The increase in shear strength can be due to toughening mechanisms which are evident from the presence of shear cusps, crack pinning and extensive plastic deformation.     

金属薄板面内压剪变形的损伤断裂行为

相变诱导塑性钢（TRansformation induced plasticity, TRIP）作为常用的先进高强钢在汽车等交通工具的轻量化方面有广泛的应用前景。而对于其复杂零件的成形过程,韧性断裂是不可忽视的问题之一。本文针对现有实验装置不易诱发薄板承受面内压剪时断裂失效,从而无法研究板料负应力三轴度区间断裂行为的问题,以高强钢TRIP800薄板为研究对象,设计了可在单向试验机完成压剪实验的试样和夹具。通过调整夹具旋转角度和试样装夹位置可以实现同一种试样在广泛的负应力三轴度范围内进行压剪断裂分析。基于ABAQUS/Explicit平台建立了三个典型加载方向20°、30°和45°对应的压剪过程有限元模型,分析表明:三种情况的试样局部变形区域的应力三轴度都小于0且断裂点的应力三轴度低至?0.485,验证了设计的装置可实现负应力三轴度区间的断裂失效分析,同时基于MMC断裂准则分析了不同应力状态的初始损伤情况及损伤扩展路径。      

Shear Failure of Pultruded Fiber-Reinforced Polymer Composites under Axial Compression

When structural elements are subjected to compressive loads, the shear forces and stresses induced by second-order effects may lead to shear failure prior to compressive failure. This is particularly likely to occur in the case of pultruded glass fiber-reinforced polymer profiles, which normally exhibit low shear strength in relation to compressive strength. This paper analyzes the effects of initial imperfection, slenderness, shear-to-compressive strength ratio, shear coefficient, and type of shear failure criterion on ultimate load and failure mode (shear or compressive failure). A formulation for predicting ultimate load based on shear failure and second-order deformation is proposed. The results obtained compare well with similar results obtained using other methods and experimental data available in literature. The proposed method is based strictly on mechanics and thus requires no fitting to experimental data.     

Surface Characteristics of Rare Earth Treated Carbon Fibers and Interfacial Properties of Composites

Effect of rare earth treatment on surface physicochemical properties of carbon fibers and interfacial properties of carbon fiber/epoxy composites was investigated, and the interfacial adhesion mechanism of treated carbon fiber/epoxy composite was analyzed. It was found that rare earth treatment led to an increase of fiber surface roughness, improvement of oxygeaa-containing groups, and introduction of rare earth element on the carbon fiber surface. As a result, coordination linkages between fibers and rare earth, and between rare earth and resin matrix were formed separately, thereby the interlaminar shear strength （ILSS） of composites increased, which indicated the improvement of the interfacial adhesion between fibers and matrix resin resulting from the increase of carboxyl and carbonyl.     

Salient shear bands and second-phase addition interactions of bulk metallic glass matrix composite

This article discusses Charpy impact testing and fracture morphology of the Zr_41.25Ti_13.75Cu_12.5Ni₁₀Be_22.5 bulk metallic glass matrix composite with long tungsten fibers. Energy to failure was measured via the impact test as well as by integrating the compressive stress-strain curves, and compared for various fiber fractions. Failure energy increased with fiber volume fraction by both measures. Observation of fracture surfaces was made by using scanning electron microscopy. The results show that the fracture surface of the unreinforced bulk metallic glass (BMG) exhibits three different regions, i.e., the impact zone, the transition zone, and the ridged zone, which have different morphology. The composites present uneven or jagged morphology on macroscopic scale, while the microstructure exhibits salient shear bands and second-phase addition interactions. Bridge formation between tungsten fibers is interpreted as evidence that the shear band propagation in the matrix is suppressed by the fibers. Furthermore, shear lips were observed for the composites containing over 50 pct, fiber volume fraction, showing a great improvement in toughness.     

In-situ microfracture observation of strip-cast Zr-Ti-Cu-Ni-Be bulk metallic glass alloys

In this study, Zr-Ti-Cu-Ni-Be bulk metallic glass (BMG) alloys containing a small amount of crystalline phase particles were fabricated by strip casting, and their improvement of mechanical properties and fracture toughness was explained by direct observation of the microfracture process. The compressive and fracture toughness test results indicated that strength, strain to failure, and fracture toughness of the strip-cast BMG alloy containing coarse crystalline particles were higher than those of the as-cast monolithic BMG alloy or the strip-cast BMG alloy containing fine crystalline particles. From in-situ microfracture observations, the improvement of overall mechanical properties of the strip-cast BMG alloy containing coarse crystalline particles could be interpreted by taking consideration of both the existence of coarse crystalline particles and the role of the particles to block crack propagation and to form multiple shear bands. Such property improvement suggests new applicability of the strip-cast BMG alloys containing coarse crystalline particles, which can work as toughening and strengthening reinforcements, to structures and components requiring excellent mechanical properties.     

Microbuckle propagation in carbon fibre-epoxy composites

Observations of microbuckle propagation in uni-directional carbon fibre-epoxy material are described. The fibres buckle either in the plane of the specimen or out-of-plane, depending on the constraints on the free surface. Large scale bridging models of in-plane and out-of-plane microbuckles are reported. The in-plane and out-of-plane microbuckles are modelled as mode II and mode I cracks, respectively. Sliding behind the microbuckle tip is resisted by a constant shear stress of 90 MPa for the in-plane microbuckle, and by a constant normal stress of 220 MPa for the out-of-plane microbuckle. For both the in-plane and out-of-plane microbuckles a microbuckle tip toughness in the range 10–17 KJ/m² is inferred from the experiments. The observed relative displacements across an out-of-plane microbuckle agree with theoretical values using the mode I bridging model. Micrographs of the propagating microbuckle tip show that the details of the failure mechanism are similar for both in-plane and out-of-plane microbuckling. Both develop kink bands with a width of between 25 and 70 μm and with a propagation angle β of between 25° and 30°. A process zone extends about 250 μm ahead of the kink band tip, wherein the fibres buckle and break. Fibres in this region become almost straight again on unloading. When the deduced large scale bridging model of microbuckling failure for unidirectional material is applied to failure at a sharpened slit in multi-directional laminates, reasonable agreement is found between the theoretical and the observed compressive fracture toughnesses.     

The strength,fracture toughness,and low cycle fatigue behavior of 17-4 PH stainless steel

The influence of microstructure on the strength, fracture toughness and low cycle fatigue behavior of 17-4 PH stainless steel has been examined. Aging hardening involves initial formation of coherent copper-rich clusters which transform to incoherent fee ∈-copper precipitates upon further aging. The changes in strength level and strain hardening rates observed during aging are consistent with previously suggested models for precipitation hardening based on differing elastic moduli. The fracture toughness and fatigue crack growth rates were shown to be a function of microstructure and environment. At equivalent strength levels overaging resulted in a higher fracture toughness than did underaging. The fatigue crack growth rates increased with increasing strength level and humidity but were not a function of toughness level. Attempts to correlate the fatigue crack growth rates with monotonie tensile properties were unsuccessful. However when final failure obeyed a critical strain criteria, the fracture toughness behavior could be reasonably described and related to preferential void nucleation and growth at δ-ferrite-matrix interfaces.     

Structural Behavior of Single Key Joints in Precast Concrete Segmental Bridges

A group of five full-depth male–female shear key specimens were match cast and tested to examine the shear capacity of epoxy-jointed single keys. Another group of four specimens were match cast using full-scale dimensions of a segmental construction bridge deck system for testing the fatigue and water tightness at a segment joint. Both cold-weather and hot-weather epoxy types were used to join the specimens. In addition to the experimental testing, finite-element analysis was also used to model the static response of the joint specimens. The observed failure mode of all shear-key specimens was fracture of concrete along the joint with shearing of the key. Good agreement was observed between the experimental test results and the finite-element analysis in terms of the failure mode of unreinforced specimen and the load of crack initiation of the specimens. Fatigue loading had a minor effect on the behavior of the posttensioning bars. The contribution of either the cold-weather or hot-weather epoxies to the joint shear strength was significant knowing that for similar concrete properties, the hot-weather epoxy specimens showed an increase of about 28% in the shear capacity, in comparison to the cold-weather epoxy specimens. The excellent performance of the epoxy-jointed shear keys was verified by field application on a prototype model simulating a portion of the Wacker Drive Bridge system. It was concluded that implementing AASHTO procedures result in conservative estimates of the shear strength of the single keyed joint since it neglects the contribution of the epoxy and underestimates the strength of the key itself.     

Interlaminar Fracture of Composites under Tension and Compression

Interlaminar fracture of AS4∕3502 graphite∕epoxy material system is investigated using a double cracked‐lap‐shear (DCLS) specimen and a single cracked‐lap‐shear (SCLS) specimen. A fundamental feature of the designed specimens is their ability to be tested under net tensile and compressive loadings. The specimens exhibit mixed‐mode or mode II behavior depending on the loading direction. The specimens are designed to precipitate crack growth at a designed‐in site in a gage section. In the specimen design process, overall dimensions of the specimens are selected so that local disturbances in the stress field will not interact, there is adequate length to permit crack growth, and overall buckling will not occur under compressive loading. The experimental results confirm that the specimens and tests perform as designed, It is observed that: (1) There is an increasing resistance to crack growth under tensile loading; (2) interlaminar fracture under compression is a totally unstable process; and (3) tension and compression behaviors are considerably different. Fracture surfaces in the unstable regions from short beam shear and DCLS specimen tests exhibit similar characteristics.     

WC颗粒尺寸对超音速火焰喷涂WC–10Co4Cr涂层组织及力学性能的影响

采用超音速火焰喷涂技术(high velocity oxygen-fuel, HVOF)制备了纳米结构、亚微米结构及常规结构的WC-10Co4Cr涂层, 研究了沉积过程中颗粒尺寸对WC脱碳行为的作用, 分析了WC颗粒尺寸对复合涂层微观组织、硬度、断裂韧性及界面结合强度的影响。结果表明: 随着WC颗粒尺寸的增大, WC脱碳率和涂层孔隙率先增大后减小, 而涂层硬度和断裂韧性先减小后增大, 界面结合强逐渐降低。在100 g压痕载荷下, 亚微米和常规结构涂层硬度的Weibull分布呈双峰特征, 而在300 g压痕载荷下, 3种结构涂层硬度的Weibull分布均呈单峰特征, 这是3种结构涂层的WC脱碳程度、层间结合力和孔隙率综合作用结果。WC-10Co4Cr纳米结构涂层呈现出低脱碳率、高硬度、高界面结合强度和适中断裂韧性的优异综合性能。     

Fracture Mechanics of Rubber Epoxy Composites

The effect of the addition of rubber micro-particles to epoxy matrix on the mechanical properties and the fracture toughness were investigated. Rubber epoxy composites were prepared with different weight percentages namely, 0, 5, 10, 15, 20, 25, 30 wt pct of rubber. Both quasi-static and dynamic ultrasonic measurements of the elastic modulus were found to decrease by 60 pct, and the critical value of the stress intensity factors was found to increase by approximately 45 pct for the rubber epoxy composites. This was also confirmed with the finite element analysis that had the same increasing trend. The fracture surface morphology reveals rough cleavage fracture in the epoxy matrix with brittle intergranular decohesion caused by the impurity segregation that exhibits relatively high micro-roughness of the fracture surfaces.     

Z-Pin Composites: Aerospace Structural Design Considerations

Z-pins are increasingly used to enhance the delamination toughness and impact damage tolerance of composite aircraft structures. An important consideration in the design of z-pinned structures is the deterioration of the in-plane mechanical properties of the composite material because of the pins. Experimental property data presented in this paper reveal that large improvements to the delamination toughness of carbon-epoxy composite gained with z-pins also result in an unavoidable reduction to the in-plane tension, compression, bending, interlaminar shear, and fatigue properties. The data show that increasing the volume fraction of z-pins in carbon-epoxy to increase the delamination resistance causes a corresponding deterioration to the in-plane properties, and this is a key consideration in the design of z-pinned aircraft structures for damage tolerance. The data reveal that the reduction to the in-plane mechanical properties caused by z-pins is usually modest (typically less than 5–15%) compared to the very large improvements in delamination toughness (up to nearly 500%).     

Uniaxial compressive failure of unidirectional composites with small imperfections

In this investigation, it was observed that the presence of a small defect such as a tiny hole can significantly reduce the compressive strength of composites by up to 60 pct. The extent of this reduction in compressive strength depends on the size of the hole, with smaller holes resulting in higher compressive strength. The failure mode is either (1) microbuckling or (2) microbuckling with attendant splitting, both originating from the cylindrical hole. The attendant splitting has no significant effect on the compressive failure strength, suggesting that microbuckling is the dominant failure mode in the compressive failure process. The implication of these findings is that even “minor defects” such as matrix voids should be taken seriously, and measures should be taken to reduce their extent during composite manufacture via better processing methods and stringent quality control.     

Effect of Using Carbon Nanotubes on ILSS of Glass Fiber-Reinforced Polymer Laminates

Carbon nanotubes have been considered as good filler materials for enhancing the characteristics of advanced nano-composites due to their excellent properties. In z-axis property, especially interlaminar shear strength (ILSS) is of prime concern for FRP laminates undergoing lateral loads. The present work highlights the effect of MWCNTs on the ILSS of GFRP composites manufactured by vacuum-assisted hand layup technique. Result shows that the addition of 0.5 wt% MWCNTs into the unfilled GFRP composite decreases ILSS by 8.35%. The fracture surface of glass fiber-reinforced polymer composites is analyzed by field-emission scanning electron microscope, and micrographs are used to delineate the ILSS outcomes.     

Dynamic deformation and fracture behaviors of two Zr-based amorphous alloys

Dynamic deformation and fracture behaviors of Zr-based amorphous alloys were investigated in this study. Quasi-static and dynamic compressive tests were conducted using a universal testing machine and a compressive Kolsky bar, respectively, and then the test data were analyzed in relation to microstructure and fracture mode. Quasi-static compressive test results indicated that the compressive strength of the amorphous alloy containing dendritic β phases was similar to that of the amorphous alloy, while the ductility was better. Under dynamic loading, the maximum shear stress and ductility of the amorphous alloys were considerably lower than those under quasi-static loading because of the decreased resistance to fracture. The deformation and fracture behaviors occurring under quasi-static and dynamic loading conditions were explained by fracture mechanisms observed on fractured surfaces.     

Ductile-phase toughening in V-V3Si in situ composites

This article describes the room-temperature fracture behavior of ductile-phase-toughened V-V₃Si in situ composites that were produced by arc melting (AM), cold-crucible induction melting (IM), and cold-crucible directional solidification (DS). Composites were produced containing a wide range of microstructures, interstitial impurity contents, and volume fractions of the ductile V-Si solid solution phase, denoted (V). The fracture toughness of these composites generally increases as the volume fraction of (V) increases, but is strongly influenced by the microstructure, the mechanical properties of the component phases, and the crystallographic orientation of the (V) phase with respect to the maximum principal stress direction. For eutectic composites that have a (V) volume fraction of about 50 pct, the fracture toughness increases with decreasing “effective” interstitial impurity concentration, [I]=[N]+1.33 [O]+9 [H]. As [I] decreases from 1400 ppm (AM) to 400 ppm (IM), the fracture toughness of the eutectic composites increases from 10 to 20 MPa √m. Further, the fracture toughness of the DS eutectic composites is greater when the crack propagation direction is perpendicular, rather than parallel, to the composite growth direction. These results are discussed in light of conventional ductile-phase bridging theories, which alone cannot fully explain the fracture toughness of V-Si in situ composites.