Temperature, moisture and mode-mixity effects on copper leadframe/EMC interfacial fracture toughness

A systematic investigation and characterization of the interfacial fracture toughness of the bi-material copper leadframe/epoxy molding compound is presented. Experiments and finite element simulations were used to investigate delamination and interfacial fracture toughness of the bi-material. Two dimensional simulations using virtual crack closure technique, virtual crack extension and J-integral proved to be computationally cheap and accurate to investigate and characterize the interfacial fracture toughness of bi-material structures. The effects of temperature, moisture diffusion and mode-mixity on the interfacial fracture toughness of the bi-material were considered. Testing temperature and moisture exposure significantly reduce the interfacial fracture toughness, and should be avoided if possible.     

Effect of interfacial mobility on flexural strength and fracture toughness of glass/epoxy laminates

Mechanical testing and surface fractography were used to characterize the fracture of E-glass fiber reinforced epoxy composites as a function of the silane coupling agent used. -Aminopropyltriethoxysilane (APS) and -aminobutyltriethoxysilane (ABS) were used because these have been shown to have different interfacial mobilities at multilayer coverage. The values of the properties studied generally increased from untreated c, as determined from a Mode I translaminar fracture toughness tests, for the untreated composites (10.5 ± 0.4 kJ/m²) was lower than that for the ABS-treated composites (14.3 ± 2.1 kJ/m²) which was lower than that for the APS-treated composites (17.1 ± 2.4 kJ/m²). Macroscopic observations showed that a larger fiber debonding area was formed in the crack tip region for the untreated glass composites, suggesting poorer bonding compared to those treated with coupling agents. Since these silanes have similar chemistry, the differences were attributed to differences in the interfacial mobility of the coupling agent layers.     

A framework to correlate a/W ratio effects on elastic-plastic fracture toughness (J c )

Single edge-notched bend (SENB) specimens containing shallow cracks (a/W < 0.2) are commonly employed for fracture testing of ferritic materials in the lower-transition region where extensive plasticity (but no significant ductile crack growth) precedes unstable fracture. Critical J-values J _c ) for shallow crack specimens are significantly larger (factor of 2–3) than the J _c )-values for corresponding deep crack specimens at identical temperatures. The increase of fracture toughness arises from the loss of constraint that occurs when the gross plastic zones of bending impinge on the otherwise autonomous crack-tip plastic zones. Consequently, SENB specimens with small and large a/W ratios loaded to the same J-value have markedly different crack-tip stresses under large-scale plasticity. Detailed, plane-strain finite-element analyses and a local stress-based criterion for cleavage fracture are combined to establish specimen size requirements (deformation limits) for testing in the transition region which assure a single parameter characterization of the crack-tip stress field. Moreover, these analyses provide a framework to correlate J _c )-values with a/W ratio once the deformation limits are exceeded. The correlation procedure is shown to remove the geometry dependence of fracture toughness values for an A36 steel in the transition region across a/W ratios and to reduce the scatter of toughness values for nominally identical specimens.     

Miscibility, morphology and fracture toughness of tetrafunctional epoxy resin/poly (styrene-co-acrylonitrile) blends

Poly(styrene-co-acrylonitrile) (SAN) was found to be miscible with the tetraglycidylether of 4,4'-diaminodiphenylmethane (TGDDM), as shown by the existence of a single glass transition temperature (T _g) over the whole composition range. However, SAN was found to be immiscible with the 4,4-diaminodiphenylmethane (DDM)-cured TGDDM. Dynamic mechanical analysis (DMA) shows that the DDM-cured TGDDM/SAN blends have two T _gs. A scanning electron microscopy (SEM) study revealed that all the DDM-cured TGDDM/SAN blends have a two-phase structure. The fracture toughness K _IC of the blends increased with SAN content and showed a maximum at 10 wt% SAN content, followed by a dramatic decrease for the cured blends containing 15 wt% SAN or more. The SEM investigation of the K _IC fracture surfaces indicated that the toughening effect of the SAN-modified epoxy resin was greatly dependent on the morphological structures.     

Mechanisms for rate effects on interlaminar fracture toughness of carbon/epoxy and carbon/PEEK composites

The objective of this study was to investigate strain-rate dependent energy absorption mechanisms during interlaminar fracture of thermosetting (epoxy) and thermoplastic (PEEK) uni directional carbon fibre (CF) composites. A simple model addressing the translation of matrix toughness to mode I and mode II interlaminar toughness of the composite is presented, in conjunction with a fractographic examination of the fracture surfaces and the fracture process. The observed rate dependency of composite fracture toughness is attributed to the rate dependent toughness of the viscoelastic matrix and the size of the process zone around the crack tip. Other important factors identified are the roughness of the fracture surface and fibre bridging.     

The effect of hygrothermal conditions on the fracture toughness of epoxy/poly(styrene-co-allylalcohol) blends

The influence of water content on the fracture toughness of epoxy/poly(styrene-co-allylalcohol) (PScoPA) blends with different amounts of modifier has been investigated. The water ingress has a double consequence: a considerable plasticization effect of the matrix and a significant weakening of the interphase between the matrix and the thermoplastic particles. The addition of thermoplastic modifier reduces, in the aged samples, the deterioration in fracture toughness. Analytical models, based on plastic voids growth around the dispersed phase particles, have been applied with reasonable agreement.     

Fracture toughness and fracture mechanisms of PBT/PC/IM blends: Part V Effect of PBT-PC interfacial strength on the fracture and tensile properties of the PBT/PC blends

To investigate the effect of PBT-PC interfacial strength on the fracture toughness and toughening mechanisms of the PBT/PC system, a series of PBT/PC blends with different content of in situ formed PBT-PC copolymers were made by melt blending. The in situ copolymer was separately prepared via reactive blending of the PBT and PC in the presence of a transesterification catalyst in a twin-screw extruder for a few minutes. The reactive extrudate (RE) was studied using a DSC and the existence of the PBT-PC copolymer in the RE was confirmed. Microstructure characterizations of the PBT/PC/RE blends revealed that the domain sizes of the PBT and PC decrease and the PBT-PC interfacial strength increases with the RE content. Compared with the PBT/PC blend, all the PBT/PC/RE blends have higher yield strength, elongation at break as well as tensile modulus. The quasi-static fracture tests show that fracture toughness of the blends increases with the RE content. Since the highest toughness was obtained with the blend having the highest RE content (7.5%), it is not certain at this stage whether adding more than 7.5% RE will further improve the fracture toughness. The impact toughness of the PBT/PC/RE blends was found to decrease with the increase of the PBT-PC interfacial strength, which confirms the failure mechanisms proposed in the Part-4 of this series.     

Experimental results of fracture energy and fracture toughness of Radiata Pine laminated veneer lumber (LVL) in mode I (opening)

The load-carrying capacity of notched timber beams can be predicted using linear elastic fracture mechanics (LEFM). Material properties such as fracture toughness and energy are needed for the analysis. The micro and macroscopic complexity of wood and its anisotropic nature give different fracture properties in the longitudinal, radial and tangential grain directions. This complexity and the infrequent use of LEFM mean there is little data available. While wood is highly anisotropic, fracture analysis can use a subset of the possible material properties because wood normally cracks parallel to its grain due to its low tensile strength perpendicular to grain. This allows a significant reduction in the number of tests required to measure fracture properties, with considerable saving of resources. This paper presents the results of an experimental study investigating the fracture energy and fracture toughness of Radiata Pine laminated veneer lumber in mode I (opening). A more efficient test apparatus is proposed and shown to produce identical results to the test apparatus used by others. Results are presented for the fracture toughness properties in the grain direction, and include fifth percentiles and coefficients of variation. The influence that the specimen size has on the fracture toughness is also presented. Numerical analyses using the ABAQUS software package show good agreement with the experimental test results. The experimental results are within the range of experimental values reported in the literature for solid wood.     

Strength,fracture toughness and Vickers hardness of CeO2-stabilized tetragonal ZrO2 polycrystals (Ce-TZP)

Dense Ce-TZP ceramics containing about 7 to 16 mol% CeO₂ were fabricated using fine powders prepared by the hydrolysis technique. The mechanical properties of these ceramics were evaluated. The bending strength of sintered bodies with 10 to 12 mol% CeO₂ content and small grain-size was about 800 MPa. Fracture toughness was measured by two different methods; a micro-indentation technique and the chevron notched beam technique. A high fracture toughness was obtained for sintered bodies with 7 to 10% CeO₂ content and large grain-size. Fracture toughness and hardness were dependent on CeO₂ content and grain-size. These mechanical properties are discussed on the basis of the stability of the metastable tetragonal phase depending on CeO₂ content and grain-size.     

Ti(C,N)/Ni扩散焊接界面的组织和性能

以铜和铌作为中间夹层,真空扩散焊接Ti(C,N)/Ni,研究温度和时间等主要工艺参数对Ti(C,N)/Ni界面微观组织和性能的影响.结果表明,当扩散焊接的温度低于1273 K时,界面的夹层材料基本保持不变,界面的微观组织为Cu/Nb层状物,铜在镍中有少量扩散;而当扩散焊接的温度为1523K时,界面微观组织在初期为Ni8Nb的金属问化合物 离散析出的CuNi固溶体,到后期变为靠近Ti(C,N)侧为(Ti,Nb)(C,N) NbT(Ni,Ti,Cu)6 NbNia层,靠近Ni侧为NiCu NbNis层.这表明,液态Cu为过渡液相,通过Ni的溶解而形成CuNi过渡液相,加速了Nb在CuNi过渡液相中的溶解.由此产生的NiNbCu过渡液相能浸润Ti(C,N),并在界面处形成少量的(Ti,Nb)(C,N)固溶体合金,从而提高了界面的结合性能,界面剪切强度可达到140 MPa.     

Effect of yield stress and specimen size on the position of fracture toughness peak (FTP) in Ji-a/W curves

Early work showed that there was a fracture toughness peak (FTP) as the fracture toughness changed with crack length/specimen width (a/W). It could be thought of as a “safe crack” for the cracks whose length is smaller than that where the FTP is located. In the present paper, it is indicated that the crack length of FTP isJ_i-a/Wcurves decreases with increasing yield stress of material and specimen size and decreasing test temperature. The reason for the fracture toughness being insensitive to the (a/W) for the ultra-high strength and brittle material is explained.     

Effects of microstructure on crack tip fields and fracture toughness in PC/ABS polymer blends

Numerical simulations are performed in order to gain a better understanding of the effects of various microstructural features and toughening mechanisms in amorphous PC/ABS polymer blends. Crack tip loading under global small-scale yielding conditions is considered with the blend microstructure explicitly resolved in the near-tip process zone. Constitutive models are employed which account for large visco-plastic deformations, the characteristic softening- rehardening behavior of glassy polymers, as well as the effect of plastic dilatancy in the ABS phase due to rubber particle cavitation. The influence of blend composition and morphology on the local stress distribution and the development of the plastic zone at a stationary crack tip are analyzed. Furthermore, crack propagation and the evolution of fracture toughness are studied using different cohesive surface models for failure in the different phases of the blend microstructure.     

Determination of fracture toughness and energy dissipation of SiO2-poly(methyl metacrylate) hybrid films by nanoindentation

In this paper we have applied different methods based on nanoindentation techniques to measure the toughness of SiO₂-poly methyl-methacrylate hybrid films on organic acrylic substrates. The hybrid films were deposited by the Sol-Gel method from precursor solutions containing tetraethyl-orthosilicate, methylmethacrylate and 3-trimethoxysilyl propyl methacrylate (TMSPM) as the coupling agent. The influence of TMSPM content in the hybrid precursor solution on the fracture behavior of the hybrid films was studied. The classical indentation crack length method was applied from nanoindentation tests to determine the stress intensity factor by direct measurement of crack length from atomic force microscopy images. A second method, based on the pop-in analysis allowed the separation of crack formation from film delamination from multiple pop-ins. Finally, a third method based on energy methods is also reported and discussed. The amount of TMSPM in the precursor solution showed a strong influence on the toughness of the hybrid films.     

Mode I and mode II fracture toughness of E-glass non-crimp fabric/carbon nanotube (CNT) modified polymer based composites

In this study, mode I and mode II interlaminar fracture toughness, and interlaminar shear strength of E-glass non-crimp fabric/carbon nanotube modified polymer matrix composites were investigated. The matrix resin containing 0.1 wt.% of amino functionalized multi walled carbon nanotubes were prepared, utilizing the 3-roll milling technique. Composite laminates were manufactured via vacuum assisted resin transfer molding process. Carbon nanotube modified laminates were found to exhibit 8% and 11% higher mode II interlaminar fracture toughness and interlaminar shear strength values, respectively, as compared to the base laminates. However, no significant improvement was observed for mode I interlaminar fracture toughness values. Furthermore, Optical microscopy and scanning electron microscopy were utilized to monitor the distribution of carbon nanotubes within the composite microstructure and to examine the fracture surfaces of the failed specimens, respectively.