Key Engineering Materials Vols. 306-308

Abstract: Cutting tool wear is a major problem in machining processes. It has a great effect on the quality of a workpiece. Thus, monitoring cutting tool wear is very important in order to maintain the workpiece quality as well to reduce production rate and production time. The use of a single sensor in a monitoring system may not be accurate to detect cutting tool wear. In this paper, sensor fusion technology is introduced for monitoring cutting tool wear.

Abstract: This research aims to investigate strain rate effect on transverse compressive strength of unidirectional fiber composites. Both glass/epoxy and graphite/epoxy composites were taken into account in this study. To demonstrate strain rate effect, composite brick specimens were fabricated and tested to failure in the transverse direction at strain rate ranges from 10-4/s to 500/s. For strain rate less than 1/s, the experiments were conducted by a hydraulic MTS machine. However, the higher strain rate tests were performed using a Split Hopkinson Pressure Bar (SHPB). Experimental observations reveal that the transverse compressive strengths increase corresponding to the increment of the strain rates. A semi-logarithmic function was employed to describe the rate sensitivity of the transverse compressive strength. SEM photographic on the failure surfaces depicts that for glass/epoxy composites, the failure mechanism is mainly due to the matrix shear failure, however, for the graphite/epoxy composites, it becomes the fiber and epoxy interfacial debonding which could dramatically reduce the transverse compressive strengths of the fiber composites.

Abstract: The impact response and energy absorbing characteristics of laminated, foam sandwich and honeycomb sandwich composites under ballistic impact have been studied in this investigation. An improved model is proposed in this paper to predict the ballistic property of the laminated composites. In this model, the material structures related to fiber lamination angles are designed in terms of their anti-impacting energy absorption capability. The ballistic limit speed and energy absorption per unit thickness of the three composites under different conditions are calculated. It is shown that honeycomb sandwich composite has the best ballistic resistance capability and energy absorption property among the three composites.

Abstract: The dynamic fracture tests were carried out for a glass fiber reinforced plastic specimen with a crack and dynamic fracture toughness was evaluated by examination of cracking at an initial slit root. Before the crack initiated at the slit root, a whitened damage zone was created surrounding the slit tip. The damage zone consists of micro cracking in the matrix, debonding between a fiber and the matrix, and fracture of the fiber. The comparison of the dynamic fracture toughness and the static fracture toughness value shows that the former is around 12 MPa√m and apparently higher than the later, which is 7 MPa√m. To understand those experimental results and mechanics of the damage zone, a dynamic debonding test was carried out and dynamic bonding strength was estimated as around 70 MPa.

Abstract: The tensile, fatigue and creep properties of carbon fiber filled poly(ether-ether-ketone) or PEEK were compared by employing injection and compression molding processes. The specimens with different initial fiber lengths were fabricated using compression and injection molding methods. The effects of fiber length on mechanical properties are evident in cases of specimens made by compression molding. But, the injection molded samples show no differences in tensile and fatigue strengths contingent on the initial fiber length. Similarly, the effects of fiber volume fraction on tensile and fatigue strengths are more evident for the compression molded specimens than the injection molded specimen. For the creep properties, even the injection molded specimens show some improvement for long reinforcing fibers especially as temperature is increased. When the influence of time and temperature is considered, the effects of fiber length on mechanical properties are very significant. Also, these results represent the significant fiber damage during injection molding.

Abstract: This paper presents a damage detection of surface crack in composite laminate. Carbon/epoxy composite AS4/PEEK was used to fabricate a quasi-isotropic laminate [0/90/±45]2s. Surface crack was created by using laser cutting machine. Modal analysis was performed to obtain the mode shapes of the laminate before and after damage. The mode shapes were then adopted to compute the strain energy, which was used to define a damage index. Consequently, the damage index successfully predicted the location of surface crack in the laminate. Differential quadrature method (DQM) was introduced to calculate the partial differential terms in strain energy formula.

Abstract: Theoretical formulas for effective elastic modulus and Poisson's ratio of honeycomb core materials were proposed considering the bending, axial and shear deformations of cell walls. Theoretical results obtained by the formulas showed orthotropic elasticity and large Poisson’s ratio, which were comparable to results by finite element analysis(FEA). Tensile test of honeycomb sandwich composite(HSC) plates was performed for analysis of their deformation behaviors and interlaminar stresses. Equivalent plate model using the theoretical results of honeycomb core layer show that interlaminar shear stress occurring due to large difference of Poisson’s ratio between skin and honeycomb core layers led to the delamination in HSC plate under tensile loading. Load-displacement behavior of HSC specimen simulated by equivalent plate model coincided fairly with that of detailed FEA model similar to experimental results.

Abstract: Bending deformation and energy absorption characteristics of aluminum-composite hybrid tube beams have been analyzed for improvement in the bending performance of aluminum space frame by using experimental tests combined with theoretical and finite element analyses. Hybrid tube beams composed of glass fabric/epoxy layer wrapped around on aluminum tube were made in autoclave with the recommended curing cycle. Basic properties of aluminum material used for initial input data of the finite element simulation and theoretical analysis were obtained from the true stress-true strain curve of specimen which had bean extracted from the Al tube beam. A modified theoretical model was developed to predict the resistance to the collapse of hybrid tube beams subjected to a bending load. Theoretical moment-rotation angle curves of hybrid tube beams were in good agreement with experimental ones, which was comparable to the results obtained from finite element simulation. Hybrid tube beams strengthened by composite layer on the whole web and flange showed an excellent bending strength and energy absorption capability.

Abstract: Effective thermal conductivities of composites consisting of curvilinearly anisotropic inclusions with Kapitza thermal contact resistance between the constituents are considered. We show that the effect of these curvilinearly anisotropic inclusions can be exactly simulated by certain equivalent isotropic or transversely isotropic inclusions. Three different micromechanical models are employed to estimate the effective thermal conductivity of the composite. Interestingly, all these methods result in the same simple, closed-form expression.

Abstract: Strain Invariant Failure Theory (SIFT) is a newly-developed failure criterion for composite structures [8, 9]. SIFT comprises two main features, namely micromechanical finite element modification and critical strain invariant parameters. Micromechanical finite element modification is performed to incorporate residual strains between fibers and matrix into homogenized finite element lamina solution. The presence of residual strains takes into account the mechanical and thermal (environmental) effects. Critical strain invariant parameters can be obtained from simple tensile test by carefully observing the occurrence of damage initiation in composite constituent. Strain tensors extracted from experiment are substituted into strain invariant parameters of i J1 (first invariant of strain; i = f, m—fiber, matrix) and i vm ε (equivalent strain or von Mises strain; subscript vm stands for von Mises). As noted in [8, 9], three critical strain invariants were found to be the onset of composite failure for carbon-fiber/epoxy system; they are m J1−cr , f vm−cr ε and m vm−cr ε . Micromechanical characterization parameters aim to provide general insight on the critical state of strains in which damage initiation locus may be determined by using SIFT critical parameters. Micromechanical model was subjected to six different loading conditions (three normal deformations and three shear deformations) and strain tensors (ε1, ε2, ε3, ε12, ε13, ε23) were extracted from finite element analysis. Micromechanical characterization parameters were obtained by normalizing the strain invariants of micromechanics analysis with respect to critical strain invariant. Important issues such as effect of fiber volume fraction and fiber packing arrangement are briefly discussed.