Damage in carbon fibre composites due to repetitive low-velocity impact loads

The effect of repetitive impacting with increasing impact energy on unsupported thermoset matrix and thermoplastic matrix carbon fibre laminates was studied. In the case of the thermoset laminates, there were two abrupt losses in stiffness, the first corresponding to through-the-thickness damage and the second to the damage extending to the specimen edges. The thermoplastic matrix laminates exhibited a continuous decline in stiffness but again the damage sequence was through-the-thickness followed by extension to the specimen edges. Static bend testing of these composites resulted in the same type and extent of damage as was observed for impact loading. The effect of increasing the unsupported area and laminate thickness were investigated. Impacting with increasing impact energy was compared to repetitive impacting at a constant impact energy.     

A fast algorithm for the elastic fields due to interacting fibre breaks in a periodic fibre composite

Monte Carlo simulations of the failure of unidirectional fibre composites typically require numerous evaluations of the stress-state in partially damaged composite patches. In a simulated composite patch comprised of N fibres, of which \(N_b\) fibres are broken in a common cross-sectional plane transverse to the fibre direction, the stress overloads in the intact fibres are given by the weighted superposition of the unit break solutions associated with each of the breaks. Determining the weights involves solving \(N_b\) linear equations, and determining overloads in the intact fibres requires matrix-vector multiplication. These operations require \(O(N_b^3)\), and \(O(N N_b)\) floating point operations, respectively. These costs become prohibitive for large N, and \(N_b\); they limit Monte Carlo failure simulations to composite patches of only a few thousand fibres. In the present work, a fast algorithm to determine the overloads in a partially damaged composite, requiring \(O( N_b^{1/3} N \log N)\) floating point operations, is proposed. This algorithm is based on the discrete Fourier transform. The efficiency of the proposed method derives from the computational simplicity of weighted superposition in Fourier space. Computations of the stress state ahead of large circular clusters of breaks in composite patches comprised of about one million fibres are used to demonstrate the efficiency of the proposed algorithm.     

Number of phase levels of a talbot array illuminator

The number of phase levels of a Talbot array illuminator is an important factor in the estimation of practical fabrication complexity and cost. We show that the number (L) of phase levels of a Talbot array illuminator has a simple relationship to the prime number. When there is an alternative pi-phase modulation in the output array, the relations are similar.     

Modelling and characteristics of actively Q-switched Er-doped fibre laser with Tm-doped fibre saturable absorber

A theoretical model concerning FSA-based active Q-switching is developed. And, characteristics of the active Q-switching operation are investigated and compared with the traditional passive Q-switching operation. Numerical results demonstrate that the active control of the repetition rate of FSA-based Q-switching is attainable. And, stable active Q-switching could be obtained from both no lasing state and stable passive Q-switching state.     

激光对碳纤维及碳纤维/环氧树脂复合材料性能影响

采用高能激光束对聚丙烯腈（PAN）基碳纤维进行表面改性。利用SEM、EDS、FTIR、XRD、万能试验机等表征手段,对改性前后碳纤维微观形态、成分变化、物相结构、力学性能进行表征,系统地研究了激光束对碳纤维微观组织变化、性能变化等的影响规律,探索激光束对碳纤维的作用机制。结果表明,碳纤维经激光表面改性后,其表面的粗糙度和比表面积增加,碳纤维的浸润性得到提升,且激光束的功率越高、扫描速度越低,碳纤维浸润性越好。改性后的碳纤维化学成分、微观结构及官能团种类没有改变;改性后的碳纤维官能团种类没有改变,说明激光改性过程主要以物理过程为主;激光改性没有改变碳纤维的微观结构,改性后微晶尺寸略有减小,有利于改善碳纤维与环氧树脂的界面黏结性能。激光表面改性碳纤维/环氧树脂复合材料的拉伸强度和冲击强度均有不同程度的提高,当碳纤维质量分数为0.2wt%、激光改性功率为150 W时,碳纤维/环氧树脂复合材料的拉伸强度提高了59%,冲击强度提高了52%。     

Passively Q-switched fibre laser utilizing erbium-doped fibre saturable absorber for operation in C-band region

In this paper, a stable and robust all-fibre passively Q-switched erbium-doped fibre laser (EDFL) emitting at 1558?nm is described. The proposed laser utilizes an 11?cm long erbium-doped fibre as saturable absorber (SA). The fibre SA features a linear optical absorption of about 13?dB in the Q-switched EDFL operating regime. By elevating the input pump power from the threshold of 60?mW to the maximum available power of 142?mW, a pulse train with a maximum repetition rate of 86?kHz, minimum pulse width of 3.39?µs, maximum average output power of 10.5?mW, maximum pulse energy of 122?nJ and maximum peak power of 36?mW are obtained. The signal to noise ratio (SNR) of the spectrum is measured to be around 70?dB. This fibre SA is simple, reliable, compact and alignment free. Thus it is suitable for making a portable pulse laser source.     

Limitations of interferometry due to the flicker noise of laser diodes

Interferometry with laser diodes is a cost-effective way to perform displacement measurement. The tunability of laser diodes is also of great interest in multiple-wavelength interferometry. However, the additional flicker noise in the frequency-noise spectrum of semiconductor lasers may become a limiting factor. Investigations on the limitations due to the 1/f noise of laser diodes are presented for both classical and multiple-wavelength interferometry. Measurements at the limit of the coherence length of laser diodes with the corresponding phase fluctuations are reported. The theoretical results are verified experimentally.     

Stress disturbance due to broken fibres in metal matrix composites with non-uniform fibre spacing

Premature fracture of weaker fibres causes stress disturbances in composites. These disturbances are affected by non-uniformity of fibre spacing. In order to evaluate quantitatively how the disturbances in metal matrix composites are affected by the extent of non-uniformity of fibre spacing, a method of calculation is presented on the basis of two-dimensional shear lag analysis. Static tensile stress concentrations in the intact fibres to broken fibres, tensile stress distribution along the fibre axis in the broken and intact fibres and shear stresses between broken and intact fibres were calculated by the method presented, using some examples. It is shown quantitatively that the spacing between broken and intact fibres and that between intact and next fibres has a significant influence on tensile stress concentrations in intact fibres and also on the shear stresses between broken and intact fibres: the narrower the former spacing and the wider the latter spacing, the higher become both tensile and shear stress concentrations. This tendency is enhanced when the number of broken fibres is large and when the strain hardening of the matrix is high.     

Fractal laser mode structure due to iterated diffractive multiple imaging

A variation on iterated multiple imaging in optical resonators, a mechanism that creates fractal structure in the resonator's eigenmodes, is presented. During each round trip, side-by-side images of the intensity distribution in a plane are placed into the same plane by a grating in the resonator. It is shown that this leads to fractal structure in the eigenmodes. Interestingly, a very similar mechanism can be said to be at work in almost every canonical resonator; however, it creates fractal eigenmode structure only over a negligible range of length scales.     

In-phase supermode selection in a multicore fiber laser array by means of a self-Fourier external cavity

A procedure is developed to determine the transverse-mode structure of a cavity consisting of a dense, evanescently coupled, waveguide laser array, which, in addition, is externally coupled by feedback from an external cavity. The formalism is used to determine the loss and phasing properties of a multicore fiber array coupled to an external self-Fourier cavity. Best performance is predicted for linear arrays of up to five cores, or two-dimensional arrays of up to 25 cores. A low-loss, in-phase, fundamental array mode is predicted, which achieves better than 30 dB discrimination against higher-order modes at periodically spaced values of the array length. However, we show that a shift in operating wavelength of typically a few nanometers can bring about near-perfect phasing and loss operation over a continuum of fiber lengths. With increased fill factor, significantly more of the output power can be concentrated in the central lobe of the far field but at the penalty of increased loss in the fundamental eigenmode.     

Carbon fibre composites with improved fatigue resistance due to the addition of tin-lead alloy particles

The addition of tin-lead (60 wt% Sn) alloy particles (about 21–25 μm in diameter, in an amount up to 37 wt% or 7.2 vol%) between continuous unidirectional carbon fibre layers in an epoxy-matrix composite was found to improve the fatigue life by over 100 times. The alloy addition had little effect on the tensile strength, tensile modulus, compressive strength, compressive modulus (with the compressive force parallel and perpendicular to the fibres), but increased the electrical resistivity. The composites were fabricated by compression moulding at 185–200°C and 1 MPa for 30 min. The heating allowed the alloy to melt while the epoxy cured. The fatigue life enhancement is probably due to the hindering of the fatigue crack growth by the alloy particles.     

Study on fibre laser spot welding of TRIP980 steel

A 980 MPa transformation induced plasticity (TRIP) steel was fibre laser spot welded by different Argon (Ar) shielding conditions, laser power (1000 up to 2500 W) and defocusing distances (?8 up to +8 mm). The surface appearance, cross-section macrostructure, microstructure, hardness, tensile shear properties and fatigue properties of laser spot welds were evaluated. The results showed that the welds with Ar shielding had larger weld appearance and bonding sizes, better tensile shear properties compared with the welds without Ar shielding. With the increase in laser power, the laser welding mode changed from conduction to keyhole, which improved the bonding size and mechanical properties. The bonding size and mechanical properties increased in the order of defocusing distances of +8, ?8, +4, ?4 and 0 mm. During the fatigue tests of laser spot weld, the fusion zone pullout and sheet transverse fracture failure modes were observed.     

Narrow-linewidth passively Q-switched fibre laser based on microsphere resonator

We experimentally demonstrate a stable narrow-linewidth passively Q-switched fibre laser based on a microsphere resonator (MSR) and graphene saturable absorber (SA). The MSR made by the arc discharge method has the characteristics of easy fabrication, broad free spectral range (FSR) and flexibility. And it acts as a narrow band-pass filter to ensure narrow-linewidth operation. A stable passively Q-switched pulse with 0.016?nm narrow spectral linewidth is successfully achieved. The output pulse has the pulse width of 5.2 µs, repetition frequency of 28 kHz and high signal to noise ratio (SNR) of ～60?dB. The results demonstrate that our works may provide an effective way to achieve narrow-linewidth pulsed fibre lasers.     

Hyperbolic heat conduction due to a mode locked laser pulse train

In situations which involve repetitive pulsing of a material with a mode locked Nd:YAG laser, the pulse duration can be sufficiently small (i.e. in the picosecond range) that the classical parabolic heat conduction equation fails to adequately predict the resulting temperature distribution in the material. In such cases, the hyperbolic heat conduction equation, which accounts for the finite time to the commencement of heat flow, is appropriate. In the present work, the hyperbolic heat conduction equation is used to predict the temperature distributions in both semi-infinite and finite isotropic media due to a train of temporally rectangular pulses which approximate the Gaussian temporal profile of mode locked laser pulses. The energy carried in the pulses is assumed to be absorbed in the surface plane of the material. The spatial profile of the pulses can be either Gaussian, doughnut or a combination of the two. The parabolic and hyperbolic models are examined for selected pulse frequencies.     

Mutual alignment errors due to wave-front aberrations in intersatellite laser communications

We analyze mutual alignment errors due to wave-front aberrations. To solve the central obscured problem, we introduce modified Zernike polynomials, which are a set of complete orthogonal polynomials. It is found that different aberrations have different effects on mutual alignment errors. Some aberrations influence only the line of sight, while some aberrations influence both the line of sight and the intensity distributions.     

Room-temperature dual-wavelength erbium-doped fibre laser based on a sampled fibre Bragg grating and a photonic Robin Hood

With the assistance of a kind of photonic Robin Hood that is originated from four-wave mixing in a dispersion-flattened high-nonlinearity photonic-crystal fibre, a novel dual-wavelength erbium-doped fibre (EDF) laser is proposed and demonstrated by using a sampled fibre Bragg grating. The experiments show that, due to the contribution of the photonic Robin Hood, the proposed fibre laser has the advantage of excellent uniformity, high stability and stable operation at room temperature. Our dual-wavelength EDF laser has the unique merit that the wavelength spacing remains unchanged when tuning the two wavelengths of laser, and this laser is simpler and more stable than the laser reported by Liu et al. [Opt. Express, 13 142 (2005)].     

Transportable optical frequency comb based on a mode-locked fibre laser

A new transportable frequency comb has been developed at the National Physical Laboratory. The transportable frequency comb is based around a MenloSystems FC1500 optical frequency synthesizer, which is capable of measuring any frequency between 500 and 2100 nm. Measurements made by the comb while off-sites are referenced to the SI second via a global positioning system (GPS)-disciplined Rapco 2804AR rubidium oscillator. All components of the system (except for the GPS antenna) have been integrated into a single mobile unit. Measurements of high-stability narrow-band optical sources using this system, with either the GPS-disciplined oscillator or a hydrogen maser as a reference, are described. Some of these measurements were made simultaneously with a hydrogen-maser-referenced Ti:sapphire-based comb. It was found that the GPS-referenced transportable comb has a fractional inaccuracy of better than 4 10^-12 (averaged over several hours) when compared with the maser-referenced Ti:sapphire comb, and a fractional frequency instability of 1times10^-12 for an averaging time of 10 s. It was also demonstrated that continuous frequency measurements are possible with this system over a period of 60 h or more.