Destructive Mechanical Testing Research Articles

Comparison between gas flame welding and shielded gas welding in case of aluminium alloys

Although the assembly by welding of aluminum alloys is a common assembly process, it is done in quite difficult conditions, being forced to take into account a number of shortcomings, such as: high thermal conductivity which translates into losses of high heat in the welding area; formation of aluminum oxide films that melt at high temperatures, well above the melting temperature of the alloy; large expansion coefficients that result in the appearance of large deformations, the appearance of cracks in the joint at high temperatures due to loss of mechanical strength of the alloy, but also the appearance of pores, inclusions of aluminum oxide and corrosion in various aggressive environments. Despite all these shortcomings that occur in welding, aluminum alloys can be joined indestructibly by welding. The present paper aims, is to make a comparative study, based on experimental data, of two welding assembly processes, one with gas flame and another with shielded inert gas environments-WIG, for aluminum alloys of group 6000. The base material will be presented, the preparation of the basic material for welding through the two mentioned welding procedures and the steps taken to make the welded joints. The comparative study will include the parameters of welding regimes, non-destructive defectoscopy methods with which certain discontinuities can be identified and the destructive mechanical tests. The experimental results will be able to lead to pertinent conclusions on the possibilities of aluminum alloys welding assembly, using those two welding processes mentioned above, the results being easily generalized in the industrial fields.

Theoretical and experimental research on the mechanical tests of 3D printed samples from various materials

At this moment, a real industrial, scientific and technological revolution is being discussed worldwide, consisting in the development of Additive Manufacturing AM technologies through the use of 3D printing. The FDM (Fused Deposition Modelling) 3D printing technology consists in the deposition of fusible material layer by layer in the XOY plane, the thickness of the 3D printed layer is g = 0.12 ÷ 0.24 mm, the resolution of 3D printing is r = 0.1mm, while the print head advances vertically on the OZ axis. The purpose of the work is to determine the breaking moment through destructive mechanical bending tests of some samples made by 3D printing from the following materials: ABS+, PETG and PLA. 5 samples were made in 3 printing directions: horizontal, vertical and lateral, resulting in 15 samples for each material. The total number of samples used is 45. Knowing these values, depending on the requirements of the specifications, a certain material and a certain printing position can be used.

Quantitative Nondestructive Evaluation of Cold Spray Manufactured Aluminum Alloy 6061 and Copper Samples

Cold spray (CS) is a solid-state process for depositing thick layers of material via the successive high-velocity impact of powder particles onto a solid surface, which leads to high rates of deformation, interparticle bonding, and coating build-up. Although CS is finding commercial utilization in non-load-bearing repair and coating applications, clear nondestructive characterization procedures are necessary to realize its potential in load-bearing structural applications. In this study, the viability of electrical conductivity and through thickness ultrasound wave velocity measurement methods was studied to serve as a means for nondestructive quantitative measurement for quality control in CS and potentially other additive manufacturing (AM) methods. Eddy current, ultrasound, porosity, hardness, and uniaxial tensile strength tests were conducted on CS deposited layers of aluminum alloy 6061 and copper on aluminum alloy 6061 and commercially pure copper substrates, respectively. CS gas dynamic parameters were intentionally and systematically varied to result in corresponding discrete differences in mechanical properties of deposits. Ultrasound measurements of longitudinal wave velocity and eddy current electrical conductivity measurements showed good correlation with process conditions, microstructural characterization results, and destructive mechanical tests (hardness, tensile). The results of this work show that ultrasound wave velocity and electrical conductivity correlate well with increased particle impact velocity in CS deposited aluminum and copper blocks, which evidently show an incremental decrease in porosity, increase in hardness, and increase in tensile strength. The outlined ultrasound and eddy current nondestructive testing methods present effective means for quantitative assessment of cold spray deposited structures while intact with the substrate.

Determining the Level of Structural and Mechanical Degradation of Steel in the Supporting Structure of Mining Excavation Machinery

The steels from which the components of mining machinery are constructed must be based primarily on knowledge of their behaviour under environmental conditions and of the excavation technology. Secondly, the structural changes undergone by the materials due to mechanical and thermal processing and the stresses generated by the action of excavation forces that can induce internal stresses at a level that is difficult to determine must be taken into account. Determining the level of structural and mechanical degradation involves sampling the load-bearing structure elements of the excavation and storage machinery for analysis of the mechanical and chemical properties of the component materials. The paper focuses on issues related to the structural and mechanical degradation of steel as a material for parts of mining machinery. In this paper, a methodology for evaluating the degree of structural degradation of three types of bucket wheel excavators with different operating hours (ERc 1400-30/7-07 with 50,400 h, ERc 1400-30/7-08 with 69,264 h, and SRs 1300-26/3.5 with 112,000 h) is proposed. The methodology aims to provide a quantitative assessment of the structural degradation by considering the specific elastic limit of the material used in the machines and the number of hours of operation. This method uses a minimum set of destructive mechanical tests, such as tensile, resilience for three working temperatures, chemical analysis, and durometry on resistance elements taken from the machine, as well as the use of Weibull analysis. By combining this information with the number of hours of operation, a comprehensive evaluation can be made to determine the extent of degradation and potential maintenance requirements. The use of a minimum set of mechanical tests ensures an efficient and cost-effective approach to assessing the structural integrity of these machines. The metallographic analysis highlighted a predominant and defined form of grain rearrangement in the E14-07 machine after only 50,400 h. The constituents identified in the samples taken from the three machines are ferrite and pearlite. The highest hardness of the samples was recorded for the E14-08 machine, with values between 162–165 HV10. For bucket wheel excavators E14-07 and E14-08, the material structure showed elongated grains in the direction of deformation, compared to the material structure of E13-04, which has finely equiaxed grains. As a result of the research, final conclusions were formulated.

In vivo characterisation of field pea stem wall thickness using optical coherence tomography

BackgroundModern field pea breeding faces a significant challenge in selecting lines with strong stems that resist lodging. Traditional methods of assessing stem strength involve destructive mechanical tests on mature stems after natural senescence, such as measuring stem flexion, stem buckling or the thickness of dry stems when compressed, but these measurements may not correspond to the strength of stems in the living plant. Optical coherence tomography (OCT) can be used as a noncontact and nondestructive method to measure stem wall thickness in living plants by acquiring two- or three-dimensional images of living plant tissue.ResultsIn this proof-of-principle study, we demonstrated in vivo characterisation of stem wall thickness using OCT, with the measurement corrected for the refractive index of the stem tissue. This in vivo characterisation was achieved through real-time imaging of stems, with an acquisition rate of 13 milliseconds per two-dimensional, cross-sectional OCT image. We also acquired OCT images of excised stems and compared the accuracy of in vivo OCT measurements of stem wall thickness with ex vivo results for 10 plants each of two field pea cultivars, Dunwa and Kaspa. In vivo OCT measurements of stem wall thickness have an average percent error of − 3.1% when compared with ex vivo measurements. Additionally, we performed in vivo measurements of both stem wall thickness and stem width at various internode positions on the two cultivars. The results revealed that Dunwa had a uniform stem wall thickness across different internode positions, while Kaspa had a significantly negative slope of − 0.0198 mm/node. Both cultivars exhibited an increase in stem width along the internode positions; however, Dunwa had a rate of increase of 0.1844 mm/node, which is three times higher than that of Kaspa.ConclusionsOur study has demonstrated the efficacy of OCT for accurate measurement of the stem wall thickness of live field pea. Moreover, OCT shows that the trends of stem wall thickness and stem width along the internode positions are different for the two cultivars, Dunwa and Kaspa, potentially hinting at differences in their stem strength. This rapid, in vivo imaging method provides a useful tool for characterising physical traits critical in breeding cultivars that are resistant to lodging.

Biomechanics of plate fixation following traditional olecranon osteotomy versus novel proximal ulna osteotomy for visualizing a distal humerus injury.

After a distal humeral injury, olecranon osteotomy (OO) is a traditional way to visualize the distal humerus for performing fracture fixation. In contrast, the current authors previously showed that novel proximal ulna osteotomy (PUO) allows better access to the distal humerus without ligamentous compromise. Therefore, this study biomechanically compared plating repair following OO versus PUO. The left or right ulna from eight matched pairs of human cadaveric elbows were randomly assigned to receive OO or PUO and repaired using pre-contoured titanium plates. Destructive and non-destructive mechanical tests were performed to assess stability. Mechanical tests on OO versus PUO groups yielded average results for ulna cantilever bending stiffness at a 90° elbow angle (29.6 vs 30.5 N/mm, p = 0.742), triceps tendon pull stiffness at a 90° elbow angle (28.2 vs 24.4 N/mm, p = 0.051), triceps tendon pull stiffness at a 110° elbow angle (61.9 vs 59.5 N/mm, p = 0.640), and triceps tendon pull failure load at a 110° elbow angle (1070.1 vs 1359.7 N, p = 0.078). OO and PUO elbows had similar failure mechanisms, namely, tendon tear or avulsion from the ulna with or without some fracture of the proximal bone fragment, or complete avulsion of the proximal bone fragment from the plate. The similar biomechanical stability (i.e., no statistical difference for 4 of 4 mechanical measurements) and failure mechanisms of OO and PUO plated elbows support the clinical use of PUO as a possible alternative to OO for visualizing the distal humerus.

Effect of controlled surface roughness and biomimetic coating on titanium implants adhesion to the bone: An experiment animal study

IntroductionLaser micromachining of titanium and its alloys can create micro-grooves with sizes similar to cell diameter of about 10 μm. Its coating with arginine-glycine-aspartic acid (RGD) may enhance cellular spreading and adhesion. This study aimed to evaluate the effect of laser micro-grooving and laser micro-grooving combined with RGD coating on the strength of the dental implants/bone interface using destructive mechanical pullout testing in experimental animals. Materials and methodsIn this study, the test groups consisted of 1.5-mm diameter, 5-mm long laser-grooved and laser-grooved/RGD coated titanium alloy (Ti–6Al–4 V) rods, and the control group included plain titanium alloy (Ti–6Al–4 V) rods. These rods were implanted in the mandibles of New Zealand white rabbits for 2, 4, and 6 weeks. After sacrifice, the test and control specimens were retrieved for mechanical pullout testing. The DMA 7-e was used to pull the titanium rods out of the bone, the probe position was plotted versus time graph to monitor the test progression, and the static modulus versus time graph was viewed; such graphs was then transformed into tables. The results were analyzed using the Mann-Whitney test. ResultsThe laser-grooved/RGD coated rods had significantly higher pull-out strength than the laser-grooved and control rods. Additionally, the laser-grooved rods had significantly higher pull-out strength than control rods. ConclusionTwo novel surface treatments were used: laser micro-grooving and tri peptide RGD coating, both of which had different effects on the dental implant interface. Laser grooving improved peri-implant bone healing, whereas RGD coating facilitated earlier bone–implant adhesion and better mineralization.

Investigating impacts of FDM printing parameters and geometrical features on void formation in 3D printed automotive components

Additive manufacturing (AM) has opened a world of new possibilities in the automotive industry by offering high flexibility in design, shortening production time, and producing lightweight and customizable parts. Despite these advantages, the high risk of defect formation associated with AM processes and the difficulty of defect detection in quality control stages have hindered the rapid adoption of AM in the automotive industry. Mechanical tests are destructive tests which are usually used for quality control of conventionally fabricated components. However, they are not practical for quality control of AM parts since, due to the higher risk of defects, a higher number of AM parts should be mechanically tested which is time-consuming and expensive. Hence, rather than destructive mechanical tests, automakers require non-destructive imaging techniques for AM parts to characterize defects for a high number of samples at the quality control stage. This study aims to examine the impact of printing parameters and geometrical features on the void formation of an automotive component (car window holder). These outcomes will be helpful in the establishment of non-invasive quality control for the window car holder which is based on imaging techniques. For this purpose, a window car holder was printed with varying printing parameters using the FDM technique. Based on the geometry of the component, two regions with distinct geometrical features were specified and analyzed using Micro-CT. Results indicate that by increasing printing speed (30 to 60 mm/min) and temperature (230 °C to 250 °C), voids fraction decreased. Conversely, when increasing the printing layer thickness (0.1 to 0.3 mm), the void fraction increased. It was also concluded that void formation in regions with curved surfaces and overhang structures is significantly higher compared to edges with an angle of 90 ° to 60 °. These results offer a platform to reduce the risk of defect formation in 3D-printed automotive parts, improving quality control.

Acoustic Micro-Tapping Optical Coherence Elastography to Quantify Corneal Collagen Cross-Linking

To evaluate changes in the anisotropic elastic properties of exvivo human cornea treated with ultraviolet cross-linking (CXL) using noncontact acoustic micro-tapping optical coherence elastography (AμT-OCE). Acoustic micro-tapping OCE was performed on normal and CXL human donor cornea in an exvivo laboratory study. Normal human donor cornea (n=22) divided into 4 subgroups. All samples were stored in optisol. Elastic properties (in-plane Young's, E, and out-of-plane, G, shear modulus) of normal and ultraviolet CXL-treated human corneas were quantified using noncontact AμT-OCE. A nearly incompressible transverse isotropic model was used to reconstruct moduli from AμT-OCE data. Independently, cornea elastic moduli were also measured with destructive mechanical tests (tensile extensometry and shear rheometry). Corneal elastic moduli (in-plane Young's modulus, E, in-plane, μ, and out-of-plane, G, shear moduli) can be evaluated in both normal and CXL treated tissues, as well as monitored during the CXL procedure using noncontact AμT-OCE. Cross-linking induced a significant increase in both in-plane and out-of-plane elastic moduli in human cornea. The statistical mean in the paired study (presurgery and postsurgery, n=7) of the in-plane Young's modulus, , increased from 19 MPa to 43 MPa, while the out-of-plane shear modulus, G, increased from 188 kPa to 673 kPa. Mechanical tests in a separate subgroup support CXL-induced cornea moduli changes and generally agree with noncontact AμT-OCE measurements. The human cornea is a highly anisotropic material where in-plane mechanical properties are very different from those out-of-plane. Noncontact AμT-OCE can measure changes in the anisotropic elastic properties in human cornea as a result of ultraviolet CXL.

Systematic studies on behaviors of ultra-high performance concrete subject to freezing and thawing cycles combining DIC technology

Freezing and thawing alternation is one harsh weather condition for the concrete structures serving in seasonal frozen areas. As an emerging construction material, the frost-resistance of ultra-high performance concrete (UHPC) is highly necessary to be understood for propelling its practical application. In this study, the frost-resistance of UHPC was systematically evaluated based on the mechanical properties and the cracking behaviors. The fast freezing and thawing (FT) cycles were chosen as the experiment condition, along with the water-immersion condition for comparison. A combination of destructive mechanical and non-destructive resonance frequency test was used to characterize the behaviors. The results showed that, after undergoing 300 FT cycles, the tensile initial cracking strength, the tensile strength, the flexural strength, and the compressive strength exhibited a certain of degradations. The corresponding effects of the water-immersion action were more positive. The tensile strain capacity under the FT condition almost kept constant, whereas that under the water-immersion condition decreased. The two indices (including strain and strength) could reasonably reflect the magnitude of tensile strain. The digital image correlation (DIC) analysis unveiled that there may exist an additional plastic deformation in the tensile full-field deformation, apart from the elastic deformation and crack opening. Compared with the water-immersion action, the FT action resulted in a less zigzag cracking path, shorter cracking time, and larger crack opening under flexural loading. In addition, the variations of dynamic modulus of elasticity and re-hydration reaction degree under the FT condition were not coincident with those of mechanical properties. The findings could provide some valuable references for estimating the bearing capacity of the UHPC structures served in practical engineering.

Engineering of hydrogel electrolyte for aqueous Zn||LiFePO4 battery on subzero-temperature adaptability, long cycles and mechanical safety

Aqueous Zn||LiFePO4 (Zn||LFP) hybrid batteries are gaining increasing interest because of their stable charging/discharging, non-flammability, low cost, and environmental friendliness. However, aqueous hybrid batteries still face the challenges of sluggish charge transfer and ion diffusion processes due to electrolyte freezing at subzero temperatures. Herein, we designed an antifreeze hydrogel electrolyte for low-temperature Zn||LFP battery. This hydrogel electrolyte is comprised of double-network κ-carrageenan/PAAm containing moderately concentrated electrolyte (1 M ZnCl2 and 7 M LiCl), possesses high elasticity with a fracture strain of 834% and high ionic conductivity of 14.8 mS cm−1 at −40 °C. It also enables a stable Zn plating/stripping efficiency with reduced interface resistance and alleviates undesired H2 evolution. The corresponding flexible Zn||LFP battery with hydrogel electrolyte displays outstanding cycle stability with capacity retentions of 95.2% and 80.5% after 2000 cycles at −20 °C and −40 °C, respectively. In addition, the battery exhibits low self-discharge rate and retains good electrochemical stability under different destructive mechanical testing including bending, folding, and puncturing. Owing to the ion-orientation effect, cooperative solvation effect, and interface wettability, the hydrogel electrolyte endows Zn||LFP battery with prolonged cyclability, subzero-temperature operation, and mechanical safety as compared to the liquid electrolyte counterpart.

Noncontact Acoustic Micro-Tapping Optical Coherence Elastography for Quantification of Corneal Anisotropic Elasticity: In Vivo Rabbit Study.

The purpose of this study was to demonstrate accurate measurement of corneal elastic moduli in vivo with noncontact and noninvasive optical coherence elastography. Elastic properties (in-plane Young's modulus, E, and both in-plane, μ, and out-of-plane, G, shear moduli) of rabbit cornea were quantified in vivo using noncontact dynamic acoustic micro-tapping optical coherence elastography (AµT-OCE). The intraocular pressure (IOP)-dependence of measured mechanical properties was explored in extracted whole globes following in vivo measurement. A nearly incompressible transverse isotropic (NITI) model was used to reconstruct moduli from AµT-OCE data. Independently, cornea elastic moduli were also measured ex vivo with traditional, destructive mechanical tests (tensile extensometry and shear rheometry). Our study demonstrates strong anisotropy of corneal elasticity in rabbits. The in-plane Young's modulus, computed as E = 3μ, was in the range of 20 MPa to 44 MPa, whereas the out-of-plane shear modulus was in the range of 34 kPa to 261 kPa. Both pressure-dependent ex vivo OCE and destructive mechanical tests performed on the same samples within an hour of euthanasia strongly support the results of AµT-OCE measurements. Noncontact AµT-OCE can noninvasively quantify cornea anisotropic elastic properties in vivo. As optical coherence tomography (OCT) is broadly accepted in ophthalmology, these results suggest the potential for rapid translation of AµT-OCE into clinical practice. In addition, AµT-OCE can likely improve diagnostic criteria of ectatic corneal diseases, leading to early diagnosis, reduced complications, customized surgical treatment, and personalized biomechanical models of the eye.

Integrated use of unconditional mining raw materials

The development of the mining industry is accompanied by an increase in the volume of overburden, enclosing and substandard rocks located in dumps, which negatively affects the ecological situation. The purpose of this study is to establish a possibility and offer ways for the integrated use of mining raw materials. A number of experiments were conducted to achieve the goal. Using the methods of gravimetry and atomic absorption spectroscopy, the chemical analysis of basalts of the Baikal region was carried out; using the destructive method, physical and mechanical tests of the samples were performed. The possibility of using these rocks as a coarse aggregate for concrete was studied; the dependence of the strength of the materials on the chemical composition of crushed stone was established; in particular, it was determined that the addition of crushed stone with a high content of magnesium and iron oxides into the composition of concrete improves the mechanical properties of the samples. The rock acidity modulus was within the regulated limits, which indicates the possibility of obtaining high-quality melt that contributes to good fiber formation. The working temperature for the basalt melt and its relationship with the chemical and mineralogical composition of the basalts were established. Basalts which are of little use for producing concrete are high-quality raw materials for producing mineral fiber. It is possible to use mining raw materials in a complex way and reduce the amount of substandard rocks transferred to dumps, thereby improving the environmental situation in the areas of subsoil use.

Effect of Aspergillus Flavus Fungi Infection and Aflatoxin Contamination on Single Corn Kernel Mechanical Strength

HighlightsKernels were artificially inoculated with toxigenic and non-toxigenic strains of Aspergillus flavus in the lab.Corn kernel mechanical strength was assessed as single kernel compression rupture force (CRF).Fungus-infected kernels had lower mechanical strength than the controls.Kernels with high aflatoxin contamination had lower mechanical strength than others.ABSTRACT. One persistent food safety issue associated with corn is aflatoxin contamination. Aflatoxins are secondary metabolites produced mainly by the fungi Aspergillus flavus (A. flavus) and A. parasiticus. Under environmental conditions suitable for fungal growth and aflatoxin production, these fungi are capable of infecting corn kernels in the field and in storage. When corn kernels are infected, the fungi use nutrients from the kernels to grow and propagate. In general, moldy and contaminated kernels are discolored and lighter in weight. These features could aid in the identification and removal of infected kernels. The objective of this research was to investigate the relationship between the mechanical strength of corn kernels and fungal infection, as well as aflatoxin contamination as a consequence of fungal infection. Corn kernels were infected with aflatoxin producing (AF13) and non-toxin-producing (AF36) A. flavus strains in lab inoculations. A total of 900 kernels were assigned to three groups, non-inoculated control, AF13-inoculated, and AF36-inoculated. One hundred kernels from each group were incubated for 3, 5, and 8 days. After incubation each kernel was cleaned and dried, then subjected to a destructive mechanical test. The compression rupture force (CRF) of each kernel was measured using a Mark-10 material test gauge to assess its kernel strength. The results show that kernel strength was significantly different between the control and treatment groups, with the control kernels being the strongest and the AF36-inoculated kernels the weakest. The results also indicated that kernels with aflatoxin contamination of 300 ppb and above were significantly weaker than control kernels and those less contaminated in the AF13-inoculated group. It is expected that the results of this research could benefit the corn industry by mitigating the aflatoxin contamination problem in post-harvest management. Keywords: Aflatoxin, Aspergillus flavus, Compression rupture force, Corn, Fungus-infected, Inoculation, Mechanical strength.

Fractography analysis into low-C steel undergone through various destructive mechanical tests

The present work deals with a critical fractographic analysis into low carbon (0.18%-C) steel samples which were used for three different mechanical tests: tensile test; shear test; and toughness test. These mechanical tests were performed in standard sized specimens as recommended by ASTM. In each category of test, there were two different specimens with different physical states according to heat treated conditions. First specimen was in ‘as received’ condition and another was annealed. For annealing, sample was first heated up to austenitic temperature and inserted inside the sand for slow rate of cooling. As these two categories of samples were undergone through destructive tests, the variation in fracture behaviour of the samples was analysed by FESEM, XRD. A significant variation in fractographic images could be observed in different heat-treated samples. Micro-pores, dimples, cleavage facet, peaks, valleys, and cave formation were observed in the samples.

An experimentally validated model for predicting the tensile modulus of tubular biaxial and triaxial hybrid braids

AbstractBraiding is a versatile and cost‐effective approach to generating various composite structures for mechanical, sports, and biomedical engineering applications, which are different from each other. For example, a stent is a braided structure and blood penetration in it is essential as much as the right function. Predicting the tensile responses is a prerequisite for the success of implementation of braided structures, and it is usually performed via destructive mechanical testing that might be costly and/or time‐consuming. Therefore, it is essential to provide a model that can accurately predict the tensile modulus. In this research, a theoretical model is developed using the simplification of a braided structure and is validated via testing of biaxial and triaxial braids composed of polyester, glass, and basalt yarns on a 16‐carrier vertical braiding machine. Experimental results not only are used for the model validation but also show the effectiveness of axial yarn presence, hybridization, and the presence of high‐performance yarn in the braided structures on the tensile properties. A good correlation between theoretical values and experimental results is observed approving the high accuracy of the proposed model. This paper is likely to fill a gap in the state of the art and provide pertinent results that are instrumental in the design of hybrid‐braided structures with minimum computational/experimental effort. The research innovation centers on the use of two different yarns to make hybridization, simplicity of the model to be used for biaxial and triaxial braided structures, and a start to omit destructive tests.

Study on anisotropy of 3D printing PVA fiber reinforced concrete using destructive and non-destructive testing methods

To further understand the mechanical properties and anisotropy characteristics of 3D printing PVA fiber reinforced concrete (PFRC). This study first obtained 3D printing PFRC mixes with superior mechanical properties through slump tests, penetration resistance tests, printability tests, and flexural tests. Second, the fiber reinforcement mechanisms and anisotropy characteristics of PFRC were analyzed by nondestructive ultrasonic pulse velocity tests, destructive mechanical property tests, and scanning electron microscope tests. Third, the correlation between mechanical properties and ultrasonic signals was analyzed, and the conversion criteria were further established. Moreover, the application range of conversion criteria was also determined. The test results showed that the optimum dosage of PVA fibers is 0.5 wt% for 3D printing FRPC, and the highest cube compressive strength and flexural strength are 98.2 MPa and 6.8 MPa, respectively. The mechanical properties and acoustics of 3D printed specimens showed significant anisotropy. The conversion criteria show a good agreement with the test data and are applicable to 3D printing fiber concrete with a small aggregate size.

PVA increases efficiency of bacterially-induced self-healing in cement mortars

Different strains of bacteria capable of calcite precipitation can be incorporated into fresh concrete or cement mortars to efficiently promote self-healing of microcracks after hardening. To date, all bacterial strains investigated have required protection against the highly alkaline environments and high pressures in cementitious matrices. The core objective of this study was to eliminate the inconveniences related to protecting (typically encapsulating) bacteria. Poly(vinyl alcohol) (PVA) doped with spores of Bacillus pseudofirmus and nutrients were added to mixing water when cementitious mortars were prepared. The hardened mortar specimens were cracked and studied using microscopy, resonance measurements, X-ray microtomography (μ-CT), and destructive mechanical tests to assess the rate and efficiency of the healing processes. PVA was efficient in promoting remediation in all mortar specimens, both with and without bacteria. Specimens containing PVA plus bacteria showed the most efficient self-repair processes, while autogenous remediation in reference samples lacking PVA or bacteria were least efficient in healing microcracks.

Latent cure epoxy resins for reliable joints in secondary-bonded composite structures

In high-performance polymer matrix composite assemblies, adhesive bonding is generally superior to mechanical fastening in structural performance and manufacturing efficiency. However, adhesive bonds are susceptible to minute levels of contamination accumulated during assembly that can lead to unpredictable, weak bonds. Current methods of measuring bond strength are all destructive mechanical tests. To overcome these challenges, redundant load paths (e.g., mechanical fasteners) are often implemented in secondary-bonded, primary-structures, which can greatly reduce structural performance. This study investigated reformulated aerospace epoxy matrix resins with stoichiometric offset to inhibit cure of the matrix resin prior to assembly. Inhibited resins can reflow and mix across the joint interface , which eliminates the material discontinuity and forms a homogenous joint with reliable fracture properties . The goal of this study was to develop and demonstrate secondary composite assemblies that are mechanically and microscopically indistinguishable from a co-cured composite joint. This article describes the development of latent epoxy resins, the fabrication of test articles, and the mechanical properties measured from experimental joints compared with conventional, co-cured laminates. Methods of in-line quality control using and infrared spectroscopy and post-assembly forensics are also described. The final mode-II fracture toughness measured from precracked AERoBOND specimens was similar to that measured from co-cured laminates indicating that later cure epoxy materials could be a suitable replacement for secondary bonding. • Methods enable rapid assembly of thermoset composites with the simplicity of secondary bonding and reliability of co-cure. • Epoxy based resins were formulated with offset stoichiometry: epoxy rich and hardener rich. • End-notched flexure tests indicated that joints with offset stoichiometry matched or exceeded co-cured joint performance. • Infrared spectroscopy and ultrasonic inspection were viable means of inline quality control before and after assembly.

Relationship between Thermal Diffusivity and Mechanical Properties of Wood

This paper describes an experimental study of the relationships between thermal diffusivity and mechanical characteristics including Brinell hardness, microhardness, and Young’s modulus of common pine (Pinus sylvestris L.), pedunculate oak (Quercus robur L.), and small-leaf lime (Tilia cordata Mill.) wood. A dependence of Brinell hardness and thermal diffusivity tensor components upon humidity for common pine wood is found. The results of the measurement of Brinell hardness, microhardness, Young’s modulus, and main components of thermal diffusivity tensor for three perpendicular cuts are found to be correlated. It is shown that the mechanical properties correlate better with the ratio of longitude to transversal thermal diffusivity coefficients than with the respective individual absolute values. The mechanical characteristics with the highest correlation with the abovementioned ratio are found to be the ratio of Young’s moduli in longitude and transversal directions. Our technique allows a comparative express assessment of wood mechanical properties by means of a contactless non-destructive measurement of its thermal properties using dynamic thermal imaging instead of laborious and material-consuming destructive mechanical tests.