Peak Failure Load Research Articles

Numerical simulation of crack propagation and coalescence in pre-cracked rock-like Brazilian disks using the non-ordinary state-based peridynamics

In the non-ordinary state-based peridynamics (NOSB-PD), the maximum tensile stress criterion and Mohr-Coulomb failure criterion, which are common stress-based failure criteria in rock mechanics and rock engineering, are applied to distinguish the tensile cracks and shear cracks, respectively. The initiation and propagation and coalescence of one single crack or double cracks in the Brazilian Disk specimens under compressive line loads are investigated using NOSB-PD. The effects of the specimen sizes and the crack inclination angles on the peak failure loads of the pre-cracked Brazilian Disk specimens are studied. Mode I fracture toughness and mixed Mode I/II fracture toughness are evaluated using the suggestions by ISRM. Moreover, the effects of horizon size and material points spacing on crack propagation in Brazilian Disks are investigated to prove the accuracy of NOSB-PD. The present numerical results are in good agreement with the previous experimental and numerical results.

Comparative effects of Al-Si and galvannealed coatings on the properties of resistance spot welded hot stamping steel joints

Al-Si coated and galvannealed (GA) hot stamping steels were resistance spot welded, and the welding current, properties and mechanical performance of the joints produced were compared. The coating significantly affected the welding current required for attaining the conventionally acceptable weld fusion zone size of 4t, and the welding current range. The coating also influenced the energy absorption and failure mode transition but had no effect on the peak failure load provided the stack consisted of sheets having similar coating. For dissimilar combination of Al-Si and GA coated sheet stack, the joint mechanical performance was significantly deteriorated.

Investigation and improvement in peak failure load of hot staked joints in starter motor armatures using complementing problem-solving tools

Hot staking is the preferred process for conductor-commutator joints in the DC motor armatures as it is versatile and cost effective. But however its application is limited by the peak failure load of the joint which depends only on the mechanical contact as there is no amalgamation or diffusion of metals in the faying surface. This study investigates the mechanical joint strength and found its variation has a positional pattern which is significantly related to similar pattern in electrode heat. A hypothesis for the similarity is reasoned and validated, which enabled evolution of a feasible solution to have differential input of energy compensating the variation in heat. This resulted in overcoming the limitation of joint strength in introduction of hot staked armatures to new applications. The use of Shainin clue generating tool, Relations diagram and visualizing the physical mechanism complemented the problem solving approach in causal clue exploration, hypothesis reasoning and solution development.

Evaluation of the Load-bearing Capacity of Fractured Incisal Edge of Maxillary Permanent Central Incisors restored with a Glass Fiber-reinforced Nanocomposite: An in vitro Study.

ABSTRACTObjectives: The aim of this study was to evaluate and compare the load-bearing capacity of fractured incisal edge of maxillary permanent central incisors restored with a nanocomposite and a glass fiber-reinforced nanocomposite.Materials and methods: Thirty-six extracted sound maxillary central incisors randomly divided in three groups were used for the present study. Group I (control) contained untreated teeth. Samples in experimental groups II and III were prepared by cutting the incisal (one-third) part of the crown horizontally and subjected to enamel preparations and restored with a nanocomposite and a glass fiber-reinforced nanocomposite respectively. All restored teeth were stored in distilled water at room temperature for 24 hours. Fracture resistance was evaluated as peak load at failure (Newton) for samples tested in a cantilever-bending test using Hounsfield universal testing machine. Failure modes were microscopically examined.Results: Highest mean peak failure load (Newton) among experimental groups was observed in glass fiber-reinforced nano composite group (863.50 ± 76.12 N) followed by nanocomposite group (633.67 ± 40.14 N). One-way analysis of variance (ANOVA) revealed that the restoration technique significantly affected the load-bearing capacity (p < 0.001). Scheffe’s post-hoc comparison test (subset for α = 0.05) revealed that there was significant difference in the mean peak failure load values of nanocomposite and glass fiber-reinforced nanocomposite groups when considered together (p < 0.001). Experimental groups showed similar types of failure modes with majority occurring ascohesive and mixed type. Fifty-eight percent of the teeth in glass fiber-reinforced nanocomposite group fractured below the cementoenamel junction.Conclusion: By using fiber-reinforced composite substructure under conventional composites in the repair of fractured incisors, the load-bearing capacity of the restored incisal edge could be substantially increased.How to cite this article: Kumar PSP, Srilatha KT, Nandlal B, Dhull KS. Evaluation of the Load-bearing Capacity of Fractured Incisal Edge of Maxillary Permanent Central Incisors restored with a Glass Fiber-reinforced Nanocomposite: An in vitro Study. Int J Clin Pediatr Dent 2015;8(1):22-29.

Influence of Etching Protocol and Silane Treatment with a Universal Adhesive on Lithium Disilicate Bond Strength.

To measure the effects of hydrofluoric acid (HF) etching and silane prior to the application of a universal adhesive on the bond strength between lithium disilicate and a resin. Sixty blocks of lithium disilicate (e.max CAD, Ivoclar Vivadent) were sectioned into coupons and polished. Specimens were divided into six groups (n=10) based on surface pretreatments, as follows: 1) no treatment (control); 2) 5% HF etch for 20 seconds (5HF); 3) 9.5% HF etch for 60 seconds (9.5HF); 4) silane with no HF (S); 5) 5% HF for 20 seconds + silane (5HFS); and 6) 9.5% HF for 60 seconds + silane (9.5HFS). All etching was followed by rinsing, and all silane was applied in one coat for 20 seconds and then dried. The universal adhesive (Scotchbond Universal, 3M ESPE) was applied onto the pretreated ceramic surface, air thinned, and light cured for 10 seconds. A 1.5-mm-diameter plastic tube filled with Z100 composite (3M ESPE) was applied over the bonded ceramic surface and light cured for 20 seconds on all four sides. The specimens were thermocycled for 10,000 cycles (5°C-50°C/15 s dwell time). Specimens were loaded until failure using a universal testing machine at a crosshead speed of 1 mm/min. The peak failure load was used to calculate the shear bond strength. Scanning electron microscopy images were taken of representative e.max specimens from each group. A two-way analysis of variance (ANOVA) determined that there were significant differences between HF etching, silane treatment, and the interaction between HF and silane treatment (p<0.01). Silane treatment provided higher shear bond strength regardless of the use or concentration of the HF etchant. Individual one-way ANOVA and Tukey post hoc analyses were performed for each silane group. Shear bond strength values for each etch time were significantly different (p<0.01) and could be divided into significantly different groups based on silane treatment: no silane treatment: 0 HF < 5% HF < 9.5% HF; and RelyX silane treatment: 0 HF < 5% HF and 9.5% HF. Both HF and silane treatment significantly improved the bond strength between resin and lithium disilicate when used with a universal adhesive.

Clinical and Biomechanical Evaluation of an All-Arthroscopic Suprapectoral Biceps Tenodesis.

Background:Pathology of the long head of the biceps (LHB) is a well-recognized cause of shoulder pain in the adult population and can be managed surgically with tenotomy or tenodesis.Purpose:To compare the biomechanical strength of an all-arthroscopic biceps tenodesis technique that places the LHB distal to the bicipital groove in the suprapectoral region with a more traditional mini-open subpectoral tenodesis. This study also evaluates the clinical outcomes of patients who underwent biceps tenodesis using the all-arthroscopic technique.Study Design:Controlled laboratory study and case series; Level of evidence, 4.Methods:For the biomechanical evaluation of the all-arthroscopic biceps tenodesis technique, in which the biceps tendon is secured to the suprapectoral region distal to the bicipital groove with an interference screw, 14 fresh-frozen human cadaveric shoulders (7 matched pairs) were used to compare load to failure and displacement at peak load with a traditional open subpectoral location. For the clinical evaluation, 49 consecutive patients (51 shoulders) with a mean follow-up of 2.4 years who underwent an all-arthroscopic biceps tenodesis were evaluated using the American Shoulder and Elbow Surgeons (ASES) score preoperatively and at last follow-up, as well as the University of California, Los Angeles (UCLA) Shoulder Score at last follow-up.Results:On biomechanical evaluation, there was no significant difference in peak failure load, displacement at peak load, or displacement after cyclic testing between the arthroscopic suprapectoral and mini-open subpectoral groups. In the clinical evaluation, the mean preoperative ASES score was 65.4, compared with 87.1 at last follow-up. The mean UCLA score at last follow-up was 30.2. Forty-eight (94.1%) patients reported satisfaction with the surgery. In subgroup analysis comparing patients who had a rotator cuff repair or labral repair at time of tenodesis with patients who did not have either of these procedures, there were no significant differences in UCLA or ASES scores.Conclusion:The excellent biomechanical strength as well as the high rate of satisfaction after surgery and high ASES and UCLA postoperative scores make this technique a novel option for treatment of biceps tendon pathology.

Three-point bending test investigation of the fracture behavior of siltstone after thermal treatment

The influence of temperature on the fracture behavior of siltstone is investigated in detail by mode I fracture toughness tests under three-point bending in situ SEM observations. A total of 27 specimens subjected to thermal pre-treatment have been tested. Experimental results indicate that effects of temperature on siltstone fracture behavior are obvious, not only on failure mechanism, but also on mechanical parameters like peak failure loads, fracture toughness and modulus of elasticity. The failure mechanism changes from intergranular fracture to mixed intergranular and transgranular fractures, and finally to intergranular fracture and thermal cracking with temperature from 25 to 60°C. Fracture toughness KIC decreases slightly from room temperature 25 to 100°C, and then increases significantly from 100 to 125°C, and then gradually declines from 125 to 600°C. A new numerical elastic modulus estimation method is proposed, considering a series of fluctuated experimental data. The variation of the elastic modulus with the temperature is similar with that of fracture toughness.

Biomechanical Evaluation of a Knotless Barbed Suture Repair in a Human Achilles Tendon Rupture Model

Knotless barbed sutures are seeing more common use in wound closures and small tendon repairs. The purpose of this study was to evaluate the strength and resistance to gap formation of these sutures in larger tendons, such as the Achilles, compared with conventional repair. Six matched pairs of fresh-frozen human cadaver Achilles tendons were transected to simulate a rupture. Six tendons were repaired using a double Bunnell over-the-top technique with Fiberwire, and the matched pairs were repaired using a double Bunnell knotless technique with barbed suture. The tendons were tested to failure and stiffness, peak failure load, percentage elongation, load at initial gap and load at 5-mm gap (clinical failure) were compared. The mean peak failure load for the Fiberwire group (459 N) was significantly higher (P = .029) compared with the barbed suture group (184 N). Percentage elongation at peak load was significantly lower in the barbed suture group (P = .014), as was percentage elongation at initial gapping (P = .007) and percent elongation at 5 mm of gapping (P = .004). While the knotless barbed suture is attractive for its design and resistance to gap formation, low failure loads compared with conventional suture are concerning for large tendon repairs. Therapeutic, Level IV: Bench testing.

Biomechanical and anatomical considerations in lumbar spinous process fixation—an in vitro human cadaveric model

Background contextAlthough multiple mechanisms of device attachment to the spinous processes exist, there is a paucity of data regarding lumbar spinous process morphology and peak failure loads. PurposeUsing an in vitro human cadaveric spine model, the primary objective of the present study was to compare the peak load and mechanisms of lumbar spinous process failure with variation in spinous process hole location and pullout direction. A secondary objective was to provide an in-depth characterization of spinous process morphology. Study designBiomechanical and anatomical considerations in lumbar spinous process fixation using an in vitro human cadaveric model. MethodsA total of 12 intact lumbar spines were used in the current investigation. The vertebral segments (L1–L5) were randomly assigned to one of five treatment groups with variation in spinous process hole placement and pullout direction: (1) central hole placement with superior pullout (n=10), (2) central hole placement with inferior pullout (n=10), (3) inferior hole placement with inferior pullout (n=10), (4) superior hole placement with superior pullout (n=10), and (5) intact spinous process with superior pullout (n=14). A 4-mm diameter pin was placed through the hole followed by pullout testing using a material testing system. As well, the bone mineral density (BMD) (g/cm3) was measured for each segment. Data were quantified in terms of anatomical dimensions (mm), peak failure loads (newtons [N]), and fracture mechanisms, with linear regression analysis to identify relationships between anatomical and biomechanical data. ResultsBased on anatomical comparisons, there were significant differences between the anteroposterior and cephalocaudal dimensions of the L5 spinous process versus L1–L4 (p<.05). Statistical analysis of peak load at failure of the four reconstruction treatments and intact condition demonstrated no significant differences between treatments (range, 350–500 N) (p>.05). However, a significant linear correlation was observed between peak failure load and anteroposterior and cephalocaudal dimensions (p<.05). Correlation between BMD and peak spinous processes failure load was approaching statistical significance (p=.08). 30 of 54 specimens failed via direct pullout (plow through), whereas 8 of 54 specimens demonstrated spinous process fracture. The remaining cases failed via plow through followed by fracture of the spinous process (16 of 54; 29%). ConclusionsThe present study demonstrated that variation in spinous process hole placement did not significantly influence failure load. However, there was a strong linear correlation between peak failure load and the anteroposterior and cephalocaudal anatomical dimensions. From a clinical standpoint, the findings of the present study indicate that attachment through the spinous process provides a viable alternative to attachment around the spinous processes. In addition, the anatomical dimensions of the lumbar spinous processes have a greater influence on biomechanical fixation than either hole location or BMD.

Advanced Age Diminishes Tendon-to-Bone Healing in a Rat Model of Rotator Cuff Repair

Background: Advanced patient age is associated with recurrent tearing and failure of rotator cuff repairs clinically; however, basic science studies have not evaluated the influence of aging on tendon-to-bone healing after rotator cuff repair in an animal model. Hypothesis/Purpose: This study examined the effect of aging on tendon-to-bone healing in an established rat model of rotator cuff repair using the aged animal colony from the National Institute on Aging of the National Institutes of Health. The authors hypothesized that normal aging decreases biomechanical strength and histologic organization at the tendon-to-bone junction after acute repair. Study Design: Controlled laboratory study. Methods: In 56 F344xBN rats, 28 old and 28 young (24 and 8 months of age, respectively), the supraspinatus tendon was transected and repaired. At 2 or 8 weeks after surgery, shoulder specimens underwent biomechanical testing to compare load-to-failure and load-relaxation response between age groups. Histologic sections of the tendon-to-bone interface were assessed with hematoxylin and eosin staining, and collagen fiber organization was assessed by semiquantitative analysis of picrosirius red birefringence under polarized light. Results: Peak failure load was similar between young and old animals at 2 weeks after repair (31% vs 26% of age-matched uninjured controls, respectively; P > .05) but significantly higher in young animals compared with old animals 8 weeks after repair (86% vs 65% of age-matched uninjured controls, respectively; P < .01). Eight weeks after repair, fibroblasts appeared more organized and uniformly aligned in young animals on hematoxylin and eosin slides compared with old animals. Collagen birefringence analysis of the tendon-to-bone junction demonstrated that young animals had increased collagen fiber organization and similar histologic structure compared with age-matched controls (53.7 ± 2.4 gray scales; P > .05). In contrast, old animals had decreased collagen fiber organization and altered structure compared with age-matched controls (49.8 ± 3.1 gray scales; P < .01). Discussion: In a rat model of aging, old animals demonstrated diminished tendon-to-bone healing after rotator cuff injury and repair. Old animals had significantly decreased failure strength and collagen fiber organization at the tendon-to-bone junction compared with young animals. This study implies that animal age may need to be considered in future studies of rotator cuff repair in animal models. Clinical Relevance: With increasing age and activity level of the population, the incidence of rotator cuff tears is predicted to rise. Despite advances in rotator cuff repair technique, the retear rate remains specifically high in elderly patients. The findings of this research suggest that aging negatively influences tendon-to-bone healing after rotator cuff repair in a validated animal model.

Primary stability of two uncemented acetabular components of different geometry: hemispherical or peripherally enhanced?

ObjectiveThis study compared the primary stability of two commercially available acetabular components from the same manufacturer, which differ only in geometry; a hemispherical and a peripherally enhanced design (peripheral self-locking (PSL)). The objective was to determine whether altered geometry resulted in better primary stability.MethodsAcetabular components were seated with 0.8 mm to 2 mm interference fits in reamed polyethylene bone substrate of two different densities (0.22 g/cm3 and 0.45 g/cm3). The primary stability of each component design was investigated by measuring the peak failure load during uniaxial pull-out and tangential lever-out tests.ResultsThere was no statistically significant difference in seating force (p = 0.104) or primary stability (pull-out p = 0.171, lever-out p = 0.087) of the two components in the low-density substrate. Similarly, in the high-density substrate, there was no statistically significant difference in the peak pull-out force (p = 0.154) or lever-out moment (p = 0.574) between the designs. However, the PSL component required a significantly higher seating force than the hemispherical cup in the high-density bone analogue (p = 0.006).ConclusionsHigher seating forces associated with the PSL design may result in inadequate seating and increased risk of component malpositioning or acetabular fracture in the intra-operative setting in high-density bone stock. Our results, if translated clinically, suggest that a purely hemispherical geometry may have an advantage over a peripherally enhanced geometry in high density bone stock.Cite this article: Bone Joint Res 2013;2:264–9.

The changes in flexural properties and microstructures of carbon fiber bismaleimide composite after exposure to a high temperature

Recent increase in the use of carbon fiber reinforced polymer composites, especially for high temperature applications, has created new challenges to predict their service life. This paper examines the changes in weight loss and flexural properties along with the changes in microstructures of unidirectional carbon fiber reinforced bismaleimide with [016] and [9016] specimens, after exposure to 260°C for 3000h in air. The percentage of fiber end area exposed to air in a specimen end section significantly influences the extent of these changes. The [9016] specimens have an order of magnitude higher fiber open ends than the [016] specimens, and their weight loss rate and loss in flexural modulus and strength are significantly higher than that of the [016] specimens. In three point bending, interlaminar shear leads to delamination preceding the final failure in compressive mode. A viscoelastic cohesive layer model has been implemented to simulate interlaminar delamination. Viscoelastic regularization of the constitutive equations of the cohesive layer used in this model not only mitigates numerical instability, but also predicts load–deflection behavior beyond peak failure load. The model is in a good agreement with experimental results and has been able to simulate the delamination failure successfully.

Biomechanical Evaluation of Physeal-Sparing Fixation Methods in Tibial Eminence Fractures

Background: Tibial eminence fractures occur most commonly in skeletally immature children. Several techniques using physeal-sparing fracture fixation have been described, but their structural properties have not been evaluated. Purpose: To determine the strength and resistance to displacement of physeal-sparing techniques used to fix tibial eminence fractures. Study Design: Controlled laboratory study. Methods: Skeletally immature porcine knees were randomized into 4 treatment groups: (1) ultra–high molecular weight polyethylene suture–suture button (UHMWPE/SB), (2) suture anchor, (3) polydioxanone suture–suture button (PDS/SB), and (4) screw fixation. A prospective analysis of bone mineral density using dual-energy x-ray absorptiometry was performed on all specimens. Fracture fragments were created in a standardized manner and measured for size comparison. After fracture fixation, biomechanical testing was performed with cyclical and load-to-failure protocols by loading the tibia with an anterior shear force. Results: In load-to-failure testing, screw fixation had a significantly lower median peak failure load (186.4 N; lower quartile [LQ], 158.4 N; upper quartile [UQ], 232.6 N) than did UHMWPE/SB (465.8 N; LQ, 397.8 N; UQ, 527.8 N), suture anchors (440.5 N; LQ, 323.0 N; UQ, 562.3 N), and PDS/SB (404.3 N; LQ, 385.9 N; UQ, 415.6 N). UHMWPE/SB demonstrated a significantly higher median yield load (465.8 N; LQ, 397.8 N; UQ, 527.8 N) than did PDS/SB (306.7 N; LQ, 271.4, N; UQ, 405.7 N) and screw fixation (179.0 N; LQ, 120.2 N; UQ, 232.5 N). During cyclical testing, screw fixation demonstrated significantly lower percentage survival of specimens (0%) compared with the other groups (UHMWPE/SB, 100%; suture anchor, 78%; PDS/SB, 78%). After 1000 cycles of loading, PDS/SB fixation had significantly more median creep (6.76 mm; LQ, 6.34 mm; UQ, 8.28 mm) than did UHMWPE/SB (4.43 mm; LQ, 3.80 mm; UQ, 4.73 mm) and suture anchor fixation (3.06 mm; LQ, 2.59 mm; UQ, 4.28 mm). The lowest median stiffness was observed in the PDS/SB group (48.6 N/mm; LQ, 45.3 N/mm; UQ, 54.2 N/mm). UHMWPE/SB fixation demonstrated a significantly higher median peak failure load after cyclic testing (469.0 N; LQ, 380.6 N; UQ, 507.2 N) than did PDS/SB (237.7 N; LQ, 197.3 N; UQ, 298.3 N) and screw fixation (132.4 N; LQ, 123.7 N; UQ, 180.9 N). Suture anchor fixation had significantly more variance, as demonstrated by width of interquartile range, in peak failure load, yield load, and creep than did other techniques. Conclusion: Physeal-sparing fixation of tibial eminence fractures with UHMWPE suture–suture button is biomechanically superior to both PDS suture–suture button and a single screw at the time of surgery and provides more consistent fixation than do suture anchors. Clinical Relevance: Suture anchors provide inconsistent fixation for tibial eminence fractures.

Evaluation and comparison of the effect of enamel preparation designs on fracture resistance of micro-filled and nano-filled composite resin: Anin vitrostudy

Background and Objectives: The main objective of this study was to compare and evaluate the effect of enamel preparation designs; bevel and stair step chamfer, on fracture resistance of composite resins (micro-filled and nano-filled) using incremental technique. Materials and Methods: The sample group consisted of 60 non-carious maxillary permanent central incisors. They were divided into four groups of 15 teeth each. According to the group, the teeth were prepared and restored with corresponding design and material. The teeth were subjected to compression bending test under instron testing machine then were examined under stereomicroscope. The results were subjected to statistical analysis. Results: Mean peak failure load (Newton) of among experimental groups was observed in the order; Group I (Bevel with micro-filled material) was 255.6 ± 65.0 N and Group II (bevel with nano-filled material) l was 282.1 ± 67.7 N. Group III (Stair-step chamfer with micro-filled material) was 249.8 ± 78.0 N. and Group IV (Stair-step chamfer with nano-filled material) was 283.3 ± 52.0 N. Results of the analysis of variance revealed there was no significant difference in the mean peak failure load values of the four different groups. Failure mode evaluation revealed, majority of the failures occurred as cohesive and mixed type for all the experimental groups. Adhesive type failure was observed maximum (47%) in the bevel group. Stair-step chamfer preparation showed the greatest potential for application, but also involved sacrificing less amount of tooth structure adjacent to fractured edge. Conclusion: This study concludes that both the preparations, bevel as well as stair-step chamfer are equally good to resist fracture when restored either with micro-filled or nano-filled composite resins.

The Overlay Tester (OT): Using the Fracture Energy Index concept to analyze the OT monotonic loading test data

In this study, the concept of the Fracture Energy (FE) Index was explored as a fracture parameter to characterize and differentiate the cracking resistance potential of hot-mix asphalt (HMA) subjected to the Overlay Tester (OT) test under monotonic loading in the laboratory; denoted herein as the OTM test. Mathematically, the FE Index was derived and defined as a parametric ratio of the total FE to the HMA tensile strength and tensile strain at peak failure load per unit crack length under the OT monotonic loading testing, with a tensile loading rate of 3.125mm/min at a test temperature of 25°C. Various HMA mixes, commonly used in Texas, with cracking resistance potential ranging from poor to good, were evaluated. Corresponding results indicated that the FE Index has promising potential to be used as a fracture parameter to discriminate and rank the cracking resistance potential of HMAs in the laboratory. While variability was generally within the acceptable tolerance (i.e., coefficient of variation of less than 30%), the FE Index exhibited sensitivity to the mix-design variables and was able to capture the effects of changes in the asphalt-binder content and temperature variations. Thus, this concept should be investigated further with more mixes and validated with field data for subsequent consideration in routine mix-designs and screening for adequate HMA cracking resistance in the laboratory.

A mechanism-based multi-scale model for predicting thermo-oxidative degradation in high temperature polymer matrix composites

This paper describes a mechanism-based multi-scale model for life prediction of high temperature polymer matrix composites (HTPMC) under thermo-oxidative aging conditions. The multi-scale model incorporates molecular level damage such as inter-crosslink chain scission in a thermoset polymer due to thermo-oxidative aging of the polymer resin. The degradation of inter-laminar stress depends on remaining inter-crosslink density of thermo-set polymer in fiber/matrix interface region subjected to thermo-oxidative aging environment. The degradation of inter-laminar shear stress of thermo-oxidatively aged unidirectional IM-7/PETI-5 composite specimens at 300 °C was modeled using an in-house test-bed FEA code (NOVA-3D). A micromechanics based viscoelastic cohesive layer model was used to model delamination. The model is fully rate dependent and does not require a pre-assigned traction-separation law. Viscoelastic regularization of the constitutive equations of the cohesive layer used in this model not only mitigates numerical instability, but also enables the analysis to follow load–deflection behavior beyond peak failure load. The model was able to successfully simulate delamination failure in thermo-oxidatively aged unidirectional IM-7/PETI-5 composite, and the model predictions were verified using test data.

Augmentation of Achilles Tendon Repair With Extracellular Matrix Xenograft

Background: Achilles tendon rupture is a frequent injury in athletes and the general public. Cases of chronic rupture or poor tendon quality secondary to tendinopathy are challenging to repair primarily. Commercially available extracellular matrix materials have been utilized in recent years to augment tendon repair. Hypothesis: Augmentation of Achilles tendon with extracellular matrix xenograft results in reduced repair site gapping and increased peak failure load in a cadaveric model featuring simulated physiologic loads. Study Design: Controlled laboratory study. Methods: Ten matched pairs of fresh-frozen human lower extremities amputated just below the knee were obtained and each Achilles tendon was sharply tenotomized. One randomly selected specimen from each matched pair underwent Achilles repair using a 4-strand Krackow technique with extracellular matrix xenograft augmentation (TissueMend Soft Tissue Repair Matrix), while the opposite tendon underwent suture repair alone as a control. Each tendon was then subjected to 1000 sinusoidal tensile loading cycles to 86 N during which repair site gapping was monitored, followed by distraction to failure. One pair was used to evaluate the effects of graft orientation and not included in the analysis. Results: Significantly less gapping was noted in the augmented tendon group at all time points after the 10th load cycle (P < .05). The mean repair site gapping after 1000 cycles of loading was 4.0 mm (range, 3.1-5.0 mm) in the augmented group and 6.5 mm (range, 4.1-8.6 mm) in the suture-only group. The ultimate failure load was 821 N (range, 613-1021 N) in the augmented group and 392 N (range, 322-481 N) in the suture-only group (P < .01). Conclusion: The augmentation of Achilles tendon repair with extracellular matrix xenograft decreases gapping and increases load to failure immediately after surgery in a cadaveric model. Clinical Relevance: Tendon repair augmentation may allow more aggressive early rehabilitation, particularly in cases of chronic rupture or poor tendon quality. Further work is necessary to define indications for extracellular matrix graft augmentation of tendon repairs.

The Influence of Castration on Collagen Expression and Anterior Cruciate Ligament Strength in Rats

Sex specific responses to specific steroid sex hormones have been suggested as a potential cause for the disparate anterior cruciate ligament (ACL) injury rate between male and females athletes. Type-I (T1C) and type-3 collagen (T3C) are crucial structural components that define the ligament's ability to withstand tensile loads. Transcription of mRNA is an important mediator of downstream collagen synthesis and remodeling. Sex differences in collagen mRNA expression and ACL strength have been reported but the specific impact of testosterone on collagen mRNA expression and ACL strength is not clear. PURPOSE: To examine the influence of castration on T1C and T3C mRNA expression and mass normalized peak failure load in the ACLs of skeletally mature male rats METHODS: Seventeen 12 week old male Sprague-Dawley rats were separated into castrated (C) (389.1 + 25.9g) or non-castrated control (CNTL) groups (417.7 + 35.4g). After 16 days rats were euthanized and real time polymerase chain reaction was used to determine T1C and T3C mRNA expression and to calculate the T1C:T3C ratio in tissue from the right ACL. The left ACL was isolated and attached to a hydraulic materials testing device in order to measure ACL load to failure normalized for animal weight. Non-parametric Wilcoxon rank sum tests were used to compare the means of mRNA expression and failure load between the C and CNTL groups. RESULTS: Castrated rats had lower expression of T3C mRNA (p=0.001), a higher T1C:T3C ratio (p=0.015) and higher load to failure (p=0.021) than CNTL rats. The C rats also had a higher expression of T1C mRNA that approached significance (p=0.066). CONCLUSIONS: Castrated male rats had significantly higher T3C mRNA expression, the ratio of T1C:T3C collagen mRNA expression and downstream ACL load to failure in male Sprague-Dawley rats. Supported by the National Institute of Child Health and Human Development Grant 5 K12 HD-043489, National Institute of Arthritis and Musculoskeletal and Skin Diseases Grant 1R03AR54573-1A1

Sex, Collagen Expression, and Anterior Cruciate Ligament Strength in Rats

Sex-specific responses to steroid sex hormones have been suggested as a potential cause for the disparate anterior cruciate ligament (ACL) injury rates between male and female athletes. Type 1 collagen (T1C) and type 3 collagen (T3C) are crucial structural components that define the ligament's ability to withstand tensile loads. Messenger RNA (mRNA) is an important mediator of downstream collagen synthesis and remodeling, but the sex-specific mechanisms of collagen mRNA expression and ACL strength are unknown. To examine the influence of sex on T1C and T3C mRNA expression and mass-normalized stiffness and peak failure load in the ACLs of skeletally mature rats. Observational study. Basic sciences and biomechanical testing laboratories. Nineteen 12-week-old male (n = 9) and female (n = 10) Sprague Dawley rats. We used real-time polymerase chain reaction to determine T1C and T3C mRNA expression and a hydraulic materials testing device to measure ACL stiffness and failure load. Nonparametric Wilcoxon rank sum tests were used to compare the groups. Female rats had lower amounts of T3C mRNA expression and higher normalized ACL tangent stiffness and failure load than male rats. These findings suggest that sex-specific differences in T1C and T3C mRNA expression may play an important role in the downstream mechanical properties of the ACL.

Size reduction of high- and low-moisture corn stalks by linear knife grid system

High- and low-moisture corn stalks were tested using a linear knife grid size reduction device developed for first-stage size reduction. The device was used in conjunction with a universal test machine that quantified shearing stress and energy characteristics for forcing a bed of corn stalks through a grid of sharp knives. No published engineering performance data for corn stover with similar devices are available to optimize performance; however, commercial knife grid systems exist for forage size reduction. From the force–displacement data, mean and maximum ultimate shear stresses, cumulative and peak mass-based cutting energies for corn stalks, and mean new surface area-based cutting energies were determined from 4–5 refill runs at two moisture contents (78.8% and 11.3% wet basis), three knife grid spacings (25.4, 50.8, and 101.6 mm), and three bed depths (50.8, 101.6, and 152.4 mm). In general, the results indicated that peak failure load, ultimate shear stress, and cutting energy values varied directly with bed depth and inversely with knife grid spacing. Mean separation analysis established that high- and low-moisture conditions and bed depths ≥ 101.6 mm did not differ significantly ( P < 0.05) for ultimate stress and cutting energy values, but knife grid spacing were significantly different. Linear knife grid cutting energy requirements for both moisture conditions of corn stalks were much smaller than reported cutting energy requirements. Ultimate shear stress and cutting energy results of this research should aid the engineering design of commercial scale linear knife gird size reduction equipment for various biomass feedstocks.