The Need for Speed

In the race to get products to market, does risk-mitigation get enough time in the winner’s circle?

indy-blog-cover-imageWhat do aerospace, medical device manufacturers, and auto racing all have in common?  Answer: the need to minimize risk of premature/unexpected component failure while crossing the finish line first.  While these industries each have vastly different stakeholders, goals, and success metrics, all look to avoid costly breakdowns in the field.  And speed is key.  Being the first across the finish line in auto racing gives you the largest share of the purse, not to mention first choice of lucrative endorsement deals.  Being the first to market with an innovative or more reliable medical implant or a lighter aircraft component helps in marketing, product launch success, or company profitability and growth.  However, pushing the design limits to gain this speed advantage must be weighed against the possible failure of the component in an unforeseen manner.

Speed in Design

Auto racing is one of the world’s most expensive sporting endeavors.  A recent USA Today article puts the price tag of prepping and running a car in the Indianapolis 500 at nearly $1 million, and that is already assuming that the car is owned outright.  While the bulk of this cost is sunk into parts, staffing, and off-track expenses, a not-insignificant 4.5% of that is spent on controlled testing.  For example, one day at a rolling wind tunnel costs $35,000…more than the MSRP of the average production vehicle on US roadways today.  While the finances of auto racing and the commercial automotive industry may differ, their goals are similar: to create lighter-weight components that will aid (or at least not hinder) aerodynamic performance.  Designers are constantly being tasked with pushing the envelopes of their designs, while still attempting to maintain reliability and risk targets.  These designs, in turn, lead to more expensive and detailed manufacturing/machining techniques and the use of more exotic material alloys.  The uncertainties in every design usually manifest themselves as restrictive knock-down or safety factors that inevitably detract from performance.  Governing bodies in the various auto racing categories (F1, NASCAR, drag racing, to name a few) place additional restrictions in the form of specification limits on components such as engines, body shapes, and spoilers to maintain competitive balance.  Regardless of the type of restriction, if a way to reduce the uncertainty in a design is found, it can be advantageous.  VEXTEC’s Virtual Life Management (VLM) simulation technology can be that solution.  Through rigorous computational analysis of design, load-induced stress, and material, VLM can efficiently identify and quantify those design uncertainties.  VEXTEC has provided VLM support to many of the industry’s leading manufacturers of heavy duty engine connecting rods, engine blocks, and turbochargers.  This new insight has offered engineers the ability to understand where they really are on their design envelope, and how far they can push certain parameters, even before the first test piece is built.

Speed in Optimizing Maintenance

Weight savings and aerodynamics are, arguably, even more critical in the aerospace industry, where the civilian maintenance repair & overhaul (MRO) market is expected to be $56 billion this year.  Engine maintenance alone will take up about 40% of this valuation.  The cost of an unexpected catastrophic failure is much higher here than in the auto world.  But in order to reduce this risk, aircraft must be maintained and repaired.  And while they’re being maintained, they are not in the air delivering passengers, hauling freight, or making money.  So minimizing the downtime is crucial to keeping viable profit margins. VEXTEC has partnered extensively with civil and military aircraft users, employing VLM on a multitude of issues including: unitized wing structures (US Air Force), certifying weld-repaired engine blades (EB Airfoils) and resolving premature bearing failure (American Airlines).  The bearing study, for example, saved American Airlines about $4 million per year by avoiding the repair/replacement of their APU bearings.  The results from these and other studies provide our clients with knowledge they would not have otherwise been able to acquire, and allow for sound financial decisions to be made on fielded components.

Speed in Reliability

One of the fastest-changing industries is the medical device industry.  Technology is racing forward, minimizing invasiveness is driving the miniaturization of implantable devices (especially in heart rhythm monitors), while manufacturing methods are still trying to catch-up.  It seems that no other industry is as heavily scrutinized in terms of reliability and risk, at least in public perception. Medical implants are exposed to harsh internal environments, unpredictable stress and strain cycles, and oftentimes difficult installation procedures.  Yet these devices are counted-on to reliably elevate our quality of life on a daily basis.  The variability observed in material, vendor supply, and manufacturing all play a part in the reliability of the components that make up a medical device.  Through industry-directed capability studies, the VLM technology pioneered by VEXTEC has effectively modeled these sources of variability, virtually tested millions of components, and delivered reliability answers to as many “what-if” scenarios as design and materials engineers saw fit to explore.  The VLM approach reduces the number of blind alleys (ineffective combinations of material, design, and operational limits) that companies would have to travel down through the traditional design-build-test method, and focuses internal R&D resources on the combinations most likely to succeed in both manufacturing cost and operational reliability.

These three high-risk/high-reward sectors are not the only sectors that have benefited from VLM technology.  Indeed, any company looking to speed-up their design phase, reduce their warranty reserves, or just wanting to make more-informed decisions on how their products can best be sourced, manufactured, and used would benefit from a conversation with us.  The green flag has dropped…where are you in the field?

VEXTEC: Meeting the Need for Speed.

Structural Design Concepts: Safe Life, Damage Tolerance and VEXTEC’S Virtual Life Management® (VLM®)

Computer_Structural_Design_OverviewOver the last few weeks, VEXTEC has explored the two main philosophies of modern structural design: safe life design and damage tolerant design.  Today we share with you a summary of these methods, including their benefits and limitations. We will also look at other considerations that are commonly encountered when implementing either method in a manufacturing environment, and how VEXTEC’s Virtual Life Management® (VLM®) technology can help.

Safe Life Design

Safe life design is a common method to predict the durability of a design in many industries, including automotive, pressure vessels, bearings, and portions of jet engines and aircraft landing gear.  It is employed by manufacturers when it is understood that regular structural inspection of their products would not be possible or practical.  Components are removed from service when their calculated safe life expires, regardless of the condition of the component.

Safe life is based on laboratory testing of simple specimens that are cyclically-loaded to create test points in a S-N curve.  Damage initiates naturally within the material microstructure and grows to final failure.  The resulting S-N curve has three major limitations: 1) due to the large scatter in lifetime, many tests must be performed to get a statistically-confident dataset; 2) the simple test specimens are not similar to the design of complex components (lack of similitude); and 3) the analysis usually considers only the peak stress that the component should see.  The design method does not account for any unforeseen rogue flaws in the material, nor rare stress spikes in operation.  Thus, large safety factors are applied to the design allowable life to theoretically ensure that any naturally-initiated damage will not pose a threat to the acceptable reliability for the life of the structure.  The safety factors may be in terms of life or load, or in the case of pressure vessels, both life and load.  While the goal of safety factors (or ‘knockdown factors’ or ‘margins of safety’, as they are also known) is admirable, their implementation is arbitrary at best.  Using them leads to structures that are heavy and overly conservative, and add to the cost of manufacturing (in terms of necessary raw material, processing, and machining).  There is no standardized way of implementing these factors across industries, because institutional knowledge will often trump hard physics when it comes to design.

Damage Tolerant Design

Damage tolerant design is a methodology which uses the physics-based principles of fracture mechanics (in particular, the stress intensity factor) to account for and measure the initiation and growth of damage in a structure in operational service.  Industries which employ this methodology include military and civil aerospace and rotorcraft, heavy duty cranes, and bridges.  Manufacturers who use this method can design lighter products or more severe loading cycles, as routine periodic inspection protocols are used to observe, detect, and repair fatigue cracks before reaching critical sizes.

Traditional fracture mechanics has less scatter in the developing of a S-N curve as opposed to safe life design, and can account for the geometry and stress distributions of complex designs.  Residual stress can also be accounted for in the stress intensity factor, and complex load missions can also be assessed.  The main limitation in damage tolerant design is that a crack must be assumed to be present in the component because fracture mechanics predicts the cycles for the crack to grow from one size to another. Linear elastic fracture mechanics, the most commonly used form of fracture mechanics, assumes the crack is large relative to the material’s microstructure.  By assuming this initial crack size, the percent of the lifetime that would have actually been spent developing that initial crack in reality is not accounted for in the durability of the design.  This conservative assumption can greatly reduce the design allowable life.  Advanced fracture mechanics methods have been developed in recent decades to both reduce the size of the assumed initial crack and to understand fatigue crack closure effects.  Additional testing to develop these methods can prove to be expensive, and the cost of these paired with the additional costs of routine end user inspections must be weighed during the design and manufacturing processes.

VLM-Assisted Design

VEXTEC’s VLM technology can reduce assumptions and cost in either type of design, or can be used to combine the best aspects of safe life with damage tolerance to create a “total life” approach:

a) In safe life design, VLM can be used to predict the stress and strain vs. fatigue life (S-N curve) test results of simple test specimens, as well as components with complex geometry and loading.  As VLM uses finite element methods, the simple and complex geometries drive the differences in the fatigue results (using the same set of empirical material parameters), so the similitude issue in safe life design is avoided.  Virtual Life Management can be implemented without the need for large safety factors, as it has been shown to predict the mean and scatter in fatigue life of a final component’s design.  Virtually testing many designs first can result in huge cost savings in money and time compared to manufacturing and testing of a multitude of “possible design” components.

b) The VLM approach can also be used in damage tolerance analysis methods without the need for the large initial crack assumption.  The use of Monte Carlo statistical techniques advance fracture mechanics to predict actual fatigue response and its scatter.  The method simulates fatigue crack growth starting at the naturally-occurring microstructural features (rather than an assumed large initial crack size) using the appropriate fracture mechanics processes (crack nucleation, short-crack growth, long-crack growth) to more accurately predict the fatigue durability of complex components experiencing complex loading.  This analysis can provide a more realistic (and less-conservative) life prediction, providing more value to manufactured components.

c) By integrating safe life with fracture mechanics, VLM creates a total lifetime prediction from the initially unflawed component to final failure. VLM simulates the material at the fundamental level to include all of the individual grains, voids and inclusions that exist in the material microstructure of the component. As viewed under a microscope, the microstructure of each component and each location in the component is different. VLM uses probabilistic methods to capture these differences by representing the material as space filling 3-D statistical volume elements. Structural finite element analysis methods are used to capture the complex stress and temperature variations throughout the 3-D component along with geometry and residual stresses. The finite element stresses and temperatures are combined with the material volume elements to simulate damage as it initiates and grows through the component. This process captures the similitude needed for safe life and predicts the number of cycles spent initiating a crack for damage tolerance; all using probabilistic methods to capture the uncertainties that exist in a fleet of components, and thus predicting the true probability of when components fail in the field.

We hope you have found this series on structural design principles informative.  Please contact us if you feel that your company could benefit from more-informed decision making capabilities in your products’ design process.