How do applied stresses and residual stresses interact?

Paper from ECOROLL AG Tool Technology

Surface properties - Subsurface properties - Residual stresses - Information/Know-how.

How to estimate the effect of residual stresses.

It is not always easy to estimate the effect of residual stresses during the design phase. It is generally known that residual stresses can significantly influence the service life of components. For this reason, processes such as deep rolling, machine hammer peening, and shot peening are used repeatedly. They generate the necessary compressive residual stresses in the subsurface area and are thus largely responsible for extending the service life of a dynamically loaded component. However, it is still difficult for design departments to take this into account in their calculations. In the following, we will explain how an initial rough estimate can be made and how residual stresses actually affect dynamic strength.

Residual stresses are internal stresses in the structure.
First, we need to understand what residual stresses actually are. Residual stresses are stresses in the microstructure of a component that are present even when no external forces, torques, or temperature gradients are acting on the component, i.e., when the component is completely free of any load. They can occur anywhere in the manufacturing chain and are the result of mechanical and thermal loads during the individual manufacturing steps. For example, so-called casting stresses can arise during casting because the component cools at different rates in different areas. They can arise during machining due to the strong thermo-mechanical stresses caused by the cutting edge, or they can be generated specifically by means of mechanical solidification processes.

As with residual stresses, intrinsic stresses are generally divided into tensile and compressive stresses. Tensile stresses are described mathematically by a positive figure, while compressive stresses are described by a negative figure. In general terms, it can be said that compressive stresses extend the service life of components, while tensile stresses shorten it.

The reason for this is the initiation of cracks by stress peaks. If the tensile stress in one part in a component is too great, a crack will form at this point. Initially, this crack is small, but it grows larger and larger with further loading until the component finally fails. In simple terms, tensile residual stresses pull on the crack, causing it to grow faster. Compressive residual stresses counteract crack propagation and thus slow down crack growth.

Superposition of load stresses and residual stresses.
Like all stresses, residual stresses can also be superimposed with load stresses. This can be done by simple superposition. In other words, the load and residual stresses are simply added together. The result is then a resulting stress.

We can easily understand this using the example of a uniaxial stress state, i.e., a bar. If a uniaxial tensile stress of sload = 600 MPa is applied to this component and it has no residual stress, then the resulting stress is still sres = 600 MPa. If, on the other hand, the bar is subjected to a compressive residual stress of sESP = -200 MPa in the same direction, then mathematically the 200 MPa is subtracted from the 600 MPa and the resulting stress is sRes = 400 MPa. A bar that would fail at 550 MPa, for example, could therefore be used in the second case, but not in the first case. This explains the effect of residual stresses in a comprehensible way.

But what about a multi-axial stress state? Here, of course, the concept must be applied to the entire stress tensor. In this case, the corresponding residual stress values must be used for each component of the stress tensor. For example, in the x-direction, the principal stress of s(x) must be calculated with the principal stresses in the x-direction. If this is done for all components as well as the shear stresses, the result is a complete stress tensor with principal stresses.*

In order to estimate the effect on strength, the concepts of equivalent stress can be applied. This compresses the stress tensor to a single stress value and allows it to be compared with the strength characteristics from the stress-strain diagram.

Of course, the method presented here is not a complete service life assessment, and further calculations or tests must always be carried out to estimate dynamic strength. However, the method presented here allows the effect of residual stresses to be roughly estimated. Effects such as additional strengthening of the microstructure or residual stress reduction during loading are not taken into account, however.

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* Mörke, T.: Randzonenanalyse zur Bestimmung mechanischer Belastungen im Lebenszyklus spanend gefertigter Bauteile. Doctoral thesis, Leibniz University Hannover, 2016.

@PresseBox @UnnGmbh #Ecoroll #Stress #Manufacturing

Control for hydrostatic propel applications.

Combines the behaviors of load-dependent and load-independent controls into one unit.

The launch of Hybrid Load Control, a displacement control designed for the company’s H1P and MP1P hydraulic pumps has been announced by Danfoss Power Solutions. Intended for dual-mode hydrostatic propel applications in mobile machinery, Hybrid Load Control combines the behaviors of two control types into one unit: load-independent behavior (also known as electric displacement control or EDC) and load-dependent behavior (also known as non-feedback proportional electric control or NFPE). The patented control eliminates performance compromises, increasing operator comfort and productivity.

“Hybrid Load Control is a groundbreaking addition to our H1P and MP1P family of controls, combining the best of EDC and NFPE. You no longer have to choose between the two — it's truly a 'no compromises' solution,” said Abhijit Patwa, Director, Medium Power Portfolio Management, Hydrostatics, Danfoss Power Solutions. “Hybrid Load Control offers unparalleled flexibility and performance that no other control or system on the market can match.” 

Many hydrostatically propelled machines — telehandlers, wheel loaders, street sweepers, and forestry equipment, for example — perform multiple tasks or have different operational modes. These machines benefit from both load-independent and load-dependent pump controls. Load-independent control delivers consistent speed and responsiveness regardless of the load or the terrain. Load-dependent control adapts to actual load conditions to deliver smooth hydrostatic braking and optimise power delivery. Until now, machine designers had to select only one control type for the machine’s hydrostatic pump. Danfoss’ Hybrid Load Control eliminates that compromise, delivering the best driving behavior regardless of the task. It improves hydrostatic braking smoothness in load-dependent mode, and it provides precise speed and instant response in load-independent mode. This superior driving performance increases operator comfort and productivity.

Hybrid Load Control is an integrated hardware/software solution offering machine designers the flexibility to define and optimise the vehicle’s driving behavior. The control works with the Danfoss PC036 microcontroller as part of the PC-PRO software package. The PLUS+1® compliant package enables the designer to set key parameters that determine the behavior, resulting in simple configuration.

Compared to conventional methods of achieving this dual behavior, Danfoss’ Hybrid Load Control improves system robustness, simplifies machine architecture, and minimises costs. The solution does not require additional sensors, thereby eliminating potential points of failure and additional costs. Reducing the number of components simplifies system design. In particular, the omission of sensors for complicated control strategies, such as swashplate angle or pressure sensors, eliminates the wire harness and calibration, thereby reducing startup time and service efforts.

Additionally, by optimising hydrostatic braking, the system minimises reliance on mechanical brakes. This reduction in mechanical brake usage decreases wear and further reduces total costs.

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@Danfoss @NapierPR #PAuto #Loads

Weighing the load!

The handling of dry bulk materials used to be a completely manual process, but it is now being increasingly automated in a drive to reduce both the costs and the risks of injury, while also increasing throughput and accuracy. Tony Ingham of Sensor Technology reports on some of the new technologies behind these developments.

The demands of today’s highly competitive commerce and modern industry are such that speed, accuracy and efficiency are required within every process. This includes the handling of dry bulk materials, such as grain, coal, aggregates, minerals and chemicals through every stage of their logistical journey from producer through processor and distributor to eventual end-user.

Not surprisingly, major handling facilities are now highly computerised, and often have a control room with a bank of computers, which would not look out of place on the USS Enterprise. Their job can be summed up as collecting information from many, many different sources, collating it, and then calculating the optimum procedures for every stage of the overall process.

The computers represent a well-developed technology and their data processing abilities, while hugely impressive, are based on relatively simple logic algorithms. The technology developments that are driving the advancement of bulk handling are at the sensing end of the system, where the prevailing conditions of the moment are detected and converted into data signals for the computer to use.

"Sensors are the eyes and ears of the computerised system,” says Mark Ingham of Sensor Technology Ltd, one of the companies at the forefront of developments. “They constantly monitor what is going on and feed real-time data to the computer." One of their latest products is a load sensor with a difference. Called LoadSense, it is wireless, so is ultra easy to deploy in situations like docks and grain banks where installing cables would be difficult – and maintaining them even harder!

LoadSense is online permanently and constantly sends real-time load value signals to either the central computer or its own local computer or receiver for preliminary analysis. It can be used in all types of materials handling operations, but is particularly useful for measuring loads in augers and on conveyors, where materials are ‘in flight’, so the load varies constantly with time.

LoadSense can be thought of as a two-part system. First, is a strain gauge-based stainless steel tension type sensor, with a twin antenna transmitter built into it for transferring the load data the instance it is generated. The second part of LoadSense is the receiver, which can be either fixed in place or handheld. The receiver reads, displays and records the data and can pass it onto the central control system for integration with other data streams for analysis.

The receiver includes an in-built 32MBit memory, which can hold up to 280 hours of data, thus providing both a local control station and a backup databank for the wider system. It is also notable that each receiver can collect data from several nearby load sensors simultaneously, in a range of up to 100m.

Importantly, the LoadSense transmits using the worldwide licence-free frequency of 2.4GHz, so can be installed and operated freely without disrupting other radio-based equipment. In operation, data is transmitted at up to 10 times a second, so LoadSense is constantly updating and providing real-time information so that operations can be optimised for best results.

The key benefits LoadSense brings to materials handling installations include: the simplicity of wireless installation, easy reconfiguration, its long battery life (which is complemented by easy recharging), and the dual antennas which represent just one part of the overall rugged design.

Technology transfer.
In fact, Sensor Technology originally developed LoadSense for use with helicopters, as Mark explains: “Helicopters often carry cargo in nets slung from cargo hooks on their underside, and it is important that the pilot knows its weight. A conventional load sensor in the hook could provide this data, but wiring it back to a readout in the cockpit would invalidate the craft’s Certificate of Airworthiness, implying the need for expensive recertification. When we heard about this dilemma, we instantly realised that a wireless solution was the answer!”

Since its development LoadSense has been adopted by many other industries, too. For instance, in forestry tree harvesting is often best done by pulling the trees over with a large tractor – this uproots them, so that the whole tree is recovered and also the ground is left stump-free so is in better condition for reuse – a LoadSense on the pulling cable will provide the tractor driver with vital live information and also collect data on harvest volumes for commercial analysis. They have also proved popular in the fly towers of theatres, allowing scenery and backdrops to be raised and lowered quickly and safely, for materials monitoring in road laying and civil engineering, in food production and processing, etc.

Sensor Technology has also transferred another of its technologies, TorqSense into the world of dry bulk materials handling. Like LoadSense, TorqSense uses a radio frequency signal transfer technique , but its sensing head measures the rotary torque in a turning shaft.

"Consider a screw conveyor or auger, both of which are driven by a rotating motor shaft," says Mark. "If this is rotating empty, it requires little power from its drive shaft. If it is half full, it requires rather more, and if it is completely full its needs a lot more.

"The same goes for speed – the faster, the more power consumed. Also, the denser the material being conveyed, the more power required. By constantly measuring the torque in the driveshaft, we can determine the volume and weight of material being conveyed."

Both LoadSense and TorqSense have been successfully used in dry bulk materials handling installations and have proved their worth many times over. As handling inevitably becomes more automated, the need for such technologies will become more and more crucial.

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@sensortech #MaterialHandling #Load

High Power Regenerative DC Load.

The ELR9000 HP Series of regenerative, programmable electronic DC loads has been introduced by Intepro Systems. This latest version of their ELR Series is designed to accommodate three-phase mains between 360V – 528V AC. The increased power density of HP Series units provides load ratings of up to 15kW in a single 3U rack-mounted instrument. A master-slave bus can be used to connect as many as 16 units in parallel to test loads of up to 240 kW. A unique feature of the ELR 9000 HP Series is an integral, grid-synchronized inverter designed to return up to 94.5% of the load test energy back to the grid. 

“The wide input voltage of the new ELR 9000 HP Series of electronic DC loads is especially important to our customers who have manufacturing facilities in multiple global locations,” said Gerard Sheehan, Intepro's Chief Technical Officer. “In addition, the regenerative output is even more valuable in these higher power density units, eliminating the need for expensive cooling systems used by conventional air- and water-cooled loads to dissipate energy as heat.”

The ELR 9000 HP Series offers four common regulation modes: constant voltage, constant current, constant resistance and constant power. The FPGA-based operational controls include a function generator and a table-based regulation circuit for the simulation of non-linear internal resistances. Intepro’s PowerStar 6 software package enables the creation of test routines using a drag-and-drop menu where coding can be performed without programming.  

@InteproSystems  #PAuto @proactivefleet

Load instrumentation for vehicle testing.

Mantracourt Electronics  has helped its Australian distributor to develop an exciting vehicle testing project.  This particular project is typical of how engineers at Mantracourt, in partnership with their customers, can provide working solutions to a wide variety of weighing and measurements applications.

The project was collaboration between Mantracourt and Applied Measurement Australia who were approached by the Australian Automotive Research Centre (AARC) to help renovate and update a Tilt Table that was used to test vehicle stability by tilting to an angle of 45º.

Mantracourt’s T24 Wireless Telemetry system was chosen for modernising the out-dated measurement system.  Initially it was the ease of set up and operation that attracted Applied Measurement to T24 but a wireless system was also more suitable for the application environment, bearing in mind that the operators have to perform tests outside in all weather conditions.

T24 is a wireless telemetry system for multiple data acquisition in real time. The range comprises a versatile set of products, based around proprietary acquisition devices that can be mixed and matched to provide high quality remote readings for critical applications. The low power, battery equipped, radio units can allow data collection over many years without externally supplied power and so are considerably more cost-effective than hardwired systems.

As the vehicles under test were on the tilt table, data from the load cells, which were connected to a T24-ACMi-VA (voltage acquisition module) and situated on the weigh pads under each wheel, was sent back wirelessly to a laptop equipped with a T24-BSue (USB receiver base station) and Mantracourt’s logging and mapping software. 

Applied Measurement also attached inclinometers on the tilt bridge.  Each inclinometer, which is integrated with a T24-ACMi-VA, measures the returned voltage which is proportional to the angle of the beam. Each beam was calibrated using a laboratory tilt table to + 45 degrees.  This information was also displayed during testing. 

All data from the test was logged every second during testing to allow for analysis and compilation of a report to be given to the end customer. 

Throughout the project technical support was available from Applied Measurement Australia and from the British based Mantracourt office.  In addition a Mantracourt engineer was also available onsite to assist with initial setup and calibration.

@mantracourt #Automotive #Australia

DC Electronic Load with Energy Recovery to AC Mains.

Intepro Systems has introduced the ELR9000 Series of electronic dc loads. A unique feature of the ELR9000 Series is an integral, grid-synchronized inverter designed to return up to 95% of the load test energy back to the grid. The rack-mounted unit is available, off-the shelf, with output ratings of 3.5 kW, 7 kW and 10.5 kW — scalable to 105 kW.

The ELR9000 Series offers four common regulation modes: constant voltage, constant current, constant resistance and constant power. The FPGA-based operational controls include a function generator and a table-based regulation circuit for the simulation of non-linear internal resistances. Intepro’s PowerStar 6 software package enables the creation of test routines using a drag-and-drop menu where coding can be performed without programming.

“Our new ELR9000 Series of electronic DC loads offers all the necessary features of today’s electronic loads — plus the added benefit of energy recovery to mains,” said Gerard Sheehan, Intepro's Chief Technical Officer. “Recovering the load energy reduces energy costs. It also offers a cost-effective alternative to the expensive cooling systems used by conventional air- and water-cooled loads to dissipate energy as heat.”

Intepro Systems offers electronic loads as stand-alone units or as complete test systems. Maintenance contracts and onsite calibrations are also available for its test systems.